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Sustainable Material Solutions for Solar Energy Technologies: Processing Techniques and Applications (Solar Cell Engineering) [1 ed.]
 0128215925, 9780128215920

Table of contents :
Front Cover
Sustainable Material Solutions for Solar Energy Technologies
Copyright Page
Contents
List of contributors
Preface
I. Trends in Materials Development for Solar Energy Applications
1 Bismuth-based nanomaterials for energy applications
1.1 Introduction
1.2 Photovoltaics
1.2.1 Solar Cell Operation
1.2.2 Nanoengineering
1.2.3 Bismuth-Based Nanomaterials
1.2.3.1 Bismuth-based Perovskites and Bismuth Halides
1.2.3.2 Bismuth Chalcogenides
1.2.4 Summary
1.3 Thermoelectric devices
1.3.1 Thermoelectric Devices Operation
1.3.2 Nanoengineering
1.3.3 Bi-Based Nanomaterials
1.3.3.1 Metallic bismuth
1.3.3.2 Bi2Te3 and (Bi,Sb)2(Te,Se)3 alloys
1.3.3.3 Bi2Se3 and Bi2S3
1.3.3.4 Ternary materials
1.3.4 Summary
1.4 Batteries & Supercapacitors
1.4.1 Battery Operation
1.4.2 Supercapacitor Operation
1.4.3 Bismuth-Based Electrodes
1.4.4 Nanoengineering
1.4.5 Coating or Mixing with Conductive Materials
1.4.6 Bismuth Perovskite Supercapacitors
1.4.7 Summary
1.5 Solar-hydrogen production
1.5.1 Fundamentals of photocatalysis for hydrogen production
1.5.2 Nanoengineering
1.5.3 Bi-based nanomaterials
1.5.3.1 Bismuth chalcogenides Bi2E3 (E = S, Se, Te)
1.5.3.2 Ternary Bismuth Chalcogenides (I-Bi-VI2)
1.5.3.3 Bismuth-based composite oxides
1.5.3.3.1 Bismuth oxides
1.5.3.3.2 Bismuth Oxyhalides BiOX (X= Cl, Br, I)
1.5.3.3.3 BiMO4 (M = P, V, Nb and Ta)
1.5.3.3.4 Aurivillius oxides Bi2MO6 (M = Cr, Mo and W)
1.5.4 Summary
1.6 Conclusions
Acknowledgements
References
2 Emergent materials and concepts for solar cell applications
2.1 Introduction
2.2 Perovskite solar cells
2.2.1 Historical review
2.2.2 Solar cells
2.2.3 Stability
2.2.4 Scaling up and possibilities for commercialization
2.3 III–V semiconductor materials for multijunction solar cells applications
2.3.1 Historical review
2.3.2 Some basics of multijunction solar cells
2.3.3 III–V materials for photovoltaic applications
2.3.4 Selected examples
2.3.4.1 Bonded lattice matched structures
2.3.4.2 Inverted metamorphic lattice mismatched structures
2.3.5 Discussion
2.4 Final remarks and future perspectives
References
3 Novel dielectrics compounds grown by atomic layer deposition as sustainable materials for chalcogenides thin-films photov...
3.1 Introduction
3.2 Atomic layer deposition technique
3.2.1 Requirements for ideal precursors and atomic layer deposition signature quality
3.2.2 Commercial and research tools
3.3 Atomic layer deposition applied on chalcogenides thin films technologies
3.3.1 Absorber layers: Cu(In,Ga)Se2, Cu2ZnSnS4, and Cu2ZnSn(S,Se)4
3.3.1.1 Chalcopyrite thin films: mature level
3.3.1.2 Kesterite thin films: under development level
3.3.2 Sustainable buffer layers based on atomic layer deposition
3.3.3 Sustainable passivation layers based on atomic layer deposition
3.4 Final remarks
Acknowledgments
References
4 First principles methods for solar energy harvesting materials
4.1 Introduction
4.2 Fundamental concepts
4.2.1 Crystalline representation
4.2.2 The multielectron system
4.2.3 The variational principle
4.2.4 The universal functional of the density
4.2.5 The auxiliary Kohn-Sham system
4.3 Selected materials with solar energy harvesting implementations
4.3.1 The input file
4.3.2 A supercell of zinc oxide
4.3.3 Structural stability of FAPbI3 perovskites
4.3.4 Charge order and half metallicity of Fe3O4
4.3.5 Optimization of anatase titanium dioxide
4.3.6 A conventional and a reduced representation of mBiVO4
4.3.7 A template structure for chalcopyrite
4.4 Conclusion
References
II. Sustainable Materials for Photovoltaics
5 Introduction to photovoltaics and alternative materials for silicon in photovoltaic energy conversion
5.1 Introduction
5.2 Current status of photovoltaics
5.3 Fundamental properties of photovoltaics semiconductors
5.3.1 Crystal structure of semiconductors
5.3.2 Energy band structure
5.3.3 Density of energy states
5.3.4 Drift-motion due to the electric field
5.3.4.1 Drift velocity
5.3.4.2 Mobility of carriers
5.3.4.3 The resistivity of charge carriers
5.3.5 Diffusion-due to a concentration gradient
5.3.6 Absorption coefficient
5.4 Physics of solar cell
5.4.1 Homojunction and heterojunction structure
5.4.2 p-n junction under illumination
5.4.3 I-V equations of solar cell
5.4.3.1 Short circuit current Isc
5.4.3.2 Open circuit voltage Voc
5.4.3.3 Fill factor
5.4.3.4 Efficiency
5.5 Categories of the photovoltaic market
5.6 Commercialization of Si solar cells
5.7 Status of alternative photovoltaics materials
5.8 Thin film technology
5.9 Material selection in thin film technology
5.10 Thin film deposition techniques
5.10.1 Physical deposition
5.10.1.1 Evaporation techniques
5.10.1.1.1 Vacuum thermal evaporation
5.10.1.1.2 Electron beam evaporation
5.10.1.1.3 Laser beam evaporation/pulsed laser deposition
5.10.1.1.4 Arc evaporation
5.10.1.1.5 Molecular beam epitaxy
5.10.1.2 Sputtering techniques
5.10.2 Chemical deposition
5.10.2.1 Sol-gel technique
5.10.2.2 Chemical bath deposition
5.10.2.3 Spray pyrolysis technique
5.10.2.4 Chemical vapor deposition
5.10.2.4.1 Low pressure and atmospheric pressure chemical vapor deposition
5.10.2.4.2 Plasma enhanced chemical vapor deposition
5.10.2.4.3 Hot wire chemical vapor deposition
5.10.2.4.4 Ion assisted deposition
5.11 Copper indium gallium selenide-based solar cell
5.11.1 Alkali metal postdeposition treatment on copper indium gallium selenide based solar cells
5.12 Cadmium telluride solar cells
5.13 Multijunction solar cells
5.14 Emerging solar cell technologies
5.14.1 Organic solar cells
5.14.2 Dye-sensitized solar cells
5.14.3 Perovskite solar cells
5.14.4 Quantum dot solar cells
5.15 Summary, conclusions, and outlook
Acknowledgment
References
6 An overview on ferroelectric photovoltaic materials
6.1 Overview
6.2 Ferroelectric materials
6.3 Photovoltaic effect
6.3.1 Mechanism of ferroelectric photovoltaic
6.3.2 History and current status of ferroelectric photovoltaic
6.4 Barium titanate
6.4.1 Crystal structure
6.4.2 Dielectric properties
6.4.3 Ferroelectric phenomena in BaTiO3
6.4.4 Optical properties
6.4.5 Various techniques of depositing BaTiO3 thin film
6.4.6 Potential applications of BaTiO3
6.5 Bismuth ferrite
6.6 Conclusion
Acknowledgments
References
7 Nanostructured materials for high efficiency solar cells
7.1 Introduction
7.2 Nanostructures and quantum mechanics
7.3 Quantum wells in solar cells
7.4 Quantum wires (nanowires) in solar cells
7.5 Quantum dots in solar cells
7.5.1 InAs quantum dots on GaAs
7.5.2 In(Ga)As or InAsP quantum dots on wide bandgap material barriers
7.6 Conclusions
Acknowledgments
References
8 Crystalline-silicon heterojunction solar cells with graphene incorporation
8.1 Heterojunction solar cells and graphene
8.1.1 Heterojunction solar cells
8.1.2 Graphene
8.2 Fabrication of silicon heterojunction solar cell
8.2.1 Surface patterning and surface cleaning
8.2.2 Deposition of a-silicon:H layers
8.2.3 Deposition of transparent conductive oxide
8.2.4 Metallization
8.2.5 Thermal treatment
8.3 Synthesis of graphene
8.3.1 Incorporating graphene into silicon heterojunction solar cells
8.4 Conclusion
Acknowledgment
References
9 Tin halide perovskites for efficient lead-free solar cells
9.1 Introduction
9.2 Halide perovskite solar cells: why tin?
9.2.1 Perovskite structure
9.2.2 Carrier transport and tin halide perovskite defects
9.2.3 Tin perovskite bandgap
9.2.4 Tin oxidation
9.2.5 Tin toxicity
9.3 ASnX3: a brief historical excursus
9.4 Toward efficient and stable ASnX3 PSCs
9.4.1 Additives
9.4.1.1 Tin containing additives: SnX2 and Sn
9.4.1.2 Reducing agents
9.4.2 Passivation
9.4.3 Low dimensional perovskites
9.4.4 Solvent
9.5 Conclusion
References
III. Sustainable Materials for Photocatalysis and Water Splitting
10 Photocatalysis using bismuth-based heterostructured nanomaterials for visible light harvesting
10.1 Introduction
10.2 Fundamentals of heterogeneous photocatalysis
10.2.1 Heterogeneous photocatalysis applied to environmental engineering processes
10.2.2 Factors affecting the photocatalytic process
10.2.2.1 Physical properties
10.2.2.2 (Photo)electrochemical properties
10.2.2.3 The matrix composition
10.2.3 Insights of physicochemical characterization of nanophotocatalysts
10.3 Bismuth-based heterostructures for photocatalytic applications
10.3.1 Semiconductor-semiconductor heterostructures using bismuth-based materials
10.3.2 General strategies for synthesis of bismuth-based semiconductors
10.3.2.1 Sol-gel synthesis
10.3.2.2 Hydrothermal/solvo thermal synthesis
10.3.2.3 Ball milling process
10.3.2.4 Sputtering process
10.3.3 Applications of bismuth-based heterostructures
10.3.3.1 Water treatment
10.3.3.2 Self-cleaning
10.3.3.3 Water splitting
10.4 Conclusions
Acknowledgments
References
11 Recent advances in 2D MXene-based heterostructured photocatalytic materials
11.1 Introduction
11.2 Synthesis of 2D-MXenes
11.2.1 Functionalization and electronic properties of MXene
11.3 Photocatalytic applications
11.3.1 H2 evolution by H2O splitting
11.3.1.1 Water splitting activity of MXenes
11.3.1.2 MXene-based heterojunctions
11.3.1.2.1 2D/2D composites
11.3.1.2.2 2D/3D composites
11.3.1.2.3 Doped MXene
11.3.1.2.4 Tertiary composite system
11.3.1.2.5 Electrochemical water splitting
11.3.2 Photocatalytic CO2 reduction to fuel
11.3.3 Environmental applications
11.3.3.1 Organic degradation
11.3.3.2 Photoreduction process
11.3.3.3 MXene for antimicrobial activity
11.4 Conclusion and future prospects
Acknowledgments
References
12 Atomic layer deposition of materials for solar water splitting
12.1 Introduction
12.2 Solar energy
12.3 Photoelectrochemical cells
12.4 Hydrogen generation from water photoelectrolysis
12.5 Materials for photoelectrode
12.6 Atomic layer deposition technique: process and equipment
12.6.1 Atomic layer deposition process
12.6.2 Atomic layer deposition reactors: types and characteristics
12.7 Final remarks
Acknowledgments
References
IV. Sustainable Materials for Thermal Energy Systems
13 Solar selective coatings and materials for high-temperature solar thermal applications
13.1 Introduction
13.1.1 Concentrated solar power: facts
13.1.2 Concentrated solar power: basics
13.2 CSP efficiency considerations: the concept of solar selectivity
13.3 State-of-the-art review of solar absorber surfaces and materials for high-temperature applications (%3e 565°C in air)
13.3.1 Absorber paints
13.3.2 Solar selective coatings
13.3.2.1 Intrinsic absorber
13.3.2.2 Metal-semiconductor tandem stack
13.3.2.3 Textured surface absorber
13.3.2.4 Multilayer absorber
13.3.2.5 Metal-cermet coatings
13.3.3 Volumetric receivers
13.4 Current trends and issues
13.4.1 Durability studies of solar absorbers
13.4.2 Lack of standardized characterization protocols
13.5 Roadmap for concentrated solar power absorbing surfaces and materials
13.5.1 Alternative concentrated solar power absorbing surfaces: selectively solar-transmitting coatings
13.5.2 Industrialization of high-temperature solar selective coatings
Acknowledgments
References
14 Applications of wastes based on inorganic salts as low-cost thermal energy storage materials
14.1 Introduction
14.2 Thermal energy storage
14.2.1 Sensible, latent and thermochemical heat storage
14.2.1.1 Sensible heat storage
14.2.1.2 Latent heat storage
14.2.1.3 Chemical reaction/thermochemical heat storage
14.2.2 Basic concepts for thermal energy storage materials
14.2.3 Overview of thermal energy storage system types
14.2.4 Comparison of energy storage density for different thermal energy storage materials
14.3 Overview of industrial waste studied as thermal energy storage materials
14.4 Inorganic salt-based products and wastes as low-cost materials for sustainable thermal energy storage
14.4.1 Availability and abundance of inorganic salts in Northern Chile
14.4.2 Economic analysis of inorganic salts as low-cost thermal energy storage materials
14.4.3 State-of-art of currently proposed by-products and wastes as thermal energy storage materials
14.4.3.1 Sensible heat storage materials
14.4.3.2 Latent heat storage materials
14.4.3.3 Thermochemical storage materials
14.5 Challenges for the application of waste and by-products in thermal energy storage systems
14.5.1 Proposed uses of wastes as thermal energy storage materials
14.5.2 Challenges for the application of inorganic salt-based wastes in thermal energy storage systems
14.5.3 Optimization of thermal properties of thermal energy storage materials based on inorganic salt wastes
14.5.3.1 Encapsulation of latent heat storage materials
14.5.3.2 Use of additives
14.5.3.3 Graphite, enhancing thermal conductivity
14.6 Conclusion
References
15 Nanoencapsulated phase change materials for solar thermal energy storage
15.1 Introduction
15.1.1 Selection criteria of phase change materials
15.1.2 Working principle of phase change material
15.1.3 Encapsulation in phase change materials
15.1.4 Advantages of micro or nanoencapsulation of phase change material
15.2 Brief review of the work done
15.3 Results and discussion
15.4 Applications
15.4.1 Need for phase change material-based solar air heaters
15.4.1.1 Phase change materials in solar air heaters
15.4.1.2 Construction and working principle of solar-air heating systems
15.4.1.3 Deliverables: Performance criteria for solar-air heating
15.4.2 Need for phase change material-based building materials for rural houses
15.4.2.1 Phase change materials for building applications
15.4.2.2 Deliverables: performance criteria for phase change materials for building applications
15.4.3 Need for phase change material-based textiles
15.4.3.1 Phase change materials in textiles
15.5 Challenges ahead
15.6 Conclusions
Acknowledgments
References
Further reading
V. Sustainable Carbon-Based and Biomaterials for Solar Energy Applications
16 Carbon nanodot integrated solar energy devices
16.1 Introduction
16.2 Carbon nanodot integrated solar energy devices
16.2.1 Dye-sensitized solar cells
16.2.1.1 Carbon dots as sensitizer in dye-sensitized solar cells
16.2.1.2 Carbon dots modified photoanodes in dye-sensitized solar cells
16.2.1.3 Carbon dots as counter electrode in dye-sensitized solar cells
16.2.2 Quantum dot solar cells
16.2.3 Organic solar cells
16.2.4 Polymer solar cells
16.2.5 Perovskite solar cells
16.3 Summary and future aspects
Acknowledgments
References
17 Solar cell based on carbon and graphene nanomaterials
17.1 Introduction
17.2 Carbon and its derivatives
17.2.1 Fullerene
17.2.2 Carbon nanotube
17.2.3 Graphene
17.3 Solar cells based on carbon nanomaterials
17.3.1 Carbon in dye-sensitized solar
17.3.2 Carbon in organic solar cells
17.3.3 Carbon in perovskite solar cells
17.4 Challenges and prospects
References
18 Sustainable biomaterials for solar energy technologies
18.1 Introduction
18.2 Structural properties of biomaterials
18.3 Biomaterials used in biophotovoltaics
18.3.1 Living organism based solar cell systems
18.3.1.1 Algae and cyanobacteria
18.3.1.2 Plants
18.3.1.3 Bioengineered bacteria
18.3.2 Light-harvesting proteins
18.3.2.1 Green fluorescent protein
18.3.2.2 Bacteriorhodopsin
18.3.2.3 Artificial photosynthetic devices
18.3.2.4 Protein pigment complexes from Rhodopseudomonaspalustris CQV97 and Rhodobacter azotoformans R7
18.3.2.5 Peptide
18.3.3 Natural pigments
18.3.3.1 Carotenoids
18.3.3.2 Lycopene
18.3.3.3 Flavin
18.3.3.4 Xanthophylls from Hymenobacter sp. (Antarctica bacteria)
18.3.3.5 Chromatophores from Rhodospirillum rubrum S1 biological redox
18.3.3.6 Chlorophyll a derived Spirulina xanthin carotenoid in Spirulina platensis
References
19 Bioinspired solar cells: contribution of biology to light harvesting systems
19.1 Introduction
19.2 Methodologies for engineered biomimicry
19.2.1 Bioinspiration
19.2.1.1 Function
19.2.1.2 Simplicity
19.2.1.3 Dissipation
19.2.1.4 Soft matter
19.2.1.5 Scientific impact
19.2.2 Biomimetic
19.2.3 Bioreplication
19.3 Bioinspired solar cells
19.4 Bioinspired structures and organisms
19.4.1 Dyes
19.4.2 Wettability and superhydrophobic dyes
19.4.3 Organisms
19.4.3.1 Common rose butterfly
19.4.3.2 Leaf
19.4.3.3 Lotus
19.4.3.4 Firefly
19.4.3.5 Human eye
19.4.3.6 Beetle
19.4.3.7 Dipteran
19.4.3.8 Crab
19.5 Biological processes for bioinspiration
19.5.1 Photosynthesis
19.5.1.1 Artificial photosynthesis
19.5.2 Cyanobacteria
19.5.3 Bioinspired chromophores
19.6 Physics in biological systems
19.6.1 Coherence effects in biological systems
19.6.2 Excitation energy transfer
19.6.3 Charge transfer
19.7 Structures
19.7.1 Origami structures
19.7.2 Graphene
19.7.3 Multijunction solar cells
19.7.4 Perovskite solar cells
19.7.5 Silicon-based solar cell
19.7.6 Dye-sensitized solar cell technology
19.7.7 Thin film solar cell
19.8 Conclusions
References
Index
Back Cover

Citation preview

Sustainable Material Solutions for Solar Energy Technologies Processing Techniques and Applications

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Solar Cell Engineering

Sustainable Material Solutions for Solar Energy Technologies Processing Techniques and Applications

Edited by

Mariana Amorim Fraga Instituto de Cieˆncia e Tecnologia, Universidade Federal de Sa˜o Paulo, Sa˜o Jose´ dos Campos, Brazil

Delaina Amos University of Louisville, Louisville, KY, United States

Savas Sonmezoglu Karamanoglu Mehmetbey University, Karaman, Turkey

Velumani Subramaniam Department of Electrical Engineering (SEES), Centro de Investigacio´n y de Estudios Avanzados del IPN (CINVESTAV-IPN), Mexico City, Mexico

Elsevier Radarweg 29, PO Box 211, 1000 AE Amsterdam, Netherlands The Boulevard, Langford Lane, Kidlington, Oxford OX5 1GB, United Kingdom 50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States Copyright © 2021 Elsevier Inc. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein). Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the Library of Congress ISBN: 978-0-12-821592-0 For Information on all Elsevier publications visit our website at https://www.elsevier.com/books-and-journals

Publisher: Matthew Deans Acquisitions Editor: Christina Gifford Editorial Project Manager: John Leonard Production Project Manager: Sojan P. Pazhayattil Cover Designer: Mark Rogers Typeset by MPS Limited, Chennai, India

Contents List of contributors Preface

xv xix

Section I Trends in materials development for solar energy applications 1.

Bismuth-based nanomaterials for energy applications

3

Nichole C. Cates, Jessica C. Ramirez de la Torre, Sergio Aina, M. Pilar Lobera and Mar´ıa Bernechea 1.1 Introduction 1.2 Photovoltaics 1.2.1 Solar Cell Operation 1.2.2 Nanoengineering 1.2.3 Bismuth-Based Nanomaterials 1.2.4 Summary 1.3 Thermoelectric devices 1.3.1 Thermoelectric Devices Operation 1.3.2 Nanoengineering 1.3.3 Bi-Based Nanomaterials 1.3.4 Summary 1.4 Batteries & Supercapacitors 1.4.1 Battery Operation 1.4.2 Supercapacitor Operation 1.4.3 Bismuth-Based Electrodes 1.4.4 Nanoengineering 1.4.5 Coating or Mixing with Conductive Materials 1.4.6 Bismuth Perovskite Supercapacitors 1.4.7 Summary 1.5 Solar-hydrogen production 1.5.1 Fundamentals of photocatalysis for hydrogen production 1.5.2 Nanoengineering 1.5.3 Bi-based nanomaterials 1.5.4 Summary 1.6 Conclusions Acknowledgements References

3 4 4 6 7 9 9 9 11 12 16 16 16 17 17 18 20 21 21 22 22 24 24 29 29 30 30 v

vi

2.

Contents

Emergent materials and concepts for solar cell applications

37

Mar´ıa Dolores Perez and Juan Pla´ 37 40 40 41 45 49

2.1 Introduction 2.2 Perovskite solar cells 2.2.1 Historical review 2.2.2 Solar cells 2.2.3 Stability 2.2.4 Scaling up and possibilities for commercialization 2.3 IIIV semiconductor materials for multijunction solar cells applications 2.3.1 Historical review 2.3.2 Some basics of multijunction solar cells 2.3.3 IIIV materials for photovoltaic applications 2.3.4 Selected examples 2.3.5 Discussion 2.4 Final remarks and future perspectives References

3.

50 50 53 55 58 61 62 63

Novel dielectrics compounds grown by atomic layer deposition as sustainable materials for chalcogenides thin-films photovoltaics technologies

71

William Chiappim Junior, Leandro X. Moreno, Rodrigo Savio Pessoa, Anto´nio F. da Cunha, Pedro M.P. Salome´ and Joaquim P. Leita˜o 3.1 Introduction 3.2 Atomic layer deposition technique 3.2.1 Requirements for ideal precursors and atomic layer deposition signature quality 3.2.2 Commercial and research tools 3.3 Atomic layer deposition applied on chalcogenides thin films technologies 3.3.1 Absorber layers: Cu(In,Ga)Se2, Cu2ZnSnS4, and Cu2ZnSn(S,Se)4 3.3.2 Sustainable buffer layers based on atomic layer deposition 3.3.3 Sustainable passivation layers based on atomic layer deposition 3.4 Final remarks Acknowledgments References

4.

First principles methods for solar energy harvesting materials

71 78 79 82 84 85 87 88 89 90 90

101

J.J. R´ıos-Ram´ırez and Velumani Subramaniam 4.1 Introduction

101

Contents

4.2 Fundamental concepts 4.2.1 Crystalline representation 4.2.2 The multielectron system 4.2.3 The variational principle 4.2.4 The universal functional of the density 4.2.5 The auxiliary Kohn-Sham system 4.3 Selected materials with solar energy harvesting implementations 4.3.1 The input file 4.3.2 A supercell of zinc oxide 4.3.3 Structural stability of FAPbI3 perovskites 4.3.4 Charge order and half metallicity of Fe3O4 4.3.5 Optimization of anatase titanium dioxide 4.3.6 A conventional and a reduced representation of mBiVO4 4.3.7 A template structure for chalcopyrite 4.4 Conclusion References

vii 103 103 107 111 113 116 118 118 121 122 122 123 125 126 127 127

Section II Sustainable materials for photovoltaics 5.

Introduction to photovoltaics and alternative materials for silicon in photovoltaic energy conversion 131 Ganesh Regmi and Velumani Subramaniam 5.1 Introduction 5.2 Current status of photovoltaics 5.3 Fundamental properties of photovoltaics semiconductors 5.3.1 Crystal structure of semiconductors 5.3.2 Energy band structure 5.3.3 Density of energy states 5.3.4 Drift-motion due to the electric field 5.3.5 Diffusion-due to a concentration gradient 5.3.6 Absorption coefficient 5.4 Physics of solar cell 5.4.1 Homojunction and heterojunction structure 5.4.2 p-n junction under illumination 5.4.3 I-V equations of solar cell 5.5 Categories of the photovoltaic market 5.6 Commercialization of Si solar cells 5.7 Status of alternative photovoltaics materials 5.8 Thin film technology 5.9 Material selection in thin film technology 5.10 Thin film deposition techniques 5.10.1 Physical deposition 5.10.2 Chemical deposition

131 133 136 136 137 139 142 143 144 145 146 147 149 151 152 153 154 157 158 158 160

viii

6.

Contents

162

5.11 Copper indium gallium selenide-based solar cell 5.11.1 Alkali metal postdeposition treatment on copper indium gallium selenide based solar cells 5.12 Cadmium telluride solar cells 5.13 Multijunction solar cells 5.14 Emerging solar cell technologies 5.14.1 Organic solar cells 5.14.2 Dye-sensitized solar cells 5.14.3 Perovskite solar cells 5.14.4 Quantum dot solar cells 5.15 Summary, conclusions, and outlook Acknowledgment References

163 164 165 165 166 166 168 168 169 170 170

An overview on ferroelectric photovoltaic materials

175

Savita Sharma 6.1 Overview 6.2 Ferroelectric materials 6.3 Photovoltaic effect 6.3.1 Mechanism of ferroelectric photovoltaic 6.3.2 History and current status of ferroelectric photovoltaic 6.4 Barium titanate 6.4.1 Crystal structure 6.4.2 Dielectric properties 6.4.3 Ferroelectric phenomena in BaTiO3 6.4.4 Optical properties 6.4.5 Various techniques of depositing BaTiO3 thin film 6.4.6 Potential applications of BaTiO3 6.5 Bismuth ferrite 6.6 Conclusion Acknowledgments References

7.

175 176 178 179 186 187 187 190 190 191 191 192 194 195 196 196

Nanostructured materials for high efficiency solar cells 201 Daniel N. Micha, Roberto Jakomin, Rudy M.S. Kawabata, Mauricio P. Pires, Fernando A. Ponce and Patr´ıcia L. Souza 7.1 7.2 7.3 7.4 7.5

Introduction Nanostructures and quantum mechanics Quantum wells in solar cells Quantum wires (nanowires) in solar cells Quantum dots in solar cells 7.5.1 InAs quantum dots on GaAs 7.5.2 In(Ga)As or InAsP quantum dots on wide bandgap material barriers

201 203 205 210 214 216 221

Contents

8.

ix

7.6 Conclusions Acknowledgments References

222 223 223

Crystalline-silicon heterojunction solar cells with graphene incorporation

229

Recep Zan, Ali Altuntepe, Tolga Altan and Ayse Seyhan 8.1 Heterojunction solar cells and graphene 8.1.1 Heterojunction solar cells 8.1.2 Graphene 8.2 Fabrication of silicon heterojunction solar cell 8.2.1 Surface patterning and surface cleaning 8.2.2 Deposition of a-silicon:H layers 8.2.3 Deposition of transparent conductive oxide 8.2.4 Metallization 8.2.5 Thermal treatment 8.3 Synthesis of graphene 8.3.1 Incorporating graphene into silicon heterojunction solar cells 8.4 Conclusion Acknowledgment References

9.

Tin halide perovskites for efficient lead-free solar cells

229 229 232 234 235 237 240 242 243 244 249 250 251 251

259

Giuseppe Nasti, Diego Di Girolamo and Antonio Abate 9.1 Introduction 9.2 Halide perovskite solar cells: why tin? 9.2.1 Perovskite structure 9.2.2 Carrier transport and tin halide perovskite defects 9.2.3 Tin perovskite bandgap 9.2.4 Tin oxidation 9.2.5 Tin toxicity 9.3 ASnX3: a brief historical excursus 9.4 Toward efficient and stable ASnX3 PSCs 9.4.1 Additives 9.4.2 Passivation 9.4.3 Low dimensional perovskites 9.4.4 Solvent 9.5 Conclusion References

259 263 263 266 267 269 271 272 274 274 277 279 280 281 281

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Contents

Section III Sustainable materials for photocatalysis and water splitting 10. Photocatalysis using bismuth-based heterostructured nanomaterials for visible light harvesting

289

´ ´ lvarez, Araceli Romero-Nun˜ez, K.T. Drisya, Juan Carlos Duran-A ´ Myriam Sol´ıs-Lopez and Velumani Subramaniam 10.1 Introduction 10.2 Fundamentals of heterogeneous photocatalysis 10.2.1 Heterogeneous photocatalysis applied to environmental engineering processes 10.2.2 Factors affecting the photocatalytic process 10.2.3 Insights of physicochemical characterization of nanophotocatalysts 10.3 Bismuth-based heterostructures for photocatalytic applications 10.3.1 Semiconductor-semiconductor heterostructures using bismuth-based materials 10.3.2 General strategies for synthesis of bismuth-based semiconductors 10.3.3 Applications of bismuth-based heterostructures 10.4 Conclusions Acknowledgments References

289 291 294 295 297 299 301 303 309 320 321 321

11. Recent advances in 2D MXene-based heterostructured 329 photocatalytic materials Sudeshna Das Chakraborty, Pallab Bhattacharya and Trilochan Mishra 11.1 Introduction 11.2 Synthesis of 2D-MXenes 11.2.1 Functionalization and electronic properties of MXene 11.3 Photocatalytic applications 11.3.1 H2 evolution by H2O splitting 11.3.2 Photocatalytic CO2 reduction to fuel 11.3.3 Environmental applications 11.4 Conclusion and future prospects Acknowledgments References

329 331 333 334 336 345 347 355 356 356

Contents

12. Atomic layer deposition of materials for solar water splitting

xi

363

Rodrigo Savio Pessoa, William Chiappim Junior and Mariana Amorim Fraga 12.1 12.2 12.3 12.4 12.5 12.6

Introduction Solar energy Photoelectrochemical cells Hydrogen generation from water photoelectrolysis Materials for photoelectrode Atomic layer deposition technique: process and equipment 12.6.1 Atomic layer deposition process 12.6.2 Atomic layer deposition reactors: types and characteristics 12.7 Final remarks Acknowledgments References

363 366 367 368 369 373 373 375 376 376 376

Section IV Sustainable materials for thermal energy systems 13. Solar selective coatings and materials for high-temperature solar thermal applications

383

Ramo´n Escobar Galindo, Matthias Krause, K. Niranjan and Harish Barshilia 13.1 Introduction 13.1.1 Concentrated solar power: facts 13.1.2 Concentrated solar power: basics 13.2 CSP efficiency considerations: the concept of solar selectivity 13.3 State-of-the-art review of solar absorber surfaces and materials for high-temperature applications ( . 565 C in air) 13.3.1 Absorber paints 13.3.2 Solar selective coatings 13.3.3 Volumetric receivers 13.4 Current trends and issues 13.4.1 Durability studies of solar absorbers 13.4.2 Lack of standardized characterization protocols 13.5 Roadmap for concentrated solar power absorbing surfaces and materials 13.5.1 Alternative concentrated solar power absorbing surfaces: selectively solar-transmitting coatings 13.5.2 Industrialization of high-temperature solar selective coatings Acknowledgments References

383 383 388 392 395 395 397 402 405 405 407 409 409 413 417 418

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Contents

14. Applications of wastes based on inorganic salts as low-cost thermal energy storage materials

429

Svetlana Ushak, Yanio E. Milian, Paula E. Mar´ın and Mario Grageda 14.1 Introduction 14.2 Thermal energy storage 14.2.1 Sensible, latent and thermochemical heat storage 14.2.2 Basic concepts for thermal energy storage materials 14.2.3 Overview of thermal energy storage system types 14.2.4 Comparison of energy storage density for different thermal energy storage materials 14.3 Overview of industrial waste studied as thermal energy storage materials 14.4 Inorganic salt-based products and wastes as low-cost materials for sustainable thermal energy storage 14.4.1 Availability and abundance of inorganic salts in Northern Chile 14.4.2 Economic analysis of inorganic salts as low-cost thermal energy storage materials 14.4.3 State-of-art of currently proposed by-products and wastes as thermal energy storage materials 14.5 Challenges for the application of waste and by-products in thermal energy storage systems 14.5.1 Proposed uses of wastes as thermal energy storage materials 14.5.2 Challenges for the application of inorganic salt-based wastes in thermal energy storage systems 14.5.3 Optimization of thermal properties of thermal energy storage materials based on inorganic salt wastes 14.6 Conclusion References

15. Nanoencapsulated phase change materials for solar thermal energy storage

429 431 431 434 438 439 440 442 442 444 446 453 453 455 456 461 461

467

Jyoti Saroha, Sonali Mehra, Mahesh Kumar, Velumani Subramaniam and Shailesh Narain Sharma 15.1 Introduction 15.1.1 Selection criteria of phase change materials 15.1.2 Working principle of phase change material 15.1.3 Encapsulation in phase change materials 15.1.4 Advantages of micro or nanoencapsulation of phase change material 15.2 Brief review of the work done 15.3 Results and discussion 15.4 Applications

467 470 473 474 476 476 479 483

Contents

15.4.1 Need for phase change material-based solar air heaters 15.4.2 Need for phase change material-based building materials for rural houses 15.4.3 Need for phase change material-based textiles 15.5 Challenges ahead 15.6 Conclusions Acknowledgments References Further reading

xiii

484 486 488 490 490 491 491 494

Section V Sustainable carbon-based and biomaterials for solar energy applications 16. Carbon nanodot integrated solar energy devices

497

¨ zge Ala¸s and Ru¨kan Genc¸ Melis O 16.1 Introduction 16.2 Carbon nanodot integrated solar energy devices 16.2.1 Dye-sensitized solar cells 16.2.2 Quantum dot solar cells 16.2.3 Organic solar cells 16.2.4 Polymer solar cells 16.2.5 Perovskite solar cells 16.3 Summary and future aspects Acknowledgments References

17. Solar cell based on carbon and graphene nanomaterials

497 500 500 509 511 515 519 524 529 529

537

Abdellah Henni, Nesrine Harfouche, Amina Karar and Djamal Zerrouki 17.1 Introduction 17.2 Carbon and its derivatives 17.2.1 Fullerene 17.2.2 Carbon nanotube 17.2.3 Graphene 17.3 Solar cells based on carbon nanomaterials 17.3.1 Carbon in dye-sensitized solar 17.3.2 Carbon in organic solar cells 17.3.3 Carbon in perovskite solar cells 17.4 Challenges and prospects References

537 538 538 540 540 541 541 543 544 547 549

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Contents

18. Sustainable biomaterials for solar energy technologies 557 Yakup Ulusu, Numan Eczacioglu and Isa Gokce 18.1 Introduction 18.2 Structural properties of biomaterials 18.3 Biomaterials used in biophotovoltaics 18.3.1 Living organism based solar cell systems 18.3.2 Light-harvesting proteins 18.3.3 Natural pigments References

557 558 562 563 570 575 584

19. Bioinspired solar cells: contribution of biology to light harvesting systems

593

B. Gopal Krishna and Sanjay Tiwari 19.1 Introduction 19.2 Methodologies for engineered biomimicry 19.2.1 Bioinspiration 19.2.2 Biomimetic 19.2.3 Bioreplication 19.3 Bioinspired solar cells 19.4 Bioinspired structures and organisms 19.4.1 Dyes 19.4.2 Wettability and superhydrophobic dyes 19.4.3 Organisms 19.5 Biological processes for bioinspiration 19.5.1 Photosynthesis 19.5.2 Cyanobacteria 19.5.3 Bioinspired chromophores 19.6 Physics in biological systems 19.6.1 Coherence effects in biological systems 19.6.2 Excitation energy transfer 19.6.3 Charge transfer 19.7 Structures 19.7.1 Origami structures 19.7.2 Graphene 19.7.3 Multijunction solar cells 19.7.4 Perovskite solar cells 19.7.5 Silicon-based solar cell 19.7.6 Dye-sensitized solar cell technology 19.7.7 Thin film solar cell 19.8 Conclusions References Index

593 595 595 596 597 597 601 601 603 603 611 611 614 616 616 616 617 618 620 620 620 620 620 621 622 623 623 625 633

List of contributors Antonio Abate Department of Chemical, Materials and Production Engineering, University of Naples Federico II, Naples, Italy Sergio Aina Instituto de Nanociencia y Materiales de Arago´n (INMA), CSICUniversidad de Zaragoza, Zaragoza, Spain; Department of Chemical and Environmental Engineering (IQTMA), University of Zaragoza, Zaragoza, Spain ¨ zge Alas¸ Chemical Engineering Department, Faculty of Engineering, Mersin Melis O University, Mersin, Turkey ¨ mer Tolga Altan Nanotechnology Application and Research Center, Nig˘de O Halisdemir University, Nig˘de, Turkey; Department of Mechanical Engineering, ¨ mer Halisdemir University, Nig˘de, Turkey Nig˘de O ¨ mer Ali Altuntepe Nanotechnology Application and Research Center, Nig˘de O Halisdemir University, Nig˘de, Turkey Harish Barshilia Nanomaterials Research Laboratory, Surface Engineering Division, CSIR-National Aerospace Laboratories, Bangalore, India Marı´a Bernechea Instituto de Nanociencia y Materiales de Arago´n (INMA), CSICUniversidad de Zaragoza, Zaragoza, Spain; Department of Chemical and Environmental Engineering (IQTMA), University of Zaragoza, Zaragoza, Spain; Networking Research Center on Bioengineering, Biomaterials and Nanomedicine, Madrid, Spain; ARAID, Government of Aragon, Zaragoza, Spain Pallab Bhattacharya Functional Material Group, AMP Division, CSIR-National Metallurgical Laboratory, Jamshedpur, India Nichole C. Cates Smart Material Solutions, Inc, Raleigh, NC, United States of America William Chiappim Junior i3N and Department of Physics, University of Aveiro, Aveiro, Portugal; Plasmas and Processes Laboratory, Aeronautics Institute of Technology, Sa˜o Jose´ dos Campos, Brazil Anto´nio F. da Cunha i3N and Department of Physics, University of Aveiro, Aveiro, Portugal Sudeshna Das Chakraborty Functional Material Group, AMP Division, CSIRNational Metallurgical Laboratory, Jamshedpur, India K.T. Drisya Departamento de Ingenier´ıa Ele´ctrica, Centro de Investigacio´n y de Estudios Avanzados del Instituto Polite´cnico Nacional, Mexico City, Mexico ´ lvarez Instituto de Ciencias Aplicadas y Tecnolog´ıa, Juan Carlos Dur´an-A Universidad Nacional Auto´noma de Me´xico, Mexico City, Mexico

xv

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List of contributors

Numan Eczacioglu Department of Bioengineering, Faculty of Engineering, Karamanoglu Mehmetbey University, Karaman, Turkey Mariana Amorim Fraga Institute of Science and Technology, Federal University of Sa˜o Paulo, Sa˜o Jose´ dos Campos, Brazil Ramo´n Escobar Galindo Applied Physics I Department, Higher Polytechnic School (EPS), University of Seville, Spain Ru¨kan Genc¸ Chemical Engineering Department, Faculty of Engineering, Mersin University, Mersin, Turkey Diego Di Girolamo Department of Chemical, Materials and Production Engineering, University of Naples Federico II, Naples, Italy Isa Gokce Department of Bioengineering, Faculty of Natural Sciences and Engineering, Tokat Gaziosmanpasa University, Tokat, Turkey B. Gopal Krishna Photonics Research Laboratory, School of Studies in Electronics & Photonics Pt. Ravishankar Shukla University, Raipur, India Mario Grageda Center for Advanced Study of Lithium and Industrial Minerals (CELiMIN), University of Antofagasta, Antofagasta, Chile Nesrine Harfouche Polymer Materials Interfaces Marine Environment, University of South Toulon, Toulon, France Abdellah Henni Laboratory Dynamic Interactions and Reactivity of Systems, Kasdi Merbah University, Ouargla, Algeria Roberto Jakomin Campus Duque de Caxias, Universidade Federal do Rio de Janeiro, Duque de Caxias, Brazil Amina Karar Laboratory Dynamic Interactions and Reactivity of Systems, Kasdi Merbah University, Ouargla, Algeria Rudy M.S. Kawabata Semiconductor Laboratory, Pontifı´cia Universidade Cato´lica do Rio de Janeiro, Rio de Janeiro, Brazil Matthias Krause Helmholtz-Zentrum Dresden-Rossendorf, Institute for Ion Beam Physics and Materials Research, Dresden, Germany Mahesh Kumar Council of Scientific and Industrial Research (CSIR)-National Physical Laboratory (NPL), New Delhi, India; Academy of Scientific and Innovative Research (AcSIR), Ghaziabad, Uttar Pradesh, India Joaquim P. Leita˜o i3N and Department of Physics, University of Aveiro, Aveiro, Portugal M. Pilar Lobera Instituto de Nanociencia y Materiales de Arago´n (INMA), CSICUniversidad de Zaragoza, Zaragoza, Spain; Department of Chemical and Environmental Engineering (IQTMA), University of Zaragoza, Zaragoza, Spain; Networking Research Center on Bioengineering, Biomaterials and Nanomedicine, Madrid, Spain Paula E. Marı´n Sustainable Thermal Energy Technologies (STET), University of Warwick, Coventry, United Kingdom

List of contributors

xvii

Sonali Mehra Council of Scientific and Industrial Research (CSIR)-National Physical Laboratory (NPL), New Delhi, India; Academy of Scientific and Innovative Research (AcSIR), Ghaziabad, Uttar Pradesh, India Daniel N. Micha Department of Physics, Centro Federal de Educac¸a˜o Tecnolo´gica Celso Suckow da Fonseca, Petro´polis, Brazil Yanio E. Milian Center for Advanced Study of Lithium and Industrial Minerals (CELiMIN), University of Antofagasta, Antofagasta, Chile Trilochan Mishra Functional Material Group, AMP Division, CSIR-National Metallurgical Laboratory, Jamshedpur, India Leandro X. Moreno Department of Physics, Institute of Geosciences and Exact Sciences (IGCE), Sa˜o Paulo State University “Ju´lio de Mesquita Filho” (Unesp), Rio Claro, Brazil Giuseppe Nasti Department of Chemical, Materials and Production Engineering, University of Naples Federico II, Naples, Italy K. Niranjan Nanomaterials Research Laboratory, Surface Engineering Division, CSIR-National Aerospace Laboratories, Bangalore, India Marı´a Dolores Perez Institute for Nanoscience and Nanotechnology (INN)— National Atomic Energy Commission (CNEA)—National Council for Scientific and Technical Research (CONICET), Buenos Aires, Argentina; Solar Energy Department— National Atomic Energy Commision, Av. General Paz 1499, San Martin, Buenos Aires, Argentina Rodrigo Savio Pessoa Plasmas and Processes Laboratory, Aeronautics Institute of Technology, Sa˜o Jose´ dos Campos, Brazil Mauricio P. Pires Institute of Physics, Universidade Federal do Rio de Janeiro, Rio de Janeiro, Brazil Juan Pla´ Institute for Nanoscience and Nanotechnology (INN)—National Atomic Energy Commission (CNEA)—National Council for Scientific and Technical Research (CONICET), Buenos Aires, Argentina; Solar Energy Department— National Atomic Energy Commision, Av. General Paz 1499, San Martin, Buenos Aires, Argentina Fernando A. Ponce Department of Physics, Arizona State University, Tempe, AZ, United States Jessica C. Ramirez de la Torre Instituto de Nanociencia y Materiales de Arago´n (INMA), CSIC-Universidad de Zaragoza, Zaragoza, Spain; Department of Chemical and Environmental Engineering (IQTMA), University of Zaragoza, Zaragoza, Spain Ganesh Regmi Department of Electrical Engineering (SEES), Centro de Investigacio´n y de Estudios Avanzados del IPN (CINVESTAV-IPN), Mexico City, Mexico J.J. Rı´os-Ramı´rez Departamento de Ingenierı´a Ele´ctrica (SEES), Centro de Investigacio´n y de Estudios Avanzados del Instituto Polite´cnico Nacional (CINVESTAV-IPN), Ciudad de Me´xico, Mexico

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List of contributors

Araceli Romero-Nun˜ez Departamento de Ingenierı´a Ele´ctrica, Centro de Investigacio´n y de Estudios Avanzados del Instituto Polite´cnico Nacional, Mexico City, Mexico Pedro M.P. Salome´ International Iberian Nanotechnology Laboratory, Braga, Portugal Jyoti Saroha Council of Scientific and Industrial Research (CSIR)-National Physical Laboratory (NPL), New Delhi, India; Academy of Scientific and Innovative Research (AcSIR), Ghaziabad, Uttar Pradesh, India ¨ mer Halisdemir University, Nig˘de, Ayse Seyhan Department of Physics, Nig˘de O ¨ mer Turkey; Nanotechnology Application and Research Center, Nig˘de O Halisdemir University, Nig˘de, Turkey Savita Sharma Physics Department, Kalindi College, University of Delhi, Delhi, India Shailesh Narain Sharma Council of Scientific and Industrial Research (CSIR)National Physical Laboratory (NPL), New Delhi, India; Academy of Scientific and Innovative Research (AcSIR), Ghaziabad, Uttar Pradesh, India Myriam Solı´s-Lo´pez Departamento de Ingenierı´a Ele´ctrica, Centro de Investigacio´n y de Estudios Avanzados del Instituto Polite´cnico Nacional, Mexico City, Mexico Patrı´cia L. Souza Semiconductor Laboratory, Pontifı´cia Universidade Cato´lica do Rio de Janeiro, Rio de Janeiro, Brazil Velumani Subramaniam Department of Electrical Engineering (SEES), Centro de Investigacio´n y de Estudios Avanzados del IPN (CINVESTAV-IPN), Mexico City, Mexico Sanjay Tiwari Photonics Research Laboratory, School of Studies in Electronics & Photonics Pt. Ravishankar Shukla University, Raipur, India Yakup Ulusu Department of Bioengineering, Faculty of Engineering, Karamanoglu Mehmetbey University, Karaman, Turkey Svetlana Ushak Center for Advanced Study of Lithium and Industrial Minerals (CELiMIN), University of Antofagasta, Antofagasta, Chile ¨ mer Halisdemir University, Nig˘de, Recep Zan Department of Physics, Nig˘de O ¨ mer Turkey; Nanotechnology Application and Research Center, Nig˘de O Halisdemir University, Nig˘de, Turkey Djamal Zerrouki Laboratory Dynamic Interactions and Reactivity of Systems, Kasdi Merbah University, Ouargla, Algeria

Preface Over the past 20 years, the world has witnessed the growing development of renewable energy technologies, especially solar energy systems. Initial research efforts have mainly been directed to the theoretical understanding, design, modeling, simulation, and fabrication of devices for solar energy conversion and storage based on traditional materials. These traditional materials include c- and a-silicon, cadmium telluride, copper indium gallium selenide, gallium arsenide, and many others. Recent progress in synthesis of micro- and nano-structures, processing techniques, and applications of sustainable materials have opened new opportunities in this field providing solutions that allow the development of devices with improved performance and efficiency. This book covers a wide range of topics in sustainable materials for solar energy technologies organized into five sections: (1) trends in materials development for solar energy applications, (2) sustainable materials for photovoltaics, (3) sustainable materials for photocatalysis and water splitting, (4) sustainable materials for thermal energy systems, and (5) sustainable carbon-based and biomaterials for solar energy applications. It is comprised of 19 chapters written by experts in the field with diverse research backgrounds, nationalities, and specialties including physicists, engineers, material scientists, and chemists. Each chapter is essentially a detailed overview of relevant and current topics in sustainable materials for solar energy systems including emerging materials, such as bismuth-based nanomaterials, ferroelectric materials, 2D MXene materials, novel dielectric compounds, tin halide perovskites, atomic layer deposited thin films, selective coatings, waste materials based on inorganic salts, carbon-based materials (nanodot and graphene), and biomaterials. Also covered are a wide range of potential technological applications in fields such as photovoltaics, thermal energy systems, photocatalysis, and water splitting. Furthermore, there is one chapter devoted to a significant theoretical topic based on first principle calculations for solar energy harvesting materials to motivate readers to compare their experimental results with theoretical calculations to understand the intricacies involved. Each chapter contains a comprehensive list of references. Finally, research regarding solar energy processes and devices based on sustainable materials is an area worthy of study and attention. The coauthors of this text present a combination of innovative and timeless approaches,

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Preface

producing a valuable resource for students, researchers, and professionals interested in or already working in these fields. Furthermore, it is an appropriate reference or text for interdisciplinary courses devoted to solar energy materials and/or devices across different programs and departments such as Chemistry, Physics, Materials Science, and Engineering. We trust that you will enjoy reading this book. Mariana Amorim Fraga Instituto de Cieˆncia e Tecnologia, Universidade Federal de Sa˜o Paulo, Sa˜o Jose´ dos Campos, Brazil Delaina A. Amos University of Louisville, Louisville, KY, United States Savas Sonmezoglu Karamanoglu Mehmetbey University, Karaman, Turkey Velumani Subramaniam Department of Electrical Engineering (SEES), Centro de Investigacio´n y de Estudios Avanzados del IPN (CINVESTAV-IPN), Mexico City, Mexico

Section I

Trends in Materials Development for Solar Energy Applications

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Chapter 1

Bismuth-based nanomaterials for energy applications Nichole C. Cates1, Jessica C. Ramirez de la Torre2,3, Sergio Aina2,3, M. Pilar Lobera2,3,4 and Mar´ıa Bernechea2,3,4,5 1

Smart Material Solutions, Inc, Raleigh, NC, United States of America, 2Instituto de Nanociencia y Materiales de Arago´n (INMA), CSIC-Universidad de Zaragoza, Zaragoza, Spain, 3 Department of Chemical and Environmental Engineering (IQTMA), University of Zaragoza, Zaragoza, Spain, 4Networking Research Center on Bioengineering, Biomaterials and Nanomedicine, Madrid, Spain, 5ARAID, Government of Aragon, Zaragoza, Spain

Dedicated to the memory of our colleague and friend Guillem Xercavins who passed away in February 2020

1.1

Introduction

Human development is facing a serious dilemma. Energy consumption is rising due to increased worldwide industrialization and a continuously growing population. Traditionally, energy is obtained from fossil fuels that are limited resources, are restricted to precise geographical areas, have rising prices, and in some cases are associated with economic and political instability. Moreover, air pollution and greenhouse gases are generating health and environmental problems that need to be solved in the immediate future. Therefore, it is urgent and necessary to change to clean, zero-emissions renewable energy. In this sense, the development of energy-harvesting and energy-storage devices based on renewable sources will be fundamental for the deployment of autonomous or isolated systems like emerging internet-ofthings (IoT) applications. In this chapter we focus on the use of bismuth nanomaterials in energyharvesting devices like solar cells and thermoelectrics, electrochemical energy-storage devices such as batteries and supercapacitors, and chemical energy storage in the form of hydrogen obtained from photocatalytic processes. Cationic bismuth (Bi31) has been suggested as an excellent candidate for defect-tolerant compounds, i.e. materials with good electric and optoelectronic properties despite the presence of defects. These defects are limited to Sustainable Material Solutions for Solar Energy Technologies. DOI: https://doi.org/10.1016/B978-0-12-821592-0.00001-7 © 2021 Elsevier Inc. All rights reserved. 3

4

SECTION | I Trends in Materials Development for Solar Energy Applications

shallow states at band edges thanks to the active ns2 lone pair that creates antibonding interactions at the valence band maximum (Ganose et al., 2017). Moreover, this lone pair can also lead to distorted bonding and thus to a low thermal conductivity, a very convenient aspect for thermoelectric applications (Nielsen et al., 2013). Moreover, the relatively low price and abundance of Bi is attractive for its large-scale applications (Go´mez-Vel´azquez et al., 2018; Miller and Bernechea, 2018). Additionally, despite being a heavy metal, bismuth is considered non-toxic and is even used in common medicines such as Pepto-Bismol (Mohan, 2010; Yang et al., 2015). Indeed, very recently, some reviews about the use of bismuth-based materials in biomedicine have been published, pointing to low or no cytotoxicity of these materials even at high doses, although more research is needed in this sense (Badrigilan et al., 2020; Shahbazi et al., 2020). In this chapter we will put special attention to nanostructured materials because they offer additional attractive features like tuning of light absorption or electrical properties with size and/or shape (Lu et al., 2020; Bernechea et al., 2015; Carey et al., 2015). Moreover, in this type of materials the ligands on the surface can modulate properties such as band edge positions, carrier density, charge mobility, or thermal transport (Ong et al., 2013; Liu et al., 2015), which has proven to be key for improving performance in solar cells, or photocatalytic processes (Brown et al., 2014; Yang et al., 2012). For each technology, we will discuss the state of the art, challenges, and the focus areas for current research. However, we would like to point out that it is not our aim to provide an exhaustive compilation of all the materials that can be employed in these applications, but to offer an overview of the vast and versatile bismuth-based candidates and the advantages provided by nanotechnology for such uses.

1.2

Photovoltaics

A photovoltaic (PV) solar cell is a device that directly transforms (solar) light into electricity by means of a semiconductor. Usually they are used in solar panels for primary energy production converting solar light into electricity, but they can also find applications as (indoor) light harvesters for self-powered systems, implantable or wearable electronics, and IoT applications.

1.2.1

Solar Cell Operation

If a semiconductor is illuminated with light having an energy equal or higher than the difference in energy between the valence band (VB) and conduction band (CB), also known as the band gap (Eg), this light can be absorbed promoting an electron from the valence band to the conduction band and leaving a hole in the valence band. By introducing adequate contacts, these charge

Bismuth-based nanomaterials for energy applications Chapter | 1

5

carriers (electrons and holes) can be separated and extracted to an external circuit. These contacts will depend on the semiconductor(s) used as light absorbers. In a typical solar cell, a n-type semiconductor (Fermi level close to the conduction band, good electron-transport material) is put in contact with a p-type material (Fermi level close to the valence band, good holetransport material). When these two semiconductors are in contact, their Fermi levels (μF) equilibrate and the relative position of their valence and conduction bands shift. The formation of this p-n junction creates a favorable energy path for the electrons and holes generated in both semiconductors to flow in opposite directions (Fig. 1.1). If an intrinsic semiconductor is used as the light absorber, it is usually sandwiched between two selective contacts, a hole-transport layer (HTL) and an electron-transport layer (ETL). The efficiency of a solar cell (η, in %), also known as power conversion efficiency (PCE), is given by: η5

VOC ISC FF P0

where VOC is the open circuit voltage, ISC is short circuit current, FF is the fill factor, and P0 is the light power used to illuminate the device (P0  1000 W/m2 if the sun is the light source). The VOC is related to the difference in energy of the materials Fermi levels (Fig. 1.1), the ISC makes reference to the actual number of electrons and holes generated in the external

FIGURE 1.1 Illustration of solar cells operation based on a p-n junction (left) or an absorber with selective contacts (right).

6

SECTION | I Trends in Materials Development for Solar Energy Applications

circuit, and the FF reflects the resistance to charge conduction through the material itself or among different materials involved in the whole device. The efficiency limit for a solar cell using a single semiconducting material or a single p-n junction is around 33%, a value also known as the ShockleyQueisser limit, and the ideal band gap (Eg) of a semiconductor for photovoltaics is between 1.1 and 1.4 eV (Shockley and Queisser, 1961). Traditional silicon cells achieve high efficiencies, close to the 33% limit, but employ thick films (500-200 μm) that require expensive, multistep processes conducted at high temperatures (. 1000  C) in high vacuum and special clean room facilities. Several alternatives have appeared, among them the “emerging PV” technologies, which have already reached efficiencies over 25% for perovskite solar cells, and over 15% for organic or quantum dot solar cells. At the same time these alternative technologies offer reduced costs due to the use of new materials that are often composed of abundant elements and low-cost fabrication processes like solution processing (Polman et al., 2016; Lee and Ebong, 2017). Moreover, these “emerging PV” technologies use thinner active layers (500-200 nm), giving in some cases semitransparent devices (adequate for their integration in windows), are efficient at a wider range of light intensities, and can be fabricated on flexible substrates, widening the scope for applications. Indeed, when comparing different PV technologies it might be more important to look at the price or the energy required for device fabrication compared to the output energy produced during their lifetime (the Energy payback time  EPBT, or Energy return on energy invested ratioEROI), than to look only at differences in efficiency (Bhandari et al., 2015). In this sense the “emerging PV” technologies show promising prospects.

1.2.2

Nanoengineering

Colloidal semiconductor nanocrystals, used in quantum dot solar cells, are an interesting option because they can be obtained in solution and can be composed of abundant elements. Solution processing allows fabrication just by painting, spraying, or printing the semiconductor on a substrate, simplifying the process, without the need for high temperatures or vacuum. Moreover, as compared to other PV materials, nanomaterials show unique features due to their novel size- and shape-dependent properties (Lu et al., 2020; Carey et al., 2015; Polman et al., 2016; Buonsanti and Milliron, 2013; Hanna and Nozik, 2006): 1. Their band gap can be tuned over the solar spectrum. 2. Devices can surpass the theoretical limit of efficiency because of multiple exciton generation (MEG). 3. Their behavior as n- or p-type semiconductors can be modified by introducing foreign atoms, like in traditional silicon technologies, but also by

Bismuth-based nanomaterials for energy applications Chapter | 1

7

modifying the ligands on the surface of the nanocrystals, which is a unique feature of these materials and a very powerful tool.

1.2.3

Bismuth-Based Nanomaterials

We (Miller and Bernechea, 2018), and others (Wu et al., 2019; Lee et al., 2018), recently reviewed the use of bismuth-based materials in solar cells, therefore in this chapter we will focus on some recent developments with extra attention to the use of bismuth-based nanostructured materials.

1.2.3.1 Bismuth-based Perovskites and Bismuth Halides Most of the bismuth-based perovskites and bismuth halide compounds used in photovoltaics have been inspired by the amazing results obtained with lead-based materials. Although they still show limited efficiencies as compared with the lead counterparts, in general bismuth materials offer enhanced robustness and stability. As an alternative to the standard (CH3NH3)PbI3 hybrid lead halide perovskites, (CH3NH3)3Bi2I9 perovskite solar cells fabricated by vapor deposition have shown a maximum efficiency of 3.17% (Jain et al., 2018). In general, better results are obtained with all-inorganic materials. For example, Cs3Bi2I9 films, as analogous material to CsPbI3, has reached a maximum efficiency of 9.2% when sandwiched between a ZnO electron-transport layer and a CuI hole-transport layer (Heo et al., 2019). Considerable progress has been achieved also for the silver bismuth iodide family showing efficiencies of 2.12.6% for Ag2BiI5 and AgBi2I7 (Zhu et al., 2017; Jung et al., 2018; Ghosh et al., 2018), and 4.3% for Ag3BiI6 (Turkevych et al., 2017). Moreover, the efficiency of this family can be improved up to 5.44% by partial anionic substitution of iodide with sulfide dianion (Pai et al., 2019). Very recently, a bulk heterojunction perovskite solar cell with the photoactive layer consisting of Cs3Bi2I9 and Ag3Bi2I9 components has led to a 3.6% efficiency (Hu et al., 2020). Also double perovskites such as Cs2AgBiBr6 or Cs2NaBiI6 have delivered promising results of up to 2.43% or 0.42%, respectively (Greul et al., 2017; Zhang et al., 2018). Recently, Cs2AgBiBr6 films deposited by vacuum sublimation led to an efficiency of 1.41% that was improved to 2.51% when Cs2AgBiBr6 nanocrystals were synthesized, following a solvothermal route, and subsequently spin-coated onto a TiO2 layer (Igbari et al., 2019). This last study shows the promise of bismuth halide nanomaterials for solar cells, however, there are very few examples of this approach, and all of them are quite recent. Cs3Bi2I9 nanocrystals of  2 nm were synthesized using 1-dodecanol as solvent and capping ligand. Solar cells were fabricated using the Cs3Bi2I9 nanocrystals as the active layer, SnO2 as the electron-transport layer, and Spiro-OMeTAD as the holetransport layer. The highest efficiency achieved was only 0.0103%, which

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can be explained by the small size of the nanocrystals, the presence of the insulating long organic ligands, and the transport layers (Yao et al., 2019). Data analytics and high throughput density functional theory calculations predicted KBaTeBiO6 inorganic double perovskite oxide as a potential photovoltaic material. Nanopowders were synthesized following a wet-chemistry route, and they showed an indirect band gap of 1.88 eV. Subsequently they were used as sensitizers in Dye Sensitized Solar Cells (DSSC) leading to a maximum efficiency of 0.06% (Thind et al., 2019). The simplest halide compound, bismuth triiodide (BiI3), has also shown interesting efficiencies up to 1.0% (Hamdeh et al., 2016). When BiI3 is obtained by gas-phase iodination of Bi2S3 the solar cells led to an efficiency of 1.2% and a record open-circuit voltage above 600 mV rationalized in terms of carrier lifetimes longer than 1 ns as probed by time-resolved photoluminescence spectroscopy (Tiwari et al., 2018). Later, the introduction of a bismuth sulfide iodide (BiSI) interlayer between the electron transport layer and the BiI3 absorber layer promoted efficient charge separation and provided a power conversion efficiency of 1.21%. Indeed, the use of mixed sulfur/iodide anions seems to be a good strategy. Bi13S18I2 rods with diameters of 50 nm and lengths of several microns were synthesized following a solvothermal method using high S/Bi mol ratios. The rods exhibit a strong absorbance in the wide wavelength range from UV to NIR (band gap of 0.75 eV) and n-type behavior. They have been used as sensitizers in DSSC with an efficiency of 0.85% (Li et al., 2020).

1.2.3.2 Bismuth Chalcogenides At the moment, ternary bismuth chalcogenides are the most-studied systems for bismuth-based photovoltaics, among them copper, silver and alkaline derivatives. Co-evaporation of bismuth and CuS in a vacuum system followed by an annealing process led to the deposition of copper bismuth sulfide (CBS) layers consisting of a mixture of Cu3BiS3 and CuBiS2 phases. A photovoltaic device based on CdS/CBS heterojunction was fabricated and an efficiency of 1.7% was obtained (VOC 5 0.32 V, JSC 5 18.2 mA/cm2, and FF 5 30.3%) (Yang et al., 2019), surpassing previous reports (Miller and Bernechea, 2018). Recently, sodium bismuth dichalcogenides have been proposed as promising semiconductors for photovoltaics (Zhong et al., 2019; Rosales et al., 2018), however, initial reports using NaBiS2 quantum dots grown on the surface of TiO2 following the SILAR technique yielded an efficiency of 0.05% (Zumeta-Dube´ et al., 2014), indicating that more studies are needed with these semiconductors. Ternary AgBiS2 nanocrystals have shown great promise for their use in solution-processed solar cells as a certified efficiency of 6.3% was reported for an active layer thickness of only 35 nm (Bernechea et al., 2016). A

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modified synthesis route to obtain AgBiS2 nanocrystals at room-temperature and in ambient-air has been reported and the solar cells made of ethanethiolexchanged nanocrystals yield a promising power conversion efficiency (PCE) of 5.5% for devices with a 40 nm active layer (Akgul et al., 2020). The stability of AgBiS2 nanocrystals and the TMAI-exchanged films fabricated with them has been recently demonstrated in ambient conditions and in direct contact with water. This allowed for the fabrication of thin film solar cells that exhibited a PCE of 5.75%, that was maintained after a water treatment (5.71%), showing the potential of this material for water-resistant, lead-free, and air-stable solar cells (Oh et al., 2019). Charge photogeneration and transport studies on TMAI-treated AgBiS2 nanocrystal films have determined an electron diffusion length of 60 nm, therefore thicker active layers could be grown if optical interference effects were solved, or nanocrystals were treated with different ligands (Diedenhofen et al., 2019).

1.2.4

Summary

Although bismuth materials are not yet a competitive alternative to lead containing photovoltaics in terms of efficiency, in the last years, new materials have appeared that exhibit promising results and improved stability. Moreover, they are relatively abundant and are composed of non-toxic elements.

1.3

Thermoelectric devices

Thermoelectric devices are able to directly convert heat to electricity and vice versa. Moreover, they can also convert light-induced heat energy to electrical energy by absorbing the photothermal energy of sunlight or the heat generated by electric light bulbs. These devices find applications in the field of energy harvesting, cooling, temperature sensing, and, very importantly, as power generators from waste heat recovery in industrial processes or automobiles, therefore opening the door to improving energy generation and utilization efficiency. More recently with the appearance of micro and nanodevices, sometimes on flexible substrates, new applications have emerged like self-powered systems, implantable or wearable electronics, and IoT applications.

1.3.1

Thermoelectric Devices Operation

In thermoelectric materials, a difference in temperature on different sides of the sample produces a diffusion of charge carriers from the hot side to the cold side. Macroscopically this carrier flow is observed as a difference of voltage (Seebeck effect). When two different materials subjected to these different temperatures are connected, the generated voltage drives the migration

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FIGURE 1.2 Schematic drawing of thermoelectric devices used for power generation (left) and heating or cooling applications (right, represented in the case of a cooling system).

of charge carriers, forming a circuit (Fig. 1.2). The conversion of thermal energy to electrical energy is given by ΔV 5 SΔT, where ΔV is the generated voltage, S is the Seebeck coefficient or thermopower, and ΔT is the magnitude of the temperature difference. Similarly, if a voltage is applied to a thermoelectric material, a temperature gradient is observed (Peltier effect), which can be used in solid heating or cooling applications (Fig. 1.2). By changing the direction of the applied current, the device can be switched from heating to cooling. The heat flow in the system is given as Q 5 ΠI, where Q is the heat absorbed or emitted, Π is the Peltier coefficient, and I is the applied current. The performance of a thermoelectric material is defined by its dimensionless figure of merit, ZT, which gives an idea of the maximum energy conversion efficiency, for both power generation and heating or cooling, at a given temperature. The figure of merit is defined as: ZT 5

S2 σT κ

where S is the Seebeck coefficient, σ is the electrical conductivity, T is the absolute temperature, and κ is the thermal conductivity. To obtain good performance, a high power factor (S2 σ) and a low thermal conductivity (κ) are required, but these material properties are usually interconnected. Good electric conductors tend to be good thermal conductors as well, which makes it especially difficult to find good materials and optimize their properties. For example, in degenerate semiconductors high electrical conductivity goes with high thermal conductivities as well, and high carrier concentrations increase electrical conductivity (σ), but decrease the thermopower (S). This

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interdependence of transport properties and Seebeck coefficient seems to limit ZT to 1 for bulk materials, while a ZT higher than 1.5 is needed to be competitive with already-commercialized solutions (Szczech et al., 2011; Urban, 2015; Rathore et al., 2019). The traditional materials for thermoelectrics are heavily doped semiconductors with low band gaps (, 1 eV), composed of elements having large atomic masses, with anisotropic or complex crystal structures, or crystalline structures where atoms do not have well-defined lattice positions, in which dopants are introduced or alloys are formed to create point defects that scatter heat-carrying phonons. Typical materials used in thermoelectric devices, ordered from higher to lower temperature applications, are rare-earth chalcogenides like La3-xTe4, PbTe, and Bi2Te3 and its (Bi,Sb)2(Te,Se)3 alloys (Szczech et al., 2011; Rathore et al., 2019; Chen et al., 2012; Beretta et al., 2019; Han et al., 2016).

1.3.2

Nanoengineering

Nanostructuring and the use of nanomaterials instead of bulk materials are strategies that have shown to increase the efficiency of the devices. Nanoengineering can lead to a reduction in thermal conductivity due to increased interface scattering of phonons on the grain boundaries, and to an increase in the power factor by quantum confinement of electrons, but sometimes at the expense of carrier transport (Szczech et al., 2011; Urban, 2015; Beretta et al., 2019; Han et al., 2016; Xu et al., 2019). High interface density, higher for smaller nanoparticles, appears to be the key factor for enhancing ZT, together with low thermal conductivity, predicted to be equal for periodically aligned or randomly ordered nanocomposites but increasing as the nanomaterial size decreases (Szczech et al., 2011). In general, most of the highest ZT nanostructured thermoelectric materials have been obtained using top-down methods. However, more recent bottom-up methods have shown their promise and offer additional features like tailoring of surfaces or price reduction. Indeed, apart from reaching a high ZT, any synthetic approach should be capable of producing high quantities of thermally and mechanically stable materials at low cost. In that sense traditional preparation methodologies of thermoelectric materials require very high temperatures (T . 1000  C), which impact on the fabrication costs. The synthesis of nanomaterials (T , 200  C), followed by densification into nanostructured bulk materials, usually by cold/hot pressing or spark plasma sintering, can reduce the price of thermoelectric devices, as long as material abundance, cost of production, and toxicity are considered as well (Chen et al., 2012; Han et al., 2016; Bux et al., 2010; Yazdani and Pettes, 2018). The use of nanomaterials, especially those that can be obtained as colloidal solutions, makes the fabrication of hybrid structures or nanocomposites

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easier by combining different materials or embedding thermoelectric materials in a matrix that facilitates electron transport while hindering thermal transport (Bux et al., 2010; Cho et al., 2015; Kim, 2015). The interface type will influence the transport properties and thermoelectric performance of nanocomposites. In general, it is assumed that ideal interfaces will have coherent structures, so that the electronic structure is minimally disrupted while still providing a site for phonon scattering (Szczech et al., 2011; Medlin and Snyder, 2009). Additionally, the use of nanomaterials that can be dispersed in solvents allows the use of solution-processed fabrication methods like inkjet printing, additive manufacturing like 3D printing, and the development of flexible thermoelectric generators (Yazdani and Pettes, 2018; Cho et al., 2015; Peng et al., 2019; Li et al., 2017; Chen et al., 2019).

1.3.3

Bi-Based Nanomaterials

1.3.3.1 Metallic bismuth Elemental bismuth is an interesting material system for thermoelectrics. It shows different surface and bulk properties, its surface allows for the observation of classical and quantum size effects even at relatively large length scales and high temperatures, its heavy mass efficiently scatters phonons, it exhibits strong interface scattering effects, and it shows a transition from semimetal to semiconductor when its dimensions are reduced. By ballmilling and subsequent spark plasma sintering, a polycrystalline bulk sample was prepared showing a ZT  0.13 at 300 K, which is a six-fold improvement as compared to the starting commercial material (Puneet et al., 2013). Doping bismuth with antimony, to increase the semiconducting properties of bismuth, and introducing carbon nanotubes (CNT), to enhance the electronic confinement effects and as an energy filter for the electronic transport, enhanced ZT up to 0.42 at 300 K for the composite consisting of Bi0.8Sb0.2 nanoparticles and 0.3 wt % CNT (Gu¨ne¸s et al., 2017). 1.3.3.2 Bi2Te3 and (Bi,Sb)2(Te,Se)3 alloys Bi2Te3 and its alloys, Bi0.5Sb1.5Te3 as p-type and Bi2Se0.3Te2.7 as n-type materials, are considered the best room temperature thermoelectric materials. They have a low band gap (0.20.3 eV), crystallize in a layered structure, have a ZT of 0.61 at 300 K, and quite low thermal conductivity of 23 W/m  K (Szczech et al., 2011; Han et al., 2016). In one of the first studies of the use of nanostructures to improve ZT, stoichiometric n-type Bi2Te3 nanocrystals were cold pressed and sintered in an Ar atmosphere at different temperatures. The samples composed of 30 nm grains showed a ZTmax of 0.94 at 398 K, which was higher than ZT of the state-of-art materials reported at that point. The results suggest that the enhancement of ZT comes from a large reduction in the phonon thermal

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conductivity (κ  0.6 W/m  K) (Yu et al., 2009). The ZT was further increased to 1.16 at 420 K by hot-pressing Bi2Te3 nanopowders with different morphologies obtained following a solvothermal method (Wu et al., 2013). A microwave-assisted reduction of Bi and Te precursors in the presence of hydrazine led to the formation of 5070 nm nanoparticles that were consolidated using spark plasma sintering. The measured ZT was 1.18 at 300 K thanks to an enhanced Seebeck coefficient (Pradhan et al., 2017). Bi2Te32xSex nanoplates prepared by a microwave-assisted, surfactant-free solvothermal method led to a ZT of 1.23 at 480 K. Nanostructuring together with the introduction of Se to tune anion vacancies enhances the thermoelectric properties (Hong et al., 2016). The beginning of nanostructuring thermoelectric materials began with the study published in 2008 by Poudel et al. They obtained a p-type nanocrystalline Bi0.5Sb1.5Te3 bulk alloy with a high density of grain boundaries by hot pressing nanopowders obtained from ball-milled crystalline ingots. The nanobulk sample exhibited a ZT of 1.2 at room temperature and a ZTmax of 1.4 at 373 K. Importantly, the thermal conductivity was reduced to κ  1.0 W/m  K, as compared to κ  1.3 W/m  K of the bulk ingot at 373 K. The sample was polydisperse which likely resulted in efficient scattering of a broad distribution of phonon wavelengths (Poudel et al., 2008). A different approach consisted of synthesizing Bi2Te3 and Sb2Te3 nanopowders using a solvothermal technique. A composite consisting of Bi2Te3 and Sb2Te3 nanolayers with thicknesses varying between 5 and 50 nm was formed after mixing the nanopowders and consolidating via hot pressing. The maximum ZT measured was 1.47 at 440 K, mostly due to an improvement of the Seebeck coefficient (S) and electric conductivity, probably because of quantum confinement effects (Cao et al., 2008). In another study, a p-type Bi0.52Sb1.48Te3 bulk nanostructured material was obtained by a melt spinning technique combined with a subsequent spark plasma sintering process. The final structure consisted of an amorphous structure, 515 nm fine nanocrystalline regions, and coherent interfaces between the resulting nanocrystalline regions and exhibited a ZT 5 1.56 at 300 K. This structure can adjust the transport of phonons and electrons enhancing the thermoelectric performance, mostly due to a reduction in the thermal conductivity that decreases from 1.4 W/m  K of the bulk material to 0.9 W/m  K for the hot pressed nanopowder sample, and to 0.67 W/m  K for the amorphous with nanostructures sample (Xie et al., 2009). After this study, and following a similar fabrication method, a liquid-phase compaction process using Te excess led to the formation of Te-coated grains of Bi0.5Sb1.5Te3. Apparently, interference effects in the coated grains due to Bragg reflection destroy the mid-wavelength phonons leading to a very low thermal conductivity and a very high ZT of 1.86 at 320 K (Kim, 2015; Il Kim et al., 2015). Following a similar approach, but using BixSb22xTe3 nanocrystals with controlled composition as precursor building blocks, a maximum ZT of 1.96 at 420 K was

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obtained for the Bi0.5Sb1.5Te3 composition. The nanocrystalline samples have some Te excess that is fundamental to obtaining high-performance materials because when hot pressing above 450 C it melts creating a solid 2 liquid interface and an adequate crystallographic texture (Liu et al., 2018). Enhancing ZT for n-type materials has proven to be more difficult, but a ZT 5 1.1 at 300 K for n-type Bi2Te3 was measured for bulk nanostructured samples prepared by the assembly and sintering of nanoplate building blocks synthesized by a microwave-stimulated wet-chemical technique. The high ZT found (also for p-type materials) were obtained thanks to a combination of a high Seebeck coefficient (S) and electrical conductivity, attributed to the sulfur-doping induced by the surfactants used in the synthesis, and low thermal conductivity (κ), attributed to nanostructuring (Mehta et al., 2012). More recently, undoped n- and p-type bismuth telluride based materials reached values of a record ZT 5 1.2 at 445 K for n-type polycrystalline Bi2Te2.3Se0.7 alloys, and a high ZT 5 1.3 at 380 K for p-type polycrystalline Bi0.3Sb1.7Te3 alloys, by controlling the concentration of defects. This was tuned by adjusting the composition and applying hot deformation to the samples, leading to optimized compositions and to improvements in electrical properties and lattice thermal conductivity (Hu et al., 2014). In a similar method to that used for p-type BixSb22xTe3, the ZT of n-type Bi2Te32xSex has been improved up to 1.31 at 438 K for the Bi2Te2.7Se0.3 composition. Disk-shaped Bi2Te32xSex colloidal nanocrystals were hot pressed in the presence of an excess of tellurium at temperatures above the melting point of Te. This results in bulk nanomaterials with marked crystallographic texture with a preferential orientation (Liu et al., 2018).

1.3.3.3 Bi2Se3 and Bi2S3 The toxicity and high cost of tellurium, due to its scarcity, has forced researchers to look for alternatives to Bi2Te3. In that sense, Bi2Se3 and Bi2S3 appear as natural options. Bi2Se3 has a narrow band gap (0.35 eV) and a similar layered structure to Bi2Te3, but its thermoelectric properties are worse because the lower atomic weight of selenium increases the thermal conductivity (Han et al., 2016). Using a high-temperature technique, good quality bulk single crystals of p-type Bi2Se3 were prepared and their maximum reported ZT was 0.75 at 423 K (Gupta et al., 2015). Using a different approach, compacting nanoplatelets using spark plasma sintering, n-type nanostructured bulk Bi2Se3 samples led to a ZT of 0.41 at 533 K (Ali et al., 2014). Highly-crystalline Bi2Se3 ultrathin nanosheets with thicknesses between 1 and 13 nm were obtained using a microwave-stimulated solvothermal method. The pellet fabricated with the thinnest nanosheets showed a maximum ZT of 0.46 at 425 K, attributed to broadened bandgap and optimized Fermi level (Hong et al., 2015).

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Contrary to Bi2Te3 and Bi2Se3, Bi2S3 crystallizes in an orthorhombic structure and has a higher band gap (1.31.7 eV). It has high a Seebeck coefficient (400 μV/K) and low thermal conductivity (, 1 W/m  K) at room temperature, but shows poor electrical properties because of the high electronegativity of S and, as result, shows low thermoelectric properties with a ZT  0.1 at 573 K (Han et al., 2016; Liu et al., 2014). Nanostructuring has helped in improving these properties: Bi2S3 nanonetworks synthesized using a solvothermal method were hot pressed after a surface treatment to remove ligands leading to a record ZT of 0.5 at 623 K. The use of nanostructures lowers the thermal conductivity while the surface treatment reduces energy barriers at grain boundaries enhancing the electrical conductivity (Liu et al., 2014). Another strategy that has helped to improve the performance is the introduction of dopants. The use of silver led to a maximum ZT of 0.25 at 573 K for a n-type Bi1.99Ag0.01S3 sample (Yu et al., 2011). Chloride provided a ZT value of 0.6 at 760 K for n-type Bi2S3  0.5% mol BiCl3 (Biswas et al., 2012). The combined doping with bromide and copper led to a maximum ZT of 0.72 at 773 K for the 0.5 mol% CuBr2 doped Bi2S3 sample (Liu et al., 2015). Finally, exploring n-type Bi2S3 alloys a maximum ZT of 0.8 was found at 773 K for Bi2SeS2, and at 553 K for Bi2Te2S compositions (Liu et al., 2013).

1.3.3.4 Ternary materials AgBiS2 nanocrystals crystallize in the high temperature pseudo-cubic rock salt structure instead of the hexagonal phase. In the cubic structure, the position of S atoms is fixed and Ag and Bi atoms are disordered in the cation sublattice. The lone 6s2 pair electrons of bismuth distort the crystal lattice and lead to strong anharmonicity in the Bi-S bond, which is effective in phonon scattering and produces low thermal conductivity in the material. In this context, Biswas et al. reported the synthesis of n-type AgBiS2 nanocrystals (  11 nm), removal of capping ligands and subsequent densification, obtaining a maximum ZT of 0.2 at 810 K. Moreover, at 600 K changes in Seebeck coefficient and electrical conductivity were observed due to an order 2 disorder transition, which optimized the electronic charge transport and led to a minimal thermal conductivity (Guin and Biswas, 2013). Studies in bulk n-type AgBiSe2 have shown a ZT up to 0.5 at 773 K that can be improved up to ZT  1 when doping the material with Nb (Pan et al., 2013). AgBi3S5 has a low band gap of 0.6 eV and crystallizes in the monoclinic system. Recently, Cl doped n-type AgBi3S5 has shown a ZT  1.0 at 800 K. The high performance originates from the low thermal conductivity, generated by the unusual vibrational properties generated by the different phonon modes associated with Ag and Bi atoms, and the optimized electrical properties provided by chlorine doping (Tan et al., 2017).

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CsBi4Te6 has a layered anisotropic structure, has shown a ZT of 0.20.5 at room temperature, and a calculated band gap of 0.050.11 eV. The ZT was increased up to 0.8 at 225 K and 0.65 at room temperature by introducing 0.05% mol of SbI3 which produced p-type doped materials (Chung et al., 2000).

1.3.4

Summary

Bismuth-based materials are still the reference for low-temperature thermoelectric applications. The use of nanoscale and nanostructured materials has been demonstrated to increase the thermoelectric figure of merit (ZT) relative to the corresponding bulk materials. This increase is mainly attributed to a lower thermal conductivity as interface density increases, and to quantum size effects. Inexpensive bulk nanocomposites have also demonstrated impressive ZT relative to bulk crystals.

1.4

Batteries & Supercapacitors

Batteries and supercapacitors are both electrochemical energy storage devices, but they have different electrochemical mechanisms and thus different strengths and weaknesses (Simon et al., 2014). We are familiar with batteries used to power our cell phones, laptops, watches, and even electric vehicles. Batteries supply energy over time to keep our portable electronics powered for extended periods between charges. This is because batteries have high energy densities, meaning that they can store large amounts of energy per unit mass and volume. In contrast, supercapacitors tend to store less energy, but they can charge and discharge very quickly due to their high power densities. Supercapacitors are therefore desirable in applications such as regenerative braking that require rapid power delivery and recharging.

1.4.1

Battery Operation

Batteries generally consist of a separator and an electrolyte sandwiched between two electrodes as illustrated in Fig. 1.3. The separator allows ions to flow freely through the electrolyte while preventing electrical contact between the electrodes. Ions and electrons flow through the electrolyte and an external circuit, respectively. During charge, a charger causes the ions and electrons flow from the cathode to the anode. During discharge, the ions and electrons flow in the opposite direction to supply a current to a load. The ions must be able to reversibly incorporate themselves into the anode and cathode materials without compromising electrical or ionic conductivity or causing undesirable side reactions for a battery to maintain high performance throughout the thousands of charge-discharge cycles a typical battery experiences in its lifetime.

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FIGURE 1.3 Illustration of the operation of batteries (left) and supercapacitors (right).

1.4.2

Supercapacitor Operation

Whereas ions are stored inside the electrodes in batteries, they are stored on the surface of the electrodes in supercapacitors. In its discharged state, a supercapacitor has positive and negative ions dispersed throughout the electrolyte (Fig. 1.3). When charging, an external circuit forces positive and negative charges into the anode and cathode, respectively. To compensate the buildup of charge in the electrodes, the oppositely charged ions migrate to the electrodes to form an electrolytic double layer. Storing ions on electrode surfaces rather than inside electrodes makes ion movement much faster, since the ions do not have to migrate through the bulk of the electrodes during charging and discharging. This leads to higher power densities and longer lifetimes of supercapacitors relative to batteries. However, supercapacitors can suffer from lower energy densities since only a small amount of energy can be stored through the adherence of ions to the electrode surface.

1.4.3

Bismuth-Based Electrodes

Finke et al. first brought attention to bismuth-based electrodes through demonstrations of lithium cycling with nanostructured bismuth electrodes (Finke et al., 2008). Although these electrodes were plagued by substantial capacity fade, they highlighted the promise of bismuth-based electrodes by demonstrating a high initial volumetric capacity approaching 4000 mAh/cm3. The theoretical volumetric capacity of lithium storage in bismuth is 3765 mAh/cm3, almost five times that of graphite (756 mAh/cm3) (Finke et al., 2008; Zhong et al., 2018). Finke also demonstrated an initial specific capacity approaching 400 mAh/g, which is comparable to that of graphite (372 mAh/g) (Finke et al., 2008; Park et al., 2009). Moreover, the redox potential of bismuth (B0.8 V vs Li1/Li) is low enough to ensure a high working

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voltage, yet high enough to prevent lithium plating that can result in catastrophic failure (Wu et al., 2020; Ni et al., 2017; Sun et al., 2016). These desirable properties have raised interest in bismuth electrochemistry for supercapacitors (Devi et al., 2019; Lu et al., 2017) as well as Li-ion (Finke et al., 2008; Jin et al., 2017), Na-ion (Jin et al., 2017; Xu et al., 2019; Sun et al., 2018; Liu et al., 2016), K-ion (Zhang et al., 2018; Lei et al., 2018; Huang et al., 2018), Mg-ion (Jin and Wang, 2018; Kravchyk et al., 2018; Shao et al., 2014), and aqueous batteries (Zuo et al., 2015; Zeng et al., 2016; Zhang et al., 2018). Still, Finke’s battery electrodes were limited by their inferior cyclability due to capacity loss during charge-discharge cycles (Finke et al., 2008). Capacity fade in batteries is generally caused by the large volume changes that occur during uptake and release of a large quantity of metal ions during cycling. Bismuth, for example, exhibits a volume expansion of 110% after complete uptake of lithium to form Li3Bi, while Bi2S3 exhibits an even larger volume expansion of 230% upon complete uptake of lithium to form Li2S and Li3Bi (Ni et al., 2017). These extreme volume changes result in mechanical stress that can lead to electrode cracking, which cuts off electrical connectivity to portions of the electrode and renders these regions inoperable. Several methods have been developed to limit the impact of volume change during cycling of batteries while improving electrical and ionic conductivity of the electrodes. These methods include nanoengineering, alloying, doping, and coating or mixing the active materials with conductive materials such as carbon (Park et al., 2009; Sun et al., 2016). Many of these methods will be discussed in detail below. Other contributions to capacity fade include undesirable side reactions that consume the electrolyte, cause irreversible reactions within the electrode, or coat the electrode with a nonconductive material. Supercapacitors do not tend to suffer from extreme volume changes during cycling, since ions are stored on the surfaces of supercapacitor electrodes rather than inside their electrodes. Nevertheless, some of the methods described above to improve battery performance can also improve supercapacitor performance. For instance, nanoengineering can increase the electrode surface area available for ion storage and mixing of electrode materials with highly conductive materials can improve the electrical conductivity of supercapacitor electrodes.

1.4.4

Nanoengineering

Nanoengineering of battery electrode materials allows the materials to accommodate larger volume changes without cracking, thus maintaining good electrical and ionic connectivity through numerous charge-discharge cycles. Nanoengineered electrode materials have the additional advantage of high surface area, which lowers the barrier to current flow, increases the

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available power, and decreases the charge/discharge time. In supercapacitors, increasing surface area also increases the energy density since ions are stored on supercapacitor electrode surfaces. Researchers have therefore nanoengineered electrode materials in the form of nanoparticles (Jin et al., 2017; Liu et al., 2016; Yin et al., 2017; Chai et al., 2019), nanofibers (Jin et al., 2017; Yin et al., 2017; Wang et al., 2015), and nanosheets (Zhai et al., 2018; Li et al., 2016; Xiang et al., 2018). Many promising electrodes combine more than one of these nanostructures to achieve optimal performance. Some examples are the inclusion of bismuth nanoparticles in carbon nanofibers (Jin et al., 2017; Yin et al., 2017) or the coating of carbon nanofibers with bismuth oxide nanosheets (Li et al., 2016). Bismuth nanoparticles with a variety of shapes including spherical nanoparticles (Finke et al., 2008; Devi et al., 2019), nanorods (Liu et al., 2016; Chai et al., 2019), and complicated branched nanoparticles (Kumari et al., 2019; Noordeen et al., 2018; Fang et al., 2017; Gujar et al., 2006) have been studied as electrode materials for both batteries and supercapacitors. For example, in 2016 Liu et al. fabricated sodium-ion-battery anodes comprised of bismuth nanorod bundles by chemical dealloying of Al30Bi20 ribbons (Liu et al., 2016). These anodes exhibited an initial discharge capacity of 667.3 mAh/g and a high reversible capacity of 301.9 mAh/g after 150 cycles at 50 mA/g. More recently, Chai et al. synthesized Bi2S3 nanorods and Bi2S3/C nanorods for lithium-ion batteries with impressive specific capacities of 765 and 603 mAh/g, respectively, after 100 cycles at 100 mA/g (Chai et al., 2019). More complicated nanostructures like Kumari’s flower-like Bi2S3 for lithium-ion anodes significantly outperform bulk Bi2S3 to reach a discharge capacity of 375 mAh/g after 50 cycles (Kumari et al., 2019). Similar flowerlike Bi2S3 structures (Noordeen et al., 2018; Fang et al., 2017) and other complicated nanostructures like hierarchically rippled Bi2O3 nanobelts (Gujar et al., 2006) have also shown promise as supercapacitor electrodes. The rippled Bi2O3 nanobelts, which were fabricated by oscillating the potential during an electrodeposition process, exhibited a stable capacitance of 250 F/g compared to only 61 F/g for the smooth sample. Many groups have also explored nanofiber electrodes. Electrospinning stands out as a promising nanofiber-fabrication technology since it is a scalable nanoengineering method that has already been proven in industrial-scale manufacturing (Wang et al., 2015). The North Face, for example, uses electrospinning (which it coins nanospinning) to manufacture its breathable waterproof material, FUTURELIGHT (DeAcetis, 2019). Electrospinning involves the application of a high voltage between a metal collector and a syringe tip that is extruding a precursor solution or melt (Teo and Ramakrishna, 2006). The high voltage causes charge to build up on the surface of the extruded material, resulting in rapid elongation of the solution into a nanofiber and its ultimate deposition onto the metal collector. Zhu et al. used electrospinning to incorporate bismuth nanoparticles into carbon

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nanofiber anodes for lithium-ion and sodium-ion batteries (Yin et al., 2017). The carbon nanofibers prevent the aggregation of bismuth nanoparticles and buffer the volume changes during cycling. Around the same time, Jin et al. used electrospinning to fabricate binder-free bismuth/carbon nanofiber anodes for lithium-ion batteries with a high reversible capacity of 483 mAh/ g after 200 cycles at 100 mA/g (Jin et al., 2017). The same electrodes exhibited a reversible capacity of 346 mAh/g for sodium-ion batteries. Other groups have demonstrated high energy densities, high power densities, and outstanding cycling stability in supercapacitor electrodes made by electrospinning solutions of carbon nanotubes mixed with bismuth chalcogenides like Bi2S3 or Bi2O3 (Li et al., 2016; Zong et al., 2018).

1.4.5

Coating or Mixing with Conductive Materials

Coating nanostructured bismuth or bismuth alloys with conductive materials such as carbon or Al2O3 can increase the electrical and ionic conductivity of the electrodes while further buffering the extreme volume changes that occur during battery cycling. For instance, Park et al. created nanostructured Bi/C and Bi/Al2O3/C battery electrodes using high-energy mechanical milling that outperformed the Bi control electrodes by exhibiting both higher initial capacities and better cycle retention (Park et al., 2009). The Bi/Al2O3/C electrodes exhibited an initial discharge capacity of 423 mAh/g with 95% capacity after 10 cycles and 74% capacity after 100 cycles. For comparison, the control Bi electrode exhibited an initial discharge capacity of 196 mAh/g and only 88% capacity after 10 cycles. Li et al. also explored carbon-coating of Bi2O3 on a nickel foam, which exhibited an impressive initial discharge capacity of 1923 mAh/g (Li et al., 2013). The lower second discharge capacity of 1346 mAh/g indicated that 30% of the initial discharge capacity was irreversible; this irreversible capacity is likely due to degradation of the electrolyte to form a solid-electrolyte interphase during the first cycle. Despite substantial capacity fade to 782 mAh/g after 40 cycles, the observed capacity is still above the theoretical capacity of 690 mAh/g. The additional capacity may be due to either the storage of lithium in the carbon or nickel, a difference in the lithium-storage mechanisms in nanoscale and bulk Bi2O3, or the presence of a side reaction during cycling. Sun et al. addressed the capacity fade by creating bismuth nanoparticles in an “ion conductive solid-state matrix” of Li3PO4 (Sun et al., 2016). These electrodes, which were prepared in situ through electrochemical conversion of bismuth phosphate (BiPO4), exhibited a reversible capacity of 280 mAh/g and a low capacity decay rate of 0.071% per cycle for 500 cycles. The success of this electrode can be attributed to the Li3PO4 matrix maintaining physical, ionic, and electrical connectivity while preventing undesirable side reactions by isolating the Bi nanoparticles from the liquid electrolyte.

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The addition of highly conductive materials can also improve the performance of supercapacitor electrodes by improving their electrical conductivity and in some cases providing additional capacitive material and surface area. Carbon-based materials like graphene and carbon nanotubes often perform well because they are conductive, chemically robust, and mechanically flexible. Supercapacitor anodes made from Bi2S3 anchored to graphene sheets, for example, exhibited a large specific capacitance of 396 F/g (Lu et al., 2017), and a core-shell structure with Bi2S3 nanorods coated with amorphous carbon demonstrated a specific capacitance of 333 F/g and over 97% capacity retention after 1000 cycles at 4 A/g (Vattikuti et al., 2018). In another study, a supercapacitor based on binder-free graphene/BiVO4 demonstrated a specific capacitance of 479 F/g (Deng et al., 2018). In all cases, the addition of carbon-based materials improved the supercapacitor performance by increasing electrical conductivity.

1.4.6

Bismuth Perovskite Supercapacitors

Inspired by the impressive performance of perovskite solar cells (Lyu et al., 2016), researchers have also begun to study perovskite materials for other applications including supercapacitors. The first studies focused on methylammonium bismuth iodide ((CH3NH3)3Bi2I9), which has shown promise for photovoltaic applications (Miller and Bernechea, 2018). These studies highlighted the promise of bismuth perovskite supercapacitor electrodes by demonstrating electrochemical storage with a modest areal capacitance of 5.5 mF/cm2 and 84.8% capacitance retention after 10,000 cycles at 2 mA/cm2 (Pious et al., 2017). Exploration of other bismuth perovskite systems has improved performance, resulting in an increase in the areal capacitance by several orders of magnitude to 3.3 F/cm2, equivalent to a specific capacitance of over 1000 F/g, for electrodes made from the hybrid bismuth-halide complex (CN2SH5)3BiI6 (TBI) (Li et al., 2019). These results are particularly impressive considering that relatively little research has been devoted to bismuth perovskite supercapacitors thus far.

1.4.7

Summary

Bismuth-based electrodes have demonstrated promise for electrochemical storage in both batteries and supercapacitors. Bismuth electrodes offer a variety of advantages such as exceptionally high volumetric capacities, but they can be plagued by short lifetimes. Nanoengineering and the addition of conductive materials have substantially improved both the performance and lifetime of bismuth-based electrodes, but further research is needed to bring these relatively young technologies to their full potential.

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1.5

Solar-hydrogen production

Chemical reactions promoted by sun energy have received significant attention as drivers for sustainable development of society, solving global energy and environmental issues. Photocatalytic technologies have many practical applications in disinfection, water treatment, hydrogen production, photoreduction of CO2 and air purification (Chou et al., 2013; Anku et al., 2018; Liu et al., 2020). Among them, in this chapter, we focus on solar-hydrogen production as a renewable and carbon-free technology that helps solve the world energy crisis (Anku et al., 2018; Ganguly et al., 2019). However, this technology is only in lab-scale, and there is still a long journey for its transfer to the market. Given that visible-light energy constitutes about 43% of solar energy, the key challenge to industrialization of photocatalysis technologies is the search for an ideal visible-light-responsive catalysts (Liu et al., 2020), which should possess four trademarks: high photocatalytic efficiency, large specific surface area, full utilization of sunlight, and recyclability (Zhang et al., 2019).

1.5.1

Fundamentals of photocatalysis for hydrogen production

Heterogeneous photocatalytic hydrogen production can be divided into two categories: photocatalytic hydrogen production with a sacrificial agent and overall water splitting. When using a sacrificial agent, the cost and the stability of the photocatalyst should be considered. For this reason the overall water-splitting process is considered the most suitable and competitive solar hydrogen production technology (Fang and Shangguan, 2019). The water splitting reaction on a semiconductor photocatalyst can be summarized in three steps: 1. The photocatalyst absorbs photons with energy equal to or greater than the bang gap (Eg) of the semiconductor to generate photo-excited electron/hole (e-/h1) pairs. 2. The photo-excited e-/h1 pairs are separated to the conduction (CB) and valence band (VB) without recombination. 3. The chemisorbed water molecule (or sacrificial agent) on the photocatalyst is oxidized by transferring its electrons to the holes in the VB: 2H2 O 1 4h1 !O2 1 4H 1

ð1:1Þ

Then CB electrons reduce the hydrogen ions to produce hydrogen gas: 4H 1 1 4e2 !2H2

ð1:2Þ

In both directions, the net flow of electrons is null and the catalyst remains unaltered (Anku et al., 2018). The basis for the performance is the hole-mediated water oxidation (1.1), because it consumes holes, to eliminate

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the e-/h1 recombination, and helps in the extraction of protons from water to H2 evolution (Li et al., 2017). The band gap of the photocatalyst should be $ 1.23 eV (i.e., the difference between the energy levels needed to achieve the redox potential for H1/H2 and O2/H2O), but lower than 3.0 eV to be active under visible light (Fig. 1.4) (Chou et al., 2013; Liu et al., 2020). In 1972, a TiO2 electrode was first demonstrated to assist photocatalytic water splitting (Fujishima, 1972). Since then many wide band (Eg $ 3 eV) semiconductors such as TiO2, ZnO, ZnS, SrTiO3 and CuO have been tested, but they show low photocatalytic activity under visible light (Anku et al., 2018; Wu and Lee, 2018). With the aim of using visible or solar light, semiconductors with Eg # 3 eV, called visible-light-responsive photocatalysts, have been developed. Among them we can find Ag2O, Bi2WO6, InTaO4, CoO, BiVO4, Fe2O3, Cu2O, Ag3VO4, TaON, CdS, Ta3N5, CdSe, Bi2S3, SiC, g-C3N4, and Si (He et al., 2018). Regarding the catalyst, there are two ways to perform overall water splitting under visible light: (1) using a single photocatalyst system, where adequate band gap and efficient utilization of solar energy are necessary simultaneously. (2) Through a heterojunction structure, mainly in Z-scheme, where H1/H2 and O2/H2O reactions will take place on different photocatalysts, mimicking the natural photosynthesis system (Fig. 1.4) (Xu et al., 2018). This last option allows the use of semiconductors with lower band gap that can absorb a wider range of wavelengths (Fig. 1.5). Moreover, in this kind of system, a mediator is widely used to perform the redox cycle between H2 and O2 photocatalysts (Li and Li, 2017).

FIGURE 1.4 Illustration of the photocatalytic water splitting process at pH 5 0 using a single semiconductor (left) or using a Z-scheme or heterojunction (right).

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FIGURE 1.5 Energy levels at pH 5 0 of TiO2 and some bismuth-based materials used as photocatalysts (left) (Lee et al., 2018; Sun et al., 2014; Wang et al., 2014; Almeida et al., 2014; Adhikari et al., 2015; Yang et al., 2017; Lai et al., 2013; Li et al., 2015; Zhang et al., 2015; Yu et al., 2019). Solar spectrum on earth crust, visible region indicated with a rainbow, and comparison of the wavelengths absorbed by TiO2 and other semiconductors with lower band gaps (right).

1.5.2

Nanoengineering

Nanomaterials possess several advantageous properties, such as controllable structures, higher specific surface area and higher photo-quantum yields compared to early bulk photocatalytic materials, namely TiO2. Moreover due to their small size, the physicochemical properties of the nanomaterials can be tuned from the synthesis changing the metal precursors, solvents, surfactants, and synthesis pathways (Wu and Lee, 2018). Accordingly, newly developed nanostructured semiconductors have been widely studied for solar-hydrogen production and various modifications have been introduced to enhance their performance (Ni et al., 2017; Chou et al., 2013; Anku et al., 2018; Fang and Shangguan, 2019; Wu and Lee, 2018; Li and Li, 2017; Roduner, 2006). For example, improving the crystallinity of the semiconductor reduces the presence of structural defects, which act as trapping and recombination centers of the photo-generated e-/h1 pairs. In addition, smaller particle size decreases the recombination rate of the photo-generated carriers because the distance they need to travel to the surface becomes smaller.

1.5.3

Bi-based nanomaterials

Among the many semiconductors, bismuth based materials exhibit excellent photocatalytic properties under visible light due to their adequate band gap and hierarchical structure (Sun et al., 2020). The band gaps of Bi compounds are usually smaller than 3.0 eV, which make them good candidates for solar hydrogen production. Moreover, their light response range, charge-transfer properties, and redox potential are influenced by the band structure (Liu

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et al., 2020), and their intrinsic Eg and CB/VB (Conduction/Valence band) edge positions can be adjusted and fine-tuned (Wu and Lee, 2018). The CB of mixed bismuth oxides is determined by the d-orbitals of the transition metals present in the mixed oxide, while the hybridization of the Bi 6s and O 2p levels makes the VB largely dispersed, which favours the mobility of photo-induced holes in the VB and is beneficial to the oxidation reaction (Shang et al., 2014). Last but not least, they are attractive semiconductors due to their cost-effectiveness, high stability, low toxicity and earth abundant materials (Anku et al., 2018).

1.5.3.1 Bismuth chalcogenides Bi2E3 (E 5 S, Se, Te) Bismuth sulfide (Bi2S3) has an adequate band gap for photocatalytic applications, ranging from 1.3 to 1.7 eV. There are different morphologies for Bi2S3 such as nanowires, nanotubes, nanorods and nanoflowers in which quantum confinement enhances the photocatalytic efficiencies (Anku et al., 2018). Researchers have also reported a Bi2O3/Bi2S3/MoS2 p-n heterojunction that exhibits 1.5 times the water oxidation under solar irradiation compared to pure Bi2S3 (1.1). The heterojunction has been developed through a facile and practical hydrothermal method, with an O2 production of 529 μmol/h  gcat (Wu and Lee, 2018). Bismuth selenide (Bi2Se3) has a stacked layered structure and an adjustable narrow bandgap (0.3  1.2 eV) (Anku et al., 2018; Wang et al., 2018). Decoration of Bi2Se3 nanosheets with a thin layer of Bi2SeO2 to form Bi2Se3/Bi2SeO2 nanocomposites showed a band gap of 1.34 eV and allowed a hydrogen production rate of 1360 μmol/h  gcat (Wang et al., 2018). Rajamathi et al. reported bismuth telluride (Bi2Te3) nanosheets that led to a hydrogen production of 921.2 μmol/h  gcat. The better performance of nanosheets is attributed to their higher surface and greater number of active sites (Rajamathi et al., 2017). 1.5.3.2 Ternary Bismuth Chalcogenides (I-Bi-VI2) Ternary bismuth chalcogenides, I-Bi-VI2 compounds are an interesting class of photocatalysts due to their high absorption coefficient, suitable band gap for light harvesting and unique electro-optical properties (Ganguly et al., 2019; Choi et al., 2017; Hu et al., 2018). Ganguly et al. reported AgBiS2-TiO2 heterostructures as photocatalyst for hydrogen production. Pristine AgBiS2 showed an Eg 5 3.19 eV and the position of the energy bands were CB 5 -0.9417 eV and VB 5 2.248 vs. NHE. In order to evaluate the AgBiS2-TiO2 solar-hydrogen production, methanol ([CH3OH] 5 1 0 vol.%) was used as a sacrificial agent, and the highest value of hydrogen production rate was 1310 μmol min-1 (Ganguly et al., 2019).

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1.5.3.3 Bismuth-based composite oxides A variety of bismuth-based composite oxides are being actively investigated as promising high efficiency photocatalysts for solar water splitting; these mainly include: Bi2O3 (Anku et al., 2018; Fang and Shangguan, 2019; Wu and Lee, 2018; Chitrada et al., 2016; Walsh et al., 2006), BiOX (X 5 Cl, Br and I) (Lee et al., 2018; Yang et al., 2018), BiMO4 (M 5 P, V, Nb and Ta) (Huang et al., 2014; Pan et al., 2011; Lv et al., 2013; Wang et al., 2012; Sun et al., 2014; Hu et al., 2012; Lee et al., 2003; Wang et al., 2014; Zhai et al., 2013; Almeida et al., 2014; Hou et al., 2011; Pan and Zhu, 2011; Pan and Zhu, 2010; Xie et al., 2020; Tokunaga et al., 2001; Kudo et al., 1998; Zhao et al., 2014; Muktha et al., 2006; Nagabhushana et al., 2013), Bi2MO6 (M 5 Cr, Mo and W) (Brown et al., 2014; Zhang et al., 2010; Tian et al., 2011; Liu et al., 2008; Fu et al., 2006), BiFeO3 (Luo and Maggard, 2006), BiYO3 (zeng Qin et al., 2009) and (BiO)2CO3 (Madhusudan et al., 2011). 1.5.3.3.1

Bismuth oxides

Bismuth oxide (Bi2O3) can exhibit both n and p-type conductivity depending on the synthesis conditions. It is an ideal material for water splitting due to its high refractive index, dielectric permittivity, exceptional photoconductivity and band gap (2.402.80 eV) (Anku et al., 2018). Although few studies have reported the performance of the binary Bi2O3, a variety of Bi2O3-based materials have shown to improve solar-hydrogen production. Bi2O3/TaON and Ta3N5 heterojunctions have been developed and the results show an improvement in hydrogen production against the pristine materials (TaON and Ta3N5). Specifically, the hydrogen production goes from 17 to 23 μmol/ h  gcat for Bi2O3/TaON and from 0.75 to 1.75 μmol/h  gcat for Bi2O3/Ta3N5. The better performance of Ta3N5 against TaON was explained by their VB positions. The VB of Ta3N5 (2.12 eV vs. NHE) has a lower energy than TaON VB (3.11 eV vs. NHE), making the injection of electrons from Bi2O3 CB to Ta3N5 VB more energetically favorable than to TaON VB in the Zscheme (Anku et al., 2018; Adhikari et al., 2015). Also, Bi2Ga3.6Fe0.4O9 loaded with RuOx has proper CB and VB potentials to enhance the solar water splitting performance and shows a greater hydrogen production of 41.5 μmol/h  gcat as compared to Bi2Ga4O9 alone (19.3 μmol/h  gcat) (Yang et al., 2017). Cr2O3/Pt/RuO2:Bi2O3 nanocomposite enhances the solarhydrogen production to 17.2 μmol/h  gcat against the pristine Bi2O3 with μmol/h  gcat production (Wu and Lee, 2018). 1.5.3.3.2 Bismuth Oxyhalides BiOX (X 5 Cl, Br, I) BiOX (X 5 Cl, Br, and I) are layered semiconductors that crystallize in a tetragonal matlockite form. This arrangement enables BiOX to possess unique optical, mechanical, electrical and catalytic properties and chemical stability (Anku et al., 2018).

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The experimental band gaps for the p-type semiconductor: BiOCl, BiOBr, and BiOI are 3.22, 2.64, and 1.77 eV, respectively. The band gaps decrease with the increase of halogen atomic number moving from the UVlight region towards the visible-light region (Yang et al., 2018). According to Lee et al., BiOI and BiOBr achieved a hydrogen evolution rate of 1316.9 μmol/h  gcat and 750 μmol/h  gcat, respectively (Lee et al., 2018). Black ultrathin BiOCl nanosheets with expanded {001} facet spacing and abundant oxygen vacancies, loaded with Pt, showed an hydrogen production rate of 79.2 μmol/h  gcat (Anku et al., 2018; Lee et al., 2018; Yang et al., 2018). Even though the CB potential of BiOX cannot satisfy the reduction potential of H1 to H2, recent studies clearly corroborated their solar H2-production achieved by several strategies such as heterojunction creation (e.g. with α-Fe2O3), cocatalysts introduction, use of photosensitizers, doping with foreign elements, tailoring crystal facets, utilizing defects, applying strains, or reducing thickness (Yang et al., 2018). These demonstrate the feasibility of the BiOX photocatalysts for H2 production under visible light with the help of some strategies. 1.5.3.3.3

BiMO4 (M 5 P, V, Nb and Ta)

BiPO4 has been reported to be active for photocatalytic solar-hydrogen production (Pan et al., 2014). BiPO4 possess a conduction band edge (-0.7 V vs SCE) (Pan et al., 2011), sufficiently more negative than the reduction potential of water. It has a suitable band structure, good photocatalytic performance, and considerable chemical stability (Pan and Zhu, 2011). Also, the bismuth phosphate family allow the fine-tuning of lattice parameters through oxygen defect engineering. This strategy not only can adjust the bandgap structure but also can act as an electron trap for photoinduced e-/h1 separation, which play an important role in photocatalytic activity (Xie et al., 2020). Pan et al. reported a hydrogen production of 3060 μmol/h  gcat for BiPO4/reduced graphene oxide nanocomposite (Pan et al., 2014). An important member of the family of bismuth-related materials is the bismuth vanadate (BiVO4) because it is environmentally friendly, highly resistant to photocorrosion, non-toxic and low cost (Anku et al., 2018). It crystallizes in three main phases but only the monoclinic scheelite, presenting absorption in the UV and the visible region (Eg 5 2.4 eV), is active in photocatalysis (Tokunaga et al., 2001; Kudo et al., 1998). In 1998, the Kudo’s group first reported BiVO4 as a promising photocatalyst for water splitting, and since then, many studies have focused on their applications in solar-hydrogen production (Sun et al., 2014; Zhao et al., 2014; Nagabhushana et al., 2013). Nagabhushana et al. reported a hydrogen production rate of 48.9 μmol/h  gcat using BiVO4 nanocrystals (Nagabhushana et al., 2013).

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BiNbO4 and BiTaO4 are investigated as catalysts in both contaminant photodegradation and H2 generation (Hu et al., 2012; Lee et al., 2003; Wang et al., 2014; Muktha et al., 2006). In these compounds, the photocatalytic performance is mainly linked with the crystalline phase and specific surface area. In general, the orthorhombic (α phase) samples exhibited much higher activity than that of triclinic ß phases (Zhai et al., 2013). Although, these compounds possess narrow band gap energy (Eg , 3 eV) and suitable VB and CB positions to be good photocatalysts for solar-hydrogen production, they show poor photocatalytic activity above 400 nm, probably due to the low absorption intensity of BiTaO4 in the visible region (Almeida et al., 2014). Doping and/or phase transitions have been explored for tailoring band-gap energies and band edge potentials. Some wet-chemical synthesis routes, such as the citrate method at low temperature, can result in stoichiometric nanosized powders of both simple and complicated oxides that leads to interesting results for solar-hydrogen production (Wang et al., 2014; Zhai et al., 2013; Almeida et al., 2014). Almeida et al. prepared pure and Cr(III) and Mo(V)-doped BiNbO4 and BiTaO4 by the citrate method. While Cr(III)-doped BiTaO4 and BiNbO4 are more selective for solar-hydrogen production, Mo(V)-doped materials are more selective for CO2 generation. Theoretical calculations show that there is a slight shift of the conduction band minimum (CBM) potential in Cr(III)doped BiTaO4 and BiNbO4 increasing their reduction potential (Almeida et al., 2014). Also, Hou et al. reported that Cr-doping of bismuth titanate enhanced the light absorption from UV to the visible region with an improvement in the hydrogen production rate. Bi4Ti2.6Cr0.4O12 composition led to a hydrogen production of 98 μmol/h  gcat. Furthermore, NiOx was used as co-catalyst improving the hydrogen rate up to 140 μmol/h  gcat (Hou et al., 2011). 1.5.3.3.4 Aurivillius oxides Bi2MO6 (M 5 Cr, Mo and W) Aurivillius phases are formulated as Bi2An-1BnO3n13 consist of n perovskite layers (An-1BnO3n13)2- between bismuth oxides sheets. Aurivillius oxides are an effective photocatalyst for water splitting and they have gained much attention due to their layered structure and unique properties (Kendall et al., 1996; Lai et al., 2013; Li et al., 2020). Aurivillius oxides exhibit optical and shape-dependent photocatalytic properties (Zhang et al., 2007), but up to now, the overall water splitting to simultaneously produce H2 and O2 is still a challenging task due to the inadequate position of the energy levels. In 2006, Shimodaira et al. first reported the potential of Bi2MoO6 as a visible light active photocatalyst for O2 evolution (Shimodaira et al., 2006). Bismuth molybdates have an exclusive layered structure, narrow band gap and visible-light response. They have been widely studied and reported as excellent photocatalysts in water splitting and degradation of organic

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compounds (Shimodaira et al., 2006; Pandit et al., 2018; Stelo et al., 2020). Li et al. synthesized Bi2MoO6/C3N4 composites for photocatalytic hydrogen evolution with triethanolamine as a sacrificial reagent. The composite of C3N4 with a 20 %wt. of Bi2MoO6 exhibited a hydrogen generation rate of 563.4 μmol/h  gcat (Li et al., 2017). Several works revealed Bi2WO6 (with a Eg 5 2.8 eV) as an excellent photocatalytic and solar-energy-conversion material (Liu et al., 2008; Zhang et al., 2007; Zhang et al., 2014). Liu et al. first found that BiYWO6 oxide solid solution present a good performance as a photocatalyst for overall water splitting under the visible light when loading co-catalysts such as RuO2, Cr2O3, Pt, and Au. BiYWO6 with 0.5 wt.% RuO2 as co-catalyst has the best activity, with a hydrogen production rate of 13.7 μmol/h  gcat. These results show that the formation of BiYWO6 solid solution was a suitable way to adjust the CB and VB to obtain a visible-light-driven photocatalyst (Liu et al., 2008). Recently, Wu et al. reported that 2D Bi2WO6 nanosheets sensitized with Rhodamine-B can reach a hydrogen production rate of 56.9 μmol/ h  gcat, under solar light, while no activity was found for Bi2WO6 nanocrystals (Wu et al., 2020). Although Bi2CrO6 has been rarely reported in comparison with Bi2MoO6 or Bi2WO6, a recent study shows that it can absorb a wide range of wavelengths of the solar spectrum (Eg 5 1.99 eV). However, the apparent quantum yield (0.0104%) was too low for practical applications and more study is needed (Li et al., 2020).

1.5.4

Summary

Nanostructured semiconductors are a good alternative to convert solar irradiation into hydrogen since they can be easily tuned. The key in solarhydrogen production is to seek nanostructured photocatalysts capable of efficient and cost-effective conversion of sunlight into hydrogen from overall water splitting. Among others, bismuth based semiconductors such as BiVO4, Bi2O3, Bi2WO6, BiOl, Bi2MoO6, Bi2Ga4O9, and BiPO4 are being actively investigated as promising high efficiency photocatalysts for solar water splitting applications (Chitrada et al., 2016; Zhao et al., 2014).

1.6

Conclusions

In this chapter we have shown that many different bismuth-based materials: from metallic bismuth to halides, chalcogenides or more complex formulations, are excellent candidates for energy applications. We have focused on photovoltaics, thermoelectrics, batteries, supercapacitors, and solar-hydrogen production. Apart from the functional properties of the bismuth-based materials, they offer the advantage of being composed of abundant, non-toxic elements offering the option for large-scale commercial applications without

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compromising human health or the environment. Moreover, we have shown that the use of nanomaterials can be beneficial for improving the performance.

Acknowledgements Authors acknowledge Fundacio´n Iberdrola Espan˜a; Programa Operativo FEDER Arago´n 20142020, “Construyendo Europa desde Arago´n” (Ref. LMP35_18); and M-ERA.NET network, project "NOEL" (reference PCI2019103637) funded by the Agencia Estatal de Investigacio´n through the PCI 2019 call.

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Chapter 2

Emergent materials and concepts for solar cell applications Mar´ıa Dolores Perez1,2 and Juan Pla´ 1,2 1

Institute for Nanoscience and Nanotechnology (INN)—National Atomic Energy Commission (CNEA)—National Council for Scientific and Technical Research (CONICET), Buenos Aires, Argentina, 2Solar Energy Department— National Atomic Energy Commision, Av. General Paz 1499, San Martin, Buenos Aires, Argentina

2.1

Introduction

Solar photovoltaics (PV) is already an economically competitive alternative energy source in many markets, giving the cost of energy generation plumbed over the last decade while the cost for other energy sources like fossil fuel based or nuclear remained relatively constant (Haegel et al., 2019). As an example, the average selling price of PV modules have had a drastic fall, as much as 18 times since 2009 (Green, 2019). The usual interpretation assigns this favorable price drop to diverse factors; among these, the active government policies to open markets and technology dissemination, technological advance given by Research and Development (R&D) investment, and economic scale at production level. A quite rigorous approach can be found in the article of Kavlak et al. (2018). Green (2019) revised this issue and gave a very interesting and deep inside vision related to serendipity and particular political decisions or historical facts, like the launch of the project independence by the former US president Nixon as a consequence of the 1973 oil crisis, the Chernobyl nuclear accident, and the establishment of a long term feed-in-tariff scheme in Germany in 1998. Accompanying the aforementioned factors, there was an impressive rising of the installed PV power. The evolution of the cumulative PV power can be found in Ren21, reaching 627 GW at the end of 2019. The historical dominant technology in the PV industry is based on crystalline silicon (c-Si, mono or multicrystalline), accounting of about the 96% Sustainable Material Solutions for Solar Energy Technologies. DOI: https://doi.org/10.1016/B978-0-12-821592-0.00010-8 © 2021 Elsevier Inc. All rights reserved. 37

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of the global PV market by 2018. Particularly, the multicrystalline Si technology represented 51% of the market during that year (Burger et al., 2020). At the same time, the sum of the thin film technologies saw a consistent decrease along the years, reaching just an about 4% of the market at 2018 (Burger et al., 2020). These technologies were mainly based on CdTe and chalcopyrite Cu(In,Ga)Se2, with a minor contribution of amorphous Si. Other noncommercial technologies are under R&D, although they can be considered in very specific applications; among them DSSC (dye sensitized solar cells, or Gra¨tzel’s cells), based on organic materials (OPV, organic PV), quantum dot solar cells, and finally those based on perovskite solar cells (PSCs), one of the main issues of this chapter. Interest in solar cells based on PSC materials have soared since this particular semiconductor was first applied successfully in a solar cell in 2009. By that time, the emerging PV technology based on organic solar cells was laying short in terms of efficiencies. Even though OPVs were promising in terms of reducing processing costs and the use of abundant and environmentally friendly materials, the PV performance was relatively low due to the inherent characteristic of organic materials: high exciton recombination and low carrier mobilities. Hybrid organicinorganic PSC found a fertile ground: many researchers with long experience in OPVs started working in PSCs and were able to apply much of the knowledge and technology allowing the field to grow rapidly. The interdisciplinary work is signature of the advances achieved in PSCs. Chemists, physicists, theoreticians, and engineers collaborate and contribute in search of knowledge from the basic understanding of the material properties to the correct design, fabrication, and testing of efficient working devices. This has been shown in the exceptional time evolution of the efficiency of PSCs as can be observed in Fig. 2.1. In less than a decade, PSCs have almost reached the best reported laboratory c-Si solar cell which has more than four decades of development. The stability and toxicity are two main issues that are still under study and must be resolved to make PSC a PV technology viable for commercialization (Jena et al., 2019; Urbina, 2020). An increase in the device area is needed for effective product placement and this will only be cost effective if stability and the toxicity issues are minimized. Hybrid PVs are rather liable to ambient conditions, they have been shown to decompose under humidity and ultraviolet light. Also, the toxicity of Pb is an issue that will not necessarily pose an extra concern if devices are carefully encapsulated and properly disposed or recycled; however, many efforts in replacing the Pb cation are underway. The toxicity of Pb is also dependent on the stability under water of the perovskite material that will make the toxic Pb21 environmentally available. On the other side, multijunction (MJ) solar cells based on IIIV semiconductor materials are the standard in the industry for space applications, and there are also commercial modules for terrestrial applications that make use of the very high efficiency and stability of these devices using optical

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39

FIGURE 2.1 Lab efficiency records for selected solar cell technologies. The traditional technology based on c-Si show maturity and will hardly evolve further, while IIIVs and PSCs demonstrate a noticeable development. MM stands for metamorphic, IMM for inverted metamorphic, and LM for lattice matched (this will be clarified later). In the case of measurements under concentration the data of the number of suns is included. c-Si, Crystalline silicon; PSC, perovskite solar cell. Data were extracted from Burger, B., et al., 2020. rFraunhofer ISE: photovoltaics report. https://www.ise.fraunhofer.de/en/publications/studies.html; NREL 2020, Best researchcell efficiency chart. https://www.nrel.gov/pv/cell-efficiency.html.

concentration of the sunlight, providing a better economical and performance equation. IIIV materials run with several historic advantages. GaAs is a semiconductor widely studied for several decades, with a direct bandgap that potentially allows very thin devices; it also presents an excellent stability and can be grown with crystallinity and purity of electronic devices with high carrier mobility. Until about 1990 GaAs devices competed with c-Si in space applications, and MJ cells began to be developed. 10 years later, IIIV cells displaced those based on c-Si from the space market because of their superior conversion efficiency and radiation resistance performance. At the same time, research associated to the development of concentrating PV (CPV) taking advantage of their high conversion efficiency came forward. In the following sections, we will analyze in detail the main topics of the two technologies based on perovskite and IIIV materials. They share the potential to be considered the best candidates to compete in the medium and long term with the well-established c-Si based PV technology.

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2.2 2.2.1

Perovskite solar cells Historical review

Perovskites are a family of materials that share the crystal structure with the mineral CaTiO3 that was named after the famous Russian mineralogist Lev Perovskiy in (17921856). The formula is of the type ABX3, where A is a large cation (organic for hybrid perovskites), B is an inorganic cation, and X is a halide anion. Typically, the phase with the best properties for PVs is the cubic phase, however, depending on the ionic sizes, some combinations of ions do not yield the cubic perovskite structure and the material is mostly unstable. In the ideal cubic structure, the A cation is located at the cube corner positions and has a 12-fold coordination. The B cation sits at the body center positions with a sixfold coordination and surrounded by octahedron X anions that occupy face-center positions. The most widely studied material is methylammonium (MA) lead iodide (MAPbI3) which is most commonly obtained in the cubic crystalline structure. The ionic nature of halide hybrid perovskites must be underlined; this characteristic offers unique properties when compared to other classical semiconductors for PV applications like Si, GaAs, CdTe, that are essentially covalent. The simultaneous ionic and semiconductor nature allows to easily modify the bandgap energy (Eg) and optical absorption by varying the perovskite composition (Eperon et al., 2014; Suarez et al., 2014). The incorporation of Cl2 and Br2 in the X-site greatly affects the perovskite properties. While it is not clear that the Cl2 incorporates into the crystal structure, it greatly affects the morphology enhancing the cell efficiency. On the other hand, the inclusion of the smaller Br2 ions, increase the Eg by contracting the lattice in a quadratic relationship with respect to the concentration of Br. Many mixed cations and anions perovskites are under study for their application in solar cells (Ono et al., 2017). Usually, the A cation is partially replaced by other organic cations like formamidinuim (FA1) to yield highly efficient devices. For example, the solar cell containing 40% FA1 and 60% MA1 (FA0.4MA0.6PbI3) are among the most efficient cells (Pellet et al., 2014). If the replacing cation is larger than MA1, then the lattice is expanded and the bandgap is reduced. If the cation is smaller, like Cs1 or Rb1, then the Eg is increased due to lattice contraction. The latter perovskites replace the A-site by an inorganic cation and are therefore called inorganic perovskites. Many researchers are directing their efforts toward the use of inorganic perovskites for PVs due to the enhanced stability but so far, the efficiencies have not excelled. While solution processing, either in one step or two steps, is the most widespread method for perovskite synthesis, thermal vapor deposition has also received a lot of attention (Shi and Jayatissa, 2018). Other techniques like blade coating, slot-die coating, meniscus coating, spray coating, inkjet printing, screen printing, and electrodeposition will need to be put in place to

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scale up device fabrication for commercialization (Li et al., 2018a). The synthesis is adjusted to provide good quality films, that is, cubic crystalline structure, large grains in the order of 100 nm and flat and dense pin-hole free films. The solution synthesis often involves antisolvent dripping to promote good crystallization (Xiao et al., 2014; Jeon et al., 2014), solvent-assisted vapor annealing in a “two-step” method (Yu et al., 2015; Liu et al., 2015) and inclusion of additives in the precursor solution (You et al., 2017; Mabrouk et al., 2017). There are a number of extensive studies of the dependence of the quality of the fabricated materials with a variety of synthesis parameters, a neat control of the fabrication is required for reproducibility and to ensure optimal optoelectronic properties like high optical absorption coefficient, large carrier diffusion length and ambipolar carrier mobility (Salim et al., 2015; Li et al., 2018b).

2.2.2

Solar cells

Solar cells are fabricated in different configurations that contain a perovskite layer as the optoelectronically active material. The most efficient cells place the perovskite film sandwiched between two layers that provide efficient charge extraction, a hole transporting material (HTM) and an electron transporting material (ETM). The mechanism of charge extraction is presented in Fig. 2.2A. The different architectures are construed in the standard physics of solar cells interpretation as n-i-p and p-i-n depending on the construction orientation towards illumination, where the n-type layer is the ETM and the p-type layer is the HTM. Fig. 2.2B shows the most typical configurations which include the use of mesoporous ETM like TiO2 that provides a scaffold

FIGURE 2.2 (A) Schematics of energy level alignment of the active layers and charge extraction, (B) different architectures of perovskite devices. TCO, Transparent conducting oxide.

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for enhanced crystalline growth and more efficient electron extraction. Also, due to the long carrier diffusion length and slow recombination of perovskites, reasonable PV performance can be obtained without one of the layers (Jena et al., 2019). As mentioned in Section 2.1, the efficiency of PSCs has increased notoriously since the first publication. MAPbI3 was first employed as a PV material in 2009 when applied as a sensitizer in a typical DSSC (or Gra¨tzel cell) configuration employing an electrolyte based on a Li redox couple in an organic solvent for hole extraction but it could only operate for a few minutes due to perovskite dissolution into the organic solvent (Kojima et al., 2009). By that time, big efforts were being made in developing materials that could act as solid electrolytes HTMs that would prevent the perovskite dissolution into the liquid electrolyte, such as the molecule spiro-OMeTAD, so it did not take too long before the n-i-p configuration was successfully introduced with outstanding efficiency of 10.9% in 2012 (Lee et al., 2012). The use of mesoscopic TiO2 was also translated from the DSSC field; both materials, TiO2 and spiro-OMeTAD, are still being applied as standards in the PSC research community due to their optimum results. The first few years after the 2012 publication boomed, there was research dedicated to the study of the materials properties, exploration of different synthetic routes for improved crystallization, and understanding of the impact of the experimental conditions on the cell performance. The development of new perovskite materials for solar cells was the focus of the road to boost the cells’ efficiency. As mentioned above, hybrid mixed composition perovskites are the materials with record efficiencies. Asite cations and anions are varied stoichiometrically within ranges that allow the cubic structure according to the geometrical tolerance factor. Table 2.1 presents the PV performance of a representative number of solar cells fabricated with different perovskite materials, HTMs, and ETMs. The ETM must be a substance with a band alignment that places the lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) higher that the perovskite active layer. In that sense, ETMs must be adjusted to the particular perovskite used in the chosen cell architecture. It must also have a high UVvis transmittance so as not to absorb part of the light and most importantly, the electron mobility must be high to efficiently extract carriers and prevent recombination losses. While TiO2 is the most widespread material used as ETM it also presents stability issues due to the induced photodegradation (Leijtens et al., 2013). Another drawback for the use of TiO2 is the high annealing temperatures employed for anatase crystallization that prevents the use of flexible substrates for unique applications. Mesoporous titania yields the best efficiencies as compared to planar TiO2 because the material porosity provides a 3D scaffold for improved crystallization of the perovskite resulting in larger grains and reduced defects. Other metal oxides are also applied as the electron

TABLE 2.1 A few examples of best efficiencies for different PSC configurations and materials. Perovskite

HTL

ETL

FF

Jsc (mA/cm2)

Voc (V)

PCE (%)

Ref.

MAPbI3

Spiro-OMeTAD

mTiO2

0.741

21.64

1.056

17.01

Im et al. (2014)

MAPbI3

Spiro-OMeTAD

TiO2 planar

0.648

19.7

0.974

12.66

Huang et al. (2016)

MAPbI3

Spiro-OMeTAD

ZnO

0.749

20.4

1.03

15.7

Liu and Kelly (2014)

MAPbI3

P3HT

mTiO2

0.66

19.1

0.98

12.4

Guo et al. (2014)

MAPbI3

CuSCN

mTiO2

0.62

19.7

1.016

12.4

Qin et al. (2014)

MA0.6FA0.4PbI3

Spiro-OMeTAD

mTiO2

0.7

21.2

1.003

14.9

Pellet et al. (2014)

Cs0.05FA0.81MA0.14PbI2.55Br0.45

PTAA

PCBM

0.796

22.51

1.07

19.16

Yang et al. (2019)

(FAI)0.81(PbI2)0.85(MAPbBr3)0.15

Spiro-OMeTAD

mTiO2

0.78

23.7

1.14

21.6

Bi et al. (2016)

FA0.85MA0.15PbI2.55Br0.45

PTAA

mTiO2

0.73

22.0

1.08

17.3

Jeon et al. (2015)

ETL, Electron transporting layers; FF, fill factor; HTL, hole transporting layer; Jsc, short circuit current; PCE, power conversion efficiency; Voc, open circuit voltage.

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transporting layer (ETL). SnO2 has been successfully applied with elevated efficiencies. It presents high electron mobilities with deep conduction band that promotes charge transfer and reduces charge accumulation at the ETL/ perovskite interface (Mahmood et al., 2017). ZnO doped with Al has also yielded good results. Organic molecules also have proper functionality as ETM with the added condition that can be flexible and do not need high temperature processing, being the most used fullerene molecule and its derivatives. Fullerenes can be applied from solution as phenyl-C60-butyric acid methyl ester (PCBM) which is soluble in organic solvents. It has been applied both in n-i-p and p-i-n, also called inverted, devices and it yields high performance devices due to the high electron mobilities and can passivate grain defects reducing charge accumulation and hysteresis. The original C60 form is also used as ETM but it must be applied from its vapor phase due to the lack of solubility (Zheng et al., 2019). Other nonfullerene molecules can be applied as electron transport layer but their use remains marginal. The HTM is responsible for the hole extraction, therefore the HOMO level must lie above the perovskite valence band and the LUMO level above the conduction band to block electrons and prevent charge recombination. HTMs that yield highly efficient devices present high hole mobility and provide a good interface that prevents charge accumulation with efficient charge extraction (Rakstys et al., 2019). A number of organic molecules and inorganic compounds have been applied as the HTM, but the most widespread materials are spiro-OMeTAD and the polymer PTAA (poly[bis(4-phenyl) (2,4,6-trimethylphenyl)amine]). Both molecules are extremely expensive and despite the fact that the spiro-OMeTAD is the most commonly used material, it is itself a poor hole conductor and requires doping that in turn adds stability issues to the device that will be discussed later. Inorganic compounds like CuI, CuSCN, MoO3, NiOx, to name a few, offer an inexpensive and more robust alternative but they still lag behind in terms of efficiency (Rakstys et al., 2019; Pashaei et al., 2020). The Pb toxicity issue is assessed by the investigation of materials alternatives with Pb-free perovskites. A great number of researchers have embarked on this quest; however, the results have not been encouraging. The best candidates for Pb replacement, taking into account the ionic size and PV properties, are Sn21, Ge21, Mg21, Mn21, Ni21, and Co21 (Hoefler et al., 2017). ˚ ) to Pb21 (1.49 A ˚ ) and Sn-based perSn21 has a very similar radius (1.35 A ovskites present a very attractive lower bandgap and high carrier mobility, that make them an exceptional option for replacing lead (Hoefler et al., 2017), however, they are mostly unstable due to rapid oxidation of Sn21 to Sn41. The oxidized ion causes self-doping of the perovskite and limits the power conversion efficiency. Also, uncontrolled crystallization yields poor films morphology that is detrimental for device operation (Hao et al., 2014). The stability has been found to improve slightly by including additives like

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SnF2, SnCl2, and H3PO4 to the precursor solution. It is believed that the additives compensate the missing Sn21 from oxidation but more research is needed to completely understand the mechanism of stabilization to find new routes and strategies for further improvement (Bin Song et al., 2017). It has also been described that the Sn perovskites do not form an efficient interface with the commonly used Spiro-OMeTAD HTM and other materials must be used for stability. Most devices presented hysteresis during the JV (currentvoltage) testing that was attributed to the intrinsic ionic movement within the perovskite films. Hysteresis is an undesired effect for solar cells performance and many efforts were dedicated to its understanding and resolution (Kang and Park, 2019). Protocols for device testing were implemented to account for the hysteresis and the selective forward and reverse testing performance (Dunbar et al., 2017; Zimmermann et al., 2016; Nemnes et al., 2018). Hysteresis was observed to depend strongly on the crystalline quality and the interfaces since moving ions accumulate and provoke a capacitance effect. It is well known that TiO2 poses a large hysteretic behavior due to charge accumulation at the MAPbI3/TiO2 interface whereas for other materials like PCBM or PEDOT there is minimum capacitive current (Zhang et al., 2015). Also, mesoscopic TiO2 presents a reduced hysteresis compared to planar titania since the more intricate interface offers more surface area for charge extraction and diminishes charge accumulation as can be observed in Fig. 2.3 (Kim and Park, 2014). Capacitance can arise from grain boundaries and material defects that accumulates charges, therefore, strategies for defect passivation, crystallinity enhancement and 2D/3D structures offer devices with reduced JV hysteresis (Kang and Park, 2019; Liu et al., 2019). Nowadays, hysteresis is no longer considered an issue to resolve as long as the quality of the films and interfaces is ensured; other issues like stability are currently at the spotlight.

2.2.3

Stability

Solar cells must offer stable power at the operation conditions (real sun irradiation, atmospheric moisture and O2) for a long period of time. In the case of silicon PVs this time is established as 25 years. However, hybrid perovskites have been found to degrade upon water and UV light exposure (Jena et al., 2019; Boyd et al., 2019), as presented below: Degradation by moisture mechanism: 4CH3 NH3 PbI3 1 4H2 O24½CH3 NH3 PbI3 H2 O2 2ðCH3 NH3 Þ4 PbI6 2H2 O 1 3PbI2 1 2H2 O ðCH3 NH3 Þ4 PbI6 2H2 O-4CH3 NH3 I 1 PbI2 1 2H2 O

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FIGURE 2.3 Side view SEM images of devices with mesoporous TiO2 (A) and (B); and planar device without mp-TiO2. JV curves in FS and RS at AM1.5 G one sun (100 mW/cm2) below correspond to the top devices. FS, Forward scan; RS, reverse scan. Reprinted with permission from Kim, H.-S., Park, N.-G., 2014. Parameters affecting IV hysteresis of CH3NH3PbI3 perovskite solar cells: effects of perovskite crystal size and mesoporous TiO2 layer. J. Phys. Chem. Lett. 5 (17), 29272934, doi: 10.1021/jz501392m. Copyright 2014 American Chemical Society.

Degradation by light mechanism: light

CH3 NH3 PbI3 - CH3 NH3 PbI3 O2

CH3 NH3 PbI3

-





O2 2

CH3 NH3 PbI3 1 O2 2 -CH3 NH2 1 PbI2 1 1=2I2 1 H2 O While decomposition of the perovskite layer due to water or O2 can be mitigated by physical barriers that prevent insertion into the device, illumination is the essential part of the operation of a solar cell. Degradation by light cannot be easily solved and requires rational architectural device design and chemical modification of the active layers. Many strategies are designed to prevent degradation of the perovskite layer. These include sealing and encapsulation of the final solar cell, the optimized design of the device architecture to avoid moisture penetration through addition of hydrophobic layers, passivation of perovskite by surface molecular addition, and 2D perovskite design with hydrophobic groups. On the other hand, O2 incorporation inside the device inner layers is not easily averted, being encapsulation the most feasible strategy. The device performance degradation by light is mainly caused by the UV portion of the spectrum and may occur through photocatalytic degradation of

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TiO2, photoinduced generation of trapped states and phase segregation or ion redistribution. Whereas encapsulation is not an option for the light issue, UV blocking filters can be added to the device architecture to mitigate the photodegradation effect. Also, the TiO2 could be easily replaced by organic layers that do not present photocatalytic properties, like PCBM and C60. The modification of the perovskite layer morphologically or chemically was proven to be a successful strategy to prevent moisture degradation (Boyd et al., 2019; Ono et al., 2020). For instance, a thin layer of a polymer polyvinylpyridine coating the MAPbI3 surface in contact with the HTL or over the TiO2 to promote crystallization of the perovskite, shows increased resistance to water and improved Voc due to reduced recombination (Duan et al., 2020; Chaudhary et al., 2017; Yavari et al., 2018). Other materials, for example, thiols, oleic acid, and triblock copolymers, have also been applied as a surface interfacial layer that have shown good response to moisture and enhanced device parameters due to reduced grain boundaries (Ono et al., 2020; Abdelmageed et al., 2018; Zong et al., 2018; Cao et al., 2015b; Liu et al., 2020). Another strategy is the development of 2D/3D structures that include internal layers of molecules within the perovskite structure. The 2D perovskites contain a large hydrophobic organic cation as a spacer between the octahedral perovskite planes. It is well established that 2D perovskites present higher stability and resistance to moisture and heat due to their unique crystal structure but they lack in efficiency because the insulating cation reduces carrier mobility and increases charge accumulation and recombination (Cao et al., 2015a; Smith et al., 2014; Chen et al., 2018; Wygant et al., 2019). Therefore a combination of both structures with a careful dimensionality engineering are implemented to increase efficiency and improve the stability (Etgar, 2018; Gao et al., 2018; Krishna et al., 2019). For instance, anilinium cation was used as a small conjugated organic spacer that demonstrated high stability and maximum efficiency of 5.96% for the material prepared at 110 C and 7.63% for 190 C annealing (Rodr´ıguezRomero et al., 2018). The increase in efficiency with annealing temperature was assigned to enhanced crystallization. The resulting material applied for device fabrication used was Any2MAn21PbnI3n11 with n 5 5. With larger n, a higher efficiency is expected since the material is closer to a 3D structure and lower n increases stability with diminishing performance. Later, the improved design of a 3D to 2D perovskite ratio of 8:1, demonstrated an efficiency of 15.96%. The addition of a small amount of the anilinium cation (PbI2:MAI:AnI 5 1:0.8:0.2) provided a controlled 2D to 3D phase without a significant change in the bandgap (Abbas et al., 2019). The intrinsical stability of the perovskite is also affected by the inherent crystal structure defined by the stoichiometry. The Goldschmidt tolerance factor is an empirical index that allows the prediction of the different crystalline structures for ABX3 perovskites. Its value depends on the different ionic pffiffiffi radii and it is calculated according to the formula τ 5 RA 1 RX = 2ðRB 1 RX Þ

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where RA, RB, and RX are the radius of the ions A, B, and X, respectively. Ideal cubic perovskite structures are obtained when the tolerance factor lies in the range 0.8 , τ , 1. When τ . 1.0 or τ , 0.8, there is a diminished possibility of perovskite phase formation. For organic A cations where the ionic radius is hard to determine accurately, the tolerance factor carries an amount of uncertainty but it is still a widely used parameter for structure prediction as can be observed in Fig. 2.4 for the typically used APbI3 system. The tolerance factor was calculated for the most commonly used cations, A 5 Na, K, NH4, Rb, Cs, MA, FA, ethylamine, and ethylenediamine. Cs, MA, and FA appear in the cubic perovskite zone with FAPbI3 near the upper boundary and Cs in the lower boundary. The other cations lie outside the perovskite phase zone and can be added to the cubic perovskites to produce stabilized structures and passivate defects. For instance, large ionic size difference is preferred for phase stabilization while small ionic size difference is selected for defects passivation. While the tolerance factor is a useful tool to predict structures, it does not necessarily apply for all cases or MAPbI3 with a τ 5 0.9 would be more stable than mixed perovskites and that is experimentally not accurate. The approach of ion mixing has shown great results for both efficiency and stability, with the triple cation perovskite (MA/FA/Cs)Pb (I/Br)3 being the most popular for solar cells applications (Ono et al., 2017). The quadruple cation-based perovskite has also received a lot of attention

FIGURE 2.4 Calculated tolerance factors (τ) for different cations (A) in APbI3 perovskite system (Han et al., 2018). EA, Ethylammonium; EDA, ethylenediamine; FA, formamidinuim; MA, methylammonium. Reprinted with permission from Han, G. et al., 2018. Additive selection strategy for high performance perovskite photovoltaics. J. Phys. Chem. C. 122 (25), 1388413893, doi: 10.1021/acs.jpcc.8b00980. Copyright 2018 American Chemical Society.

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due to its high cell efficiency and stability (Kubicki et al., 2017; Jacobsson et al., 2018). The HTM is also responsible for the device stability issues. A number of materials have been applied as the HTM, but the molecule spiro-OMeTAD is still the most common due to the exceptional efficiency despite its stability issues. Inorganic materials like NiOx, MoO3, CuOx, Cu2O, CuI, and CuSCN are normally more robust materials and show longer lifetimes (Jiang et al., 2017). Other polymeric materials like P3HT, PTAA, and PEDOT:PSS have also shown good stability and efficiencies. Spiro-OMeTAD presents the highest efficiencies but displays the shortest lifetimes. This molecule is not intrinsically a good hole conductor and it must be doped to yield good performance. The most common dopant is lithium bis(trifluoromethanesulfonyl) imide (LiTSFI) which oxidizes the spiro-OMeTAD and enhances the hole mobilities. LiTSFI is hygroscopic and promotes permeation of water inside the device. Degradation by crystallization and photooxidation of spiroOMeTAD as well as Au diffusion into the HTM has also been observed as responsible for device degradation due to the HTL (Domanski et al., 2016; Sanchez and Mas-Marza, 2016; Zhao et al., 2017).

2.2.4

Scaling up and possibilities for commercialization

To be considered for commercialization, the efficiencies of laboratory solar cells must exceed 10%. Perovskites solar cells have long surpassed the minimum performance requirements and are being considered for real applications in the short term. Still, there are some issues that must be accounted for; stability and toxicity were described above, but scale up fabrication and reproducibility must also be resolved before the technology can be placed into market. Tandem cells (not covered in this chapter) that include perovskite cells associated with Si cells are nowadays a real strategy for commercialization due the installed capacity for fabrication and high efficiencies. Single junction cells on the other hand, offer simpler and energy efficient processes and reduced materials usage at the same time that ensure high efficiencies, therefore there is a lot of room for the technology to become commercially available in the following years. Increasing the device area often results in an increase of the sheet resistance, a loss in the active area due to charge collectors and a reduction in the film homogeneity that together results in the device performance degradation. A starting point towards industrialization is then the development of scale up techniques, fabrication methods and designs that minimize losses and improve cell performance. Upscale methods for perovskite deposition include blade coating, slot-die coating, ink-jet printing, screen printing, spray coating, vapor phase deposition and electrodeposition. Some of these methods can be integrated into roll-to-roll printing and other can only be done through screen-to-screen fabrication (Jena et al., 2019).

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An important challenge is to reproduce the crystallization strategies in large area perovskite deposition. For instance, antisolvent dripping, commonly used to promote controlled perovskite crystallization and to yield good quality films, cannot be easily introduced in methods like doctor blade or slot-die coating. Instead, antisolvent spraying and bathing can be introduced for large area manufacturing (Kim et al., 2017). Annealing must also be adapted successfully from the hot plate laboratory technique used in small area scale to large area techniques and include photonic flash annealing and IR light sintering among other rapid thermal treatments. In the same manner, the precursor solution formulation must be adapted for the desired morphology, crystal size and defects formation (Hu et al., 2017). All these aspects must be carefully studied to ensure the required reproducibility needed for product commercialization. Currently, efficiencies as high as 14% have been obtained for large areas single junction PSCs, and even though it is a decent performance, it is still far from the small area device efficiency and more effort should be dedicated to improve all the relevant parameters for upscaling, that is, stability, reproducibility and high efficiency (Roy et al., 2020). Recently, a number of companies have embarked on resolving the issues for commercialization and manufacturing large area devices; for example, Microquanta Semiconductor, Solliance, Saule Technologies, Greatcell Solar, and Oxford PV (Tandem). Different manufacturing processes and device architectures are currently being explored and there are still uncertainties of which method will excel and become the forefront of the technology (Qiu et al., 2018; Rong et al., 2018).

2.3 IIIV semiconductor materials for multijunction solar cells applications 2.3.1

Historical review

The interest on MJ IIIV solar cells began in the late 80s, when it was found that competitive devices respect to the traditional based on c-Si were possible to be fabricated for space applications. Replacement of the GaAs substrate by Ge wafers enabled fabrication of lighter large area devices, due to the superior mechanical strength of Ge that allowed using thinner substrates. At the same time, IIIV semiconductor materials assure higher conversion efficiencies and radiation damage resistance respect to c-Si. Moreover, the activation of the Ge substrate opened the potentiality of a better covering of the solar spectrum given its bandgap allows absorbing low energy photons complementary to the other IIIV materials grown monolithically onto that substrate. The first fabricated devices were GaAs/Ge single junctions and were naturally followed by MJ InGaP/GaAs/Ge (two or three junctions whether Ge is active or not). The choice of Ge is also based on the possibility of the epitaxial

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growth of IIIV crystalline materials (GaAs for instance) of very good quality, due to lattice parameter match and similar thermal expansion. This is not the case of the other natural candidate, the c-Si (Chang et al., 1987). The information about the characteristics of these solar cells, like subcells’ thickness, dopants, and tunnel junction structure, is in general terms disperse. In the first works (Cavicchi et al., 1988; Gillanders, 1992; Tobin et al., 1988b; Chang et al., 1987; Tobin et al., 1988a; Huggins et al., 1992), some problems associated with the unawareness of the junction formation in Ge were found, such as the anomalous J-V characteristics (there was still no understanding about the spectral matching necessity due to subcell series connection), and the autodoping of GaAs from Ge in relatively high temperature processes (B700 C). It must be mentioned that low pressure metalorganic vapor phase epitaxy (MOVPE) is the main technique to grow these semiconductor materials. The first papers describing double junctions appeared at the early 90s, first on GaAs substrates (Olson et al., 1990; Bertness et al., 1994a, 1994b), and later on Ge substrates (Chiang et al., 1994). The Ge activation was considered, and the first triple-junction solar cell was reported in the mid-90s (Chiang et al., 1996). By using lattice matched (LM) IIIV semiconductor materials it is possible to obtain InGaP/Ga(In)As/Ge triple-junction solar cells for space applications with efficiencies up to about 30% (AM0, 28 C) at industrial level (Solaero; Spectrolab; AZUR SPACE Solar Power GmbH; CESI S.p.A.), with lab prototypes reaching 37.9% (AM1.5, 25 C) for the structure InGaP/GaAs/InGaAs with inverse growing (Green et al., 2020; Yamaguchi and Luque, 2013). The main characteristics of the traditional commercial IIIV triplejunction cells are: monolithic structure, LM IIIV semiconductor materials grown by MOVPE using Ga0.99In0.01As for the middle cell, passivation layers at the front surface (named window) and rear surface (BSF, Back Surface Field) for each subcell, antireflection multilayer covering the full response spectrum, Ge junction optimized, tunnel junctions for internal connection, and thicknesses optimized for the solar spectrum. Relatively closer in time, several institutions and companies, like NREL (National Renewable Energy Laboratory, United States), Spectrolab, SolAero (formerly Emcore), and Fraunhofer Institute of Solar Energy (FhG-ISE, Germany), proposed semiconductor structures named metamorphic (MM) (Geisz et al., 2007, 2008; Guter et al., 2009; King et al., 2009; Stan et al., 2010), where the semiconductor gap is engineered for a better match with the solar spectrum changing the alloy composition, for instance Ga0.44In0.56P/ Ga0.92In0.08As/Ge. In this case the layer’s structure is grown in conditions of lattice constant mismatch, adapting the substrate using appropriate buffer layers to minimize the huge defects density introduced by that mismatch. Although theoretical simulations predict an important increase in the conversion efficiency using the approach just mentioned, the fabricated devices

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performed similarly to the LM devices under light concentration (see Fig. 2.1). This plot is based on data extracted from the NREL chart (NREL). Additional data for the 4-J (four-junction) records were retrieved from Dimroth et al. (2016) and France et al. (2015). Furthermore, IMM (inverted MM) devices perform clearly better under one sun conditions. The technical challenge to obtain good performing MM devices was later overcome with other approaches, as discussed later. The advances in IIIV solar cells have a parallel behavior for space (one sun) as well as terrestrial applications (under concentration). The parallel evolution is graphically presented in the publication from Bett et al. (2013), where an absolute increment of about 1% per year in efficiency can be noticed. While in space solar cells IIIV compound is the largely main technological option, the terrestrial PV market continues to be dominated by cSi, however, this fact does not stop the expectation of the very high efficiency and functional IIIV devices to finally share a significant portion of this market. As it is known, the electronic quality of the IIIN-V compounds (diluted nitrides), despite having the proper bandgap (about 1 eV), is not enough to provide a fourth junction in LM devices. However, an outstanding result was obtained for the structure GaInP/GaAs/GaInNAs(Sb) grown onto GaAs, giving an efficiency of 44.0% under concentration (AM1.5d, 942 suns) (Sabnis et al., 2012). Nevertheless, in the application of this concept, it is necessary to consider that the structure was grown using Molecular Beam Epitaxy, and therefore its economic feasibility should be assessed respect to industrial competitive MOVPE processes (Bett et al., 2013). The application of diluted nitrides was already explored in 4 (King et al., 2009), 5, and 6 (Dimroth et al., 2005) junction monolithic devices with a certain success, but their perspective for industrial processes seems hard to imagine and simpler structures provide better results. The most recent innovations tend to overcome the technical limitations to obtain four-junction devices or more, using two strategies. One of them is based on the concept of wafer bonding. This idea looks for the feasibility of the combination of junctions performed on materials with a better approximation to the ideal bandgaps, just as the MM devices, but using LM materials. MJs are grown in separate processes, where component materials with different lattice constant are joined in a second step of the fabrication process (bonding) with a proper electrical contact and mechanical strength. These structures accomplish two successive efficiency records for four-junction devices (Dimroth et al., 2016, 2014), as well as the record for five-junction (Chiu et al., 2014). In the first case (Dimroth et al., 2014) a conversion efficiency of 44.7% (AM1.5d, 297 suns) is reported for a device formed by two junctions grown inverted on a GaAs wafer (InGaP/GaAs) bonded to another two junctions grown upright on a InP wafer (GaInPAs/GaInAs), resulting in the decreasing bandgap structure 1.88/1.45//1.10/0.73 eV. While each double junction is LM,

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they do not result LM between them after bonding. This structure was later refined, achieving the record of 46.0% (AM1.5d, 504 suns) (Dimroth et al., 2016). The five junctions device reported in Chiu et al. (2014) follows a similar concept, in this case with a gap structure 2.2/1.7/1.4//1.05/0.73 eV (AlGaInP/ AlGaInAs/GaInAs//GaInPAs/GaInAs), but there are no major details about the materials. This device holds the five junction record with 38.8% (AM1.5 G, 1 sun) efficiency (Green et al., 2020) (not showed in Fig. 2.1). The other strategy follows the monolithic structure. For instance, four junctions are obtained growing inverted LM materials for the first two cells, and MM materials to obtain proper bandgaps for the other two cells. France et al. report (France et al., 2015) the best four-junction solar cell by using this concept reaching 45.6% (690 suns), very close from the record established by the bonding LM cells strategy (see Fig. 2.1). Finally, an outstanding result was recently published (Geisz et al., 2020), where the last record for MJ devices (and beyond of any other solar cell technology) is reported, reaching 47.1% (143 suns) for a six-junction solar cell. This device is monolithic and carefully optimized, with a bandgap structure 2.1/1.7/1.4/1.2/0.95/0.69 eV obtained using inverse growing LM materials for the three first subcells, and then MM materials, with proper buffers grading the lattice constant, for the other three subcells. We will later consider this particular case in more detail.

