Composites-Based Perovskite Solar Cells 9783527352937

Table of contents :
Cover
Half Title
Composites‐Based Perovskite Solar Cells
Copyright
Contents
Preface
1. Introduction – Why Composites-Based Perovskite Solar Cells?
1.1 Need to Develop Composites‐Based Perovskite Solar Cells
1.2 Fabrication Strategy for Composites‐Based Perovskite Solar Cells
References
2. Hybrid Perovskites and Solar Cells
2.1 Perovskite Materials
2.1.1 Three‐Dimensional Perovskites
2.1.1.1 Lead‐Based Perovskites
2.1.1.2 Lead–Tin‐Mixed Perovskites
2.1.1.3 Tin‐Based Perovskites
2.1.1.4 All Inorganic Perovskites
2.1.2 Low‐Dimensional Perovskites
2.1.2.1 Ruddlesden–Popper (RP) 2D Perovskites
2.1.2.2 Dion–Jacobson (DJ) 2D Perovskites
2.1.2.3 One‐/Zero‐Dimensional (1D/0D) Perovskites
2.1.3 Single‐Crystal Perovskites
2.1.4 Dynamics of Perovskite Crystal Growth
2.2 Perovskite Solar Cells
2.2.1 Working Principles of Perovskite Solar Cell
2.2.2 Configurations of Perovskite Solar Cell
2.2.2.1 n‐i‐p‐Based Traditional Structure
2.2.2.2 p‐i‐n‐Based Inverted Structure
2.2.2.3 Hole/Electron‐Transport‐Free Simple Structure
2.2.2.4 Flexible Perovskite Solar Cells
2.2.2.5 Semitransparent Perovskite Solar Cells
2.3 Limitations and Improvements of Energy Conversion in Perovskite Solar Cells
2.3.1 Limitation Parameters
2.3.1.1 Energy Gap
2.3.1.2 Interface Defects
2.3.2 Improvement of the Efficiency of Solar Cells
References
3. Fundamentals and Benefits of Functional Composite Materials
3.1 Introduction to Composite Functional Materials
3.1.1 Definition of Composite Material
3.1.2 Properties of Composite Materials
3.1.3 Advantages of Composites for Perovskite Solar Cells
3.2 Development of Composites‐Based Perovskite Solar Cells
3.2.1 Alloy Structure in A, B, or X Site
3.2.2 Composite Perovskites
3.2.3 Composite‐Based Charge Transport Layers
3.2.4 Composite‐Based Electrodes
References
4. Stability and Efficiency Loss Issues of Perovskite-Based Devices
4.1 Materials Instability
4.1.1 Moisture‐Induced Perovskite Degradation
4.1.2 Photo‐Induced Perovskite Degradation
4.1.3 Heat‐Induced Perovskite Degradation
4.1.4 The Point Defects Induced Perovskite Degradation
4.1.5 Defects at Perovskite Film Surface/Buried Interfaces
4.1.6 Strain‐Induced Perovskite Lattice Distortion and Phase Instability
4.1.7 Ions Migration of Perovskites
4.1.8 Device Efficiency Loss Induced by Materials Instability
4.2 Device Heterointerface Instability
4.2.1 Heterointerface Defects of Perovskite/ETL
4.2.2 Heterointerface Defects of Perovskite/HTL
4.2.3 Interaction with Metal Electrodes
4.2.4 Efficiency Loss Induced by Heterointerfaces Instability
4.3 Solutions for Instability Problems
4.3.1 Development of Perovskite Composites
4.3.2 Design of Device Structures
4.3.3 Robust Design of Device Encapsulation
References
5. Composites-Based Charge-Transport and Interfacial Materials
5.1 Organic‐Based Composites
5.1.1 ETL Materials
5.1.2 HTL Materials
5.2 Inorganic‐Based Composites with Metal and Metal Oxide
5.2.1 ETL Materials
5.2.2 HTL Materials
5.3 Carbon‐Based Composites
5.3.1 ETL Materials
5.3.2 HTL Materials
5.3.3 Carbon‐Based Composites for Interfacial Layer
References
6. Composite-Based Pb-Perovskite Materials as Absorbers
6.1 Organic Additives‐Based Perovskite Composites
6.1.1 Organic Ammonium Halides
6.1.2 Organic Small Molecules
6.1.3 Polymer‐Based Materials
6.2 Inorganic Additives‐Based Perovskite Composites
6.2.1 Metal Oxides
6.2.2 Semitransparent Perovskite Solar Cells with Metal Oxide‐Based Composites
6.2.3 Carbon, Graphene, and Its Derivatives
6.2.4 Alkali Halide Additives
6.2.5 Others
6.3 Low‐Dimensional (LD)/Three‐Dimensional (3D) Heterostructure Perovskite Composites
6.3.1 2D–3D Composites
6.3.2 1D‐3D Composites
6.3.3 0D–3D Composites
6.4 Quantum Dot (QD) Additives‐Based Perovskite Composites
6.4.1 Perovskite QD‐Based Composites
6.4.2 Carbon QD‐Based Composites
6.5 Reduced Film Strain by Composites‐Based Perovskites
6.5.1 Reduce Lattice Strain by Compositional Design
6.5.2 Control Crystallization by Chemical Interaction
6.5.3 Facilitate Strain Release by Heterostructure Interfaces
References
7. Composites-Based Pb-Free Perovskite Materials as Absorbers
7.1 Inorganic Additives‐Based Perovskite Composites
7.1.1 SnF2 Additive
7.1.2 SnCl2 Additive
7.1.3 Hydrazine Additive
7.1.4 Acidic Additive
7.1.5 Other Additives
7.2 Organic Additives‐Based Perovskite Composites
7.3 Carbon Additives‐Based Perovskite Composites
References
8. Composite-Based Perovskite Materials in Tandem Solar Cells
8.1 Introduction
8.2 Configuration of Perovskite‐Based Tandems
8.2.1 Perovskite/Si Tandems
8.2.2 All Perovskite Tandems
8.2.3 Perovskite/Organic Tandems
8.2.4 Perovskite/CIGS Tandems
8.3 Perovskite Alloy‐Based Composites as Absorbers
8.3.1 A‐Site Alloy‐Based Composites
8.3.2 X‐Site Alloy‐Based Composites
8.3.3 B‐Site Alloy‐Based Composites
8.4 Additives‐Based Perovskite Composites as Absorbers
8.4.1 Additive‐Based Wide‐Bandgap Perovskite Composites
8.4.2 Additive‐Based Narrow‐Bandgap Perovskite Composites
8.4.3 2D‐3D‐Based Wide‐Bandgap Perovskite Composites
8.4.4 2D‐3D‐Based Narrow‐Bandgap Perovskite Composites
8.5 Composite‐Based Interconnection Layers (ICLs)
8.5.1 Composite‐Based Interconnection Layers (ICLs) in Perovskite/Si Tandems
8.5.2 Composite‐Based Interconnection Layers (ICLs) in All Perovskite Tandems
8.5.3 Composite‐Based Interconnection Layers (ICLs) in Perovskite/Organic Tandems
8.6 Composite‐Based Charge Transport Layers
8.6.1 Composite‐Based Hole Transport Layers in Tandems
8.6.2 Composite‐Based Electron Transport Layers in Tandems
8.7 Composite‐Based Interfacial Layers in Tandems
8.7.1 Composite‐Based Buffer Layers
8.7.2 Composite‐Based Passivation Layer
References
9. Issues for Commercialization of Perovskite Solar Cells
9.1 Introduction to The Current Status of Perovskite Solar Cells
9.2 Solutions to Stability Issues
9.2.1 Evaluation Standards
9.2.2 Internal Encapsulation
9.2.3 External Encapsulation
9.3 Upscaling, Commercialization, and Challenges
9.3.1 Scalable Fabrication Methods
9.3.2 Module Design and Process
9.4 Status of Solar Modules Production
9.4.1 Module Efficiency
9.4.2 Market Prospect
9.4.3 The Toxicity Issues of Lead in Modules
References
10. Characterization Methods for Composite-Based Perovskite Solar Cells
10.1 Composite‐Based Perovskite Films Characterization
10.1.1 Growth Dynamics of Composite‐Based Perovskites
10.1.2 Optical and Electrical Properties of Composite‐Based Films
10.1.3 Heterogeneity of Composite‐Based Films
10.1.4 Chemical Interactions and Simulations
10.1.4.1 Chemical Interactions
10.1.4.2 Simulations
10.2 Devices Characterization
10.2.1 Carrier Mobility and Dynamics
10.2.2 Trap Densities
10.2.3 Stability Characterization
References
11. Perspectives and Future Work of Composites-Based Perovskite Solar Cells
11.1 Perspectives of Composites‐Based Perovskite Solar Cells
11.2 Future Work for Composites‐Based Perovskite Solar Cells
11.2.1 Scale‐Up Processing Technology
11.2.2 Green Production Technology
11.2.3 Cyclic Utilization of Lead Components for Perovskite Precursors
References
Index

Citation preview

Composites-Based Perovskite Solar Cells

Composites-Based Perovskite Solar Cells Yoon-Bong Hahn, Yousheng Wang, and Tahmineh Mahmoudi

Authors Yoon-Bong Hahn

Jeonbuk National University 567 Baekje-daero, Deokjin-gu Jeonju 54896, South Korea

All books published by WILEY-VCH are carefully produced. Nevertheless, authors, editors, and publisher do not warrant the information contained in these books, including this book, to be free of errors. Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate.

Yousheng Wang

Jinan University 855 Xingye Avenue East Panyu District Guangzhou 510632, China

Library of Congress Card No.: applied for

Tahmineh Mahmoudi

Bibliographic information published by the Deutsche Nationalbibliothek

RMIT University 124 La Trobe Street Melbourne VIC 3001, Australia Cover Image: © sutadimages/Shutterstock

British Library Cataloguing-in-Publication Data

A catalogue record for this book is available from the British Library.

The Deutsche Nationalbibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data are available on the Internet at . © 2025 WILEY-VCH GmbH, Boschstraße 12, 69469 Weinheim, Germany The manufacturer’s authorized representative according to the EU General Product Safety Regulation is WILEY-VCH GmbH, Boschstr. 12, 69469 Weinheim, Germany, e-mail: [email protected]. All rights reserved (including those of translation into other languages, text and data mining and training of artificial technologies or similar technologies). No part of this book may be reproduced in any form – by photoprinting, microfilm, or any other means – nor transmitted or translated into a machine language without written permission from the publishers. Registered names, trademarks, etc. used in this book, even when not specifically marked as such, are not to be considered unprotected by law. Print ISBN: 978-3-527-35293-7 ePDF ISBN: 978-3-527-84452-4 ePub ISBN: 978-3-527-84453-1 oBook ISBN: 978-3-527-84454-8 Typesetting

Straive, Chennai, India

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Contents Preface 1 1.1 1.2

2 2.1 2.1.1 2.1.1.1 2.1.1.2 2.1.1.3 2.1.1.4 2.1.2 2.1.2.1 2.1.2.2 2.1.2.3 2.1.3 2.1.4 2.2 2.2.1 2.2.2 2.2.2.1 2.2.2.2 2.2.2.3 2.2.2.4 2.2.2.5 2.3 2.3.1 2.3.1.1

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Introduction – Why Composites-Based Perovskite Solar Cells? 1 Need to Develop Composites-Based Perovskite Solar Cells 1 Fabrication Strategy for Composites-Based Perovskite Solar Cells 3 References 5 Hybrid Perovskites and Solar Cells 7 Perovskite Materials 7 Three-Dimensional Perovskites 7 Lead-Based Perovskites 7 Lead–Tin-Mixed Perovskites 8 Tin-Based Perovskites 9 All Inorganic Perovskites 10 Low-Dimensional Perovskites 10 Ruddlesden–Popper (RP) 2D Perovskites 10 Dion–Jacobson (DJ) 2D Perovskites 11 One-/Zero-Dimensional (1D/0D) Perovskites 12 Single-Crystal Perovskites 13 Dynamics of Perovskite Crystal Growth 14 Perovskite Solar Cells 16 Working Principles of Perovskite Solar Cell 16 Configurations of Perovskite Solar Cell 17 n-i-p-Based Traditional Structure 18 p-i-n-Based Inverted Structure 18 Hole/Electron-Transport-Free Simple Structure 19 Flexible Perovskite Solar Cells 19 Semitransparent Perovskite Solar Cells 20 Limitations and Improvements of Energy Conversion in Perovskite Solar Cells 21 Limitation Parameters 21 Energy Gap 21

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Contents

2.3.1.2 2.3.2

Interface Defects 22 Improvement of the Efficiency of Solar Cells 22 References 23

3

Fundamentals and Benefits of Functional Composite Materials 27 Introduction to Composite Functional Materials 27 Definition of Composite Material 27 Properties of Composite Materials 27 Advantages of Composites for Perovskite Solar Cells 30 Development of Composites-Based Perovskite Solar Cells 31 Alloy Structure in A, B, or X Site 31 Composite Perovskites 33 Composite-Based Charge Transport Layers 34 Composite-Based Electrodes 35 References 36

3.1 3.1.1 3.1.2 3.1.3 3.2 3.2.1 3.2.2 3.2.3 3.2.4

4 4.1 4.1.1 4.1.2 4.1.3 4.1.4 4.1.5 4.1.6 4.1.7 4.1.8 4.2 4.2.1 4.2.2 4.2.3 4.2.4 4.3 4.3.1 4.3.2 4.3.3

5 5.1 5.1.1 5.1.2 5.2

Stability and Efficiency Loss Issues of Perovskite-Based Devices 41 Materials Instability 41 Moisture-Induced Perovskite Degradation 41 Photo-Induced Perovskite Degradation 42 Heat-Induced Perovskite Degradation 44 The Point Defects Induced Perovskite Degradation 44 Defects at Perovskite Film Surface/Buried Interfaces 46 Strain-Induced Perovskite Lattice Distortion and Phase Instability 48 Ions Migration of Perovskites 50 Device Efficiency Loss Induced by Materials Instability 51 Device Heterointerface Instability 52 Heterointerface Defects of Perovskite/ETL 52 Heterointerface Defects of Perovskite/HTL 54 Interaction with Metal Electrodes 56 Efficiency Loss Induced by Heterointerfaces Instability 58 Solutions for Instability Problems 60 Development of Perovskite Composites 60 Design of Device Structures 60 Robust Design of Device Encapsulation 62 References 63 Composites-Based Charge-Transport and Interfacial Materials 71 Organic-Based Composites 71 ETL Materials 71 HTL Materials 72 Inorganic-Based Composites with Metal and Metal Oxide 76

Contents

5.2.1 5.2.2 5.3 5.3.1 5.3.2 5.3.3

ETL Materials 76 HTL Materials 79 Carbon-Based Composites 81 ETL Materials 81 HTL Materials 82 Carbon-Based Composites for Interfacial Layer 84 References 85

6 6.1 6.1.1 6.1.2 6.1.3 6.2 6.2.1 6.2.2

Composite-Based Pb-Perovskite Materials as Absorbers 93 Organic Additives-Based Perovskite Composites 93 Organic Ammonium Halides 93 Organic Small Molecules 96 Polymer-Based Materials 99 Inorganic Additives-Based Perovskite Composites 103 Metal Oxides 103 Semitransparent Perovskite Solar Cells with Metal Oxide-Based Composites 104 Carbon, Graphene, and Its Derivatives 105 Alkali Halide Additives 110 Others 112 Low-Dimensional (LD)/Three-Dimensional (3D) Heterostructure Perovskite Composites 114 2D–3D Composites 114 1D-3D Composites 118 0D–3D Composites 118 Quantum Dot (QD) Additives-Based Perovskite Composites 119 Perovskite QD-Based Composites 120 Carbon QD-Based Composites 120 Reduced Film Strain by Composites-Based Perovskites 121 Reduce Lattice Strain by Compositional Design 122 Control Crystallization by Chemical Interaction 124 Facilitate Strain Release by Heterostructure Interfaces 124 References 127

6.2.3 6.2.4 6.2.5 6.3 6.3.1 6.3.2 6.3.3 6.4 6.4.1 6.4.2 6.5 6.5.1 6.5.2 6.5.3

7 7.1 7.1.1 7.1.2 7.1.3 7.1.4 7.1.5 7.2 7.3

Composites-Based Pb-Free Perovskite Materials as Absorbers 133 Inorganic Additives-Based Perovskite Composites 133 SnF2 Additive 133 SnCl2 Additive 134 Hydrazine Additive 134 Acidic Additive 135 Other Additives 136 Organic Additives-Based Perovskite Composites 137 Carbon Additives-Based Perovskite Composites 142 References 146

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Contents

8 8.1 8.2 8.2.1 8.2.2 8.2.3 8.2.4 8.3 8.3.1 8.3.2 8.3.3 8.4 8.4.1 8.4.2 8.4.3 8.4.4 8.5 8.5.1 8.5.2 8.5.3 8.6 8.6.1 8.6.2 8.7 8.7.1 8.7.2