2.3.2

Some basics of multijunction solar cells

A solar cell makes use of the internal potential generated by a semiconductor junction to separate the carriers generated in the material by the absorbed photons of the sunlight. The generated carriers diffuse to the electrical contacts of the device, but there are several physical processes such as thermalization and recombination that imply energy losses and limit the conversion efficiency. A schematic diagram of this process is presented in Fig. 2.5A). The main observed facts are as follows: (1) the semiconductor absorbs photons with energy greater than or equal to the bandgap energy; (2) the difference of energy between the incident photon and the bandgap is lost by thermalization. Other losses include the Joule dissipation because of the finite resistance of bulk semiconductors, metallic contacts and interfaces, carrier recombination at the surfaces and interfaces, and nonzero reflectivity at the front surface. MJs divide the solar spectrum in bands, using a sequence of decreasing bandgaps, so that the difference between photon energy and energy gap is kept constrained. Fig. 2.5B shows, as an example, the case of the standard triple-junction InGaP/GaAs/Ge. The absorption for each junction is obtained multiplying the photon flux by the quantum efficiency (QE, number of electrons

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FIGURE 2.5 (A) Schematic of a one semiconductor junction band diagram, showing the main processes of energy loss: thermalization and recombination. Photons with energy less than gap energy are not absorbed and then do not generate carriers. (B) Photon flux of the AM1.5 G solar spectrum. The absorption determined by the QE for a standard InGaP/GaAs/Ge 3-J solar cell is shown in color. QE, Quantum efficiency.

generated per photon) of the device. The vertical lines mark the wavelengths corresponding to the gap of each semiconductor. The penetration of the GaAs subcell response in the InGaP subcell zone is due to the low thickness of the latter one, optimized to obtain the current match and so avoiding electrical losses. This is because of the monolithic architecture of the MJs: given the junctions are series connected, the current is limited by the subcell that generates the lowest photocurrent. As the number of junctions is increased, the absorption bands become narrower and hence lower the photogenerated current, while the voltage increases. Working with lower currents is particularly advantageous in

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terrestrial concentrating applications, when the flux of photons is consistently high: more junctions minimize resistive losses (I2R), and also tunnel junctions do not have to support very high current densities. In the limit of infinite junctions, and considering just the fundamental losses (thermalization, radiative recombination), a conversion efficiency of 65% at one sun is obtained, while the increment of the photogenerated carriers under concentration implies a further voltage increment reaching 85% in this limit (Mart´ı and Arau´jo, 1996). How IIIV compounds can approach this limit is the matter of the following section.

2.3.3

IIIV materials for photovoltaic applications

IIIV compounds have key properties to be applied as PV materials. Among these properties, we can identify the availability of bandgaps to match the absorption of the solar spectrum, the possibility of modulating the energy gap changing the composition of the constituent elements, sometimes called gap engineering, excellent stability and electronic properties, and a growth process capable to scale to industry manufacture, as it is the MOVPE. These semiconductor compounds can be binary, ternary, and also quaternary in the search for the necessary characteristic according to their function in the device. In the Fig. 2.6 are presented a few possibilities, as stoichiometric compounds or families of graded stoichiometries. In this figure the particular case of the standard InGaP/GaAs/Ge triple junction [to be precise Ga0.51In0.49P/Ga0.99In0.01As/Ge, for a truly LM conditions (Takamoto et al., 2000)] is color highlighted (blue, green, red, on the left). The vertical alignment in the lattice constant is clearly visible. Other compounds used in standard 3-J are the family of AlGaAs, according to the need of different bandgaps in the structure of the device, and AlInP (not showed in Fig. 2.6). In all cases the fulfillment of the LM condition assures good electronic properties and hence good electrical performance. An example of the standard 3-J structure can be seen, for instance, in the early work of Takamoto et al. (2000) and showed here in Fig. 2.7. As described earlier, each subcell has its own n1/p junction, and the heterostructure defined by the window and BSF layers acts as surface passivation to prevent carrier recombination (Pl´a et al., 2007). Tunnel junctions allow internal low resistance contact, while the buffer layer acts as nucleation layer for the epitaxial growth. Considering each junction individually, higher bandgap materials are used for the passivation and electrical connection to prevent photon absorption useful for carrier generation. The thicknesses in this layer structure are optimized to maximize light absorption and current matching (Friedman et al., 2011). The layers associated to passivation and tunnel junctions are kept as thin as possible to conserve their function. Typical values for a 3-J structure can be found in

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FIGURE 2.6 Energy gap versus lattice constant for selected IIIV compounds.

Friedman et al. (2011), thicknesses are of the order of 100 nm for emitters, BSFs, and tunnel junctions, 30 nm for window layers, 1 and 3 μm for the InGaP and GaAs base subcell, respectively. The heteroepitaxy is a complex chemical process, and its detailed description is beyond the scope of this work. The early paper of Stringfellow (Stringfellow, 1982) is a good reference in this issue. As a brief description, the process of our interest takes place in the chamber of a metalorganic chemical vapor deposition reactor, under controlled substrate temperature, pressure, and gas fluxes. A carrier gas transports the phase vapor of the chemical species in a given fine-tuned concentration. The usual carrier gas is H2, and metalorganic precursors (in liquid phase at ambient temperature) are currently used for Ga (trimethylgallium) and In (trimethylindium), while hydride gas sources are used as precursors for P (phosphine, PH3) and As (arsine, AsH3). For the dopant species the current choices are Zn (diethylzinc, DEZ), Si (disilane, Si2H6), and C (carbon tetrachloride, CCl4) (Friedman et al., 2011). H2 carries the vapor of MOs, and together with H2 diluted gas precursors in determined proportions are injected into the reaction chamber. Chemical reaction of precursors takes place on the substrate surface, kept at a constant temperature, followed by diffusion and nucleation in a dynamical process. The lattice mismatch in the epitaxy of a IIIV semiconductor heterostructure induces the appearance of strain that, beyond a critical layer thickness, is relieved by dislocations. This extended defect acts as nonradiative

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FIGURE 2.7 Layers structure for a standard triple-junction InGaP/GaAs/Ge solar cell. AR stands for antireflection. Dopant elements are indicated between brackets, aside the IIIV compounds.

recombination center, degrading the electronic quality of the material. This is a challenge for MM (non LM) materials, in the search for a close to ideal bandgap semiconductors for PV devices. For instance, in the paper of Guter et al. (2009), a close to ideal 3-J structure (current matched) based on a Ge substrate is proposed: Ga0.35In0.65P/ Ga0.83In0.17As/Ge, with bandgap energies of 1.67/1.18/0.66 eV, but having a large lattice mismatch of 1.1% with respect to the Ge substrate. To overcome this issue, a graded Ga12yInyAs buffer layer is instrumented by the authors, in such a way that the upper two subcells are LM and free of defects, but lattice mismatched with the Ge substrate. The parameter y is ranged from 0.01 to 0.17, along seven 200 nm buffer layers, to finally match with the middle cell lattice constant. The lattice stress is absorbed by the transparent and nonactive buffer layer, concentrating all the dislocations and allowing a free defect growth of the middle and top subcells.

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The other approach of interest to obtain close to ideal materials is the inverted growth to fabricate the already mentioned IMM solar cells. In contrast to the example of upright growth given in the last paragraph, the layer’s structure is grown from the top subcell to the bottom subcell. After growing, the layer structure is detached from the substrate using a selective etching of a sacrificial layer, a technique named epitaxial lift-off. This approach has two advantages: (1) the structure can be attached to a lightweight substrate and the original one can potentially be reused, with huge cost savings; (2) the bottom cell has not constrained by the nonideal and too narrow Ge bandgap. An example for 3-J cells can be found in the paper of Geisz et al. (2007). Using a GaAs substrate, LM InGaP/GaAs junctions are grown, combined with a MM 1.0 eV In0.3Ga0.7As third junction. In this case, a graded GaxIn12xP buffer layer is used, ranging from x 5 0.51 (matched to GaAs) to x 5 0.25 (matched to In0.3Ga0.7As). These ideas will be developed in more detail below, where we describe the later developments in MJ solar cells with two specific examples.

2.3.4

Selected examples

To gain a deeper insight into the development of novel materials and concepts related to MJ solar cells we will consider two examples, one concerning LM bonded structures, and the other with MM monolithic structures. These cases offer a good perspective of the state-of-the-art on the IIIV solar cells research activity.

2.3.4.1 Bonded lattice matched structures The already mentioned 4-J cell record based on LM materials (Dimroth et al., 2016), was preceded by a very similar structure Ga0.51In0.49P/GaAs// Ga0.16In0.84As0.31P0.69/Ga0.47In0.53As, with bandgaps 1.88/1.42//1.12/0.74 eV, developed by the same authors (Dimroth et al., 2014). We will consider this case. The two top junctions are fabricated with the well-known used in the standard 3-J devices, LM to the Ge (or GaAs) substrate, while the two bottom junctions are LM to InP; these materials are particularly highlighted in Fig. 2.6. Epitaxial growth is performed in two different MOVPE processes, the top junctions in an inverted growth on a GaAs wafer, and the bottom junctions in an upright growth on an InP wafer. Lattice match assures dislocations free materials in each case, and hence excellent electronic properties. There are no further details about the layers structure, but each subcell has the usual structure already discussed, a p-n junction with passivation layers, as well as tunnel junctions for internal connection, but with the feature of having highly doped n-type layers (with doping concentration

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ND . 1018 cm23) of GaAs and InP at the bond interface (Dimroth et al., 2014). One of the keys in this proposal is the bonding process. Among the bonding processes available, a proper one for this application should have a low temperature thermal load (to avoid dopant diffusion, for instance in the heavily doped tunnel junctions), enough mechanical strength to support the device process steps, and low contact resistance, allowing a good electric contact between mismatched structures. A description of a bonding process applied to PV devices is presented in the published development by Bhusari et al. (2011). Among its main features are: (1) Very flat surfaces; as grown surfaces have a few nm of average roughness that must be decreased to about 0.5 nm; (2) low particle count on the surface; particles are identified as the origin of defects in the surface bonding (Dimroth et al., 2014). After a chemical mechanical polishing process of 10 minutes, they obtained 0.36 nm of average surface roughness and ,1000 particles on a 40 3 40 μm2 area. Detailed data about the process parameters, such as temperature, pressure, temperature ramp rates and time, are not given. The authors claim their process produces a very high strength bond (bond energy higher than 4.1 J/m2), good transparency (97%), and low resistance (0.3 Ω cm2). In the work of our interest a resistance ,0.01 Ω cm2 is reported (Dimroth et al., 2014). The device fabrication process continues with the GaAs substrate removal by chemical etching, front contact deposition, MgF2/TaOx two layer antireflection coating, and mesa etch to define the devices area. The record bonded MJ solar cell (Dimroth et al., 2016) presents a similar structure to that already considered, although a theoretical optimization is performed. Optimum bandgap combination is achieved maximizing the sum of the product of gap energy and photocurrent, under the hypothesis of QE equals one in all cases (one electron-hole pair generated by each photon). Optical properties of the ternary and quaternary materials are obtained by interpolation, and the transfer matrix method is applied to calculate the absorption in each layer (Dimroth et al., 2016). Tunnel junctions are considered transparent and they are not inserted in the structure. The subcells’ thicknesses were further corrected to achieve close current matching for all subcells. A theoretical limit on the conversion efficiency for this structure of 53.8% at 500 suns is determined, showing that devices with .50% efficiency are possible. MJ device characterization is a tricky issue, and we will not extend here about it. Electrical characterization must take into account the spectral content of the light source; eventually the mismatch respect to the standard AM1.5d solar spectrum could lead to an erroneous result. Also, MJ devices have no contacts to access the subcells, so as QE measurements require a careful light and electrical bias (Garcia et al., 2017). A review about this matter can be found in Osterwald and Siefer (2016). This is the reason why different conversion efficiency results are presented in the work, depending on spectral content of the illumination source.

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As already commented, an independent confirmed record of 46.0% efficiency at 508 suns was obtained for this LM bonded 4 J solar cell, while the authors measured at the Fraunhofer ISE calibration lab peaks of 46.1% at 312 suns concentration and 47.4% at 389 suns, using two different illumination sources. Furthermore, excellent external quantum efficiencies in the order of 90% for all subcells were obtained (Dimroth et al., 2016).

2.3.4.2 Inverted metamorphic lattice mismatched structures The most recent record, accomplished by an IMM six-J device by Geisz et al. (2020), shows the huge potential of this approach. The layer’s structure begins with compounds LM to GaAs, forming the three first junctions inverted grown, followed by another three lattice mismatched junctions, to achieve an optimal bandgap mix. How to put under control dislocations and to avoid phase segregation in metastable IIIV compounds in the growth of the MM structure is the big challenge to overcome. The structure presented for the 6-J device is Al0.18Ga0.33In0.49P/ Al0.23Ga0.77As/GaAs, for the three top junctions, using Al0.6Ga0.4As tunnel junctions, and Ga0.84In0.16As/Ga0.66In0.34As/Ga0.42In0.58As for the three bottom junctions, using compounds mismatched to the GaAs substrate by 1.1, 2.4 and 4.2%, respectively (Geisz et al., 2020). All main materials used are indicated in Fig. 2.6, with smaller colored symbols to distinguish them from the other compounds there considered. The lattice constant is modulated successively using GaInP compositionally graded buffers, indicated in Fig. 2.6 as blue arrows, allowing the growth of the desired MM materials with low threading dislocation density. Bottom junctions are connected by MM C-doped GaAsSb and Se-doped GaInAs tunnel junctions (Geisz et al., 2020). Many details concerning the growth strategies and materials properties are presented, as well as in a previous publication from the same authors (Geisz et al., 2018). Cross section TEM and cathodoluminescence characterizations show the low concentration of dislocations in the MM junctions, assuring their good performance. The complexity of this device can be noticed in the detailed layer structure presented in the supplementary information from Geisz et al. (2020). Temperature process is varied between 500 C and 700 C approximately, over a complete time process of about 7.5 hours. The concentration of solar radiation favors the economic rating of costly devices as MJ solar cells through the increment of the conversion efficiency and areal device reduction. However, as a consequence, the devices that operate under these conditions have to support high current density, a special issue for the tunnel junctions. In this sense, one of the advantages of the sixJ solar cell is that solar spectrum is divided in more bins respect, for instance, the traditional three-J cells, so as the current generated by each junction decreases and hence the current of the overall device. Anyway, the

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ultimate practical limitation on the concentration is the finite series resistance, which pushes the efficiency down because of the increment of the Joule loss. Among the problems faced in the work of Geisz et al., there are two particularly mentioned and related with this issue. One of them is the low electron mobility of the high bandgap Al0.18Ga0.33In0.49P material used as top cell, problem associated to the incorporation of Al in this quaternary compound (Geisz et al., 2020). This fact influences the lateral sheet resistance between front contact fingers where the current is collected, contributing to the increment of series resistance. The solution proposed is to reduce the Al content from 18% at the base to 6% at the emitter, giving place to the named reverse heterojunction, where the emitter is a material with lower bandgap respect to the base. The other problem mentioned is the diffusion of Zn from heavily doped layers during long and high temperature growing process, creating resistive internal barriers (Schulte et al., 2019). This problem arose between the GaInP BSF of the fourth junction and the subsequent tunnel junction. This was mitigated using an optimized (AlGa)0.84In0.16As Zn doped diffusion barrier (Geisz et al., 2020).

2.3.5

Discussion

Here we reviewed the progress of MJ IIIV solar cells, the PV technological option of highest conversion efficiency. It is notorious the constant advance in this field, topped by the recent ideas involving materials and device architectures and processes to overcome the practical limitations of the available IIIV LM compounds. In this sense, we observe a certain competition between the two approaches: bonded structures using LM materials, and monolithic IMM growth. The first one has the advantage of relatively simpler growth process to obtain good electronic quality materials, but using three processes (two epitaxial growths and one bonding) and costly substrates, while the second involves just one growth run, but during a long time and with really challenging issues to obtain good performance materials. A fine economic calculation is necessary to assess the possible industrial application of these very high efficiency approaches, as well as the feasibility to scale them efficiently. IIIV MJ solar cells are already an industrial product from 20 years ago, covering essentially the space market. These include an IMM cell, 32% efficiency under one sun AM0, offered by SolAero (Solaero). This cell outperforms the previous LM products, with advantages also in the weight per watt performance and radiation hardness. There are also commercial MJs for CPV terrestrial applications available from Spectrolab and Azur (Spectrolab; AZUR SPACE Solar Power GmbH).

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CPV is a technology that makes use of MJ devices and already has commercial products, although its penetration in the PV market has been difficult. The report of reference (Wiesenfarth et al., 2017), no further updated, states a globally installed capacity of 370 MW, yet marginal among the already established PV technologies. As the possible reasons of this, we can identify its relatively recent entry in the PV market (about 2005), with the consequent lack of heritage and concrete on-field data about the reliability of systems, as well as the continuous plumbing of c-Si technology prices. As a consequence, the CPV technology cannot scale, and its industry components are hardly standardized. However, new growth and efficient processes like dynamic hydride vapor phase epitaxy and the huge potential of very high conversion efficiencies keep CPV expecting its opportunity.

2.4

Final remarks and future perspectives

It is largely accepted that PV technology will have a central role in the transition to a future 100% clean energy scenario. Its wide availability, modularity, and marginal cost of operation and maintenance are very attractive characteristics of this technology. The competitive cost of the energy generation respect to conventional sources is already a reality in many markets around the world, and this trend is expected to hold in view of the drastic fall of PV related component prices. Future issues to face for a PV TW scale era will be the material’s availability, which will require an aggressive recycling policy, and the development of accumulation options to compensate the intermittent character of the PV generation. The mature PV technology based on c-Si is a reliable and consolidated industry to scale further. In this context, new technological options of potential lower costs, environmentally friendly, and capable to cover particular niches are on the way. Among these, we identify two which show a huge potential, although for different reasons and at different stages of development: those based on perovskite and IIIV semiconductors. In this chapter we reviewed these options, covering both the historical development and the state-of-the-art. PSCs surged few years ago, outperforming consolidated technologies like a-Si, or previously promising approaches like OPV and DSSC. PSC had an impressive and fast development, and with soft chemistry methods it is positioned to be scaled as a lowcost option and as candidate to be applied in flexible substrates. Nowadays, intense worldwide research is devoted to solve the main issues, like Pb-free alternatives, UV and moisture induced degradation, material stability, and areal uniformity. On the other hand, IIIV-based solar cells are already at commercial stage and consolidated in the space market. This technology possesses the highest energy conversion efficiency, but also the highest production cost. In

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this sense, its application in the terrestrial market is constrained to use concentrating optics and sun tracking mechanisms, enhancing conversion efficiency and minimizing the device cost in the whole system. For certain niches like regions with high direct irradiance availability, it competes well with c-Si, with the additional advantage of reduced surface occupation. Among the challenges of this technology we can mention the industrial implementation of new very high efficiency concepts and lower cost of the fabrication processes. Recent research is aimed to face the main sources of the high IIIV cells cost, which are the epitaxial growth technique, the metallization scheme, and the substrate. The roadmap is addressed in the recent paper by Wilson et al. (2020) and references there in. Both material families, IIIV and perovskites, were also tested as tandem cells using c-Si as a substrate and bottom cell with quite good results. This is an interesting option given that it proposes a lower cost substrate, in the case of IIIV, and a boosted conversion efficiency, in the case of perovskites. In summary, we have reviewed the more promising emergent PV technologies, which having very different characteristics, share a wide place for R&D improvement and the challenge of a future market deployment.

References Abbas, M.S., et al., 2019. Orientationally engineered 2D/3D perovskite for high efficiency solar cells. Sustain. Energy Fuels 4 (1), 324330. Available from: https://doi.org/10.1039/ c9se00817a. Abdelmageed, G., Sully, H.R., Bonabi Naghadeh, S., El-Hag Ali, A., Carter, S.A., Zhang, J.Z., 2018. Improved stability of organometal halide perovskite films and solar cells toward humidity via surface passivation with oleic acid. ACS Appl. Energy Mater. 1 (2), 387392. Available from: https://doi.org/10.1021/acsaem.7b00069. AZUR SPACE Solar Power GmbH, SPACE solar cells. http://www.azurspace.com/index.php/en/ products/products-space/space-solar-cells. Bertness, K.A., Kurtz, S.R., Friedman, D.J., Kibbler, A.E., Kramer, C., Olson, J.M., 1994a. 29.5%-efficient GaInP/GaAs tandem solar cells. Appl. Phys. Lett. 65 (8), 989991. Available from: https://doi.org/10.1063/1.112171. Bertness, K.A., Kurtz, S.R., Friedman, D.J., Kibbler, A.E., Kramer, C., Olson, J.M., 1994b. High-efficiency GaInP/GaAs tandem solar cells for space and terrestrial applications. In: Conf. Rec. IEEE Photovolt. Spec. Conf., vol. 2, pp. 16711678. Available from: https://doi. org/10.1109/wcpec.1994.520540. Bett, A.W., et al., 2013. Overview about technology perspectives for high efficiency solar cells for space and terrestrial applications. In: 28th Eur. Photovolt. Sol. Energy Conf. Exhib., pp. 16. Available from: https://doi.org/10.4229/28thEUPVSEC2013-1AP.1.1. Bhusari, D., et al., 2011. Direct semiconductor bonding technology (SBT) for high efficiency IIIV multi-junction solar cells. In. Conf. Rec. IEEE Photovolt. Spec. Conf. 1, 001937001940. Available from: https://doi.org/10.1109/PVSC.2011.6186332. Bi, D., et al., 2016. Polymer-templated nucleation and crystal growth of perovskite films for solar cells with efficiency greater than 21%. Nat. Energy 1 (10), 16142. Available from: https://doi.org/10.1038/nenergy.2016.142.

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Wilson, G.M., et al., 2020. The 2020 photovoltaic technologies roadmap. J. Phys. D: Appl. Phys. 53, 493001. Available from: https://doi.org/10.1088/1361-6463/ab9c6a. Wygant, B.R., Ye, A.Z., Dolocan, A., Vu, Q., Abbot, D.M., Mullins, C.B., 2019. Probing the degradation chemistry and enhanced stability of 2D organolead halide perovskites. J. Am. Chem. Soc. 141 (45), 1817018181. Available from: https://doi.org/10.1021/jacs.9b08895. Xiao, M., et al., 2014. A fast deposition-crystallization procedure for highly efficient lead iodide perovskite thin-film solar cells. Angew. Chem. 126 (37), 1005610061. Available from: https://doi.org/10.1002/ange.201405334. Yamaguchi, M., Luque, A., 2013. Outline of Europe-Japan collaborative research on concentrator photovoltaics. In: Conf. Rec. IEEE Photovolt. Spec. Conf. 16621665. Available from: https://doi.org/10.1109/PVSC.2013.6744464. Yang, S., et al., 2019. Stabilizing halide perovskite surfaces for solar cell operation with widebandgap lead oxysalts. Science 365 (6452), 473478. Available from: https://doi.org/ 10.1126/science.aax3294. Yavari, M., et al., 2018. Reducing surface recombination by a poly(4-vinylpyridine) interlayer in perovskite solar cells with high open-circuit voltage and efficiency. ACS Omega 3 (5), 50385043. Available from: https://doi.org/10.1021/acsomega.8b00555. You, S., et al., 2017. Additive-enhanced crystallization of solution process for planar perovskite solar cells with efficiency exceeding 19 %. Chem.—Eur. J. 23 (72), 1814018145. Available from: https://doi.org/10.1002/chem.201704181. Yu, H., et al., 2015. Room-temperature mixed-solvent-vapor annealing for high performance perovskite solar cells. J. Mater. Chem. A 4 (1), 321326. Available from: https://doi.org/ 10.1039/c5ta08565a. Zhang, Y., et al., 2015. Charge selective contacts, mobile ions and anomalous hysteresis in organic-inorganic perovskite solar cells. Mater. Horiz. 2 (3), 315322. Available from: https://doi.org/10.1039/c4mh00238e. Zhao, X., Kim, H.S., Seo, J.Y., Park, N.G., 2017. Effect of selective contacts on the thermal stability of perovskite solar cells. ACS Appl. Mater. Interfaces 9 (8), 71487153. Available from: https://doi.org/10.1021/acsami.6b15673. Zheng, S., Wang, G., Liu, T., Lou, L., Xiao, S., Yang, S., 2019. Materials and structures for the electron transport layer of efficient and stable perovskite solar cells. Sci. China Chem. 62 (7), 800809. Available from: https://doi.org/10.1007/s11426-019-9469-1. Zimmermann, E., et al., 2016. Characterization of perovskite solar cells: Towards a reliable measurement protocol. APL Mater. 4 (9), 091901. Available from: https://doi.org/10.1063/ 1.4960759. Zong, Y., et al., 2018. Continuous grain-boundary functionalization for high-efficiency perovskite solar cells with exceptional stability. Chem 4 (6), 14041415. Available from: https:// doi.org/10.1016/j.chempr.2018.03.005.

Chapter 3

Novel dielectrics compounds grown by atomic layer deposition as sustainable materials for chalcogenides thin-films photovoltaics technologies William Chiappim Junior1,2, Leandro X. Moreno3, Rodrigo Savio Pessoa2, Anto´nio F. da Cunha1, Pedro M.P. Salome´4 and Joaquim P. Leita˜o1 1 i3N and Department of Physics, University of Aveiro, Aveiro, Portugal, 2Plasmas and Processes Laboratory, Aeronautics Institute of Technology, Sa˜o Jose´ dos Campos, Brazil, 3 Department of Physics, Institute of Geosciences and Exact Sciences (IGCE), Sa˜o Paulo State University “Ju´lio de Mesquita Filho” (Unesp), Rio Claro, Brazil, 4International Iberian Nanotechnology Laboratory, Braga, Portugal

3.1

Introduction

According to the “2030 Agenda for Sustainable Development goals of the United Nations (UN)”, one in five people in the world still does not have access to modern electricity and about 2.6 billion people in the developing world have difficulties in accessing some kind of electricity (Sustainable Development Goals, 2020). The global population is expected to reach 9.8 billion by 2050, leading the world to an energy collapse. To solve this problem, it is necessary to invest in renewable energy sources, which are inexhaustive and clean. Market and industry trends for renewable energy sources in electricity production focus on (1) hydroelectric power, (2) wind energy, (3) bioenergy, (4) photovoltaic solar energy (PV), and (5) geothermal energy. However, Fig. 3.1 shows that from 2017 to 2018 there was a 0.3% increase in the use of nonrenewable electricity; this caused a 2.9% growth in CO2 emissions (Renewables and 2019, 2020; International Energy Agency, 2019). On the Sustainable Material Solutions for Solar Energy Technologies. DOI: https://doi.org/10.1016/B978-0-12-821592-0.00020-0 © 2021 Elsevier Inc. All rights reserved. 71

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FIGURE 3.1 Annual share of renewable energy in global electricity production and jobs in photovoltaic solar energy, end of 2017 and 2018.

other hand, there was a 0.3% drop in the share of global renewable electricity, except for solar PV, which grew 0.5% and helped to control the evolution of CO2 emissions. At the end of 2019, the International Renewable Energy Agency (IRENA) presented data on the number of global jobs in PV energy. It was estimated that 3.5 million jobs are in the PV manufacturing and market. IRENA introduced the statistical data concerning the raise of 250,000 posts between 2017 to 2018. This increase in the supply of jobs showed that for every 0.1% increase in the share of global renewable electricity in PV, there is an increase of 50,000 jobs worldwide (see Fig. 3.1) (International Renewable Energy Agency IRENA, 2020). Therefore, solar PV technologies emerge as a sustainable alternative to supply the greatest demand for electricity, to solve the issue of global climate change by reducing CO2 emissions and to raise the employability, directly or indirectly. Currently, solar cells and PV modules made from different semiconductors and divided into subcategories such as crystalline, polycrystalline, amorphous and thin film are reported. The main single-junction solar cells are classified as: G

Silicon wafer-based PV: The most used semiconductor in PV technology, with 95.0% of total global production (GP), has two significant types of the silicon wafers, monocrystalline silicon (c-Si) and polycrystalline silicon, also called multicrystalline silicon (mc-Si). The mc-Si corresponds to 62% of the GP and c-Si to 33%. As the noncrystalline form stands out, amorphous silicon (a-Si), it can be used in solar cells based on thin-film (Photovoltaics Report, 2019). As a thin film, a-Si has a versatility of

Novel dielectrics compounds grown by atomic layer Chapter | 3

G

G

G

G

73

growth on glass, plastic and metal, being used in flexible PV devices. Due to low efficiency and investments, this PV technology corresponds to 0.34% of the market (Photovoltaics Report, 2019). Table 3.1 summarizes the theoretical solar cell efficiency limit (ShockleyQueisser limit) and the maximum efficiency for the dominant PV technologies (cells and modules). Chalcogenide thin-film-based PV: The second most used semiconductor in PV technology, with 4.66% of the total GP, is based on chalcogenides absorbers. The principal absorbers are CdTe (cadmium telluride), CIGS (Cu(In,Ga)Se2), CZTSSe (Cu2ZnSn(S,Se)4) and CZTS (Cu2ZnSnS4). CdTe-based PV corresponds to 2.56% of GP with a power conversion efficiency (PCE) of 21%, and CIGS-based PV technology is responsible for 2.1% of the GP with a PCE of 23.35%. All the chalcogenides mentioned are considered a thin-film technology, because the active layers are approximately one tenth the diameter of a hair, that is, a few microns thick (Photovoltaics Report, 2019; Britt and Ferekides, 1993; Repins et al., 2008; Guo et al., 2010; Leita˜o et al., 2011). IIIV based PV: The IIIV PV technology is based on gallium (Ga) or indium (In), which have three valence electrons, and arsenic (As) or phosphorus with five valence electrons. The junction of these IIIV elements leads to PV materials, such as gallium arsenide (GaAs) (Bauhuis et al., 2009), gallium phosphide (GaP) (Lu et al., 2012) and indium phosphide (InP) (Yamamoto et al., 1984). Under concentrated solar and standard AM 1.5 conditions, these materials showed high conversion efficiencies. The GaAs as a thin-film cell has a current PCE of 29.1% and the InP as a crystalline cell has a PCE of 24.2%. Perovskite-based PV: The perovskite has been the material that has increased the PCE in the last years from 2.2% in 2006 to 21.6% in 2020 (Green et al., 2020). Its general formula is ABX3, where A and B are cations, and X is an anion. The most common perovskite is based on organicinorganic materials, where a large cation A is based on organic materials (CH3NH1), B is lead (Pb) or tin (Sn) and X is a halogen; frequently iodine (X 5 I) is used. A significant issue with this PV technology is the low stability and high toxicity (Pb); this is a fundamental problem to solve (Yin et al., 2014). Dye-sensitized based PV: Dye-sensitized solar cells (DSSCs) are devices based on organic dye particles sandwiched between titanium dioxide (TiO2) nanoparticles, an electrolyte and platinum-based (Pt) contacts (Moraes et al., 2016). The current PCE of the DSSCs technology is 11.9%, with the advantage of low production cost. However, the electrolyte is unstable, can freeze at low temperatures and expand at high temperatures, making encapsulation difficult. Another issue is the high cost of Pt-based contacts. These challenges hinder the expansion of the commercialization of this PV product.

TABLE 3.1 Single-junction cell and module efficiencies measured and theoretical solar cell efficiency limit (Green et al., 2020; Anttu, 2015; McCarthy and Hillhouse, 2012). Technology

Material

level

Current PCE

Current PCE

ShockleyQueisser

for small lab cells (%)

for the modules (%)

limit for cells (%)

Test center, date

Percentage of global market production

Silicon Mature

c-Si(crystalline cell)

26.7 6 0.5

22.4

33.5

AIST, 3/17

33%

Mature

mc-Si (multicrystalline cell)

23.2 6 0.3

18.5

33.5

ISFH, 9/19

62%

Emerging

a-Si -thin-film (amorphous cell)

10.2 6 0.3

12.2

29.0

AIST, 7/14

0.34%

Thin film chalcogenide Mature

CIGS (cell—Cd free)

23.35 6 0.5

17.5

33.6

AIST, 11/18

2.1%

Mature

CdTe (cell)

21.0 6 0.4

18.6

32.8

Newport, 8/14

2.56%

Under development

CZTSSe (cell)

11.0 6 0.3

n.a.

33.6

Newport, 10/18

n.a.

Under development

CZTS (cell)

10.0 6 0.2

n.a.

32.8

NREL, 3/17

n.a.

Manufacturing level

GaAs (thin film cell)

29.1 6 0.6

24.1

33.2

FhG-ISE, 10/18

n.a.

Under development

InP (crystalline cell)

24.2 6 0.5

n.a.

31.0

NREL, 3/13

n.a.

IIIV cells

Measurements of efficiency carried out under the global AM 1.5 spectrum (1000 W/m2) at 25 C. AIST, Japanese National Institute of Advanced Industrial Science and Technology; Fraunhofer Institut fu¨r Solare Energiesysteme; ISFH, Institute for Solar Energy Research, Hamelin; NREL, National Renewable Energy Laboratory; n.a., not available.

Novel dielectrics compounds grown by atomic layer Chapter | 3 G

G

75

Organic-based PV: The organic PV technology uses (1) organic polymers or (2) organic molecules as an absorber; these organics materials are based on carbon, which can form a linear, cyclic, acyclic or mixed structure. The current PCE of the organic cell is 13.45%. Low production costs and the possibility of integration into flexible substrates are advantages. On the other hand, low efficiency, low stability, low performance and expensive encapsulation materials to protect the organic materials from moisture limit marketing (Gu¨nes et al., 2007). Quantum dot solar cells: Quantum dots solar cells (QDSCs) are in the embryonic phase with CdSe-based QDSCs being well studied and promising materials emerging such as silver sulfide (Ag2S) (Su´arez et al., 2017; Sousa et al., 2020) and a mixed cesium and formamidinium lead triiodide perovskite system (Cs12xFAxPbI3) (Hao et al., 2020). However, these technologies need to be improved and have the potential to evolve.

Despite the many types of PV technologies, the commercially available PV market is shared by the first generation of solar cells (Si-based) and second generation (thin-film-based) (as shown in Table 3.1). Among the second generation technologies, the chalcogenide thin films have an enormous advantage over Si-based solar cells, which is the high absorption coefficient of these materials, resulting from their direct band nature (Kumar et al., 2009; Calixto et al., 1999). This nature allows a reduction in the manufacturing costs of the modules, as it will allow us to reduce the thickness of the absorbent layer. On the other hand, Si technology has a low absorption coefficient due to the nature of the indirect band (Bermel et al., 2007), significantly limiting the reduction in production costs. Even with this benefit, investors lack confidence in thin-film PV technology due to the low efficiency of the modules in relation to Si and the scarcity and toxicity of some materials present in the alloys used in thin-film technology (Polman et al., 2016). To attract these investors, it is necessary to improve the PV market through additional strategies to reduce production costs, increase the PCE and sustainability. Thus, the most viable way is to invest in alloys labeled as composed of nontoxic and abundant elements. In this sense, CIGS, CZTS, and CZTSSe thin films are an excellent alternative, being able to be adopted in flexible materials, which will allow the manufacture of roll-to-roll, which further reduces costs. The CdTe is excluded from this list due to the cost and scarcity of tellurium (Fthenakis et al., 2009), besides the high toxicity of cadmium, which becomes a significant limitation of this PV technology. In addition to the absorbent layers, chalcogenide thin film devices have other layers that also need to be sustainable, environmentally friendly, and abundant. CIGS, CZTSSe, and CZTS cells are based on a pn junction formed between the p-type absorber and the n-type CdS buffer layer (BF) (Rondiya et al., 2018; Chen et al., 2019; Sousa et al., 2017; Nakada and Kunioka, 1999; Salome´ et al., 2017a,b). Fig. 3.2A illustrates the conventional architecture of these solar cells,

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SECTION | I Trends in Materials Development for Solar Energy Applications

FIGURE 3.2 Schematic view of different layers in (A) conventional chalcogenide solar cells with (B) illustrating the front and (C) the back passivated chalcogenide solar cells. It worth highlighting that the thickness of the layers is not at scale.

which consist of (1) a substrate, usually soda-lime glass; (2) a back metallic contact layer, usually molybdenum (Mo); (3) a p-type absorber layer, than can be CIGS, CZTSSe, or CZTS; (4) a n-type CdS BF layer; (5) a window layer divide into an intrinsic ZnO (i-ZnO) layer, and an optical window consisting of a transparent conductive oxide, generally based on the ZnO/Al or SnOx/In layer; and 6) a metallic grid at the top to guide carrier extraction. As can be seen, the CdS BF layer is well established for CIGS, CZTSSe, and CZTS. Even so, to comply with the environmental regulations suggested by the UN (Chemical and waste, 2020) and proposed by the European Union (EU) (Towards a Non-toxic Environment Strategy, 2020), there is a strong appeal for the development of the Cd-free BF layer solar cell. In addition to toxicity, CdS has some disadvantages, such as the loss of absorbed light due to the high recombination of the holes (minority carries), which reduces the external quantum efficiency, consequently decreasing the PCE (Kanevce et al., 2017; Becque and Halliday, 2019). Hence, it is necessary to develop different BF layers to suit the current legislation. Also, in order to reach its ShockleyQueisser limit and to be deployed on a large scale, further improvements in the chalcogenide solar cells need to be developed. Fig. 3.2B and C shows the interface passivation that is of utmost importance for the design of new chalcogenide device architectures, allowing them to reach even higher levels of PCE performance (Salome´ et al., 2017c; Cunha et al., 2018; Bose et al., 2019; Lee et al., 2012; Ranjbar et al., 2017). The passivation layer (PSL) technology was adopted based on silicon PV technology (Dingemans and Kessels, 2012; Christiano and dos Santos Filho,

Novel dielectrics compounds grown by atomic layer Chapter | 3

77

2015; Watanabe et al., 2018; Santos et al., 2019; Watanabe et al., 2019). It has been expanded to all types of solar cells, such as perovskite solar cells (Jiang et al., 2019), CdTe solar cells (Liang et al., 2015) and DSSC solar cells (Tehare et al., 2018). This technology can reduce the interface defects and increase the recombination rate, important to improve the PCE for all PV technologies. Therefore, to overcome the present and future challenges related to chalcogenides, it is necessary to invest in sustainable materials to change the traditional CdS BF layer and, at the same time, adopt a PSL to reduce the interface defects. In this context, the atomic layer deposition (ALD) process, discovered in the 1960s in the Soviet Union, and established in the 1970s in Finland (Persons et al., 2013; Puurunen, 2014; Chiappim Junior et al.), emerges with unique characteristics, such as (1) conformal film growth; (2) homogeneous film along substrate surface; (3) excellent adhesion to a complex and large substrate area; (4) excellent repeatability; and (5) subnanometer control of the thin-film thickness (Pessoa et al., 2014; Pessoa et al., 2018a,b; Chiappim et al., 2020b; George, 2010). ALD is a superior technique for obtaining dielectric thin films with higher quality, being the primary candidate to achieve the goals and points shown in Fig. 3.3. This chapter focuses on ultrathin ALD dielectric oxide films used to fabricate nanometer-thick passivation and BF layers. Essential advances in chalcogenide PV technologies implemented by the ALD technique are presented. A discussion of the influence of ALD thin film dielectric properties on the BF layer, PSL and consequent improvement on CIGS, CZTSSe and CZTS solar cell performance is also provided.

FIGURE 3.3 Goals, points and candidates required to improve chalcogenide technology.

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SECTION | I Trends in Materials Development for Solar Energy Applications

Atomic layer deposition technique

As mentioned, ALD is a disruptive deposition technique derived from the chemical vapor deposition (CVD) technique, being an ideal tool for the production of nanometer thickness ultrathin films. Unlike CVD, where a metallic precursor vapor and an oxidant gas reach the chamber simultaneously and dissociate above a heated substrate, in ALD, the two precursors are alternately exposed to the sample with a temperature low enough to avoid thermal decomposition (Chiappim et al., 2016a,b,c, 2020a; Pessoa et al., 2015). The deposition chamber is purged with an inert gas (usually N2 gas) between the exposures of the metal precursor and the ligand precursor. These steps are called the ALD cycle and consist of a sequence of metal reactant vapor exposure, purge, ligand reactant vapor exposure, and another purge, as shown in Fig. 3.4A. Hence, the film grows through chemisorption reactions between the metal precursor or ligand gas and reactive functional groups on the surface (i.e., hydroxyl or chemisorbed organometallic species). This pulsed process provides self-limited film growth (Aarik and Siimon, 1994; Aarik et al., 1995; Sammelselg et al., 1998; Mitchell et al., 2003). Due to the ability to coat extremely complex shapes and high aspect ratio surfaces with high-quality conforming materials (Elam et al., 2003; Rauwel et al., 2011; Spende et al., 2015), ALD can be applied in the most diverse areas, for example, in biomedical engineering (Pessoa and Fraga, 2019;

FIGURE 3.4 Fundamental characteristics of ALD: (A) ALD cycle, (B) growth per cycle versus pulse time, (C) growth per cycle versus purge time and (D) linear growth of the film.

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79

Pessoa et al., 2017), protection against corrosion in packaging of beverages or food (Dias et al., 2019, 2020), in fuel cells (Pessoa et al., 2018b), chemical passivation in solar cells (Cunha et al., 2018), heterostructure for photonto-electron conversion applied in solar cells (Zazpe et al., 2018), in PV and silicon-based microelectronic technology (Chiappim et al., 2020b; Macoo et al., 2017), in the antitarnish coating on gemstone and metals for jewelry (Paussa et al., 2011), and in textile technology (Atasanov et al., 2014; Khan et al., 2020).

3.2.1 Requirements for ideal precursors and atomic layer deposition signature quality An ALD precursor must have the following requirements: (1) the chemisorption must be extremely exothermic with massive activation of desorption to guarantee a sticking coefficient close to the unit; (2) the by-products must not be chemically reabsorbed in the vacant sites subsequent to the chemisorption; and (3) by-products should not etch the growing film by reverse reactions (George, 2010; Chiappim et al., 2020a; Elliot et al., 2016). Table 3.2 shows a wide range of ALD-grow materials and precursors used by ALD technology that comply with the requirements above. Table 3.3 compares advantages and disadvantages of precursor groups. It worth noting due to the effort of all scientific communities, these tables can be updated continuously. Metallic reagents require suitable oxidizing precursors to meet the basic requirements of ALD. Commonly in ALD operating in thermal mode, H2O, O3, O2, and H2O2 are used as ligands (Johnson et al., 2014; Puurunen, 2005; Testoni et al., 2016). Plasma enhanced atomic layer deposition (PEALD) has emerged in recent years and uses O2 plasma as an oxidant precursor, which creates highly reactive radicals (such as atomic oxygen, excited species and oxygen ions) from a plasma source (Chiappim et al., 2016b, 2020a,b). Although O3 is used at low process temperatures due to its high reactivity, in addition to allowing the use of less reactive metal reagents, these oxidizing precursors have the undesired effect of oxidizing the sample surface and, consequently, creating a problematic oxidized interface (Johnson et al., 2014; Puurunen, 2005; Ha et al., 2005). The most widely used oxidant is H2O, due to the decomposition occurring at higher temperatures and because it does not harm the surface of the samples. It is worth mentioning that for nitrides and sulfides are used hydrides with similar behavior to H2O, NH3 and H2S, respectively (Johnson et al., 2014; Puurunen, 2005). And finally, despite its toxicity to selenides, H2Se is used as a ligand (Johnson et al., 2014; Puurunen, 2005). In addition to the characteristics of the aforementioned precursors, a real ALD growth mechanism must comply with a signature quality to avoid the “parasitic CVD effects” in the process. It is worth highlighting that three

TABLE 3.2 List of atomic layer deposition (ALD)-grow materials and metallic precursor groups used by ALD (Johnson et al., 2014; Puurunen, 2005; Miikkulainen et al., 2013; Knisley et al., 2013; Kim, 2003; Leskela¨ and Ritala, 2002; Ha¨ma¨la¨inen et al., 2011; Yum et al., 2011; Uusi-Esko and Karppinen, 2011; Povey et al., 2006; Park et al., 2003; Pore et al., 2009; Zazpe et al., 2020; Chua et al., 2019). Types of materials grown by ALD technique Elemental materials

Oxide materials

Nitride materials

Sulfide materials

Nanolaminates, ternary, and quaternary compounds

Al, Ag, C, Co, Cu, Fe, Ga, Ge, Ir, Mo, Ni, Os, Pd, Pt, Rh, Ru, Si, Ta, Ti, W, Zn

Al, B, Ba, Be, Bi, Ca, Ce, Co, Cr, Cu, Dy, Er, Eu, Fe, Ga, Gd, Ge, Hf, Ho, In, Ir, La, Li, Lu, Mg, Mn, Mo, Nb, Nd, Ni, P, Pb, Pd, Pr, Pt, Rh, Ru, Sb, Sc, Se, Si, Sm, Sn, Sr, Tb, Ti, Tm, V, W, Y, Yb, Zn, Zr

Al, B, Cu, Ga, Hf, In, Mo, Nb, Si, Ta, Ti, W, Zr

Ba, Ca, Cd, Cu, In, La, Mn, Sb, Sn, Sr, Ti, W, Y, Zn

Al, As, B, Ba, Bi, Ca, Cd, Co, Cr, Cu, Ga, Ge, Hf, In, La, Li, Lu, Mg, Mn, Nd, P, Pr, Sb, Si, Sn, Sr, Ta, Te, Ti, W, Y, Zn

Types of metallic precursors groups Metal organic

Metal inorganic

Metal organic

Alkyls

Cyclopentadienyls

Elemental precursors

Halides

β-diketonates

Other precursors

Al, Be, Cd, Ga, Ge, Hg, In, Mo, Se, Si, Sn, Zn

Lu, Mg, Ni, Os, Pt, Ru, Sc, Sr, Y, Zr

Cd, Ga, In, Mg, Mg, Sn, Zn

B, C, Cd, Cr, Cu, Ga, Ge, Hf, In, Mn, Mo, Nb, Pb, Sb, Sn, Ta, Ti, V, W, Zn, Zr

Ba, Ca, Ce, Co, Cr, Cu, Dy, Er, Eu, Fe, Ga, Gd, Ho, In, Ir, La, Mg, Mn, Nd, Ni, Pb, Pd, Pt, Ru, Sc, Sm, Sr, Tb, Tm, V, Y, Zr

Ag, As, Bi, Co, Cu, Fe, Ge, Hf, Ir, La, Li, Ni, P, Pb, Pr, Pt, Rh, Sb, Ta, Te, Ti, Yb, W, Zn, Zr

Notes: In this table were used abbreviations of all chemical elements.

Novel dielectrics compounds grown by atomic layer Chapter | 3

81

TABLE 3.3 Advantages and disadvantages of metallic precursors groups used in ALD (Chiappim et al., 2016b,c, 2020b; Johnson et al., 2014; Puurunen, 2005; Miikkulainen et al., 2013; Knisley et al., 2013; Kim, 2003; Leskela¨ and Ritala, 2002; Ha¨ma¨la¨inen et al., 2011; Yum et al., 2011; UusiEsko and Karppinen, 2011; Povey et al., 2006; Park et al., 2003; Pore et al., 2009; Zazpe et al., 2020; Chua et al., 2019; Testoni et al., 2016). Groups

Advantages

Alkyls

G G

Disadvantages

Very reactive; A direct metal-carbon bond that allows the use of H2O vapor as a ligand precursor

G

G

At higher temperatures suffers decomposition; Limited for a small number of elements

Cyclopentadienyls

G

Similar to alkyls

G

Similar to alkyls

Elemental precursors

G

Contamination free; Steric hindrance not affect the growth

G

A small selection of metals suitable as elemental precursor

Halides

G

Very reactive; Low steric hindrance; Used over a wide range of deposition temperatures

G

By-products are reactive; Incorporation of the some halide atoms inside the material

Highest growth rates when used a more reactive ligand as O3; A large variety of elements;

G

G

G G

β-diketonates

G

G

Other precursors: (a) Alkoxides, (b) Amides, (c) Amidinates, (d) Silyls

(b), (d): good reactivity; (c): promising alternative with a large group of elements; good thermal stability and reactivity.

G

G G

Bulky materials; Low reactivity; Low growth rates

(a): poor thermal stability; (b), (d): decomposition at low temperature.

Notes: Oxygen precursors include O2 plasma, H2O, O3, O2, and H2O2 vapor; Nitride, sulfide, and selenide ligands include NH3, H2S, and H2Se, respectively.

essential characteristics must be followed simultaneously for a real ALD process (Chiappim et al., 2020a): 1. The most underestimated mechanism is related to a saturation of the growth per cycle (GPC) as a function of pulse time, as shown in Fig. 3.4B. This characteristic is responsible for the behavior of the deposition to be self-limited for each cycle. As can be seen in Fig. 3.4B, before the saturation point, ALD behaves like CVD, losing the self-limited growth. Fig. 3.4C shows the optimal purging time with an inert gas in the reactor, which is essential to maintain a self-limiting growth by extracting

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SECTION | I Trends in Materials Development for Solar Energy Applications

unreacted metal precursors before introducing the ligand precursor. Therefore, an ideal GPC needs a perfect purge time too. It is worth highlighting that, for short purge period, a mixture of precursors in the vapor phase occurs on the surface of the substrate, extinguishing the selflimited growth. This is due to the remaining massive by-products, such as hydrogen ions, carbon group, HCl and excess reactants, which causes more precursors to stick to the surface, interfering with the reaction of precursor in the next cycle. As can be seen in Fig. 3.4C, this behavior leads to thickness gradients and higher GPC (Sonsteby et al., 2020). When a long purge time is used, productivity is limited and impurities in the carrier gas may increase, that is, the levels of oxidant concentration in the ALD films may vary with the purge time, given the oxidant impurities in the carrier gas. Due to these issues, the purge time should be optimized. An excellent purge time is specific for each reactor, ranging from 0.1 to 10 s for commercial and research tools. The essential characteristics that affect the purging time are the reactor geometries. Hence, the fundamental point is to optimize the pulse time and purge time through saturation studies to achieve self-limited growth. It is important to note that, for a given ALD process, the purge times specified for a material, reactor and precursor may not apply before a saturation study specific for each item mentioned (Chiappim et al., 2020a; Sonsteby et al., 2020); 2. The thickness of the films should increase linearly with the increase in the number of reaction cycles, as shown in Fig. 3.4D (Fei et al., 2015); 3. For each growth process in ALD, an ideal temperature “window” is crucial for each reactant where the growth is saturated in a monolayer, thus preventing decomposition of the precursor in the chamber (Puurunen, 2005). Therefore, a real ALD free of parasitic CVD is obtained when these three requirements are satisfied.

3.2.2

Commercial and research tools

Fig. 3.5 shows schematically the fundamental geometries of the ALD reactors, where the reactors applied in research and industry are highlighted. The geometry of an ALD reactor used in research is relatively simple due to the nature of the process that is self-limited at the substrate surface. Depending on the application, these reactors do not require a strictly uniform gas distribution and temperature. However, due to frequent gas changes during the process, system automation is essential. The geometries of the research reactors generally have a substrate holder for a single sample (single wafer reactor—SWR). SWRs can be classified due to the gas flow as stagnant flow reactor (SFR) or cross-flow reactor (CFR), and both, SFR and CFR can be used on thermal mode (e.g., water vapor as an oxidant) or plasma mode (e.g., O2 plasma as an oxidant), as shown in Fig. 3.6 (Chu, 2014).

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FIGURE 3.5 Main ALD reactors geometries used in research and industry. ALD, Atomic layer deposition.

FIGURE 3.6 ALD single-wafer and multiwafers reactors: (A) thermal mode at cross-flow, (B) plasma mode at stagnant-flow, (C) spatial mode at atmospheric pressure, and (D) carousel mode at low pressure. ALD, Atomic layer deposition.

Generally, commercial tools use the geometry of the preferred ALD reactor for each type of application, based on economic and technical considerations. The primary geometries of commercial reactors can be divided by the number of deposited samples and specified pressure applied, as presented in Fig. 3.6. Although the ALD technique has requirements of significant importance for industrial applications, such as (1) uniformity in deposition, (2) conformity of the films in high-aspect areas, (3) low defects in the as-deposited

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films, (4) high reproducibility of the process, (5) thickness in magnitude of a few nanometers, the tool suffered over time with two crucial limitations. The first one is the high cost of the equipment due to the low operating pressure and the dominance of the technology in the hands of a few industries, and the second one is the low deposition rate. Fig. 3.6 shows the schematic of two of the main SWRs used in research and development. In Fig. 3.6A we have the cross-flow SWR that operates in thermal mode at a temperature from 100 C to 600 C. This type of configuration is the most used in research laboratories and is probably the most sold due to its excellent cost-benefit. On the other hand, Fig. 3.6B shows the scheme of a PEALD in remote mode, that is, an ALD that uses plasma as a ligand precursor where there is a plasma generation zone separated from the zone plasma process (Pessoa et al., 2018b; Chiappim et al., 2020b). However, this reactor has been gaining popularity and has three more basic configurations, namely: (1) radical-enhanced ALD, (2) direct plasma ALD and (3) direct plasma ALD with grid (Heil et al., 2007). Based on the single-wafer reactors mentioned above that have a deposition rate between 100 and 300 nm/h (Dias et al., 2020; Johnson et al., 2014), new technologies are emerging that can increase the deposition rate to up to 3600 nm/h (Johnson et al., 2014) and, consequently, eliminating the main limitation for large-scale application in the industry. As can be seen in Fig. 3.6C, the Spatial ALD operating at atmospheric pressure is an excellent alternative, since in addition to increasing the deposition rate by more than ten times, it also eliminates the limitation of low-pressure operation. In this way, two barriers can be overcome by the same equipment. An additional alternative is shown in Fig. 3.6D, where is presented an ALD in a carrousel that, despite operating at low pressure, is able to deposit in a large number of samples, accelerating the deposition process. Another point to highlight is that there are currently a large number of companies that produce ALD equipment for research and industrial applications; thus, the associated cost tends to reduce rapidly, making the tool accessible for large-scale industrial use.

3.3 Atomic layer deposition applied on chalcogenides thin films technologies After an overview of the leading PV technologies and a more in-depth view of ALD technology, this section focus on the advances of ALD applied in the technology of chalcogenides absorber layers. It is important to remember that the ALD technique used without obeying the quality signature will behave like a CVD technique, where should occur a nonuniformity on the growth of the films with a drastic gradient of thickness and low coverage of substrate.

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3.3.1 Se)4

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Absorber layers: Cu(In,Ga)Se2, Cu2ZnSnS4, and Cu2ZnSn(S,

The chalcogenide-absorbing layers can be divided based on the crystalline structure, with the dominant CIGS structures called chalcopyrite corresponding to a tetragonal structure (Chen et al., 2009). For the quaternary compounds CZTS and CZTSSe, the dominant structure called kesterite structure result from a reduction in symmetry in chalcopyrite by the replacement of group III cations by two elements from groups II and IV (Khare et al., 2012). Below we discuss the technological level of each technology.

3.3.1.1 Chalcopyrite thin films: mature level CIGS absorber layers are a mature technology that allows it to manufacture industrial-scale PV systems. CIGS are an alloy formed by CIS (CuInSe2) and CIG (CuGaSe2) being both in the chalcopyrite structure. Due to their nature of direct bandgap, low material consumption, low energy consumption, light weight, low energy payback times, PCE comparable to Si technology, and flexibility to grow on flexible substrates, the CIGS-based PV technology reached a slice of the total GP the order of 2.1% with laboratory size solar cells reaching a PCE of 23.35%, and as can be seen in Table 3.1, the commercial modules reach 17.5% of PCE (Green et al., 2020; Siebentritt, 2002; Siebentritt, 2017; Teixeira et al., 2019, 2020). The last 6 years were a period of consolidation and maturation of CIGS technology, in which the alkali treatment that increased the PCE of these PV devices stands out (Chirila et al., 2013). It was observed that for postdeposition treatments in-situ and ex situ, there was an improvement in PCE (Chirila et al., 2013; Jackson et al., 2016; Kamada et al., 2016; Lundberg et al., 2016; Reinhard et al., 2015; Pistor et al., 2014; Mansfield et al., 2014). This treatment was designed to introduce Na in CIGS films grown at low temperatures (Rudmann et al., 2005). However, the technological leap occurred with treatment with K or heavier alkalis at low and high temperatures that further improved the PCE and VOC of CIGS solar cells with laboratory sizes (Jackson et al., 2016; Kamada et al., 2016; de Wild et al., 2019) and also in industrial production in marketable sizes (Lundberg et al., 2016). This improvement in PCE caused by the alkaline treatment is related to the increase in voltage in the open circuit, which originates in the formation of a different layer on the surface caused by the addition of Na, K or heavier Alkalis, which leads to an exchange of ions within the absorbent layer of the CIGS and directly influences the electronics properties. Another current trend is to implant silver into alloy CIGS to change the Cu(In,Ga)Se2 to (Ag, Cu)(In,Ga)Se2 (ACIGS) (Chen et al., 2014; Thompson et al., 2015; Edoff et al., 2017; Kim et al., 2018; Soltanmohammad and Shafarman, 2018; Larsson et al., 2020a; Keller et al., 2020). The incorporation of Ag produce a raised the efficiency of wide bandgap CIGS cells,

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that is, devices with high Ga/(Ga 1 In) ratio (Hanket et al., 2012; Lei et al., 2014; Tauchi et al., 2013; Yamada et al., 2006). This occurs due to a change in optoelectronic properties that lowering the valence and conduction band edge (Boyle et al., 2014). Hence, for advanced development chalcopyrite absorbers, thin-film solar cells need novel selective contacts (BF layers) with suitable electronic band structures, that is, with variable band alignments, which eases the band-structure building of full device. It worth highlighting a trend of made the chalcopyrite absorbers thinner to reduce costly with In. However, for chalcopyrite absorber thickness lower than 0.5 mm occurs a decrease in short circuit current (Lundberg et al., 2003; Dahan et al., 2012). To overcome this issue is needed optical confinement based on reflection and absorption properties (Dahan et al., 2012; Li-Kao et al., 2012). This is achieved by structures that optimizing the light trapping or backside illumination, as an alternative it can be used PSLs (Werner et al., 2020).

3.3.1.2 Kesterite thin films: under development level Kesterites absorber layers are under development technology being free of critical raw materials, like indium, gallium, and tellurium (Fthenakis et al., 2009; Wadia et al., 2009; Andersson, 2000; Zuser and Rechberger, 2011). According to the European Commission (Communication from the Commission to the European Parliament, the Council, 2017), thin-film PV technologies needs rapidly substitute these critical raw materials to develop a novel technology sustainable. The family of kesterites as such CZTS, CZTSe, and CZTSSe with laboratory size solar cells reached a PCE in the range of 10%11% (Green et al., 2020). These materials are closely related to the close cousin called CIGS. For example, the CZTSSe absorber is obtained by replacement of two gallium (Ga13) or indium (In13) atoms into the CIGS absorber structure by one zinc (Zn12) and one tin (Sn14) atoms (Schorr et al., 2009). As kesterite materials belong at the same family of chalcopyrites, that is, a family of chalcogenides absorber, they demonstrate similarity on the following properties: (1) suitable range of bandgaps, (2) very high light absorption coefficient, and (3) nature of conductivity type p (Siebentritt and Schorr, 2012). It worth highlighting that this technology is free of critical raw materials, fully inorganic, low cost, and still has enormous potential to reach the ShockleyQueisser Limit for cells (up to 33%). As in the CIGS absorbers, the current trend in kesterites is to substitute some elements in the alloy. This strategy is used to reduces the deficit of VOC in kesterites PV devices, consequently improving the PCE (Giraldo et al., 2019). For example, Cu can be substituted by Ag (Gershon et al., 2016; Qi et al., 2017; Yu et al., 2018), Zn replaced by Cd (Yan et al., 2017; Rondiya et al., 2020), and Sn substituted by Ge (Collord and Hillhouse, 2016; Kim et al., 2016; Brammertz et al., 2019). Other substitutions were explored in the literature as Si, Mg, and Mn (Giraldo et al., 2019). Similarly,

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to chalcopyrite absorbers, the kesterites need novel selective contacts with suitable electronic band structures and PSLs to help to increase the PCE of this technology.

3.3.2

Sustainable buffer layers based on atomic layer deposition

Selective contacts that are acting as an optical and electrical buffer sandwiched between the p-type absorber (CIGS, CZTS, CZTSe, and CZTSSe) and the n-type ZnO layers are called BF layer. This BF layer, when deposited on chalcogenides, defines the quality of the pn junction and plays a vital role in interdiffusion, recombination, and performance of the device (Rau and Schmidt, 2001; Naghavi et al., 2010). An excellent BF layer needs to fulfill the following requisites: (1) be n-type to form an ideal pn junction, (2) have a large bandgap to prevent overdone light absorption, (3) provide perfect alignment of the conduction band between chalcogenide absorber and the window layer, (4) have a small positive conduction band offset (CBO), (5) the deposition should have the capability to passivate the absorber layer and be high conformal (Naghavi et al., 2010; Niemegeers et al., 1995). The history of BF layers deposited by the ALD technique on chalcopyrite started in 1994, where Lujala et al. (Lujala et al., 1994) grown a binary compound of ZnO on CIS. However, due to the high recombination of the CBO, the performance was lacked. In 2003, Naghavi et al. (Naghavi et al., 2003) used ALD to grow a binary compound based in In2S3. This BF layer showed an excellent performance with the CIGS solar cells reach a PCE up to 16% (Naghavi et al., 2003). The first ternary ALD compound develops as a BF layer was used by Yousfi et al. (Yousfi et al., 2000) in 2000. They developed a compound system based on Zn(O,S). In 2007, To¨rndahl et al. (To¨rndahl et al., 2007) grown by ALD a ternary compound based on (Zn,Mg)O. Both works showed the ZnO could be modified by adding S or Mg, thus create a favorable CBO on CIGS solar cells. These ternary compounds were responsible for producing devices with a PCE comparable or better than their benchmark devices using CdS (Johnson et al., 2014). Due to promising Cd-free BF layers mentioned above, the scientific community increased the interest in use ALD for chalcopyrite BF layers, where both layers were systematically studied (Ramanathan et al., 2012; Nakada et al., 2013; Naghavi et al., 2011; Lee et al., 2011; Rousset et al., 2011). These promising materials have led to the development of new ternary compounds such as ZnInS (Geneve´e et al., 2013) and Zn12xSnxOy (ZnSnO) (Kapilashrami et al., 2012; Lindahl et al., 2012). Among the ternary mixtures earlier highlighted it the BF layers of ZnSnO showed a better performance. An essential study that showed the power of ALD in tuning BF layers on CIGS was carried out by Hultqvist et al. (2012) in 2012. They varied the composition of the Zn12xSnxOy BF in chalcopyrite devices to find a

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favorable CBO. They showed that for x 5 0.2 occurs a favorable CBO with a decreased of recombination, consequently occurred a high open-circuit voltage led to a higher PCE (Hultqvist et al., 2012). Lindahl et al. (2012) tuned the composition and thickness of the ZnSnO BF layers, thus improved the performance of the CIGS cells above 18%. They demonstrated, too, that it is possible to omit the intrinsic-ZnO (i-ZnO) layer using a sufficiently thick of ZnSnO BF layer without reducing the PCE (Hultqvist et al., 2012). This reduction of one process stage can reduce the manufacturing costs of the PV devices. In 2017 Salome´ et al. (2017a,b) compared the influence of the CdS and ZnSnO on photoluminescence of CIGS devices. These studies demonstrated a lower fluctuating potential on the ZnSnO BF layers. Recently, Ribeiro et al. (2020) studied the influence of ZnSnO BF layers using an implanted positive muons as a probe. They demonstrated a reduce the width of a defect layer present at the CIGS surface to about half its original value (Ribeiro et al., 2020). This shows the importance of ZnSnO BF layer growth by ALD to improve the performance of the CIGS solar cells. Most of these papers showed that ALD-grown BF layers for CIGS devices to give comparable PCE as CdS and higher Jsc to CdS. Due to the more upper interface recombination attributed to CIGS interaction with CdS, the VOC of the ALD BF layer is lower in comparison with CdS. Most of these mentioned papers showed that ALD-grown BF layers for CIGS devices to give comparable PCE as CdS and higher Jsc to CdS. A more recent study novel BF layers on CIGS absorber are presented by Larsson et al. (2020b), where they showed two novel ternary compounds, namely, Sn12xGaxOy and Zn12xSnxOy. In 2020, Keller et al. (2020) and Larsson et al. (2020a) applied these BF layers on ACIGS. Keller et al. showed an efficiency of up to 15.1% when used Zn12xSnxOy on ACIGS (Keller et al., 2020), and Larsson et al. showed a PCE up to 17.0% when used Sn12xGaxOy (Larsson et al., 2020a). This constant search for new BF layers growth by ALD taking advantage of their ability to tune the electrical and optical properties of the BF layer spurred interest for ALD of many groups and this lead an application of these BF layers in other technologies as kesterite absorber (Barkhouse et al., 2012; Yan et al., 2014; Gosh et al., 2017; Cui et al., 2018, 2019; Larsen et al., 2019; Gour et al., 2020), SnS (Sinsermsuksakul et al., 2013), and Cu2O (Lee et al., 2013), which have similar challenges as in case of CIGS.

3.3.3 Sustainable passivation layers based on atomic layer deposition PSL was the first strategy used in silicon technology to reducing the number of defects at the interface of Si-based solar cells. However, according to Salome´ et al. (2017c), the PSL applied on chalcopyrite and kesterites solar

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cells leads to three beneficial: (1) as back passivation (see Fig. 3.2) increase the reflection and maintaining an excellent electrical contact ensuring a high fill factor values in devices; (2) acting as chemical passivation, the PSL reduces the interface defect density being a photoelectric effect. The chemical passivation decrease the total number of electrically active defects (Salome´ et al., 2017c); (3) acting as field-effect passivation, the PSL it drives minority carriers away from the highly recombinative back contact into the space charge region (Salome´ et al., 2017c). As in Si-based solar cells, the most used PSLs on chalcogenides are aluminum oxide (Al2O3) (Cunha et al., 2018; Bose et al., 2019; Lee et al., 2012; Werner et al., 2020; Cui et al., 2019), silicon nitride, and silicon oxide (SiOx) (Bose et al., 2019) and TiO2 (Ranjbar et al., 2017). In conjunction with strategies such as different BF layers, ultrathin absorber layers, novel alloys of chalcopyrite, and kesterite, the PSL tends to expand quickly into chalcogenides solar cells. By using all the advantages of the ALD technology presented in Section 1.2, the ALD-grown PSL strategy can help chalcopyrite and kesterite PV technology to overcome problems and improve the PCE up to the ShockleyQueisser limit (as shown in Table 3.1).

3.4

Final remarks

Chalcogenides absorbers are an essential renewable energy system with nonpolluting, which can achieve high PCE for the production of electricity. Because they are structured materials in thin films is a possible growth in many types of substrates improving the technological appeal. Furthermore, there is a strong effort for the development of the Cd-free BF layer at a low cost. This occurs because many strategies are imported from other PV technologies and due to the applications of ALD tools. Current R&D into chalcogenides tends to focus on how to improve their performance, reducing costs and stages on the production process, to make this technology use viable for large production scale. Progress in chalcopyrite and kesterite PV technology depends on developments in material components, as well as, of the junction of the different interfaces, especially pn junction between the absorber and BF layer. It is known that the use of thin or ultrathin chalcogenides absorber is one way to reduce the costs; however, this is possible using adequate PSLs. Over the past years, the number of papers concerning chalcopyrites and kesterites solar cells using ALD has increased considerably. The growth of the ultrathin buffer and the PSL by ALD has a significant role, and their applications in this PV technology have been investigated. It has been observed that ALD technology already provides adequate performance in both absorbers, chalcopyrite, and kesterite, in addition to expanding in other PV technologies.

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Acknowledgments This work was supported in part by individual grant financed by the SusPhotoSolutions project CENTRO-01-0145-FEDER-000005, individual grant financed by FAPESP grant no. 20/10450-7, FAPESP grant no. (18/01265-1, and 19/05856-7), CNPq (grant no. 446545/2014-7, 308127/2018-8 and 437921/2018-2) and the Brazilian Space Agency (AEB/Uniespac¸o), NovaCell (028075) and InovSolarCells (029696) and in part by Fundac¸a˜o para a Cieˆncia e a Tecnologia and the ERDF through COMPETE2020. Fundac¸a˜o para a Cieˆncia e a Tecnologia (FCT) is further acknowledged through IF/00133/ 2015. The European Union’s Horizon 2020 research and innovation programme ARCIGSM project (Grant agreement 720887) is also acknowledged. The financial support by National Funds through the FCT - Fundac¸a˜o para a Cieˆncia e a Tecnologia, I.P., under the scope of the projects UIDB/50025/2020 and UIDP/50025/2020  Program´atico, are acknowledged.

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Chapter 4

First principles methods for solar energy harvesting materials J.J. R´ıos-Ram´ırez1 and Velumani Subramaniam2 1

Departamento de Ingenier´ıa Ele´ctrica (SEES), Centro de Investigacio´n y de Estudios Avanzados del Instituto Polite´cnico Nacional (CINVESTAV-IPN), Ciudad de Me´xico, Mexico, 2 Department of Electrical Engineering (SEES), Centro de Investigacio´n y de Estudios Avanzados del IPN (CINVESTAV-IPN), Mexico City, Mexico

4.1

Introduction

The density functional theory (DFT) methods represent one of the strongest approximations to the quantum phenomena of electrons in material science. This valuable intellectual heritage is constantly implemented in the development of materials for solar energy harvesting, in specific, for organic (Cheema et al., 2019) and inorganic (Zhaoa et al., 2017) electroactive solar cells along with nanoscale photocatalysts (Kong et al., 2019). Experimentally, such a thoroughly work demands expensive labor hours in the laboratory, frequent handling of hazardous substances, characterization techniques with their own scopes and availabilities, among other highly relevant surrounding conditions. On the other hand, the possibility to perform calculations demands only computing infrastructure. However, the obtained outputs are rarely straightforward correlated with the experimental results, therefore, in order to provide meaningful insights into the discussion, it is the task from the numerical theoretician to find the right correspondence defining both suitable material models and a proper selection of the computational scheme. The number of atoms in the structural models is only limited by the memory capabilities from each modeling facility, however, the design of its shape is far from any other boundary. Regularly, they are constructed as a supercell, which is a crystalline segment that repeats more than one conventional unit cell along a certain direction, in order to provide both a larger distance among atomic sites and a reduced ratio of defects such as dopants. These rebuilt structures could be found either filling all the space with the atomic sites or Sustainable Material Solutions for Solar Energy Technologies. DOI: https://doi.org/10.1016/B978-0-12-821592-0.00003-0 © 2021 Elsevier Inc. All rights reserved. 101

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including a vacuum layer, known as bulk or slab supercells, respectively. As some of the main examples, it is possible to find constructions such as crystalline surfaces (Dickens et al., 2019), two dimensional nanosheets (Lehtinen et al., 2015), nanowires and nanotubes (Nagarajan and Chandiramouli, 2019), punctual structures such as clusters of atoms (Abdel-Mageed et al., 2019), carbon allotropes (Bi et al., 2019) or quantum dots (Montejo-Alvaroa et al., 2019), heterostructures (Ren et al., 2019), dopants (Yin et al., 2019) and alloys (Ikeda et al., 2019). On the other hand, a continuously improved set of calculation schemes, offer a closer description with respect to the chemical accuracy as the so-called Jacob’s ladder is climbed (Tran et al., 2016). With local density approximations in the first step (Rehman et al., 2018), immediately followed by the generalized gradient approximation (GGA) (Saı¨la et al., 2015), meta GGA as the third rung (Patra et al., 2019), up to the hybrid approximations on top of it (Vin˜es et al., 2016). All the latter theory levels consolidate a vast diversity of calculation schemes, each of them with their own scope, computational cost, usage complexity, and implementation level in the available codes. Meanwhile, another set of methodologies are devoted to solve specific issues in the electronic structure such as the modified Becke and Johnson potential (Ramay et al., 2017) and the perturbative Green’s-function approximations (Bacaksiz et al., 2017) which represent highly cited methodologies for the bandgap correction in semiconductors. On the other hand, a higher approximation to the standard electronic correlation could be included in all the welldefined Hubbard procedures (DFT 1 U) (Guo et al., 2019), along with all the set of relativistic approximations, available by means of either scalar (SR) (Fraccarollo et al., 2016) or spin-orbit methods (Hern´andez-Haro et al., 2019). Furthermore, the dispersion interactions responsible for the van der Waals forces are found in another set of schemes known as vdW (Fischer et al., 2019). Among the most requested set of properties to calculate mentioned are the band structure, electronic density of states, optical properties, electronic and spin density distributions, the set of adiabatic elastic constants, the phonon dispersion, the specific heat, formation energies, the chemical interactions among a surface and an adsorbed molecule, and reaction paths. Several software options are currently available with their own approximation to DFT. It is possible to characterize each of them by the way in which they build the electrostatic interaction among the nucleus and electrons, so it is common to observed, in the computational methods section from reports, codes such as: pseudopotentials 1 PW, PAW 1 PW (Projector Augmented Wave 1 Plane Waves), or All-Electron approximations, to describe the most common schemes to this regard. A highly accurate and computational affordable scheme are in fact the pseudopotentials, with an immense catalog for almost every element in the periodic table. In the concept of pseudopotential only the most important set of electrons are solved by the DFT formalism, usually the ones responsible of the chemical bonds, along with a substitution by a soft function (or pseudo-wave) of all the remaining interactions not relevant for the cohesion of the compound, that is, frozen or core charges. Therefore, the selection of the cut-off ratio to separate

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core and valence electrons is a relevant parameter in the selection of a suitable pseudopotential. On the other hand, the complement of the pseudopotential formalism is the expansion in a plane wave series, in terms of energy, which describes the valence region from the atom. As the cut-off energy of the latter expansion becomes higher, the expected result will be more accurate increasing the computational cost at the same time. To avoid the selection of a pseudopotential or for cases in which the core electrons are in fact relevant for the description of the compound, the all-electron approximation to DFT represent a consolidated option. Known by the high accuracy, the all electron methodologies are based on the Full Potential Linearized Augmented Plane Waves (FPLAPW), or Linearized Augmented Plane Waves (LAPW), that stands for full-potential linearized augmented plane waves, or simply, linearized augmented plane waves. In which the concept of pseudopotential is followed, dividing the spatial region among core and valence electrons by demarcation spheres around every atomic site. Within that region, also known as muffin-tin sphere, the electrons are described by an expansion of solutions to the Schro¨dinger equation plus their first energy derivative, therefore, no pseudo-function is needed since every state from the electronic configuration of the atom is expanded, within the spheres, and as plane waves in the interstitial region. The electronic solution of the entire atom converged into highly accurate results with probably the highest computational budget from all the available approximations. From either FPLAPW or LAPW methodologies, the most important parameters are the muffin-tin sphere radius, and the expansion of plane waves in the interstitial region. A highly popular methodology comprehended the brightest features from both pseudopotential 1 PW and LAPW, right in the middle point between computational cost and result accuracy. The PAW, which stands for Projected Augmented Waves, all electron wave function is linearly transformed into soft functions just as the pseudopotential formalism, however, the potential file possesses the accuracy of all electron calculation avoiding its computational cost. This chapter is divided into two sections facilitating readers with no background on theoretical calculations and motivates undergraduate and graduate students into theoretical calculations and materials modeling. In the first section, the reader is familiarized to the fundamental concepts of the theory, in the second section a set of materials, synthesized by our group, is described in terms of their theoretically determined properties, based on DFT methodologies. The most important goal of the present chapter contribution is to incorporate undergraduate students into material modeling, to encourage their interests in computational material science topics.

4.2 4.2.1

Fundamental concepts Crystalline representation

One of the most frequently used representations to describe the structure of a sample in material science is the solid-state. Under this structural model the

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most relevant property is the translational symmetry since every feature from the material is determined by the way in which its atomic species are periodically distributed in space. A period is any physical magnitude that defines the extension of an event right before the immediate start of the afterward, therefore, it characterizes the length that sections an infinite sequence into identical components that continuously repeat. Any periodical distribution of atoms in the three dimensions defines a perfect crystal. Within this highly symmetric matter the period limits a certain distribution of atomic sites determining the crystalline lattice. This group of atoms is represented in Cartesian coordinates known as base vectors, and it is said to be irreducible since it contains the precise number of positions in space that could build an infinite crystal when they are periodically repeated, consolidating this beautiful and elegant property of the symmetry that allows the filling of infinite space by the repetition in three dimension only of a known set of atoms. The second major component needed to fully define a crystal is the set of lattice vectors. In three dimensions, there are only seven geometries capable to fill the entire space and those with lattice boundaries closer to orthogonality possess the higher symmetry. The most familiar way in which an ordered 3D grid of atoms could split is in cubic segments or cells. There are three different lattices that could be represented within a cubic symmetry known as primitive (P), body centered (I), and face centered (F) depending upon the distribution of the positions. Any primitive lattice contains a single site written as 0,0,0 in reduced coordinates, meanwhile, I and F symmetries include one more at 1/2,1/2,1/2, and three more at 1/2,1/2,0; 1/2,0,1/2 and 0,1/2,1/2, for the body centered and face centered positions, respectively (Fig. 4.1). Although, it seems that the repetition of the lattice sites exceeds the number of irreducible coordinates, the symmetry simplifies the representation. For the primitive case, the four spheres appearing in the corners define a single position, since every vertex of the cube may be assigned to the origin of the coordinate system. Therefore, the 0,0,0 site strictly represent one eighth of a single position due to the partition among eight adjacent cells. The body centered cubic structure (bcc) shows a lattice site with all the reduced coordinates different from zero, which means that the current position is found away from any faces or vertices of the unit cell. One of the most important benefits of the usage of the reduced notation is that, the editing of a structure by means of fractional positions allows the model developer any adjustment of the lattice parameters without the modification of the n-sites since the atomic positions remain in proportion. The values are the result of the quotient of the absolute position by the magnitude of the lattice parameter for the three crystalline directions. For a centered site within a cube (a/2,a/2,a/2) gives one half for every reduced coordinate. For the case of the face centered cubic (fcc) structure, the sites are located exactly centered over all the faces of the cube. A simple association shows that the number of sites needs to be equal to six, however, the

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FIGURE 4.1 Cubic Bravais lattices with primitive (P), body centered (I), and face centered (F) site distributions. The Cartesian coordinates are written with the reduced notation, which is the quotient of the absolute position by the magnitude of the lattice vector for each direction.

translational symmetry reduces the number of atomic sites. In a fcc structure a single site located right at the boundary of the lattice will be spitted among two adjacent cells. Therefore, any site located at the lattice faces will be included in the adjacent unit cell too, representing one half of a single position as the unit cells are repeated in the direction parallel to such lattice plane. For instance, the site with the x coordinate equal to zero (0,1/2,1/2) along with its mirror image, located exactly one lattice parameter away (1,1/2, 1/2) represents the same position. The addition of the rest of the face centered cases and the primitive position will give the four sites found in the fcc structure. The rest of the forms of the 14 Bravais lattices in three dimensions are brought by the trigonal, hexagonal, tetragonal, orthorhombic, monoclinic, and triclinic systems. The trigonal case is represented by a primitive cell with the same magnitude for the three lattice parameters (a,b,c) and all angles equal to 60 degrees. In an fcc structure, the trigonal lattice may be obtained as its primitive representation, that is, an equivalent unit cell that reduces the position display without the symmetry of the conventional representation. In order to see the trigonal cell within the fcc structure it is necessary to choose two primitive positions that define the longest diagonal of the cube. The two segments that link those positions with faced centered sites generate alternate interior angles with respect to the diagonal of the cube and give the lattice

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parameter from the trigonal cell. Two more axis parallel to the former segments connect two face centered sites. The resulting diamond shape faces will have three faced shared positions with one primitive site. For an fcc structure, the confined volume of the trigonal representation (or primitive representation) is equal to one quarter of the volume from the conventional unit cell. Another primitive representation is seen in hexagonal unit cells where the ratio c/a is characteristic for this structure. Meanwhile, both a and b lattice parameters possess the same magnitude and the gamma angle, defined as the arc in between a and b, is equal to 120 degrees (Fig. 4.2). As it was previously mentioned, the structure tends to lose symmetry as far as it gets from a cubic representation. Whether the c magnitude is changed, the resulting system is called tetragonal and possesses two Bravais lattices, one primitive and a second one body centered. Considering all lattice parameters with a different value, the resultant structure becomes orthorhombic and the Bravais symmetries could be P, B, F, or C where the last one is a particular face centered case with a single shared site at the reduced coordinates 1/2,1/2,0. Changing the beta angle, that is, the arc in between a and c lattice, the resulting Bravais lattice could be P or C monoclinic and finally, the strongest complexity is found for all oblique angles called triclinic (Fig. 4.2). In practice, the definition of a unit cell with all the possible symmetries is always desirable. Since the calculations consider a lesser number of equations the

FIGURE 4.2 Noncubic Bravais lattices with the lattice magnitudes and angles with the number of sites for each translational symmetry. The titanium dioxide in the anatase phase belongs to the body centered tetragonal system with the space group: I41/amd.

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delivery of the results is sped up. On the other hand, the translational symmetry is a very useful tool for the primary analysis of a freshly modeled structure. For instance, the chemical stoichiometry of titanium dioxide (TiO2) establishes that, for every single metallic site, there must be two oxygen positions. In the anatase structure (Fig. 4.2) it is possible to see a couple of faced shared sites, along with two more titanium atoms completely inside the unit cell. Therefore, there is a total of four metallic sites that must be complemented with eight oxygens. The reader may be able to identify a plane of atoms right in the middle of the unit cell, over such a plane, in which there are two oxygen positions appearing very close to the center of the unit cell, along with two more bonding the faced sheared titanium atoms, that is four oxygens, the other half lay at the unit cell faces as what it seems to be eight O sites, however, the factional representation for face shared sites tells us that such positions bear only 0.5 from each site. Therefore, the stoichiometry is fulfilled with eight oxygen position within the unit cell (Ti4O8).

4.2.2

The multielectron system

The essential microscopic representation of matter cohesion involves the interactions among charged particles. In DFT, such fundamental physical interactions present in atoms, molecules, polymers, solid state, and amorphous matter, are modeled by means of an approximation to the many particle Schro¨dinger equation, which is a theoretical framework that generalizes the problem of a single particle under an electrostatic potential, written in atomic units as:   1 1 2 r2 1 ψ 5 εψ: ð4:1Þ 2 r For a set of electrons embedded in an electrostatic nuclear field, the instantaneous Hamiltonian that considers all the Coulomb interactions is written as summations of both kinetic and potential operators. ^ Total 5 2 H

N X M XN 1 XM X 1 ZA 2 2 r 2 r 2 i A i51 2 A51 2M j r 2 RA j A i i51 A51

1

N X N X i51

M X M X 1 ZA ZB  1 : ri 2 rj  j R A 2 RB j j.i A51 A . B

ð4:2Þ

The latter notation is written in relative Cartesian coordinates since the interaction in between two particles is inversely proportional to the vector parallel to their relative position (Fig. 4.3). Following the assumption that, nuclear motion is much slower than the movement of the electron, due to the mass difference among these two particles. The expected value of the Schro¨dinger equation defined by Eq. (4.2) will show two separable contributions. This fundamental reference of motion, known as

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FIGURE 4.3 Schematic representation of all the relative positions defining the external, electron-electron, and nuclei-nuclei potential operators, written as the third, fourth, and fifth summation terms in Eq. (4.2). The position vector in capitals expand the M total nuclear positions, meanwhile, the r-vectors centered each of the ith electronic distributions.

Born-Oppenheimer approximation, solves the so-called electron problem by separable wave equations, one of them in terms of electrons coordinates ({ri}), called electronic equation, and the second one, with a total Hamiltonian operator considering both particle spatial coordinates ({ri};{RA}). This rearrangement of variables shows a full dependency upon fri g in both kinetic ðT e ðfri gÞÞ and electronelectron repulsion ðV e2e ðfri gÞÞ operators, with the latter summation expansion explicitly showing no self-interaction among electrons. However, the external potential ðV ext ðfri g; fRA gÞÞ of the electronic Hamiltonian keeps the dependency upon the reference position to the ath nucleus (Szabo and Ostlund, 1989). ^ Elect 5 T e ðfri gÞ 1 V e2e ðfri gÞ 1 V ext ðfri g;fRA gÞ H   1 2 1 2 1 2 r 1 ? 1 ri ? 1 rN 52 2 1 2 2 1

N X i51

2

! 1 1 1     ri 2 rj11  1 ri 2 rj12  1 ? 1 jri 2 rN j

N  X i51

 Z1 Z2 ZM 1 1?1 : jri 2 R1 j jri 2 R2 j jri 2 RM j

ð4:3Þ

Therefore, the electronic expectation energy possesses now a parametric dependency upon the nuclear configuration, which means that, under any change in the nuclear positions, considered either in the volume of the

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crystalline unit cell or in molecular bond length, will lead immediately to the modification of the electronic energy (Szabo and Ostlund, 1989). D E ^ Elect jΨelect 5 εelect ðfRA gÞ; ð4:4Þ Ψelect jH Finding a solution of the nuclear Schro¨dinger equation, defined by the full Hamiltonian in Eq. (4.2), generates an energy n-dimensional surface as a function of the variation of nuclear positions. ^ Total 5 T n ðfRA gÞ 1 V n2n ðfRA gÞ 1 εelect ðfRA gÞ H   1 1 1 2 2 2 52 r 1?1 r ?1 r 2M1 1 2MA A 2MM M  M  X ZA ZB11 ZA ZB12 ZA ZM 1 1 1?1 1 εelect ðfRA gÞ: jRA 2 RB11 j jRA 2 RB12 j jRA 2 RM j A51 ð4:5Þ Which is solved by the total wave function written as a product of both electronic and nuclear wave functions: Ψðfri g;fRA gÞ 5 Ψelect ðfri gÞΨnucl ðfRA gÞ;

ð4:6Þ

Writing the expectation value of the total Hamiltonian under the BornOppenheimer approximation by the separable wave function in Eq. (4.6) and considering an orthonormal set of wave functions per each particle, we obtain: D E   ^ Total jΨ 5 Ψnucl ðfRA gÞjT n ðfRA gÞ 1 V n2n ðfRA gÞjΨnucl ðfRA gÞ 1 ΨjH 

 Ψelect ðfri gÞjT e ðfri gÞ 1 V e2e ðfri gÞ 1 V ext ðfri g;fRA gÞjΨelect ðfri gÞ ;

ð4:7Þ

that finally shows both nuclear and electronic contributions to the total value of energy. The construction of the electronic component of Eq. (4.6) needs to fulfill the Pauli’s exclusion. Physically, such a fundamental statement demands that the particles remain indistinguishable, as they distribute in a space of configurations defined by both spatial and spin coordinates. This relevant principle of nature is found in the antisymmetry of a many particle wave function of fermions that includes the z-projection of the electronic spin into the formalism. Neglecting the electron-electron interaction (Ve-e), the total Hamiltonian is written as a summation of n-terms containing both T e ðfri gÞ and V ext ðfri g; fRA gÞ operators: N X

^ 1 1H ^ 2 1?1H ^ i 1 ?H ^ N: ^ i 5H H

ð4:8Þ

i51

The solution associated to Eq. (4.8) has the form of a total wave function represented as the product of spin orbitals, that is, single particle wave

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functions in terms of both spatial ({ri}) and spin coordinates ({σ i}) (Zhaoa et al., 2017), known as Hartree products (in lower case notation). Increase the description of the equations: ΨHP ðfri ; σ i gÞ 5 ψ1 ðr1 ; σ 1 Þψ2 ðr2 ; σ 2 Þψ3 ðr3 ; σ 3 Þ?ψN ðrN ; σ N Þ: ð4:9Þ However, under the latter formalism, each particle is centered around distinguishable points in space. Therefore, a specific linear combination of Hartree products is required in order to properly build an antisymmetric wave function. For the case of a pair of electrons we have: ΨHP ðr1 ; σ 1 ;r2 ; σ 2 Þ 5 222 ½Ψ12 ðr1 ; σ 1 ;r2 ; σ 2 Þ 2 Ψ21 ðr2 ; σ 2 ;r1 ; σ 1 Þ 1 5 222 ψ1 ðr1 ; σ 1 Þψ2 ðr2 ; σ 2 Þ 2 ψ2 ðr1 ; σ 1 Þψ1 ðr2 ; σ 2 Þ ; ð4:10Þ 1

Where the total wave function possesses two Hartree products separated by a negative sign. This expression now shows a vanishing probability whenever more than one electron occupies the same spin orbital, fulfilling the most familiar statement of the Pauli exclusion, and since each electron could be represented by every spin orbital [see the second term of Eq. (4.10)] the indistinguishable representation of the electrons is also achieved. Following the latter expansion, the Slater function accomplishes the generalization of the exclusion principle for N-electrons, and it is written as a determinant with N-rows and N-columns expanded for every electron and spin orbital, respectively. 

1 ΨSlater ðr1 ; σ 1 ;r2 ; σ 2 ;?;rN ; σ N Þ 5 N!

 12  ψ1 ðr1 ; σ 1 Þ ψ2 ðr1 ; σ 1 Þ  ψ1 ð r 2 ; σ 2 Þ ψ2 ð r 2 ; σ 2 Þ  ^ ^   ψ1 ðrN ; σ N Þ ψ2 ðrN ; σ N Þ

? ψN ðr1 ; σ 1 Þ ? ψN ðr2 ; σ 2 Þ & ^ ? ψ N ðrN ; σ N Þ

    ;  

ð4:11Þ Where N! is a normalization factor representing all the possible permutations of a pair of electronic coordinates. By means of this mathematical object, every electron is contained in all N-orbitals, as shown by the column expansion from each raw (undistinguished particles). The exchange of a pair of electronic coordinates (fri ; σ i g) in Eq. (4.11) modifies the resultant sign of the determinant (antisymmetry), meanwhile, the repetition of two columns results in a total wave function equal to zero (Pauli exclusion).

Furthermore, under any exchange coordinates, the total probability jΨj2 remains independent (Fiolhais et al., 2003). ð 1 X Pðr1 ; r2 ; . . . ; rN ;σ 1 ; σ 2 ; . . . ; σ N Þd3 r1 d3 r2 . . . d3 rN N! σ ð  2 1 X 5 N!Ψðr1 ; r2 ; . . . ; rN ;σ 1 ; σ 2 ; . . . ; σ N Þ d 3 r1 d 3 r2 . . . d3 rN 5 1 N! σ ð4:12Þ

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Returning to the two particles model, the representation with opposite spin shows:   12  1  ψ1 ðr1 Þαðσ 1 Þ ψ2 ðr1 Þβ ðσ 1 Þ  Slater Ψ ðr1 ; σ 1 ;r2 ; σ 2 Þ 5 ; ð4:13Þ 2  ψ1 ðr2 Þαðσ 2 Þ ψ2 ðr2 Þβ ðσ 2 Þ  Expanding this determinant, as in the case of Eq. (4.10), the associated probability to find an electron in a differential volume segment d 3 r1 centered at r1 along with a particle within d3 r2 localized at r2 is written as:     Ψðr1 ; r2 ;σ 1 ; σ 2 Þ2 5 1 ψ1 ðr1 Þαðσ1 Þψ2 ðr2 Þβ ðσ 2 Þ2ψ1 ðr2 Þαðσ2 Þψ2 ðr1 Þβ ðσ 1 Þ2 2 5

1  1    ψ1 ðr1 Þψ1 ðr1 Þψ2 ðr2 Þψ2 ðr2 Þ 1 ψ1 ðr2 Þψ1 ðr2 Þψ2 ðr1 Þψ2 ðr1 Þ: 2 2

ð4:14Þ

However, for the case of the interaction among electrons with the same spin functions, the latter representation adds the so-called Pauli or exchange correlation into the probability value.     Ψðr1 ; r2 ;σ 1 ; σ 2 Þ2 5 1 ψ1 ðr1 Þαðσ1 Þψ2 ðr2 Þαðσ2 Þ2ψ1 ðr2 Þαðσ2 Þψ2 ðr1 Þαðσ1 Þ2 2 5

1  1    ψ1 ðr1 Þψ1 ðr1 Þψ2 ðr2 Þψ2 ðr2 Þ 1 ψ1 ðr2 Þψ1 ðr2 Þψ2 ðr1 Þψ2 ðr1 Þ 2 2

1  1    2 ψ1 ðr1 Þψ2 ðr1 Þψ2 ðr2 Þψ1 ðr2 Þ 2 ψ2 ðr1 Þψ1 ðr1 Þψ1 ðr2 Þψ2 ðr2 Þ: 2 2

ð4:15Þ

Where both negative extra terms represent the correlated component of the electronic motion, seen for the parallel spin case only. For electrons under no exchange correlation, that is, antiparallel spin cases, their behavior would be uncorrelated or spatially symmetric. Although, the current scheme possesses the fundamental requirements necessary for the description of a set of N-electrons, an important construction is still pending for the relevant electron-electron potential of Eq. (4.3). Within the HartreeFock approximation, such interactions are brought by the antisymmetric nature of the many particle wave function, determining a classical term along with a repulsion interaction that rises, as in the case of Eq. (4.15), due to the exchange correlation.

4.2.3

The variational principle

In quantum chemistry the fundamental magnitude involved in calculations is the total energy, therefore, every methodology aims at determining the ground state of the system of particles, which is defined as the lowest energy state and represents the most likely behavior regime of all the particles. When the variation of the total energy is equal to zero it is said that

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expression is minimized, just as any function when its derivative is represented by a flat line at its minimum point. The parameters that are allowed to vary during the minimization process are present in the wave functions, known as variational parameters, and the ground state is determined at the minimum energy with optimized wave functions. However, there is a fundamental assumption regarding the reached state, since it still represents an upper limit to the exact minimum energy, which is only available by the analytic, and yet unknown, solutions to the many particle system, since the varied wave functions are approximations. As more precise the shape of the varied wave functions is, the solution will appear closer to the exact ground state. In order to reach such a minimum energy configuration, the quantum variational principle formally states that, any system in its minimum energy level reaches a stationary state. Since it shows a first order variation equal to zero in its expected value of energy with respect to a certain parameter ðδE 5 0Þ. Therefore, by means of the modification of such a variational parameter, it is possible to assign a specific expectation value of energy, to iteratively reach the ground state of the system of particles. The expectation value of energy in quantum mechanics shows a single dependency upon Ψ, which is itself a many particle wave function for the present case. This mathematical object is known as functional, a generalized definition of a function in terms of the variational parameter Ψ written in the Dirac notation as: D E ^ ΨjHjΨ E ½ Ψ 5 : ð4:16Þ hΨjΨi Where the squared bracket defines that E is a functional of Ψ, and the suffix of both total Hamiltonian and Slater functions were omitted for simplicity. A first order variation to the current parameter, Θ 5 Ψ 1 δΨ;

ð4:17Þ

defines a modified many particle wave function Θ. By substitution of Eq. (4.17) for Eq. (4.16), all the first order terms vanish, considering the variational principle at the ground state, leading to a functional in terms of a second order variation of the energy functional of Ψ, D E D E ^ ^ ΘjHjΘ E½Ψ 1 δΨjHjδΨ E ½Θ  5 5 : ð4:18Þ hΘjΘi hΨ 1 δΨjΨ 1 δΨi The restrictions as the orthogonality of the wave function set or the fixed number of particles from the system are introduced by means of the undetermined Lagrange multipliers method in a renamed functional F ½Θ: ^ D½Θ 5 ΘjHjΘ 1 ζ ðΘjΘ 2 1Þ:

ð4:19Þ

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Where the second term contains the multiplier factor ζ appearing due to the orthonormalization condition from the wave function set. Applying a first order variation to the latter functional of Θ: n o n o ^ ^ δD½Θ 5 δΘjHjΘ 1 ζδΘjΘ 1 ΘjHjδΘ 1 ζΘjδΘ 1 δζ ðΘjΘ 2 1Þ 5 0; ð4:20Þ Where the wave function Θ was optimized until the condition of stationarity, δD½Θ 5 0, is reached. Shows that the variational principle is fulfilled whether both expressions in brackets are equal to zero, taking the term that contains the conjugated variations from the wave function, it is possible to define the expected set of equations from Θ, returning the many particle framework into the Eigen equation formalism, with ζ as the self-value of the total Hamiltonian. ^ 5 ζΘ: HΘ

4.2.4

ð4:21Þ

The universal functional of the density

The electronic density represents a SR magnitude determined as the resultant probability to find an electron with a spin σ i centered at ri . Since it shows a dependency upon the position vector from the ith particle, it is considered a local property. Therefore, there are spatial distribution chances at every selected point in real space, generating a 3D plot as in Fig. 4.4. The electronic density associated to the particle ðr1 ; σ1 Þ is written as (Fiolhais et al., 2003): ð X ð  2 1 d 3 r2 . . . d3 rN N!Ψðr1 ; r2 ; . . . ; rN ;σ 1 ; σ 2 ; . . . ; σ N Þ nσ1 ðr1 Þ 5 ðN 2 1Þ! σ2 ;σ3 ;...;σN ð4:22Þ Which resembles Eq. (4.12). Since the density of the ith particle from the system is the result of the simultaneous probability to find the rest of the electrons centered at every undistinguished coordinate from the N electrons, respectively. Integrating this probability distribution with respect to r1 gives (Szabo and Ostlund, 1989): ð Xð Xð  2 3 3 d r1 nσ1 ðr1 Þ 5 N d r1 . . . d3 rN Ψðr1 ; r2 ; . . . ; rN ;σ 1 ; σ 2 ; . . . ; σ N Þ 5 N; σ1

σ

ð4:23Þ that recovers the total number of particles N by the product of the normalization factor from a N 2 1 particle expansion into N! coordinate permutations from the inner product of Ψ.

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FIGURE 4.4 Calculated electronic density from fcc aluminum crystal. The 3D isosurface is obtained from a real space grid of local density points. This result was obtained by means of the Castep module form the Materials Studio Package (Clark et al., 2005).

In order to define a complete energy functional of the density, capable to describe a set of interacting particles ðE½nðrÞÞ, it is necessary to write each term from Eq. (4.3) in a density functional form. On the first hand, a functional F is determined containing both kinetic (T e ) and repulsive (V e2e ) terms from the electronic Hamiltonian, conserving an independent behavior from the external potential (V ext ). Such a functional is said to be universal, since both of its components are present in every possible electronic distribution, F ½Ψ 5

hΨjT e 1 V e2e jΨi ; hΨjΨi

ð4:24Þ

where the integrals from Eq. (4.22) are implicit in the Dirac notation of the latter expression. On the other hand, the dependency of the external interactions with respect to the electronic density is found outside F, as in the separation of equations in Eq. (4.3), found in a second functional that integrates the product of nðrÞ into the external potential V ext Fiolhais et al. (2003): ΨjV ext jΨ 5 ΨjΨ

Ψj

N P i

vðri ÞjΨ

ΨjΨ

5

Xð σ

d3 rvðrÞnσ ðrÞ:

ð4:25Þ

Both expressions in Eq. (4.24) and Eq. (4.25) determine a template of E½nðrÞ written as: ð hΨjT e 1 V e2e jΨi X 3 ½  1 d rvðrÞnσ ðrÞ: E nðrÞ 5 ð4:26Þ hΨjΨi σ

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Where a functional of Ψ and a density dependent integral conduct towards a two-stage minimization process to find the ground state of E½nðrÞ Fiolhais et al. (2003). In the first step, as in the previous section, the stationary state of the universal functional is reached by means of the variation of Ψ, restricting the determination of a fixed nðrÞ. F ½nðrÞ 5

hΘjT e 1 V e2e jΘi ; hΘjΘi

ð4:27Þ

As in Eq. (4.18), Θ denotes a varied wave function that determines nðrÞ by means of the same formulation of Ψ, but this time represents the total wave function that minimize the expected value F, which is from this point said to be a universal functional of the density, meanwhile, the latter optimization leaves the second term of Eq. (4.26) unchanged, due to the fixed nðrÞ condition. Rewriting in terms of the fixed nðrÞ gives an expression fully dependent upon the density: ð E½nðrÞ 5 F ½nðrÞ 1 d3 rvðrÞnðrÞ; ð4:28Þ where both spin label and σ summation is included in a single variable of density as: nðrÞ 5 nm ðrÞ 1 nk ðrÞ. The Eq. (4.28) is subsequently minimized to reach the stationary state of E½nðrÞ at the ground state electronic density. This process known as constrained search includes the restriction upon a fixed number of particles N, as Eq. (4.20), by means of a Lagrange multiplier factor μ. The latter constrain defines the following functional to be minimized: ð

ð 3 3 E½nðrÞ 5 F ½nðrÞ 1 d rvðrÞnðrÞ 2 μ d rnðrÞ 2 1 ; ð4:29Þ where the last term includes the value of the probability function integration in Eq. (4.23) for the constrained number of particles. Proceeding with the variation of Eq. (4.29), by means of the functional derivative, at the stationary state of E½nðrÞ is found:   ð δF ½nðrÞ 3 1 vðrÞ 2 μ δnðrÞ 5 0; ð4:30Þ δE½nðrÞ 5 d r δnðrÞ where the terms within brackets fulfill the variational principle whether: δF ½nðrÞ 1 vðrÞ 5 μ: δnðrÞ The latter expression shows determined by the ground state the stationary point of E½nðrÞ. dependency upon nðrÞ and the

ð4:31Þ

that, the external potential vðrÞ is uniquely density, except for a constant μ, that is, at Since the functional F possesses a single external potential is kept fixed during the

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minimization process, due to the Born-Oppenheimer approximation. In the paper published in 1964 by Hohenberg and Kohn (Hohenberg and Kohn, 1964), the fundamental dependency of the external potential with respect to the ground state density is enunciated by a keystone theorem proven by reductio ad absurdum, that demonstrates that it is not possible to have more than one wave function that determines the same density, instead, a single ground state density generates all possible solutions to any total Hamiltonian, and hence for any external potential. Excluding the case in which the total Hamiltonian is defined by a constant vðrÞ.

4.2.5

The auxiliary Kohn-Sham system

Once the main foundation of the theory has been established, it is necessary to introduce a model to solve a single particle wave equation. The elegant substitution of a real set of electrons with a concept in which the charges behave as independent particles, was introduced by Kohn and Sham (1965) and both many particle systems show the same ground state nðrÞ as a keystone restriction. Therefore, for a system of noninteracting particles the universal density functional is written as:  KS   Ψ jT e jΨKS F KS ΨKS 5  KS KS  ; ð4:32Þ Ψ jΨ where the KS labels stand for the Kohn-Sham system of independent particles, at the stationary state from F KS , the varied total wave function define a noncorrelated density expressed as: XX  ψσ ðrÞ2 : nð r Þ 5 N ð4:33Þ σ

i

i

An expression that resembles the difference in between Eq. (4.14) and Eq. (4.15), where the interaction among electrons is not considered. If the subtraction of the two universal functionals of the density is imposed, the result will define the well-known exchange-correlation functional of the density, written as:

ð4:34Þ Exc ½n 5 F ½nðrÞ 2 F KS ½nðrÞ 1 U ½nðrÞ ; where, the exchange-correlation functional contains only both kinetic and potential interactions of electrons which are not present neither in the KS system or in the classical repulsion interaction among two electronic densities (U ½nðrÞ), also known as Hartree interaction, a further development shows that the subtraction takes place one to one, in between the kinetic and potential components of the functional. Under the variational of the energy functional containing the KS universal functional of the density, exchange,

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and correlation functional and Hartree interaction, under the restriction of a fixed number of particles the Kohn-Sham equations are defined as:   1 ð4:35Þ 2 r2 1 VKS ψσi ðrÞ 5 εi ψσi ðrÞ: 2 Where the Kohn-Sham potential contains the functional derivatives of the Hartree and Exchange correlation functional, along with the external potential. To find the solution of this set of equations it is necessary a group of one particle wave functions (Kohn-Sham orbitals) for all the occupied i-states. the density  However,  defines all the terms in the potential, therefore, to find ψσi ðrÞ is necessary an iterative procedure until the converged density is finally reached (Fig. 4.5). The process involved in the iterative solution of the KS equations is called self-consisted filed (SCF). During this loop an initial density, usually provided by atomic calculations is generated as a first estimate to construct the KS potential. Once the KS Hamiltonian is complete the wave equation is solved and the new

FIGURE 4.5 The SCF cycle flow chart. SCF, Self-consisted filed.

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ψσi ðrÞ construct the electronic density of the first iteration. At the end of every step the result is compared in order to measure the deviation among two consecutive iterations. This comparison decides whether or not another SCF cycle is performed. Whenever the deviation is large compared to the tolerance factors in either energy or charge, the present iterative density is used as an initial expression to start the cycle all over again. Finally, when all the tolerance factors are fulfilled it is said that the cycle found the ground state density, therefore the total energy of the system could be determined.

4.3 Selected materials with solar energy harvesting implementations 4.3.1

The input file

In the following section some representative examples for energy harvesting materials will be discussed (Table 4.1), currently developed by the MREB research group (materials for renewable energy and biomedical applications, https://mreb-en.cinvestav.mx/mreb/Research), which is specialized mainly in photocatalysis and photovoltaics material research. Although, every DFT-based software possesses its own methodological approach to the theory, it is possible to visualize the main components of a generic input file. For every code, all the parameters for the preparation of the calculation along with the numerical coordinates and base vectors from the crystalline representation is found within that file. Therefore, the

TABLE 4.1 Brief description of the main solar energy harvesting application from the selected materials. Sec.

Selected materials

Solar energy harvesting applications developed in the MREB group

3.2

ZnO

Transparent conducting thin films in photovoltaic devices.

3.3

FAPbI3 perovskites

Organo-lead halide based perovskite solar cell.

3.4

Fe3O4

Heterostructure material suitable for adsorption and degradation.

3.5

TiO2 anatase

Photocatalytic material with engineered bandgap

3.6

mBiVO4

Nano heterostructure material with photodegradation capabilities

3.7

Cu (In, Ga) Se2 (CIGS)

Absorber layer for photovoltaic devices.

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necessary time needs to be spent in the planification of every single line of it, since many of the errors and calculation failures rise due to the requested calculation scheme in the input file. The following display of lines comes from a template file found in the abinit first principle code (Gonze et al., 2002), for further definitions regarding the input variables the readers is encouraged to consult the user manual, open to the public at www.abinit.org. The geometrical optimization procedure is defined, offering the user several combinations to relax either atomic positions, lattice parameters or both, with the aid of well tested algorithms, with the definition, as in the SCF cycle, of tolerances in terms of force, which needs to be fulfilled in order to leave the optimization cycle, since the tolerance is not fixed for any case, a convergency test is recommended in order to avoid either poorly optimized structures or a nonfeasible number of iterations. #Name of the compound: BiVO4 ionmov 2 optcell 2 ntime 100 tolmxf 5.0d-4

One of the possible ways to write the lattice parameter from the crystal is the real space matrix, Within this notation, the rectangular components from a, b, and c vectors are contained, written as atomic units, however, a group of acceptable definitions are available with a different combination of input variables. The code also offers the possibility to write the number of the space group. rprim

09.729456 -00.065717 00.000000 -00.065717 09.729456 00.000000 00.000000 00.000000 21.945042

Once the lattice parameters are defined, the atomic positions are writing defining the total number of chemical species characterized by the atomic number z. ntypat 3 znucl 83 23 8

On the other hand, the total number of atoms in the unit cell followed by the distribution for every atom type needs to be in close agreement with the list of pseudopotentials and atomic coordinates. The option for either Cartesian or reduced coordinates are available for the input file writing, choosing the latter notation due to the previously discussed advantages in the definition of the unit cell. In the reduce coordinates list, the first four lines corresponds to bismuth positions, followed by four vanadium sites and the final sixteen lines represent oxygen coordinates. natom 24 typat 1 1 1 1 2 2 2 2 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 xred 0.500000000000 0.750000000000 0.125000000000 0.000000000000 0.750000000000 0.375000000000

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0.250000000000 0.250000000000 0.250000000000 0.250000000000 0.750000000000 0.750000000000 0.750000000000 0.387499980000 0.498499990000 0.001500000000 0.498499990000 0.001500000000 0.387499990000 0.112499990000 0.612500010000 0.887500000000 0.501500010000 0.998499990000 0.501500010000 0.998500000000 0.612500010000 0.887500000000

0.675000000000 0.875000000000 0.125000000000 0.375000000000 0.625000000000 0.875000000000 0.875000000000 0.046599990000 0.203400000000 0.203400000000 0.296600010000 0.296600010000 0.453399990000 0.453399990000 0.546599980000 0.546599980000 0.703400020000 0.703400020000 0.796600000000 0.796600000000 0.953400020000 0.953400020000

Numerical value in Ha of the cut off energy of the plane wave expansion, with a double grid option whenever the PAW 1 PW scheme is implemented in the calculation. To obtain accurate result a previous process of convergency is mandatory in order to choose the optimal value. ecut 30.0 pawecutdg 30.0

Definition of the reciprocal grid of allowed electronic states under the periodical potential formalism, the number of k-points changes for every geometry, however a similar distancing is recommended to improve the quality of the result, along with a convergency test from a coarse to a finer grid to determine optimal conditions. kptopt 1 ngkpt 664 nshiftk 2 shiftk 0.25 0.25 0.25 -0.25 -0.25 -0.25

For the conventional calculation, the number of iteration steps in the SCF cycle is defined along with the energy tolerance to eventually accept the solution of the KS equations. nstep 100 toldfe 1.0d-6

First principles methods for solar energy harvesting materials Chapter | 4

4.3.2

121

A supercell of zinc oxide

One of the most important tools in DFT modeling is the construction of supercells to represent a point defect, such as a vacancy or a dopant substitution. In a supercell, there is a crystal rebuild process that repeats the conventional unit cell a certain number of times into a tailored cell, which could carry an approximated atomic percent of a defect with respect to that of the pristine sites. For instance, the Wurtzite structure of ZnO possesses only two metallic sites within the conventional unit cell. Therefore, if it is necessary to include a metallic substitution into one of those sites, the atomic percentage of the defect will be 50%, which is a very large value for a dopant. A supercell transformation of the structure will repeat the pristine wurtzite structure a certain number of times with two main objectives, on one hand, the amount of crystalline planes will isolate the structural effects from the defect during the optimization and secondly, the atomic percentage will approximately reach sample conditions. For a 2 3 2 3 2 supercell of ZnO (Fig. 4.6A), the structure is repeated two times, along x, y, and z, generating a supercell with 16 available sites for substitution (6.25 at.%), a scheme that is more realistic with respect to the dopant conditions. In a contribution from 2018 (Karthick et al., 2018), a model substitution as the one depicted was solved by DFT in order to see the structural and

FIGURE 4.6 (A) Hexagonal unit cell of ZnO and its 2 3 2 3 2 supercell, extended two times along each unit cell axis, (B) formamidinium lead iodine perovskites for photovoltaic applications, the orientation from the organic cation determines the structural stability, and (C) conventional unit cell of the inverse spinel magnetite.

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electronic effects from the substitution, comparing with sputtered samples used as window layers for solar cell devices. The result determines that the optical transmittance, calculated by the real and imaginary parts of the refractive index, increases with the substitution of aluminum atoms at Zn sites, in agreement with the experimental measurements.

4.3.3

Structural stability of FAPbI3 perovskites

One of the most intensively studied materials in recent years is the hybrid organic-inorganic perovskites since their applications as an absorber layer in photovoltaic devices result in spectacular efficiencies. However, the phase instability represents a major flaw. The Formamidinium lead iodine perovskite possesses a cubic phase suitable for solar cell applications (Fig. 4.6B), with a carbon-based cation centered in an inorganic cage. In a study implementing Van der Waals interactions, it was determined that the major interaction is carried out among the hydrogen terminations from the cation and the iodine sites at the edges of the cell. One of the methodologies to determine the mechanical stability of a structure is the calculation of the elastic constants by means of a finite deformation method. In this procedure the ground state structure is strained modifying both lattice and base vectors by a transformation tensor: 0 01 0 01 x x @ y0 A 5 ðDðeÞ 1 I Þ@ y0 A; ð4:36Þ z0 z0 Where, the strain tensor DðeÞ determines a single strain-energy path as the deformation takes place before a limit or strain amplitude is reached. For every strain energy curve, a single elastic constant from the cij set is determined, depending upon the symmetry from the ground state energy. The number of strains changes along with the structural stability conditions. For the ground state from the FAPbI3 structure there are nine independent elastic to determine and it was found that an specific deformation path, with strain components that modify the main charge distribution, could turn the structure unstable if a strain amplitude is exceeded. At such a state, the cation goes through an unexpected tilting which is not seen below the instability threshold.

4.3.4

Charge order and half metallicity of Fe3O4

As it is well known, above the Verwey temperature (Tv 5 120K), the inverse spinel crystalline structure of magnetite (Fd-3m 227) is built by two alternating sublattices of iron cations with an octahedral and a tetrahedral oxygen coordination, respectively. Due to the reduced number of chemical bonds in the tetrahedral iron sites (Fe31), a higher degree of electronic localization

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compared with any of the octahedral-coordinated sublattice sites is expected. On the other hand, the latter atomic layers of iron under an eightfold oxygen coordination is the primary responsibility for the electronic conduction mechanism of magnetite. Compared to Fe31tet, the octahedral sites possess twice the number of iron positions in the unit cell, with a site distribution of one half from both ferric (Fe31oct) and ferrous (Fe21oct) oxidation sates. Finally, the spin projections from tetrahedral and octahedral sublattices are antiparallel to each other. The tetrahedral-coordinated iron sites (A-sites) possess five valence electrons with a minority spin projection, that is, antiparallel with respect to the net magnetization of magnetite, distributed into two eg and three t2g orbitals occupations. On the other hand, there is only a single electron, in the octahedral-coordinated sublattice (B-sites), with a spin minority projection, differentiating Fe21 from Fe31 (Fig. 4.6C). In the current work, three models of atomic plane stacking are studied under a DFT 1 U scheme with a fixed U 5 4.00 eV and a variable J parameter, both values representing the high-correlation parameters defining the degree of localization of a certain electronic distribution, with no constrains either for the geometrical optimization nor on the symmetry of the k-mesh. For the present case, the charge disproportionation is established by means of the magnitude of the onsite spin moment, and the selection of the appropriate local magnetization was based on the electronic localization/itinerance degree determined based on the oxygen coordination and oxidation state for each A and B site. Our results show three different electronic spectra, one of which presents the previously reported formation of a bandgap, losing the t2g degeneracy at the Fermi level, meanwhile, the remaining accessible states attain two different half-metallic configurations. However, none of the former results belongs solely to a certain model of spatial distribution of the Fe21octa and Fe31octa. Instead, the proposed arrangements switch among these behaviors, showing modified occupations in the t2g orbitals at the spinminority projection, along with a full exchange in the occupation among B-sites. Hence, the presence of both short range ordering and mixed-valence configuration, appearing with a very small energy difference in between them as a result of a slight variation in the correlation parameter, suggests a possible recovery of the cooperative effect in the electronic conduction mechanism.

4.3.5

Optimization of anatase titanium dioxide

The geometrical optimization process represents the beginning for a computation of any property of the crystal. Once the structure is said to be geometrically relaxed, the crystal reaches the ground state energy, that is, the minimum energy allowed by the degrees of freedom of every structure. For the case of the tetragonal unit cell from anatase, the structure possesses two variables to optimize the lattice vectors, the volume of the unit cell and the

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c/a ratio. The first feature is varied by isotropic deformation of the unit cell, modifying uniformly the magnitude of the lattice parameters in such a way that, the c/a ratio remains unchanged, defining percentual changes along with limits, usually symmetrical with respect to the initial volume. In practice, it is really uncommon to find that the theoretically optimized volume match perfectly the experimental measurements, and this fact should not be a major concern since the calculations are based around approximations and numerical schemes that may not consider all the conditions of the sample measurement. However, acceptable values around 23% of variations represent good results. On the other hand, the c/a ratio variation need to have a fixed unit cell volume. With a constant volume the structure could be varied by a transformation of both c and a lattice parameter, in such a way that, an expansion of c is followed by a reduction of the magnitude and vice versa. The methodology to optimize the crystalline lattice of anatase was found in a two stage process, from initial volumes and a c/a ratio, a two dimensional grid of possible geometries was established, with a set of isotropic deformations distributed as rows and linear variations of the c/a ratio as columns. The results (Fig. 4.7) show that, for every column (constant c/a ratio) the fitting of equation of state shows a total energy in terms of the volume with a minimum value around a variation of the c/a ratio of 20.6%.

FIGURE 4.7 Conventional unit cell from the anatase phase of TiO2, along with a two-stage optimization process for tetragonal structures.

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125

A conventional and a reduced representation of mBiVO4

The monoclinic phase of bismuth vanadate is a structure with interesting applications in photocatalysis, currently studied by the group mixed with TiO2 as a nanocomposite. However, the crystalline structure possesses an extraordinary complexity in the coordination of atoms and in their spatial distribution in the unit cell. The structure was initially model with the conventional representation of 24 atoms. And the reported crystalline planes for a heterojunction show a favorable match with the unitary thickness from the model, but the number of sites turn the calculation slow for a bulk case. It was aimed to build a model with the space group C2/c in order to reduce the number of irreducible sites. The transformation needs to consider a b’ parameter as a resultant from the lattice vectors a and b from the conventional lattice (Fig. 4.8A). The complexity of the structure demands the optimization of four degrees of freedom, three lattice parameter and the gamma angle.

FIGURE 4.8 (A) The conventional and reduced representation of monoclinic BiVO4, the structured with one half of the atomic positions is obtain by a transformation of the lattice vectors, (B) the structures of chalcopyrite of CIGS at both gallium and indium rich conditions. CIGS, Cu (In, Ga)Se2.

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The process then needs a grid of four dimensions with a constant step in between the variations. For instance, if a step of one percent is chosen a total of 81 structures will be generated each of them with a different percentual modification (21,0,1). The results of the nanocomposites show, as the load of one of the constitutes increases, the electronic transitions take place reassembling the behavior from the major component. However, the optimal degradation conditions appear for samples closer to the monoclinic phase. Therefore, it is proposed to study the current model as a surface with substitution of titanium atoms as a heterostructure model.

4.3.7

A template structure for chalcopyrite

The Cu(In, Ga)Se2 (CIGS) is an IIIIVI2 semiconductor material, which crystallizes in a tetragonal chalcopyrite structure. The quaternary system CuInGaSe is based on the CuInSe and CuGaSe ternary systems. CIGS and CGS have the same crystal structure, except as a matter of fact that some In atoms are replaced by Ga atoms. The chalcopyrite structure of CIS is obtained from the zinc blende structure with the insertion of an additional ordering of the cation sublattice, requiring a doubled primitive cell (tetragonal structure). The structure can be imagined as two interpenetrating face-centered cubic (fcc) lattices: the first anion lattice consisting of group VI atoms (Se22) and the other being an ordered array of group (Cu1) and (In31) cations. Each of the Cu or In atoms is bonded tetragonally with four Se anion atoms, whereas each of the Se atoms is coordinated with two Cu and two In atoms. The transition from CIS to CIGS is accomplished by the partial aleatory substitution of indium for gallium atoms. One of the most noticeable effects on the CIS structure with the addition of gallium is the decrease of the lattice parameters. This decrease represents a distortion of the crystal structure and is directly related to the size difference of indium and gallium atoms (atomic radii ratio rGa/rIn  3/4). The value of tetragonal distortion is given by Δ 5 2c/a and is depending linearly on Ga content in Cu(In12xGax)Se2. The value Δ is negative for x , 0.23 and positive for x . 0.23. It could be due to the change in electronegativities of In and Ga. For a pure CIS, the c/a ratio is close to 2. However, the c/a ratio diverges toward lower values along with grain refinement due to the substitution of In by Ga atoms. Considering the band structure, the valence band of CIGS is derived from the weak CuSe bond group (IVI) due to the hybridization of Cu-d and Se-p orbitals. The bottom of the conduction band is mainly contributed from the In and Ga atoms (group III-s orbitals). The CIGS solar cell technologies are based on chalcopyrite structures, with alternating atoms of Ga or In at the same crystalline sites (0,0, 1/2) and (0, 1/2, 3/4) (Fig. 4.8B). The proposed theoretical study uses all the tools early described, to build a mechanically stable model, with substitutions of Ga and In in proportional

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amounts to verify one experimental condition observed in the c/a ratio as the load of Ga/In varies.

4.4

Conclusion

In this chapter, the principal concepts regarding the DFT implementation in solid state were briefly discussed. Along with examples of the determination of realistic properties from energy harvesting materials. The present document represents an introductory reference aimed for readers that show interest in computational material science. Since the very basic formalisms needed to start the train into DFT were written in detail, in order to encourage the reader to build new crystalline models as possible instruments to study open research topics by theoretical methodologies, as brand new members of a continuously growing community, aimed to first principle calculations, and encouraged by the availability of a rich variety of computational codes, such as castep (Clark et al., 2005), among several more.

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Guo, Y., Chen, S., Yu, Y., Tian, H., Zhao, Y., Ren, J.-C., et al., 2019. Hydrogen-location-sensitive modulation of the redox reactivity for oxygen-deficient TiO2. J. Am. Chem. Soc. 141, 8407. Hern´andez-Haro, N., Ortega-Castro, J., Martynov, Y.B., Nazmitdinov, R.G., Frontera, A., 2019. DFT prediction of band gap in organic-inorganic metal halide perovskites: an exchangecorrelation functional benchmark study. Chem. Phys. 516, 225. Hohenberg, P., Kohn, W., 1964. Inhomogeneous electron gas. Phys. Rev. B 136, B864. Ikeda, Y., Grabowski, B., Ko¨rmann, F., 2019. Review ab initio phase stabilities and mechanical properties of multicomponent alloys: a comprehensive review for high entropy alloys and compositionally complex alloys. Mater. Charact 147, 464. Karthick, S., R´ıos-Ram´ırez, J.J., Chakaravarthy, S., Velumani, S., 2018. Electrical, optical, and topographical properties of RF magnetron sputtered aluminum-doped zinc oxide (AZO) thin films complemented by first-principles calculations. J. Mater. Sci.: Mater Electron. 29, 15383. Kohn, W., Sham, L.J., 1965. Self-consistent equations including exchange and correlation effects. Phys. Rev. B 140, A1133. Kong, Z., Chen, X., Ong, W.-J., Zhao, X., Li, N., 2019. Atomic-level insight into the mechanism of 0D/2D black phosphorus quantum dot/graphitic carbon nitride (BPQD/GCN) metal-free heterojunction for photocatalysis. J. Appl. Surf. Sci. 463, 1148. Lehtinen, O., Komsa, H.-P., Pulkin, A., Whitwick, M.B., Chen, M.-W., Lehnert, T., et al., 2015. Atomic scale microstructure and properties of Se-deficient two-dimensional MoSe2. ACS Nano 9, 3274. Montejo-Alvaroa, F., Olivab, J., Zaratec, A., Herrera-Trejoa, M., Hdz-Garc´ıad, H.M., MtzEnriqueza, A.I., 2019. Icosahedral transition metal clusters (M13, M 5 Fe, Ni, and Cu) adsorbed on graphene quantum dots, a DFT study. J. Phys. E 110, 52. Nagarajan, V., Chandiramouli, R., 2019. Detection of trace level of hazardous phosgene gas on antimonene nanotube based on first-principles method. J. Mol. Grap. Mod. 88, 32. Patra, A., Peng, H., Sun, J., Perdew, J.P., 2019. Rethinking CO adsorption on transition-metal surfaces: effect of density-driven self-interaction errors. Phys. Rev. B 100, 035442. Ramay, S.M., Hassan, M., Mahmood, Q., Mahmood, A., 2017. The study of electronic, magnetic, magneto-optical and thermoelectric properties of XCr2O4 (X 5 Zn, Cd) through modified Becke and Johnson potential scheme (mBJ). Curr. Appl. Phys. 17, 1038. Rehman, S.U., Butt, F.K., Haq, B.U., Faify, S.A., Khan, W.S., Li, C., 2018. Exploring novel phase of tin sulfide for photon/energy harvesting materials. Sol. Energy 169, 648. Ren, K., Wang, S., Luo, Y., Xu, Y., Sun, M., Yu, J., 2019. Strain-enhanced properties of van der Waals heterostructure based on blue phosphorus and g-GaN as a visible-light-driven photocatalyst for water splitting. RSC Adv. 9, 4816. Saı¨la, K., Bassou, G., Gafour, M.H., Miloua, F., 2015. PBEDFT theoretical study of organic photovoltaic materials based on thiophene with 1D and 2D periodic boundary conditions. J. Exp. Theor. Phys. 121, 1015. Szabo, A., Ostlund, N.S., 1989. Modern Quantum Chemistry: Introduction to Advanced Electronic Structure Theory. McGraw Hill, New York. Tran, F., Stelzl, J., Blaha, P., 2016. Rungs 1 to 4 of DFT Jacob’s ladder: extensive test on the lattice constant, bulk modulus, and cohesive energy of solids. J. Chem. Phys. 144, 204120. Vin˜es, F., Bernechea, M., Konstantatos, G., Illas, F., 2016. Matildite versus schapbachite: firstprinciples investigation of the origin of photoactivity in AgBiS2. Phys. Rev. B 94, 235203. Yin, J., Ahmed, G.H., Bakr, O.M., Bre´das, J.-L., Mohammed, O.F., 2019. Unlocking the effect of trivalent metal doping in all-inorganic CsPbBr3 perovskite. ACS Energy Lett 4, 789. Zhaoa, Y., Lib, D., Liua, Z., 2017. Structural and elastic DFT study of four structures for Cu2ZnSnS4 under high pressure. J. Alloys Compd. 696, 86.

Section II

Sustainable Materials for Photovoltaics

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Chapter 5

Introduction to photovoltaics and alternative materials for silicon in photovoltaic energy conversion Ganesh Regmi and Velumani Subramaniam Department of Electrical Engineering (SEES), Centro de Investigacio´n y de Estudios Avanzados del IPN (CINVESTAV-IPN), Mexico City, Mexico

5.1

Introduction

The production of energy and its consumption has been a foundation factor for our civilization’s socio-economic development. The crucial components of good quality of life in modern society correlate with its energy consumption. The necessity to increase energy production to fulfill the basic requirement becomes one of the most challenging tasks that our world facing today. To maintain a balance in energy consumption, energy sources’ production must be increased, which subsequently increases the harmful collateral effects. Energy is vital for many applications in transportation, industrial, household, agricultural, and office applications (Pasten and Santamarina, 2012). It exists in different forms, such as heat, light, chemical, nuclear, electrical, etc. Firewood was the primary source of energy during ancient times and replaced by coal after discovering a steam engine. The invention of the internal combustion engine ensued petroleum products like petrol, diesel, and natural gas to fulfill the demand for energy consumption. Coal, oil, and gas that are considered fossil fuels are recently consumed as a primary energy source. It is to be remembered that these sources are nonrenewable and getting depleted every year. When these sources are exhausted, it will be inevitable to face the energy crisis and the huge vacuum in energy production with the high increase of prices leading to detrimental effects on the entire world population’s. Electrical energy is the most versatile and appropriate form of energy, converted into various forms when considering power transmission, power distribution, and power control. Sustainable Material Solutions for Solar Energy Technologies. DOI: https://doi.org/10.1016/B978-0-12-821592-0.00004-2 © 2021 Elsevier Inc. All rights reserved. 131

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FIGURE 5.1 (A) Human development index (HDI) vs annual per capita electricity consumption, (B) estimated renewable share of total final energy consumption (2019).

Fig. 5.1A shows the Human Development Index (HDI) for different countries versus the per capita annual electricity consumption (data taken for the year 2019) (UNDP, 2020). The HDI is calculated based on life expectancy, mean years of schooling, per capita gross national income. It is seen from the HDI data that the necessity is to increase the energy consumption per year is obligatory for improving the quality of life. The supply of energy should be secure and sustainable with economic, eco-friendly, and acceptable consumption because energy consumption has become an integral part of human life. However, the current status is neither secure nor sustainable. The tremendous use of fossil fuels, coupled with greenhouse gas emissions, threatens our life span, and environmental degradation that could cause irreversible climate change. Considering the growth in population the need for energy production, with a reasonable distribution of energetics, and a nonthreatened climate, the use of renewable energy sources is obligatory as a stepping stone for mankind. Rapid growth in the world’s population and the techno-economic growth of countries are the main reasons for the increase in energy demand. The global energy requirement is ever-increasing, which puts pressure on the exhaustible nonrenewable fossil fuels. Renewable energy has been established worldwide as a mainstream source of electricity generation for several years. The estimated contribution of renewables to global electricity generation was more than 28% by the end of 2019, as shown in Fig. 5.1B (REN21, 2019). Renewable power is cost-effectively increasing when compared to conventional fossil

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fuels. The data by the end of 2019 has been demonstrated that the electricity generated from wind and PV plants had become more economical than fossil fuel. The purpose of PV energy conversion stands basically in two areas of applications. One is the power distribution of off-grid professional devices, including the home appliances supply system. The other is a large-scale electricity generation suitable as a complement to the present nonrenewable energy yield. The potential of renewable utilization does not account strictly, because of the technical restriction (land, efficiency, storage, policy, etc.). Direct conversion of PV energy is a technologically agile process avoiding conventional thermodynamic and mechanical intermediate methods. On the other hand, the production of PV modules and systems includes material transformation and the wastage of products, which grabs the attention of environmentally benign fabrication schemes. The average power density of solar radiation is approximately 100 W/m2, which signifies that solar energy’s global harvesting inevitably requires large-area production of energy converters. Therefore, recycling is an inevitable strategy and thus will be mandatory for reliable power by PVs applications. Business policy and modularity are very important concerning developing an electricity supply system in remote and rural areas where the grid connection is economically not viable. Implementing a low power installation of a PV module to a suitable design will effectively deal with the expanding energy demands. The absence of moving parts, friendlier with the environmental concern, less maintenance effort, and the lack of fuel supply requirements makes PVs a well-suited energy harvesting technology (Vidhya et al., 2011). The rapidly growing semiconductor industry is another positive impact on the PV market, facilitating the substantial technology shift from mature industry to the emerging PV industry which opens various employment opportunities and escalates business. This chapter aims to represent a general overview of PVs and energy harvesting materials, especially solar cells. A coherent compilation of the most recent progress and findings on thin film and emerging solar cell technology is presented.

5.2

Current status of photovoltaics

An accord about developing alternative energy and the reduction of fossil fuel consumption has been reached globally. Solar energy is one of the new energy sources which is considered as a potential technology to convert light energy directly into electrical energy via photovoltaic (PV) devices using semiconductor materials that exhibit a PV effect. The PV effect refers to creating electric current and voltage in a material when light is incident on it. The solar cell generates electrical power as long as the light incident on it. The negative impact of conventional energy sources can be overcome using

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solar energy to clean green energy. The cost-effective energy conversion technologies need to be developed to replace the existing technology of the conventional way of energy harvesting. Most renewable energy sources cannot be stored for future use; therefore, an effective storage mechanism and efficient recovery are essential (Rohini et al., 2015). Another challenge of renewables is the location dependency due to the geographical structure and transportation facilities. The colossal investment requirement is another task for solar renewable energy conversion because of the daunting cost of creating a new one and replacing the current infrastructure. The advantages and disadvantages of PVs are listed in Table 5.1. In the starting phase of PV research during the 196080s, the focus was devoted to making the product enhancement by improving efficiency to produce more power. It has reported that a significant improvement in cell and module efficiency was made, and the drastic reduction in the fabrication cost, as solar cells transformed from pilot scale to semiautomated production. Fig. 5.2 shows the prices ($/W) and cumulative solar module installation in GW over PVs’ commercial history. Until the year 2019, these values

TABLE 5.1 Advantages and disadvantages of photovoltaics. Advantages G

G

G

G

G

G

G

Solar electricity is clean and ecofriendly. Because it does not use fuel other than sunlight, with no emission, no combustion, or radioactive fuel for disposal, it does not contribute noticeably to global climate change. PV systems are quiet and visually unnoticeable, with no moving parts. Rooftop installation of small-scale solar plants can take advantage of unused space. PV is a dynamic technology utilized for terrestrial to space shuttle applications. Solar energy is a locally existing, renewable resource. This lessens the environmental influences associated with transportation and reduces the reliance on imported oil. PV system can be constructed from a mini scale to a large-scale solar plant based on energy requirements. The operating cost of the PV system is relatively low.

Disadvantages G

G

G

G

Cadmium, selenium, and arsenic are used in the PV production process, which are toxic in nature. Solar energy is to some extent more expensive to manufacture than conventional sources of energy due to the cost of manufacturing PV devices. Solar power is a variable energy source. Solar capabilities may produce no power at all some of the time, especially cloudy and snowy time. Solar energy is more insufficient reliability of auxiliary elements, including storage.

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FIGURE 5.2 Historical trends of cost per Watt for the solar module and solar PV global installation capacity. PV, Photovoltaics.

represent mostly c-Si solar cell technology (IEA, 2019). From 200307, there was a shortage of polysilicon, which led to an increase in solar modules’ price. But subsequently the global economic crisis of 2008, the cost of solar modules toppling down, thereby scaling-up the production (ReyesFigueroa et al., 2015). These two curves (Fig. 5.2) are characteristic of most new technologies. Initially (2010), prices are high since the production quantity is low, so expansion and startup costs are stretched with few modules selling. Enhancement in cell efficiency, reduction in wafer thickness, manufacturing yields, diamond wire cutting, and technological advancement have brought down the use of silicon from 15 g/W in 2000 to about 5.2 g/W in 2014. This, in turn, has aided in the falling cost of PV modules (Battaglia et al., 2016). The technology is now within the economic reach of a broader market, and demand grows swiftly as consumers with a moderate income can afford the product. The small decrease in cost opens broader markets and utilization reliability to the consumers. The arrival of new investment in the market widens the use of PV products in a cost-effective way. In 2019, the gross cost of the solar module was lowered to 2.4 $/W, while the global installation reached 126 GW annually. The successful development of alternative materials for Si and an increase in the demand are the reason for such a drop in PV module prices. With this type of growth, it is believed that the average cost of electricity will start to compete with the cost of grid electricity by 202021. It signifies that the PV technology is now within the economic

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grasp of broader markets, and demand expands swiftly as consumers with a moderate income can afford the products.

5.3 Fundamental properties of photovoltaics semiconductors The material properties are crucial for the PV energy conversion, which requires an increase in the electron’s potential energy that possesses different energy bands separated from each other. The energy levels’ difference should be more significant than the room temperature energy of the electrons given by kT (B26 meV at 27 C) (Sze and Ng, 2006). The energy band difference keeps the electrons in an excited state for more extended periods than its relaxation time in the ground state. This will increase the probability of charge separation and hence the output from the device. Semiconductors’ choice is genuine because metals have continuous energy bands and are not suitable for this application. The photons reaching the earth’s surface from the sun do not have enough energy to excite the electrons to higher energy levels in the case of insulators, possessing a very high band gap. The separation of energy bands in semiconductor differ from each other. Hence, the conductivity can be varied depending upon the type of impurities added, the so-called doping. For example, the energy band gap of InAs is B0.36 eV; the energy gap for cadmium telluride is about 1.56 eV, while the energy band gap for ZnO is B3.3 eV. The photons’ energy in the sun’s spectrum is in the range between 0.3 and 4.5 eV, high enough to excite the electron in the semiconductors (Rudan, 2015). Such a unique feature of controlling conductivity makes them suitable for solar PVs.

5.3.1

Crystal structure of semiconductors

Various semiconductor materials arranged from group-II to VI of the periodic table are of great interest to solar PVs. These semiconductors can be used in different forms like elemental (Si, Ge), binary [GaAs, InAs, cadmium telluride (CdTe), CdS, ZnO], ternary (AlGaAs), or quaternary compound (CuInGaSe, CuZnSnS, CuInAlSe, InAlGaAs), which provides the better optical (absorption, radiation, reflection, transmission, etc.) and electrical (conductivity, mobility, carrier lifetime, drift velocity, etc.) properties (Babu et al., 2018). The choice of suitable energy band gap material with electrical properties’ plays a pivotal role in high-efficient solar cell engineering. Not that silicon is in group-IV, indicating that it has four valence electrons that can be shared with neighboring atoms to form a covalent-bonds. The arrangements of atoms in a solid can be periodic or random depending upon the parameters such as the synthesis of solid, intermolecular forces, and so on. In a single crystal lattice (one type of atomic arrangement) or monocrystalline, atoms’ periodic structure follows specific rules in all

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directions with a small volume (namely unit cell), which can be a representative of the entire crystal system. On the other hand, solids, where the periodic arrangement of atoms in different solid regions is different, are called polycrystalline or multicrystalline. The simplest possible unit cell crystal lattice is cubic. Other shapes that define the crystal lattice are hexagonal, tetragonal, orthorhombic, tetrahedral, etc. In crystalline silicon, the atoms are arranged in a diamond lattice with tetrahedral bonding, where the angle between any two bonds is 109.5 degrees (Gilkes et al., 1995). These arrangements can be characterized by two interpenetrating face-centered cubic (fcc) unit cells where the second fcc unit cell is shifted to one-fourth of the distance along the diagonal body of the first fcc unit cell. In the case of CdTe and GaAs binary compound, a zincblende lattice with interpenetrating fcc unit cells occurs. In the zincblende lattice of GaAs, if Al and P are replaced by both Ga and As for the formation of a quaternary compound, AlGaAsP occurs. This property of semiconductor is significant for getting band gap grading in solar cell applications. A similar pattern can be obtained for chalcopyrite based copper indium gallium selenide (CIGSe) compound semiconductor with a tetragonal crystal structure.

5.3.2

Energy band structure

When two atoms are brought enough together such that their outer subshell interacts with each other, the similar energy level of two atoms will be split to satisfy Pauli’s exclusion principle. This principle indicates that no two electrons can have the same quantum numbers, which implies that a single energy level cannot occupy more than two electrons (Ashoori, 1996). The split energy levels due to two atoms belong to both the atoms. In this manner of two atoms, more atoms can be added. As more and more atoms are added, the splitting of energy levels continues to satisfy Pauli’s exclusion principle. Hence, the splitting of energy levels outcomes in a continuous energy level set known as an energy band. When atoms are close to each other, the force of attraction increases due to the interaction between the atom’s nucleus and the other atom’s electron. Still, there exists a repulsion force between the nuclei of two atoms when their inter-atomic distance is small enough. The equilibrium between the force of attraction and repulsion occurs at the inter-atomic distance equal to each material’s lattice constant, which is a characteristic constant of each ˚ , respectively material. For Si and Ge, the lattice constant is 5.43 and 5.65 A (Haddara et al., 2017). In Si, at an inter-atomic distance corresponding to the lattice constant, two energy bands exist separated by a gap where no energy states are available. This gap is referred to as the energy gap or band gap (Eg) of the material. Among two energy bands, the upper energy band is called the conduction band, and the lower energy band is the valence band. Valence electrons occupy the valence band, and the conduction of current

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takes place because of electrons in the conduction band. In Si crystals, each atom will be surrounded by trillions of atoms, and the electronic arrangement of a single atom obeys the 2n2 rule, that is, each shell can have 2n2 energy levels. According to the quantum number, Si has 4 valence electrons. If we consider N isolated Si atoms, there will be 4N valence electrons. Out of the total 8N states, only 4N can be occupied while the other 4N remains empty. At absolute 0K, the electrons will occupy the lowest energy level. This signifies that in the Si at absolute 0K, all 4N valence band states will be occupied by 4N electrons, while the conduction band will be empty. The relation between energy and momentum (in terms of wave vector or propagation constant k) gives an idea about the materials’ direct and indirect band gap. The wave vector is related to an electron’s wave function in an infinite lattice and represents the electron momentum in the lattice. If the valence band maximum and the conduction band minimum exist at the same value of k, then the semiconductor type is called the direct band gap. This gives an idea that to excite an electron from the valence band to the conduction band, only a photon of energy Ec 2 Ev 5 Eg is required. Photon is considered high energy and low momentum particle so that the electronic transition is vertical in the E-k diagram (Fig. 5.3) for direct band gap semiconductor. In the reverse process, the excited electrons jump back to its ground state with photon energy (Eg) emission. The excitation of electrons to the conduction band due to the absorption of a photon is known as carrier generation. In contrast, the reverse process, falling back an excited electron to the valence band ground state, is referred to as carrier recombination (Sah and Lindholm, 1977). In indirect band gap semiconductors, the conduction band minima do not occur directly below the valence band maxima in the Ek diagram (Fig. 5.3). Excitation of an electron from the valence band to the

FIGURE 5.3 Energy-momentum diagram for direct/indirect band gap semiconductor.

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conduction band occurs with the absorption of a photon, and the absorption of another particle termed phonon. A phonon is believed to be a particle of low energy and high momentum related to lattice vibration. Instead of a photon, two particles (photon and phonon) are involved in the generation and recombination of electrons in indirect band gap semiconductors. Therefore, both generation and recombination are less probable in indirect band gap semiconductors (Sah et al., 1957).

5.3.3

Density of energy states

When surrounded by many other atoms, the energy levels of single atoms do not lie at the same energy level due to Pauli’s exclusion principle. Still, they are distributed in the energy band as a function of energy. The distribution of such energy levels in the conduction and valence band as a function of energy is given by the density of state g(E). The density of states in the valence band gv(E) and conduction band gc(E), at an energy level E, can be expressed as: qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi  mp 2mp ðEv 2 EÞ cm23 eV21 for E # Ev ð5:1Þ gv ðEÞ 5 π2 ℏ3  pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi mn 2mn ðE 2 Ec Þ 23 21 cm eV for E $ Ev ð5:2Þ gc ð E Þ 5 π2 ℏ3 



Here mp and mn are the effective masses of holes and electrons in a semiconductor, Ev and Ec are the corresponding energy level at valance and conduction band, respectively. The effective mass is different from the mass of electrons in a vacuum (mo) because electrons’ mass is in a complex field in semiconductors. The classical formula (F 5 maÞ for a particle with mass m  and acceleration a can be written as F 5 mn a. The effective mass of holes and electrons in Si at room temperature is 0.81m0 and 1.18mo, respectively. The expression of Eqs. (5.1) and (5.2) shows that the density of states increases as we go away from the conduction band edge and the valence band edge. The carrier distribution function is also known as the Fermi function or Fermi-Dirac distribution function, which explains how many of the available energy states are occupied. The probability that an electron will occupy an available energy state at energy level E at absolute temperature T under the equilibrium condition (absence of both an electric field and light illumination), which can be written as (Phillips, 1987): f ðE Þ 5

1 1 1 eðE2EF Þ=kT

ð5:3Þ

where k is the Boltzmann’s constant and T is the absolute temperature. The critical parameter EF is termed as Fermi level, which gives valuable insight for understanding the charge distribution in a semiconductor. The value of f

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(E) can vary between 0 and 1 (Irac, 1926). The unity value of f(E) at energy level E signifies a 100% probability of occupancy by electrons in that energy level. Moreover, suppose the value of f(E) at energy level E is zero. In that case, there is a 100% probability that the energy state will be empty, that is, an electron will occupy in that energy state is 0%. Similarly, the probability of having a hole at energy level E is given by [1 2 f(E)]. At absolute zero, the Fermi function is a step function (rectangular shape), and all the states below EF are filled with electrons, and all the states above EF are empty (Phillips, 1987). As the temperature increases, some states below EF are empty due to thermal excitation, and the corresponding number of states above EF will be filled with excited electrons. When an energy level E is equal to EF, the f(E) value is half; therefore, the Fermi level can be defined as the energy level at which the probability of an occupied electron state is 1/2. In an n-type semiconductor, the number of electrons is more than holes of an intrinsic semiconductor, which leads to a higher probability of occupied energy states in the conduction band. This is the reason why the Fermi function shifts towards the conduction band. The larger is the electron concentration in a semiconductor, and the EF will be closer to EC. The arguments that are given for n-type semiconductors also hold for p-type semiconductors. The equilibrium concentration of electron no (#/cm23) in terms of integral function is the product of the density of states in the conduction band and the probability of electron occupying these states, which can be expressed as: ðN no 5 f ðEÞgc ðEÞdE ð5:4Þ Ec

Similarly, for equilibrium hole concentration po (#/cm23) can be written as: po 5

ð Ev 2N

½1 2 f ðEÞgc ðEÞdE

ð5:5Þ

The Fermi function becomes very small at higher conduction band energy levels. The conduction band and valence band effective densities of state, Nc and Nv, respectively, are given by  3=2  2πmn kT Nc 5 2 ð5:6Þ h2 

2πmp kT and Nv 5 2 h2

!3=2 ð5:7Þ

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From the above expression Eqs. (5.4)(5.7), the intrinsic electron and hole concentration ni and np can be expressed as: ni 5 Nc e2ðEc 2Ei Þ=kT and pi 5 Nv e2ðEi 2Ev Þ=kT

ð5:8Þ

Replacing no -ni and EF -Ev n0 5 ni eðEF 2Ei Þ=kT and p0 5 pi eðEi 2EF Þ=kT

ð5:9Þ

ni pi 5 n2i 5 OðNc Nv Þe2Eg =2kT and n0 p0 5 n2i

ð5:10Þ

The Fermi energy in intrinsic semiconductor (Ei 5 Ef) can be written as:

  E c 1 Ev kT Nv ln 1 Ei 5 2 2 Nc

ð5:11Þ

The intrinsic carrier concentration is minimal compared to the density of states and typical doping densities and is fixed for a given semiconductor. The semiconductor becomes extrinsic by the introduction of specific impurities with either acceptor atoms or donor atoms. The doping densities of acceptor and donor atoms are usually expressed in Na (atoms/cm3) and Nd (atoms/cm3), respectively. An acceptor atom accepts an electron from the valence band. It becomes a negatively charged ion to create a hole in the valence band whereas, a donor atom donates an electron to the conduction band and becomes a positive ion. But both positive and negative ions will be present in the semiconductor if it is doped with both donor and acceptor atoms (Reyes-Figueroa et al., 2015; Ashoori, 1996). Thus, the charge neutrality condition can be represented as follows: p0 1 Nd1 5 n0 1 Na2

ð5:12Þ

where Nd1 and Na2 are the number of ionized donors and acceptors. If a semiconductor is doped with n-type dopants, for an assumption of complete ionization of donor impurities at room temperature, it gives n0  Nd . Similar be the case for p-type dopants, which implies that p0  Na . The Fermi energy in a p-type semiconductor can be written as:   Nd EF 5 Ei 1 kTln ð5:13Þ ni And for n-type semiconductor material

  Na EF 5 Ei 2 kTln ni

ð5:14Þ

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Drift-motion due to the electric field

Semiconductor devices are designed by considering the motion of charge carriers (electrons and holes) under various forces like electric and magnetic fields, that is, charges in semiconductor devices are not in the steady-state due to thermal excitation. This motion due to thermal energy is genuinely random in direction. When an electric field is applied to a semiconductor, the holes experience the force in an electric field direction. In contrast, the electron experiences the force in the direction opposite to the electric field. So, an electron’s motion is the sum of motions due to its thermal energy and influence of electric force, which gives a net movement of electron in the opposite direction of the electric field. This movement of charge carrier under the influence of the electric field is called drift.