9 9.1 9.2 9.2.1 9.2.2 9.2.3 9.3 9.3.1 9.3.2 9.4 9.4.1 9.4.2 9.4.3

Composite-Based Perovskite Materials in Tandem Solar Cells 151 Introduction 151 Configuration of Perovskite-Based Tandems 151 Perovskite/Si Tandems 152 All Perovskite Tandems 154 Perovskite/Organic Tandems 155 Perovskite/CIGS Tandems 156 Perovskite Alloy-Based Composites as Absorbers 156 A-Site Alloy-Based Composites 157 X-Site Alloy-Based Composites 158 B-Site Alloy-Based Composites 160 Additives-Based Perovskite Composites as Absorbers 161 Additive-Based Wide-Bandgap Perovskite Composites 162 Additive-Based Narrow-Bandgap Perovskite Composites 162 2D-3D-Based Wide-Bandgap Perovskite Composites 164 2D-3D-Based Narrow-Bandgap Perovskite Composites 166 Composite-Based Interconnection Layers (ICLs) 167 Composite-Based Interconnection Layers (ICLs) in Perovskite/Si Tandems 167 Composite-Based Interconnection Layers (ICLs) in All Perovskite Tandems 170 Composite-Based Interconnection Layers (ICLs) in Perovskite/Organic Tandems 171 Composite-Based Charge Transport Layers 173 Composite-Based Hole Transport Layers in Tandems 173 Composite-Based Electron Transport Layers in Tandems 176 Composite-Based Interfacial Layers in Tandems 178 Composite-Based Buffer Layers 178 Composite-Based Passivation Layer 179 References 180 Issues for Commercialization of Perovskite Solar Cells 185 Introduction to The Current Status of Perovskite Solar Cells 185 Solutions to Stability Issues 186 Evaluation Standards 186 Internal Encapsulation 187 External Encapsulation 189 Upscaling, Commercialization, and Challenges 190 Scalable Fabrication Methods 190 Module Design and Process 192 Status of Solar Modules Production 194 Module Efficiency 194 Market Prospect 196 The Toxicity Issues of Lead in Modules 200 References 200

Contents

10 10.1 10.1.1 10.1.2 10.1.3 10.1.4 10.1.4.1 10.1.4.2 10.2 10.2.1 10.2.2 10.2.3

11 11.1 11.2 11.2.1 11.2.2 11.2.3

Characterization Methods for Composite-Based Perovskite Solar Cells 205 Composite-Based Perovskite Films Characterization 205 Growth Dynamics of Composite-Based Perovskites 205 Optical and Electrical Properties of Composite-Based Films 207 Heterogeneity of Composite-Based Films 211 Chemical Interactions and Simulations 215 Chemical Interactions 215 Simulations 217 Devices Characterization 218 Carrier Mobility and Dynamics 218 Trap Densities 220 Stability Characterization 222 References 222 Perspectives and Future Work of Composites-Based Perovskite Solar Cells 225 Perspectives of Composites-Based Perovskite Solar Cells 225 Future Work for Composites-Based Perovskite Solar Cells 226 Scale-Up Processing Technology 226 Green Production Technology 228 Cyclic Utilization of Lead Components for Perovskite Precursors References 231 Index 233

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Preface To meet the rapidly growing global energy demand, building an ecofriendly sustainable energy conversion and storage system is one of the great challenges of our time, leading both academia and industry to seek alternative and renewable energy solutions. Solar energy is the richest and the most green energy source on the planet, thus evaluating that photovoltaic (PV) technology is the most effective and promising technology. Among solar cells, recently perovskite PVs have emerged as a promising candidate for the next-generation solar cell industry with an unprecedented rise of efficiency over 26%. However, the lack of stability of perovskite materials is a well-known bottleneck in the commercialization of perovskite solar cells (PSCs). To solve this problem, many research groups around the world have studied various methods to develop reliable PSCs in terms of efficiency and stability. It is worth noting that both efficiency and stability are related to materials used to fabricate PSCs. Hence, in order to resolve the instability problems of PSCs with high efficiency, development of robust materials that can improve their chemical, electrical, optical, and thermal properties is of critical importance. Recently, it has been reported that composite materials are suitable for solving the stability issues of PSCs. This book addresses the principles and materials for the development of composites-based PSCs (CPSCs) and provides detailed descriptions of the functional composite materials that can be used for light-absorbing, charge-conducting, and interfacial layers of CPSCs. It will appeal to graduate-level students and researchers interested in the practical use of perovskite PVs. Chapter 1 introduces why the CPSCs are needed to solve problems related to power conversion efficiency and device stability, as well as the development strategy of CPSCs and the manufacturing strategy of highly stable and efficient CPSCs with perovskite-based composite and interface engineering. Chapter 2 presents the optical and electrical properties of perovskites including three-dimensional, two/low-dimensional and single-crystal perovskites, the typical growth dynamics of 3D perovskite crystals, the development history, the working principles, and device configurations of perovskite PVs. Chapter 3 describes fundamentals and benefits of functional composite materials and introduces various kinds of composite materials for applications to active layer, hole- and electron-transport layers, and electrodes of PSCs. Chapter 4 explains how the external factors such as moisture, light, and heat cause degradation of

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Preface

perovskite and interface, then discusses the defects formation mechanism in the solution-processed perovskite thin films. This chapter further explores how the defects induce the perovskite instability and efficiency loss, followed by methods to improve materials and device stability without sacrificing efficiency. Chapter 5 introduces composite-based charge-transport and interfacial materials. Efficient interface engineering methods with organic-, inorganic-, and carbon-based composite materials for charge-transport layers and interfacial layers are presented in detail. Chapter 6 presents a concept of composite-based hybrid Pb-perovskites, including organic additive-based perovskite composites, inorganic additive-based perovskite composites, 2D/3D heterostructure perovskite composites, quantum dot additive-based perovskite composites, etc. Moreover, the mechanism of additives-induced growth of perovskite crystals, mitigation of lattice distortion and ions migration, and passivation of film defects as well as the optical and electrical properties of composite-based Pb-perovskite films are discussed in detail. Chapter 7 introduces the strategic implementation of additive engineering technique, which is a key to unlocking the full potential of Sn-based PSCs, paving a way for sustainable high-performance PV technologies. Chapter 8 provides information on the composites-based hybrid perovskite materials for application to tandem solar cells (TSCs). The theoretical efficiency limit for two-terminal and four-terminal TSCs, the device configurations of a perovskite-based TSC, and the progress and prospect of TSCs are presented. Chapter 9 presents a comprehensive analysis of the path to commercialization of PSCs. First, briefly introduce the stability and encapsulation issues of perovskite-based modules in their current state of development. Then up-scaling, device design, and assessment methods of modules are discussed, with respect to scalable fabrication of high-quality perovskite films, module fabrication process and characterization, as well as the status of solar module production, market prospect, and future challenges. Chapter 10 introduces the characterization methods for CPSCs, which are indispensable tools in the advancement of CPSCs, offering critical insights into their optical, electrical, and structural properties toward sustainable and efficient PV applications. Chapter 11 presents perspectives of composites-based PSCs and explores future work on developing green, highly efficient, and large-scale PV products, which include scalable deposition techniques for all device layer stacks, green production technology, and recycling of perovskite solar products, etc. Last but not least, our special thanks to the Wiley-VCH GmbH for their continuous support and guidance. We also thank the publishers and authors who kindly permitted reprint of their work and acknowledge the National Research Foundation of Korea (2021R1A2C2013426) for supporting our CPSC research, which produced many useful data for writing this book. YW acknowledges partial support by Graduate textbook project of Jinan University (2023YJC009). July 2024

Yoon-Bong Hahn Yousheng Wang Tahmineh Mahmoudi

1

1 Introduction – Why Composites-Based Perovskite Solar Cells? 1.1 Need to Develop Composites-Based Perovskite Solar Cells Global electricity demand has continued to increase at a higher rate than the rate of global energy production. Compared to fossil fuels that produce huge amounts of carbon and cause global warming, the development of technologies related to energy conversion and storage for various clean and renewable energy resources such as solar, wind, hydro, and biomass is a big challenge of our time. Among the renewable energy resources, solar energy is the richest and the most eco-friendly energy source on the planet that can supply the growing electricity demand, evaluating that photovoltaic (PV) technology is the most effective and promising technology. The solar PV industry is a vast field with various solar cells divided into several generations. Silicon-based solar cells, classified as first generation, are the most common type of PV with a market share of 95%. Crystalline silicon solar cells have many benefits, such as high efficiency of more than 20%, nontoxic material, good photoconductivity and stability, resistant to corrosion, long lifetime span of over 25 years, low maintenance, and versatile applications. However, due to the complex processes of manufacturing crystalline silicon and the use of pricey and high quality of silicon, the silicon solar cell panels are quite expensive. By contrast, thin-film solar panels, classified as second generation, are lighter, less expensive, and more flexible than the silicon solar panels, allowing for easier installation in versatile applications. Among the thin-film solar cells, made with newer and less established materials, are classified as third generation or next generation. The emerging third-generation solar cells include innovative technologies, such as perovskite, dye-sensitized, quantum-dot, organic, and semiconducting compound-based (e.g. CZTS, CZTSe, CIGS, and CdTe) thin-film solar cells. Among the third-generation solar cells, perovskite solar cells (PSCs) have emerged as a promising candidate for the next-generation solar cell industry with an unprecedented rise of power conversion efficiency (PCE) exceeding 26%. It is also worth noting that the best PCEs of tandem cells are 29.1% and 33.9%, respectively, with perovskite tandem cell and perovskite/Si (two terminals) tandem cell [1]. In PSCs, light-harvesting material is the perovskite that has the same crystal structure as the naturally occurring mineral calcium titanium oxide (CaTiO3 )

2

1 Introduction – Why Composites-Based Perovskite Solar Cells?

with an ABX3 crystal structure (see Chapter 2). It is worth noting that perovskites have remarkable characteristics for PV applications, such as direct bandgap, broad light-harvesting ability, high defect tolerance ability, long charge carrier diffusion length, and cost-effective easy fabrication. However, the lack of stability of perovskite materials is a well-known problem that degrades the performance of PSCs. One of the main reasons for instability is that the perovskite materials contain unstable elements due to extra weak interactions, such as van der Waals force and weak hydrogen bonds [2]. Moreover, the stability of perovskite technology depends on its environmental factors, such as humidity, heat accumulation, and continuous irradiation of sunlight [3]. Thus, the degradation of device performance is caused by external and internal factors. The former includes air-, thermal-, and photo-induced instability, and the latter includes intrinsic factors such as ions migration and interfacial recombination attributed to grain boundaries, contact interface, and vacancies. Solution-processed polycrystalline perovskite thin films present parasitic bulk and interface defects during the crystal growth process. In addition, their bulk and interface trap densities are higher than that of single-crystal perovskites. These bulk and interface defects often cause undesirable deep-level traps: undercoordinated Pb2+ ions, undercoordinated halide ions, metallic lead clusters, and intrinsic point defects (such as ion vacancies and Pb-I antisite defects) [4, 5]. Therefore, defects at the bulk grain boundaries (GBs) and at interfaces of perovskite polycrystalline thin films become major sources to induce shallow trap states and localize charge carriers through nonradiative recombination, which are detrimental to the efficiency and stability of PSCs. To overcome these defects, achieving a high-quality perovskite film and its defect passivation is crucial. Thus, tremendous efforts have been dedicated toward minimizing the perovskite GBs and surface/interface defects by additive engineering to induce the formation of perovskite-based composites. As the stability issues are mostly related to materials in terms of chemical, optical, and mechanical properties, to resolve the instability problems of PSCs, robust materials that can improve their chemical, electrical, optical, and thermal properties should be developed. The development of perovskite-based composites with composition engineering has been considered an efficient strategy to stabilize the structures of perovskite and further improve their optical and electronic properties. Recently, it has been reported that composite materials are efficient for solving or alleviating the stability issues of PSCs [6–20]. A composite material is a combination of two or more materials having different chemical and physical properties. Compared to traditional materials, composites can improve the properties of base materials and can be applied in many situations. Composite materials have advantages such as design flexibility, specialized chemical and physical properties, and resistance to a wide range of chemicals. Therefore, they may give benefits to solve critical issues related to the efficiency and operational stability of PSCs. Figure 1.1 illustrates the development strategy of composites-based perovskite solar cells (CPSCs) schematically, in which composite materials can be used for active layer (AL) and charge-transport layers. The incorporation of composite materials can significantly improve the PCE and stability of single-junction solar

1.2 Fabrication Strategy for Composites-Based Perovskite Solar Cells

Figure 1.1 Schematic illustration of the development strategy of composites-based perovskite solar cells.

cells as well as of tandem cells. This strategy is applicable to the development of both Pb-based and Pb-free CPSCs. To enhance the PCE and stability, it is crucial to design optimal light-absorbing and charge-transport materials along with interface engineering and additive engineering.

1.2 Fabrication Strategy for Composites-Based Perovskite Solar Cells In addition to stabilizing the AL with composite-based perovskites, interface engineering plays a crucial role because the interface contacts between the light-absorbing and charge-conducting materials are worthy of further study toward commercialization of PSCs in terms of interfacial energetics, charge transfer, and recombination kinetics, and interfacial degradation [17, 21, 22]. A typical PSC configuration consists of a perovskite film sandwiched between electron-transport layer (ETL) and hole-transport layer (HTL), which can form ETL/perovskite and perovskite/HTL interfaces, respectively. The carrier transport contacts and their interfaces determine device performance, including PCE, long-term stability, and J–V hysteresis. The separated holes and electrons have to transport across the interfaces in the device, but charge loss often occurs because of possible interfacial defects. GBs within the AL separating perovskite grains also induce recombination and provide moisture and oxygen penetration pathways, resulting in J–V hysteresis, device performance loss, and deterioration. Particularly, the interfacial degradation between absorber and contact materials has become a critical intrinsic factor, resulting in poor stability of PSCs. The ETL/AL and AL/HTL contacts may induce interfacial collapse of the perovskite structures because groups

3

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1 Introduction – Why Composites-Based Perovskite Solar Cells?

Figure 1.2 Schematic illustration of an efficient strategy for the fabrication of highly stable and efficient PSCs: inclusion of perovskite-based composites and interface engineering.

of perovskite atoms or interfaces may move or be disordered under real operation conditions such as humidity, air, heat, and UV [17, 23, 24]. Therefore, interfacial engineering is important to improve interfacial contact by interface modifications such as interlayers or multilayers, which form a stabilized interface between active and charge-collecting layers, ultimately improving the device performance and stability. The purpose of interface engineering is to modify perovskite contact and crystal growth, tune energy band alignment for reducing electron or hole transport barrier, lower recombination, enhance charge carriers transfer, and suppress interfacial defects and ions migration. Figure 1.2 presents an efficient strategy for fabricating highly stable and efficient CPSCs with the inclusion of functional perovskite composite materials and interface engineering. The composite materials include mixed cations and halides perovskite with composition engineering, 2D/3D perovskite composites, organic material–perovskite composites, and inorganic material–perovskite composites. The perovskite-based composites can significantly regulate the optical and electronic properties of perovskites and facilitate the growth of perovskite grains and carriers transport, but they inhibit ion migration and reduce defect formation. Besides, the organic or inorganic materials incorporated in perovskite composites also remarkably enhance stability because of their protective effects and strong chemical interaction and cross-linking behavior. The 2D/3D perovskite composites significantly enhance device stability without sacrificing the considerable performance of PSCs. Furthermore, the organic or inorganic substances contained in perovskite composites significantly improve their stability thanks to their protective

References

effects and strong chemical interactions and crosslinking with perovskite molecules. In addition, the strategy of interface stabilization can improve interfacial contact, assist the growth of perovskite grains, passivate perovskite film surface, facilitate charge transport, suppress ions migration, and protect perovskite films from water and oxygen molecules. Thus, it can be concluded that the strategy utilizing both perovskite-based composites and interfacial engineering is one of the most efficient ways to achieve high-efficiency and stable perovskite photovoltaics.

References 1 Best Research-Cell Efficiency Chart, NREL. https://www.nrel.gov/pv/cellefficiency.html (accessed 1 July 2024). 2 Min, H., Lee, D.Y., Kim, J. et al. (2021). Perovskite solar cells with atomically coherent interlayers on SnO2 electrodes. Nature 598 (7881): 444–450. 3 Kung, P.K., Li, M.H., Lin, P.Y. et al. (2018). A review of inorganic hole transport materials for perovskite solar cells. Adv. Mater. Interfaces 5: 1800882. 4 Chen, B., Rudd, P.N., Yang, S. et al. (2019). Imperfections and their passivation in halide perovskite solar cells. Chem. Soc. Rev. 48: 3842. 5 Ju, H., Ma, Y., Cao, Y. et al. (2022). Roles of long-chain alkylamine ligands in triple-halide perovskites for efficient NiOx-based inverted perovskite solar cells. Sol. RRL 6: 2101082. 6 Wang, Y., Rho, W.-Y., Yang, H.-Y. et al. (2016). Air-stable, hole-conductor-free high photocurrent perovskite solar cells with CH3 NH3 PbI3 -NiO nanoparticles composite. Nano Energy 27: 535–544. 7 Mahmoudi, T., Seo, S., Yang, H.-Y. et al. (2016). Efficient bulk heterojunction hybrid solar cells with graphene-silver nanoparticles composite synthesized by microwave-assisted reduction. Nano Energy 28: 179–187. 8 Wang, Y., Mahmoudi, T., Rho, W.-Y. et al. (2017). Ambient-air-solution-processed efficient and highly stable perovskite solar cells based on CH3 NH3 PbI3-x Clx -NiO composite with Al3 O3 /NiO interfacial engineering. Nano Energy 40: 408–417. 9 Mahmoudi, T., Wang, Y., and Hahn, Y.-B. (2018). Graphene and its derivatives for solar cells application. Nano Energy 47: 51–65. 10 Wang, Y., Mahmoudi, T., Yang, H.-Y. et al. (2018). Fully-ambient-processed mesoscopic semitransparent perovskite solar cells by islands-structure-MAPbI3-x Clx -NiO composite and Al2 O3 /NiO interface engineering. Nano Energy 49: 59–66. 11 Yang, H.-Y., Rho, W.-Y., Lee, S.K. et al. (2019). TiO2 nanoparticles/nanotubes for efficient light harvesting in perovskite solar cells. Nanomaterials 9: 326. 12 Mahmoudi, T., Wang, Y., and Hahn, Y.-B. (2019). Stability enhancement in perovskite solar cells with perovskite/silver−graphene composites in the active layer. ACS Energy Lett. 4: 235–241. 13 Wang, Y., Mahmoudi, T., Rho, W.-Y., and Hahn, Y.-B. (2019). Fully-ambient-air and antisolvent-free-processed stable perovskite solar cells with perovskite-based composites and interface engineering. Nano Energy 64: 103964.