5.3.4.1 Drift velocity The charge carriers interact with the lattice atoms and the ionized impurities in the trajectory. During the motion, acceleration and deacceleration of charge carriers occur, which can be taken as an average for further analysis. The average velocity of the charge carriers under the electric field’s presence is called drift velocity (υd). The current flowing through a given semiconductor with a cross-section area A having a concentration of electrons n moving with drift velocity υd can be given as: In;drift 5 2 ð2 qÞnυd A 5 qnυd A

ð5:15Þ

And the drift current density Jn, drift can be defined as: Jn;drift 5 qnυd

ð5:16Þ

where q is the charge of an electron. The expression for drift current density Jp;drift for hole concentration can be represented as follows: Jp;drift 5 qpυd

ð5:17Þ

5.3.4.2 Mobility of carriers Higher drift velocity is related to faster acceleration due to the collision of charge carriers under the influence of electric fields. The drift velocities do not increase with the electric field above 105 V/cm and get saturated to a level of 107 cm/s. At a lower electric field (103 V/cm), the linear relation between drift velocities and electric field can be obtained. The linearity constant at the lower electric field is termed as mobility of charge carriers and is denoted by μ. Symbols can represent the mobility of the electrons and holes μn and μp , respectively. Mobility (μ 5 υd =EÞ can be defined as the drift velocity experienced by a charge carrier per unit applied electric field for a given semiconductor. The SI unit of charge carrier mobility is m2/V s. For

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instance, the mobility of electrons and holes for Si at 300K for acceptor and donor level of 1014 atoms/cm3 is 1350 and 480 cm2/V-s, respectively (Sampaio and Gonz´alez, 2017). The mobility of electrons for most semiconductors is typically more than that of a hole because of having a higher effective mass of holes than electrons.

5.3.4.3 The resistivity of charge carriers The drift current density is the sum of the electron and hole drift current densities when both electron and holes are present in a semiconductor. The expression of drift current density in terms of the electric field can be written as: Jn;drift 5 qnμn E and Jp;drift 5 qpμp E

ð5:18Þ

  1 Jdrift 5 qnμn 1 qpμp E 5 σE 5 E ρ

ð5:19Þ

where E is the electric field, σ and ρ is the conductivity and resistivity of a semiconductor, respectively. The resistivity of semiconductors is the ability to resist the current flow and is expressed in terms of Ω m or Ω cm.

5.3.5

Diffusion-due to a concentration gradient

Due to nonuniformity or gradient in the spatial distribution, particles’ free motion is referred to as diffusion. The particles move from a region of high carrier concentration to a lower concentration region to become uniform distribution. According to Fick’s first law of diffusion, the carrier flux Φ is proportional to the concentration gradient dn(x)/dx (Palmes and Lindenboom, 1979). It implies that the larger the concentration gradient, the larger will be the flux, that is, a more significant number of charge carriers can pass through a given cross-section area per unit time. Let n(x) be the concentration profile of electrons along the x-axis and Dn be the diffusion coefficient of an electron, then the flux of electron Φn can be written as: Φn 5 2 Dn

dnðxÞ dx

ð5:20Þ

The diffusion coefficient is a material parameter related to the action of charge carriers in the semiconductor and has a unit of cm2/s. The negative sign in the above expression [Eq. (5.20)] is due to the charge carrier’s movement in the negative carrier gradient direction. The expression of electron current density due to diffusion Jn, drift along the x-axis can be written as: Jn;drift 5 qDn

dnðxÞ dx

ð5:21Þ

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Similarly, the expression for current density due to the diffusion of holes drift can be written as: Jp;drift 5 2 qDp

dpðxÞ dx

ð5:22Þ

where Dp is the diffusion coefficient of holes and dp(x)/dx is the hole concentration gradient along the x-axis.

5.3.6

Absorption coefficient

Absorption coefficient (α) is a material parameter indicating how strongly a particular photon gets absorbed in a material. The value of the absorption coefficient depends on the wavelength of the light and varies from material to material. The unit of α is cm21. This parameter is crucial for understanding the performance of the solar cells. Its value is a determining parameter in deciding the thickness of the material required to absorb most of the sunlight. The inverse of the absorption coefficient is known as absorption length La, which can be expressed mathematically as: La 5

1 cm α

ð5:23Þ

For direct band gap semiconducting material, the typical value of the absorption coefficient is high ( . 104 cm21) for most solar radiation wavelengths having the photon energy greater than band gap energy (Mattheis et al., 2007). The high absorption coefficient’s value represents the higher probability of absorption of light by the direct band gap semiconductor because it involves only a photon but no phonon for absorption to occur. The total energy and the momentum of all particles involved in the absorption process must be conserved. The absorption coefficient for given photon energy, hυ, is proportional to the probability of the transition of an electron from energy level E1 to energy level E2 (P12), the density of states gv(E1) and gc(E2), and then summed over all possible transitions between states where Ev 2 E1 5 hυ, X ð5:24Þ αðhυÞα p12 gv ðE1 Þgc ðE2 Þ assuming that all the valence band states are full and all the conduction band states are empty. The absorption process results in electron-hole pair creation since a free-electron is excited to conduction band leaving behind a hole in the valence band. Since we have a parabolic band in transition, Ev 2 E 1 5

p2 2mp

ð5:25Þ

Introduction to photovoltaics and alternative materials Chapter | 5

E2 2 Ec 5

p2 2mn

Combining Eqs. (5.25) and (5.26) yields p2 1 1 1  hυ 2 Eg 5 mp 2 mn

145

ð5:26Þ !

and the absorption coefficient for direct transitions is 1=2 

αðhυÞ  A hυ2Eg

ð5:27Þ

ð5:28Þ

where A is a constant.

5.4

Physics of solar cell

When p-type semiconductor and n-type semiconductors sandwiched each other, the diffusion of electrons from the n-side to the p-side and diffusion of holes from the p-side to the n-side takes place due to the difference in carrier concentration. As holes diffuse from p-side to n-side, they leave behind a fixed negative charge in the form of ionized acceptor impurity atom. Acceptor impurities become negatively charged after donating a positively charged hole, as shown in Fig. 5.4A. Similarly, electrons diffuse from n-side to p-side, leaving behind a fixed positive charge in the form of ionized donor impurities. Such diffusion of charge carriers gives rise to diffusion current, which flows from p-type to ntype. In this manner, a layer of positive charges and a layer of negative charges appear during p-n junction formation. The charged region in a pn junction is known as the space charge region or depletion region. An electric field is directed from the n-side to the p-side due to the separation of positive and negative charge carriers in the space charge region. At equilibrium, the drift current due to the electric field is equal to the diffusion current due to the concentration gradient, so the net current in a device is zero. The region outside the space charge region is electrically neutral at both p and n-type of the semiconductors, that is, the electric field is zero called a quasi-neutral part. Semiconductors can absorb the photons and convert the partial energy of absorbed photons to transport charge carriers (electrons and holes). Sunlight is an incident from the top of the solar cell. A metallic grid as a front electric contact allows sunlight to fall on the semiconductor and thus be absorbed and converted into electrical energy (Latha et al., 2018). An antireflective layer between the grid lines reduces the optical loss from transmitted light to the semiconductors. A solar cell diode is designed when a p-type semiconductor and an n-type semiconductor are brought together to form a metallurgical

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FIGURE 5.4 Schematic diagram of (A) formation of a space charge region in a p-n junction, and (B) heterojunction p-n structure.

junction (Fig. 5.4B). The presence of the electric field in the space charge region is the indication of a voltage drop across it by the following relation: E ð xÞ 5 2

dVðxÞ dx

ð5:29Þ

In equilibrium condition, this voltage drop is known as built-in-potential or junction potential Vo .

5.4.1

Homojunction and heterojunction structure

A p-n homojunction is formed in the same semiconductor material with a different type of doping. The doping can be done by introducing various materials or induced by defects. In homojunction structure, holes and electrons act as mobile charge carriers in the p- and n-type material. The photon travels through the window layer in a solar cell structure and is absorbed in the p-side, where photogeneration carriers form (Melehy, 1978). The maximum absorption and maximum photogeneration in solar cells occur close to the surface of materials. During the illumination of light through the n-side material, most charge carriers are generated in the neutral region far from

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SCR, leading to the increasing recombination on the surface. An approach to solve this issue is utilizing a heterojunction structure that permits the use of a wide band gap window layer and hence reduces the surface recombination (Fig. 5.4B). The use of different materials in the junction forms two significant effects on device performance. First, the presence of discontinuities in the valence and conduction band along the device structure representing the possible formation of energetic barriers for the charge carrier transport is beneficial (Kronik et al., 1995). Second, the presence of an interface could act as recombination centers and thus reduce the device’s performance. Therefore, the decoupling of the physical and electronic junction by forming a buried link could mitigate the effects mentioned above. The inversion of the interface could implement another approach by developing an n1p interface in the absorber layer. By doing this, the minority carriers in the p-type absorber layer become majority carriers at the adjacent interface zone, thereby decreasing the recombination probability (Pantelides, 1978; Palmes and Lindenboom, 1979).

5.4.2

p-n junction under illumination

A solar cell is a p-n junction diode under the light illumination with a broad surface area. There are mainly three steps involved in the operation of solar cells. These are photogeneration of charge carriers by absorbing photon, separation, and collection of those charge carriers. A semiconductor material can absorb the photon with an energy Eλ greater than the band gap Eg . The absorbed photon excites an electron from the valence band to the conduction band of the absorber material. The generated charge carriers can either recombine or be separated and then collected. The number of absorbed photons depends on the thickness and absorption coefficient of the absorber material. The photon supplies sufficient energy to the junction to create several electron-hole pairs. The free electrons in the depletion region can move to the n-type side of the junction. Similarly, the holes in the depletion region can quickly move to the p-side of the junction. Once the newly created free electrons come to the n-side, they cannot further cross the junction due to the junction’s barrier potential. Likewise, newly created holes once come to the p-side cannot cross the junction because of the junction’s same barrier potential. A voltage is set up because of concentration accumulation on either side, that is, the concentration of electrons becomes more in the n-side of the junction and concentration of holes on the p-side of the junction. The current can flow through the circuit after connecting a load across the junction. The carriers that are generated in the space charge region will immediately be swift away due to the electric field. The probability of recombination of charge carriers is relatively less because of the electric field. The generated electron-hole pairs in the quasi-neutral region will wander

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FIGURE 5.5 Schematic diagram of a solar cell (A) I-V curve under dark (I) and illuminated (II) and (B) typical solar cell I-V curve showing its parameters.

randomly so that some of the generated minority charge carriers will come near to the space charge region edge, where they will experience an electric force and will be pulled out at the other side. In this manner, the movement of minority electrons from p-side to n-side and minority holes from n-side to the p-side takes place. In this way, there is a net increase in the positive charges at the p-side, and a net increase in the negative charges at the n-side causes a difference in potential across the p-n junction due to the incidence of light. This generation of voltage upon exposure to sunlight is known as the PV effect. It is important to note that not all the minority carriers that are generated in the quasi-neutral region can cross the junction. It means they travel an average distance Lp or Ln before they recombine with an opposite type of charge carrier. The symbol Lp and Ln are known as diffusion length of holes and electrons, respectively. In p-n junction diode, drift current and diffusion current due to electrons and holes are four different current components in equilibrium condition. When light is incident on solar cells, it leads to a large drift current due to minority electrons and holes, and the generated current is denoted by IL (light generated current) (Fig. 5.5A). The generated photovoltage reduces the junction’s potential energy barrier, and there is a diffusion current flowing in the opposite direction to the light-generated current. But the magnitude of IL is larger than the forward-biased diffusion current, and hence net current flows from the n-side to the p-side. Therefore, when the light incident on a solar cell, the current flows opposite to that of the generated voltage. So, there is a downward shift in the I-V curve, as shown in Fig. 5.5. In the fourth quadrant of the I-V curve, voltage is positive, and the current is negative, resulting in negative power. The negative power means that the power can be generated from the device rather than consuming.

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5.4.3

149

I-V equations of solar cell

The continuity equation assuming that in the space charge region, generation occurs, but the recombination is zero, which can be written as (ReyesFigueroa et al., 2015; Haddara et al., 2017; Phillips, 1987): @Δp @2 Δp Δp 5 0 5 Dp 2 1G @t @x2 τp

ð5:30Þ

where, G is the generation rate, Dp is the diffusion coefficient, Δp 5 p 2 p0 and Δn 5 n 2 n0 are excess carrier concentration, τ p and τ n represent the carrier lifetime of holes and electrons, respectively. The solution of Eq. (5.30) provides the excess minority carrier concentration along the x-axis under the illumination, which can be expressed as: Δp 5 Gτ p 1 Aex=Lp 1 Be2x=Lp

ð5:31Þ

Hence, the I-V equation for the solar cell under illumination can be written as:   

Dn Dp Itotal 5 qA np0 1 pn0 eqV=kT 2 1 2 qAG Ln 1 Lp 1 W ð5:32Þ Ln Lp where pn0; represents the electron concentration at p-side in equilibrium, np0 represents the hole concentration at the n-side in equilibrium; W is the width of the depletion region. Eq. (5.32) can be simplified as:   ð5:33Þ Itotal 5 I0 eqV=kT 2 1 2 IL

where Itotal 5 qAG Ln 1 Lp 1 W represent the light generated current generated within the volume of cross-sectional area A.

5.4.3.1 Short circuit current Isc It is the maximum current provided by a solar cell when its terminals at the p-side and n-side are shorted with other, that is, V 5 0. Putting V 5 0 in Eq. (5.33) gives that short circuit current is the light generated current, that is, Isc 5 2 IL . Short circuit current is generally expressed in current per unit area (current density), and its unit commonly is defined by mA/cm2. 5.4.3.2 Open circuit voltage Voc The maximum voltage generated across the terminals of a solar cell when they are kept open, that is, I 5 0. After putting this value in Eq. (5.33), the expression becomes dependent on the light generated current and reverse saturation current and is given by:

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Voc 5

  kT IL ln 11 q I0

ð5:34Þ

For higher Voc , the value of I0 should be lower. The lowest value of I0 is obtained when the recombination rate is equal to the thermal recombination rate. It is generally given in terms of mV or V.

5.4.3.3 Fill factor The fill factor (FF) is the I-V curve’s squareness and mainly related to the resistive losses in a solar cell. It is also the largest rectangle, which will fit in the I-V curve. The value of FF is 100% to the ideal case. Mathematically, it is the maximum power ratio that can be extracted from the solar cell to the ideal power. Thus, FF 5

Pm Vm I m 5 P0 Voc Isc

ð5:35Þ

As short-circuit current and open-circuit voltage are the maximum current and voltage respectively from the solar cells. However, the power from the solar cell is zero at both operating points. As FF measures the “squareness,” a solar cell with a higher voltage has a larger possible FF because the I-V curve considers less area in the rounded portion. The maximum theoretical FF from a solar cell can be determined by differentiating the power from a solar cell concerning voltage and finding where this is equal to zero.

which gives,

dðIVÞ 50 dV

ð5:36Þ

  qV mp nKT ln 11 Vmp 5 Voc 2 q nKT

ð5:37Þ

After utilizing the Lambert function, the FF can be expressed empirically as FF 5

ν oc 2 lnðν oc 1 0:72Þ ν oc 1 1

ð5:38Þ

oc where ν oc 5 qV nKT is defined as normalized Voc. This expression also demonstrates the importance of the ideality factor (n-factor) of a solar cell. The n-factor value measures the junction quality and recombination type in a solar cell and will be 1 for the simple recombination mechanism. A high n-factor degrades not only the FF but also it lowers the Voc.

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5.4.3.4 Efficiency The efficiency (ηÞ of a solar cell is the ratio of the power output to power input. The output power is the maximum power Pm of a solar cell, and input power is the power of the solar radiation Prad . The value of Prad is considered 100 mW/cm2 or 1000 W/m2 according to the international standard for the characterization of solar cells (air mass AM1.5). η5

Pm Vm Im Voc Isc FF 5 5 Prad Prad Prad

ð5:39Þ

The efficiency of a solar cell is expressed in terms of percentage. In 1960, Shockley and Queisser pointed out that the upper limit reachable with a single-junction solar cell is 33.7% (Shockley and Queisser, 1961). This value is relatively low if we take into consideration the limiting efficiency of the Carnot engine. The Shockley-Queisser limit provides a theoretical upper limit for single-junction solar cells, but practical cells are often much lower in conversion efficiency due to imperfections in semiconducting materials and parasitic resistance (Polman et al., 2016). Another PV characteristic is quantum efficiency (QE), which is the ratio of the number of carriers collected by the solar cells to the number of incident photons. It is the function of either wavelength or energy. The QE curve is ideally square, but practically it cannot be square because of recombination effects.

5.5

Categories of the photovoltaic market

The PV market is generally divided into different technologies depending mainly on the type of absorber material used in the fabrication of solar cells

FIGURE 5.6 Schematic illustration of different solar cell technologies.

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(Fig. 5.6). It is possible to observe that solar cells could be classified into three generations: first generation, second generation, and third generation. The first generation belongs to the bulk crystalline silicon-based solar cell (monocrystalline and multicrystalline), which exhibited the most mature technology with conversion efficiencies of .26% (cell) and a share B90% in the global annual PV production. The second generation consists of thin film solar cell technology, which can reduce the cost of final modules by developing new growth and deposition techniques with a reduction in thickness. The thin film solar cell technology involves amorphous Si, CdTe, copper zinc tin sulfide (CZTS), CIGSe, etc. The third generation includes all the new approaches to reduce the manufacturing costs by offering high efficiency. Multijunction cells, hot carriers use, organic solar cells, and dyesensitized cells are examples of third generation technology solar cells.

5.6

Commercialization of Si solar cells

As of today, more than 90% of the world’s solar PV modules are produced in Si wafers (Regmi et al., 2020). Maturity, performance, reliability, and abundance are the benefits of Si solar cells. However, Si solar cell technology is not cost-reliable because of expensive material processing. Therefore, it is obligatory to understand the production way of Si for various applications (Fig. 5.7). The electronic grade of Si is traditionally produced for the microelectronics industry. The electronic quality Si manufacturers could supply Si to the solar cell industry as per the requirement and can be obtained by the Czochralski technique. Si is used in solar cells in different forms like thin film amorphous Si to monocrystalline/multicrystalline Si wafers (Shah

FIGURE 5.7 Various paths for purification of Si.

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et al., 2004). The production of Si wafer can be divided into different paths. The expensive path-I for making mono-crystalline wafer-based Si solar cells is decreasing worldwide. The path-II produces polycrystalline Si wafers using the block casting process, which is cheaper than path-I. More than 50% of the world’s solar cells are made from this polycrystalline Si wafer. The path-III is an alternative to expect the lower production cost of Si wafer by avoiding Si ingots’ sawing. The solar cell from Si sheets has relatively lower efficiency due to high-level defect density. The path-IV is the deposition of Si thin film, where Si wafers’ production is completely avoided to bring down a solar cell’s fabrication cost. The solar cell from amorphous Si has been the most successful thin film technology. The path-V differs from all other paths because the purification of metallurgical Si grade is performed differently. Tremendous efforts are being applied to lower the production cost of Si solar cells. After the oxygen, silicon is the second most commonly existing element in the earth’s crust. As of today, more than 90% of the world’s solar PV modules are produced in Si wafers. Crystalline Si has a fundamental indirect band gap (1.1 eV) and a direct band gap above 3 eV at 300K. These features determine the variation of optical properties of Si with wavelength, including the absorption coefficient for the generation of charge carriers for near band gap photons. On one side at short wavelengths in the solar spectrum, one photon’s generation of two electron-hole pairs seems possible, though quantitatively, it sees a small effect. On the other side of high wavelength, band to band generation occurs. At high carrier densities, the band structure is altered, leading to an increase in the effective intrinsic concentration, which may degrade the PV device’s efficiency.

5.7

Status of alternative photovoltaics materials

Since the inception of development, alternative PV materials have been gaining acute concern. However, cost reliability and competitiveness are required to attain the same level in the commercial field with traditional Si technology. Successful fabrication of efficient solar cells using alternative absorber materials will significantly enrich the PV industry and reduce the market gap with dominated Si solar panels. Besides Si technology, few other thin film technologies (see Section 5.8 in detail) such as CdTe, CIGSe, III-V materials have been receiving an exciting PV market position. Recently, emerging solar cell technologies (see Section 5.14 in detail) becomes a center of attraction to the PV specialist to make it more reliable and efficient cost-effective. As a highlight of some development in the PV field, some of the alternative absorber materials for Si are listed in Table 5.2 (Green et al., 2019). Besides the scarcity of In and Te, both CIGSe and CdTe contain toxic material (Se in CIGSe and Cd in CdTe), Also, emerging solar cell technology have a concern of stability and large-area production. However, some

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TABLE 5.2 Efficiencies of different solar cell technologies, measured under global AM1.5 spectra (1000 W/m2) at 25 C (Green et al., 2019). Categories

Efficiency (%)

Voc (V)

Jsc (mA/ cm2)

FF

Description

GaAs thin film (cell)

29.1 6 0.6

1.127

29.78

86.7

Alta Devices

GaAs multicrystalline

18.4 6 0.5

0.994

23.20

79.7

RTI

InP crystalline cell

24.2 6 0.5

0.939

31.15

82.6

NREL

CIGSe (cell) Cdfree

23.35 6 0.5

0.734

39.58

80.4

Solar Frontier

CIGSe (module)

19.2 6 0.5

48.0

0.456

73.7

Solar Frontier

CdTe (cell)

21.0 6 0.4

0.875

30.25

79.4

First Solar

CdTe (module)

19.0 6 0.9

227.8

2.560

76.6

First Solar

CZTSSe (cell)

11.3 6 0.3

0.533

33.57

63.0

DGIST

CZTS (cell)

10.0 6 0.2

0.708

21.7

65.1

UNSW

Perovskite (cell)

21.6 6 0.6

1.193

21.64

83.6

ANU

Perovskite (minimodule)

17.5 6 0.6

1.070

20.66

78.1

Microquanta

Dye (cell)

11.9 6 0.4

0.744

22.47

71.2

Sharp

Dye (minimodule)

10.7 6 0.4

0.754

20.19

69.9

Sharp

Organic (cell)

13.4 6 0.2

0.842

23.28

68.6

Uni Potsdam

Organic (minimodule)

12.6 6 0.2

0.831

21.32

71.1

ZAE Bayern

other promising alternating absorber materials are listed in Table 5.3 (Rabaia et al., 2021).

5.8

Thin film technology

The prime objective of thin film technologies is to reduce PV devices’ cost significantly lower than the existing wafer-based solar modules. According to Chopra et al., the definition of the thin film is “random nucleation and growth process of individually condensing/reacting atomic/ionic/molecular species on a substrate” (Chopra et al., 2004). The characteristic properties of the thin film strongly depend upon the deposition conditions. These

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TABLE 5.3 List of device structure and band gap considered alternative absorber materials (Rabaia et al., 2021). Material

Device structure

Band gap (eV)

Cuprous oxide (Cu2O)

Heterojunction/homojunction/Schottky

2.17

Cupric oxide (CuO)

Nanostructured heterojunction

1.40

Iron pyrite (FeS2)

Heterojunction/nanostructured heterojunction

1.30

Tin sulfide (SnS)

Heterojunction/quantum dot sensitized solar cell

1.30

Di-bismuth trisulfide (Bi2S3)

Heterojunction/Schottky/ photoelectrochemical cell

1.25

Tungsten disulfideWS2

Photoelectrochemical cell

1.29

Molybdenum disulfide (MoS2)

Photoelectrochemical cell/Schottky

1.29

Tungsten diselenide (WSe2)

Heterojunction

1.14

Molybdenum diselenide (MoSe2)

Photoelectrochemical/Schottky

1.14

Lead sulfide (PbS)

Planar heterojunction/Schottky/ nanostructured heterojunction

0.41

Lead selenide (PbSe)

Heterojunction/Schottky

0.38

Zinc phosphide (Zn3P2)

Schottky

1.38

B-Iron silicide (β-FeSi2)

Heterojunction

0.87

Aluminum antimonide (AlSb)

Heterojunction

1.62

properties may also depend upon the thickness of the film. The film is thin if the thickness varies from a few nanometers to several tens of micrometers. Si wafer-based solar cell technologies are expensive because of the high purification cost of Si used in the cells, which accounts for almost 50% of the price at the module. Si wafer-based modules’ current value is in the range of 2.43.8 $/Wp. In comparison, thin film solar cell technologies can lower the cost to a level of 0.5 $/Wp, which can be assessed from the fact that the current value of thin film solar modules is in the range of 0.6 $/Wp (Solar Technology Cost Analysis et al., n.d). This is possible because of the materials processing technologies in terms of fabrication compared to the wafer-based technologies. The following are the generic advantages of thin

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film technologies compared to market dominated wafer-based solar technologies. G

G

G

G

G

G

G

Minimum utilization of material: The amount of material used in the fabrication of thin film is about 25 μm, significantly lower than the waferbased technology (200300 μm). The lower consumption of the material leads to lower production costs, hence the PV module’s low cost. Shorter energy payback time: The energy payback time (EPBT) of a solar cell is the time required to generate as much energy as is consumed during production and lifetime operation of the solar cell. The energy payback time has continuously been decreasing since the inception of PV systems in the energy market due to the improving production technologies. The energy payback time of thin film technology is approximately one year, which is better than Si wafer-based solar cells (34 years) (Grant and Hicks, 2020). Tunable material properties: The characteristics properties like the thin film’s optical and electrical properties depend on the films’ structure and chemical composition. The deposition parameters can control these properties at the time of deposition for most of the thin film technologies. The property of flexibility offers the thin film in realizing the multijunction solar cells for harnessing a large portion of incident solar radiation. This property is not possible with Si wafer-based solar cells. Monolithic integration: It is a technique that allows both electronic and optical devices to be connected in the same semiconductor material in a single growth process. The process of joining solar cells in series during the fabrication of modules is referred to as monolithic integration. Most of the thin film technologies offer monolithic integration possibilities, which is a more challenging task in Si wafer-based solar cells. Low-temperature processes: The utilization of process temperature during thin film growth can be done below 600 C, whereas about 800 C1000 C is used for wafer-based Si solar cells. The overall thermal budget of thin film technologies is relatively low in comparison to Si solar cells. Moreover, low processing temperature offers different substrates like glass, polymer, and stainless-steel foils. Broad area modules: The current PV modules based on Si solar cells are limited to around 225 cm2. But the area of the thin film module is enlarged several times in comparison to Si solar module. Large area fabrication expertise also gives the cost advantages within the limit of fabrication tools. Fabrication of transparent modules: Thin film technologies allow the deposition of material on a glass substrate with controlled thickness. The thickness of the film less than the optimum required thickness to absorb the solar radiation will be transparent. Such transparent modules can be utilized in rooftop building-integrated PV modules.

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157

Material selection in thin film technology

Different Semiconducting materials are discovered and researched extensively for the generation of PV devices. These materials are used to form the junction, which facilitates the separation of charge carriers. The junction can be created between two similar materials as in wafer-based Si solar cell, called homojunction. The junction formed between two dissimilar materials is called heterojunction. Thin film technologies mostly use inorganic materials, except emerging thin film technology, where the polymer is also used. There are only a few materials which can provide reasonable efficiency for the conversion of light into electricity with the following requirements: G G

G G G

should have a direct band gap between 1 and 1.5 eV. The absorption coefficient of the material should be very high in the order of 104106 cm21. Low recombination rate of generated charge carriers High diffusion length of generated charge carriers Widely available, reproducible, and nontoxic materials

Si satisfies the above requirements, except that it is an indirect band gap material. Si, with a thickness of around 50 μm, has been deposited nowadays for the fabrication of solar cells. Other semiconducting materials CdTe and CISe/CIGSSe of interest, belong to the II-VI group of periodic. CIGSe thin film solar cell has attained an acute concern due to competitive advantages over c-Si solar cells among the various thin film solar cells. The dramatic increment of efficiency from its inception to now made CIGSe a strong candidate than the c-Si solar cell for solving the problem of the energy crisis. The maximum reported efficiency of CIGSe ever is 23.35% by solar Frontier (Nakamura et al., 2019), which is comparable to a maximum reported c-Si solar cell (27.6%) in a lab-scale (Green et al., 2019). The utilization of material for typical CIGSe absorber layer is about 13 μm, which is around 100 times thinner than the thickness required for c-Si, due to the high absorption coefficient ( . 104 cm21). CIGSe is a ternary p-type semiconducting material belonging to the I-III-VI2 family. That crystallizes in a tetragonal chalcopyrite structure. It has a tunable direct bandgap ranging from 1.04 (CISe) to 1.68 eV (CGSe) in the visible region (Peng et al., 2018). The high thermal stability and durability have led to increased interest among researchers for CIGSe thin film solar cell employment. Among the II-IV compound semiconductors, CdTe is one of the most promising material for the fabrication of thin film solar cells owing to its ideal direct band gap of 1.45 eV at room temperature, high absorption coefficient ( . 104 cm21) in the visible range of the solar spectrum (Metzger et al., ˚ and has a zinc blende cubic 2019). The lattice constant of CdTe is 6.481 A structure. These materials are impressive because their internal surface and external surface are well passivated, resulting in low-charge carrier

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recombination at the surfaces and higher solar cell efficiencies. Compound semiconductor materials from III-V group like InP, GaAs, InGaAs have a direct band gap. They are suitable for solar cell applications even though the deposition techniques are expensive. GaAs has a high conversion efficiency compared to mono-c-Si and poly-c-Si cells. But it is not commercially matured due to high fabrication costs. It has more applications in the space shuttle and concentrated PV modules because of having a high-temperature coefficient. The efficiency of solar cell absorbers doping with phosphorous (P), indium (I), aluminum (Al), or antimony (Sb) increases due to the formation of a multijunction structure.

5.10 Thin film deposition techniques Nowadays, most of the technologies are adopted to achieve a low dimension of the material in thin films or coatings, ranging from nanometer to several micrometers. The thin films’ properties and structure depend on the deposition technique, which can be divided into two broad categories: physical and chemical depositions.

5.10.1 Physical deposition The atoms’ physical transportation obtains the thin film’s deposit from the source to the substrate in gas phase. Several deposition techniques have been developed in physical vapor deposition, which is as follows:

5.10.1.1 Evaporation techniques 5.10.1.1.1 Vacuum thermal evaporation This technique can be used to deposition many PV materials, including metals, dielectrics, and amorphous semiconductors under a high vacuum. The source materials to be deposited should be solid, and the energy is supplied via heat (thermal evaporation). The solid phase transition to the gas phase, transport of vapor from source to the substrate, and vapor condensation on the substrate are three main steps involved in the thermal evaporation process. The heat for melting the source pellets is provided by resistive heating. The metal molecules should travel in a straight line from source to substrate without colliding with any other gas molecules, called a mean free path. A high vacuum (1026 Torr) is generally required to be evaporated the source material. CIGSe absorber materials are obtained through this technique (Feurer et al., 2017). 5.10.1.1.2

Electron beam evaporation

The high quality and pure thin films can be achieved by electron beam evaporation. The electron’s intensive beam is generated from the filament and

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maneuvered through electric and magnetic fields to hit the target and vaporize under a vacuum environment. This technique can deposit various types of materials such as amorphous and crystalline solids, oxides, metals. For the front metal contact in the PV device, this is most useful (Feurer et al., 2017). 5.10.1.1.3

Laser beam evaporation/pulsed laser deposition

Different kinds of laser sources are applied to ablate the material for depositing the thin films under a vacuum atmosphere in this deposition technique. The most common laser sources are neodymium-doped yttrium aluminum garnet (Nd-YAG), krypton fluoride (KrF), Xenon monochloride (XeCl). The plume containing neutral and ground-state atoms with ionized species will be produced when a laser strikes the target. The thin films’ quality depends on various parameters such as laser energy, wavelength, gas pressure, substrate temperature, pulse duration, and the substrate to target distance. The ablation process is monitored using laser-induced fluorescence, laser ablation molecular isotopic spectroscopy, and optical emission spectroscopy. Three different growth modes viz., Volmer-Weber, Frank-van der Merwe, and StranskiKrastanov are followed by this deposition process (Grabow and Gilmer, 1988). This technique has been used to synthesize nanostructure of hybrid metal-organics, coordinative and complex compounds, biomaterials, and polymers. 5.10.1.1.4

Arc evaporation

A discharge of electricity between two electrodes is used for thin film deposition in arc evaporation. This process initiates with the hitting of a high current on the target’s surface that gives rise to a small energetic emitting area referred to as a cathode spot. The cathode spot is active for a short time, where the temperature reached up to 15,000 C, which leads to a highvelocity jet of vaporized materials. When a reactive gas is introduced, three different phenomena occur dissociation, ionization, and excitation, leading to thin films. A magnetic field shifts the cathode spot to guarantee that it does not stay too long in one place. 5.10.1.1.5

Molecular beam epitaxy

It is an atomic layer crystal growth technique based on the reaction of molecular or atomic beams with a heated crystalline substrate under an ultra-high vacuum (typically below 10210 Torr) environment. This technique is used to produce ultra-thin films as high-quality epitaxial layers with very sharp interfaces and reasonable control of thickness, doping, and composition.

5.10.1.2 Sputtering techniques This technique (commonly used radio frequency and direct current sputtering) uses plasma for thin film deposition. Plasma is a mixture of ionized gas,

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electrons, and radicals defined as the ionized gas showing quasi-neutral behavior. An inert gas, generally Ar, is used for creating the plasma. Due to the high voltage, electrons in the chamber get accelerated. The high-speed electrons can again collide with the Ar atom and produce Ar1 ions. It travels towards the target and gives momentum to the target atoms because it gets dislodged (Regmi et al., 2018). It also results in the generation of secondary electrons from the target. These secondary electrons are functional in maintaining the plasma. The dislodged atoms from the target get condensed on the substrate. High DC voltage in the range of 15 kV is applied between the substrate and the target. Simultaneously, at radio frequency (particularly 13.6 MHz) sputtering, an electron in the plasma oscillates with the frequency, but heavy ions do not respond to the signals. The use of a magnetic field increases the ionization of the gas. This technique is widely used for the deposition of back contact, absorbers, and window layers,

5.10.2 Chemical deposition Although highly expensive physical depositions have been utilized for thin film production with excellent quality and functionalize properties, the need to manufacture cost-effective thin films is necessary to revolutionize the efficient device fabrication. The film’s deposition results from the chemical reaction at the surface of the substrate in either gas or liquid phase. Several techniques have been developed within these categories, which are as follows:

5.10.2.1 Sol-gel technique This technique operates under low-temperature processing and gives homogenous thin films for multicomponent materials in a colloidal suspension state. It is broadly used for the synthesis of oxide PV materials. Inorganic precursors’ preparation via inorganic salts in aqueous solution and the metal alkoxide precursors via metal alkoxides in nonaqueous solvents are two routes of transition metal oxides. Once the precursor solution is formed by the hydrolysis and condensation process in the sol-gel technique, there are two processes for producing thin films: dip-coating and spin-coating techniques. Dip-coating is used to prepare transparent layers of oxides with a high degree of surface quality. In the spin-coating approach, the solution is dripped onto a spinning substrate and spreads orderly. 5.10.2.2 Chemical bath deposition One of the oldest methods to deposit thin films is chemical bath deposition, also known as the solution growth technique. In this method, metal ions’ precursor solution must be added to complexing agents such as ammonia solution, triethanolamine, citric acid, ethylene-diamine-tetra acetic acid, and so

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on. Anions such as thiourea, thioacetamide, thiosulfate, sodium sulfide solutions, and selenourea can be added. This method is mostly used to prepare chalcogenide thin films and metal oxide films. Substrates are immersed in a vertical, horizontal, or specific position inside the solution until the desired thickness is obtained. The pH of the solution, bath temperature, and deposition duration controls parameters to get expected thin films. For CIGSe based solar cells, the buffer layer is deposited by adopting this technique.

5.10.2.3 Spray pyrolysis technique The deposition of thin films using spray pyrolysis involves spraying a metal solution onto a heated substrate. The droplet formation, substrate temperature, the droplets’ momentum, and the precursor solution’s concentration are deposition parameters in this technique. The steps needed to fabricate the thin films include dissolution of inorganic precursors, forming a mist from the liquid source, utilizing a carrier gas to carry the fog into a preheated chamber, and vaporizing the droplets, and selective reduction of the metal oxides to produce metallic materials. Depending on the atomizer used for the atomization process, spray pyrolysis can be defined into electrostatic, ultrasonic, and air blast (pneumatic) spray pyrolysis. The deposition of metal oxides and metallic precursor layer can be obtained by spray pyrolysis technique. Usually, metal chlorides or nitrides (CuCl2, InCl3, GaCl3) and thiourea or N, N-dimethyl selenourea (sulfur and selenium source) are used for the deposition of CIGSe by spray pyrolysis because these precursor materials can easily be dissolved in water and alcohol that helps to deposit film effectively (Babu et al., 2015). 5.10.2.4 Chemical vapor deposition Chemical vapor deposition (CVD) technique consists of chemically reacting volatile compounds with other gases to produce a nonvolatile solid that sticks at the substrate’s atomic level. The vapor supersaturation affects the nucleation rate of thin films, and substrate temperature influences the growth rate. Low vapor supersaturation and high substrate temperature lead to a single crystal film’s growth on the substrate. Simultaneously, high vapor supersaturation and low substrate temperature promote the development of polycrystalline nanocrystalline films. The most established processes on CVD are discussed briefly as: 5.10.2.4.1 Low pressure and atmospheric pressure chemical vapor deposition The deposit of dielectric layer occurs due to the reactant gases’ chemical reaction at the substrate’s surface. The energy for the reaction is supplied by thermal heating, obtained by lamp heating, resistive heating, or inductive

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heating consisting of a quartz tube. The reactant gases flow over the surface of the substrate with high velocity. 5.10.2.4.2 Plasma enhanced chemical vapor deposition Plasma is used to provide the energy to reacting gases for thin film deposition in two parallel plates acting as electrodes, where RF signal is applied for the formation of glow discharge. This technique is a low-temperature process for the deposition of various amorphous and crystalline semiconductors. 5.10.2.4.3

Hot wire chemical vapor deposition

This technique involves thermal decomposition of precursor gases at the filament surface to form radicals. The energy for the dissociation of sources gases maintained by the catalytic reaction is supplied by the hot filament (tungsten or tantalum) heated to more than 1600 C keeping a substrate at the close vicinity of the filament. Thus, formed radicals form other species and get absorbed on the heated substrate (200 C800 C). The lower processing temperature promises the HWCVD a useful technique for the epitaxial thin high-quality thin films. This technique favors the deposition of chalcogenide materials due to low processing temperature and pressure. The thin film from this technique on whatever the substrate is plasma free will prevent damaging the film by energetic ions. 5.10.2.4.4 Ion assisted deposition This technique facilitates low-temperature epitaxy with a high growth rate. The surface mobility of the atoms to be deposited should be increased to have epitaxial layer growth and is achieved with ionized ions. The electron gun is used to evaporate the material, and a bias voltage of 220 to 2150 V of two electrodes is used. In this deposition technique, epitaxial layers are formed at the temperature range between 500 C and 700 C.

5.11 Copper indium gallium selenide-based solar cell Among the different thin film solar cell materials, the highest efficiency devices have been obtained employing CIGSe absorber layers. Solar Frontier has recently acquired the record efficiency for a CIGSe-based solar cell with 23.35% (for a cell area of 1 cm2) (Regmi et al., 2020; Nakamura et al., 2019). Before this, for a cell area of 0.5 cm2, the efficiency of 22.6% was recorded by ZSW (Jackson et al., 2016). State of the art CIGSe solar cell in a substrate and superstrate configuration is depicted in Fig. 5.8, consisting of a multilayered structure of thin film layers deposited onto the soda-lime glass substrate. In substrate configuration, molybdenum (B0.5 μm) as electrical back contact is generally formed on SLG via DC sputtering. For the p-n

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FIGURE 5.8 Schematic illustration of thin film solar cells in (A) substrate, and (B) superstrate configuration.

junction formation, p-type CIGSe absorber layer (23 μm) and n-type CdS buffer layer (5060 nm) are deposited by coevaporation/sputtering and chemical bath deposition techniques, respectively (Ashok et al., 2020). The window layer is formed by a bilayer of i-ZnO and AZO (Al-doped ZnO) with the approximate thickness (50100 nm) and (150200 nm), respectively as the barrier and conductive layer, deposited by RF sputtering. Ultimately, Nickel-Aluminum (Ni-Al) grids as top metal contacts are deposited onto transparent conducting oxide (TCO) to enhance the charge collection. An antireflective (AR) coating onto the full solar cell structure is then evaporated (MgF2, 105115 nm) to avoid optical reflection losses.

5.11.1 Alkali metal postdeposition treatment on copper indium gallium selenide based solar cells Many researchers have shown that the crystalline structure and device performance of CIGSe based absorber grown on SLG substrate is better than that developed on borosilicate glass. The main reason for this advantageous effect is the incorporation of diffused sodium (Na), which enhances the Voc and FF, and hence increases cell efficiency (Sun et al., 2017). Na’s inclusion either diffused from SLG or introduced by other method leads to increased conductivity when the concentration of Na reached 1015 cm23. However, it is worth to reveal that the device performance worsens as the Na concentration becomes one at%. Na’s optimal concentration in a high-efficient solar cell ranges from 0.05 to 0.5 at.%. The incorporation of alkali elements passivates the defects at the p-type CIGSe based absorber surface or grain boundaries. It decreases the compensating donor concentration instead of a significant change in acceptor concentration. The p-type carrier concentration increases; as a result, the Fermi level (EF) is lowered. The differential EF value will help to produce higher Voc and FF.

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There are different ways of alkali metal incorporation, namely, predeposition, coevaporation, and postdeposition. The diffusion of Na from SLG substrate during CIGSe absorber growth, without intentional incorporation, is the right way of Na doping. Sodium containing molybdenum (MoNa) and a thin layer of NaF on Mo back contact is another way of Na incorporation to ensure sufficient supply throughout the film. In the three-stage coevaporation process, the absorber grown with NaF evaporation at the third stage shows the best performance because Na incorporation during the first and second stages degrades the crystallinity of CIGSe absorber. The enhancement in crystallinity and passivation at surface and grain boundaries is crucial for higher free carrier concentration (Li et al., 2017). The incorporation of alkali metal (Na, K, Rb, Cs) into the CIGSe under Se atmosphere after absorber growth is called PDT. Several experiments have reported that the sheet resistance decreases, and carrier concentration improves as the substrate temperature increases. In literature, a substrate temperature between 350 C and 450 C is chosen for alkali metal PDT. Moreover, heavier alkali elements (Rb, Cs) are more effective than K in upgrading the cell efficiency. The main reason is the reduced diode quality factor. The summary of Na’s supplementary effect on CIGSe absorber can be presented to the following points. G

G

G

G

G G

G

The presence of Na reduces the diffusion of metals in the growing film, especially preventing the Ga gradient. The Na doping leads to accelerated surface oxidation in the form of Se oxides. Na promotes the growth of MoSe2 at the back contact. This is an intermediate layer act as Ohmic contact. Na diffusion appears to be maximal for stoichiometric films via grain boundaries. Na distributes in the film with smaller grain size. The diffusion of Na from the absorber seems to be promoted by exposure to air. Na facilitates the widening of chalcopyrite phase formation.

5.12 Cadmium telluride solar cells For state of the art CdTe solar cell in superstrate configuration, glass is often used as the substrate with an alkali diffusion barrier (Carron et al., 2019). A several hundred nanometers of TCO and a buffer layer (generally tens of nanometers thick) such as intrinsic SnO2, MgZnO, or CdS is deposited on glass. These layers are n-type, transparent, and nanocrystalline to allow most of the incident solar radiation. A p-type photoconversion with several micrometers thick layer (CdTe or CdSeTe) can be deposited by different deposition technique (more common closed spaced sublimation). Before depositing

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back contact, the photoconversion/absorber layer is usually deposited without intentional doping impurities. It results in a carrier lifetime and hole density in the range of tens of picoseconds and 1013 cm23, respectively. Moreover, CdCl2 heat treatment and back surface etching can be done to recrystallize and densify the grains. It leads to the interdiffusion of CdS and CdTe at the interface to release the interface stress.

5.13 Multijunction solar cells In tandem (double junction) solar cells (Fig. 5.9a), the high band gap material (top cell) collects the high energy photons. In contrast, low energy photons are collected by low band gap materials (Uhl et al., 2018). As the number of junctions in a cell increases, the complexity of solar cell fabrication increases. A critical issue in a multijunction solar cell structure is the pn junction that separates the individual cells. The electrons are generated in the top cell flow to the p-n junction, where they must recombine with holes from the corresponding bottom cells. As a result, recombination takes place via tunneling. The space charge accumulation phenomenon will occur when the recombination rate is not balanced with the supply charge carriers. This results in a negative impact on the electric field in the adjacent cell with the highest generation rate. The multijunction solar cell structure demands the current generated in each cell should be equal. The connection between cells should have low optical and electrical losses, controlled by controlling the thickness and band gap of absorber material. A current mismatch can be quickly revealed by measuring the spectral response. The quantum efficiency is flat over a wide range of wavelengths for the current matched cells. Moreover, the losses can be minimized by making a tunnel junction between the solar cells. The tunnel junction can be achieved by a p-type layer and ntype layer, which acts as a p-n junction diode and operates in the reverse direction when the individual cell functions in the forward direction. The recombination of electrons and holes at the tunnel junction keeps the current flowing, considering the low ohmic losses due to heavily doped p and n layers.

5.14 Emerging solar cell technologies The emerging solar cell technologies yet to show their potential in electrical power generation beyond the conventional wafer-based Si solar cell and thin film solar cell technologies (Stranks and Snaith, 2015; Park, 2015; Lee et al., 2018; Ranabhat et al., 2016). These technologies are different in terms of materials used and the operation principle of the PV device. In this category, technologies like dye-sensitized solar cells, organic solar cells, hybrid organic/inorganic solar cells, etc. are included (Celik et al., 2020).

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5.14.1 Organic solar cells The solar cell based on an organic semiconductor can provide a low-cost alternative for solar power conversion. The thickness of the active layer in organic solar cells is around 100 nm, approximately a thousand times thinner than c-Si solar cells, and 10 times thinner than thin film solar cells. There is a huge potential for low cost-effective organic solar cell fabrication due to minimal material usage and more straightforward cell processing at low temperature (An et al., 2016). The improvement in efficiency is required because the champion efficiencies of organic solar cells showed so far are about two to three times lower than the wafer-based Si solar cells. The selection of organic materials used in solar cell applications has very different properties than inorganic solar cells. Firstly, the electron and hole mobilities are much lower as compared to Si. Due to this reason, the thickness of the active layer should be minimal for the charge generation and separation effectively. The absorption coefficient of organic semiconductors is relatively high ( . 105 cm21) (Yang and Ma, 2019). Secondly, an electron’s excitation to the lowest unoccupied molecular orbital (LUMO) occurs when a photon is absorbed in an organic semiconductor. The excited carrier leaves behind a hole in the highest occupied molecular orbital (HUMO). Columbic forces bound these holes and electrons, so they are not free charge-carriers called an exciton. The binding energy of this exciton is higher than that of Si. The thermal energy at room temperature is not enough to make the charge carriers free due to high exciton binding energy. Thirdly, organic and inorganic semiconductors have different bonding systems. The semiconducting nature of organic semiconductors arises from the p-electron bonds which exist when molecular are fully conjugated. The material is said to be conjugated if it has an alternate single and double bond. It is important to note that efficient charge separation of exciton and free carrier formation can occur if the junction is made of two different organic materials, consisting of a donor type and an acceptor type (Fig. 5.9B). The possibility of charge separation occurs if the potential difference between the donor’s ionization potential and electron affinity is more massive than exciton binding energy. Organic PV materials are characterized by low specific weight, ruggedness, and mechanically flexible solar cell fabrication. Durability is a concern in achieving more reliable organic PV devices, although its PV conversion efficiency is yet to be accomplished comparable with thin film technologies.

5.14.2 Dye-sensitized solar cells The dye-sensitized solar cell is somehow considered thin film solar cells that are not yet commercialized but are on the verge of commercialization. It is a photoelectrochemical device involving a photon, an electron, and a chemical reaction in its operation. In DSSC solar cells’ performance, the functions of

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FIGURE 5.9 Schematic illustration of (A) tandem solar cells, (B) organic solar cells, (C) dyesensitized solar cells, (D) perovskite solar cells, and (E) quantum dot solar cells.

light absorption and charge transport are done by two different materials. Unlike in other semiconductor solar cells, the same materials do both features mentioned above. In DSSC, the light is absorbed by dye materials, and the carriers are transported by wide band gap semiconductor (Fig. 5.9C). The three dye types are commonly used: ruthenium-based metal-organic complexes (N-3 red dye, N-749 black dye, and Z-907) with polypyridyl ligands (Gong et al., 2017). These dyes should have a wide range of spectrum and higher absorption coefficient. Commonly, a wide band gap material is chosen as TiO2 because of suitable band edge levels for charge injection and extraction, long lifespan of excited electrons, exceptional resistance to photo corrosion, nontoxicity, low cost and ease of synthesis. It has an optical band gap of 3.2 eV at room temperature. The operation of DSSC can be explained in the following steps: G

G

G

Step 1: After the absorption of a photon by the dye, the dye molecule goes into the excited state. The electron is given off to the wide band gap semiconductor, and the excited dye molecule gets oxidized. For increasing the effective absorption of a photon, the surface morphology of the semiconductor-dye contact should be improved. Step 2: The excited electron is given off to the conduction band of the semiconductor. A transparent conducting oxide material (generally SnO2: F) is used to collect the conduction band’s electrons. The electron is then flowing through the external load to the counter electrode. Step 3: The oxidized dye molecule is reduced to the original form by regaining the electrons from the organic electrolyte solution. The

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electrolyte system contains the redox system, which is being mixed to form tri form ions. Generally, iodide ðI 2 Þ=triiodideðI32 Þ is taken as an electrolyte. The catalytic presence of Pt at the electrode functions to restore the iodine ions.

5.14.3 Perovskite solar cells In the emerging solar cell technology, perovskite solar cells (PSC) has emerged in recent years, which has attained tremendous interest and quickly revived research. Within a short duration of research and development, single-junction PSC’s power conversion efficiency has gained a rapid pace from 3.8% in the first version by Kojima et al., 2009 to a certified 25.2% in 2019 (You et al., 2020). In the early stages of PSC development, Pb-based organic/inorganic halide perovskite having a general formula of ABX3, where A 5 CH3NH31 or HC(NH2)2FA1; B 5 Pb21; and X 5 I2, Br2 or Cl2 has been widely studied. A thick layer of semiconducting perovskite material is inserted between an electron transport layer (ETL) and a hole transport layer (HTL), a perovskite material is inserted to make a full device (Fig. 5.9D). The front contact is accomplished by a transparent conducting oxide (TCO) and a back contact by an opaque metal. Electrons are excited from the valence band to the conduction band leaving behind a hole in the valence band under the illumination of light. The difference in work function between the ETL and HTL supports photo-generated charge carriers to move off in opposite directions, creating a current in an external electrical load connected between the front and back contacts. The problem with this technology is the hysteresis, low sunlight stability of the perovskite material, scaling up issue, and environmental concern, especially lead based material.

5.14.4 Quantum dot solar cells A quantum dot is a semiconductor crystal having nanometric size. Semiconductor QDs demonstrate unique optical properties due to the combination of their band gap energy and quantum well phenomena. Electrons generated in the QD by the absorption of the photon are confined in an infinite potential well. The band gap of these dots can be changed by changing their size (Lee et al., 2020). The quantum confinement effects occur due to the nano-size effect increasing the band gap of materials that help design materials with a different band gap. The change of the band gap changes the range of the solar spectrum radiation absorbed by the material. Hence, it is an attractive technology for multilayer PV cells (Fig. 5.9E). These cells are easy to synthesize and cost-effective. Quantum dot solar cells (QDSC) is a promising technology and has the potential of theoretical PCE that surpassing the Shockley-Queisser limit of 44% due to the possibility of extraction of hot electrons and multiple exciton generations. Until now, a maximum

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PCE of 16.6% has been reported for optimized QDSC. QDs have a discrete density of states with a band gap inversely proportional to the size of QD. Quantum confinement effect and edge effect takes place during the operation mechanism of QDSC.

5.15 Summary, conclusions, and outlook PVs is a promising and suitable technology to harvest the clean energy that the consumers highly appreciate due to its unique features for many applications of elevated social value, such as providing electricity to people who are far apart from electricity access. Its consumption is increasing rapidly to produce electricity in grid-connected houses and buildings in industrialized countries. PV technology is primed to become a sizeable high-tech industry in developed countries. Public research and development and the government and entrepreneurs should be aware of this endeavor to take to lead. Silicon is one of the most abundant elements in the Earth’s crust, but the Si material used in solar cell standard is obtained primarily as off-grade poly-Si and wafers from the microelectronics industry. Thus, there are some concerns regarding the shortage of purified Si for the PV industry, which increases device fabrication cost. Hence, low-cost alternative and high-efficiency novel concepts as emerging PV technology, to compete with matured Si technology, are needed. Thin film technologies are expected to grow as new generation solar cells to revolutionize the solar energy harvest. It consists of CdTe, CZTS, CIASe, and CISe/CIGSe, which reduces the production cost by developing new growth and deposition methods of material with a few micrometers range of thickness. A variety of deposition techniques, including physical, chemical, electro-chemical, mechano-chemical, plasma-based, and hybrid, can be utilized to grow amorphous to highly oriented films on different substrate configurations attaining .22% PCE. Compositional grading across the film’s thickness can be effortlessly carried out during the growth process even by doping and alloying to obtain high quality of optoelectronic properties. Surface passivation is a crucial tool to alter the film’s characteristics, which can be performed after the deposition. Furthermore, a flexible thin film PV module can be installed on the nonrigid and curved surface, broadening its applicability. Apart from metallic foils, plastic films, flexible glass, paper substrates such as cellulose papers, security papers, and planes, copying papers can be used for flexible solar cells. Hybrid organic/inorganic materials are expected to boom the PV market through emerging solar cell technology. Thus, the thin film and emerging PV technology’s reliability with the availability of materials gaining favorable optical and electrical properties will certainly suppress the Si solar market’s dominancy shortly for harnessing green energy.

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Acknowledgment Authors would like to acknowledge CONACyT-Mexico for the financial support.

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Chapter 6

An overview on ferroelectric photovoltaic materials Savita Sharma Physics Department, Kalindi College, University of Delhi, Delhi, India

6.1

Overview

Harnessing solar energy as an alternative to compensate the depleting nonrenewable energy resources is a technological field with great potential. Silicon-based photovoltaic (PV) technology has dominated the solar cell industry over the past years and continues to foster to date. Over several decades, researchers have been striving to open different corridors for the production of renewable energy to curb the evolving demands for inexhaustible energy and clean fuel sources. In the junction-based PV effect, the internal electric field only exists in a very thin space charge region of a pn junction or Schottky barrier which separates the charge carriers. Without the internal field in the bulk material region, the photo-generated charge carriers sweep out of the depletion region and diffuse to the cathode or anode. Thus the charge transportation is often limited by diffusion in the junction-based PVs. The open circuit photovoltage (Voc) cannot exceed the energy barrier height of the junction, which is usually lower than 1 V for Si. To overcome these limitations, another mechanism was discovered in noncentrosymmetric materials, such as ferroelectrics and is called the Ferroelectric photovoltaic effect (FEPV), which differs from the conventional junction-based interfacial PV effect in semiconductors, such as pn junction or Schottky junction. FEPV is a fascinating phenomenon with many unique features such as extremely large photovoltage, where a photocurrent is proportional to the polarization magnitude and charge carrier separation in homogeneous media. Here, the charge transportation is not limited by diffusion, and Voc is not restricted to the energy barrier (energy band gap). As a result, the photovoltage observed here is significantly higher than the electronic band gap and proportional to the inter electrode distance in the polarization direction, which is found to be originated from the engineered domain walls of the ferroelectric material. Ferroelectric photovoltaic effect (FE-PV) was originally investigated in Sustainable Material Solutions for Solar Energy Technologies. DOI: https://doi.org/10.1016/B978-0-12-821592-0.00002-9 © 2021 Elsevier Inc. All rights reserved. 175

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several ferroelectric perovskite oxides, such as in BaTiO3, PbTiO3, Pb(Zr,Ti)O3, PLZT, and LiNbO3. Exploitation of suitable ferroelectric materials having narrow-band gap useful for visible region are promising for their potential application in both novel optoelectronic and the solar energy devices. Current and future research strategies are focused around megatrends like enhanced energy harvesting, energy conversion, renewable energy, environmental and health science, etc. Advancements in material science and technologies for sustainable, resource-efficient products and processes make a significant contribution to improve living and health standards. Multifunctional materials and designs possess the ability to perform multiple functions by employing different combinations of materials, their structural properties, and at least one supplementary functional capability as required by the system application (Gibson, 2010). The noble contexts of multifunctional systems include (1) “self-governing” structures focused toward sensing, diagnosing, and selectively responding toward some analyte with minimal external interference, (2) “adaptive” structures allowing changes in functional, mechanical, and structural properties through reconfiguration or rearrangement of materials as per application, and (3) “self-sustaining” systems with efficient energy harvesting/storage/detection/transmission capabilities. Nanostructures of multicomponent materials have an edge over other systems owing to their tunable functionality, unique structural and mechanical properties, enhanced stability, etc. and thus offer various multifunctional applications (Chopra, 2010). In this context, heterostructures find diverse applications in the field of nanoelectronics, sensors, optoelectronics, solar cells, batteries, biomedical devices and other analytical devices (Wang et al., 2010). Recently, the integration and/or coupling of ferroelectric materials with other functional components has resulted in the development of various kinds of multifunctional systems exhibiting unique ferroelectric-photo (Kreisel et al., 2012), ferroelectricelastic (Hwang and Lynch, 1995), ferroelectricelasticmagnetic (Spaldin and Fiebig, 2005), ferroelectricmechanical (Lu et al., 2012; Wang et al., 2000) properties and exploited for diverse applications such as photovoltaics (Kreisel et al., 2012) and spintronics (Spaldin and Fiebig, 2005), opticalelectricalmechanical actuators, and sensor applications (Lu et al., 2012; Wang et al., 2000).

6.2

Ferroelectric materials

An ideal dielectric material on application of the electric field gets polarized and acquires dipole moment due to rearrangement of charges. Atomic polarization refers to the electron displacement relative to the nucleus while ionic polarization refers to the relative displacement of cation and anion sublattices in ionic materials. Another type of polarization is space-charge polarization which involves the limited movement of charges resulting in the alignment of charged dipoles under applied field. Thus an applied field does not

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directly affect individual atoms or ions in a dielectric but a local field is produced by lattice polarization under certain conditions which tend to stabilize the polarization further (feedback mechanism). This shows the existence of “spontaneous polarization” that is, lattice polarization in the absence of an applied field and the ferroelectric materials constitutes an important class of such spontaneously polarized materials (Moulson and Herbert, 1990). The spontaneous polarization varies with temperature and the temperature at which the transition from spontaneous polarization state to a state that is originally electrically neutral occurs is called as the Curie temperature (Pardo and Ricote, 2011). The crystallographic phase above Curie temperature (Tc) is known as paraelectric while below Tc is called ferroelectric. Amongst 32 point groups or classes of crystals, 21 classes are noncentrosymmetric and 20 are piezoelectric (Kao, 2004). One class of crystals is not piezoelectric although it lacks center of symmetry, which is because of other combined symmetry elements. Out of 20 piezoelectric crystal classes, there are 10 crystal classes which are pyroelectric (Kao, 2004). Pyroelectric materials possess permanent polarization in a given temperature range which is due to the development of spontaneous polarization which forms permanent dipoles in the structure. This polarization is temperature dependent and thus termed as pyroelectricity. Ferroelectric materials are a subclass of the spontaneously polarized pyroelectric materials (Gonzalo and Jimenez, 2005). They possess spontaneous polarization like pyroelectrics but the direction of polarization can be changed by application of the electric field of magnitude smaller than that at which dielectric breakdown of the material occurs. Below the Curie temperature, ferroelectric crystals generally develop a random domain structure, leading to a net zero polarization. Within each ferroelectric domain, the polarization is in the same direction with a domain wall separating regions with different polarization directions (Xu, 1991). This structure is necessary to minimize free energy due to the development of anisotropic strains and depolarization fields below critical temperature. Domain walls are characterized by the angle between the polarization directions on either side of the wall. Thus a 180 degrees domain wall demarks a boundary between antiparallel domains, while a 90 degrees wall would be formed at the boundary between domains for example, pointed “up” and “left.” The allowed angles for domain walls depend on the orientation of the spontaneous polarization allowed by symmetry (Guyonnet, 2014). For example, in rhombohedrally distorted perovskites, there are no 90 degrees domain walls like a tetragonal perovskite, but instead 71 degrees and 109 degrees walls are present. In the early 1940s, the turning point in ferroelectricity came with discovery of the unusual dielectric properties of a number of simple mixed oxides that crystallize with the perovskite structure (Cross and Newnham, 1987). BTO is the most widely used ferroelectric perovskite oxide and is exploited commercially in multilayer ceramic capacitors due to high dielectric constant

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and low loss. Also, BTO is promising for the field induced piezoelectric transducers due to its large polarization, large permittivity and the large induced strains. Lead-free BTO has become one of the most important electroceramic materials among ferroelectrics because of good functional properties along with being an environmentally-friendly material.

6.3

Photovoltaic effect

Ferroelectrics have shown potential as promising materials for future PV applications. Observance of high open circuit voltages in ferroelectric thin films, has generated considerable interest in the field of ferroelectric PV in recent years (Yuan et al., 2013). The field of ferroelectric PV is evolving and not yet completely understood compared to the semiconductor-based PV technology. PV materials and devices, commonly known as solar cells, convert sunlight into electrical energy. Generation of electricity in a clean, quiet, and reliable way is one of the major attractions of PV technology. On illumination of a conventional semiconductor-based PV cell, generation of charge carriers (electronhole pairs) takes place at the depletion region (pn junction) of the semiconductor. The carriers thus produced are responsible for the generation of photocurrent, which can be harvested in a useful manner. The electronic band gap of the semiconductor determines the photovoltage produced by the PV cell. The PV technologies can be classified on the basis of the material used, which in turn is responsible for the determination of the current voltage (IV) characteristics and the efficiency of PV device. High conversion efficiencies have been demonstrated by PV cells based on silicon and IIIV semiconductor compounds (Yuan et al., 2011; Yang et al., 2012). The highest conversion efficiencies are shown by single and multi-junction derivatives of these materials and find applications in space and terrestrial technologies. Inorganic, organic, and quantum dot cells are some emerging PV cell technologies (Park, 2015, Thesis: Bennett, 2014). Though the efficiencies of ferroelectric PV cells are lower as compared to their semiconductor counterparts, but they have found a place in applications (Fig. 1.4) due to their cost effectiveness. Fig. 6.1 shows the review of various companies, universities, and research institutes behind the advancements of solar cells based on silicon. It is impressive to see how National Renewable Energy Limited (NREL) has been heavily involved in technological advancements since the 90s. Other heavyweights have been research labs like Fraunhofer and (Boeing)Spectrolab. Focusing on the corporate giants, we notice that not only Sharp, but also Panasonic, Sanyo and Siemens have contributed to the field with their own R&D departments. There is an ongoing pursuit for further improvement of the PV cell efficiency along with reduction in the fabrication cost. The PV industry and research is driven by the quest of maximizing the power generated per rupee. The past decade has seen the momentum in

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FIGURE 6.1 Review chart showing the work done on solar cells [Website 2].

exploring the viability of alternate materials to increase the efficiency of PV cells. Lately, ferroelectric materials have been used for PV technology and this research area is generally referred to as ferroelectric PV.

6.3.1

Mechanism of ferroelectric photovoltaic

Charge carriers (electronhole pairs) are generated when a ferroelectric material is illuminated with a light of wavelength corresponding to its energy band gap (Eg). A PV output is generated when the photogenerated carriers are separated and driven to the electrodes by the polarization induced by the internal electric field (Fig. 6.2B). On the other hand, for a junction-based semiconductor PV device, the charge carriers are separated by the electric field which exists in the depletion layer at the pn junction interface (Fig. 6.2A). Thus in ferroelectrics the PV effect is a bulk-based effect, which is different from the junction-based semiconductor PV effect. In FEPV, the PV responses can be generated without forming complex pn junction structures since the internal electric field is not limited to an interfacial region. A simplified schematic of this mechanism in conventional semiconductor pn junction and ferroelectric photovoltaic is shown in Fig. 6.2A and B. Researchers worldwide are working very hard to open different corridors for the production of renewable energy to meet the evolving demands of the inexhaustible energy and clean fuel sources (Yang et al., 2009a,b,c, 2010; Zhang et al., 2013; Chen et al., 2011; Cai et al., 2011). Harnessing solar energy as an alternative to compensate the depleting nonrenewable energy resources is a technological field with great potential. Silicon-based PV technology (Yang et al., 2010) has dominated the solar cell industry over the past many years and continues to foster to date. Typically PV effect involves

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FIGURE 6.2 Simplified schematics of PV mechanism in (A) semiconductor pn junction and (B) ferroelectric thin film. PV, Photovoltaic.

v Eg

eVOC

VB hʋ

n-type

Depletion layer

p-type

FIGURE 6.3 Band diagram of conventional pn junction showing the conduction band (CB), valence band (VB), band gap (Eg), and open-circuit voltage (Voc). Here, hν is the incident photon energy and e is the elementary electric charge (Ji et al., 2010).

two processes, creation of electronhole pairs as the electrical charge carriers, and formation of electric current due to the motion of separated electrons and holes. In the junction-based photovoltaic effect, the internal electric field only exists in a very thin space charge region of a pn junction or Schottky barrier, which separates the charge carrier as shown in Fig. 6.3. Without the internal field in the bulk material region, the photo-generated charge carriers swept out of the depletion region and diffuses to the cathode or anode (Qin et al., 2008a,b). Thus the charge transportation is often limited by diffusion in the junction-based photovoltaics. The open circuit photovoltage (Voc) cannot exceed the energy barrier height of the junction, which is usually lower than 1 V for Si (Qin et al., 2008a,b; Shah et al., 1999; Peumans et al., 2003). Moreover, a pn junction is not a prerequisite for the photovoltaic effect. The photovoltaic effect can originate from a variety of

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other mechanisms such as gradient in a chemical potential (Shah et al., 1999) or spin polarization (Peumans et al., 2003). Another mechanism was discovered in noncentrosymmetric materials, such as ferroelectrics and is called the bulk photovoltaic effect (BPVE), which differs from the conventional junction-based interfacial photovoltaic effect in semiconductors, such as pn junction or Schottky junction (Ji et al., 2010). Bulk ferroelectric photovoltaic effect (BFPVE) is a fascinating phenomenon with many unique features, such as extremely large photovoltage, where a photocurrent is proportional to the polarization magnitude and charge carrier separation in homogeneous media (Ji et al., 2010). In BFPVE, the remnant polarization and the polarization-induced internal electric field exists over the whole bulk region of the ferroelectric rather than a thin interfacial depletion layer. Here, the charge transportation is not limited by diffusion, and Voc is not restricted to the energy barrier (energy band gap) (Glass et al., 1974; Ji et al., 2010). As a result, the photovoltage observed here is significantly higher than the electronic band gap and proportional to the inter electrode distance in the polarization direction, which is found to be originated from the engineered domain walls of the ferroelectric material (Yang et al., 2010). FE-PV was originally investigated in several ferroelectric perovskite oxides, such as in BaTiO3 (Chynoweth, 1956; Brody and Crowne, 1975), PbTiO3 (Fridkin, 1979), Pb(Zr,Ti)O3 (Kholkin et al., 1998; Poosanaas et al., 1998; Poosanaas and Uchino, 1999; Li et al., 2004a), PLZT (Qin et al., 2007), and LiNbO3 (Chen, 1969). Brief literature on the PV application of ferroelectric material is presented in Table 6.1. Oxide materials are cheap, abundant, stable, highly light absorbing and their properties, such as band gap and conductivity can be systematically tuned through chemical substitutions, making them a promising candidate for thin film ferroelectric PVs (Basu et al., 2008). Much excitement was generated due to the anomalously large open circuit photovoltage (in some case, Voc . 102 V), which is produced when crystal was subjected to illumination (Ji et al., 2011). However, the light-to-electricity conversion efficiency (power conversion efficiency) of the bulk PV effect in ferroelectric thin film based solar cell is reported to be significantly lower (,1024) than that of commercially available silicon-based solar cell (Glass et al., 1974; Yang et al., 2009a,b,c; Alexe and Hesse 2011). The origin of the photocurrent enhancement is related to more photogenerated charges and their fast transportation between the electrodes. Most of the ferroelectric PV devices have been designed in sandwiched structure, and their output has been limited by the thickness of fabricated ferroelectric films. Therefore to design highly efficient ferroelectric device with enhanced output, ferroelectric device with different styles of electrode configurations should be optimized. Ma et al. suggested that electrode-configuration-design is an attractive route for manipulating the photosensitivity and an enhanced photocurrent was achieved by them on the planar-structured BaTiO3 device (Ma et al., 2018).

TABLE 6.1 Brief summary on photovoltaic study carried out on ferroelectric samples. S. no.

Sample type

Deposition technique

Light source

Important results

References

1.

BFO/SRO/(111) STO, (110) DSO

PLD

λ 5 550 nm with (100 mW/cm2)

Jsc 5 13.4 μA/cm2

Basu et al. (2008)

2.

BFO/SRO/(001) STO

MOCVD

White-light (285 mW/cm2)

Voc 5 0.9 V Jsc 5 1.5 mA/cm2 η 5 10%

Yang et al. (2009a,b,c)

3.

Au/BFO crystal/Au, Ag/BFO crystal/Ag



Green and red light (532 and 650 nm)

Jsc(green) 5 7.35 μA/cm2 Jsc(red) 5 2.6 nA/cm2 η 5 3 3 1023%

Choi et al. (2009)

4.

BFO/(110) DSO

MOCVD

White-light (285 mW/cm2)

Open circuit voltage Voc 5 16 V η 5 7 3 1022%

Yang et al. (2010)

5.

BFO/SRO/(001) STO

RF magnetron sputtering

Incident light (λ 5 435 nm) with (750 mW/cm2)

Voc 5 0.3 V η 5 7 3 1024%

Ji et al. (2010)

6.

ITO/Poly-BFO/Pt/Ti/ SiO2/Si

Solgel

Incident light of 450 μW cm22

1000-fold photoconductivity, η 5 0.125%

Chen et al. (2011)

7.

BFO/SRO/(111), (100) STO

RF magnetron sputtering

λ 5 435 nm with (20 mW/cm2)

5 order large bulk photovoltaic tensor coefficient β22

Ji et al. (2011)

8.

BFO/(001) Nb-STO

Laser molecularbeam epitaxy

He-Cd laser (λ 5 325 nm) and pulsed Nd:YAG laser (λ 5 355 nm)

Voc 5 136 mV

Wang (2011)

9.

BFO/(001) Nb-STO

PLD

White-light (285 mW/cm2)

Voc 5 40 mV η 5 3 3 1022%

Qu et al. (2011)

10.

ITO/BFO ceramic/ Au

Solid state reaction

Diode lasers (λ 5 373, 532 nm)

Jsc 5 1.2 μA/cm2

Hung et al. (2012)

11.

BFO/FTO

Modified CSD

Solar simulator (100 mW/cm2)

Jsc 5 0.13 3 1023 A/cm2 Voc 5 0.65 V η 5 5.34 3 1023%

Dong et al. (2012)

12.

Au/BFO ceramic/ ITO

Solid state reaction

Diode lasers (λ 5 405,532 nm)

Jsc 5 0.23 A/m2 Voc 5 0.7 V

Tu et al. (2012)

13.

Carbon nanotube/ BFO/Pt with CdSe

Spin coating

Solar simulator (100 mW/cm2)

Jsc 5 2.1 μA/cm2 Voc 5 0.47 V

Zang et al. (2012)

14.

AZO/BFO/FTO

Modified CSD

Solar simulator (100 mW/cm2)

Jsc 5 0.13 3 1023 A/cm2 Voc 5 0.63 V η 5 2.08 3 1022%

Dong et al. (2013)

15.

BFO/Pt/Ti/SiO2/Si (100)

Sputtering

λ 5 405 nm laser

Jsc 5 11.7 μA/cm2

Chang et al. (2013)

16.

BFO/TbScO3

PLD

λ 5 405 nm laser

Voc 5 7.6 V

Bhatnagar et al. (2013)

17.

BFO

Solgel

λ 5 2001100 nm laser

A At temp 5 550 C; Jsc 5 4:327 3 1025 cm 2 ; V oc 5 0:11 V

A At temp 5 600 C; Jsc 5 2:031 3 1025 cm 2 ; Voc 5 0:075 V

Lin et al. (2013a,b) (Continued )

TABLE 6.1 (Continued) S. no.

Sample type

Deposition technique

Light source

Important results

18.

In2O3-SnO2/ BiFe0.6Sc0.4O3/ LaNiO3 (ITO/BFSO/ LNO)

RF magnetron sputtering

19.

BFO

RF magnetron sputtering



Voc for 750 nm thickness BFO film is 0.35, 0.22, 0.18 and 0.06 V at temperature of RT, 180, 340 and 500 C, Jsc for this film is 0.1 μA/cm2 in dark and 13.5, 5.0, 1.2 and 0.6 μA/cm2 in purple light at temperature of RT, 180 C, 340 C and 500 C

20.

Bi12xKxFeO3

Solgel



ISc 5 1.32 lA/cm2 and Voc 5 0.45 V for 20%K substituted film

21.

Bi0.9La0.1FeO3

PLD



Thin films cooled at 500, 20 and 0.5 Pa, Voc 5 0.12, 0.15 and 0.18 V, respectively, Jsc 5 2.25, 22, and 80 lA/cm2

22.

(Bi1-xSrx)FeO32δ



λ 5 405nm

Voc and Jsc are 0.62 V and 0.04 A/m2 in ITO/BFO ceramic/ Au, 0.47 V and 0.44 A/m2 in ITO/BFO5Sr/Au, 0.57 V and 0.17 A/m2 in ITO/BFO10Sr/Au, and 0.47 V and 0.04 A/m2 in ITO/BFO15Sr/Au

References

BFSO film in negatively poled state, efficiency is improved by 5 times with an enhanced switchable Voc up to 0.6 V

Tu et al. (2012)

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It has also been reported that photocurrent can be tuned by the depolarization field, that is, different polarizations will produce different photocurrents. Gao et al. observed that the polarization in their material along (110) direction was larger than that of (001) and (010) directions, thus, the depolarization filed along (110) direction was found to be larger than that of (001) and (010) directions and so does the photocurrent (Gao et al., 2018). Thus we can conclude that depolarization field influences the transverse PV effects. One of the other major obstacle in achieving high conversion efficiency is the intrinsically low bulk conductivity of ferroelectric domains. Indeed, any effort to increase the bulk conductivity of ferroelectric domains has proved to be ineffective, because leaky domains cannot withstand strong electric ~ P) ~ a domain wall will polarization and charge density associated with (r: therefore be reduced for a leaky ferroelectric material, resulting in a low open circuit voltage. Moreover, the large energy band gap of ferroelectric materials allows strong absorption of light in UV region only. Recently, a different mechanism for ferroelectric PV phenomenon is proposed for BFO, where the observed high PV voltage is related to the domain walls of the ferroelectric material instead of the bulk, which provides important insight into the PV effect in BFO (Yang et al., 2009a,b,c). Detailed optical studies using absorption spectroscopy and spectroscopic ellipsometry have shown that BFO has a direct band gap in the visible region with relatively large absorption coefficient (Kumar et al., 2008; Allibe et al., 2010). Basu et al. (2008) have shown appreciable photoconductivity in BFO under visible light illumination. Yang et al. (2010) was able to achieve above band-gap photovoltages. A simple possible explanation of FE-PV is schematically illustrated in Fig. 6.4B. When a light of source wavelength corresponding to the energy band gap of the ferroelectric material is incident, photons are absorbed with the generation of charge carriers-electrons and holes. The photo-generated

+

+ + + + + + + + + + + + + + + + + + + + + + +

------ ---------- -------------- -- Au

-

+ +

+ + + + + + + + + + + + +

BFO/BTO

Pt/Si

+

-

(A)

Electric dipoles

(B)

FIGURE 6.4 Schematic illustration of mechanism of photovoltaic effect in ferroelectric thin film.

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electrons and holes are driven by the polarization induced internal electric field in opposite directions, and thus contribute to the PV output. In other words, this effect is associated with the absence of inversion symmetry in the distribution of defects, impurities, space charges and polarizations in ferroelectric materials. The output signal of solar cell made up of ferroelectric material can be tuned by various other means including mechanical, electrical, and magnetic functionality. Designing of materials having enhanced magnetoelectric (ME) properties can be achieved using the principle of strain engineering employed to multiplayer structures and superlattices (SLs). In this regard, recent study of a number of multilayer heterostructures containing alternating ferroelectric and multiferroic layers, such as Pb(Zr,Ti)O3/CoFe2O4 (Ortega et al., 2009, 2006; Zhou et al., 2006), (Pr0.88Ca0.15MnO3)/(Ba0.6Sr0.4TiO3) (Murugavel et al., 2005), CoFe2O4/BaTiO3 (Zheng et al., 2004), BiFeO3/ BaTiO3 (Toupet et al., 2008), (Bi,Nd)FeO3/(Ba,Sr)TiO3 (Ivanov et al., 2012), BiFeO3, and BiMnO3 (Chen et al. (2013)) has been useful in observing the enhanced ME properties. Upon investigation of BiFeO3/BaTiO3 SLs, it was reported that samples with smaller modulation periods showed larger magnetization due to increasing number of interfaces (Toupet et al., 2008). Similarly, examination of (Bi,Nd)FeO3/(Ba,Sr)TiO3 multilayers with 3 and 6 nm layer thicknesses (Ivanov et al., 2012) revealed saturation magnetization of about 5 3 1046 3 104 A/m, having an order of magnitude higher than that reported for BiFeO3/BaTiO3 SLs (Toupet et al., 2008). Composites consisting of ferroelectric (FE) and ferromagnetic (FM) materials (Chen et al., 2013a,b) and multilayered ceramic samples (Yang et al., 2009a,b,c) have been investigated. These composite/multilayers were shown to be exhibiting larger ME effect as compared to single phase materials, and have been exploited for applications in the field of energy harvesting. By using suitable FM electrodes and clever design of the device structure, it may also be possible to control the spin polarization of the output photocurrent favoring device miniaturization with novel applications such as optical microsensing, light controlled elastic micromotion and microactuation in microelectromechanical systems (Catalan and Scott, 2009). Exploitation of suitable ferroelectric materials having narrow-band gap useful for visible region are promising for their potential application in both the novel optoelectronic and the solar energy devices.

6.3.2

History and current status of ferroelectric photovoltaic

The PV effect in ferroelectric ceramics (bulk) and single crystals such as BaTiO3, Pb(Zr,Ti)O3, and LiNbO3 were observed earlier (Glass et al., 1974; Brody, 1973; Nonaka et al., 1995). The noncentrosymmetric nature of the unit cell gives rise to this effect (Fridkin, 1979; Vladimir and Popov, 1978). However, owing to small current densities of the order of

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nanoamperes/cm2 due to large band gap in the ferroelectric materials, the PV efficiency was limited. The existence of photovoltage higher than the band gap in thin films of Bismuth ferrite (BiFeO3) has led to a renaissance of research in PV ferroelectric materials (FE-PV). An open circuit voltage (Voc) of B20 V was reported for BiFeO3 (BFO) thin film, which has a band gap (Eg) of 2.67 eV (Yang et al., 2009a,b,c, 2010). It was further shown that the photovoltage could be enhanced or reversed by controlling the ferroelectric polarization, which in turn is controlled by electrical poling in lead-based thin films (Meng et al., 2009). Poling is the process of applying an electric voltage higher than the coercive field to a ferroelectric material, while cooling it from transition temperature to room temperature. Poling results in orienting the ferroelectric domains in one particular direction, leading to maximum polarization. The internal polarization (PS) is not uniform throughout a ferroelectric. For instance, at the surfaces the crystal terminates and polarization becomes zero and in the neighborhood of defects, the value of PS may differ from that of the perfect crystal. This variation in PS gives rise to depolarizing field. This field can be compensated by the flow of free charge in the crystal or by the free charge in the surrounding medium at the crystal surface. The depolarization field energy (WE) is zero for a totally compensated crystal in equilibrium. However, for insulators this is not readily attained. Typical examples of ferroelectric materials are Barium Titanate (BaTiO3), Bismuth Ferrite (BiFeO3), Potassium Niobate (KNbO3), Barium Strontium Titanate (BaxSr12x)TiO3 (BST), and Lead Zirconate Titanate Pb (ZrxTi12x)O3 commonly referred as PZT. Ferroelectric materials, which exhibit strong bulk PV effect, have been widely explored since 1956 for harvesting light energy (Chanussot et al., 1977; Dharmadhikari and Grannemann, 1982). Compared to other ferroelectric materials like BTO (Chynoweth, 1956), LiNbO3 (Chen, 1969), (PbLa)(ZrTi)O3 (Qin et al., 2007) with a wide energy band gap, the smaller band gap of BFO (2.22.8 eV) can generate a significant current in response to the interaction with visible light photons coming from the sun, as an abundant renewable clean energy. The above mentioned discussion clearly points toward the advantages of BFO over other multiferroic and ferroelectric materials. BFO crystallizes in the perovskite structure and is compatible with many other functional compounds such that its ferroic property and electric behavior can be tailored for the desired applications.

6.4 6.4.1

Barium titanate Crystal structure

BTO is a member of the perovskite family with chemical formula ABO3 and primitive cubic crystal structure (Fig. 6.5). The larger cation A (Ba) is placed

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FIGURE 6.5 Cubic perovskite-type structure ABO3 (Vijatovi´c et al., 2008).

at the corners, the smaller cation B (Ti) is placed in the center and the oxygen anion is placed in the center of the face edges of the cube (Fig. 6.5). This crystal structure derives its name from the mineral perovskite, CaTiO3. The crystal structure is determined by the geometrical packing of ions in the lattice. The perovskite structure can be considered as a three-dimensional framework of BO6 octahedra (Fig. 6.5A), but it can also be regarded as a cubic close packed arrangement of A and O ions, with the B ions filling the interstitial positions (Fig. 6.5B) (Jona and Shirane, 1993; Haertling, 1999; Filho, 2006; Lazarevic et al., 2005; Duran et al., 2002). It can be seen from Fig. 6.5 that the coordination number of larger cation A is 12 and for the smaller cation B is 6. The ferroelectric properties of perovskite BTO are connected with a series of three structural phase transitions. Fig. 6.6 depicts the changes in the crystallographic structure of BTO (Jona and Shirane, 1993) as a function of temperature. The Curie point Tc of BTO is 120 C, above which the original cubic cell is stable up to 460 C and BTO attains hexagonal structure above this temperature (Cho, 1998). Below Tc 5 120 C, the crystallographic structure changes and ferroelectric phase transitions between the cubic, paraelectric, and ferroelectric phase of tetragonal structure takes place (Fig. 6.6). At ,5 C, the transition from tetragonal (5 C120 C) to orthorhombic structure (290 C to 15 C) occurs and at , 2 90 C, the transition to trigonal structure takes place (Jona and Shirane, 1993; Koelzynski and Tkacz-Smiech, 2005). The dotted lines in Fig. 6.6B, C, and D define the primitive cubic cell at the Curie point (Jona and Shirane, 1993) where Ti-ions are in equilibrium positions at the center of their octahedra. Below Curie point (Tc 5 120 C), Ti-ions jump between energetically favorable positions out of the octahedron center as shown in Fig. 6.7 (Buchanan, 1990).

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FIGURE 6.6 Unit cells of the different phases of BaTiO3: (A) Cubic ( . 120 C), (B) Tetragonal (120 C5 C), (C) Orthorhombic (15 C to 290 C) and (D) Rhombohedral (, 2 90 C).

FIGURE 6.7 Ti ion positions in tetragonal BaTiO3 (Buchanan, 1990).

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SECTION | II Sustainable Materials for Photovoltaics

Dielectric properties

BTO, owing to its high dielectric constant and low dielectric loss, was the first material used for manufacturing dielectric ceramic capacitors, multilayer capacitors, etc. The dielectric constant of a material depends on the synthesis route (which defines the purity, density, grain size, etc.) dopant and on the parameters like temperature and frequency. The temperature dependence of the dielectric constant of BTO synthesized by different routes has been reported in a number of papers. The influence of grain size on the dielectric constant of BTO had been studied and the inverse relation between them was reported (Kim and Han, 2004). The dielectric constant of 4500 and 1800 at the Curie point were reported for 0.86 and 10 μm grains, respectively. The influence of grain size on the dielectric constant of BTO ceramics prepared by the hydrothermal method was also studied (Boulos et al., 2005). Benlahrache et al. showed the frequency dependence of the dielectric constant of BTO prepared by the conventional milling and calcination process (Benlahrache et al., 2006). Under ambient conditions, the dielectric constant increases for frequencies below 1 kHz and tends toward a constant value for higher frequencies. To modify the electrical properties of BTO, A and B site dopants are used (Buscaglia et al., 2000). To make BTO a p-type semiconductor, acceptor dopants (monovalent, divalent and trivalent ions) are used which substitute Ba21 ions. On the other hand, substitution of Ti41 ions with donor dopants (trivalent, tetravalent and pentavalent ions) makes BTO a n-type semiconductor. Low concentration of donor dopants leads to room-temperature semiconducting ceramics whereas high doping concentration results in insulating materials.

6.4.3

Ferroelectric phenomena in BaTiO3

BTO is the most extensively studied ferroelectric material since its discovery by Wul and Goldman in 1945 (Kwei et al., 1993). BTO possesses classic perovskite structure at high temperatures, which is a centrosymmetric cubic structure with Ba21 at the corners, Ti41 at the center and the oxygen at the face centers (Kwei et al., 1993). As the temperature is lowered, BTO undergoes successive transitions to three different ferroelectric phases and each phase involves small distortions from the conventional cubic structure (Fig. 6.7). Each of these distortions can be thought of as elongations of the cubic unit cell along an edge ([00l] or tetragonal), along a face diagonal ([0l11] or orthorhombic), or along a body diagonal ([1111] or rhombohedral). This results in a net displacement of the Ba21 and Ti41 ions with respect to the oxygen octahedra along these directions, which gives rise to the spontaneous polarization in the ferroelectric phases. The previous theories suggest the existence of structural instabilities due to the rattling of Ti41

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ions in the octahedral oxygen cage (Slater, 1950). However, this is inconsistent as the bond length of TiO is approximately equal to the sum of the ionic radii of Ti41 and O22 ions (Shannon, 1976).

6.4.4

Optical properties

To meet the increasing demand of efficient integrated optical devices, smart materials with multifunctional properties are desirable. For optical communication purposes, thin films of the ferroelectric materials are the promising candidate. Thus the fabrication of ferroelectric thin films based electrooptical (EO) devices has indulged a large group of science community. The high optical transparency and the large EO coefficients of BTO make it a suitable choice for the fabrication of EO modulators based on thin films (Zgonik et al., 1994). BTO can be easily grown as thin film and under ambient conditions, it has high ordinary (no) and extraordinary (ne) refractive indices in its tetragonal phase. In particular, the no and ne values of undoped BTO at 20 C are 2.412 and 2.360, respectively (Nikogosyan, 1997) and the birefringence of the bulk BTO crystal is Δn 5 ne 2 no 5 20.052. The no value is practically independent of temperature and the temperature derivative of the ne, that is, dne/dt 5 140 3 1026/K at a wavelength of 514.5 nm, that is, the temperature coefficient of ne is very small (Nikogosyan, 1997).

6.4.5

Various techniques of depositing BaTiO3 thin film

During the last few years, various synthesis methods such as solid-state reaction, hydrothermal, solgel, pulsed laser deposition (PLD), rf sputtering, etc. have been adopted for the preparation of BTO ceramics and thin films. Among these, the solid-state reaction method is a simple, inexpensive process for fabrication of ceramics but the wide size distribution and the susceptibility to aggregation restricts its ability to be used for fabrication of reliable electronic components (Wu et al., 2009). Also, BTO prepared by solid-state reaction process requires high calcination temperature, thus limiting the use of this technique (Pavlovi´c et al., 2007). On the contrary, the hydrothermal method can be used for the low-temperature fabrication of highly reactive BTO powder or thin films (Xu and Gao, 2004). Also, this technique offers good control over particle shape, size, and stoichiometry (Lee et al., 2003). Solgel processing route is the extensively used technique for producing highly pure BTO thin films with improved surface area, pore volume, and grain size through the careful control of chemistry (Li and Shih, 1997). The PLD technique has an edge over other techniques as it facilitates the epitaxial or crystalline growth of thin films at low temperatures and allows the facile deposition of materials of high melting point with complex stoichiometry

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(Inam et al., 1998). Hence, BTO thin films can be prepared using different techniques for versatile applications. Hydrothermal method: Hydrothermal reaction technique is the method of preparation of BTO in the form of powder and thin films at low-temperature conditions. This technique produces highly reactive powder on sintering and also produces thin films with controlled grain size and stoichiometry, and in some cases controlled shape. The growth kinetics of BTO thin films deposited using hydrothermal method on the surface of single-crystal rutile TiO2 have been studied, and morphology was reported to be influenced strongly by the impurities (Lisoni et al., 2002). High purity hydroxide allows the deposition of pronounced crystals of BTO with octahedral grains. Chemical solution deposition (CSD) technique: CSD is a low temperature processing route extensively used for producing ceramic powder and thin film of high purity and uniform grain size. In this method, when a stable colloidal sol prepared from the organometallic precursors (usually alkoxides) is dried, it forms a gel-like network onto the substrate. This method offers several advantages like good control over the composition of materials, ease of surface modification, and low temperature processing (which permits the use of thermally fragile substrates). Several investigators used this technique for the formation of multicomponent oxide thin films including BTO (Dixit et al., 2002; Harizanov et al., 2004; Silva`n et al., 2002; Vitanov et al., 2003; Xu et al., 2002). PLD technique: PLD is a simple thin film deposition technique which offers several advantages over other methods like facile deposition of materials having high melting points or ability to form metastable microstructures, epitaxial growth of materials at relatively low substrate temperatures with fast turn-around time, coherent deposition of materials with complex stoichiometries, etc. (Norton et al., 1992). Especially for the growth of multicomponent oxide materials like BTO, PLD is the most preferred growth technique. The deposition system consists of a vacuum chamber, a target carousel, a substrate holder, and a high-power pulsed laser which vaporizes the target material to be deposited as thin film onto the substrate. Norton et al. demonstrated the epitaxial growth of BTO thin film with their c-axis perpendicular to the film-substrate interface on (001) magnesium oxide (MgO) single crystal (Norton et al., 1992). The morphology of the thin films can be tailored through the control of various deposition parameters. The controlled microstructures and desirable properties can be obtained through the careful optimization of the deposition conditions (Scarfone et al., 1990). There are a number of reports on growth of ABO3 type perovskite thin films using PLD technique for a number of applications (Norton et al., 1992).

6.4.6

Potential applications of BaTiO3

Fig. 6.8 shows the possible applications of BTO thin film diagrammatically. For decades, BTO is widely being investigated for several practical

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193

FIGURE 6.8 BTO as a multifunctional material for various applications.

applications owing to its high chemical and mechanical stability, good ferroelectric property at and above room temperature, high dielectric constant, low loss, etc. These unique characteristics make BTO a good multifunctional material to be used in applications, such as ceramic and multilayer capacitors, memory elements, ferroelectric PVs, gas sensors, etc. Doped BTO finds widespread application in semiconductor and piezoelectric devices, and has become one of the most important ferroelectric material (Vijatovi´c et al., 2008). Ceramic materials with perovskite structure are important electronic materials and the largest class of ceramic capacitors produced, in number and in value, is the multilayer type. High values of dielectric constant make BTO ceramic a popular choice for use in capacitors. The positive temperature coefficient of resistance was reported in doped BTO and is a grain boundary controlled phenomenon. For two decades, a lot of efforts have been made for the fabrication of reliable photodetectors for efficient detection of harmful ultraviolet (UV) radiations (Zomorrodian et al., 2005; Kozielski et al., 2013; Pintilie et al., 1998). A few reports are available on the realization of UV photodetectors using Lead Zirconate Titanate (PZT) ferroelectric thin films by exploiting its unique optical properties (Zomorrodian et al., 2005; Kozielski et al., 2013). However, there is an urgent need for the identification of suitable eco-friendly materials for UV detector applications. BTO, a lead-free ferroelectric material may be useful for UV photodetector applications due to its favorable band gap

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(Jalali and Fathpour, 2006; Bi et al., 2011). The perovskite oxides (ABO3) have also been exploited as gas sensing materials due to their high thermal and chemical stability in the atmosphere (Fergus, 2007). ABO3 type of perovskite materials as sensing elements exhibit highly sensitive and selective response characteristics. Focus of the present work amongst these applications of BTO is on the ferroelectric PVs, UV photodetection, and gas sensing which are discussed in the following sections.

6.5

Bismuth ferrite

The lower optical band gap (2.22.8 eV) of multiferroic BiFeO3 (BFO) has attracted resurgent interest of the research community since ferroelectric, FM, and ferroelastic orderings interact with each other. The coupling among these ferroic orderings provides multiple degrees of freedom for controlling the PV effect, and this can be used to endow next generation solar cells with new additional functionality. Physical mechanism of PV effect in multiferroic BFO system is not completely understood yet. It is clearly evident from Table 6.1 that PV effect has been observed in BFO in different forms including single crystal (Alexe and Hesse, 2011; Kundys et al., 2010), thin films (Wang, 2011; Yang et al., 2009a,b,c; Chen et al., 2011) and ceramics (Hung et al., 2012). However, enhanced values of PV parameters are reported either in single crystals or epitaxially grown thin films only. It is well known that thin films are largely preferred over single crystal or ceramics for lowvoltage electronic device applications, since the poling voltage required for reversing the direction of the spontaneous polarization in thin films is significantly reduced. However, the main limitation of using BFO thin film for PV applications is its leakage current that degrades its multiferroic and hence PV properties. As discussed in previous section, BTO is an equally attractive perovskite ferroelectric material employed in various devices including capacitors, ferroelectric memories, etc. due to its unique dielectric, piezoelectric and ferroelectric properties (Cai et al., 2011; Jiang et al., 2013; Chen et al., 2010). However, BTO is a wide band gap (B3.3 eV) material and hence cannot be employed for FE-PV applications, in the visible spectral region. Efforts are being continuously made to reduce the optical band gap of BTO retaining the good ferroelectric properties for possible FE-PV application in visible region (Nechache et al., 2015). As we already discussed, multilayered structures possess superior ferroelectric properties compared to the individual parent materials in terms of high polarization, high dielectric constant and relatively lower losses (Murari et al., 2008). Thus in order to improve the ferroelectric properties of BFO while simultaneously reducing the band gap of BTO, fabrication of the multilayered structure of BFO and BTO was found beneficial. There are several reports where band gap engineering has been demonstrated by fabricating multilayered structures (Yadav et al., 2007, 2010a,b). Some research groups have

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also studied BFO/BTO multilayered structures. Significant improvement in the magnetic property of the BFO/BTO multilayered structure has been reported conclusively reported an improvement in the magnetic properties of the system (Lorenz et al., 2014a,b; Toupet et al., 2008; Yang et al., 2009a,b,c). Significant efforts have also been made to study the PV response of BFO/BTO multilayered structures by Sharma et al. (2003). BFO/BTO multilayer structures having different numbers of individual layers had been reported to prepared by CSD technique as well as PLD technique. The multiferroic properties and the ferroelectric PV response of multilayer structures were studied and influence of different number stacking layers of BFO and BTO has been investigated. Both the ferroelectric and FM properties were found to enhance with increase in number of individual layers and thereafter showed degradation after a particular number of layers. The degradation in multiferroic properties after a certain number of layer in BFO/ BTO system was attributed to the smearing of the sharp interfaces into mix phase of BFO and BTO layers. The BFO/BTO multilayer structure with stacking layers were found to exhibit lower leakage current density in comparison to pure BFO thin films. Multilayer BFO/BTO structures demonstrated significant PV response owing to the preferred (110) growth and interface coupling induced strain between the adjacent layers with good retention and high stability of transient current response.

6.6

Conclusion

This chapter gave a brief introduction to different multicomponent, multiferroic, and ferroelectric materials with special emphasis on their multiferroic properties for ferroelectric PV applications. Recent research on multiferroic materials to achieve multifunctional properties is presented. Different composite materials and multilayer structures are reviewed and suitable materials that is, BTO and BFO are identified for desired applications in the present study. Importance and issues related to the fabrication of BTO and BFO thin films by different techniques (PLD, CSD and Solgel hydrothermal (SGHT)) work are discussed. Criterion for achieving FEPV are emphasized and the present state-of-art in the field of multiferroics is in relation to the development of single-phase thin films and artificial multilayer structure is highlighted. A brief review about the fabrication of BFO/BTO multilayer structures for ferroelectric PVs is presented. Designing of ferroelectric materials having enhanced multiferroic responses can be achieved using the principles of strain engineering employed for multilayer structures and SLs. Reports on ferroelectric PV properties indicated that the BFO/BTO multilayer structure with six stacking layers exhibited highest value of both the open circuit voltage and short circuit current density with high on to off ratio. The enhanced PV response is due to good ferroelectric properties related to significant interface coupling between the six stacking layers of BFO and BTO. Further, increase in the number of stacking layers to seven in

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the BFO/BTO structure resulted in a degradation in all the ferroic properties due to poor interface between BFO and BTO. Thus we may conclude that strain engineering in multilayer structure of the suitable ferroelectric/multiferroic materials may pave the way toward realization of new device functionalities in ferroelectric PV cell.

Acknowledgments The author is thankful for Prof. Vinay Gupta, Department of Physics & Astrophysics, University of Delhi for providing research facilities. The author is also thankful for Dr. Monika Tomar, Physics Department, Miranda House, University of Delhi for the support and guidance.

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Chapter 7

Nanostructured materials for high efficiency solar cells Daniel N. Micha1, Roberto Jakomin2, Rudy M.S. Kawabata3, Mauricio P. Pires4, Fernando A. Ponce5 and Patr´ıcia L. Souza3 1

Department of Physics, Centro Federal de Educac¸a˜o Tecnolo´gica Celso Suckow da Fonseca, Petro´polis, Brazil, 2Campus Duque de Caxias, Universidade Federal do Rio de Janeiro, Duque de Caxias, Brazil, 3Semiconductor Laboratory, Pontif´ıcia Universidade Cato´lica do Rio de Janeiro, Rio de Janeiro, Brazil, 4Institute of Physics, Universidade Federal do Rio de Janeiro, Rio de Janeiro, Brazil, 5Department of Physics, Arizona State University, Tempe, AZ, United States

7.1

Introduction

Since the early 1950s, semiconductor heterostructures have given mankind the opportunity to develop artificial crystals [or “Man-made crystals”, according to Leo Esaki (Alferov, 2001)]. From the combination of bulk materials, which is the very base of the classical pn junction, to the formation of nanostructures, such as quantum wells (QWs), quantum wires (also called nanowires—NWs) and quantum dots (QDs), the semiconductor heterostructures have allowed to build novel and better transistors, lasers, photodiodes, photodetectors, solar cells, and much more (Alferov, 2001). Furthermore, charge carrier confinement achieved in the so-called nanostructures, which allows for further bandgap engineering, has been the key to improve performance in several applications, especially the ones involving optoelectronic processes. Embedding nanostructures into a solar cell creates or generates optical changes that lead to enhancement in the power conversion efficiency (PCE). Several novel concepts for photovoltaics have been proposed recently based on such new properties. Single junction solar cell’s (SJSC) PCE is naturally limited by thermalization and transmission, the two main sources of loss in photovoltaics. The impact of each loss source on the figures of merit of a solar cell depends on the active material bandgap energy, the energy threshold for absorption in a semiconductor. Whereas decreasing the bandgap energy leads to an increase in short circuit current (ISC) due to reduced photon transmission, the open circuit voltage (VOC) decreases as the charge carriers thermalize to the edges of the bands. This trade-off imposes an optimum bandgap energy to achieve Sustainable Material Solutions for Solar Energy Technologies. DOI: https://doi.org/10.1016/B978-0-12-821592-0.00016-9 © 2021 Elsevier Inc. All rights reserved. 201

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the highest PCE. Fig. 7.1 shows the PCE of a SJSC as a function of the bandgap energy under 1 sun illumination (black solid line) and under full concentration of 46,000 suns (red dashed line) (Arau´jo and Mart´ı, 1994). In addition, it also shows the maximum theoretical achievable PCE as a function of the relevant bandgap energy for several photovoltaic concepts discussed in the next paragraphs, such as Intermediate Band Solar Cell (IBSC), Multiple Exciton Generation Solar Cell (MEGSC), Hot Carrier Solar Cell (HCSC), and Multijunction Solar Cell (MJSC). The raw data were extracted from various references, as indicated in the figure caption. The benefits of inserting nanostructures into solar cells include the increase in ISC, such as in the Quantum Well Solar Cell (QWSC), Quantum Dot Solar Cell (QDSC), IBSC, MEGSC and MJSC, or the increase in the VOC, such as in the HCSC (Luque and Hegedus, 2011). QWSC and QDSC are based in the bandgap engineering of (nano) structures to achieve bandgap energies closer to the optimum value (Nelson and Ekins-Daukes, 2014; Sogabe et al., 2016). Their operation principle does not differ from that of SJSC made of bulk materials, however, they lead to an increase in ISC without degradation in VOC, as has already been shown (Barnham et al., 1997). Therefore, using proper configurations of QWSC or QDSC as individual junctions of a series-connected MJSC leads to a more balanced solar photon

FIGURE 7.1 Maximum theoretically achievable PCE for several photovoltaics concepts. The raw data were taken from various references for SJSC (Arau´jo and Mart´ı, 1994), IBSC (Lee and Honsberg, 2011; Luque and Mart´ı, 1997), MEGSC (Luque and Hegedus, 2011), HCSC (Green, 2006), and MJSC (Micha and Silvares, 2019). The abbreviation 6JSC refers to a MJSC with six junctions and the bandgap energy for this case refers to the bottom-most junction. IBSC, Intermediate band solar cell; HCSC, hot carrier solar cell; MEGSC, multiple exciton generation solar cell; MJSC, multijunction solar cell.

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distribution among the junctions, a strategy for achieving high efficiencies in this type of device (Adams et al., 2011; Toprasertpong et al., 2015). Charge carrier confinement in nanostructures can bring other advantages to photovoltaic conversion, as well. The QD based IBSC, or QD-IBSC, relies on the double-step absorption of low energy photons using the quantum levels of QDs as an intermediate energy band created below the matrix bandgap energy (Okada et al., 2015). By a proper choice of materials, VOC can be kept the same while ISC increases, thus increasing PCE further up to 63% under full concentration (Luque and Mart´ı, 1997). Multiple exciton generation can dramatically increase ISC in MEGSC. The generation of more than one electron-hole pair per incident photon can be enhanced using nanostructures. Such effect has already been observed in QDs, nanorods, carbon nanotubes, and nanoribbons (Siemons and Serafini, 2018). Finally, hot carriers generated in QDs can be transported through the mini bands formed by the quantum levels, thus avoiding thermalization. This effect is used to increase VOC and enhance PCE in QD based HCSC (Sakho and Oluwafemi, 2019; Nozik, 2002). Along with the cited benefits, the use of nanotechnology in the industry, in general, is expected to improve the manufacturing processes by sparing natural resources and energy during material fabrication and by substituting rare and toxic materials (Rickerby, 2013; Wadia et al., 2009; Fiedeler, 2008).

7.2

Nanostructures and quantum mechanics

Wave mechanics is the description of matter in a low dimensional scale. This scale ranges from the diameter of the smallest atom with subnanometer dimensions to large molecules with hundreds of nanometers in size. The dynamics of particles, such as electrons and holes in a semiconductor crystal, for example, can also be described by wave mechanics being governed by a wave function ψð~ r Þ, which is the solution of the time-independent Schro¨dinger Eq. (7.1): -

-

-

-

r2 ψð r Þ 1 Vð r Þψð r Þ 5 Eψð r Þ

ð7:1Þ

in which Vð~ r Þ is the potential energy to which the charge carrier is subjected in the crystal and E are the possible energies it can assume. Details of the use of the Schro¨dinger equation to understand the electrical properties of different materials can be seen, for example, in Chuang (1995). We will illustrate the effect of the results of wave mechanics in a simple example. Consider a particle (e.g. an electron in a QW) in a one-dimensional potential energy well of a narrow-bandgap energy material A with length Lx surrounded by material B with a larger bandgap energy, let us assume infinite for the moment. This situation is exemplified in Fig. 7.2A(i) in which

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FIGURE 7.2 (A) Confinement in nanostructures: (a) 1D—quantum well, (b) 2D—quantum wire, (c) 3D—quantum dot. (B) Energies and wave functions for the infinite quantum well case. (C) Density of states (DOS) for structures of different dimensionality.



Ly ; Lz cLx Bnm. In this well, the particle is subjected to the potential energy described in Eq. (7.2a): 0; 0 , x , Lx ð7:2aÞ V ðxÞ 5 N; elsewhere ħ2 π2 5 n2 E1;L ; nAN  2mLx 2 rffiffiffiffiffi   2 nπx ϕn ðxÞ 5 sin ; nAN  Lx Lx En 5 n2

ð7:2bÞ ð7:2cÞ

Solving Eq. (7.1) with the potential energy given in Eq. (7.2a) leads to the solutions (7.2b) and (7.2c). The particle energy spectrum is discrete (horizontal lines in Fig. 7.2B) and not continuous, as it would be in a classical situation (a bulk material is not a classical system, but the energy level distribution is essentially continuous). This is due to the confinement that the well creates for the particle. In this system, the optical transitions are only allowed for photons with energy matching the difference between two levels. The particle’s wave function ψðxÞ can always be written as a linear  2combination of the eigenstates ϕn ðxÞ and the square of its absolute value ψðxÞ is the probability density distribution, which represents the probability of finding the particle between x and x 1 dx. Additionally, another important quantity is the density of states (DOS) of the system (Fig. 7.2C), which describes the number of allowed states per unit volume within an energy interval between E and E 1 dE. The DOS directly impacts the optical transition rates. In Fig. 7.2C, it is possible to see how the DOS is affected by the dimensionality of the nanostructure.

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The results described above are similar to the real case in which a charge carrier is confined in a well for which the potential energy barrier is finite. The potential energy barrier in this situation is a value V0 which is normally related to the difference in the QW and barrier material’s bandgap energies and is not the same for electrons and holes. Electrons notice a potential energy barrier given by the offset in the QW and barrier’s conduction band (CB), hereafter named CBO. For holes, the offset in the QW and barrier’s valence band (VB) is named hereafter VBO. Additionally, such results can be extrapolated for the cases with confinement in two dimensions, structures called quantum wires [Fig. 7.2A(ii)], when Lx c Ly ; Lz Bnm, or three dimensions, structures called QDs [Fig. 7.2A(iii)], when Lx ; Ly ; Lz Bnm. Controlling the nanostructure dimensions, which can be achieved by several growth techniques, allows for bandgap engineering. The optical characteristics of a nanostructured material are then modified when compared to the bulk counterpart and can in many cases lead to improvement in solar cell PCE, as it will be shown in next sections.

7.3

Quantum wells in solar cells

A QW structure is formed when the charge carriers (electrons in CB and/or holes in VB) are confined in 1D by a potential well with dimensions in the range of the carrier’s de Broglie wavelength. This can be achieved by material stacks in which the material with the lower bandgap energy has a thickness in the nanometer scale and is surrounded by the others, as shown in Fig. 7.2A(i). In this configuration, the allowed energy levels for the charge carriers are changed due to the appearance of discrete levels in the QW material. Therefore, bandgap engineering in QW structures can be achieved by materials’ choice and system’s geometry. Fig. 7.3A represents the energy band diagram of a stack of materials named QW and B, representing the QW and the barrier in a multiple QW system (MQW), respectively. The energy level distribution in the CB and VB (red horizontal lines in Fig. 7.3A) is altered due to the confinement of the particles (electrons in CB and holes in VB) in the QW material, as exemplified in Fig. 7.2B. The CBO and VBO, the QW and barrier widths (LQW and LB , respectively), the material lattice parameters (aQW for the QW and aB for the barrier) and other material properties define an effective bandgap energy EgEf that can be tuned for a specific application, according to Eq. (7.3) (Sayed and Bedair, 2019). EgEf 5 EgQW 1 ΔEgstrain 1 ΔEgQSE 1 ΔEgQCSE

ð7:3Þ

In Eq. (7.3), EgQW is the QW material bandgap energy, ΔEgstrain is the QW material bandgap energy shift due to elastic strain, ΔEgQSE is the shift due to quantum size effects, and ΔEgQCSE is the shift due to the quantum confined

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FIGURE 7.3 (A) Energy band profile of a stack of materials QW and B indicating the relevant energy definitions, as described in the text. (B) and (C) represent the same system under different values of LB leading to multiple quantum well (MQW) and superlattice (SL), respectively.

Stark effect under applied bias. For a detailed description of each term, please refer to (Sayed and Bedair, 2019). The insertion of QWs in a solar cell structure decreases the energy threshold for absorption with respect to the barrier material, as EgEf # EgB , allowing for the absorption of additional solar photons. A larger number of charge carriers causes ISC to increase. On the other hand, the existence of multiple levels in the QWs and the lack of charge carrier confinement in the QW plane allows for reduction of the VOC (Arau´jo et al., 1994; Luque et al., 2001). For sufficiently small values of LB the wave functions of the successive QWs are superimposed, and the MQW (Fig. 7.3B) becomes a superlattice (SL in Fig. 7.3C). In this regime, the discrete energy levels in the SL are close enough to be treated as mini bands. SL mini bands can be used to transport hot carriers before they thermalize to the bottom of the bands (the fundamental energy levels in MQWs or the lowest mini band in SLs), thus increasing the energy delivered by the charge carriers to the contacts during operation. Furthermore, this causes VOC to increase, which is the benefit used in HCSC for PCE optimization (Luque and Hegedus, 2011). The use of MQW in solar cells has been first explored in the 1990s with the AlGaAs/GaAs and strained InGaAs/GaAs systems (Barnham et al., 1997). However, only the latter material combination led to EgEf in the range of interest and has been further developed (Ekins-Daukes et al., 2001). The ISC increase in this system is compensated by a VOC reduction as more QW are stacked and strain accumulates, thus limiting the PCE gain (Ekins-Daukes et al., 2001). Strain compensation has been proposed to avoid the creation of misfit dislocation during growth, which are responsible for charge carrier trapping and consequent PCE reduction (Ekins-Daukes et al., 2001). In this technique, a material with smaller lattice parameter aB, such as GaAsP or GaInP, is used as barrier material to compensate the strain ε between the InGaAs QW, with lattice parameter aQW, and the host substrate, with lattice parameter aS. Eq. (7.4)

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correlates the average strain ε to the parameters of the stacked structure and the host substrate (Ekins-Daukes et al., 2001): ! AQW LQW aQW a2B 1 AB LB aB a2QW a 2 aS 5 2 aS =aS ð7:4Þ ε5 aS AQW LQW a2B 1 AB LB a2QW in which a is the averaged lattice parameter, which is calculated as the geometric mean between barrier and well materials’ lattice parameters in the zero stress method developed by Ekins-Daukes et al. (2002). The weights involve the thicknesses, relaxed materials’ lattice parameters and elastic stiffness coefficients AQW (for the QW) and AB (for the barrier) of the layers. It is important to note that the layer thicknesses in a strained system are limited to the critical thickness of each material in the system (Matthews and Blakeslee, 1974). The critical thickness is the maximum thickness that a strained material can have before relaxing. Table 7.1 presents results on the figures of merit obtained with QWSC in the last decades. Ekins-Daukes et al. (2001) studied both systems (strained InGaAs/GaAs and strain compensated InGaAs/GaAsP) showing a huge improvement for the one in which strain balancing was applied. Fujii et al. (2014) were able to stack up to 100 period of QW and barrier including interlayers in between them to relieve strain during growth, showing an increase of up to 20% in ISC, with further improvement in PCE. In another study (Toprasertpong et al., 2015), the same group was able to show a broad tunability in the absorption threshold in the strain compensated InGaAs/GaAsP

TABLE 7.1 Literature results for quantum well solar cells (QWSCs). The results are always compared with the proper reference. System

Ref ISC =ISC

Ref VOC =VOC

η=ηRef

Ref.

10 MQW InGaAs/GaAs (strained)

1.03

0.88

0.89

Ekins-Daukes et al. (2001)

23 MQW InGaAs/GaAs (strained)

1.04

0.67

0.65

Ekins-Daukes et al. (2001)

20 MQW InGaAs/GaAsP (strain compensated)

1.02

0.98

0.99

Ekins-Daukes et al. (2001)

70 MQW InGaAs/GaAsP (strain compensated)

1.11

0.91

0.96

Fujii et al. (2014)

100 MQW InGaAs/GaAsP (strain compensated 1 interlayer)

1.20

0.88

1.02

Fujii et al. (2014)

30 MQW InGaAsP/InGaP (strain compensated)

1.16

0.90

1.04

Hashem et al. (2016)

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system from 1.42 eV down to 1.15 eV. Hashem Sayed et al (Hashem et al., 2016) started studying the InGaAsP/InGaP system for a higher EgEf range of 1.651.82 eV, whereas Farrell et al. (2011) studied the system InGaN/GaN for extending the threshold absorption from 3.26 eV down to 2.38 eV in nitride compounds. MJSC is a photovoltaic concept that can benefit from MQW structures. In this concept, two or more single junctions are stacked, allowing for the use of different portions of the solar spectrum by each junction, reducing losses and increasing the conversion PCE. When the proper bandgap energy combination is used in a MJSC, the solar spectrum is absorbed in a more balanced way by the junctions leading to the highest possible PCE. QW can aid in the MJSC optimization process by the fine tuning in EgEf with the control of the nanostructure properties. Fig. 7.4 presents the MJSC maximum PCE as a function of the number of junctions and incident spectrum, as calculated by detailed balance theory (Micha and Silvares, 2019). The inset in each bar is the best bandgap energy combination leading to such efficiencies. It is possible to see that for a MJSC with more than two junctions, the best bandgap energy combination always shows at least one junction with a bandgap energy between 1.0 and 1.4 eV. For conventional systems based on Ge or GaAs substrate, suitable combinations for the intermediate junctions are normally not possible due to the absence of materials with compatible lattice parameters and bandgap energies within this range. The insertion of QW structures in the intermediate junctions brings a solution to this well-known problem. Recently, a commercial triple junction solar cell using the strain

FIGURE 7.4 Theoretical limiting PCE of MJSC as a function of the number of junctions for different incident spectra (Micha and Silvares, 2019). The values inserted in each bar are the best bandgap energy combination leading to such PCE (Green et al., 2020).

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compensated InGaAs/GaAsP MQW system with a PCE of 41% has been demonstrated reinforcing the potential of such strategy (Browne et al., 2013). The strain compensated InxGa12xAs QW on InyGa12yP system for use in the low EgEf range between 1.01.4 eV was investigated to a lesser extent. The expected advantage of using such material combination in MJSC is the high radiation hardness for space applications due to the use of InGaP as barrier material (Dharmarasu et al., 2001). By varying the layer widths LQW and LB, and the alloy compositions x and y, Winter et al. (2017) reported a theoretical study showing EgEf ranging from 0.71.3 eV. With such tunability, it was possible to indicate in which applications this strain balanced system could be used without exceeding the InGaAs critical thickness. Here, we update such results, expanding them to the cases in which a GaAs interlayer (of thickness LI) is used to relieve the strain between barrier and QW materials avoiding the formation of misfit dislocations, as described in Winter et al. (2018). Table 7.2 shows the alloy compositions

TABLE 7.2 Parameters of the strain compensated InyGa12yP/GaAs/ InxGa12xAs/GaAs/ InyGa12yP QW system leading to EgEf in the range of the optimum values for application in MJSC. Optimum values for x and y are presented for different number of junctions and solar spectra. LI, LQW and LB were fixed as 2, 10 and 10 nm, respectively. # Junctions

Spectrum

Junction (energy [eV])

x/y

1

AM0

1st (1.26)

0.171/0.334

AM1.5 g

1st (1.34)

0.094/0.398

AM1.5d ( 3 1000)

1st (1.12)

0.313/0.230

AM0

2nd (1.21)

0.221/0.296

AM1.5 g

2nd (1.36)

0.075/0.414

AM1.5d ( 3 1000)

2nd (1.18)

0.251/0.273

3

4

5

6

AM1.5 g

2nd (1.12)

0.313/0.230

AM1.5d ( 3 1000)

3rd (1.37)

0.066/0.422

AM0

3rd (1.22)

0.211/0.303

AM1.5 g

3rd (1.31)

0.123/0.373

AM1.5d ( 3 1000)

3rd (1.22)

0.211/0.303

AM0

4th (1.35)

0.084/0.406

AM1.5 g

3rd (1.20)

0.231/0.288

AM1.5d ( 3 1000)

3rd (1.07)

0.366/0.195

4th (1.37)

0.066/0.422

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x and y and the layer thicknesses LI, LQW and LB that shall be used in the InyGa12yP/GaAs/InxGa12xAs/GaAs/ InyGa12yP QW system to achieve the ideal EgEf for several MJSC applications. Additionally, they showed experimentally the tunability of EgEf from 1.4 eV down to 1.28 eV (Winter et al., 2018). Within the achieved energy range, it is possible to see from Table 7.2 that the developed QW system could be used as the active region of a junction showing the ideal bandgap energy for several applications. As an example, an 10/2/10 nm In0.221Ga0.779As/GaAs/ In0.296Ga0.704P structure shows an EgEf of 1.21 eV, which is the optimal bandgap energy for the second junction of a 3JSC operating under AM0.

7.4

Quantum wires (nanowires) in solar cells

As previously stated, for some time, there has been much interest in the study and production of nanostructured semiconductor devices for optoelectronic applications to broaden or improve what has already been achieved with thin film structures. One route of specifically interesting studies has been devices using semiconductor NWs. For NWs one can find distinct delimitations regarding its dimensions, but commonly one can define a NW as a structure with nanometer size in two of the three dimensions and at least micrometer scale in the third direction (see Fig. 7.2A(ii)). The higher the ratio between the length and the lateral dimensions (cross section), the higher the aspect ratio of the NW. Due to their morphology being so unique in comparison with thin film devices and even to QD based devices, it is possible to envision NWs acting in diverse ways within a solar cell. One can study and build a device using the semiconductor NW as the main active material for the absorption of radiation or as an embedded accessory to improve the absorption of an active matrix material (such as polymers and other type of materials). For the latter case, the discussion surrounding the production of the NWs is based mostly on homoepitaxy (growth of just the active absorption semiconductor, e.g. GaAs) and removal of the NWs from the substrate (if a substrate is even used (Barrigo´n et al., 2018)). But for the former case, one can make an analogy (see Fig. 7.5A) to the aforementioned discussion involving thin film solar cells (SJSC, MJSC, etc.) in terms of growing NWs with the same mature techniques of semiconductor epitaxy (e.g. metalorganic vapor phase epitaxy—MOVPE, molecular beam epitaxy—MBE, chemical beam epitaxy, and others). For this approach, there are several challenges regarding the production of the devices. The question which is often raised is if their promised improvement in solar cell PCE compensates their production costs. Regarding the benefits that the NW geometry can theoretically deliver, one should mention the flexibility in the material composition for heteroepitaxial structures. Material composition for planar growth is commonly

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FIGURE 7.5 (A) Schematic of a SJSC in NW technology. Scanning electronic microscopy images of nanowires grown by (B) selective area epitaxy (SAE) and by (C) vapour-liquid-solid (VLS).

strongly limited by the difference of lattice parameter between different materials due to stress accumulation in the layers. Even a standard way of growing the so-called self-assembled QDs relies on this stress accumulation to generate 3D islands. For NWs in an axially stacked structure, the entire lateral surface of the NW can be used to relax the stress and so inhibit the surge of defects in the structure (Ertekin et al., 2005). As an example of this benefit of using a NW structure, one can mention IIIV semiconductor MJSCs that frequently need to cope with a nonoptimal energy gap for lattice match purposes. The most conventional triple junction solar cell used in space applications is formed by a germanium bottom junction for the least energetic photons, an (In)GaAs junction for the middle range energy photons and an InGaP junction for the most energetic photons. (In)GaAs is chosen for the middle junction because of the lattice constant mismatch problem, since there is no bulk IIIV semiconductor that fulfills both conditions of having the correct absorption energy threshold (Fig. 7.4) and the same lattice parameter of a germanium substrate. On the other hand, it is possible, and it has been already done in multiple occasions, to grow IIIV semiconductor NWs with a lattice parameter highly different from that of the substrate. As an example, InAs NWs are commonly grown on GaAs substrates (and other substrates like Si) for optoelectronic devices since years (Bjork et al., 2002), even though the mismatch between InAs and GaAs is around 7% at 300K. Considering the cost of substrates, there is a high industrial interest to develop structures that could be grown on Si wafers. Both in the case of the Si wafer being part of the final device and in the case of the NWs being removed to be embedded in another material, savings involving this exchange of substrates reaches at least 10 times, if wafer reuse is not applied. Multiple publications have already shown that growth of IIIV semiconductor NWs directly on Si substrates is possible even with lattice mismatches of

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the order of 12% in the case of InAs NWs on Si (Martensson et al., 2004). So, as a prospect, one could build a MJSC of IIIV semiconductors entirely based on the NW technology and using the Si wafer solely as the base for device integration. Despite the cited advantages, there are some general technological limitations for the production of NWs that arise from their unusual morphology. Firstly, to grow NWs homogenously on the surface of a substrate, one typically aims at vertically grown structures. This means that fundamental changes have to be made to the growth procedures such as substrate selection, surface preparation and growth parameters, which are very far from the conditions typically used for thin film technology. For the substrate orientation the most basic change is substituting the usual (001) oriented wafers by (111) oriented wafers. This comes from the fact that the (111) orientation under typical growth conditions by MOVPE and MBE is the one that has the higher growth rate. So, under these circumstances, instead of obtaining a thin film growth, there is the emergence of high aspect ratio structures perpendicular to the surface. The wafer choice is still not sufficient for NW growth. There is a need for thorough wafer surface preparation in a way to catalyze and funnel the flow of adatoms to very narrow areas on which the NWs will begin to grow. There are multiple ways to achieve this catalysis. The two most studied ones are the socalled vapor-liquid-solid (VLS) method, mainly with gold nanoparticles, and selective area growth or epitaxy (SAG or SAE) with a pregrowth mask (both are shown in Fig. 7.5B and C). Each have their own advantages and disadvantages and have been investigated in parallel in the past decades. As of 2020, typically higher aspect ratio NWs with lower occurrence of twinning (Kratzer et al., 2012) are achieved with the VLS method, while higher homogeneity among NWs with a lower impurity incorporation (Spirkoska et al., 2009) are typically achieved with the SAE method. As for the growth conditions of NWs, the challenge is usually to control the incorporation of the adatoms on the NWs either axially or radially. In regard to growing heteroepitaxial structures, one can think of axially stacked or radially enveloped materials. Even though there are devices based in radially structured NWs (nanopillars/nanorods depending on the final aspect ratio of the structure), the aim for solar cells has been mostly to build axially structured heteroepitaxial NWs. Therefore, if one thinks about these stacked NWs, the growth conditions should account for inhibiting phenomena like tapering and kinks that are inherent of NW morphology, but could lower important characteristics such as homogeneity and crystal quality. In addition to the bottom-up approach discussed above, there is also the possibility of the top-down approach. This technique consists in growing planar structures and use postgrowth procedures to build the NWs. This technique has, as main advantage, the opportunity to still use the same mature growth technology for planar structures, but having to obey the same

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composition limitations previously commented. Still as a disadvantage, there is the investment needed on the development of the technology for the postgrowth procedure, considering that it requires nanolithography for some steps (e.g., etching of the NWs). Furthermore, yet thinking about axially grown NW solar cell, some considerations can also be made devicewise. The transport properties in optoelectronic devices are as important as the previously discussed material engineering to maximize their PCE. Specifically, in the case of a nanostructure with high surface to volume ratio, preventing loss of the photogenerated current is necessary. One general area of research for optoelectronic devices is mesa passivation to avoid leakage current through defects on the surface. Fortunately, it has been demonstrated that when dealing with pn junctions in NWs the current is channeled through the axis of the NW by the depletion region formed on the NW surface due to the elevated refractive index of the usual IIIV semiconductors (Kupec et al., 2010). This phenomenon ends up being also beneficial for the further device fabrication, consisting of planarizing the structure, filling the regions between NWs and for the subsequent metallic contacts deposition. As for now, the main limitations for the devices fabricated using NWs are transport based, so boosting the devices’ response in this respect is key. Likewise for the transport properties, there is an optical virtue that the NW morphology bears, which is its behavior as an antennae for the incident radiation (Haverkort et al., 2018). Through optical models for the absorption and emission angles, the NW could theoretically boost the open-circuit voltage to beyond the planar solar cell limit. This means that the NW devices can benefit from its geometry to work as a natural concentrator for the incident light and thus achieve higher efficiencies. For industrial purposes, the devices should not be thought of as based on only one single NW but on an array of them working in parallel. As mentioned above, the production of an array of NWs introduces the challenge of obtaining a homogeneous distribution. But it also introduces two important parameters, besides diameter and length of the NW, namely the type of array (usually honeycomb distribution) and the distance between the NWs (pitch). Both parameters are defined during the surface preparation phase, whether it is the VLS or the SAE method, and these two parameters can be tuned to enhance the solar cell performance. The ratio of surface area that is covered by NWs can be called the filling factor. One way to use these parameters to enhance the device’s performance is to use the array of NWs as a way to trap incident radiation and increase the interaction between the radiation and the active material. The role of these parameters has been investigated not only for solar cells but also for photodetectors and lasers (Anttu and Xu, 2010). As a theoretical example, using models for InP NWs, if one builds an array with NWs with 180 nm diameter and 2000 nm length, and a filling factor of only 16%, the array will be able to absorb around 90% of the incident radiation.

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Analyzing the advantages mentioned in the previous paragraphs, theoretical studies have been made over the years and some could be given as examples of the potential of NWs as active material for these devices. Mokkapati and Jagadish (2016) showed through numerical calculations that using InP as the active material (with energy gap of 1.34 eV at 300K) under AM1.5 solar spectrum, a NW array solar cell has PCE of 32.5%, which is slightly above (31%) the case of a bulk solar cell with same material and measurement conditions. Still, using InP as the material of interest, Kupec et al. (2010) showed that, for an array of InP NWs with 180 nm diameter and with pitch of 360 nm (20% surface filling factor), one could theoretically reach 30% PCE, which is double of what would be achieved with a planar solar cell using equivalent amount of material. Besides the theoretical studies, some experimental results have also been presented and they show that it is feasible to build an array of NW SJSC that has equivalent characteristics to a planar SJSC, but with a lower amount of total material used. As an example, Hultin et al. (2016) showed that an InP NW SJSC reached 86% of the short-circuit current density in comparison with the best planar InP SJSC (Wanlass, 2017) on the market having used only 6% of the total volume of deposited material. Up to now, SC based on NWs has been developed in prototypical laboratory conditions mostly as SJSC. The step of introducing tunnel junctions to connect different junction into a MJSC is being actively investigated in the past decade (Zeng et al., 2018).

7.5

Quantum dots in solar cells

QDs, formed by a material with a bandgap energy lower than the embedding matrix (barrier) and nanometer size in all dimensions, create 3D potential wells producing charge carrier localization and discrete electronic states, as represented in Fig. 7.2A(iii) and 7.2B. Depending on the band alignment between the barrier and the QD material, different electron and hole spatial distribution can be achieved which affect their lifetimes, defining type-I or type-II QDs. While for the type-I QDs electron-hole pairs are both confined in the QD material, for type-II only one type of charge carrier is confined within the QD, whereas the other remains in the barrier material. The control of these 3D structures, via material composition and QD dimensions (Lx ; Ly ; Lz ), can be used to tune the transition energies and to enhance the charge carrier lifetimes, influencing optoelectronic properties of the system, which is particularly important for photovoltaic applications. In QD solar cells, additional electron-hole pairs can be produced by absorption of subbandgap energy photons, which does not occur in a standard SJSC, inducing an ISC increase. Differently from the QW, the QD deltafunction like DOS (Fig. 7.2C) is isolated from the CB, with the consequence of possibly maintaining the high Voc of the host material. This concept is

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explored in QD-IBSC, which is one of the proposed alternatives to overcome the Shockley-Queisser limit in SJSC (Luque and Mart´ı, 1997). In QD-IBSCs an intermediate band (IB), between VB and CB, is formed via the overlap of the QD confined states (Fig. 7.6A). The formation of this IB in QD-IBSC with an individual and isolated charge carrier population provides possible transitions not only from the VB to the CB (EG), but also from VB to IB (EM) and from IB to CB (EL). From detailed balance theory, A. Luque and A. Mart´ı showed that a combination of EG 5 1.95 eV, EM 5 1.24 eV and EL 5 0.71 eV maximizes the theoretical PCE to 63% under full concentrated solar spectrum (Luque and Mart´ı, 1997). As discussed, this can be achieved by controlling the QD system characteristics. The InAs/GaAs system (Fig. 7.6B shows a typical structure) has been the most extensively studied one for QD-IBSCs over the last years, due to wellestablished growth conditions, especially for other optoelectronic device applications, as diode lasers and photodetectors. Thus, even if the GaAs barrier bandgap (EG 5 1.42 eV at room temperature) differs from the ideal value, making this system not ideal for IBSC, it has been considered as a good proof of concept. Nonetheless, InAs/GaAs QD-IBSC have been generally failing in outperforming SJSC references structures without QDs. Increasing the solar cell PCE, associated to the incorporation of QDs, has been demonstrated in few cases, such as by Bailey et al. (2012), that applied strain compensation strategies to guarantee a dislocation-free structure. It is clear that in order to achieve an increase in Isc, in comparison with a conventional SJSC reference, a stack of several high density QD layers is necessary. On the other hand, the fabrication of high density QD stacked layers preserving high crystal quality and long carrier lifetime is a critical issue (Mart´ı et al., 2006). Generally, in QD-IBSC the presence of recombination centers or misfit dislocations associated to high strain fields induce

FIGURE 7.6 (A) Simplified IBSC bandgap energy diagram and (B) schematic diagram of an InAs QDs/GaAs solar cell. Adapted from Weiner, E.C., Jakomin, R., Micha, D.N., Xie, H., Su, P.-Y., Pinto, L.D. et al., 2018. Sol. Energy Mater. Sol. Cell, 178, 240248 with permission.

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a degradation in VOC when compared to reference cells without QDs (Okada et al., 2015; Mart´ı et al., 2007; Tutu et al., 2012). Another approach, explored in the last years, is the use of wide bandgap matrix material (EG around 1.81.9 eV), such as AlGaAs or InGaP, allowing more appropriate EG, EM, EL absorption threshold energy values for QDIBSCs. The drawback in this case is the need to use ternary or even quaternary materials during the QD-IBSC structure growth, which, especially at low growth temperature cycles, required in QD growth, can create several undesirable recombination centers and structural defects. Recently, alternative structures such as InP QDs on InGaP, with more appropriate band offsets and band alignment, have been studied. In this case it is possible to achieve type-II QDs, resulting in a spatial separation between electrons and holes, which potentially increases the photocarrier lifetimes (Okada et al., 2015; Kum et al., 2018; Tayagaki and Sugaya, 2016). In this section, we present results on the investigation of InAs QDs/GaAs system’s growth methods to minimize the formation of defects around the nanostructures during the stacking of QD layers. Then, we present alternative structures of In(Ga)As or In(As)P grown on wide bandgap matrices for typeI and type-II QDs.

7.5.1

InAs quantum dots on GaAs

The production of self-assembled InAs QDs on GaAs is commonly obtained by the Stranski-Krastanov (SK) growth mode, established in the 1990s using MBE, MOVPE and chemical beam epitaxy for different optoelectronic devices such as lasers (Alferov, 2001; Pearsall, 2013). In the SK method, a QD material is deposited on a barrier material with a large lattice mismatch. As a first step a thin two-dimensional wetting layer is formed, followed by islands (QD) in a transition between 2D and 3D growth mode (Torelly et al., 2016). The MOVPE growth technique, in SK mode, presents great advantage for large scale industrial applications. However, in comparison to MBE, it is characterized by a faster QD formation that can induce island coalescence, bimodal QD size distributions and postgrowth morphology evolution as ripening or interdiffusion. Another crucial aspect, in device applications, is that the lattice mismatch strain, necessary for the QD formation, accumulates as the QD layers are stacked, possibly leading to the generation of misfit dislocations and strain-induced defects. Since in QD-IBSCs ten or more stacked QD layers, interposed with spacer layers of higher energy gap materials, are necessary in order to reach a sufficient absorption volume, a great amount of residual strain can be expected. The strategies to avoid plastic relaxation and limit strain-induced defects in the QD structure are various. In the InAs/GaAs case, the compressive strain accumulation can be attenuated using thick spacer layers between the dot layers ( . 80 nm), which may also prevent photocurrent tunnel escape

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directly from the IB to the CB through the barrier layers (Antolin et al., 2010). Other methods contemplate strain compensation layers, as GaP layers within the intrinsic region of a p-i-n InAs QDs/GaAs solar cell. In the case of Hubbard et al. (2008), a tensile strained GaP layer of 1.4 nm has been grown within the 10 nm GaAs spacer layer in order to compensate for the InAs compressive strain and reach an average neutral strain. The authors have demonstrated, for 5 layers QD solar cells, an extraordinary increase in PCE from 3.7 to 10.8 %, for 1 sun AM0 conditions, in the case of the use of compensation layers. In addition to strain-balancing techniques, a reduced InAs QD layer coverage of 1.8 monolayers has been used by Bailey et al. (2012) to avoid island coalescence and bimodal QD growth distributions, obtaining a high density of improved optical quality QDs. In this way, they demonstrated a 14.3% PCE for the 40-period QD cell, showing an improvement of 3.6% in comparison to the GaAs SJSC reference. Other approaches are based on partial QD covering followed by annealing. The partial capping followed by annealing, also known as an Indiumflush method, has been firstly described by Wasilewski et al. (1999) and by Fafard et al. (1999). During the annealing step, or the growth of the spacer layer at high temperature (around 700 C), the tip of the QDs that exceeds the capping layer thickness can be removed inhibiting the formation of QDs higher than the cap layer. This allows to convert the lens shaped QD into disks with almost equal height, leading to a homogeneous size distribution and preventing the formation of QDs with heights above the critical dimension for plastic relaxation, thus reducing the compressive strain during QDs stack fabrication. Kalyuzhnyy et al. (2016) grew 57 nm thick GaAs capping layers at low temperatures (520 C) after QD growth followed by spacer layers of 35 nm grown at 700 C, used to attenuate strain effects. Therefore, for a 10 period QD solar cell an excellent quantum efficiency with no degradation in comparison with the reference cell has been obtained. Similarly, in our previous studies (Weiner et al., 2018; Micha et al., 2016; Weiner et al., 2016) we have shown the influence of some growth conditions, namely capping layer thickness and annealing temperature during the In-flush process, on the structural quality of InAs QDs/GaAs and, as a consequence, on the device performance. The QD solar cell structure investigated by Weiner et al. (2018) is schematically shown in Fig. 7.6B. To grow such structure, TMAl, TMGa, TMIn, and AsH3 were used as aluminum, gallium, indium and arsenic sources, respectively, in an Aixtron AIX200 horizontal reactor at 100 mbar. CBr4 and SiH4 were used for p-and n-type doping, respectively. The 1 μm thick active region consists of ten InAs QD layers, grown at 490 C with a density of 1.8 3 1010 cm2 and Si doped (n-type) at B 2 3 1017 cm23. After the QD deposition, a GaAs capping layer 3 or 6 nm thick was grown at the same temperature. At this point, the In-flush step was performed by raising the temperature to 630 C in samples 3630 and 6630 or to 700 C in samples 3700 and 6700. The sample numerical labels C-T represent the

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thickness of the capping layer (C) and the annealing temperature (T). Finally, the 90 nm thick GaAs barriers were grown at the same temperature of the In-flush step and the cycle was repeated for the subsequent QD layers. Solar cells were processed by selectively depositing a double layer MgF/TaO antireflective coating and metal ohmic contacts to the n-type (Ti/Ge/Pd/Au) and p-type (Pd/Zn/Au) contact layers. High-angle annular dark field (HAADF) images (Fig. 7.7) obtained from the samples show no evidence of plastic relaxation and threading dislocations in the layer structures. On the other hand, it is possible to notice that the annealing temperature influences the QD morphology. Generally, the maximum QD height is the same as that of the capping layer, since the In-flush technique removes the tips of the QDs. As can be seen in Fig. 7.7B, besides the wetting layer, a top In(Ga)As layer is also observed for samples grown with a 6 nm capping layer. For sample 6630, QDs may present different shapes: lens, spool and disk, indicating that the QD morphology and dimensions are not uniform. Differently, for annealing at 700 C (Fig. 7.7C and D), only disk shaped QDs are present. The top In(GaAs) layer is thicker for samples annealed at 700 C (around 0.9 nm). When a 3 nm capping layer is used, the HAADF STEM

FIGURE 7.7 HAADF-STEM (scanning transmission electronic microscopy) images: (A) QD region of sample 6630 (B) QD with higher magnification of sample 6630 (C) QD region of 6700 (D) QD with higher magnification of sample 6700 (E) QD for 3700 sample. Adapted from Weiner, E.C., Jakomin, R., Micha, D.N., Xie, H., Su, P.-Y., Pinto, L.D. et al., 2018. Sol. Energy Mater. Sol. Cell, 178, 240248 with permission.

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images indicate coalescence of QDs forming a continuous 3 nm layer including a diffuse region above each QD layer (the 0.9 nm top layer) and a wetting layer of about 0.6 nm (Fig. 7.7E). The disk shape is then a consequence of the closeness between the top In(Ga)As layer and the wetting layer. The capping layer thickness and annealing temperature also influence the optical quality of the QD solar cell structures. As shown by Weiner et al. (2016), in low temperature (15K) photoluminescence (PL) spectra, both the reduction of the capping layer thickness and the increase of the In-flush temperature induce a blue shift of the peak energy (as shown in Fig. 7.8A), a reduction of the peak full width at half maximum and a higher relative peak intensity, indicating better material quality and an increased homogeneity of the QD size distribution. The increased intensity can be attributed to lower density of nonradiative recombination centers for samples subjected to higher annealing temperatures and having thinner capping layers. On the other hand, by means of theoretical calculations of the QD electronic structure [performed with the Nextnano software tool (Birner et al., 2007)], we have explained the PL blue shift arguing that the In-flush step at higher temperatures accentuates the QD shape modification via lateral migration of indium from the top regions of the QDs, which occurs also during the capping layer deposition. The higher annealing temperature intensifies the degree of indium/gallium intermixing due to lateral indium migration from the QDs to the capping layer regions, so that the QDs annealed at 700 C interconnect forming a thin In(Ga)As layer with a Ga content which has been estimated to be around 50% (In0.5Ga0.5As layer). In fact, calculations of compressive strain, obtained from high resolution X-ray diffractometry, indicate that also the strain decreases for lower capping layer thicknesses and higher annealing temperatures. This suggests that the reduced nonradiative recombination, observed in PL analysis, could be

FIGURE 7.8 (A) Photoluminescence (PL) spectra at 15K of samples with 3 nm thick capping layers, and In-flush at 630 C (black line) and 700 C (red line) (B) J-V curves of the solar cells obtained at 25 C and AM1.5 G. Adapted from Weiner, E.C., Jakomin, R., Micha, D.N., Xie, H., Su, P.-Y., Pinto, L.D. et al., 2018. Sol. Energy Mater. Sol. Cell, 178, 240248 with permission.

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attributed to the decrease in accumulated compressive strain in the QD structures with 3 nm capping layers and annealed at 700 C. The thinner capping layer influence on the average strain can be associated to the truncated height of the QDs, while at higher annealing temperatures the interdiffusion mechanism tends to attenuate the misfit strain energy, since In0.5Ga0.5As has a much smaller lattice parameter than InAs. Fig. 7.8B shows J-V curves of the fabricated QD solar cells under standard test conditions. The most remarkable result is the strong increase in PCE associated with the annealing temperature of 700 C in comparison to the samples annealed at 630 C (η 5 4.4% and 5.6% respectively for 6630 and 3630, η 5 11.5% and 15.9% respectively for 6700 and 3700). Therefore, this result is in agreement with the better optical quality observed from PL measurement indicating once more the reduction of the amount of strain, which is expected to lead to a lower density of nonradiative recombination centers, especially at the QD interfaces of the 3700 and 6700 structures. We also observe an enhancement in PCE for the sample with thinner capping layer induced by a quite high increase in Voc shown by sample 3700 in comparison with 6700. This can be associated to the higher uniformity of QDs, the complete removal of the normally defective QD top and by the reduction of strain promoted by the thinner capping layers. The importance of the capping layer deposition procedure is evident since the energy conversion efficiencies can be enhanced by up to 83% when this procedure is well controlled. Such benefits can also be noticed in the External Quantum Efficiency (EQE) (Weiner et al., 2016), which represents the ratio between the charge carriers collected by the solar cell and the incident photons, and is obtained from spectral response measurements at 25 C under bias light and short circuit conditions. Weiner et al. (2016) observed that EQE is lower for wavelengths shorter than 800 nm for the samples grown with an annealing temperature of 630 C compared to the reference. This result can be associated with degradation in the upper layers (top of the intrinsic and the emitter layers) due to the accumulation of strain during the stacking of QD layers. However, this effect is almost indistinguishable for an annealing temperature of 700 C, indicating once more that in this case the capping layer procedure positively influences the structural and optical material quality. For subbandgap absorption of wavelengths longer than 870 nm, we observe a significant photocurrent signal from QDs, which is though not sufficient to compensate for the typical VOC loss of the QD solar cells, therefore the GaAs reference cell still presents higher PCE (η 5 20%). This is probably also due to the limited QD density and number of layers, noting that the cases in which the QD solar cells exhibited a higher PCE than the reference, the number of QD layers was 40 (Bailey et al., 2012). It should be noted that the introduction of quantum dots can also increase the concentration of electrically active point defects, which affect the solar cell peformance. The low temperature required for the deposition of quantum dots is

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responsible for the enhanced incorporation of such defects, as discussed in (Collazos et al., 2021).

7.5.2 In(Ga)As or InAsP quantum dots on wide bandgap material barriers For applications in QD-IBSCs, materials with larger CBO (and consequently higher EL values) with respect to InAs QDs (such as InGaP or AlGaAs) to be used as barriers have obtained a growing attention in the last years. These material systems present energies closer to the ideal combination, offering the possibility to reduce the photocarrier thermal escape. For example, in InAs QDs on InGaP the intersubband energy is enlarged over 400 meV (Ramiro et al., 2015). Lam et al. (2016) studied the InAs growth on InGaP buffer, observing that the QDs coalescence was inhibited by the use of an Al0.33Ga0.67As thin layer between the InAs and the InGaP materials, which also limited As-P interdiffusion. Within the InGaP matrix, InAs QDs were grown at 495 C on a 3 nm Al0.33Ga0.67As layer and capped by a 6 nm layer of the same material. The AlGaAs, caused a reduced average size of the QDs compared to that obtained for quantum dots deposited on the GaAs surface and, consequently, a higher maximum QD density of 1.1 3 1011 cm2. Though the subband energy combination was demonstrated to be near the ideal values and the solar cell performance was improved by rapid annealing treatments, especially at 800 C, QD solar cells exhibited much lower PCE than the reference cell. In a previous work, we have also shown an InAs/AlGaAs system with the energy combination close to the ideal one (Jakomin et al., 2014), in which the optical properties are dramatically dependent on the chosen capping procedure. We have observed that samples subjected to the Indium-flush process exhibited a strong improvement of their optical properties in comparison with samples in which no Indium-flush was applied. Wide bandgap host materials are also exploited as interesting alternative structures for IBSC, including type-II QDs. The interest in this kind of system is the possibility to separate photogenerated carriers, the electrons being confined in the QDs, while the holes are located outside the QDs. As a consequence, the radiative recombination events should be limited, permitting longer photocarrier lifetimes and the appropriate characteristics for two-step photon absorption in IBSCs. Additionally, it is possible to tailor the structure to have an almost null VBO and, at the same time, a large CBO to approach the ideal transition energy combination. Tayagaki and Sugaya (2016) studied InP QDs grown by MBE on InGaP, demonstrating type-II InP QDs and a CBO of 0.35 eV. Time resolved PL in this case show a portion of QDs with slow decay times, longer than 30 ns for QDs with different nominal thicknesses and average height between 10 and 20 nm. Kum et al. (2018) presented an InP/InGaP solar cell with 3 InP QDs layers with 7.9% PCE, which decreased to 6.8% for 5 layers: a PCE still

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much lower than the SJSC reference of 17.3%. In that work all the QD cells were affected by an EQE decrease for wavelengths shorter than 700 nm in comparison with the reference, indicating the increase in strain-induced defects. For this reason, the authors claim that a strain-balancing method is crucial for future optimization of the QD structures. With the aim of finding a system with a EL, EM, EG combination near the ideal and type-II QDs, we have considered In(As)P/InGaP structures with different QD sizes and As concentrations (Rocha et al., 2019) using calculations of QD energy transitions with the Nextnano software [database taken from Birner et al. (2007)]. The IB-CB energy transition increases with the QD diameter: for a fixed 50% concentration of As and 20 nm height we calculated a 0.425 eV IB-CB energy for an 80 nm wide dot. On the other hand, for larger dot diameters the VB-IB energy transition decreases and deviates from the ideal value (1.15 eV for a dot of 40 nm width to 1.1 eV for a dot of 80 nm). Maintaining the QD height of 20 nm and its width of 40 nm, while changing the As concentration in the InAsxPx21 alloy, we observed that the higher the As concentration, the lower the VB to IB transition energy is, reaching a value near to the ideal energy of 1.2 eV for an As concentration of 50%. Inversely, the IB-CB transition energy increases with As concentration causing a CBO (which is an undesired trend because a large CBO is crucial to limit thermal carrier escape from IB to CB). It is clear that the ideal EL, EM, EG energies cannot be approached simultaneously in this system, but an appropriate compromise is a 60 nm wide dot with an InAs0.5P0.5 alloy. The presence and the origin of the type-II confinement can be studied calculating the confinement potential of the QD structure, including strain effects, as well as the probability density for electrons and holes. VB edges of the QD along the x axis, show that the VB of the heavy holes exhibits two potential energy peaks at the border regions of the QD. This band deformation is due to the compressive strain of the InAsP QD on the InGaP barrier, which is much larger at the dot edge than in the internal QD region (Rocha et al., 2019). For this reason, the hole probability density for a 20 nm high, 60 nm wide QD with 50% P content in the xy plane is concentrated at the border and at the base of the dot, as shown by Rocha et al. (2019). Thus, the holes are confined just outside the QD region while the electron probability density is spread inside the QD region, creating a spatial separation between holes and electrons. This effect is intrinsically associated to the compressive strain between the InP or InAsP QD and the InGaP barrier (in calculations with no strain we observe no spatial hole-electron separation). In fact, the biaxial strain for zinc blend lattice induces an anisotropy which would be absent in the case of isotropic strain (He et al., 2004).

7.6

Conclusions

Nanostructured solar cells can overcome the Shockley-Queisser limit and offer a low-cost solution for photovoltaics. QWs, NWs and QDs added to a

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solar cell alter its optoelectronic properties improving its interaction with sunlight, hence increasing its PCE. While some nanostructured photovoltaic concepts lead to an increase in short-circuit current (QWSC, QDSC, MJSC, MEGSC, IBSC), others (HCSC) show an increase in the open circuit voltage. Fundamentally, the enhancements are a consequence of the charge carriers confinement in the nanostructures. The introduction of QWs in solar cells allows for bandgap engineering by controlling the nanostructure dimensions. In this chapter, we discussed the use of strain compensated MQW systems as a solution to the well-known problem of current mismatching in MJSC. We showed theoretical and experimental results that demonstrate the benefits of the use of QWs, especially in raising the short-circuit current and hence the PCE of solar cells. Moreover, commercial QW based MJSC is already available. On the other hand, the use of NWs as the active material of solar cells in comparison with the traditional thin film structures is in its initial stages. Such system has already shown flexibility in terms of compositions, but reproducibility and processing still need further development for mass production. IIIV semiconductor based QD structures are promising for high efficiency solar cells. However, such system has not yet fully shown their potentiality, especially due to structural defects associated to the QD growth which are detrimental for solar cell performances. Strain compensated structures and/or In-flush technique are fundamental in order to attenuate strain-induced defects, especially in stacked QD layer structures. Another promising approach is the use of wide bandgap matrix, allowing more appropriate subbandgap absorption energies, and the formation of type-II QDs which could increase the photocarrier lifetimes. These different strategies can also be combined to increase the PCE of QD solar cells, especially preserving the VOC. With further development of the cited concepts and/or integration with others, nanostructured solar cells can contribute to improve the efficiencycost ratio of IIIV photovoltaics, enabling its use in various terrestrial applications in the near future.

Acknowledgments The authors acknowledge the PUC-Rio Semiconductor Laboratory’s staff in the development of the work on nanostructured solar cells and the support of FAPERJ, CNPq, CAPES, and FINEP. The authors also acknowledge Stefan Birner from Nextnano GmbH and his staff for the support in the last years.

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Micha, D., Weiner, E., Jakomin, R., Pinto, L., Pires, M., Souza, P., 2016. Influence of the quantum dot capping procedure on the density of defects in InAs/GaAs quantum dot intermediate band solar cells. In: 32nd European PV Solar Energy Conference and Exhibition (EUPVSEC), Munich. Mokkapati, S., Jagadish, C., 2016. Review on photonic properties of nanowires for photovoltaics. Opt. Express 24 (15), 1734517358. Nelson, J., Ekins-Daukes, N., 2014. Chapter 10: quantum well solar cells. In: Archer, M.D., Green, M.A. (Eds.), Clean Electricity from Photovoltaics. Imperial College Press, London, pp. 453489. Nozik, A.J., 2002. Quantum dot solar cells. Phys. E: Low-Dimen. Syst. Nanostruct. 14 (12), 115120. Okada, Y., Ekins-Daukes, N., Kita, T., Tamaki, R., Yoshida, M., Pusch, A., et al., 2015. Intermediate band solar cells: recent progress and future directions. Appl. Phys. Rev. 2, 021302. Pearsall, T., 2013. Quantum Semiconductor Devices and Technologies, 6. Springer Science & Business Media. Ramiro, I., et al., 2015. Wide-bandgap InAs/InGaP quantum-dot intermediate band solar cells. J. Photovolt. 5 (3), 840845. Rickerby, D.G., 2013. Nanotechnology for more sustainable manufacturing: opportunities and risks. In: Shamim, N., Sharma, V.K. (Eds.), Sustainable Nanotechnology and the Environment: Advances and Achievements. ACS, pp. 91105. Rocha, B.V., Jakomin, R., Kawabata, R.M., Dornelas, L.P., Pires, M.P., Souza, P.L., 2019. Transition energy calculations of Type II In(As)P/InGaP quantum dots for intermediate band solar cells. In: 34th Symposium on Microelectronics Technology and Devices (SBMicro), Sa˜o Paulo, Brazil. Sakho, H.M., Oluwafemi, O.S., 2019. Chapter 11—Quantum dots for solar cell applications. In: Thomas, S., Sakho, H.M., Kalarikkal, N., Oluwafemi, S.O., Wu, J. (Eds.), Nanomaterials for Solar Cell Applications. Elsevier, pp. 377415. Sayed, I., Bedair, S.M., 2019. Quantum well solar cells: principles, recent progress, and potential. J. Photovolt. 9 (2), 402422. Siemons, N., Serafini, A., 2018. Multiple exciton generation in nanostructures for advanced photovoltaic cells. J. Nanotechnol. 7285483. 2018. Sogabe, T., Shen, Q., Yamaguchi, K., 2016. Recent progress on quantum dot solar cells: a review. J. Photon. Energy 6 (4), 040901. Spirkoska, D., Arbiol, J., Gustafsson, A., Conesa-Boj, S., Glas, F., Zardo, I., et al., 2009. Structural and optical properties of high quality zinc-blende/wurtzite GaAs nanowire heterostructures. Phys. Rev. B 80, 245325. Tayagaki, T., Sugaya, T., 2016. Type-II InP quantum dots in wide-bandgap InGaP host for intermediate-band solar cells. Appl. Phys. Lett. 108, 153901. Toprasertpong, K., et al., 2015. Absorption threshold extended to 1.15 eV using InGaAs/GaAsP quantum wells for over-50%-efficient lattice-matched quad-junction solar cells. Prog. Photovol.: Res. Appl. Torelly, G., Jakomin, R., Pinto, L., Pires, M., Ruiz, J., Caldas, P., et al., 2016. Early nucleation stages of low density InAs quantum dots nucleation on GaAs by MOVPE. J. Cryst. Growth 434, 47. Tutu, F., Sellers, I., Peinado, M., Pastore, C., Wilis, S., Watt, A., et al., 2012. Improved performance of multilayer InAs/GaAs quantum-dot solar cells using a high-growth-temperature GaAs spacer layer. J. Appl. Phys. 111, 046101. Wadia, C., Alivisatos, A.P., Kammen, D.M., 2009. Materials availability expands the opportunity for large-scale photovoltaics deployment. Environ. Sci. Technol. 43 (6), 20722077.

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Chapter 8

Crystalline-silicon heterojunction solar cells with graphene incorporation Recep Zan1,2, Ali Altuntepe2, Tolga Altan2,3 and Ayse Seyhan1,2 ˘ O¨mer Halisdemir University, Nigde, ˘ Department of Physics, Nigde Turkey, 2Nanotechnology ˘ O¨mer Halisdemir University, Nigde, ˘ Application and Research Center, Nigde Turkey, 3 ˘ O¨mer Halisdemir University, Nigde, ˘ Department of Mechanical Engineering, Nigde Turkey 1

8.1 8.1.1

Heterojunction solar cells and graphene Heterojunction solar cells

Silicon (Si) is the most common material used in photovoltaic applications among semiconductor materials (Lucen˜o-S´anchez et al., 2019; Battaglia et al., 2016). Crystalline Si (c-Si) solar cells hold more than 90% of the global photovoltaic market share (Sampaio and Gonz´alez, 2017). The two most important reasons why silicon-based solar cells dominate the majority of the photovoltaic market are: Si is the most abundant material in nature, and the production cost of Si-based solar cells has been decreasing due to the improvement of solar cell manufacturing technologies (Talavera et al., 2019; https://www.energytrend.com/pricequotes/20201021-19671.html; Kuhlmann, 1963). With these advantages and scientific studies in recent years, tremendous progress has been observed in c-Si solar cells in terms of efficiency, repeatability, and stability (https://www.nrel.gov/pv/cell-efficiency.html; https://itrpv.vdma.org/web/itrpv/download; Green, 2009). While c-Si-based solar cells continue to dominate the PV market, efforts are made to use more advanced technological developments to reduce the cost of electricity from photovoltaics (Sampaio and Gonz´alez, 2017; Green et al., 2019). These developments offer solutions such as higher power generation, longer module life, and less cost per unit area. In recent years, one of the most critical developments in c-Si solar cells has been noted to be the increase in photovoltaic parameters in terms of high open-circuit voltage, efficiency, lowtemperature processes, and low-temperature coefficient (Masuko et al., 2014; Sustainable Material Solutions for Solar Energy Technologies. DOI: https://doi.org/10.1016/B978-0-12-821592-0.00007-8 © 2021 Elsevier Inc. All rights reserved. 229

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Yoshikawa et al., 2017b; De Wolf et al., 2012). The technology that offers the highest efficiency and the lowest cost among c-Si solar cell technologies is the undisputed silicon heterojunction solar cell technology with an Intrinsic Thin layer (HIT). The first c-Si heterojunction solar cell with amorphous hydrogenated silicon (a-Si: H) layers on c-Si surface was fabricated in 1983 by Hamakawa et al. (1983), Okuda et al. (1983). In the early 1990s, Sanyo (Panasonic as of December 2009) started working on HIT solar cells (Taguchi, 1990). The first silicon heterojunction solar cell structure fabricated by Sanyo is in the form of p-type a-Si: H layer deposited on the n-type c-Si substrate. The cell was found to have a relatively low Voc and FF value due to the high defect-state density at the a-Si: H/c-Si interface. In addition, the use of the thick p-type a-Si: H layer increased optical absorption, which caused a decrease in Jsc. As a result, it has been understood that the p-type a-Si: H layer should be as thin as possible, provided that it creates a good p/n junction structure. It was also observed that the Voc for this structure did not change depending on the thickness of the p-type a-Si: H layer. In this solar cell structure, the conversion efficiency of 12.3% was achieved with a thickness below 10 nm for p-type a-Si: H layer (Wakisaka et al., 1991; Tanaka et al., 1992). Later, Sanyo named the solar cell structure obtained by adding a thin intrinsic (i) a-Si: H layer between the n-type c-Si substrate and p-type a-Si: H layer as “HIT solar cell”. With the addition of an intrinsic a-Si: H layer, the dangling bonds on the c-Si surface were passivated, thereby significantly reducing the a-Si: H/c-Si interface defect-state density and the carrier recombination. The addition of a 4 nm thick intrinsic a-Si:H layer allowed the Voc and FF values to be improved, and a conversion efficiency of 14.8 % was achieved (Wakisaka et al., 1991). The next step toward increasing the HIT solar cell efficiency was the application of the surface patterning process. The purpose of this process was to create a light-trapping structure. The random pyramid structure used for surface patterning helps to reduce surface reflection and increases the average optical path length in the cell, thereby increasing Jsc. It was shown that the Jsc value of the surface patterned cell increased by 20% compared to the standard cell. However, with this structure, it was observed that Voc value decreased, which may be attributed to the increase in surface defect density as the surface patterning process may increase the total surface area (Wakisaka et al., 1991). In the implementation of this structure, difficulties such as the challenge of depositing the a-Si layer uniformly on the patterned surfaces, and the c-Si surface not being adequately cleaned before the a-Si:H layer was deposited were encountered. The deposition conditions were optimized by reducing the coating speed and diluting the silane gas. Effective cleaning of the substrate was achieved by exposing the substrate surface to hydrogen plasma before

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the film growth. With this cell, a conversion efficiency of 16.8% was reached (Wakisaka et al., 1991). In the next step, the performance of the HIT solar cell was further improved by adding a back surface field (BSF, densely n 1 a-Si: H layer) to the back of the solar cell in addition to the surface patterning process. The decrease in the Voc value due to the surface patterning process was compensated by this structure. The inclusion of surface patterning and BSF provided a high HIT solar cell efficiency of 18.1% (Tanaka et al., 1992) It is a fact that the HIT structure provides excellent passivation on the cSi surface. Thus, the efficiency was additionally improved by adding (i) a-Si to both surfaces of the substrate. Besides the (i) a-Si structure, which provides passivation on both sides of the substrate, applying a highly reflective rear electrode and optimizing other conditions increased the efficiency up to 20% (Sawada et al., 1994). Then, Sanyo introduced the first commercial HIT cell in 1997 and started the mass production of 180 W solar panel named HIT Power 21t with a 15.2% conversion efficiency (Taguchi et al., 2000). In the early 2000s, Sanyo published the details of the cells with doublepassivated symmetrical structure and announced the efficiency value of solar cells that are not affected by thermal and mechanical stresses and could generate power from both sides of the solar cell (Taguchi et al., 2000), which were 20.7%. Sakata et al. (2000), and 21.3% (Tanaka et al., 2003), respectively. After these developments, Sanyo has continuously been working on HIT solar cell optimization and focusing on approaches which are geared towards developments regarding Heterojunction structure, metallization, properties of the TCO layer, and the reduction of the thickness of Si substrate to increase efficiency (Tsunomura et al., 2009; Taguchi et al., 2009; Fujishima et al., 2010; Inoue et al., 2009; Tohoda et al., 2012; Taguchi et al., 2014). In January 2014, Panasonic achieved a conversion efficiency of 24.7% with 98 μm thickness and 1018 cm2 surface area (Voc 5 750 mV, Jsc 5 39, 5 mA /cm2, FF 5 83.2%) (Tohoda et al., 2012). Finally, a record efficiency value of 25.1% (Voc 5 738 mV, Jsc 5 40.8 mA/cm2, FF 5 83.5%), in 2015, was achieved with160 μm thick, 6-inch n-type c-Si substrate with HIT solar cell by Kaneka (Adachi et al., 2015). Until now, Panasonic, Kaneka, CIC, and Supreme are among the leading companies that have industrialized HIT solar cells (Liu et al., 2018). For the standard HIT solar panel (with front surface metallization), Panasonic and Kaneka have achieved 22.5% and 22.2% efficiency respectively (Ohshita et al., 2017). Kaneka and Panasonic (https://www.kaneka.co.jp/topics/uploads/2017/ 06/1479120629_101.pdf, https://news.panasonic.com/global/press/data/2016/03/ en160302-2/en160302-2.html) have also studied heterojunction back contact (HBC) type solar panels with conversion efficiencies of 24.37% and 23.8%, consecutively.

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The highest efficiency value in the c-Si HIT solar cell has been achieved with HBC solar cell technology. The HBC solar cell technology is the application of IBC solar cell technology to HIT solar cell technology. In IBC solar cells, there is no shading loss due to the lack of metallization on the front surface, and by reducing the optical losses, high short circuit current density is obtained (Lammert and Schwartz, 1977; Nakamura et al., 2014; Franklin et al., 2016; Smith et al., 2016; Yoshikawa et al., 2017a). This HBC technology aims to produce high-efficiency cells by combining the advantages of both IBC and HIT technologies. Such HBC solar cell production steps generally start with a low surface recombination and a high transparent passivation layer deposited on the patterned c-Si substrate. Then, the dielectric antireflection coating is deposited. To the back surface, i: a-Si: H/p: aSi: H and i: a-Si: H/n: a-Si: H structures are deposited with an interdigitate distribution. Finally, metallization is carried out to ensure optimum electrical performance. Currently, many researchers and groups are contributing to the development of HBC solar cell efficiency (Masuko et al., 2014; Yoshikawa et al., 2017b; Nakamura et al., 2014; Tomasi et al., 2014; Harrison et al., 2016; Stang et al., 2017). Kaneka has achieved the world’s highest conversion efficiency as 26.63 % with 179.74 cm2 area, 740.3 mV open-circuit voltage, 42.5 mA/cm2 short circuit current, and 84.65% fill factor (2017).

8.1.2

Graphene

Graphene has become one of the top research areas in recent years because of its remarkable properties such as high optical transmittance (97%), high thermal conductivity (5000 W/mK), high charge mobility (200,000 cm2/V per second) and high durability (1 Tpa Young modules) (Geim and Novoselov, 2010; Liu et al., 2019). Graphene is the new member of the sp2 bonded carbon family that was isolated and characterized in 2004. The outstanding properties enable graphene to be employed in many applications, namely transistors, sensors, batteries, and solar cells. Besides its attractive properties, graphene has some handicaps such as high sheet resistance and absence of the energy bandgap, and those are quite important for graphene to be employed in semiconductor technologies. However, these issues can be overcome by chemical doping or multilayer synthesis of graphene (Wei et al., 2009). The doping can be performed using different elements and sources via a chemical doping approach. Boron or nitrogen are commonly used elements to dope graphene. Nitrogen doping is more frequently employed because it is easier than that of boron in graphene for both in-situand postsynthesis. This is related to the atomic sizes of carbon, nitrogen, and boron atoms as nitrogen atomic size (65 pm) is closer to that of carbon (70 pm) compared to boron’s atomic size (85 pm) (Matsoso et al., 2016; Liu et al., 2011a). Therefore, nitrogen atoms can easily substitute those of carbon in graphene when compared with the ones in boron.

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Many theoretical and experimental studies showed that graphene film has various applications in solar cells (Choi et al., 2010; Singh and Nalwa, 2015; Abdullah and Hashim, 2019; Lancellotti et al., 2020). It can be used as a transparent conductive electrode (TCE), catalytic counter electrode, active layer, and charge transport layer. Therefore, graphene can be employed in various types of solar cells as a different component. For example, platinum (Pt) can be used as a catalyst for dye synthesized solar cell (DSSC), but because Pt is expensive and its sources are limited, graphene can be used instead of Pt as a catalyst in DSSC (Roy-Mayhew et al., 2010). Additionally, graphene is one of the best candidates for Schottky junction and tandem solar cells if the energy band structure of the materials is matched. The work function of graphene, as stated above, can be amended/adjusted via doping or other techniques (Ye et al., 2011). Xinming Li and co-workers, and Qin and co-workers revealed that graphene can be combined with Si to form Schottky junctions and efficient solar cells since graphene has a much higher mobility when compared to silicon (Si) (1000 cm2/V per second) (Li et al., 2010; Lee et al., 2018). Electrons and holes are diffused to graphene film in addition to photogenerated carriers that are separated via the built-in field in graphene-based solar cells. These make graphene suitable to be employed in solar cells as hole transport and electron transport layers. Besides the applications of graphene mentioned above, graphene is one of the best promising materials to be employed as TCE. Indium thin oxide (ITO) is one of the most commonly used TCE in Si-based solar cells. ITO has some superior properties such as low sheet resistance, high transmittance in the visible region, and stability over time in relation to the other TCEs. However, ITO also has some limitations which are brittleness, a low mechanical flexibility, and a scarce source in nature. Although pristine (undoped) graphene sheet resistance is much higher than that of ITO, graphene sheet resistance can be reduced to become comparable with ITO via chemical doping. Thus, graphene can be one of the best alternatives for Sibased heterojunction solar cells as TCE thanks to its high light transmittance, conductivity, and flexibility (Yin et al., 2014). Chemical doping can modify the electronic, chemical, and magnetic properties of graphene. Boron and nitrogen can be used to dope graphene by modifying the Fermi level of graphene (Guo et al., 2010). Dirac point is located at the Fermi level in pristine graphene, and it goes below the Fermi level for p-type doping and goes up for n-type doping (Yadav and Dixit, 2017). Different boron or nitrogen sources such as ammonia (NH3), pyridine (C5H5N), diborane (B2H6) can also be used for graphene doping (Liu et al., 2011a; Lee et al., 2018). There are two common approaches to synthesize doped graphene. These are called surface doping (temporary) and substitutional doping (permanent). When the dopant atoms don’t replace carbon atoms in the graphene lattice, these atoms hold on to the surface or edge of the surface. Thus, this doping

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is known to be temporary as the doping degrades in time. However, despite being temporary, graphene can exhibit p- or n- type behavior in this type of doping (Wei et al., 2009). On the other hand, in substitutional doping, nitrogen or boron atoms replace the carbon atoms in the graphene lattice. Due to the differences in the atomic radius and the electronegativity between the carbon and dopant atoms, the dopants powerfully polarize the carbon network in the graphene film (Lee et al., 2018; Zhang et al., 2011). Nitrogen doping creates three different configurations named pyridinic N, pyrrolic N, and quaternary N (graphitic) in the graphene lattice. Generally, pyridinic N is observed at the edges or on the defective sides of the lattice. In this type of configuration, two carbon atoms at the edge or on the defect side contribute one p electron to the π system. In the Pyrolic N, nitrogen atoms contribute to the π system with two p electrons. In the Quaternary N structure, which is also called graphitic N, nitrogen atoms replace carbon atoms in the hexagonal ring of the graphene lattice. The substitutional doping can be determined with the presence of quaternary N configuration in XPS measurements. Therefore, if quaternary N configuration is present in the graphene lattice, it means the substitutional doping is achieved (Choi et al., 2010; Bulusheva et al., 2020).

8.2

Fabrication of silicon heterojunction solar cell

The production process of a standard silicon heterojunction solar cell consists of two main stages: chemical processes and solar cell fabrication. Before cell fabrication, a series of chemical processes are applied to the substrate surface. These processes can be listed as using a chemical solution to remove the saw damage which occurs during the production of the substrate, applying a chemical alkaline solution for patterning the surface in the form of a pyramid, and the application of RCA and HF for surface cleaning before coating. The c-Si substrate surface, which becomes ready for coating after the cleaning process, has surface energy states that cause current loss due to recombination, which is minimized by (i) a-Si: H layer coating of the surface. After coating the intrinsic a-Si:H layers on both surfaces of the substrate, the p-type a-Si: H (for n-type substrate) is deposited as an emitter. Ntype a-Si: H layer, which acts as BSF, is deposited at the rear side of the cell. Due to the low conductivity of a-Si: H layers, the TCO layers are coated on both surfaces. With surface patterning and TCO, reflection is almost completely suppressed. The full backside metallization is generally performed using PVD. Since electrical connections are needed for the completion of the cell, the front side metallization process is performed using screen printing with a pattern consisting of fingers and busbars. Fig. 8.1 shows the cross-sectional view of HIT solar cell structure and band diagram of HIT solar cell, which was used in this study.

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FIGURE 8.1 (A) Cross-sectional view of c-Si HIT solar cell (not to scale) and (B) Schematic band diagram of HIT solar cell based on n type c-Si wafer. The material of each layer is labeled in the diagram and the direction of flow in both electrons and holes is represented.

C-Si HIT solar cells are formed by combining Si-based materials with different bandgaps. The optical band gaps of Si and a-Si:H layers are 1.12 and 1.80 eV, respectively. Since bandgap energies are different, a mismatch occurs on the conduction and valance band edges, as illustrated in Fig. 8.1B. A barrier for the electrons at the a-Si:H (n region) and c-Si interface forms. Holes accumulate at the barrier which is located at a-Si:H (p region) and cSi interface and has a height of ΔeV. These holes drift either by tunneling through the barrier or by thermionic emission.

8.2.1

Surface patterning and surface cleaning

The substrates used in silicon heterojunction solar cells should be as thin as possible (below ,200 μm) to reduce material costs and to get high efficiency. However, light absorption is greatly reduced in thin substrates. The application of light-shielding techniques has become an important topic in silicon heterojunction solar cells (Van Sark et al., 2012). The role of the light-trapping structure is to maximize the optical thickness while keeping the physical thickness of the absorber layer as thin as possible. For single cSi substrates, a random pyramid structure is widely used. Patterning of c-Si substrate is commonly performed with the help of a chemical solution. ,111. and ,100. oriented crystal silicon substrates are etched at different rates. However, this process is not suitable for polycrystalline substrates due to the crystalline orientation of the surface (Van Sark et al., 2012). Chemicals such as TMAH (tetramethylammonium hydroxide) (Tabata et al., 1992; Papet et al., 2006) and NaOH (Vazsonyi et al., 1999; Barrio et al., 2012) are used for the patterning process; KOH or KOH/IPA chemical solution (Barycka and Zubel, 1995; Sato et al., 1999; Hayashi et al., 2006; Munoz et al., 2009; Chu et al., 2009) is widely used in the patterning process since it has advantages such as simplicity, ease of use, low cost and homogeneous etching rate (Munoz et al., 2009; Chu et al., 2009; Mahmoud Al and Lahlouh, 2017).

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Besides the advantages of surface patterning, there are also disadvantages such as coating a-Si: H layer inhomogeneously and the surface cleanliness not being at the desired level. To create a perfect hetero-interface, it is necessary to use a well-cleaned c-Si substrate surface and remove the natural oxide on the substrate surface before loading the substrate into the PECVD [plasma-assisted chemical vapor deposition (CVD)] system (Fahrner, 2013). Various cleaning techniques can be applied prior to the deposition of thin films. Usually, after cleaning with the standard RCA (Radio Corporation of America) method, a chemical mixture of hydrofluoric acid (HF) and Nitric Acid (HNO3), an isotropic etchant, is used to round off the sharp ends of the pyramids on the substrate surface and remove the natural oxide. This process is followed by the PECVD coating process. Standard RCA-1 and standard RCA-2 are two versions of the RCA cleaning method. Ammonium hydroxide (NH4OH), hydrogen peroxide (H2O2), and pure water (H2O) are used for RCA 1. RCA 2 uses a mixture of hydrochloric acid (HCI), hydrogen peroxide, and purified water (Kern, 1990). The effects of various chemical cleaning procedures on cell performance can be investigated by measuring the carrier lifetime. Carrier lifetime indicates surface recombination; therefore, the quality of the cleaning process. In this realm, three different cleaning methods, for instance, were compared in the study carried out by Delft University of Technology (Van Sark et al., 2012). In this study, the cleaning processes were performed before coating the a-Si: H layers on the ,100. oriented FZ c-Si substrate. The first substrate was cleaned using a standard RCA cleaning method, while the second was cleaned employing standard DIMES cleaning procedures containing concentrated nitric acid. As for the third substrate, it was cleaned only by the HF dip method, and no external cleaning method was used. HF was used to remove the natural oxide layer for all three substrates. After the cleaning, a 120 nm-thick intrinsic a-Si: H layer was deposited on both sides of the substrates using the same deposition conditions for each run. The carrier lifetime was also measured to evaluate passivation quality. The substrate cleaned using the standard RCA process showed the highest carrier life, and therefore, the best passivation. The lowest carrier lifetime was observed for the substrate subjected only to HF. The last step of the cleaning process is to immerse the substrate shortly in HF solutions in order to remove the natural oxidation layers from the surface and provide surface bonding with hydrogen bonds. Hydrogen bond passivation prevents the formation of natural oxide for a short time, but it cannot be effective for a long time (Fahrner, 2013). The time between HF dipping and the next process coating with PECVD is limited. This period was tested by Fahrner (2013) They found out that as the immersion time in HF solution (1%) increases, so does the effective lifetime. It was observed that the effective lifetime decreases as the exposure time to the air increases between the HF dipping process and the coating (Fahrner, 2013).

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In addition to wet chemical cleaning, there is also dry cleaning for substrates (Tucci et al., 2001, 2002). Dry cleaning costs lower than wet chemical cleaning for the mass production of solar cells. Gases such as H2 and CF4 /O2 are generally used for plasma treatment applied on the substrate surface. However, as the plasma treatment has a potential to damage the crystal surface, this method may have a negative effect on the electrical properties of the solar cell (Tucci et al., 2001, 2002). In this study, standard RCA-1 and RCA-2 processes were performed to clean the surface of the Si wafer. Following the RCA process, samples were rinsed under DI water to remove chemical residues from the Si wafer surface. Rinsing is a critical step to prevent the surface from contamination coming through each step. The oxide removal of Si wafers was carried out with a 1% HF solution, which was required to remove oxide and get a clean surface. After the HF dip, wafers were rinsed and dried under N2 flow as a final step of the cleaning process.

8.2.2

Deposition of a-silicon:H layers

After cleaning the substrate, the intrinsic layer is deposited on the c-Si surface, which is followed by the deposition of doped a-Si: H layer. Each atom of a single crystal silicon is covalently bonded to four adjacent atoms in a diamond structure at the same angle and bond length (Fig. 8.2A). This is continuously repeated unit cells come together to form a crystal lattice. The amorphous silicon has an irregular lattice structure, as can be observed in Fig. 8.2B. On the other hand, a-Si cannot exhibit a continuous structural order due to minor deviations in its bond lengths and angles. Larger variations lead to weak bonds that can easily break, resulting in defect formation within the lattice structure. As seen in Fig. 8.2B, the defects in the a-Si are the silicon atoms that are covalently bonded to only three silicon atoms and have an unbounded electron. These bonds are called dangling bonds (Poortmans and Arkhipov, 2006). In general, the defect density in the a-Si structure results from dangling bonds (about 1021 defects per cm3). These defects fall to 10151016 cm3, as a result of the strong hydrogen-silicon

FIGURE 8.2 Atomic structures: (A) single crystalline Si, (B) a-Si (B) a-Si passivated with hydrogen.

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bonds, by incorporating hydrogen into the atomic structure (Poortmans and Arkhipov, 2006; Street, 1991). The hydrogenated amorphous silicon (a-si: H) structure is given in Fig. 8.2C. The high defect density of a-Si structure has negative effects on doping, photoconductivity, and other desired properties of a semiconductor. The passivation of dangling bonds with hydrogen has improved the a-Si properties, namely photoconductivity (Street, 1991). a-Si:H is a promising semiconductor for solar cells. Some advantages of a-Si: H thin film compared to c-Si are exemplified in some studies (Street, 1991; Cahyono et al., 2017). Depending on the deposition conditions, the properties of a-Si: H layers such as bandgap, thickness, and doping can be easily controlled. Low fabrication temperature (B200 C) provides advantages in terms of production cost and ease of substrate selection. Intrinsic and doped a-Si: H layer is critical to fabricate high efficiency HIT solar cells. Several deposition techniques are used for the fabrication of HIT solar cells and these are HWCVD (hot wire CVD) (Jadkar et al., 2007), spraying (Hossain et al., 2006), chemical annealing (Futako et al., 1999), and PECVD (plasma-assisted CVD) (Van Sark et al., 2012). Among others, PECVD is widely used for the production of HIT solar cells (Hanyecz et al., 2011). The chemical reaction is created or supported by a plasma in the PECVD system. The advantages of PECVD over the other techniques are a higher deposition speed and a lower process temperature (Rostan, 2010). Plasma is produced by an alternating electric field applied between two electrodes. The source gas silane (SiH4) is decomposed into SiHx radicals and atomic hydrogen in plasma. Then, hydrogenated amorphous silicon (a-Si: H) begins to grow on the substrate surface. Hydrogen is an additional gas that plays a vital role in a-Si:H deposition. a-Si: H films are typically deposited at about 200 C at a pressure range of 10200 Pa. Doped a-Si:H layers play a role in forming p-n junction structure in HIT solar cells and prevent surface recombination by providing the structure called BSF. Just like intrinsic a-Si: H films, doped a-Si:H layers are deposited by PECVD with a mixture of SiH4 and H2. While trimethyl boron (TMB) or diborane [(B2H)6] gases are mixed with the SiH4 and H2 gas flow for p-type a-si: H layers, phosphine (PH3) gas is mixed for n-type a-si:H layers. Attention should be paid to cross-contamination in the deposition process. If intrinsic and doped a-Si:H layers are deposited sequentially in a single PECVD, the dopant gases also deposit on the walls of the process chambers or substrate carriers, and this can cause cross-contamination. This type of contamination leads to poor p/i or n/i a-Si:H interface properties (decrease in Voc due to reduced passivation quality) and adversely affects solar cell performance. Therefore, multichamber systems (separate chambers for each layer) or chamber coating following chamber cleaning procedures should be used to fabricate an efficient solar cell (De Wolf et al., 2012; Willeke and Weber, 2013).

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The quality of a-Si: H layers is directly related to deposition conditions, so it is important to control the deposition parameters during the process. Deposition parameters such as pressure, temperature, plasma power, and silane concentration affect the a-Si:H layers’ properties such as thickness, hydrogen bonding ratio, optical band gap, and lifetime. In this respect, coating conditions are very important for optimum performance. Some critical process parameters are as follows; G

G

G

G

G

Plasma excitation frequency: 13.56 MHz plasma excitation frequency (Schulze et al., 2010; Descoeudres et al., 2011; Mun˜oz et al., 2012; Seyhan et al., 2017) is frequently used in the PECVD method. Besides, there are successful uses of very high frequencies (VHF) such as 40.68 MHz (Zicarelli et al., 2011) or 70 MHz (De Wolf et al., 2010). Gas concentration: Gas concentration is expressed as dilution rate (R 5 [H2]/[SiH4]) or silane density (SC 5 [SiH4]/[H2] 1 [SiH4]). The density of the silane decreases as the dilution rate increases. If the rate of hydrogen used during a-Si:H growth exceeds a certain dilution rate, this can lead to the transition from amorphous structure to microcrystalline structure. Deposition pressure (Fahrner, 2013): Deposition pressure mainly affects growth rate and homogeneity. If the deposition pressure is too high, it supports the formation of defects in the deposited material. Moreover, it can cause the formation of high pressure polysilane powder. Both effects are undesirable. Plasma power (Van Sark, 2002): Plasma power affects deposition rate. The rate of deposition increases, when the plasma power increases and this causes several disadvantages such as low film quality. The optical bandgap increases with increasing power due to the increase of hydrogen content. However, very high power levels cause microcrystalline silicon formation rather than amorphous growth. Therefore, low deposition rate and low plasma power are required to obtain high-quality a-Si:H thin films. Temperature (Van Sark, 2002): Temperature changes the nucleation properties and affects the defect density of the films. The deposition rate is almost independent of the temperature. Temperatures higher than 250 C cause the drop of hydrogen content in the film and lead to higher defect densities.

In view of the above discussion in the present study, two PECVD chambers are used for the deposition of intrinsic and doped a-Si:H layers to get rid of contamination coming from doping agents. The power used in PECVD chambers has a radiofrequency of 13.56 MHz. The plasma is formed between anode and cathode. Depending on gasses, the thin films are deposited on texturized n-type Si wafers. The heaters are located behind the anodes and cathodes. During the deposition, the process temperature is kept

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at 200 C. To identify the relationship between deposition parameters and passivation quality, deposition power, the thickness of a-Si:H layers, and uniformity were investigated. The cross-sectional view of the device structure of HIT solar cells is given in Fig. 8.1A. After the deposition of (i) a-Si:H layer on both sides, samples were characterized by Sinton lifetime tester. The thickness of (i) a-Si:H layers was found by ellipsometry. To understand the relationship between the deposition power and passivation quality of (i) a-Si:H layers, four different deposition power (30, 40, 45, and 50 W) was used. The effective lifetime was measured to understand the quality of (i) aSi:H layers. The results show that decreasing deposition power results in increasing of effective lifetime values 2.8, 2.5, 2.4, 2.0 ms, respectively. The effect of (i) a-Si:H thickness on passivation quality was investigated for three different thickness values (10, 8, 6, and 5 nm). The optimum thickness was found 8 nm with a 2.6 ms effective lifetime. The same experimental procedures were performed by doped a-Si:H layers. The optimum plasma power was found 150 W for both n and p-type a-Si:H layers with 1.5 ms effective lifetime, Fig. 8.3C.

8.2.3

Deposition of transparent conductive oxide

TCO is a layer coated between doped a-Si: H layer and metallization in HIT solar cells due to its good optical transmittance and conductive properties. The lateral conductivity of doped a-Si: H layers is quite poor, so a transparent conductive oxide (TCO) layer deposition is required on a-Si: H films (De Wolf et al., 2012). TCO materials must have properties such as good antireflection, electrical conductivity, and being processible at low temperatures. Indium doped tin oxide (ITO), indium doped zinc oxide (IZO), aluminumdoped zinc oxide (AZO), gallium doped zinc oxide (GZO), and magnesiumdoped zinc oxide (MZO) are some TCOs used in solar cells. ITO ((In2O)3: Sn) films have low resistivity and are widely used in the HIT solar cell as they can be easily deposited at low temperature with high transparency at the visible wavelength (Van Sark et al., 2012). Optimizing the optical and electrical properties of the TCO layer is a complicated process that includes metallization design and optimizing the properties of other layers (Van Sark et al., 2012). Methods used for ITO deposition are PVD, CVD (Maruyama and Fukui, 1991), Electron beam evaporation method (Raoufi, 2009), Ion beam deposition (Han et al., 2001), Sol-gel method (Biswas et al., 2006), and Pulsed Laser Deposition (Zheng and Kwok, 1993). PVD is the most widely used and effective method (Zhu et al., 2000; Her and Chang, 2016), because the temperature (Meng and Dos Santos, 1998; He et al., 2013; Boonyopakorn et al., 2010), DC-RF power (Her and Chang, 2016; Terzini et al., 2000; Kurdesau et al., 2006), and the O2 gas flow rate (Bender et al., 1998; Zhang, 2010; Hussain et al., 2014) can be easily controlled by changing sputtering

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FIGURE 8.3 Schematic view of the standard PECVD (A) and PVD (B) chamber, the lifetime of a-Si:H layer of the HIT solar cell (C), image (inset of D) and ellipsometer result of 100 nm ITO layer (D), front metallization (E), backside metallization by PVD (F), I-V graph of Si HIT solar cell and full HIT solar cell image (G).

conditions. The process conditions have an impact on the electrical and optical properties of the ITO film, such as resistance, carrier density, carrier mobility, transmittance, and reflectance. The optical and electrical properties of ITO were examined by keeping the process parameters constant and changing the plasma power value by Chang et al. Her and Chang (2016). With increasing power value, carrier mobility, and sheet resistance increase, while carrier density decreases. Experimental results showed that the optical transmittance decreases as the power increases, and 80% optical transmittance is obtained in the visible range for ITO films prepared at a power value of 50 W. As a result, a low power rate has been proposed for good ITO performance. Hussain et al. (2014) examined the effect of the O2 flow rate on ITO films with samples coated on a glass substrate. It was shown that a higher the O2 flow rate increases the carrier mobility and the sheet resistance, and

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decreases the carrier density. The resistance had the lowest value for the oxygen flow rate of 0.1 sccm. In this study, the maximum transmission of ITO (93%) was obtained for the oxygen flow rate of 0.1 sccm. The study of Boonyopakorn et al. (2010) examined the effect of substrate temperature on the ITO performance. With this study, it was observed that the increase in temperature improved Sn substitution in the film structure during the crystallization process, resulting in a higher carrier concentration and, consequently, a decrease in sheet resistance value. Although high temperatures improve ITO performance, it should be remembered that this study was carried out on a glass substrate, and the high temperatures could have negatively affect HIT solar cell performance. It also showed that the additional annealing process affects ITO performance. It was also understood that a 300 C temperature is critical for the ITO deposition process. Thermal treatments at higher temperatures was observed to cause deterioration of the ITO/Si interface and the electrical properties of the films (Balasundaraprabhu et al., 2009). In the present study, ITO was sputtered as a TCO layer by PVD on both sides of the HIT solar cell schematically as illustrated in Fig. 8.3B. The chamber working pressure was 5 3 1026 mbar, and the chamber pressure was maintained at 1 3 1022 mbar with the helping of Ar and O2 gas flow during deposition. Ar and O2 flow rate and deposition power were studied for the optimization of the ITO layer. 200 and 3.5 sccm were the optimum flow rate of Ar and O2 to get low resistance. During optimization, the temperature of the substrate kept at 200 C to get rid of the deterioration of the interface at TCO/a-Si:H layer. The thickness of the ITO layer was measured by ellipsometer. The optimum thickness of ITO layers was found 50 and 100 nm for the front and backside of HIT solar cells, respectively. The transmission of the ITO layer for 100 nm is shown in Fig. 8.3D with high transparency around 90 %.

8.2.4

Metallization

Screen printing (Ebong et al., 2012) aerosol printing (Mette, 2007), and Cu electroplating (Mun˜oz et al., 2012; Geissbu¨hler et al., 2014; Aguilar et al., 2016) are some of the techniques for metallization of solar cells. Although there are many techniques, screen printing is the most widely used metallization technology in the market as it is simple, fast, and suitable for mass production. Ink-jet printing technology (Ebong et al., 2012; Gizachew et al., 2011) is a promising alternative technique to the screen printing method. However, there are disadvantages such as lower printing speed and the problems caused by the ink used (Ho¨rteis and Glunz, 2008; Panek et al., 2017). Another promising metallization technique is stencil printing for highefficiency solar cells (Panek et al., 2017; Hannebauer et al., 2015). Metallization based on copper electrolysis coating seems to be the best

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alternative to screen printing technology in terms of material cost and cell performance. However, its implementation at an industrial scale is still limited by several obstacles. Difficulties such as adhesion, and complexity of process have been solved to a certain extent, but long-term reliability is seen as the most important barrier to sustainable production (Geissbu¨hler et al., 2014; Aguilar et al., 2016; Lee, 2014; Yu et al., 2017; Daisuke et al., 2017; Hernandez et al., 2012). In the present study, the commonly used screen printing technique and low-temperature Ag paste were used for the front metallization of HIT solar cells. First, the finger design was transferred, and thermal treatment was subjected to the cell. Immediately after the thermal treatment, the five busbar design was transferred to the cell. The final firing process was performed for busbars at 200 C. Resulting front side HIT solar cell metallization can be visualized in Fig. 8.3E. The backside metallization of HIT solar cells was performed by PVD. 250 nm Ag deposited on the back side ITO layer, Fig. 8.3F.

8.2.5

Thermal treatment

The cell is subjected to heat treatment after the front side metallization is done by the screen printing method. With a heat treatment below 250 C, the solvents of the silver paste are removed and hardened. The applied heat treatment temperature and time affect the contact resistance at the ITO/Ag interface and the electrical properties of the silver electrodes. Therefore, heat treatment is also a step that affects the efficiency of the cell. Heat treatment should be carried out at low temperatures (,250 C) for HIT solar cells. Therefore, metal pastes used for HIT solar cells and standard c-Si solar cells are different. Since a-Si: H layers are damaged at high temperatures, the production steps of HIT solar cells take place at a low temperature. Lee et al. (2014) made a detailed examination of the heat treatment stage for HIT solar cells with their study. In the study that Lee et al. done, the effect of temperature between 160 C240 C and heat treatment times between 10 and 50 minutes on cell performance were investigated. The effects of heat treatment temperature and time on contact resistivity (ρc) were analyzed. The results show that the firing time affects ρc, and the best ρc value is obtained by 20 minutes of heat treatment at 200 C. Also, a process at 200 C appears to provide a solar cell with the highest FF and efficiency with the lowest Rs. In the present study, the contact resistance of the ITO/Ag interface was performed to find the optimum heat treatment temperature by using the TLM measurements. Various drying temperatures were used as 180, 200, 220, and 250 C. The contact resistivity was found 0.6, 0.5, 1.2 and 2 mohm  cm2, respectively. The optimum heat treatment temperature was found as 200 C, with the lowest contact resistivity of 0.5 mohm  cm2, similar to the literature.

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The best cell efficiency was achieved as 19.7 % on 243.4 cm2 cell area with 703 mV Voc, 8.64 mA/cm2 Isc, and 77.4 % FF as shown in Fig. 8.3G.

8.3

Synthesis of graphene

In the first step of the experimental procedure for graphene synthesis, single layer pristine graphene was synthesized using a thermal CVD system. The CVD system has a 3-zone furnace and 6 mass flow controllers. Additionally, high purity (%99.9995) methane (CH4) was used as carbon source, and high purity (%99.9999) hydrogen (H2) was used as decomposition gas during the synthesis. The pristine graphene was synthesized at 1000 C with 30 sccm CH4 and 20 sccm H2 along with 30-minutes growth time that was optimized in our previous studies. These growth parameters were employed during the doped graphene synthesis processes. Then, the experiment proceeded with the doping phase. During that phase, ammonia (NH3) was employed as a doping source for nitrogen-doped graphene synthesis. The effects of growth time and amount of ammonia flow were investigated in order to obtain high-quality large scale nitrogen-doped graphene. The doping of graphene was performed using 5, 10, and 15 sccm ammonia flow, which enabled a smooth flow through to the system only during the growth process. Then, the growth time was investigated for 10, 15, 20, 25 minutes. After these investigations and growth optimizations, the nitrogen-doped graphene films were applied to 2.5 3 2.5 cm size solar cells, whose fabrication details were provided above. In order to characterize the synthesized graphene films, Raman and XRay photoelectron spectroscopy (XPS) were employed. Raman spectroscopy is one of the straightforward and non-destructive techniques to characterize graphene in terms of both quality and doping. Graphene shows mainly three bands that are called D, G, and 2D in the Raman spectra. The D band can be observed on the defective regions and edge sites, and it cannot be observed in perfect pristine single-layer graphene film. It is located at around 13201350 per cm in the defective graphene or graphite in the Raman spectra. The G band represents the crystallinity of graphene, and the peak occurs because of the double degenerate zone center. The G band is located around 15801586 cm21, and it shifts 35 cm21 due to doping. Therefore, this band helps to determine the doping in the graphene film as well (Ferrari et al., 2006). 2D band is another prominent band/peak in the Raman spectra of graphene. It can be called as trace band to determine the thickness of graphene. Additionally, 2D and G intensity ratios and full-width half-maximum value (FWHM) of 2D ease the process regarding the evaluation of quality and thickness of graphene film. I2D/IG ratio is known to be bigger than 2 for single graphene; however, few layer graphene I2D/IG ratio is smaller than 2 as stated in the literature. Furthermore, the average FWHM of a single-layer graphene is 27.5 6 3.8 per cm, and if the FWHM is bigger than 30 per cm,

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the film is considered to be multilayer. However, the FWHM evaluation regarding the graphene layer (thickness) is only valid with the presence of 2D band on the right position along with an accurate I2D/IG ratio in the Raman spectra. Moreover, the doped graphene has an extra peak called D’, which occurs around B1620 per cm in the spectra; thus, the presence and position of the D’ peak can be employed for the evaluation of doping in graphene (Liu et al., 2011b; Hao et al., 2010). XPS is another prominent characterization technique to evaluate doped graphene. XPS also helps to reveal the bonding and doping ratio in the doped graphene film by monitoring C1s and N1s configurations. There are three components in C1s spectra of XPS measurements. The main peak is placed at B284 eV and represents the graphite-like sp2 bonded carbon (substitutional doping), and it means that carbon atoms conjugated in the nitrogendoped graphene lattice. The other peaks, which appear as the shoulder of the main peak, are placed at B285 and B287 eV, and these peaks represent Nsp2 carbon and N-sp3 carbon bonds, respectively. Additionally, the substitution of carbon atoms with nitrogen can be considered as defect formation in the graphene lattice, and nitrogen atoms can also be bonded to the carbon atoms, which are at the edge of the graphene film. Thus, N-sp2 C and N-sp3 C bonds can be obtained. N1s spectra also have three components similar to C1s in XPS measurement, and these components are known to be pyridinic N, pyrrolic N, and quaternary N, which are located at B398, B400 and B401 eV consecutively. The presence of a quaternary N band in the spectra confirms substitutional doping in the graphene structure. In other words, the nitrogen atoms replace carbon atoms in the quaternary N configuration in the honeycomb structure of the graphene film. Following these characterization details, in this study, HIT solar cells that were fabricated without ITO and metal contacts were used for graphene applications. Three different types of cells were fabricated and compared. Initially, 80 nm ITO was deposited onto n-type (2.5 3 2.5 cm) HIT solar cells in a Physical Vapor Deposition (PVD) system. The film thickness was the optimum, which was determined from the previous experiments (Altuntepe et al., 2020). Then, in turn, pristine graphene and nitrogen-doped graphene were individually and separately transferred onto the solar cell. The front and back contacts were prepared using silver paste by the screen printing technique. Finally, three different cells which are Si-HIT, Si-HIT/pristine graphene and Si-HIT/doped graphene were obtained. The fabricated solar cells were characterized using a solar simulator to find out the photovoltaic parameters such as efficiency and I-V characteristics. Regarding the transfer procedure, the wet-transfer method was used. This was based on using polymer on graphene surface and etching the copper foil where graphene film was grown. The grown graphene on copper foil was initially coated with polymethyl methacrylate (PMMA) using a spin coater. The PMMA layer was used as support material during the transfer process. Then,

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ammonium persulphate (NH4)2S2O8 was used to remove the graphene film from the copper surface. To do this, the PMMA coated graphene film was placed in the 1 molar (NH4)2S2O8 solution to allow the PMMA/graphene stack to get separated from the copper foil within a few hours. The stack was then taken into deionize (DI) water bath for a few times to get rid of any possible residues that could have come from the etcher. In order to complete the transfer process, PMMA/graphene stack floating on the DI water was transferred onto the cell structure and dried at low temperature on a hot plate. Finally, the support material, PMMA, was dissolved with the aid of acetone. The same transfer method was used to transfer both pristine and doped graphene films onto the solar cell. After the graphene film transfer, the whole structure was annealed under Ar/H2 gas flow at 300 C temperature to remove the contamination such as PMMA residues that could remain on the surface of the graphene film during transfer. Moreover, the dry transfer method could also have been applied for the transfer of graphene films, which is regarded to be a cleaner way. However, the transfer success rate is low for the dry transfer approach in comparison to the wet transfer one. Pristine graphene average G and 2D band positions, which were gathered from many points on many samples, were determined as 1586 and 2661 per cm via Raman spectroscopy measurements. The change in these positions, in particular the shift in G band, helps to determine whether graphene is doped. NH3 was used as a doping source to synthesize nitrogen-doped graphene, and it was found that the graphene film quality was severely affected with the increase in the NH3 flow ratio. 5 sccm NH3 provided single-layer graphene synthesis. However, few layer and multilayer graphene were synthesized via 10 and 15 sccm NH3, respectively. A decrease in graphene film quality was expected with the increase of NH3 flow, which is in line with the literature (Guo et al., 2010; Feng et al., 2016). The presence of more dopant atoms when increasing NH3 flow could replace carbon atoms in the graphene structure, and this could be the reason for the deterioration of the graphene film quality. Further, due to the decomposition of NH3 during growth, more hydrogen was available, and this could cause more CH4 to disassociate, which meant that more carbon atoms were available in the system during the growth. These along with increasing the partial pressure could explain the growth of the multilayer graphene film. Therefore, we can conclude that chemical doping impacts graphene lattice negatively. The Raman spectra of nitrogen-doped graphene are given in Fig. 8.4A. When the doped graphene band positions and those of pristine graphene band were compared, 7, 2, and 4 per cm band shifts occurred in the G band position for graphene synthesized with 5, 10 and 15 sccm NH3 flow, respectively. These shifts confirm the synthesis of nitrogen-doped graphene; however, Raman spectroscopy is solely not enough to ensure the success of the doping. Based on the results obtained from Raman spectroscopy measurements, 5 sccm NH3 doped graphene exhibited the best quality among three ammonia flows, which were

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FIGURE 8.4 Raman spectroscopy of graphene film grown with different NH3 flow ratio (A), Raman spectra of the doped graphene film grown with different growth times (B), XPS spectra N1s of graphene film fabricated with 5 sccm NH3 30 mins. growth time (C), XPS spectra C1s of graphene film grown with 5 sccm NH3 30 mins. growth time (D).

10 and 15 sccm. Suffice to say, 5 sccm NH3 flow was accepted as an optimum value for next stage of the research. Additionally, to further confirm and evaluate the success of the doping, XPS characterization was employed for 5 sccm NH3 nitrogen-doped graphene. Following the optimization of NH3 flow, the growth time effect (10, 15, 20 25, and 35 minutes) was investigated for the doped graphene synthesis. It was revealed that the quality of the doped graphene films increased with the increase of the growth time up to 30 minutes. Also, multilayer graphene was obtained for 10 and 15-minute growth times. Additionally, a significant decrease was observed in the graphene quality upon decreasing the growth

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time. When the 30-minute growth time was employed, the graphene film quality was affected negatively. As for the 25 and 30-minute growth times, high-quality graphene was obtained. These results are coherent with the literature as the doping rate is expected to increase upon lowering the growth time (Matsoso et al., 2016; Zan and Altuntepe, 2020). Moreover, the G band position was found to be 1590, 1592, 1592, 1588 per cm for 10, 15, 20 25 and 35-minute growth times respectively. The G peak positions upshifted for all the growth times in comparison to the pristine graphene. The Raman spectra of the doped graphene film grown with different growth times are given in Fig. 8.4B. On the other hand, contrary to these growth times for the doped graphene, pristine graphene could not be synthesized below 20 minutes. and above 30-minute growth times in our previous studies. Based on the above results, the 30-minute growth time was determined the optimum value for the doped graphene synthesis with the flow of 5 sccm NH3. Following the Raman spectroscopy measurements, XPS analyses were performed, and the spectrums for C1s and N1s are presented in Fig. 8.4C,D. The main peak was located at 284.4 eV that represented graphite-like sp2 bonding and other peaks were located at 286.1, 288.3 eV, and these peaks represented N-sp2, N-sp3 C bonds in C1s spectra respectively. In the N1s spectrum, the main peak was located at around 401.1 eV, which corresponds to quaternary N, and it represents substitutional doping. Other peaks were located at around 399.4 and 398.5 and represented pyrrolic N and pyridinic N, respectively. The nitrogen amount was found to be %1.8 in the doped graphene film (5 sccm NH3) in XPS measurements. The optical transmission of the film was measured using an ellipsometer. The transmission is one of the most important parameters for a film to be considered as TCE. 5, 10 and 15 sccm NH3 doped graphene films exhibited 96%, 93%, and 85% optical transmission, consecutively. Increasing the doping rate reduced the transparency of the film as expected because the film became thicker with more NH3 flow. Based on the transmission rate, nitrogen-doped graphene could have been employed as TCE, but the best results were obtained for graphene film that was synthesized with 5 sccm NH3 ratio. The electrical characterization of the graphene films was performed for pristine graphene and 5 sccm NH3 doped graphene by four-point probe system. The sheet resistance value is another crucial parameter to evaluate whether the film is suitable to be used as TCE or not. So, the sheet resistance was measured for both pristine and NH3 doped (5 sccm) graphene. Pristine graphene had approximately 550 Ω/sq sheet resistance, while nitrogen-doped graphene had a sheet resistance of about 350 Ω/sq. This result further confirms the success of the doping in the graphene film as the sheet resistance value decreased with the doping as expected. The graphene film grown with a higher NH3 rate can result in a lower sheet resistance value. However, the quality of the film and the optical transmission can deteriorate. For this

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reason, in our experiment, graphene films doped with 5 sccm NH3 were employed for solar cell applications.

8.3.1

Incorporating graphene into silicon heterojunction solar cells

In this study, the cell fabricated using PECVD system was in an unfinished form as TCE and metal contacts were intentionally not made. This type of cell were then taken to the PVD system for ITO film coating. An 80 nm ITO film was deposited onto the cell via DC sputtering technique. Following this procedure, we transferred pristine and nitrogen-doped graphene onto a oneinch size HIT solar cell via wet transfer technique. Then, the metal contacts were formed on the cell structure and to do this, screen-printing technique was applied to the front and back contacts using silver paste. These were followed by the characterization of the cell via a solar simulator. Initially, the cell with only ITO was analyzed. After that, the cell with ITO and pristine graphene, and finally the one with ITO and nitrogen doped graphene were measured. The solar cell with only ITO (80 nm) conversion efficiency (η) was measured to be 9.5%, and the one with ITO and pristine graphene was measured as %10.2. Open-circuit voltage (Voc) 0.53 V, a short circuit current density (Jsc) 0.035 A/cm2 and a fill factor (FF) 0.747 were measured for only ITO coated solar cell. We also analyzed ITO and pristine graphene coated solar cell (Voc 0.379, Jsc 0.035 A/cm2 and FF 0.773). We found out that FF increase was not significant because graphene sheet resistance was not comparable with that of ITO at that time. Despite limited FF improvement, an increase in the cell efficiency was achieved thanks to the work function of graphene. Since graphene work function (4.654.70 eV) is greater than that of ITO (4.584.63 eV), graphene has the potential to improve Voc. This is thanks to some of the properties of graphene like high mobility and transmission nature, which can altogether enhance solar cell efficiency (Filleter et al., 2008). The application of nitrogen-doped graphene film to solar cell enhances the efficiency further in comparison to the cell with ITO and pristine graphene, and only with ITO. In this study, the cell efficiency was measured as 10.8%, which is higher than that obtained with only ITO and ITO and pristine graphene. The other cell parameters for the cell were Voc 0.420, Jsc 0.035 A/cm2, and FF 0.757 for 80 nm ITO coated solar cell along with nitrogen doped graphene. However, it should be noted that the increase in the cell efficiency was lower than we expected. An increase in the efficiency when nitrogen-doped graphene was implemented was foreseen thanks to the decrease in the sheet resistance with doping compared to the undoped graphene. Still, the doping ratio in the graphene film was found to be about 1.8%, and this relatively low doping ratio could be the limiting factor for further reducing the sheet resistance. This could be attributed to a lower increase in the cell efficiency. The cell efficiency is mainly determined by

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TABLE 8.1 Photovoltaic parameters of the cells with pristine graphene, nitrogen-doped graphene and without graphene. Voc(V)

Vmp(V)

Jsc(A/ cm2)

Jmp(A/ cm2)

Filling factor

Efficiency %

Cell with 80 nm ITO

0.363

0.303

0.035

0.031

0.747

9.5

Cell with 80 nm ITO 1 G

0.379

0.321

0.035

0.032

0.773

10.2

Cell with 80 nm ITO 1 N doped G

0.420

0.349

0.035

0.031

0.757

10.8

the dissipation of power across internal resistances. Increasing the series resistance will suppress Jsc and reduce FF. Enhancing graphene properties and further reducing the series resistance may boost the cell performance (Li et al., 2012) (Table 8.1).

8.4

Conclusion

In this chapter, c-Si HIT solar cells with graphene incorporation were studied. First, the literature of c-Si HIT solar cells was reviewed, and the solar cell was fabricated successfully. The heterojunction solar cell showed good photovoltaic behavior with an energy conversion efficiency of 19.7 % on 6" n-type c-Si. The results showed that c-Si HIT solar cells are the most promising solar cell technology with the ability to adapt with innovative materials such as graphene. Second, the literature of graphene was reviewed in brief, and pristine and doped graphene were successfully synthesized using thermal CVD. The synthesized films were characterized using Raman and XPS in terms of quality, thickness, homogeneity, and doping rate. The permanent doping of the high-quality graphene films was obtained thanks to the in-situ doping approach, and this was confirmed via XPS measurements. This confirmation was crucial for the success and applicability of the doped films in various fields and applications. Nitrogen doping partially helped to overcome single layer pristine graphene handicaps like high sheet resistance, which limits the usage of graphene films in semiconducting technology. Following the successful growth and doping optimization, the films were transferred on the Si-HIT solar cells over the transparent conductive layer, and the cell was finished off by the deposition of the front and back contacts using silver paste. The presence of both pristine and doped graphene films on the solar cell improved the cell performance as photovoltaic parameters, such as Voc and FF, increased. Therefore, the cell efficiency increased in comparison to

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the cell without graphene. The presence of graphene may have increased the cell efficiency thanks to its high mobility and antireflective nature. However, the increase in the cell efficiency was higher for a solar cell with nitrogendoped graphene in comparison to the cell with pristine graphene as expected due to the reduction in graphene film sheet resistance with doping. Having graphene on the cell could also contribute to protecting the cell from external conditions, and this makes the cell more durable and stable over a long period. Moreover, the graphene usage over existing TCE in the cell could also challenge the traditional applications of TCE in solar cells, which could open a new direction by balancing the film thicknesses between graphene and TCE. This in return could lower the cost and thickness of the cell without losing efficiency. Graphene can also be used solely as TCE in the solar cells hopefully in the near future without sacrificing the efficiency by further developing the homogeneity of doping and the graphene film quality itself.

Acknowledgment The authors gratefully acknowledge the funding from The Scientific and Technological ¨ B˙ITAK-117M401). Research Council of Turkey (TU

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Chapter 9

Tin halide perovskites for efficient lead-free solar cells Giuseppe Nasti, Diego Di Girolamo and Antonio Abate Department of Chemical, Materials and Production Engineering, University of Naples Federico II, Naples, Italy

9.1

Introduction

Quality of life is an abstract concept and it is hard to quantify it. Despite this, it is possible to show that there is a direct correlation between the yearly amount of energy used by a population, in units of kg of oil equivalent per capita, with respect to the Human Development Index (HDI) (Fig. 9.1). The HDI is an index estimated by the United Nations Development Programme combining measurements of life expectancy, education, and per capita income that has become a common tool to estimate the quality of life in different nations. For this reason the answer to the world’s need for energy is not only in the interest of the over nourished developed countries, but firstly for the underdeveloped populations that could find, in a clean and abundant source of energy, the opportunity to improve their style of life. Nowadays the amount of energy consumed by the world is gigantic. In 2018 it was estimated to be around 14 Billion Tons Oil Equivalent (Gtoe, 1 toe 5 11.63 MWh) that, converted, is equal to 1:6 3 1011 MWh. Almost all this energy is produced using fossil fuels and only 10% using renewable resources. Moreover, of the world’s total electricity production, that is the form of energy usually produced by renewable resources, only one fourth is obtained by renewable sources and solar photovoltaic is not the leading technology (despise being the most growing department in terms of yearly installed power) accounting for only 9% of it, with silicon photovoltaics representing a share of about 95%. In this picture the emerging of hybrid organic-inorganic metal halide perovskites solar cells (PSC, perovskite solar cell) represents a breakthrough that could boost the production and installation of a new era of solar power plants. In only a decade, PSCs have approached the record power conversion efficiency of siliconbased photovoltaics (Fig. 9.2) and surpassed well established thin film

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FIGURE 9.1 The quality of life of a country, estimated by the human development index, is related linearly with the amount of energy available and consumed per capita by its citizens. Based on IEA data from IEA, 2014. Energy use (kg of oil equivalent per capita). ,www.iea.org/ statistics., All rights reserved; as modified by the Author and United Nations Development Programme, 2019. Human Development Report 2019. Beyond income, beyond averages, beyond today: Inequalities in human development in the 21st century. New York ,http://hdr.undp.org/ en/content/human-development-report-2019..

FIGURE 9.2 “Best Research-Cell Efficiencies” chart published and constantly updated by National Renewable Energy Laboratory showing how PSC technology (orange line with yellow circles) has reached in 10 years the efficiency of much “mature” technologies, which development goes back to 1979. Modified from: https://www.nrel.gov/pv/cell-efficiency.html.

photovoltaic technologies such as CIGS (Copper Indium Gallium Selenide) and CdTe (Cadmium Tellurium). The term “perovskite” identifies a vast class of compounds with ABX3 stoichiometry, that shares the crystal lattice with the calcium titanium oxide mineral (CaTiO3), firstly discovered and investigated by the Russian

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mineralogist Lev Perovski. Oxide-perovskites (with O22 at the X-site) are ubiquitous in nature, however metal halide perovskites (Fig. 9.3A) increased exponentially its notoriety only in the last 10 years, after the first application in photovoltaics. Notably, lead halide perovskites were known even before 1900 (Wells, 1893) (and from the 70s in the hybrid organic-inorganic configuration (Weber, 1978)), with first applications as semiconductors for optoelectronics by Saito and Mitzi between 1990 and 2000 (Era et al., 1994; Mitzi, 2001). The first application of lead halide perovskites in a solar cell was reported in 2009 by Kojima and Miyasaka which used methylammonium (MA) lead iodide perovskite (MAPbI3) as a light absorber in a dye sensitized solar cell (DSSC) configuration (Kojima et al., 2009). MAPbI3 was presented as a promising inorganic dye with the power conversion efficiency approaching 4%. An improvement toward 6.54% in power conversion efficiency was demonstrated by the group of Park in 2011 (Im et al., 2011). However, the breakthrough was the outstanding PCE (Power Conversion Efficiency) of 9.7% demonstrated in 2012 by the groups of Gratzel and Park, (Kim et al., 2012) which adopted spiro-OMeTAD as a solid-state state HTM (Hole Transporting Layer), replacing the liquid electrolyte. In 2012 the group

FIGURE 9.3 (A) Cubic crystal lattice of an ABX3 metal halide perovskite; the A position, represented with the blue spheres, can be occupied be large monovalent cations as MA1, FA1, and Cs1, while Rb1 and K1 are usually not included in the lattice but populate interstitial positions; the B species is a divalent cation-like Pb21 and Sn21; the X species is an halide, or a mixture of different halides, usually I2, Br2 or Cl2; (B) the device structure (left) and the mechanism (right) of the experiment used by Lee et al. to demonstrate how the perovskite could be used directly to harvest and transport the photogenerated charges to selective contacts.

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of Snaith clearly demonstrated that the perovskite was not acting merely as a conventional dye (Lee et al., 2012b). In fact, by replacing the semiconducting TiO2 with the insulating Al2O3 power conversion efficiencies above 10% were obtained, proving that the perovskite itself could transport of the photogenerated charges to the selective contacts (Fig. 9.3B). From these pioneering works the efficiencies of PSC cells has grown steadily up to 25.5% in 2019 (Burschka et al., 2013; Eperon et al., 2014; Heo et al., 2013; Im et al., 2014; Jeon et al., 2018, 2014; Liu et al., 2013; Saliba et al., 2016; Xu et al., 2019; Yang et al., 2017, 2015) and the field appears to be closer and closer to commercialization. (Roy et al., 2020) A crucial drawback regarding the environmental and human health risks related to the lead content of PSCs has not been satisfactorily addressed yet. Lead is a well-known toxic element as it can imitate, in its 12 oxidation state, the role of many different ions like Ca, Fe, and Zn in fundamental biological processes. Lead tends to be accumulated in many different human organs, like the brain, liver, kidney, and bones (Lidsky and Schneider, 2003). Moreover, it is well established that it can be the source for miscarriages and fetus malformations if assumed by pregnant women (IPCS, 1995). For these reasons, the lead content in PSC and the consequences of its dispersion in the environment, due to possible accident in a PSC solar plant, must be addressed carefully. A strategy under investigation to minimize the environmental impact of lead is the smart encapsulation of solar modules. Jiang and coworkers demonstrated an epoxy resin encapsulation method, whose selfhealing properties after the mechanical failure, reduce the Pb leakage rate by a factor of 375 in respect to encapsulation based on UV-cured resin (Jiang et al., 2019). A different approach has been developed by Li and coworkers, who implemented two functional coatings based on lead adsorbing materials (exploiting strong lead complexing agents, such us phosphates) on the solar cell, measuring a reduction of lead leakage of 96% (Li et al., 2020c). These approaches might represent an important path toward a safe installation of lead halide perovskite solar modules; however, one must be cautious in assessing the long-term fate of leaked lead. Moreover, it has been clearly shown that the regulation currently adopted for lead-containing electronics cannot be simply extended to lead-based perovskite (Li et al., 2020a). In fact, the bioavailability of lead from perovskite is higher than that from other lead sources. The higher uptake increases the amount of lead accumulated in all the different parts of the plant (i.e., roots, stems, and leaves), which could more efficiently enter into the human food chain. Therefore the substitution of the lead cation in the perovskite represents a necessary condition for the development of sustainable and safe perovskite-based photovoltaics. The most direct approach for the elimination of lead from PSC is the use of tin (Sn21) that has a similar electronic configuration and also a similar ionic radius. However, the low stability of Sn21 due to its easy oxidation to Sn41 is a major limitation for its application for PSC (Abate, 2017). In this

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chapter, we will give a comprehensive picture of this subject dividing it into three sections. In the first we will introduce tin halide perovskites (ASnX3), illustrating the main pros and cons of these compounds. In the second part we will describe the most recent progress in the performances of ASnX3 devices. Finally, in the third section we will present the most promising approaches for the stabilization of such systems that could lead to stable and efficient ASnX3 devices.

9.2 9.2.1

Halide perovskite solar cells: why tin? Perovskite structure

Concerning the ABX3 stoichiometry of metal halide perovskites for photovoltaic application, the A-site cation is a bulky monovalent cation, organic or inorganic, such as methylammonium (MA, CH3NH31), formamidinium (FA, NH2CHNH21), and cesium (Cs1). The X-site ion is a halide, usually iodide (I2), bromide (Br2), and chloride (Cl2). Notably, alloying the A-site cation or the halide allows a fine tuning of the bandgap of the perovskite and impact optoelectronic properties and stability towards environmental or thermal stress. The B-site ion is a divalent metal cation. The perovskite lattice is cubic (even if the tetragonal and the orthorhombic lattice might be found depending on the A-site and X-site composition), with the B metal sixfold coordinated by the halides, X, forming a 3-dimensionale corner sharing BX6 octahedra network. The large A-site cation is located in an AX12 cubooctahedra and is therefore 12-fold coordinated. Lead is the archetypal B-site cation, with lead halide perovskites attaining the best photovoltaic performances so far. In order to find suitable divalent cations to replace lead in the halide perovskite, the first criterion is the ionic radius size. The ionic radius ˚ , is very similar to that of Pb21, B119 A ˚ , (Kour et al., of Sn21, B1.10 A 2019) and, considering that they belong to same group (the 14th) of the periodic table, it represents the most natural choice for substituting the lead (Ke and Kanatzidis, 2019). For example, the ionic radius of Ge21, that belongs ˚ ). Given the close similarity, the to same group, is much lower (0.730.77 A crystal structure of tin halide perovskites can be described within the same framework of lead halide perovskite. Useful parameters to estimate the possibility to form the perovskite structure based on geometrical considerations are the Goldschmidt’s tolerance factor Eq. (9.1) and the octahedral factor Eq. (9.2): t5

ðrA 1 rB Þ O2ðrB 1 rX Þ

ð9:1Þ

rB rX

ð9:2Þ

μ5

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where r is the ionic radius of the subscript A, B, or X species. The optimal tolerance factor to obtain a stable cubic structure (Fig. 9.4A) is t 5 1 and the octahedral factor, μ, comprised between 0.44 and 0.9. If the tolerance factor is comprised between 0.8 and 1.0 it is still possible to obtain an α cubic stable phase. For lower values of t above 0.7 and below 0.8, the cubic structure can be formed only at high temperatures while, at ambient temperature, it transforms, due to the deformation of BX6 octahedra, into tetragonal β phase (Fig. 9.4B) or into orthorhombic γ phase (Fig. 9.4C). If it lowers below 0.7 the perovskite cannot be formed as the structure becomes too much deformed. For values of t higher than 1, that means that A-cation is larger than the commonly used MA or FA cations, the crystal lattice becomes hexagonal and it forms 2D layered structures (Fig. 9.4D) (Xu et al., 2018). Also considering Cs1, the larger nonradioactive cation of the first group of the periodic table, the tolerance factor assumes a value around 0.8, both for lead and tin-perovskites and with all three possible halides (i.e., Cl2, Br2, and I2). In fact, CsBX3 perovskites are not cubic at room temperature. In the case of CsPbI3 the cubic phase is stable only above 300 C, while at room temperature this stoichiometry leads to the yellow phase (δ-CsPbI3),

FIGURE 9.4 Phase change among four CsSnI3 polymorphs showing the equilibrium crystal structures at different temperatures. (A) At 500K black cubic α-phase; (B) at 380K the β-tetragonal phase; (C) at 300K the black orthorhombic γ-phase; (D) at the same temperature, 300K, but after exposure to air the yellow γ-phase. Reprinted with permission from Chung, I., Song, J.-H., Im, J., Androulakis, J., Malliakas, C.D., Li, H., et al., 2012. J. Am. Chem. Soc. 134, 85798587. Copyright (2012) American Chemical Society.

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where the corner sharing BX6 network is lost Li et al. (2016a). The yellow phase has a large bandgap ( . 3 eV) and optoelectronic properties not suitable for photovoltaics application. A similar situation holds for FAPbI3. The room temperature phase of CsPbBr3 is orthorhombic, with a transition to tetragonal around 88 C and the cubic phase stable only above 138 C. Notably, the three CsPbBr3 phases are based on the BX6 network, thus imparting interesting optoelectronic properties to this material, however the large bandgap around 2.35 eV makes this material scarcely attractive for photovoltaics (Rodov´a et al., 2003). The transition between low to high symmetry (tetragonal or orthorhombic to cubic) by increasing the temperature described for CsPbBr3 is typical of most halide perovskite composition. Chung and coworkers characterized in great details the phase transitions of CsSnI3 via temperature dependent single crystals and powder XRD (X-Ray Diffraction, Chung et al., 2012). The cubic α-CsSnI3 is stable above 160 C170 C, while the tetragonal β-CsSnI3 transits to the orthorhombic γ-CsSnI3 around 80 C90 C. Notably, conversion to the yellow phase δ-CsSnI3 was observed upon exposure to air (moisture and oxygen) for 1 hour at room temperature. Heating the yellow phase at 150 C leads to the recovery of the black α-CsSnI3. The lowering of symmetry following α-β-γ phase upon cooling holds in the case of MA tin iodide (MASnI3) and FA tin iodide (FASnI3) as well. However, the α phase, the high temperature phase with the higher symmetry (pseudo-cubic), is the room temperature phase for the two hybrid organic-inorganic tin iodide perovskites (Stoumpos et al., 2013) (Fig. 9.5).

FIGURE 9.5 Formation energy of defects in MASnI3 perovskite as a function of the iodine excess concentration during the film formation creating an (A) I-rich, (B) I-medium, and (C) Ipoor chemical conditions. Reprinted (adapted) with permission from Meggiolaro, D., Ricciarelli, D., Alasmari, A.A., Alasmary, F.A., De Angelis, F., 2020. J. Phys. Chem. Lett. Copyright (2020) American Chemical Society.

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Carrier transport and tin halide perovskite defects

In the effort of finding a different and less toxic element than Pb, many different metallic cations have been tested, like the homovalent cations Sn21 and Ge21 but also the heterovalent ions as Bi31 and Sb31. All these cations have in common the ns2 electronic configuration in the outermost orbital shell. This configuration plays a key role in the performances of perovskite materials. The energy of the s-state is lower than the p-state forming a socalled “lone pair state” (Drago, 1958). These s-electrons hybridized with the p orbital of the X anion forming an antibonding orbital that lowers the valence band (VB) maximum. At the same time the high spin-orbit coupling pushes the conduction band (CB) minimum downward. The peculiar nature of the band edges of lead halide perovskites was considered one of the reasons behind the “defect tolerance” of these semiconductors, that is the characteristic of being scarcely sensitive in term of nonradiative recombination to the presence of defects. In fact, early computational investigations indicated that the defects with low formation energy (e.g., Ai, VB, Xi, VX, VA) are shallow, with energy levels around 0.05 eV above (and below) the VB maximum (and the CB minimum). At the same time, the defects that would have been positioned deep into the bandgap and that would have acted as efficient Shockley-Read-Hall nonradiative recombination centers, have high formation energies (e.g., XB, XA, Bi, BX) and therefore are not easily formed (Kim et al., 2014; Yin et al., 2014). Recent reconsideration pointed out that interstitial iodide induces deep states into the bandgap of CH3NH3PbI3, however remains not effective in term of nonradiative recombination for kinetic reasons (Meggiolaro et al., 2018). Despite the picture of the defect chemistry of lead halide perovskites is not complete yet, it can be stated that the relatively low concentration of deep trap states is responsible for the high carrier lifetime and mobility of perovskites crystals. Lead halide perovskites have shown the most remarkable performances in this sense with a diffusion length that reached 3 mm under low illumination and almost 200 microns under simulated sun illumination, AM 1.5 (Dong et al., 2015). Tin-based perovskites do not reach the same efficiency in the transport of charges through the crystal as the measured diffusion length is usually in the range 200500 nm (Abdelaziz et al., 2020; Yang et al., 2019; Wu et al., 2017). This difference is usually related to the much higher carrier density showed by tin-perovskites (10161018 cm23) compared to lead-perovskites (10141016 cm23) due to typical the self-doping of tin halide perovskites (Lim et al., 2019; Shao et al., 2019). The computational analysis of the defect formation energies (DFEs) for tin halide perovskites revealed that the low DFE of the tin vacancy (VSn) and interstitial iodine (Ii) is the main one responsible for the p-type self-doping of tin halide perovskites. This has been obtained by Chung and coworkers concerning CsSnI3 (Chung et al., 2012) and Meggiolaro and coworkers

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concerning MASnI3 (Meggiolaro et al., 2020; Alkauskas et al., 2011). The DFE can be computationally obtained by the following equation: X DFE½X q  5 ETOT ½X q  2 ETOT ½bulk 2 n μ 1 qEF 1 Ecorr ð9:3Þ i i i where ETOT ½X q  is the energy obtained for a supercell containing the defect and P ETOT ½bulk is the analog for the respective perfect crystal. The term i ni μi account for the chemical potential (μ) of the atom removed (n , 0) or introduced (n .0). The term qEF account for the chemical potential of the electrons, introduced via the Fermi energy (EF) and Ecorr is a computational term introduced to avoid artifacts due to the finite size of the supercells andPother computational aspects. The i ni μi is particularly interesting. In fact, in Sn-rich condition (which could be experimentally achieved via non stoichiometric precursor ink formulation) the DFEs of the VSn and of the Ii increase markedly, as calculated for CsSnI3 and MASnI3, suggesting a strategy to mitigate the tin halide perovskite self-doping. Another source of p-doping might be the accidental inclusion of Sn41, as discussed in detail in the following (Fig. 9.6).

9.2.3

Tin perovskite bandgap

The bandgap of metal halide perovskites is determined by the energetic difference between the valence band maximum (VBM), constituted by an antibonding hybridization of the p orbital of the iodine and the s orbital of the tin (lead) and the conduction band minimum (CBM), arising from the antibonding hybridization of the p orbital of the iodine and the empty p orbital of the tin (lead) (Goyal et al., 2018). The orbital composition of the band edges and the antibonding nature of both conduction and VB is at the basis of a set of peculiar properties, such as the increase of the bandgap with increasing temperature, the opposite behavior of silicon and conventional semiconductors. Moreover, the broad shape of the band edges induces low effective masses of both electrons and holes (about 0.3me) and a shallow excitonic binding energy of few meV (Manser et al., 2016). As a result, the free carriers generation efficiency approaches unity and the mobility of those carriers is very high (within the range 1100 V/cm2s). However, the most important feature is the possibility to tune the bandgap by modifying the composition of the perovskite, especially when modifying the B-site metal and the halide, whose orbitals are directly involved in the VBM and CBM. The A-cation on the other hand, has a much smaller impact on the properties of the perovskite (Bernal and Yang, 2014). The tailoring of the bandgap is a very important tool to produce more efficient PSC (and to implement metal halide perovskites in a vast set technological application such as LEDs, lasing application, photocatalysis, and more). Stemming from the bandgap

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FIGURE 9.6 (A) Scheme showing the origin of the bandgap from the hybridization of the atomic orbitals of lead and tin in pure-lead (left), mixed-lead-tin (center) and pure-tin (right) perovskites. (B) ShockleyQueisser limit and maximum predicted efficiencies, as a function of temperature, for different semiconductors. (C) Bandgaps various of various perovskites. (Figure 9.6A) Reprinted and adapted from Ke, W., Kanatzidis, M.G., 2019. Nat. Commun. 10, 965. By Nature Springer licensed under Creative Commons (2019). (Figure 9.6B) Reprinted and adapted from Dupre´, O., Vaillon, R., Green, M.A., 2015. Sol. Energy Mater. Sol. Cell 140, 92100, copyright (2015), with permission from Elsevier. (Figure 9.6C) Reprinted and adapted with permission ´ V., from Goyal, A., McKechnie, S., Pashov, D., Tumas, W., Van Schilfgaarde, M., Stevanovic, 2018. Chem. Mater. 30, 39203928. Copyright (2018) American Chemical Society.

tunability, metal halide perovskites can be successfully tailored for an efficient implementation in single-junction photovoltaics (also for application as building integrated photovoltaics) or tandem photovoltaics. The vast majority of lead halide perovskite compositions employed in high efficient PSCs has a bandgap above 1.5 eV. FAPbI3 would have a bandgap of 1.4 eV, fitting perfectly with the wavelength dependence of the ShockleyQueisser limit of the maximum achievable power conversion efficiency for a single absorber solar cell. Unfortunately, at room temperature FAPbI3 is stable in the “yellow phase” (or δ-phase, as discussed before). Therefore alloying of FA with MA and Cs1 at the A-site and of iodide with bromide is required to stabilize the “black phase,” but also increases the bandgap (and brings about issues in term of phase segregation due to light, current, electric field). Therefore the bandgap range spanned by lead halide perovskites is close but not perfectly tailored for the maximum theoretical

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efficiency, for a single junction solar cell. With a bandgap comprised between 1.1 and 1.4 eV, tin iodide perovskites can theoretically produce devices with superior performances (Koh et al., 2015; Ru¨hle, 2016; Shockley and Queisser, 1961; Stoumpos et al., 2013). Furthermore, theoretical calculation shown that the optimal tandem stack should have a top bandgap in the range 1.71.9 eV and the bottom bandgap in the range 0.91.2 eV, (Leijtens et al., 2018) which indicate that tin halide perovskites are a valid option as bottom-cell, thus enabling all-perovskite tandem solar cells. Both lead and tin-perovskites are direct bandgap semiconductors and possess very high absorption coefficients, respectively 105 and 104 per cm at the respective bandgap (De Wolf et al., 2014; Jiang et al., 2015; Shum et al., 2010). It is worth to be noted that this extremely high absorption coefficient plays a crucial role in the elevated performances of these devices. In fact, it permits the deposition of a thin layer of the absorbing material that can absorb almost all the incident radiation but with a thickness comparable to the diffusion length of the charge carriers, in the order of 500 nm. In this way, most of the photogenerated carriers have a high probability of being collected by the two terminals of the device. (Stranks et al., 2013)

9.2.4

Tin oxidation

In the previous section the ionic radius similarity was invoked to justify the identification of tin, rather than germanium, as the most promising replacement of lead. Here, we focus on the main difference between the two metals, which is the stability of the 12 oxidation state. In fact, Ge21 is so unstable that it is rarely found in this oxidation state, while Pb21 is on the other hand very stable and does not oxidize into the 14 state (due to relativistic stabilization called “inert pair” effect) (McKelvey, 1983). Tin exhibits both of 12 and 14 as stable oxidation states Comparing the standard redox potential for the Pb21/Pb41 redox couple, 11.67 V versus NHE (Normal Hydrogen Electrode), to the Sn21/Sn41 redox couple, 10.15 V versus NHE, it isimportant to note that the oxidation in air is spontaneous only for the lat ter EO0 2 =H2 O 5 1:23 V vs NHE . The easy oxidation of Sn21 to Sn41 is a serious problem during the formation of the perovskite as it induces the formation of defects in the crystal (Lanzetta et al., 2020). At the same time, the ready oxidation of tin when exposed to air is the reason for its lower environmental toxicity as it forms SnO2 which has very low water solubility and, therefore it cannot be easily absorbed by plants (Granjeiro et al., 2020). The origin of the detrimental self-doping of ASnX3 crystal is not related to the oxidation of tin after the formation of the lattice, as noted by Konstantakou and Stergiopoulos (2017). In fact, in this case the substitution Sn41 defect should be associated to the generation of two free electrons, Sn21 leading to n-doping of the material, while ASnX3 perovskites usually show p-doped characteristic (Kontos et al., 2017). In the work of Gupta et al.

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(2018, 2) the authors tried to list all the possible defects that could form in a tin perovskite. Using the Kro¨ger-Vink notation we can describe a defect formation specifying with XY that a particular species X is located in the lattice position Y, and with a superscript a 0 or a  if the defect is respectively associated with a negative or positive charge, or with a superscript x if both species have the same charge. Vacancies are denoted with a capital V and interstitial positions are denoted with an i. Starting with the defect that we have already illustrated we can write: 2 SnxSn -Sn Sn 1 2e

ð9:4Þ

Using this notation we can clearly see that the oxidation “in situ” of Sn (II) to Sn(IV) is associated with the release of two electrons and should lead to the formation of an n-type material. We can list other three different defect formation processes that should lead to n-doping and, therefore they should not be dominant during the formation of the crystal: 00

VSn -VxSn 1 2e2 0

ð9:5Þ

2Xi -2Xxi 1 2e2

ð9:6Þ

2VxX -2VUX 1 2e2

ð9:7Þ

More interesting we can consider the processes that can lead to p-doping of the material. For example, a halide vacancies is associated with the formation of free holes: VX -VxX 1 h1 The presence since the beginning of the perovskite formation of Sn41 species can lead to p-doping by releasing two holes or by trapping two free electrons: x 1 Sn Sn -SnSn 1 2h

ð9:8Þ

2 x Sn Sn 1 2e -SnSn

ð9:9Þ

The identification of the leading mechanism in the self-doping of tinperovskites is a crucial step in order to mitigate the problem and produce efficient tin-PSC. A strong indication that the formation of Sn(IV) can happen during the formation of ASnX3 crystal has been shown recently by Saidaminov et al. (2020) In their work they showed how dimethylsulfoxide (DMSO), commonly used for the solvent deposition of perovskite thin films, can act as an efficient oxidizing agent for Sn21 leading to the formation of Sn41 right before the deposition of the film (Fig. 9.7).

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FIGURE 9.7 (A) Pictures of mint plants growth on control soil (left) and with 250 mg/kg21 Pb21 perovskite-contaminated soil (right). (B) Lead, and (C) tin uptake ability of plants. Reprinted and adapted from Li, J., Cao, H.-L., Jiao, W.-B., Wang, Q., Wei, M., Cantone, I., et al., 2020. Nat. Commun. 11, 15. By Nature Springer licensed under Creative Commons (2020).

9.2.5

Tin toxicity

Heavy metal cations as Sn21 (and Pb21) are usually toxic for animals as they can mimic other cation of atoms as Zn, Cu, Ca or Fe interfering with the activities of crucial enzymes. The leakage of Sn21 or Pb21 salts would therefore represent a dramatic environmental hazard. However, the risks associated to tin compounds are strictly related to the specific counter anion, which can greatly modify the efficiency of uptake by plants and animals. Tin halides (i.e., SnBr2, SnCl2, SnI4, and SnI2) are easily hydrolyzed by water and, in conjunction with air/oxygen exposure, they readily form the SnO2 and SnO oxides which have very low solubility and are therefore not easily absorbed by living beings (Graf, 1996). Organic salts of tin are instead very toxic as they are liposoluble and can attack the central nervous system. A general formula for an organo-tin compound is RnSnX42n with the toxicity increasing with the increase of the organic substitution of the molecule (i.e., highest for n 5 4) (de Carvalho Oliveira and Santelli, 2010). The direct accumulation of Sn in the organs of plants, animals, and human is not the only source of toxicity that needs to be taken into account. As demonstrated by

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Babayigit et al. (2016), the accumulation of tin halides in the soil can induce the formation of strong acids as, for example, hydroiodic acid (HI). The strong acidification of the soil can be the cause for malformations and death of zebra fish. On the other hand the toxicity of tin, compared to Pb, was not related to the bioaccumulation of the heavy metal in the animal, but only to the formation of HI. As already pointed out, the fast oxidation of tin limits the uptake level by plants and animals. In the work of Li et al. (2020a), it was clearly shown that the amount of tin uptaken by mint plants from a tinperovskite contaminated soil is the same as that uptaken from a standard soil. Lead-perovskite dispersion in soil, otherwise, increases by an order of magnitude the amount of lead uptaken by the plants leading to much higher environmental and health risks. In their work they used MA as organic cation for the tin and lead perovskite; similar results are expected from FA, but a direct measurement would be preferable for a complete scenario.

9.3

ASnX3: a brief historical excursus

The first use of a tin perovskite to produce a solar cell goes back to 2012 when Chen et al. used CsSnI3 in a Schottky solar cell (Chen et al., 2012). A Schottky solar cell is based on the metal/semiconductor junction to produce the band bending required for the splitting and extraction of photogenerated charges. In their case the structure of the device was an ITO/CsSnI3/Au/Ti configuration. The performances were very poor, achieving only 0.9% in power conversion efficiency. They also measured an “unintentional doping” of the perovskite leading to a p-type semiconductor. This problem of the undesired p-type doping of tin-perovskites was already know and reported in previous works (Scaife et al., 1974). For instance, the group of Kanatzidis obtained a 10% PCE a solid-state DSSC configuration employing CsSnI3 as hole transport layer in 2014 (Lee et al., 2012a). In 2014 independently, the works of Hao et al. (2014) and Noel et al. (2014) showed for the first time a completely lead-free MASnI3 PSC with a best efficiency of around 6% under 1 sun illumination. A very interesting result of Noel’s work was that the measured open circuit voltage (VOC) was 0.88 V, with a loss of merely 0.35 V in respect to the bandgap of the perovskite (1.23 eV). Much lower performances were reached by the works of Kumar et al. (2014) and Koh et al. (2015), which substituted the MA respectively with Cs and FA. In both works they found that using a stoichiometric composition for the perovskite absorber lead to not-working devices (i.e., efficiency close to 0%). After the addition of 20% of SnF2 to the precursor solution they managed to reduce the defect concentration and increase the efficiency up to 2%. Using a similar approach by Lee et al. (2016) fabricated FASnI3 PSC with a top performance of 4.8% by the addition of a SnF2-pyrazine complex. Using XPS to measure the ratio between the amount of Sn21 and Sn41 in

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the surface, they found the addition of pyrazine to the precursor solution lead to a sensible reduction of oxidized tin. Zhao et al. (2017), using an inverted configuration, prepared a tin-PSC with a mixed composition of MA and FA. Their best performing device reached 8.12% efficiency and 0.6 V open circuit voltage achieved with the (FA)0.75(MA)0.25SnI3 composition. Gu et al. (2018) tried to solve the problem of the fast oxidation of the tin to Sn41 adding to the precursor solution SnF2 and a dispersion of metallic tin powder. Since the precursor solution of tin perovskite exhibited a dark red color, suggesting the formation of the oxidized form of tin, the addition of metallic tin had the scope of purifying the solution thanks to the comproportionation reaction: Sn41 1 Sn0 -2Sn21 : To avoid contamination of the perovskite with residual metallic tin, the solution was filtered before the deposition removing all the residual Sn pod were dispersed in the solvent. Using this approach, they reached a 6.75% PCE and a VOC of 0.56 V. Still Ito et al. (2018), added small amount of Ge into the FA0.75MA0.25SnI3 precursors solution improving the performances and stability of the mixes SnGe PSC due to the formation, on the surface, of a protective layer of GeO2, as also showed by Chen et al. (2019). An original approach was tried by Wang et al. (2018). By addition of ammonium thiocyanate (NH4SCN) they were able to control the crystal growth of a FASnI3 absorbing layer forming what they called a “2D-quasi-2D3D hierarchy structure perovskite” with a champion device reaching 9.41% efficiency and retaining 90% of the initial performance after 600 hours. Jokar et al. (2019) used the nonpolar guanidinium (GA) cation in various ratio with respect to FA in a tin iodide PSC and added 1% of EDA. For a ratio of 1:4 of GA to FA they obtained 8.5% efficiency for freshly prepared devices. After storage, they measured a sensible increase in the performance of the devices reaching a record PCE of 9.6% after 2000 hours of storage in the glovebox. However, the loss of more than 0.8 eV from the bandgap of the absorbing materials to the measured VOC showed that, despite all the efforts in controlling the crystallization and the defect formation, the recombination in the bulk and on the surface was still a critical issue. Trying to passivate the surface defects, Kamarudin et al. (2019) introduced a chemical posttreatment using spin-coating to spread a solution of chlorobenzene and edamine (i.e., a bidentate amine, ethane-1,2-diamine) on the surface of the perovskite. This treatment increased the VOC of the device from 0.47 to 0.59 V reaching a record efficiency of 10.18% after one week of storage. They also measured a decrease in the amount of Sn21 oxidized to Sn41 on the surface but, as they stated in the paper, the decrease “is not enough to explain the large improvement of VOC.” Nishimura et al. (2020) combined many of the approaches described so far, with a “GeI2 doped (FA0.9EA0.1)0.98EDA0.01SnI3” perovskite, with the aim of tailoring the energy levels of the perovskite absorber. Additionally, the

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FIGURE 9.8 J-V curve of the record efficiency (PCE 13.24% and VOC 0.84 V) for a tinperovskite solar cell, showing how the trap passivation and the tailoring of the band alignment can improve the charge extraction. Reprinted from Nishimura, K., Kamarudin, M.A., Hirotani, D., Hamada, K., Shen, Q., Iikubo, S., et al., 2020. Nano Energy 104858. Copyright (2020), with permission from Elsevier.

substitution of a small amount of FA with the larger ethylammonium (EA) cation reduces the lattice distortion (Nishimura et al., 2019). Overall, the better match between the energy level of the perovskite and the energy level of the charge transport layers improved the VOC up to 0.84 V and lead to a record efficiency of 13.24%, with a measured bandgap of 1.42 eV (Fig. 9.8).

9.4 9.4.1

Toward efficient and stable ASnX3 PSCs Additives

9.4.1.1 Tin containing additives: SnX2 and Sn The addition of an excess of Sn(II) in the perovskite precursor solution is the main approach employed by the majority of research groups to improve the quality of the tin halide perovskite semiconductor. Tin halides (SnI2, SnCl2 and, in particular, SnF2) are the primary source for Sn(II) introduced in the perovskite precursor solution. The first use of tin fluoride as additive in CsSnI3 was reported by Chung et al. (2012). (Lee et al., 2012a) What they found was that the addition of 20% of SnF2 could optimize the charge transport of a DSSC in which CsSnI3 was used as a solid-state HTM, instead of a liquid electrolyte. The efficiency was remarkably high reaching 10.2% (8.51% using a mask) due to improved charge transport but they also supposed that “dye as well as CsSnI3 may take part in light absorption and sensitization in a concerted way, providing a synergic effect.” The positive impact of SnF2 on the performances of tin halide perovskites in solar cells probably arises from a combination of a multitude of

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mechanisms (Gupta et al., 2018; Lee et al., 2016). In first place, SnF2 allows the deposition of perovskite films with a better morphology in term of grain size and substrate coverage. Avoiding pinholes and an excessive grain boundaries density is always beneficial in lead halide perovskites, and this consideration can be reasonably applied to tin counterparts. Additionally, XPS investigations revealed a reduced content of Sn(IV) on the surface of perovskites when SnF2 is added (Hartmann et al., 2020). This observation suggests that the presence of fluoride might have an impact on the oxidation of Sn(II) or on the inclusion of Sn(IV) in the perovskite film, but more investigation is required to clarify this point. Finally, considering the defect chemistry discussed in the Chapter 8, Crystalline-silicon heterojunction solar cells with graphene incorporation, the addition of SnF2 could improve the perovskite by bringing the system into Sn-rich conditions and enhancing the DFE for tin vacancies. Unfortunately, the addition of SnF2 alone is not enough to bridge the gap with lead halide perovskites and reduce the metallic behavior of the tin halide perovskite materials. In fact, Kumar et al. (2014) prepared a lead-free tin-PSC using CsSnI3 as the absorber material and SnF2 as additive obtaining very poor performances. The additive free device did not work at all and the best efficiency, only 2%, was obtained by the addition of 20% of SnF2. Similar results were obtained by other researchers showing that SnF2 can mitigate the metallic behavior of tin perovskite, reducing the carrier density from B1019 to B1017 cm23, but that it does not represent a definitive solution (Liao et al., 2016; Sabba et al., 2015; Xing et al., 2016). Beside tin fluoride, tin chloride (SnCl2) and tin iodide (SnI2) have been exploited as additives to the precursor solution. In two different works published by Marshall et al. (2016), tin(II) chloride and tin(II) iodide were used as additive in CsSnI3 PSC. In both cases the additives improved the performances of the cells by ensuring a tin rich environment during the growth of the perovskite crystal, but the performances were still ranging between 2% and 3% (Marshall et al., 2016, 2015). Gu et al. complemented the introduction of SnF2 with the addition of metal Sn powder, which establishes a redox equilibrium between Sn, Sn21 and Sn41. In detail, metallic tin enables the disproportionation reaction: Sn21 $Sn41 1 Sn0

ð9:10Þ

This reaction has a standard redox potential, ΔE 5 2 0:29 V, as it could be calculated by combining the standard redox potential for the reduction of 0 Sn21 to metallic tin (ESn 5 20.14 V vs NHE) and those pertaining the 2 =Sn 41 21 0 0 Sn /Sn redox couple (ESn 2 =Sn4 5 20.15 V vs NHE). The value of ΔE 41 allows to calculate an equilibrium molar fraction of Sn in the order of 0.11 ppb (when Sn21 is 1 M), which, assuming that the concentrations in solution equal those in the film, would translate in a Sn41 density in the perovskite below 1014 per cm3. However, XPS analysis usually reveals a 0

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much higher content, in the range of few percent (even if this finding might be surface limited and it is not clear whether the experimental conditions of high-vacuum and X-ray exposure induce variation in the Sn oxidation state distribution). It is important to note that the ΔE0 is evaluated in standard condition and might change in different solvents or in the presence of ligands. Nonetheless, with the addition of metallic Sn the precursor solution color turned from red (indicating the presence of Sn41 species) to yellow. The devices prepared using this solution showed a remarkably improved efficiency from 0.09% to 6.19% (Gu et al., 2018). We remark that the addition of metallic tin prevents the accumulation of Sn41 in the precursor solution. However, the solution is not stabilized and will continue to react (dissolving the metallic tin) if exposed to oxidants, thus changing its formulation over time. Moreover, during the film deposition, for example, while spin-coating the perovskite precursor solution, the metallic tin is not anymore in contact with the precursor solution, implying that the concentration of Sn41 might increase due to accidental oxidation, such as that caused by DMSO.

9.4.1.2 Reducing agents The concentration of Sn41 in the precursor solution can be minimized by introducing reducing agents, as demonstrated by several research groups. In principle, the metallic tin we discussed in the previous section is an ideal reducing agent, since its byproduct is Sn(II). However, a broad set of organic or molecular reducing agents has been demonstrated so far and here we revise the more relevant. Hypophosphorous acid (HPA) is an acid with a strong reducing power. Interestingly, this acid have been already used for lead containing PSC by Zhang et al. (2015), which improved the quality of a MAPbI3 film reducing the amount of trap states and improving the quality of the crystalline grains. In fact, acid additives have been exploited to control the morphology of the perovskite film by modifying the colloidal state in the precursor solution (McMeekin et al., 2017). HPA ideally combine its reducing activity with the control of the morphology. For this reason, it was selected by Li et al. for the preparation of CsSnIBr2 PSC. They demonstrated that HPA in solution forms a complex species with the tin increasing the speed of the nucleation, during the formation of the film, and limiting its oxidation. Despite all these interesting properties the measured efficiency was only 3% (Li et al., 2016b). Song et al. tried to use hydrazine (N2H4), a reducing agent much stronger than HPA, usually used to reduce metal salts to their pure metal form. In order to avoid the complete transformation of Sn(II) to metallic Sn, hydrazine could not be used directly in the precursor solution, but they had to use it as vapor during the crystallization process. Hydrazine vapors had a twofold role, reducing the amount of available oxygen in the deposition atmosphere ðN2 H4 1O2 -N2 1H2 OÞ and reducing the oxidized Sn(IV) to Sn(II)

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22 2SnI22 6 1N2 H4 -2SnI4 1N2 1 4HI : Despite a sensible improvement of the recombination lifetimes, the power conversion efficiencies of devices remained relatively low. For the three tin-PSC they prepared, MASnI3, CsSnI3, and CsSnBr3, they obtained PCEs respectively of 3.89%, 1.83%, and 3.04% (Song et al., 2017). Trying a more aggressive approach and, at the same time, avoiding the complications of using hydrazine vapors during the crystal growth, Kayesh et al. added hydrazinium chloride (N2H5Cl) directly into the precursor solution. The FASnI3 PSC devices they obtained showed a uniform morphology without pinholes. The carrier lifetime was sensibly improved and the measured efficiency reached 5.4%, limited by the low VOC (0.455 V) still suggesting the presence of numerous defects in the crystal structure (Kayesh et al., 2018). Using an environmentally friendly reducing agent, the 2,2,2-trifluoroethylamine hydrochloride (TFEAC), together with SnF2, Yu et al. have detected multiple improvements in the fabrication of a FASnI3 PSC. In fact, TFEAC improved the morphology of the perovskite film by suppressing the segregation of the SnF2 additive, induced a better alignment of the electronic bands and reduced the oxidation to Sn41. As consequences of the enhancements of the perovskite properties they registered an increment of the PCE from 3.6% to 5.3% (Yu et al., 2019). Tai et al. tested three different antioxidants for the fabrication of FASnI3 PSCs, namely, phenolsulfonic acid (PSA), 2-aminophenol-4-sulfonic acid (APSA), and the potassium salt of hydroquinone sulfonic acid (KHQSA). For all the cases, the authors introduced the antioxidant along with an excess of SnCl2. They found that the increase of the additive concentration led to a reduced grain size, and observed sharper grain boundaries with top-view scanning electron microscopy, suggesting that the additives were accumulating at the grain boundaries. The optimal amounts of PSA and APSA were 3% with measured PCEs respectively of 4.7% and 4.3% compared to the 2.2% measured for the pristine device. The best performance was obtained with 1.5% of KHQSA reaching 5.7% efficiency (Tai et al., 2019). Li et al. used a cation displacement approach in order to convert a thin film of hydrazinium tin iodide (HASnI3) into a MASnI3 film, obtaining a uniform film with large grains, a very high PCE of 7.13% but still with a low VOC (0.486 V) (Li et al., 2019).

9.4.2

Passivation

As in the case of many semiconductors, the surfaces of perovskite crystals, including interfaces with the selective contacts and the grain boundaries, are sources of nonradiative recombination due to the high concentration of undercoordinated bonds, which might act as traps for the electronic carriers. For this reason the passivation of these trap states is an important strategy

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for the improvement of the performance of PSCs (Abate et al., 2014) and also for the reduction of the photocurrent hysteresis (Shao et al., 2014). Moreover, the poor stability of PSCs might stem from defects at the interfaces and grain boundaries, hence the passivation might also improve the lifetime of PSCs (Boyd et al., 2018; Wang et al., 2019). SnX2 additives, described in a previous section, have been extensively used by many researchers trying to minimize the formation of defects in the perovskite film. Despite the main mechanism behind the positive effect of SnX2 additives is supposed to involve the diminishment of the tin vacancies concentration, (Meggiolaro et al., 2020; Song et al., 2017) Heo et al. suggested instead that this is not always the case, with SnX2 improving the optoelectronic performances through the passivation of the perovskite surface. In particular, in their work, SnBr2 showed the best passivating performances (Heo et al., 2018). An important additive that can effectively passivate the surface of the tin perovskite is germanium, introduced via germanium halides in the precursor solution. Germanium precedes tin and lead in the same group on the periodic table but, due to the lower atomic number, the “lone pair stabilization” of the ns2 electrons in Ge is less effective and therefore Ge(II) is much prone to oxidation even when compared to Sn(II). For this reason, Ito et al. used GeI2 as additive in a FA0.75MA0.25Sn12xGexI3 perovskite order to act as sacrificial agent and protect the Sn21 cations (Ito et al., 2018). The role of Ge in mixed Ge-Sn perovskites was clarified by Chen et al. that observe the accumulation of an oxidized germanium specie, GeO2, on the surface of the perovskite. The presence of the GeO2 overlayer was shown to passivate the surface defects and to improve the stability of the devices. The Ge12 to Ge41 oxidation act as a protective mechanism in which Ge represents a sacrificial species. A mixed Sn-Ge perovskite, CsSn0.5Ge0.5I3, showed the best performance with 7.11% efficiency and maintaining 90% of the PCE after 500 hours of operation in a nitrogen atmosphere and AM 1.5 (Chen et al., 2019). A different approach was ideated by Kamarudin et al. (2019). Instead of preventing the formation of Sn41 in the precursor solution they optimized a passivating posttreatment performed after the formation of the perovskite film. They spin coated a solution of 1,2-diaminothane, a Lewis base, in chlorobenzene on the surface of a FA0.98EDA0.01SnI3 (with the addition of SnF2) to passivate the Sn21 and Sn41 dangling bonds. Interestingly, the authors propose that the main reason for the high surface recombination is the halide deficiency on the surface, rather than the presence of Sn41, deviating from the classical picture. This consideration further confirms that more efforts are required to characterize the surface chemistry and the photo-physics of tin halide perovskites. Nevertheless, the passivation approach proposed resulted quite successful as they obtained a record efficiency of 10.18%, the first surpassing 10% PCE. Jokar et al. explored bulky ammonium cation, such as ethylenediammonium diiodide (EDAI2) and butylammonium iodide (BAI), to passivate the

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surface defects of a FASnI3 PSC. The addition of BAI to the precursor solution leads to a marked orientation of the crystals during the perovskite thin film growth along with an improved connectivity of the grains. The quality of the film was the limiting factor of the performances, in fact, the formation of pinholes during the process lead to a PCE of 5.5% (already improved with respect to the pristine film, 4.0% PCE). The further addition of EDAI2 to the solution solved the pinhole problem and passivated the defect states, improving the device performances up to 7.4% PCE. Moreover, the phenomenon of the “slow surface passivation” was observed, consisting in the improvement of device performances up to 8.9% PCE after more than 30 days. This was attributed to a relaxation of the film strain, as revealed by the shift to lower angle of certain XRD peaks. The correlation between crystal strain and optoelectronics is an aspect of great relevance also for lead halide perovskite, and strain relaxation might explain the improvements. Interestingly, the crystal structure stabilized within 5 days, while the PL lifetime and the PCE of devices increased up to a month. Therefore a surface passivation mechanism was proposed. Notably, the EDA21 cation is likely involved in this passivation mechanism, since with BA1 the same effect was not observed (Jokar et al., 2018). However, further investigation will be required to unveil a phenomenon observed also by other researcher (Li et al., 2020b).

9.4.3

Low dimensional perovskites

One of the most promising approach to improve the performances and stabilities of lead halide PSCs is to form an overlayer of low-dimensional perovskites on the surface of the conventional film (Etgar, 2018; Grancini et al., 2017; Etgar, 2018). The dimensionality here refers to the connectivity of the BX3 octahedra. Within this definition, conventional perovskites are 3dimensional. 2D layered perovskites include large A-site cations, usually constituted of a long alkylic or aromatic chain. The crystal structure is based on (single or multiple) inorganic metal-halide octahedra sheets comprised between the A-cation spacers (Grancini and Nazeeruddin, 2019). The formation of a 2D structure, with respect to the classic cubic lattice of the 3D perovskite, is usually verified using XRD spectra. The formation of diffraction peaks at low angles indicates the presence of elongated unit cells characterized by (0k0) reflections, that are highly related to the thickness of the inorganic layers (Ma et al., 2018). 2D perovskites reduce the oxygen permeation and increase the hydrophobicity of the film, thus are responsible for an improvement of the environmental stability of PSCs. Moreover, the Voc of the devices usually increases thanks to a reduced interface recombination. Efficiencies above 20% have been demonstrated for lead halide PSCs, and this strategy is one of the most explored also in lead-free PSCs (Li et al., 2020b). Using 2-phenylethylammonium (PEA), Liao et al. showed that it is possible to induce the formation of a 2D layered structure in a (PEA)2(FA)n21SnnI3n11

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perovskite, where n is the number of tin halide octahedra layers, sliced in the direction (100), separated by bilayers of PEA. Using 20% of PEA they obtained the best orientation of the perovskite domains perpendicularly to the substrate. The formation of the 2D perovskite reduces the permeation of oxygen, improving the stability, in two ways. Firstly, it improves the directionality of the crystals’ growth leading to more compact layers (which is harder to diffuse though for oxygen molecules compared to the rougher 3D perovskite). Secondly, the formation of a compact layer of organic molecules at the boundary of perovskite nanolayers can block the oxygen diffusion. The so formed 2D perovskite exhibited enhanced stability to oxygen and humidity, with respect to their standard 3D perovskite control, and 5.94% efficiency (Liao et al., 2017). Optimizing the deposition procedure Shao et al. prepared a mixed 2D/3D perovskite film. Adding a small amount of PEA, only 0.08 M, to the FASnI3 precursor solution they observed a clear improvement in the crystallinity of the perovskite and a well-define orientation perpendicularly to the substrate. These characteristics lead to three positive effects: a lower number of grain boundaries, a reduction in the number of Sn21 vacancies and a longer lifetime for the charge carriers, which resulted in an enhancement of the efficiency from 6% of the control to 9% of 2D/3D device (Shao et al., 2018). Xu et al. selected a different molecule, 5-ammoniumvaleric acid, to induce the formation of a 2D phase and direct the orientation of the perovskite film during the crystallization. They prepared different precursor solutions containing SnI2, FAI, AVAI (5-Ammonium valeric acid iodide), and SnF2 in molar ratios 1:0.8:0.2:0.1 and different amounts of NH4Cl. They demonstrated through GI-WAXS (Grazing-Incidence Wide-Angle X-ray Scattering) that ammonium chloride influences the texture of the film, inducing a clear vertical orientation of the crystals which is beneficial for the charge transport towards the contacts. The mechanism behind the influence of these additive on the vertical orientation of the quasi-2D perovskite is still under debate and not fully understood. It is probably related to the coordinating interaction between the Lewis base Cl2 and the Lewis acid Sn21. The addition of 10% of NH4Cl can improve the performances of the device from 4.2% to 8.7% (Xu et al., 2019).

9.4.4

Solvent

A critical issue has been highlighted recently by the work of Saidaminov et al. which identified in the solvents used for the preparation of the perovskite precursor solution a possible source of oxidation for the Sn(II) (Saidaminov et al., 2020). They showed that DMSO oxidizes Sn21 to Sn41 at temperature compatible with the annealing of the perovskite film. DMSO is the solvent most broadly employed for the fabrication of ASnX3 perovskites, due to its chemical interaction with the perovskite precursor complexes, that allows a good

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control over the crystallization of the perovskite and improves the quality of the film morphology. The DMSO-Sn(II) reaction results in the formation of dimethylsulfide, that is also a product of the degradation reaction of DMSO in slightly acidic conditions (Monga et al., 2018). Upon heating of the DMSO/ ASnX3 solution the color turned from yellow to red showing the formation of Sn(IV) due to the redox reaction: 2SnI2 1 2ðCH3 Þ2 SO-SnO2 1SnI4 1 2ðCH3 Þ2 S Despite the reaction might proceed though different paths in solution or in the wet film during the thermal annealing, it is important to consider that a very small quantity of Sn41 might compromise the performance of the solar cell. Therefore it will be crucial develop new DMSO-free processes for the preparation of highly efficient tin PSCs, approaching their theoretical efficiencies.

9.5

Conclusion

Tin-based lead-free PSCs are promising candidates for the upcoming development of more environmentally clean and sustainable energy production. Compared to lead, tin was demonstrated to be potentially less toxic with a lower biological impact. The performances of these devices is still much lower than the lead counterpart and numerous issues still need to be solved. Completely new strategies need to be implemented for these materials as the common techniques and compounds used for producing lead PSCs seem to not work properly with tin, leading to underperforming devices (with respect to the theoretical potentials).

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Section III

Sustainable Materials for Photocatalysis and Water Splitting

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Chapter 10

Photocatalysis using bismuth-based heterostructured nanomaterials for visible light harvesting Araceli Romero-Nun˜ez1, K.T. Drisya1, Juan Carlos Duran-A ´ ´ lvarez2, 1 3 Myriam Sol´ıs-Lo´pez and Velumani Subramaniam 1

Departamento de Ingenier´ıa Ele´ctrica, Centro de Investigacio´n y de Estudios Avanzados del Instituto Polite´cnico Nacional, Mexico City, Mexico, 2Instituto de Ciencias Aplicadas y Tecnolog´ıa, Universidad Nacional Auto´noma de Me´xico, Mexico City, Mexico, 3Department of Electrical Engineering (SEES), Centro de Investigacio´n y de Estudios Avanzados del IPN (CINVESTAV-IPN), Mexico City, Mexico

10.1 Introduction Essentially, photocatalysis refers to a chemical reaction mediated by photogenerated species. Analogous to chemical catalysis, a photocatalyst accelerates the rate of a photoreaction with the help of photons without being getting exhausted. The photocatalysts act under irradiation of light forming the reactive species which will drive the reaction. Utilization of the sunlight, which is the ultimate renewable energy source, is advantageous for the process. Solar energy harvesting and photovoltaics are vast fields and its development is on the go, and the idea of utilization of the photon energy for the chemical reactions is also a promising technology. The photocatalysis is identified as a safe, green, clean, and sustainable process for the environmental remediation (Guo et al., 2019). Semiconductor photocatalysis got a start when Fujishima and Honda performed photocatalytic water splitting in 1970 (Imtiaz et al., 2019). Photocatalysts are chosen by considering their chemical stability, hydrophilicity, durability, toxicity, response to sunlight irradiation and cost effectiveness (Nakata and Fujishima, 2012). Major applications of photocatalysis are spread over a variety of fields such as environmental applications in the such as contaminant degradation and water treatment, health care and medical applications including cancer treatment, drug Sustainable Material Solutions for Solar Energy Technologies. DOI: https://doi.org/10.1016/B978-0-12-821592-0.00008-X © 2021 Elsevier Inc. All rights reserved. 289

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delivery and self-cleaning, medical instrumentation, and also in structural applications such as self-cleaning glass, paints tiles, etc. The process of photocatalysis can be carried out in ultraviolet (UV) light as well as the visible region (Vis-region) of the solar spectra. Vis-light photocatalysis is preferred to make use of the major portion of the spectral light, which constitutes 47% whereas compared to the UV-region is just 3%4% of the spectra. For the basic understanding of the process of catalysis, consider a stained white cloth soaked with detergent and placed in the sunlight, in a few minutes you can see the stain is completely gone. In this example, the stain is the contaminant, and the detergent acts as the photocatalyst. Note that the detergent alone can remove the stain, but the irradiation makes the process faster. There is a strong interest in developing clean and sustainable technologies to address our society’s current environmental issues and energy demands. The constant increase in energy consumption and the prominent use of nonrenewable sources have been causing pollution, environmental degradation, and global greenhouse gas emissions. In terms of fundamental and applied sciences, photocatalysis appears as a central element when dealing with energy shortage and environmental contamination; additionally, it also represents a powerful modern synthetic chemistry tool. Among the wide range of developed and potential photocatalytic nanomaterials, there is still a great need to develop a mature photocatalyst that uses sunlight as a sustainable, renewable, and cost-effective energy source. Sunlight is considered a source of energy for long-term sustainability, and appropriate photoresponsive materials are used in different devices for its collection and harvesting. Bismuth-based functional materials are a well-established group in the realm of photocatalytic reactions in the visible spectral region, which are getting attention considering the nontoxicity, low-cost, and easily tunable morphologies (Merupo et al., 2015). Despite their excellent visible-light responses, the formation of bismuth-based systems (formed combining bismuth compounds) provides an excellent opportunity to reduce recombination of photogenerated electrons and holes, thereby enhancing stability. Careful design and proper synthetic methods will help develop bismuth-based heterostructured photocatalysts for suitable technological applications. For instance, the morphology can be controlled by an adequate synthetic route to obtain 0D, 1D, or 2D architectures, which will yield different optical, physicochemical, and mechanical properties. A tailored tuning of the system properties will address improvements in specific applications for clean energy production and environmental remediation, as depicted in Fig. 10.1. This chapter aims to offer a general overview of nanostructured photocatalysts for visible light-harvesting, particularly bismuth-based heterostructures. We present the fundamentals of photocatalysis, basic features on how they are synthesized, and a brief review on characterization and synthetic strategies. Finally, this chapter presents recent advances on bismuth-based heterostructured photocatalysts for applications in the water treatment, self-cleaning surfaces, and water splitting. The intended audience is undergraduate and

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FIGURE 10.1 The development of photocatalysis using bismuth-based heterostructured nanomaterials for visible light-harvesting is expected to continue to grow in the fields like water splitting, and air and water purification etc.

graduate students, technicians, researchers, and anyone interested in learning about visible light (solar) photocatalysis. It provides useful information for a better understanding of the fundamentals and heterojunction properties of bismuth- containing photocatalysts.

10.2 Fundamentals of heterogeneous photocatalysis Photocatalysis process are generally of two types: 1. Homogeneous catalysis 2. Heterogeneous catalysis In the homogeneous catalysis, both the catalyst and reactants exist in the same phase, hence the recovery of the catalyst from the system will be very difficult. And in the heterogeneous system, both are in different phase and

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this is the widely used methodology. Heterogeneous photocatalysis is a widely known process in which a photon-excited crystalline semiconductor absorbs enough energy to promote the electrons from the highest occupied molecular orbital (HOMO) up to the unoccupied molecular orbital (LUMO), located in the valence band and conduction band (Fig. 10.2), respectively (Amenta and Amenta, 2017). The amount of energy necessary for the electrons’ promotion is the bandgap energy (Eg) and varies from one semiconductor to another. Upon migration and concentration of the photoelectrons in the conduction band, the “voids” in the valence band result in positive charges, creating the “electron-hole” pair (Linsebigler et al., 1995). Finally, these charge carriers can either recombine to achieve the electrons’ background energy level or migrate through the semiconductor’s crystalline network, reaching the surface of the photocatalytic particle to interact with the adsorbed species (Low et al., 2017). When the photocatalytic process is carried out in an aqueous medium, water molecules and dissolved oxygen are the most abundant chemical species adsorbed on the semiconductor surface, followed by the dissolved components in the water matrix (Kabra et al., 2004). Photoholes, electrophilic in nature, oxidize the water molecules producing hydroxyl and hydroxyl-radical activated species (OH2 and OH, respectively); the latter is known to be highly electrophilic and thus oxidizing (Eo 5 2.8 vs NHE) (Amenta and Amenta, 2017). Other minor reactions can% also occur with the aqueous matrix’s dissolved components, such as the carbonate ions to produce carbonate activated species ( CO22 3 , Eo 5 1.59 V vs NHE). The dissolved %

FIGURE 10.2 Illustration of possible molecular orbitals in a semiconductor photocatalysis.

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organic matter (DOM) can be thoroughly oxidized into CO2. However, midoxidation is more common, resulting in a plethora of oxidized species, like carboxylic acids, alcohol, and in some cases excited DOM (Kabra et al., 2004), which is also a strong electrophile. All the reactive oxygen species (ROS) produced by photoholes are able to further oxidize other organic molecules, producing a cascade of chemical reactions that ends with the mineralization of the DOM, that is, the production of CO2, H2O and other min22 eral species, like NO2 3 or SO4 . On the other hand, the photoelectrons accumulated in the conduction band with the dissolved oxygen (O2) to produce the superoxide radical

 2react O2 , a very oxidative species that is able to start a chain reaction generating H2O2 and OH species (Low et al., 2017). Less spontaneous is the interaction of photoelectrons with water molecules, splitting them into H1 1 OH2. When photoelectrons own a high reduction potential, a further reduction step occurs, producing H2 by splitting water molecules. This process demands a very reductive environment (EH1/H2 5 0 V vs normal hydrogen electrode (NHE), while caution must be taken to avoid the re-oxidation of H2 by interaction with photoholes and other oxidizing species (Ismail and ´ lvarez et al., 2014). The schematic representaBahnemann, 2014; Dur´an-A tion of the photocatalytic process is shown in Fig. 10.3. Heterogeneous photocatalysis is a surface process, which can be ´ lvarez et al., 2014). described in the following steps (Dur´an-A 1. The reagents are adsorbed on the surface of the photocatalyst. 2. The photocatalyst is photoexcited, producing the electron/hole pairs. 3. When the recombination rate is low, the charge carriers move through the crystalline network to the surface of the photocatalyst. 4. The charge carriers reaching the surface of the photocatalysts react with the adsorbed molecules. 5. Redox reactions produce ROS, such as OH or  O22 radicals and H2O2, which react with other organic/inorganic adsorbed molecules. 6. The intermediaries produced by the adsorbed organic molecules’ oxidation are desorbed from the photocatalyst surface, releasing active sites for the adsorption of other molecules, which can be used for a new cycle to begin. G

Heterogeneous photocatalysis can be perceived as a limited process compared with other light-driven processes, given that the chemical reactions occur only on the photocatalyst’s surface. However, this drawback can be easily overcome via the synthesis of photocatalysts displaying high surface area. Several works in this area have reached important milestones with respect to developing photocatalysts with outstanding surface area values, some of them with hundreds of square meters per gram of solid (Lindstrom et al., 2007). Additionally, heterogeneous photocatalysis will pave way for improved processes and ultimately the sustainability (Mun˜oz et al., 2005, 2006; Gonzalez-Perez et al., 2018): (1) truly efficient photocatalysts can be

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FIGURE 10.3 Photocatalytic process for removing organic compounds, such ibuprofen, in water using semiconductors materials in the form of (A) powders and (B) thin films.

reused through several reaction cycles; (2) there is no need to use hazardous reagents to perform the reaction; (3) there is a growing number of synthetic routes to obtain functional photocatalysts utilizing principles of green chemistry; and (4) the new photocatalysts are produced to achieve the harvesting of sunlight, producing highly oxidizing and reducing charge carriers under low-energy irradiation (i.e., visible and near-infrared light).

10.2.1 Heterogeneous photocatalysis applied to environmental engineering processes In the last few decades, efforts have been made to implement heterogeneous photocatalysis for environmental engineering processes, such as utilizing renewable energy sources and the abatement of water and air pollution

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(Low et al., 2017; Di Paola et al., 2012). Further, heterogeneous photocatalysis has the potential to remove organic pollutants by oxidation (Zhu and Zhou, 2019), inactivate pathogens (Laxma Reddy et al., 2017; Zhang et al., 2019a), destroy antibiotic-resistant genes (Dunlop et al., 2015), and turn heavy metals from deleterious to innocuous species (e.g., the oxidation of As13 to As15 or the reduction of Cr16 into Cr13) (Lee and Park, 2013; Barakat, 2011). Environmental photocatalysis is carried out mostly in aqueous media; therefore the matrix’s specific characteristics heavily impact the efficiency of the process. The photocatalytic capability of the material and light source will play an important role in the photocatalytic performance.

10.2.2 Factors affecting the photocatalytic process Physical, chemical, optical, and electrical properties of the semiconductors strongly impact their photocatalytic performance. In this section, a nonexhaustive description of such impacts is presented; for a more profound description, the reader can call on the cited references.

10.2.2.1 Physical properties The crystalline architecture of the semiconductor plays a crucial role in its photoactivate capacity under visible light irradiation. A semiconductor can display different photocatalytic activities under visible light irradiation depending on its crystalline phase during the synthesis process. When the crystallite size is small, the nanocrystals’ bandgap tends to shrink. As the crystallite size approaches the value of the quantized Bohr radius, the associated confinement effects can increase the bandgap energy. Therefore, the photocatalyst synthesis must produce those polymorphs displaying the lowest bandgap energy value in nanocrystals with a size above the Bohr radius. In terms of the secondary size (i.e., at the particle scale), the 3D nanostructures, like microspheres, have demonstrated maximum photocatalytic performance since such architectures have high specific surface area values, increasing the adsorption capacity of the material. Some 2D structures, such as multilayer nanotubes, are also useful in photocatalysis because of their broad specific surface area distributions that are efficient for separating the charge carriers (Paramasivam et al., 2012). From an environmental engineering perspective, 2D structures, such as thin films supported on static or fluidized beds, are the most efficient presentation of the photocatalyst for a continuous process. Other textural properties of the nanomaterials, such as pore size and volume, also impact the photocatalytic performance. They modulate the adsorption of the target reagents and the scattering of light within the photocatalyst particles. In this sense, high adsorption and low light scattering are sought desirable.

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10.2.2.2 (Photo)electrochemical properties The semiconductor’s chemical composition has a crucial role in the valence and conduction bands’ energy levels, and thus the overall band gap energy value. For example, in metallic oxides, the O 2p orbitals and their hybridizations consist of the band structure, providing a highly oxidizing valence band and a moderately reducing conduction band. Alternatively, for chalcogenides, like sulfides, selenides, and tellurides, the orbitals forming the conduction band tend to be more reductive, which results in suitable materials for the water-splitting process. The cations’ orbitals are also crucial in the valence and conduction bands’ energetic position of the photocatalyst, contributing with d and f orbitals. The electrons’ path from the valence band to the conduction band upon irradiation can be direct when the HOMO and LUMO are aligned in the valence and conduction bands, respectively. In this case, the semiconductor’s activation requires the energy equivalent to one photon to produce one photoelectron, although the recombination of the photoelectron is highly spontaneous. In contrast, when the HOMO and LUMO positions are not aligned, the electronic transition is indirect; thus the energy requirements are higher (Dong et al., 2015). The insertion of traces of foreign atoms within the crystalline network, known as doping, can notably impact the Eg value and the potential of the semiconductor’s valence and conduction bands. For example, when TiO2 is doped with nonmetallic atoms, such as nitrogen, carbon, or boron, the bandgap value is reduced due to the insertion of electronic states within the inter-band space, decreasing the energy necessary to promote an electron toward the LUMO state. In contrast, when fluoride is used as a doping agent, energetic states are formed below the valence band, raising the Eg value. Similarly, the substitution of atoms within the crystalline network leads to the formation of vacancies (e.g., oxygen vacancies in oxides), accelerating the separation of the charge carriers and their migration toward the crystal surface. Nevertheless, caution should be taken, as large numbers of such (oxygen) vacancies within the crystal can change the photocatalyst architecture by acting as recombination centers. Finally, the semiconductors’ superficial modification to form semiconductor-semiconductor or semiconductor-conductor heterostructures strongly impacts the light absorption spectrum and the potential charge carriers formed in the new photocatalyst (Chen et al., 2016; Moniz et al., 2015; Drisya et al., 2020). 10.2.2.3 The matrix composition Dissolved substances and suspended particles provide water with its actual and apparent color, respectively. Both parameters can define the rate of conversion in the photocatalytic process, interfering in the dispersion of light within the aqueous matrix, the adsorption of the reagents, the interactions between the charge carriers and the adsorbed molecules on the photocatalyst surface, and even turning the reaction toward the production of undesirable products or intermediaries. In the case of DOM, when the photocatalysis

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process’s objective is to remove target molecules, for instance, microcystins and microorganisms in surface water sources, natural DOM acts as a scavenger of charge carriers and ROS that otherwise remove the target pollutants. Natural DOM can also promote a screening effect, reflecting the light and reducing photons’ density reaching the photocatalyst. Dissolved inorganic salts abundant in surface and ground waters, such as nitrate, carbonate, chloride, sulfate, can also scavenge photoholes to produce active species

  22  22 owing to their lower oxidation potential than NO2 ; CO ; and SO 3 3 4 that of the OH radicals, resulting in both the drop of the photocatalytic rate and the generation of unwanted intermediaries. Other metallic ions can also negatively impact the photocatalytic process (Espinoza et al., 2010). Finally, pH plays a decisive role in heterogeneous photocatalysis, as it defines the speciation of the target molecules—either protonated or deprotonated—as well as the superficial charge of the photocatalyst due to the density of OH, O2 or OH1 2 terminal groups on the surface of the crystalline particles. It is essential to keep a constant charge on the catalyst particles to avoid the agglomeration of the particles and to manage the pH of the aqueous media to encourage electrostatic interactions between the target contaminants. Thus, the photocatalyst can significantly improve the efficiency of the photocatalytic process. So far, it is well known that the highest reaction yields can be achieved under mild acid conditions, as the generation of ROS is favored. In contrast, in basic media, the OH can be scavenged by the excess OH2 species produced by the H2O. As well, the CO2 produced by the complete oxidation of the organic matter will be turned into carbonates, CO22 3 , therefore increasing the water hardness (Hern´andez-Ram´ırez et al., 2015).

10.2.3 Insights of physicochemical characterization of nanophotocatalysts Physicochemical characterization of photocatalytic nanomaterials is essential for the development of this research field. Understanding the fundamental properties of the nanocatalysts will lead to a more representative correlation with their functional and technical performance in photocatalytic applications. The goal is to obtain enough information about the system so that the catalyst behavior. Essential characterization and analysis of nanostructured materials includes determining the following properties: crystal structure, phase, elemental composition, size and morphology, and topology. Specific characteristics such as defect features, exposed facets, distribution of phases, the formation of interfaces, particle size distribution, surface area, porosity, and surface reactivity properties become essential when working with nanostructured photocatalysts. This chapter’s intention is not structural and physicochemical analysis; however, a brief list of common techniques for analysis of nanomaterials is included (Table 10.1). Measurement of certain characteristics may not be

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TABLE 10.1 Summary of some of the main experimental techniques used for the characterization of nanomaterials (Hernandez-Ram´ ´ ırez and Medina-Ram´ırez, 2015; Zhang, 2009; Dent and Smith, 2005). Characteristics/ properties

Technique

Main information derived

Structural analysis

X-ray diffraction

Structure arrangement, including unit cell angles and parameters, crystallite size, preferential orientation, and quantitative phase determination when refined.

Transmission electron microscopy (TEM)a

Morphology, crystal phases, exposed surfaces, direction growths, and point defects on areas of about some nanometers.

High resolution (HR-TEM)

All information obtained with conventional TEM plus crystalline defects, interfaces, and grain boundaries.

Raman spectroscopy

Surface composition, defects such as oxygen vacancies.

X-ray photoelectron spectroscopy

Quantitative chemical composition as well as chemical bonding information and oxidation state of the surface elements.

Secondary ion mass spectroscopy

Analyze surface layers of about 12 nm in static mode. The dynamic mode can provide information on the composition and depth distribution of trace elements detected from subnanometer to tens of nanometer.

Scanning electron microscopy (SEM)b

Crystal size and shape, grain boundaries, and topography.

Atomic force microscopy (AFM)

Topographic 3D image of the sample in which vertical resolution is about 0.1 nm, while the lateral resolution is about 1 nm.

N2 adsorptionc

Specific surface area, presence of pores, volume, and size distribution of the pore.

Elemental analysis

Morphology characterization

Textural properties

a Energy dispersive X-ray spectroscopy (EDS), electron energy loss spectroscopy, and high angle annular dark field, also known as Z-contrast, are commonly adapted in TEM microscopes and used for composition analysis. b EDS detectors are commonly implemented into SEM microscopes for elemental composition analysis. c Other techniques for estimating surface area are include mercury porosimetry and, small-angle scattering of X-rays or neutrons.

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possible using only one technique; consequently, use of multiple analyses is recommended to characterize complex system characteristics. Additionally, some techniques can deliver only local information about a certain part of the sample, while other analyses may offer average measurements. Therefore the selection of techniques and analysis path depends mainly on the information sought for the material/system and on their complexity.

10.3 Bismuth-based heterostructures for photocatalytic applications Bismuth-based semiconductors have caught the attention of the scientific community since most of them are photocatalytically active under visible light irradiation and obtained by simple or “soft” synthesis procedures, like precipitation or solvothermal method. In terms of sustainability and their capacity to get photoactivated under visible light irradiation, the spontaneous formation of crystalline self-assembled 2D and 3D structures facilitates the development of synthetic methods that follow the principles of green chemistry (Silvestru et al., 2002; Chen et al., 2016; Meng and Zhang, 2016; Lin et al., 2014). Another remarkable feature that makes the bismuth-based nanomaterials more sustainable is their innocuous properties. Some nanostructured photocatalysts, such as Ag2O, TiO2, ZnO, or CeO2, have been shown to cause deleterious effects on a variety of aquatic and soil organisms, which even in their ionic form, can be micronutrients for plants and fauna. In contrast, the occurrence of bismuth-based materials, like Bi2O3, Bi2 S3, or BiVO4, has not been related to harmful effects in any biota studied? In some cases, the formation of biofilms has been observed in bismuth-based thin films. Based on the advantages mentioned, bismuth-based nanomaterials can become a new generation of sustainable photocatalysts. The valence band in bismuth-based semiconductors comprises the hybridization of the oxygen 2p orbitals and the bismuth 6s orbitals. This hybridization results in a significant property common to the bismuth-based semiconductors. The valence band’s oxidation potential, given by the Bi15/ Bi13 pair (1.6 eV vs NHE), is considerably lower than that of the OH/H2O pair (2.1 eV vs NHE). Hence, the generation of OH radicals via the oxidation of water molecules by photoholes is restricted. The resulting oxidation potential displayed by the bismuth-based semiconductors’ valence band relies on the energy of the orbitals of the other atoms in the HOMO, such as oxygen in Bi2O3 and sulfur for Bi2S3. As depicted in Fig. 10.4, the presence of oxygen in the semiconductor takes the potential of the valence band toward more oxidizing potentials, while the opposite phenomenon occurs in the case of Bi2S3. A very efficient strategy fostering the oxidation potential of the valence band in the bismuth-based semiconductors is the incorporation of a third atom, either a metal, alkali, or halides in ionic form- conforming ternary structures, like Bi2WO6, BiOI, BiVO4, and bismuth titanates. However, caution

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FIGURE 10.4 Bandgap values and band potentials of some bismuth-based semiconductors. Information from R. He, S. Cao, P. Zhou, J. Yu, Recent advances in visible light Bi-based photocatalysts, Chin. J. Catal. 35 (2014) 9891007. https://doi.org/10.1016/S1872-2067(14)60075-9.

must be taken since the modification of the band potentials directly impacts the band gap value, usually increasing it (Fig. 10.4) (He et al., 2014). Moreover, it drops the photoactivity under visible light irradiation, as occurs with the bismuth oxychloride (BiOCl) and bismuth oxyfluoride (BiOF). Even though ternary structures are synthesized, the formation of the OH species by the oxidation of the water molecule is still limited for most of the bismuth-based materials compared to other semiconductors, like TiO2. Also, the valence band’s potential in the ternary bismuth oxides will depend on the crystalline structure and the chemical composition of the material; for instance, the occurrence of halide vacancies in the bismuth oxyhalide materials. The low generation of OH radicals is crucial in terms of the bismuth-based materials’ photocatalytic activity since photoholes will mainly drive the oxidation reactions. Thus nucleophilic molecules (e.g., phenolic compounds, polyaromatic hydrocarbons, and basic compounds) will be the primary reagent in the process. In photohole-driven reactions, the positive charge carriers reaching the crystalline semiconductor’s surface will “suck” the electrons from the absorbed compounds, leading to molecular rearrangements that can be interpreted as degradation and possibly mineralization. The trend of bismuth-based semiconductors to generate photoholedriven reactions could be seen as a restriction in the photocatalytic process; nevertheless, it offers an essential window of opportunities in the field of fine chemistry. The bismuth 6p orbitals form the conduction band of bismuth-based materials with the atoms’ high energy orbitals added to the ternary oxides, such as the halides. The reduction potential of the conduction band of some bismuth-based materials suggests important limitations of these semiconductors to produce H2 via the water-splitting reaction. Similar to that mentioned

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for the valence band, the reduction potential of the conduction band can be tailored by modifying the crystalline architecture and the chemical composition of the semiconductor, mainly in terms of atomic vacancies. In practical terms, the midstability of bismuth relative to other crystalline semiconductors should be considered when these materials are used as photocatalysts. Bismuth can be reduced into its metallic form upon prolonged UV irradiation or by thermal treatment at temperatures above 200 C. Conversely, bismuth is prone to react with dissolved ions in aqueous media. For instance, a thin layer of BiOCl can form on the surface of Bi2O3 when used as a photocatalyst in tap water; the same happens with BiOI turning into BiOCl. Moreover, bismuth can form coordination bonds with organic molecules; as well as pincer bonds and π-π interactions, which are common in medicine’s organometallic compounds. The formation of strong (irreversible) bonds between bismuth and organic compounds may lead to the loss of active sites on the semiconductor surface, which have an undesirable effect on the photocatalytic activity. Considerable efforts have been developed to address those drawbacks and enhance the photocatalytic properties of bismuth-based systems. One of the most efficient approaches to further upgrade their photocatalytic performance is the formation of semiconductor-semiconductor heterostructures, which will be further discussed in the next section.

10.3.1 Semiconductor-semiconductor heterostructures using bismuth-based materials When semiconductor-semiconductor heterostructures are synthesized, additive or synergistic effects are sought, compared to the separated components’ photocatalytic performance (Jiang et al., 2018; Dong et al., 2015). In order to achieve this, the simultaneous photoactivation of both semiconductors is a condition sine qua non. Otherwise, the heterostructure functions as a support/photocatalyst system rather than a photocatalyst/cocatalyst approach (Del Angel et al., 2018). By coupling two semiconductors in a heterostructure arrangement, the alignment of both semiconductors’ valence and conduction bands will occur spontaneously. Such arrangement may be classified as (B) “straddling gap” (type I), (B) “staggered gap” (type II), and (C) “broken gap” (type III) (Low et al., 2017), as shown in Fig. 10.5. The type I heterounion consists of the semiconductor B, which displays a more oxidizing valence band and a more reducing conduction band than semiconductor A. Under this arrangement, photoelectrons in the conduction band of semiconductor B migrates to the conduction band of semiconductor A, while photoholes are mobilized to the valence band of semiconductor A. In this scheme, the charge carriers are accumulated in semiconductor A, and then recombination occurs, disrupting the photocatalytic process (Low et al., 2017; Moniz et al., 2015).

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B B A

B

A

A

Type I

Type II

Type III

straddling gap

staggered gap

broken gap

FIGURE 10.5 Schematic representation of semiconductor-semiconductor heterostructures.

In the heterostructure type II, the potential of the valence and conduction bands owning to semiconductor B presents more negative values than semiconductor A. Upon irradiation, photoelectrons migrate to the conduction band of semiconductor A, while the photoholes move from the valence band of semiconductor A to the valence band of semiconductor B. This migration leads to the effective separation of the charge carriers, increasing their lifetime and encouraging their displacement toward the surface of the photocatalyst and thus increasing the conversion rate. Nevertheless, it is vital to notice how the potential of the photoholes and photoelectrons gets reduced upon the migration from one semiconductor to another (Low et al., 2017; Del Angel et al., 2018; Moniz et al., 2015; Sharma et al., 2019). In the type III heterounion, a more prominent impairment in the potential is observed than the type II heterounion. In this case, the bands are not overlapped, so the charge carriers’ migration is not spontaneous. Due to this, the heterostructure does not provide a good separation of the charge carriers. Additive or synergistic effects are not observed in the photocatalysis process than the single components (Low et al., 2017; Moniz et al., 2015). Type II heterounions own the highest potential to boost the semiconductors’ photocatalytic activity by increasing the charge carriers’ separation. The decrease of the redox potential of these charge carriers could be tackled by the construction of semiconductor n-semiconductor p heterounions (Wang et al., 2014). As shown in Fig. 10.6, upon the photoexcitation of the heterostructure, the electrons accumulated in the conduction band of the semiconductor type p mobilize to the conduction band of the semiconductor type n, passing through the n-p interphase. At the same time, photoholes are transferred from the valence band of the semiconductor type n to that of the semiconductor type p. This process results in the accumulation of photoholes in the semiconductor type p, and the type n semiconductor acts as a capacitor (Low et al., 2017). The charge carriers’ diffusion continues until the Fermi levels’ equilibration in the heterounion, creating an inner electric field as the charge carriers are transferred from one semiconductor to another (Low et al., 2017; Moniz et al., 2015; Wang et al., 2014). The formation of n-p heterostructures increases the transfer of the photocreated charge carriers, enlarging the

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Electric field Type p semiconductor Conducon band

Type n semiconductor Fermi level

Conducon band

Valence band

Valence band

FIGURE 10.6 Schematic representation of the charge carrier mobilization in an n-p heterostructure.

lifetime of the hole/electron pair and increasing the composite’s photocatalytic performance.

10.3.2 General strategies for synthesis of bismuth-based semiconductors In general, the nanomaterials are synthesized by two basic approaches as given in Fig. 10.7. In both cases the band gap engineering is possible that allows for (1) better and adequate bandgap and photoelectrochemical characteristics, (2) enhanced electron-hole pair lifetime, and (3) a high number of reactive sites. For the synthesis of the photocatalytic materials, the majority of the techniques utilize a bottom up synthesis strategy; mostly liquid-phase methods, which include sol-gel, hydrothermal synthesis, also called wet-chemical methods. These methods consist of the crystallization/precipitation of metallic ions, carried out using a metal salt and a precipitating agent as precursors. These methods have been employed to obtain Bi-based materials with high crystallinity, several morphologies, and controllable particle size by regulating solvents, reaction temperature, aging time, pH value, and additives. Those methodologies can also be a strategy for active facet preparation of crystals, surface defects, and 3D hierarchical microstructures. The exposed active facet of a photocatalyst plays a vital role in the photocatalytic activities due to the following reasons: (1) the adsorption and activation of reactants are determined by the arrangement of surface atoms, thereby tuning catalytic activity and selectivity, (2) the facets are determined by surface electronic structures or surface states, which provide the photoinduced

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FIGURE 10.7 Main approaches for the synthesis of nanostructures; (1) a bottom-up approach where, aggregation of basic building blocks for the heterojunction structure, such as atoms or molecules form the desired nanostructures, and (2) a top-down approach, cutting down of a bulk system to form the desired nanostructures by different tools/methodologies.

charge carriers with adjustable oxidation and reduction abilities for photocatalytic reactions, (3) crystal orientations determine the transfer and separation efficiency of charge carriers, leading to different charge densities for surface reactions. Whereas, solid-phase methods for synthesis of photocatalysts utilize a solventless reaction process where two solids are being mixed in different stoichiometry. These methods include physical vapor deposition (PVD), chemical vapor deposition (CVD) and molecular beam epitaxy (MBE) etc. Gas phase reaction yields nanoparticle during the cooling process. High energy ball milling is a top down approach of the synthesis, also a solid-phase method which is an example for the mechanochemical milling. This route is a well-accepted technology considering some factors such as; low cost, low temperature requirement, comparatively simple execution, and also the higher reaction rates. Normally, the precursors used for the milling process are bismuth oxide (Bi2O3) and vanadium oxide (V2O5). It is well established as an industrial technology. The balls used are usually made from tungsten carbides or ceramic materials. The reaction is executed in a closed jar which containing the balls and the precursor particles. The ball to powder ratio (BPR), time of execution and the speed of rotation are the factors to be considered during the process. Repeated fracturing and welding mechanism is happening inside which ensures the solid state reaction that further ensures the formation of the desired products (Merupo et al., 2016).

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The catalysts might either be immobilized or suspended. Hence different methodologies for the synthesis of nanoparticles and films can be considered. Hence we discuss different synthesis methodologies of Bi based photocatalysts and further details can be obtained from the corresponding literatures.

10.3.2.1 Sol-gel synthesis Sol-gel synthesis is a wet chemical route for the synthesis of the nanomaterials. It has several advantages since it is a low-temperature process, and ensures high homogeneity and purity of the final product. Furthermore, the properties of the final product (such as size, morphology etc.) can be modified by manipulating the process parameters. The process comprises of following significant steps; 1. Hydrolysis—Hydrolysis and partial condensation of the alkoxides results in the formation of the Sol. 2. Gelation/polymerization—formation of the M(metal)-O-M or M-OH-M bonds by polycondensation to form gel-like structure. 3. Ageing—Continued condensation of the molecules that further results in the removal of the solvents. 4. Drying—Collapses the porous network to form xerogel. 5. Dehydration/sintering/calcination—For the removal of the surface M-OH groups and other residual solvents at higher temperature. During the hydrolysis, the precursors reacts with the solvents such as water or alcohol, reaction between adjacent molecules takes place and metal oxide linkages are formed during the polymerization. These polymeric links are further linked through the process of gelation to form a gel network and this aggregation results in the ageing of the gel. During the process of drying the gel gets shrink since it is drying under elevated temperatures. And finally, by the process of dehydration or also known as calcination, the chemically bound water as well as other chemical residues are eliminated. Certain experimental parameters to be taken care during this process are; pH of hydrolysis, amount of water solvent used, temperature of gelation and calcination temperature. Victor et al synthesized the BiVO4 nanoparticles by sol-gel methodology using ammonium metavanadate (NH4VO3) and bismuth nitrate penta hydrate Bi(NO)3.5H2O and Citric acid precursors dissolved in water and nitric acid (Merupo, 2016). The general route for the sol-gel synthesis method is illustrated in Fig. 10.8. The nanostructures can be of different shapes, from spherical, to rod-like to other geometries. Also, the nanomaterials can be made into films using dip coating spin coating, and even by the drop casting of the formed gel during the sol-gel process.

10.3.2.2 Hydrothermal/solvo thermal synthesis The hydrothermal/solvo-thermal route is another method for the synthesis of the nanostructures. Several researchers have synthesized Bi-based nanomaterials

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FIGURE 10.8 Sol-gel synthesis of the nanomaterials.

such as bismuth ferrite, bismuth titanate, BiVO4 etc. by this method (Wei et al., 2013) and its references. In this method, the nanomaterials are synthesized at elevated temperature and pressure in a sealed container. Here water is used as the solvent. If the process uses alcohol as the solvent, the process will be known as a solvo-thermal process. Here the precursor reactants will be dissolved in water or other solvents in a sealed vessel. By this method, nanomaterials produced under high pressures can be synthesized without much loss of the material. The composition of the final product can be controlled efficiently (Yang et al., 2018). Fig. 10.9 illustrates the scheme of a general hydrothermal reactions. The general route for the hydro/solvo thermal process involves; 1. Addition of the reactants into the solvent to form the ions or molecular groups. 2. They undergo aging in the solution and later transferred to a Teflon container (Hydrothermal reactor), where the process takes place at an elevated temperature and pressure. 3. The final product is collected and undergoes drying/sintering to form the final product (nanostructures).

10.3.2.3 Ball milling process Ball milling (Fig. 10.10) is a mechanochemical methodology for the synthesis of Bi-based photocatalytic materials. The ball mill works basically like a mixer grinder, which is an example of a top-down approach (Merupo et al., 2015; Venkatesan et al., 2012). In this method, the bulk particles get grinded by the balls to form nanoparticles. It can be used for oxides or nanocomposite synthesis and/or structure/phase composition optimization [14,41]. Mechanical activation by ball milling is known to increase the material reactivity and uniformity of spatial

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FIGURE 10.9 Hydro/solvothermal process. The reaction, in aqueous or other medium, proceeds under conditions above 100 C and 1 bar into the sealed vessel.

FIGURE 10.10 Illustration of the ball-mill process. (A) Sample and balls inside the rotation chamber. (B) The breaking phase, where repeated fracturing of bulk reactants causes formation of composite particles with desired compositions. (C) The welding phase, where small agglomeration of particles forms the final morphology of the powder.

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distribution of elements. Milling process takes place in a closed container which will be loaded with balls, usually of tungsten carbide, and the precursor materials either with or without addition of liquid, such as propan-2-ol. Critical parameters to consider when using ball mills are: rotational speed, load, density and size of the ball and compositional ratio. Repeated fracturing and welding happen due to the high energy collision between the balls and also the collision between the inner wall of the container and balls. The properties of the final products depend upon the ball-to-powder ratio (BPR), chemical properties of the grinding material, milling atmosphere, milling time, nature of the milling material etc. (Merupo et al., 2015).

10.3.2.4 Sputtering process In some of the photocatalytic applications, the use of a powder form is not practical. For example, in processes like water treatment, the use of the photocatalyst powder and its further recovery makes the process more complex. Hence, films of the photocatalysts are utilized. There are several existing methodologies such as PVD, CVD, and MBE etc. PVD is a thermal evaporation technique where, the atoms are ejected from a target by thermal energy and they get deposited onto a substrate. In contrast, the CVD is the process where the precursors are directly introduced onto a heated substrate to form the film. All these methodologies are useful for the fabrication of the Bi based as well as other nanomaterials photocatalysts. Victor, et al has developed films of BiVO4 by a PVD method using Radio-Frequency sputtering (RF-Sputtering) (Fig. 10.11). When a radio frequency power is applied onto the cathode (where the target is situated and the substrate holder is connected to the anode), under the high voltage, the gas gets ionized and then forms plasma inside the chamber between the target and substrate. The Ar1 ions bombard with the target surface which causes the migration of ions from the target surface to the substrate and hence get deposited over the substrate surface. Argon and Oxygen are chosen as the inlet gasses. Argon has used considering its wide availability and nonreactivity. Deposition can be done at low temperature (even at room temperature). This method ensures good adhesion between the film and substrate, uniformity, stability and controllability of deposition parameters. This process can be scaled to large areas (Venkatesan et al., 2012). A large number of publications exist on synthesis of photocatalysts (Merupo et al. 2015, 2016), well-defined structures (Lin et al., 2014; Chen et al., 2020; Li et al., 2016a, b), 2D layered materials (Liu et al., 2020; Zhao et al., 2020), hierarchical nanostructures (Lin et al., 2014; Xu et al., 2019; Li et al., 2020a), thin-films (Di et al., 2017; Liu et al., 2020; Zhao et al., 2020), heterojunctions (Chen et al., 2016; Wang et al., 2019a; Sun et al., 2018; Drisya et al., 2020), and green-synthesized quantum dots of Bi-based photocatalysts (Bhoi et al., 2018; Venkatesan et al., 2018).A great variety of

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FIGURE 10.11 Schematic of radio-frequency sputtering.

bismuth-based heterostructured powders and thin films can be obtained using the techniques mentioned above. Essential aspects to highlight are (1) the previous design of the material directed toward the enhancement of required properties, (2) development of optimum conditions to improve reproducibility and quantity, and (3) preference for using local and abundant materials leading to a sustainable and cost-effective process.

10.3.3 Applications of bismuth-based heterostructures Three of the most useful applications involving vis-light environmental photocatalysis are included in terms of the required characteristics of bismuth-based heterostructures.

10.3.3.1 Water treatment Water treatment is an environmental remediation application of photocatalysis. The schematic of the experimental set up for the photocatalytic reaction is illustrated in the Fig. 10.12. This application requires a simplified experimental setup. Usually the contaminant solution will be kept under stirring once the catalyst is added to make the homogeneous distribution of the catalyst in the whole solution. The initial concentration and pH of the starting solution is recorded. Before irradiating the liquid-catalyst mixture has to be kept under dark with continued stirring. This is to ensure the adsorption desorption equilibrium between the catalyst and the contaminant solution (adsorption favors the degradation). Once it is achieved, we can have the

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FIGURE 10.12 Schematic of a photocatalytic reactor set up for a water-treatment application.

irradiation on to the solution. The intensity has to be checked prior to beginning the experiment. After ensuring that the sample is properly irradiated and that all other conditions are appropriate, samples are collected and the absorbance is measured at each time interval of the irradiation. There exists a certain absorbance wavelength for every light-sensitive contaminant. So, the absorbance (A) at that wavelength has to be measured for each interval. Considering Beer-Lambert law, which states that the quantity of light absorbed by a substance is directly proportional to the concentration, the rate of the degradation reaction can be calculated using the following equation. ln

A 5 e2kt ; k is the reaction rate constant A0

The percentage degradation of the contaminant can be found using another expression c0 2 c % degradation 5 3 100 c0 where c0 is the initial concentration of the contaminant and c is the concentration after time t. The equations mentioned below gives the basic reaction happening within the system during the photocatalysis. In a wastewater treatment plant, heterogeneous photocatalysis can be used as a polish treatment after the secondary effluent. Once the suspended solids, which cause turbidity to water are removed, light can pass through the liquid matrix allowing the photocatalytic reaction (Ahmed and Haider, 2018). Thus photocatalyst can oxidize DOM for the removal of recalcitrant

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Primary treatment

Secondary treatment

Primary sludge

Secondary sludge

Sand remover

Sand

311

to disinfecon

Photocatalyc reactor

FIGURE 10.13 The allocation of the photocatalysis process in a wastewater treatment plant. The wastewater is passed through several tanks and filters that separate water from contaminants, on each stage the degree of effluent quality is increased.

from the previous stages (Rizzo et al., 2009), pathogens remaining in secondary effluent are inactivated (Imtiaz et al., 2019; Ibhadon and Fitzpatrick, 2013). Also, the antibiotic resistance genes are destroyed (Dunlop et al., 2015), Fig. 10.13. Barkul et al. (2017) tested the removal of dyes in secondary effluents using TiO2 as a photocatalyst under sunlight irradiation. Compared to the chlorination process, the photocatalytic reactions reduced the risk of formation of trihalomethane compounds (Liu et al., 2008). The effluent from the photocatalytic process in wastewater treatment plants can be discharged to surface water bodies with indirect or no human contact, like ornamental lakes and offshore seawater (Tsoumachidou et al., 2016). Also, treated water can help support urban needs, such as melting the snow on the streets during the winter. From the water quality viewpoint, heterogeneous photocatalysis improves the capacity of indigenous biota to cope with organic compounds in the effluent, lowering the risk of eutrophication in the water, due to exhaustive oxidation of organic matter leading to the production of CO2, leaving the water via volatilization (Wu et al., 2018). However, photocatalysis cannot remove nitrogen and phosphor. Given that heterogeneous photocatalysis is so efficient in removing DOM, its implementation is not recommended when effluents are planned to be used in agricultural irrigation. Furthermore, a disinfection step is necessary according to the precautionary principle, and UV light irradiation or oxidation with performic or peracetic acid can be applied to avoid the formation of trihalomethane compounds. In drinking water treatment systems, heterogeneous photocatalysis is an excellent option to remove organic matter since biological processes are unsuitable. Similar to the established procedure for wastewater treatment, the photocatalytic process should be located after the coagulation-flocculationsedimentation stages to avoid the occurrence of suspended particles (Ahmed and Haider, 2018) depicted in Fig. 10.14. Heterogeneous photocatalysis has shown high efficiency in treating water from superficial sources, removing organic compounds that confers color, taste, and odor to water, such as mycrocystins (Zhang et al., 2019b;

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Coagulaon Flocculaon Sedimentaon

Filtraon

to disinfecon

Heterogeneous photocatalysis

FIGURE 10.14 Localization of the heterogeneous photocatalytic process in a drinking water treatment plant.