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14 Wang, Y., Mahmoudi, T., and Hahn, Y.-B. (2020). Highly stable and efficient perovskite solar cells based on FAMA-Perovskite-Cu:NiO composites with 20.7% efficiency and 80.5% fill factor. Adv. Energy Mater. 10: 2000967. 15 Mahmoudi, T., Wang, Y., and Hahn, Y.-B. (2020). SrTiO3 /Al2 O3 -graphene electron transport layer for highly stable and efficient composites-based perovskite solar cells with 20.6% efficiency. Adv. Energy Mater. 10: 1903369. 16 Mahmoudi, T., Wang, Y., and Hahn, Y.-B. (2021). Highly stable perovskite solar cells based on perovskite/NiO-graphene composites and NiO interface with 25.9 mA/cm2 photocurrent density and 20.8% efficiency. Nano Energy 79: 105452. 17 Wang, Y., Arumugam, G.M., Mahmoudi, T. et al. (2021). A critical review of materials innovation and interface stabilization for efficient and stable perovskite photovoltaics. Nano Energy 87: 106141. 18 Mahmoudi, T., Rho, W.-Y., Kohan, M. et al. (2021). Suppression of Sn2+ /Sn4+ oxidation in tin-based perovskite solar cells with graphene-tin quantum dots composites in active layer. Nano Energy 90: 106495. 19 Mahmoudi, T., Kohan, M., Rho, W.-Y. et al. (2022). Tin-based perovskite solar cells reach over 13% with inclusion of N-doped graphene oxide in active, hole-transport and interfacial layers. Adv. Energy Mater. 12: 2201977. 20 Kohan, M., Mahmoudi, T., Wang, Y. et al. (2023). SnO2 /BaSnO3 electron transport materials for stable and efficient perovskite solar cells. Appl. Surf. Sci. 613: 156068. 21 Fakharuddin, A., Schmidt-Mende, L., Garcia-Belmonte, G. et al. (2017). Interfaces in perovskite solar cells. Adv. Energy Mater. 7: 1700623. 22 Cho, A.-N. and Park, N.-G. (2017). Impact of interfacial layers in perovskite solar cells. ChemSusChem 10: 3687–3704. 23 Berhe, T.A., Su, W.-N., Chen, C.-H. et al. (2016). Organometal halide perovskite solar cells: degradation and stability. Energy Environ. Sci. 9: 323–356. 24 Bai, Y., Meng, X., and Yang, S. (2017). Interface engineering for highly efficient and stable planar p-i-n perovskite solar cells. Adv. Energy Mater. 8: 1701883.

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2 Hybrid Perovskites and Solar Cells 2.1

Perovskite Materials

A perovskite is a material that has the same crystal structure as the naturally occurring mineral calcium titanium oxide (CaTiO3 ). It was first discovered by German mineralogist Gustav Rose in 1839 and was named after Russian mineralogist L. A. Perovski. Generally, a perovskite has a three-dimensional (3D) structure ABX3 , where the cation A is stabilized in a cubo-octahedral cage formed by the 12 nearest X anions, and the cation B coordinates with the six nearest X anions to form octahedral geometry [1]. Usually, artificial organic–inorganic hybrid perovskites are a broad class of materials adopting a general chemical formula of ABX3 (A = monovalent cation, such as FA+ , MA+ , Cs+ ; B = metallic divalent cations, such as Pb2+ and Sn2+ ; X = halide anions such as I– , Br– , and Cl– ) as shown in Figure 2.1a. To establish a stable and cubic phase, the tolerance factor (t) of perovskite should be in a range of 0.8 ≤ t ≤ 1.0. √ The value of t can be calculated by the Goldschmidt equation t = (rA + rX )∕ 2(rPb + rX ), where r is the ionic radius [3].

2.1.1

Three-Dimensional Perovskites

2.1.1.1 Lead-Based Perovskites

As discussed above, the hybrid perovskites have a general chemical formula of ABX3 . The B site metallic divalent cations such as Pb, Sn, and Ge ions but Pb-based perovskites are still indispensable to ensure excellent photoelectric properties. Although molecular orbitals of APbI3 and ASnI3 are similar as shown in Figure 2.1b, the band edges in APbI3 are more strongly bound than those of ASnI3 , thus leading to the difference of the band edge positions (band offsets) [4]. Since the energy levels of the Pb 6s and Pb 6p atomic orbitals are both lower than those of the Sn 5s and Sn 5p orbitals, the CBM and VBM of APbI3 are both higher. Compared to the inert electrons in Pb 6s orbitals, the active electrons in high-energy Sn 5s orbitals are unstable and easy to lose. Besides, the redox potential of Sn2+ /Sn4+ (0.15 V) is lower than that of Pb2+ /Pb4+ (1.60) [2]. That is why Sn2+ is easily oxidized to Sn4+

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(a)

(b)

Figure 2.1 (a) Typical 3D hybrid perovskite structure ABX3 (A = organic cation, B = metal cation, and X = halide anion) and (b) molecular orbitals for Pb- and Sn-based perovskites. Source: Adapted from [2].

under ambient conditions, leading to high trap densities, short charge carriers, and diffusion lengths in ASnI3 . As it is well known, pure Pb-based hybrid perovskites show intriguing optical and electronic properties, such as high carrier mobility and absorption coefficient, tunable band gap, long charge diffusion lengths, low exciton binding energy, and broad light absorption wavelength [5–8]. With the advent of solid-state pure Pb-based perovskite solar cells (PSCs) since 2012 [9, 10], it has achieved great success in the field of third-generation/emerging photovoltaics [11–15]. 2.1.1.2 Lead–Tin-Mixed Perovskites

Due to the similar outer electronic structure, Sn can be used to partially replace Pb to develop a narrow bandgap Pb–Sn-mixed perovskite (∼1.17 eV). Low-bandgap Sn–Pb perovskites with mixed organic cations (MA/FA) also show high-quantum efficiency and decent storage stability because of the low bandgap of ∼1.2 eV and long carrier lifetime over hundreds of nanosecond (ns) [16]. The band gap and absorption wavelength of Pb–Sn-mixed perovskites can be remarkably adjusted by Sn-doping [17]. Among various compositions, (FASnI3 )1-x (MAPbI3 )x showed good optoelectronic properties with the highest performance for low bandgap stable perovskite solar cells [18]. Halide (Cl and Br) incorporation in Pb–Sn-based perovskite helps to passivate grain boundary via improved electronic properties and reduced recombination, leading to high power conversion efficiencies (PCEs) [19]. Although the stability of Pb–Sn-mixed perovskites is poor compared to pure Pb-based perovskites, they are promising to combine with wide-gap perovskites yielding all-perovskite tandem solar cells with higher efficiency. For example, 2-T monolithic all-perovskite tandem cell using low-bandgap Sn–Pb PSCs was first reported by using a FA0.83 Cs0.17 Pb(I0.7 Br0.3 )3 (Eg = 1.8 eV) top cell and FA0.75 Cs0.25 Sn0.5 Pb0.5 I3 (Eg = 1.2 eV) bottom cell, achieved a PCE of 19.1% with high V oc of 1.81 V and J sc of 14.8 mA/cm2 [20]. More importantly, all-perovskite tandem cells based on Sn-Pb

2.1 Perovskite Materials

PCE (%)

25

Si

2.7

ASnl3

2.4

ASnl3 APbl3

60 APbl3

50

2.1 VOC (V)

30

3.0 32.1%

20 15

1.8 1.5 1.2

0.9

10

40

32.9 29.0

30

1.1 1.2

20

0.6

5

10

0.3

0

0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0 2.1 2.2 2.3 2.4 2.5

Eg (eV)

JSC (mA/cm2)

33.4%

35

0

0.0

0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0 2.12.2 2.3 2.4 2.5

Eg (eV)

Figure 2.2 Shockley–Queisser efficiency limit, and V oc and Jsc . Reproduced with permission from [22]. Copyright 2021, American Chemical Society.

perovskites with an ideal bandgap can obtain a theoretical efficiency of over 40%, which is higher than single-junction PSCs with around 32% efficiency limit [21, 22]. 2.1.1.3 Tin-Based Perovskites

Although Pb or Pb–Sn-based perovskites have shown great potential for high-performance single-junction or multi-junction solar cells, the lead leakage from PSCs can be toxic to the environment, humans, and other species, which may impede the commercialization of this technology. With this in mind, Pb-free perovskites, such as Tin (Sn), germanium (Ge), antimony (Sb), and bismuth (Bi), have been widely studied for solar cell applications though they have shown poor efficiency and stability. Among them, Sn has a crystal structure similar to that of Pb and is considered the most promising candidate to replace Pb. According to the Shockley– Queisser (S–Q) limit, pure Sn-based PSCs also have a theoretical PCE of 33.4%, even higher than Pb-based PSCs with a PCE of 32.1% (Figure 2.2, left) [23]. Due to the excellent optoelectronic properties including large carrier mobility and strong light absorption coefficients, the theoretical efficiency of Sn-PSCs can be >33% with an ideal V oc of 1.1 V and a J sc of 32 mA/cm2 (Figure 2.2, right). Unluckily, Sn-PSCs have achieved a certified record PCE of 14.6% so far [23], which is only 43.7% of the theoretical maximum PCE. The lower PCEs are mainly attributed to the following reasons: (1) Ease oxidation of Sn2+ as discussed in Section 2.1.1.1. (2) Uncontrollable crystallization of ASnI3 : due to the active 5s electrons in Sn, SnI2 has a higher Lewis acidity than that of PbI2 , thus the reaction speed between SnI2 and Lewis bases such as MAI and FAI is faster than that of PbI2 [22]. (3) Sn vacancy defects: because of the higher energy level of Sn 5s, the Sn-I bonds are easy to break, leading to the formation of Sn vacancy defects [24]. Meanwhile, the weak bond of Sn−I also facilitates the formation of H−I and Sn−O bonds under ambient conditions (H2 O and O2 ) [25].

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(4) Interface recombination: due to the easy oxidation of Sn2+ , the film surface defects are easily formed. Thus, a protection layer is needed to suppress the interface recombination [26]. (5) Band alignment mismatch: The unique band structure of Sn-perovskite requires a distinctive structure design to reduce energy band alignment mismatch. An inverted structure is highly desired due to better compatibility and band alignment [27]. 2.1.1.4 All Inorganic Perovskites

All-inorganic halide perovskitescan be formed when organic cation site A is replaced by inorganic cation Cs in ABX3 , such as CsPbI3 , CsPbBr3 , and CsPbCl3 . Note that the cubic α-phase CsPbI3 is the most prominent candidate for photovoltaic applications due to the appropriate Eg of 1.73 eV. However, the cubic α-phase CsPbI3 is only stable at a high temperature, and it rapidly transforms into a yellow orthorhombic non-perovskite δ-phase with Eg of 2.82 eV, resulting in poor photovoltaic properties at room temperature [28]. Although CsPbBr3 presents the best phase stability among all Cs-based all inorganic perovskites, the wide Eg of 2.3 eV limits the photocurrent and thus makes it difficult to achieve a higher PCE in solar cells. As reported, partial substitution of iodine (I) with bromide (Br) can build mixed-anion CsPbI2 Br (Eg = 1.92 eV) and CsPbIBr2 (Eg = 2.05 eV). However, such higher bandgaps severely limit the photocurrent, leading to lower PCEs [29]. Therefore, in-depth experimental and theoretical studies on the chemical and physical properties of all inorganic perovskites are needed to further improve the efficiency and the long-term stability of inorganic photovoltaics under operational conditions.

2.1.2

Low-Dimensional Perovskites

The 3D crystal perovskites present poor stability when exposed to ambient conditions. The instability is mainly attributed to organic compounds and phase transformation. However, perovskites are endowed with dimensional tailoring properties for more stable low-dimensional structures. For instance, 2D perovskites with natural multi-quantum-well structures have become a class of emerging optoelectronic materials due to their unique properties, such as impressive chemical diversity, tunable optoelectronic properties, and superior structure stability. 2D perovskites have the general formula R2 An-1 Bn X3n+1 , where R is a bulky organic cation that plays the role of spacer between the inorganic framework, A is a small monovalent cation (such as FA and MA), B is a divalent metal cation (such as Pb2+ and Sn2+ ), X is a halide anion, and n is the thickness of inorganic perovskite layers between adjacent organic spacer layers. 2.1.2.1 Ruddlesden–Popper (RP) 2D Perovskites

Adopting alkylammonium cations (such as butylammonium (BA) and phenylethylamine (PEA)) in the R position results in a layered two-dimensional (2D) perovskite structure (RP phase). As shown in Figure 2.3 (top) [30], a van der Waals gap and hydrogen bonds can be formed in the spacer layer in the RP phase (when n = 3).

2.1 Perovskite Materials

Figure 2.3 Band gaps of 2D (n = 1) and quasi-2D BA2 MAn−1 Pbn I3n+1 (BA: butylammonium) for different n values and corresponding to their alignment of the energy levels and band gaps. Source: Adapted from [30].

The spacers used in 2D RP perovskites mainly include aliphatic and aromatic spacer cations. Properties of 2D RP perovskites such as the energy levels, exciton binding energy, and charge transport mobility can be tuned by changing the organic spacers and the n-values. As shown in Figure 2.3 (bottom), the quasi-BA2 MAn-1 Pbn I3n+1 can give various bandgaps and optical properties depending on the number of layers. When n ≤ 2, the 2D perovskites have a high bandgap and high exciton binding energy up to hundreds of meV. Thus, the 2D perovskites are difficult as light absorbers and unsuitable for PVs. However, they exhibit strong excitonic behavior attributed to quantum confinement effects, which can yield high photoluminescence and make them suitable for LED applications. 2.1.2.2 Dion–Jacobson (DJ) 2D Perovskites

In contrast to 2D RP perovskites with monovalent spacers (Figure 2.4, top), DJ type perovskite structure has a divalent interlayer spacers (such as propane-1,3-diammonium (PDA) and butane-1,4-diammonium (BDA)) to bridge the neighboring inorganic slab with hydrogen bonding by monolayer bivalent organic cations, removing the van der Waals gap (n = 3, Figure 2.4, bottom) [8]. The divalent spacers with

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Figure 2.4 The 2D structure of Ruddlesden–Popper (RP) (top) and Dion–Jacobson (DJ) perovskites, n = 3, bottom). Source: [31]/John Wiley & Sons/CC BY 4.0.

similar lengths in 2D-DJ perovskites exhibit shortened interslab distance and form a more rigid structure, which could reduce the charge transport barrier. Also, 2D-DJ perovskites have less lattice distortion than the 2D-RP type, leading to better optoelectrical performance. Therefore, DJ perovskites possess better stability compared to RP perovskites. For example, to remove the van der Waals gap in 2D perovskites, Li et al. developed DJ phase 2D perovskites by incorporating diammonium cations (PDA) into MAPbI3 [32]. Compared to RP 2Dperovskites incorporated propylamine (PA) cations-based solar cells (PCE = 8.8%), DJ phase 2D perovskites-based solar cells showed improved efficiency of 13.3%. Also, DJ-2D PSCs showed better device stability than the RP counterpart-based device due to strengthening the 2D layered perovskite structure by the hydrogen bonding interactions. 2.1.2.3 One-/Zero-Dimensional (1D/0D) Perovskites

1D/0D perovskites can be synthesized by exceptional structural tunability with proper components. As shown in Figure 2.5, stable 1D structures have the metal halide octahedrons connected in a chain by means of shared corners, edges, or faces, and surrounded by organic cations; 0D perovskites have individual metal halide octahedral anions or metal halide clusters which are completely surrounded and isolated by the organic cations [33]. Such 1D/0D perovskites show unique photophysical properties owing to strong quantum confinement, high exciton binding energy, and largely Stokes-shifted broadband emissions from self-trapped excitons,

2.1 Perovskite Materials

Figure 2.5 The structure of 0D and 1D perovskites. Source: [33]/John Wiley & Sons/CC BY 4.0.

resulting in great potential for LEDs, photodetectors, etc. [34]. For example, Yuan et al. reported 1D-perovskites C4 N2 H14 PbBr4 , in which the edge-sharing halide octahedral chains [PbBr4 2− ] are surrounded by the organic cations C4 N2 H14 2+ to form quantum wires. Such 1D-based LEDs give efficient bluish white light emissions with 20% photoluminescence quantum efficiencies [35]. Besides, 1D/0D perovskites also are selected as a passivated or protected layer for highly efficient and stable PSCs due to their excellent material stability properties against humidity, light exposure, and heat stress [36, 37].