Choi et al., 2007) and humic acids (Ljubas, 2005). Additionally, disinfection can be achieved (Ahmed and Haider, 2018), along with the removal of organic pollutants (Ayekoe et al., 2017), oxoanions (Marks et al., 2016), and other organic compounds regulated by water quality standards. For groundwater purification, heterogeneous photocatalysis has been used since the 1990s to remove oil-related organic compounds (Marks et al., 2016; Mehos and Turchi, 1993). Recently, this technology is used not only to deal with polyaromatic hydrocarbon (Vela et al., 2012) but also, to treat aquifers polluted by pathogens (De Vera et al., 2018), nitrates (Hirayama and Kamiya, 2014), pharmaceuticals, and other trace pollutants coming from leakages in the sewerage in urban areas (Paredes et al., 2019). Water decontamination by heterogeneous photocatalysis is commonly performed in batch systems, either at laboratory scale (Ahmed and Haider, 2018) or using microreactors (Asha et al., 2015), presents a low-efficiency treatment for continuous flow. In contrast, the compound parabolic concentrator systems have been successfully used in the sunlight-driven photocatalysis process (Tanveer and Guyer, 2013). This approach can treat high volumes of water under dynamic flow conditions, displaying higher efficiencies than batch systems in terms of the amount of water treated and the removal of organic contaminants, and the inactivation of pathogenic bacteria (Malato et al., 2016). This approach is mostly under the experimental phase, and laudable efforts have been made worldwide for industrialized application and commercialization (Spasiano et al., 2015). When heterogeneous photocatalysis is integrated into a water treatment process, the use of nanopowders becomes a significant drawback, as the particles must be recovered before release the effluent, making necessary the integration of a filtration step. This drawback increases the cost of the whole depuration treatment and raises the production of greenhouse gasses, discouraging the use of heterogeneous photocatalysis as a sustainable operation in water treatment (Mun˜oz et al., 2005). The photocatalyst can be used either as a thin film or deposited on high-density porous support to tackle this challenge, such as zeolites or activated carbon (Ismail and Bahnemann, 2014).

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In these systems, the photocatalyst is deposited on the support surface for producing ROS when irradiated. The water pollutants are then adsorbed on the high-density porous support and oxidized due to ROS. Finally, the supported photocatalyst is separated at the end of the reaction 1 and recovered for a new reaction cycle, while the purified effluent is released from the reactor. Many efforts on the development of suitable visible light photocatalyst based on bismuth functional materials have been made. By forming bismuthbased heterostructures, a remarkable variety of photocatalysts have been investigated for water treatment applications in pollutant degradation through disinfection processes. Some of the most recent vital advances are displayed in Table 10.2.

10.3.3.2 Self-cleaning Self-cleaning property is relevant to maintain a surface free of undesirable agents like organic matter, microbial pathogens and air pollutants. Photocatalytic self-cleaning represents one of the most important ways to achieve this purpose. When a photocatalyst is deposited on a surface, their photooxidizing capacity contributes to the degradation of contaminants, either gaseous, liquids or solid ones, thus improving and maintaining the surface’s characteristics for a longer time. Potential applications of self-cleaning coatings go from building surfaces such facades, windows, decorating panels, and cement-based coverings, to textiles and hospital items. The self-cleaning mechanism follows the conventional steps of gas-solid heterogeneous photocatalysis. When the photocatalyst is irradiated with the proper energy, it generates electron-hole pairs that lead to the formation of highly oxidizing species, such H2O2 and OH, which then start the degradation of organic pollutants to transforming them into mineralized products as CO2, mineral acids, and H2O (Pichat, 2013). Additionally, indoors photocatalytic coatings can also purify the air from volatile organic compounds and perform disinfection processes based on the same photodriven degradation mechanism. The second aspect regarding self-cleaning property relies on water wettability behavior, which refers to the degree of water attraction or repulsion by a surface. Control over surface wettability is a critical aspect of self-cleaning surfaces. The extreme wettability cases are known as super-hydrophobicity and super-hydrophilicity, exhibiting self-cleaning features through different mechanisms, Fig. 10.15. Super-hydrophobic coatings allow water droplets to carry out soluble contaminants as they slide out the surface. This behavior can remove water from vertical structures, permitting the cleaning of building exteriors using raindrops or the removal of fogging from mirrors, lenses, and shower screens. It is important to note that in this case no photocatalytic property is needed. In contrast, super-hydrophilic surfaces favor the formation of the water-coating interface, which, in combination with photocatalytic activity, can degrade dirt and give surfaces an all-over wash once water naturally flows down.

TABLE 10.2 Recent vital advances in functional bismuth-based heterojunction materials for the removal of water pollutants. Heterojunction

Synthesis method

Pollutant

Light resource

Photocatalytic activity

Reference

Bi2O3/Bi2SiO5

Solid state

2,4dichlorophenol

500 W Xe lamp35 mW/cm2

k 5 1.0/h

Lu et al. (2018)

Ag/AgCl/ BiOCOOH

Solvothermalprecipitationphotoreduction

Rhodamine B

300 W Xe lamp

kapp 5 0.1353/min near 100% degradationC0 5 10 mg/L

Li et al. (2019a)

ZnO/BiVO4

Hydrothermal-wet chemical

Reactive black 5

15 W cool daylight bulb8.5 mW/cm2

92% degradation after 180 minkapp 5 0.0132/min

Sen Chang et al. (2020)

WS2-QDs/BiOBr

Hydrothermal

Ciprofloxacin

500 W Xenon lamp40 mW/cm2

92% degradation

Fu et al. (2020)

Ag/p-Ag2S/nBiVO4

Ion-exchange deposition

Oxytetracycline/ hydrochloride

500 W Xe lamp, .420 nm

kapp 5 0.041/min 99.8% degradationC0 5 20 mg/L

Wei et al. (2018)

Ag6Si2O7/ Bi2WO6

Precipitation

Ciprofloxacin

300 W Xe lamp, .420 nm

kapp 5 0.0227/min 94.8% degradationC0 5 20 mg/L

Li et al. (2020b)

Bi4NbO8Cl gC3N4

Ball-milling

Tetracycline

(300 W Xe lamp, λ . 420 nm)

kapp 5 0.0125/min 78%C0 5 10 mg/L

Wu et al. (2018)

BiOI/ZnWO4

Hydrothermalprecipitation

Nitrous oxide

Simulated sunlight

48.24% removal

Gong et al. (2020)

LaFeO3/ Bi3NbO7

Hydrothermal/solgel

Cr (VI)

300 W Xe lamp, . 420 nm

kapp 5 0.0212/min85% of reduction to Cr (III)C0 5 10 mg/L

Xu et al. (2020b)

CeO2/Bi2MoO6

Molten salt

Cr (VI)

5 W white LED light

kapp 5 0.04113/min 97% reduction to Cr (III)

Yang et al. (2019)

BiOI@Bi5O7I

Solvothermal

PFOA

800 W Xe lamp, simulated sunlight

kapp 5 0.247/h81% degradation60% mineralizationC0 5 15 mg/L

Wang et al. (2019b)

CuS/Bi2W2O9

Combustionhydrothermal

Diuron

150 W Xe lamp, . 420 nm

kapp 5 0.016/min95% degradation after 3 h.C0 5 10 ppm

Li et al. (2020a)

Bi2WO6/Ag/ AgBr

Solvothermal/ photodeposition/ precipitation

Ciprofloxacin

25 W Xe lamp, . 420 nm

100% degradation in 1 h and complete mineralization upon 3 h. C0 5 50 ppb in tap water

´ lvarez A et al. (2019)

kapp, degradation rate constant; PFOA, perfluorooctanoic acid; QD, quantum dots.

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FIGURE 10.15 Self-cleaning mechanisms on (A) superhydrophobic surface and (B) superhydrophilic surface.

As with other photocatalytic applications, the use of suitable materials, which are active in the region of visible light, is highly desirable. Generally, the research in materials design has been directed toward favoring this feature. In these terms, bismuth-based compounds arise again as great alternatives and complements of the extensively studied UV-active photocatalysts TiO2. Gnayem et al. present the self-cleaning properties of a composite of BiOCl0.8Br0.2 solid solution with gypsum (Gnayem et al., 2015); this is a commercially available construction material based on calcium. Clean-up of naphthalene and rhodamine B stains over the surface of the composite were acquired on less than 5 minutes of irradiation, demonstrating the outstanding self-cleaning potential of the composite. Degradation was attributed mainly to a direct photooxidation reaction of the organic testers with photogenerated holes. A super hydrophobic system was presented by Yu et al., who studied the dendritic Bi/Bi2O3/ZnO heterostructure (Yu et al., 2018). They demonstrated the system’s feasibility to self-cleaning by spreading white chalk powder over the surface, placed in a vertical position, and washed down with water droplets; the process resulted in a clean surface without any residuals on the rolling path of the droplets. Unlike Bi/Bi2O3/ZnO heterostructure, other bismuth-contained super hydrophobic systems, such BiOCl (Li et al., 2011), Bi2WO6 (Yang et al., 2015), and Bi2S3 (Xiao et al., 2010), require the use of sensitizers to enhance the super hydrophobic property.

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Finally, Singh and Vaish synthesized fluorinated SrO-Bi2O3-B2O3 transparent glass-ceramics within different fluorination degrees and suggests the system can be used as self-cleaning structural applications. Their results exemplified both photoinduced hydrophilicity and hydrophobic behavior toward fluorination degree. Better photodegradation under a methylene blue solution is observed for moderated fluorination, while higher hydrophobicity was found when fluorination increases (Singh and Vaish, 2018). It is noteworthy that bismuth-based heterostructures have diverse potentials to fulfill the criteria for solar cleaning utilizing photodegradation of adsorbed organic compounds and as well as wettability feature, either superhydrophobicity or super-hydrophilicity. A wide range of bismuth-based photocatalysts expect to meet the self-cleaning characteristics under solar irradiation; therefore they are waiting to be tested over their wettability, durability, and corrosion-resistant features in order to continue with the development of viable self-cleaning surfaces.

10.3.3.3 Water splitting Water splitting is another major application of photocatalysis. In this process, the goal is fuel production, where the water is split into (H2) and (O2) gas. Hydrogen a carbon-free fuel burned with oxygen, making it a clean, renewable and environmentally friendly fuel. Regarding renewable energy production, heterogeneous photocatalysis has been mostly used in the production of H2 by the water-splitting reaction (Ismail and Bahnemann, 2014). As mentioned before, this process is performed only under highly reductive conditions. Thus most of the published works are carried out using batch systems, while a few works have raised the idea of moving this procedure toward hydrodynamic conditions (Wei et al., 2017). Photocatalytic water-splitting process follows the same stages of photocatalytic reactions. It firstly involves the excitation of a semiconductor by the appropriate light irradiation and generation of the electron-hole pairs, followed then by their corresponding separation and transportation to the surface where it will react with surface adsorbed species. The electrons in the conduction band reduce the adsorbed H+ to H2 and the holes in the valence band oxidize water into oxygen according to the next reactions: Reduction : 2H1 1 2e2 -H2

E0 5 0:00 eV

Oxidation : 2H2 O-O2 1 4H1 1 4e2 Overall : 2H2 O-O2 1 2H2

E0 5 1:23 eV

E0 5 2 1:23 eV

In order to obtain a proper photocatalytic material for water splitting, the conduction band energy level should be lower than 0 V vs NHE, more negative than the reduction potential. And the valence band energy level should

TABLE 10.3 Recent critical advances in functional bismuth-based heterojunction materials for photo(electrochemical) water splitting. Heterojunction

Synthesis method

Electrolyte

Photocurrent density

Photocatalytic activity

Reference

Bi2O2.33/Bi2S3TiO2

Electrodeposition

0.1 M Na2S0.1 M Na2SO3

5.9 mA/cm2

H2:62.61 μmol/hO2: 0.5 μmol/h

Ma et al. (2020)

CuAl2O4/ Bi2WO6

Electrospinning/ solvothermal



B 0.24 mA/ cm2

H2: 17.8 μmol/g/hO2: 5.8 μmol/g/ h47.78% charge separation

Zhang et al. (2019c)

2D Black phosphorus/ BiVO4

Solvent exfoliation/ hydrothermal

0.1 M Na2SO4

B 0.60 mA/ cm2

H2: 0.16 mmol/g/hO2: 0.10 mmol/g/ h

Zhu et al. (2018)

Bi/Bi2O2CO3

Solvothermal

0.25 M Na2SO30.35 M Na2S

B 0.03 mA/ cm2

H2: 81.5 μmol/g/h

Sun et al. (2020)

Bi2O3/MoS2

Hydrothermal





H2: 10 mmol/g/h

Khalid et al. (2020)

Ba5Nb4O15/gC3N4

Hydrothermal8Wet chemical

0.025 M oxalic acid

B 0.88 mA/ cm2

H2: 2.67 mmol/g/h

Wang et al. (2020)

CuS/BiVO4

Electrochemical deposition, dip coating

0.5 M Na2SO4

B 1.2 mA/cm2

H2: B0.82 μmol/h

Li et al. (2019b)

Bi6Fe2Ti3O18/ BiOBr

Hydrothermal/ion exchange

0.5 M Na2SO4

B 0.85 mA/ cm2

O2: 50 μmol/g/h

Gu et al. (2019)

Bi3O4Cl/Bi2O2Cl

Hydrothermal/ exfoliation

0.2 M Na2SO4

B 0.065 mA/ cm2

O2: 58.6 mmol/g/h

Ning et al. (2019)

Black phosphorus/ BiOBr

Hydrothermal/ solvothermal



B 0.030 mA/ cm2

O2: 89.5 μmol/g/h

Li et al. (2020c)

YF3:Yb, Tm@BiOCl

Hydrothermal





O2: 0.50 mmol/g/h λ . 780 nm

Wang et al. (2019c)

Au/BiVO4/ZnO

Spin coating

0.5 M sodium phosphate buffer

2.87 mA/cm2

40% IPCE

Kim et al. (2018)

2D, two dimensional; IPCE, incident photon-to-current efficiency.

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be higher than 1.23 V versus NHE, more positive than the oxidation potential. Additionally, the following requirements might be considered: adequate band gap to be used unvisible range, enhanced electron-hole pair lifetime and high number of reactive sites. The design of heterostructures and the incorporation of additional cocatalysts represent another way to achieve efficient water splitting systems. For instance, the TiO2/BiVO4 heterostructure was used to support silver nanoparticles by S. Ali et al. and tested as a photoanode for water splitting. Simple addition of silver (Ag) nanoparticles further decreases the heterostructure’s band gap and photoluminescence intensity via a surface plasmon resonance (SPR) effect. Tong et al. develop a TiO2/BiVO4/Co-Pi nanorod array on a FTO substrate, where TiO2 nanorods performed as one-dimensional (1D) electron-transport tunnel, BiVO4 as light harvester, and Co-Pi (CobaltPhosphate) as cocatalyst. Electrochemical impedance spectroscopy results reveal that Co-Pi significantly decreases the charge transfer resistance at the photoanode/electrolyte interface. The band alignment of the TiO2/BiVO4 1D nanoarray heterostructure was beneficial for the charge separation; a hydrogen production of B7.31 mmol/cm2/h was achieved for this efficient water splitting system (Tong et al., 2017). Some of the latest works focused on the development of bismuth-based functional heterostructures for water splitting are summarized in Table 10.3. Currently, hydrogen production via photocatalytic water splitting is a process under investigation, and very few attempts have been made to scaleup this process (Singh and Dutta, 2018). The apparent slow pace of this development is due to the relatively high economic costs vis-´a-vis the modest fuel production, in addition to the safety issues linked to handling this flammable gas. Furthermore, upon the generation of H2, an efficient storage device is necessary; Several approaches have been developed for these storage devices, based either on adsorption or chemisorption of the gas molecule; a comprehensive review of such methods can be found in references (Durbin and Malardier-Jugroot, 2013; Lim et al., 2010; Rusman and Dahari, 2016).

10.4 Conclusions This chapter is intended for students, technical people, and researchers who wish to learn more about the visible light photocatalysis and materials explored in the 2010s around the world by various research groups. Bismuthbased heterostructures represent novel alternatives to titanium dioxide-based nanomaterials; due to their unique properties such as (photo)electrochemical characteristics, the possibility to synthesize with controlled morphology, toxic innocuity, and photocatalytic efficiency. They offer a vast potential for the generation and commercial application of these materials extending into other green technologies. This chapter briefly presents insights on general and integrative perspectives of the field covering fundamentals, materials synthesis and characterization, and some relevant photocatalytic applications.

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The role of bismuth-based visible light photocatalysts in practical applications like chemical-free water treatment systems, clean energy generation, and selfcleaning devices to reduce pollution are detailed. In water treatment, these structures reduce the usage of large amounts of toxic chemicals in various places, such as the sea, freshwater, and soils leading to water treatment systems that significantly reduce the level of processing chemicals. Energy generation by water splitting devices has already attracted attention and gained importance as a renewable source of clean fuel. Visible-light H2 production can be a boost to this field. Minimizing and avoiding contamination is an essential route; Self-cleaning systems represent an emerging opportunity to ablate air contaminants in closed sites and buildings as well as usage in medical disinfection applications. These applications can help integrate a broader strategy to eliminate the complex environmental issues that prevalent in the early 21st century. Based nano heterostructures should receive increased attention from the scientific and technological community to fully comprehend and optimize these sustainable, high-efficiency photocatalytic systems.

Acknowledgments The authors wish to gratefully acknowledge CONACYT SENER Project no.263043 and ´ lvarez thanks SEP-CINVESTAV Project no. 200 for the financial support. J. C. Dur´an-A the Secretar´ıa de Ciencia, Tecnolog´ıa e Innovacio´n de la Ciudad de Me´xico grant (SECITI/047/2017). And K.T. Drisya, A. Romero-Nun˜ez, and M. Sol´ıs-Lo´pez wish to thank CONACYT for their Ph.D. and postdoctoral grants.

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Chapter 11

Recent advances in 2D MXenebased heterostructured photocatalytic materials Sudeshna Das Chakraborty, Pallab Bhattacharya and Trilochan Mishra Functional Material Group, AMP Division, CSIR-National Metallurgical Laboratory, Jamshedpur, India

11.1 Introduction Energy, environment, and health are the most critical global issues faced by mankind and hence need immediate attention (Chu and Majumdar, 2012; Lubchenco, 1998). For all these issues solar photocatalysis is in the forefront as the most desirous green process of the future. At present the observed quantum efficiency and lack of stability of the catalytic materials are limiting the process. However, the right material as photocatalysts is needed to mitigate the impending fossil fuel crisis by utilizing these photocatalysts for efficient water splitting, accelerates the CO2 reduction to fuel and fasts the pollution degradation kinetics. In the recent years, two-dimensional (2D) materials including metal chalcogenides (Haque et al., 2018; Lu et al., 2016), graphene and graphene-based composites (Stoller et al., 2008; Zhu et al., 2011), metal organic frameworks (MOF) (Farha et al., 2010; Li et al., 2009) and MXenes/MXene-based composites (Ghidiu et al., 2014; Lukatskaya et al., 2013) have stimulated international research in this area. These 2D materials are of interest because of their unique physicochemical properties. At this juncture, emphasis on the development of new 2D materials has the potential to accelerate advancement in the field of solar light utilization. Graphene-based 2D materials show promise for inactivating viruses (Ye et al., 2015) due to sharp edges of their sheet-like structure. Can 2D materials bring new innovations in critical applications in the areas of energy, health and the environment? By definition, 2D materials have either single-atomic or few-atomic layers with thicknesses typically less than 5 nm. Whereas, the lateral dimension of these 2D sheets may extend from 100 nm to a few micrometers (Tan et al., 2017; Tan and Zhang, 2015). Sustainable Material Solutions for Solar Energy Technologies. DOI: https://doi.org/10.1016/B978-0-12-821592-0.00005-4 © 2021 Elsevier Inc. All rights reserved. 329

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After the renewal of graphene research in the mid-2000s, at least 18 new varieties of 2D-nanosheets have been reported. Among this extended family of 2D materials, MXenes, which belong to the family of 2D transition-metal carbides, nitrides and carbonitrides, are one of these new materials introduced in 2011 by Gogotsi et al. Since this time, MXene material research is growing rapidly due to its exceptional properties including: large surface area, excellent chemical and thermal stability, high melting point, improved oxidation resistance, high electrical and thermal conductivity and hydrophilic nature (Ghidiu et al., 2014). In general, MXenes are described as Mn11XnTx, where “M” represents (3d and 4d) a transition metal (such as V, Ti, Nb, and Mo), “X” represents the C/N ratio having n 5 1, 2, or 3, and Tx represents a surface functional group (hydroxyl, oxygen, or fluorine) (Dillon et al., 2016). MXenes are generally created by selective removal of the metallic layers of “A” from their parent MAX phases. According to the above formula, MXenes may be of different structures like M2X, M3X, and M4X (Khazaei et al., 2013; Naguib et al., 2014). Moreover, MXenes may have one or more different “M” elements in two different forms such as repetitive phases and solid solutions. In the solid solutions, two different kinds of “M” layers may be present with random arrangements. These variations show the abundance of materials present in the 2D-MXene family. As of 2020, almost seventy MXenes having various compositions have been calculated theoretically. However, a few of these variants have been practically synthesized. Variations in composition and functionality give rise to unique surface properties for different applications, which is the driving force behind its high demand among researchers. Unlike metal chalcogenides, most of the MXenes are not semiconductors, and hence cannot act directly as photocatalysts. However, it is noticed that Ti3C2, the most popular MXene, acts as a photocatalyst due to the formation of doped TiO2 on the surface during processing. The unique structures and exciting surface chemistry of MXenes exhibit a number of desirable properties like abundance of variable reaction sites, tunable band gap, high surface area, hydrophilicity, high conductivity, good mechanical strength, etc., that are beneficial in improving the efficiency of any photocatalyst. Since 2016, researchers are looking into the different modifications of MXenes and their use as photocatalysts. For the hydrogen evolution reaction (HER), MXenes are found to be much better than other 2D photocatalysts, such as MoS2 and g-C3N4, owing to their higher charge-transfer rates and Gibbs free energy changes close to 0 eV with respect to hydrogen adsorption (Khazaei et al., 2019). Only a few pure MXenes have shown semiconducting properties with the capability to act as direct photocatalysts. So to improve the photocatalytic performance of these materials, it is necessary to develop variations in heterostructured 2D materials to tune the properties such as band gap, electrical

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conductivity, surface area, porosity, and variety of active sites and study them as photocatalysts. This chapter extensively deals with 2D-MXenebased photocatalyst materials and their modification to attract a broad audience from among the scientific community, for example, material scientists, chemists, physicists, and engineers. Herein, we review the synthetic strategies adopted to develop 2D-MXene-based heterostructures and their photocatalytic properties in energy conversion. These materials have use in applications such as water splitting, CO2 to fuel conversion and environmental remediation including antimicrobial properties. In doing this, new challenges and possible future directions with MXene-based materials for future photocatalytic technology will be emphasized.

11.2 Synthesis of 2D-MXenes In 2011, the first, 2D-MXene, Ti3C2Tx, was successfully synthesized by eliminating the aluminum (Al) layer from Ti3AlC2 (MAX phase) using hydrofluoric (HF) acid as the etching agent (Naguib et al., 2011). Afterwards, the syntheses of various 2D-MXenes from their corresponding MAX phases have received significant attention from various fields including catalysis. 2D-MXenes with specific qualities such as surface termination (Hope et al., 2016), size (Lipatov et al., 2016), number of layers, amount of surface defects, etc. Sang et al. (2016) can be achieved for a specific application by controlling properties such as the etchant type and concentration, time and temperature during the etching process. In general, “A” layer of MAX phase is etched using a specific etchant of desired concentration at a particular temperature with a different set of time to develop MXenes. Mostly, hydrogen fluoride from 3 to 50wt.% (Naguib et al., 2014) or the appropriate amount of various fluoride-based salts (Ghidiu et al., 2014; Halim et al., 2016; Liu et al., 2017). The chemistry of the etching of MXenes with HF (Hong Ng et al., 2017) can be tuned to control the surface terminations like fluorides and hydroxides. Depending on the requirement, the nature and amount of functionalities like fluorides and hydroxides on the MXene surfaces can be tuned by controlling etchants and etching parameters. For example, to increase the fluorine content on MXenes, highly concentrated fluoride-based etchant can be used at 50 C55 C for a few days. To enhance the hydroxide termination on MXene surface one can perform the etching in the presence of water for a short time with moderate HF concentration. As witnessed, the amount of hydroxyl functional groups may reduce the metal-like electronic conductivity of MXenes and induce semiconducting nature in it with a narrow band gap (Anasori et al., 2016). Depending on the variety of M of a specific MAX the etching condition needs to be modified to develop MXenes. It is observed that the increase in the number of layers requires stronger etchant to operate for a longer period

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of time to develop the required MXenes. Etching of MAX phases to develop MXenes may also be carried out using various fluoride salts in place of only HF. The main advantage of using salts for etching in place of bare HF is the intercalation of various cations like Li1 or small molecules like H2O during the etching, which creates additional spaces inside the flakes to generate exfoliated MXenes (Mashtalir et al., 2013). Basically, MXene flakes withstand with hydrogen bonding and Van der Waals interactions which may be weakened by such intercalation and help in exfoliation. As a result of exfoliation, one may generate more surface area for MXenes used in various applications. It is experimentally proven that during the etching process using salts, cations intercalate inside the layers thus minimizing the force between two layers (Hong Ng et al., 2017; Sun et al., 2018). In Fig. 11.1A, the delamination process is schematically presented which includes the steps like

FIGURE 11.1 (A) Schematic representation for the MXenes delamination by organic bases through swelling, shaking and centrifuging to result a stable colloidal solution. (B) Dispersions of Ti3C2Tx in different solvents. (A) Reprinted with the permission from Naguib, M., Unocic, R. R., Armstrong, B.L., Nanda, J., 2015. Large-scale delamination of multi-layers transition metal carbides and carbonitrides “MXenes.” Dalton Trans. 44 (20), 93539358. Copyright 2015 The Royal Society of Chemistry; (B) Maleski, K., Mochalin, V.N., Gogotsi, Y., 2017. Dispersions of two-dimensional titanium carbide MXene in organic solvents. Chem. Mater. 29 (4), 16321640. Copyright 2015 7 American Chemical Society.

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swelling, shaking and centrifuging of MXenes in a suitable solvent (Naguib et al., 2015). Depending on the type of solvent, quality of MXene dispersion obtained through etching and delamination varies as depicted for Ti3C2 MXene in Fig. 11.1B (Maleski et al., 2017). After delaminating the MXene flakes, its inner surface gets exposed and can be further used for developing different MXene based heterostructures. In that way, the desired MXene structure with required functionalities may be achieved for any specific application.

11.2.1 Functionalization and electronic properties of MXene Usually, etching of MAX to produce MXene using various etching system leads to the development of three different functionalities, that is, -O, -F, -OH. As per the theoretical studies, such functionalities on MXene surface are effective to control the physicochemical properties like electronic, optical, and mechanical properties (Sun et al., 2018). Optical and electronic properties are very important to control during the photocatalysis reactions. Several studies attempted to reveal the effect of various surface functionalities on controlling the optical and electronic properties of MXenes. For example, the most studied MXene, that is, Ti3C2 showed metallic properties in pristine form; however, functionalization with -OH, -F, or -I results in semiconducting properties having narrow band gaps (Tang et al., 2012). This might happen due to the surface dipoles and electronic redistribution in relation to surface functionalization that shifts the Fermi level and hence electronic work-function to alter the electronic conduction between pure and functionalized Ti3C2 MXenes (Jiang et al., 2020). In many such works, researchers have successfully used the surface functionalization technique to achieve optimized band gap or hence electronic conductivity for high performing applications (Jiang et al., 2019; Cheng et al., 2019). Not only the surface functionalities but the presence of different transition metals “M” in MXenes may also alter the optical and electronic properties. Depending on the nature of “M,” MXene may be of metallic, semiconductor or insulators in nature (Tang et al., 2012). In one study, Ti3C2Tx has appeared as metallic; however partial replacement of Ti by Mo makes it semiconductor (Tang et al., 2012). We have summarized many such studies related to the band gap of various MXenes with different functionalities in Table 11.1, which reflects the diversity in the band gap of different MXene systems necessary for photocatalytic applications. Until 2019, such experimental studies are mostly localized on the first-ever developed MXene Ti3C2 which indicates that plenty of scopes are available in studying and understanding the mechanism of electronic conduction and optical properties of different MXenes. A good catalyst should always absorb light from a wide region of the electromagnetic spectrum with suitable energy which must be greater than its band gap to generate electron-hole pair to conduct photocatalysis. Bare

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TABLE 11.1 Variation of band gap in functionalized MXenes calculated using density functional theory method. MXene

Bandgap (eV)

Reference

Sc2CO2

2.87

Zha et al. (2015)

Sc2CF2

1.85

Zha et al. (2015)

Sc2C(OH)2

0.845

Zha et al. (2015)

Mo2CF2

0.858

Zha et al. (2015)

Ti2CO2

0.917

Zha et al. (2015)

Zr2CO2

1.70

Zha et al. (2015)

Hf2CO2

1.66

Zha et al. (2015)

Mo2TiC2O2

0.17

Li (2016)

Cr2CF2

1.105

Je et al. (2016)

Cr2C(OH)2

0.396

Je et al. (2016)

Cr2CF(OH)

0.383

Je et al. (2016)

Mo2TiC2(OH)2

0.05

Anasori et al. (2016)

MXenes are not a good catalyst because of its low light absorption capability and zero band gap as well; thus it is important to control optical properties to achieve efficient catalytic performances. In different studies, surface functionalizations of MXenes have appeared as an effective technique to control its optical properties. For example, -O functionalized Ti2C and Ti3C2 showed higher in-plane absorption coefficients as compared to the same for -F and -OH functionalized Ti2C and Ti3C2 (Bai et al., 2016; Berdiyorov, 2016). Further studies have also exhibited that -O functionalized M2C type MXenes show optimized optical absorptions (Jiang et al., 2020; Wang et al., 2017). MXenes containing different metals also show diversified optical properties. In a typical example, the absorption coefficients are observed in the order like Ti2CO2 . Zr2CO2 . Hf2CO2 (Zhang et al., 2016). Thus it can be concluded that both electronic and optical properties of MXene can be regulated through surface functionalization and partial metal substitution. Based on some of the attractive features, MXene can be used as photocatalyst and cocatalyst with various heterostructure for different application as presented in Fig. 11.2.

11.3 Photocatalytic applications The photocatalytic application depends on the fabrication of the proper catalyst system. Most of the new generation catalyst depends on the innovative

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FIGURE 11.2 Advantages and possible photocatalytic application of MXenes.

FIGURE 11.3 Different types of possible hetrojunction photocatalysts.

designing of heterostructures with the proper combination of two or more semiconductor systems as single material-based photocatalyst is not able to deal with the fast electron-hole recombination and limited solar light absorption. Overall MXene-based composite photocatalyst can be categorized as (1) Z scheme system, (2) Schottky junction system, and (3) the Type I & Type II heterostructure system, as presented in Fig. 11.3. All type has typical

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electron and hole flow pattern leading to better charge separation. However, advantages and disadvantages depend on the proper fabrication of heterostructure with the choice of catalyst and cocatalyst. Natural photosynthesis by plant inspired the Z-scheme photocatalyst system which is a proven approach for improved photocatalysis with photoinduced charge separation (Maeda, 2013). This system is based on two semiconductors with proper band alignment as shown in Fig. 11.3 where electron flow from the first semiconductor to the second semiconductor thus delaying the recombination. Typically in Z-scheme electron flow from conduction band (CB) of the first semiconductor to the valence band (VB) of the adjacent semiconductor. Fabrication of Schottky junction at interface of two semiconductors is another approach to delay the recombination of electron-hole pairs (Ran et al., 2014). In this system electrons generally flow from a higher Fermi level of a photocatalyst to Fermi level of cocatalyst through the interface (Ran et al., 2017). In the interface, this creates excess negative charges on the cocatalyst side and positive charges on the semiconductor thus creating a space charge layer at the interface having upward bent structure of the CB and VB of the semiconductor. As formed Schottky junction creates an electron trap for stabilizing photoinduced electrons efficiently. In Type-I heterojunction both the holes and electrons flows to the second semiconductor as the CB and VB of the second semiconductor is more positive and negative, respectively than that of the first semiconductor (Jiang et al., 2015). On the other hand type II has similarity in band alignment with the Z-scheme system having different electron transfer directions as presented in Fig. 11.3. Typically Type II system is observed in p-n junction which is composed of alternate p- and n-type of semiconductor in the composite photocatalysts (Moniz et al., 2015) (Fig. 11.3).

11.3.1 H2 evolution by H2O splitting Burning of fossil fuel to satisfy the overall need of modern civilization is a major cause of global warming followed by unexpected weather changes, the outbreak of pandemic diseases, etc. In such a distressing situation, a green fuel in place of fossil fuels is of sole importance. H2 has been predicted as the pollution-free future fuel. Availability of H2 in the earth and its atmosphere is very low and hence the production of H2 through a green process from abundantly available H2O is an attractive possibility. Splitting of water to oxygen and hydrogen is thermodynamically an energy-intensive four-electron process with ΔG of 237 kJ/mol. So nearly 50% of the available energy of sunlight can be utilized for this reaction. However, surface over potentials for the HER and oxygen evolution reaction requires extra energy. Therefore both kinetic and thermodynamic factors need to be considered. Therefore we need an appropriate catalyst to perform the above reaction smoothly. The basic requirement of a material to be an effective photo-catalyst toward H2O

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splitting reaction is that the VB potential of the material should be higher than the O2 oxidation potential (1.23 eV) and the CB potential should be lower than the H1 reduction potential (0 eV). In this category, a lot of reported materials based on MXene as composites and Z scheme photocatalyst are studied with improved efficiency. In the Z-scheme catalyst, the CB of the cocatalyst becomes lower in energy than the H1 reduction potential and the VB of the catalyst remain in higher energy than the O2 oxidation potential. So, a semiconductor material with appropriate potential and large surface area is a good choice. The interface of two materials in a composite or Z scheme catalyst is a better choice as it exposes more active sites and high energy crystal facets and helps in charge separation to enhance the catalytic activity. Among 2D materials, MXenes are considered to be better than other 2D catalysts such as metal chalcogenides and carbon nitrides due to their higher charge transfer abilities. Most of the MXenes are not good semiconductors when used by themselves. Hence, MXene heterostructures that include other semiconductor materials are promising. Such heterostructures utilize the better electron mobility and charge separation of MXenes, typically Ti3C2, in combination with the electronic properties of more traditional semiconductor materials. Since 2016, different heterostructure architecture including heterojunctions and Z-scheme catalysts have been studied in order to more precisely tune the band gap and facilitate charge transfer as required for some reactions. In this review, we shall discuss in detail about the MXenes and MXene based heterostructured materials reported till date toward photocatalysis as presented in Table 11.2.

11.3.1.1 Water splitting activity of MXenes In a theoretical study, ab initio calculations of forty eight types of MXene which includes transition metal carbides having the formula of M2C, M3C2, and M4C3 were systematically analyzed by Guo et al. (2016). The calculation was based on both pristine (Sc2C, Ti2C, Zr2C, Hf2C, V2C, Nb2C, Ta2C, Mo2C, Ti3C2, Ti4C3, V4C3, and Nb4C3) MXenes and the corresponding surface-functionalized ones by O, OH or F group to understand the semiconductor nature and variation of band gap for possible photocatalytic use. From the result, they have predicted the presence of band gap in a few semiconductor-based MXenes as shown in Table 11.1. In particular thin layers of Hf2CO2 and Zr2CO2 can be used as an efficient photocatalyst for water splitting reaction as these materials shows appreciable optical absorption in the approximate wavelength range of 300500 nm having the required CB and VB potentials. In addition, these materials exhibit high anisotropic carrier mobility which can facilitate the separation and migration of photoinduced electron-hole pair with required stability in liquid water medium justifies their practical applicability (Guo et al., 2016). The H2O adsorption and H2 formation on the MXene surface is energetically favorable, which facilitates photocatalytic hydrogen evolution 2D Mo2C derived

TABLE 11.2 Photocatalytic water splitting in MXene-based photocatalysts. Photocatalysts

Reaction parameters

Scavenger

H2 rate/important findings

Reference

TiO2 nanofibers/ Ti3C2 (1D/2D)

300 W Xe lamp, RT

Methanol

6.98 mmol/g/h, 3.8 times of TiO2 nanofibers

Zhuang et al. (2019)

TiO2/Ti3C2

200 W Hg lamp RT

Methanol

H2 rate 400% than only TiO2

Wang et al. (2016)

g-C3N4/Ti3C2 quantum dots (0D/2D)

300 W Xe lamp, RT

Triethanolamine (TEOA)

5111.8 μmol/g/h, 2.5 times of pt/g- C3N4

Li et al. (2019b)

ZnO nanorod/Ti3C2

300 W Xe lamp, RT

Ethanol

456 μmol/g/h

Liu and Chen (2020)

CdS nanorod/Ti3C2

300 W Xe lamp, 6 C

Lactic acid

2407 μmol/g/h

Xiao et al. (2020)

TiO2/Ti3C2 (F & OH group)

350 W Xe arc lamp

Glycerinum/water

Quantum efficiency of 0.9% under 365 nm, higher HER by F functionalized MXene.

Li et al. (2019a)

Ti3C2/g-C3N4 (2D/ 2D)

Visible light

TEOA

72.3 μmol/g/h, 10 times more than g-C3N4

Su et al. (2019)

Ti2C/g-C3N4

Visible light

TEOA

47.5 μmol/h, 14.1 times higher than g-C3N4

Shao et al. (2017)

Nb2O5/C/Nb2CTx Composites

200 W Hg lamp: λ 5 285325 nm, RT

Methanol

7.81 μmol/g/h,

Zuo et al. (2020)

TiO2/Ti3C2/ amorphous carbon

350 W Xe Lamp (λ $ 420 nm)

Ascorbic acid

33.4 μmol/g/h, (Sensitization by Eosin Y)

Su et al. (2018)

CdS/Ti3C2

300 W Xe lamp: λ $ 420 nm, RT

Lactic acid

14,342 μmol/h/g, quantum efficiency: 40.1% at 420 nm

Ran et al. (2017)

ZnS/Ti3C2

300 W Xe lamp: λ $ 420 nm, RT

Lactic acid

502.6 μmol/g/h, 4 times higher than ZnS

Tie et al. (2019)

ZnIn2S4/Ti3C2

420 nm

TEOA

3475 μmol/h/g, 6.6 times higher than ZnIn2S4

Zuo et al. (2020)

C doped TiO2/Ti3C2

Simulated solar light, RT

TEOA

9.7 times than commercial P25

Jia et al. (2018)

Ti3C2/TiO2/UiO-66NH2 hybrid

300 W Xe Lamp

Na2S and Na2SO3

1980 μmol/g/h, 2 times than UiO-66-NH2 hybrid

Tian et al. (2019)

Cu/TiO2/Ti3C2Tx

300 W Xe lamp/RT

Methanol

860 μmol/g/h

Peng et al. (2018)

CdS/MoS2/MXene

300 W Xe lamp (λ $ 420 nm)

Sodium sulfide & sodium sulfite

9679 μmol/g/h, Rate is 251.3% of CdS-MoS2

Chen et al. (2019)

g-C3N4/Ti3C2Tx/Pt

300 W Xe lamp, RT

TEOA

5100 μmol/g/h

An et al. (2018)

1T-WS2/TiO2/ Ti3C2Tx

300 W Xe arc Lamp, RT

TEOA/acetone

3409.8 μmol/g/h, rate 50 times of TiO2

Li et al. (2020a)

Ti3C2Tx/TiO2/g-C3N4

300 W xenon lamp, 25 C

TEOA

1620 μmol/g/h, 3 cycle stability.

Zhang et al. (2018b)

Ti3C2Tx/TiO2/MoS2 (2D/2D/2D)

300 W Xe lamp, 25 C

TEOA

6425.3 μmol/g/h, 7 times of Ti3C2Tx/TiO2

Li et al. (2019c)

TiO2/Ti3C2Tx

150W Xe lamp (light intensity: 100 mW/cm2)

Photoelectrochemical experiment (PEC), RT

Current Efficiency 1.47%, six-fold higher than pristine TiO2

Yu et al. (2019)

Mo2TiC2Tx/single Pt atom

HER activity in 0.5M H2SO4

PEC, RT (EC)

Over potentials 30 and 77 mV; Current density 10 and100 mA/cm2

Zhang et al. (2018a)

Ti3C2Tx/single Ru atom

Visible light

PEC, RT

Over potential of 76 mV, For Photocurrent density of 10 mA/cm2

Ramalingam et al. (2019)

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from Mo2GaC showed appreciable HER activity from computational screening followed by experiment without further modification (Seh et al., 2016). HER activity of Mo2C was found to be many folds in comparison to that of Ti2C due to the availability of catalytically active sites in basal planes of Mo2C, unlike MoS2. On the other hand, the most common MXene, Ti3C2, forms carbondoped TiO2 on its surface due to a mild oxidation process. This creates a MXene/TiO2 interface that gives rise to the Schottky barrier (SB) formation thus promoting separation of photoinduced charge carriers (Wang et al., 2016). Density functional theory (DFT) calculations on different MXene polymorphs (M2X, M3X2, and M4X3) by Pandey et al. showed their applicability as electrocatalysts for the HER reaction. The calculation was based on an activity descriptor like the hydrogen adsorption free energy at equilibrium coverage on 2D sheet (Pandey and Thygesen, 2017). The results confirmed that hydrogen adsorption energy is completely dependent on the number of layers in the sheet which indicate that the degree of catalytic activity of MXenes can be regulated by controlling the number of layers or thickness.

11.3.1.2 MXene-based heterojunctions MXene based heterostructures coupling with other semiconductor materials having 1D/2D, 0D/2D, 3D/2D interface as presented in Figure 11.4 show more interest toward water splitting over MXene itself. In any heterojunction the contact interface between two materials is important and that decides the mechanism of the reaction process. In this context, active quantum dot photocatalyst on MXene sheet (0D/2D) attracts the attention of researchers due to the direct contact interface between the quantum dot photocatalysts and large surface area MXene substrate (Figure 11.4A). Strong bonding of 0D photocatalyst on MXene surface is possible due to the functionality developed (-F, -OH and -O) during the processing of MAX phase. Typically strain-dependent electronic properties can be generated on MXene surface due to the short interface distance between the 0D particles and 2D sheets. The efficient interface between 0D and 2D sheet gives rise to a Schottky junction formation thus improving the overall photocatalytic activity (Li et al., 2018). Photocatalysts having zero-dimensional size obtained through the surface oxidation of the MXene through mild oxidation process can provide the heterojunction interface with 2D MXene which will facilitate the charge separation due to excellent conductivity of sheets (Jia et al., 2018).

FIGURE 11.4 Schematic representation of (A) 0D/2D, (B) 1D/2D, (C) 2D/2D, (D) 3D/2D heterostructure involving MXene sheet.

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Ti3C2 quantum dot with 2D g-C3N4 nanosheets acts as an attractive catalyst for HER in visible light. g-C3N4 acts as cocatalyst substituting platinum thus improving the quantum efficiency (Li et al., 2019b). Strong adsorption of TiO2 on the Ti3C2 hinders the restacking of sheets. Different types of heterojunction formation make MXene as a sink for photogenerated electrons. However yet there is a little report on the 0D/2D composites based on MXene. TiO2 fiber/Ti3C2 composite (1D/2D) gives very high rate of hydrogen evolution under visible light (Zhuang et al., 2019). The heterogeneous interface between TiO2 NFs and Ti3C2 nanosheets improved photocatalytic H2 production of TiO2/Ti3C2 with higher cycle stability. Similarly reports on 1D/2D heterojunction involving MXene is limited for photocatalytic application. Other than TiO2, ZnO with MXene has also been studied for water splitting reaction (Liu and Chen, 2020). It was experimentally demonstrated that the ZnO nanorods/Ti3C2Tx heterostructure (1D/2D) showed photocatalytic H2 evolution activity which is better than the pure ZnO nanorods or MXene showing the importance of heterostructure. However, more studies are required on ZnO based composites. Solvothermally fabricated (1D) CdS nanorod/(2D) Ti3C2 nanosheet heterojunctions was also reported recently with improved HER activity (Xiao et al., 2020) under solar light illumination. In total there is a very little report on these two heterostructures and need more investigation.

11.3.1.2.1

2D/2D composites

The majority of reported work on 2D/2D MXene-based photocatalytic system has already proved beneficial in enhancing photocatalytic water splitting activity and stability. Recently Xie et al. reported 2D/2D in-plane CdS/Ti3C2 heterojunction by using Ti3C2 as 2D sheets (Xie et al., 2018). 2D CdS nanosheets are grown over Ti3C2 which worked as an electron mediator through the interlayer interaction of CdS/Ti3C2 heterostructures, resulting in improved photocatalytic stability of CdS nanosheets. Another report of 2D/2D heterojunction by Yang Li et al. where they have synthesized a Ti3C2/TiO2 hybrid with a multilayered Ti3C2 and truncated octahedral bipyramidal structure of TiO2 with exposed (001) facets. In the composite Ti3C2 acts as a cocatalyst and increases the photocatalytic water splitting activity by capturing photo-generated electrons from TiO2 surface. Ti3C2 shows good cocatalytic activity because of its electron reservoir capability and suitable position of the Fermi level energy. The highly active crystal facet (101) and (001) of TiO2 at the heterojunction further accelerated the photogenerated carriers separation. In this study, two types of MXene has been synthesized by calcination method, that is, F-terminal and O-terminal MXene due to the treatment of MAX with HF followed by H2O and NaOH treatment respectively. In this case, F-functionalized Ti3C2/TiO2 heterostructure showed photocatalytic hydrogen generation two times higher than that of heterostructure having OH-functionalized Ti3C2 (Li et al., 2019a).

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Su et al. reported a g-C3N4/Ti3C2 based 2D/2D composite (Su et al., 2019) synthesis by employing an electrostatic self-assembly approach thus creating a large contact interface which ultimately facilitates the photogenerated charge carrier migration for improved photocatalytic H2 production. A similar conclusion was reported (Shao et al., 2017) based on g-C3N4 and Ti2C composite which justifies the beneficial effect of the 2D/2D structure for achieving ultrafast charge separation. For Z-scheme photocatalysis, the lower the energy gap between the CB of the catalyst and the Ef of the cocatalyst (Schottky barrier), the higher will be the electronic transition probability from catalyst to cocatalyst surface and lower will be the recombination rate which ultimately increases the photocatalytic efficiency. A theoretical study has shown a strategy to make a SB free hole contact to MoS2 with MXene (V2CO2, Cr2CO2, Mo2CO2, V4C3O2, Cr2NO2, and V2NO2). They have proposed that SB free hole contacts at MoS2/MXene interfaces mostly dependent on the high work functions of the MXenes and the absence of the interfacial gap states formation (You et al., 2019). Ultrathin ZnIn2S4 grown on Ti3C2 creates the 2D/2D heterostructure with high surface area and improved H2 evolution activity (Zuo et al., 2020). Till date, most of the studied MXene is confined within Ti3C2 and hence the combination of new MXene materials with various other 2D semiconductor materials need to be further investigated for exploring the new possibilities and improving overall water splitting efficiency. 11.3.1.2.2 2D/3D composites Niobium pentoxide/carbon/niobium carbide (MXene) heterostructured materials (Nb2O5/C/Nb2C) prepared through controlled oxidation of Nb2C as photocatalysts (Su et al., 2018) shows improved hydrogen generation rate with fourfold increase compared to Nb2O5. The enhancement of H2 evolution in Nb2O5/C/Nb2C is due to the proper heterostructure formation in between Nb2O5 and conductive Nb2C which facilitates the photogenerated charge carriers separation at the interface. The catalytic activity of a heterostructured MXene can also be enhanced via the sensitization by a dye molecule as in the case of Eosin Y (EY)-sensitized partially oxidized Ti3C2 thus creating TiO2/Ti3C2. The hydrogen generation rate reported by TiO2/Ti3C2/amorphous carbon composite with the sensitization by EY is 110 times more than that of TiO2/Ti3C2 without EY (Sun et al., 2019). As reported by Ran et al., Ti3C2 coupled with CdS (3D) shows a highest photocatalytic hydrogen production rate of 14,342 mmol/h/g under visible light with a quantum efficiency of 40.1% at 420 nm whereas pure Ti3C2 is not an eligible candidate due to its inefficient band gap and unfavorable Fermi level (Ghidiu et al., 2014). Desirable Fermi level is observed with O functionalized Ti3C2 whereas F functionalized is not a good candidate for the same. Here O-terminated Ti3C2 acts as a cocatalyst on the CdS catalytic surface. This high performance is mostly due to the favorable Fermi level position and high electrical

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conductivity of Ti3C2 nanoparticles (Ran et al., 2017). Ti3C2 MXene is also a good cocatalyst for ZnS and Zn0.8Cd0.2S catalytic surface. Design of a heavy metal-free heterojunction photocatalyst through the in-situ decoration of ZnS nanoparticles on Ti3C2 nanosheets (Tie et al., 2019) with enhanced photocatalytic H2 efficiency is reported recently. The presence of Ti3C2 sheets mainly promotes better charge transfer thereby extending the lifetime of photogenerated charge carriers which results in an improved H2 efficiency under optimum conditions. 11.3.1.2.3 Doped MXene To enhance the photocatalytic activity of MXenes, several authors have reported on doped MXene materials. Jia et al. have shown that the effective in-situ carbon doping of titanium dioxide can significantly enhance the lifetime of photogenerated carriers and widen the light absorption range for effective photocatalytic reactions. They have synthesized a highly carbon-doped TiO2 with a hierarchical structure and good crystallization with exfoliated MXene. The high carbon-doped TiO2 derived from MXene through oxidation process boosts the water-splitting activity by creating a VB tail state which promotes effective separation of photoinduced carriers and reduced band gap. This ultimately improved the light utilization for desired photocatalytic reactions. The photocatalytic hydrogen production rate of the carbon-doped TiO2 was found to be 9.7 times than that of commercial TiO2 (P25) under simulated sunlight (Jia et al., 2018). In addition to that, a 2D heterostructure based on nitrogen-doped TiO2 nanoparticles and nitrogen-doped carbon (N-TiO2/NC) was developed using layered NTi3C2 as the template for effective photocatalytic use. The N-TiO2/ NC heterostructure showed enhanced visible light absorption, improved charge separation and transport capability thus increasing the hydrogen production rate under visible-light without any additional cocatalyst (Kong et al., 2020). 11.3.1.2.4 Tertiary composite system To improve the photocatalytic efficiency a number of researchers adopted the tertiary composite systems involving MXene material. In a typical system, Zr-MOF is grown on the as-prepared Ti3C2/TiO2 composite and used for the water-splitting catalyst. This tertiary system (Zr-MOF/Ti3C2/TiO2) enhanced the water-splitting activity more than twofold in comparison to only Zr-MOF/Ti3C2 system due to the presence of dual photo absorber and enhanced charge separation activity (Tian et al., 2019). Similarly, MoS2/TiO2/Ti3C2 system showed highly improved results of 10.505 mmol/g/h of H2 production rate because of the perfect formation of the tertiary system with defect rich MoS2 (Li et al., 2020b). To elongate the lifetimes of charge carriers and enhance the activity of semiconductor photocatalyst, a ternary complex (TiO2/Ti3C2/Cu2O) where TiO2 sheets are deposited on layered Ti3C2 exposing (001) surfaces and Cu2O nanodots is deposited on TiO2. In the composite, the photogenerated electrons on TiO2-Ti3C2Tx

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accumulate and transferred to Cu2O forming elemental Cu in the composite. The resulting Cu/TiO2/Ti3C2 photocatalyst increased the hydrogen production rate. In this system, elemental Cu facilitates the electron transport as a reduction cocatalyst and Ti3C2 promotes the hole transport to enhance the charge separation through a dual mechanism (Peng et al., 2018). A tertiary composite CdSMoS2-MXene showed high H2 generation rate under visible light (λ $ 420 nm). Obtained H2 generation rate is 2.51 fold increase in comparison with only CdSMoS2 because of synergistic effect (Chen et al., 2019). This outline the importance of MXene based material for improved water splitting activity. In the recent time similar other ternary heterostructures have been fabricated to improve the photocatalytic water splitting activity mainly C3N4/ Ti3C2Tx/Pt (An et al., 2018), WS2/TiO2/Ti3C2Tx (Li et al., 2020a), TiO2/ Ti3C2/gC3N4 (Zhang et al., 2018b), and MoS2/TiO2/Ti3C2 (2D/2D/2D) (Li et al., 2019c). Use of two metallic cocatalysts in the form of Ti3C2 and tetrahedral phase of WS2 with titania catalyst improves the hydrogen evolution efficiency to 50 times of TiO2 nanosheet (Li et al., 2020a). The exceptional increase is due to the proper electron-hole separation and electron segregation in the CB for the reduction reaction. On the other hand deposition of MoS2 layer on the 101 facets of TiO2 helps in the trapping of electron for efficient water reduction reaction (Li et al., 2019c). Proper heterojunction formation with high electron mobility improves the water splitting efficiency to a high level. However, work on tertiary composites of MXene with other semiconductor cocatalysts are in an early stage and need more investigation to conclude the importance of MXene-based heterostructures. 11.3.1.2.5

Electrochemical water splitting

MXene heterostructures are good for photo-electrochemical water splitting also. In a recent report, TiO2/Ti3C2 heterostructures photoanode showed six-fold enhancement of photoactivity for PEC water splitting over TiO2 itself proving the importance of MXene in the photoelectrochemical (PEC) process. The enhanced PEC performance was corroborated to the strong interfacial interactions of TiO2 with Ti3C2 and presence of Ti3C2 as cocatalysts for oxygen evolution (Yu et al., 2019). Recently there is a report on the synthesis of Mo, Ti containing bimetallic MXene nanosheets (Mo2TiC2) with maximum exposed basal planes. Interestingly Mo vacancies formed on the sheets are used to mobilize single Pt atoms on the sheets (Zhang et al., 2018a). Obtained catalyst was found to enhance the MXene’s HER activity. The obtained photocatalyst exhibits a high catalytic activity with low over potentials of 30 and 77 mV so as to achieve a photocurrent density of 10 and 100 mA/cm2. Overall activity is about 40 times more than the commercial Pt/carbon catalyst. Exceptional catalytic activity and stability originate from the covalent interactions of positively charged Pt single atom with the Mo2TiC2. In another report synthesized 1D/2D Schottky heterojunction (CdS/Ti3C2) showed improved PEC activity with accelerated charge separation

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having lower SB in presence of solar light. Interestingly the above photocatalyst showed sevenfold increased efficiency HER in comparison to pristine CdS nanorod which is due to the synergistic effects of n-type semiconductor CdS and highly conductive 2D Ti3C2 nanosheets (Xiao et al., 2020). MXene (Ti3C2Tx) is used as an effective support to host ruthenium single atom coordinated with nitrogen and sulphur resulting in an efficient catalyst toward the HER activity. The obtained catalyst showed low over potential (76 mV) so as to achieve 10mA/cm2 current density and very high photocurrent density of 37.6 mA/cm2 which is exceptionally higher than the reported results having noble metals coupled photocathode (Ramalingam et al., 2019).

11.3.2 Photocatalytic CO2 reduction to fuel In recent years photocatalytic reduction of carbon dioxide to high-value green fuel under solar light has attracted tremendous attention so as to address both energy problems and environmental concerns of CO2. Unlike the HER, CO2 reduction can end up with several products or a mixture of the products depending on the reaction condition. A suitable photocatalyst for CO2 reduction must have a CB potential less than the CO2 reduction potential, that is, for CO2 to CH4 (20.24 eV) and CO2 to CH3OH (20.38 eV). Therefore the product formation and yield of the reaction can be changed depending on the band gap and potential of the photocatalyst or by tuning the band gap by forming heterostructured material. Liquid phase CO2 reduction reaction generally follows first and second pathway (Scheme 11.1) ΔE: -0.24 V

ΔE: -0.61 V

ΔE: -0.38 V

ΔE: -0.48 V

Possible mechanism 1st

2nd

3rd

SCHEME 11.1 CO2 reduction mechanism with required reaction potential.

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resulting in CH3OH as the major and HCOOH, HCHO, C2H5OH, CH3COOH, and CH3CHO as side products. Whereas CO and CH4 are the most favorable product in the gas phage system following the third pathway on metal-doped photocatalysts (Ali et al., 2019). Tuning the band gap of the photocatalyst by doping through substitution of C atom by F, N, S, B, Si, Ge, Sn in Sc2CF2 (Balcı et al., 2017) or formation of heterostructured catalyst is possible with MXene material also. Various pure MXenes like Ti2CO2, V2CO2, and Ti3C2O2 were investigated based on a computational study. Interestingly the study revealed that the CO2 reduction preferentially takes place at the oxygen vacancies (Zhang et al., 2017) which can be generated in situ due to the presence of H2 and CO. On the other hand a possible reaction pathway for CH4 synthesis from CO2 reduction on MXene was calculated using DFT method which shows that Cr3C2 and Mo3C2 are the two promising photocatalysts (Li et al., 2017). The obtained result gives insight into the atomic level understandings with possible electrochemical mechanisms for CO2 reduction to hydrocarbonbased green fuel generation. The fast recombination of the photoinduced electron and hole pairs is the major challenge on the way to achieve the true potential of carbon dioxide reduction reaction (CRR). In this regard, a composite material with high surface area, tunable band gap, the presence of suitable cocatalyst is the need of the hour. MXene-based heterostructures are showing a good performance toward CRR as a photocatalytic material (Zhan et al., 2020). Though Ti3C2 is not a semiconductor, TiO2 nanoparticles can be created on conductive MXene through different oxidation techniques. In this context in situ grown TiO2 with unique rice crust-like structure was reported (Low et al., 2018) exhibiting a higher CO2 reduction activity for CH4 production in comparison to commercial titania (P25). This extraordinary photocatalytic activity was corroborated to the unique morphology and proper distribution of TiO2 in the optimized TiO2/Ti3C2 composite. Development of noble metal free cocatalysts for the photocatalytic reaction is of high demand. In this regard, surface-functionalized MXene (Ti3C2) is proven to be an efficient noble metal-free cocatalyst with commercial TiO2 (P25) for CO2 reduction. Alkaline treatment of Ti3C2 creates the functionalization for improved CO2 adsorption which ultimately enhances the CH4 formation (Ye et al., 2018). Dramatic enhancement of CO2 reduction can be attributed to enough activation sites of surface alkalinized Ti3C2 and charge carrier separation ability. In a typical material deigning Ti3C2 quantum dots are incorporated inside Cu2O nanowires through a self-assembly process making 0D/1D composite (Zeng et al., 2019). Photocatalytic performance and stability of Cu2O nanowires are significantly improved by the presence of Ti3C2 quantum dots through enhanced charge transfer, carrier density, light adsorption, and decreased charge recombination. The newly developed 0D/1D heterostructure showed improved photocatalytic conversion of CO2 to methanol.

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2D/2D heterojunction is fabricated by in situ growth of Bi2WO6 nanosheets on the surface of Ti3C2 nanosheets. The resultant Ti3C2/Bi2WO6 heterostructure exhibits a large interface area having short charge transport distance thereby facilitating excellent interfacial charge transfer which results in exceptional improvement in the photocatalytic performance of CO2 reduction to CH4 and CH3OH under simulated solar light (Cao et al., 2018). The total conversion yield is nearly 4.6 times than that of pristine Bi2WO6 nanosheets which justifies the importance of 2D/2D heterostructure having Ti3C2 as cocatalyst. Formation of simple 2D/2D composites using graphitic carbon nitride (g-C3N4) and alkalized Ti3C2 was demonstrated by Tang et al. (Tang et al., 2020). The photocatalytic carbon monoxide (CO) evolution rate of the said composite was 5.9 times higher in comparison to only g-C3N4. Large CO2 adsorption capacity and high electrical conductivity of alkalized Ti3C2 is the driving force for the excellent photocatalytic activity. Similarly, in another report, an ultrathin Ti3C2/g-C3N4 (2D/2D) heterojunction was synthesized by heat treatment of Ti3C2 in the presence of urea. Here urea is used as the template for exfoliation of Ti3C2 and the precursor of g-C3N4 to form Ti3C2/g-C3N4 heterojunction (Yang et al., 2020). Total CO2 conversion on the composite is found to be one-fold higher than only g-C3N4. Fabrication of Schottky-junction using hydrothermal deposition of cube-shaped CeO2 and Ti3C2 so as to improve the photocatalytic CO2 reduction has been recently reported (Shen et al., 2019). Creation of electric field between CeO2 and Ti3C2 due to the Schottky junction formation results in segregation of the electrons in Ti3C2 for improved photocatalytic activity. Recently, the inorganic perovskite materials are explored a lot for solar energy conversion application. In this context, CsPbBr3 nanocrystals deposited on Ti3C2 sheets as presented in Figure 11.5 showed promising photocatalytic activity for CO2 reduction to CO and CH4 because of their appropriate band gap, good photostability, and efficient charge transfer (Pan et al., 2019). Comparative performance of different heterostructures or modified MXenes for photocatalytic CO2 reduction reaction is compiled in Table 11.3.

11.3.3 Environmental applications The rapid pace of industrialization gives rise to hazardous waste which has negative impacts on the overall environment and health. Typically soluble pollutants such as heavy metals, antibiotics, pesticides, dyes and organic compounds in wastewater or drinking water create a global environmental challenge as most of them are highly toxic to human and other living organisms. MXene has a potential role for the betterment of environmental issues through which we are going through in recent time. MXene can be used as a photodynamic therapeutic agent, the photothermal and chemo-photothermal substrate in addition to the photocatalytic decomposition of dye and drug molecules. MXene can also be used as an antibacterial and antiviral agent through photocatalysis to keep our environment safe.

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FIGURE 11.5 Schematic presentation synthesis and high resolution TEM image of CsPbBr3/ MXene nanocomposite at different magnifications (a, b & c). Reprinted with the permission from Ref. Pan, A., et al., 2019. CsPbBr3 perovskite nanocrystal grown on MXene nanosheets for enhanced photoelectric detection and photocatalytic CO2 reductionin en J. Phys. Chem. Lett. 10 (21), 65906597. Copyright (2019) American chemical society.

In this context, a recent review highlights the different aspects of MXenes applications as efficient adsorbents, electrodes materials and photocatalytic environmental application including antibacterial activity for water decontamination and remediation processes (Rasool et al., 2019). Here we are focused on the environmental remediation through photocatalysis using MXene and modified MXene.

11.3.3.1 Organic degradation The initial study shows that only MXene is effective in photocatalytic organic decomposition. Ti3C2 based MXene shows a very high affinity toward dye adsorption and subsequent enhanced photocatalytic decomposition activity. Having many negatively charged groups, MXene shows strong adsorption toward positively charged dye like methylene blue (MB). From Langmuir and Freundlich isotherms data it is observed that Ti3C2 can adsorb around 39 mg/g MB. Among the adsorbed MB 81% gets degraded in the presence of UV irradiation in 5 h on Ti3C2Tx surface. In addition, the same material can degrade up to 62% of Acid Blue dye in the presence of UV light irradiation in 5 h (Mashtalir et al., 2014). It is observed that in-situ surface oxidation of Ti3C2 to TiO2 takes place in the presence of water and dissolved O2 thereby making Ti3C2 a photocatalyst. However,

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TABLE 11.3 MXene-based photocatalyst for CO2 reduction to green fuel. Photocatalyst

Product

Performance

Reference

Cr3C2 and Mo3C2 (DFT studies)

CH4

OCHO and HOCO radical species formation

Li et al. (2017)

Ti2CO2 (first-principle computation)

HCOOH

Ti2CO2 is better than Ti3C2O2 and V2CO2

Ti3C2/P25

CO and CH4

CH4: 0.22 μmol/h, 3.7 times higher than (P25)TiO2

In situ grown TiO2 on Ti3C2 by heat treatment

CH4

CO (11.74 μmol/g/h) and CH4 (16.61 μmol/g/h) 277times higher than P25

Ti3C2 quantum dots/ Cu2O nanowires (0D/1D)

Mainly MeOH

Improved activity in comparison to Cu2O

Zeng et al. (2019)

MXene/Bi2WO6 (2D/2D)

CH4 and CH3OH

4.6 times of Bi2WO6

Cao et al. (2018)

g-C3N4/alkalinized Ti3C2 (2D/2D)

CO and CH4

5.9 times of g-C3N4

Tang et al. (2020)

g-C3N4/Ti3C2 (2D/2D)

CO and CH4

5.19 and 0.044 μmol/h/g for CO and CH4

Yang et al. (2020)

CeO2/Ti3C2(3D/2D) Schottky junction

CO

1.5 times of CeO2

Shen et al. (2019)

CsPbBr3 perovskite/ MXene

CO and CH4

Enhanced CO2 reduction[R (CO) and R(CH4): 32.15 & 14.64 μmol h21 g21.

Pan et al. (2019)

Zhang et al. (2017) Low et al. (2018) Ye et al. (2018)

surface oxidation behavior needs to be investigated to ascertain the stability of material typically in aqueous solution. In continuation of heterojunction interfaces designing using TiO2 and Ti3C2 Peng et al. (2016) fabricated a composite having exposed 001 facets of TiO2 using controlled hydrothermal oxidation of Ti3C2 which ultimately results in Schottky junction formation with n-type TiO2. Creation of SB effectively reduces the electron-hole recombination. It is well established that the photocatalytic activity of titania depends preferably on the exposed facets, particle shape, and size. Preferably 001 and 101 facets provide oxidation and reductive sites respectively during the photocatalytic process. Possibly exposed active facets and SB formation dramatically boost the photocatalytic decomposition of methyl orange. MXene-based composite heterostructures were investigated by a number of researchers toward dye decomposition reactions (Liu et al., 2019a; Liu et al., 2018; Tariq et al., 2018). In a recent report (2D/2D) interface was developed using sheet-like BiOBr and Ti3C2 having similar layered structures so as to facilitate efficient photoinduced electrons and holes separation and transfer (Liu et al., 2019a). Thus the obtained heterostructure (BiOBr/

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Ti3C2) showed the improved visible light-mediated photocatalytic degradation of RhB dye than pure BiOBr due to the synergetic effects of improved visible light absorption and better charge separation. Another approach based on a new type of heterostructure material of Gd and Sn codoped bismuth ferrite (BiFeO3) nanoparticles deposited on metal carbide sheets (MXene) through coprecipitation method can be projected as a promising future photocatalyst. The material (Bi0.9Gd0.1Fe0.8Sn0.2O3) shows low electron-hole pair recombination compared to that of bismuth ferrite itself, which makes the material suitable for 100% photo-degradation of Congo red dye in 120 min (Tariq et al., 2018). In this context, Wang et al. (2018) reported a quasi core-shell In2S3/TiO2-Ti3C2 heterostructure by a hydrothermal method for the enhanced degradation of methyl orange. The enhanced photocatalytic activity of ternary nanohybrid is corroborated to band alignment due to type-II heterostructure and Schottky junction formation without noble metal which facilitates higher charge transfer. Overall synergistic effect of improved visible-light absorption by In2S3, upward bending of TiO2 band and the remarkable electrical conductivity of Ti3C2 is the main driving force for the enhanced activity. In a 0D/2D strategy, TiO2 nanoparticles are in-situ grown on Ti3C2 nanosheets through hydrothermal technique making it a semiconducting 2D material. Subsequently, the surface of Ti3C2 sheets is decorated with black phosphorus quantum dots (0D) making it a typical 0D/2D heterostructure. The obtained photocatalyst exhibits remarkable enhancement in photocatalytic decomposition of methyl orange (Yao et al., 2020b). The optimum degradation efficiency of 93% in 60 min reaction is remarkably higher than only Ti3C2TiO2 under visible light. The beauty of the material lies on the tunable band gap of black phosphorus quantum dots (BP) and the intimate heterojunction formation with Ti3C2-TiO2, which promote the electrons transfer from BP to TiO2 CB. Ultimately the quick mobility of electrons from TiO2 to the Ti3C2 nanosheets having excellent electronic conductivity prolongs the lifetime of electrons and delayed the photogenerated carrier recombination. In addition, the enhanced visible light absorption and specific surface area of BP-Ti3C2TiO2 further accelerate the photocatalytic activity. 2D/2D heterojunction based on R-scheme Ti3C2/MoS2 composites (Yao et al., 2020a) was fabricated by simple hydrothermal technique. Obtained Ti3C2/MoS2 photocatalyst showed remarkable improvement in catalytic activity toward methyl orange degradation of 97.4% within 30 min in comparison to only 2D MoS2. Other than dye there are certain organic compounds found in the wastewater stream causing concerns and hence need to be decomposed. Reports on the use of MXene based catalyst for the organic chemical decomposition is limited in the literature. Recently Iqbal et al. fabricated a nanohybrid material involving bismuth ferrite nanoparticles deposited on Ti3C2 processed through a solvothermal technique. Obtained photocatalyst was studied for the oxidative decomposition of organic pollutants

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like acetophenone and Congo red dye in water (Iqbal et al., 2019). Material having a large surface area of 147 m2/g is capable of 100% degrading the dye and acetophenone in 42 and 150 min, respectively. Ti3C2 modified with TiO2, Ag, Ag2O, Pd, PdO, and Au photocatalyst was evaluated for the degradation of salicylic acid under UV light (Wojciechowski et al., 2019). Obtained degradation efficiency up to 97% in 3 h was quite encouraging. Synthesis of CdS-Ti3C2-TiO2 (2D/2D) nanohybrids Z-scheme photocatalyst by facile calcination followed by the hydrothermal process is reported recently (Liu et al., 2019b). The importance of CdS on the photocatalytic activity toward degradation of environmental contaminants was evaluated. The optimum ternary nanostructures exhibited enhanced degradation of phenol, sulfachloropyridazine and several dyes in the presence of visible light. So the improved photocatalytic activity mainly credited to the enhanced electron-hole separation efficiency. Pharmaceutical components are biologically active to the human body and resistant to biodegradation. Pharmaceuticals are used for some specific reason for some specific diseases. Contamination of the pharmaceutical composition with wastewater may harm our immune, nervous, circulatory, and hormonal systems due to unnecessary uptake to the drugs. So management of the wastewater containing drugs is equally important in addition to a specific use of drugs. Drug carbamazepine generally decreases nerve impulses causing nerve pain and seizures. Shahzad et al. showed that TiO2/Ti3C2 based heterostructure can degrade the carbamazepine photocatalytically (Shahzad et al., 2018). The obtained rate constant for the decomposition of carbamazepine under UV light was found to be 0.0304/min. In a recent report a new composite photocatalyst of Ti3C2-Bi/BiOCl involving Bi/BiOCl microspheres and Ti3C2 sheets which was synthesized by solvothermal technique. The obtained composite exhibits an improved photocatalytic degradation than both BiOCl and Bi/BiOCl toward the degradation of ciprofloxacin (antibiotic) under visible light (Wu et al., 2020). Enhanced activity of Bi/ BiOCl-Ti3C2 can be attributed to the significant broadening the light adsorption in the visible region. This ultimately acts as an electron trap for promoting photogenerated electrons-holes separation. Improved degradation of tetracycline on CeO2/Ti3C2 Schottky junction was also reported under visible light (Shen et al., 2019).

11.3.3.2 Photoreduction process Photocatalysis is accompanied by both oxidation and reduction process. In addition to the oxidation process, photocatalytic reduction was also reported on MXene-based composites. Superficial oxidation of Ti3C2 gives rise to oxygen functionalized MXene which can be used as a good cocatalyst with SrTiO3 for reductive removal of U(VI) under solar light (Deng et al., 2019).

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Improved photocatalytic activity is mainly due to electron separation in the CB due to the SB formation. Similarly, the reduction of nitrate and nitrogen to ammonia is also reported on the MXene based photocatalysts (Chen et al., 2018). However, the exact mechanism of N2 adsorption on MXene for photoreduction process is not clear and hence needs more investigation.

11.3.3.3 MXene for antimicrobial activity Easy and quick control of microbial activity on, any surface is very challenging and hence there is a need for proper materials to maintain contamination-free surfaces. A 2016 (Rasool et al., 2016) report showed that surface nanomaterials help in inhibiting bacterial growth via nanobio interaction. Moreover to better understand the environmental impact of MXene, it is highly desirable to investigate the antimicrobial activity of these materials. It was observed that TiO2 grown on a MXene surface enhanced the growth inhibition activity (97%99%) against Bacillus subtilis and Escherichia coli in the membrane under UV light (Rasool et al., 2016; Rasool et al., 2017). Interestingly, the only MAX phase (Ti3AlC2) that shows poor activity of 14%18%. In a similar line, Rasool et al. show that MXene can be used in water desalination/purification membranes as it is resistant to biofouling (Rasool et al., 2017). In addition, MXenes have shown antibacterial activity toward gram-negative E. coli and gram-positive B. subtilis and 98% cell viability is lost after 4 h exposure (Rasool et al., 2016). Ti3C2 can destroy the bacteria cells via the stress-induced interaction pathway. Jastrze˛bska et al. have explained the atomic origin of the antibacterial activity of MXene Ti3C2. It is notable that Ti2C does not show antibacterial activity toward E. coli whereas Ti3C2 can. The reason behind this can be explained by the difference in atomic structure revealed from their TEM ˚ which is thicker than Ti3C2 study. In Ti2C, atomic layers of Ti-C are 9.76 A ˚ ˚ (9.23 A) by 0.53 A. So it is clear that the individual sheet thickness of Ti2C is higher in comparison to that of Ti3C2 and this supports that the atomic structure is the main factor in the difference in bioactivity between Ti3C2 and Ti2C, which have the same chemical composition but different stoichiometry (Jastrze˛bska et al., 2019). Cu2O anchored Ti3C2 heterojunction exhibits an excellent photocatalytic antibacterial activity against S. aureus and P. aeruginosa under visible light (Wang et al, 2020). Increased efficiency is due to the better charge transfer by Ti3C2 thus increasing the photo-oxidation activity. Improved antibacterial property is attributed to the synergistic effect of bacteriostatic action of Ti3C2 (MXene) and photocatalytic antibacterial ability of Cu2O. However more such investigations are expected in the future to strengthen the photocatalytic antimicrobial activity of MXene nanosheets. All the available results on the environmental application of MXene-based heterostructure are presented in Table 11.4.

TABLE 11.4 Comparative results on the environmental applications of MXene-based photocatalysts. Photocatalyst

Pollutants

Reaction condition

Main findings

Reference

Ti3C2Tx

Methylene blue (MB) and acid blue 80 (AB80)

UV irradiation for 5 h

81% adsorbed MB and 62% adsorbed acid blue dye degraded.

Mashtalir et al. (2014)

TiO2/Ti3C2 Hybrids

Methyl orange (MO)

Ultraviolet (300 W mercury lamp), 160 C Time: 12 h

Catalytic activity: 97.4%

Peng et al. (2016)

Co3O4/MXene composite

Bisphenol A

Conc: 20 (mg/L) catalyst dosage 0.1(gm/L)

95% removal within 7 min.

Liu et al. (2018)

BiOBr/Ti3C2 composites

RhB

UVVis light absorption (300-W Xe lamp)

79.5% of RhB degraded in 50 min.

Liu et al. (2019a)

Gd31, Sn41 doped bismuth ferrite Bi12xGdxFe12ySny/MXene.

Congo dye

300 W Xenon lamp, visible light source (400 2 700 nm).

100% degradation in 120 min

Tariq et al. (2018)

Core-shell In2S3/TiO2/ Ti3C2

Methylene orange

300 W Xenon lamp, Intensity: 300 mW/cm2.

Degradation rate: 0.04977/min, 3.2 and 6.2 times of In2S3 and Ti3C2.

Wang et al. (2018)

BPQDs/Ti3C2/TiO2 composites

Methylene orange

400 W metal halide lamp, light intensity: 80 mW/cm2

93% in 60 min

Yao et al. (2020b)

Ti3C2/MoS2(2D/2D)

Methylene orange

400 W metal halide lamp

97.4% within 30 min

Yao et al. (2020a)

BiFeO3/Ti3C2 nanohybrid

Congo red and acetophenone

Visible light 420 nm 400 , λ , 780 nm

Congo red:100% in 42 min, acetophenone:100% in 150 min.

Iqbal et al. (2019) (Continued )

TABLE 11.4 (Continued) Photocatalyst

Pollutants

Reaction condition

Main findings

Reference

Ti2C/3%TiO2 doped with Ag2O, Ag, PdO, Pd, Au.

Salicylic acid (SA) photodegradation

UV-reactor, medium pressure Hg lamp (150 W), 18 C 2 20 C.

degradation efficiency: 97% in 3 h

Wojciechowski et al. (2019)

CdS/Ti3C2/TiO2 nanohybrids

Methylene blue (MB, 20 mg/L), Rhodamine B (RhB, 20 mg/L)

Visible light source (λ 5 4001050 nm), Temp 225 C

Decolorization rate: MB 5 48%, RhB Ti3C2 was 30%, 180 min

Liu et al. (2019b)

TiO2/Ti3C2Tx

Carbamazepine

Both solar light and UV light, 25 C

Degradation rate: 98.67%, only Ti3C2Tx: B60%

Shahzad et al. (2018)

Ti3C2-Bi/BiOCl

Antibiotic ciprofloxacin

Visible light 300 W Xe lamp (λ . 420 nm).

89% of CIP degradation, 22.3 times of BiOCl and 1.76 times of Bi/BiOCl.

Wu et al. (2020)

Ti3C2/SrTiO3 heterostructure

U(VI)

300 W Xe lamp, RT

UO221 removal rate of 77%, 38 times higher than pristine SrTiO3

Deng et al. (2019)

Ti3C2Tx membrane

Escherichia coli and Bacillus subtilis

24 h incubation with Ti3C2Tx modified membranes.

The antibacterial rate: Ti3C2Tx $ 73% for B. subtilis, and 67% for E. coli, with 99% growth inhibition

Rasool et al. (2017)

Ti2C and Ti3C2

E. coli and B. subtilis

Ti3C2 concentration: 100 μg/mL, exposed for 4 h.

98% bacterial cell viability loss.

Jastrze˛bska et al. (2019)

Cu2O/Ti3C2

S. aureus, and P. aeruginosa

Visible light, RT

Antibacterial efficiencies against S. aureus and P. aeruginosa is 90.04% and 95.59%

Wang et al. (2020)

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11.4 Conclusion and future prospects Presently, the design of heterostructured photocatalysts based on MXenes is mainly concentrated on Ti3C2 even though more than twenty MXenes are already synthesized and around seventy are theoretically reported. The whole MXene-based heterostructure family is an important and attractive subject for future research in the area of photocatalysis and photoelectrocatalysis. Though studies of Ti3C2 have demonstrated its advantages as a photocatalyst, research has shown that there remain several critical challenges. In general, the titanium carbide and nitride nanoparticles are unstable in nature primarily because they can be easily oxidize to TiO2 during synthesis which ultimately compromises their advantage in promoting charge separation. Thus extensive work involving other MXene materials needs to be accelerated in order to derive better photocatalysts in terms of stability and charge transport capability. However, the expansion of experimental research to other materials in the MXene family depends on the widespread availability of the corresponding MAX phase to the majority of researchers. For example, some of the MXenes having inherent bandgaps need to be studied more extensively. In recent times, mostly heterostructures based on a 2D/2D composition involving Ti3C2 have been studied. This research needs to be extended to other MXenes. The beneficial effect of the 2D/2D structure for achieving ultra-fast charge separation needs more investigation in combination with other 2D structures like metal chalcogenides and boron nitrides. In metal chalcogenides, mostly MoS2 has been studied in combination with MXene. Research on these heterostructures involving MXenes needs to be extended to include other metal chalcogenides. In terms of heterostructures, more lowdimensional heterostructures involving 0D/2D and 1D/2D should be explored. In addition, 3D architecture at the nanoscale needs to be rationally designed with interfacial and geometrical engineering of the MXene-based nanocomposites. In addition, the prospects of using MXene-based heterostructures for novel photocatalytic applications beyond water splitting and environmental degradation is needed to be explored more comprehensively. In general, for green fuel generation, the proposed catalyst should have high activity, stability, and selectivity for targeted commercial applications in the long run. Limited work on the use of MXene-based photocatalysts has been reported but the true catalytic pathways and involved mechanisms are still not explored. Thus an expansion of applications of these materials requires further investigation in order to design new catalysts. To date, predictive research on MXene-based material development has been mostly confined to theoretical studies. Therefore it is imperative to increase experimental investigations on novel heterostructures in order to validate these simulations in the development of more practical applications. Still, a collective effort is necessary for a substantial breakthrough in designing MXene-based composites

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to solve the renewable energy problem at the earliest. Moreover, the use of MXene-based nanomaterials could also be extended for other photocatalytic applications like organic synthesis and viral disinfection, such as against COVID-19 and other viruses.

Acknowledgments The authors are thankful to Dr I. Chattoraj, Director, CSIR-NML for his permission and encouragement to carry out this work. Authors are also thankful to CSIR, India for the financial support under NCP project (MLP-3111).

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Chapter 12

Atomic layer deposition of materials for solar water splitting Rodrigo Savio Pessoa1, William Chiappim Junior2 and Mariana Amorim Fraga3 1

Plasmas and Processes Laboratory, Aeronautics Institute of Technology, Sa˜o Jose´ dos Campos, Brazil, 2i3N and Department of Physics, University of Aveiro, Aveiro, Portugal, 3Institute of Science and Technology, Federal University of Sa˜o Paulo, Sa˜o Jose´ dos Campos, Brazil

12.1 Introduction Energy is the key to accessible, clean, safe, and livable conditions necessary for a better standard of living (Thakur et al., 2020). Among the renewable energy sources, the sun is the cleanest because it emits no carbon dioxide (CO2). It is also the most abundant renewable energy source. The total energy resources available worldwide are generally classified as nuclear, fossil, or renewable. According to the United States Energy Information Administration, in 2013, the estimated global energy consumption exceeded 150,000 TW and a large part of this energy came from fossil fuels, and although they have many environmental disadvantages, their research is still very active, especially in some parts of the world (https://www.iea.org/reports/global-energyco2-status-report-2018, accessed in June 01, 2020). The consumption of these nonrenewable reserves is expected to promote serious environmental problems, whose effects can cause irreversible damage to the planet. In this way, since the 1970s oil crisis, renewable energy sources, such as geothermal, wind, biomass, solar, etc., have been the focus of many research and technological developments due to their sustainability advantages (Ghosh and Ghosh, 2020; Costa-Silva et al., 2019). Fossil fuel consumption trend and global warming scenario have been covered in a recent review article, which showed data that evidence a continuous reduction of fossil fuel energy consumption since 1980 due to the rise of renewable energy technologies (Ghosh and Ghosh, 2020). This indicates that renewable energies are starting to play a significant role in power generation, replacing fossil fuels. It is estimated that renewable energy Sustainable Material Solutions for Solar Energy Technologies. DOI: https://doi.org/10.1016/B978-0-12-821592-0.00015-7 © 2021 Elsevier Inc. All rights reserved. 363

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sources may exceed about 35% of the total energy produced in the world in the year 2025 (Thakur et al., 2020). Therefore in order to meet daily energy demands and help solve intrinsic environmental problems, the search for economically viable, sustainable, and renewable alternative energy sources has received great attention from scientists and technologists. As a result, the exponential increase in the accumulated installed capacity of renewable energy sources has been observed for at least 15 years (Thakur et al., 2020). Although the solar energy is considered to be the most available renewable source on Earth, its efficient harnessing, storage, and use are not still meeting the growing global demand for energy needs because with the improving standards of living energy consumption grows faster than the world’s population (Thakur et al., 2020; Ghosh and Ghosh, 2020; Graetzel, 2001). Two main approaches are taken for photochemical conversion and storage of solar energy: (1) the direct conversion of solar energy to electricity, which can then be used for various needs or (2) the direct generation of high energy fuels, such as molecular hydrogen (H2), from the photoelectrolysis of water, also called “photoelectrochemical water splitting.” These approaches are based on the “artificial photosynthesis” process, which is a biomimetic approach in which the important structural elements characteristic of the natural photosynthesis reaction are used in conceptually simpler systems to obtain natural photosynthesis results (Ftoy and Critchley, 2005). Such systems, also known as solar or photoelectrochemical cells (PECs), based on photovoltaic or PEC technology, evolve each year and owe their impressive growth rate to a combination of decreasing production costs, due to scaling economies, and the development of new solar energy systems and less expensive technologies like those based on thin films. A practical example was reported by Tan et al., which demonstrated unassisted PEC water splitting driven by the photovoltage of series-connected silicon heterojunction with intrinsic thin layer cells, in which ALD-grown TiO2 protection layers inhibit corrosion during the hydrogen evolution reaction (HER) and electronically couple the silicon photocathode to an efficient HER catalyst. This ALD-protected silicon heterojunction solar cells exhibited efficiency in excess of 10% (Tan et al., 2019). Despite significant efforts to date, the production of semiconductor materials stable in contact with the electrolyte (water) is limited, leading a growing scientific community to more complex photoelectrode structures. In the last decade, thin films studied and applied for use as photoelectrodes in PEC devices are mainly from the following groups of materials: (1) metal oxides such as titanium dioxide (TiO2) (Pessoa et al., 2015), zinc oxide (ZnO) (Dom et al., 2020; Shet, 2010), niobium oxide (Nb2O5) (Sajiki et al., 2015), hematite (α-Fe2O3) (Tamirat et al., 2016), among others; (2) nitrides/oxynitrides such as copper nitride (Cu3N) (Han et al., 2018) and titanium oxynitride (TiOxNy) (Mohamed et al., 2020); (3) silicon-based compounds such as silicon carbide (SiC) and silicon nitride (Si3N4) (Pessoa et al., 2015; Varner, 2002); and (4) chalcogenides (Ros et al., 2020).

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The stability of these devices in harsh acidic or alkaline electrolytes, that is in a complete pH range (0 < pH < 14), has become a key issue, driving the research in metal oxides electrodes for PEC applications due to their chemical stability and good semiconducting properties. However, generally these materials exhibit wide band gaps and therefore only absorb ultraviolet (UV) radiation, which makes them less suitable for terrestrial solar energy conversion. A large number of experimental and theoretical studies have been devoted to discuss how to control the properties of metal oxide semiconductors to use them as efficient light collection materials. In this regard, a variety of papers and reviews have been published on the PEC properties of wide band gap materials giving an idea of the importance of this issue (Thakur et al., 2020; Tamirat et al., 2016; Ros et al., 2020; Li et al., 2020; Bagal et al., 2019; Joe et al., 2019; Sinha et al., 2018). Despite the vast amount of literature on these materials, there are many important aspects that still have not properly been addressed. Atomic layer deposition (ALD) is a CVD-type technique developed in the 1970s to grow conformal, homogeneous, and adherent thin films over large and complex areas with precise control on film thickness and excellent repeatability (Pessoa et al., 2015; Pessoa et al., 2018; Pessoa and Fraga, 2019). In the last two decades, ALD has emerged as an outstanding technique to obtain high-performance electrode materials due to its efficacy to deposit pin-holefree metal oxide thin films (Pessoa et al., 2018). Furthermore, other types of ceramics synthetized by ALD have shown suitable characteristics, such as composition, structure, and morphology, to be used as transparent, conductive, and protective layers in PEC devices. In relation to other ALD applications in solar energy technologies, in their review article, Hossain et al. discussed the application of ALD layers as a surface passivation layer, buffer layer, window layer, absorber layer, electron/hole contact, or transparent conductive oxide in a wide range of different solar cell types, such as industrial silicon, organic, thin film, and quantum dot. They show that ALD layers are already applied high-volume manufacturing of silicon solar cells, which currently dominate the photovoltaic industry. It was highlighted that this is a result of ALD intrinsic benefits including digital control of the film thickness and thin film composition, conformal deposition of pin-hole-free thin films on challenging 3D structures and on large areas and the precursor utilization yield higher compared to other deposition techniques (Hossain et al., 2020). This chapter aims to review the atomic layer deposited thin films based mainly on metal oxides and nitrides for PEC application for solar water splitting process. Particular emphasis is given to thin film materials characteristics and the ALD synthesis method. In topic 1.2 the basics about solar energy generation are presented. In topic 1.3, the photocatalytic conversion of solar energy into electricity or chemical energy is presented, more specifically hydrogen generation from a PEC water splitting process. Topic 1.4 presents the concepts of hydrogen generation from water-electrolysis. In topic

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1.5, the characteristics of thin films, mainly oxides and nitrides, used for water splitting process are presented. In topic 1.6 the basic concepts and characteristics of ALD technique and water splitting materials are discussed. Finally, in topic 1.7 the final remarks are presented.

12.2 Solar energy The radiation from the sun is due to the conversion of four hydrogen nuclei into a helium nucleus, where every 6 3 1011 kg of hydrogen is converted into helium every second. If constant, the amount of energy per second produced by the sun is around 5.4 3 1028 J (Aldabo´, 2002). That energy rate may be stable for the next 10 billion years (Aldabo´, 2002). However, the ozone layer absorbs a large part of this energy, so that annually around 3 3 1024 J reaches the surface of the planet, or approximately 10,000 times more than the global population currently consumes, in one year (Graetzel, 2001). Therefore it is estimated that covering 0.1% of the Earth’s surface with solar cells with 10% efficiency in converting solar energy into electricity could satisfy the current global need (Graetzel, 2001). However, this depends on each region of the planet, since the incident radiation is not constant for all continents. Fig. 12.1 presents the solar radiation map of long-term average of global horizontal irradiation (GHI) (World Bank, 2017). GHI is the total amount of shortwave radiation received from above by a surface horizontal to the ground. This value is of interest to photovoltaic installations and includes both Direct Normal Irradiance (DNI) and Diffuse Horizontal Irradiance (DIF). DNI is solar radiation that comes in a straight line from the direction of the sun at its current position in the sky. DIF is solar radiation that does not arrive on a direct path from the sun, but has been scattered by molecules

FIGURE 12.1 Solar radiation map of long-term average of global horizontal irradiation. This map was published by the World Bank Group and prepared by Solargis [World Bank, 2017. Global Solar Atlas. ,https://globalsolaratlas.info. (accessed 22.12.20)].

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FIGURE 12.2 Geometry and working principle of photoelectrochemical cells based on n-type semiconductors. (A) regenerative type cell for producing electric current from sunlight; (B) cell that generates a chemical fuel, hydrogen, through the photodissociation of water. Adapted from Graetzel, M., 2001. Photoelectrochemical cells. Nature, 414, 338344.

and particles in the atmosphere and comes equally from all directions. The average annual solar radiation arriving on a horizontal surface at ground level is about 1120 W/m2 (World Bank, 2017; Coddington et al., 2016). On the other hand, according to Fig. 12.1, regions of South America, Africa, Middle East, and Oceania present average GHI values above 228 kW/m2 (or 2000 kWh/m2 per year), making the development of solar energy applications in these regions even more attractive.

12.3 Photoelectrochemical cells Since the French scientist Edmond Becquerel discovered the photoelectric effect, researchers and engineers have been encouraged by the idea of converting sunlight into electrical energy or chemical fuels (Becquerel, 1839). The common dream is to capture the energy that is freely available from sunlight and turn it directly into electricity, a strategically important and valuable asset, or use it to generate chemical fuels such as hydrogen. Photovoltaic science takes advantage of the fact that photons, which come into contact with a semiconductor, can create electron (e2)—hole (h1) pairs in the semiconductor energy bands. For a junction between two different semiconductors, this effect can configure a difference in electrical potential through the interface and, consequently, the generation of a photocurrent (Graetzel, 2001). In the last few decades, PV technology has been dominated by devices where the junction is made with inorganic semiconductor materials. Typically, these materials are forms doped with crystalline or amorphous silicon, which benefit from the experience and availability of silicon-based

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semiconductor industry. However, in recent years, the field, which is dominated by these solid-state inorganic junction devices, faces new challenges. Increasingly, the advantages of PV devices based on nanostructured materials and conductive polymers have been highlighted. For example, these allow them to be relatively inexpensive to manufacture and can be used on flexible substrates or be shaped according to home appliances, architectural, or decorating applications (Graetzel, 2001; Sanders, 2002). With these new materials it has been possible to completely exit the classic solid-state junction device, replacing the phase in contact with the semiconductor by an electrolyte (gel, liquid, or organic solid), forming a PEC also known as a Gra¨etzel cell (Costa-Silva et al., 2019; Graetzel, 2001). Investigations have focused on two main types of PECs, whose operating principles are shown in Fig. 12.2. The first type is a regenerative cell, which converts sunlight into electrical energy, causing no chemical change in the system during its operation. Its structure is of the sandwich type, composed of a transparent conductive electrode (photoanode) that has a thin layer (of the order of μm) of wide band-gap semiconductor, whose surface is sensitized by an organic dye that aims to expand the light absorption range for the visible spectrum, thus promoting a greater injection of electrons in the semiconductor conduction band and, consequently, increasing the photocurrent generated in the cell. Generally, materials such as titanium dioxide (TiO2), zinc oxide (ZnO), etc. Graetzel (2001) and Kalyanasundaram and Graetzel (2010) are used in photoanodes. And, a counter electrode (cathode) that is usually coated with a platinum layer is used to catalyze the reduction process in the electrolyte. The two electrodes are joined by a spacer and the intermediate space is filled with an organic electrolyte containing an ox reductive mixture, usually iodide/triiodide, which is responsible for the chemical process of regeneration through redox reactions. In the second type, cells operate on a similar principle to the previous one, except that the electrolyte is water and there are two redox systems: one reacting with the holes (h1) on the surface of the semiconductor electrode and the second reacting with the electrons (e2) entering the counter electrode.

12.4 Hydrogen generation from water photoelectrolysis Hydrogen is a key solar fuel as it allows you to store solar energy and can be used directly in combustion turbines or fuel cells (Clark et al., 2005; Winter, 2009; Mandal and Gregory, 2010). There are two methods used for its generation: (1) electrolysis of water or (2) reform of fossil fuels. The second option, of course, is somewhat inappropriate when it comes to renewable sources of energy, since it is the use of fossil fuel as a source of H2. The first option remains, but it needs electricity for its occurrence and the question

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remains: Where will the electric energy come from to perform the electrolysis of water? Since Fujishima and Honda demonstrated in the 1970s that water could be decomposed into H2 and O2 by a PEC with a TiO2 photoanode, the photocatalytic division of water using semiconductors has been considered as an ideal method to convert solar energy into hydrogen energy (Fujishima and Honda, 1972). This process is illustrated in Fig. 12.2B, which shows the division of water through the oxidation processes for the formation of oxygen (O2), which occurs in the semiconductor (photoanode), and the reduction generating hydrogen (H2) in the cathode. In summary, the reactions that take place on the electrodes are as follows (Graetzel, 2001; Sun et al., 2010):1 -Anodeðn-type semiconductorÞ: 2H2 Oðl´ıquidoÞ 1 sunlight-O2ðgasÞ 3 1 4H1 Ered 5 1; 23V versus NHE ðaq:Þ 1 4e; -Cathode or contra-electrodeðplatinumÞ: 4H1 ðaq:Þ 3 1 4e-2H2ðgasÞ ; Ered 5 0V versus NHE

ð12:1Þ

ð12:2Þ

For a material to perform the photocatalytic division of water: (1) it must be stable under photolysis conditions, that is, have the maximum valence and minimum conduction potentials between 1.6 and 2.0 eV, respectively; (2) it must absorb a significant portion of the solar spectrum (UV 1 visible); (3) resist degradation in a liquid environment; (4) scalable for device fabrication techniques; (5) cost-effective; and (6) Earth abundant (Minggu et al., 2010). The review of Ros et al. presents in detail more requirements for PEC materials for water splitting applications (Ros et al., 2020). The main problem of PECs for photoelectrolysis of water is related to how to increase the efficiency and stability of photoactive materials. The goal in overcoming these challenges is to achieve the required solar-to-hydrogen (STH) efficiencies of 10% or higher to increase the chances of developing new materials and solar water splitting technology.

12.5 Materials for photoelectrode One of the most studied areas in the development of PECs is related to the coating of its photoactive electrode. As highlighted by Ros et al. (2020), the number of published works containing “photoelectrochemical water splitting” in the Scopus database was increased considerably in recent years, totaling 875 works published in 2019, with many of them focused on the issue of photoactive electrode material. Only by way of comparison, Ghobadi et al. evidenced the growing interest in PEC devices leading to more than 3000 published scientific papers in Web of Science database from 2014 to 2018 (Ghobadi et al., 2018). 1. NHE 5 Normal Hydrogen Electrode.

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As observed in literature, for the water photoelectrolysis process, materials with low bandgap and high chemical inertia such as TiO2 and CuO2, for example, are needed in order to increase the efficiency of conversion from solar energy to hydrogen. Depending on the type of semiconductor, there is a certain configuration of the PEC electrodes and, consequently, the inversion of the photocurrent path in the PEC circuit (Minggu et al., 2010). For n-type semiconductors covering the photoactive electrode the current direction is illustrated in Fig. 12.2, in this case the photoactive electrode is called a photoanode. When a p-type semiconductor is used, the current is reversed and, therefore the redox reactions in the electrodes, in this case the photoactive electrode is called a photocathode (Ros et al., 2020; Ghobadi et al., 2018). Among all possible metal oxides studied and applied for PEC water splitting, the n-type oxide TiO2 has been extensively investigated as a photoanode due to its favorable position of energy bands, strong optical absorption, excellent chemical stability and resistance to photo corrosion, and low cost (Pessoa et al., 2015; Paracchino et al., 2011; Lee et al., 2009). However, the efficiency of transforming solar energy into hydrogen from TiO2 is substantially limited due to its wide band gap (B3.2 eV: anatase; B3.1 eV: rutile), which allows only a small fraction (approx. 5%) of the solar energy spectrum to be absorbed. Numerous attempts to change the spectral response from TiO2 to the visible have been the subject of research in the last decade. One of the most conventional are the anchors of organic dyes, which red shifts the absorption range. However, the dyes commonly used are rutheniumbased which are expensive and whose long-term stability may be questionable (Wang et al., 2011; Lindgren et al., 2004). Furthermore, such dyes undergo degradation in the presence of oxygen (Lindgren et al., 2003), a fact that limits its use for the water photoelectrolysis process. Another way to absorb wavelengths in the visible region without the use of dyes is to improve the photocatalytic activity of the semiconductor material through the insertion of dopants in its structure. In the case of TiO2, doping materials such as: (1) transition metal cations like V, Cr, Mn, Fe and (2) anions of N, C or F gas atoms (Kisch and Macyk, 2002). An example is the doping of TiO2 with nitrogen. Depending on the amount of nitrogen incorporated in the TiO2 film, it can form a ternary alloy, known as titanium oxidenitride (TiOxNy) whose band gap can be less than 2.0 eV. To perform this doping or ternary alloy formation, plasma-assisted deposition processes are quite efficient, and the magnetron sputtering process using the pulsed gas injection method (gas pulsing technique) proved to be effective (Hĕrman et al., 2006). Another frequently used technique for growth of TiOxNy films is the plasma enhanced atomic layer deposition (PEALD) technique (Martin et al., 2007). Another interesting plasma treatment that has been used to shift the absorption of TiO2 film to red, is the hydrogenation of the TiO2 film, making it black-colored, a fact that led it to be called “black TiO2” with

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band gap energy of the order of 1.0 eV, favoring a much superior PEC efficiency (Kot and Łobaza, 2018). A recent work of Wang et al. reviewed the application of black TiO2 for solar hydrogen conversion, indicating it as a promising candidate for photoelectrocatalytic hydrogen generation via water splitting (Godoy et al., 2020). On the other hand, according to the theoretical prediction of Lee et al. (2009) and Paracchino et al. (2011), binary copper semiconductors, such as copper oxides, are attractive because they are photoactive and abundant. Cu2O is an attractive p-type oxide for the PEC production of hydrogen since it presents a bandgap between 2.0 and 2.4 eV. It can be used as photo or counter electrode. The main limiting factor for the use of Cu2O as a counter electrode for water reduction is its low stability in aqueous solutions, such as water, because the Cu2O redox potentials are between 0 and 1.23 eV (Paracchino et al., 2011). A solution found to increase this stability is the deposition of ultrathin films of type-n oxides on the surface of the Cu2O film, which also aims to protect against the corrosion process of the counter electrode (Paracchino et al., 2011). ALD technique has shown good results through the use of passivation metal oxide ultrathin films, for example, as discussed in Hossain et al. (2020). The review of Bagal et al. presented that the improvement in the Cu2O photocathodes was achieved by finding elegant solutions such as forming thin overlayers by ALD technique (Bagal et al., 2019). To protect Cu2O from reductive decomposition, ALD overlayers of Al:ZnO and TiO2 were deposited with different thicknesses (Bagal et al., 2019). Paracchino et al. deposited TiO2 at different temperatures to enhance crystallinity of the top layer for improvement in the stability. They found that the performance of Cu2O photocathode was dependent on ALD deposition temperature, which determines energy band level and crystallinity (Bagal et al., 2019; Wang et al., 2017). To enhance the photostability of PEC devices based on Cu2O, Azevedo et a. deposited SnO2 overlayers (20, 50, and 100 nm thick) via ALD on Cu2O (Bagal et al., 2019; Paracchino et al., 2012). After analyzing the J-V curves and stability they found that higher thickness provides higher photocurrent density with higher stability. The stability retention was found about 80% of its initial plateau after 45 hours. Another metal oxide that is used in photo electrodes for solar water splitting is the n-type zinc oxide (ZnO) of energy band gap of 3.3 eV (Azevedo et al., 2016). ZnO has been intensively studied as an efficient photocatalyst because of its abundance, high electron mobility, and lifetime (Asha et al., 2019). However, ZnO readily undergoes photocorrosion in aqueous electrolytes. This shortcoming of ZnO can be overcome by depositing a thin film of TiO2 as a second passivation layer on it (Wang et al., 2019). The theoretical maximum STH conversion efficiencies of the n-type Fe2O3 is 16.8% (Kim et al., 2019; Huang et al., 2014). The use of Fe2O3 for water splitting is attractive because of its high potential STH efficiency,

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abundance, and low cost. CVD-grown Fe2O3 can be used as a water-splitting photoanode at wavelengths greater than 400 nm. Fe2O3 is a nontoxic and abundant metal oxide with a bandgap of 2.1 eV. This moderate band gap allows it to absorb 40% of incident solar radiation. Unfortunately, many limitations of Fe2O3 have kept it from realizing this potential. For example, the lifetimes of the photogenerated electrons and holes in Fe2O3 are short (Rahman and Joo, 2013; He et al., 2019). Moreover, Fe2O3 suffers from poor hole diffusion kinetics, which limit its photoanode performance. Efforts have been made to develop nanostructures that promote faster transport of holes to the Fe2O3 surface. However, when the excited holes reach the nanostructured Fe2O3 surface, defects arising from oxygen vacancies at the electrode/ electrolyte interface promote carrier recombination. Hence, thin overlayers of Al2O3, Ga2O3, or TiO2 have been explored as passivation layers to protect the Fe2O3 film (Rahman and Joo, 2013; Gurudayal et al., 2018; Hisatomi et al., 2011). TiO2 has been coated as a passivation overlayer on Fe2O3 using various processes (Rahman and Joo, 2013; Jeon et al., 2017). Davide et al. (2015) and Jeon et al. (2017) coated TiO2 on a Fe2O3 layer by ALD; the Fe2O3 itself had been deposited by plasma enhanced chemical vapor deposition (PECVD). They observed that the TiO2/Fe2O3 heterojunction protected the photoelectrodes from corrosion and thus enhanced the photocurrent density. Further, when a ZnO layer is sandwiched between the Fe2O3 and TiO2 layers, the valence band of ZnO facilitates generation of photoexcited holes. Other metal oxide candidates have been widely studied for the PEC water splitting are: WO3 (n-type, 2.6 eV) (Liu et al., 2013), NiO (p-type, 3.4 eV) (Cen et al., 2019) and SnO2 (n-type 3.7 eV) (Hu et al., 2014). Also, we can cite the metal oxide materials that have been tested for PEC water splitting, such as PbO, Ta2O5, and ternary oxides such as FeTiO3, BaTiO3, CuWO4, Co3O4, BiFeO3 (Ros et al., 2020). Concerning metal nitride films, cuprous nitride (Cu3N) material is one of the most studied for PEC water splitting (Ghobadi et al., 2018; Paracchino et al., 2011). However, in a general way, studies of metal nitrogenated thin films for PEC water splitting are scarce in literature, especially for ALDbased processes. Among group IV, group III-V semiconductors, that is Si, GaAs, GaN, GaP, and GaInP2 have feasible potentials for water reduction, and GaN an InP have proper potential for water oxidation. The review of Joe et al. (2019) presents a detailed discussion about the use of transition metal (di)chalcogenide catalysts in conjunction with silicon photoelectrode as Earth-abundant material systems (Azevedo et al., 2016). The growth mechanism of metal (di)chalcogenide material using ALD and its implication of low-temperature growth on defect chemistry are featured. They highlighted the multi/mixed-phase metal (di)chalcogenide overlayers on Si, their operation principles, and provided challenges and directions regarding future research for achieving the theoretical PEC performance of Si-based photoelectrodes (Azevedo et al., 2016).

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12.6 Atomic layer deposition technique: process and equipment 12.6.1 Atomic layer deposition process The ALD is a vapor-phase deposition technique in which ultrathin films can be synthesized over large and complex substrate areas with precise control on film thickness, composition, and excellent repeatability (Pessoa et al., 2015; Joe et al., 2019; Cassir et al., 2010; Marin et al., 2012; Chen et al., 2020). The ALD technique is similar to chemical vapor deposition (CVD), except that the deposition is carried out in cycles, each of which is a sequence of two self-limited surface reactions between gaseous precursor molecules and a substrate (Chen et al., 2020; Chiappim et al., 2016a). Consequently, when sufficient precursor species and reactants are dosed, the growth of the ALD film does not dependent on the gas flow, as is the case of CVD and physical vapor deposition techniques (Chiappim et al., 2016b). Because of the surface control, it is possible to grow ALD films with excellent conformability (up to 100%) on surfaces with complex roughness and topography, large area uniformity and high quality (Profijt et al., 2011). Moreover, the thickness of growing film can be precisely tuned at the atomic level by varying the number of ALD cycles (Marin et al., 2012). In addition, ALD processes are generally performed at significantly lower temperatures than related CVD processes, which normally depend on high temperatures to thermally decompose the precursor on the substrate surface. In ALD, the processes can be carried out at temperatures as low as 100 C, thus allowing the covering of thermally sensitive substrates such as polymers (Chiappim et al., 2016b; Lua et al., 2016). Furthermore, the dependence of the growth rate (usually called growth per cycle, GPC) with the temperature variation is generally weak (Marin et al., 2009). Nominally the GPC should be one monolayer per cycle, in practice however the GPC can vary from 0.01 to 0.2 nm/cycle depending on the precursor reactivity and molecular size, process parameters, and the progress of nucleation on the substrate surface (Miikkulainen et al., 2013). Currently, a variety of materials, including metal oxides, metal nitrides, metal sulphides, metals, polymers and inorganicorganic hybrid materials, have been successfully grown using ALD. An updated list of the synthesized materials can be found in the review of Miikkulainen et al. (2013) and Marin et al. (2009). The unique attributes and flexibility in material choice have fostered a rapid expansion in ALD research for energy applications, particularly for nanomaterial synthesis and functionalization, in areas such as photovoltaics, catalysis, fuel cells, batteries, supercapacitors, solar water splitting, and optoelectronics (Chen et al., 2020). In special, the ALD strategy to the efficient energy transfer in PEC systems should be a powerful tool not only for coating functional materials such as metals, semiconductors, and insulators but also for incorporating dopant elements in a controlled fashion (Azevedo et al., 2016).

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The ALD of metal oxides typically involves two self-saturating half-reaction cycles, one consisting of a metal precursor such as a halide, metalorganic, or an organometallic, which is followed by exposure of the surface to an oxidant, such as deionized water (H2O), ozone (O3), hydrogen peroxide (H2O2), or O2-plasma (Chen et al., 2020; Chiappim et al., 2016a). For the case of metal nitrides, ammonia (NH3) or N2 1 H2-plasma are often used (Marin et al., 2009). Fig. 12.3 shows the scheme of a typical ALD cycle for metal oxide synthesis, using a metal precursor and an oxidant precursor. More specifically, a classic case of metal oxides can be exemplified by the ALD process of Al2O3 using trimethyl aluminum (TMA, Al(CH3)3) and water (H2O) as the precursors (Gregorczyk and Knez, 2016; Meng et al., 2017; Shim et al., 2013). Basically, one ALD cycle usually consists of four steps: (1) supplying the first precursor to introduce the first surface reactant (TMA) into a reaction chamber; (2) purging the unreacted precursor from the reaction chamber; (3) supplying the second precursor (H2O) to introduce the second reactant into the reaction chamber, and (4) purging the reaction chamber again. The growth cycles should be repeated as many times as necessary to achieve the desired film thickness. At each half-cycle, precursor molecules adsorb onto the underlayer and react to form a new layer. After saturation, no further adsorption or chemisorption takes place. During ALD, precursors are strictly separated using active purging and valving between one halfcycle and the next. The strict separation of reactants enables only precursors

FIGURE 12.3 Schematic model of the thermal atomic layer deposition deposition process. (A) Functionalized substrate containing active sites with hydroxyl radical on the surface of the initial substrate or forming film. (B) Precursor “trimethyl aluminum” is pulsed and reacts with the surface. (C) Purge—excess of the precursor and reaction products (CH4) are purged with inert gas N2. (D) Precursor “H2O” is pulsed and reacts with the surface. (E) New purge is performed. (F) The cycle is repeated until reaching the desired layer thickness. Adapted from Shim, J.H., Kang, S., Cha, S.W., Lee, W., Kim, Y.B., Park, J.S., et al., 2013. Atomic layer deposition of thin-film ceramic electrolytes for high-performance fuel cells. J. Mater. Chem. A 41, 1269512705.

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and substrates to react, preventing the gas-phase reactions of precursors (Puurunen, 2005).

12.6.2 Atomic layer deposition reactors: types and characteristics The ALD reactors can be divided in two types: those that operate in thermal mode and those that operate in energetic mode, where the most common is the plasma mode. The ALD reactor operating in thermal mode has a heated chamber (depending on the equipment operates between 100 C and 600 C) (Lownsbury et al., 2017), where the substrate is positioned. It has a geometry where on one side are inserted the process gases/vapors in crossed way (crossflow) and on the other side is the vacuum system. The ALD technique operating in thermal mode is applied to the deposition of thin films on 2D and 3D surfaces, mainly metal oxides are deposited for technological applications (Marin et al., 2009; Aarik et al., 2013). The ALD reactor operating in plasma mode also known as PEALD reactor has been gaining rapid popularity (Potts et al., 2013). In PEALD the surface of the substrate is exposed to species generated by the plasma, acting as the ligand precursor. Usually, the plasmas used during the PEALD process are the gases O2 for metal oxide, N2 and N2O for metal nitrides, and H2 for metals (Potts et al., 2013; Knoops et al., 2019). There are four PEALD configurations. In the configuration of type “a” called Radical-enhanced ALD, a plasma generator is set to a thermal ALD reactor. There are technical limitations in this type of reactor, because the plasma is generated far from the zone of reaction. Therefore the plasma species must flow through the reactor tube between the plasma source and the reaction chamber. This allows many surface collisions, where electrons and ions are lost before reaching the surface of the substrate due to recombination. Surface collisions in the reactor tube significantly reduce the flow of radicals reaching the substrate, hence the importance of harmonizing tube material with plasma. For example, H radicals have a low probability of recombination on quartz surfaces, but have a high probability of recombination with most metals. The type “b” configuration called direct plasma ALD stems directly from the PECVD. In this case a capacitively coupled plasma is generated by a radio frequency (RF) power source of 13.56 MHz between two parallel electrodes, where one of which is grounded. Where the substrate is positioned on the grounded electrode. In the “c” configuration called remote plasma ALD, the plasma source is located remotely, that is the substrate is not in contact during the generation of plasma species. This configuration differs from the “a” configuration because the plasma is still present after surface deposition, that is the density of electrons and ions does not fall to zero. The type “d” configuration called direct plasma ALD with grid is derived directly from the PECVD. In this case a capacitively coupled plasma is generated by a RF source of

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13.56 MHz between two parallel electrodes, one of which is grounded. Between the electrodes there is a polarized grid with direct current, alternating current or RF, creating a triode. Where the substrate is positioned on the grounded electrode. It was reported that for the resulting negative voltage both the temperature of the electrons and the density of the plasma increased, resulting in the higher flow of ions and radicals to the substrate. PEALD reactor can be applied in the deposition of thin films on 2D and 3D surfaces, mainly oxides are deposited for technological applications and “d” type reactors can also be used for atomic layer etching process (Jia et al., 2019).

12.7 Final remarks With the sun being one of the most abundant energy sources, PEC water splitting is postulated as one of the most interesting technologies due to worldwide water availability and carbon-free products, enabling hydrogen as a clean energy source. Currently, most hydrogen gas feedstock is being produced by methane steam reforming, producing CO2 as a byproduct, so “clean” methodologies are of great impact in this field. Consequently, scientists, researchers, and technologists have devoted efforts to develop and characterize optimized materials for PEC water splitting. Extensive knowledge of synthesis of novel materials with desired properties is important in increasing the hydrogen generation efficiency by overcoming limitations of traditional materials. ALD has emerged and is developing rapidly to produce high quality thin films with improved performance properties for PEC electrode application. As presented and discussed in this chapter, ALD metal oxides thin films, have demonstrated remarkable characteristics for PEC electrode production, especially because their composition and thickness can be atomically controlled. Several challenges and opportunities for solar water splitting solutions based on ALD materials are still open, such as active materials, surface state passivation, and corrosion protection. More research and development needs to be conducted before commercially exploiting the vast potential of this technology.

Acknowledgments The financial support of the Brazilian agency program FAPESP/MCT/CNPq-PRONEX (grant no. 2011/50773-0), FAPESP (grant no. 2018/01265-1), CNPq (grant no. 446545/ 2014-7 and 437921/2018-2) and the Brazilian Space Agency (AEB/Uniespac¸o) is also gratefully acknowledged.

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Section IV

Sustainable Materials for Thermal Energy Systems

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Chapter 13

Solar selective coatings and materials for high-temperature solar thermal applications Ramo´n Escobar Galindo1, Matthias Krause2, K. Niranjan3 and Harish Barshilia3 1

Applied Physics I Department, Higher Polytechnic School (EPS), University of Seville, Spain, Helmholtz-Zentrum Dresden-Rossendorf, Institute for Ion Beam Physics and Materials Research, Dresden, Germany, 3Nanomaterials Research Laboratory, Surface Engineering Division, CSIR-National Aerospace Laboratories, Bangalore, India 2

13.1 Introduction 13.1.1 Concentrated solar power: facts In recent years, use of renewable sources of energy (mainly solar, wind, hydropower, and geothermal) has become a crucial task in order to reduce man-made greenhouse gas emissions, which are considered the main cause for global atmospheric warming (Intergovernmental Panel on Climate Change, 2013). However, the world’s total primary energy supply (TPES) still relies predominantly on carbon-based fossil sources. The absolute energy production from oil, coal, and natural gas was doubled from approximately 5000 Mtoe1 to approximately 12,000 Mtoe in the period from 1971 to 2018, and its share on the world’s TPES decreased only moderately from 87% to 81% in the same period of time (International Energy Agency/OECD, 2020). Nuclear energy, biofuels, and waste2 currently contribute 14% of the world’s TPES, while only 4.5% is provided by the renewable energy sources (RES), viz., hydro, wind, and solar. Hence, RES have an enormous development potential, and this holds specifically for solar energy (International Energy Agency/OECD, 2020; Renewables 2019: Global Status Report, 2019; International Energy Agency/ OECD, 2017). To foster this technology, particularly in relation to 1. Mtoe: million tons of oil equivalent 2. Biofuels and waste are comprised of solid biofuels, liquid biofuels, biogases, industrial waste and municipal waste. Sustainable Material Solutions for Solar Energy Technologies. DOI: https://doi.org/10.1016/B978-0-12-821592-0.00011-X © 2021 Elsevier Inc. All rights reserved. 383

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concentrated solar power (CSP), rapid development of novel materials and innovative engineering solutions are urgently needed. CSP technology harnesses the sun’s power to generate electricity (Lovegrove and Stein, 2012; Boretti et al., 2019). A heat collector element receives a concentrated radiation flux reflected from a heliostat field that is transformed into heat via a heat-transfer fluid (HTF), for example, steam or molten salts. This heat is then converted into electrical energy using a turbine and a generator (see Fig. 13.1) https://commons.wikimedia.org/wiki/ Category:PS20_Solar_Power_Plant#/media/File:PS20andPS10.jpg. The main technological advantage of CSP compared to other renewable energy technologies lies on its intrinsic storage capacity. This capacity enables solar thermal electricity to be used on demand and dispatched at the request of power grid operators. Nowadays, the commercial CSP technologies are parabolic trough collector (PTC) and central receiver solar power plants. The other two configurations (Fresnel lenses and parabolic dish) have, so far, no relevance for electricity generation (Weinstein et al., 2015). The PTC is currently the more mature CSP technology with a maximum operating temperature of 400 C in the absorber tube and an expected service lifetime of 25 years (Weinstein et al., 2015; National Renewable Energy Laboratory). The central receiver solar power plants, namely, Gemasolar (Spain, in operation since 2011) and Cerro Dominador/Atacama—1 (Chile, under operation since April 2021) with 15 and 17 hours heat storage capacity, designed and constructed by the Spanish companies SENER and Abengoa, respectively, define the cutting edge of CSP technology. Tower plants currently in development or under construction in China, Chile, the MENA countries and Australia still rely on the materials and processes used thereby, namely on pigment-based absorber paints and molten salts as HTF (National Renewable Energy Laboratory) (see Table 13.1). While CSP has a great potential as part of “solar age electricity supply”, technological and economic reasons prevented it to keep on track with the

FIGURE 13.1 (left) PS20 and PS10 CSP plants in Spain. (right) Schematic illustration of a CSP plant showing three key components: (1) heliostat field, (2) solar receiver and (3) thermal storage unit. (Left) picture of public domain from Wikimedia ,https://commons.wikimedia.org/ wiki/Category:PS20_Solar_Power_Plant#/media/File:PS20andPS10.jpg. (accessed 30.11.20.).

TABLE 13.1 CSP tower plants in operation, under construction or in development in alphabetic order. Name

Power (MW)

HTF

T ( C)

Storage (h)

Country

Status

ACME Solar Tower

2.5

Water/ steam

440

None

India

Operational since 2011

Ashalim Plot B

121

Water/ steam

ns

None

Israel

Operational since 2019

Atacama—1

110

Molten salt

550

17.5

Chile

Aurora Solar Energy Project

135

Molten salt

ns

8

Australia

Under development (start 2020)

Copiapo´

260

Molten salt

ns

14

Chile

Under development (start 2019)

Crescent Dunes Solar Energy Project

110

Molten salt

565

10

USA

Operational since 2015

1

Water/ steam

400

1

China

Operational since 2012

DEWA CSP Tower Project

100

Molten salt

ns

15

UAE

Under construction (start 2021)

Gemasolar Thermosolar Plant

19.9

Molten salt

565

15

Spain

Operational since 2011

Golden Tower 100 MW Molten Salt Project

100

Molten salt

ns

8

China

Under development

Golmud

200

Molten salt

ns

15

China

Under construction (start 2018)

Dahan Power Plant

Operational since 2021

(Continued )

TABLE 13.1 (Continued) T ( C)

Storage (h)

Country

Status

Water/ steam

ns

yes

Turkey

Operational since 2012

50

Molten salt

ns

8

China

Operational since 2019

Ivanpah Solar Electric Generating System

377

Water/ steam

565

none

USA

Operational since 2014

Jemalong Solar Thermal Station

1.1

Liquid sodium

560

3

Australia

Operational since 2017

Ju¨lich Solar Tower

1.5

Air

680

1.5

Germany

Operational since 2008

Khi Solar One

50

Water/ steam

ns

2

South Africa

Operational since 2016

Likana Solar Energy Project

390

Molten salt

ns

13

Chile

Under development (start 2021)

Luneng Haixi 50 MW Molten Salt Tower

50

Molten salt

ns

12

China

Operational since 2019

MINOS

52

ns

ns

5

Greece

Under development (start 2020)

NOOR III

134

Molten salt

ns

7

Morocco

Operational since 2018

Planta Solar 10 (PS10)

11

Water/ steam

250300

1

Spain

Operational since 2007

Planta Solar 20 (PS20)

20

Water/ steam

250300

1

Spain

Operational since 2009

Name

Power (MW)

Greenway CSP Mersin Tower Plant

1

Hami 50 MW CSP Project

HTF

Qinghai Gonghe 50 MW CSP Plant

50

Molten salt

ns

6

China

Operational since 2019

Redstone Solar Thermal Power Plant

100

Molten salt

565

12

South Africa

Under development (start 2018)

Shangyi 50 MW DSG Tower CSP Project

50

Water/ steam

ns

4

China

Under development

Shouhang Dunhuang 10 MW Phase I

10

Molten salt

ns

15

China

Operational since 2016

Shouhang Dunhuang 100 MW Phase II

100

Molten salt

ns

11

China

Operational since 2018

Sundrop CSP Project

1.5

Water/ steam

ns

none

Australia

Operational since 2016

SUPCON Delingha 10 MW Tower

10

Molten salt

ns

2

China

Operational since 2013

SUPCON Delingha 50 MW Tower

50

Molten salt

ns

7

China

Operational since 2013

Tamarugal Solar Energy Project

450

Molten salt

ns

13

Chile

Under development (2021)

Yumen 50 MW Molten Salt Tower CSP Project

50

Molten salt

ns

6

China

Under construction

Power, peak power capacity; HTF, heat transfer fluid; T, receiver outlet temperature; ns, not specified. Source: Data obtained from Koebrich, S., Bowen, T., Sharpe, A., 2018. National Renewable Energy Laboratory (NREL). ,https://www.nrel.gov/docs/fy20osti/75284. pdf. (accessed 20.06.20.).

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SECTION | IV Sustainable Materials for Thermal Energy Systems

roadmap of the International Energy Agency (International Energy Agency/ OECD, 2017; International Energy Agency, 2014). On the one hand, the maximum temperature of the central receiver solar power plants is limited to 565 C at present. This temperature is below the optimal one as we will discuss later in Section 13.1.2. Materials and components stable under harsher operation conditions (i.e. T . 800 C in air) are currently the object of intense research and development (R&D) activities. Moreover, high investment costs together with a constant decrease in oil prices delayed the construction and commissioning of new CSP plants. Hence, in this chapter, we will present and summarize the latest innovative materials science approaches devoted to increase the CSP plant efficiency by implementing higher operation temperatures and reducing the levelized costs of electricity (LCOE). The chapter is organized as follows: The first section provides statistical data (Section 13.3.1) and the basic knowledge (Section 13.3.2) necessary to understand the state-of-the-art and the recent R&D directions of the field “high-temperature solar thermal applications”. In the second section 13.2 we introduce the concept of solar selectivity. Based on realistic operational parameters of CSP plants, their potentials and limitations are discussed and graphically illustrated. State-ofthe-art results from the last decade are briefly reviewed in the third section for: absorber paints (Section 13.3.1), solar selective coatings (SSCs) (Section 13.3.2), and volumetric receivers (Section 13.3.3). The fourth Section 13.4 focuses on the need of comparable stability studies of newly developed SSCs and materials along with the demand and the criteria for standardized characterization protocols.

13.1.2 Concentrated solar power: basics Among the main CSP plant components the solar absorber, consisting of the receiver tube as substrate and the absorber paint as coating, plays a significant role in defining the overall efficiency. The solar absorber determines in particular the conversion efficiency of solar to heat energy (Kennedy, 2002). The thermal-to-electrical energy conversion efficiency, or solar performance (η) can be decomposed into 3 main components (Lovegrove and Stein, 2012): η 5 ηreceiver Uηmechanical Uηgenerator :

ð13:1Þ

The factors ηreceiver , ηmechanical and ηgenerator represent the efficiencies of the solar receiver (i.e., fraction of the concentrated incident light converted into thermal energy), of the turbine (conversion of thermal energy into mechanical energy) and of the electric generator (conversion of mechanical energy into electrical energy), respectively. The efficiency of the generator is not directly related to the development of CSP materials and will not be taken into account in the rest of the chapter. The mechanical efficiency is

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389

usually referred to the maximum value of the Carnot cycle at the working temperature T (Boretti et al., 2019): ηCarnot 5 1 2

Tamb T

ð13:2Þ

Finally, the efficiency of the receiver can be expressed as:   Qabs 2 Qloss ηreceiver 5 Qsolar

ð13:3Þ

with Qabs ; Qloss being the fluxes absorbed and lost by the CSP receiver, respectively, and Qsolar being the incident solar flux. These fluxes can be expressed in terms of optical constants: Qsolar 5 ηoptics UIUCUAabs

ð13:4Þ

Qabs 5 αUQsolar

4 Qloss 5 εðT ÞUAemi UσU T 4 2 Tamb

ð13:5Þ

ð13:6Þ

where, ηoptics is the fraction of incident light concentrated onto the receiver (i.e., the efficiency of the solar field heliostats), I is the incident solar flux density [W/m2], C is the solar concentration ratio, Aabs and Aemi are the absorbing and emitting areas, respectively, σ is the Stefan-Boltzmann constant, T and Tamb are the receiver and ambient temperatures (K), respectively and α and ε are the solar absorptance and the thermal emittance of the receiver, respectively. The solar absorptance of the receiver is defined as the ratio of the amount of absorbed solar flux to the incident solar flux on the surface. Similarly, the thermal emittance is the ratio between the radiant flux emitted by the receiver surface per unit area (also known as radiant emissivity, typically measured in W/m2) and the one emitted by a BB. In the literature emittance and emissivity (and to a lower extent absorptance and absortivity) are sometimes misused as synonyms (Cverna, 2002). But one should note that both absorptance and emittance are dimensionless quantites (in the range from 0 to 1) while emmissivity and absorptivity have irrandiance units (Worthing, 1940). The solar absorptance α and thermal emittance ε are experimentally calculated from the spectral reflectance by the following equations (Lovegrove and Stein, 2012; Duffie and Beckman, 2005): Ð 2:5μm 0:3μm ½1 2 RðλÞGðλÞdλ α5 ð13:7Þ Ð 2:5μm 0:3μm GðλÞdλ

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SECTION | IV Sustainable Materials for Thermal Energy Systems

Ð λ2 εðTÞ 5

λ1

½1 2 Rðλ; TÞBðλ; T Þdλ Ð λ2 λ1 Gðλ; T Þdλ

ð13:8Þ

where, R(λ) is the spectral reflectance of the sample, G(λ) is the solar radiation power at AM1.5, and B(λ, T) is the spectral BB emissive power at temperature T (ASTM, 2008; N. R. E. Laboratory, 2019). The emittance integration range of Eq. 13.8 reported in the CSP literature covers usually from λ1 5 2.5 μm to λ2 5 25 μm, as this is the typical measurement range of Fourier Transform Infrared spectrometers (FTIR) used for the experimental determination of ε (Xu et al., 2020). However, it should be noted that for very high-temperature applications (T . 873K) a wider range of 125 μm, based on European standard EN-673:2011 (European Standard, UNE-EN 673:2011, 2011), should be employed to consider the overlap of the reflectance spectrum with the BB emission [see the displacement of BB maxima to lower wavelengths with increasing temperature in the left panel of Fig. 13.2, adapted from Heras (2016). One should note that the emittance values reported by many authors calculated using Eq. (13.8) may not correspond to the actual performance of an absorber at high temperatures (Lovegrove and Stein, 2012; Ro¨ger et al., 2017). Theses values are often

FIGURE 13.2 (left) Normalized black body (BB) radiation spectrum (dashed lines) and spectral reflectance of an ideal solar selective coating (solid lines) with different cut-off wavelengths (λC) at different temperatures (100 C, 450 C, 650 C and 800 C). The solar spectral irradiance (ASTMG173-03) is also plotted to show the overlapping with the BB spectrum at increasing temperatures. (right) Variation of CSP efficiency, calculated from Eq. (13.9), with temperature for a black body (BB, α 5 1, ε 5 1) at different solar concentration factors corresponding to the different solar thermal technologies. (Left) Adapted from Heras, I., 2016. Multilayer solar selective coatings for hightemperature solar applications: from concept to design. Ph.D. Thesis, Universidad de Sevilla, Spain. https://idus.us.es/handle/11441/47789 (accessed 4.06.20.). (Right) All insert figures are of public domain from Wikimedia, ,https://commons.wikimedia.org/wiki/File:Solar_heating_syste m_-_Thermosolaranlage_-_M%C3%B6rfelden-Walldorf_-_Germany.jpg. (accessed 30.11.20.); ,https://commons.wikimedia.org/wiki/File:Boden_000144_172854_518011_4578_(3664388064 1).jpg. (accessed 30.11.20.); ,https://commons.wikimedia.org/wiki/File:SolarStirling_Engine. jpg. (accessed 30.11.20.); ,https://commons.wikimedia.org/wiki/File:Diss.jpg. (accessed 30.11.20.).

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calculated assuming that the spectral characteristics of the receiver material (i.e., reflectance) do not change with increasing temperature (i.e., does not oxidize or undergo any phase change). In this approximation Rðλ; T Þ 5 R½ðλÞ the emittance is typically calculated from room temperature FTIR data, and its temperature dependence is only introduced by the Planck function describing the BB (Echaniz et al., 2015). Hence, it is very important to compare the results of this method with direct hightemperature emittance measurements (Del Campo et al., 2006; Gonz´alez de Arrieta et al., 2019). Assuming that the ηoptics is maximum (equals to 1), that is the absorbing and the emitting areas are the same, and that we are working at high tem4 peratures (T 4 cTamb ), the remaining elements of Eq. (13.1) constitute the most common figure of merit or ηCSP ðTÞ used to describe the solar-tomechanical efficiency of a CSP system at high temperature in terms of the receiver materials (Hall et al., 2012).     εðT ÞUσUT 4 Tamb ηCSP ðT Þ 5 ηreceiver UηCarnot 5 α 2 U 12 ð13:9Þ CUI T Hence, the key parameters describing the performance of the CSP system are the solar absorptance, the thermal emittance, the solar concentration factor and the working temperature of the receiver. According to Eq. (13.9) the receiver temperature T has two opposing influences on the final efficiency. On the one hand, the higher the working temperature, the higher is the Carnot efficiency, and therefore, the CSP performance. However, for high working temperatures, the thermal losses increase (proportional to T4), thus reducing the efficiency significantly. Hence, ηCSP ðT Þ will have a maximum, (dη/dT) 5 0, at an optimal temperature Topt that would depend on the optical variables in the following way: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 5 CUIUαUTamb ð13:10Þ Topt  4UεðT ÞUσ where, the approximation Topt cTamb has been employed to eliminate the 4 term ð3=4Topt Tamb Þ.3 In the right panel of Fig. 13.2 the evolution of the CSP efficiency with temperature for the specific case of a pure BB (α 5 1; ε 5 1Þ at different solar concentration factors is shown, corresponding to the different solar thermal technologies4. Above this optimal value, any increase of the working temperature will result in a decrease in the CSP receiver

3. An error of ,8% in Topt was estimated after using this approximation for C $ 50 compared to values obtained from Eq. (13.9) and plotted in Fig. 13.2 right. 4. In order to compute this calculation an incident solar flux density I 5 892 W/m2 and Tamb 5 298K were estimated.

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efficiency until it reaches a zero value for a maximum temperature Tmax equals to: sffiffiffiffiffiffiffiffiffiffiffiffiffiffi 4 CUIUα ð13:11Þ Tmax 5 εðT ÞUσ

13.2 CSP efficiency considerations: the concept of solar selectivity According to Eq. (13.9) the solar absorptance of the receiver should be as high as possible (ideally α 5 1) in order to maximize the efficiency of the CSP system, while keeping the thermal emittance as low as possible (ideally ε 5 0). However, in real practice, the predominant factor is the solar absorptance. This is due to the working temperature (T # 565 C caused by the lack of thermal stability of solar absorber materials) and by the concentration factors (C , 500) of the current operational tower plants. Recently, Lungwitz et al. (2019) proposed a graphical way of representing the solar-tomechanical efficiency as a function of the working temperature and the solar concentration factor for fixed values of solar absorptance and thermal emittance. In Fig. 13.3 the efficiency maps are plotted for the cases of a pure BB

FIGURE 13.3 CSP solar-to-mechanical efficiency (ηCSP) maps calculated from Eq. (13.9) of (left) a nonselective black body (α 5 1, ε 5 1), and (right) an ideal solar selective coating (α 5 0.98, ε 5 0.05) as a function of the concentration factor C and the operating temperature T. Adapted from Lungwitz et al. (2019)

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absorber (α 5 1; ε 5 1Þ and an ideal SSC. Values of α 5 0:98; ε 5 0:05 were chosen following Kennedy’s definition of ideal SSC for PTC technology operating at 500 C (Kennedy, 2002). Following Fig. 13.3, it can be clearly observed that for 100 , C , 500, typical values for current solar tower technology, no clear efficiency improvement can be obtained by reducing the emittance of the absorber unless temperature higher than the current operating ones are reached (i.e., T . 800 C). In (Lo´pez-Herraiz et al., 2017), Lopez-Herraiz et al. conducted a numerical study of the thermal performance of different types of CSP receivers, namely from the superheated cavity receiver of Khi Solar One and the external molten salt receiver of Crescent Dunes (Table 13.1). The authors show why and by how much the effect of the absorptance on the efficiency exceeds that of the emittance. They found a factor 7 for the superheated steam cavity receiver and an even higher factor (. 66) for the molten salt external-type receiver. Therefore, all the commercial CSP tower receivers are currently using absorber paints with high emittance values (i.e., Pyromark 2500 paint). In Section 13.3.1 a state-of-the-art of these solar absorber paints will be detailed. For lower concentration factors (C , 100) or higher working temperatures (T . 800 C) the reduction of the receiver emittance enhances significantly the efficiency of the energy conversion (see Fig. 13.3). Such concentration values are typical for parabolic trough plants, so the use of SSC is commercial deployed in this type of CSP technology (Rioglass). In Fig. 13.4, the ηCSP ðT Þ are plotted as a function of the solar absorptance and thermal emittance for different working temperatures and concentration factors. For T 5 873K (close to a current real situation) the efficiency is only defined by the absorptance (and it is practically independent of the emittance) for higher concentration factors (C 5 500 and 1000). This would correspond to the case of a current solar tower plant. For the same temperature, but at a lower concentration factor (C 5 100), the efficiency could be enhanced by a factor of 3 by reducing the emittance from 0.9 (typical value of solar absorber paints as Pyromark 2500) to 0.15 (typical values of commercial SSC in PT plants). The use of SSCs to increase the operating temperature of the receiver (i.e., T 5 1473K in Fig. 13.4), getting closer to the Topt defined in Eq. 13.10, enhances the solar-to-mechanical efficiency both for mid and high solar concentration factors. However, the development of SSC thermally stable at such high temperatures is still in R&D stage. In Section 13.3.2 we present selected examples of this exciting area of research challenge. Finally, in Section 13.3.3 we present a review of the current issues and challenges of volumetric solar receivers. These receivers, fabricated using porous structures with inclusion of metals and dielectric materials in the main solar absorber, enhance the optical as well as thermal properties at elevated temperature.

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FIGURE 13.4 CSP solar-to-mechanical efficiency (ηCSP) maps, calculated from Eq. (13.9), as a function of solar absorptance α and thermal emittance ε for different concentration factors (C) and operating temperatures (T). (A) T 5 873 K, C 5 100; (B) T 5 873 K, C 5 500; (C) T 5 873 K, C 5 1000; (D) T 5 1473 K, C 5 100; (E) T 5 1473 K, C 5 500; (F) T 5 1473 K, C 5 1000. The values for Pyromark 2500 (open square, α 5 0.97, ε 5 0.86), an ideal SSC (open circle, α 5 0.98, ε 5 0.05) and a commercial SSC (open triangle, α 5 0.95, ε 5 0.15) are shown as examples.

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13.3 State-of-the-art review of solar absorber surfaces and materials for high-temperature applications (. 565 C in air) 13.3.1 Absorber paints Several attempts to develop absorber paints for solar thermal applications have been reported in the literature (see Orel et al., 2007 and references therein). Some of the most relevant absorber paints that have been extensively studied are silicone-based paints such as Pyromark (series 2500 and 1200) (Ambrosini et al., 2019), SOLKOTE, etc (Rojas et al., 2020). Among these, Pyromark 2500 has been used as the standard absorber coating in CSP central receivers since the 1980s due to its high solar absorptance (αB0:97) (Schiel et al., 1987; Radosevich, 1988; Ho et al., 2014; Thappa et al., 2020). It was originally developed as a refractory paint by Tempil Corporation over 50 years ago to be used in aerospace applications by NASA (Wade et al., 1962). Currently, it is still the most commonly used solar absorber paint based on the available performance and reliability data obtained from operational tower projects in these last four decades (Ambrosini et al., 2019; Coventry and Burge, 2017; Boubault et al., 2014a,b). The thermal emittance of Pyromark is as high as 0.87 for the typical working temperatures of the current solar towers (, 600 C) (Ambrosini et al., 2019; Ho et al., 2014; Coventry and Burge, 2017). Hence, as discussed previously, this is not a problem regarding the CSP performance because, at such temperatures and concentration factors, the solar-to-mechanical efficiency of the receiver is driven by the solar absorptance with little or no influence of the thermal losses (see Fig. 13.4). Besides its high α value, the main benefits of Pyromark 2500 include the virtual independence of the solar absorptance from the incidence angle [i.e., α  0:8 at 80 degrees (Ho et al., 2014)], the low levelized cost of coating (LOOC) (Boubault et al., 2016) and its ease of application and replacement on different substrates: steel, aluminum, alloys, ceramics surfaces (Ambrosini et al., 2019). However, Pyromark was originally not designed to withstand the harsh conditions which, during its operational lifetime, a solar tower receiver is subjected to (i.e., thermal cycling between extreme temperatures, sand storms, and high humidity). In addition, there is an enormous influence of a myriad of parameters such as the selection of the deposition method, the curing process parameters of the paint, or even the homogeneity of the coating thickness that jeopardize the on-site performance of Pyromark 2500 (Ambrosini et al., 2019; Coventry and Burge, 2017). This results in a degradation of the paint and requires subsequent periodic maintenance (typically every 2 years) of the CSP receiver. According to Ho et al., the absorptance reduction of “Solar One” plant was found to be of 3% in the first year and 2% in the subsequent years (Ho et al., 2014). Coventry and Burge reported that the absorptance of the receiver of the Solar One plant coated with Pyromark was reduced from 95% to 88%

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over the first 4 years of testing and operation. Then after repainting, it decreased from 97% to 96% over the following 18 months (Coventry and Burge, 2017). The maintenance process includes the shutdown of the plant for several days to remove the receiver panels, followed by a mechanical sandblasting, repainting, and re-curing of the coated tubes. It causes significant economic losses due to the downtime of the plant. Moreover, the curing of the tubes is performed using external furnaces fed with fossil fuels, thus, increasing the carbon footprint of the CSP technology. Recently, a novel solar absorber coating was developed by Brightsource Energy (Brightsource Innovates a Solar-cured Coating for DEWA Tower CSP). It retains a similar high solar absorptance as Pyromark (αB0:97), but with a longer operational lifetime (3 years). Moreover, once the coating needs to be replaced it can be recoated in situ and subsequently re-cured using the sunlight without the use of nonrenewable furnaces. This coating will be implemented in the DEWA CSP Tower Project (under construction, see Table 13.1) and it is expected to decrease the operation and maintenance costs to record-like low levels. In order to further increase the efficiency of the solar receiver the operating temperature should be increased (see right panel of Fig. 13.2). The next generation of molten salt solar receivers are expected to operate at least at 800 C and finally aiming to 1000 C, temperatures closer to Topt (see Eq. 13.10) (Ho, 2017; Harzallah et al., 2019). At such high temperatures, Pyromark 2500 (or similar absorber paints) in its present application and processing technology may not be the optimal solar absorber solution. This is due to the fact that, the thermal losses will start to play a significant role in reducing drastically the efficiency of the receiver (Ambrosini et al., 2019). In this respect, the use of SSCs would contribute to enhance the efficiency at high temperatures. It has been calculated that a reduction of the emittance from 0.87 (Pyromark value) down to 0.4 would result in a 4% increase of the thermal efficiency at 650 C. This increase would rise to 7% if the working temperature of the receiver is elevated to 800 C (Gray et al., 2015; Project High-Temperature Solar Selective Coating Development for Power Tower Receivers). Furthermore, very high working temperatures require long term thermally stable materials that conserve the optical performance up to 25 years of operation. In this respect, it has been reported that Pyromark 2500 degrades when exposed for a few days to temperatures exceeding 700 C (Ambrosini et al., 2019; Radosevich, 1988; Ho et al., 2014; Coventry and Burge, 2017). This is much faster than under the current maximum solar tower temperature of 565 C (see above). The origin of such degradation has been related to a phase change in the crystalline structure of the coating that results in a reduction of the solar absorptance after thermal annealing treatments of . 750 C (Ho et al., 2014). Hence, the optimization of the siliconebased Pyromark 2500 is a subject of current research. Martinez et al. studied the effect of the solidification process on its mechanical stability. The measured wear rates differed by 5 orders of magnitude. It was concluded that the

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precise control of curing (2 hours @ 125 C) and vitrification (1 hour @ 250 C) parameters could contribute to a higher mechanical stability of the paint (Martinez et al., 2020). Tsuda et al. proposed solar absorbers based on bare carbon nanotubes (with an extremely high αB0:99; but only stable until 500 C) and on black pigments embedded in a porous structure to enhance the durability (Tsuda et al., 2018). They proved the stability of the latter coating at 850 C after 100 hours of aging, while keeping a solar-weighted absorptance value of αB0:97. More recently the Belgium company CMI designed several new solar absorber coatings: (1) a high performance CoteRillTM750 “Siliconbased Black Paint”, with high solar absorptance (0.97) and an increased lifetime (3 years), and (2) a patented “Plasma Sprayed Coating” that will provide good optical performances (similar absorptance and lower emissivity than the current technology) with higher durability (. 3 years) useful in the next generation of molten salt solar receivers (Harzallah et al., 2019). Kim et al. developed tandem solar absorbers using copper-alloyed spinel black oxide nanoparticles for solar tower applications. The samples showed excellent thermal durability, after long-term isothermal annealing of 500 hours at 750 C in air as well as rapid thermal cycling between room temperature and 750 C (Kim et al., 2016). Similarly, Rubin et al. reported the thermal stability of copper containing spinel oxide nanoparticles ðαB0:97Þ after 2000 hours of isothermal annealing at 800 C (Rubin et al., 2019). However, the development of paints with high absorption along with high temperatures stability to replace Pyromark as the standard solar absorber is still a very challenging task (Guillon, 2019; Boubault et al., 2017).

13.3.2 Solar selective coatings For the reasons explained in Section 13.2, the substitution of nonselective absorber paints by SSCs is a generally accepted concept to overcome the current limitations of CSP receivers in relation to operation temperature, efficiency and costs. In the spectral range of 0.32.5 μm the energy for utilization is about 8.03% in the UV, 46.41% in the visible and 46.40% in the NIR regions. An ideal absorber has zero reflectance (0%) in the solar spectral region and high reflectance (100%) in the IR region. The real spectrally selective solar absorber coating should have certain properties such as: high solar absorptance of α $ 0.950 (0.252.5 μm), very low thermal emittance of ε # 0.10 (125 μm) and high thermal stability at elevated temperatures (T $ 565 C) (Zhang et al., 2017). The coating properties on any substrate are measured with respect to their reflectance spectra over a wavelength range of 0.2530 μm. However, the transition between high absorptance and low emittance should possess a steep change over and this cut-off wavelength also depends on the operating temperature (Dan et al., 2017) (see Fig. 13.2). The selectivity depends on material-specific light-matter

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interactions. For example, metals, the usual materials of absorber pipes, exhibit a high reflectance for visible and IR radiation and are bad absorbers. In order to increase the solar absorptance of the substrate material, a thin film can be deposited on top of the metals with different material properties forming a tandem or multilayer stack. Typical SSCs can be classified into: (1) intrinsic absorber, (2) metal-semiconductor tandem stack, (3) textured surface absorbers, (4) multilayer interference absorber and (5) ceramic-metal composites (Zhang et al., 2017; Dan et al., 2017).

13.3.2.1 Intrinsic absorber Intrinsic absorbers are the class of materials in which solar selectivity is an inherent property. Their absorption originates primarily from strong interband transition of metal atoms with d electrons and semiconductors. Moreover, metals possess inherent plasmon frequency in the UV region due to free electrons and in the visible region there will be a slight increase in absorptance. No bulk material possesses ideal selectivity and doping would gradually increase the absorption. The intrinsic absorbers are structurally stable, whereas, optically less selective at longer cut-off wavelength which leads to higher emissivity of the material. Their optical properties can be improved by modifying their composition and lattice structure. Doping with suitable materials would increase the optical performance in two ways: (1) donor atoms will induce an electron plasma and (2) the donors will act as scattering sites and extend the optical path of incident radiation, thereby increasing the absorption in the coating (Dan et al., 2017). Examples for intrinsic absorbers are: TaC, HfC, ZrC (Sani et al., 2012), HfB2, ZrB2, SiC (Sani et al., 2011), V2O5 (Ehrenreich and Seraphin, 1975), LaB6 (Touloukian et al., 1974), and MoO3 doped Mo (Savelli and Bougnot, 1979). A downside of most intrinsic absorbers is their relatively low selectivity in the solar spectral range and the changeover from low to high wavelengths, that is, the shallow slope at the cut-off wavelength, which increases the emittance. 13.3.2.2 Metal-semiconductor tandem stack In metal-semiconductor tandem stacks the underneath metal layer acts as an IR reflector to reduce the emittance and the top semiconductor layer as the solar absorber layer. Suitable semiconductors exhibiting bandgaps (Eg) in the range of 0.5 eV (2.5 μm) to 1.26 eV (1.0 μm) absorb most of the radiation in the solar spectrum. In general, semiconductors are transparent at longer wavelengths (IR region) below their bandgap (Eg) and strongly absorb the shorter wavelengths (above Eg) of solar radiation. In case of metal/semiconductor tandem stack, semiconductor material absorbs the solar radiation in the Vis-NIR region while the metal layer reflects the longer wavelengths (i.e., thermal radiation). Hence, the combination of these materials provides solar selectivity. In solar thermal applications, some potential semiconductor

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absorber materials are: Si (1.1 eV), Ge (0.7 eV) and PbS (0.4 eV) (Gupta and Agnihotri, 1981). Moon et al. reported selective semiconductors with high refractive indices, for which nanostructured texturing is used to avoid an antireflective layer (Moon et al., 2014). A Si0.8Ge0.2 based novel absorber with 1.04 eV band gap showed a solar absorptance of 0.900.95 with a relatively low emittance of less than 0.3. Yet in another case, Xiao et al. reported a copper oxide-based p-type semiconductor, that is, CuO and Cu2O with 1.4 and 2 eV bandgaps, respectively (Xiao et al., 2011). The coatings, fabricated using a one-step chemical method, exhibited a high solar absorptance (0.94) and low thermal emittance (0.08). The downside of metalsemiconductor tandem stack coatings is the high refractive index, which induces a significant reflectance loss and, thus, a reduction in the solar absorptance (Zhang et al., 2017). By providing an antireflection layer on top of the semiconductor absorber, the efficiency of these coatings can be enhanced. Moreover, low bandgap semiconductors often have insufficient thermal stability for high-temperature applications. Their stability in air was reported to be of the order of 200 C to 300 C (Kennedy, 2002), and are not suitable for solar tower technology.

13.3.2.3 Textured surface absorber Surface texture is a typical method for achieving spectral selectivity through the optical trapping of solar energy. By designing textured surfaces that use porous, granular or needle like metal structures, high solar absorption in metals can be attained by multiple reflections that effectively trap the solar radiation. Rephaeli and Fan developed a tungsten-based nano-pyramid absorber and estimated a high solar absorptance of 96.5% and relatively low thermal emittance of 19.8% at 727 C (Rephaeli and Fan, 2009). Ungaro et al. predicted a nanostructured tungsten cone absorber using computational electrodynamics simulations (FDTD) to attain a high solar absorptance of B0.99 (Ungaro et al., 2013). A structured tungsten based selective solar absorber was developed using laser sintering technique, with a solar absorptance of 87% and thermal stability in air up to 650 C (Shah et al., 2015). Similarly, 1D CuO nanostructures such as nanofibers and nanoneedles, developed using chemical oxidation of Cu, exhibited a high selectivity, that is, an absorptance of 0.95 and an emittance of 0.07 (Karthick Kumar et al., 2013). Another textured SSC was designed based on 2D Mo photonic crystals with submicrometer periodicity using a Rigorous Coupled-Wave Analysis method (Wang et al., 2010). However, it would help to trap short wavelength radiation without increasing absorption for longer wavelengths if structures were designed with multiple reflections or gradation in the refractive indices. It is to be emphasized that the textured surface at high temperature should be protected from external damages and oxidation by providing a protective layer for better performance, thereby, enhancing the service life of

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the coating. The optical properties of these coatings are excellent at moderate temperatures, whereas, at high temperatures, the coatings deteriorate mainly due to oxidation, diffusion of the substrate material and diffusion of constituent elements (intra and interlayer) due to a large amount of thermal stresses developed at the interfaces (Konttinen et al., 2003).