2.1.3

Single-Crystal Perovskites

Compared with polycrystalline perovskites, single-crystal perovskites without grain boundaries (GBs) show better optoelectronic properties, that is, extended absorption spectrum, lower band gaps, longer carrier diffusion length, lower trap densities, and suppressed ion migration effect [38]. Due to the well-aligned lattice structures and no GBs effect, the single-crystal perovskites also show much-enhanced device stability compared to their polycrystalline counterparts. More importantly, single-crystal PSCs present a higher theoretical PCE and J sc than polycrystalline PSCs [39], as shown in Figure 2.6. However, the efficiency of single-crystal PSCs is lower than that of polycrystalline PSCs partly because of challenges in crystal growth, thickness control, and solar cell device integration. Besides, single-crystal perovskites also present great potential in other fields, such as photodetector, laser, and LED [40].

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Figure 2.6 The S–Q theoretical limit for the PCE and Jsc of single-junction solar cells as a function of the bandgap for single-crystal GaAs and FA0.6 MA0.4 PbI3 , and polycrystalline FAPbI3 . Source: Reproduced with permission from [39]/Royal Society of Chemistry.

2.1.4

Dynamics of Perovskite Crystal Growth

As it is well known, the crystal morphology of the perovskite absorber layer remarkably affects the photovoltaic performance of PSCs. Unluckily, 3D perovskite films that are generally obtained by a simple solution-processed deposition technology always present poor morphologies with incomplete surface coverage and many voids, which are detrimental to solar cell efficiency and stability because of inefficient light-harvesting, leakage paths, moisture invasion, etc. [41]. Such film defects also provide a route to accelerate ion migration of halide anions (I− ) and escape of organic cations (MA+ and FA+ ) from the perovskite absorber layer. Those factors not only lead to notorious J-V hysteresis in solar cells but also erode top electrodes and finally burn in the structure of perovskites [42]. Furthermore, the inhomogeneity of perovskite films becomes worse while enlarging area of films by scalable fabrication methods [43, 44]. Thus, understanding the growth and formation mechanisms of perovskite crystals is of paramount importance to obtain high-quality and homogenous large-area perovskite films by scalable fabrication technology. The formation of polycrystalline perovskite films from precursor solutions generally relates to three important steps, that is, nucleation, growth and diffusion of the crystals. Figure 2.7a systematically presents the process of perovskite film formation from a wet film to a complete film during heat treatment. A La Mer mechanism has been applied to elaborate the nucleation and crystallization of perovskite films by solution coating methods, as shown in Figure 2.7b. During the evaporation of the solvents (such as DMF and DMSO) in stage I, the solution concentration increases

2.1 Perovskite Materials

(a)

(b)

(c)

Figure 2.7 (a) The nucleation and growth processes of perovskite films. (b) Lar Mer model for nucleation and growth of perovskite films, C s , C c , and C limit are supersaturation concentration, critical concentration, and limiting supersaturation concentration of a precursor solution, respectively. (c) Free energy diagram of nucleation, ΔG, ΔGs , ΔG𝜈 , ΔGc , and r c are total free energy, surface free energy, bulk-free energy, critical free energy, and critical radius of nucleus. Source: Adapted from [45].

to reach supersaturation concentration (Cs ) at a time of t1 ; meanwhile, the nuclei start to take place until the critical concentration (Cc ) at a time of t2 ; then, both nucleation and crystal growth occur in stage II; in stage III, the solvent evaporation is slower than the consumption of solute, thus the solution concentration is lower than the Cs , where only growth of the formed nuclei happens without formation of additional nuclei [45]. According to the classical theory of homogenous nuclei formation, the nucleation rate can be determined based on a critical free energy (ΔGc ), as shown in Figure 2.7c. The nucleation rate depends on the number of nuclei (N), which can be described by using an Arrhenius equation [45, 46]: ( ) ΔGc dN = A exp − (2.1) dt kB T where N, t, A, kB , and T represent the number of nuclei, nuclei time, pre-exponential factor, Boltzmann’s constant, and temperature, respectively. The critical free energy (ΔGc ) is further determined by surface energy 𝛾, mole volume 𝜈, and supersaturation of solution S according to the following equation [45, 46]: ( ) 16𝜋𝛾 3 ν2 dN = A exp (2.2) dt 3kB3 T 3 (lnS)2

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During the film formation, the free energy of heterogeneous nucleation is equal to the energy formation of homogenous nucleation and a correction term ∅ [45, 46]. ΔGhetero = ∅ΔGhomo c c

(2.3)

The correction term ∅ is determined by the contact angle of the solution on a substrate (𝜃) according to the following: equation [45, 46]: (2 + cos𝜃)(1 − cos𝜃)2 (2.4) 4 According to the above equations, perovskite nucleation depends on surface energy, temperature, supersaturation level, and wettability of the substrate. According to previous research experience on the fabrication of perovskite films, the supersaturation level is an important factor that remarkably affects the perovskite crystal size and growth quality. ∅=

2.2 Perovskite Solar Cells Compared with traditional PVs and PSCs based on earth-abundant organometal halide perovskites have shown superb potential for the fabrication of cost-benefited, simple solution-processed, and high-performance next-generation solar cells. The advent of PSCs has opened up great PV developments due to impressive PCE beyond 25% in single solar cells and above 30% in tandem solar cells. Undoubtedly, PSCs have a great opportunity to become a game-changer in the photovoltaics market.

2.2.1

Working Principles of Perovskite Solar Cell

The original structure of the PSC is derived from DSSC, but there are some differences in operating principles. Usually, DSSCs require a semiconductor oxide as a scaffold, but PSCs based on insulating mesoporous Al2 O3 or planar heterojunction with compact TiO2 also work well, suggesting that electrons and holes can be transported through the perovskite layer. As widely accepted today, the working principles of PSCs are more like solid-state p-n junction solar cells. The presence of a continuous perovskite layer in charge-transport layers is important for enhancing light harvesting and conversion efficiency of photons to electrons. The operation principle of PSC is more consistent with those of n-i-p and p-i-n solar cells, where perovskites act as intrinsic absorbers sandwiched between two transport layers (electron-transport layer (ETL) and hole-transport layer (HTL)). As shown in Figure 2.8, the working principles of PSCs mainly include (i) the perovskite layer absorbs photons to generate electrons and holes (carrier generation process); (ii) the electrons and holes are selectively collected and transported by the n-type ETL and p-type HTL (charge separation and transport process), respectively; (iii) then current is produced by an external circuit connect (charge collection). Besides, the quasi-Fermi levels (QFLs) between transport layers and perovskite layer for n-i-p or

2.2 Perovskite Solar Cells

Figure 2.8 Perovskite solar cell working mechanism: charge generation, charge transport, and charge recombination.

p-i-n type PSCs show different influences on V oc . Thus, a mismatch in energy band diagram levels severely affects the interface charge-transport efficiency, resulting in lower PCEs. In other words, the CB of n-type ETL and the VB of p-type HTL have to match the CB and VB of the perovskite layer, respectively.

2.2.2 Configurations of Perovskite Solar Cell After decades of development for PSCs, there have been four types of configurations: traditional n-i-p, inverted p-i-n, traditional hole-conductor free (HCF), and inverted HCF PSCs, as shown in Figure 2.9. Currently, both traditional n-i-p and inverted p-i-n structures can achieve over 25% efficiency for lab-scale cells developed by many research groups. In the following, the structures, performances, and advantages of those four types of solar cell configuration will be elaborated.

Figure 2.9

Typical configurations of perovskite solar cell reported.

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2.2.2.1

n-i-p-Based Traditional Structure

As discussed earlier, the first solid-state PSC was an n-i-p architecture, but there was no perovskite capping layer at that time. In fact, the traditional n-i-p configurations have two types of architecture, that is, mesoporous- and planar-based structures. In general, mesoporous type PSC comprises an electron-transport layer (ETL) scaffold (such as TiO2 and Al2 O3 ) with a compact ETL, where perovskite precursor can infiltrate into the scaffold, forming a perovskite-metal oxide composite layer. Thus, the light absorber layers include both perovskite capping and composite layers, leading to high light-harvesting ability. The traditional planar n-i-p type architecture does not have an ETL mesoporous scaffold or uses a full-organic charge-transport layer. The first n-i-p planar type PSCs were reported by Snaith et al., which only used compact TiO2 (c-TiO2 ) as the ETL and followed by deposition of the perovskite layer by a dual-source vapor method. Interestingly, compact SnO2 ETL-based planar PSCs have been considered the most promising solar cell architecture to achieve over 30% efficiency. For example, You et al. developed SnO2 ETL-based planar solar cells with a surface-passivation strategy, achieving a certificated efficiency of 23.32% by NREL in 2019 [47]. Afterward, Seo et al. reported SnO2 ETL-based planar solar cells with a certified efficiency of up to 25.2% by improved carrier management [48]. Subsequently, Soek et al. achieved a PCE of 26.08% (certified 25.73%) SnO2 ETL-based planar solar cells by interface modification strategy between SnO2 ETL and perovskite active layer [49]. Such results suggest that SnO2 can be selected as one of the most effective ETLs in perovskite planar solar cells, which is mainly ascribed to the excellent optical and electronic properties of SnO2 . 2.2.2.2 p-i-n-Based Inverted Structure

Similarly, the inverted p-i-n structure also has two kinds of architectures: mesoporous and planar-based structures. Usually, NiOx nanoparticles (NPs) can be used as scaffolds in inverted mesoporous PSCs. The perovskite precursor can infiltrate into the scaffold, forming the perovskite-NiOx NPs composite layer. Wang et al. first reported NiOx -based mesoscopic PSC in 2014, which resulted in 9.51% efficiency [50]. Afterward, efficiencies over 17% were achieved by rational design of mesoporous NiOx layer and metal-doped NiOx layer, respectively [51, 52]. However, the efficiency is limited (1.7 eV)

Low bandgap perovskites (1.5–1.6 eV)

PCE(%)

i-PCE (%)

45 Area: 0.14cm2 (Flexible, 41.23%)

40

35

Area: 12.96 cm2 (minimodule, 43.54%)

Area: 0.08cm2 (40.1%)

Area: 0.1 cm2 (36.2%)

Area: 0.105 cm2 Area: 0.0725cm2 (35.6%) (Flexible, 31.85%)

Area: 0.09cm2 (35.2%)

Area: 12.96 cm2 (minimodule, 36.36%, Certified)

30 SCU

(e)

SNNU

SKKU

SCU

CityU

HKU

JNU

JNU/MEC

JNU/MEC

Organization

Figure 6.19 (a) The configuration with a cross-sectional view of a solar module; (b) photograph and microscope images of the P1, P2, and P3 scribe lines in the dead area of PSMs; (c) current–voltage (I–V ) curves under reverse and forward scans for control (Mc) and passivated (Mp) perovskite solar minimodules under LED illumination (1000 lx and 3000 K); (d) the Shockley–Queisser (S–Q) limit of PCE of an ideal photovoltaic device as a function of band-gap energy under LED illumination (1000 lx and 3000 K); and (e) summarized i-PCE of representative works using different band-gap perovskites. Source: Reproduced with permission from [61]/Elsevier.

corresponding microscope images of the P1, P2, and P3 scribe lines in the dead area. The P1, P2, and P3 designs and their laser scribing technology play important roles in obtaining excellent interconnection and high geometric filling factor (GFF) for overcoming cell-to-module (CTM) efficiency losses [62]. After optimizing line patterns and scribing parameters, a narrow dead area of around 260 μm was achieved, leading to a high GFF of 94%. Thus, the passivated module based on 2D–3D perovskite heterostructures under AM1.5G 100 mW/cm2 irradiation shows a

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6 Composite-Based Pb-Perovskite Materials as Absorbers

PCE of 19.20% with a high V oc of 7.40 V (average V oc of 1.233 V for each subcell) and FF of 80% (Figure 6.19c). Furthermore, the indoor performance of WBG-perovskite solar minimodules without encapsulation was further carried out under LED illumination (1000 lx, 3000 K, and 301.9 μW/cm2 ) in ambient conditions. As shown in Figure 6.19d, the Mp is capable of achieving a record i-PCE of 43.54% with a high V oc of 6.49 V (average V oc of 1.08 V for each subcell) and FF of 80%, which are the best indoor photovoltaic parameters so far (Figure 6.19e).

6.3.2

1D-3D Composites

The success of 2D–3D composite-based perovskites in the fabrication of highly efficient and stable PSCs has motivated researchers to develop 1D or 0D perovskites due to their excellent material stability properties against humidity, light exposure, and heat stress. Fan et al. first reported 1D–3D hybrid perovskite heterojunctions for the fabrication of PSCs, which exhibited a high PCE of 18.1% with excellent air and thermal stability under an ambient environment of 50% humidity and heat treatment at 85∘ C [63]. Besides, compared to the rapid degradation of pristine 3D perovskite-based devices, the 1D-3D perovskite-based cells showed thermodynamic self-healing capability. The 1D–3D perovskite composite films dramatically passivate interface traps, suppress ion migration in heterojunction regions, and protect the 3D perovskite crystal grains from moisture. Furthermore, they reported a layer-by-layer arrangement of 3D–1D–3D heterostructures that can significantly improve charge transport and reduce non-radiative recombination [64].

6.3.3

0D–3D Composites

Inspired by the 2D perovskites for defect passivation of 3D perovskites, Chen et al. [65] prepared 1D ([p-(C8 H14 N2 )]Pb2 I6 ) and 0D ([m-(C8 H14 N2 )]2 PbI6 ) perovskitoids on the 3D perovskite films to form 1D–3D and 0D–3D heterostructures by reacting 4-bis(aminomethyl)benzene dihydroiodide (p-PBAI2 ) and 1,3-bis(aminomethyl) benzene dihydroiodide (m-PBAI2 ) with extra PbI2 , respectively, as shown in Figure 6.20a. In detail, p- or m-PBAI2 salts were dissolved in the isopropyl alcohol (IPA) as co-antisolvent to build intermediate 1D–3D or 0D–3D composite-based perovskite phases. It can be clearly seen that plate-like morphology with a characteristic size of several hundred nanometers on the 3D surface (Figure 6.20b–d), indicating the formation of 1D and 0D perovskitoids. GIWAXS results with obvious 1D and 0D phases (Figure 6.20e) further confirm this conclusion. Powder HRTEM images of 3D–1D and 3D–0D show that both 3D perovskite, 1D, and 0D perovskitoids were successfully formed and closely linked with each other (Figure 6.20f,g). More importantly, 1D or 0D perovskitoids present low defect densities and can withstand relatively high lattice strains, which can serve as blocking channels for undesired interface non-radiative recombination and material degradation. Simultaneously, 1D and 0D perovskitoids can passivate the surfaces and GBs in 3D perovskite films.

6.4 Quantum Dot (QD) Additives-Based Perovskite Composites

(a)

Perovskite precursor solution

3D/1D or 3D/0D

Intermediate phase

perovskite film

IPA + organic salt antisolvent

Substrate

Annealing

Substrate

(b)

Substrate

(c)

(d)

2 μm (e)

0D phase (11–1)

IPA + m-PBAI2

1D phase (002)

2 μm (f)

3D phase (100)

3D phase (100)

6

δ phase PbI2 8 10 q (nm–1)

(g) 3D 1D

IPA + p-PBAI2

IPA

2 μm

0D 3D

3D phase (100)

12

2 nm

5 nm

Figure 6.20 (a) Formation processes of 3D/1D or 3D/0D heterostructures, (b–d) SEM of 3D, 3D/1D and 3D/0D surface morphologies, (e) GIWAXS of 3D, 3D/1D and 3D/0D, and (f,g) HRTEM images of 3D/1D and 3D/0D. Source: Reproduced under Creative Commons Attribution 4.0 International License from [65]/American Association for the Advancement of Science.

Sum et al. prepared zinc-based 0D perovskites (PEA2 ZnX4 , X = Cl, I) instead of lead by reacting PEA organic cation and zinc halides [66]. The energy levels of the 0D PEA2 ZnX4 align better with the 3D absorber than the Pb-based perovskites (PEA2 PbX4 ), thus facilitating electron transfer across the perovskite/contact layer junction. The 0D PEA2 ZnX4 also has a deep valence band edge that effectively blocks the transfer of holes, which helps charge separation. Moreover, the 0D PEA2 ZnX4 can also passivate the defects presented at 3D perovskites and thus suppress trap-assisted recombination [67]. Finally, as-fabricated PSCs based on such 0D–3D composite-based perovskites achieved a PCE of 24.1% (certified PCE of 23.25%). Further, the devices also showed long-term operational stability, retaining 94.5% of the initial PCE after 1009 h of operation at MPP tracking under continuous 1 sun illumination.