13.3.2.4 Multilayer absorber Multilayer absorber coatings comprise very different coating designs, all of which use destructive interferences between multiple reflected light beams to improve the light trapping throughput to the absorber (Zhang et al., 2017). The incoming light is trapped in the multilayer stack, and in this way, the overall solar absorptance is enhanced and the thermal emittance is suppressed. Multilayer absorbers consist of metal/dielectric multilayers with identical or different material combinations and of multilayer combinations of metals, semiconductors and dielectrics. Usually an antireflective top layer is applied to minimize the reflection losses at the air/coating interface. Absorption in these films is usually attributed to transitions within the metal or/and the semiconductor layers. Different to the cermet type absorbers, the individual layers of the multilayer absorbers are structurally homogenous. The recent interest for multilayer absorber coatings, based on transition metal nitrides/oxynitrides/oxides, has its origin in their tuneable optical properties such as reflection, absorption, refractive index, and their high-temperature thermal stability. Towards this, Barshilia et al. developed a TiAlN/TiAlON/Si3N4 (Barshilia et al., 2006) tandem absorber coating deposited on Cu substrate with a solar absorptance of 0.950 and a low emittance of 0.07, which exhibits thermal stability at 550 C in air for 2 hours. Similarly, Niranjan et al. reported a multilayer SSC absorber stack of W/ WAlSiN/SiON/SiO2 with a solar absorptance of 0.955 and low thermal emittance of 0.10 (Niranjan et al., 2019). The samples exhibited excellent thermal stability in air (500 C for 100 hours) and in vacuum (700 C for 200 hours) (Niranjan et al., 2021). The high-temperature thermal stability of the coating is ascribed to the formation of fine nano-multilayers of W2N and AlSiN in the WAlSiN layer, and to the presence of two antireflection top layers (SiON and SiO2). This structure supresses the diffusion of atmospheric oxygen at hightemperatures. Escobar Galindo et al. recently reported AlyTi12y(OxN12x) oxynitride multilayer SSCs (α  0.88; ε  0.1), the most stable of them fulfilling the performance criterion (PC) for high-temperature SCCs after 900 hours of thermal cycling between 300 C600 C in air (Heras et al., 2018; EscobarGalindo et al., 2018). Similarly, Nuru et al. deposited a solar absorber coating based on MgO/Zr/MgO using e-beam evaporation which exhibited a solar absorptance of 0.92 and a thermal emittance of 0.09. It was thermally stable up to 400 C (Nuru et al., 2015). In another work, Liu et al. reported a CrAlO based absorber coating with Mo layer as metal interlayer. This absorber consists of CrAlO nano-multilayer with lower oxygen content, CrAlO amorphous layer

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with middle oxygen content and top high oxygen content CrAlO. This coating exhibits a solar absorptance of 0.92 and a low thermal emittance of 0.15 (Liu et al., 2015). In this regard, a large number of similar multilayer coatings have been developed by several groups, namely: TiAlSiN/TiAlSiON/SiO2 (Rebouta et al., 2012), TiAlCrN/TiAlN/AlSiN (Valleti et al., 2014), TiAlN/CrAlON/Si3N4 (Selvakumar and Barshilia, 2012), AlSiN/AlSiON/AlSiOy (Rebouta et al., 2015), AlCrSiN/AlCrSiON/AlCrO (Zou et al., 2016), WAlN/WAlON/Al2O3 (Dan et al., 2016), W/WAlSiN/SiON/SiO2 (Niranjan et al., 2021), CrAlSiNx/ CrAlSiOyNx/SiAlOx (AL-Rjoub et al., 2019). The major disadvantage of these multilayer coatings are high fabrication cost and up-scaling on a larger substrate, as we will discuss in Section 13.5.2, and the relatively low long-term thermal stability of the coatings above 600 C in air.

13.3.2.5 Metal-cermet coatings The metal-cermets or ceramic-metal composite coatings (cermets) comprise one or more layers with metal nanoparticles embedded in a ceramic matrix or a porous oxide impregnated with metal, a metallic bottom layer as thermal reflector and an antireflective top layer. The embedded nanoparticles play a significant role in the optical performance of these coatings. The metal nanoparticles are responsible for the strong absorption of visible and near-infrared light due to interband transitions and small particle shape resonances (Zhang et al., 2017). Moreover, multiple scattering of incident solar radiation occurs at the boundaries between the metal nanoparticles and the dielectric cermet matrix, enhancing the effective path of the light through the layer. Due to its low refractive index, the dielectric phase of the cermets effectively reduces the surface reflection and increases the solar absorption of the coating. The spectral selectivity of the coating can be fine-tuned by various parameters such as the constituent materials, thickness, particle concentration, particle size, shape, and particle orientation. The optical performance of the coatings may be further enhanced, for example, by introducing complex structures like graded cermet layers or multilayers, in which the metal nanoparticle sizes are gradually controlled throughout the thickness, or by providing a graded antireflection coating on the top. Among of the materials often used as ceramic matrix are: AlN, Al2O3, Cr2O3 and SiO2. Typical metal inclusions in the matrix are: W, Al, Cr, Ni, V, Mo, SS and some noble metals (Dan et al., 2017). In this view, a large number of cermet-based absorbers have been developed such as: W-Al2O3 (Antonaia et al., 2010), PtAl2O3 (Nuru et al., 2012), Ag-Al2O3 (Barshilia et al., 2011), Mo-Al2O3 (Xinkang et al., 2008), Mo-SiO2 (Zheng et al., 2015), AlNi-Al2O3 (Xue et al., 2013), W-Ni-YSZ (Cao et al., 2015a,b), TixAl12x/(TiN-AlN)H/(TiN-AlN)L/AlN5 (Hao et al., 2010), and Ta:SiO2 (Bilokur et al., 2020). 5. H refers to High Metal Volume Fraction (HMVF) and L refers to Low Metal Volume Fraction (LMVF)

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Cao et al. reported a W-Ni:Al2O3 based SSC deposited on stainless steel with a solar absorptance of 0.90 and total hemispherical emittance of 0.15 at 500 C. Its high temperature thermal stability at 600 C for 7 days is attributed to the W interlayer, which acts as infrared reflector (Cao et al., 2015a,b). Wang et al. developed and investigated the cermet absorber based on W/TiAl2O3, which exhibited a solar absorptance of 0.93 and a very low thermal emittance of 0.103 at 500 C even after annealing at 600 C for 840 hours in vacuum. At high temperature, the impregnated Ti atoms in the cermet matrix oxidize partially, form a protective layer that shields outward diffusion of W, interlayer diffusion and agglomeration of nanoparticles, thus enhancing the thermal stability of the coating (Wang et al., 2017). Similarly, Heras et al. reported SSCs for high-temperature applications based on metal carbides, that is, a-C:MeC (Me 5 transition metal; V, Mo) with experimental and simulated data. The SSCs demonstrated a high solar absorptance of 0.96 and thermal emittance of 0.05 and 0.15 at room temperature and 600 C, respectively (Heras et al., 2016). Due to high operating temperatures, higher efficiency and low manufacturing cost all ceramic solar absorbers and transition metal oxides could be candidate materials for the next generation CSP (Xu et al., 2020). However, thermal stability above 700 C is necessary as most of the receiver tubes in solar tower would operate at such high temperatures.

13.3.3 Volumetric receivers At high temperatures (. 700 C), most of the multilayer absorber and cermet-based absorber coatings fail due to oxidation, diffusion of constituent elements (intra/interlayer), and also degrade over long-term thermal cycles mainly because of high thermo-mechanical stresses. To overcome these issues, volumetric receivers are promising candidates for the nonlinear concentrator type solar plants (i.e., solar tower and parabolic dish instead of PTC). These solar absorbers consist of regular, porous metal or dielectric frameworks. The porous structure mutually affects radiation, convection, and conductive transport of thermal energy. At high temperature, a porous structure (absorber matrix) will have a higher efficiency than a tubular receiver; the reason behind this is the volumetric effect. It leads to a low temperature at the front of the absorber, reducing the radiative emission losses. Thus, volumetric receivers efficiently convert the incident radiation into thermal energy (Prattico et al., 2017). Different types of porous structures are used as solar absorbers in volumetric receivers, namely: wire mesh, ceramic or metallic foams, honeycombs, ducts and packed beds (see Fig. 13.5 for a general scheme of a volumetric absorber) (Ginley and Cahen, 2011). In general, ceramics have received the most attention regarding use as absorber materials in a volumetric air flow receiver, because they are thermally resistant and are able to withstand temperatures . 1000 C. Further advantages of using ceramic materials instead of metals, besides the high

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FIGURE 13.5 Schematic design of a typical volumetric receiver in CSP. Adapted from Ginley and Cahen, 2011. Fundamentals of Materials for Energy and Environmental Sustainability, Cambridge University Press, Cambridge, https://doi.org/10.1017/CBO9780511718786.

intrinsic thermal stability, are their very low coefficient of thermal expansion, high oxidation resistance, and very low creep at high temperatures (Gobereit et al., 2016). Some of the ceramic materials such as silicon carbide and alumina are extensively studied for volumetric receiver applications. Avila-Marin et al. developed both ceramic and metallic absorbers, the ceramic absorbers based on recrystallized SiC with an open porosity of 49.5%. The central receiver with ceramic absorber showed a mean air outlet temperature between 250 C and 650 C and the corresponding system efficiencies were 89%  60%, whereas the receivers with metallic absorbers showed an air outlet temperature in the range of 260 C to 500 C with system efficiencies between 89%  70% (Avila-Marin et al., 2014). The authors also reported volumetric absorbers with gradual porosity and optimized the thickness of the metallic absorber using six metallic wire mesh absorbers. Yet in another work, Agrafiotis et al. (2007) developed a porous monolithic multi-channeled silicon carbide (SiC) honeycomb, which exhibited superior mechanical properties and oxidation resistance (700 C), for their use as volumetric solar receivers. Ali et al. developed a new alumina honeycomb absorber and investigated the interaction between optical, thermal, and fluid flow dynamics in the microscopic scale of the alumina structure. They demonstrated the effect on total solar to thermal efficiency while comparing it to the conventional SiC honeycomb absorber (Ali et al., 2020). Alumina, a lowcost material with excellent mechanical, wear-resistant, and thermal properties, showed a better depth of penetration of the incident solar flux than the SiC absorber (Ali et al., 2020). Ultra-high temperature ceramic (UHTC) materials include borides, carbide and nitrides of hafnium, zirconium, and tantalum from group 46 of the periodic table. Sani et al. developed UHTC of ZrC, HfC, and TaC carbides by sintering the commercially available powders. Different compositions were sintered by mixing MoSi2 as a sintering aid (Sani et al., 2011). The optical properties of sintered Zr, Hf and Ta carbides at high

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temperatures were compared to conventional SiC ceramic. Among the sintered materials, porous HfC exhibited the highest solar absorptance and dense TaC exhibited the lowest. The directional thermal emittance calculated for of all the materials in the wavelength range of 2.525 μm depicted a considerably low thermal emittance as compared to SiC ceramic. The main disadvantage of crystalline carbides is their low oxidation resistance above a temperature of 700 C. Sani et al. developed ultra-refractory diborides of ZrB2 and HfB2 with 10 vol.% of MoSi2 as sintering aid. The dense diboride samples (ZrB2 and HfB2) and SiC exhibited a low hemispherical thermal emittance of 0.4 and 0.7 in the temperature range of 1100K1450K (Sani et al., 2012). These results of carbide and boride ceramics indicate that the carbide and boride ceramics have lower emittance than SiC at high temperatures. In yet another work, Fang et al. fabricated layered Ti3SiC2 and Zr3[Al(Si)]4C6 ceramics using a hot-pressed sintering method. These ceramics exhibited better thermal stability and low thermal emittance when compared to TiC and ZrC (Fang et al., 2015). MeyCloutier et al. compared a new selective ZrB2 foam with a SiC honeycomb structure by varying the pore diameter and porosity (Mey-Cloutier et al., 2016). The results indicated that the new material with a small pore diameter and with low porosity showed the best absorber efficiency. This finding is attributed to an increase in the convective heat transfer between the ceramic structure and the airflow. Similarly, Du et al. reported a gradually varied volumetric solar receiver, wherein the porosity gradually decreases from the top surface to the back surface in the structure. This design increases the solar flux distribution inside the structure, which reduces the drastic temperature gradient and thereby enhances the prolonged service life of the receiver at high temperatures (Du et al., 2017). These studies show that the material property and geometry of the structure play a crucial role in defining the solar to thermal energy conversion efficiency. The effect of these various parameters (geometric properties, thermal conductivity, and spectral selectivity of absorber) was investigated by Kribus et al. and the optimized properties depicted an increase in thermal efficiency from 70% to 90% for air heating at 1000 C under a solar radiation of 800 kW/m2 (Kribus et al., 2014). The volumetric ceramic receivers as solar absorbers in central towers are potential candidates for extremely high-temperature CSP plants due to their higher efficiency, high thermal stability, and low-cost fabrication. However, due to the complexity of the volumetric phenomena there are still difficulties in designing volumetric receivers. Moreover, aging studies should remain mandatory in any production of CSP material. In the case of solar absorbers, many researchers have investigated the advanced materials for hightemperature applications in the temperature range of 200 C  900 C, by varying various parameters such as atmosphere, duration, constant or cyclic thermal loads, etc. (Ngoue et al., 2020).

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13.4 Current trends and issues 13.4.1 Durability studies of solar absorbers As discussed in Section 13.3, in the past few years, many groups have developed different solar absorber materials with high optical performance and thermal stability. Solar selective materials based on transition metal nitrides, oxynitrides, and oxides have shown excellent optical properties, oxidation resistance, and good durability in mid- to high- temperature applications (see examples in Section 13.3.2). Some of the coatings have been commercialized for PTC technology (Rioglass). Extensive research is in progress on solar absorber coatings for central tower receivers (Zhang et al., 2017). The operation temperature of these plants is generally higher than that of PTC plants, with the current maximum of 565 C and a targeted temperature of B800 C. Higher mean operation temperatures lead automatically to larger temperature gradients with higher thermal stress due to day and night alternation as well as due to frequent thermal shocks because of transient clouds periods. The extreme environmental atmosphere of central tower receivers with oxygen, moisture, dust, wind, etc. opens new pathways for coating degradation and aging compared to PTC plants. The two main degradation mechanisms of solar absorbers at high temperatures are diffusion and oxidation of elements (Zhang et al., 2017). The former comprises both internal and external diffusion. Moreover, two types of internal diffusion, notably internal penetration and internal segregation, are distinguished. Internal diffusion is often related to the mobilization of the metals in the reflective layer of multilayer absorber stacks, for example, Cu, Mo, and Ag, but also to the interdiffusion of the components in multilayer and cermet absorbers (Zhang et al., 2017; Rodriguez-Palomo et al., 2018). Both degradation mechanisms modify the composition and the interface quality of the stacked layers as well as their refractive indices. This deterioration in the optical properties results in a decrease of the energy conversion efficiency of the absorber. Oxidationinduced degradation is closely connected with external diffusion of ambient oxygen. This case was recently reported for multilayers of AlyTi12y(OxN12x) on TiN (Escobar-Galindo et al., 2018). After 12 hours of in-air treatment at 800 C the initial fcc-structure of the layer stack had transformed, and the metallic mirror layer of TiN was oxidized into rutile-type TiO2. The TiN oxidation can be described by Eq. (13.12): 2TiN12O2 22TiO2 1N2

ð13:12Þ

The structural changes led to an optical performance loss (for definition see below) of the coating stack by 25%. They were thermodynamically driven by the much higher stability of rutile-type TiO2 (ΔGB 5 2889.4 kJ/mol) compared to that of TiN (ΔGB 5 2309.2 kJ/mol) (Lax, 1998). In this regard, it is important to evaluate the thermal stability of the solar absorbers under conditions simulating the actual environment encountered

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by the absorber in the field. During thermal cycling, multilayers would experience thermal stress at the interfaces of the coating, as different materials usually have different expansion coefficients. Furthermore, formation of microcracks, bulging, and finally peeling off can occur due to high thermal stress generated in the absorber of the multiphase structure after prolonged annealing. Stability studies of candidate absorber materials for central tower receivers should include exposure to varying external environmental conditions at different timescales (day, month, year) (Boubault et al., 2017). In addition, the long-term thermal stability of the solar absorber coating has to be validated because it is essential for the maintenance of the system performance (Raccurt and Disdier, 2018). However, there is still a lack of a consistent, appropriate standardized qualification test and service lifetime prediction models, particularly for high-temperature applications. In the accelerated aging studies, a PC is calculated from the obtained optical properties at the two temperatures used in the aging process applying the equation: (International Energy Agency IEA, 2014; Carvalho et al., 2016) PC 5 2 Δα 1 0:5Δε

ð13:13Þ

where, Δα 5 α(aged) 2 α(unaged), and Δε 5 ε(aged) 2 ε(unaged). An acceptable PC requires a value lower than 0.05 (5%), otherwise the coating is identified as failed. The service life prediction for a solar absorber is theoretically calculated using an Arrhenius relationship proposed by Carlsson et al. for low-temperature applications (Carlsson et al., 2000). This procedure is based on oxidation, hydration and atmospheric corrosion exposure of the solar absorber during constant load-accelerated lifetime tests (Carlsson et al., 2004). The method is an approximate and simplified way for calculating the activation energy, but the smaller the diffusion coefficient the higher the thermal stability of the coating. In this model the activation energies were calculated separately for absorptance and emittance from ln(Δα) or ln(Δε) versus 1/T slopes. The goal of this standard test method is to predict the potential life expectancy of the coatings for 25 years. Studies on accelerated aging due to diffusion, oxidation, constituent element, grain size changes, as well as coating micro defects were reviewed by Zhang et al. (2017). However, the prediction of service lifetime for high-temperature solar thermal coatings still needs extensive research. Boubault et al. reported a numerical model that was used to assess the thermal behavior of Pyromark 2500 on Inconel 625 substrate. Their simulation and experimental results indicated that the thickness of Pyromark 2500 greatly influences the optical properties and thermal stability (Boubault et al., 2012). An experimental solar accelerated aging facility was built and described in detail, which applies a two meters’ diameter parabola concentrating the sun radiation up to 16,000 fold. Later, the same group investigated the influence of mean solar irradiance, duration period and thermal cycle

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exposure time on the performance of a two-layer solar absorber (metal 1 paint) coating. The results demonstrated that the loss of absorptance was the main factor for the reduction of performance of the system (Boubault et al., 2014a,b). Sallaberry et al. reported the various test conditions of aging in different environments for solar absorbers to withstand temperature higher than 1000 C (Sallaberry et al., 2015). Similarly, Antonaia et al. designed a new graded WN-AlN cermet coating to operate at high temperatures with high performance and thermal stability up to 550 C (Antonaia et al., 2016). Their theoretical calculations predicted the service lifetime of 25 years with a 1.65% reduction in solar absorptance for an operation temperature of 550 C and using molten salt as HTF. According to the Arrhenius theory, the failure time at high temperatures is a function of temperature and the activation energy of the dominant degradation process. However, this does not consider the real stress parameters. Therefore, the modified test protocol for solar absorber coating includes the stress parameters in estimating the realistic service lifetime. Reliable predictions of service lifetime should be checked to compare natural and accelerated aging and ensure the reliability and pertinence of the obtained results (Ko¨hl et al., 2004).

13.4.2 Lack of standardized characterization protocols Standardized characterization protocols for newly developed solar-selective coatings are required for a reliable comparison of their properties among different research groups and for the qualified selection of the most-suitable candidate coating for scale-up to industrial size and shape. Therefore, it is necessary to define the most appropriate environmental conditions for the characterization, the quantities to be measured and the most suitable techniques. Such a standardized characterization protocol would represent a kind of quality control or label and be a key element for coating validation on the lab scale. It should include those techniques that are indispensable, not all techniques that are available from lab to lab. Moreover, the protocol should define unambiguous exclusion criteria on whether a coating is worth to be further developed or whether it fails and has to be either redesigned or discarded. The standardized characterization protocols and experimental methods (i.e., in situ measurements) have to match realistic environments. In this respect, Heras et al. described in (Heras et al., 2014) a multistep protocol for in situ characterization of CSP coatings (see Fig. 13.6). It comprises the sequential coating characterization under defined atmospheres by ion beam analysis, Raman spectroscopy and spectroscopic ellipsometry in the individual chambers of a cluster tool as a function of temperature (Fig. 13.6), and enables depth-resolved elemental, structural and optical analysis of any material. As criterion for a passed stability test the ellipsometry data should have changed by 6 5% at maximum at a given temperature; otherwise the coating was evaluated as has failed.

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FIGURE 13.6 Flow chart of the standardized characterization protocol for high temperature testing of solar absorber coatings. The different chambers represented in the diagram are the analysis chamber (AC), the environmental chamber (EC) and the ion beam analysis chamber (IBA-C). Reproduced from Raccurt, O., Disdier, A., 2018. Accelerated ageing tests for durability study of solar absorber coatings on metallic substrate for solar thermal energy (STE) application. AIP Conf. Proceed. 2033, 230011, https://doi.org/10.1063/1.5067239 with permission granted from Elsevier publisher.

With regard to the stability optimization, the homogeneity of the coatings and properties such as thickness, composition, crystallinity, microstructure, and surface roughness need to be verified. The control of the solar absorber layer thicknesses in the tolerable limit at high temperature is a significant challenge. Further, the evaluation of the optical properties such as refractive index and extinction coefficient at high-temperature [i.e., in situ high-temperature ellipsometry (Heras et al., 2014; Wenisch et al., 2018)] is another area which needs significant attention for predicting the realistic performance of the absorber coating. The measurement of optical properties (absorptance and emittance) is relatively easy after deposition. In contrast, it gets more complicated to analyze these properties after a specific time over the service life. Also, the correlation of optical properties, nanostructure and nanomechanical properties of the coatings during performing the new aging protocols needs to be explored in more detail. A better understanding of how solar radiation and nanomaterials interact as the basis to design the new absorber coatings is vital for experimental methods to test materials under utmost conditions

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(temperature and radiations). In the case of accelerated aging tests, a continuous monitoring of high temperature (thermocouples), long cycle times and different gas concentrations (a quadrupole mass spectrometer) with flexible software could record the realistic environmental data. In the past few years, extensive research in the area of advanced characterization techniques and new standard testing protocols is under development. These improved methods reduce the risk of overestimating the properties of the solar selective absorber coatings on the receiver tubes and are of utter importance in boosting up the maturity level of the CSP technology. In this context, Baron et al. reported in-air thermal emittance measurements using a Round-Robin test procedure. They measured thermal emittance from 100 C  600 C and the discrepancies between the results were analyzed in detail. The study depicts the complication of accurate thermal emittance measurements, which is a function of temperature (Le Baron et al., 2019). Yet in another work, the Nanoscale Enhanced Characterization of Solar Selective Coatings (NESCO) project in collaboration with the Spanish private research center IK4-Tekniker, emittance measurements on curved surface were carried out. Significant differences between the curved and planar surfaces were found by FTIR measurements (Barriga et al., 2014). Noˇc et al. developed a relationship between the performance and the service lifetime of the CSP coating with advanced materials characterization, providing in-depth insights into the long-term stability and degradation process (oxidation) under simulated operating conditions (Noˇc et al., 2019). To bring research and innovations to a commercial level, the dissemination and enforcement of standardized characterization protocols for new materials, components and systems are of vital importance. The National Renewable Energy Centre of Spain, has extensive experience in the solar thermal collector and system validation tests with a wide range of testing capabilities, including the test benches to classify the thermal and optical properties of the receiver tubes (Le Baron et al., 2019; Sanchez et al., 2010). Their expertise on the national level in Spain might be helpful to proceed towards the internationally accepted characterization standards for solar absorber coatings.

13.5 Roadmap for concentrated solar power absorbing surfaces and materials 13.5.1 Alternative concentrated solar power absorbing surfaces: selectively solar-transmitting coatings An alternative concept to achieve selectivity for solar thermal materials and applications consists in the use of solar selective transmitter coatings (Kennedy, 2002). These are characterized by a high transmittance in the solar spectral range and a high reflectance in the thermal emission range of the electromagnetic spectrum (see Fig. 13.7). Suitable materials for selective

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FIGURE 13.7 (A) Transmittance spectra of SnO2: Ta (1.2 at.% Ta) films before and after 12 h of in-air treatment at 450 C, 650 C and 800 C; (B) Reflectance spectra of SnO2:Ta (1.2 at.% Ta) films before and after 12 h of in-air treatment at 450 C, 650 C and 800 C; (C) Dependence of cut-off wavelength and reflectance edge on the charge carrier concentration and mobility in SnO2: Ta thin films on fused silica with different Ta concentrations. M. Krause, private communication of unpublished results.

transmitters are dielectric/metal/dielectric multilayers and transparent conductive oxides (Groth, 1966; Fan and Bachner, 1976). From the technological perspective selective transmitters can be advantageous in two configurations: (1) as backside cover of the front window in flat plate collectors (Fan and Bachner, 1976; Lampert, 1981; Giovannetti et al., 2014) or volumetric receivers (Dutta, 2017), and (2) as selectivity-providing coating on top of a nonselective absorber (Lungwitz et al., 2019; Haitjema and Elich, 1987; Orel et al., 1992; Varol and Hinsch, 1996; Tesfamichael and Roos, 1998; Shimizu et al., 2013; Shimizu et al., 2017a,b; Wang et al., 2018). The backside coating configuration decouples the thermal reflector spatially from the absorber and leads in first approximation to a constant thermal emittance of the whole receiver. The application of the solar transmitter as top coating onto a nonselective absorber (e.g., absorber paints described in 3.1) exhibits a series of advantages: - The easiness of manufacturing compared to sophisticated multilayer deposition processes (such as the ones used for SSCs described in Section 13.3.2);

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- The possibility to use standard materials as transmitter (e.g., ITO or FTO) and absorber (e.g., Pyromark or black chrome); - The additional oxidation and wear protection of the absorber and the improvement of its thermal stability; - The adaptability to specific requirements with respect to receiver temperature and solar concentration factor. Clearly, the transmitter layer on top of the BB reflects a certain fraction of the solar radiant flux and reduces the solar absorptance of the receiver by a few percent. For example, the refractive index of SnO2:Ta at 500 nm, n 5 2.0, would lead to a normal incidence reflectance of 0.11 at the air/SnO2:Ta interface (Lungwitz et al., 2019). Applying a SiO2 antireflective layer would reduce the reflection-induced losses to approximately 0.06, but not totally avoid them. This disadvantage may be responsible for the relatively limited number of scientific papers in this specific field of SSCs (see Table 13.2). As outlined in the previous sections, receiver stability up to temperatures of 800 C is required for the next generation of solar power plants. So far, this has not been demonstrated for the most advanced solar-selective cermet or multilayer absorber coatings. Recently published data for selective transmitters are promising in this respect. Shimizu and co-workers studied indium doped tin oxide (ITO) as selective transmitter on top of refractive metals, namely Ta and W (Table 13.2). These authors reported a thermal stability in vacuum up to 700 C for ITO on homogenous and micro-structured W substrates (Shimizu et al., 2017a,b). In yet another work, Lungwitz et al. demonstrated thermal stability of SnO2:Ta (1.2 at.% Ta) on fused silica in high vacuum up to 800 C by in situ Rutherford backscattering and spectroscopic ellipsometry (Lungwitz et al., 2019). Exposing the coating to 650 C and 800 C in a lab furnace for 12 hour, revealed also excellent in-air stability (see Fig. 13.7A and B). Both the transmittance and reflectance spectra show that the initial optical properties of the TCO on fused quartz did not change at all after these in-air treatments. So far the best stability for a selective transmitter on top of a bare absorber was reported by Wang et al. (2018). They investigated a complete absorber system comprising of stainless steel substrate, black chrome (5 μm) absorber layer, ITO (0.5 μm) transmitter and SiO2 (0.15 μm) antireflective layer. Thermal stability after exposure to 900 C for 120 hours was reported and represents up to now the best stability for high temperature CSP coatings. The high stability goes along with reasonable good values for solar absorptance and thermal emittance (Table 13.2). A further advantage of solar-selective transmitters on BB absorbers is their adaptability to specific operation conditions and specifications of a CSP plant. Maximum CSP efficiency is achieved by adjusting the cut-off wavelength of the receiver, where it switches from absorbing to reflecting, according to the operational temperature and the solar concentration factor.

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TABLE 13.2 Solar absorptance (α), thermal emittance (ε), α/ε ratio, and thermal stability reported for solar selective transmitter coatings for solar thermal CSP applications (n.a.: not available). Coating structure

(reference)

α

ε

α/ε

Critical temperature ( C) Vacuum

Air

Ideal black body/SnO2:Ta/SiO2 (90 nm)

(Lungwitz et al., 2019)

0.95

0.3

3.2

800

800

Corning7059/ITO (350 nm)/MgF2 (100 nm)

(Fan and Bachner, 1976)

0.90

0.08

11.1

n.a.

n.a.

Black enameled steel/SnO2:F

(Orel et al., 1992)

0.90

0.13

6.9

n.a.

n.a.

Black enameled steel/SnO2:F

(Haitjema and Elich, 1987)

0.91

0.15

6.1

, 250

400

Al/Ni-Al2O3/SnO2:Sb

(Varol and Hinsch, 1996)

0.92

0.21

4.4

n.a.

450

Al/Ni-Al2O3/SnO2: F/SiO2

(Tesfamichael and Roos, 1998)

0.94

0.15

6.3

n.a.

300

Ta/ITO

(Shimizu et al., 2013)

0.71

0.11

6.5

600

n.a.

W (1 μm)/ITO (1 μm)/SiO2 (0.2 μm)

(Shimizu et al., 2017b)

0.71

0.08

8.9

700

n.a.

Patterned W (1 μm)/ITO (1 μm)/SiO2 (0.2 μm)

(Shimizu et al., 2017a)

0.83

0.16

5.2

700

n.a.

Stainless steel/black chrome (5 μm)/ITO (0.5 μm)/SiO2 (0.15 μm)

(Wang et al., 2018)

0.90

0.40

2.3

n.a.

900

Corresponding modeling was done for T 5 800 C for C 5 200, 500, 1000 and 10,000 by Lungwitz et al. (2019). The higher the concentration factor, the more the cut-off wavelength was shifted to longer wavelengths. In the experimental practice, such shift can be achieved by the reduction of the charge carrier concentration of the selective transmitter. This effect was demonstrated already in the early days of TCO research (Groth, 1966;

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TABLE 13.3 Experimentally verified dependence of the cut-off wavelength of SnO2:Ta thin films on fused quartz as a function of the charge carrier concentration and mobility (Lungwitz et al., 2019). The cut-off wavelength was defined at a reflectnace value of 0.5. Charge carrier concentration (1020/cm3)

Charge carrier mobility (cm2/Vs)

Cut-off wavelength (nm)

Slope of the tangents

3.75

12.1

2058

6.63

2.96

11.4

2184

5.60

2.00

8.4

2248

5.12

1.53

6.7

2344

4.64

Lampert, 1981). Here the effect of the charge concentration on the cut-off wavelength or reflectance edge of the newly developed solar-selective transmitter SnO2:Ta is shown (see Fig. 13.7C and Table 13.3). Lowering the charge carrier concentration shifts the cut-off wavelength from B2060 nm to B2350 nm. Thus, the TCO can be optimized in such a way that practically the whole solar radiant flux is transmitted to the absorber. Solarselective transmitter coatings on top of a BB absorber can represent a promising alternative to multilayers and cermets for high-temperature SSCs. They are easier to fabricate and competitive with respect to the thermal stability. Future research activities in the fields should focus on the optimization of their optical constants and on their application on commercial BB-like materials or even state-of-the-art paints.

13.5.2 Industrialization of high-temperature solar selective coatings In the first section of this chapter a substantial gap was noted between the share of CSP on the energy market predicted by the IEA as necessary for the energy turnaround and its actual marketization. However, its targeted share of the global electricity production in 2050 is unchanged with a projection of 11% (International Energy Agency, 2014). To achieve this target, the globally installed CSP capacity would have to be increased from 5.5 GW (2018) (Koebrich et al., 2018) via 261 GW (2030) to 982 GW (2050) (International Energy Agency, 2014). These numbers are proposed for the most recent high renewable (hi-Ren) scenario of the IEA. Until 2030 (2050), the projected growth would thus require the commissioning of approximately 2330 (9000) new power plants of the Atacama—1 capacity or of approximately 676 (2600) new power plants of the capacity of Ivanpah (see Table 13.1). This

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projection would require a gigantic growth of the CSP plant construction capacities and the utilization of new plant areas. These numbers could be reduced by power plants operated at higher temperatures and with correspondingly higher efficiencies applying SCC-based absorbers. Assuming all these tasks will be solved, the global market for CSP plants will exponentially grow in the next decade. Starting from that consideration it is interesting to reason about the possible realization of new, either material-based or technological solutions for new CSP tower plants utilizing the currently most common tubular receiver configuration (Dutta, 2017). One concept for such predictions are technology readiness levels (TRLs), which describe the stages of technology development from the first observation of its principles (TRL 1) to its successful operation under environmental conditions (TRL 10) (NASA; European Commission). The whole process could take at least 10 years or, more realistically, to 1015 years. The most advanced materials for high-temperature CSP applications, for example, the oxynitride multilayer SSCs (Escobar-Galindo et al., 2018; Niranjan et al., 2021) and the solar selective transmitter on commercial black chrome absorber (Wang et al., 2018), have been so far manufactured in lab scale size and their functionality has been tested under operation-relevant conditions in the lab. This stage of development corresponds to TRL 4technology validation in the lab. In the following section specific steps for the development of such a successful coating to its industrial application in a CSP plants are briefly discussed (Table 13.4). In the case of PVD coatings, the challenge from TRL 4 to TRL 5 and TRL 6 consists mainly in the change of the substrate shape from planar in the lab to cylindrical for the final application. This is not critical since industrial PVD systems are equipped with sample rotation stages that ensure a high homogeneity of layer composition and thickness (Von Ardenne Advanced Coatings Equipment for Global Markets; Platit PVD Coating Units). For easiness and cost efficiency, one could accept object sizes of 1 m length for that TRL level that can be coated in standard industrial machines. This size would also be compatible with the possibilities of existing test facilities for the technology demonstration in relevant environments that is required for TRL 6 (Plataforma Solar de Almeria CSP Test Center; Institute of Solar Research DLR). It should be noted that long-term research is needed in order to guarantee the lifetime of the PVD coatings. Standardized stability tests for high-temperature CSP coatings have to be developed and applied, and accelerated aging tests would be beneficial for a rapid technological progress (Section 13.4.2). The technological bottlenecks to advance from TRL 6 to TRL 7 might consist in the manufacturing of 18 m long prototype and even more in the demonstration of the prototype in operational environment. Receiver tubes of this length are used in the Atacama—1 power plant. Moreover, an operational environment simply means that coated absorber tube prototypes should be tested in real tower receivers. Investment efforts and costs even increase for the final TRL 8

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TABLE 13.4 Proposed development steps for high-temperature stable CSP coatings from its current TRL level 4 to its possible application in solar power plants (TRL 9) for the example of a selective transmitter on a bare absorber. TRL

Definition (NASA; European Commission)

Translation of the definition in specific coating development steps

4

Technology validated in lab

Complete coating stack deposition, comprising commercially available absorbers like Pyromark and black chrome, selective transmitter ITO and antireflective layer of SiO2, is optimized and reproducibly validated (Wang et al., 2018) Solar selectivity and thermal stability at 900 C is tested and validated under application relevant conditions in the lab for 120 h (Wang et al., 2018)

5

Technology validated in relevant environment

Scale-up and validation of the deposition as industrial process for tube geometry with approximately 1 m length Validation of the solar selectivity and thermal stability of the scaled-up coating stack on tubular substrates

6

Technology demonstrated in relevant environment

Demonstration of long-term optical, thermal, and mechanical performance of coatings deposited at TRL 5 under conditions similar to tower plant operation, for example, in solar furnaces of large test facilities

7

System prototype demonstration in operational environment

Prototype manufacturing in real length of B 18 m Optical, thermal, and mechanical performance of coated absorber tube prototypes in tower receivers

8

System complete and qualified

Testing phase of solar-selective transmitter prototype tube successfully passed Long-term operation demonstrated, advantages of the new technology verified Planning stage of a CSP tower plant using new developed solar-selective transmitters for the absorber unit

9

Actual system proven in operational environment

Operating CSP tower plant using new developed solar-selective transmitters for the absorber unit

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and 9. In this respect it is instructive to study the experiences of solar plant developments in the past, for example, by Abengoa S.L. in Spain, a pioneer in CSP technology. As a real example, for the Atacama solar plant of 110 MW installed capacity approximately 10 receiver tubes are used per MW. That is a total of 1100 tubes, each of them 18 m in length and 5 cm in diameter, with a total surface of circa 3100 m2. In order to assess the PVD SSC scale-up feasibility the following consideration must be taken in account: (1) Preliminary cost benefits consist in the reduction of costs due to less maintenance (Pyromark repaint). PVD coatings are expected to survive 25 years in operation; (2) Approximately 70% cost reduction on raw materials is possible as compared to Pyromark; (3) An increase of efficiency due to higher selectivity and higher temperatures to Pyromark painted receivers is achieved. Regarding the costs involved to the PVD technology, one of the most important factors is related to the investment for a coating system. Moreover, the cost per coated tube has to be taken into account. No commercial PVD systems for 18-m-long tubes exist at present. However, preindustrial scale-up systems do exist. At IK4-Tekniker a PVD coating process for 4 m long receiver tubes, a typical length of segments for PT collector tubes, was developed. It can be considered as an intermediate step towards processing the full required length (Selective coating for parabolic cylinder collector pipes). For single tube coating a throughput of few units per day is expected. Such a coating system would cost in the range of 1 Mh when scaled to an industrial product. Those systems should work at full capacity during a year in order to be able to give service and coat the tubes needed for a solar plant like in the example mentioned above. If an industrial coating system is scaled up for an increased productivity, a system which will be able to coat up to 20 tubes of 18 m length per batch should be considered. The coating work for a solar plant like the one above described would involve around 60 coating batches. Such a system would produce enough receiver tubes for the construction of several solar plants. The cost for the system is estimated to be of at least 10 Mh. Additionally, the cost for coating these tubes with PVD technology would also require considerable investments in a suitable cleaning unit plant. The cleaning technology should guarantee the best surface condition for the tubes and the production environment should be controlled to maintain the cleanliness of the surfaces before coating with appropriate handling systems for these tubes. All these investment costs should be taken into consideration for an alternative PVD coating process for these tubes. Levelized cost of energy has become one of the essential criteria in the identification of the most cost-efficient technology in energy production (Boubault et al., 2016). Ho and Pacheco developed an innovative way to evaluate annual performance and illustrated that LCOE is responsible for annualized performance, costs, and durability (Ho and Pacheco, 2014).

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LCOC, is defined as the ratio of the total annual coating costs to the annual thermal energy absorbed (MWhth): LCOC 5 Cannual =Ethermal

ð13:14Þ

where, Cannual is the total annualized cost and Ethermal is the average energy absorbed. Cannual 5 Cinitial coating coat and service life 1 Crecoat coat and interval for recoat 1 ð13:15Þ 1 Cmiscellaneous annual cost Ethermal 5 Ethermal energy absorbed 2 Eenergy loss due to degradation 2

ð13:16Þ

2 Eenergy loss due to recoat In addition to LCOC, maintenance, and durability must be considered when considering the cost of a receiver. Efforts are underway in the CSP industry to develop more efficient and durable coatings that will be used in next-generation receivers. While large CSP plants with their higher efficiency but huge real consumption are suited for desert areas in the solar belt, small-scaled solar thermal power units are compatible with urban regions (e.g., in Europe). They can provide a substantial contribution to grid and power stability due to their inherent storage capability and dispatchability of energy. One technology used for such application utilizes Fresnel lenses collectors and organic Rankine cycles (ORC) power blocks. The Ottana pilot plant in Sardinia (Italy) consists of a hybrid power/heat generation system with a 630 kWel CSP and a 400 kWel concentrated photovoltaic power block, and possesses a thermal and an electrical storage unit (Camerada et al., 2015; Cocco et al., 2016; Petrollese et al., 2020). Still, the plant is in the test phase to optimize the control regime of either being weather forecast or combined storage state of charge/weather forecasts driven. An already successful example for the use of small-sized CSP based on Fresnel lenses is a district heating facility installed by the company Heliac, and owned and operated by E.ON in Denmark (Heliac’s Panels Generate Heat Using Lenses that Focus Sunlight Exactly Like Magnifying Glasses).

Acknowledgments The authors would like to thank Dr. Irene Heras (CATEC, Spain), Frank Lungwitz (HZDR, Germany) and Alvaro Mendez (Nano4Energy) for the preparation of some of the figures shown in the work. Technical assistance by A. Schneider and Ilona Skorupa (HZDR) are gratefully acknowledged. REG and MK acknowledge the financial support of H2020 RISE project “Framework of Innovation for Engineering of New Durable Solar Surfaces (FRIENDS2, GA-645725)”. HCB thanks Indo-French Centre for the Promotion of Advanced Research (CEFIPRA), New Delhi, India and Department of Science and Technology, New Delhi, India for partial funding support to carry out high-temperature solar absorber coating work at CSIR-NAL.

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Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect those of the host institutions or funders.

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Chapter 14

Applications of wastes based on inorganic salts as low-cost thermal energy storage materials Svetlana Ushak1, Yanio E. Milian1, Paula E. Mar´ın2 and Mario Grageda1 1

Center for Advanced Study of Lithium and Industrial Minerals (CELiMIN), University of Antofagasta, Antofagasta, Chile, 2Sustainable Thermal Energy Technologies (STET), University of Warwick, Coventry, United Kingdom

14.1 Introduction Nowadays, a critical commitment of science is responding not only to human necessities, but emphatically to environmental health, in order to heal our unique home. Climate change, driven mainly by global warming, has reached a point where all areas of the planet are being damaged or under threat, from glaciers melting to uncontrolled burning of jungles, savannas, and forests. Greenhouse gas emission is one of the main concerns related to climate change and is bound to have an irreversible impact if not immediately reduced. The total number of tons of CO2 emitted over some long horizon is the key factor to be reduced, mainly by counting on low-cost and zero- or low-carbon emission alternatives to fossil fuels (Gillingham and Stock, 2018), since these emissions into the atmosphere are directly related to fuel combustion. Meeting energy and electricity demands in a clean and environmentally friendly way, such as using electrical vehicles and solar energy, are among alternatives to moderate CO2 emissions. Currently, the important role played by renewable energy in this aspect is clear, through natural energy sources such as sun, wind, or water, which are available in abundance. The sun is the main source of unlimited free energy, and new technologies are being engaged to produce electricity from gathered solar energy considerably reducing and relieving problems related to climate change, energy security, joblessness, etc. (Kabir et al., 2018). For this reason, a logical great effort is ongoing in scientific communities to develop technologies based on Sustainable Material Solutions for Solar Energy Technologies. DOI: https://doi.org/10.1016/B978-0-12-821592-0.00009-1 © 2021 Elsevier Inc. All rights reserved. 429

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solar energy usage. Theoretically, solar energy potential could meet all electricity demands. Additionally, solar technologies could generate electricity in ways not requiring water, and avoiding generation of fuel by-products or radioactive waste. Flexibility and ruggedness of solar power systems are guaranteed by individual arrays and the possibility to add other models, if needed, in order to improve energy generation capacity and long-continuous working time (Kabir et al., 2018). Hotspot is a term employed for areas with potential to install energy solar systems, due to its relatively high temperatures in comparison to its surroundings (Fig. 14.1). Throughout the Americas, there are viable lands that need to be considered by their solar energy potential at utility-scale and economically feasible levels (Fig. 14.1A, based on (Viviescas et al., 2019)). Specifically, high development potential indices for solar energy are concentrated in the central-west zone of South America (Fig. 14.1B, based on (Oakleaf et al., 2019)); where, additionally, areas of high mineral deposits are located (Fig. 14.1C, based on (Oakleaf et al., 2019)), with the benefits of applying these materials for development of solar TES systems. Limitations are the obstacles that humans face to take advantage of the sun’s energy. These limitations of solar energy technologies can be classified as “system dependent factors” and “external factors”. The first classification contains factors such as high initial installation cost, energy efficiencies, performance limitations of system components, system maintenance (installation, maintenance, repair and evaluation). and irregular usage. Weather or climate conditions, air pollution levels, huge plots of land requirements, and

FIGURE 14.1 (A) Solar energy installations Hotspots, (B) Development potential indices for solar energy, and (C) Major mineral deposits in South America. (A) From Viviescas, C., Lima, L., Diuana, F.A., Vasquez, E., Ludovique, C., Silva, G.N., et al., 2019. Contribution of Variable Renewable Energy to increase energy security in Latin America: complementarity and climate change impacts on wind and solar resources. Renew. Sustain. Energy Rev. 113, 109232. (B) From Oakleaf, J. R., Kennedy, C. M., Baruch-Mordo, S., Gerber, J. S., West, P. C., Johnson, J. A., and Kiesecker, J., 2019. Mapping global development potential for renewable energy, fossil fuels, mining and agriculture sectors. Sci. Data 6(1), 117.

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the fact that solar energy can only be harnessed during the daytime and works most efficiently when sunny are in the “external factors” category. High initial installation costs and solar energy intermittence are the most significant shortcomings for solar energy development. The industry faces imperative long-term challenges related to energy, constituting the second largest segment of total operating cost in mining because energy-related costs represent between 20% and 40% of total mining processes cost (Nasirov and Agostini, 2018). In this regard, the use of low-cost materials reduces the investment cost of solar systems. Solar energy instability depends not only on day hours but also on sustainable weather or climate conditions and air contamination levels at the area of interest (Kabir et al., 2018). In this aspect, solar energy intermittence and instability could be solved by supplementing this technology with thermal energy storage (TES) systems. Likewise, storage materials represent a third part of the overall cost of a TES system; low-cost materials are required to promote construction of cost-effective thermal and sustainable storage systems. In the sections that follow, industrial wastes to be considered low-cost materials for TES systems are reviewed. First, it is necessary to discuss what thermal energy storage is, mechanisms through which is made possible and which parameters and phenomena are associated with this emerging technology.

14.2 Thermal energy storage 14.2.1 Sensible, latent and thermochemical heat storage TES systems, also commonly referred to as heat and cold storage, allow heat or cold storage to be used later. In order to recover stored heat or cold, the storage method must be reversible (Sarbu and Sebarchievici, 2018). Fig. 14.2A shows a simple process classification. These can be divided into physical and chemical processes. Thermal energy can be stored in the form of sensible heat, latent heat, by chemical/thermochemical processes, or a combination of both.

14.2.1.1 Sensible heat storage In sensible heat storage (SHS) systems, energy is stored through temperature changes in the storage medium, for example water, air, rocks, bricks, among others. The amount of energy that penetrates into the storage medium is proportional to temperature increase, heat capacity, and storage medium mass. Each material has associated advantages and disadvantages, but specific selection is based mainly on heat capacity and space availability (Abedin and Rosen, 2011). The amount of sensible heat stored in a material, Q, can be calculated according to the following equations:

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FIGURE 14.2 Storage processes description showing: (A) reversible thermal energy storage processes classification and (B) difference between sensible heat and latent heat storage (LHS) with a solid-liquid and liquid-gas phase change of water. (A) From Sarbu, I., and Sebarchievici, C., 2018. A comprehensive review of thermal energy storage. Sustainability 10(1), 191.

Q 5 m 3 Cp 3 ΔT

ð14:1Þ

Q 5 ρ 3 Cp 3 V 3 ΔT

ð14:2Þ

where m is storage material mass (kg), Cp is storage material specific heat (J/kg C), ΔT is the temperature change ( C), ρ corresponds to storage material density (kg/m3), and V is the storage material volume (m3).

14.2.1.2 Latent heat storage When a solid material is supplied with heat, its temperature increases until reaching its melting point, going then through a phase change from solid to liquid state, thus calling it a phase change material (PCM). Heat absorbed by

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the solid material, instead of being used in increasing temperature, is used in producing a phase change, so material temperature remains relatively constant. Thus, we speak of latent (hidden) heat. Similarly, when the phase change process is reversed, that is from a liquid to a solid-state, stored latent heat is released again at an almost constant temperature (Crespo et al., 2019). The process of solid-liquid phase change by melting and solidification can store large amounts of energy if a suitable material is selected. As the material melts, heat is transferred to the storage material, the material maintains its temperature constant at the melting temperature. This is called the phase change temperature (Crespo et al., 2019). The main difference with SHS is shown in Fig. 14.2B. Solid-liquid phase change is the most studied, but some solid-solid phase changes are also of interest in certain applications. Fig. 14.2B shows the difference between sensible heat and latent heat, through a graph divided into five parts: G

G

G

G

G

First, solid temperature increases in proportion to supplied thermal energy: Sensible heat. Second, supplied energy amount continues to increase, but the material has reached melting temperature and therefore uses this amount of energy in making a phase change. Material is kept at a constant temperature during phase change: Latent heat. Third, material is in a liquid state, and its temperature increases in proportion to thermal energy supplied by the medium: Sensible heat. Fourth, supplied energy amount continues to increase, this time, material has reached vaporization temperature, so the amount of energy is used to generate a phase change. Material is kept at a constant temperature during phase change: Latent heat. Fifth, material is in a gaseous state, and its temperature can increase in proportion to supplied thermal energy by the medium: Sensible heat.

Storage capacity or the amount of latent heat stored in a material is calculated according to the following equation: Q 5 m 3 Δh

ð14:3Þ

where Δh is the enthalpy of phase change (J/kg), also called enthalpy or heat of fusion, and m is storage material mass (kg).

14.2.1.3 Chemical reaction/thermochemical heat storage Thermal storage by chemical processes category includes sorption and chemical/thermochemical reactions. In this type of storage system, energy is stored after a dissociation reaction and recovered in a chemically reverse reaction. Thermochemical storage models for low and medium temperature applications are established on very well-known reversible chemical

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reactions involving water (and/or CO2) and solid substances. In fact, only direct reactions are genuine chemical reactions (since new compounds are formed), but frequently even physical phenomena as adsorption and absorption are signified as a thermochemical reaction in this system. Thermochemical heat storage (TCS) systems have higher storage densities compared to physical processes (sensible and latent), allowing large amounts of energy to be stored using small amounts of storage substance (Farulla et al., 2020). Chemical reaction-based TCSs is characterized by a change in chemical bonds of the compound involved in the reaction (dissociation and recombination), energy can be stored through an endothermic reaction and released by a reverse exothermic reaction. This energy storage system is particularly characterized by energy losses reduction throughout storage period, making it suitable for long-term storage applications. As explained before, sorption thermal storage systems (adsorption and absorption), even though, not involving a chemical reaction, base their principle on chemical processes. Sorption storage is defined as a phenomenon of binding of a gas or vapor by a sorbent substance in a condensed state (solid or liquid) through less intense interactions. Adsorption happens when an adsorptive accumulates on the surface of an adsorbent and forms a molecular layer, while absorption is a process that occurs when a substance is distributed in a liquid or solid and forms a solution (Abedin and Rosen, 2011). Heat stored in a chemical/TCS material depends on the amount of storage material, endothermic heat of reaction, and extent of conversion given by the following equation: Q 5 ar 3 m 3 Δhr

ð14:4Þ

where ar is fraction reacted, m is storage material mass (kg), and Δhr is the reaction endothermic heat. Thermal energy storage finds its justification in reduction of both energy consumption and CO2 emissions (Farulla et al., 2020).

14.2.2 Basic concepts for thermal energy storage materials A material must meet a series of requirements in order to be considered a thermal storage material. Required thermophysical properties are the following: G

G

Thermal properties: melting and crystallization point within the range of operating temperature, relatively high latent heat per unit volume and specific heat. Physical properties: congruent melting point of material, high density so material exhibits high-energy storage density at operating temperature. It

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G

G

G

435

is necessary that vapor pressure of the material is low and material shall undergo small volume change. Kinetic properties: an ideal material must have high nucleation and adequate crystallization rates and no supercooling. Chemical properties: material required to have long-term chemical stability, be compatible and non-corrosive with container material, reversible freeze/melt cycle and to ensuring safety is necessary. In addition, material must be non-toxic, non-flammable and non-explosive. Finally, TES material must be readily available in large quantities at low cost.

Main terms used to explain thermophysical properties are reviewed below: Energy storage density (esd): is the amount of heat (MJ) that can be stored in 1 m3 of material, which is calculated with the following equation: esd 5 ρ 3 Q

ð14:5Þ

where ρ is storage material density (kg/m3) and Q is the amount of heat stored in a material, as given for sensible heat by Eqs. (14.1) and (14.2), for latent heat by Eq. (14.3) and for thermochemical heat by Eq. (14.4). Thermal stability: refers to material mass conservation measured over time as temperature changes. A thermal stability test is useful to ensure that the material is stable at established working temperature range, for later application in sensible, latent and/or TCS processes. The maximum temperature the material can stand is known as degradation temperature. Thermal stability can be studied in an oven or through Thermogravimetric analysis (TGA). Fig. 14.3A (based on (Ushak et al., 2015b)) shows an example of a thermal stability study for two materials: bischofite and MgCl2  6H2O. Appropriate stability is observed in both analyzed samples when closed crucibles are used. However, when open crucibles are used, MgCl2  6H2O thermal stability is much better than that of bischofite, with temperature stability ranges of up to 90 C and 50 C for MgCl2  6H2O and bischofite respectively. Cycling stability: refers to a requirement for materials which determines their reproducibility for subsequent applications in both latent and thermochemical heat processes. Further details related to cycling stability of latent and thermochemical heat are explained separately. Cycling stability for latent heat storage process: also referred to as long term stability, refers to ability of a PCM to remain stable after several repeated melting/freezing cycles, as required by an application. One of the main problems of cycling stability is phase segregation, specially observed for salt hydrate and mixtures of compounds. PCM cycling stability is commonly studied through Differential Scanning Calorimetry analysis. Fig. 14.3C (data from (Mar´ın, 2017)), shows how PCM RT-25 exhibits cyclic stability after 50 heating and cooling cycles, and in similarly,

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FIGURE 14.3 Basic concepts in TES material characterization: (A) Thermal stability study for two materials: bischofite and MgCl2  6H2O using TGA. (B) Temperature vs. time curve for a sample showing subcooling phenomenon obtained by T-history method. (C) Cycling stability curves for PCM RT-25 after 50 cycles using DSC. (D) Cycling stability curves for PCM LiNO3 after 4 cycles using DSC. (E) Cycling stability curves of dehydration/hydration of carnallite at 25 kPa water pressure using STA-modular humidity generator. (F) Cycling stability curves of dehydration/hydration of carnallite at 4.0 and 1.3 kPa water pressure for dehydration and hydration process respectively. (A) From Ushak, S., Gutierrez, A., Galleguillos, H., Fernandez, A. G., Cabeza, L. F., and ´ Grageda, M., 2015b. Thermophysical characterization of a by-product from the non-metallic industry as inorganic PCM. Sol. Energy Mater. Sol. Cells 132, 385391. (B) From Rathgeber, C., Schmit, H., Miro´, L., Cabeza, L. F., Gutierrez, A., Ushak, S. N., and Hiebler, S., 2018. Enthalpy-temperature plots to compare calorimetric measurements of phase change materials at different sample scales. J. Energy Storage 15, 3238. (C) From Mar´ın, P. E., 2017. Implementacio´n de PCMs comerciales en la climatizacio´n pasiva de construcciones modulares a escala piloto en clima costero (Doctoral thesis). Universidad de Antofagasta, Antofagasta, Chile. (D) From Milian, Y. E. ´ (2018). Encapsulamiento de materiales de cambio de fase inorganicos. Influencia en sus propiedades termof´ısicas (Doctoral thesis). Universidad de Antofagasta, Antofagasta, Chile. (E and F) From Mamani, V., Gutie´rrez, A., ´ Fernandez, A. I., and Ushak, S., 2020. Industrial carnallite-waste for thermochemical energy storage application. Appl. Energy 265, 114738.

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Fig. 14.3D (data from (Milian, 2018)) shows PCM cyclic stability of LiNO3. However, Fig. 14.3D indicates there is no crystallization cycle repetition, probably due to material phase segregation, as these salt hydrates are known to exhibit incongruent melting. Phase segregation—congruent and incongruent melting: Salt hydrates are generally described by the formula AB  nH2O, where n represents number of water molecules. Salt dehydration occurs during phase transformation, as expressed by: AB  nH2 O-AB 1 nH2 O

ð14:6Þ

AB  nH2 O-AB  mH2 O 1 ðn 2 mÞH2 O

ð14:7Þ

According to phase transition performance, salt hydrates can be divided into salt hydrates with congruent and incongruent melting behavior. If an anhydrous salt is considered, which turns out to be completely soluble in the water contained within the crystalline structure of the salt hydrate, it is said that the salt hydrate melts congruently: as a result, fusion is a homogeneous salt water solution. AB 1 nH2 O-AB  nH2 O:

ð14:8Þ

Nonetheless, a number of salt hydrates with large latent heat which could be considered PCMs would not melt congruently. If an anhydrous salt is only partially soluble, the salt hydrate melts incongruously, resulting in a heterogeneous mixture consisting of a saturated aqueous solution and a second solid phase of the anhydrous salt or a low salt hydrate. The solid phase deposits on the bottom due to large density—usually referred to as sediments—leading to phase separation. With continuous melting/freezing cycles, sediments continue to grow, showing a significantly lower capacity for heat storage. One of the common methods to minimize or avoid phase separation is the addition of thickener agents into salt hydrates, increasing PCM viscosity and causing different phases not to separate considerably until all PCM becomes solid. Subcooling: or so-called supercooling is a common phenomenon among hydrated salts due to their poor nucleation capacity. This phenomenon consists of material temperature reduction below melting temperature without phase change, causing a delay of the beginning of solidification, that is the difference between nucleation temperature (Tn) and crystallization temperature (Tcr) (Fig. 14.3B, data from (Rathgeber et al., 2018)). If the material is not solidifying at all, only sensible heat is stored. To overcome subcooling problems, various methods have been suggested. One of the most effective and cheapest approaches is adding nucleating agents, which initiate heterogeneous nucleation (Honcov´a et al., 2020). Additionally, PCM encapsulation and salts impregnation in porous materials, used as support material (SM), are also applied to overcome subcooling.

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A common method used to determine stored heat as a function of time in compounds that exhibit subcooling effect is the T-history method due to its large sample size capacity, especially interesting for volume-dependent behavior materials. Cycling stability for TCS process: refers to the ability of a thermochemical material to have appropriate reversibility (for example, carbonation, hydration, sulfation, oxidation and peroxidation, hydrogenation of organic, dehydrogenation reactions) without side reaction after several repeated cycles. Cycling stability or reversibility of thermochemical reactions can be studied with simultaneous thermal analysis (STA). Reactors are also used for cycling stability studies, where molar reaction fraction change, heat storage capacity of reagent at different decomposition temperatures and mass variation condition are explored, for example, in dehydration/hydration processes. Fig. 14.3E,F (from (Mamani et al., 2020)) show examples of dehydration/ hydration cycling stability curves made with STA coupled to a modular humidity generator for potassium carnallite at 25 kPa water pressure (Fig. 14.3E), and after modifying operating conditions (Fig. 14.3F). Fig. 14.3E shows how potassium carnallite begins to decompose as cycles occur, while in Fig. 14.3F, the material, under different operating conditions, exhibits different behavior. In this case, carnallite is not undergoing significant decomposition after studied cycles.

14.2.3 Overview of thermal energy storage system types TES is also divided into active and passive systems, direct active systems blend the function of heat transfer fluids (HTF) and storage material. For active systems, the storage material is streamed through a heat exchange subsystem, while remaining stationary for passive systems. Regenerators (passive systems) are simple arrangements displaying maximum work temperatures. These systems exhibit external losses and moderated longduration storage efficiency produced by a temperature gradient within storage mass. Moreover, diverse materials posses the ability to absorb and release heat later as passive systems; requiring optimization control of peak heat delivery reliant on specific system (Bastien and Athienitis, 2018). Active systems require additional equipment and are impacted by parasitic losses. Nevertheless, “They allow for stationary temperature conditions, for a long storage duration and an easy decoupling between Energy and Power of the thermal storage system” (Haider and Werner, 2013). Alva et al. exhaustively reviewed different TES systems categorized according to storage material, storage cycle frequency, delivery scheme, storage mechanism (passive and active systems), and operating temperature range (Alva et al., 2018). Among categories of storage cycle frequency, the following are included: diurnal TES systems (Concentrating Solar Power (CSP) Thermal Plants, domestic solar hot water supply, hybrid photovoltaic-

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thermal with PCMs systems) and seasonal and long duration TES systems (underground TES systems, solar ponds and deep-water source cooling systems). A diurnal TES system is charged during daytime and discharged at night. Meanwhile, in seasonal TES systems, excess heat storage is preferred in summer for later usage by domestic demand during winter, due to extreme weather at high latitudes. Large quantities of low-cost TES materials, such as industrial waste salts, are needed for this type of systems. In general, high initial capital cost is a prerequisite barrier to TES proliferation. Hence, “continued research effort is needed to reduce cost through the use of alternative cheap TES materials from renewable bio-sources, naturally occurring earth materials, industrial waste materials etc.” (Alva et al., 2018). However, thermal storage performance of these low-cost materials requires high energy storage density, according to the applied storage mechanism. Additional details related to energy storage density for different TES materials are further discussed in the following section.

14.2.4 Comparison of energy storage density for different thermal energy storage materials Different thermal properties like heat capacity, latent heat or melting point are particularly desirable according to the thermal storage mechanism, but energy storage density (esd) is always expected to be as high as possible. Materials used in SHS systems are divided into gas (air), liquid (water, mineral oil, molten salts and liquid metals and alloys) and solid storage (rocks, concrete, sand and bricks) media. Materials for SHS systems are commonly categorized as organic (paraffin, fatty acids, esters, alcohols and glycols), inorganic (salts, metal and metal alloys) and eutectic mixtures (compositions of two or more components: organic-organic, organicinorganic and inorganic-inorganic, with a defined melting/freezing point) (Alva et al., 2017). Storage efficiencies and esd differ for each class of material (Fig. 14.4). TES systems offer a esd ranging from 10 to 50 kWh/t for SHS mechanism (with storage efficiencies between 50% and 90%). Materials for latent heat storage (LHS), denominated PCMs, deliver higher esd than SHS materials (around 100 kWh/m3 and storage efficiencies from 75% to 90%). While TCS systems can provide esd of up to 250 kWh/t (efficiencies from 75 to closely 100%) (Fig. 14.4) (Khan et al., 2017; Sarbu and Sebarchievici, 2018; https://iea-etsap.org/ETechDS/PDF/E17IR% 20ThEnergy%20Stor_AH_Jan2013_final_GSOK.pdf). Different materials obtained as wastes or by-products from industrial processing have already been evaluated for TES applications requiring high esd values or high porosity. Therefore, they are proposed as SM to stabilize and preserve specific TES materials.

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14.3 Overview of industrial waste studied as thermal energy storage materials A broad diversity of materials have been identified with potential for sensible, latent or thermochemical energy storage. Application of TES materials depends on desired features: operating temperature range, power, esd and additionally low-cost. However, among the main drawbacks in of TES materials progress are high-costs and economical unfeasibility. Utilization of waste materials for TES has been recognized to provide enhanced economic and positive social effects on civilization, production and management of energy, reducing the total cost of TES systems. Moreover, revalorization of waste materials generates immediate environmental benefits, decreasing accumulation of landfill waste (Ghani et al., 2020) and avoiding harmful ecological impact when decomposing or reacting with atmospheric gases or water. Furthermore, an essential factor to reuse industry wastes or byproducts is their availability or yearly production avoiding product supply deficiency (Gutierrez et al., 2013)(Gutierrez et al., 2016a). Gutierrez et al. (2016b) reviewed several industrial waste materials characterized and projected as potential low-cost TES materials (see Table 14.1). In that study, emphasis was on several materials applied for TES systems both TES media and SMs. These last few allow heat storage materials (like PCMs) to remain stable over time and avoiding leakages. Among low-cost TES materials based on wastes, a number of materials are found: asbestos, municipal solid waste incinerator residues, fly ash, concrete, and slag from metallurgic industry. These are mainly applied for SHS (Gutierrez et al., 2016b) in active systems. Glass and ceramics, materials obtained from melted asbestos-containing wastes (ACW) on high-energy consuming melting-based processes at high-temperature (about 1400 C), have been thermophysically analyzed by several groups The compatibility of the ACW ceramics with different HTF, currently used in TES applications like molten salts, oils, hot water and atmospheric steam, has already been tested. Fly ashes from industrial combustion, such as those released from municipal solid wastes incinerators or coal-fired power plants, are microparticles mainly composed by SiO2, Al2O3 and CaO. Municipal waste components, like glass and nylon, are among major contributors to solid wastes. All these materials could be used as raw materials for ceramic manufacturing and, those ceramics could be employed for TES systems Gutierrez et al., 2016b. A very important human activity is metallurgic industry, which produces several by-products attractive for sensible TES applications, due to their high thermal conductivity, high heat capacity and thermal stability over 1000 C. For instance, steel production involves steels-lag accumulation, one of the major by-products from iron ore melting processes at high-temperatures, generated by the copper industry and a residue from the aluminum industry.

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FIGURE 14.4 Energy storage density esd (denoted here as storage capacity) as related to temperature for TES applications. From Khan, M.M.A., Saidur, R., Al-Sulaimana, F.A., 2017. A review for phase change materials (PCMs) in solar absorption refrigeration systems. Renew. Sustain. Energy Rev. 76, 105137.

An example of inactive and low-cost recycled material from asbestos waste, in the form of vitrified waste, is Cofalit. This product was assessed as a solid filler for SHS in thermocline TES, as part of a pilot-scale system. In this way, the two-tank TES system was replaced by a single tank, and as a result, a high volumetric heat capacity was achieved. Waste material was compared with a reference material of alumina spheres, commonly employed as filler material. Synthetic oil was employed as HTF, solar molten salt (60 wt.% sodium and 40 wt.% potassium nitrates) and ternary nitrates mixture (42 wt.% calcium, 15 wt.% sodium, 40 wt.% potassium) were also identified as compatible HTF with this vitrified rock product. Results showed that charge and discharge duration were lower in 22% and 16%, respectively, for Cofalit when compared to alumina. Additionally, charge and discharge efficiencies were, to some extent, higher in Cofalit (82% and 90%, respectively) compared with the reference material (78% and 80%, respectively). These results were due to lower volumetric heat capacity of Cofalit (2.9 MJ/m3/K), contrasted with alumina (3.73 MJ/m3/K). Good thermal

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SECTION | IV Sustainable materials for thermal energy systems

performance of Cofalit as filler material in thermocline TES, cost-saving and positive environmental impact of using Cofalit (contributing to reduce asbestos waste gathering) made this waste a potential filler material in TES systems (Keilany et al., 2020). Finally, several inorganic salts and hydrates have been investigated as TES materials, accounting for important advantages, such as their wide availability and apparent inexistence of a requirement for treatment prior to TES applications. Interestingly, many of these salts have been found to be waste materials produced by various mining activities. Moreover, to date, these compounds have been proposed for TES systems: SHS; LHS, and TCS. Thus, countries with high mining activity might occupy a relevant role in TES materials production. In this context, Chile bears an important potential as producer of TES materials based on this sort of wastes.

14.4 Inorganic salt-based products and wastes as low-cost materials for sustainable thermal energy storage 14.4.1 Availability and abundance of inorganic salts in Northern Chile Chile has increasingly become more active in exploration and environmental innovation since 2011, conducing to mining activity as the main source of income (Fernandez, 2020). The importance of mining in the Chilean economy is vastly substantial, currently representing around 55% of total exports. Nitrates, carbonates, sulfates, chlorides and other salts from the Atacama Desert salt flat and mineral rocks (caliche) are obtained on a large scale due to a large extent to the non-metallic industry in North of Chile. In this context, the country has been producing from brine sources more than 60,000 t of lithium compounds and 2,087,800 t of potassium compounds per year, among others (Gutierrez et al., 2017; Anuario de la Miner´ıa de Chile, http://www.sernageomin.cl/pdf/mineria/). Additionally, the production of nitrate compounds from caliche sources has reached 772,170 t. A product mainly used as fertilizer and solar salt in CSP plants (see Fig. 14.5) (SERNAGEOMIN ANNUARY, 2016; Evans, 2009). Production processes in the non-metallic mining industry consist on use of solar evaporation ponds, where leaching solution of mineral rocks (caliche) (Fig. 14.6A)or brines (Fig. 14.6B) are deposited. Solar evaporation, with a high evaporation rate (3200 mm/year), leave salts (o mixture of salts) as by-products or waste materials (see Fig. 14.6). Interest in using these substances as TES materials is increasing mainly due to their cost being close to zero. Additionally, usage of solar evaporation process allows for considering non-metallic mining wastes as zero carbon footprint materials. Since they have been already extracted and have low embodied energy.

TABLE 14.1 Industrial waste materials used or proposed for TES. TES classification

SHS

Industrial sources of TES materials Asbestos with wastes

Fly ashes (FA)

Salt industry

Metal industry

 Ceramic  Glasses

 Coal fired powerplant FA  Municipal solid waste FA

   

 Steel slag  Copper slag  Aluminium black dross

NaCl Kainite Bischofite Astrakanite

LHS

 Kainite  Bischofite  Astrakanite

TCS

 Bischofite  Astrakanite  Potassium carnallite

 Electrical arc furnace dust (as SM)

Municipal waste

 Glass (as SM)  Nylon (as SM)

LHS, latent heat storage; SHS, sensible heat storage; TCS, thermochemical heat storage; TES, thermal energy storage. Source: Based on Gutierrez, A., Miro´, L., Gil, A., Rodr´ıguez-Aseguinolaza, J., Barreneche, C., Calvet, N., et al. 2016b. Advances in the valorization of waste and byproduct materials as thermal energy storage (TES) materials. Renew. Sustain. Energy Rev. 59, 763783.

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Recently, Albemarle Company, one of the major non-metallic producers, has been permitted to raise its corresponding quota in Salar de Atacama to 82,000 t/year of lithium carbonate equivalent from 2018 to 2025; implying an increase in by-products and waste materials with associated availability of low carbon footprint materials (see Fig. 14.5). Thus, in non-metallic industry, many materials such as halite, bischofite, astrakanite, kainite, carnallite are considered as by-products or wastes, which are consequently discarded (Fig. 14.6). All these inorganic salts could be evaluated as potential low-cost TES materials. As already explained, the use of these by-products and wastes could provide better management of inorganic salt production sites for a more sustainable future, decreasing natural environmental stressors exerting degradation.

14.4.2 Economic analysis of inorganic salts as low-cost thermal energy storage materials Nowadays, salt industry landfills contain high amounts of inorganic compounds that have been proposed as TES materials. Halite (with NaCl as the main compound), a by-product from the potash industry, has been tested as SHS material; bischofite (with MgCl2  6H2O as the main compound), astrakanite, potassium and lithium carnallites and kainite (by-products or waste materials, accumulating from non-metallic mining industry in Northern Chile) have been tested as latent heat and/or TCS materials. These waste compounds have been considered as low-cost TES materials, for example, in 2015 the cost to store one MJ of energy stored with bischofite was US$ 1.28, which is low-cost compared with those of synthetic Mn(NO3)2  6H2O and MnSO4  7H2O (US$ 15.0 and US$ 12.4, respectively) (Gutierrez et al., 2015). Similarly, Ushak et al. compared the cost (h/kg) of several PCMs applied at low temperature. In this study, the lowest cost was estimated for bischofite (0.14 h/kg), comparable with NaCl (0.15 h/kg) (Miro´ et al., 2014), followed by commercial MgCl2.6H2O and acetamide (0.46 and 2.2 h/kg, respectively) (Ushak et al., 2016b). In general, lithium compounds are expensive materials but have been widely proposed to improve thermal properties of molten salts (SHS materials) or to obtain new SHS eutectic mixtures with lower melting point (Ushak et al., 2015a). Nevertheless, Chile is among the biggest producers of Li compounds from brine, with lower production costs compared with other nations (Cabeza et al., 2015). In mid-2015, Li spot price was around 6000 US$/t, nonetheless, “batterygrade” Li2CO3 spot price was appreciated over US$ 10,500, depending on conditions of sale, contract and region to which it was traded. However, aiming a cost reduction, lithium compounds were usually mixed with waste materials available in large quantities, like halite or bischofite, in order to

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FIGURE 14.5 Mining production in the North of Chile: (A) non-metallic industry production in Chile; (B) potassium and nitrate compounds; (C) boron and lithium compounds and projections of production increasing until 2025. (A) Based on SERNAGEOMIN ANNUARY, 2016. ,https://www.sernageomin.cl/wp-content/ uploads/2017/Anuario-2016-sernageomin.pdf.. (B and C) Based on (p)1 Production projection: Evans, R.K., 2009. Production projection. Lithium resources are they adequate?. Presentation on “Lithium Supply Markets 2009”. Santiago Chile, (p)2 Estimation from the publication “La Tercera: Pulso” February 2018, (p)3 Own estimate based on data from the magazine “Chilean Mining” February 2018.

improve its thermal properties and, at the same time, achieve low final TES material cost (Milian et al., 2020). Additionally, temperature is an important factor in energy cost considering a specific thermal energy storage material. In 2018, energy storage cost for bischofite was 0.36 US$/MJ while commercial MgCl2  6H2O was 1.17 US$/MJ at 70 C. However, these values decreased at higher temperatures; for example, at 100 C the energy storage cost for bischofite and MgCl2  6H2O were 0.19 and 0.59 US$/MJ (Mamani et al., 2018), respectively, considerably lower than those at 70 C. Thermal energy storage process must also be considered since the mechanism of TES system impacts price. For instance, energy cost, using bischofite as latent and TCS were 1.28 and 0.915 US$/MJ, respectively (Mamani et al., 2018). In other study, price reported for synthetic KCl  MgCl2  6H2O was 0.05 h/MJ, when estimated through the price of low-quality bischofite 40 US

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FIGURE 14.6 Schematic process to obtain: (A) nitrate salts from caliche and (B) KCl and lithium concentrated brines. From Ushak, S., Gutierrez, A., Galazutdinova, Y., Barreneche, C., ´ Cabeza, L. F., and Grageda, M., 2016a. Influence of alkaline chlorides on thermal energy storage properties of bischofite. Int. J. Energy Res. 40(11), 15561563.

$/t (31.1 h/t, February 2018) as a reference, followed by CaCl2  2H2O and Al2(SO4)3  18H2O, both with 0.21 h/MJ (Gutierrez et al., 2018). Nevertheless, the price for a potassium carnallite waste, applied as thermochemical storage material (TCM), was recently reported as close to zero (Mamani et al., 2020). Consequently, feasibility of using non-metallic mining industry wastes, as low-cost materials is supported. Nevertheless, further detailed investigations on thermophysical properties and scalability are required to obtain more and better strategies for TES application of these salts. Even thought, the necessity for additional research to reach such goal is essential, it is evident a great potential arises for primarily mining countries, such as Chile.

14.4.3 State-of-art of currently proposed by-products and wastes as thermal energy storage materials This section presents a review of by-products or wastes from non-metallic mining industry that have already been proposed and studied as TES materials at lab or pilot-plant scale. Salt industry is one of the main producers of

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potential candidates for TES systems. Several materials have been already proposed and analyzed with such focus (Table 14.2). Different thermal properties are provided: application temperature (Temp., C), latent heat (ΔH, kJ/kg), density (ρ, kg/m), heat capacity (Cp, kJ/kg/K), thermal conductivity (λ, W/m/K), TCMs dehydration and reaction enthalpies (ΔDH, J/g and ΔRH, kJ/mol, respectively) and energy storage density (esd kWh/m3). Sulfate, carbonate and chloride based salts are among the most common available compounds from non-metallic mining industry. Therefore, other studies related to similar commercial salts are included. As seen in Table 14.2, bischofite and carnallites are among the most studied compounds for the three alternative TES proccesses, including possible mixtures using these wastes. Specifically, bischofite esd values are 170 J/cm3 (Ushak et al., 2015b) and over 666.0 J/cm3 (Mamani et al., 2018) when applied for LHS and TCS, respectively (Table 14.2). Sensible storage using inorganic salt-based waste has been reported between 0 C and 200 C. LHS has been reported for low (i.e. 0 C100 C) and high (i.e. 400 C1100 C) temperature, with no inorganic salt-based waste material applied for medium temperature (i.e. 100 C400 C). Lastly, TCS process temperature range, where use of waste materials has been proposed, is between 70 C 400 C. In order to visualize inorganic salt-based waste potential for TES materials, the following sections provide detailed discussion of their thermophysical properties, expounded for each type of thermal energy storage process.

14.4.3.1 Sensible heat storage materials According to Ushak et al. (2014), astrakanite (Na2SO4  MgSO4  4H2O) and kainite (KCl  MgSO4  3H2O), both waste materials resulting from nonmetallic industry extraction processes in Northern Chile (see Fig. 14.6A), were suitable for use as TES materials at low-temperatures (0 C100 C). Halite was proposed as sensible TES material for industrial heat recovery between 100 C and 200 C, at laboratory and pilot plant scale. At laboratory scale, specific heat capacity, density and cycling stability were determined (Table 14.2). At pilot plant scale, 59 kg of NaCl-based waste material were used for a thermal cycle study between 100 C and 200 C varying parameters such as flow rate and HTF type (Miro´ et al., 2014). A comparison of bischofite with other SHS materials (see Table 14.2) showed highest specific heat values compared to reinforced concrete, silica firebricks, and NaCl (Miro´ et al., 2014). Besides, in terms of cost per unit of mass, bischofite is among the cheapest ones. A pilot plant assembled at University of Lleida (Spain) was employed to analyze bischofite at large scale in two different charging processes (CP 1 and CP 2). First, a sensible heat process was carried out by increasing temperature from 50 C to 80 C, while a second charging process was performed between 80 C and 120 C to observe latent progression influence. Evaluated bischofite (204 kg) was able

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to store 3.07 and 11.97 kWh in sensible and latent periods, respectively (Fig. 14.7A). Finally, the first charging process needed 67% of time, whereas the second one needed only 55.6% of time to store 90% of total energy (Fig. 14.7B) (Gasia et al., 2015). Consequently, the same material is considered for both TES processes: sensible and LHS. In general, inorganic salt-based wastes and by-products presented heat capacity (Cp) values in the range of 0.73.0 kJ/kg/K, exhibiting suitability for SHS (see Table 14.2). Another important parameter to be considered for application in SHS materials was thermal conductivity (λ). Among all the materials studied (see Table 14.2), this property has only been measured for halite (0.33 W/m/K), which continues to be a weak point of these investigations given the importance of λ values for SHS systems.

14.4.3.2 Latent heat storage materials Recently, inorganic salt-based wastes and by-products studied as LHS materials, called also PCMs, have been proposed as TES materials. Some of them exhibit high latent heat values (around 100 kJ/kg). An anhydrous phase of astrakanite (Na2SO4  MgSO4) was found to be a potential latent TES material in a reversible melting—solidification behavior in the temperature range between 570 C and 680 C (Ushak et al., 2014; Gutierrez et al., 2017). However, a latent heat of 35.0 kJ/kg is relatively low for a practical application. Regardless, this compound could still be employed, due to its low cost for developing eutectic mixtures enhancing this property (ΔH). Thermal characterization of bischofite as LHS material was compared with commercial synthetic MgCl2  6H2O, resulting in 100 C and 115 kJ/kg for bischofite, and 114.5 C and 135 kJ/kg for commercial salt (melting temperature and latent heat, respectively). Other thermal properties were determined for both samples, recognizing that bischofite was a potential PCM with similar thermophysical characteristics to magnesium chloride hydrate (Table 14.2), but at a lower cost. However, this waste salt presents some adverse effects, as subcooling (around 35 C) and phase segregation that needs to be addressed further (Ushak et al., 2015b). In a consecutive work, a verification was achieved that using an inorganic eutectic mixture based on 40 wt.% bischofite and 60 wt.% Mg(NO3)2  6H2O, with a melting point of 58.2 C and heat of fusion around 117.0 kJ/kg (Table 14.2), made possible to reduce bischofite subcooling level to 23.4 C (Galazutdinova et al., 2017). Finally, eutectic mixtures: 54.5 wt.% Na2SO4 1 45.5 wt.% MgSO4, and 52 wt.% Na2SO4 1 48 wt.% MgSO4 were validated to be used as PCM at high temperature due to their high latent heat (close to 100 kJ/kg) (see Table 14.2) (Gutierrez et al., 2017). Similarly, the use of bischofite/Mg (NO3)2  6H2O mixture for passive thermal regulation of lithium-ion batteries, absorbing internal heat and therefore extending battery life and enhancing performance, was discussed and compared with paraffin and commercial

TABLE 14.2 By-products and inorganic salts-based wastes from non-metallic mining industry. Material

TES

Temp. [ C]

ΔH [kJ/ k/g]

ρ [kg/m3]

Cp [kJ/kg/K]

λ [W/ m/K]

ΔDH [J/g] and reaction kinetics

ΔRH [kJ/ mol]

esd [kWh/ m]

Ref.

Halite (NaCl)

Sensible

100200

n.a.

1384

0.738

0.33

n.a.

n.a.

n.a.

Miro´ et al. (2014)

0100

n.a.

n.a.

0.91.2

n.a.

n.a.

n.a.

n.a.

Latent (dehydrated astrakanite)

550700

35.0

n.a

1.231.54

n.a

n.a.

n.a.

n.a.

Gutierrez et al. (2017)

TCS

110.6

n.a.

n.a.

n.a.

n.a.

506.2

n.a.

n.a.

Sensible

0100

n.a.

n.a.

0.985

n.a.

n.a.

n.a.

n.a.

Astrakanite (Na2SO4  MgSO4  4H2O)

Kainite (KCl  MgSO4  3H2O)

Ushak et al. (2014)

(Continued )

TABLE 14.2 (Continued) Material

TES

Temp. [ C]

ΔH [kJ/ k/g]

ρ [kg/m3]

Cp [kJ/kg/K]

λ [W/ m/K]

ΔDH [J/g] and reaction kinetics

ΔRH [kJ/ mol]

esd [kWh/ m]

Ref.

52 wt.% Na2SO4 1 48 wt.% MgSO4

Latent

600700

103.1

n.a.

n.a.

n.a.

n.a.

n.a.

n.a.

Gutierrez et al. (2017)

Lithium carnallite (LiCl  MgCl2  7H2O)

No TES

n.a.

n.a.

n.a.

n.a.

n.a.

n.a.

n.a.

n.a.

Potassium carnallite (KCl  MgCl2  6H2O)

TCS

127.1

n.a.

n.a.

n.a.

n.a.

330

n.a.

n.a.

100150

n.a.

n.a.

n.a.

n.a.

n.a.

191.1

303.9

Gutierrez et al. (2018)

100150

n.a.

1710

n.a.

n.a.

n.a.

B 58.0

B 313.6

Mamani et al. (2020)

LiNO3Mg(OH)2

250400

n.a.

n.a.

n.a.

n.a.

ka: 1.06  1022

174.0

n.a.

Li et al. (2020)

Bischofite (MgCl2  6H2O)

70100

n.a.

1513 (50 C)

n.a.

n.a.

440.0827.7

96.4

666.4 1250.9 J/ cm3

Mamani et al. (2018)

100.0

116.0

16861481 (30 C115 C)

1.63.0

n.a.

n.a.

n.a.

170 J/ cm3

Ushak et al. (2015b)

40 wt.% bischofite 1 60 wt.% Mg (NO3)2  6H2O

58.2

116.0

n.a.

1.22.1 (25 C45 C) 1.51.875 C125 C

n.a.

n.a.

n.a.

n.a.

Galazutdinova et al. (2017)

41 wt.% bischofite 1 58 wt.% Mg(NO3)2  6H2 1 expanded graphite (EG)

53.0

100.7

1190

n.a.

25.5 (inplane) and 9.3 (throughplane)

n.a.

n.a.

n.a.

Galazutdinova et al. (2020)

Latent

k a. Reaction velocity [s21] at 300 C. TES, thermal energy storage.  It has no potential to be applied as TES material under the conditions reported by Gutierrez et al. (2017).

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MgCl2  6H2O/ Mg(NO3)2  6H2O, impregnated in expanded graphite (EG) (Galazutdinova et al., 2020). Results showed that both inorganic mixtures were suitable for thermal control in Li-ion battery packs. Higher thermal conductivity values were obtained for bischofite composite when compared to commercial salts, while melting temperature and latent heat were below 60 C and over 100 kJ/kg, respectively, for both mixtures (Table 14.2) (Galazutdinova et al., 2020).

14.4.3.3 Thermochemical storage materials Inorganic salt-based wastes studied as TCS materials included astrakanite, potassium/lithium carnallites and bischofite, where enthalpies of dehydration and/or reaction were determined. Astrakanite (Na2SO4  MgSO4  4H2O) and potassium carnallite (KCl  MgCl2  6H2O), waste salts produced during the process of obtaining NaNO3 and concentrated lithium brines, respectively (Gutierrez et al., 2017), were synthesized and analyzed by thermogravimetry coupled to mass spectroscopy. Astrakanite and potassium carnallite showed release of water below 300 C, displaying potential as thermochemical material at low-medium temperature (,300 C) (Gutierrez et al., 2017). Future research was deemed required in order to demonstrate reversibility of dehydration/hydration reactions for these salts. As it was mentioned, bischofite is one of the most investigated byproducts from non-metallic mining as TES materials. In this regard, bischofite ore dehydration reaction, studied as a low-cost TCS material, was characterized by Mamani et al. (2018). Authors determined bischofite reaction kinetic parameters by isothermal method at 70 C, 80 C, 90 C and 100 C, applying a heating rate of 20 K/min, and applying the Arrhenius linearized equation. Parameters values were: Ea/R/103: 11.597, ln A: 27.762, A: 1.141  1012 s21, Ea: 96 414.8 J/mol and a linearity characterized by R 5 0.954. These values indicated that 96.415 kJ/mol was the minimum energy required to induce bischofite dehydration reaction for losing 2.11 H2O moles, while it was needed 103.74 kJ/mol to initiate dehydration reaction to reach loss of 2.34 H2O moles (Mamani et al., 2018). Four dehydration stages were observed for bischofite, between 80 C and 240 C, corresponding to loss of one, three, five, and finally the last water molecule at each stage, respectively (Fig. 14.8A). Such values occurred at a longer time and higher temperatures as compared to commercial MgCl2  6H2O. Bischofite weight loss occurred more rapidly at 80 C than at 90 C and 100 C (Fig. 14.8B). Chemical reactions at each temperature and some additional thermal factors were reported as shown in Table 14.3. Reaction velocity was affected by particle size and impurities (NaCl, KCl, calcium and Li compounds) detected in bischofite and typically presented in this mineral (Ushak et al., 2015B), obtained during evaporation and brine concentration processes (see Fig. 14.6B). Bischofite dehydration occurred by the R2 model (cylindrical particle contraction) at 70 C and 80 C and F1 (first order reaction) at 90 C,

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FIGURE 14.7 Energy balance analysis of bischofite (204 kg) in a pilot plant, during two charging processes (CP 1, CP 2) considering (A) different system components and ambient losses and (B) bischofite energy accumulation rate at different parts of storage system. From Gasia, J., Gutierrez, A., Peiro´, G., Miro´, L., Grageda, M., Ushak, S., and Cabeza, L. F., 2015. Thermal performance evaluation of bischofite at pilot plant scale. Appl. Energy 155, 826833.

which were based on a mechanism controlled by chemical reactions in limit phase (Mamani et al., 2018). Bischofite is not the only material that has been proposed as TCM. Alternatively, a high carnallite-bearing material, comparable to natural waste (see Fig. 14.6B), was prepared by crystallizing a ternary equilibrium solution of KCl 2 MgCl2 2 H2O at 35 C (Gutierrez et al., 2018). The obtained material showed gradual decomposition and low reversibility of hydration reaction above 150 C. Then, in the same study, the reversibility of thermochemical storage reaction based on hydration/dehydration was improved under nitrogen and water vapor (pH2O 5 25 kPa), additionally reducing decomposition at the interval between 100 C and 150 C. Authors proposed a long-term heat storage application for this material with dehydration below 150 C and rehydration close to 40 C (Gutierrez et al., 2018). Similarly, in order to improve cyclic stability of potassium carnallite samples, working conditions for dehydration were experimentally optimized to 110 C, applying water vapor at 4.0 kPa, while hydration was established at 40 C with pH2O 5 1.3 kPa (Mamani et al., 2020). Research performed by Gutierrez et al. (2017) using lithium carnallite (LiCl  MgCl2  7H2O) with no current application (Table 14.2), showed a high release of HCl at high temperatures, as well as simultaneous decomposition along with melting and solidifying processes. Lithium carnallite was discarded as TCS material under the conditions assessed in this work considering HCl release as very corrosive for TES systems and due to the lithiumbased waste degradation (Gutierrez et al., 2017). On the other hand, LiNO3, a pure lithium compound, has been extensively investigated, displaying excellent performance for medium temperature TES applications (Milian et al., 2020). In addition, lithium compounds have been employed in the formulation of binary and ternary eutectic salts,

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453

FIGURE 14.8 (A) Bischofite and MgCl2  6H2O thermogravimetry and (B) Mass loss versus time for bischofite isothermal dehydration at different temperatures. From Mamani, V., Gutie´rrez, A., and Ushak, S., 2018. Development of low-cost inorganic salt hydrate as a thermochemical energy storage material. Sol. Energy Mater. Sol. Cells 176, 346356.

based on lithium fluorides, nitrides and/or chlorides. As an example of the latest, a recent work employing LiNO3Mg(OH)2 has presented a high potential in TCS applications between 250 C and 400 C, fine-tuning chosen temperature array by addition of different LiNO3 wt.% (Li et al., 2020). Although several wastes based on inorganic salts were studied as promising TES materials, there are several challenges that must be overcome for their implementation in real scale TES systems.

14.5 Challenges for the application of waste and byproducts in thermal energy storage systems 14.5.1 Proposed uses of wastes as thermal energy storage materials Before going through improvements that have been made to TES technologies by applying different waste materials, it is necessary to know some common applications developed for TES, in order to understand reasons behind the necessity of such improvements. SHS materials have been applied with air or liquid HTF based solar systems, solar collectors (such as flat plate solar air heaters and liquid flat collectors), in greenhouses, agricultural crop drying, solar desiccants systems for buildings, space-heating/cooling systems and CSP (Almendros-Ib´an˜ez et al., 2019). Likewise, concrete, as SHS storage medium, has been investigated for industrial process heat storage and CSP plants (Laing et al., 2008). Moreover, a new solid-state concrete-like storage medium, designated as HEATCRETE has been tested in a 2 3 500 kWhth SHS system at temperatures up to 380 C over 20 months, being suitable for industrial waste heat recovery, thermal power plants and CSP applications (Hoivik et al., 2019).

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TABLE 14.3 Bischofite isothermal dehydration (Mamani et al., 2018). T ( C)

wt. %

H2O (mol)

Reactions

70

16.5

1.86

MgCl2  6H2O (s) - MgCl2  4.14H2O (s) 1 1.86H2O (g)

80

20.7

2.34

MgCl2  6H2O (s) - MgCl2  3.66H2O (s) 1 2.34H2O (g)

90

19.2

2.17

MgCl2  6H2O (s) - MgCl2  3.83H2O (s) 1 2.17H2O (g)

100

20.7

2.34

MgCl2  6H2O (s) - MgCl2  3.66H2O (s) 1 2.34H2O (g)

Similarly, solid urban demolition wastes have been analyzed as SHS material (Koc¸ak and Paksoy, 2019). Packed and fluidized beds for solar collectors, buildings (active and passive TES systems), solar thermal plants powering on Organic Rankine Cycle and cascade latent heat thermocline tanks, industrial heat recovery, liquefied natural gas, electric power peaking regulation, greenhouse agriculture, textiles, health care and aerospace are among the many applications of LHS materials (Almendros-Ib´an˜ez et al., 2019; Huang et al., 2019). Recently, use of PCMs on thermal management of Li-ion batteries, temperature management of microelectronics and photovoltaic thermal applications have been also described (Nazir et al., 2019; Galazutdinova et al., 2020). Meanwhile, TCS materials have expanded in applications, including their integration on systems based on a fluidized bed for solar collectors and as a way to increase maximum temperature and enhancing power cycle efficiency in CSP (Almendros-Ib´an˜ez et al., 2019). TES systems could be installed as either centralized plants or distributed devices for application in the building sector (e.g., space heating, and airconditioning) and in the industrial sector (Sarbu and Sebarchievici; 2018). Chemical, food and brewery industries are among the potential industrial targets to integrate TES systems. However, procedures for enhanced integration of TES must be developed further. In more depth, LHS systems, compared with SHS materials, have displayed more suitable characteristics for industrial application, due to higher heat storage capacity and energy storage density, besides storing heat at nearly constant temperature (Crespo et al., 2019). Availability of systems applying TCS technology is low, and several scientific challenges still need to be overcome, at material- and prototype-scale, mainly the development of suitable materials with considerably high energy density, stability and cyclability (Scapino et al., 2020).

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Once again, the high potential of development of solar energy in countries in the central west region of the American continent is evidenced and justified, especially considering high solar radiation levels and multiple nonmetallic mining wastes accumulated and available. These resources could also be used as low-cost TES materials. As a consequence, many research activities are focusing on developing methods to enhance thermal characteristics of these low-cost compounds making them suitable for TES systems.

14.5.2 Challenges for the application of inorganic salt-based wastes in thermal energy storage systems Non-metallic industry wastes and by-products are inorganic compounds, specifically, salts and salt hydrates. A key factor to consider when used as TES material is thermal stability. For example, lithium carnallite is discarded as TES material since it starts to decompose at low temperature (below 100 C) (Gutierrez et al., 2017). Once materials show thermal stability between temperature ranges of interest, according to desired applications, other thermophysical properties play a decisive role, as discussed below. SHS materials are thermally stable, have relatively low esd. But, considering their wide operating temperature intervals, high specific heat (Cp) and high density (ρ), sensible TES systems achieve higher amount of heat stored values (Alva et al., 2018). Molten salts have relative low cost, operating temperatures up to 700 C, low vapor pressures, and adequate physical properties. Therefore, they are highly applied for sensible TES, mainly in CSP plants. However, low thermal conductivity, relatively high melting point and corrosivity limit their applications for heat storage (Mohamed et al., 2017). For example, one waste material proposed and studied, halite (NaCl), shows low thermal conductivity and high corrosion levels (Miro´ et al., 2014). Miro´ et al. have achieved an increase in thermal conductivity of NaCl by changing material shape through water treatment: addition of a small amount of water to obtain a saturated solution and eliminate air gaps, followed by an evaporating process, producing a continuous solid block of salt. In addition, authors have achieved an increase in specific heat and storage capacity of salt (Miro´ et al., 2014). In CSP plants, lithium nitrate has been proposed to decrease the melting point of molten salts, reducing maintenance and operational costs (Cabeza et al., 2015). LHS materials have been reviewed by Mohamed et al. (Mohamed et al., 2017), showing main concerns related to inorganic materials. Salt and salt hydrates have shown phase segregation and subcooling, which reversibly affect esd, as well as displaying a negative influence on discharging stability and cycling life of TES (Zhao and Wang, 2019). Among the most challenging features of inorganic salts are low thermal conductivity (nearby 1 W/ mK) and all factors affecting phase transition (poor nucleation, presence of subcooling and volume change due the difference between solid and liquid

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SECTION | IV Sustainable materials for thermal energy systems

densities). In addition, some salts are rather costly (e.g. fluorides and their composite salts) and lack of industrial applications at medium or high temperatures based on assessment of energy savings and environmental impact, particularly the CO2 equivalent emissions, hamper their progress. Furthermore, there is no unanimity in the scientific community on environmental evaluation methods to be used; consequently, their research results might not be compared (Mohamed et al., 2017). Other problems are leakage, flammability and cycling instability. Bischofite, when evaluated as PCM, is one of the less expensive candidates among inorganic salts retrieved from non-metallic mining waste. However, bischofite shows a relatively low energy storage density (174 MJ/m3) (Gasia et al., 2015; Ushak et al., 2016a), slight cycling stability (Gutierrez et al., 2015) and evidence of two main drawbacks: a high subcooling degree (Ushak et al., 2015b; Ushak et al., 2016b) and phase segregation (Ushak et al., 2016a). Thermochemical energy storage has benefits such as high stored thermal energy durability (with small heat losses) and highest esd (both per-unit volume and mass). Nevertheless, low porosity of TCM during charging, when dehydration (or other charging process depending on thermochemical selected reaction) occurs, due to TCM sintering and grain growth and low velocity of dehydration reaction, directly impacts TCS system efficiency and life cycle. For example, a lower dehydration reaction rate and lower esd for bischofite compared with synthetic MgCl2  6H2O (see Fig. 14.8) has been found (Mamani et al., 2018). Also, gradual decomposition and poor reversibility of hydration reaction above 150 C for carnallite-beared synthetic material have been detected (Gutierrez et al., 2018). Moreover, TCS is still at laboratory stage and commercial applications involve further optimizations (Alva et al., 2018), to mainly demonstrate reversibility of dehydration/hydration reactions of proposed waste salts with high TCS systems potential, such as astrakanite and carnallite (Gutierrez et al., 2017). Recently, a natural carnallite-waste material (73.54 wt.% of KCl  MgCl2  6H2O and impurities such as NaCl (23.04 wt.%) and KCl (1.76 wt.%)) has been analyzed as lowcost TCM. Application temperatures and partial pressures for this material for seasonal heat storage have been optimized and reaction reversibility has been enhanced over 10 cycles, representing usage for 10 years (with a decrease in chemical reversibility around 8.5%). Energy storage density of this waste salt results in 1.129 GJ/m3 after the tenth cycle of hydration/dehydration, being equivalent and competitive with materials such as K2CO3 and MgCl2 (Mamani et al., 2020).

14.5.3 Optimization of thermal properties of thermal energy storage materials based on inorganic salt wastes Some of the challenges for the application of low-cost materials obtained from non-metallic mining industry are (1) low thermal conductivity for sensible

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457

storage; (2) subcooling and poor cycling stability for latent storage; and (3) low reversibility and decomposition, which implies low cyclic stability, for thermochemical storage (Table 14.4). As it has been observed, NaCl, bischofite and carnallite are the only waste salts that have been improved so far, due to low thermal and cycling stability, low thermal conductivity and subcooling. Different methods have been employed in order to improve performance of inorganic salt waste materials from non-metallic mining industry. There exist various methods available to overcome these challenges. For sensible and LHS, methods mainly focus on improving thermophysical properties of storage material by using additives, such as nucleating and thickener agents for subcooling and phase segregation elimination, or synthesis of mixed composites that could improve a particular property. For example, to increase thermal conductivity, incorporation of graphite and creation of a composite material are applied (Fig. 14.9B). Other methods are directed at optimizing external conditions of the TES system, atmosphere and vapor pressure, similar to those proposed for TCS systems (Fig. 14.9A). In the following section, main methods applied to enhance thermal properties of inorganic compounds and wastes are presented: encapsulation of LHS materials (e.g. avoiding leakages and decreasing subcooling degree), use of additives and graphite usage (enhancing thermal conductivity).

14.5.3.1 Encapsulation of latent heat storage materials Encapsulation techniques provide an increase in LHS materials heat transfer area, reduce reactivity with outside environment, decrease subcooling degree and regulate phase transition volume change of PCMs. Encapsulated PCMs are classified as core-shell materials (particles of storage material (core) covered by a shell-material) and shape-stabilized PCMs (SS-PCMs), which retain TES material by capillarity into a porous solid compound. Encapsulation methods developed for inorganic compounds are emulsion, in situ polymerization, interfacial polymerization, electroplating, sol-gel process, mechanical packaging and a combination of mechanical-electroplating techniques (Milian et al., 2017; Galazutdinova et al., 2017). Core-shell macrocapsules have been produced for bischofite and acrylic polymer by fluidized bed method, resulting in a decrease of subcooling and avoiding PCM leakage when melting (Ushak et al., 2016c). Recently, shape stabilized thermal energy storage materials based on bischofite and carnallite have been obtained by a direct sol-gel method, using tetraethyl orthosilicate as the monomer (Milian and Ushak, 2020). The resulting material, based on carnallite/SiO2, exhibits potential as a thermochemical storage medium for applications under 100 C. 14.5.3.2 Use of additives Addition of compounds into TES materials are mainly targeted to improve thermal conductivity, reduce subcooling degree and avoid phase segregation.

TABLE 14.4 Summary of studies, performed for enhancement of by-product and inorganic salt-based wastes by different methods. Waste Material

Disadvantages/ mechanism

Enhance method

Optimized material/working conditions

Ref.

Halite

Low thermal conductivity / SHS

Water treatment

Compacted NaCl

Miro´ et al. (2014)

Bischofite

Poor cycling stability/ LHS

Adding polyethylene glycol (PEG)

95 wt.% bischofite 1 5 wt.% PEG2000

Gutierrez et al. (2015)

Subcooling (ΔT 5 36 C)/ LHS

Adding nucleating agents

99 wt.% bischofite 1 1 wt.% Sr(OH)2

Ushak et al. (2016b)

97 wt.% bischofite 1 3 wt.% Sr(OH)2 97 wt.% bischofite 1 3 wt.% SrCO3

Poor cycling stability. Subcooling (ΔT 5 36 C)/ LHS

Microencapsulation by a fluidized bed method

MgCl2  6H2O  acrylic microcapsules

Ushak et al. (2016c)

Low thermal conductivity/LHS

Immersion

Composite material based on expended graphite and eutectic mixture (41 wt.% bischofite 1 59 wt.% Mg (NO3)2  6H2O)

Galazutdinova et al. (2020)

Subcooling and low heat transfer (low thermal conductivity)/LHS

Vacuum and ultrasound impregnations and immersion

Composite material based on expended graphite (flakes and matrix) and eutectic mixture (40 wt.% MgCl2  6H2O 1 60 wt.% Mg(NO3)2  6H2O)

Galazutdinova et al. (2018)

High carnallitebeared material

Low reversibility and decomposition/TCS

Changing of external operation condutions

100 C and 150 C under 25 kPa

Gutierrez et al. (2018)

Potassium carnallite waste

Low cyclic stability/TCS

Changing of external operation condutions

Dehydration at 110 C and 4.0 kPa Hydration at 40 C and 1.3 kPa

Mamani et al. (2020)

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FIGURE 14.9 Optimization methods for TES materials: (A) external parameters modification or (B) improvement on TES material. Illustrations: (A) Percentage mass changes due to dehydration/hydration reactions of potassium carnallite and derivatives (pH2O 5 25 kPa). (B) Assembly of lithium battery packs with composite material [42 wt.% bischofite 1 58 wt.% Mg(NO3)2  6H2O 1 expanded graphite (EG)]. (A) From Gutierrez, A., Ushak, S., and Linder, M., 2018. High carnallite-bearing material for thermochemical energy storage: thermophysical characterization. ACS Sustain. Chem. Eng. 6(5), ´ 61356145. (B) From Galazutdinova, Y., Al-Hallaj, S., Grageda, M., and Ushak, S., 2020. Development of the inorganic composite phase change materials for passive thermal management of Liion batteries: material characterization. Int. J. Energy Res. 44(3), 20112022.

Two possible solutions to overcome subcooling are mixing nucleation agents with storage materials and cold finger technique (external tool kept at a temperature below subcooling), helping nucleation process by crystals creation (Mohamed et al., 2017). Addition of nucleating agents, such as nanoparticles, is performed by seven primary techniques separated into two groups, aqueous stirring and dry blending. First group includes classic sonication, sonication microprobe, magnetic stirring and mechanical agitation. Second group contains hand-mix, planetary ball mill and freezer/mill methods (Wong-Pinto et al., 2020). Therefore, numerous approaches are available in order to obtain a high level homogeneity in mixing nucleating agents with TES materials. Low contents of LiCl in bischofite has been shown to decrease its phase change point, which not only might allow use of this material in LHS at lower temperature (Ushak et al., 2016a), but could directly impact on subcooling degree. In fact, bischofite subcooling has been reduced from 36 C to 0 C and 1.7 C by adding Sr(OH)2 and SrCO3, respectively. Authors proved that crystal structure lattice parameters (cell longitude or angle) must be considered when selecting a nucleating agent for a salt hydrate (Ushak et al., 2016b). Adding 5% PEG 2000 to bischofite improves PCM cycling stability without affecting other thermal properties, such as Cp and ΔH, but subcooling issue is not eliminated (Gutierrez et al., 2015). A study demonstrating improvements in cyclic stability of synthetic magnesium chloride hexahydrate, MgCl2  6H2O, considered as TCM, was

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performed by Zhang et al. (2019). In this study, hydrolysis of salt hydrate during hydration reaction was inhibited preparing a mixture of magnesium chloride hexahydrate and ammonium chloride (1:1), forming dehydrated ammonium carnallite (below 300 C) and the conversion of hydrolysate (above 400 C) (Zhang et al., 2019). Therefore, bischofite, where the main component is MgCl2  6H2O, could be further optimized by adding NH4Cl and used as low-cost TCM. Another example of improvement of cycling stability of TCM by adding of NaCl was reported by Mamani et al. (2020). This research showed that, reaction reversibility of carnallite was improved by adding 23.04 wt.% NaCl, and no sign of decomposition by hydrolysis was observed (Mamani et al., 2020).

14.5.3.3 Graphite, enhancing thermal conductivity Thermal conductivity is generally enhanced by increasing TES material heat transfer area, by creating a composite material with TES and another one with high thermal conductivity, like graphite, by using fins or by encapsulation of the TES material (Mohamed et al., 2017). An inorganic mixture of 40 wt.% bischofite (MgCl2  6H2O) and 60 wt.% Mg(NO3)2  6H2O and two types of EG as flakes and matrix have been used to obtain different composite materials by multiple methods: direct blending, ultrasonic impregnation, immersion and vacuum impregnation (Galazutdinova et al., 2018). In this work, not only thermal conductivity was improved, but also subcooling phenomena was reduced by EG addition. Recently, thermal conductivity of another mixture, based on bischofite (41.3 wt.% bischofite 1 58.7 wt.% Mg (NO3)2  6H2O) was enhanced by an impregnation method with EG matrix, resulting in a composite with a melting point of 53 C and latent heat around 100 C (Galazutdinova et al., 2020). Moreover, direction-dependency of thermal properties of final materials in PCM penetration rate into EG pores, was determined. Results showed higher thermal conductivity for in-plane direction (cut in parallel to the graphite compaction layers) than that for throughplane direction (cut done perpendicularly). Authors explained that in future works, arrangement of PCMEG composites are planned to be included into lithium-ion battery packs to evaluate hitches of corrosion and possible shortcircuiting, as well as electrical cycling studies and nail penetration tests for inflammability control (Galazutdinova et al., 2020). Additionally, EG is available commercially or is obtained from graphite by simple procedures. Therefore, resulting in a cheaper support porous material, compared with worm-like structure and nano-clay like. Finally, EG shows high electrical, mechanical and thermal properties, making it a relevant material to improve TES material performance. Several pathways to improve wastes and by-products performance as TES materials have been presented, depending on a specific property that requires enhancement. Selection of available materials from non-metallic industry, supported by a proper enhancing method, could guarantee

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expansion of solar technologies in an economic and sustainable pathway, bounded to policies to be further implemented around the world, being essential to decrease CO2 emissions.

14.6 Conclusion America has always been characterized by on the weal of natural resources. When looking at the central-west zone of South America, high solar radiation represents an opportunity for thermal storage materials application. Moreover, the existence of areas of high levels of mineral deposits and nonmetallic mining industry provide low-cost materials that have been proposed for thermal energy storage for several applications, such as CSP plants, solar collectors, buildings, and thermal control devices. Among discussed materials, astrakanite and kainite show potential for SHS low-temperature applications. One of the most promising low-cost TES materials based on inorganic salt wastes is bischofite. It precipitates on solar evaporation ponds during lithium brine concentration with MgCl2  6H2O as major component. This mineral has been analyzed for three thermal energy storage processes (SHS, LHS, and TCS), showing promising results for sensible and LHS systems. Potassium carnallite-waste with high content of KCl  MgCl2  6H2O constitutes as a better candidate to be applied as TCM. However, to apply these waste salts as TES materials, improvements of their thermophysical properties are required, especially due to some important drawbacks, such as subcooling, relatively low thermal conductivity, and low reversibility, among others. In this regard, encapsulation, addition of nucleating agents and development of graphite-based composites are the most promising techniques to improve waste salts thermal performance as TES materials. As discussed in this chapter, a great opportunity opens up for research and development of technologies based on use of these abundant waste materials resulting from non-metallic mining activity. These technologies might reduce installation and maintenance costs of solar systems and/or thermal energy storage systems as well as becoming an important contribution to lowering mining industry environmental impact and favoring alternative and more ecofriendly solar energy technologies.

References Abedin, H., Rosen, M., 2011. A critical review of thermochemical energy storage systems. Open Renew. Energy J. 4 (1), 4246. Almendros-Ib´an˜ez, J.A., Fern´andez-Torrijos, M., D´ıaz-Heras, M., Belmonte, J.F., Sobrino, C., 2019. A review of solar thermal energy storage in beds of particles: packed and fluidized beds. Sol. Energy 192, 193237. Alva, G., Liu, L., Huang, X., Fang, G., 2017. Thermal energy storage materials and systems for solar energy applications. Renew. Sustain. Energy Rev. 68, 693706.

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Chapter 15

Nanoencapsulated phase change materials for solar thermal energy storage Jyoti Saroha1,2, Sonali Mehra1,2, Mahesh Kumar1,2, Velumani Subramaniam3 and Shailesh Narain Sharma1,2 1

Council of Scientific and Industrial Research (CSIR)-National Physical Laboratory (NPL), New Delhi, India, 2Academy of Scientific and Innovative Research (AcSIR), Ghaziabad, Uttar Pradesh, India, 3Department of Electrical Engineering (SEES), Centro de Investigacio´n y de Estudios Avanzados del IPN (CINVESTAV-IPN), Mexico City, Mexico

15.1 Introduction The rapid increase in greenhouse gas emissions from fossil fuels and the associated high prices remain among the major contributing factors for the need of alternative and environmentally friendly renewable energy sources (Hasnain, 1998). As the energy requirements in different parts of the globe continues to increase daily, the dependence on fossil fuels (e.g., oil, coal, natural gas) or nonrenewable resources will create a huge crisis in the future (Anisur et al., 2013; Mofijur et al., 2019). Some of the emitted toxic gases include CO2, CO, sulfur dioxide, lead, nitrogen oxide, and even radioactive materials, which are very harmful to humans and the Earth’s atmosphere (Gorle et al., 2016). Another challenge is the limited availability of these fossil fuels and nonrenewable energy sources for the increasing population. Hence, researchers are working to find alternative and renewable sources of energy that fulfill people’s needs without any hazard to humans, and solar energy promises to be an effective and prospective renewable source. However, solar radiation has a low density on the earth’s surface with variation in different times and seasons, making limited sunlight available during certain hours of the day, especially during the rainy season (Farid et al., 2003). Consequently, there is a need to find a method and technology to efficiently store thermal energy during periods of bright sunshine, preserved and later released for further utilization during the night or other periods. The thermal energy storage (TES) systems offer a broad range of engineering Sustainable Material Solutions for Solar Energy Technologies. DOI: https://doi.org/10.1016/B978-0-12-821592-0.00014-5 © 2021 Elsevier Inc. All rights reserved. 467

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applications and an important energy utilization process (Gurulingam and Alagumurthi, 2011) whose implementation helps to maximize energy consumption and ease to utilize clean and renewable energy sources, for example, solar energy (Lin et al., 2018). The need for maximally tapping and storing solar energy for use at night and off-peak periods has increased TES’s demand, which required the development of energy storage devices to meet the demand and supply of renewable sources of energy (Sharma et al., 2007). Therefore it is a task for the researchers to find suitable energy storage options and optimized materials for energy storage. In addition to minimizing the inconsistency between supply and utilization, the materials will also improve thermal energy systems’ performance and reliability, leading to energy conservation (Liu et al., 2016). TES depicts the technological systems that store thermal energy by heating and cooling some storage medium for later use, described by the basic types (FatihDemirbas, 2006): 1. Sensible heat storage (SHS) 2. Thermochemical storage (TCS) 3. Latent heat storage (LHS) The diagram (Fig. 15.1) describes the classification of TES systems (Ioan Sarbu and Sebarchievici, 2018). 1. SHS SHS systems operate with the heat capacity and the change in temperature of the solid or liquid material being heated during charging or cooled during discharging to store the thermal energy without any change in its phase storage medium. The storage material’s temperature increases when energy is absorbed and decreases when extracted, and the storage temperature varies according to the amount of energy stored (Sharma et al., 2007). Examples of storage materials include molten salts, water,

Thermal energy storage(TES) Sensible heat storage (SHS) Liquid

Latent heat storage (LHS)

Chemical heat storage (CHS)

Solid Liquid-solid Liquid-gas Solid-solid

Molten salts Water Rocks/sand Metals

Hydroxides Granite Ceramics

PCMs H2 Gas Gasoline Fossil Gas H2 liquid (Oxidation) (Oxidation)

FIGURE 15.1 Types of thermal energy storage systems (Cabeza et al., 2015; Rathod and Banerjee, 2012; Riffat et al., 2013; Mishra et al., 2015).

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rocks, oils, and metals (Cabeza et al., 2015). The amount of energy stored by the material is related to the temperature changes by the equation: ð T2 E5m Cp dT ð15:1Þ T1

where Cp gives the specific heat at constant pressure, m is the mass of the material, and T1 and T2 represent the initial and final temperatures, respectively. An alluring feature of the SHS systems is the complete reversibility of charging and discharging operations during the storage’s life-span for an unlimited number of cycles (Sharma et al., 2007). 2. TCS Thermochemical energy storage employs reversible chemical reactions to store energy through the formation and dissociation of chemical bonds (Cabeza et al., 2015). These include reversible endothermic chemical reactions, where energy can be stored and then released back reversibly without any energy losses. The storage mechanisms range from physical adsorption to reversible chemical reactions, and therefore several storage materials are available. The system reportedly yields higher storage capacity with minor thermal losses during the storage period. They provide high storage densities with minor thermal losses, making the systems attractive for lowtemperature long-term storage and high-temperature storage. Examples of some prominent working materials include iron carbonate, iron hydroxide, silicon oxide, calcium sulfate, and some working pairs of materials considered and incorporated include silica gel/water, magnesium sulfate/water, lithium bromide/water, lithium chloride/water, and NaOH/water. 3. LHS LHS uses the phase transition of the material whereby the temperature remains constant during releasing or absorbing heat or energy by a chemical substance or a thermodynamic system during phase change (Ioan Sarbu and Sebarchievici, 2018). The systems commonly use the solid-liquid phase change by melting and solidifying a material whereby heat is transferred to the material during melting, storing large amounts of heat at constant temperature and the heat released when the material solidifies. When the system absorbs energy (endothermic process) during state change from solid to liquid to gas, the change is exothermic (the process releases energy) in the opposite direction. It produces a change in phase of the substance (from solid to liquid or vapor, from liquid to vapor/from vapor to liquid or solid, from liquid to solid) without any change of temperature (Rathod and Banerjee, 2012). The amount of heat, in this case, is measured by (Riffat et al., 2013) ΔQ 5 mL

ð15:2Þ

m represents the mass of the substance, and L is the energy absorbed/given out per unit mass, called latent heat of fusion/solidification (solid to liquid

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changes), latent heat of sublimation (solid to gas changes), and latent heat of vaporization/condensation (liquid to gas changes). LHS systems offer an attraction and comparative advantage over the other TES methods due to the high-energy storage at a constant temperature corresponding to the storage material’s phase transition temperature. Phase change materials (PCMs) used in LHS systems absorb or release large amounts of latent heat when the materials change from solid to liquid or vice versa, with small temperature variations for different applications. PCM have a high heat of fusion and can store or release energy or heat by melting and solidifying at a certain temperature (Ioan Sarbu and Sebarchievici, 2018). Heat can be absorbed or released when the material changes its state from one (solid) to other (liquid) or vice versa. The recent development of encapsulation of PCMs serves as an effective solution for the above problems inherent in inorganic and organic PCMs. The encapsulation process ensures that the liquid PCM does not leak out of the granulate, and the polymer or inorganic shell can form macro-, micro-, and nanoencapsulated PCMs. Some considerations for the choice of solidliquid PCM for heat storage in PCMs instead of liquidgas phase change for heat storage include (Cabeza et al., 2015; Rathod and Banerjee, 2012): 1. the requirement of very high pressure to store heat in the gas phase; 2. the large volume needed to store material when it stays in the gas phase; 3. a higher heat of transformation. For comparison, the same amount of stored thermal energy, an ice storage unit would require eight times less volume than a typical water storage unit storing with a 10 C temperature change. Some properties of PCM include (Cabeza et al., 2015; Rathod and Banerjee, 2012; Mishra et al., 2015): G G

G

High latent heat of fusion Provides good stability after many phase changes cycles Easily customized for desired temperature range

G

G

G G

Reversibility over phase changes Nontoxic and noncombustible Easy to handle Noncorrosive

Table 15.1 presents the classifications and properties of the PCMs (Sharma et al., 2007; Liu et al., 2016; FatihDemirbas, 2006; Ioan Sarbu and Sebarchievici, 2018; Mishra et al., 2015):

15.1.1 Selection criteria of phase change materials The selection of PCMs is vital in designing the LHS systems as the physical properties can influence the systems’ efficiency and applications. Although

TABLE 15.1 Different types of phase change materials. S. no.

Types of PCM

Occurrence/constitution

Applications

1

Organic Paraffin (CnH2n12)

Derived from petroleum and have waxy consistency at room temperature and melting temperature ranges between 28 C and 40 C.

G G

G G

2

Inorganic Salt hydrates (MnH2O)

Consist of inorganic salts and water

G G

G

Advantaged

TES Passive storage in bioclimatic building using HDPE Solar power plants Space craft thermal system

G

TES Cooling: food beverages, coffee, wine, milk products Passive storage in bioclimatic building

G

G

G

Disadvantages

Good thermal storage capacity and freeze without super cooling Chemical stability over many heating and freezing cycles Noncorrosive and compatible with most encapsulation material

G

Low material costs, high LHS capacity, precise melting point, high thermal conductivity and inflammability

G

G G

G

G

G

Limited range of melting points Cost is linked to unstable prices of petroleum Several paraffins are hazardous to health and environment and may cause narcotic effects (if inhaled), for example, Hexadecane Volume change in solid/liquid phase change hydrate is up to 10%, which require special packaging to accommodate the changing volume Poor nucleating properties make them vulnerable to super cooling but in some cases nucleating agents must be added Cannot recrystallize completely and thus lose all latent heat capacity Toxic and corrosive to metals

(Continued )

TABLE 15.1 (Continued) S. no.

Types of PCM

Occurrence/constitution

Applications

3

Hygroscopic materials Wool insulation, earth/clay render finishes and waterbased ice & gel packs

Known for keeping materials cold around 0 C

G

Biobased Fatty acids [CH3(CH2)2nCOOH]

Organic compounds derived from fatty acids, animal fat & plant oils and melt point temperatures range between 240 C and 151 C.

4

Advantaged G

G

TES Medical application such as transportation of blood, operating tables, hot-cold therapies. Textiles

G

TES

G

G

G G G G

G

G G

G G G

LHS, latent heat storage; PCM, phase change materials; TES, thermal energy storage.

Disadvantages

Nontoxic Low-cost Nonflammable Environmental friendly and Easy to use

Useful only in applications required a temperature of 0 C and vulnerable to microbial growth

Nontoxic Experience minimal volume change between phases Stable Higher efficiency than salt hydrates and petroleum-based PCMs Last for decades High latent heat Fire-resistant and cheaper than petroleum-based PCMs

Not suitable for eutectics and materials other than fatty acids

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no material may possess the perfect combination of properties profiles to yield an ideal efficiency in the performance of 100%, the chosen materials yield unique thermal boundaries and operation goals. Furthermore, a combination of PCMs may offer additional advantages to single ones in matching TES systems’ thermal conditions and requirements. The summarized properties serving as criteria for selecting materials include (Lin et al., 2018; Sharma et al., 2007; Liu et al., 2016; FatihDemirbas, 2006; Ioan Sarbu and Sebarchievici, 2018; Cabeza et al., 2015; Rathod and Banerjee, 2012; Riffat et al., 2013; Zalba et al., 2003): 1. Thermodynamic properties a. Desired operating range of melting temperature b. High latent heat of fusion per unit volume c. High specific heat and density d. Small volume changes on phase transformation and small vapor pressure at operating temperatures to reduce the contamination problem e. Congruent melting 2. Kinetic properties a. High nucleation rate to avoid supercooling of the liquid phase b. A high rate of crystal growth so that the system can meet demands of heat recovery from the storage system 3. Chemical properties a. Chemically stable b. Reversibility in freezing/melting cycles c. Very less degradation after a large number of freezing/melting cycles d. Noncorrosiveness, nontoxic, nonflammable, and nonexplosive materials 4. Economic properties a. Cost-effective b. Easily available

15.1.2 Working principle of phase change material TES systems operate based on the difference between two systems’ temperatures to store or release heat. The PCM plays an important role in the dynamic thermal and moisture buffering process, extending the desired temperature duration to last for a definite period. Hence, thermal properties are very important during phase change because once the PCM fabric’s temperature goes out of the phase change range, it is no longer effective as active thermal wear. Fig. 15.2A shows the flow of heat in PCM; initially, when the PCM is at low temperature, it absorbs heat from its surroundings or a heat source, and then it remains heat stored in PCM. Later on, when desired, the stored heat gets released from PCM by providing temperature difference or

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FIGURE 15.2 Schematic of the working principle of phase change materials; (A) flow of heat; (B) graph temperature versus energy content (Riffat et al., 2013; Mishra et al., 2015).

release after some time. The principles involved may be summarized by the steps: 1. at a certain constant temperature, the heat absorption melts the material; 2. the material absorbs significant amounts of heat from the environment during melting, causing the environment to become cooler; 3. when the temperature drops, the material solidifies and releases heat to the environment, making it warmer; 4. storing the PCMs in insulated buffers allows the “latent heat or cold” for later reuse. Fig. 15.2B presents the phase change processes of PCM graphically. It represents a system of solid-liquid PCM. At the initial stage, PCM is in the solid-state, and it starts absorbing heat (sensible heat) slowly as and when the temperature of its surroundings increases. After some time, the temperature of the surrounding becomes equal to that of the melting point of PCM (melting process), then it reaches the maxima where PCM absorbs maximum heat at a constant temperature. Heat stored at this temperature is in the form of latent heat. This process continues until PCM melts completely into a liquid state, allowing the PCM to maintain an optimum temperature. Again, when the temperature of the surrounding drops, PCM starts releasing stored energy and again returns to the solid phase (crystallization process) (Mishra et al., 2015; Zalba et al., 2003; Anusha, 2016).

15.1.3 Encapsulation in phase change materials As described earlier, PCMs change in phase with temperature gradients, which happens due to an increment in the atom’s mobility in segments (Ramesh Babu and Arunraj, 2018). However, for specific thermal storage applications, PCM needs to change its states several times, leading to a decrease in efficiency due to thermal cycles’ repetition. Therefore a

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protective layer of polymer or different material is formed outside the PCM, making it suitable for different applications and improving efficiency and durability. Thus encapsulation is a synthesis process where a material or compound gets enclosed by a shielding layer of a different material (Biswas, 2016). The inner material is the core, while the outside layer is called a shell (or membrane or shield). The resulting product obtained after encapsulation is known as core-shell material or capsules. The commonly used encapsulation methods (Table 15.2) and processes used for PCMs are: 1. Microencapsulation The material’s encapsulation process in this method has one or two solid or liquid PCMs acting as a core and surrounded by a protective polymer shell or coating in size range of 1100 μm resulting in the production of microcapsules. For example, paraffin can be microencapsulated with various polymers of μm range, while it is not possible to use salt-hydrates for the microencapsulation (Mondal, 2008; Salunkhe and Shembekar, 2012). 2. Nanoencapsulation The nanoencapsulation process is similar to microencapsulation, except in this process, the shell’s size lies in the nanoscale range (less than 1 μm) (Iqbal et al., 2019) (Fig. 15.3).

TABLE 15.2 Types of encapsulation (micro or nano) methods (Ramesh Babu and Arunraj, 2018). S. no.

Chemical methods

Physical methods

Physicomechanical methods

1

Polymerization (emulsion, interfacial, in situ, mini-emulsion)

Sol-gel encapsulation

Spray-drying

2

Suspension

Coacervation

Multiple nozzle spraying

3

Polycondensation

Layer by layer assembly

Fluid bed coating

4

Dispersion

Supercritical method

Centrifugal techniques

5

Vacuum encapsulation

6

Electrostatic encapsulation

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Core (PCM)

+ PCM

Shell (NPs) Nanoparticles

Core-shell

FIGURE 15.3 Schematic of micro or nanoencapsulation of phase change materials.

15.1.4 Advantages of micro or nanoencapsulation of phase change material Many studies focused on enhancing the thermal conductivity of PCMs by nanoencapsulation methods since they predicted a higher heat release rate of nanoPCMs compared to conventional PCMs. They also present the potential for diverse thermal storage applications of nano-PCMs (Mondal, 2008; Salunkhe and Shembekar, 2012). Some of the advantages enumerated below include: 1. 2. 3. 4.

reduces reaction ability with surrounding material; provides stability and durability; eliminates the problem of change in volume while the phase changes; enhancement in thermal properties (e.g., heat transfer, thermal conductivity, heat storage).

15.2 Brief review of the work done The numerous advantages and potentials of PCMs have attracted interest from several researchers intending to improve TES systems’ efficiency by improving the PCM properties. Lin et al. briefly reported increased thermal conductivity by adding high thermal conductivity filters and encapsulated PCMs using graphite, CNTs, carbon fibers, metal oxide, and metal foams as C-based and metal-based filters. The C-based additives are more prominent because of their better density and stability than oxide-based additives. In core-shell encapsulated PCMs, the shell prevents the PCM from damage and leakage, thus improving the thermal conductivity with an extra advantage leading to increased heat transfer rate (Lin et al., 2018). Fang et al. explained the methods for preparing nanocapsules, which can be used in PCM using in situ polymerization methods. N-tetradecane served as core material and urea or formaldehyde as shell material, focusing on the size of nanocapsules, which helps in LHS. Due to N-tetradecane’s high thermal stability, encapsulation is easier with different concentrations of NaCl by mass, and the encapsulated as-synthesized NPs used for various heat storage applications (Fang et al., 2009). Ho et al. studied the changes in latent heat of fusion, density, thermal conductivity, and dynamic viscosity of nanoparticles implanted with PCM where nanoparticles of Al2O3 were emulsified in n-octadecane (paraffin)

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using the emulsion technique. Studies on the different weight concentrations of NPs in PCMs (5 and 10wt.%) thermophysical properties showed nonlinear changes in melting and freezing points. Changes in thermal properties, for example, conductivity and dynamic viscosity, compared to pure paraffin, showed they could be very effective for heat storage applications (Ho and Gao, 2009). Murugan et al. prepared nanoenhanced phase change materials (NPCM) using paraffin as PCM and multiwalled carbon nanotubes (MWCNTs) as nanoparticles and experimentally studied their behavior during the melting & solidification process. Paraffin/MWCNTs NPCM were prepared by two-step method using different volume % as 0.3%, 0.6%, 0.9% of MWCNTs in PCM. Latent heat measurements made using Differential Scanning Calorimetry (DSC) disclosed a maximum increase of 12.5% and 8% in latent heat of melting and solidification for 0.9% compared to bare PCM. After adding MWCNTs, the melting onset temperature decreases, and solidification onset temperature increases while the maximum reduction was approx. 30% in case of 0.3wt.% (Murugan et al., 2018). Zhao et al. experimentally investigated the melting and solidification process, observing a significant increase in heat transfer due to metal foams in paraffin. The heating rate can also increase 310 times depending on the metal foam’s porosity and pore density. In the solidification process, faster cooling was observed due to natural and forced convection conditions (Zhao et al., 2010). Darzi et al. studied numerically the heating and cooling rate for different configurations of two concentric annuli in which PCM filled the inner ring and the outer ring with nanoparticles. They used different volume percent of copper NPs encapsulated with PCM and examined the heat flow rate after adding N-eicosane (PCM), observing that natural convection plays an important role during the melting process. The rate of melting is higher at the top section of the annulus compared to the lower section. However, with the increase in the volume percent of NPs, the melting and solidification rate also increased, which was more efficient during solidification because of the suppression of natural convection during the melting process (Ali Rabienataj Darzi et al., 2016) (Fig. 15.4). Liu et al. reviewed the enhancement in the efficiency of solar energy utilization via microencapsulation of PCM. The composites prepared contained highly conductive graphene nanosheets and microcapsules of N-eicosane (PCM) as core and brookite TiO2 as the shell. The graphene nanosheets formed another shell around the microcapsule with H-bonding, increasing the microcapsule’s stability, durability, and solar photocatalytic activity. The results showed that incorporating 5wt.% of graphene sheets leads to an increase in thermal conductivity from 0.64 to 0.98 W/m/K. Thus the composite proved a potential material for several applications such as solar thermal storage, detoxification of water, and solar photodegradation (Liu et al., 2017).

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(A)

(B)

(C)

(D)

FIGURE 15.4 Schematic of different configurations; (A) circular shaped tube (B) elliptical tube (vertical) (C) elliptical tube (horizontal) (D) circular tube (finned).

Gupta et al. analyzed the behavior of PCM nanocomposite prepared by magnesium nitrate hexahydrate (MNH) (an inorganic salt hydrate PCM) dispersed with metal (Fe and Cu) nanoparticles using melt blending techniques with characterizations to study the chemical and thermal behavior of bare PCM and PCM-nanocomposite. The studies showed that thermal conductivity increased from 0.4 to 0.61 W/m/K and 0.63 W/m/K after adding 0.5wt.% of Fe and Cu NPs into MNH (PCM), respectively. Also, charging and discharging rates increased from 7.8% and 35%, and 5.6% & 30% for Fe and Cu-nanocomposites, respectively. The study showed that metal nanoparticles lead to an efficient increase in thermal conductivity, which is very beneficial for TES and other applications (Gupta et al., 2020). Salunke et al. studied PCM’s encapsulation thoroughly, reviewing the nano- and microencapsulation of PCM and discussed important properties of the core-shell materials. The study showed that the combination of organic core PCM and inorganic shell provided the best mechanical and thermal stability while other factors such as shell size, material type, shell thickness, geometry, and core-to-coating ratio play significant roles in providing overall stability to the core-shell particles. However, a higher core-to-coating ratio makes the shell thin, which decreases the stability and lower core-to-coating ratio resulting in less amount of PCM, and decreases the heat storage capacity. Further studies on the melting, solidification, and thermal conductivity of core-shell PCMs showed that melting and solidification dominated heat transfer processes through natural convection and conduction, respectively. In microcapsules, the results showed that microencapsulated phase change slurry (PCS) plays a vital role in transferring heat between PCM and heat transfer fluid (HTF) by making surface contacts leading to enhancement in the thermal conductivity. Also, the high temperature of HTF and high

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thermal conductivity of shell surrounding PCM result in faster melting of PCM core with increased efficiency of PCM core-shell nanoparticles (Salunkhe and Shembekar, 2012). Saw et al. investigated the performance of the solar radiation collector with PCM and nanoencapsulated PCM. They used paraffin as PCM, and copper nanoparticles were added to paraffin to enhance its properties in three different conditions- no PCM, bare PCM, and nano-PCM. The experimental conditions included an inclination angle of 10 C with a flow rate of 0.5 kg/min, and the observed water temperatures were 40.2 C, 40.8 C, and 35.2 C for PCM, Nano-PCM, and without PCM, respectively. The solar collector system’s efficiency reportedly increased to 6.9% and 8.4% using PCM and Nano-PCM (Saw et al., 2013.). Kaviarasu et al. reviewed the changes in PCM’s thermal properties after incorporating nanoparticles (Al, Cu, SiO2, TiO2, carbon nanotubes, carbon nanofibers, Al2O3, NaOH, KaOH) in PCM (water, paraffin, salt hydrates and bio-based) for different engineering applications. The results showed that nano-PCM shows good results for TES applications, but with a high concentration of nanoparticles, viscosity increases, which is not viable and thus the need to optimize the volume% of NPs. For microelectronics, nanofluid and nanocomposites acted as the coolants, reducing the overheating problem in electronic systems. Also, micro or nanoencapsulated PCMs are very useful for buildings and textiles applications (Kaviarasu and Prakash, 2016) (Table 15.3).

15.3 Results and discussion Previous literature showed that commonly used PCMs are paraffin wax, sugar alcohols, salt hydrates, and paraffin wax used for various applications as it has a wide melting temperature range. Our group in CSIR-NPL procured commercial paraffin wax of different compositions for PCM applications for the present study. Paraffin wax used as PCM faces several problems, including low thermal conductivity, and we will synthesize different NPs [Titanium dioxide (TiO2), silver (Ag), gold (Au)] for incorporation with paraffin wax depending on their melting properties, which helps to increase the durability and thermal conductivity of PCMs and hence TES capacity will also be enhanced. The salt hydrates are available commercially and may be synthesized too with some salt hydrate-nanomaterial composite mixture, mixed with cement and other building materials in buildings’ ceilings. Similarly, the synthesized PCM (paraffin)-TiO2 nanocomposites can be incorporated in textiles during textile designing to maintain the different textiles’ temperature according to the requirements (Figs. 15.5 and 15.6). We performed several experiments using bare Paraffin (PCM) and paraffin incorporated with TiO2 nanoparticles. The DSC characterization gives an idea about the accurate melting temperature of the bare and PCM

TABLE 15.3 Brief literature survey on phase change materials s encapsulated with nanoparticles for thermal energy storage. PCMs

Nanoparticles

Paraffin

G G G

Cu Al C/Cu

Results/conclusions

Applications

Reference

Paraffin/Cu showed good thermal stability as compare to Al &C/Cu Enhancement in thermal conductivity Heat transfer rate increased on addition of NPs For 1 wt.% heating and cooling time decreased by 30.3% and 28.2%, respectively

LHS

Wu et al. (2010)

G

Thermal conductivity increased by 2.5 times

LHS

Yavari et al. (2011)

G

Studied with different wt.% NPs Maximum increment in thermal conductivity was 47.85% for 3wt.% Maximum Latent Heat was 165.1 & 167 j/g for 1% & 3% Thermal stability increased on addition of surfactant

TES

Sami et al. (2017)

Thermal conductivity increased 5 times

Recovery of solar heat and industrial waste

Karthik et al. (2015)

G

G G G

1-octadecanol (stearyl -alcohol)

Graphene (nanosheets)

Paraffin

G

TiO2

G

G

G

Erythriol (sugar alcohol)

G

Graphite Foam

G

Myristic acid

G

Silver

Improved thermal properties with different concentration of NPs

Heat storage applications

Sivasamy et al. (2019)

Laurie acid (LA)

G

Mesoporous silica nanomicrospheres (MSMN) Activated carbon (AC)

Comparison study: Enthalpy of LA/MSNM was higher than that of LA/ AC G LA/MSNM better for thermal energy storage.

TES

Zhang et al. (2019)

4.5% Increment in thermal conductivity

Thermal storage applications

Zeng et al. (2010)

G

1-Tetradecanol

Ag Nanowires

G

Water, ethylene glycol, engine oil, vacuum pump fluid

G G

Al2O3 CuO

G

G

G

G

Paraffin wax (PW) Oleic acid

G G

Bulk graphite Nanographite

G G G G

OM-35 Eicosane

G

G

G

Paraffin wax

G

Silver nanobase organic ester

G

G

Thermal conductivity (TC) increases with decrease in particle size Increase in case of ethylene glycol and engine oil was highest than pump fluid and water TC increases 26% and 40% on addition of Al2O3NPs in Ethylene Glycol by volume fraction 5% and 8%. Increase in TC depends on dispersion techniques

Wang and Xu (1999)

Melting rate decreases Ultrafast thermal charging High light to heat conversion Enhancement in thermal conductivity after adding nanographite and bulk graphite in paraffin from 0.182 to 0.662 W/m/K & 0.286 W/m/K respectively

Solar water heating system ultrafast solar chargeable gloves

Shankara Narayanan et al. (2016)

Om-35 is better than eicosane in performance and cost Heat reduction in 24 h is 16% and 24% for single slot and for double slot of bricks PCM thickness of 11.3 cm can be able to reduce temperature up to 6 C

Passive conditioning in buildings

Saxena et al. (2020)

Freezing and melting improved 41% and 45.6% respectively Thermal conductivity increase from 0.257 to 0.284 2 0.765 with incorporation of 0.15wt.%

Heat storage

Parameshwaran et al. (2013)

(Continued )

TABLE 15.3 (Continued) PCMs

Nanoparticles

Paraffin wax

G G

G G

Carbon fibers Expanded Graphite Graphite foam High thermal conductive particles

Results/conclusions G G G

Reduces heat losses Improvement in efficiency of system Increment in thermal conductivity

LHS, latent heat storage; PCM, phase change materials; TES, thermal energy storage.

Applications

Reference

Solar dryers

Shalaby et al. (2014)

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Solid PCM taken in beaker

Addition of nanoparticles into molten PCM

Heating upto melting temp

Heating & stirring continuously

Monitoring temp. during experiment

Cooling upto

Nanocomposite (PCM+ NPs) formed

483

Homogeneous solution obtained

room temp

FIGURE 15.5 Method of preparation of phase change materials-nanocomposites. 75 70

Melting point ( oC)

Bare 3% 5% 20%

Heat flow [mW]

4 3 2 1 0 -1 -2 -3 -4 -5 -6 -7 -8 -9 -10 10

65 60 55 50 45

20

30

40

50

60

o

70

Temperature[ C]

80

90 100

0

2

4

6

8

10 12 14 16 18 20 22

Concentration(%)

FIGURE 15.6 Results of phase change materials-nanocomposites; (A) differential scanning calorimetry graph of bare Paraffin and different wt.% of nanocomposites; (B) Melting point versus TiO2 concentration graph.

nanocomposite and compared it with the commercial PCM’s reported values. We incorporated different wt.% of TiO2 NPs in PCM and then compared the change in melting temperature with an increase in the NPs concentration. The results showed that an increase in NPs concentration decreases the PCM nanocomposite’s melting temperature, as shown in the DSC results (Fig. 15.6A and Table 15.4). The plot of melting point versus TiO2 concentration (Fig. 15.6B) shows that melting temperature decreases linearly with an increase in the concentration of TiO2 NPs incorporated in PCM. Further, we are also working to evaluate the bare PCM and PCM nanocomposites’ thermal conductivity to comply with the increase in thermal conductivity of PCM encapsulated NPs. Moreover, an optimized concentration of TiO2 NPs encapsulated in PCM is currently being sought for the solar air heaters (SAH) application since PCM-TiO2 nanocomposites in SAHs reportedly improve thermal energy conservation.

15.4 Applications Nowadays, PCM has diverse TES applications, for example, SAHs, buildings, textiles, solar water heaters, and solar dryers. That plays a very

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TABLE 15.4 Measured value of melting temperature from differential scanning calorimetry for paraffin and paraffin-TiO2 nanocomposites. S. no.

Wt.(g) (phase change materials)

Wt.% (nanoparticles)

Melting point ( C) (differential scanning calorimetry)

1

2

0

70.00

2

2

3

69.10

3

2

5

67.58

4

2

20

50.00

important role in the day-to-day life of humankind. Our main focus is on three important applications in this chapter, that is, SAHs, buildings, and textiles. We will discuss how various researchers are working to implement the nanoencapsulated PCMs, their principle, and working for energy storage applications because energy storage improves energy systems’ performance that is a beneficial factor for energy conservation. A brief discussion, along with the working of nanoencapsulated PCMs in buildings, SAHs, and textiles, will be explained.

15.4.1 Need for phase change material-based solar air heaters Natural gas, coal, and wood, widely used for winter space heating (Begum et al., 2009; Colbeck et al., 2007), have inefficient combustion, thereby releasing carbon monoxide, smoke, and particulates that affect the indoor air quality and cause hazardous health problems (Fullerton et al., 2008). That suggests the urgent need for environmentally friendly and clean techniques because of health issues. In this regard, PCM-based SAH offers comfortable conditions inside rooms or houses during winter or cold places. It reduces the risks of air pollution inside the room caused by fossil fuel or biomass burning in combustion devices. Since solar energy is recurring in nature, an efficient and reliable (thermal) energy storage system becomes necessary to ensure continuous operation during nonsunshine hours and cloudy days.

15.4.1.1 Phase change materials in solar air heaters Here, the PCM energy storage unit for the solar-air heating system would be selected based on PCM’s melting point rather than its latent heat. The widely used paraffin wax is a cost-effective PCM with wider availability within different melting point ranges (Ghoneim, 1989) given in Table 15.5.

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TABLE 15.5 Melting temperature and latent heat of different types of paraffin wax. Paraffin wax

Latent heat (J/kg)

Melting temp ( C)

C36H74

223

7276

C32H66

261

6670

C36H62

249

5966

Ambient air

Solar air collector

PCM storage unit

Room FIGURE 15.7 Working of solar air heaters in sunshine hours.

Ambient air

PCM storage unit

Room

FIGURE 15.8 Working of solar air heaters in nonsunshine hours.

15.4.1.2 Construction and working principle of solar-air heating systems Solar-air Heating systems consist of solar air collector, storage unit, fan for forced ventilation through solar collector and storage unit, analyzing the air temperature primarily at storage unit outlet (Waqas and Kumar, 2013). The basic working of this setup is as follows: 1. During sunshine hours, the solar air collector heats the air, which is then delivered to the PCM storage unit and charges it (changing the storage material to liquid phase) and is then delivered to the room as given below in A (Fig. 15.7) 2. During nonsunshine hours, cool, ambient air is passed through the PCM storage unit, gets heated, and delivered to the space for conditioning. During this process, PCM is converted from a liquid phase to a solid phase, as shown below in B (Fig. 15.8). 1. Sunshine hours In sunshine hours from 8.00 in the morning till 4.00 in the evening, the storage unit is coupled with a solar air collector and operated at a higher airflow rate. During this period, the storage unit receives hot air from a solar

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air collector to change the PCM phase from solid to liquid. This stage is named “charging mode operation.” This mode analysis will provide information about the outlet air temperature from the solar collector, outlet air temperatures from the storage unit, and the liquid phase’s PCM percentage. 2. Nonsunshine hours In nonsunshine hours from 4:00 in the evening till 8:00 in the morning, air flows through the storage unit at a reduced flow rate compared to the charging mode operation. The cold air extracts heat from the PCM as it flows through the storage unit and the heated air at the exit of the storage unit used for space heating. It is known as the “comfort mode operation” of the storage unit. It tells about the outlet air temperature from the storage unit (comfort range of temperature) and PCM concentration in the solid phase.

15.4.1.3 Deliverables: Performance criteria for solar-air heating 1. The ability to work in nonsunshine hours in all weather conditions 2. Ease of mobility, accessibility, and affordability for rural people 3. Environment-friendly features 4. Ease of installation, operation maintenance, low maintenance costs, and safe handling 15.4.2 Need for phase change material-based building materials for rural houses In rural areas residential houses during peak summer or winter time, PCMs are helpful passive methods that absorb excess heat when the room is hot and release this stored heat when the room becomes cool, as explained in Fig. 15.2A. Similarly, in the winter time, PCM helps maintain the optimum room temperature and thus provides the comfort zone of human beings, especially for children and older adults, by reducing the alteration in the rate of internal air temperature. It also helps maintain the required temperature of indoor air for a longer period.

15.4.2.1 Phase change materials for building applications In general, while constructing buildings, paraffin should be used in walls, while salt hydrates used in ceilings as PCMs. Some of them are classified as follows: 1. PCM as middle layer in wall The different analyses involved different configurations of external building walls for typical building walls by changing various parameters such as the PCM layer location, ambient conditions (Barrientos et al., 2012), the

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orientation of walls, and PCM phase transition temperature. According to standard Spanish construction, this construction should consist of five layers: 1. 2. 3. 4. 5.

first cement layer (15 mm thick) thick brick wall (115 mm thick) insulation of PCM material between the layers of bricks (40 mm) a thin layer of bricks (40 mm) plaster layer placed on the interior of the building (15 mm)

There was no optimum reduction in the heat loss observed during winter, irrespective of the wall orientation or PCM transition temperature. In comparison, some modifications showed that heat absorbed during the summer period due to the elevated flux of solar radiation and air temperature in the room with PCM decreases to 4.2 C. The appropriate location for placement of PCM within the building premises depends on the resistance in between the PCM layer and the external boundary conditions, and the PCMnanoparticles composite also helps to reduce the energy usage in summer and winter and shift the peak electricity load in the summer (Mirzaei and Haghighat, 2012) as well as also enhances the natural convection in the room. (Fig. 15.9). 2. PCM act as internal layer in wall The thermodynamic models of building structures using PCMs studied the effect of PCMs on buildings’ energy performance (Huang et al., 2006) under different conditions. It compared the two rooms; in the first room with PCM using lateral walls and ceiling, while in the second room with no PCM. The results showed that the first room’s temperature (with PCM) was near to humans’ thermal comforts due to air temperature and the walls’ radiative effects. The new PCM with nanoparticle composite wall system (Diaconu

FIGURE 15.9 Schematic diagram of phase change materials (PCM) used in building walls; (A) a middle layer in wall; (B) Composites PCM wall system (as internal layer in wall) (Akeiber et al., 2016; Huang et al., 2006; Diaconu and Cruceru, 2010).

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and Cruceru, 2010) introduced in three functional layers (I, II, and III) performed these particular functions; 1. The outer layer (I) consists of a building material integrated with a PCM. 2. The internal layer (II) also consists of PCM. 3. The middle layer (III) consists of conventional thermal insulation. The results showed that the outer PCM layer prevents an excessive increase in the insulation interface temperature due to the variable melting temperature of incorporated PCMs.

15.4.2.2 Deliverables: performance criteria for phase change materials for building applications When integrated into buildings, PCM without proper encapsulation causes various problems such as leakage from the wall surface and chemical composition changes during humid conditions. Nanoencapsulated PCM’s incorporated with different building materials such as cement mixtures and mortars, wallboards, panels, and slabs would increase the building materials’ thermal capacity while there was a reduction in the compressive strength and density. 1. New buildings with light-weighted construction materials and structures built using low density encapsulated PCM’s to avoid temperature intolerance and environmental impacts like CO2 emission. 2. Different nanoparticles like CuO, Al2O3, and SiO2 dispersed in the binary base fluid ethylene glycol/water mixture acting as a PCS, increasing the heat transfer rate of PCM materials integrated with building materials. 3. In building heating systems, the same heat transfer rate, volume flow rate, mass flow rate, and pumping power decreased while using nanofluids phase-change slurries compared to conventional HTF.

15.4.3 Need for phase change material-based textiles Nowadays, many people work in different areas such as industries, mines, space, manufacturing labs, intensive hot or cold places. There is a huge temperature variation and to overcome this variation human needs smart textiles materials that help to change or maintain temperature under ambient conditions according to human comforts (Mondal, 2011). Also, military and navy people have to stay in extreme weather conditions during wars or other operations. They cannot carry a large number of clothes, so they need some lightweight, comfortable and thermoregulating suits that can prevent them from abrupt temperature conditions. To fulfill these needs of the human society, PCMs offer the best option used in textiles products, for example, clothing, insulated out-fits, footwear, sports-wear, sleeping bags, space-suits, lab

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coats, and other textile materials (Ramesh Babu and Arunraj, 2018; Biswas, 2016; Mondal, 2008; Salunkhe and Shembekar, 2012; Iqbal et al., 2019).

15.4.3.1 Phase change materials in textiles In textiles, the main PCM used are n-paraffin, hydrocarbons like hexadecane, heptadecane, octadecane, nonadecane, and eicosane. The melting temperature range of PCM used in textiles should lie within the ambient temperature or human comfort range, that is, 18 C35 C (Mondal, 2011) (Table 15.6). Researchers’ main problem in using PCMs for textiles is that they can melt on heating, and in the case of salt hydrates, they get mixed with water on washing, giving rise to the need for some improvements in PCM used in clothing and other textiles products. Hence, to overcome this problem, microencapsulation or nanoencapsulation is adapted. PCM serves as the core, and nanoparticles as the shell provide several protection layers over PCMs during thermal cycles. Some factors considered during encapsulation include: 1. 2. 3. 4. 5.

Appropriate core to shell ratio The thickness of the shell wall Size of material used for shell High thermal conductivity and thermal capacity Durability of material

TABLE 15.6 Commonly used phase change materials for textiles applications. S. no.

Phase change materials

Melting temperature ( C)

Crystallization temperature ( C)

References

1

Paraffin wax

18.0

10.0

Gopalakrishnan et al.

2

n-Heptadecane

22.9

16.5

Mondal (2008), Salunkhe and Shembekar (2012), Iqbal et al. (2019)

3

n-Octadecane

28.2

25.2

Mondal (2008)

4

n-Nonadecane

32.1

26.4

Ramesh Babu and Arunraj (2018)

5

n-Eicosane

36.1

30.4

Mondal (2008), Salunkhe and Shembekar (2012), Iqbal et al. (2019)

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These factors improve the stability and efficiency of thermoregulating PCMs.