6.4 Quantum Dot (QD) Additives-Based Perovskite Composites Quantum dots (QDs) have emerged as promising functional materials for improving the stability and efficiency in PSCs due to their high photoluminescence

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6 Composite-Based Pb-Perovskite Materials as Absorbers

(PL) quantum yields and low excitation energies. QDs as additives in perovskite precursors or interlayers not only can significantly induce the growth of perovskite crystals but also passivate perovskite film defects. Especially, inorganic perovskite QDs (CsPbX3 ) with similar experimental characteristics and lattice parameters, and better chemical stability are more suitable to be incorporated into perovskite layers.

6.4.1

Perovskite QD-Based Composites

Perovskite quantum dots (PQDs) are a class of solution-processable semiconductors with tunable and strong light absorption spectrum and high quantum yields. Due to their excellent optical and electronic properties, PQDs also have been used to passivate defects presented in 3D perovskites. For example, inorganic PQDs (CsPbBrCl2 ) with a size ∼7 nm uniformly crossed a surface of MAPbI3 perovskite film to form composite-based MAPbI3 -PQDs film (Figure 6.21a) [68]. Such a strategy can effectively passivate defects at the perovskite/HTL interface and interfacial recombination. The influences of quasi-Fermi level from the passivation strategy of CsPbBrCl2 PQDs effectively mitigate the energy disorders of MAPbI3 , narrow the band-tail electronic states, and reduce the mid-gap states of MAPbI3 (Figure 6.21b). As a result, this strategy-based device exhibits an improved V oc of 1.15 V and PCE of over 21% with negligible J-V hysteresis and excellent operational stability (Figure 6.21c). Besides, the CsPbBrCl2 PQDs films also have a surface hydrophobic property. Some relevant examples of inorganic PQDs as passivators for forming composite-based heterostructures have also been widely studied to enhance solar cell efficiency and long-term stability, such as CsPbBr3 [69] and CsPbI3 [70]. Except for the main function of surface passivation, the inorganic PQDs are resistant to thermal stress, which enables thermal stability of perovskite-based stacked PSCs.

6.4.2 Carbon QD-Based Composites Carbon or graphene QDs have been widely used to build composite-based perovskite heterostructures for the fabrication of stable PSCs [71, 72]. As a worthy work, Yavari et al. [46] developed perovskite composites with CNPs, i.e. CNPs-Cs0.05 (MA0.17 FA0.83 )0.95 Pb(I0.83 Br0.17 )3 , which were successfully used in PSCs, resulting in high performance and excellent thermal stability. The CNPs in perovskite composite film not only can assist the growth of perovskite grain size but also tremendously improve thermal stability, which was proved by no change in UV-absorption and XRD spectra (Figure 6.22a). Plus, this perovskite-based composite film also shows more hydrophobicity than that of the pristine perovskite film. In this case, the enhanced stability may be mostly ascribed to the strong chemical interaction between CNPs and perovskite molecules because of the presence of hydrogen bonds between FA fragments within the perovskite and the functional groups of CNPs (Figure 6.22b).

6.5 Reduced Film Strain by Composites-Based Perovskites

OA coated CsPbBrCl2 QDs

MAPbl3 precursor

C O H MAPbl3:CsPbBrCl2

Oleic acid

(a) MAPbl3

MAPbl3 with QDs

–1.8 CB

–3.9

Tail states

ΔVOC

Mid-gap states

EFn qVOC EFp

–5.1 PTAA

VB

–5.4 MAPbl3

Density of states N (E)

Normalized PCE

(b) 1.0 0.8 0.6 0.4 0.2 0.0

Pristine With QDs Under 1-sun illumination 0

(c)

100

200

300

400

500

Time (h)

Figure 6.21 (a) Schematic illustration of CsPbBrCl2 PQDs well-distributed on MAPbI3 film surface, (b) effect of V oc by CsPbBrCl2 PQDs, and (c) stability test for pristine and CsPbBrCl2 PQDs composite-based devices. Source: Reproduced with permission [68]. Copyright 2019, Elsevier.

6.5 Reduced Film Strain by Composites-Based Perovskites The solution-processed perovskite films usually require an annealing process to evaporate organic solvents, transition intermediate phases, and induce crystal growth. During the annealing process, the intermediate phase transition generally causes distortion of the perovskite lattices. Besides, the difference in thermal

121

6 Composite-Based Pb-Perovskite Materials as Absorbers

4

120 °C

CsM/CNP 2 hr

CsM/CNP CsM/CNP,120 °C, 2 hr Intensity / a.u.

0 hr

O.D. / a.u.

2

0 550

600

650 700 Wavelength (nm)

750

800

11

12

13 2θ (°)

14

15

(a)

FA+

FAI + 0.25 mg/ml CNP

8.16

2000

9.26

FA

Perovskite Reference

4

FAI + 0.164 mg/ml CNP

1500

3

8.03 DMF

+

8.98

122

1000

FAI + 0.082 mg/ml CNP

2

500

0.1 FAI

Perovskite + 0.05 mg/ml CNP

1

0 3.78 12

11

10 9 Chemical shift (ppm)

1.00 8

9.4

9.2

9.0

8.8 8.6 8.4 Chemical shift (ppm)

8.2

8.0

7.8

(b)

Figure 6.22 (a) Thermal stability of CsM-0.1 CNP composite film by UV–vis spectra and XRD analysis. (b) 1H NMR of CsM and CsM-CNP composite solutions. Source: Reproduced with permission [46]. Copyright 2018, John Wiley & Sons.

expansion coefficients between the perovskite and its direct contact interface layer can also cause residual stress in perovskite films. Simultaneously, the presence of residual stress further causes unnecessary degradation of the perovskite films. These factors have a negative effect on the performance and stability of PSCs. Thus, it is highly desirable to reduce the strain of perovskite films through material innovation.

6.5.1

Reduce Lattice Strain by Compositional Design

Perovskite crystal distortion, such as lattice strain and defects, is induced by soft perovskite lattices, which greatly determines the dynamics of charge extraction, transfer, and recombination. It is important to stabilize the perovskite lattice and regulate the grain growth dynamics for releasing residual strain. The 2D transition metal carbides (MXenes, Ti3 C2 Tx ) with different chemical groups are highly desirable to develop composite-based perovskites for optimizing perovskite properties [73, 74]. Tang et al. proposed a method to reduce the perovskite lattice

6.5 Reduced Film Strain by Composites-Based Perovskites

60

5.94 w/o Ti3C2Clx

40

fitting curve fitting curve Raman shift

20

0 270 (a)

w/o Ti3C2Clx

Ti3C2Clx

d-spacing (Å)

Counts

w/

Strain release

280 290 300 308 Peak position (cm–1)

310

5.92

w/

Ti3C2Clx

5.90 5.88

5.86 0.0 0.2 0.4 0.6 0.8 1.0 (b) Grazing incidence angle (°)

Tensile-strain

(c)

Strain-released

(d)

Figure 6.23 (a) Raman peaks of CsPbBr3 perovskite and Ti3 C2 Clx MXene-based CsPbBr3 composite films. (b) d-spacing values obtained from the GIXRD patterns as a function of the incidence angle. Schematic diagram of residual strain in the (c) CsPbBr3 grains and (d) Ti3 C2 Clx MXene-based CsPbBr3 composites. Source: [73]/John Wiley & Sons/CC BY 4.0.

tensile strain by incorporating an inorganic 2D Cl-terminated Ti3 C2 (Ti3 C2 Clx ) MXene into the bulk and surface of CsPbBr3 film [73]. As shown in Figure 6.23a, the pristine CsPbBr3 perovskite film shows a lower average wavenumber around 295 cm−1 for Cs+ vibrational mode, indicating the presence of a serious tensile strain. However, the CsPbBr3 film incorporated Ti3 C2 Clx MXene shows a homogeneously distributed Raman peak for Cs+ vibrational mode at 309 cm−1 . Besides, this Raman shift also indicates the released tensile strain in the perovskite film. Moreover, the CsPbBr3 film incorporated Ti3 C2 Clx MXene has a significantly reduced shift of diffraction peaks and slope compared to

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6 Composite-Based Pb-Perovskite Materials as Absorbers

the pristine perovskite film, as shown in Figure 6.23b. By theoretical calculation, 2D Ti3 C2 Clx MXene can tightly adhere to the surface of CsPbBr3 grains, like a lattice “tape” to heal the soft perovskite lattice and to relieve the lattice expansion due to the strong electron-withdrawing property of terminated Cl atoms to bond with under-coordinated Pb2+ ions in the perovskite (Figure 6.23c,d). In particular, the Pb-Cl bond plays a role of “glue” and the 2D Ti3 C2 immobilizes the lattice. Thus, PSCs based on CsPbBr3 -Ti3 C2 Clx composites obtained a record Voc of 1.702 V and a PCE of 11.08%, with good operational stability.

6.5.2 Control Crystallization by Chemical Interaction To reduce the lattice strain of perovskite crystals, He et al. [75] introduced a kind of π-conjugated organic amines, that is, 4-pyrene oxy butylamine (PYBA), into perovskite precursor to control the crystallization of FAPbI3 perovskites. The PYBA pairs at the GBs serve as a template to assist the growth of crystals in the vertical direction by a hydrogen bond. Note that the PYBA, as a significant inducer for FAPbI3 perovskite crystallization, assisted orientation growth in the crystals (Figure 6.24a). Meanwhile, the strong interaction force between the PYBA cation and [PbI6 ]4− octahedron decelerated the crystallization rate of the perovskite films, thus guaranteeing the high phase purity and high orientation of the α-FAPbI3 films. In particular, the strong hydrogen bonding interaction force is an important basis for regulating the crystallization kinetics of FAPbI3 perovskite films. It was also found that the formation energy of α-phase perovskite incorporated PYBA is lower than that of δ-phase perovskite, indicating that the δ-FAPbI3 phase can be significantly suppressed. Compared to the growth of random crystal orientation in the pristine perovskites, the FAPbI3 -PYBA composite film can achieve a highly oriented growth of α-FAPbI3 perovskite crystals (Figure 6.24b). Besides, the PYBA pairs with strong π–π interactions provide a solid fulcrum for external compression strain, thus compensating for the inherent tension strain of FAPbI3 crystals. As shown in Figure 6.24a, the amino groups in the tails of the PYBA pairs are linked to the [PbI6 ]4− octahedrons of the FAPbI3 perovskite via hydrogen bonds, which compensates for the inherent tensile strain of the FAPbI3 crystals. The GIWAXS results further confirm this conclusion, as shown in Figure 6.24c. Compared to the pristine perovskite film with severe lattice distortion, the PYBA-based perovskite composite film effectively relieves residual tensile strain attributed to the increase of the π–π interaction force between the π-conjugated cationic pairs. The reduced strain also pushes up the valence band of the perovskite crystals, thereby decreasing the bandgap and trap density. Thus, PSCs based on FAPbI3 -PYBA composites showed a high PCE of 24.76% and improved operational stability that maintained more than 80% of its initial PCE after 1500 h under MPP tracking conditions.

6.5.3

Facilitate Strain Release by Heterostructure Interfaces

The residual stress can also be released by building efficient interface contacts [10, 76]. Wang et al. developed a graded configuration based on both interface-

Lattice strain regulation

Intensity (a.u.)

π-π interaction

Hydrogen bond 3D Perovskite

PYBA Ψ = 50° Ψ = 40° Ψ = 30°

30.5

31.0

3D Perovskite

(a)

31.5 2θ (°)

Ψ = 50°

Intensity (a.u.)

Pristine R-NH3+

+H N-R 3

Ψ = 40° Ψ = 30°

Ψ = 20°

Ψ = 20°

Ψ = 10°

Ψ = 10°

32.0

32.5

30.5

31.0

31.5 2θ (°)

32.0

32.5

(c)

FA+

Pristine

Precursor solution

Disordered crystallizaion

[Pbl6]4–

Random orientation

PYBA+

Vertical orientation

SnO2

PYBA

Ordered crystallization

Precursor solution

(b)

Figure 6.24 (a) Schematic illustration of lattice strain regulation via a π-conjugated organic amine (PYBA+ ), (b) schematic illustration of in situ crystallization kinetics for the pristine and PYBA-composited perovskite precursor solutions, and (c) GIXRD patterns of (012) plane at different tilt angles for the pristine (left) and PYBA-composited (right) perovskite films. Source: Reproduced with permission [75]. Copyright 2023, John Wiley & Sons.

6 Composite-Based Pb-Perovskite Materials as Absorbers

F4TCNQ-2D-3D/2D

Intensity (a.u)

Intensity (a.u)

3D Perovskite

Tensile strain Ψ = 40° Ψ = 30° Ψ = 20° Ψ = 10° Ψ = 0°

30

31

32

31.60 31.56 31.52

(c)

30

0.1

0.2 Sin2(Ψ)

31

0.3

32

33

2θ (°)

(b)

3D F4TCNQ-2D-3D/2D

0.0

Ψ = 40° Ψ = 30° Ψ = 20° Ψ = 0°

33

PMPPT (Normalized)

31.64

Reduced tensile strain

Ψ = 10°

2θ (°)

(a)

2θ (°)

126

1.0 T90

0.8 0.6 0.4 3D F4TCNQ-2D-3D/2D

0.2 0.0

0.4

0 (d)

300

600

900

1200

Light soaking (h)

Figure 6.25 (a–c) GIXRD spectra at different tilt angles for 2θ at 31.58∘ and linear fit of residual strain as a function of sin2ψ for pristine 3D perovskite and 2D-3D-F4TCNQ composite-based heterostructures, respectively. (c) Operational stability test of power out at maximum power point under continuous one-Sun illumination in ambient-air conditions for two types of encapsulated devices based on pristine 3D and composite-based F4TCNQ-2D-3D/2D heterostructures. Source: [10]/Springer Nature/CC BY 4.0.

cascaded structures and p-type molecule-doped composites with two−/threedimensional perovskites (i.e. F4TCNQ-2D-3D heterostructures) [10]. The residual stress in both pristine 3D and composite-based F4TCNQ-2D-3D films was investigated by grazing-incidence XRD (GIXRD) spectra at different tilt angles for 2θ at 31.58∘ . It can be observed that the crystallographic plane at 31.58∘ shifts toward lower 2θ positions, indicating the presence of lattice distortion and tensile stress within the pristine 3D perovskite film (Figure 6.25a). However, the composite-based F4TCNQ-2D-3D film demonstrates almost the same 2θ position for the crystallographic plane at 31.58∘ when ψ angles change from 0∘ to 40∘ . Furthermore, the linear fit curve for 2θ position as a function of sin2ψ in the composite-based F4TCNQ-2D-3D film has a slower slope than the pristine 3D perovskite film (Figure 6.25c), indicating efficient suppression of the residual stress and lattice distortion. The reduced lattice strain is attributed to efficient heterointerface contacts and a strong chemical interaction through halogen bonding and coordination bonding. More importantly, F4TCNQ-2D-3D composite-based cells with encapsulation showed better operational stability under continuous one-Sun illumination in ambient-air conditions (Figure 6.25d). In detail, the 3D perovskite-based control devices showed a large degradation of almost 100% of the

References

initial power after 1000 hours, but F4TCNQ-2D-3D perovskite composite-based cells are highly stable with T90 (the time as a function of PCEs decrease to 90% of its initial value) lifetime measurement over 1200 h.

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15 Liu, C., Huang, Z., Hu, X. et al. (2018). Grain boundary modification via F4TCNQ to reduce defects of perovskite solar cells with excellent device performance. ACS Appl. Mater. Interfaces 10: 1909–1916. 16 Cavallo, G., Metrangolo, P., Milani, R. et al. (2016). The halogen bond. Chem. Rev. 116: 2478–2601. 17 Fu, X., He, T., Zhang, S. et al. (2021). Halogen-halogen bonds enable improved long-term operational stability of mixed-halide perovskite photovoltaics. Chem 7: 3131–3143. 18 Wu, W.-Q., Yang, Z., Rudd, P.N. et al. (2019). Bilateral alkylamine for suppressing charge recombination and improving stability in blade-coated perovskite solar cells. Sci. Adv. 5: eaav8925. 19 Girolamo, D.D., Matteocci, F., Kosasih, F.U. et al. (2019). Stability and dark hysteresis correlate in NiO-based perovskite solar cells. Adv. Energy Mater. 9: 1901642. 20 Liu, C., Yang, Y., Rakstys, K. et al. (2021). Tuning structural isomers of phenylenediammonium to afford efficient and stable perovskite solar cells and modules. Nat. Commun. 12: 6394. 21 Han, T.-H., Lee, J.-W., Choi, C. et al. (2019). Perovskite-polymer composite cross-linker approach for highly-stable and efficient perovskite solar cells. Nat. Commun. 10: 520. 22 Cao, Q., Li, Y., Zhang, H. et al. (2021). Efficient and stable inverted perovskite solar cells with very high fill factors via incorporation of star-shaped polymer. Sci. Adv. 7: eabg0633. 23 Wu, Y., Xu, G., Xi, J. et al. (2023). In situ crosslinking-assisted perovskite grain growth for mechanically robust flexible perovskite solar cells with 23.4% efficiency. Joule 7: 398–415. 24 Wang, Z., Liu, L., Wang, Y. et al. (2023). Green antisolvent-mediators stabilize perovskites for efficient NiOx -based inverted solar cells with V oc approaching 1.2 V. Chem. Eng. J. 457: 141204. 25 Lee, J.-W., Kim, H.-S., and Park, N.-G. (2016). Lewis acid–base adduct approach for high efficiency perovskite solar cells. Acc. Chem. Res. 49: 311–319. 26 Auer, S. and Frenkel, D. (2001). Suppression of crystal nucleation in polydisperse colloids due to increase of the surface free energy. Nature 413: 711–713. 27 Cacciuto, A., Auer, S., and Frenkel, D. (2004). Onset of heterogeneous crystal nucleation in colloidal suspensions. Nature 428: 404–406. 28 Zhou, Y. and Padture, N. P. (2020). Understanding and engineering grain boundaries for high-performance halide perovskite photovoltaics. 2020 47th IEEE Photovoltaic Specialists Conference (PVSC). IEEE. 29 Rothmann, M.U., Kim, J.S., Borchert, J. et al. (2020). Atomic-scale microstructure of metal halide perovskite. Science 370: 548. 30 Wang, Y., Rho, W.-Y., Yang, H.-Y. et al. (2016). Air-stable, hole-conductor-free high photocurrent perovskite solar cells with CH3 NH3 PbI3 -NiO nanoparticles composite. Nano Energy 27: 535–544.