15.5 Challenges ahead PCM in TES applications has been a major topic of discussion among researchers because of its durability and stability (Anisur et al., 2013). However, there are some major general challenges that need to be worked upon in a more practical way for energy conservation, such as: 1. Thermal properties: PCM’s thermal conductivity is a major issue that researchers face that needs improvement in recent years. Furthermore, this property improves with incorporating different nanoparticles and micro or nanoencapsulation of PCM using higher thermal conductive nanoparticles. Many researchers worked in this area and developed different nano-PCM composites and encapsulated PCM with improved thermal properties and storage capacity. However, no best combination of PCM-NPs for nanocomposite with an optimized concentration of NPs has been reported, leading to the fixed increment in thermal conductivity. Consequently, more research is still needed on PCM’s encapsulation with NPs to enhance thermal conductivity for energy storage applications. 2. Ineffective phase change properties: The phase change in PCM occurs due to the supercooling of PCMs. Because of this phenomenon, the thermal properties of PCMs get affected, as PCM is segregated in a liquid state and is not able to solidify to its original state properly, which causes inconsistency in phase change cycles. So, there is the need to overcome this problem of ineffective phase change in PCMs to store large amounts of latent heat required for various storage applications. 3. Cost: The cost of PCM is required to be manageable, and low-cost PCMs should be utilized on a commercial scale to make it a better and cheap storage material for various applications. Hence, despite the several sustainability advantages (Adesina, 2019) achieved with the help of PCMs, there is the need to overcome the several challenges allowing its universal acceptance and easy availability for commercial-scale applications.

15.6 Conclusions This chapter gives a brief idea about various PCMs, their properties, and the experimental results based on some commonly used PCMs. PCMs are the materials that provide an ecofriendly and efficient way to store the naturally available or waste energy or heat and to reuse it further for different applications. These PCMs have good storage ability but low thermal conductivity; thus different nanoparticles need to be incorporated with PCM to enhance

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thermal conductivity, capacity, and working efficiency. This chapter discussed different PCMs such as paraffin, sugar alcohol, and salt hydrates. Here, paraffin is the commonly used PCM for TES applications incorporated with various nanoparticles of high thermal conductivity. To date, there is no report published regarding the combination of nanoparticles encapsulated PCM that proves to be the best nanocomposites for TES applications. On the other hand, Nano-PCM technology is very beneficial for human comforts because it can be coated on the buildings’ walls or ceiling to maintain its ambient temperature. Many researchers were even working on developing PCM-bricks and other PCM-based construction materials for smart buildings with good thermoinsulation properties. These Nano-PCM can also be used in textile materials to manufacture sportswear, space clothes, insulating helmets, gloves, jackets, medical clothes, and lab coats.

Acknowledgments The authors would like to sincerely acknowledge the support of the Director CSIRNational Physical Laboratory, New Delhi, India, for this research work and provide the necessary characterization facilities. The author JS duly acknowledges the University Grants Commission for providing JRF fellowship, and author SM sincerely acknowledges the Department of Science and Technology for providing Women Scientist-A project grant (SR/WOS-A/CS-132/2018). There is no financial concern publishing this chapter.

References Adesina, A., 2019. Use of phase change materials in concrete: current challenges. Renew. Energy Environ. Sustain. 4, 9. Akeiber, H., Nejat, P., Majid, M.Z.A., Wahid, M.A., Jomehzadeh, F., Zeynali Famileh, I.Z., et al., 2016. A review on phase change material (PCM) for sustainable passive cooling in building envelopes. Renew. Sustain. Energy Rev. 60, 14701497 (2016). Ali Rabienataj Darzi, A., Jourabian, M., Farhadi, M., 2016. Melting and solidification of PCM enhanced by radial conductive fins and nanoparticles in cylindrical annulus. Energy Convers. Manage. 118, 253263. Anisur, M.R., Mahfuz, M.H., Kibria, M.A., Saidur, R., Metselaar, I.H.S.C., Mahlia, T.M.I., 2013. Curbing global warming with phase change materials for energy storage. Renew. Sustain. Energy Rev. 18, 2330 (2013). Anusha, A.S., 2016. Phase change materials. Int. J. Eng. Res. Gen. Sci. 4 (2), 20912730. Barrientos, M.A.I., Belmonte, J.F., Rodriguez-Sanchez, D., Molina, A.E., Ibanez, J.A.A., 2012. A numerical study of external building walls containing phase change materials (PCM). Appl. Therm. Eng. 47, 7385 (2012). Begum, B.A., Paul, S.K., Dildar Hossain, M., Biswas, S.K., Hopke, P.K., 2009. Indoor air pollution from particulate matter emissions in different households in rural areas of Bangladesh. Build. Environ. 44 (5), 898903. Biswas, K., 2016. Nano-Based PCMs for Building Energy Efficiency, pp. 132. Cabeza LF, Martorell I, Miro´ L, Fern´andez AI, Barreneche C. Introduction to thermal energy storage (TES) systems. In: Cabeza LF, editor. Adv. therm. energy storage syst.. Cambridge,

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United Kingdom: Elsevier; 2015. p. 128. Available from: https://doi.org/10.1533/ 9781782420965.1 Colbeck, I., Nasir, Z.A., Hasnain, S., Sultan, S., 2007. Indoor air quality at rural and urban sites in Pakistan. Water Air Soil. Pollut. Focus 8 (1), 6169. Diaconu, B.M., Cruceru, M., 2010. Novel concept of composite phase change material wall system for year-round thermal energy savings. Energy Build. 42 (10), 17591772. Fang, G., Li, H., Yanga, F., Liu, X., Wu, S., 2009. Preparation and characterization of nanoencapsulated n-tetradecane as phase change material for thermal energy storage. Chem. Eng. J. 153, 217221. Farid, M.M., Khudhair, A.M., Razack, S.A.K., Al-Hallaj, S., 2003. A review on phase change energy storage: materials and applications. Energy Convers. Manage. 45, 15971615. Fatih Demirbas, M., 2006. Thermal energy storage and phase change materials: an overview. Energy Sources B Econom. Plan. Policy 1, 8595. Fullerton, D., Bruce, N., Gordon, S., 2008. Indoor air pollution from biomass fuel smoke is a major health concern in the developing world. Trans. R Soc. Trop. Med. Hyg. 102 (9), 843851. Ghoneim, A.A., 1989. The effect of phase-change material properties on the performance of solar air-based heating systems. Sol. Energy 42 (6), 441447. Gorle, R.D., Wandhare, M.M., Khelkar, A.R., Bhoyar, A.S., Muley, A.S., 2016. Solar powered evaporative air cooler with cooling cabin for household food items. IOSR J. Mech. Civ. Eng. (IOSR-JMCE) 13 (2), 5356. Gupta, N., Kumar, A., Dhawan, S.K., Dhasmana, H., Kumar, A., Kumar, V., 2020. Material nanoparticles enhanced thermophysical properties of phase change materials for thermal energy storage, Mater. Today Proc. Gurulingam, S., Alagumurthi, N., 2011. Phase change materials for solar latent heat storage applications: a review. Elixir Therm. Eng. 33, 21792207. Hasnain, S.M., 1998. Review on sustainable thermal energy storage technologies, part I: heat storage materials and techniques. Energy Convers. Manage. 39 (11), 11271138. Ho, C.J., Gao, J.Y., 2009. Preparation and thermophysical properties of nanoparticle-in-paraffin emulsion as phase change material. Int. Commun. Heat Mass Transf. 36, 467470. Huang, M.J., Eames, P.C., Hewitt, N.J., 2006. The application of a validated numerical model to predict the energy conservation potential of using phase change materials in the fabric of a building. Sol. Energy Mater. Sol. Cell 90 (13), 19511960. Ioan Sarbu, I.D., Sebarchievici, C. 2018. A Comprehensive Review of Thermal Energy Storage. Sustainability 10, 191. Iqbal, K., Khan, A., Sun, D., Ashraf, M., Rehman, A., 2019. Phase change materials, their synthesis and application in textiles—a review. J. Textile Inst. 110 (4), 625638. Karthik, M., Faik, A., Blanco-Rodriguez, P., Rodriguez-Aseguinolaza, J., Aguanno, B.D., 2015. Preparation of erythritolgraphite foam phase change composite with enhanced thermal conductivity for thermal energy storage applications. Carbon 94, 266276. Kaviarasu, C., Prakash, D., 2016. Review on phase change materials with nanoparticle in engineering applications. Eng. Sci. Rev. 9 (4), 2636. Lin, Y., Jia, Y., Alva, G., Fang, G., 2018. Review on thermal conductivity enhancement, thermal properties and applications of phase change materials in thermal energy storage. Renew. Sustain. Energy Rev. 82, 27302742. Liu, L., Su, D., Tang, Y., Fang, G., 2016. Thermal conductivity enhancement of phase change materials for thermal energy storage: a review. Renew. Sustain. Energy Rev. 62, 305317.

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Liu, H., Wang, X., Wu, D., 2017. Fabrication of graphene/TiO2/paraffin composite phase change materials for enhancement of solar energy efficiency in photocatalysis and latent heat storage. ACS Sustain. Chem. Eng. 5, 49064915. Mirzaei, P.A., Haghighat, F., 2012. Modelling of phase change materials for applications in whole building simulation. Renew. Sustain. Energy Rev. 16 (7), 53555362. Mishra, A., Shukla, A., Sharma, A., 2015. Latent Heat Storage Through Phase Change Materials. Mofijur, M., Meurah Indra Mahlia, T., Susan Silitonga, A., Chyuan Ong, H., Silakhori, M., Heikal Hasan, M., et al., 2019. Phase change materials (PCM) for solar energy usages and storage: an overview. Energies 12 (3167), 120. Mondal, S., 2008. Phase change materials for smart textiles. Appl. Therm. Eng. 28, 15361550 (2008). Mondal, S. 2011. Thermo-regulating textiles with phase change materials. In: Functional Textiles for Improved Performance, Protection and Health. Murugan, P., Ganesh Kumar, P., Kumaresan, V., Meikandan, M., Malar Mohan, K., Velraj, R., 2018. Thermal energy storage behaviour of nanoparticle enhanced PCM during freezing and melting. Phase Transit. 91 (3), 254270. Parameshwaran, R., Jayavel, R., Kalaiselvam, S., 2013. Study on thermal properties of organic ester phase-change material embedded with silver nanoparticles. J. Therm. Anal. Calorim. 114, 845858. Ramesh Babu, V., Arunraj, A., 2018. Thermo regulated clothing with phase change materials. J. Text. Eng. Fash. Technol. 4 (5), 344347. Rathod, M.K., Banerjee, J., 2012. Thermal stability of phase change materials used in latent heat storage systems: a review. Renew. Sustain. Energy Rev. 18 (2013), 245258. Riffat, S., Mempouo, B., Fang, W., 2013. Phase change material developments: a review. Int. J. Ambient Energy 36 (3), 102115. Salunkhe, P.B., Shembekar, P.S., 2012. A review on effect of phase change material encapsulation on the thermal performance of a system. Renew. Sustain. Energy Rev. 16, 56035616. Sami, S., Etesami, N., 2017. Improving thermal characteristics and stability of phase change material containing TiO2 nanoparticles after thermal cycles for energy storage. Appl. Therm. Eng. 124, 346352. Saw, C.L., Al-Kayiem, H.H., Owolabi, A.L. 2013. Experimental investigation on the effect of PCM and nano-enhanced PCM of integrated solar collector performance. WIT Trans. Ecol. Environ. 179, 899909. Saxena, R., Rakshit, D., Kaushik, S.C., 2020. Experimental assessment of phase change material (PCM) embedded bricks for passive conditioning in buildings. Renew. Energy 149, 587599. Shalaby, S.M., Bek, M.A., El-Sebaii, A.A., 2014. Solar dryers with PCM as energy storage medium: a review. Renew. Sustain. Energy Rev. 33, 110116. Shankara Narayanan, S., Kardam, A., Kumar, V., Bhardwaj, N., Madhwal, D., Shukla, P., et al., 2016. Development of sunlight-driven eutectic phase change materials nanocomposite for applications in solar water heating. Resource-Efficient Technol. 15. Available from: https://doi.org/10.1016/jreffit.2016.12.004. Sharma, A., Tyagi, V.V., Chen, C.R., Buddhi, D., 2007. Review on thermal energy storage with phase change materials and applications. Renew. Sustain. Energy Rev. 13, 318345. Sivasamy, P., Harikrishnan, S., Imran Hussain, S., Kalaiselvan, S., Ganesh Babu, L., 2019. Improved thermal characteristics of Ag nanoparticles dispersed myristic acid as composite for low temperature thermal energy storage. Mater. Res. Express 6, 17.

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Wang, X., Xu, X., 1999. Thermal conductivity of nanoparticlefluid mixture. J. Thermophys. Heat. Transf. 13 (4), 474480. Waqas, A., Kumar, S., 2013. Phase change material (PCM)-based solar air heating system for residential space heating in winter. Int. J. Green. Energy 10, 402406. Wu, S., Zhu, D., Zhang, X., Huang, J., 2010. Preparation and melting/freezing characteristics of cu/paraffin nanofluid as phase-change material (PCM). Energy Fuels 24, 18941898. Yavari, F., Hafez, R.F., Pashayi, K., Rafiee, M.A., Zamiri, A., Yu, Z., et al., 2011. Enhanced thermal conductivity in a nanostructured phase change composite due to low concentration graphene additives. ACS J. Phys. Chem. 115, 87538758. Zalba, B., Marın, J.M., Cabeza, L.F., Mehling, H., 2003. Review on thermal energy storage with phase change: materials, heat transfer analysis and applications. Appl. Therm. Eng. 23, 251283. Zeng, J.L., Cao, Z., Yang, D.W., Sun, L.X., 2010. Thermal conductivity enhancement of Ag nanowires on an organic phase change material. J. Therm. Anal. Calorim. 101, 385389. Zhang, Z., Wang, J., Tang, X., Liu, Y., Han, Z., Chen, Y. 2019. Comparison study between mesoporous silica nanoscale microsphere and active carbon used as the matrix shapestabilized phase change material. Sci. Rep., 19. Zhao, C.Y., Lu, W., Tian, Y., 2010. Heat transfer enhancement for thermal energy storage using metal foams embedded within phase change materials (PCMs). Sol. Energy 84, 14021412.

Further reading Arkar, C., Medved, S. 2002. Enhanced solar assisted system using sphere encapsulated PCM thermal heat storage, In: ECES IA Annex 17, Advanced Thermal Energy Storage Techniques—Feasibility Studies and Demonstration Projects, pp. 35.

Section V

Sustainable Carbon-Based and Biomaterials for Solar Energy Applications

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Chapter 16

Carbon nanodot integrated solar energy devices ¨ zge Ala¸s and Ru¨kan Genc¸ Melis O Chemical Engineering Department, Faculty of Engineering, Mersin University, Mersin, Turkey

16.1 Introduction Technological and industrial developments in parallel to the population growth resulted in an inevitable demand for energy worldwide. Elevated concerns on global warming due to increased emission of green gases together with environmental and health impact of accelerated use of traditional energy sources, the development of new technologies for harvesting and converting energy from more sustainable sources has gradually become essential. The coronavirus (COVID-19) pandemic resulted in a considerable decline in crude oil prices to the lowest level of the last couple of years (see www.eia.gov), which emphasized the importance of building economies that are less demanding on oil for petroleum exporting countries. Meanwhile, it showed the world the importance of being self-sufficient in general in which energy self-sufficiency could be achieved by developing energy conversion and storage technologies from renewable sources. Solar energy is one of the cleanest, safest, and abundant energy sources which can be directly converted to electricity without producing contamination and environmental problems (Conibeer, 2014). An essential use of solar energy is to produce direct current electrical energy by using solar cells to convert sunlight into the flow of electron by the photovoltaic (PV) effect (Paulo et al., 2016; Lim et al., 2018; Shanks et al., 2016). During the last few decades, the development of strategies aimed at increasing PV efficiency in solar cells has led to a wide variety of research activities. Despite the fast progression in research on third-generation solar cells, siliconbased solar cells are still the most crucial player in the PV industry (Conibeer, 2014; Chowdhury et al., 2016; Ananthakumar et al., 2019; Mohan et al., 2018). The development of strategies aimed at advanced PV efficiency, performance, and stability in solar cells has led to a wide range of research activities. For this purpose, many studies have been made to improve the performance of new generation solar cells by using low-cost nanomaterials with different Sustainable Material Solutions for Solar Energy Technologies. DOI: https://doi.org/10.1016/B978-0-12-821592-0.00017-0 © 2021 Elsevier Inc. All rights reserved. 497

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physical properties such as flexible band spacing, advanced optical band, and charge carrier recombination (Johnston and Herz, 2016; Corby et al., 2019). It has been reported that carbon-based nanomaterials have the potential to overcome many problems by the integration of them in the energy device materials, interfaces, and structure. Carbon nanomaterials are widely used to advance the flexibility and high performance of solar cells due to some specific features such as abundance, low cost, good electrical and thermal conductivity, long-term stability, tunable energy levels, and high charge carrier mobility, (Hadadian et al., 2020; Zhu et al., 2009). Carbon nanodots (CDs) (also denoted as CDot, CQDs, CNP, and CND), which are the zero-dimensional hemispherical form with graphene-like structural properties (Bourlinos et al., 2008; Alas et al., 2019; Zhang et al., 2018). They are formed from either multiple layers of graphitic sheets consisting of sp2/sp3 hybrid crystalline or an amorphous arrangement of carbon and carry different functional groups (hydroxyl, carboxyl or sulfonate groups) on their surface (Yang et al., 2013; Mishra et al., 2018; Maiti et al., 2016). One of the amazing features of CDs is that they can be synthesized from a variety of naturally occurring, cheap, and sustainable carbon-containing organic sources including biomasses and bio-related wastes (i.e., molasses, lemon salt, cola, coffee, paper, food, eggshell, and sugarcane) using one-step and straightforward synthesis methods (Alas et al., 2019; Genc et al., 2017; C¸alhan et al., 2018; Jiang et al., 2014; Park et al., 2014; Ke et al., 2014; Wei et al., 2014; Thambiraj and Ravi, 2016). While graphene and derivatives were becoming the shining star of advanced materials, CDs are closing the gap with their distinct optical properties. Excitation-dependent fluorescence in a broad range within the visible spectrum, upconversion/down conversion fluorescence emission, superior photocatalytic properties, high absorption coefficients, high fluorescence quantum yields equivalent to quantum dots (QDs), electron acceptor/donor behavior, and photo-induced charge transfer are some of those properties that can expand the applications of CDs even further for designing cheaper and more effective optic devices (Jin et al., 2017; Zhu et al., 2019; Molaei, 2020). The origin of photoluminescence (PL) in CDs is generally associated with core states, surface defect states, element doping, and functional groups on the surface. In another sight, PL in CDs arises from the radiation recombination of electronic bandgap transitions of conjugated π-domains (Fig. 16.1) (Dhenadhayalan et al., 2016; Sciortino et al., 2018; Sun, 2006; Yan et al., 2019; Yu et al., 2012). By functionalizing the surface of the CDs with passivation agents, new functional groups are attached, or the molecular layer is formed to the carbonated core. In this way, the PL characteristics of CDs were significantly improved by modulating the photon-harvesting capacity and photoexcited state properties of them (Cao, 2019; Sciortino et al., 2018; Yang, 2016). Besides that, as compared with the semiconductor luminance nanodots and organic dyes, CDs also show ecofriendly features such as biocompatibility, chemical inertness, and low toxicity which allowed CDs to be integrated safely into the

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FIGURE 16.1 Schematic illustrating the (A) electronic bandgap transitions of conjugated domains of CDs, and the relation between varying surface defect states and photoluminescence characteristics of CDs, (B) mechanism of fluorescence emission from a surface passivated CD, and excitation wavelength-dependent emissions color change of CDs dispersed in water. Reproduced with permission from Yang, F., et al., 2016. Functionalization of carbon nanoparticles and defunctionalization-toward structural and mechanistic elucidation of carbon “Quantum” dots. J. Phys. Chem. C., 120 (44), 2560425611. Sun, Y.P., et al., 2006. Quantum-sized carbon dots for bright and colorful photoluminescence. J. Am. Chem. Soc., 128 (24), 77567757.

application areas such as sensor development (Rezaei et al., 2018; Bac¸ and Genc¸, 2017; Jung et al., 2016), bioimaging (Yao et al., 2017; Sachdev and Gopinath, 2015), energy device development (Alas et al., 2019; Genc et al., 2017; Mistry et al., 2019; Benetti et al., 2019) and biomedical devices (Du, 2014; Kumawat et al., 2017; Li, 2014; Yue, 2020). Lately, in parallel with the progression in graphene-based technologies, it mainly attributed to the increased number of research on CDs as a cheaper and easy to manufacture carbon material for the production of new generation solar cells. For example, CDs were used as a cheaper alternative of sensitizers in dye-sensitized solar cells (DSSCs), or while they were integrated to perovskite solar cells (PeSCs), quantum dot solar cells (QDSCs) and organic solar cells (OSCs) as photoactive material, electrolyte, or counter electrode (CE) for improving the energy and power densities

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of such energy devices (Dao et al., 2016; Guo et al., 2017; Wang, 2018; Wang et al., 2019). In this chapter, the synthesis of carbon dots made up of organic and sustainable materials and their surface properties, physicochemical properties, optical and electrochemical characterization were highlighted focusing on their applications in a varying number of solar energy devices including dyesynthesized solar cells, QDSCs, OSCs, polymer solar cells, and PeSCs. Besides, this chapter covers future developments and challenges by featuring recent articles and review papers underlying the recent progress of carbon dots in energy device development.

16.2 Carbon nanodot integrated solar energy devices 16.2.1 Dye-sensitized solar cells DSSCs are considered as one of the main types of third-generation solar cell technology which are established through a sandwich-like arrangement of dye sensitizer, photoanode, redox electrolyte, and CE where the electrolyte is placed between a dye-sensitized semiconductor anode and a CE (Fig. 16.2) (Gnanasekar et al., 2019). The dye plays a significant role in the absorption and conversion of the incoming light to electricity in the device. Today, ruthenium (Ru)-based dyes or semiconductor QDs sensitized with PbS, CdSe, and CdTe. are most frequently used. Although these materials contribute to high energy conversion, they are considered both expensive and environmentally hazardous and toxic (Shen et al., 2017; Marinovic et al., 2017). Platinum (Pt) covered tin oxide is often used as a CE in DSCCs due to its high electrical

FIGURE 16.2 C-V responses of (A) aqueous CQD-sensitized solar cell and (B) NCQDs and CQDs as sensitized TiO2-based DSSCs. Reproduced with permission from Mirtchev, P., Henderson, E.J., Soheilnia, N., Yip, C.M., Ozin, G.A., 2012. Solution phase synthesis of carbon quantum dots as sensitizers for nanocrystalline TiO2 solar cells. J. Mater. Chem., 22 (4), 12651269; Zhang, Y.Q., Ma, D.K., Zhang, Y.G., Chen, W., Huang, S.M., 2013. N-doped carbon quantum dots for TiO2-based photocatalysts and dye-sensitized solar cells. Nano Energy 2 (5), 545552, respectively.

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conductivity, stability towards corrosion, and electrocatalytic activity in the reduction. However, Pt is a rare noble metal and can be collected from limited resources, which makes it less cost-effective and limits larger-scale productions; thus, alternative and more efficient platinum-free FTO CE materials are highly required (Gnanasekar et al., 2019; Yoon et al., 2006; Wahyuono et al., 2019). In the following sections, recent research evaluating the performance of CDs in different layers of DSSC devices (sensitizer, cosensitizer, and CE) as cheap, nontoxic, and ecofriendly alternatives were presented and discussed.

16.2.1.1 Carbon dots as sensitizer in dye-sensitized solar cells CDs typically exhibit high optical absorption at a wide visible light range and photoelectric properties, they have been explored as an alternative sensitizer and cosensitizer combined with other materials for DSSCs. The very first study on the performance of CDs as a synthesizer on nanocrystalline TiO2 solar cells was shown by Mirtchev et al. in 2012 (Mirtchev et al., 2012). Water-soluble CDs were synthesized by dehydration of g-butyrolactone. The surface functional groups such as -COOH, and -OH was used for decorating the TiO2 photoanode via the -COOH groups. As depicted in Fig. 16.2A, researchers achieved a highly low power conversion efficiency (PCE) (η) 0.13% with short circuit current (Jsc) of 0.53 mA/cm2 and open-circuit voltage (Voc) of 0.38 V with fill factor (FF) of 0.64. While the short circuit current density becomes the limiting factor in this device, open circuit voltage is much lower than routinely obtained with Ru sensitizers. The authors presented a couple of explanations for this outcome: (1) Unlike to QDs, CD surface was oxidized with sulfuric acid during the synthesis, and thus, they were unlikely corrosive to I32/I2 electrolyte so decreased photocurrent could not be due to corrosion. (2) CDs contain different emissive trap regions originating from PL feature where these regions also contribute to the low current density by acting as recombination centers for the photogenerated excitons. (3) The possibly lower charge injection by CDs to the conduction band of TiO2 as compared to Ru dyes. Zhang et al. employed CQD and nitrogen-doped CQDs (NCQD) as nonmetal sensitizers in DSSCs, and examined the effect of nitrogen on cell performance (Zhang et al., 2013). NCQDssensitized TiO2-based solar cell displayed a Jsc of 0.33 mA/cm2, Voc of 0.37 V and FF of 28% with an overall PCE of 0.03%, while NCQDs sensitized TiO2based solar cell displayed a Jsc of 0.69 mA/cm2, a Voc of 0.46 V, a FF of 43%, and overall PCE of 0.13% (Fig. 16.2B). Although researchers obtained a very low PCE, with this study, they showed a successful in situ hybridization of TiO2 with CDs and reduced work function (WF) induced by nitrogen doping contributes to improved device performance of CDs. In a more recent study, a DSSC device construction was designed by using ZnO nanoparticles (ZnO-NPs) as photoanodes and N-CQDs as synthesizer (Rama Krishna et al., 2017). Researchers showed that the unique interface regions of NCQDs with charge separation and increase the concentration of

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SECTION | V Sustainable Carbon-Based and Biomaterials

charge carriers involved in photo-reactions and worked as an efficient solar light-harvesting material. The tiny size (23 nm) of 0D nanoparticles resulted in a homogenous immobilization of NCQDs on the ZnO-NPs enabled an efficient charge separation and electron transfer to the electrode surface. As a result, photoanodes based on ZnO-NCQDs showed a significantly higher photocurrent density/photoconversion efficiency than photoanode devices based on nude ZnO NPs. Authors reported 5.48 mA/cm2 Jsc, Voc value 0.343 V, FF 52% and total PCE 1.18% for ZnO-NCQDs based solar cell, and Jsc of 5.02 mA/cm2, Voc of 0.342 V, FF of 51% with a total PCE of 0.88% for naked ZnO-based solar cell. It is known that the starting precursors and passivation agents used in the synthesis of CDs change their optical, electronic, and morphological properties (Xiao et al., 2015). Guo and coworkers investigated the effect of the starting carbon type (bee pollen (B-CQDs), citric acid (C-CQDs), and glucose (G-CQDs),) on the properties of CQDs as a green synthesizer in mesoporous titanium-dioxide-based solar cells (Guo et al., 2017). Devices built by B-CQDs showed the highest open-circuit voltage (Voc, 0.461) and short circuit current density (Jsc, 0.33) with the highest PCE of 0.11% among the three of the CQDs synthesized. Authors contributed the higher the Voc value of the B-CQDs with the quantum size effect dominated, and the higher the Jsc value due to stronger light absorption capacity within broad light spectrum range and the extraordinary electron transfer ability of B-CQDs. C-CQDs with the same size of B-CQDs showed a poorer performance which was associated with surface defects and internal recombination caused by limitation of transfer of photogenerated electrons to the opposite electrode. Solar cells designed using CQDs have been found to successfully improve the photoelectric conversion efficiency of all CQDs compared to the native TiO2. Zhu et al. designed a cosensitizing photoanode by combining polyethylene glycol (PEG) modified carbon quantum dots (PEG-m-CQDs) with N719 Ruthenium dye (Fig. 16.3A) (Zhu et al., 2017). Due to upconversion, broad spectral absorption, and hole-transporting properties of PEG-m-CQDs with more rapid charge extraction, the standard Jsc of N719 increased from 15.77 to 17.01 mA/cm2, and Voc increased from 0.703 to 0.708 V when the dyes mixed with the PEG-m-CQDs. Moreover, the maximum photoelectric conversion efficiency under sunlight was increased from 7.80% to 9.89%. These findings highlight the beneficial role of PEG-m-CQDs in expanding the absorption spectral, facilitating electron-hole separation for solar cells, and preventing backward recombination reactions between oxidized species (I32) in the electrolyte and photogenerated/electrolyte interface. In line with the study of Zhu et al., Zhao and colleagues designed DSSC with the TiO2/green emitting long persistence phosphor anode, and cosensitized N-doped CQDs with N719 dye showed an increase in broad spectral absorption and rapid charge extraction (Zhao et al., 2018). Fig. 16.3B shows the design of the DSSC and energy level distribution at photoanode/electrolyte interface and JaV characteristics of the designed solar cell. Under the sun illumination,

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FIGURE 16.3 Schematic diagrams (A, B, and C) showing varying device constructions of dye: CQDs cosensitized hybridized solar cells and mechanism of energy level distribution and charge transfer processes at photoanode/electrolyte interface. Corresponding characteristic JaV curves for various photovoltaics under simulated solar irradiation (100 mW/cm2, AM 1.5) (D, E, and F). Reproduced with permission from Zhu, W., Zhao, Y., Duan, J., Duan, Y., Tang, Q., He, B., 2017. Carbon quantum dot tailored counter electrode for 7.01%-rear efficiency in a bifacial dye-sensitized solar cell. Chem. Commun. 53 (71), 98949897; Zhao, Y., Duan, J., He, B., Jiao, Z., Tang, Q., 2018. Improved charge extraction with N-doped carbon quantum dots in dyesensitized solar cells. Electrochim. Acta, 282, 255262; Bora, A., Mohan, K., Dolui, S.K., 2019. Carbon dots as cosensitizers in dye-sensitized solar cells and fluorescence chemosensors for 2,4,6-trinitrophenol detection. Ind. Eng. Chem. Res. 58 (51), 2277122778.

a 14.83-fold enhancement of PCE from 8.09% (Jsc of 17.3 mA/cm2, Voc of 0.721 V, FF of 64.9%) to 9.29% (Jsc of 18.6 mA/cm2, Voc of value 0.736 V, FF of 67.9%) was achieved when CQD was used as cosensitizer. Lately, in 2019, Bora and coworkers have applied the synthesized green-emitting CDs (G-CDs) together with N719 dye as cosensitizers in DSSCs (Bora et al., 2019). The cosensitive DSSC device (Fig. 16.3C) achieved a 6.9% conversion efficiency with a high Jsc value of 14.23 mA/cm2 and a Voc value of 0.763 V (sensitized DSSC efficiency without G-CDs was 6.28%). Table 16.1 summarizes device

TABLE 16.1 Dye-sensitized solar cell devices with CD integrated (Section 16.2.1). DSSC Year

Device architecture

2012

FTO/TiO2:CQDs/I32:I2/Pt

2013

FTO/TiO2: CQDs/I32: I2/Pt FTO/TiO2:NCQDs/I32:I2/Pt

2013

JSC (mA/ cm2)

VOC (V)

FF

PCE (%)

Role of CD

Ref.

0.532

0.380

0.640

0.13

Sensitizer

Mirtchev et al. (2012)

0.330 0.690

0.370 0.460

0.280 0.430

0.03 0.13

Sensitizer

Zhang et al. (2013)

FTO/RhB/CQDs/TiO2/I32:I2/Pt

0.640

0.510

0.15

Sensitizer

Ma et al. (2013)

2015

FTO/ZnO/CT-CQD/CuSCN/Au FTO/ZnO/CS-CQD/CuSCN/Au FTO/ZnO/G-CQD/CuSCN/Au

0.530 0.500 0.153

0.175 0.275 0.255

0.350 0.440 0.440

0.032 0.061 0.017

Sensitizer

Briscoe et al. (2015)

2015

FTO/TiO2/ N719 /I32:I2/PANI:CND

13.80

0.770

0.700

7.45

CE

Lee et al. (2015)

2016

FTO/TiO2/CQDs/N719/ I32:I2/Pt

21.26

0.690

0.610

8.70

Modify the photoanode

Shi et al. (2016)

2017

FTO/TiO2/C-CQDs/I32: I2/Pt FTO/TiO2/G-CQDs/I32:I2/Pt FTO/TiO2/ B-CQDs/ I32:I2/Pt

0.082 0.148 0.330

0.414 0.375 0.461

0.603 0.535 0.726

0.020 0.0297 0.110

Sensitizer

Guo et al. (2017)

2017

FTO/TiO2/CNDs/I32:I2/Pt

0.97

0.660

0.570

0.36

Sensitizer

Marinovic et al. (2017)

2017

FTO/TiO2/C-120/I32:I2/Pt

1.110

0.600

0.730

0.48

Sensitizer

Shen et al. (2017)

2017

FTO/ ZnO:NCQDs/I32:I2/Pt

5.80

0.343

0.510

1.18

Sensitizer

Rama Krishna et al. (2017)

2017

FTO/ CD doped PANI/N719/I32:I2/graphite

9,50

0.690

0.600

3.65

Photoanode

Kundu et al. (2017)

2017

FTO/TiO2/N719/I32:I2/CQDs/CoSe

17.64

0.733

0.702

9.08

CE

Zhu et al. (2017)

2017

FTO/TiO2/N719/PEG-m-CQD/I32:I2/ CoSetLPP

19.59

0.717

0.704

9.89

Cosensitizer

Zhu et al. (2017)

2018

FTO/TiO2/N719/PCD/ I32:I2/Pt

9.55

0.892

0.703

6.05

Polymer gel electrolyte

Mohan et al. (2018)

2018

FTO/TiO2/N719/I32:I2/CQDs12-CoSe/Pt

16.79

0.732

0.694

8.54

Counter electrodes

Duan et al. (2018)

2018

FTO/TiO2/N719/CCQDs/I32:I2/Pt

16.60

0.721

0.725

8.68

Cosensitizer

Dou et al. (2018)

2018

FTO/TiO2/N719/I32:I2/S-CQDs:CoSe FTO/ TiO2/N719/I32:I2/S-CQDs:Pt FTO/TiO2/ N719/I32:I2/S-CQDs:RuSe

16.60 16.20 17.60

0.771 0.765 0.754

0.714 0.673 0.689

9.15 8.34 9.15

CEs

Zhang et al. (2018)

2018

FTO/TiO2/N719/N-CQDs/LPP/I32:I2/Pt

18.60

0.736

0.679

9.29

Cosensitizer

Zhao et al. (2018)

2019

FTO/TiO2/CQD/I32:I2/Pt

0.360

0.510

0.653

1.20

Sensitizer

Mistry et al. (2019) (Continued )

TABLE 16.1 (Continued) DSSC JSC (mA/ cm2)

VOC (V)

FF

PCE (%)

Role of CD

Ref.

Year

Device architecture

2019

ITO/ZnO(20)@Cdot(2)/N719/I32:I2/Pt

2.340

0.670

0.590

5.92

Modify the photoanode

Efa and Imae (2019)

2019

FTO/TiO2/N719/CD/I32:I2/Pt

14.23

0.763

0.630

6.90

Cosensitizer

Bora et al. (2019)

2019

FTO/TiO2:CDs/N719/I32:I2/Pt

16.94

0.788

0.740

7.32

Modify the photoanode

Rezaei et al. (2019)

2019

FTO/TiO2/N719/CQDs/I32:I2/Pt

15.00

0.730

0.749

8.20

Coatings CQDs on DSSC

Riaz et al. (2019)

2020

ITO/NiO/C-dots/N719/I32:I2/Pt

23.60

0.580

0.360

9.85

Modify the photoanode

Etefa et al., 2020

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arrangements and conclusions of the recent articles in the literature covering the use of carbon dots as synthesizer or cosynthesizer in DSSCs.

16.2.1.2 Carbon dots modified photoanodes in dye-sensitized solar cells Fluorescence CDs can be successfully absorbed by dyes as they are easily coabsorbed by the TiO2 film. Shi et al. applied, for the first time, fluorescent CQDs to DSSC by incorporating them onto N719 sensitized TiO2 photoanodes as the electron-transport material (Fig. 16.4A) (Shi et al., 2016). Comparison studies with solar cell devices constructed with and without CQD were shown to reveal 32% and 21% improvement for short circuit current density (Jsc) and photoelectric conversion (PCE) efficiency, respectively. Following device results were reported; Jsc 5 21.26 mA/cm2, Voc 5 0.69 V, FF 5 0.61, and η 5 8.2% for cell device designed using CQDs, Jsc 5 16.12,

FIGURE 16.4 (A) Schematic representation of CDs decorated on TiO2 film as a photoanode bearing DSSC construction, and JaV curves showing the increased current density of designed DSSCs with increased CD addition (wt %). (B) JaV characteristics of DSSCs with Pt, PANI, and PANI-CND CEs. (C) Illustration of the bifacial DSSC device with the representation of photo-induced charge-transfer processes with the JaV curves of bifacial DSSC. (A) Reproduced with permission from Rezaei, B., Irannejad, N., Ensafi, A.A., Kazemifard, N., 2019. The impressive effect of eco-friendly carbon dots on improving the performance of dye-sensitized solar cells. Sol. Energy 182, 412419. (B) Reproduced with permission from Lee, K., et al., 2015. Highly porous nanostructured polyaniline/carbon nanodots as efficient counter electrodes for Pt-free dye-sensitized solar cells. J. Mater. Chem. A, 3 (37), 1901819026. (C) Reproduced with permission from Zhu, W., Zhao, Y., Duan, J., Duan, Y., Tang, Q., He, B., 2017. Carbon quantum dot tailored counter electrode for 7.01%-rear efficiency in a bifacial dye-sensitized solar cell. Chem. Commun. 53 (71), 98949897.

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Voc 5 0.74 V, FF 5 0.60, and η 5 7.25% for the reference device. Rezaei and coworkers modified the TiO2 photoanode with ecofriendly and wide absorption CDs synthesized from rosemary leaves (Fig. 16.4A) (Rezaei et al., 2019). They fabricated DSSCs with different weight ratios of CDs (16 wt. %) decorated onTiO2 film and used as photoanode. CD addition significantly improved photoanode dye adsorption and photo-induced electron production. The authors explained the improvement with the holes accumulated in the valance band of CDs, and an enhanced electron transfer to the conduction band of TiO2 together with effectively separated charge carriers and increased lifetime of the electrons. The PCE in the newly designed DSSC (Jsc 5 16.94 mA/cm2, Voc 5 0.788 mV, η 5 7.32%) has increased 2.2 times compared to the TiO2-based DSSC (Jsc 5 7.92 mA/cm2, Voc 5 0.765 mV, η 5 3.25%). Due to the rapid charge recombination of the Nickel oxide (NiO) semiconductor, low PCE values were reported in NiO-based DSSCs. To improve this problem, Etefa et al. designed a DSSC device using NiO NPs modified with C-Dots as photoanode and reported that the cell significantly improved PCE performance (Etefa et al., 2020). While the PCE value of NiO @ CDots based DSSC is 9.85%, it is 2.44% in NiO based DSSC. The four-fold high performance was resulted depending on several reasons, such as plentiful charge separation, reduced charge recombination owing to the reduced bandgap of the composite, and an increase in N719 adsorption of NiO@C-Dots composites after the addition of C-Dots due to enhanced electrostatic interactions.

16.2.1.3 Carbon dots as counter electrode in dye-sensitized solar cells Pt, commonly used in DSSC as a CE, is one of the significant barriers to large-scale production due to its high cost related to its being the rarest of the precious metals. Recently, materials such as carbon materials, transition metal complexes, and conductive polymers with low cost, high chemical stability, and high electrocatalytic activity have been used in DSSCs as CE. The behavior of the CDs in the CE/electrolyte interface was investigated using the photo-excitation behavior and electronic properties of the CDs. In a study conducted in 2015, researchers used CDs as nucleates to synthesize polyaniline (PANI) with higher porosity, conductivity, and electrocatalytic activity for high-performance CEs in DSSCs (Lee et al., 2015). To produce high-porous PANI film, optimization studies based on carbon nanodot (CND) weight ratio, and the performance of DSSCs fabricated with PANI: CND (5% wt) CE and Pt CE was compared. When Pt, PANI ve PANI:CND are used in DSSC as CE, following PCEs were obtained: 7.37%, 5.60%, and 7.45%, respectively (Fig. 16.4B). As a result, PANI: CND CE exhibited superior PCE compared to the traditional Pt CE. Zhu et al. designed the

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novel CE (CQDs-CoSe) with the integration of polyethylene glycol-modified CQDs with transparent CoSe to increase both the catalytic activity of CE and the back efficiency of the corresponding bifacial DSSC (Zhu et al., 2017). Owing to the resorption of unabsorbed visible light along the photoanode and electrolyte by the CQDs, and increased electron density at electrode surfaces was achieved. With the use of CQDs-CoSe CE, the front and rear efficiencies were increased to 9.08% and 7.01%, respectively (Fig. 16.4C). Zhang and coworkers have designed sulfur-doped carbon quantum dot (S-CQD) decorated Pt or metal selenide (CoSe, RuSe) CE (Zhang et al., 2018). CV measurements on CEs constructed w/wo S-CQDs showed that the presence of CQDs improves the catalytic kinetics with high-concentration electrons, and reduced the energy loss during reversible I322 I2 conversion. Researchers showed that the catalytical effect of CQDs accelerates under sunlight irradiation. The maximized front efficiency increased from 8.09% to 8.34%, while the increment in rear efficiency was from 4.34% to 5.21% in S-CQD-integrated CE as compared to the Pt-CE. CoSe and RuSe based CEs responded the same manner with enhanced maximized front and rear efficiency values of 9.15% and 5.69% for S-CQDs/CoSe CE and 9.15% and 6.26% for S-CQDs/RuSe CE, respectively. These results demonstrated the potential of CQDs as combined with common materials for designing CEs. The improved device performance mainly connected to the wide-spectral light absorption capacity, increased surface electron density, and tailorable photo-induced catalytic ability of CDs. Readers are referred to Table 16.1 for detailed information on the recent studies that could not be covered herein.

16.2.2 Quantum dot solar cells Third generation QDSCs are considered as a promising alternative to dyesensitive solar cells (DSSCs). Semiconductor QDs such as CdS, CdSe, CdTe, PbS, with low-bandgap and broad absorption in the solar spectrum are widely applied as sensitizers for efficient PV devices as they aim to replace silicon, or copper indium gallium selenide (Zhang et al., 2017; Jiao et al., 2015; Lu et al., 2017). However, QDs have a couple of disadvantages, including their cytotoxicity, the surface defects acting as electron traps, and trap-induced blinking, which limits their more extensive use. Replacing them with green and low-cost, environmentally friendly alternatives could be an answer for cheaper and safer solar cell devices with higher efficiency. At that point, carbon dots with the optical and catalytical properties assembling the semiconductive QDs may replace the QDs soon. The very first attempt at the use of CDs in solar cell devices was made by Narayanan et al. in 2013. Researchers built a novel device construction relying on the Forster resonance energy transfer driven QDSSs with ZnS/ CdS/ZnS/C-dot/CuPc. They used ZnS/CdS/ZnS QDs as current collector rotators, copper phthalocyanine (CuPc) as electrolyte, and C-dots embedded

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SECTION | V Sustainable Carbon-Based and Biomaterials

on the anode as electron transport support (Fig. 16.5A) (Narayanan et al., 2013). A facilitated charge propagation by C-dots due to faster charge transfer from CuPc to the current collector has resulted in an increased PCE with 5.76 times higher photocurrent in the blue/green region as compared to ZnS/ CdS/ZnS QDs arrangement (Jsc 5 1.88 mA/cm2, Voc 5 0.605 mV, FF 5 0.31, η 5 0.35%). In a more recent study, Dao et al. investigated the CDot-Au nano-raspberries (NRs) based on a hybrid CE in ZnO nanowire/CdS/CdSe QDSCs (Dao et al., 2016). Cdot-Au NRs exhibited improved catalytic activity against polysulphide electrolyte reduction compared to other Au or CDot CEs. Cells using Cdot-Au NR CE have 16.6 mA/cm2 JSC, 46% FF and 5.4.% PCE, while cells using Au-sputtered CE have 13.2 mA/cm2 JSC, 39% FF and 3.6% PCE while CE of Cdots alone resulted in much lower PCE of 0.18% (Fig. 16.5B). The accelerated performance of the device arrangement with

FIGURE 16.5 (A) Device construction and energy band diagram of the quasi solid-state ZnS/ CdS/ZnS/C-dot/CuPc/S22/MWCNT based solar cell and JV response of designed photoanode architectures. (B) Illustration of QDSC device designed combining Cdot-Au NR counter electrode and CdSe/CdS/ZnO-NW and corresponding comparative JaV curves that show the highest photocurrent density in the presence of Cdot-Au NR counter electrode. (C) Device structure, and light/dark JaV curve of solar cells with TiO2/N-CQDs based photo-collector layer. (D) Energy band structures and JV characteristics of the device constructed with QDSCs having different surface functional groups. The champion device was achieved with CQD-D carrying the highest number of N. (A) Reproduced with permission from Narayanan, R., Deepa, M., Srivastava, A.K., 2013. Fo¨rster resonance energy transfer and carbon dots enhance light harvesting in a solidstate quantum dot solar cell. J. Mater. Chem. A, 1 (12), 39073918. (B) Reproduced with permission from Dao, et al., 2016. Facile synthesis of carbon dot-Au nanoraspberries and their application as high-performance counter electrodes in quantum dot-sensitized solar cells. Carbon N.Y. 96, 139144. (C) Reproduced with permission from Carolan, D., Rocks, C., Padmanaban, D.B., Maguire, P., Svrcek, V., Mariotti, D., 2017. Environmentally friendly nitrogen-doped carbon quantum dots for next generation solar cells. Sustain. Energy Fuels 1 (7), 16111619. (D) Reproduced with permission from Huang, P., Xu, S., Zhang, M., Zhong, W., Xiao, Z., Luo, Y., 2019. Modulation doping of absorbent cotton derived carbon dots for quantum dot-sensitized solar cells. Phys. Chem. Chem. Phys. 21 (47), 2613326145.

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Cdot-Au NR-based CE was explained by the larger surface area of the nanomaterial produced as compared to Au-sputtered CE, which increased electrocatalytic activity, and improved charge transfer rate together with enhanced catalytic properties due to Cdot presence. Carolan et al. started their manuscript with a wishful and indeed cheering sentence, “Shine on you crazy carbon!” exploring their beliefs on the potential of CQDs for achieving more efficient solar cells (Carolan et al., 2017). They evaluated the performance of the N-CQD-based photoactive layer, and they manufactured a solar cell device without the use of polymer composites or additional sensitizer. Among the designed devices, the champion device with 0.8% PCE and an extraordinarily high open-circuit voltage (1.8 V) showed good charge separation and band alignment (Fig. 16.5C). On account of increasing the photoelectric performance of the devices, authors proposed a future modification of the synthesis procedure for obtaining CDs with a broader absorption spectrum, tunable energy-bandgap, and different functional groups for altering the PCE obtained in their study. Huang et al. have synthesized CDs from the cotton precursor using passivating agents (dopant) such as carbamide, thiourea, and 1,3-diamino propane (CQD-C, CQD-T, and CQD-D) (Huang et al., 2019). Researchers modulated the optical properties, energy band structures, and surface functional groups of the CQDs by dopant type, and investigated the effect of these changes on the photoelectric performance of QDSCs. Presence of dopants significantly improved the short-circuit density (Jsc) and open-circuit voltage (Voc) and PCE of 0.350%, 0.316%, and 0.527%, was achieved with CQD-C, CQD-T, and CQD-D, respectively, as the PCE obtained with the CQD-N, synthesized without a passivating agent was as low as 0.176% (Fig. 16.5D). When the morphological and structural features of CQD are examined in detail, there has been found that CQDs with a smaller size, smaller bandgap, and surface rich in either nitrogen or sulfur beneficial to advanced light absorption performance and photo-excitation properties that help to develop the maximized Jsc levels. The CQD-D carrying these properties among the others showed the largest Jsc (1.28 mA/cm2), and it acts as a donor due to high N-content achieving the highest Voc (0.57 V) by assisting the sensitized photoanode with a higher Fermi level. Summary of the studies discussed in this section is summarized in Table 16.2.

16.2.3 Organic solar cells Over the past few years, OPVs have received considerable attention for designing new-generation devices with green energy technologies. The largescale production techniques available, high PCE (achieved 17.3% in 2018), light-weighted material, flexibility, the low production cost of the substrates, and low toxicity are some of the crucial features of QSCs that may provide benefits directly to the manufacturers (Semeniuk et al., 2019;

TABLE 16.2 Quantum dot solar cell devices with CD integrated (Section 16.2.2). QDSC Year

Device architecture

2013

FTO/ZnS/CdS/ZnS/C-dot/ CuPc/MWCNT

2015

FTO/TiO2/CQD-C/I32/I2/Pt FTO/TiO2/CQD-T/ I32/I2/Pt FTO/TiO2/CQD-D/ I32/I2/Pt FTO/TiO2/CQD-N/ I32/I2/Pt

2015

JSC (mA/ cm2)

VOC (V)

FF

PCE (%)

Role of CD

Ref.

1.88

0.605

0.310

0.35

Photoanode

Narayanan et al. (2013)

1.13 0.99 1.28 0.76

0.560 0.480 0.570 0.470

0.550 0.660 0.720 0.490

0.350 0.316 0.527 0.176

Photoanode

Huang et al. (2019)

FTO/ TiO2/PbSe/CdS/Cdots/S22/Sn22

17.26

0.694

41.52

4.97

Photoanode

Kokal et al. (2015)

2016

FTO/ TiO2/NCQD/gel electrolyte/Pt

2.65

0.625

0.470

0.79

Photoanode

Wang et al. (2016)

2016

FTO/CdS/CdSe/ZnO NW/ S22/Sx22/Cdot-Au

16.60

0.708

0.460

5.40

CE

Dao et al., 2016

2017

FTO/TiO2/NCQD/S22/S222/ Cu2S

6.47

0.430

0.310

0.87

Photoanode

Zhang et al. (2017)

2017

ITO/ TiO2/N-CQDs/Au

0.230

1.800

0.790

0.80

Photoactive layer

Carolan et al. (2017)

2019

FTO/ TiO2/CNP/CdS/CdSE/ S22/Sx22/CuS

14.40

0.620

0.490

4.41

Photoanode

Gao et al. (2019)

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Meng et al., 2018). The organic CDs have been applied for the improvement of OSCs due to their efficient photon harvesting in the solar spectrum and photo-induced redox potentials. Lately, researches on the use of CDs as an electron transport layer (ETL), hole transport (HTL) layer, and interfacial layer material (the modification of the interface or cathode between the photoactive layer and the buffer layer) have been conducted by various researchers for developing high-performance OPVs. In 2015, Zhang et al. used CDs as an ETL in both small molecule and polymer-based OSCs and examined their effects on cell performance and life stability (Zhang et al., 2015). Small molecule-based OSCs were fabricated using CDs of various concentrations as ETL (Fig. 16.6A), and the device with the CDs at the optimum concentration (0.1 mg/mL) showed the highest performance among the devices using both without the ETL layer and LiF as ETL (Fig. 16.6A). Authors reported that the device without an ETL showed an average PCE of 7.00% (Voc of 0.895 V, Jsc of 12.88 mA/cm2, and FF of 60.8%), LiF-based ETL showed an average PCE of 7.22% (Voc of 0.91 V, Jsc of 12.92 mA/cm2, and FF of 61.8%), while an increased PCE of 7.67% with Voc of 0.904 V, Jsc of 13.32 mA/cm2, and FF of 63.7% was reported with CDs-ETL (0.1 mg/ mL). Polymer-based OSCs using CDs as ETL have exhibited a similar performance as small molecule-based OSCs. Fabricated devices employing without ETL, LiF as ETL and CDs as ETL showed 3.06%, 3.38%, and 3.42% PCE, respectively. Besides, CD devices as ETL had much better air stability as compared to the other devices built. The increased air stability is vital for an improved device lifetime. The initial PCE of the device with LiF as the ETL decreased rapidly to 54% after 1970 minutes (B33 hours) in air and 53% after 10220 minutes (170 hours) in the glove box (Fig. 16.6A). The device with CDs as ETL remained the PCE near 85% after 11150 minutes (B186 hours) in air and PCE near 90% of initial PCE in the glove box. The device with CDs as ETL had PCE upon 85% of the initial PCE in the air after 11150 minutes and PCE close to 90% of the initial PCE in the glove box. Zhang and coworkers have modified metal-oxide electron transport layer (TiO2) with CQDs and combined them with inverted organic solar cells (I-OSCs) based on PCDTBT: PC71BM and P3HT: PC60BM (Zhang et al., 2017). To reduce the natural incompatibility between the metal oxide and the active organic layers, and the energy barrier for electron transfer, the CQDs with an excellent electron carrying capacity are deposited as a selfassembled monolayer (SAM) with the TiO2 buffer layer (Fig. 16.6B). TiO2/ SAM CQD, as ETL with improved morphology and electrical properties, has improved device performance by creating a reduced energy barrier for electron extraction, reducing interfacial charge recombination, and improving FF. PCEs of the devices with TiO2/CQD for PCDTBT and P3HT configuration increased by 19.77% and 18.50%, respectively, as compared to the reference device (Fig. 16.6B). The device parameters reported by authors are

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FIGURE 16.6 (A) Fabricated organic solar cell device structure, and comparison of JV and normalized efficiency vs. storage time curves of the devices constructed using CDs or LiF as the ETL, and conventional devices without ETL. Results were obtained under air and glove box conditions. (B) Schematic illustration of I-OSCs device construction with representative energy level diagram in I-OPVs based on PCDTBT: PC71BM and P3HT: PC60BM as active layers. JV characteristics of I-OCSs designed with varying combinations of PCDTBT and P3HT active layer with TiO2/nonSAM CQD and TiO2/SAM CQD. (C) Representation of device configuration of inverted OSCs, dark/light J 2 V curves of ZnO, ZnO: N-CQDs, and ZnO: N, S-CQDs based PTB7-Th: PC71BM solar cells. (D) Device structure and enhanced JaV characteristics of fabricated OSCs using C-dots @ PEI ETL as compared to varying types of polymers as ETLs. (A) Reproduced with permission from Zhang, H., et al., 2015. Investigation of the enhanced performance and lifetime of organic solar cells using solution-processed carbon dots as the electron transport layers. J. Mater. Chem. C. 3 (48), 1240312409. (B) Reproduced with permission from Zhang, X., et al., 2017. An easily prepared carbon quantum dots and employment for inverted organic photovoltaic devices. Chem. Eng. J. 315, 621629. (C) Reproduced with permission from Wang, Y., Zhang, J., Chen, S., Zhang, H., Li, L., Fu, Z., 2018. Surface passivation with nitrogen-doped carbon dots for improved perovskite solar cell performance. J. Mater. Sci. 53 (12), 91809190. (D) Reproduced with permission from Li, Z., et al., 2018. Toward efficient carbon-dots-based electron-extraction layer through surface charge engineering. ACS Appl. Mater. Interfaces 10 (46), 4025540264.

7.33% (Jsc of 15.02 mA/cm2, Voc of 0.85 V) and 4.42% (Jsc of 13.84 mA/cm2, Voc of 0.59 V). Light (UV)-soaking processing is generally applied to improve many surface defects and incompatible energy bands with photoactive layers to

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achieve high device performance in ZnO based cells. Wang et al. used NCQDs and N, S-CQDs as the surface defect modified reagent for ZnO ETL to overcome the light soaking in PTB7 Th: PC71BM based inverted OSCs (Wang et al., 2018). The use of N, S-CQDs as surface modifiers in the ETL layer facilitated the transport and collection of photogenerated carriers by reducing the surface defects and energy of the ZnO layer. Consequently, cells using ZnO: N, S-CQDs as ETL showed the highest PCE of 9.31% (PCE) compared with ZnO (9.18%) and ZnO: N-CQDs (9.06%) and the JaV curves obtained were without an S-shape kink (Fig. 16.6C). The results showed that the modification of ZnO ETL with N-CQDs did not effectively eliminate the light-soaking effect, but the presence of the sulfur atom was the main actor playing a critical role in reducing the light-soaking effect. Li et al. employed functionalized C-dots with amino-riched polyethyleneimine (PEI) as ETL in inverted OSCs to simplify electron extraction by surface charge engineering (Li et al., 2018). High luminescence C-dots @ PEI expected to absorb ultraviolet light and then convert into the low energy photons that can be transferred to the active layer. Also, due to the local states present in the C-dots @ PEI structure, authors showed that C-dots could work as light control switches that prevent current leakage and facilitate the electron extraction of the cathode by electron filling and releasing. Li’s group manufactured a solar device with the following construction; ITO/ETL/PTB7: PC71BM/MoO3/Ag and evaluate the performance of PEI, PFN, ZnO, TiO2 and C-dots @ PEI as ETL. Among the devices made up of PEI, PFN, ZnO, and TiO2 as ETL, the highest device performance was achieved with C-dots @ PEI-based device, which exhibited a Jsc (18.37 mA/cm2) and FF (71.14%) value higher than the rest. The improvement in the performance values is attributed to the facilitated electron transport and extraction by C-dots. The champion efficiency of 9.53% for C-dots@PEI based PTB7: PC71BM device was obtained (Fig. 16.6D). Summary of the other valuable literature on the use of CDs in OPV development that could not be discussed in this section are summarized in Table 16.3.

16.2.4 Polymer solar cells To improve PCE in polymer solar cells (PoSCs) benefits from the optimization of manufacturing processes such as the use of new acceptor or donor materials with excellent electro-optic properties, interface engineering, and the application of different device architectures (Geiker and Andersen, 2009; Singh and Kushwaha, 2013). Recently, together with the increased attention to the graphene and graphene derivatives, CDs also attracted the interest of researchers as they can be tailored to be the donor or acceptor, together with previously mentioned optical and electronic properties as a green alternative material to improve the PCE of the PoSCs. For example, Choi et al. synthesized carbon-dot supported silver nanoparticles (CD-Ag NPs) using CDs as

TABLE 16.3 Organic solar cell devices with CD integrated (Section 16.2.3). OSC JSC (mA/ cm2)

FF

PCE (%)

Role of CD

Ref.

Year

Device architecture

2015

ITO/PEDOT:PSS/DR3TBDTT:PC71BM/CDs/Al ITO/ PEDOT:PSS/P3HT:PC61BM/CDs/Al

13.32 10.25

0.904 0.609

0.637 0.548

7.67 3.42

ETL material

Zhang et al. (2015)

2016

ITO/nc-TiO2/PCDTBT:PC71BM/CNDs /MoO3/Ag

14.71

0.870

0.569

7.22

Interlayer between active layer and HTL

Zhang et al. (2016)

2017

ITO/ TiO2/CQD/PCDTBT:PC71BM/MoO3/Ag ITO/ TiO2/ CQD/P3HT:PC60BM/MoO3/Ag

15.02 13.84

0.850 0.590

0.573 0.533

7.33 4.42

Modify the ETL

Zhang et al. (2017)

2018

ITO/ZnO:N-CQDs/PTB7-Th:PC71BM/MoO3/Al ITO/ ZnO:N,S-CQDs/PTB7-Th:PC71BM/MoO3/Al

16.91 17.20

0.800 0.800

0.670 0.680

9.06 9.36

Modify the ETL

Wang et al. (2018)

2018

ITO/ZnO/CNDs/PTB7-Th:PC71BM/MoO3/Ag ITO/ZnO/ CNDs/PBDB-T:ITIC/MoO3/Ag

17.00 15.80

0.770 0.910

0.728 0.635

9.60 9.10

Cathode interfacial layer

Juang et al. (2018)

2018

ITO/C-dots@PEI/PTB7-Th:PC71BM/MoO3/Ag

18.37

0.730

0.711

9.53

ETL material

Li et al. (2018)

2019

ITO/carbon dot@PEDOT:PSS/P3HT:PC61BM/LiF/Al

15.48

0.590

0.520

4.78

HTL material

Lim and Hong (2019)

VOC (V)

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517

reducing agent and evaluated their performance in both polymeric LEDs and PoSCs constructed with ITO/CDAg NPs/PEDOT:PSS/emission or active layer/(LiF)/Al (Choi et al., 2013). The group showed that CD-Ag NPs had many advantages over conventional metal nanoparticles. For instance, CDAg NPs showed an increased plasmon resonance (SPR) effect, excellent electron donation ability of photo-induced CDs, which provides the rapid reduction of metal salts on the CD surface. Furthermore, resulting NPs provided a broad light absorption due to the plasmon coupling effect resulting from the clustering of Ag on CDs. When the current density and voltage (JaV) characteristics of devices with and without CD-Ag NPs are compared, we can see that the device without CDAg NPs has lower Jsc (14.4 mA/cm2), Voc (0.75 V), and FF (0.70) values as compared to the device with CD-Ag NPs (Jsc of 16.0 mA/cm2, a Voc of 0.75 V, a FF of 0.70). The presence of CD-Ag NPs enhanced the device PCE from 7.53% to 8.31%. Also, when dark JaV characteristics were examined, it was seen that the SPR property of CD-Ag NPs was the leading cause of enhanced device performance. Yan and coworkers produced crystal graphite structure C-CQDs with chemical vapor deposition method, and using this method; they aimed to eliminate the hydrophilic terminal groups and long alkyl side chains carried by H-CQDs (CQDs produced through hydrothermal method) which negatively affect the charge transport between the layers. They evaluated the integration of it into different active layers of PoSCs (P3HT: PC61BM, PTB7: PC61BM, and PTB7-Th: PC71BM) employing the performance of fabricated cells (Fig. 16.7A) (Yan et al., 2016). When CQD was incorporated into the devices, they exhibited similar or slightly improved device performance in comparison with the LiF based device. PCE values of optimized P3HT: PC61BM, PTB7: PC61BM, and PTB7-TH: PC71BM devices integrated CQD ETLs achieved 3.11%, 6.85%, and 8.23% respectively. Furthermore, the long-term thermal stability of devices using CQD as ETL due to less molecular diffusion of CQDs has also been notably improved. In 2016, for the first time, C-dots containing amino groups on its surface had been used as a perfect interface modification layer above the ZnO or AZO interlayer for high-performance inverted polymer solar cells (i-PoSCs) by Lin et al. (2016). Lin et al. aimed to overcome some constraints of the use of ZnO and AZO layers by modifying them with C-dots. Those constraints are listed as (1) disproportion between conduction band energy level of ZnO and AZO and the LUMO level of PC71BM, (2) the poor interfacial contact with active organic layers leading to significant series resistance (Rs), (3) weak electronic coupling and severe back charge combination. The modification of the surface with the C-dots, researchers showed that both the working function and the roughness of metal oxides were successfully reduced. That way, the current losses can be prevented, and more efficient transport and collection of photogenerated charge carriers can be achieved. Besides, the luminescent downshifting effect of C-dots increased the light

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FIGURE 16.7 (A) Device structure and energy level diagram of the solar cells with CQDs as ETLs and P3HT: PC61BM, PTB7: PC61BM, or PTB7-Th: PC71BM as the active layer. (B) Schematic illustration of the inverted PoSCs modified with a C-dots layer, and JaV of characteristics of i-PoSCs constructed with PTB7: PC71BM (left) and PTB7-Th: PC71BM (right) using ZnO/C-dots under light irradiation (100 mW/cm2). (C) The schematic cross-sectional view of an inverted PoSC and JaV characteristics of PoSC with QD layers under light irradiation. (A) Reproduced with permission from Yan, L., Yang, Y., Ma, C.Q., Liu, X., Wang, H., Xu, B., 2016. Synthesis of carbon quantum dots by chemical vapor deposition approach for use in polymer solar cell as the electrode buffer layer. Carbon 109, 598607. (B) Reproduced with permission from Lin, et al., 2016. Interfacial modification layers based on carbon dots for efficient inverted polymer solar cells exceeding 10% power conversion efficiency. Nano Energy 26, 216223. (C) Reproduced with permission from Zhang, et al., 2018. Solution-processable ZnO/carbon quantum dots electron extraction layer for highly efficient polymer solar cells. ACS Appl. Mater. Interfaces 10 (5), 48954903.

conversion of near-ultraviolet and blue-violet portions of sunlight, which was also effective in device performance. Consequently, researchers successfully overcame some of the limitations of PoSCs and obtained extremely high maximum PCE values up to 10.24% by their unique i-PoSC design using AZO/C-dots interlayers (Fig. 16.7B). Following this, Lim et al. reported that the PV performance of the i-PoSC could be improved by blending the C-dots on the PV polymer active layer of PEIE polymer-based i- PoSCs using a simple spin-coating technique (Lim et al., 2018). C-dots in different doping amounts (0.01, 0.025 and 0.05 wt.%) were blended into P3HT and PC60BM, which were then used as active layers

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in the i-PoSC architecture. The JaV characteristics of the devices under AM 1.5 G illumination showed that the devices produced C-dots doping at all concentrations resulted in a higher Jsc than that of the reference device, and the highest PCE of 3.5% [which was 30% higher than that of the control device (2.7%)] was achieved when C-Dots were doped in 0.01 wt.%. In 2018, Zhang and coworkers applied bilayer ZnO/C-QD as an electron extraction layer (EEL) for inverted PoSCs (Zhang et al., 2018). PTB7: PC71BMbased PoSCs with designed bilayer EEL exhibited 9.64% PCE, which is 27% higher than that of the reference cell constructed with the pure ZnO (7.59% PCE) (Fig. 16.7C). Researchers examined the performance of bilayer ZnO/C-QD in P3HT: PC61BM based PoSCs. Likewise, a significant improvement in PCE, from 3.78 % to 4.85 %, was achieved as compared to the reference device. The authors explained the underlying mechanism the improved device performance in the presence of bilayer ZnO/C-QD EEL with a reduction in charge recombination loss, enhanced carrier collection, and more efficient charge extraction due to the passivation of ZnO surface defects and traps by C-QDs. Readers are referred to Table 16.4 for additional literature exploring the potential of CD integration to PoSCs.

16.2.5 Perovskite solar cells PeSCs have become a superstar of the next generation PV devices for the past few years, with meager production costs and high PCEs obtained so far. PeSCs exhibit the latest certified value of 25.2% PCE, that even better than commercialized multicrystalline silicon solar cells. However, PeSCs suffer from low cost and high device performance potential, as well as problems like long-term stability, lead-based toxicity, and hysteresis (Green et al., 2020; Ferguson et al., 2019). In recent years, carbon-based materials (e.g., fullerene and its derivatives, graphene, carbon nanotubes, carbon nanodot) have been used in optimization studies in perovskite absorbent layer, interface, and device structure to overcome these problems in PeSCs and have been shown to have positive effects while maintaining high PCE (Batmunkh et al., 2017; Jeng et al., 2013; Ye et al., 2016). CDs have often been used as an additive in various layers of the PeSC architecture to increase light absorption, guaranteeing more efficient charge transfer, improve electron mobility and electron extraction ability. In 2017, Li et al., for the first time, used CQDs and produced a CQDs/TiO2 composite as ETL for developing an efficient planar n-i-p heterojunction PeSC (Li et al., 2017). The produced composite showed a negligible absorption in the visible spectral range, and ETL efficiently improved electron mobility and charge carrier extraction between the perovskite and TiO2 layers. With this cleaver approach, authors developed a B19% PCE device having a Jsc of 21.36 mA/cm2 and Voc of 1.136 V with reduced photocurrent hysteresis as compared to TiO2 alone (Fig. 16.8A). In order to obtain solar cells with high performance and long-term stability, Yavari and coworkers have adjusted

TABLE 16.4 Polymer solar cell devices with CD integrated (Section 16.2.4). Polymer Solar Cell Year

Device architecture

JSC (mA/ cm2)

VOC (V)

FF

PCE (%)

Role of CD

Ref.

2013

ITO/CQD-PCBM/Al

3.19

0.617

0.370

0.73

ETL material

Chandra et al. (2013)

2013

ITO/CD-Ag NPs/PEDOT:PSS/PTB7:PC71BM/Al

16.00

0.750

0.700

8.31

Modify the HTL

Choi et al. (2013)

2014

ITO/PEDOT:PSS/P3HT/CND/Al

0.29

1.588

0.485

0.23

Photoactive layer

Kwon et al. (2014)

2014

ITO/TiO2/PCDTBT:PCBM:Cdots/MoO3/Ag

13.61

0.870

0.556

7.05

Photoactive layer

Liu et al. (2014)

2016

ITO/PEDOT:PSS/P3HT:PC61BM/C-CQDs/Al ITO/ PEDOT:PSS/PTB7:PC61BM/C-CQDs/Al ITO/PEDOT: PSS/PTB7-TH:PC71BM/C-CQDs/Al

9,49 13,24 16,23

0,630 0,758 0,792

0,510 0,670 0,640

3,11 6,85 8,23

ETL material

Yan et al. (2016)

2016

ITO/PEI:Cdots/PCDTBT:PC71BM/MoO3/Ag

14.84

0.870

0.584

7.56

Interfacial layer

Zhang et al. (2016)

2016

ITO/ZnO/C-dots/PTB7:PC71BM/MoO3/Al ITO/AZO/Cdots/PTB7:PC71BM/MoO3/Al ITO/AZO/C-dots/PTB7Th:PC71BM/MoO3/Al

17.59 17.65 18.12

0.750 0.750 0.800

0.682 0.695 0.706

9.08 9.28 10.32

Interlayers

Lin et al. (2016)

2017

ITO/C-dots/PTB7-Th:PC71BM/MoO3/Al

15.35

0.770

0.680

8.13

Cathode interlayers

Yang et al. (2017)

2018

ITO/ZnO/P3HT:C-CQDs/MoO3/Al ITO/ZnO/P3HT:CCQDs:PC61BM//MoO3/Al

0.77 9.56

0.620 0.610

0.60 0.63

0.29 3.67

Electron acceptor

Cui et al. (2018)

2018

ITO/PEIE/P3HT:PC60BM 1 Cdots/MoO3/Ag

8.00

0.650

0.671

3.50

Modification of active layer

Lim et al. (2018)

2018

ITO/PEIE/CQDs/PTB7:PC71BM/MoO3/Ag

16.70

0.760

0.655

8.34

EEL

Lim et al. (2018)

2018

ITO/ZnO/C-QDs/PTB7:PC71BM/MoO3/Al

19.60

0.750

0.664

9.64

Electron extraction layer material

Zhang et al. (2018)

2019

ITO/PEDOT:PSS 1 CQDs/P3HT:PC61BM 1 CQDs/Al

17.10

0.540

0.469

4.31

Electron and hole transfer material

Wang et al. (2019)

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SECTION | V Sustainable Carbon-Based and Biomaterials

FIGURE 16.8 (A) Schematic illustration of energy levels of CQD/TiO2 layers formed on ITO as ETL for perovskite solar cells and J 2 V curves comparing CQD/TiO2 with the reference device under AM 1.5 G 1 sun (100 mW/cm2). (B) SEM cross-sectional image of the device configuration and JaV curves of PeSCs contracted with PEDOT: PSS (1) and PEDOT:PSS: CNDs (2) devices. (C) False-color, cross-sectional SEM image and energy-band diagram with the current-voltage characteristic of CnD/TiO2 perovskite solar cell depicting increased current density in the presence of urea and CnDs. (A) Reproduced with permission from Li, H., et al., 2017. Carbon quantum dots/TiO x electron transport layer boosts efficiency of planar heterojunction perovskite solar cells to 19%. Nano Lett. 17 (4), 23282335. (B) Reproduced with permission from Li, Z., et al., 2019. Using easily prepared carbon nanodots to improve hole transport capacity of perovskite solar cells. Mater. Today Energy 12, 161167. (C) Reproduced with permission from Sidhik, S., Velusamy, J., De la Rosa, E., Pe´rez-Garc´ıa, S.A., Ramos-Ortiz, G., Lo´pez-Luke, T., 2019. Role of carbon nanodots in defect passivation and photo-sensitization of mesoscopic-TiO2 for application in perovskite solar cells. Carbon N.Y. 146, 388398.

the perovskite film morphology by adding carbon nanoparticles (CNP) at an increasing concentration to the perovskite (CsM) precursor solution. The interaction of functional groups on CNPs and perovskite components resulted in higher thermal stability, larger grain size, and more hydrophobic perovskite films (Yavari et al., 2018). It has been found that the efficiency of PeSCs produced can be maintained above 18% after the addition of CNPs, while photocurrent hysteresis is significantly reduced compared to the reference device. In another study, bamboo-derived CNDs and urea were used as doping materials for the perovskite layer in PeSCs by Hsu et al. The carboxylic and hydroxyl groups on the surface of the CNDs facilitated molecular interaction with perovskite. As a result, this interaction provides effective passivation of traps/defects at the grain boundaries (Hsu et al., 2018). The interactions between CNDs and perovskite both advanced the morphology of the perovskite film and provided longer carrier lifetimes, increasing the current density due to diminished traps, which eventually resulted in an increased device PCE from 14.48% to 16.47%. Moreover, they obtained the best performance of 19.5%, with values of Jsc of 23.25 mA/cm2, Voc of 1.06 V, and FF of 79.2% from CND/urea

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embedded PeSC devices fabricated by the addition of urea, a Lewis base. More importantly, CND/urea embedded devices exhibited perfect shelf-life and air stability by protecting their high PCE in the air at 25 C and 40% humidity for a long time of 500 hours. The most widely used hole transfer material in PCSs is PEDOT: PSS with high working function and transparent features. Nevertheless, partial binding between PSS and PEDOT material generally reduces electrical and structural homogeneity. Li et al. used sulfur and nitrogen codoped CNDs (S, N codoped CNDs) as additives in the HTL of PeSCs aiming to overcome the problems in PEDOT:PSS layer (Li et al., 2019). The addition of CNDs to HTL removed the bond between PEDOT and PSS, which as a result, improved the structural and electrical homogeneity and exhibited a relatively higher device performance compared to the PEDOT: PSS system. The optimized PEDOT:PSS:CND based PeSCs revealed improved air stability and PV performance reaching to PCE of 18.03% with Voc of 1.01 V, Jsc of 22.6 mA/cm2, and a FF of 79.06%, (Fig. 16.8B). Sidhik et al. have introduced different concentrations (0, 1,3, 5 wt.%) of CnDs to mesoscopic TiO2 and used them as electrode material in PeSC (Sidhik et al., 2019). Researchers mainly focused on the effect of the mesoscopic TiO2/CnDs composite on the optoelectronic properties of the device. The presence of CnD in the electrode led to the passivation of oxygen defects of TiO2 and increased the charge carrier mobility, and photogeneration ability was also elevated due to the absorption capacity of CDs in the UV region. As a result, TiO2: CnD-based PeSCs displayed PCE (19.45%), which is 16% higher than that of pure TiO2-based PeSC (16.20%) (Fig. 16.8C). In their recently published article, Zhang et al. showed that the abundant and easily synthesizable CDs could be used as surface modifiers, which can minimize the charge injection/extraction energy barrier and improve the interface to form transparent thin films in PeSCs (Zhang et al., 2020). Using citric acid (CA) and ethylenediamine ratios, they adjusted the number of amine or carboxylic groups on the surfaces of the CDs. The effect of synthesized CDs on the WF of indium tin oxide (ITO) substrates was examined, and it was found that amine-containing CDs showed the capacity to reduce ITO WF from 4.64 to 3.42 eV, carboxyl groups bearing CDs increase to 4.99 eV, respectively. Researchers also showed similar improvement of the external quantum efficiencies of CsPbI3 perovskite and QDotLEDs formed of CdSe/ ZnS from 4.8% to 10.3% and from 8.1% to 21.9%, respectively, when they employ the CDs to the device structure. Finally, it was shown that when CD modifiers are used as an interface between SnO2 film and the active layer of PeSCs, significant improvement with a 19.5% PCE (with Voc 5 1.1 V, Jsc 5 22.7 mA/cm2, and FF 5 0.78) could be achieved as compared to the reference device (with PEC 5 17.3%, Voc 5 1.1 V, Jsc 5 21.9 mA/cm2, and FF 5 0.72). Hui et al. have achieved the highest mobility for the modified SnO2 ETLs in PeSCs (Hui et al., 2019).

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They used carboxylic acid and hydroxyl-rich red carbon quantum dots (RCQs) as doping agents for producing doped-SnO2 composite ETL, which provides superior electron mobility. The produced SnO2-RCQs ETLs (1.73 3 1022 cm2/V/s) exhibited  20 times higher electron mobility than reported electron mobility for SnO2-based ETLs (9.32 3 1024 cm2/V/s). When these SnO2: RCQ ETLs are integrated into planar (n-i-p) PeSCs, PCE of the device improved from 19.15% to 22.77%. Besides, PeSCs with RCQs showed long-term stability to moisture, maintaining more than 95% of the initial PCE after 1000 hours under 40%60% humidity at 25 C. For a more extended list of the recent literature not discussed in this section, the readers are referred to Table 16.5.

16.3 Summary and future aspects Future efforts to improve the stability, cost, and PCE of solar cells will continue to be focused, in large part, on controlling the material properties and the responses at the device interfaces. This goal requires continued information flow and gain skills in developing new materials with improved photochemical and electrochemical properties. Among those carbon-based nanomaterials carrying most of those properties that proved their adequacy in many different fields, herein, we collected and discussed the relationship between the intrinsic properties of CDs and the efficiency of CD-integrated solar energy devices. The literature on dye synthesized solar cells, organic and polymeric solar cells, QDSCs, and finally, PeSCs were discussed from different angles taking account the effect of optical, mechanical, and physicochemical features of CDs on device performance, and other related device features affecting the performance. Since the CDs have been discovered relatively recently, use of them in solar cell technology is also a relatively new and still evolving discipline. However, in this concise term, researchers showed that CDs carry a considerable potential for improving solar cell devices when integrated into varying layers, that is, dye sensitizer, photoanode, redox electrolyte, CE, and electron transfer layer. Most of these studies obtained the highest performance values when CDs are used in combination with the materials already used in the native device. CDs enhanced the device performance in varying ways, including electron mobility and charge carrier extraction, improved structural and electrical homogeneity through the device interfaces, faster electron transfer, enhanced photo-induced catalytic properties, charge separation, reduced charge recombination, and reduced bandgap. More importantly, those effects can easily be controlled by particle size and through simple surface modification by N- and S- groups. Likely, a better understanding of structural and PL properties of CD-based interfaces, and focusing on the significance of interface recombination and engineering on the device operation will offer a more efficient solar cell architecture. Advancements of fluorescent CDs will likely shift the focus of future

TABLE 16.5 Perovskite solar cell devices with CD integrated (Section 16.2.5). Perovskite Solar Cell Year

Device architecture

JSC (mA/ cm2)

VOC (V)

FF

PCE (%)

Application

Ref.

2016

FTO/TiO2/MAPI:CQDs/Au/ Spiro-OMeTAD

7.830

0.515

0.740

3.00

Hole transport material

Paulo et al. (2016)

2017

FTO/TiO2/CQD-CsPbBr3 IO/ spiro-OMETAD/Ag

11.34

1.060

0.690

8.29

CQD-embedded perovskite layer

Zhou et al. (2017)

2017

FTO/c-TiO2/m-TiO2:CDs/ MAPbI3-xCl3-x/SpiroOMeTAD/Au

22.64

1.019

0.716

16.40

CDs for converting to ultraviolet blue light in the m-TiO2 layer of PeSCs

Jin et al. (2017)

2017

FTO/NiMgLiO/Perovskite/GPCBM:CQDs/Ag

20.60

1.080

0.766

17.00

Ion/molecule blocking and EEL between the perovskite light absorber layer and the electrode layer.

Bi et al. (2017)

2017

ITO/CQDs:TiO2/MAPbI3-xClx/ Spiro-OMeTAD/Au

21.36

1.140

0.780

18.89

Carbon quantum dot (CQDs)/TiO2 composite as the ETL material

Li et al. (2017)

2018

FTO/TiO2/perovskite:CQDs/ Carbon

16.40

0.790

0.612

7.62

Photocurrent density enhancer in HTM-free perovskite solar cell

Zou et al. (2018)

2018

FTO/bl-TiO2/ CDs@CH3NH3PbI3:ml-TiO2/ Spiro-OMeTAD/Ag

20.66

1.080

0.739

16.49

As an additive to the perovskite layer for increasing the charge transfer from perovskite to the ETL.

Wang et al. (2018) (Continued )

TABLE 16.5 (Continued) Perovskite Solar Cell Year

Device architecture

JSC (mA/ cm2)

VOC (V)

FF

PCE (%)

Application

Ref.

2018

FTO/c- iO2/mp-TiO2/CsM: CNP/Spiro-OMeTAD/Au

22.10

1.150

0.710

18.30

CNPs in triple cation perovskite films of the generic form (CsM/ CNP).

Yavari et al. (2018)

2018

ITO/NiOx/CH3NH3PbI3: CQDs/PC61BM/BCP/Ag

21.68

1.060

0.787

18.24

As an additive for grain boundary passivation

Ma et al. (2018)

2018

ITO/NiOx/CH3NH3PbI3:CND/ PC61BM/BCP/Ag

22.74

1.070

0.769

19.50

As an additive for adapting the optoelectronic properties of perovskites.

Hsu et al. (2018)

2019

FTO/cp-TiO2/mp-TiO2/MAPI (-CQDs)/carbon

21.50

1.028

0.600

13.30

As an interface modifier between the perovskite layer and carbon electrode.

Han et al. (2019)

2019

ITO/NiOx/MAPbI3/PC61BM: CNDs/BCP/Ag ITO/PEDOT: PSS/MAPbI3/PC61BM:CNDs/ BCP/Ag

21.20 21.90

0.970 0.910

0.670 0.672

13.80 13.40

As an additive in PC61BM ETL for inverted planar MAPbI3 PeSCs

Subair et al. (2019)

2019

ITO/GO/Cdots/MAPbI3/PCBM/ Bathocuproine (BCP)

21.00

0.922

0.810

16.20

C-dots for modifying the hole handling layer in planar PeSC devices

Benetti et al. (2019)

2019

ITO/NiO:CQD/CH3NH3PbI3/ PCBM/BCP/Ag

19.57

1.080

0.774

16.42

As an additive in a NiO hole transport layer (HTL)

Kim et al. (2020)

2019

ITO/NiOx/CsFAMAPb(I,Br)3/ PC61BM:CDs/Ag

22.41

1.060

0.740

17.56

A cathode interlayer (CIL).

Zhu et al. (2019)

2019

ITO/PEDOT:PSS:CNDs/ CH3NH3PbI3/PCBM 1 BCP/ Ag

22.60

1.011

0.790

18.03

As an additive to improve hole transport capacity of PeSCs

2019

FTO/PEDOT:PSS/Perovskite/ PCBM:CQDs/BCP/Ag.

22.30

0.970

0.796

18.10

PCBM ETL

Zhu et al. (2019)

2019

IT0/PTAA/MAPbI3:CQDs/Ti/Cu

23.13

1.101

0.750

19.17

As an additive in the methylammonium iodide solution for high-quality CH3NH3PbI3 (MAPbI3) films.

Wen et al. (2020)

2019

FTO/TiO2:CDs/CH3NH3PbI3/ spiro-MeOTAD/Au

23.67

1.100

0.752

19.45

As an additive for defect passivation and photo-sensitivity of mesoscopic TiO2 for PeSCs

Sidhik et al. (2019)

2019

ITO/SnO2:CNDs/perovskite/ spiro-OMeTAD/Au

23.14

1.100

0.790

20.03

As an additive to the SnO2 ETL.

Wang et al. (2019)

2020

CQD@PMMA/FTO/SnO2/ Perovskite/spiroOMeTAD/Au

23.21

1.130

0.682

17.86

CQD@PMMA as the photoactive functional film on PeSCs

Maxim et al. (2020)

2020

ITO/SnO2:CD0.33/MAPbI3/ spiro-OMeTAD/Ag

22.70

1.100

0.780

19.50

In order to tailor interface properties between metal oxide (SnO2) film and active layers.

Zhang et al. (2020)

Li et al. (2019)

(Continued )

TABLE 16.5 (Continued) Perovskite Solar Cell Year

Device architecture

JSC (mA/ cm2)

VOC (V)

FF

PCE (%)

Application

Ref.

2020

ITO/SnO2-RCQs/ CH3NH3PbI3/spiroOMeTAD/ MoO3/Au

24.10

1.140

0.829

22.77

RCQD doped SnO2 composite with enhanced electron mobility for efficient and stable PeSCs.

Hui et al. (2019)

2020

ITO/PEDOT:PSS/Perovskite:NCQDs/PCBM/Al

23.07

0.822

0.587

13.93

As an additive in the CH3NH3PbI3 solution to reduce radiation-free chargeback recombination and to inactivate trap conditions in the absorbent layer.

Kırbıyık et al. (2020)

Carbon nanodot integrated solar energy devices Chapter | 16

529

research on solar energy devices from the use of semiconductive optically active nanomaterials, which are expensive and environmentally hazardous to carbon dots, which are cheap, easy to prepare, and biocompatible.

Acknowledgments This work is financed by the Turkish Scientific and Technological Research Council ¨ B˙ITAK) with project number 117M215. (TU

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Conibeer, G., 2014. Third-generation solar cells. Solar Cell Materials. John Wiley & Sons, Ltd, Chichester, UK, pp. 283314. Corby, S., et al., 2019. Charge separation, band-bending, and recombination in WO3 photoanodes. J. Phys. Chem. Lett. 10 (18), 53955401. Cui, B., et al., 2018. Fluorescent carbon quantum dots synthesized by chemical vapor deposition: an alternative candidate for electron acceptor in polymer solar cells. Opt. Mater. (Amst). 75, 166173. Dao, V.D., et al., 2016. Facile synthesis of carbon dot-Au nanoraspberries and their application as high-performance counter electrodes in quantum dot-sensitized solar cells. Carbon 96, 139144. Dhenadhayalan, N., Lin, K.C., Suresh, R., Ramamurthy, P., 2016. Unravelling the multiple emissive states in citric-acid-derived carbon dots. J. Phys. Chem. C. 120 (2), 12521261. Dou, D., Duan, J., Zhao, Y., He, B., Tang, Q., 2018. Cubic carbon quantum dots for lightharvesters in mesoscopic solar cells. Electrochim. Acta 275, 275280. Du, F., et al., 2014. Economical and green synthesis of bagasse-derived fluorescent carbon dots for biomedical applications. Nanotechnology 25 (31), 315702315712. Duan, J., Zhao, Y., He, B., Tang, Q., 2018. Efficiency enhancement of bifacial dye-sensitized solar cells through bi-tandem carbon quantum dots tailored transparent counter electrodes. Electrochim. Acta 278, 204209. Efa, M.T., Imae, T., 2019. Effects of carbon dots on ZnO nanoparticle-based dye-sensitized solar cells. Electrochim. Acta 303, 204210. Etefa, H.F., Imae, T., Yanagida, M., 2020. Enhanced photosensitization by carbon dots coadsorbing with dye on p-type semiconductor (nickel oxide) solar cells. ACS Appl. Mater. Interfaces 12 (16), 1859618608. Ferguson, V., Silva, S.R.P., Zhang, W., 2019. Carbon materials in perovskite solar cells: prospects and future challenges. Energy Environ. Mater 2 (2), 107118. Gao, Q., et al., 2019. Carbon nanoparticle template assisted formation of mesoporous TiO2 photoanodes for quantum dot-sensitized solar cells. N. J. Chem. 43 (14), 53745381. Geiker, M.R., Andersen, M.M., 2009. Nanotechnologies for sustainable construction. In: Khatib, J. (Ed.), Sustainability of construction materials. (1 ed., pp. 254284). Woodhead Publishing. Genc, R., et al., 2017. High-capacitance hybrid supercapacitor based on multi-colored fluorescent carbon-dots. Sci. Rep 7 (1), 11222. Gnanasekar, S., Kollu, P., Jeong, S.K., Grace, A.N., 2019. Pt-free, low-cost and efficient counter electrode with carbon wrapped VO2(M) nanofiber for dye-sensitized solar cells. Sci. Rep. 9 (1), 5177. Green, M.A., Dunlop, E.D., Hohl-Ebinger, J., Yoshita, M., Kopidakis, N., Ho-Baillie, A.W.Y., 2020. Solar cell efficiency tables (version 55). Prog. Photovolt. Res. Appl 28 (1), 315. Guo, X., Zhang, H., Sun, H., Tade, M.O., Wang, S., 2017. Green synthesis of carbon quantum dots for sensitized solar cells. ChemPhotoChem 1 (4), 116119. Hadadian, M., Sma˚tt, J.-H., Correa-Baena, J.-P., 2020. The role of carbon-based materials in enhancing the stability of perovskite solar cells. Energy Environ. Sci. Han, J., et al., 2019. An excellent modifier: carbon quantum dots for highly efficient carbonelectrode-based methylammonium lead iodide solar cells. Sol. RRL 3 (9), 1900146. Hsu, H.L., et al., 2018. Carbon nanodot additives realize high-performance air-stable pin perovskite solar cells providing efficiencies of up to 20.2%. Adv. Energy Mater. 8 (34), 19. Huang, P., Xu, S., Zhang, M., Zhong, W., Xiao, Z., Luo, Y., 2019. Modulation doping of absorbent cotton derived carbon dots for quantum dot-sensitized solar cells. Phys. Chem. Chem. Phys. 21 (47), 2613326145. Hui, W., et al., 2019. Red-carbon-quantum-dot-doped SnO2 composite with enhanced electron mobility for efficient and stable perovskite solar cells. Adv. Mater. 1906374, 19.

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Chapter 17

Solar cell based on carbon and graphene nanomaterials Abdellah Henni1, Nesrine Harfouche2, Amina Karar1 and Djamal Zerrouki1 1

Laboratory Dynamic Interactions and Reactivity of Systems, Kasdi Merbah University, Ouargla, Algeria, 2Polymer Materials Interfaces Marine Environment, University of South Toulon, Toulon, France

17.1 Introduction Due to the environmental pollution and energy crisis, new energy has attracted more and more attention. Access to energy is essential for today’s societies. To face the growing needs in energy, it seems essential to focus on nonfossil renewable energies. However, renewable energies are still minor today compared to other primary energy sources such as petroleum, coal, or natural gas. Solar energy is a sustainable and available renewable energy source and its participation in the power industry is growing day by day. Among the sources of solar origin which is the most abundant energy source, the photovoltaics cells (PVs) can be especially counted on to convert photonics into electrical energy (Varshney et al., 2020; Wang et al., 2020). The PV energy comes essentially from the sector of silicon. The siliconbased solar cells are the first generation with a power conversion efficiency (PCE) of 25% for crystalline Si solar cells and 10% for amorphous Si ones (Green et al., 2015). The manufacturing of such materials is expensive, both from a manufacturing cost and the energy cost. A second generation of solar cells is based on thin film technology based on copper indium gallium selenide or cadmium telluride. However, these cells have the disadvantage of low abundant In and Te as well as toxicity of Cd (Zhang et al., 2015). This is why a third generation was born: organic solar cells (OSCs) such as dyesensitized solar cells (DSSCs) (Gra¨tzel, 2003), organic-inorganic hybrid cells perovskites (Kojima et al., 2009) and OSCs themselves. The OSCs have been attractive because of their low-temperature processing during fabrication (belowB150 C), low thicknesses, and adjustable colors Sustainable Material Solutions for Solar Energy Technologies. DOI: https://doi.org/10.1016/B978-0-12-821592-0.00012-1 © 2021 Elsevier Inc. All rights reserved. 537

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(Polman et al., 2016). Currently, research efforts in the organic sector are focused on reducing the costs of raw materials or manufacturing, the stability and lifespan of solar cells as well as increasing yields. The working principle of these cells can be described into five main steps: (1) absorption of photons and creation of excitons, (2) diffusion of excitons, (3) separation of the exciton into free charge carriers, (4) transportation of charges, and (5) extraction of charges. These solar cells are based on the optoelectronic properties and charge transport of the organic semiconductors. The semiconductors constitute the active layer of the solar cell, which is between two electrodes. However, electron transport layer and hole transport layer are transported and collected to cathode and anode, respectively (Luque and Hegedus, 2011). The performance of the PV devices is influenced by the interactions between electrodes and active layers, these interactions lead to variation on morphology and interface for all the functional layers thanks to low-temperature process (Bi et al., 2017; Tan et al., 2017). In the last few years, carbon nanomaterials, such as fullerene, carbon nanotubes (CNTs), and graphene, have attracted great interest. The Nobel Prize in Chemistry was awarded in 1996 to R. F. Curl, H.W. Kroto, and R.E. Smalley for their discovery of fullerenes, and in 2010 the Nobel Prize in Physics was awarded to A. Geim and K. Novoselov for their original experiences with two-dimensional (2D) graphene (Prezhdo et al., 2011). Carbon nanomaterials are commonly used in solar cells devices for PV applications due to its adjustable energy levels, good mechanical properties such as thermal conductivity, optical absorption, carrier mobility and mechanical strength they also have well-organized structures which procure high conductivity and long-term stability (Gu¨nes et al., 2007; Luo et al., 2018). This chapter will thus focus on third-generation solar cells and the application of carbon nanomaterials to improve their performance. In particular, DSSCs, OSCs, and perovskite solar cells (PSCs) will be discussed. The carbon and its derivatives are also described.

17.2 Carbon and its derivatives Until 1985, only nonmolecular forms of pure carbon were known: those of graphite and diamond. Since then, new forms of carbon have been discovered as shown in Fig. 17.1, such as zero-dimensional fullerenes, 1D nanotubes and 2D graphene all forms consist of similar carbon atom networks, but exhibit significantly different properties that are dictated by size, shape, and chirality (Pan et al., 2020).

17.2.1 Fullerene One of the most attractive allotropes of carbon are fullerenes, which have a cage-like structure composed of sp2-hybridized carbon atoms and conjugated systems (Xu et al., 2020). Harold Kroto, Robert Curl, and Richard Smalley

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FIGURE 17.1 Structure and hybrid orbit of several allotropes of carbon. From: Pan, Y., Liu, X., Zhang, W., Liu, Z., Zeng, G., Shao, B., et al., 2020. Advances in photocatalysis based on fullerene C60 and its derivatives: Propertiesproperties, mechanism, synthesis, and applications, Appl. Catal. B Environ. 265, 118579. doi:10.1016/j.apcatb.2019.118579.

discovered fullerenes in 1985, which earned them the Nobel Prize in Chemistry in 1996 (Mart´ın, 2006). Fullerene C60 is spherical and made of pentagons and hexagons in its structure, the pentagonal ring has only a single bond, and in the hexagonal ring, the single bond and the double bond are alternately arranged. The average bond length of the single bond is 0.145 nm, and the double bond is 0.141 nm (Zhang et al., 1991). Fullerene consists 12 pentagons and 20 hexagons with 60 bonds among them 30 conjugate double bonds (Ganesamoorthy et al., 2017). Thus fullerenes are the zero-dimensional form of graphitic carbon and come in different forms and sizes ranging from 30 to 3000 carbon atoms. Fullerene is chemically synthesized by numerous methods including electric arc discharge, electron beam ablation and sputtering (Smalley, 1992). Most of these methods employed graphite electrodes or targets as the carbon source. The preparation in quantity of fullerenes (C60 and C70) from soot particles leads to the exploration of the properties of fullerenes for various applications (Zhennan et al., 1991). The cage structure of C60 molecules remains unchanged at very high temperatures, indicating excellent high temperature stability (Quo et al., 1991). Also, fullerene is transparent and has good electrical conductivity (1024 S/ cm). Most importantly, fullerene lacks bulky side-chains and can be packed more densely, which facilitates intermolecular charge transport. Thanks to good electron affinity, high carrier mobility at room temperature, and appropriate energy levels, fullerenes are effective electron transport materials in organic PVs (Gatti et al., 2017).

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17.2.2 Carbon nanotube Discovered by Sumio Iijima in 1991, CNTs are the spearhead of nanotechnologies (Iijima, 1991). A nanotube is a sheet of graphite formed of carbon atoms arranged in a hexagonal network, like a honeycomb, and rolled up on itself like a cigar. Its diameter is around a few nanometers and length can reach several micrometers. At each of its two ends is a half-molecule of fullerene of approximately one nanometer in diameter. They can be singlesheet (SWCNTs) or multisheet (MWCNTs) (Yu et al., 2016). The SWCNTs is mainly composed of monolayer graphite rolled into seamless cylindrical tubes, while MWCNTs can be seen as coaxially composed of several SWCNT with different diameters and a spacing between individual walls is ˚ . Three methods for preparation of bulk form CNTs are known: about 3.4 A arc discharge (Journet et al., 1997), laser ablation (Thess et al., 1996), and chemical vapor deposition (CVD) (Hata, 2004). The most used method is CVD because it is economical and the easiest way to produce at batch-scale CNTs. 100,000 times thinner than a hair, a CNT is 100 times stronger and 6 times lighter than steel. Its advantages do not stop there: it has great thermal conductivity up to 3500 W/m/K and good electronic properties (Salaway and Zhigilei, 2014). Furthermore, sp2 hybridization of carbon atoms in CNTs confers them excellent mechanical properties with Young’s modulus and fracture stress of 12 TPa and 50 GPa, respectively (Zhou et al., 2015). CNT is also a material with a specific surface area of 1600 m2/g (Kim et al., 2013). In addition, individual CNT has great intrinsic mobility of the charges exceeding 105 cm2 /V/s and current carrying capacity overshoot 109 A/cm2. CNT also has high electrical conductivity up to 106 S/cm thanks to π bonds formed by p electron orbital overlap (Chen et al., 2014). These excellent properties lead to their use in solar cell applications as transparent conductors and also as the photoactive components (Jariwala et al., 2013).

17.2.3 Graphene Graphite-like nanoplatelets have recently attracted attention as a viable and inexpensive filler in composite materials that can be used in many engineering applications, given the excellent inplane mechanical, structural, thermal, and electrical properties of graphite (Liu et al., 2010). These excellent properties may be relevant at the nanoscale if graphite can be exfoliated into thin nanoplatelets, and even down to the single graphene sheet level (Stankovich et al., 2007). Graphene is an atomically thin layer of sp2-bonded carbon atoms, stoked in a 2D honeycomb lattice, forming the basic building block for carbon allotropes of any dimensionality (Geim and Novoselov, 2007). The first synthesis of graphene dates from 2004 by Andre Geim of the University of Manchester, these discoveries awarded him the physics Nobel

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Prize with Konstantin Novoselov in 2010 (Ogino, 2010). A theoretical thickness of graphene is 0.335 nm (Chen et al., 2008). Graphene can be synthesized by a bottom-up approach or top-down approach (Zheng et al., 2010). The first approach is not suitable for the production of large quantities of graphene and are rather reserved for high added value applications such as the production of devices operating at very high frequency on the other hand, the top-down approach based on exfoliation in solution is suitable for mass production of graphene sheets. The primary obstacle of achieving individual or few layer graphene is overcoming the enormous interlayer Van der Waals forces (Marcano et al., 2010). Graphene has many properties that can be used for a wide range of applications. 2D material, it is characterized by a high Young’s modulus of 1 TPa, a very high electrical conductivity of 6.103 S/cm and thermal conductivity of 5.103 W/m/K. It has great flexibility and transparency, excellent charge mobility about 2.105 cm2 /V/s, a large specific surface of 2600 m2 /g and also acts as a moisture barrier (Ray, 2015). It behaves like a semiconductor with zero gap (B00.25 eV) (Mittal, 2014). It has been observed in the literature that the transport properties strongly depend on the number of graphene sheets. More particularly, beyond 10 sheets, the material seems to have the same properties as those of graphite (Partoens and Peeters, 2006). The high conductivity and transparency of single layer graphene make it a promising material for transparent conducting electrodes in organic PVs.

17.3 Solar cells based on carbon nanomaterials 17.3.1 Carbon in dye-sensitized solar Numerous attempts have been reported to improve the pristine properties of oxide such as heterostructuring, alloying and doping (Bouznit, 2020; Bouznit and Henni, 2019; Abdellah Henni et al., 2016; A. Henni et al., 2016; Henni et al., 2015, 2019; Mahroug et al., 2018). The CNTs are promising materials that can increase the efficiency of DSSCs, as they provide good electrical conductivity to nanocomponent metal oxides. However, they can be added to either the electrolytes or the counter electrodes (Uk Lee et al., 2010). The recombination of electrons is a very important factor affecting the conversion efficiency of DSSCs. CNTs have a special structure and can conduct electrons at room temperature without electrical resistance. The TiO2CNTs compound can also easily conduct photoelectrons while electrons are generated from TiO2 under light irradiation. CNTs attach to TiO2 nanoparticles to make charge transport more efficient from the active layer to the electrodes. This would extend their lifespan and therefore improve the conversion efficiency of the cell. The principle of operation is almost the same as in a conventional DSSCs.

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In 1996, Kay and Gra¨tzel (1996) replaced, for the first time, the platine by a less expensive nanomaterials based on graphite and carbon black. Subsequently several studies have been reported to replace the Pt by carbonbased materials and the CNTs (Noureldine et al., 2012) and graphene-based composites (Wang et al., 2012; Bajpai et al., 2012) have been widely studied. The efficiency results of DSSCs based on these materials are comparable to those based on Pt electrodes. On the other hand, in order to improve the efficiency of charge collection, different types of CNTs (Chan et al., 2013; Guo et al., 2011; Dang et al., 2011) have been added to the active layer of metal oxide. According to the literature, a percentage between 0.01 and 0.3 at.% of CNT is optimal in the active layers of DSSC. Dembele et al. (2013) have shown that the efficiency increases from 7% to 9% when 0.01% of CNT is added within the porous layers. This can be associated with a variation in the Fermi level of the active layer. For example, as the conduction band of the CNTs is generally weaker than that of the TiO2, the level of Fermi TiO2 will move this toward the more negative potentials (Du et al., 2013). Mainly, the purpose for introduction of CNTs into the mesoporous electrodes is that CNTs would fill the role of the special channels to reduce the resistance of the electrodes and facilitate electron transport, as illustrated in Fig. 17.2 (Chen et al., 2012). he DSSC equipped with functionalized multiwalled carbon nanotubes (fMWCNTs) embedded with RGO has a PCE of 8.7%, which is higher than that of Pt-based CE (Khan et al., 2020). The efficiency of DSSCs increases with the graphene content up to a critical value beyond which the yield decreases. Most studies use grades less than 0.3 at.% (Kazmi et al., 2017). The Fig. 17.3 shows the conceptual diagram of the working principle of DSSC. The types of graphene studied are mainly reduced graphene oxides, but also graphene nanoobjects obtained by liquid phase exfoliation or else by electrochemical exfoliation (Su et al., 2011; Hong et al., 2008). Nanocomposites, on the other hand, were made by a mixture of two dispersions of graphene

FIGURE 17.2 Schematic diagrams for the photogenerated electron transport route in blank semiconductor electrodes (A) and carbon nanotube-incorporated electrodes (B) of dye-sensitized solar cells. From: Chen, J., Li, B., Zheng, J., Zhao, J., Zhu, Z. 2012. Role of carbon nanotubes in dye-sensitized TiO2 -based solar cells, J. Phys. Chem. C. 116 1484814856. https://doi.org/ 10.1021/jp304845t.

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FIGURE 17.3 Conceptual diagram of the working principle of DSSC using SnS2@fMWCNTs@RGO counter electrode. DSSC, dye-sensitized solar cells. From: Khan, M.W., Zuo X., Yang, Q., Tang, H., Ur Rehman, K.M., Wu, M. et al., 2020. Functionalized multi-walled carbon nanotubes embedded with nanoflakes boost the short-circuit current of Ru (II) based dyesensitized solar cells, Dyes Pigments. 108573. https://doi.org/10.1016/j.dyepig.2020.108573.

and TiO2, or else by a thermal process involving the synthesis of TiO2 nanoparticles in the presence of graphene Fig. 17.3.

17.3.2 Carbon in organic solar cells For about 30 years, the use of organic semiconductor materials in PV cells has been of great interest to scientific researchers. As OSCs are made up of several active layers surrounded by electrodes, the efficiency and performance of OSC depend on many parameters. Among the donor-acceptor type organic cells, the most promising combination is that of P3HT and the fullerene derivative PCBM (Brabec et al., 2010). Research on the use of carbonaceous materials in solar cells has made it possible to develop an alternative of flexible, light, and low-cost devices. Two types of carbonaceous material are always involved in this research, fullerene and CNTs; particularly by combining them with p-type conjugated polymers and considered as photoactive materials (Fig. 17.4) (Jun et al., 2013). The work functions of multi-and single-walled CNTs are found to be 4.95 and 5.05 eV, which is close to the valence band of P3OT/P3HT (Zhu et al., 2009). Therefore nanotubes can transport electrons and holes more efficiently in OSC. In a heterojunction between P3HT/CNT and C60 compounds, the CNTs provide more improved charge transport mobility (Pradhan et al., 2006). A complex of C60-SWCNTs has been used as a component of the photoactive layer in the heterojunction of a PV cell (Li and Mitra, 2007). C60 offers a large surface for photoexcitons dissociation and transport of electrons. The cells based on C60 and MWCNTs-P3OT (Kalita et al., 2008) respectively show a short-circuit current (Jsc 5 1.68 mA/cm2), an open circuit voltage

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FIGURE 17.4 Schematic representation of organic solar cells using N-doped graphene/P3HT: PCBM active layer. From: Jun, G.H. Jin, S.H., Lee, B., Kim, B.H., Chae, W.S., Hong, S.H. et al., 2013. Enhanced conduction and charge-selectivity by N-doped graphene flakes in the active layer of bulk-heterojunction organic solar cells, Energy Environ. Sci. https://doi.org/ 10.1039/c3ee40963e.

(Voc 5 0.24 V), a form factor (FF 5 27%), and an efficiency of 0.11%. The MWCNT ensures efficient transport of holes. The best device was obtained with the ITO/PEDOT configuration: MWNT-PSS/P3OT-CNx-functionalized/LiF/Al, showing a remarkable increase in Voc (up to 1.0 V) and Jsc. CNT films have been used as top or back electrodes in OSC (Ulbricht et al., 2006). Graphene has also been widely used as the transport layers of holes and electrons in OSC (Liu et al., 2012). In order to improve hole extraction, rGO has also been used as hole transport layers in combination with other materials such as bilayers (Chen et al., 2015). To improve the transport of charge carriers, rGO has been incorporated into the active layers of binary or ternary mixtures (Romero-Borja et al., 2015; Stylianakis et al., 2015).

17.3.3 Carbon in perovskite solar cells In the case of the PV sector based on perovskite materials, several studies highlight the benefit of CNTs for the transport of charges in PSCs (Jeon et al., 2017). Generally, CNTs have been integrated into PSCs in order to improve the holes collection, for example CNTs have been used as an interface between perovskite and Spiro-OMeTAD, or else mixed with SpiroOMeTAD (Habisreutinger et al., 2017). According to Ihly et al. (2016) the use of single-sheet nanotubes improves the rapid transfer of holes from the perovskite to the nanotubes. This promotes the separation of charges and therefore the reduction of charge recombination phenomena within cells. In addition, an improvement in PV performance as well as in the stability of perovskite cells was observed in the presence of CNTs. The reduction of charge recombination phenomena has also been observed by Bag et al. (2016), with the incorporation of MWCNTs in the active layer of PSC. In general, CNTs are mostly present in perovskite cells in the form of a thin film produced from a dispersion of CNTs and/or mixed with another material. To date, no use of vertically aligned carbon nanotube mats PSCs

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has been identified in the literature, which could however be of interest for a rapid and efficient transport of electrons along the nanotubes. Fig. 17.5 shows the graphical illustration of carbon allotropes sandwich approach (ITO/C60/CH3NH3PbI3/HTM/CNT). In this figure the perovskite active layer is sandwiched by fullerenes and CNTs entangled with a hole-transporting material (Ahn et al., 2018). Snaith and Nicholas Team is the first to integrate graphene into the mesoporous TiO2 layer of a PSCs (Wang et al., 2014). Generally, for the formation of a mesoporous TiO2 requires sintering of about 500 C. Snaith et al. and Wang et al. (2014) lowered the annealing temperature to 150 C for a composite layer of TiO2 and graphene with an efficiency of 15.6% is obtained for the PSC. The authors attribute this improvement to an increase in the resistance to recombination of electron-hole pairs in the active layer (Fig. 17.6). Similarly, we can hypothesize that a charge transfer from TiO2 to graphene is possible, and favored by better contact between the two materials. As in the case of DSSCs, studies have shown that beyond a graphene concentration the performance of PSCs decreases (Agresti et al., 2016). Han et al. (2015) attributed the improvement in the performances to the decrease in interface resistance and attributed the decrease in the performances to the degradation of films, more precisely to a decrease in transmittance (about of 9%). According to Sidhik et al. (2018), the presence of graphene nanoplates changes the optical gap of TiO2 and improves the transport of photogenerated electrons. Agresti et al. (2017) have been interested in switching from the PSC to the PSC module. For this, they have developed 12 reference cells of 0.1 cm2 for which the average efficiency is 13.5%. After incorporation of graphene

FIGURE 17.5 Carbon-sandwiched perovskite solar cell. Aitola, K., Domanski, K., CorreaBaena, J.P., Sveinbjo¨rnsson, K., Saliba, M., Abate, A., et al., 2017. High temperature-stable perovskite solar cell based on low-cost carbon nanotube hole contact, Adv. Mater. https://doi. org/10.1002/adma.201606398.

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FIGURE 17.6 (A) Scanning electron microscope micrographs showing a general schematic of perovskite solar cells. (B) Schematic illustration of energy levels of the materials used in this study. From: Wang, J.T.W., Ball, J.M., Barea, E.M., Abate, A., Alexander-Webber, J.A., Huang, J. et al., 2014. Low-temperature processed electron collection layers of graphene/TiO2 nanocomposites in thin film perovskite solar cells, Nano Lett. https://doi.org/10.1021/nl403997a.

into the mesoporous TiO2 layer of the PV cell, the cells show an average efficiency of 15.3%. Subsequently, modules composed of 8 cells of 6.32 cm2 of active surface connected in series were then manufactured. As observed in the literature, the transition to larger active surfaces implies a drop in PV efficiency. The modules composed of pure TiO2 lead here to an efficiency of 11.6% which reaches 12.5% in the presence of graphene. In addition, it seems possible to recycle perovskite PV cells without altering the TiO2 layer. The teams of Kadro et al. (2016) performed easy recycling of PSCs while retaining the mesoporous TiO2 layer. The dismantling

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process used requires only 10 minutes, and the PV cells produced from these substrates gave almost identical efficiency to the initial cells.

17.4 Challenges and prospects One of the most abundant elements of the Earth’s crust is carbon, with many allotropes. The different combinations of carbon atoms produce varieties of materials with interesting chemical, electronic and physical properties. In the beginning, researchers believed that CNTs and other carbon nanomaterials are not well suited to develop efficient PV cells. Recently, researchers have shown that it is possible to use these different materials to make entirely carbon-based compound devices (Roy et al., 2020; Su et al., 2020). For the first time, scientific researchers from Stanford University have assembled a PV cell made entirely of carbon. This is a great alternative to replace expensive materials. Ramus et al. have made a PV cell entirely based on carbon materials (Ramus et al., 2012) using different carbon allotropes for all components (the cathode, active layer, and anode). The produced PV consists of an active layer sandwiched between the anode and the cathode. Instead of using more expensive conventional electrodes such as ITO or silver, electrodes with rGO and doped SWNTs were used as the anode and cathode respectively (Fig. 17.7). For the photoactive layer, a material composed of CNTs is used.

FIGURE 17.7 Structure of the all-carbon solar cells. From: Ramus, M.P., Vosgueritchian, M., Wei, P., Wang, C., Gao, Y., Wu, Y. et al., 2012. Evaluation of solution-processable carbon-based electrodes for all-carbon solar cells, ACS Nano. 6, 1038410395. https://doi.org/10.1021/ nn304410w.

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TABLE 17.1 The power conversion efficiency of works used carbon and its derivatives in photovoltaics cells. Photovoltaics type

Device structure

Power conversion efficiency (%)

Reference

Dye-sensitized solar cells

ITO/PET/dye/SWCNT/PEDOT

6.0

Hashmi et al. (2014)

Glass/rGO/TiO2/dye/spiroOMeTAD/Au

0.26

Yin et al. (2010)

FTO/TiO2/dye/Co(III)/(II) mediated electrolyte/ graphene

9.3

Kavan et al. (2011)

ITO/MWCNT/P3HT:PCBM/ LiF/Al

2.7

Hatton et al. (2009)

ITO/P3HT:PCBM:SWCNTs/Al

3.02

Yan et al. (2014)

Glass/SWCNT/PEDOT:PSS/ P3HT:PCBM/ 13.78 Ca/Al

4.13

Barnes et al. (2010)

Quartz/rGO/PEDOT:PSS/ CuPc:C60/BCP/Ag

0.85

Park et al. (2012)

PET/rGO/PEDOT:PSS/CuPc: C60/BCP/Al

1.18

Gomez De Arco et al. (2010)

Au-graphene/PEDOT:PSS/ P3HT:PCBM/ZnO/ITO

3.04

Li et al. (2011)

FTO/c-TiO2/TiO2/ CH3NH3PbI3/SWCNT/GO/ PMMA/Au

11.7

Wang et al. (2016)

FTO/c-TiO2/TiO2/ Cs5((CH3NH3)0.17(CH4N2)0.83) Pb(I0.83Br0.17)3/ spiroOMeTAD/SWCNT

16.6

Aitola et al. (2017)

FTO/c-TiO2/CH3NH3PbI3/sSWCNTs/Au

16.5

Ihly et al. (2016)

FTO/graphene/TiO2/ CH3NH3PbI3-xClx/SpiroOMeTAD/Au

15.6

Wang et al. (2014)

FTO/TiO2/CH3NH3PbX3/ Spiro-OMeTAD/PEDOT:PSS/ PDMS/PMMA/graphene

12.37

You et al. (2015)

Organic solar cells

Perovskite solar cells

(Continued )

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TABLE 17.1 (Continued) Photovoltaics type

Device structure

Power conversion efficiency (%)

Reference

All-carbon solar cells

FTO/rGO-TiO2/rGOCH3NH3PbI3/rGO-SpiroOMeTAD/Ag

16.5

Balis et al. (2020)

BCP, bathocuproine; ITO, Indium doped tin oxide; MWCNTs, multi-walled carbon nanotubes; PCBM, phenyl-C60-butyric acid methyl ester; PMMA, polymethyl methacrylate; SWCNT, single wall carbon nanotube; PET, polyethylene terephthalate; FTO, fluorine-doped tin oxide; PDMS, polydimethylsiloxane; PEDOT, Poly(3,4-ethylenedioxythiophene); PSS, Poly (StyreneSulfonate); GO, Graphene oxide.

This invention can be used as an OSC for the production of renewable energy, with characteristics particularly well suited to environments. This research opens up the possibility to explore carbon derivatives in solar cells. On the other hand, flexible solar cells, including DSSCs, OSCs, and PSCs, have received a lot of attention worldwide. Although flexible solar cells still suffered from several obstacles before industrialization, despite the great progress made by researchers. The PCE of PV devices that use carbon-based composites have been summarized in Table 17.1.

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Chapter 18

Sustainable biomaterials for solar energy technologies Yakup Ulusu1, Numan Eczacioglu1 and Isa Gokce2 1

Department of Bioengineering, Faculty of Engineering, Karamanoglu Mehmetbey University, Karaman, Turkey, 2Department of Bioengineering, Faculty of Natural Sciences and Engineering, Tokat Gaziosmanpasa University, Tokat, Turkey

18.1 Introduction The use of biomaterials in photovoltaic technology is increasing day by day. Although these materials may have some problems such as electron transfer, electronic communication, production costs, they started to have a dramatic usage share especially in environmentally friendly and effective solar cell works. In recent years, especially in the genetic engineering, bioengineering, and synthetic biology fields, the interest in biomaterials has increased. The use of biomaterials that can be obtained thanks to biological sciences in these technologies is important in reducing the dependence on fossil fuels as well as reducing the carbon footprint due to the increasing energy need. For this purpose, the ability to absorbing the solar radiation by producing biohybrid solar cells and thus to produce efficient electricity is one of the issues that scientists have been thinking about in recent years (Schuergers et al., 2017). The most frequently used systems for this purpose are the production of biohybrid solar cells. For this, fixation of sunlight-absorbing proteins or other biomaterials on the surfaces of the materials to be produced by solar cells is the first step to be done in biohybrid systems. Protein engineering studies are making efforts to develop new modified proteins to solve problems in this regard (Ihssen et al., 2014). The use of self-repairing microorganisms in living photovoltaics can be an example of sustainable biohybrid systems. The photosynthetic reaction center protein is composed of a transmembrane pigment protein complex that acts as a charge-separating agent for e-h pairs formed by light (Lu et al., 2005). Due to the promising role of semiconductor/protein membranes in the development of bioelectronics they have been extensively studied (Lu et al., 2005; Trammell et al., 2004; Zhao et al., 2002). Based on the principles and materials of photosynthesis, the biosensitized Dye Sensitized Solar Cell (DSSC) Sustainable Material Solutions for Solar Energy Technologies. DOI: https://doi.org/10.1016/B978-0-12-821592-0.00019-4 © 2021 Elsevier Inc. All rights reserved. 557

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called photosynthesis has received extensive attention from the scientific community in the field of photovoltaics (Kay and Graetzel, 1993). Chlorophyll derivatives are taken into consideration to be an outstanding organic substitute for classic dye sensitizers in DSSC to transform solar energy into electric strength (Wang et al., 2005).

18.2 Structural properties of biomaterials Solar cells designed with the aim of using in solar energy technology contain certain layers in its design. These layers consist of two basic parts, photoanode and photocathode, where electron transfer takes place. The photoanode glass surface comprises transparent conductive oxide (FTO) and n-type semiconductor (TiO2). While the n-type semiconductor absorbs light from the UV-spectrum region, the transparent dye molecules (usually Rutheniumbased, N719) coated in addition to this region also absorbs light from the Visible-NIR spectrum regions. Solar cells made using dye molecules are called DSSCs, which are initiated the idea by O’regan and Gra¨tzel (1991). Since 1991, different improvements, modifications, and research have been made intensely on DSSCs. Ru-based (Ruthenium-based) dyes used in DSSCs are toxic and quite expensive, although high yields (14.3% efficiency, Kakiage et al., 2015) were obtained from dye-based solar cells. Some biobased materials that harvest light to make more use of sunlight can also be coated on the TiO2 surface instead of the transparent Ru-based dye molecules. Recent studies have shown that biological materials (BioBased) can be used instead of ruthenium-based dyes. Biobased molecules are complex structures in content, and they have a faster degradation rate compared to Ru-based dyes due to the production and biosensitivity of reactive oxygen species and also solar excitation values are very low. Despite this, it is obtained at a very low cost with the use of nature-friendly, autonomous and self-healing microorganisms compared to Ru-based molecules (Bradley et al., 2012). Thus the focus of the research efforts shifted to natural photosensitizers such as living organism like cyanobacteria, light-harvesting proteins, and natural pigments. What is expected from a solar cell is to convert solar energy into an external electric current. In biobased solar cells, solar energy is converted to redox energy through photosynthesis and external electricity production through extracellular electron transfer (Fig. 18.1). The basis of the use of dye molecules in solar cell systems is that the electrons in the dye molecules stimulated by the sun rays pass through the semiconductor layer and contribute to the formation of current. The biomaterial oxidized by the transition of electrons to the semiconductor layer must be reduced. The biomaterial, which regains the lost electrons by oxidation of the electrolyte as a result of the redox reaction, makes its work continuous. In biobased solar cells, the ability of biomaterials used to absorb solar rays

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FIGURE 18.1 Biobased solar cell (the colored balls may be any of the biomaterials that can be used in photovoltaic technology).

in red and near infrared spectral ranges and to generate photocurrents thanks to electron transfer. A solar cell is an electronic device used to convert solar energy to electricity and produces both a current and a voltage to generate electric power. This process requires sensitization of broad band spaced semiconductors mainly from dyes, photoelectrodes, electrolytes, counter electrodes, and transparent conductive oxide layered substrate glass. If the solar cell is exposed to light, biomaterial photosensitizers in the cell absorb some photon within a certain range of wavelengths based on their bandgap energy. The absorbed photon excites electrons in the valence band of the biomaterial to the conduction band. The excited electrons are transferred to the conduction band and this provides them with high conductivity. The photoanode is connected to the counter electrode by an external circuit so that the produced electrons can flow to the counter electrode. The opposite effects generate the virtual charges caused by electron excitation form holes in the valence band of biomaterial and transfer to the electrolyte by oxidizing the electrolyte. The oxidized electrolyte is reduced at the counter electrode by the electrons returning after cycling the external circuit, so that the cell is in its initial state. This makes it possible for electrons to circulate the cell over and over, even theoretically, infinitely (Hwang and Yong, 2015; Adedokun et al., 2016).

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Biological photovoltaic devices, also called photomicrobial fuel cells or living solar cells, are a kind of biological electrochemical system or microbial fuel cell. In biological photovoltaic systems using of biological organisms, electrons are transferred to the anode (anode) by decomposition of water into oxygen and hydrogen by photolysis. Another reaction on the cathode surface changes the potential equilibrium of the reaction chamber and makes a potential difference so that an electron flow occurs. As a result of an electron flow from anode to cathode, generation of electric energy is detected by an external circuit. One of the problems encountered when using both biomolecules and living organisms in solar cells is attaching them to the photo anodic surface. In this context, two ways are applied, either by direct application or by using an interconnecting material, to attach to the surface (Fig. 18.2). Electrons can reach the anode through indirect extracellular electron transfer (IEET), a system based on the electron transporters that spread between the cells and the electrode. Systems based on IEET have low efficiency from typically low power densities due to restrictions in mass transfer rate (in particular, overconcentration potential) between cells and electrode. As a result of direct

FIGURE 18.2 Electron transfer mechanism: (A) direct contact with the electron structure containing nanolinks and transferred throughout this network structure, (B) direct contact with surface redox proteins and electron transfer.

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application of the biological material to the photoanode surface, close physical contact occurs with the electrode, which provides a higher power density (McCormick et al., 2015). Biomaterials to be used in solar cell technology must have many structural features such as hardness, elasticity, elongation coefficient, thermal resistance point, optical properties, long-term resistance, photocurrent production, and providing effective electron transfer cycle. In fact, in terms of high efficiency, the ability to connect the internal electron transfer loop to an external electrode system comes to the fore, while its ability to maintain its stability for a long time when examined in terms of cost is also important in the selection of biomaterial to be used. For an effective biobased solar cell, absorption of solar rays in a wide wavelength range should be very high and the excited state energy level should be suitable for the conductivity band of the semiconductor used (Suhaimi et al., 2015). To minimize energy losses and maximize photovoltage, the excited state of the biomaterials attached to the surface should be slightly above the conductivity band edge of n-type semiconductor. The electron injection process from the excited state to the conductivity band of the semiconductor should be fast enough to overcome the unwanted relaxation and reaction pathways. The biomaterial must show high solubility in the solvent used to adhere and adhere well to the semiconductor surface (So¨nmezo˘glu et al., 2012). One of the most important factors affecting the efficiency and performance of biobased solar cells is the molecular structure of the biomaterial (Suhaimi et al., 2015). Performance parameters of biophotovoltaic systems: A DSSC can be described by an IV-curve, which plots the corresponding current (I) when the voltage (V) rises. At a deviation of 0 V the short circuit current (ISC) is measured and when the current reaches 0 A, it represents the opencircuit voltage. When the result of current and voltage is maximum, the maximum power output (Pmax) produced by DSSC will be reached. The maximum power can be determined by the following equation. The corresponding factors are called current at maximal power (Imp) and voltage at maximal power (Vmp). Pmax 5 Imp 3 Vmp Generally, using current density (J) [mA/cm2] instead of current (I) [mA] can better compare the measurement results of different solar cells. Therefore the ISC and Imp become JSC and Jmp, respectively, so that JSC and Jmp are independent from the photoactive area. In addition to the stability of its life time, how solar cells effectively convert the energy of incident light into electrical energy has also become the focus of attention. The conversion efficiency (η) from solar energy to electricity is calculated by the following equation: η 5 Pmax =Pin 5 Imp 3 Vmp =Pin 5 ISC 3 VOC 3 FF=Pin The incident power (Pin) is the irradiance illuminated on the DSSCs. The Pin must be measured in advance. The efficiency is between 0% and 100%.

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Open-circuit voltage (VOC), the fill factor (FF), ISC and η are the four performance parameters of a solar cell. The FF is a value between 0 and 1, which describes the shape of the IVcurve, where a higher value indicates a more preferred rectangular shape described by the following equation: FF 5 Imp 3 Vmp 5 ISC 3 VOC In order to compare different solar cell concepts and equipment, the standard spectrum (AM1.5) and measurement conditions (uniform illumination 100 mW/cm2 at 23 C) are characterized (Hug et al., 2014). In general, the use of biomaterials in solar conversion devices is very promising. While stability and low photocurrent generation prevent these systems from competing with traditional methods, there are several advantages that can make them a good alternative for certain applications. Although biomaterials have low stability, the major advantage is that most of them are very low cost, which makes the electrodes disposable and easy to replace. It is very useful to use such disposable electrodes in solar energy conversion devices, which have no significant negative impact on the environment and are a clean energy source. (Rasmussen and Minteer, 2014).

18.3 Biomaterials used in biophotovoltaics With the advances in solar cell technology, third generation DSSCs have started to attract attention with their high efficiency, diversification of selectable sensitizers, electrolytes, and electrodes (Gra¨tzel, 2003). In these systems, many Ruthenium and other metal-based inorganic compounds and nonmetal organic dyes are used as the main sensitizer thanks to their serious device efficiency and stability (Yum et al., 2009). However, the limited availability, high cost, and carcinogenic nature of ruthenium force scientists in this field to search for alternative nature-friendly and sustainable materials. For this, many biomaterials and dyes were extracted from plants, algae, and microbes and used as photosynthesizer. The advantages of biomaterials make them one step ahead of synthetic dyes. These biomaterials are abundant, renewable, sustainable, cost-effective, scalable, nontoxic, completely biodegradable, pose no health concerns to humans which makes them environmental-friendly alternatives and absorb most of the light energy due to their wide absorption spectrum (Maddah et al., 2020). In biobased solar cells, proteins, pigments, or photosynthetic systems that are isolated directly from the organism or organism can be used as biomaterials. Although isolated biomaterials do not possess respiration that competes with photosynthesis in sharing electron transfer pathways and no need require nutrients to sustain, whole cells stand out with high efficiency due to biomolecule stability, proper immobilization, electrical communication. However, for direct light-electric conversion applications, it is preferred to

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use a system that uses only water as an electron donor such as PSII (Calkins et al., 2013; Sekar et al., 2014). Thylakoid membranes are also considered a good candidate because they contain improved stability and several electron transfers paths (Hasan et al., 2017). Some of parameters to consider when selection of an ideal biomaterial to be used in biobased solar cell include; existing functional groups, chemical (covalent) and physical bonds, the number of conjugated double π-bonds (n) and the length of the organic hydrocarbon structure chain. It is preferred that the biomaterials have a hydrocarbon chain with a lot of hydroxylic (OH), carboxylic (COOH) and (1/4O) radicals to provide stronger bonding onto the semiconductor surface, resulting in lower electron resistance and easy electron injection. The possibility of incorporation of phenyl units on both donor and acceptor moieties within the biomaterials and also introduction of bulky groups in the biomaterial’s spacer segment has been reported to reduce aggregation of them, achieve higher power conversion and increase electron lifetime (Maddah et al., 2020). Construction of biobased solar cells involves several challenges such as immobilization of photosystem proteins to electrodes electron transfer connections of photosystems with electrodes and effective collection of light necessary to achieve high photon-electron efficiency. For this purpose, crosslinking molecules (nickel—nitrilotriacetic acid) (Noji et al., 2011), diffusional electron transfer mediators on mesoporous electrodes (Kato et al., 2012), the entrapment of biomaterial in a redox-active molecule (Os(III)polypyridine) linked to the electrode surface (Badura et al., 2008), some terminal electron acceptors (Co(II)/(III) complexes) (Ulas and Brudvig, 2011) or precious metals such as gold (Au nanoparticles films) (Yehezkeli et al., 2012) are used as immobilization support for immobilization of PSII reaction centers on electrodes (Calkins et al., 2013; Yehezkeli et al., 2013).

18.3.1 Living organism based solar cell systems In biobased solar cell systems photosynthetic organisms are used which Cyanobacteria is prominent. Since Cyanobacteria are a prokaryotic group, photosynthetic light reactions occur in dense packet membranes called thylakoids instead of chloroplasts, in contrast to highly structured plants. Thylakoid membranes are an intracellular membrane system that usually covers most of the cell’s cytoplasm in the form of a series of concentric cylinders aligned along the long axis of the cell. The thylakoid membrane in Cyanobacteria, quite similar to those found in green plants, contains photosynthetic electron transport chain, as well as a proton translocating ATPase, including reaction centers PSI and PSII and cytochrome b6f complex (Mullineaux, 1999). Cyanobacteria, thanks to the unique architecture of their thylakoid membrane structure, enable the conversion of solar energy into

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chemical energy with a much better efficiency than any man-made photovoltaic system (Vothknecht and Westhoff, 2001). Plants, algae and cyanobacteria take advantage of the energy production process called photosynthesis, which is used to convert light energy into chemical energy. The basic process in photosynthesis is the division of water into oxygen and hydrogen, that is, photolysis, with the effect of sunlight. The reaction center complex proteins in the thylakoid membrane (PSI and PSII) absorb the sunlight. When the PSII reaction center known as the P680 absorbs photons, its electrons rise at a higher energy level. The plastoquinone (Qa and Qb) molecules known as electron acceptors transfer the electrons they captured, to the Cytochrome b6f and then to PSI. The P700 reaction center absorbs another photon to create an electron with a higher energy level. The electron acceptors phylloquinone and ferrodoxine carry these electrons in order to reduce nicotinamide adenine dinucleotide phosphate (NADP1) to NADPH (reduced form of NADP1) (C ¸ evik et al., 2018).

18.3.1.1 Algae and cyanobacteria Algae are known as photosynthetically highly productive organisms that can produce various products such as biopharmaceuticals and bioenergy using solar energy (Lim et al., 2010). Cyanobacteria (Cyanophyta) or blue green algae are a different group of photosynthetic organisms. These are the only living creatures known as photosynthetic prokaryotes. As cyanobacteria are prokaryotes, they do not have mitochondria and chloroplasts, which provide energy conversion like vascular plants. These carry membrane packets (thylakoids) concentrated in their cytoplasm (Binder, 1982). Thanks to these systems, cyanobacteria can use 0.3%0.2% of sunlight. Cyanobacteria also provide about 25 Gt of CO2 to be recycled each year (Pisciotta et al., 2010). In the photosystems of these organisms, high-energy electrons are produced by the stimulation of sunlight, and transferred to an electron transmitter and electricity can be produced in this way (Quintana et al., 2011). In addition, electrical energy can be obtained directly from photosynthetic algae by using biophotovoltaic devices (Bombelli et al., 2011). The use of algae biofilms on the electrode surface in biophotovoltaics has been one of the most important milestones in the development of these systems for renewable and sustainable electrical energy. In these systems, when the light falls on algae, a series of reactions that can separate the water into protons, electrons and oxygen begins. These molecules are necessary to convert carbon dioxide and other inorganic materials into carbohydrates and proteins, which are necessary for algae to grow. Biophotovoltaic devices also utilize this charge separation to produce electrical energy. While algae can be considered as the most complicated element in a biophotovoltaic system containing eukaryotic algae, the systems where photosystem II complexes are directly connected to the electrode are much simpler. Here, carrying the chloroplast containing thylakoid

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membranes as a subcellular compartment, which carries out the photolysis event, brings the algae structure one step forward in biophotovoltaics. Normally, more complicated systems are less efficient due to reasons such as electrons losing energy when passing through carrier molecules and spending energy to sustain the system’s own vitality. However, more complicated systems such as cyanobacteria or algae also have the advantages of being able to repair and regenerate themselves and to generate energy thanks to some metabolites stored in the dark environment. Unlike higher plants, Cyanobacteria contain also the respiratory system, where they are able to share excess electrons produced during photosynthesis in the thylakoid membranes, thereby reducing the effects of oxidative stress. Therefore a diverse range of electron transport pathways have evolved in cyanobacteria. In addition to having their own systems to prevent photodamage at high light intensity, they also have some advantages in their use in photovoltaics such as the ability to survive under irregular CO2 levels, dryness and exposure to varying light (Hasan et al., 2014). Most cyanobacterial species have two membrane systems, a series of internal thylakoid membranes and the cytoplasmic membrane. The photosynthetic apparatus in cyanobacteria is found in the thylakoid membranes where light reactions occur. In addition to photosynthetic electron transport, the thylakoid membrane is the main site of transport of the respiratory electron (Sarma et al., 2016; Lea-Smith et al., 2016). The respiratory electron pathway involves sharing complexes, such as PQ, cytochrome b6f, and plastocyanin, with the photosynthetic electron pathway. Excessive amounts of protons released into the thylakoid lumen during the respiratory reaction lead to a proton gradient that enables adenosine triphosphate (ATP) synthesis across the membrane. In photovoltaics, it uses bacteria that can transfer electrons produced by photosynthesis and respiratory system across the cell membrane to an external electrode; the process is called extracellular electron transfer and plays a key role in collecting electrons (Yoon et al., 2014). The high-energy electrons produced by the light stimulation that takes place in the thylakoid membrane are transferred to an electron mediator, which transfers them to the electrodes, thereby generating electricity (Ng et al., 2018). In a cyanobacterium used in a solar cell, high energy electron transfer encounters two obstacles, the thylakoid membranes and the cell membrane, which seriously affect battery efficiency. These obstacles make it difficult to transport the high energy electron to the electrode. Therefore although artificial interconnecting materials are used to facilitate electron transfer, these intermediaries do not only accept electrons selectively from charge carriers involved in photosynthesis, but also accept electrons from any of the redox-active molecules in the cell that have a suitable midpoint potential. It is thought that this choice causes an increase in cell toxicity depending on the use of the agent, thereby negatively affecting the applicability of the solar cell (Schuergers et al., 2017).

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To use these organisms, they must first be produced in culture. For this, Cyanobacteria are incubated in BG-11 medium and when the cell density reaches a certain level, the cells are collected by centrifugation. With a series of washes, cell and nutrient waste is removed. Cells are frozen with liquid nitrogen and pelletized and suspended at certain concentrations for use in the solar cell. Thus studies are carried out with a computable chlorophyll concentration (Tsujimura et al., 2001). Light-dependent electrical interaction between live photosynthetic microorganisms and electrodes was reported in 1964. It was used purple nonsulfur Rhodospirillum rubrum in this study. The potential increase observed at illumination (0.6 V) is attributed to the IEET process-the photo evolution of H2, which is then oxidized on the platinum anode (Berk and Canfield, 1964). Rosenbaum et al. studied the effect of different raw material compositions on H2 production and light conversion by using another purple nonsulfur Rhodococcus sphaericus, which further developed this method. When fed a mixture of Escherichia coli fermentation, H2 produced by R. sphaericus results in a maximum power output of 183 mW/m2 and fuel cell conversion efficiency 8.5% (Rosenbaum et al., 2005). Further optimization has resulted in the highest current measured using the Rhodobacter sphaeroides culture, with a maximum output of 790 mW/m2 under light conditions and only 0.5 mW/m2 under dark conditions (Cho et al., 2008). Recently studies have shown that the metabolic purple nonsulfur species Rhodopseudomonas palustris can metabolize raw materials composed of intact Arthrospira maxima (filamentous cyanobacteria), while producing a current output of 5.9 mW/m3 (Inglesby et al., 2012). The power output of palustris is driven only by light. Rhodium palustris is also the first purple nonsulfur with direct extracellular electron transfer function after biofilm growth on the anode electrode (Morishima et al., 2007; Xing et al., 2008). In those systems, due to the use of nonmetallic anode materials (such as carbon, paper, graphite brushes or polyaniline particles). In addition, Morishima et al. used Rhodopesudomonas palustris mutants. with deletion of genes necessary for H2 production (Morishima et al., 2007). The complete genome sequence of R. palustris shows that there are a large number of potential electron export mechanisms, such as MtrA/MtrB Cytochrome: porin homolog MtoA/MtoB. At present, there is limited knowledge about the molecular components contained this possible unique electron transfer mechanism but improving our understanding and improving the development of genetic tools may greatly increase the possibility of developing this multifunctional organism. The genetically modified Shewanella oneidensis MR21 was modified to express the photo proton pump protein rhodopsin, resulting in an increase in the rate of nutrient absorption by cells. Johnson et al. showed that the current output increased significantly after illumination when the culture was inoculated into a microbial fuel cell device (Johnson et al., 2010). Rosenbaum et al. demonstrated a nonpurple nonsulfur system that uses Chlamydomonas

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reinhardtii (a green algae species), which is maintained in raw materials supplemented with acetate (Rosenbaum et al., 2005). They produced a maximum power output of 7 mW/L using conductive polymer coated platinum electrodes. It has been reported that the highest power output comes from a sediment type system inoculated with green algae Chlorella and electrochemically active bacteria (68 mW/m2) from wastewater. (Zhang et al., 2011). For plant microbial fuel cell, Wetser et al. recently achieved a maximum power output of 679 mW/m2 in the plant growing area, and an average power of 240 mW/ m2 over two weeks (Wetser et al., 2015) Although good progress has been made in improving the long-term sustainability of these systems, the total percentage of light converted to electricity is still very low (about 0.5%) (Wetser et al., 2015). Power output appears to be limited by the availability of anode substrates (i.e., plant root exudates), so improved system design and/or selection of plant species with higher rhizosphere deposition rates may increase power output. Further identification of specific homotrophic processes within the bacterial communities characteristic of exogenous electrical anode biofilms should also help improve the performance of these systems.

18.3.1.2 Plants The increasing need of humanity for energy resources is increasing the concerns about the limited fossil fuels. This situation causes the development of energy technologies based on renewable resources (Dresselhaus and Thomas, 2001) Sunlight is the most abundant and sustainable energy source for humanity. For this reason, capturing light energy and presenting it to humanity is among the most important issues of our century. The biomass produced as a result of photosynthesis reactions within one year corresponds to an energy of 4 3 1021 J, which is approximately 10 times the total global energy need (Alt, 2005). Due to its sustainability, biofuels based on plants are at the forefront of their interests in this regard. Here, it is seen as a problem for scientists that plants that can be used as biofuels occupy the production areas instead of plants used for food (Williams and Laurens, 2010). Therefore there is a need for bioenergy solutions that will not disrupt the food production chain but also contribute to meeting global energy requirements. In addition to many biomaterials used for this purpose, vascular plants are also used in biophotovoltaic technology. In some previous studies, systems have been developed in which vascular plants can generate electricity in a common process with heterotrophic microorganisms in the root rhizospheres (Helder, et al., 2012; Strik et al., 2008; Timmers et al., 2010). Vascular plant biophotovoltaics (VP-BPV) developed with this system can be called a carbon neutralizing and emission free technology and also has the potential to be used in

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rural areas where there is no central electricity network. A system where both the plant can grow and electricity can be produced through the bacteria in the root rhizosphere, just like a symbiotic life, can be considered as a remarkable development for the balance of food energy production (Strik et al., 2011).

18.3.1.3 Bioengineered bacteria Most bacteria used in systems called living photovoltaics have been introduced with bioengineering techniques and have been made more efficient, sustainable and stable. Naturally, while there is no bacteria that can convert solar energy into redox energy and transfer extracellular electrons, synthetic biology approaches have allowed the development of microorganisms that combine these two features (Schuergers et al., 2017). The most important bottlenecks in living photovoltaic systems are the integration of the organism with the electrodes, this process can be done in a nanodimension and the obstacles such as the living organism does not lose its vital features. It is especially important to be able to take the net charge from living organism out of the cell, as in exoelectrogen organisms. Exoelectrogen creatures are known as creatures capable of extracellular electron transfer. While these organisms produce ATP during oxidative phosphorylation, they can also send electrons out of the cell by exocytosis. While oxygen is normally used as the last electron acceptor in aerobic respiration, some soluble compounds perform this process in anaerobic respiration. In exoelectrogen organisms, the last electron can be a solid or liquid oxidizing agent that is available as an extracellular. In exoelectrogen organisms, the final electron acceptor can be a solid or liquid oxidizing agent that is available as an extracellular. Some of the gramnegative bacteria can continue their anaerobic respiration by reducing the mineral oxides such as Fe (III) and Mn (IV), which are insoluble in water and cannot enter the cell at neutral pH. In these microorganisms, iron (specifically Fe (III) oxides) or manganese (specifically Mn (III/IV) oxides) compounds act as final electron acceptors (Hartshorne et al., 2009; Baron et al., 2009). Energy production, which can be realized thanks to the proton motif force, is continued along the inner membrane. For the water soluble electron acceptors, the reduction of the quinones in the inner membrane to the quinoles and the reverse reoxidation of this process occurs in the inner membrane or in the periplasm; in the absence of a water-soluble end electron acceptor such as Fe (III) and Mn (IV), problems may arise in the conversion of reduced quinoles (Hartshorne et al., 2009). In this case, the electron flow in exoelectrogen microorganisms differs from the normal electron transfer system after cytochrome bc1 complex (Complex III) and transmits its electrons to the last electron recipient outside the cell via the outer membrane proteins OmcS and OmcB (Fig. 18.3).

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FIGURE 18.3 Alternative electron transport chain to move electrons to outer membrane of Geobacter sulfurreducens.

Fluorescent proteins are known to absorb light within a certain wavelength range. It has been reported that some proteins work like protein pumps in the presence of light, that is, they capture light energy and carry protons out of the cell across the membrane. Recombinant DNA technology is utilized to use such proteins in the solar cell system. Solar cells were produced with E. coli cells designed by bioengineers for use as a biogenic photovoltaic material. Proteins such as Lycopene, green fluorescent protein (GFP), bacteriorhodopsin (BR) produced in E. coli cell have been used to make more use of light energy, to reduce costs and to make an environmentally friendly solar cell. Instead of purifying protein and using it the in the solar cell, using the bacteria itself that produce the light-harvesting protein is increases the stability of the protein. It is more economical and saved from time and workload since it has no purification step. Recombinant bacteria, which gain the ability to produce proteins such as photosynthetic pigments (lycopene, fluorescent proteins etc.) as a result of bioengineering studies, were used by coating them directly on the n-type semiconductor (TiO2) surface in biophotovoltaics solar cells (BPV). Recombinant DNA technologies have been used to produce these bacteria. In order to ensure the expression of this pigment, the plasmid with the relevant gene sequence is designed and transformed into the host competent cell. Transformed cells are incubated in a sterile medium (like LB broth) containing appropriate components (such as carbon source, nitrogen source, vitamin, water, antibiotic). By using genetic regulation mechanisms, induction is performed with the appropriate inducer (such as IPTG, L-Arabinose) to start the expression process. After induction, the cells are incubated for some time for expression. At the end of the process, cells are collected with using centrifugation. Cells and nutrient waste are removed with a series of washing (Ulusu, 2020). The resulting pellet is suspended at certain concentrations for use in the solar cell. In this way, the pigment concentration to be used in

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biophotovoltaics solar cells can be adjusted according to the expression level of the cells.

18.3.2 Light-harvesting proteins Light-harvesting proteins that can absorb light at a specific wavelength, have high quantum efficiency, mediate electron transfer, and have high thermal and photochemical stability. The common feature of all these proteins is that they allow electron transfer via light energy.

18.3.2.1 Green fluorescent protein Proteins such as GFP obtained by purification methods from bioengineered bacteria are used by attaching them to the n-type semiconductor (TiO2) surface. In order to purify the protein from the Bioengineered bacteria obtained, the method is first determined according to whether the protein is intracellular or extracellular. In the case of extracellular protein secreted into the medium by centrifugation is separated from the cells. If it is an intracellular protein, it must be released. For this, the cells must be disrupted by mechanical or enzymatic methods. Besides, in order to maintain the stability of the proteins, proteases that have become free must be inactivated with inhibitors. DNase and RNase enzymes added to the medium to contribute to the purification process will prevent DNA and RNA contamination by breaking them (Ulusu, 2020). Following the lysis process, the proteins are collected in the supernatant by centrifugation method to separate the functionalized protein from other proteins. The purification steps of intra and extracellular proteins taken in the supernatant by the centrifuge method are determined according to the biophysical and biochemical properties of the protein. At this point, purification is performed by methods such as ion exchange chromatography, affinity chromatography, gel filtration chromatography, fast protein liquid chromatography. The purity of the protein is analyzed by SDS-PAGE (Acikgoz et al., 2014) Deepankumar et al. developed a next-generation sensitizer for BSSC using GFP and its design variant (GFPdopa). The nanostructure layer formed by GFPdopa and TiO2 results in a photon conversion efficiency of 0.94%, a short circuit current of 1.75 mA/cm2, and an open-circuit voltage of 0.60 V (Deepankumar et al., 2017). 18.3.2.2 Bacteriorhodopsin Among natural photosynthesizers, molecules such as BR, microbial pigments, and chlorophyll can be considered promising (Mohammadpour et al., 2016). BR is an integral membrane protein known as a purple membrane, which is large enough to cover about 50% of the cell surface area in archaea group organisms. BR, which is a light-operated proton pump, absorbs a light

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photon and performs proton pumping function by changing conformation. In general, BR captures light energy and uses it to transport protons out of the cell across the membrane. The proton motive force obtained by the proton gradient formed in the bacterial membrane as a result of this process is used by converting it into chemical energy (Hayashi et al., 2003). Known for its purple color, BR shows the highest light absorption between 500650 nm wavelengths, and when excited at 633 nm it emits between 650 and 850 nm (Ohtani et al., 1995). In fact, it is extremely important for these synthesizers to be able to create a photoinduced charge, to be thermally strong and to remain effective despite exposure to long-term solar radiation. BR is known as a protein produced by the Archaea group bacteria Halobacterium salinarum and used as a proton pump (Lozier et al., 1975). This protein has a high quantum efficiency (66%) at very high salt concentrations (up to 5 M NaCl) and even at a temperature of around 140 C. In addition, it can maintain its activity even in a wide pH range ranging from pH: 5 to pH: 11. Molaeirad et al. studied the use of another cheap bacterial pigment (bacteriorhodopsin and bacterioruberin) as a sensitizer in DSSC, called (BR). They are natural colored biomolecules of proteins and carotenoid (Molaeirad et al., 2015). BR protein is composed of 248 amino acids in seven α-helix bundles in the lipid membrane; once the photons are absorbed, the absorbed energy will trigger the isomerization of the retina, thereby converting the alltrans BR to 13-cis configuration, which triggers the light cycle by transferring a proton from the cytoplasm to the outside of the membrane (Groma et al., 1984). Bacterioruberin carotenoids contain an open chain of 50 carbons and 13 pairs of conjugated double bonds (Dworkin, 2006). Molaeirad et al. have immersed in 0.1 M Bacterioruberin 1 mg/mL and BR protein at room temperature for 12 hours to sensitize different TiO2 electrodes (Ito et al., 2008). During simultaneous adsorption, purple and red TiO2 films were observed, indicating the binding and attachment of BR biomolecules on the surface of nano-TiO2 the pigment coated TiO2 films have been examined in FTO/TiO2/bacteriorhodopsin/bacterioruberin/Electrolyte/ Pt/FTO configuration. When using two pigments at the same time, the maximum cell efficiency observed under 100 mW/cm2 radiation was 0.16%. Compared with the use of two pigments alone, its performance is much better. The combination of the two pigments can make the TiO2 film more sensitized and expand the light absorption spectrum of BR and bacterioruberin, with maximum visible light absorption at 568 and 497 nm, respectively (Molaeirad et al., 2015).

18.3.2.3 Artificial photosynthetic devices Artificial photosynthetic systems offer a solution for cheap and clean energy production in parallel with the development of energy and industrial applications day by day. Artificial photosynthesis can be defined as chemically

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performing natural photosynthesis reactions. In fact, it can also be called imitation of natural photosynthesis reactions for photovoltaic and hydrogen production systems. The development of artificial photosynthesis systems has the potential to produce bioanalogists such as clean electricity generation, photohydrogen production, and a healthy CO2 cycle. The two main processes in photosynthetic reactions are light harvesting and charge transfer by photosystem I (PSI) and photosystem II (PSII), a multimeric peptide-protein complex found in cyanobacteria, algae and vascular plants embedded in the thylakoid membranes. PSI and PSII work with chlorophyll molecules to capture photons in the visible region light spectrum. Electron hole pairs are created by using the energy of these captured solar radiations. These electron hole pairs are stored in the catalytic center of PSII and the oxygen production center oxidizes the water. However, electrons are stored by PSI in chemical bonds to reduce NADP to NADPH and to produce ATP (Ocakoglu et al., 2014). PSI is an integral membrane protein that catalyzes the transfer of electrons from plastocyanin to ferredoxin through the thylakoid membrane. PSII is the first protein complex of light reactions of photosynthesis. This protein, just like PSI, is a protein located in the thylakoid membranes of algae and vascular plants. In this system, light photons are captured and the energy levels of the electrons are increased, and after a series of reactions, the reduction of plastoquinone to plastoquinol takes place. Afterwards, molecular oxygen and hydrogen ions are formed as water oxidized. Both systems consist of a core complex and a light-harvesting complex. Each core complex contains a pigment [P700 (PSI) and P680 (PSII)] with electron acceptors and transmitters, which can be chemically oxidized (Pace, 2005). In 2012, Mershin et al. a PSI-based DSSC using Co (II/III) redox mediator on nanostructured TiO2 and ZnO electrodes was reported. Kondo et al. (2014) demonstrated PSI- and PSII-based DSSCs using a TiO2 electrode, and in 2015, Yu et al. (2015) achieved the most effective PSI- and LHCII-based DSSC to date using two different particle sizes of TiO2. Furthermore, polyaniline/TiO2 and hematite semiconductor electrodes have also been investigated as photoanodes for biophotovoltaic-based DSSCs. Ravi et al. (2017) developed a biohybrid series battery consisting of two stacked subcells connected in parallel, and the photocurrent is enhanced by the complementary absorption of two RC-LH1 units. PSI stabilized by surfactant peptides can be used as both a light trap and Co (II/III)-terpyridine as an electron transfer medium to self-assemble charge separators on nanostructured TiO2 or ZnO electrodes. The best Jsc 362 mA/cm2 and η 0.08% of PSI-TiO2 solar cells. The rugged red algae PSI and its light collection antenna (LHCI), as well as TiO2 and a-Fe2O3 nanocrystalline n-type semiconductors with electrolyte (I2/I32) as the electron transfer medium, are used as biophoto anodes, and the best Jsc 56.9 is also achieved mA/cm2 For PSILHCI/a-Fe2O3 solar cells, η is 0.17%.