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47 Chung, C.-C., Narra, S., Jokar, E. et al. (2017). Inverted planar solar cells based on perovskite/graphene oxide hybrid composites. J. Mater. Chem. A 5: 13957–13965. 48 Mahmoudi, T., Wang, Y., and Hahn, Y.-B. (2019). Stability enhancement in perovskite solar cells with perovskite/silver–graphene composites in the active layer. ACS Energy Lett. 4: 235–241. 49 Mahmoudi, T., Wang, Y., and Hahn, Y.B. (2020). SrTiO3 /Al2 O3 -graphene electron transport layer for highly stable and efficient composites-based perovskite solar cells with 20.6% efficiency. Adv. Energy Mater. 10: 1903369. 50 Mahmoudi, T., Wang, Y., and Hahn, Y.-B. (2021). Highly stable perovskite solar cells based on perovskite/NiO-graphene composites and NiO interface with 25.9 mA/cm2 photocurrent density and 20.8% efficiency. Nano Energy 79: 105452. 51 Son, D.Y., Kim, S.G., Seo, J.Y. et al. (2018). Universal approach toward hysteresis-free perovskite solar cell via defect engineering. J. Am. Chem. Soc. 140: 1358–1364. 52 Abdi-Jalebi, M., Andaji-Garmaroudi, Z., Cacovich, S. et al. (2018). Maximizing and stabilizing luminescence from halide perovskites with potassium passivation. Nature 555: 497–510. 53 Wang, Y., Ju, H., Mahmoudi, T. et al. (2021). Cation-size mismatch and interface stabilization for efficient NiOx -based inverted perovskite solar cells with 21.9% efficiency. Nano Energy 88: 106285. 54 Rana, P.J.S., Febriansyah, B., Koh, T.M. et al. (2022). Alkali additives enable efficient large area (>55 cm2 ) slot-die coated perovskite solar modules. Adv. Funct. Mater. 32: 2113026. 55 Min, H., Kim, G., Paik, M.J. et al. (2019). Stabilization of precursor solution and perovskite layer by addition of sulfur. Adv. Energy Mater. 9: 1803476. 56 Ye, S., Rao, H., Zhao, Z. et al. (2017). A breakthrough efficiency of 19.9% obtained in inverted perovskite solar cells by using an efficient trap state passivator Cu (thiourea) I. J. Am. Chem. Soc. 139: 7504–7512. 57 Smith, I.C., Hoke, E.T., Solis-Ibarra, D. et al. (2014). A layered hybrid perovskite solar-cell absorber with enhanced moisture stability. Angew. Chem. Int. Ed. 126: 11414–11417. 58 Tsai, H., Nie, W., Blancon, J.-C. et al. (2016). High-efficiency two-dimensional Ruddlesden-Popper perovskite solar cells. Nature 536: 312. 59 Mei, A., Li, X., Liu, L. et al. (2014). A hole-conductor-free, fully printable mesoscopic perovskite solar cell with high stability. Science 345: 295–298. 60 Ma, Q., Wang, Y., Liu, L. et al. (2024). One-step dual-additive passivated wide-bandgap perovskites to realize 44.72%-efficient indoor photovoltaics. Energy Environ. Sci. 17: 1637–1644. 61 Ma, Q., Ma, M., Liu, L. et al. (2023). Wide-band-gap perovskite solar minimodules exceeding 43% efficiency under indoor light illumination. Device 1: 100174. 62 Gao, Y., Liu, C., Xie, Y. et al. (2022). Can nanosecond laser achieve highperformance perovskite solar modules with aperture area efficiency over 21%? Adv. Energy Mater. 12: 2202287.

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63 Fan, J., Ma, Y., Zhang, C. et al. (2018). Thermodynamically self-healing 1D-3D hybrid perovskite solar cells. Adv. Energy Mater. 8: 1703421. 64 Liu, P., Xian, Y., Yuan, W. et al. (2020). Engineering electronic structure of stannous sulfide by amino-functionalized carbon: toward efficient electrocatalytic reduction of CO2 to formate. Adv. Energy Mater. 10: 1903664. 65 Chen, J., Yang, Y., Dong, H. et al. (2022). Highly efficient and stable perovskite solar cells enabled by low-dimensional perovskitoids. Sci. Adv. 8: eabk2722. 66 Ye, S., Rao, H., Feng, M. et al. (2023). Expanding the low-dimensional interface engineering toolbox for efficient perovskite solar cells. Nat. Energy 8: 284–293. 67 Nie, W. (2023). Stability beyond lead. Nat. Energy 8: 222–223. 68 Zheng, X., Troughton, J., Gasparini, N. et al. (2019). Quantum dots supply bulk- and surface-passivation agents for efficient and stable perovskite solar cells. Joule 3: 1963–1976. 69 Gao, Y., Wu, Y., Lu, H. et al. (2019). CsPbBr3 perovskite nanoparticles as additive for environmentally stable perovskite solar cells with 20.46% efficiency. Nano Energy 59: 517–526. 70 Zhao, Q., Hazarika, A., Chen, X. et al. (2019). High efficiency perovskite quantum dot solar cells with charge separating heterostructure. Nat. Commun. 10: 2842. 71 Najafi, L., Taheri, B., Martin-Garcia, B. et al. (2018). MoS2 quantum dot/graphene hybrids for advanced Interface engineering of a CH3 NH3 PbI3 perovskite solar cell with an efficiency of over 20%. ACS Nano 12: 10736–10754. 72 Gao, F., Liu, K., Cheng, R., and Zhang, Y. (2020). Efficiency enhancement of perovskite solar cells based on graphene-CuInS2 quantum dots composite: the roles for fast electron injection and light harvests. Appl. Surf. Sci. 528: 146560. 73 Zhou, Q., Duan, J., Du, J. et al. (2021). Tailored lattice “tape” to confine tensile Interface for 11.08%-efficiency all-inorganic CsPbBr3 perovskite solar cell with an ultrahigh voltage of 1.702 V. Adv. Sci. 8: 2101418. 74 Guo, Z., Gao, L., Xu, Z. et al. (2018). High electrical conductivity 2D MXene serves as additive of perovskite for efficient solar cells. Small 14: 1802738. 75 Wang, R., Li, X., Qi, J. et al. (2023). Lattice strain regulation enables high-performance Formamidinium perovskite photovoltaics. Adv. Mater. 35: 2304149. 76 Li, G., Song, J., Wu, J. et al. (2021). Efficient and stable 2D@ 3D/2D perovskite solar cells based on dual optimization of grain boundary and interface. ACS Energy Lett. 6: 3614–3623.

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7 Composites-Based Pb-Free Perovskite Materials as Absorbers 7.1

Inorganic Additives-Based Perovskite Composites

Rapid oxidation of Sn+2 to Sn4+ and fast nucleation and growth kinetics in perovskites result in structural defects. Hence, it is necessary to use additives that mitigate fast oxidation and enhance film morphology. Sn (tin) halides (SnF2 , SnCl2 , SnBr2 , and SnI2 ) and certain organic molecules have proven effective in preventing oxidation and boosting Sn-based perovskite solar cells (Sn-PSCs) performance. Additionally, Sn-halide additives can suppress the formation of Sn vacancies in films, leading to improved film morphology [1].

7.1.1 SnF2 Additive In 2012, Chung et al. introduced reducing agents, specifically SnF2 , into CsSnI3 perovskites as a hole transport material (HTM) for dye-sensitized solar cells. They obtained a V OC of 0.42 V and an overall power conversion efficiency (PCE) of 0.9% [2]. Kumar et al. also introduced the SnF2 additive into CsSnI3 to lower the formation energy of Sn vacancies, resulting in decreased conductivity in CsSnI3 and increased J SC of 22 mA/cm2 in Sn-PSCs [3]. X-ray diffraction (XRD) analysis definitively confirmed the significant impact of incorporating 20% mole SnF2 doping into formamidinium tin iodide (FASnI3 ). The findings from XRD revealed a noticeable reduction in the concentration of Sn4+ ions within the perovskite structure. This structural modification directly correlated with a remarkable enhancement in the current density, specifically contributing to an impressive increase of 10 mA/cm2 . This improvement underscores the effectiveness of SnF2 doping in optimizing the electronic properties of FASnI3 , thereby highlighting its potential for advancing the performance of PSCs [4]. In 2018, Xiao et al. explored the function of SnF2 and the mechanism of improving the Sn-perovskite film morphology [5]. They clearly showed that SnF2 can play an important role in the crystal nucleation process. That is, due to the limited solubility, SnF2 creates more nuclei for the crystal growth, enabling a more uniform thin film with high coverage. This innovative approach not only demonstrated the effectiveness of SnF2 in reducing Sn vacancy (V Sn ) concentrations by elevating

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their formation energy but also underscored its versatile application in optimizing perovskite film characteristics. Hartmann et al. further studied the effect on the properties of CsSnBr3 film by the addition of 20 mol% SnF2 to the precursor solution [6]. The SnF2 addition improved the coverage of the TiO2 substrate by CsSnBr3 and decreased the oxidation of Sn2+ to Sn4+ , while suppressing the formation of secondary Br and Cs species. They also found that SnF2 addition decreases the energy difference between the VB maximum of CsSnBr3 and the Fermi level. Ma et al. made a significant observation regarding the impact of SnF2 on carriers’ lifetime and diffusion length [7]. They showed that SnF2 addition in CH3 NH3 SnI3 film effectively increases the fluorescence lifetime up to 10 times (i.e. from 0.7 to 6 ns) and gives diffusion lengths exceeding 500 nm, while the electron diffusion length remains unchanged. The notable effect of SnF2 on carrier dynamics has positioned it as a common additive in most Sn-based photovoltaic systems. Its ease of optimization for Sn perovskites further solidifies its widespread adoption, highlighting its role in enhancing the performance and efficiency of Sn-PSCs [7].

7.1.2 SnCl2 Additive SnCl2 is also known as one of the commonly employed additional reducing agents in Sn perovskites. Marshall et al. reported CsSnI3 perovskite photovoltaic devices without a hole-selective interfacial layer [8]. They exhibited stability about 10 times greater than devices with the same architecture using CsSnI3 perovskite. They evaluated the effect of various Sn-halide additives (SnCl2 , SnBr2 , SnI2 , and SnF2 ) on HTM-free Sn-PSCs (ITO/CsSnI3 /PC61BM/BCP/Al). Among the tested devices, those incorporating the SnCl2 additive exhibited the highest PCE of 3.56% and demonstrated superior film stability, attributed to the SnCl2 layer acting as a desiccant, thereby enhancing the stability of CsSnI3 .

7.1.3

Hydrazine Additive

Hydrazine, a longstanding player in chemical synthesis that is recognized as a powerful reducing agent, is applied to prevent or suppress Sn2+ /Sn4+ oxidation. Its highly volatile nature makes it an ideal candidate for the introduction of reducing atmosphere in various processes. According to Song et al., a hydrazine vapor atmosphere was introduced before the spin-coating process of the perovskite precursor, resulting in film formation within this atmosphere. This approach led to a reduction in defects and oxidation, and consequently, an enhancement in overall performance [9]. In a similar vein, another study aimed to minimize the concentration of Sn4+ and suppress carrier recombination during the fabrication of FASnI3 perovskite films. Kayesh et al. introduced hydrazinium chloride (N2 H5 Cl) in a single precursor solvent system to fabricate the perovskite films [10]. The outcomes were notable, with a 20% reduction in Sn4+ concentration, significantly improved PCE, enhanced open-circuit voltage, and the attainment of pinhole-free uniform FASnI3 perovskite films.

7.1 Inorganic Additives-Based Perovskite Composites

Another innovative approach was reported by Li et al. [11]. They developed a two-step chemical process to prepare high-quality MASnI3 films: solution deposition of hydrazinium tin iodide (HASnI3 ) perovskite film using precursors hydrazinium iodide (HAI) and SnI2 , followed by transformation of the solution-deposited film into MASnI3 through a cation displacement approach. The two-step process produced dense and uniform MASnI3 films with large grain sizes and high crystallization, leading to the fabrication of mesoporous Sn-PSCs with an impressive PCE of 7.13% and reproducibility.

7.1.4

Acidic Additive

All-inorganic lead (Pb)-free perovskite materials, with Cs replacing organic cations and Sn replacing Pb, have shown great potential in achieving high thermal stability, but the Sn-based perovskites are forced to suffer from poor stability and low efficiency caused by Sn vacancies due to easy oxidation of Sn+2 and fast crystallization. Li et al. reported that the addition of hypophosphorous acid (HPA) plays an important role in obtaining quality CsSnIBr2 thin films with low V Sn [12]. In particular, when strong agents such as HI (hydroiodic acid) or SnF2 are present, HPA acts as an auxiliary reducing agent in the antioxidant process. Thus, addition of HPA as a complexant enables to speed up the nucleation process while suppressing the Sn2+ /Sn4+ oxidation during the formation of CsSnIBr2 thin film. With a mesoscopic structure, the CsSnIBr2 PSCs showed no efficiency loss for 77 days and remarkably stable power output for nine hours at high temperatures up to 473 K [12]. Yu et al. used an eco-friendly additive 2,2,2-trifluoroethylamine hydrochloride (TFEACl) to synergize with SnF2 to improve the efficiency and stability of FASnI3 PSCs [13]. TFEACl has the capability to modify the work function of perovskite films through the incorporation of chlorine (Cl). The addition of TFEACl suppressed the segregation of SnF2 , resulting in improvements in film morphology, more favorable energy band alignment, suppression of the Sn2+ /Sn4+ oxidation, and light soaking stability. As a result, reduced charge recombination and improved charge collection led to an efficiency increase from 3.63% to 5.30% with substantial enhancement in device stability. It is known that the addition of SnX2 (X = F, Cl, or Br) in precursor solutions inhibits Sn2+ oxidation and thus improves device efficiency and stability. However, SnX2 on the surface of perovskite grains tends to prohibit charge transfer across perovskite films. To address this, Wang et al. proposed co-additive engineering by introducing antioxidant gallic acid (GA) together with SnCl2 (i.e. SnCl2 –GA complex) to improve the performance of Sn-PSCs (see Figure 7.1) [14]. The characteristics of GA, derived from the hydroxyl groups (–OH) on its aromatic ring, enable electron donation and oxygen absorption through hydrogen atoms. It was found that the SnCl2 –GA complex not only protected the perovskite grains but also more effectively conducted electrons across it, leading to highly stable and efficient Sn-PSCs. The unencapsulated cells retained ∼80% of their initial PCEs over 1000 hours in ambient air (20% relative humidity).

135

136

7 Composites-Based Pb-Free Perovskite Materials as Absorbers

(a) (b)

(c)

(d)

Figure 7.1 (a) A schematic diagram of FASnI3 perovskite solar cell, highlighting the SnCl2 -based complex surrounding the perovskite grains. (b) A chemical reaction diagram illustrating the oxidation process of gallic acid (GA) to quinone upon exposure to air. (c) A schematic illustration detailing the preparation process of FASnI3 films, emphasizing the morphological variations induced by different additives. (d) A schematic representation depicting the interaction mechanism between gallic acid (GA) and SnCl2 . Source: Reproduced with permission from [14]. Copyright 2020, American Chemical Society.

Xu et al. reported for the first time that ascorbic acid (AA) can serve as a simple but effective additive to simultaneously enhance the performance and stability of Sn-PSCs [15]. AA, known as an antioxidant, was identified as an inhibitor that suppresses the Sn2+ oxidation while also modulating its perovskite crystallization by forming intermediate complexes. This approach significantly improved the optoelectronic quality of perovskite films, resulting in a prolonged photogenerated carrier lifetime (183 ns) with as-formed MA0.5 FA0.5 Pb0.5 Sn0.513 film, which was about two times longer in the film with regular SnF2 addition. Compared to the control device that incorporated the SnF2 additive (PCE = 12%), the AA-treated cells showed an impressively high PCE of 14% and stability enhancement (see Figure 7.2).