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Due to its structural properties and highly purified form, PSII of cyanobacterium Thermosynechococcus elongatus used in many studies (Badura et al., 2011; Kato et al., 2012). When PSII of T. elongatus is immobilized on the modified indium tin oxide (ITO) anode in a convenient direction, the light obtained by photolysis of light and water in the oxygen evolving complex of PSII can flow directly to the nearby quinone QA or quinone QB site, then to the circuit. The current produced by water oxidation biophoto anode is 16 mA/m2, which may be related to the oxygen (O2) release rate of about 0.18 (mol O2) (mol/PSII)/s (Kato et al., 2012).

18.3.2.4 Protein pigment complexes from Rhodopseudomonaspalustris CQV97 and Rhodobacter azotoformans R7 Yu et al. have been possible to added biological pigments in protein complexes (protein pigment complexes such as light-harvesting complex 2, lightharvesting complex 4, and reaction center) into TiO2 membranes to produce promising biophotonic anode sensitizers with visible-NIR response DSSC (Yu et al., 2013). The light-harvesting complex 2-sensitive DSSC can generate high photocurrent, which is reported to be stable for more than 300 s in multiple working cycles. The IV test was performed at 100 mW/cm2 to confirm the effect of various protein pigment complexes absorption bands (750850 nm) on Jsc and power conversion efficiency (PCE). the lightharvesting complex 2 concentration increases and the attenuation at zero bias is negligible, the photoelectric performance of the designed biosensitized DSSC has been enhanced. It has been determined that the optimal concentration of light-harvesting complex on the TiO2 film is Bacteriochlorophyll a 46.8 μg/mL, and the maximum Jsc and η are 1.46 mA/cm2 and 0.49%, respectively. The response of the sensitized TiO2 film of protein pigment complexes to visible light-NIR light energy becomes more prominent. Visible light causes NIR photoelectrons to be transferred from protein pigment complexes to TiO2, thereby successfully generating current from the biosensitized DSSC based on purple bacteria PPCs (Fu et al., 2014). 18.3.2.5 Peptide Short polymer (polyamide) sequences formed by the binding of α-amino acids to each other by the peptide bond between the carboxyl group of the first amino acid and the amino group of the adjacent amino acid are called peptides. The amino group hydrogen and carbonyl oxygen forming the bond are in the trans position. One is above the peptide plane and the other is below. The peptide bond is partially double bonded and the amino nitrogen is charged with positive charge and carbonyl oxygen with a negative charge. The peptide bond is a covalent bond, and during this bond formation, the H2O molecule is released by separating 2 OH from an amino acid’s

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SECTION | V Sustainable Carbon-Based and Biomaterials

carboxyl group and H from the amino group of the other amino acid (Akin et al., 2018). The dipeptide is formed by the bonding of two amino acids, and there is an amino group and a carboxyl group that can still bond at the free ends of these amino acids. They are named as dipeptide, tripeptide, hexapeptide, oligopeptide, polypeptide according to the amino acid number. In addition to the 21 natural amino acids that make up the peptides, the use of unnatural, synthetic amino acids provides peptide structures with very different physical and chemical properties, thus allowing them to be used in many different applications. The molecular structure of the amino acid in the peptide and its interactions with other amino acids play an important role in the conformation and structural features of the peptide. One of these structural features of peptides is electrical conductivity. Dipole moments occur between the amino acid residues along the oligopeptide. These dipole moments create an electrical field. This plays an important role in the structure and functions of proteins in the electrical field, for example, in photosynthesis, it affects the primary electron transfer event. It has been suggested that the permanent area of the first protein layer surrounding the photosynthetic reaction center promotes rapid electron transfer parallel to the electric field of the helix (Fox and Galoppini, 1997). It has been shown that helical peptides can be considered effective agents for electron transfer. In particular, the length and chemical structure of the peptide affect the electron transport rate. Electron transfer via peptides is direction dependent, which means that electron transfer can be strongly influenced by the molecular dipole of the peptide (Sek et al., 2005). (Gly-Gly, GlyL-Leu): Since the peptides contain carboxylic acid groups (2COOH), they can be adsorbed on the surface of a metal oxide by the 2 COOH with metal oxide or metal oxide surface reaction with 2 OH. Peptides designed in this way are generally purchased commercially. Synthetic chemical synthesis of peptides requires the consumption of large amounts of organic solvent [dimethylformide (DMF)] using protective groups and activating reagents. This makes the chemical synthesis of peptides a not very environmentally friendly process. The standard solid phase peptide synthesis method developed by Robert Bruce Merrifield is generally used for peptide synthesis in a laboratory setting (Merrifield, 1963). A binder carrying a reactive functional group (either present on the resin or inserted at the beginning of the synthesis) binds the first amino acid from the Cterminal to the N-terminal. The purpose of the binder is to allow the peptide to separate from the resin after the synthesis is complete. By separating alpha-amino (Na), which is the temporary protection group, the next amino acid is bound to the chain. When the peptide is completed, hydrogen fluoride (HF), a very dangerous reagent, is used to separate it from the resin (Merrifield, 1986). Another method for peptide synthesis is the proteolytic enzyme method (Fig. 18.4). Since these enzymes are very selective in hydrolyzing peptide

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575

FIGURE 18.4 Peptide bond hydrolyzed by a protease enzyme.

bonds, the products produced will be very pure. In addition, this method can be considered environmentally friendly as it does not require reactive chemicals and toxic solvents (Varnava and Sarojini, 2019). Nie et al. The report points out that ITO transparent cathodes modified with ultra-thin layers of environmentally friendly biomaterial peptides show a significant reduction in work function. Research on the device shows that after peptide modification, the PCE is significantly increased from 2.12% to 8.13%. Using GlyGly as a modifier, a reverse organic solar cell based on PBDTTT-CF: the highest Voc of PC71BM is 0.73 V, Jsc is 17.69 mA/cm2, and FF is 63% (Nie et al., 2014).

18.3.3 Natural pigments It has been nearly 45 years since photosynthesis reactions led to the production of biomass by converting the energy from sunlight to chemical bond energy in plants, and converting the energy coming from the sun into earth into electrical energy by imitating these reactions (Calvin, 1974). In this first study, a photoelectrochemical cell model based on a synthetic membrane with carotenoids is embedded. In this system, after the absorption of light quantum, an electron is transferred from the carotenoid, diffusing across the membrane and captured by an electron receiver on the other side of the membrane. This model is known as the first record in which sunlight is converted into electrical energy using membranes and pigments. In the studies carried out since then, man-made solar cells technology has been developed and new products have been added to the renewable energy sources (Mohr and Schopfer, 2013). Usually in these systems, sunlight is used to stimulate

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SECTION | V Sustainable Carbon-Based and Biomaterials

electrons in pigments, just like photosynthesis. DSSC using synthetic or natural dyes as light-harvesting pigments can be seen as candidate systems for third generation photovoltaic systems when compared to silicon-based solar cells (Yamazaki et al., 2007). As a photosensitizer for DSSCs, natural dyes have attracted the attention of many researchers due to their low costs, abundance and sustainability. At first, carotene was used as a natural dye for this purpose. It has been demonstrated in previous studies that this molecule can remain stable for up to 1 hour under continuous radiation and can be particularly well connected to TiO2. Natural pigments used as photosensitizers in DSSCs have high stability and performance as light harvesters just like conventional solar cells. In a material that can be used as a photosensitizer in a DSSC, it is first sought to show absorbance in the near-infrared and visible region of the solar spectrum and to be able to connect to semiconductors such as TiO2. To bind a biomaterial to the semiconductor, it must contain at least one carboxylic acid group or another peripheral acidic group (Gao et al., 2000; Koyama et al., 2009, 2012; Cherepy et al., 1997). Natural dyes are very attractive as DSSC photosensitizers because they are cheap, sufficient, and sustainable. Early evidence suggests that carotenoids can be used as photosensitizers in DSSC, a combination of 8-apolipoprotein-b-cartoten-8-oic acid and TiO2. The photocurrent stabilized under continuous radiation for 1 hour was measured, and an ordered monolayer structure of TiO2 combined with carotenoids was proposed.

18.3.3.1 Carotenoids Carotenoids have already been efficaciously used in DSSCs (Koyama et al., 2009, 2012). The maximum η of a single carotene is 2.6% (Wang et al., 2006a,b). The optimal length of carotenoids includes seven conjugated p bounds (Wang et al., 2005). By combining carotenoids with chlorophyll derivatives, η may be elevated to 4.2% (Wang et al., 2006a,b). Several extracts have been discovered as effective photosensitizers. The impact of a black rice extract has been attributed to an anthocyanin with an o-hydroxychinon moiety (Hao et al., 2006). Another study confirmed that among the 20 plant extracts, the highest efficiency η of mangosteen (Garcinia mangostana) peel extract was 1.17%. The active ingredient has been defined as routine (Narayan, 2012). Overall comparison results indicate that Rhodiola rosea extract with an efficiency of 1.49% can be used as an attractive source (Zhou et al., 2011). Data is usually obtained by different authors under different conditions and device settings, so in some cases it is difficult to compare directly. Photosensitizers belong to different chemical categories and mainly contain betaines, carotenoids, flavonoids or chlorophyll. Sicilian prickly pear extract has the highest η of 2.06% (Calogero et al., 2012). ´ rdenes-Aenishanslins et al. used UV-resistant Antarctic bacteria to O ´ rdenes-Aenishanslins produce pigments used as photosensitizers in DSSC (O

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577

et al., 2016). It was found that bacteria living in Antarctica can synthesize antioxidant carotenoid pigments by improving their DNA structure (Zenoff et al., 2006), thereby developing a mechanism to resist ultraviolet damage. It was determined as Chryseobacterium sp. (yellow) and Hymenobacter sp. (red) bacteria. A TiO2 film was sensitized with two pigments by direct adsorption in a pigment solution of 10 mg/mL incubated in the dark. Both pigments have maximum visible light absorption, with yellow and red pigments showing peaks at 450 and 478 nm, respectively. The sensitizing properties of carotenoids were evaluated under 100 mW/cm2 solar radiation in DSSC with FTO/TiO2/ xanthophylls/(I2/I23)/Pt/FTO structure. The determined DSSC efficiency of the red pigment DSSC and the yellow pigment DSSC were 0.0332% and 0.0323%, respectively. The light stability of the extracted carotenoids was studied from the attenuation of absorbance at various exposure times. Obviously, UV´ rdenesresistant carotenoids are very stable under long exposures (O Aenishanslins et al., 2016). Many biomaterial extracts have been reported in previous studies as effective photosensitizers (Hao et al., 2006). According to the literature data, while carotenoids are used alone, a yield of 2.6% is obtained, while the efficiency of solar cells obtained with the combination of these with chlorophyll types increased up to 4.2% (Wang et al., 2006a,b). Metal coordination compounds, such as ruthenium polypyridyl complexes, are used as synthesizers, since they can absorb most of the visible area and transfer an effective load transfer from metal to ligand in most solar cells (Hao et al., 2006). However, it is undesirable in terms of environmental health that ruthenium polypyridyl complexes can only be obtained with high costs and contain heavy metal. As an alternative to this situation, natural dyes appear as molecules that can be used as a sensitizer if they can offer an acceptable efficiency value (Tennakone et al., 1996). Natural dyes have advantages such as low cost, easy availability and sustainability. Sensitization of wide band gap semiconductors using natural pigments is often attributed to anthocyanins, which belong to the group of natural dyes in the red-blue range found in flowers, leaves and fruits (Polo and Iha, 2006). The hydroxyl and carbonyl groups in the anthocyanin molecule are capable of binding to pores on the surfaces of TiO2 and ZnO thin films. This facilitates electron transfer from the anthocyanin molecule to the conduction band of the semiconductor (Hao et al., 2006).

18.3.3.2 Lycopene Lycopene is a natural antioxidant carotenoid pigment that provides the characteristic bright orange-red colors to tomatoes and other red fruits. Thanks to its double bonds, it has many cis-trans isomers. Each double bond lowers the energy required for the transition of electrons to higher energy levels and

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SECTION | V Sustainable Carbon-Based and Biomaterials

thus appears in red because it absorbs light at longer wavelengths (Srivastava et al., 2018; Grumezescu and Butu, 2017) Genetically modified E. coli has been evolved to supply the photosensitizing pigment lycopene as a biomaterial for photovoltaic (Srivastava et al., 2018). Lycopene pigment is a stable redox mediator that may absorb light energy in the range of 380520 nm and effectively excite HOMO electrons. Therefore it is used as a photosensitizer in photovoltaic and photocatalytic (PC) applications (Richhariya et al., 2017; Zhuang et al., 2015). Srivastava et al. encapsulated E. coli (BL21) cells with TiO2 nanoparticles to produce a core-shell shape of TiO2/E. coli/lycopene for DSSC applications (Srivastava et al., 2018). E. coli/lycopene has the maximum absorbance at the visible positions of 450, 485, and 595 nm, producing 0.057% PCE, and has a significant conversion rate both high intensity in the surrounding environment and occasional light intensity are feasible (Srivastava et al., 2018). Biopigments isolated from chlorophyll a derivatives and protein complexes. Carotenoid compositions show the best biosensitized DSSC performance, many of which are studied dyes for solar power, the conversion performance is respectively 0.16 is 20.57% and 4%. In contrast, when the lutein carotenoids isolated from Antarctic bacteria were evaluated using biomolecular pigments from different studies, their bottom photoelectric conversion performance was 0.008%0.03%, of which due to the strong anchoring technology of the dye can achieve excellent charge, the use of coadsorbent can improve the performance to 0.03% supplier delivery and electronic injection. Under solar radiation (100 mW/cm2), the authors significantly test mentioned photovoltaic specification of different plant-based totally biomolecular photosensitizers to appraisal and/or report the DSSCs overall performance ascribed the use of: Mangosteen pericarp (rutin) 1.17% (Narayan, 2012), Rhoeo spathacea (rutin) 1.49% (Zhou et al., 2011), Sicilian prickly pear (betaxanthin) 2.06% (Calogero et al., 2012), black rice (anthocyanin) 3.27% (Hao et al., 2006), capsicum (carotenoid) 0.58% (Hao et al., 2006), Erythrina variegata flower (carotenoid, chlorophyll) 2.06% (Hao et al., 2006), Rosa xanthine (anthocyanin) 1.63% (Hao et al., 2006), kelp (chlorophyll) 1.18% (Hao et al., 2006), organic dye zinc phthalocyanines 4.6% (Mori et al., 2010) and 6.4% (Ikeuchi et al., 2014), cyanine dyes 4.8% (Wu et al., 2008) and 7.62% (Ma et al., 2008), Rose bengal (xanthenes) in ZnO2 based DSSCs 1.56% (Karki et al., 2013), coumarin dyes having thiophene moieties in TiO2-primarily based DSSCs 7.7% (Hara et al., 2003) and 9% (Wang et al., 2007). Pandey et al. (2019) pointed out earlier attempts to use apparently sensitive dyes (such as chlorophyll, betaine, anthocyanins, carotenoids and tannins) extracted from plant life, leaves and flower roots for DSSC. Due to the feasible expansion of the mild absorption spectrum, 2% PCE has been performed using natural dye mixtures (Pandey et al., 2019). Iqbal et al. (2019) outlined the comparison using different plant-based

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biomolecular dyes; the chlorophyll on the leaves of perilla covered TiO2, and the subsequent p-CuI deposition confirmed the highest PCE of 1.3% (Kumara et al., 2006) and 6.7% (from chlorin e6 byproduct); pomegranate pigment confirmed PCE to be 2% (Ghann et al., 2017). The PCE of Acalypha amentacea 1 Peltophorum pterocarpum in water and/or ethanol was 7.38 and/or 8.22%, respectively. (Sanjay et al., 2018; Iqbal et al., 2019). Calogero et al. (2015) mentioned the usage of vegetable-primarily based dyes (betalains, carotenoids and chlorophylls) in biosensitized DSSCs which showed a maximum efficiency of 2.06% the use of betalains from wild Sicilian prickly pear (2012) (Calogero et al., 2012) and 0.37% for carotenoids from annatto (Calogero et al., 2010; Go´mez-Ort´ız et al., 2010) and 4.6% using chlorophylls from wakame (2007) (Wang et al., 2007) under one solar irradiation as concluded in Calogero et al. (2015). The natural dyes extracted from the leaves of a vegetable are tested in TiO2 DSSC for the polymer gel electrolyte (2-methylimidazolium iodide (EMIM-1) sodium iodide (BDH)) and its PCE is 0. 61% (FF . 50)%, IPCE 4.7%5.2%) (Godibo et al., 2015). Bella et al. (2017) studied the use of (TiO2/cellulose)photoanode and cellulose as paper-based fully quasi-solid kingdom electrolytes for biosensitizing DSSC (Bella et al., 2017). Bioderived paper-based electrolytes/electrodes have a high PCE of 3.55%5.20% for a period of up to a thousand hours, while the sustainable efficiency retention rate is 96%. (Bella et al., 2017). Rapsomanikis et al. extracted laver (red algae) pigment and used it as a natural sensitizer in pre-made quasi-solid TiO2 DSSC. Its Jsc is 1.26 mA/cm2 and Voc is 0.66 V (Rapsomanikis et al., 2016). Under one sunlight, the transformation efficiency of artificial cells reached 10.1% (Suzuka et al., 2016). The quasi-strong national biosensitized DSSC (FTO/ TiO2/Dyestuff/Quasi-Solid Electrolyte/PEDOT/FTO) was established using the pigment of bougainvillea (Peugabilia spectabilis), with a PCE of 0.175% absorbance at 395750 nm (Yazie et al., 2016).

18.3.3.3 Flavin Flavins molecules contain a three-ring isoalloxazine motif. This motif is found in many biologically active molecules, such as flavin adenine dinucleotide (FAD), flavin mononucleotide (FMN) and riboflavin (RF, vitamin B2). FAD is a naturally occurring redox system, with flavin as the redox active structural unit. Flavin and RF have the ability to exchange electrons and form covalently bonded reaction intermediates. They are highly versatile chemical and biological building blocks. They play an active role in a variety of biological processes such as the respiratory tract and the catalytic flavoprotein. The study of flavin derivatives has aroused great interest. The photochemical reaction of flavins has been quite thoroughly studied, especially in natural flavins such as FAD, FMN and RF (Abbas and Sibirny, 2011). Jordan et al. (2008) have synthesized ABFL 240 (8-[[p-[bis(ethyl)amino]

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SECTION | V Sustainable Carbon-Based and Biomaterials

phenyl]azo]isobutylflavin). ABFL 240 showed strong absorption at 552 nm, which was attributed to an intramolecular charge transfer (ICT). They reported that after adding diaminopyridine, the ICT changed significantly from 552 to 584 nm, and the color changed from purple to blue. Pauszek et al. (2013) further studied the system to further understand the excited state characteristics of ABFL 240. Using Stark spectroscopy, they reported a dipole moment of 22.3, which is consistent with their hypothesis that the lowest excited state is intramolecular and the charge transfer occurs from diethylaniline donor to flavin acceptor. Pandiri et al. (2016) have used synthetic flavin molecules, which were anchored on Degussa P25 TiO2. The influence of their presence on the PC activity of TiO2 was studied. Fixing the synthetic flavin on the surface of TiO2 can improve the PC degradation kinetics of ethanol under ultraviolet light, while stabilizing the anchored flavin molecules from photodegradation. Under ultraviolet light, this stability was also found in the absence of sacrificial agents (such as ethanol), and visible light was found to be very harmful to the anchored flavin molecules. These findings may not only be related to the anchored flavin, but also to other functionalized photocatalysts, and may help implement sensitizers in PC systems (Pandiri et al., 2016).

18.3.3.4 Xanthophylls from Hymenobacter sp. (Antarctica bacteria) Montagni et al. used xanthophylls pigments, which were isolated Hymenobacter sp. UV11 bacteria. The obtained pigments were tested in DSSC (FTO/TiO2/xanthophylls/Electrolyte/Pt/FTO) to determine the performance IV curve. The conversion efficiency of RAW orange xanthophylls pigment is the highest (0.03%). Efficiency of other pigments are given in the Table 18.1 (Montagni et al., 2018). 18.3.3.5 Chromatophores from Rhodospirillum rubrum S1 biological redox Magis et al. evaluated the use of biosensitizers and bioelectrolytes as photosynthetic membrane vesicles of microorganism. It was found that the generation of the light driving current depends on the electrolyte, the bio/ bare gold electrode and the open-circuit potential; which was capable of accomplishing an excessive Jsc of 25 μA/cm2 (Magis et al., 2010). Woronowicz et al. A biological hybrid DSSC, called a chromatophores sensitized solar cell, was developed in which organic light-capturing dyes were replaced by chromatophores isolated from Spirulina spirulina strain S1. Due to the great connection between the utilized biological components and the perfect energy band agreement between the continuous interfaces, the reverse transmission rate is significantly reduced, so that

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TABLE 18.1 Summary table. Pigments (bacterial)

Jsc (μA/ cm2)

Voc (mV)

η (%)

FF

Reference

Chlorophyll a 1 carotenoids (Spx)

11,500



4



Wang et al. (2005)

Chromatophores

24.7

300

0.04

0.29

Woronowicz et al. (2012)

Lycopene carotenoids

696

289

0.057



Srivastava et al. (2018)

Xanthophylls carotenoids (Cocktail)

98

260

0.009

0.38

Montagni et al. (2018)

Xanthophylls carotenoids (PURE orange)

78

260

0.008

0.39

Montagni et al. (2018)

Xanthophylls carotenoids (red)

200

435

0.0332



´ rdenesO Aenishanslins et al. (2016)

Xanthophylls carotenoids (RAW orange)

127

460

0.03

0.51

Montagni et al. (2018)

Xanthophylls carotenoids (yellow)

130

549

0.0323



´ rdenesO Aenishanslins et al. (2016)

a-Mangostin/b-mangostin (Mangosteen pericarp)

2.55

621

0.92

0.58

Zhou et al. (2011)

Anthocyanin (black rice)

1.142

551



0.52

Hao et al. (2006)

Anthocyanin (Rosa xanthina)

0.637

492



0.52

Hao et al. (2006)

Antocyanin (Canarium odontophyllum 1 Ixora sp.)

9.8

343

1.55

0.46

Kim et al. (2013)

Antocyanin (C. odontophyllum)

2.45

385

0.62

0.59

Kumara et al. (2013)

Antocyanin (Ixora sp)

6.26

351

0.96

0.44

Kumara et al. (2013)

Antocyanin (Rhododendron sp)

0.85

544

0.33

0.72

Kim et al. (2013)

Antocyanin (Tradescantia zebrina)

0.63

350

0.23

0.55

Li et al. (2013)

Pigments (plant)

(Continued )

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SECTION | V Sustainable Carbon-Based and Biomaterials

TABLE 18.1 (Continued) Pigments (bacterial)

Jsc (μA/ cm2)

Voc (mV)

η (%)

FF

Reference

Antocyanin/carotenoid (Kapok)

0.87

360

0.3

0.49

Li et al. (2013)

Betalains (Beta vulgaris rubra)

9.5

425

1.7

0.37

Hug et al. (2014)

Betaxanthin (Prickly pear)

8.8

389

2.06

0.6

Henning et al. (2013)

Bixin (Bixa orellana L)

1.1

570

0.37

0.59

Go´mez-Ort´ız et al. (2010)

Carotenoid (Begonia)

0.63

537

0.24

0.72

Zhou et al. (2011)

Carotenoid (Capsicum)

0.225

412



0.63

Hao et al. (2006)

Carotenoid (M. pericarp)

2.69

686

1.17

0.63

Zhou et al. (2011)

Carotenoid (Perilla)

1.36

522

0.5

0.7

Zhou et al. (2011)

Carotenoid (Rhododendron)

1.61

585

0.57

0.61

Zhou et al. (2011)

Carotenoid, chlorophyll (Erythrina variegata)

0.716

484



0.55

Hao et al. (2006)

Chlorophyll (Kelp)

0.433

441



0.62

Hao et al. (2006)

Crocetin/crocin (Gardenia fruit)

2.84

430

0.56

0.46

Yamazaki et al. (2007)

Cyanidin-3-glycosides/ delphinidin-3-glycoside (Hibiscus sabdariffa L.)

1.63

404

0.37

0.57

Wongcharee et al. (2007)

Cyanin (Red Sicilian orange “Moro”)

3.84

340

0.66

0.5

Calogero and Di Marco (2008)

Delphinidin (Calafate fruit)

0.96

520



0.56

Polo and Iha (2006)

Modified chlorophyll/bcarotene

13.7

530

4.2

0.58

Wang et al. (2006a,b)

Modified chlorophyll/ lutein (Spinach)

12.5

540

4

0.59

Wang et al. (2006a,b)

Modified chlorophyll/ neoxanthin (Spinach)

11.8

550

3.9

0.6

Wang et al. (2006a,b) (Continued )

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583

TABLE 18.1 (Continued) Pigments (bacterial)

Jsc (μA/ cm2)

Voc (mV)

η (%)

FF

Reference

Modified chlorophyll/ violaxanthin (Spinach)

11.4

540

3.7

0.31

Wang et al. (2006a,b)

Nasunin (Eggplant skin)

3.4

350

0.48

0.4

Calogero and Di Marco (2008)

Peonidin (Jaboticaba skin)

2.6

660



0.62

Polo and Iha (2006)

Rutin (M. pericarp)

2.92

611

1.12

0.63

Zhou et al. (2011)

Bacteriorhodopsin (BR) protein

620



0.19



Chellamuthu et al. (2016)

BR protein

1008



0.49



Chellamuthu et al. (2016)

BR proteins and bacterioruberin carotenoids

450

435

0.16

0.62

Molaeirad et al. (2015)

Green fluorescent protein (GFP)

0.43

500

0.12

0.56

Deepankumar et al. (2017)

GFPdopa

1.75

600

0.95

0.88

Deepankumar et al. (2017)

Light-harvesting complex 2

800

590

0.27

0.58

Yu et al. (2013)

Protein pigment complexes (reaction center)

1240

840

0.57

0.55

Fu et al. (2014)

Protein pigment complexes (lightharvesting complex)

1460

620

0.49

0.54

Fu et al. (2014)

GlyGly

17.69

730



0.63

Nie et al. (2014)

GlyL-Leu

17.22

720



0.64

Nie et al. (2014)

340



0.41

Gordiichuk et al. (2014)

Proteins

Peptides

Artificial photosynthetic devices Photosystem I (PSI) (Thermosynechococcus elongatus)

2.81

(Continued )

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SECTION | V Sustainable Carbon-Based and Biomaterials

TABLE 18.1 (Continued) Pigments (bacterial)

Jsc (μA/ cm2)

Voc (mV)

η (%)

FF

Reference

PSI (Thermosynechococcus vulcanus)

15

580

0.53

0.59

Takekuma et al. (2020)

PSII (T. elongatus)

20

520

0.05

0.52

Kondo et al. (2014)

the unidirectional flow of electrons within the biological dye is observed (Woronowicz et al., 2012).

18.3.3.6 Chlorophyll a derived Spirulina xanthin carotenoid in Spirulina platensis Wang et al. a chlorophyll a derivative was used as a photosensitizer in DSSC, and it was also combined with 10% of various conjugation lengths of carotenoids as a redox spacer. The biosensitized DSSC was irradiated with a 100 mW/cm2 halogen lamp, and the short-circuit current density (Jsc) and conversion efficiency were analyzed by photoelectricity, which were determined as 11.5 mA/cm2 and 4.0%, respectively (Wang et al., 2005).

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Ulusu, Y., 2020. First vertebrates fluorescence protein UnaG: bacterial expression, purification and characterization. Rom. Biotechnol. Lett. 25 (1), 12161222. Varnava, K.G., Sarojini, V., 2019. Making solid-phase peptide synthesis greener: a review of the literature. Chemistry 14 (8), 10881097. Vothknecht, U.C., Westhoff, P., 2001. Biogenesis and origin of thylakoid membranes. Biochim. Biophys. Acta 1541 (1-2), 91101. Wang, X.F., Fujii, R., Ito, S., Koyama, Y., Yamano, Y., Ito, M., et al., 2005. Dye-sensitized solar cells using retinoic acid and carotenoic acids: dependence of performance on the conjugation length and the dye concentration. Chem. Phys. Lett. 416 (13), 16. Wang, X.F., Koyama, Y., Nagae, H., Yamano, Y., Ito, M., Wada, Y., 2006a. Photocurrents of solar cells sensitized by aggregate-forming polyenes: enhancement due to suppression of singlettriplet annihilation by lowering of dye concentration or light intensity. Chem. Phys. Lett. 420 (46), 309315. Wang, X.F., Matsuda, A., Koyama, Y., Nagae, H., Sasaki, S.I., Tamiaki, H., et al., 2006b. Effects of plant carotenoid spacers on the performance of a dye-sensitized solar cell using a chlorophyll derivative: enhancement of photocurrent determined by one electron-oxidation potential of each carotenoid. Chem. Phys. Lett. 423 (46), 470475. Wang, Z.S., Cui, Y., Hara, K., Dan-oh, Y., Kasada, C., Shinpo, A., 2007. A high-light-harvestingefficiency coumarin dye for stable dye-sensitized solar cells. Adv. Mater. 19 (8), 11381141. Wetser, K., Sudirjo, E., Buisman, C.J., Strik, D.P., 2015. Electricity generation by a plant microbial fuel cell with an integrated oxygen reducing biocathode. Appl. Energy 137, 151157. Williams, P.J.L.B., Laurens, L.M., 2010. Microalgae as biodiesel & biomass feedstocks: review & analysis of the biochemistry, energetics & economics. Energy Environ. Sci. 3 (5), 554590. Wongcharee, K., Meeyoo, V., Chavadej, S., 2007. Dye-sensitized solar cell using natural dyes extracted from rosella and blue pea flowers. Sol. Energy Mater. Sol. Cell 91 (7), 566571. Woronowicz, K., Ahmed, S., Biradar, A.A., Biradar, A.V., Birnie, D.P., Asefa, T., et al., 2012. Near-IR absorbing solar cell sensitized with bacterial photosynthetic membranes. Photochem. Photobiol. 88 (6), 14671472. Wu, W., Hua, J., Jin, Y., Zhan, W., Tian, H., 2008. Photovoltaic properties of three new cyanine dyes for dye-sensitized solar cells. Photochem. Photobiol. Sci. 7 (1), 6368. Xing, D., Zuo, Y., Cheng, S., Regan, J.M., Logan, B.E., 2008. Electricity generation by Rhodopseudomonas palustris DX-1. Environ. Sci. Technol. 42 (11), 41464151. Yamazaki, E., Murayama, M., Nishikawa, N., Hashimoto, N., Shoyama, M., Kurita, O., 2007. Utilization of natural carotenoids as photosensitizers for dye-sensitized solar cells. Sol. Energy 81 (4), 512516. Yazie, N., Worku, D., Reda, A., 2016. Natural dye as light-harvesting pigments for quasi-solidstate dye-sensitized solar cells. Mater. Renew. Sustain. Energy 5 (3), 13. Yehezkeli, O., Tel-Vered, R., Wasserman, J., Trifonov, A., Michaeli, D., Nechushtai, R., et al., 2012. Integrated photosystem II-based photo-bioelectrochemical cells. Nat. Commun. 3 (1), 17. Yehezkeli, O., Tel-Vered, R., Michaeli, D., Nechushtai, R., Willner, I., 2013. Photosystem I (PSI)/photosystem II (PSII)-based photo-bioelectrochemical cells revealing directional generation of photocurrents. Small 9 (17), 29702978. Yoon, S., Lee, H., Fraiwan, A., Dai, C., Choi, S., 2014. A microsized microbial solar cell: a demonstration of photosynthetic bacterial electrogenic capabilities. IEEE Nanotechnol. Mag. 8 (1), 2429.

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Yu, D., Zhu, G., Liu, S., Ge, B., Huang, F., 2013. Photocurrent activity of light-harvesting complex II isolated from spinach and its pigments in dye-sensitized TiO2 solar cell. Int. J. Hydrog. Energy 38 (36), 1674016748. Yu, D., Wang, M., Zhu, G., Ge, B., Liu, S., Huang, F., 2015. Enhanced photocurrent production by bio-dyes of photosynthetic macromolecules on designed TiO2 film. Sci. Rep. 5 (1), 19. Yum, J.H., Hagberg, D.P., Moon, S.J., Karlsson, K.M., Marinado, T., Sun, L., et al., 2009. A light-resistant organic sensitizer for solar-cell applications. Angew. Chem. Int. Ed. 48 (9), 15761580. Zenoff, V.F., Sin˜eriz, F., Farias, M.E., 2006. Diverse responses to UV-B radiation and repair mechanisms of bacteria isolated from high-altitude aquatic environments. Appl. Environ. Microbiol. 72 (12), 78577863. Zhang, Y., Noori, J.S., Angelidaki, I., 2011. Simultaneous organic carbon, nutrients removal and energy production in a photomicrobial fuel cell (PFC). Energy Environ. Sci. 4 (10), 43404346. Zhao, J., Zou, Y., Liu, B., Xu, C., Kong, J., 2002. Differentiating the orientations of photosynthetic reaction centers on Au electrodes linked by different bifunctional reagents. Biosens. Bioelectron. 17 (8), 711718. Zhou, H., Wu, L., Gao, Y., Ma, T., 2011. Dye-sensitized solar cells using 20 natural dyes as sensitizers. J. Photochem. Photobiol. A Chem. 219 (23), 188194. Zhuang, T., Sasaki, S.I., Ikeuchi, T., Kido, J., Wang, X.F., 2015. Natural-photosynthesis-inspired photovoltaic cells using carotenoid aggregates as electron donors and chlorophyll derivatives as electron acceptors. RSC Adv. 5 (57), 4575545759.

Chapter 19

Bioinspired solar cells: contribution of biology to light harvesting systems B. Gopal Krishna and Sanjay Tiwari Photonics Research Laboratory, School of Studies in Electronics & Photonics Pt. Ravishankar Shukla University, Raipur, India

19.1 Introduction Solar energy is clean and green energy for providing an excellent solution to complete energy demand. Light harvesting systems like solar cells are important devices for absorbing the light energy and converting that energy into electrical energy. The mostly used light harvesting system for this purpose is silicon-based solar technology. The silicon-based solar technology is more reliable in the terms of power-conversion efficiency (PCE), stability and life time, but fabricating cost is higher as compared to other PV technologies (Ushasree and Bora, 2019). The other technologies like thin film solar cells, organic solar cells (OSCs) and dye-sensitized solar cells (DSSCs) are now competing with silicon-based solar technologies to make efficient, stable and low cost solar cells (Polman et al., 2016). Material and processing costs limits the commercialization of most of the PV devices. The key solution to this problem is to mimic the biological structures like cyanobacteria or chlorophyll for better harvesting of solar energy to fulfill energy requirements (Chen et al., 2010). The advantage of the biomimicry is to have light harvesting devices of low cost material, material abundance and negative carbon footprint (Lakhtakia and Martın Palma, 2013). The bioinspired technologies can be inexpensive and ultimate alternative for producing low-cost and efficient PV devices (Swiegers, 2012). Photosynthetic bacteria and plants harvest light with high efficiency through special proteins (reaction centers) and convert it into electrochemical energy (Nagy et al., 2010). The phenomenon of photosynthesis is so efficient and occurs at incredibly high speeds. The journey of electron takes place through series of specially located pigments to create one-way path (Pullerits and Sundstrom, 1996). In artificial solar Sustainable Material Solutions for Solar Energy Technologies. DOI: https://doi.org/10.1016/B978-0-12-821592-0.00006-6 © 2021 Elsevier Inc. All rights reserved. 593

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systems like solar cell, the electron can easily bounce back across the junction or membrane due to which it loses its energy and executing the whole process with less efficiency (Mora et al., 2008). Cyanobacteria have the same solar energy converting mechanism as in plants and photosynthesis process and works exactly in the same way (Gao et al., 2012). In photosynthesis, solar energy is converted into energy rich products which are necessary to drive the biochemistry of life. In photosynthesis, excitation energy transfer and charge separation are two ultrafast processes with transfer rate of 1012 s21 (Colbow and Dunyluk, 1976). Under most favorable conditions, photon is absorbed by the photosynthetic organism and the electron transfer occurs in the system obeying fundamental quantum mechanics phenomena. These phenomena include delocalization, triggering the charge speed, efficiency and direction of the charge separation process (Barber, 2009). In natural photosynthesis, at least four design principles are possible which can be taken into consideration to mimic the development of bioinspired energy conversion systems (Blankenship et al., 2011; Brongersma et al., 2014). Man made silicon solar cells can convert only 18%25% light into electricity, but plants nearly convert all absorbed light into chemical energy (Blankenship et al., 2011; Chen et al., 2015). Diverse sets of optical phenomenon by insects and plants can inspire us to design and develop the much improved solar cells. The inspirations to develop synthetic form of antireflective structures to reduce reflectance loss in solar cells have been taken from the compound eyes of the moths. The synthetic moth eye coated solar panels have showed 33% improvement in efficiency as compared to normal solar cells (Greanya, 2016). Large area polydimethylsiloxane (PDMS) films with variably sized moth eye structures of thickness 300 nm were attached onto the glass side of perovskite solar cell with architecture ITO/PTAA/Perovskite/C60/BCP/Cu for improving its efficiency. The attachment of the moth eye inspired PDMS films could be used to tune the optical properties of the device by changing the size of the moth eye structures. The PCE of the device was improved from 19% to 21% because antireflective property of the moth eye structure improves the transmittance of light at the front of the glass from 92% to 96% as compared to bare perovskite solar cells (Kim et al., 2019). Biomimetically textured surfaces on the sub wavelength scale have showed to reduce the reflectance over the visible and the near-infrared regime in solar cells (Odobel et al., 2013; Singh et al., 2013). Lenticular compound lenses which are mimicked from insect eyes for industrial applications with great results. Therefore engineered biomimicry has been considered a scientific technology with great problem solving ability (Martın Palma and Lakhtakia, 2017). Researchers have been fabricating bioinspired solar cells (BISCs) by mimicking the organic and inorganic material properties of the biophotosynthetic structures. Bioinspired photovoltaic technology would have an improved efficiency as compared to the normal solar cell technology. Swollen induced bioinspired soft materials would provide direction for fabricating biomimetic solar cell (Mandal et al., 2016).

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Bioinspiration, biomimetic, and bioreplication are three methodologies of engineered biomimicry which can be employed to many solar technologies (Martın Palma and Lakhtakia, 2017).

19.2 Methodologies for engineered biomimicry 19.2.1 Bioinspiration Bioinspiration uses phenomena in biology to inspire research in nonbiological science and technology. Bioinspiration suggest simple techniques which lead to useful directional results for research. It helps to find new and useful routes bridging laboratories that have limited resources. Components of organism can contribute for bioinspired research. The function is a component in an organism which contributes to make organism’s fitness (Hamley, 2013). It offers examples of new structures and processes at every scale which guide us to select and design appropriate research strategies in bioinspired investigations. It is a field to observe remarkable functions that characterize organisms and try to mimic those functions for research purposes. The characteristics of biological systems can be understood by the following vectors (Hamley, 2013; Chiadini et al., 2013).

19.2.1.1 Function It defines the processes and structures due to which the organisms adjust to short term or long term changes in their environment. It possesses the ways for organism for adaptation, evolutionary fitness in animal behavior and defines goal in the philosophy of biology (Hamley, 2013; Zhang, 2003). 19.2.1.2 Simplicity Biological systems have elegance designs, complex mechanisms and structures which combine together to become a simple functions (Zhang, 2003). Therefore biology gives us the inspiration to study an integration of complex subsystems into simple, reliable, functions and mimic them for research purposes. 19.2.1.3 Dissipation Most of the biological systems are dissipative when there is a free flow of energy through them for their functioning (Love et al., 2005). The study of dissipative systems and structures can inspire us to develop reliable and efficient PV devices. 19.2.1.4 Soft matter Most of the biological systems are elastic and easy to deform. Material scientists can inspire from the biological systems to develop elastic and reliable devices (Simons et al., 1997; Soh et al., 2010). The soft matter like muscle,

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tendon, connective tissue, membranes and nerves in organisms inspires the researchers to understand the biological structure and function to stimulate new ideas for developing efficient PV devices.

19.2.1.5 Scientific impact The motivation to understand and mimic biological systems may reward researchers to solve the problems in the related scientific area. The observations of biological systems may lead to successful scientific or technological innovations. The impact of bioinspired research is to contribute to solve problems which are important to society (Soh et al., 2010; Kumar et al., 1994).

19.2.2 Biomimetic Biomimetics is formed by the words biology and mimesis (imitation). It also called as bionics. Technical biology analyses the form, structure and function relationship in living organisms by physical, chemical or engineering sciences (Purkait et al., 2018). Technical biology forms the basis for biomimetic research which allows the understanding of quantitative and technological functioning of the biological systems (Clonis et al., 2000). In biomimetic research, quantitative analyses provide the basis for creating theoretical ideas and making technical applications. Innovative biomimetic products have been achieved in recent years due to the better understanding and implementation of functional principles inspired from biology systems (Pohl et al., 2010). These quantitative analyses are the basis for theoretical knowledge and transferring ideas from biology to technical applications in biomimetic applications. Biomimetic research gives the better understanding of the biological systems which is referred to as reverse biomimetics (Zhang and Xuan, 2016; Boghossian et al., 2011; Golhani et al., 2020). Biomimetic research implements the creative re-inventive technology from the biological systems but never make blueprint of biology in nature. It requires several levels of abstraction and modification for this creative re-inventive technology. Bionanotechnology and nanobiotechnology are the two fields which are inspired by the biological systems to pave the way towards new materials and technologies. Biomolecules and their supramolecular organization have been playing a big role in biomimetics research due to the recent advances in bionanotechnology and nanobiotechnology (Golhani et al., 2020). There are several building blocks which include different molecules or molecular arrangements of synthetic or natural origin such as amino acids, lipids, carbohydrates, nucleic acids, carbon allotropes, dendrimers, and organosilanes. These building blocks are assembled in bottom up arrangement. However, it is very difficult to produce the biomimetic materials (Schulte et al., 2009; Dushkina and Lakhtakia, 2013; Martin Palma and Lakhtakia, 2013).

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19.2.3 Bioreplication Bioreplication is a process of direct replication of the structure of organism to produce new devices. Researchers have been able to replicate the biological structures of butterfly wing and corneal layer of an insect eye (Saison et al., 2008). Bioreplication is the youngest branch of engineering biomimicry which is useful in biomolecular synthesis, cloning and optical device fabrication. It also involves scaffolding approach in which the surface of the biotemplate is coated with a suitable material to enhance its functionality. Biotemplate is not a part of the final structure or device but provides an informational knowledge of biological scaffolds at the nanoscale. Biological systems have developed the structures and functions with much ease (Domingue et al., 2014). Advanced nano and micro technologies could be used for bioreplication of these complex structures up to a particular scale. A bioreplication of arrays of insect corneas as a collector layer have been used on solar cells. These bioreplicated solar cells have the large angle of view, antireflection (AR) and hydrophobic properties (Martın Palma and Lakhtakia, 2017). Bioreplication allows reproducing of the structures and associated functionalities found in natural organisms. The materials and techniques used for bioreplication are important. For example, piezoelectric materials in bioreplication could be used to allow the properties of a structure to be electrically tunable (Martin-Palma et al., 2008; Zhu et al., 2009). Photosensitive, high thermal expansion or other active materials could be used for replicas which respond to temperature and other environmental conditions. The use of ion beam techniques in bioreplication allows a biotemplate to be dimensionally scaled for tuning its optical properties in a different spectral regime. An ideal replication process has not been reported till now through bioreplication techniques. For this purpose, it would require the ability to reproduce bulk threedimensional structures by using a large number of compatible materials and fabrication techniques (Koch et al., 2008; Li et al., 2006).

19.3 Bioinspired solar cells The source of inspiration for researcher inventions and innovations has come from imitating solutions provided by natural world. Energy is a very important factor for the sustainable development of human race in many terms like economic growth. Nature can also inspire researchers to develop potential technologies for modern sustainable energy solutions (Ravi and Tan, 2015). There are bioinspired technological developments like DSSCs, bioinspired antireflective structures in solar cells which are being adopted to fulfill energy needs (Barber and Tran, 2013; Frischmann et al., 2013; Berardi et al., 2014; Huang et al., 2018). Bioinspired hierarchical AR layer was incorporated into a conventional TiO2 electrode of the DSSC by the inspiration from moth eye and grating-like structures in the outer layer of the Oriental hornet.

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The electrode coupled with a 2D patterned TiO2 AR layer in DSSC increased the PCE from 5% to 6.5% by absorbing large range of absorption bands of wavelength spectra due to the combination of AR and diffraction of the light on the front surface of device. Bioinspired AR layer consist a hierarchically patterned diffraction grating into a conventional TiO2 working electrode of DSSC. TiO2 nanowires (NWs) were grown by using a polymer template fabricated via interference lithography (IL) in the form circular holes to increase photon trapping effects in the visible region of light. A layer of nanocrystal TiO2 was coated on top of TiO2 NMs by doctor blade method. Platinum was used as counter electrode in the DSSCs (Ahn et al., 2012). Bioinspired dyes, organisms and bio structures are under investigation to design and develop efficient light harvesting systems like solar cells, solar concentrators and solar power satellite etc. For example, there are certain organisms which have the ability to capture or enhance the optical or thermal properties. The study of such biological structures (both living and nonliving structures) could boost the development of stable and efficient solar cells. These solar cells are called as BISCs (Zhang et al., 2019). The fabrication of BISCs is done by mimicking the arrangements and structure of biological systems and also combining the advancements in the field of nanotechnology for the creation of next-generation PV technologies. The improvement in various scientific technologies paves the way to create an artificial function just as accurately as in living organism. The artificial creation of biological functions and structures is important to fabricate BISCs (King et al., 2009; Sun et al., 2008). Photosynthesis and cyanobacteria energy conversion process from sunlight for generating food in plants and insects are the motivating factors to design and develop the stable and efficient solar PV systems (Makita and Hastings, 2017). Photosynthesis energy recovery process inspired the researcher’s to develop bioinspired light trapping mechanism for electrical energy conversion through PV cells (Sun et al., 2008; Makita and Hastings, 2017). There are attempts to engineer artificial biological functions and structures by means of different materials of construction to fabricate photovoltaic cells (Xu et al., 2016). Some insects have compound eyes with hexagonal shape of packed papillae called ommatidia. Ommatidia contain internal photoreceptors surrounded by support cells and pigment cells. These composed units in insect’s eyes effectively decrease optical reflectance (Stavenga et al., 2006; Kuo et al., 2016). The insect’s compound eye inspired scientists to fabricate high absorption and antireflective solar cells to restrict the reflection rate of sunlight within 35% using nanotechnology along with other technologies. A detailed study of structures on the surface of moth eyes would give the complete understanding of antireflective phenomenon (Dong et al., 2018; Leon et al., 2017). An ommatidium of moth is covered with antireflective nanostructures which reduce light reflection and enhance night vision of the organism as shown in Fig. 19.1A and B. The highly ordered AR nanostructured array of 200300 nm sized

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FIGURE 19.1 The image of complete compound eyes showing the details of subwavelength structure array in one ommatidium. The image also includes refractive index variations by incident of light on structures like flat UV-curable resin coating, columnar subwavelength structure, triangular subwavelength structure, and conical subwavelength structure. Reproduced with permission from Sun, J., Wang, X., Wu, J., Jiang, C., Shen, J., Cooper, M.A., et al., 2018. Biomimetic moth-eye nanofabrication: enhanced antireflection with superior self-cleaning characteristic. Sci. Rep. 8, 5438.

pillars on the surface of moth ommatidia would minimize the reflection of light in dark conditions and improve the vision capability of moth at night. The subwavelength structures are arranged for a gradual change in the refractive index between the air and the medium of the eye because the AR nanostructures which are smaller as compared to visible wavelength ranging from 380 to 760 nm would not capable of sudden variation in refractive index. This property of gradual change in the refractive index for the incident light involved in the biomimetic subwavelength structures is known as the moth eye effect. For UV curable resin, incident light interacted with single layer coating having no AR structure would have three different refractive indexes such as the refractive index of air (ηair), UV curable resin (ηuv), and substrate (ηs), respectively, as shown in Fig. 19.1C (a). For UV curable resin, a coating of columnar subwavelength array would have refractive index (η1) and could be changed by the ratio between the structures and channels as shown in Fig. 19.1C (b). Refractive index (ηeff) would be obtained for conical subwavelength structure with triangular profile and

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could involve a linear variation from ηair to ηuv as shown in Fig. 19.1C (c). Biomimetic moth eye structure would involve a gradual variation in refractive index ηeff with moth eye structure of parabola shaped profiles (Sun et al., 2018). Biomimetic moth eye structures can be applied to optical devices like solar cells to improve their efficiency. A facile and bioinspired approach was used to get AR coating by polymerization induced selfwrinkling technique. The wrinkle micro patterns were made spontaneously on the surface of AR coating during photo crosslinking to allow photocuring process in the device. Fluorinated copolymers with different monomers like methyl methacrylate, n-butylacrylate, styrene, poly(ethylene oxide) dimethacrylate (PMMA-F, PBA-F, PS-F and PEGMA-F) were dissolved in the acrylate crosslinked trimethylpropane triacrylate with 0.3 wt.% photo initiator The solution was coated on the surface of αSi solar cell solar cell by knife coating method. The encapsulation of αSi solar cell with AR coatings with wrinkles pattern improved photovoltaic performance improvement of more than 4%B8%.A Bioinspired strategy of antireflective photocuring coating through self-wrinkling in solar cells exhibited a 90% transmittance, 5%8% low reflection and AR efficiency of 4%7% as compared with flat bank coating. The introduction of the wrinkle pattern decreases the reflection of light and increases the optical path in the photoactive layer of the solar cell (Hou et al., 2017). The structures and compositions of various organisms have been studied for mimicking the polymers and organic materials to use them in photovoltaic cells (Siddique et al., 2017a). Bioinspired transparent conducting electrodes have been in development to get excellent physical, optical, and electrical properties for OSCs. Standard TCEs have chemical and mechanical instabilities which limit its use in photovoltaics. Graphene has excellent physical, optical and electrical properties with a hydrophobic surface (Siddique et al., 2017a,b). A thin layer of norepinephrine (amphiphilic catecholamine derivative) was applied to graphene to modify its surface as hydrophilic surface without decreasing optical transmittance or electrical conductivity. Using poly(norepinephrine) coated graphene electrode, the PCE of OSCs was 7.93% which was close to the ITO based reference device with 8.73%. This bioinspired adhesive material was an efficient surface modifier for chemically inert graphene for OSCs devices (Jung et al., 2018). Bioinspired borondifluoride complexes of curcuminoid derivatives endcapped with triphenylamine groups were synthesized for solution processed bulk-heterojunction OSCs. These complex derivatives improved PCE, chemical stability and thermal stability of OSCs (Archet et al., 2017). Similarly, biological materials added to perovskite solar cells can provide a high efficiency. The electrical properties of bR proteins and perovskite materials are same. Researchers aligned the gaps between bR proteins and perovskite materials and efficiency of perovskite solar cells was improved through the FRET mechanism. The integration of natural dye into OSC architectures improved its PCE from 14.59% to 17.02% as compared to conventional cell

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(Das et al., 2019). For instance, chlorophyll derivatives and copolymers produced with isoindigo can yield high PCEs of OSCs. Light harvesting complexes could be placed at the interface between the active layer and charge collection layers to enhance large PCE of OSCs (without the interlayer). Silver nanoprisms as interlayers integrated with natural photosynthetic systems result in high-performance OSCs to demonstrate a PCE over of 10% (Vohra, 2018).

19.4 Bioinspired structures and organisms 19.4.1 Dyes DSSCs represent one of the most promising PV technologies alternative to conventional silicon solar cells. DSSCs have low cost production methods, peculiar optical and mechanical properties with high indoor efficiency. Researchers are continuously searching materials for the fabrication of high efficient DSSCs (Dembele et al., 2013; Gra¨tzel, 2004; Nattestad et al., 2016). In DSSCs, photoanode is an important component for photon absorption and electron transport. Semiconductors like TiO2 and ZnO are the materials for photoanodes in DSSCs. At the semiconductor oxide interface, semiconductor oxides have the capability to recover loss of energy after recombination. The photo conversion efficiency of the DSSC can be done by adjusting the covalent bond of semiconductor oxide. Electrons are photoinjected at the photoanode TiO2 in the DSSC. These electrons are quickly drifted towards metallic cathode for existence of fast interfacial ion dynamics from cathode to the anode generated oxidized form of a redox couple. This process would avoid recombination between generated photoinduced charge at the electrode and redox ions in the electrolyte (Cavallo et al., 2017). The synthetic dyes are extracted by complicated synthetic routes with low yields and used in the fabrication of DSSCs. Natural dyes are now becoming popular due to many advantages over rare metal complexes (ruthenium-based complexes). Natural dyes can be extracted easily with minimal chemical procedures at low cost. These dyes have large absorption coefficients, nontoxic, easily biodegradable and wide availability like in fruits extracts (McCune et al., 2012). The dyes like anthocyanins, carotenoids and chlorophylls could be extracted from natural pigments to be used as sensitizers in DSSCs (Ko et al., 2011; O’Regan and Gratzel, 1991). In photosynthetic plants, chlorophyll (a natural pigment) absorbs specific wavelength of visible light from sun and converts it into chemical energy. The structure of chlorophyll consists of a chlorin ring in which four nitrogen atoms surround a central magnesium atom. The magnesium atom plays an important role in absorbing light of wavelength between 400 and 700 nm to make chlorophyll-a molecule to enter the first excited state. Quercetin, luteolin, and alizarin dyes extracted from plants like Hypericum perforatum, Reseda luteola and Rubia

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tinctorium L. have absorption spectra wavelengths which ranges from 250 to 650 nm. The intense peaks from absorbance spectra of Quercetin, luteolin and alizarin were 268 and 412 nm, 354, and 580 nm, respectively. Chlorophyll itself is not a good sensitizer for DSSC applications because of lack of binding sites to TiO2 (Wang et al., 2013). Therefore anthocyanin and chlorophyll dyes could be blended to be used as the photosensitizer. This binds carbonyl and hydroxyl groups of the chlorophyll to the surface of a porous TiO2 film in DSSCs. This provides easy electron transfer from the anthocyanin molecule to the conduction band of TiO2. Anthocyanins based DSSCs achieved 8.80 mA/ cm2 photocurrent, 389 mV open circuit voltage and 2.06% efficiency under simulated AM1.5 solar light (Calogero et al., 2015). Bioinspired DSSCs fabricated by the inspiration of crustose lichens. Crustose lichens have root-like fungal structure which grows within the rock. A porous carbon plate was used as catalytic layer to design a novel integrated counter electrode DSSC architecture to increase its efficiency. In DSSC architecture, vertically oriented ordered mesoporous carbon monolithic film was used as both conductive substrate and catalytic layer to improve the device performance. The integrated pure carbon counter electrode showed high conductivity of the carbon sheet of 488 mΩ/cm and PCE of 8.11% as comparable to Pt/FTO based devices with PCE 8.16%. Fig. 19.2 shows the bioinspired carbon electrode used in DSSC to improve its electrical performance (Wang et al., 2013; Calogero et al., 2015).

FIGURE 19.2 Bioinspired porous carbon plate used in DSSC to improve its electrical and optical performance. DSSC, dye-sensitized solar cell. Reproduced with permission from Wang, C., Meng, F., Wu, M., Lin, X., Wang, T., Qiu, J., et al., 2013. A low-cost bio-inspired integrated carbon counter electrode for high conversion efficiency dye-sensitized solar cells. Phys. Chem. Chem. Phys. 15, 14182.

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19.4.2 Wettability and superhydrophobic dyes The natural phenomena inspire us to develop new and advanced materials like bioinspired polymeric superhydrophilic, superhydrophobic, and superleophobic materials. These materials with desired wettability are synthesized by mimicking the structures of lotus leaf, rose petals and wings of various creatures. Such materials have properties such as very high water contact angles (WCAs) and many adhesion properties (Yuan and Lee, 2013; Lu et al., 2015). WCA is greater than 150 degrees for superhydrophobhic surface. The potential application of such material is anticorrosion coating, antibacterial coating, microfluidic devices, batteries and optical devices. Self-cleaning and stability of superhydrophobic materials have many applications in solar technology. The bioinspired coating could be done onto silicon-based solar cells by imitating morphology of leaf structures. The coating of transparent photopolymer with leaf surface morphologies onto silicon wafers was done by facile double transfer process. The open circuit voltage, short circuit current and power were 0.608 V, 27 and 12.5 mW/cm2 for bioinspired silicon solar cell. The open circuit voltage, short circuit current and power were 0.6 V, 24 and 10 mW/cm2 for bare silicon solar cell. The sunlight reflection for bioinspired silicon-based solar cells dropped significantly from more than 35% to less than 20% with 11% gain in power of photovoltaic cell as compared to bare silicon solar cells (Huang et al., 2018). The advances of bioinspired siliconbased solar cells would boost large scale industrial designs of bionic antireflective and superhydrophobic coating for future solar cells (Faraz et al., 2018). A water-repellant perovskite solar cell was fabricated on basis of antireflective and self-cleaning characteristics of lotus leaf as shown in Fig. 19.6 (Kang et al., 2014).

19.4.3 Organisms 19.4.3.1 Common rose butterfly Researchers are inspired by microscopic structure of the gossamer deepblack wings to fabricate new type of solar cells with enhanced light absorption rate over the complete spectrum at different angles of incidence (Vigneron et al., 2010; Lakhtakia, 2016; Kilic et al., 2014). These improved solar cells could lower the cost of photovoltaic installations. The butterfly wings have inspired the design and development of four geometrics of solar cells such as an unpatterned slab structure, ordered and periodically arranged 240 nm diameter holes, perturbed structure, and correlated structure with fill factor of 50.26%. Integrated absorption improved up to 93% under normal incidence of broad angular range of light due to the incorporation of group of ordered ensemble cylindrical holes into a 130 nm thin film of αSi:H slab on glass as compared flat thin film slab. Researchers designed thin film of αSi:H slab on glass to absorb 200% more integrated absorption incident

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radiation at large view angles of 50 w.r.t slab by biomimicking the wings of rose butterfly as compared flat thin film slab. Therefore the absorption of the incident radiation could be done by biomimicry (Siddique et al., 2017c). Morpho butterflies inspired to fabricate dye-sensitized solar cells (DSSCs) (1D multilayer structure) to absorb light of particular wavelengths for improvement of power-conversion efficiency. In 1D multilayer structure DSSCs, the deposition of TiO2SiO2 nanoparticle multilayers was done through spin coating to provide a brilliant metallic reflection and confining incident light of particular wavelength within the photoanode. The efficiency increased from 3.9% in the reference cell to 4.2%. The schematic and FESEM images of DSSCs is shown in Fig. 19.3 (Lou et al., 2012). The design and microstructure of a DSSC based on the 1D photonic crystal (1D PC) was shown in Fig. 19.3A. In this DSSC device, a porous and highly reflecting 1D PC was coupled to a dye-sensitized nc-TiO2 electrode. The deposition of nanoparticles layers of different composition was done spin coating. The advantage of porous mesostructure in DSSC was that it permits the proper flow of electrolyte through it without effecting charge transport. TiO2SiO2 nanoparticle multilayer in the dye-sensitized nc-TiO2 electrode was shown in Fig. 19.3B and C. 1D PC and 1D PC coupled DSSC were

FIGURE 19.3 Design of a DSSC coupled to a 1D based porous nanoparticle TiO2SiO2 multilayers. DSSC, dye-sensitized solar cell. Reproduced with permission from Lou, S., Guo,X., Fan, T., Zhang, D., 2012. Butterflies: inspiration for solar cells and sunlight water-splitting catalysts. Energy Environ. Sci. 5, 9195-9216.

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shown in Fig. 19.3EF. The color difference in Fig. 19.3E and F was due to different 1D PCs coupled to the same nc-TiO2 electrode (Lou et al., 2012).

19.4.3.2 Leaf The photosynthesis is process of producing energy from sunlight. This process is done by many living organisms. Such living organisms can be inspirational for researchers to develop efficient and low cost solar PV modules (Umar et al., 2012). The transfer kinetics of the molecules or atoms can be improved by the hierarchically porous structure of the leaf. This porous chlorophyll structure in leaf provides the H2 production activity. Polyacrylonitrile (PAN) nanofibers are polymers having excellent properties like chemical resistance and flexible. PAN nanofibers were integrated into a mat form by electrospinning method. PANMat has hierarchically porous flexible structure act just like stomata in leaves. It could be used as substrate for ZnxCd12xS. Zn0.5Cd0.5S has sphalerite phase structure and behaves like a photocatalyst for H2 production. Zn0.5Cd0.5S was deposited on PAN-Mat to form Zn0.5Cd0.5S@PANMat which has an advantage of hierarchically porous flexible structure and photo catalytic property for H2 production. Bioinspired Zn0.5Cd0.5S@PAN (polyacrylonitrile) mat-shaped photocatalyst was prepared by studying the leaf structure. A flexible bioinspired ZCS@PANMat prepared to ensure its separation and reuse with H2 generation rate of about 475 μmol/h per 50 mg of photocatalyst and quantum efficiency of 27.4% at 420 nm. Zn0.5Cd0.5S nanoparticles (ZCSNP), CdS nanoparticles (CSNP), ZnS nanoparticles (ZSNP), ZCS@PANMat, CS@PANMat and ZS@PANMat are some other photocatalyst used for H2 generation under visible light irradiation (Fu et al., 2018). Silver sulfide hierarchical structures with unique dorsal spine morphology were successfully synthesized by simple solidvapor reactions after studying the structure morphology of a leaf. The morphological control of Ag2S structures could be done by varying the reacting environment to counter the influence of time and voltage. Silver sulfide hierarchical structures inspired by leaf display with promising optical properties to develop highly efficient solar cells (Ruvalcaba et al., 2014). Han et al. reported a quasifractal structure network, a new strategy application of natural scaffoldings perfected by evolution for development of modern optoelectronics. The quasifractal structure network derived directly from a chemically extracted leaf venation system for solar cells and similar devices. The second network was obtained by metalizing a spider’s silk web for touch screens and flexible displays. These networks have an extraordinary optoelectonics and mechanical performance for proposed applications and will provide an encouraging direction in the development of more efficient optoelectronic devices (Zhu et al., 2010). Min Ju Yun et al. mimicked leaf anatomy and introduced a light trapping layer on top of the solar cells with microscale patterned photoanodes as shown in Fig. 19.4. The light trapping layer and photoanode patterning

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FIGURE 19.4 Mimicking strategies for bioinspired 3D arrays of DSSCs inspired by leaves. DSSC, dye-sensitized solar cell. Reproduced with permission from Yun, M.J., Sim, Y.H., Cha, S.I., Lee, D.Y., 2019. Leaf anatomy and 3-D structure mimic to solar cells with light trapping and 3-D arrayed submodule for enhanced electricity production. Sci. Rep. 9, 10273.

technique in three-dimensional DSSCs improved charge collection efficiency by altering light distribution. This modification in DSSC enhanced 55% electricity production as compared to conventional modular design and PCE from 4% to 8% (Yun et al., 2019). Wensheng Yan et al. reported a bioinspired ultrathin cSi solar cell inspired by water ferns as shown in Fig. 19.5. Bioinspired ultrathin cSi solar cell has thin polycarbonate nanofur film at the front side which improved an efficiency from 17.3% to 18.1% and short circuit current density (Jsc) from 35.8 to 37.4 mA/cm2 (Yan et al., 2018).

19.4.3.3 Lotus Lotus leaves have unique superhydrophobic and dirt-resistant properties. Researchers have replicated the unique properties which are exhibited by the structure of lotus leaves to make self-cleaning and antireflective surface of solar cells. Bioinspired coating approach was employed by replicating leaf structures onto silicon solar cells dimension of 1 3 1 cm2. Morphologies of leaf surface were cured on silicon solar cells by facile double transfer process. Bioinspired silicon solar cell gained maximum power of nearly 12% as

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FIGURE 19.5 A high-performance 3D ultrathin Si solar cell with the thin nanofur coupled at the top with its SEM image. Reproduced with permission from Yan, W., Huang, Y., Wang, L., Vu¨llers, F., Kavalenka, M., Ho¨lscher, H., et al., 2018. Photocurrent enhancement for ultrathin crystalline silicon solar cells via a bioinspired polymeric nanofur film with high forward scattering. Sol. Energy Mater. Sol. Cell. 186, 105110.

compared to 10% for flat Si solar cell. The reflectivity of mimicked surface of solar cells showed 15% reduction and 10% increase in overall power generation as compared to solar cells without mimicking. The self-cleaning ability of lotus inspired solar cells would reduce the manpower and maintenance cost at the site of large power plants (Huang et al., 2018). Seong Min Kang et al. mimicked antireflective and self-cleaning characteristics of lotus leaf to develop a water-repellent perovskite solar cell as shown in Fig. 19.6. Hierachical pyramidal PDMS film arrays which were attached on a FTO glass with enhanced self-cleaning and AR characteristics. This layer acted as self-cleaning and AR layer on the perovskite solar cell device to improve PCE from 13.12% to 14.01% (Kang et al., 2014).

19.4.3.4 Firefly The nature inspired firefly algorithm and behavior of fireflies can be effectively used to describe the parameters of solar cell with high accuracy. The conventional methods are incapable to estimate the nonlinear, multivariable and multimodal features of current-voltage of solar cell models with high accuracy. The algorithm was used to extract the parameters of solar cell from experimental IV characteristics. The developed modules using firefly algorithm would work at low irradiance which help to bring out the optimal parameters at different temperature variations. There are many other bioinspired computational algorithms by observing the natural behavior of animal, insects and other micro-organisms for estimation of parameters of solar cell with high accuracy (Siddique et al., 2017a). These bioinspired algorithms developed are

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FIGURE 19.6 Self-cleaning and antireflective perovskite solar cell device having hierarchical pyramid PDMS arrays with blocking layer TiO2. Reproduced with permission from Kang, S.M., Ahn, N., Lee, J.W., Choi, M., Park, N.G., 2014. Water-repellent perovskite solar cell. J. Mater. Chem. A. 47, 114.

genetic algorithm, differential evolution, particle swarm optimization (PSO), bacteria foraging algorithm, artificial bee colony, cuckoo search, and bat algorithm. The particle swarm algorithm has been appeared most efficient as compared to other algorithms. Natural pigments like polyacrylamide hydrogel were used to increase the PCE of the solar cell. Mandal et al. reported swollen induced hybrids with open circuit voltage Voc of 1.162 V, PCE of 0.59% and photocurrent density of 109 μA/cm2 (Mandal et al., 2016).

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Meiying Ye et al. proposed and applied a PSO to find out the solar cell parameters quickly, accurately and easily from illuminated current-voltage characteristics. The results of PSO have higher parameter precision to extract solar cell parameters compared with the genetic algorithms (Ye et al., 2009).

19.4.3.5 Human eye The retina of the human eye contains funnel-like inverted cone structures (fovea centralis) which are closely packed to each other. These tiny structures are capable of capturing great amount of light and light image trapping which are responsible for high acuity binocular vision. Three-dimensional vertical light-funnel silicon array with inverted cones (Fig. 19.7) are bioinspired by the properties of the fovea centralis which can be used for light-trapping in photovoltaic applications. The bioinspired light funnel arrays can work as efficient light absorbing layers in solar cell to boost solar cell efficiency and show broadband absorption enhancement of the solar spectrum. The bioinspired light funnel array can provide a new and better way to improve solar cell efficiency. In a thin film solar cell, silicon light funnel array mechanism increased 65% light absorption as compared to conventional silicon film of same thickness.

FIGURE 19.7 3D light funnel array fabricated by using nanosphere lithography and Langmuir Blodgett deposition techniques. Reproduced with permission from Shalev, G., Schmitt, S.W., Embrechts, H., 2015. Bronstrup, G., Christiansen S. Enhanced photovoltaics inspired by the fovea centralis. Sci. Rep. 5 (1), 8570.

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The light funnel array based solar cell has 60% higher PCE than nanowire array based solar cell (Shalev et al., 2015).

19.4.3.6 Beetle A scarabaeidae beetle species named Hopliacoerulea (Coleoptera) has natural photonic structure having fluorescent molecules embedded within the multilayered structured scales. The fluorescent molecules of the beetle can improve the light absorption with their respective wavelengths. The insect inspired to do synthesis of such fluorescent molecules. The bioinspired fluorescent molecules can be used in solar cells to improve the dielectric properties of solar materials, layer thickness and position of the planar emitting source. Van Hooijdonk et al. reported the simulation of the emission properties of a planar emission source embedded in a multilayer system. Scales of the scarabaeid beetle Hopliacoerulea have fluorescent molecules embedded in a multilayer structure which improves emission of certain wavelengths. The modeling of scales of the Hopliacoerulea was done by a multilayer architecture. In this multilayer architecture, a periodic stack of chitin layers, air/chitin mixed layers and planar emitting source were introduced in this system. Naturally inspired Hoplia-like system was investigated by means of optical simulation based on one-dimensional transfer-matrix formalism for understanding optical response of the system. The simulation results predicted that there could be a possibility to improve fluorescence emission abnormal wavelengths by changing the layer thickness, dielectric constant and position of the emitter source in the system. The optimized parameters would be useful in solar cells applications (Von Hooijdonk et al., 2012; Galusha et al., 2010). 19.4.3.7 Dipteran The scalloped structured of dipteran eye can capture light from all directions and is capable of almost around 270 degrees vision. The light capture property of solar cell technology can be improved by replicating the structure of compound eye of the dipteran. The fabrication of bioinspired compound lenses was inspired from the compound eye of the dipteran. BCL was fabricated with fractal scalloping to capture direct illumination and diffused light of wavelength ranging from 400 to 1100 nm. Therefore there are various application of bioinspired BCL in solar cell technology like improving the solar cell surface with double AR BCL layer, coupling of incident light through multiple reflections over the textured BCL layer of solar cell, most favorable conditions for capture of incident light through any direction and easy transmission of light through the silicon in solar cell (Jung et al., 2018). 19.4.3.8 Crab The crab shell can provide a path to improve the solar cell technology by using bioinspired nanocomposites, monomer and polymers. The lipids,

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FIGURE 19.8 Transparent crab shell after removal of matrix substances from original crab shell. Reproduced with permission from Shams, M.I., Nogi, M., Berglund, L.A., Yano, H., 2012. The transparent crab: preparation and nanostructural implications for bioinspired optically transparent nanocomposites. Soft Matter. 8 (5), 13691373.

pigments, protein, fats and other minerals of the crustaceans can be used to produce entirely transparent crab shell for optical applications. The above described substances of the crustaceans were removed and polymerized by chemical process to obtain transparent crab shell as shown in Fig. 19.8 (Shams et al., 2012; Chiadini et al., 2013). The chitin particles from heterogeneous microscale crab shell were used as nanocomposites. The polymer containing these nanocomposites has improved the transparent and thermal characteristics. The axial thermal expansion of the prepared chitin was found out to be 21 ppm/K. The synthesis of bioinspired reinforced transparent polymer of 0.3 mm chitin powdered particles was done. The transparent characteristic of these nanocomposites was stable over wide range of temperature from 280 C to 220 C, but showed variation in the reflective index w.r.t temperature (Shams et al., 2012).

19.5 Biological processes for bioinspiration 19.5.1 Photosynthesis Photosynthesis is a chemical process of converting solar energy into useful energy by plants, algae and some bacteria. The step-wise process for happening of photosynthesis is light harvesting, charge separation, water oxidation and fuel production. In light harvesting, array of protein and chlorophyll molecules embedded in the thylakoid membrane (antenna complex) absorb solar energy and transfer light energy to chlorophyll molecule at the reaction center of a photosystem where separation of positive and negative charges takes place. The positive charges are utilized to oxidize water molecules and negative charges are transferred through cytochrome b6f (Barber and Tran, 2013; Frischmann et al., 2013). The mobile electrons are excited through photosystem I (PSI) to produce carbohydrate fuel. The following chemical reactions are

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showing the water molecule oxidation, carbon dioxide reduction and total chemical reaction in photosynthesis process (Ogawa et al., 2011). Half reaction for water molecule oxidation 4hν

2H2 O - O2 1 4H1 1 4e2 Half reaction for carbon dioxide reduction to produce carbohydrate fuel 4hν

CO2 1 4H1 1 4e2 - ðH2 COÞ 1 H2 O Total chemical reaction in photosynthesis process 8hν

CO2 1 H2 O - ðH2 COÞ 1 O2 Four photons are necessary to initiate half reaction for water oxidation and carbon dioxide reduction. Therefore a total of eight photons (two photons per electron) are necessary for total chemical reaction in photosynthesis process. In photosynthesis, the chemical reactions are proportional to the number of photons absorbed. Therefore natural photosynthesis is determined by the total incident solar light plus the number of photons collected per unit time in the 400700 nm spectrum region. The wavelength range of light spectrum is an important parameter for the survival and reproducibility of plant because the composition of wavelength of light absorbed by plant pigments gives the total quantity of light used for photosynthesis process. The wavelength ranges between 400 and 700 nm is utilized by plant for its development by photosynthesis process. This range can be called as photosynthetically active radiation. The information of wavelengths from light reached to plants is gained by special pigments which are called as photoreceptors. The sensitivity of wavelength of light reached to plant depends upon the types of photoreceptors. Phototropins and cryptochromes are two photoreceptors which are sensitive to UV(A) and blue light. Phototropins are responsible for bending of stems towards light and stomata to open. These photoreceptors are also responsible for chloroplast movement in the cells of plant according to the light received by plant. Cryptochromes pigments are responsible to sense the direction of light. The inhibition of stem growth, tracking of sunlight and functioning of stomata is also due to cryptochromes pigments. Phytochrome photoreceptors are sensitive to near and far red light. Pfr and Pr are two types of interacting phytochromes. The elongation of stem and synthesis of chlorophyll are some functions controlled by phytochrome. Light absorption peak of Pr phytochrome is at wavelength 670 nm. Pr phytochrome absorbs red light of wavelength 670 nm and coverts to Pfr phytochrome. In a similar way, Pfr phytochrome absorbs red light of wavelength 730 nm and coverts back to Pr phytochrome. The greater amount of blue light between 400 and 500 nm influences the cell elongation, leave thickness and shorter stems. A very small amount of blue has a negative influence on the plant’s development. Various plants require minimum amount of blue light such

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as 530 μmol/m2/s for lettuce. Many plants grow close to each other which may cause shading. Most of the red light between 400 and 700 nm from sun is used from plants for photosynthesis. Most of the far red light ( . 700 nm) is reflected by plants. Plants in the shade region accept more far red light as compared to red light than there is a requirement of more light by plants for photosynthesis. As a consequence, plant becomes taller with Pr increases and larger distance between internodes and thinner stem. The efficiency of natural photosynthesis is very high but overall solar energy to carbohydrate conversion efficiency is low (Hall et al., 1995; Mohseni et al., 2008). Researchers can use a blueprint of natural photosynthesis for creating efficient artificial photosynthesis process to be utilized in applications like photovoltaic applications.

19.5.1.1 Artificial photosynthesis The following factors like incoming photon flux, energy and electron transfer, and catalysis can affect the efficiency of fuel conversion by solar energy because the factors can operate on different time, energy and length scales. The factors limit to build a system to match the most efficient solar-to-fuel conversion functioning near to the theoretical limits on solar energy conversion. Plants absorb light of wavelength ranging between 400 and 700 nm and utilize only half of the incoming photons. Polycrystalline silicon solar cells do not have a uniform spectral response to sunlight containing different wavelengths. It is more sensitive to red light band with measured 23.83% relative efficiency and less sensitive to green light band with relative efficiency of 19.15%. Polycrystalline silicon solar cell produces substantial portion of energy in the infrared region with relative efficiency of 13.56%.Therefore silicon solar cell utilizes more photons than plants as the photosynthetic efficiency is 3%8% for total sunlight incident on plants (Gouveˆa et al., 2017). In natural photosynthesis, two photosystems initiate the chemical reactions of water oxidation and CO2 reduction (Barber and Tran, 2013; Hall et al., 1995). The two photosystems absorb light of same energy and infrared energy photons remain unused. The same phenomena can be utilized in building the artificial photosynthesis device but in a different manner. In artificial photosynthesis, one absorber remains active in visible part of the spectrum and another in the infrared part. By adopting this process, the absorption of number of photons of sunlight can be maximized. Light harvesting, charge separation, water oxidation and fuel production are the four steps in artificial photosynthesis (Mohseni et al., 2008). The development of tandem artificial photosynthesis device (two absorbers tandem devices) would facilitate to produce water splitting and fuel production with two photons per electron by using artificial photosynthesis (Romero, 2012). An artificial photosynthesis phenomenon was utilized by Michael Graetzel to define the working of a DSSC (Gra¨tzel, 2003).

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19.5.2 Cyanobacteria Cyanobacteria are one of the oldest known living phyla which are divided into Gloeobacterales, Synechococcales, Spirulinales, Chroococcales, Pleurocapsales, Oscillatoriales, Chroococcidiopsidales, and Nostocales. Gloeobacterales are unicellular which lack thylakoids. Order Synechococcales include unicellular, colonial and filamentous forms of the organism and having 11 families. Order Spirulinales consist of a single family having category of coiled trichomes. Order Chroococcales consist of eight families which contains the coccoids containing nearly irregular thylakoids. Order Pleurocapsales is a fresh water unicellular which contain four families having fresh water unicellular. Order Oscillatoriales are filamentous prokaryotes having seven families with complex cytology (Pinevich et al., 2008). Order Chroococcidiopsidales have only one genus Chroococcidiopsisthermalis. Order Nostocales are filamentous cyanobacteria having 12 families with unique capability of cellular differentiation. Cyanobacteria have the ability to colonize in many habitats and can adapt to severe environment, store food, adjust illumination and fixing nitrogen. Cyanobacteria are the leading organisms in the marine ecosystem to produce primary biomass for building up coral reefs. The distribution of filamentous orders is mostly in brackish, marine and freshwater ecosystems. Cyanobacteria are playing an important role in the field of agriculture, pharmaceuticals, bioremediation and biofuels, bioenergy and bioelectricity. The utilization of cyanobacteria as biocatalysts for electricity, for producing biofuel is an excellent idea (Sarma et al., 2016; Lau et al., 2005). Thylakoid membranes in cyanobacteria are responsible for photosynthesis process. In thylakoid membrane, Phycobilisome (PBS) is a group of pigmented proteins which traps solar light and release the photon energy into the photosynthetic reaction centers. PBS is an ordered array of closely spaced granular structures found in the thylakoid membrane. Hemi-discoidal PBSs have composition of core and peripheral rods in cyanobacteria. Core has three cylinders composed of allophycocyanin which radiates six peripheral rods. These six peripheral rods create second sub domain of PBS. Chromophores phycoerythrin and phycocyanin constitute the peripheral rods to harvest the light energy. The harvested/absorbed light is transferred in a cascading way to allophycocyanin to PS I or PS II reaction center. The reaction centers work together to transfer electrons from water to NADP1. Then, the electron is transferred to a P680 pheophytin complex which reduces plastoquinone molecule (Lau et al., 2005; Ward et al., 1998). Later, the formation of plastoquinol takes place when plastoquinone molecule reduces a secondary quinone (semiquinone). At PS II reaction center, there is an oxidation of tyrosine residue from P680 (a chlorophyll cation radical). The oxidized tyrosine residue further oxidizes a cluster of four manganese atoms. There is a spitting of water molecule at this metal center. The metal center is able to assemble four oxidizing equivalents to facilitate the release O2 from two molecules of water. S state is termed as five successive

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oxidized forms. Reaction center have a nonheme iron atom and a stable tyrosine radical. The intermediary S state provides mechanism to link between S states and the tyrosine radical. PS II contains both integral membrane proteins and extrinsic proteins, but subjected to damage by sunlight at all light intensities. Therefore there is a repair activity within PSII to sustain its activity. The electric link between PS II and PS I is due to the electrons which are released from plastoquinol oxidation and accepted by cytochrome b6f complex. Photosynthetic electron transport (pet) genes have low potential protein (Cytb6), high energy Rieske iron sulfur protein [2Fe2S] center, Cyt f protein, two b and a c-heme moiety. The proton-pumping cytochrome b6f complex is a part of important for growth of cyanobacteria by photosynthesis or respiration (Pisciotta et al., 2010). The PS I complex utilizes biochemical machinery and oxidizes plastocyanin or other electron carriers to reduce ferredoxin. The membrane-bound components of the complex contains reaction center P700, a chlorophyll-a dimer and primary donor. The light absorbed by the complex releases electron which is captured by primary electron acceptor a chlorophyll-a (Chla) monomer and transferred to the phylloquinone. Electron from phylloquinone is transferred to three iron sulfur clusters [4Fe4S] in cascading manner rendering P700 in an oxidized state. Then, electron from iron sulfur clusters is transferred to ferredoxin and ferredoxin-NADP1 to get the end product NADPH which can be used for CO2 assimilation. The concentrating and uptake of CO2 mechanism in Cyanobacteria is excellent. Carboxysomal carbonic anhydrase converts the accumulated HCO2 3 to CO2 within icosahedral or quasiicosahedral protein micro-compartment within the cell. Carboxysome protein shell stops the elevated level of CO2 to make it available to Rubisco enzyme within the organelle. Active CO2 and HCO2 3 transporters are present in the plasma membrane and thylakoid membrane in high and low affinity form. Both oxygenic photosynthesis and aerobic respiration take place in the same compartment in cyanobacteria. Components like NADPH, plastoquinone, and cytochrome b6f complex are also part of the respiratory chain of cyanobacteria (Hall et al., 1995). BISCs could utilize the ability of utilizing water as the source of electrons by cyanobacteria to convert light energy into electrical current. Microbial fuel systems are the systems which need an organic carbon as electrode for microbial growth to convert the chemical energy of organic waste into electric energy. A thin film paper based biophotovoltaic cell which has cyanobacterial cells at the top of a conducting carbon nanotube surface. The printed cyanobacteria in the biophotovoltaic cell were designed to generate electric current in both dark and illuminated conditions. In dark conditions, the biophotovoltaic cell acts as solar bio-battery. In illuminated conditions, it acts as biosolar panel. The printed bioelectrode in a hybrid BPV cell had a power output of 0.38 mW/m2 in the light and 0.22 mW/m2 in the dark (Sawa et al., 2017).

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19.5.3 Bioinspired chromophores The chlorophyll is the primary chromophore for photosynthetic organisms to absorb solar energy. Thylakoid membrane is responsible for photosynthesis processes in plants, algae and cyanobacteria. The absorption spectra of chromophore must match with the solar irradiation spectra to drive the chemical reaction within the cell. The chromophores are an electron is created to facilitate the reaction around the enzyme. Chromophores are present in a particular pattern in photosystem II to assist electron transfer to other cofactors and to reduce plastoquinone (Barber and Tran, 2013; Frischmann et al., 2013). The structural analogs and asymmetry of the chlorophyll must be studied properly to synthesize it for solar cell applications. Porphyrins and phthalocyanines are the other types of chromophores with absorption spectra in purple and red region, respectively (Hall et al., 1995). They can be used as dyadic and triadic systems, sensitizers in charge transfer system and polymer units.

19.6 Physics in biological systems The overall understanding of photosynthesis can be done by studying the mechanism through excitation energy transfer and charge transfer process. Light harvesting stage of photosynthesis has nearly perfect quantum efficiency. Therefore the primary photosynthetic processes of energy and charge transfer show important quantum mechanical properties like delocalization, wavepackets, coherence and superradiance.

19.6.1 Coherence effects in biological systems Plants, algae and photosynthetic bacteria produce their energy from sunlight. Photosynthetic proteins involved in electron transport process are disordered. In light-harvesting complexes, coherence is observed in the primary excitations which play an important role between electronic excited states for energy transfer. Interference band was observed in the study of plants, algae and photosynthetic bacteria. This observation of band can be defined as coherence (Marcus, 1956). There is an indication of wavelike energy transfer through quantum coherence in photosynthetic systems. The spectroscopic data indicates the dependence of energy transport in the light harvesting systems on the spatial properties of the excited-state wave functions. Quantum efficiency of excitation energy transfer depends on the quantum coherence within the FennaMatthewsOlson (FMO) photosynthetic complex from the green sulfur bacterium (Engel et al., 2007). Two-dimensional Fourier transform electronic spectroscopy showed the occurrence of long-lasting quantum coherence in the excitation energy transport of light harvesting systems like bacteria and algae. The results revealed

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that higher efficiency of energy capture and charge transfer in these systems was due to quantum effects. Therefore understanding quantum coherence is very much important to successfully design and develop the efficient solar cells. Organic Photovoltaic (OPV) systems have polymers which act as lightabsorbing units. Conjugated π bonds in OPV are responsible for the charge transport in the OPVs and to generate photocurrent. Similarly in biological systems, molecules are contained by a protein scaffold with disordered mixture of donors and acceptors forming a thin film and the photo-generated electrons are transferred in the reaction center around enzymes to reduce plastoquinone (Engel et al., 2007).

19.6.2 Excitation energy transfer Franck et al. suggested a quantum coherent mechanism to understand the excitation energy transfer process in photosynthesis (Franck and Teller, 1938). In this process, diffusion of a Frenkel exciton is a coherent superposition of electronic excitations of an individual photosynthetic pigments. The techniques like femtosecond transient absorption spectroscopy, long-lived vibrational coherences and two-dimensional electronic spectroscopy have been used to detect the bacterial and plant light-harvesting complexes. These techniques have been used to study the decay of coherent superpositions of vibrational states and mixed excitonicvibrational states in light-harvesting complexes (Marcus, 1956; Engel et al., 2007). The 2-D spectra showed the presence of cross peaks at oscillated time. The study of the cross peaks with couplings between exciton states and their oscillations were allocated to coherent superposition between exciton states. Fleming et al. reported a particular coherences lasted long for 660 fs when the study was conducted on the FMO complex of green sulfur bacteria (Ishizaki and Fleming, 2009). In 2009, the oscillatory signals revealed quantum-coherent energy transfer for the main light-harvesting complex (LHC) of higher plants. Engel et al. reported similar oscillatory signals in FMO and light-harvesting complexes of two species of marine cryptophyte algae at physiological temperatures (Engel et al., 2007). At physiological temperature, observed quantum coherence is fragile as compared for cryogenic temperatures. The reason for fragile quantum coherence is due to an increase in environmental noise with increase in temperature, leading to shorter decoherence times. Therefore there is a major challenge to maintain quantum coherence in systems which inevitably interact with an environment (Plenio and Huelga, 2008). This result also explores the fundamental aspects of explanation of the functions of living systems by quantum mechanics only. Many models have been proposed to explain environment-assisted quantum coherence to understand efficient energy transfer systems. The objective of these models is to relate the process of quantum coherence across multiple chromophoric sites in photosynthetic

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pigmentproteins and to understand the extreme efficiency of the excitation energy transfer process, typically within approximate spin-boson models of the system. The interaction of system with environment can give increased transport efficiency and doesn’t depend upon the noise. There are number of pigments in light harvesting complexes with different site energies. Therefore the energy difference is larger than the pigmentpigment coupling and then transitions will be suppressed (Novoderezhkin et al., 2004). The dephasing of noise can shift the site energies which would help to overcome the energy gaps between pigmentpigment couplings. Due to this there would be improvement in transport between sites without the loss of excitations from the system. 2D-ES studies on the FMO complex showed that long live coherences created mainly from ground state vibrations and have no possibility of creation from excitonexciton superposition. The investigations of results for environmentally assisted transport and environment characteristics with respect to system can be utilized to design highly efficient quantum transport systems. The natural selection of parameters by quantum systems set to result for a right quantum coherence to attain maximum efficiency with optimal control (May and Kuhn, 2008). But the classical interpretation of the system cannot be ruled out only by observing its oscillatory dynamics. Recent research has shown that excitation energy transfer in photosynthetic LHCs may be carried due to nonclassical properties of environmental vibrational motions on the subpicosecond time scale at room temperature. The above concepts can be verified by testing if original source of genetic variation in photosynthesis LHCs would change the degree of quantum coherence, the coherence lifetime, and energy-transfer efficiency (Novoderezhkin et al., 2015). In natural photosystems, the energy migration depends on the balance between coherence and dephasing irrespective of packing geometry of the system. A noticeable photocurrent response was observed with the introduction of hybrid material like chlorophyll-a entrapped polyacrylamide hydrogel into bioinspired light harvesting system. The behavior of this system could be defined by environment-assisted transport of photosynthesis. The swollen induced arrangement of chlorophyll-a for efficient energy migration in this bioinspired system can give new directions to fabricate a new class of biomimetic solar cell (Romero et al., 2017).

19.6.3 Charge transfer One of the most efficient processes in photosynthesis is charge separation with a quantum efficiency nearly equal to unity. The charge separation process involves interaction between disorder and coherence facilitated by mixtures of vibrational and excitonic states and happens from subpicoseconds to milliseconds (ultra-time scale). The quantum effects for charge separation are not appeared at such a macroscopic level, but charge separation sets

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a good example for understanding the role of quantum physics in biology (Delor et al., 2014). Charge separation is a chain of different processes on ultra-time scale and total efficiency depends on each one of the step. DeVault et al. reported that the temperature dependence of the charge transfer in purple bacterial reaction center could not be explained by classical physics but explained by the behavior quantum mechanics. This research laid to the foundation for the concept of electron and nuclear tunneling in biology (De Vault and Chance, 1996). Semiclassical Marcus theory based on nonradioactive transitions give a better estimation for the increasing rate of charge separation with decreasing temperature, but Marcus’ theory of electron transfer neglects nuclear tunneling and give less importance to the electron transfer rate at low temperatures. Complete vibrational relaxation does not occur in ultrafast photoinduced electron transfer because electron transit from the donor to the acceptor is so quick. Vibrational relaxation occurs on a time scale of picosecond or subpicosecond. In photosystem II of higher plants, 2D-ES experiments on the reaction center have shown revealed longlived oscillations of specific cross peaks. The presence of these cross peaks is due to electronic coherences between excitons as well as between exciton and charge-transfer states. Therefore a strong correlation was present between the degree of coherence and ultrafast charge separation. Quantum coherent model simulations showed observed cross-peak oscillations and can be maintained by specific vibrational modes. These vibrational energy modes support and match the fundamental resonant transfer of the pigment (Novoderezhkin et al., 2004; May and Kuhn, 2008; Novoderezhkin et al., 2015). The tuning of pigment excitonic transitions is mainly due to protein scaffold. Biological mechanism for charge separation is influenced by nontrivial quantum effects. Single vibrational modes in a statically disordered landscape support the transport in both light harvesting and charge separation in photosynthetic organisms. The nuclearelectronic (vibronic) coupling mechanism in the light-induced function of molecular systems and specific designed system has a special importance. Infrared light excitations of intermolecular vibrations can drastically change the outcome of light-induced electron transfer in molecular systems (Hildner et al., 2013). Chlorophyll-a derivatives having chlorins like dodecyl ester group were used as sensitizers in DSSCs to get a highest PCE of 8%. The large photocurrent was achieved by using chlorin-3 as sensitizer due to least impedance in the electrolyte-dye-TiO2 interface. The electron injection and charge recombination dynamics in the dye-TiO2 complex has been improved and free from the type and concentration of dye sensitizer for Chlorophyll-a derivatives based DSSCs. New developments can be achieved in dye excitation from exciton annihilation and charge recombination phenomena using Chlorophyll-a derivatives. High open circuit photocurrent and longer electron life time can be achieved by using dyes with larger ester groups in DSSCs (Romero et al., 2017).

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19.7 Structures 19.7.1 Origami structures Famous Miura folding is an ancient Japanese tradition of origami which was used for construction of solar panels in space vehicles. A two-way folding of beetle wings could inspire the structural design of solar panels in space vehicles (Qiao et al., 2018).

19.7.2 Graphene The unique two-dimensional honeycomb structures of carbon atoms in Graphene have been used to fabricate BISC (Nine et al., 2015). A new type of 3D honeycomb-like structure graphene sheet was synthesized by a chemical reaction between LiO2 and CO. Graphene sheets performed as better catalytic performance as counter electrode in DSSC with PCE of 7.8% as compared to Pt as counter electrode with 9.34%. The results showed that 3D honeycomb-like structure graphene is a promising material for solar cell applications (Wang et al., 2013; Song et al., 2012). Relatively highly efficient organic and siliconbased solar cells have been fabricated containing honeycomb structures of graphene. There would be an increase of flexibility, recyclability and photocatalyst properties of graphene based solar cells (Kim et al., 2018; Thekkekara, 2015).

19.7.3 Multijunction solar cells Multifunction solar cells have relatively higher power-conversion efficiencies with less material usage. The incorporation of bioinspired antireflective structures was done in multifunction solar cell architecture (Ga0.5In0.5P/In0.01Ga0.99As/Ge solar cell) (Yu et al., 2014). High efficient multifunction solar cell containing biologically inspired moth eye structures were fabricated with a pitch of about 600 nm and depth of about 900 nm. This resulted in total increase of ultraviolet reflection of about 35% at 300 nm wavelength (Thavasi et al., 2009; Chellamuthu et al., 2016). Bioinspired antireflective structures were incorporated into monolithically grown Ga0.5In0.5P/In0.01Ga0.99As/Ge multifunction solar cells. Ga0.5In0.5P/ In0.01Ga0.99As/Ge triple-junction solar cell was inspired by moth eye structures and showed an extraordinary decrease of optical reflection in the UV regime. The photocurrent and PCE of fabricated triple-junction solar cell was 11.6 mA/cm2 and 25.3%, respectively, under one-sun Air Mass 1.5, Globe (AM1.5 G) illumination. These PCE results are excellent as compared to single layer ARC device with 24.4% PCE (Yu et al., 2014).

19.7.4 Perovskite solar cells Light harvesting in perovskites solar cells can be enhanced by integrating bioinspired moth eye nanostructures into back electrode through soft

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nanoimprint lithography. This type of lithography can be done by solution process at low temperature with prepatterned PDMS molds (Wei et al., 2014). The CH3NH3PbI32xClx based PSCs integrated with bioinspired moth eye nanostructure have the broad spectral response of 350800 nm wavelength range with polarization independence. There was an enhancement of photocurrent with value of 21.90 mA/cm2 and PCE of 16.31% for this bioinspired device. The photocurrent and PCE of flat device were 19.16 mA/cm2 and 14.27%, respectively. The improvements of photocurrent and PCE for the patterned devices with moth eye nanostructures were 14.3% and 16.31% without sacrificing the open circuit voltage and fill factor as compared to reference flat device (Wei et al., 2017). The concept of photon recycling can be used to improve the performance of PSCs. The novel approach has been achieved by incorporating TiO2/bR layers in perovskite solar cells. The new class of bioperovskite solar cells has improved performance and stability. There is a light energy conversion by transfer of photogenerated electrons from the perovskite absorber layer to bR molecules happened through the Foo¨rster resonance energy transfer process. The similar optical properties of bR and perovskite are responsible for FRET process. The chemical bonding between bR and TiO2 can be adjusted to assist charge transfer. Perovskite solar cell with bR(TiO2/bR 2 PSC) showed open circuit voltage, short circuit current and PCE of 1.05 V, 22.61 mA/cm2 and 17.02%, respectively. Conventional perovskite solar cell (TiO2PSC) showed open circuit voltage, short circuit current and PCE of 1.02 V, 22.59 mA/cm2 and 14.59%, respectively. Therefore the incorporation of biomaterials in perovskite solar cells would open new directions for improving the charge transfer and photovoltaic performance (Das et al., 2019; Griep et al., 2010). The aloe-vera processed carbon hole-conductor played an important role to develop high air-stable properties for perovskite solar cells. The fabricated PSCs having Glass/FTO/mpTiO2/ZrO2/Perovskite/AVC configuration showed open circuit voltage, short circuit current and PCE of 0.965 V, 20.5 mA/cm2, and 12.3%, respectively, for MAPbI3 as perovskite layer. The perovskite solar cell device having spiro-MeOTAD as HTM showed open circuit voltage, short circuit current and PCE of 1.046 V, 21.25 mA/cm2, and 15.8%, respectively (Mali et al., 2018). The developed natural C-HTL has moderate PCE efficiency, but gives an idea to develop fully printable, low cost and highly stable perovskite photovoltaic technology to cross 20% efficiency (Kang et al., 2014).

19.7.5 Silicon-based solar cell The development of integrated energy storage devices with flexible thin-film solar cells is required for environmental sustainability. Thekkekara et al. reported a new design concept of laser scribed graphene micro-supercapacitors

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(LSG-MSCs) using bioinspired electrodes (Thekkekara and Gu, 2017). The design concept was based on the internal structure of natural fern leaves Polystichummunitum (Yan et al., 2018). These leaves are efficient in biological process like photosynthesis for energy storage and also for information compression. The new design of biomimetic LSG-MSCs for solar energy which have the structure of Fern leaves characterized by the geometric family of space filling curves of fractals. The surface area to volume of bioinspired electrodes reduces the electrolyte ionic path with reported energy density of B1021 Wh/cm3. The energy density is 30 times more than the planar electrodes and B95% coulombic efficiency of the solar energy storage. The efficiency of the Hilbert BFE-MSC integrated thin film amorphous silicon solar cell was 10%.Solar charging and self-discharge performance of the integrated BFE-MSC and planar MSC of integrated planar were 2 and 1.5 V at 6 minutes and 1.8 and 1.4 V at 16 hours, respectively (Thekkekara and Gu, 2017). The new design concept opens the new prospects to make efficient solar-powered wearable, flexible and portable applications (Chiadini et al., 2013). The bioinspired pit and hillock texturing of the surface of a silicon solar cell was inspired by compound eyes of insects to improve the lightharvesting capability. The bioinspired pit texture configuration which consists of an array of semicircular grooves showed a good performance over the free-space wavelength range of 4001100 nm by using the GO approximation. The calculated figure of merit values for bioinspired pit and hillocktextured surfaces were 0.631 and 0.626, respectively. The light-coupling efficiency for bioinspired pit and hillock-textured surfaces were 0.52% and 0.50%, respectively, as compared to flat silicon surface with 0.40% efficiency. The figure of merit of pit textured configuration improved by 24% and efficiency by 30% with respect to flat silicon surface solar cell (Chiadini et al., 2013). Therefore bioinspired surface texturing could improve the lightcoupling efficiency of solar cells, but economy of technology needed to be explored (Chen and Yang, 2016; Soderstrom, 2010).

19.7.6 Dye-sensitized solar cell technology DSSC technology is bio mimicked from the process of photosynthesis. The light absorbing dyes convert sunlight into electrical current. In DSSC, ITO/ FTO glass is coated by nanocrystalline titania with a dye which acts as a light absorber. TiO2 surface acts as an anode and platinum electrode acts as cathode. A suitable electrolytic solution is present between the two electrodes for electronic conduction (Ito et al., 2018). Zhang et al. reported a photoanode structure inspired by quasihoneycomb like structure (QHS) and synthesized onto FTO of DSSC by separately using butterfly wings as biotemplates. The current density for the QHS based device are nearly 0.4 μA/cm2 as compared to 0.6 μA/cm2 for anatase-based device (Zhang et al., 2009).

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Therefore bioinspired DSCC can be fabricated by the inspiration of butterfly wing scales (Fan et al., 2011; Mao et al., 2014). Bio-dyes of A. platensis PSI and spinach light-harvesting complex II (LHCII) were sensitized on TiO2 film of DSSC to measure the effect of pigment-protein complex on the device. The photoelectric conversion efficiency of DSSC with these two bio-dyes was 0.47% and 0.52%, respectively (Yu et al., 2015). Therefore the macromolecular pigment-protein complexes influence the performance of DSSC and have the potential for better utilization of solar energy (Wang et al., 2013).

19.7.7 Thin film solar cell There are micro and nanostructured scales on the wings of the black butterfly such as Pachliopta aristolochiae to gather solar energy over a wide spectral and angular range (Kaufmann et al., 2005). The design of self-assembled biophotonic nanostructured thin photovoltaic absorbers of disordered nanoholes has 90% increase in relative integrated absorption at a normal incident angle of light and 200% at large incident angles for bioinspired thin PV absorbers which were made of hydrogenated amorphous silicon (αSi:H) (Siddique et al., 2017c). The properties of black butterfly structures are of great importance to develop efficient thin film solar cells (Li et al., 2007; Hu¨nig et al., 2016).

19.8 Conclusions Bioinspired research is a field to observe the functions which characterize living organisms and try to imitate their function to create bioinspired materials or devices. Initially, it is not focused on molecular structure, complex synthesis or high-resolution spectroscopy. But the research focused on the observation of the functions of organisms of millennia of Darwinian evolution and mimicked or imitated the applicable prospects of those functions without restrictions imposed by biology as per requirements of life. Now, bioinspired research is all about mixing molecular science, biology, robotics, zoology and all other areas of science. This research is all about replicating and mimicking the biological functions to provide a connection between the laboratory and the biological structures. Bioinspired research helps to design and develop the materials, structures and devices for low cost with better performance. Bioreplication is a branch of engineering biomimicry used to develop advanced and efficient micro and nano structures and devices through fabrication techniques by replication of biological structures and their functions. Convectional fabrication techniques do not allow synthesizing of required micro or nano structures. But natural process has developed excellent techniques to synthesize and develop the structures according to the needs.

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The adaptation of better techniques for fabrication of micro and nano structures to replicate biological structures can have promising results through bioreplication engineering. There is a place of bioreplication in the fabrication of photo sensitive, high thermal conductive and high elastic materials with innovative scientific techniques. The bottom up approach can be adopted to make controlled synthesis and assembly of tiny building blocks for creating better and reproducible biomimetic materials. These materials are of great importance to develop electronic materials, ceramics, solar materials and other relevant fields like biomedical, tissue engineering, nanomedicine, environment and biomineralization. Bioinspired and biomimetics materials are used to make solar technology more efficient and less costly technology day by day. The synthesis of biomimetics and biomimetic materials will enhance the further development in various fields of sciences such as engineering, physics, material science, biology, biochemistry, chemistry and other disciplines. Bioinspired thin film, nano and organic polymers materials would support the fabrication of solar cells with better performances with low cost. Biological materials such as biopolymers, polymer composites, biological ceramics, ceramic based composites, and cellular biological materials (foams) have multifunctional properties. These materials have high compressive strength, low resistance to crack propagation, excellent tensile properties. Biological and bioinspired materials are under investigation to discover and develop new structures and concepts. Development of bioinspired materials for solar cells will be possible by novel synthesis and processing methods. Many flowers contain UV absorbing and reflection pigments in their petal cells which are localized in vacuoles. The structural difference within flower petals gives UV reflecting and UV absorbing areas. The surface structure of pigments and its reflective properties can be used in structured solar panels, biomimetic AR and absorption coatings. OPVs have many advantages like flexibility, low cost fabrication process as compared to inorganics photovoltaics. Researchers have to work out on their stability and efficiency of OPVs. OPVs have more similar working principle to biological systems. Biological systems are well efficient energy harvesting systems which can be applied to develop low cost, efficient and stable light harvesting systems. The understanding of quantum coherence in biological systems helps to achieve long range photocarrier transport system. The long range photocarrier transport system would help to minimize the loss in open circuit voltage. The charge diffusion process and charge transport towards the selective electrodes without much dependence on built-in field within the active layer could be achieved by quantum coherent driven solar cell. Biological energy-harvesting systems also offer self-organized, self-sustained and self-repairing growth process which were applied to get efficient BISCs. Overall, there has been a continuous growth in the use of these materials in solar technology to design and develop solar modules with better

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antireflective, flexible, and charge and energy transfer. There are many practical challenges with bioinspired and biomimetics materials for using them in solar technologies and other living systems like material interfacing and compatibility, stability, scaling effects and interaction between different components. Bio-inspired butterfly wings increased the light absorption in the solar cells. Graphene-doped quantum dot solar cells have a comparatively increased efficiency and power conversion. The growth of bio-inspired hybrid materials with inherent properties of specific organic and synthetic polymers can lead to the fabrication of higher light absorption and better power-conversion efficient solar cells. Therefore BISCs show self-cleaning properties, anticorrosive properties, lower reflectance of incident light, shape changing properties, improved optical and electrical properties, biodegradability, and maximum power point tracking. To develop BISC, there is a need to integrate the fields like biology, nanoscience, physics, and quantum mechanics to effectively develop bio-inspired applications for mass fabrication of solar cells. There are continuous efforts to increase the absorption wave length range from ultraviolet to infrared, temperature range, mechanical properties for the PV cells. The research in bioinspiration can be applied to the PV technology for desired output. The bioinspired photoanodes in DSSC, doping of graphene in quantum dot solar cells are some efforts to increase power conversion. The board area in BISC is the execution of physical and chemical materials to perform similar to nature. The environmental issues, recycling process, and reusability of solar cell and stability can be taken care by bio-inspired solar cells. The development of highly biodegradable materials for solar cells is of importance for present researchers toward the sustainable energy production.

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Index Note: Page numbers followed by “f” and “t” refer to figures and tables, respectively.

A Absorber layers, 85 87 chalcopyrite thin films, 85 86 kesterite thin films, 86 87 paints, 395 397 Absorption coefficient, 144 145 length, 144 Active layer, 233 Adaptive structures, 176 Additives, 274 277, 457 460 reducing agents, 276 277 tin containing additives, 274 276 Adenosine triphosphate (ATP), 565 Agenda for Sustainable Development goals of United Nations (2030), 71 Air pollution, 3 Algae, 564 567 Alkali metal postdeposition treatment on CIGSe-based solar cell, 163 164 Alternative CSP absorbing surfaces, 409 413 Alternative photovoltaics materials, status of, 153 154 Aluminum-doped zinc oxide (AZO), 240 2-aminophenol-4-sulfonic acid (APSA), 277 Ammonia (NH3), 233 Ammonium thiocyanate (NH4SCN), 273 5-ammoniumvaleric acid, 280 Amorphous silicon (a-Si), 72 73, 151 152 deposition, 237 240 Anatase titanium dioxide, optimization of, 123 124 Antimicrobial activity, MXene for, 352 354 Antireflection (AR), 597 Arc discharge, 540 Arc evaporation, 159 Arthrospira maxima, 566 Artificial photosynthesis, 364, 613 Asbestos-containing wastes (ACW), 440

Astrakanite, 447 Atmospheric pressure CVD, 161 162 Atomic layer deposition (ALD), 77 84, 365 applied on chalcogenides thin films technologies, 84 89 absorber layers, 85 87 sustainable buffer layers based on ALD, 87 88 sustainable passivation layers based on ALD, 88 89 fundamental characteristics of, 78f commercial and research tools, 82 84 requirements for ideal precursors and ALD signature quality, 79 82 process, 373 375 reactors, 375 376 Atomic polarization, 176 177 Aurivillius oxides (Bi2MO6), 28 29 Auxiliary Kohn-Sham system, 116 118

B Back surface field (BSF), 51, 231 Bacteriorhodopsin (BR), 569 571 Ball milling process, 306 308 Ball to powder ratio (BPR), 304 Band gap, 4 5, 137 138 Barium titanate (BaTiO3), 187 194 crystal structure, 187 189 dielectric properties, 190 ferroelectric phenomena in, 190 191 optical properties, 191 potential applications, 192 194 various techniques of depositing BaTiO3 thin film, 191 192 Barium titanate (BTO), 177 178, 187 188 Base vectors, 103 104 Batteries, 16 21 bismuth-based electrodes, 17 18 coating or mixing with conductive materials, 20 21

633

634

Index

Batteries (Continued) nanoengineering, 18 20 operation, 16 Bee pollen (B-CQDs), 502 Beetle, 610 (Bi, Sb)2(Te, Se)3 alloys, 12 14 Biobased molecules, 558 Biobased solar cells, 562 563 Bioelectronics, 557 558 Bioenergy, 71 72 Bioengineered bacteria, 568 570 Bioinspiration, 595 596 dissipation, 595 function, 595 scientific impact, 596 simplicity, 595 soft matter, 595 596 Bioinspired chromophores, 616 Bioinspired solar cells, 597 601. See also Crystalline-silicon heterojunction solar cells bioinspired structures and organisms, 601 611 dyes, 601 602 organisms, 603 611 wettability and superhydrophobic dyes, 603 biological processes for bioinspiration, 611 616 methodologies for engineered biomimicry, 595 597 bioinspiration, 595 596 biomimetic, 596 bioreplication, 597 physics in biological systems, 616 619 charge transfer, 618 619 coherence effects in biological systems, 616 617 excitation energy transfer, 617 618 structures dye-sensitized solar cell technology, 622 623 graphene, 620 multijunction solar cells, 620 origami structures, 620 perovskite solar cells, 620 621 silicon-based solar cell, 621 622 thin film solar cell, 623 Biological photovoltaic devices, 560 Biological processes for bioinspiration bioinspired chromophores, 616 cyanobacteria, 614 615 photosynthesis, 611 613

Biomaterials, 561 electron transfer mechanism, 560f in photovoltaic technology, 557 558 structural properties of, 558 562 used in biophotovoltaics, 562 584 Biomimetics, 596 moth eye structure, 598 601 Biophotovoltaics (BPV), 569 570 biomaterials used in, 562 584 light-harvesting proteins, 570 575 living organism based solar cell systems, 563 570 natural pigments, 575 584 solar cells, 569 570 Bioreplication, 597 Bismuth (Bi) applications, 309 320 self-cleaning, 313 317 water splitting, 317 320 water treatment, 309 313 Bi-based nanomaterials, 16 Bi2Se3 and Bi2S3, 14 15 Bi2Te3 and (Bi, Sb)2(Te, Se)3 alloys, 12 14 metallic bismuth, 12 ternary materials, 15 16 Bi-based nanomaterials in solar-hydrogen production, 24 29 bismuth-based composite oxides, 26 29 aurivillius oxides, 28 29 BiMO4, 27 28 bismuth oxides, 26 bismuth oxyhalides, 26 27 bismuth-based electrodes, 17 18 bismuth-based functional materials, 290 bismuth-based heterostructures for photocatalytic applications, 299 320 bismuth-based nanomaterials, 7 9 batteries & supercapacitors, 16 21 for energy applications, 3 4 photovoltaics, 4 9 solar-hydrogen production, 22 29 thermoelectric devices, 9 16 bismuth-based perovskites, 7 8 general strategies for synthesis of bismuthbased semiconductors, 303 309 ball milling process, 306 308 hydrothermal/solvo thermal synthesis, 305 306 sol-gel synthesis, 305 sputtering process, 308 309

Index semiconductor-semiconductor heterostructures using bismuth-based materials, 301 303 Bismuth chalcogenides, 8 9, 25 Bismuth ferrite (BiFeO3), 194 195 Bismuth halides, 7 8 Bismuth oxides, 26 Bismuth oxychloride (BiOCl), 299 300 Bismuth oxyfluoride (BiOF), 299 300 Bismuth oxyhalides (BiOX), 26 27 Bismuth perovskite supercapacitors, 17 Bismuth selenide (Bi2Se3), 14 15 Bismuth sulfide (Bi2S3), 14 15 Bismuth sulfide iodide (BiSI), 8 Bismuth telluride (Bi2Te3), 12 14 Bismuth triiodide (BiI3), 8 Body centered cubic structure (bcc structure), 104 Bonded lattice matched structures, 58 60 Bottom-up approach, 212 213 Buffer layer (BF), 75 76 Building applications PCM for, 486 488 performance criteria for, 488 Bulk ferroelectric photovoltaic effect (BFPVE), 181 Bulk photovoltaic effect (BPVE), 181 Bulk supercells, 101 102 Butylammonium iodide (BAI), 278 279

C Cadmium telluride (CdTe), 73, 151 152 solar cells, 164 165 Calcium titanium oxide (CaTiO3), 260 262 Carbon allotropes sandwich approach, 544 545 and derivatives, 538 541 CNTs, 540 fullerene, 538 539 graphene, 540 541 Carbon dioxide reduction reaction (CRR), 346 Carbon dots as counter electrode in DSSCs, 508 509 modified photoanodes in DSSCs, 507 508 as sensitizer in DSSCs, 501 507 Carbon nanodots (CDs), 498 integrated solar energy devices DSSCs, 500 509 organic solar cells, 511 515 perovskite solar cells, 519 524 polymer solar cells, 515 519 quantum dot solar cells, 509 511

635

Carbon nanomaterials, 497 498, 538 solar cells based on, 541 547 Carbon nanotubes (CNTs), 538, 540 Carotenoids, 576 577 Carrier distribution function, 139 140 generation, 138 139 recombination, 138 139 transport and tin halide perovskite defects, 266 267 Catalytic counter electrode, 233 Cationic bismuth, 3 4 Chalcogenide thin film chalcogenide thin-film-based PV, 73 devices, 75 76 Chalcopyrite template structure for, 126 127 thin films, 85 86 Charge carriers, resistivity of, 143 Charge transfer, 618 619 Charge transport layer, 233 Chemical bath deposition, 160 161 Chemical deposition, 160 162. See also Physical deposition chemical bath deposition, 160 161 CVD technique, 161 162 sol-gel technique, 160 spray pyrolysis technique, 161 Chemical doping, 233 Chemical reaction/thermochemical heat storage, 433 434 Chemical solution deposition technique (CSD technique), 192 Chemical vapor deposition (CVD), 78, 161 162, 236, 303 304, 308, 373, 540 hot wire, 162 ion assisted deposition, 162 low pressure and atmospheric pressure, 161 162 plasma enhanced, 162 Chlamydomonas reinhardtii, 566 567 Chlorophyll derivatives, 557 558 derived Spirulina xanthin carotenoid in Spirulina platensis, 584 Chromatophores from Rhodospirillum rubrum S1 biological redox, 580 584 Chroococcales, 614 Chroococcidiopsidales, 614 Citric acid (CA), 523 Citric acid-carbon quantum dots (C-CQDs), 502

636

Index

Classical pn junction, 201 Clean energy, 3 Climate change, 429 Coal, 131 132 Coherence effects in biological systems, 616 617 Colloidal semiconductor nanocrystals, 6 Commercialization of Si solar cells, 152 153 Common rose butterfly, 603 605 Concentrated solar power (CSP), 383 392 absorbing surfaces and materials, 409 417 alternative CSP absorbing surfaces, 409 413 industrialization of high-temperature solar selective coatings, 413 417 efficiency considerations, 392 394 Concentrating PV (CPV), 39, 62 Conduction band (CB), 4 5, 205, 266, 334 336 Conduction band minimum (CBM), 267 268 Conduction band offset (CBO), 87 Copper bismuth sulfide (CBS), 8 Copper indium gallium selenide (CIGSe), 136 137, 151 152 absorber layers, 85, 126 127 alkali metal postdeposition treatment on, 163 164 CIGSe-based solar cell, 162 164 Copper phthalocyanine (CuPc), 509 510 Copper zinc tin sulfide (CZTS), 151 152 Coronavirus pandemic (COVID-19 pandemic), 497 498 Crab, 610 611 Cross-flow reactor (CFR), 82 Crystalline Si solar cells (c-Si solar cells), 37 38, 229 230, 235 Crystalline-silicon heterojunction solar cells. See also Bioinspired solar cells fabrication of silicon heterojunction solar cell, 234 244 graphene, 232 234 heterojunction solar cells, 229 232 synthesis of graphene, 244 250 Curie temperature, 176 177 Cyanobacteria, 563 567, 614 615 Cycling stability, 435

D Defect formation energies (DFEs), 266 267 Deionize water (DI water), 245 246 Delft University of Technology, 236

Density of energy states, 139 141 universal functional of, 113 116 Density functional theory (DFT), 101 Diamond lattice, 136 137 Diborane (B2H6), 233 Differential scanning calorimetry (DSC), 477 Diffuse horizontal irradiance (DIF), 366 367 Diffusion coefficient, 143 144 current, 148 diffusion-due to concentration gradient, 143 144 DIMES cleaning procedures, 236 Dimethylsulfoxide (DMSO), 270, 280 281 Dipteran, 610 Dirac notation, 112 Direct band gap, 138 139 Direct normal irradiance (DNI), 366 367 Direct plasma ALD, 84 with grid, 84 Dissolved organic matter (DOM), 292 293 1-dodecanol, 7 8 Doped a-Si:H layers, 238 Doped MXene, 343 Doping, 136, 296 Drift current, 148 Drift velocity, 142 Drift-motion due to electric field, 142 143 drift velocity, 142 mobility of carriers, 142 143 resistivity of charge carriers, 143 Durability studies of solar absorbers, 405 407 Dye molecules in solar cell systems, 558 559 Dye sensitized solar cells (DSSC), 7 8, 38, 166 168, 233, 260 262, 498 509, 537, 557 558, 561, 575 576, 593 595, 604 605, 622 623 carbon dots as counter electrode in, 508 509 modified photoanodes in, 507 508 as sensitizer in, 501 507 carbon in, 541 543 Dyes, 601 602

E Economic analysis of inorganic salts as lowcost TES materials, 444 446 Efficiency, 151 Electric field, drift-motion due to, 142 143

Index Electrical energy, 131 132 Electrical vehicles, 429 Electricity, 71 Electro-optical devices (EO devices), 191 Electrochemical energy storage devices, 3 4 Electrochemical water splitting, 344 345 Electron beam evaporation method, 158 159, 240 241 Electron transport layer (ETL), 4 5, 42 44, 168, 511 513 Electron transporting material (ETM), 41 42 Electron’s motion, 142 Electrospinning, 19 20 Elemental bismuth, 12 Emerging PV technologies, 6 Emerging solar cell technologies, 165 169 dye-sensitized solar cells, 166 168 organic solar cells, 166 PSC, 168 QDSC, 168 169 Encapsulation in phase change materials, 474 475, 475t Energy, 3, 363 364, 537, 597 598 band gap, 175 176 structure, 137 139 consumption, 3, 131 crisis, 537 gap, 137 138 harvesting devices, 3 4 production, 131 sources, 37 supply, 132 133 Energy payback time (EPBT), 6, 156 Energy return on energy invested ratio (EROI), 6 Energy storage density (ESD), 435 Engineered biomimicry bioinspiration, 595 596 biomimetic, 596 bioreplication, 597 methodologies for, 595 597 Environmental applications of MXene, 347 354 comparative results on, 353t MXene for antimicrobial activity, 352 354 organic degradation, 348 351 photoreduction process, 351 352 Environmental engineering processes, heterogeneous photocatalysis applied to, 294 295 Environmental pollution, 537

637

Eosin Y (EY), 342 343 Equilibrium concentration of electron, 140 Escherichia coli, 566 Ethylenediammonium diiodide (EDAI2), 278 279 Evaporation techniques arc evaporation, 159 electron beam evaporation, 158 159 laser beam evaporation/pulsed laser deposition, 159 molecular beam epitaxy, 159 vacuum thermal evaporation, 158 Excitation energy transfer, 617 618 Exciton, 166 External Quantum Efficiency (EQE), 220 221 Extracellular electron transfer, 565

F Fabrication of silicon heterojunction solar cell, 234 244 Face centered cubic structure (fcc structure), 104 FAPbI3 perovskites, structural stability of, 122 Feedback mechanism, 176 177 Fermi function. See also Carrier, distribution function Fermi-Dirac distribution function. See Carrier, distribution function Ferroelectric materials (FE materials), 176 177, 186 Ferroelectric perovskite oxides, 175 176 Ferroelectric phenomena in BaTiO3, 190 191 Ferroelectric photovoltaic effect (FEPV), 175 176 Ferroelectric photovoltaic materials, 179 186 barium titanate, 187 194 bismuth ferrite, 194 195 ferroelectric materials, 176 178 history and current status of, 186 187 photovoltaic effect, 178 187 Ferromagnetic materials (FM materials), 186 Fick’s first law of diffusion, 143 Fill factor (FF), 150, 561 562 Firefly, 607 609 Firewood, 131 Flavin, 579 580 Flavin adenine dinucleotide (FAD), 579 580 Flavin mononucleotide (FMN), 579 580 Fluorescence CDs, 507 508 Fluorescent proteins, 569

638

Index

Formamidinium (FA), 263 lead iodine perovskite, 122 Formamidinium tin iodide (FASnI3), 264 265 Fossil fuels, 467 energy consumption, 363 364 Fraunhofer Institute of Solar Energy (FhGISE), 51 Full Potential Linearized Augmented Plane Waves (FPLAPW), 102 103 Fullerene, 538 539

G Gallium arsenide (GaAs), 73 InAs quantum dots on, 216 221 Gallium doped zinc oxide (GZO), 240 Garcinia mangostana. See Mangosteen (Garcinia mangostana) Gas, 131 132 Generalized gradient approximation (GGA), 101 102 Geometrical optimization procedure, 119 Geothermal energy, 71 72 Germanium, 278 Global horizontal irradiation (GHI), 366 367 Global warming, 429 Gloeobacterales, 614 Glucose, 502 Goldschmidt’s tolerance factor, 263 264 Graphene, 232 234, 329, 538, 540 541, 620 doping, 233 graphene-based composites, 329 synthesis of, 244 250 Graphite, enhancing thermal conductivity, 460 461 Gra¨tzel’s cells, 38 Green fluorescent protein (GFP), 569 570 Green’s-function approximations, 101 102 Greenhouse gas(es), 3 emissions, 429, 467 Growth per cycle (GPC), 373 Guanidinium (GA), 273

H Halide perovskite solar cells carrier transport and tin halide perovskite defects, 266 267 perovskite structure, 263 265 tin oxidation, 269 270 tin perovskite bandgap, 267 269 tin toxicity, 271 272 Halobacterium salinarum, 570 571

Hartree products, 109 110 Heat and cold storage. See Thermal energy storage (TES) Heat transfer fluids (HTF), 384, 438, 478 479 Heavy metal, 3 4 Heteroepitaxy, 56 Heterogeneous catalysis, 291 292 Heterogeneous photocatalysis, 291 299 applied to environmental engineering processes, 294 295 factors affecting photocatalytic process, 295 297 (photo)electrochemical properties, 296 matrix composition, 296 297 physical properties, 295 physicochemical characterization of nanophotocatalysts, 297 299 Heterogeneous photocatalytic hydrogen production, 22 Heterojunction back contact (HBC), 231 Heterojunction solar cells, 229 232 Heterojunction structure, 146 147 High energy ball milling, 304 High renewable scenario (hi-Ren scenario), 413 414 High-angle annular dark field (HAADF), 218 Highest occupied molecular orbital (HOMO), 42 44, 166, 291 292 Hole transporting material (HTM), 41 42 Hole-transport layer (HTL), 4 5, 168, 511 513 Homogeneous catalysis, 291 292 Homojunction structure, 146 147 Hot carrier solar cell (HCSC), 201 202 Hot wire CVD (HWCVD), 162, 238 Hotspot, 430 Hubbard procedures, 101 102 Human Development Index (HDI), 131 132, 132f, 259 Human eye, 609 610 Hybrid organic inorganic PSC, 38 Hydrazine (N2H4), 276 277 Hydrazinium tin iodide (HASnI3), 277 Hydroelectric power, 71 72 Hydrofluoric acid (HF), 236 Hydrogen (H2) evolution by H2O splitting, 336 345 MXene-based heterojunctions, 340 345 water splitting activity of MXenes, 337 340 generation from water photoelectrolysis, 368 369

Index layers, 237 240 photocatalysis for hydrogen production, 22 23 Hydrogen evolution reaction (HER), 330, 364 Hydrothermal method, 192, 303 304 Hydrothermal/solvo thermal synthesis, 305 306 Hypophosphorous acid (HPA), 276

Inverted polymer solar cells (i-PoSCs), 517 518 Ion assisted deposition, 162 Ionic polarization, 176 177 Iron(II, III) oxide (Fe3O4) charge order and half metallicity of, 122 123

I

J

I-V equations of solar cell, 149 151 efficiency, 151 fill factor, 150 open circuit voltage, 149 150 short circuit current, 149 Ideal band gap, 5 6 III V based PV, 73 III V semiconductor materials for MJ solar cells applications, 50 62 In-flush process, 217 219 In(Ga)As on wide bandgap material barriers, 221 222 InAs quantum dots on GaAs, 216 221 InAs/GaAs system, 215 216 InAsP QDs on wide bandgap material barriers, 221 222 Indirect extracellular electron transfer (IEET), 560 561 Indium doped zinc oxide (IZO), 240 Indium tin oxide (ITO), 233, 240, 523 Indium-flush method, 217 Industrial waste studied as TES materials, 440 442 Industrialization, 3 of high-temperature solar selective coatings, 413 417 Inorganic electroactive solar cells, 101 Inorganic salt-based products and wastes as low cost materials for sustainable TES, 442 453 Inorganic salt-based wastes in TES systems, 455 456 Interference lithography (IL), 597 598 Intermediate Band Solar Cell (IBSC), 201 203 International Renewable Energy Agency (IRENA), 72 Intrinsic absorber, 398 Intrinsic thin layer (HIT), 229 230 Inverted metamorphic lattice mismatched structures, 60 61 Inverted organic solar cells (I-OSCs), 513 514

Jacob’s ladder, 101 102

639

K Kainite, 447 Kesterite thin films, 86 87 KHQSA. See Potassium salt of hydroquinone sulfonic acid (KHQSA) Kohn-Sham equations (KS equations), 116 117 Krypton fluoride (KrF), 159

L Laser ablation, 540 Laser beam evaporation/pulsed laser deposition, 159 Laser scribed graphene micro-supercapacitors (LSG-MSCs), 621 622 Latent heat storage (LHS), 432 433, 469 cycling stability for latent heat storage process, 435 437 materials, 448 451 encapsulation of, 457 Lead, 262 263 lead-free tin-PSC, 275 Lead Zirconate Titanate (PZT), 193 194 Leaf, 605 606 Levelized cost of coating (LCOC), 416 417 Levelized cost of energy, 416 417 Levelized costs of electricity (LCOE), 388, 416 417 Light harvesting proteins, 569 575 artificial photosynthetic devices, 571 573 bacteriorhodopsin, 570 571 green fluorescent protein, 570 peptide, 573 575 protein pigment complexes from Rhodopseudomonas palustris CQV97 and Rhodobacter azotoformans R7, 573 systems, 593 595

640

Index

Light-harvesting complex (LHC), 617 Light-harvesting complex II (LHCII), 623 Linearized Augmented Plane Waves (LAPW), 102 103 Liquid phase methods, 303 304 Living organism based solar cell systems, 563 570 algae and cyanobacteria, 564 567 bioengineered bacteria, 568 570 plants, 567 568 Living solar cells. See Biological photovoltaic devices Lotus, 606 607 Low dimensional perovskites, 279 280 Low pressure CVD, 161 162 Low-cost TES materials economic analysis of inorganic salts as, 444 446 latent heat storage materials, 448 451 sensible heat storage materials, 447 448 thermochemical storage materials, 451 453 Lowest unoccupied molecular orbital (LUMO), 42 44, 166, 291 292 Lycopene, 569, 577 579

M MA tin iodide (MASnI3), 264 265 Magnesium chloride hexahydrate, 459 460 Magnesium nitrate hexahydrate (MNH), 478 Magnesium-doped zinc oxide (MZO), 240 Magnetoelectric properties (ME properties), 186 Man-made crystals, 201 Mangosteen (Garcinia mangostana), 576 Mangosteen pericarp, 578 579 Materials for high-temperature applications, 395 404 selection in thin film technology, 157 158 Materials for renewable energy and biomedical applications research group (MREB research group), 118 mBiVO4 conventional and reduced representation of, 125 126 Melt spinning technique, 13 14 Metal chalcogenides, 329 Metal organic frameworks (MOF), 329 Metal-cermet coatings, 401 402 Metal-semiconductor tandem stack, 398 399 Metallic bismuth, 12 Metallic grid, 145 146

Metallic reagents, 79 Metallization, 242 243 Metal organic vapor phase epitaxy (MOVPE), 51 Metamorphic structure (MM structure), 51 Methylammonium (MA), 40, 260 263 Methylammonium lead iodide perovskite (MAPbI3), 260 262 Methylene blue (MB), 348 350 Microencapsulation, 475 of phase change material, 476 Mining wastes, 442 Miura folding, 620 Mobility of carriers, 142 143 Molecular beam epitaxy (MBE), 52, 159, 303 304 Monoclinic phase of bismuth vanadate, 125 126 Monocrystalline silicon, 72 73 Monolithic integration, 156 Monomers, 598 601 Muffin-tin sphere, 102 103 Multicrystalline silicon (mc-Si). See Polycrystalline silicon Multielectron system, 107 111 Multifunctional systems, 176 Multijunction solar cells (MJSC), 38 39, 165, 201 203, 208 209, 620 III V semiconductor materials for, 50 62 growth and efficient processes, 62 historical review, 50 53 III V materials for photovoltaic applications, 55 58 progress of MJ III V solar cells, 61 selected examples, 58 61 some basics of multijunction solar cells, 53 55 Multilayer absorber, 400 401 Multiple exciton generation (MEG), 6 Multiple exciton generation solar cell (MEGSC), 201 203 Multiple QW (MQW), 206 Multiwalled carbon nanotubes (MWCNTs), 477 MXene-based heterojunctions, 340 345 doped MXene, 343 electrochemical water splitting, 344 345 tertiary composite system, 343 344 2D/2D composites, 341 342 2D/3D composites, 342 343 MXenes/MXene-based composites, 329

Index

N N-eicosane, 477 N-tetradecane, 476 n-type semiconductors, 145 146 Nanoencapsulation, 475 of phase change material, 476 Nanoengineering of battery electrode, 18 20 photovoltaics and, 6 7 and solar-hydrogen production, 24 thermoelectric devices and, 11 12 Nanoenhanced phase change materials (NPCM), 477 Nanomaterials, 11 12 Nanophotocatalysts, physicochemical characterization of, 297 299 Nanoscale Enhanced Characterization of Solar Selective Coatings project (NESCO project), 409 Nanostructured materials for high efficiency solar cells nanostructures and quantum mechanics, 203 205 quantum dots in solar cells, 214 222 quantum wells in solar cells, 205 210 quantum wires in solar cells, 210 214 Nanostructured photocatalysts, 290 291, 299 Nanostructured semiconductors, 29 Nanostructures, 201 of multicomponent materials, 176 Nanotube, 540 Nanowires (NWs), 597 598 National Renewable Energy Limited/ Laboratory (NREL), 51, 178 179 Natural dyes, 576 Natural photosensitizers, 558 Natural photosynthesis, 334 336 Natural pigments, 575 584 carotenoids, 576 577 chlorophyll derived Spirulina xanthin carotenoid in Spirulina platensis, 584 chromatophores from Rhodospirillum rubrum S1 biological redox, 580 584 flavin, 579 580 lycopene, 577 579 Xanthophylls from Hymenobacter sp., 580 Neodymium-doped yttrium aluminium garnet (Nd-YAG), 159 Nicotinamide adenine dinucleotide phosphate (NADP1), 564 Nitric acid (HNO3), 236 Nitrogen doping, 234

641

Nitrogen-doped CQDs (NCQD), 501 Non-metallic mining wastes, 442 Nonpurple nonsulfur system, 566 567 Northern Chile, inorganic salts in, 442 444 Nostocales, 614

O Octahedral factor, 263 264 Oil, 131 132 Oil crisis (1973), 37 Open circuit photovoltage, 175 176 Open circuit voltage (VOC), 149 150, 561 562 Organic degradation, 348 351 Organic electroactive solar cells, 101 Organic PV (OPV), 38 Organic Rankine cycles (ORC), 417 Organic solar cells (OSCs), 166, 498 500, 511 515, 537, 593 595 carbon in, 543 544 Organic-inorganic hybrid cells perovskites, 537 Organisms beetle, 610 common rose butterfly, 603 605 crab, 610 611 dipteran, 610 firefly, 607 609 human eye, 609 610 leaf, 605 606 lotus, 606 607 Origami structures, 620 Oscillatoriales, 614 Oxide materials, 181 186

P p-n homojunction, 146 147 p-n junction, 175 176 under illumination, 147 148 p-type semiconductor, 145 146 Pachliopta aristolochiae, 623 Parabolic trough collector (PTC), 384 Passivation, 277 279 Passivation layer technology (PSL technology), 76 77 Pauli’s exclusion principle, 110 111, 137, 139 PEG modified carbon quantum dots (PEG-mCQDs), 502 Peltophorum pterocarpum, 578 579 Peptide, 573 575

642

Index

Pepto-Bismol, 3 4 Perovskite solar cells (PSCs), 38, 40 50, 168, 260 262, 498 500, 519 524, 538, 620 621 carbon in, 544 547 historical review, 40 41 scaling up and possibilities for commercialization, 49 50 solar cells, 41 45 stability, 45 49 Perovskite-based PV, 73 Perovskites, 260 262 Phase change materials (PCMs), 432 433, 470 advantages of micro or nanoencapsulation of, 476 applications, 483 490 need for PCM-based building materials for rural houses, 486 488 need for PCM-based solar air heaters, 484 486 need for PCM-based textiles, 488 490 challenges, 490 encapsulation in phase change materials, 474 475 review of work done, 476 479 selection criteria of, 470 473 types, 471t working principle of, 473 474, 474f Phase change slurry (PCS), 478 479 Phase segregation, 437 Phenolsulfonic acid (PSA), 277 Phenyl-C60-butyric acid methyl ester (PCBM), 42 44 2-phenylethylammonium (PEA), 279 280 Phonon, 138 139 Photocatalysis, 289 290 bismuth-based heterostructures for photocatalytic applications, 299 320 development using bismuth-based heterostructured nanomaterials, 291f fundamentals of heterogeneous photocatalysis, 291 299 for hydrogen production, 22 23 Photocatalytic (PC), 578 applications, 334 354 environmental applications, 347 354 H2 evolution by H2O splitting, 336 345 photocatalytic CO2 reduction to fuel, 345 347 CO2 reduction to fuel, 345 347 technologies, 22

Photoelectrochemical cells (PECs), 344 345, 364, 367 368 Photoelectrochemical water splitting, 364 Photoelectrode, materials for, 369 372 Photomicrobial fuel cells. See Biological photovoltaic devices Photon, 138 139 Photoreceptors, 612 613 Photoreduction process, 351 352 Photosynthesis, 557 558, 611 613 artificial, 613 Photosynthetic pigments, 569 570 Photosynthetic prokaryotes, 564 565 Photosynthetic systems, 562 563 Photosynthetically active radiation, 612 613 Photosystem I (PSI), 571 572, 611 612 Photosystem II (PSII), 571 572 Photovoltaics (PV), 4 9, 133 134, 289 290, 578 advantages and disadvantages, 134t bismuth-based nanomaterials, 7 9 bismuth chalcogenides, 8 9 bismuth-based perovskites and bismuth halides, 7 8 cells, 537 current status of, 133 136 effect, 147 148, 178 187, 497 498 fundamental properties of photovoltaics semiconductors, 136 145 absorption coefficient, 144 145 crystal structure of semiconductors, 136 137 density of energy states, 139 141 diffusion-due to concentration gradient, 143 144 drift-motion due to electric field, 142 143 energy band structure, 137 139 nanoengineering, 6 7 solar cell, 4 operation, 4 6 solar energy, 71 72 technology, 175 176 Physical deposition, 158 160. See also Chemical deposition evaporation techniques, 158 159 sputtering techniques, 159 160 Physical vapor deposition (PVD), 303 304, 308 Piezoelectric materials, 176 177 Pigments, 562 563 Plants, 564, 567 568

Index Plasma enhanced atomic layer deposition (PEALD), 79 Plasma enhanced CVD, 162 Plasma techniques, 159 160 Plasma-assisted chemical vapor deposition (PECVD), 236, 238 Plastoquinone, 564 Platinum (Pt), 500 501 Pleurocapsales, 614 Poling, 186 187 Polyaniline (PANI), 508 509 Polycrystalline silicon, 72 73 Polydimethylsiloxane (PDMS), 593 595 Polyethylene glycol (PEG), 502 Polymer solar cells (PoSCs), 515 519 Polymethyl methacrylate (PMMA), 245 246 Potassium salt of hydroquinone sulfonic acid (KHQSA), 277 Power conversion efficiency (PCE), 5 6, 8 9, 201, 537, 593 595 Pristine graphene, 246 247 Protein engineering, 557 558 Proteins, 562 563, 569 Proteolytic enzyme method, 574 575 Pulsed laser deposition (PLD), 191 192 Purple membrane, 570 571 Pyridine (C5H5N), 233 Pyroelectric materials, 176 177 Pyrolic N, 234 Pyromark 2500, 406 407

Q Quality of life, 259, 260f Quantum chemistry, 111 112 Quantum dot solar cells (QDSCs), 38, 168 169, 202 203, 498 500, 509 511 Quantum dots (QDs), 201, 498 QD-IBSC, 214 215 in solar cells, 214 222 In(Ga)As or InAsP QDs on wide bandgap material barriers, 221 222 InAs quantum dots on GaAs, 216 221 Quantum efficiency (QE), 53 54, 151 Quantum mechanics, nanostructures and, 203 205 Quantum phenomena of electrons, 101 Quantum well solar cell (QWSC), 202 203, 207t Quantum wells (QWs), 201 in solar cells, 205 210

643

Quantum wires, 201 in solar cells, 210 214 Quasi-neutral part, 145 146 Quasihoneycomb like structure (QHS), 622 623 Quaternary N structure, 234

R Radical-enhanced ALD, 84 Radio Corporation of America method (RCA method), 236 RCA-1 process, 237 RCA-2 process, 237 Radio-Frequency sputtering (RF-Sputtering), 308 Raman spectroscopy, 244 245 Reactive oxygen species (ROS), 292 293, 558 Recombinant bacteria, 569 570 Recombinant DNA technology, 569 Regenerative cell, 368 Renewable energy, 3, 132 133 Renewable energy sources (RES), 383 384, 467 Research and Development (R&D), 37 Resistivity of charge carriers, 143 Reverse biomimetics, 596 Rhodiola rosea, 576 Rhodium palustris, 566 Rhodobacter azotoformans R7 protein pigment complexes from, 573 Rhodococcus sphaericus, 566 Rhodopseudomonas palustris, 566 Rhodopseudomonas palustris CQV97 protein pigment complexes from, 573 Rhodospirillum rubrum, 566 Rhoeo spathacea, 578 579 Riboflavin (RF), 579 580 Ruthenium (Ru), 500 501 polypyridyl complexes, 577 Ru-based dyes, 558

S Scarabaeidae, 610 Schottky junction, 175 176 Schro¨dinger equation, 102 103, 107 108 Seebeck coefficient, 13 14 Selective area epitaxy (SAE), 212 Selective area growth (SAG), 212 Selectively solar-transmitting coatings, 409 413

644

Index

Self consisted filed cycle (SCF cycle), 117f, 120 Self-assembled monolayer (SAM), 513 514 Self-cleaning, 313 317 Self-governing structures, 176 Self-repairing microorganisms, 557 558 Self-sustaining systems, 176 Semiconductors crystal structure of, 136 137 devices, 142 heterostructures, 201 photocatalysis, 289 290 semiconductor-semiconductor heterostructures using bismuth-based materials, 301 303 Sensible heat storage (SHS), 431 432, 468 469 materials, 447 448 Shewanella oneidensis, 566 567 Shockley Queisser limit, 5 6, 151 Short circuit current, 149 Silicon (Si), 229 230 fabrication of silicon heterojunction solar cell, 234 244 a-silicon deposition, 237 240 metallization, 242 243 surface patterning and surface cleaning, 235 237 thermal treatment, 243 244 transparent conductive oxide deposition, 240 242 in photovoltaic energy conversion cadmium telluride solar cells, 164 165 categories of photovoltaic market, 151 152 CIGSe-based solar cell, 162 164 commercialization of Si solar cells, 152 153 current status of photovoltaics, 133 136 emerging solar cell technologies, 165 169 fundamental properties of photovoltaics semiconductors, 136 145 material selection in thin film technology, 157 158 multijunction solar cells, 165 physics of solar cell, 145 151 status of alternative photovoltaics materials, 153 154 thin film deposition techniques, 158 162 thin film technology, 154 156

silicon-based photovoltaic technology, 175 176, 179 181 silicon-based solar cells, 497 498, 621 622 silicon-based solar technology, 593 595 wafer-based PV, 72 73 Simultaneous thermal analysis (STA), 438 Single crystal lattice, 136 137 Single junction solar cell’s PCE (SJSC PCE), 201 202 Single wafer reactor (SWR), 82 Slab supercells, 101 102 Slater function, 110 111 Sodium bismuth dichalcogenides, 8 Sodium containing molybdenum (MoNa), 164 SolAero, 51 Solar absorbers durability studies of, 405 407 state-of-the-art review of solar absorber surfaces, 395 404 Solar air heaters, PCM in, 484 Solar cells, 3 4, 41 45, 364, 559, 593 595. See also Bioinspired solar cells applications III V semiconductor materials for multijunction solar cells applications, 50 62 perovskite solar cells, 40 50 based on carbon nanomaterials, 541 547 carbon in dye-sensitized solar, 541 543 carbon in organic solar cells, 543 544 carbon in perovskite solar cells, 544 547 carbon and derivatives, 538 541 challenges, 547 549 fabrication, 234 operation, 4 6 physics of, 145 151 homojunction and heterojunction structure, 146 147 I-V equations of solar cell, 149 151 p-n junction under illumination, 147 148 power conversion efficiency of works used carbon and derivatives, 548t quantum dots in, 214 222 quantum wells in, 205 210 quantum wires in, 210 214 Solar energy, 133 134, 366 367, 429, 497 498, 537, 593 595 Solar energy harvesting materials, 289 290 auxiliary Kohn-Sham system, 116 118 crystalline representation, 103 107

Index cubic Bravais lattices with primitive bcc and fcc structure, 105f noncubic Bravais lattices with lattice magnitudes and angles, 106f multielectron system, 107 111 selected materials with solar energy harvesting implementations, 118 127 charge order and half metallicity of Fe3O4, 122 123 conventional and reduced representation of mBiVO4, 125 126 input file, 118 120 optimization of anatase titanium dioxide, 123 124 structural stability of FAPbI3 perovskites, 122 supercell of zinc oxide, 121 122 template structure for chalcopyrite, 126 127 universal functional of density, 113 116 variational principle, 111 113 Solar photovoltaics, 37 Solar selective coatings (SSCs), 388, 397 402 CSP, 383 392 CSP absorbing surfaces and materials, 409 417 durability studies of solar absorbers, 405 407 industrialization of high-temperature, 413 417 intrinsic absorber, 398 lack of standardized characterization protocols, 407 409 metal-cermet coatings, 401 402 metal-semiconductor tandem stack, 398 399 multilayer absorber, 400 401 state-of-the-art review of solar absorber surfaces and materials, 395 404 absorber paints, 395 397 textured surface absorber, 399 400 volumetric receivers, 402 404 Solar water splitting atomic layer deposition process, 373 375 hydrogen generation from water photoelectrolysis, 368 369 materials for photoelectrode, 369 372 photoelectrochemical cells, 367 368 solar energy, 366 367 Solar-air heating

645

construction and working principle of, 485 486 performance criteria for, 486 Solar-hydrogen production, 22 29 Bi-based nanomaterials, 24 29 bismuth chalcogenides, 25 bismuth-based composite oxides, 26 29 fundamentals of photocatalysis for hydrogen production, 22 23 nanoengineering, 24 nanostructured semiconductors, 29 ternary bismuth chalcogenides, 25 Sol gel processing, 160, 191 192, 303 305 Solvent, 280 281 Sorting Nexin 2 (SnX2), 274 276 Spectrolab, 51 Spiro-OMeTAD, 42, 260 262 Spirulina platensis, chlorophyll derived Spirulina xanthin carotenoid in, 584 Spirulinales, 614 Spontaneous polarization, 176 177 Spray pyrolysis technique, 161 Sputtering techniques, 159 160, 308 309 Stagnant flow reactor (SFR), 82 Stranski-Krastanov growth mode (SK growth mode), 216 Subcooling, 437 Substitutional doping, 233 234 Sun, 429 430 Sunlight, 145 146, 289 290, 558 Super hydrophobic system, 316 Supercapacitors, 16 21 bismuth perovskite supercapacitors, 17 operation, 17 Supercell, 101 102 of zinc oxide, 121 122 Superhydrophobic dyes, 603 Superlattices (SLs), 186 Surface cleaning, 235 237 Surface doping, 233 234 Surface patterning, 235 237 Sustainable buffer layers based on atomic layer deposition, 87 88 Sustainable passivation layers based on ALD, 88 89 Synechococcales, 614 Synthesized graphene films, 244 245

T Technology readiness levels (TRLs), 413 414 Ternary AgBiS2 nanocrystals, 8 9

646

Index

Ternary bismuth chalcogenides, 25 Ternary materials, 15 16 Tertiary composite system, 343 344 Tetramethylammonium hydroxide (TMAH), 235 Textiles, PCM in, 488 Textured surface absorber, 399 400 Thermal energy storage (TES), 430 431, 467 468 basic concepts for TES materials, 434 438 challenges for application of waste and byproducts in inorganic salt-based wastes in TES systems, 455 456 optimization of thermal properties of, 456 461 uses of wastes as TES materials, 453 461 chemical reaction/thermochemical heat storage, 433 434 comparison of energy storage density for different TES materials, 439 industrial waste studied as TES materials, 440 442 inorganic salt-based products and wastes as low cost materials for sustainable, 442 453 latent heat storage, 432 433 sensible heat storage, 431 432 TES system types, 438 439 Thermal stability, 435 Thermal treatment, 243 244 incorporating graphene into silicon heterojunction solar cells, 249 250 Thermochemical storage (TCS), 433 434, 469 cycling stability for TCS process, 438 materials, 451 453 Thermoelectrics, 3 4 Bi-based nanomaterials, 12 16 devices, 9 16 nanoengineering, 11 12 operation, 9 11 Thermogravimetric analysis (TGA), 435 Thermosynechococcus elongates, 573 Thin film deposition techniques, 158 162 chemical deposition, 160 162 physical deposition, 158 160 solar cells, 593 595, 623 technology, 154 156 material selection in, 157 158

Thylakoids, 563 564 membranes, 562 563 Tin (Sn), 274 276 oxidation, 269 270 perovskite bandgap, 267 269 toxicity, 271 272 Tin chloride (SnCl2), 275 Tin halide perovskites (ASnX3), 262 263, 272 274 “best research-cell efficiencies” chart, 260f cubic crystal lattice of an ABX3 metal halide perovskite, 261f efficient and stable ASnX3 PSCs additives, 274 277 low dimensional perovskites, 279 280 passivation, 277 279 solvent, 280 281 halide perovskite solar cells, 263 272 quality of life, 259, 260f Top-down approach, 212 213 Total primary energy supply (TPES), 383 384 Transparent conducting oxide (TCO), 162 163 deposition, 240 242 Transparent conductive electrode (TCE), 233 2,2,2-trifluoroethylamine hydrochloride (TFEAC), 277 Trimethyl boron (TMB), 238 Two-dimension (2D) Fourier transform electronic spectroscopy, 616 617 graphene, 538 materials, 329 MXene-based heterostructured photocatalytic materials, 330 331 photocatalytic applications, 334 354 synthesis of 2D-MXenes, 331 334 functionalization and electronic properties of MXene, 333 334 2D/2D composites, 341 342 2D/3D composites, 342 343 Type I heterounion, 301 Type II heterounion, 302 Type III heterounion, 302

U Ultra-high temperature ceramic (UHTC), 403 404 Ultraviolet (UV), 289 290 radiation, 193 194, 365 UV-resistant Antarctic bacteria, 576 577

Index United Nations Development Programme, 259 Universal functional of density, 113 116

V Vacuum thermal evaporation, 158 Valence band (VB), 4 5, 266, 334 336 Valence band maximum (VBM), 267 268 Van der Waals interactions, 122 Vapor liquid-solid method (VLS method), 212 Vascular plant biophotovoltaics (VP-BPV), 567 568 Verwey temperature, 122 123 Vis-region. See Visible region (Vis-region) Visible region (Vis-region), 289 290 Voltage drop, 146 Volumetric receivers, 402 404

W Wastes based on inorganic salts challenges for application of waste and byproducts in TES, 453 461 industrial waste studied as TES materials, 440 442 inorganic salt-based products and wastes as low cost materials for sustainable TES, 442 453 availability and abundance of inorganic salts in Northern Chile, 442 444

647

economic analysis of inorganic salts as low-cost TES materials, 444 446 state-of-art of currently proposed byproducts and wastes as TES materials, 446 453 solar energy installations hotspots, 430f TES, 431 439 Water contact angles (WCAs), 603 Water decontamination, 347 348 Water splitting, 330 331 activity of MXenes, 337 340 Wave mechanics, 203 Wet chemical method, 303 304 Wettability, 603 Wind energy, 71 72 Wurtzite structure of ZnO, 121

X X-ray photoelectron spectroscopy (XPS), 244 245 Xanthophylls from Hymenobacter sp., 580 Xenon monochloride (XeCl), 159

Z 0D/2D strategy, 350 Zero-emissions, 3 Zinc blende lattice, 136 137 Zinc oxide, supercell of, 121 122