7.1.5

Other Additives

Recently, a multitude of additives have been investigated to improve the performance of Sn-PSCs [16–19]. However, this focused review highlights on the latest and most impactful additives that have exhibited remarkable device performance in Pb-free PSCs over the past two years. Table 7.1 provides a meticulous and

7.2 Organic Additives-Based Perovskite Composites –30 –25

J (mA/cm2)

–20 –15 –10 –5 0 0.0

0.2

0.4

0.6

0.8

U/V

Figure 7.2 Ascorbic acid was introduced for the first time as an effective antioxidant additive to enhance the efficiency and stability of Sn-based perovskite solar cells. Source: Reproduced with permission from [15]/Elsevier.

detailed description of these cutting-edge additives, underscoring their pivotal role in advancing the field and offering a consolidated resource for researchers and practitioners keen on harnessing the power of additives to enhance Pb-free PSCs performance. Moreover, for those exploring the realm of mixed Pb-Sn perovskites with a lead content of 50% or less, Table 7.2 provides a thorough compilation that emphasizes the role of additives. This table not only encapsulates devices incorporating perovskite additives and surface/interface modifiers but also sheds light on the structural details and reported stability of these innovative systems. It stands as an indispensable guide for those keen on leveraging additives to unlock the full potential of mixed Pb-Sn perovskites, emphasizing the impact of these compounds on both stability and performance.

7.2

Organic Additives-Based Perovskite Composites

In the context of both Pb and Sn-based PSCs, Lewis base emerges as a frequently employed additive. Typically, Lewis bases incorporate functional groups such as C–O and P–O. These groups have the unique ability to interact with Sn2+ or FA+ within perovskite structures, forming Lewis acid–base adducts. Such interaction plays a pivotal role in significantly enhancing the quality of perovskite films and consequently boosting device performance. Focusing on specific research activities seeking the introduction of Lewis base additives, Table 7.3 summarizes the efficiency and stability of Sn-based PSCs with different additives. Addressing challenges associated with low film coverage, suboptimal film crystal quality resulting from the rapid reaction between SnI2 and methylammonium iodide (MAI)/formamidinium iodide (FAI), and uncontrolled growth

137

Table 7.1 Efficiency and stability of Sn-PSCs with different structures and additives. Stability (period, conditions, percentage from original efficiency)

Refs.

7.1

42 d, 100+% – 6 h, air, 60%

[20]

7.5

60 d, N2 , 60% – 5 d, air, 76.5% – 5 d, 1 sun, 58.4%

[21]

FM+

7.7

367 d, N2 , 100%

[22]

FTO/SnO2 /Al2 O3 -Gr/FA0.8 MA0.2 SnI3 /spiro/Au

rGO

7.7

30 d air, 42% – 30 d, 85%, dry Ar

[23]

ITO/PEDOT/FA0.75 MA0.10 SnI2 Br/PCBM/BCP/Ag

PEA+

8.0

63 d, N2 , 100% – 13 d, air, 100%

[24]

Structure

Additive

ITO/PEDOT/FA0.92 PEA0.08 SnI3 /PCBM/Al

MACl

FTO/c-TiO2 /mp-TiO2 /CsSnI3 /P3HT/Au

MBAA

ITO/PEDOT/FASnI3 /C60/BCP/Ag

PCE (%)

ITO/PEDOT/PCBM/PCB/Ag

F-PDI

9.5

125 d, 1 sun, N2 , 80%

[25]

ITO/PEDOT/MASnI3 /PCBM/BCP/Ag

EABr

9.6

30 d, N2 , 93%

[26]

FTO/PEDOT/FASnI3 /C60 /BCP/Ag

FAAc

10.0

67 d, 1 sun, N2 , 82%

[27]

PET/ITO/NiOx /FASnI3 /4AMPI2 /PCBM/BCP/Ag

Ge/GeO2

10.4

29 d, 1 sun, N2 , MPP, 80%

[28]

ITO/PEDOT/FASnI3 /C60 /BCP/Ag

DipI and NaBH4

10.6

54 d, N2 , MPP, 96%

[29] [30]

ITO/PEDOT/FASnI3 /PCBM/BCP/Ag

EABr

10.8

84 d, N2 , 82%

ITO/PEDOT/FASnI3 /PAI/C60 /BCP/Ag

PEA+

12.1

21 d, 1 sun, MPP, encapsulated, 94%

[31]

ITO/PEDOT/MASnI3 /ICBA/BCP/Ag

CsPbI3 QDs

12.5

40 d, N2 , 96% – 23 d, 1 sun, 62%

[32]

ITO/PEDOT/FASnI3 /ICBA/BCP/Ag

CsPbI3 QDs

13.0

40 d, N2 , 83%

[32]

ITO/PEDOT/FASnI3 + PHCl-Br/C/BCP/Ag

PhNHNH3 + and Ph-Cl− Br−n

13.4

14 d, 1 sun, 82%

[33]

ITO/PEDOT/FASnI3 /ICBA/BCP/Ag

4A3HA

13.4

83 d, N2 , 98% – 42 d, at 82∘ C, 80%

[34]

ITO/PEDOT/PEAFASn(IBr)3 /ICBA/BCP/Ag

GAA

13.7

50 d, N2 , 93%

[35]

ITO/PEDOT/FASnI3 /BCP-ICBA/Ag

3T

14.0

30 d, N2 , 100% – 9 h, air, 85%

[36]

ITO/PEDOT/FASnI3 /ICBA/BCP/Ag

PEABr

14.6

100 d, N2 , 96%

[37]

ITO/PEDOT/FASnI3 /ICBA/BCP/Al

FPEABr

14.8

19 d, N2 , 80%

[38]

Table 7.2 Efficiency and stability of Sn-PSCs with lead content ≤50% with different structures and additives.

PCE (%)

Stability (period, conditions, percentage from original efficiency)

K-SCN

14.5

5 d, air, 55%

[39]

PEAI

17.3

33 h, air, 85% – 45 d, N2 , 87%

[40]

ITO/FA0.85 Cs0.15 Sn0.5 Pb0.5 I3 /PCBM/PCB/Cu

FSA and PEAI

17.4

20 d, air, 81%

[41]

ITO/PEDOT/FA0.5 MA0.5 Pb0.5 Sn0.5 I3 /PCBM/C60/BCP/Ag

IMBF4

19.1

42 d, N2 , 90% – 2 d, 1 sun, 80%

[42]

Structure

Additive

ITO/PEDOT/FASn0.5 Pb0.5 I3 /C60/BCP/Ag ITO/PEDOT/FA0.8 MA0.15 Cs0.05 Pb0.5 Sn0.5 I3 /C60/BCP/Ag

Refs.

ITO/PEDOT/FA0.83 Cs0.17 Pb0.5 Sn0.5I3 /C60/BCP/Ag

PEAI

19.1

4 d, N2 , 1 sun, MPP 82%

[43]

ITO/NiOx/FA0.5 MA0.5 Sn0.5 Pb0.5 I3 /PCBM/BCP/Ag

PFN

19.8

20 d, air, 68%

[44]

ITO/PEDOT/FA0 .7MA0 .3Pb0 .5Sn0.5 I3 /PCBM/BCP/Cu.

CA

19.9

21 d, N2 , 90%

[45]

ITO/Cs0.05 MA0 .45FA0.5Pb0.5Sn0.5 I3 /PCBM/C60/BCP/Ag

Cu-SCN Gly HCl

20.1

ITO/PEDOT/FA0.7 MA0.3 Pb0 .5Sn0.5 I3 /PCBM/BCP/Ag

[PNA]BF4

20.1

42 d, N2 , 90% – 4 d, 1 sun, MPP, 72% 10 d, N , 85∘ C, 80% – 50 d, N , 90.8%

[47]

ITO/PEDOT/FA0.7 MA0.3 Pb0 .5Sn0.5 I3 /C60/BCP/Ag

PhDMADI

20.5

29 d, N2 , 95%

[48]

ITO/PEDOT/FA0.7 MA0.3 Pb0 .5Sn0.5 I3 /PCBM/BCP/Ag

GUA and HAI

20.5

[49]

FTO/PEDOT/Cs0.025 FA0.475 MA0.5 Sn0.5 Pb0.5 I2.925 Br0.075 / PCBM/C60/BCP/Ag

RbI

21.0

6 d, N2 , 1 sun, 60% 6 d, N2 , at 85∘ C, 75% – 30 d, N2 , 99%

[51]

2

2

[46]

[50]

ITO/PEDOT/FA0.7 MA0.3 Pb0 .5Sn0.5 I3 /C60/BCP/Ag

HZBA

21.1

8 d, N2 , 90%

ITO/PEDOT/Cs0.2 FA0.8 Pb0.5 Sn0.5 I3 /C60/BCP/Cu

BaI2

21.2

15 d, encapsulated, 1 sun, MPP, 95%

[52]

FTO/PEDOT/FA0.6 MA0.4 Sn0.6 Pb0.4 I3 /C60/BCP/Ag

N, Cl-GQDs

21.5

[53]

ITO/PEDOT/Cs0.05 FA0.7 MA0.25 Sn0.5 Pb0.5 I3 /C60/BCP/Ag

BBMS + SnF2

22.0

42 d, N2 , 90% 111 d, N , 60∘ C, 98%

ITO/PEDOT/FA0.6 MA0.4 Sn0.6 Pb0.4 I3 /C60/BCP/Ag.

PEAI and guanidinium-SCN

22.1

76 d, N2 , MPP, 82%

[55]

ITO/CzAnp/PMMA/FA0.8 Cs0.2 Sn0.5 Pb0.5 I3 /PCBM/ C60/BCP/Cu

CzAn HTM and BHC

22.6

7 d, encapsulated, MPP, 1 sun, 90% – 42 d, encapsulated, 96%

[56]

FTO/Cs0.025 FA0.475 MA0.5 Sn0.5 Pb0.5 I2.925 Br0.075 /EDA r/PCBM/C60/BCP/Ag

2PACz and MPA

23.3

42 d, N2 , 1 sun, 100%

[57]

FTO/PEDOT/Cs0.1 FA0.6 MA0.3 Sn0.5 Pb0.5 I3 /C60/BCP/Ag

EDAI2 and GlyHCl

23.6

8 d, N2 , 1 sun, MPP, 80%

[58]

2

[54]

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7 Composites-Based Pb-Free Perovskite Materials as Absorbers

Table 7.3 Efficiency and stability of Sn-based PSCs with different Lewis base additives. Device structure

Additive PCE enhancement (%) Stability

Refs.

ITO/SO2 /C60/FASnI3 / Spiro-OMeTAD/Ag

TMA

4.2 → 7.1

1 mo, glove box, 80%

[59]

ITO/PEDOT, PSS/FASnI3 /PC61BM/ BCP/Ag

PMMA

2.8 → 3.6

1 mo, glove box, 62%

[60]

ITO/NiOx/FASnI3 / C60/BCP/Ag

PTN-Br

5.1 → 7.9

5 h UV irradiation, 66%

[61]

during the formation of Sn-perovskite films, Zhu et al. introduced trimethylamine (TMA) in FASnI3 [59]. By taking advantage of the relatively large size of TMA and its weaker affinity with SnI2 compared to FA ions, they developed a two-step process: the addition of TMA as the additional Lewis base in the Sn halide solution to form SnY2 -TMA complexes (Y = I− and F− ), followed by intercalating with FAI to convert into FASnI3 . XRD analysis of as-formed films showed a significant decrease in peak intensity in the order of SnI2 , SnI2 -SnF2 , and SnI2 -SnF2 -TMA, validating TMA’s role as a Lewis base, capable of interacting with SnX2 in the perovskite precursor solution, forming a Lewis acid–base adduct. This strategic approach allowed TMA ions to be replaced with FA ions in the precursor solution, resulting in the formation of a dense FASnI3 film, leading to a substantial reduction in defects and carrier recombination in Sn-perovskite films and thus remarkable enhancement in PCE (Table 7.3) and J-V characteristic. Deng et al. also used PMMA as an additive in FASnI3 , achieving an efficiency of 3.6% [60]. As illustrated in Figure 7.3a,b, the C=O bonds originating from PMMA interacted with FA+ within FAI, forming hydrogen bonds. This interaction effectively mitigated hole accumulation at the grain boundaries of the perovskite film, reducing both leakage current and the occurrence of defect states within the film. Consequently, the inclusion of PMMA led to a notable increase in the open-circuit voltage from 0.32 to 0.48 V. Moreover, perovskite films incorporating PMMA exhibited improved long-term stability. In contrast to Lewis base additives, PTN-Br plays a distinct role as a holetransport medium bridging the perovskite film and the hole transport layer (HTL). Liu et al. introduced a semiconducting molecule, polytetraphenylethene3,3′ -(((2,2diphenylethene-1,1-diyl)bis(4,1-phenylene))bis(oxy))bis(N,N-dimethylpropan-1amine)tetraphenylethene (PTN-Br), into a Pb-free perovskite precursor to form bulk heterojunction film [61]. In PTN-Br, the dimethylamino component interacted with uncoordinated Sn anions in the perovskite, forming a SnI2 -PTN-Br Lewis adduct. This interaction significantly enhanced charge transport efficiency and facilitated more balanced carrier transport (Table 7.4). Moreover, PTN-Br boasted the highest occupied molecular orbital (HOMO) of −5.41 eV, establishing a favorable band alignment between the perovskite layer (−5.59 eV) and the HTL

7.2 Organic Additives-Based Perovskite Composites

(a)

(b)

Figure 7.3 (a) Depiction outlining the interaction between PMMA and FASnI3 at the grain boundaries. (b) Dark current curves comparing FASnI3 films with and without PMMA. Source: Reproduced with permission [60]. Copyright 2018, IOP Publishing.

Table 7.4 Effect of PTN-Br on hole and electron mobilities, evaluated through hole-only and electron-only devices, respectively.

Device

Hole mobility (cm2 /V1 /s1 )

Electron mobility (cm2 /V1 /s1 )

W/O PTN-Br

1.2 × 10−2

7.3 × 10−1

With PTN-Br

−2

7.2 × 10−1

2.5 × 10

Source: [61]/John Wiley & Sons.

PEDOT:PSS (−5.0 eV). Consequently, PTN-Br could be strategically positioned at grain boundaries, acting as a hole transport medium between the FASnI3 perovskite film and the PEDOT:PSS layer. These positive interventions resulted in substantial increases in V oc from 0.43 to 0.57 V and in FF from 0.58 to 0.67, yielding PCE of 7.9% with minimal hysteresis. The challenge of precisely controlling the crystallization process in Sn-perovskites poses a risk of yielding unexpected film morphologies and defects, consequently lowering the PCE. In a recent development, a distinctive polymer, poly

141

142

7 Composites-Based Pb-Free Perovskite Materials as Absorbers

(ethylene-co-vinyl acetate) (EVA), was introduced into an antisolvent during the spin coating of an FASnI3 precursor solution [62]. The C=O groups within EVA formed robust Lewis acid–base complexes with uncoordinated Sn atoms in perovskite grains. This innovative approach significantly enhanced grain size, optimized grain orientation, and reduced surface defects in FASnI3 films. The outcome was an impressive PCE of 7.72% with remarkable reproducibility. The fabricated PSCs based on FASnI3 –EVA absorbers demonstrated self-encapsulation, exhibiting excellent resistance to moisture and oxygen. Even after aging for 48 hours in air with a relative humidity of 60%, the PSCs retained 62.4% of their initial efficiency. This convenient strategy not only yielded high performance but also provided a new avenue for establishing stable, Pb-free, Sn-based PSCs. The crystal fragility and suboptimal crystallinity of perovskite stretchable substrates have historically resulted in performance losses. Significantly, grain boundary (GB) defects emerge as the “Achilles’ heel” affecting both optoelectronic and mechanical stability. The incorporation of large-volume amines (LVAs) in Pb-free Sn-based perovskite films has shown the potential to enhance the PCEs of PSCs by effectively reducing GB defects. However, commonly used LVAs, such as phenylethylammonium (PEA) and butylammonium (BA), are insulating and may hinder charge extraction within the perovskite film. Introduction of conjugated LVA, 3-phenyl-2-propen-1-amine (PPA) into FASnI3 perovskite revealed that the incorporation of PPA leads to enlarged grain sizes, reduced trap density, preferential orientation, efficient charge extraction, and enhanced structural stability of the FASnI3 film [63]. These positive effects contributed to the achievement of efficient PSCs (PCE = 9.61%) with negligible hysteresis and facilitated a self-healing action in PSCs. Notably, the large-area (1 × 1 cm2 ) Sn-based PSCs with a PCE of 7.08% demonstrated the potential scalability and stability of LVA introduced in Pb-free PSCs technology.

7.3

Carbon Additives-Based Perovskite Composites

To resolve the chemical instability of Sn-PSCs, attributed to Sn2+ /Sn4+ oxidation, Hahn group developed a pioneering approach by incorporating reduced graphene oxide (rGO) and tin quantum dots (Sn QDs) into mixed-organic-cation Sn-perovskite. The inclusion of rGO-Sn QDs resulted in the formation of a functional composite of FA0.8 MA0.2 SnI3 /rGO-Sn in the active layer of mesoporous n-i-p Sn-PSC [23]. Figure 7.4a exhibits the cell structure and J-V characteristics of the composite-based Sn-PSCs (i.e. FTO/SnO2 /mp-AG/(FA0.8 MA0.2 SnI3 /rGO-Sn)/SpiroOMeTAD/Au). The inclusion of graphene as a carbon-based additive is pivotal, as it enhances the carrier transport and passivates trap states, leading to a reduction of recombination loss, which is in good agreement with the J-V and IPCE measurements (Figure 7.4a,b). The Sn QDs under the built-in energy field generate an abundance of Sn2+ that effectively suppresses Sn2+ /Sn4+ oxidation. In addition, the conductive graphene sheets probably provide fast conduction paths of electrons so that Sn4+ is readily reduced to Sn2+ . Thus, the rGO-Sn QDs in AL not only inhibit the

20

FA0.8MA0.2Snl3 FA0.8MA0.2Snl3/Sn-rGO

IPCE (%)

J (mA/cm2)

16 12 8 4 0 0.0 (a)

0.2 0.4 Voltage (V)

16 12

40

8

20

4

0

0.6

FA0.8MA0.2Snl3/Sn-rGO

60

(b)

0.7

400 500 600 700 800 900 Wavelength (nm)

0

16 JSC (mA/cm2)

VOC (V)

FA0.8MA0.2Snl3

80

Integrated JSC (mA/cm2)

7.3 Carbon Additives-Based Perovskite Composites

0.6

14

0.5 Reference

Champion

(d)

7

64

6

60

Reference

Champion

Reference

Champion

5 4

56 (e)

12

68 PCE (%)

FF (%)

(c)

Reference

Champion

(f)

Figure 7.4 (a) J-V characteristics (inset, the device configuration of composite-based Sn-PSC); (b) IPCE spectra of FA0.8 MA0.2 SnI3 and FA0.8 MA0.2 SnI3 /rGO-Sn based cells; (c–f) reproducibility test with photovoltaic parameters distribution for of FA0.8 MA0.2 SnI3 and FA0.8 MA0.2 SnI3 /rGO-Sn based cells. Source: Reproduced with permission [23]. Copyright 2021, Elsevier.

destructive Sn2+ oxidation and recombination but also accelerate charge transport, improve charge-carrier lifetime, and reduce trap state density [23]. Compared to Sn-PSCs without rGO-Sn QDs, the composite-based Sn-PSCs showed a 55% increase in PCE, remarkable reproducibility, and stability improvement (Figure 7.4c–f). This strategic integration of graphene as a carbon-based additive represents a significant leap forward in the development of Pb-free PSCs, highlighting its crucial role in achieving both environmental sustainability and stable high-performance operation.

143

144

7 Composites-Based Pb-Free Perovskite Materials as Absorbers

(a)

(b)

(c)

Figure 7.5 (a,b) FESEM images of Sn-PS and Sn-PS:N0.12 GO films with size distribution histograms of crystalline grains. (c) Time-dependent photographs showing crystallization of Sn-PS and Sn-PS:N0.12 GO films. Source: Reproduced with permission from [64]/John Wiley & Sons.

Hahn group further developed a more advanced approach, for the first time, incorporating nitrogen-doped graphene oxide (Nx GO), into the HTL, interface layer (IL), and electron transport layer (ETL) [64]. This strategic integration of Nx GO has proven highly effective in slowing the crystallization rate of tin–lead perovskite (Sn-PS) and suppressing Sn2+ /Sn4+ oxidation. Thick perovskite films with large grains are crucial for high-efficiency PSCs. Film thickness depends on the nucleation and growth rate of perovskite crystals. However, unlike Pb-perovskite, the crystallization rate of Sn-perovskite is too rapid to allow thick film growth. Compared to the pristine Sn-PS film with grains of 0.5–1.6 μm (Figure 7.5a), the Sn-PS:Nx GO composite film showed pinhole-free, dense structure with large grains ranging from 0.8–2.42 μm (Figure 7.5b), due to slower crystallization rate facilitated by Nx GO inclusion. The crystallization of pristine Sn-PS completed in approximately 12.5 s, while the Sn-PS:Nx GO film crystallized in about 25.4 s, twice as slow as the former (Figure 7.5c), confirming that the Nx GO in AL plays a critical role in the nucleation and growth process. To evaluate the effect of Nx GO-based composites on device performance, four devices with different configurations were fabricated, and their photovoltaic parameters are summarized in Table 7.5, and J-V plots and external quantum efficiency (EQE) spectra are presented in Figure 7.6. Compared to the pristine Sn-PS cell (i.e., reference cell), all the Nx GO-incorporated cells showed substantial improvement in V OC , J SC , and FF. Notably, the Nx GO-based composites cells (i.e. controls 1 and 2, and champion cell) showed high V OC values of 0.94–0.96 V (Figure 7.6a), which is much higher than the typical range of 0.4–0.8 V reported previously

7.3 Carbon Additives-Based Perovskite Composites

Table 7.5

Photovoltaic parameters of Sn-PS solar cells with and without N0.12 GO inclusion.

Device

Device configuration

V OC (V)

J SC (mA/cm2 )

FF (%)

PCE (%)

Reference

FTO/PEDOT:PSS/Sn-PS/ PCBM/BCP/Au

0.897

19.39

57.54

10.01

Control 1

FTO/PEDOT:PSS-N0.12 GO/ Al2 O3 /Sn-PS/PCBM/BCP/Au

0.939

20.54

60.39

11.64

Control 2

FTO/PEDOT:PSS-N0.12 GO/ Al2 O3 -N0.12 GO/Sn-PS/ PCBM/BCP/Au

0.957

20.70

63.44

12.57

Champion

FTO/PEDOT:PSS-N0.12 GO/ Al2 O3 -N0.12 GO/

0.961

21.21

65.05

13.26

Sn-PS:N0.12 GO/PCBM/ BCP/Au Source: [64]/John Wiley & Sons.

(a)

(b)

Figure 7.6 (a) Current density–voltage plots and (b) EQE spectra and integrated current density for the Sn-PSCs with different configurations. Source: Reproduced with permission [64]. Copyright 2022, John Wiley & Sons.

[64]. These enhanced photovoltaic parameters were primarily attributed to the inclusion of Nx GO in HTL, IL, and AL, which promoted thicker films with larger grains, reduced non-radiative recombination, and improved band energy alignment between layers. Also, the Nx GO-incorporated devices yielded more light harvesting than the pristine cell (Figure 7.6b), suggesting that the Sn-PSCs compositing with Nx GO in HTL, IL, and AL can substantially improve solar cell performance. More importantly, compared to the reference cell, the champion device with the inclusion of Nx GO in HTL, IL, and AL exhibited outstanding long-term stability, retaining 91% of the initial PCE over 60 days without encapsulation (Figure 7.7a). The Nx GO-incorporated champion device also showed remarkable photo-stability, maintaining 99% of the initial PCE to light exposure for 30 min and then 95% after 60 min (Figure 7.7b). This work provides an effective strategy for fabricating efficient and stable Sn-based PSCs.

145

146

7 Composites-Based Pb-Free Perovskite Materials as Absorbers

(a)

(b)

Figure 7.7 Comparison of (a) long-term stability of Sn-PSCs without encapsulation in Ar and (b) photo-stability under continuous illumination in ambient air condition (25∘ C, 45%–50% humidity) with configurations of FTO/PEDOT:PSS/Al2 O3 /Sn-PS/PCBM/BCP/Au (reference) and FTO/PEDOT:PSS-N0.12 GO/Al2 O3 -N0.12 GO/Sn-PS:N0.12 GO/PCBM/BCP/Au (champion). Source: Reproduced with permission [64]. Copyright 2022, John Wiley & Sons.

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55 Tong, J., Jiang, Q., Ferguson, A.J. et al. (2022). Reducing energy disorder for efficient and stable Sn− Pb alloyed perovskite solar cells. Nat. Energy 7: 642–651. 56 Wang, J., Yu, Z., Astridge, D.D. et al. (2022). Carbazole-based hole transport polymer for methylammonium-free tin-lead perovskite solar cells with enhanced efficiency and stability. ACS Energy Lett. 7: 3353–3361. 57 Kapil, G., Bessho, T., Sanehira, Y. et al. (2022). Tin-lead perovskite solar cells fabricated on hole selective monolayers. ACS Energy Lett. 7: 966–974. 58 Hu, S., Kento, O., Richard, M. et al. (2022). Optimized carrier extraction at interfaces for 23.6% efficient tin-lead perovskite solar cells. Energy Environ. Sci. 15: 2096–2107. 59 Zhu, Z., Chueh, C.-C., Li, N. et al. (2018). Realizing efficient lead-free formamidinium tin triiodide perovskite solar cells via a sequential deposition route. Adv. Mater. 30: 1703800. 60 Deng, L., Wang, K., Yang, H. et al. (2018). Polymer assist crystallization and passivation for enhancements of open-circuit voltage and stability in tin-halide perovskite solar cells. J. Phys. D. Appl. Phys. 47: 475102. 61 Liu, C., Tu, J., Hu, X. et al. (2019). Enhanced hole transportation for inverted tin-based perovskite solar cells with high performance and stability. Adv. Funct. Mater. 29: 1808059. 62 Liu, G., Liu, C., Lin, Z. et al. (2020). Regulated crystallization of efficient and stable tin-based perovskite solar cells via a self-sealing polymer. ACS Appl. Mater. Interfaces 12: 14049. 63 Ran, C., Gao, W., Li, J. et al. (2019). Conjugated organic cations enable efficient self-healing FASnI3 solar cells. Joule 3: 3072–3087. 64 Mahmoudi, T., Kohan, M., Rho, W.Y. et al. (2022). Tin-Based Perovskite Solar Cells Reach Over 13% with Inclusion of N-Doped Graphene Oxide in Active, Hole-Transport, and Interfacial Layers. Adv. Energy Mater. 12: 2201977.

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8 Composite-Based Perovskite Materials in Tandem Solar Cells 8.1 Introduction The photovoltaic market is currently dominated by crystalline silicon (c-Si) technology due to its low cost and technological stability. Single-junction c-Si solar cells have achieved the highest power conversion efficiency (PCE) of 26.81%, approaching their Auger recombination-constrained S-Q theoretical limit of 29.4% [1, 2]. It is a great challenge to further improve their PCEs in a single-junction architecture. Tandem solar cells (TSCs) consist of two or more independent subcells stacked together, which can broaden the spectral response range of solar cells, reduce thermal energy losses, and maximize the conversion of light energy into electricity. Typically, TSCs have two representative connection structures: two-terminal (2T) and four-terminal (4T) configurations, which will be discussed in Section 8.2. The theoretical maximum PCEs for 2T and 4T TSCs can be evaluated using the detailed balance principle as a function of the bandgaps of the top and bottom cells [3, 4]. Figure 8.1 shows the theoretical maximum efficiency contour maps for TSCs in the 2T and 4T configurations, with the dashed lines depicting the peak efficiency for a wide range of top cell bandgaps and the dots showing the global maximum PCEs [5]. The theoretical PCEs for both 2T and 4T tandem configurations can exceed 46%, which is remarkably higher than the S-Q theoretical limit of 33% for single-junction solar cells under one-sun illumination. Besides, to obtain a PCE over 33% for both 2T and 4T tandem configurations, it provides great flexibility for bandgap selection of the top and bottom cells.

8.2 Configuration of Perovskite-Based Tandems TSCs are stacks of p-n junctions, each of which is formed from a semiconductor of different bandgap energy. Each junction using different bandgaps absorbs a different light wavelength of the solar spectrum. Multiple single-junction cells are stacked on top of each other with the highest bandgap on the top and the lowest at the bottom. Figure 8.2 shows that for typical TSCs, there are two configurations of mechanically integrated 4T and monolithically integrated 2T. In the 4T configuration, the

8 Composite-Based Perovskite Materials in Tandem Solar Cells 2T tandem

4T tandem

PCE (%) 46.0 40.9

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Figure 8.1 Theoretical efficiency limit for 2T and 4T tandems with different bandgap combinations. Source: [5]/John Wiley & Sons/CC BY 4.0. 4T

2T

Perovskite top cell

Perovskite top cell

Bottom tandem cell

Bottom tandem cell

Recombination layer

Figure 8.2

Schematic diagrams of 4T and 2T TSCs.

subcells can be fabricated separately and mechanically connected later. The subcells with their own active layer operate individually up to their highest efficiency. In this configuration, the optical losses are high due to the presence of transparent conductive oxide (TCO) layers at interfaces. However, 4T tandems are not commercially developed due to their complex manufacturing process. Furthermore, the operating individual subcells require additional units, such as inverters, wirings, and connections from individual cells, which greatly increase the system cost. In the 2T TSCs, subcells are fabricated on the same substrate and the whole device has two electrodes. The 2T TSCs require an interconnecting layer that can either be a highly doped tunnel junction or a TCO layer to provide optical and electrical connections of subcells. The 2T TSCs contain fewer electronic circuits with lighter weight and lower cost. However, the 2T TSCs have several challenges because they require careful optimization of current matching, compatibility of subcells in the fabrication process, and proper recombination and tunneling junctions.

8.2.1

Perovskite/Si Tandems

The efficiency of c-Si-based photovoltaic devices can be further enhanced by stacking WBG absorber materials on silicon cells. Numerical calculations demonstrate that combining perovskite with a bandgap of 1.72 eV and silicon with a bandgap of 1.12 eV in a 2T configuration can theoretically achieve a high efficiency

8.2 Configuration of Perovskite-Based Tandems

(a)

(b)

Figure 8.3 Schematic of widely used perovskite/silicon tandem structures: (a) single-polished silicon as the bottom subcell and (b) fully textured silicon as the bottom subcell.

of 43%, indicating a wide range of applications of this technology [6]. The first 2T monolithic perovskite/Si TSC was reported in 2015. The TSCs were made of MAPbI3 as the top cell and Si as the bottom cell connected by a recombination junction, enabling the high V OC of 1.65 V. In the first stage, the most reported 2T monolithic perovskite/Si TSCs are designed based on float-zone (FZ) silicon heterojunction (SHJ) bottom cells with a polished or low-textured front surface to be compatible with solution-processed perovskite films, as shown in Figure 8.3. It is relatively easy to achieve a uniform interconnection layer (ICL), WBG perovskite, and other charge transport or functional layers on the polished or low-textured Si surface through all-solution processes. By modifying the interface at WBG-perovskite layer and fullerene (C60 ) ETL with piperazinium iodide (PI), Albrecht et al. achieved a certified record PCE of up to 32.5% in the monolithic 2T perovskite/Si tandem device stack with a rear-textured and polished front SHJ cell [7]. However, the FZ silicon with double-polished or single-polished and low-textured front surface cannot provide sufficient light-trapping. Besides, it requires extra complicated manufacturing for chemical–mechanical polishing surfaces [8]. More importantly, the FZ silicon cells are not widely used in PV mass production, but Czochralski (CZ) silicon is a preferred fabrication method for silicon ingots due to its high throughput and low cost. It is highly desired to develop perovskite/silicon TSCs based on production-line compatible silicon bottom cells using CZ-silicon wafers with randomly distributed

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micrometer-sized pyramids on their surface. However, it is challenging to obtain uniform ICLs, charge transport layers, and the perovskite absorber layer on the fully textured silicon cells when using fully solution-based processes. In order to completely cover such micrometer-sized pyramids, it is necessary to combine both evaporation and solution-based deposition methods. For example, as shown in Figure 8.3b, the ITO and indium zinc oxide (IZO) layers are deposited by sputtering method; the ICLs generally contain NiOx and self-assembled monolayers (SAMs) prepared by sputtering and solution methods, respectively [9, 10]; the perovskite layer can be prepared by two methods: fully thermal evaporation or combination evaporation (first formation of inorganic frames, such as PbI2 and PbBr2 ) and solution processes (formation of perovskite layer by organic solution post-treatment, such as FAI and MABr solutions) [11, 12]; finally, Ag electrodes are prepared by thermal evaporation method. Note that such 2T TSCs based on production-line compatible silicon bottom cells with a record-high PCE of 34.6% have been achieved by Longi in 2024. Within only ten years, the PCE of 2T perovskite/Si TSCs has increased from 13.7% to 34.6%, demonstrating excellent commercial prospects.

8.2.2

All Perovskite Tandems

Another promising perovskite-based tandem technology is all perovskite tandems due to their exceptional properties such as low-temperature processes for whole devices, flexible and lightweight, and potentially low manufacturing costs. In 2T all perovskite-based TSCs, the front WBG absorber can be generally organic–inorganic lead halide perovskites (Eg > 1.65 eV, such as FACsMAPbBrx I3−x ) or all-inorganic perovskites (Eg > 1.70 eV, such as CsPbBrx I3−x ); lead-tin halide perovskites (Sn-Pb, Eg ∼ 1.20 eV) are generally as the back NBG absorber. The interconnecting layers (ICLs) play a crucial role in connecting top WBG and bottom NBG subcells, as well as for protecting the bottom subcell from damage caused by processing the top cell. As shown in Figure 8.4, most reported ICLs generally contain two structures: Normal ICLs PEDOT:PSS SnOx/Au (TCO) C60

Metal ETL

ICLs

NBG HTL CRL ETL WBG HTL TCO

Figure 8.4

Schematic of all perovskite tandem solar cells.

Simplify ICLs SnO2–x C60

8.2 Configuration of Perovskite-Based Tandems

(i) normal ICLs of C60 /SnOx /(Au or TCO)/PEDOT:PSS and (ii) pristine ICLs of C60 /SnO2−x [13, 14]. Most of the reported highly efficient all-perovskite TSCs have used normal ICLs of C60 /SnOx /(Au or ITO or IZO)/PEDOT:PSS [13, 15–18]. Recently, Tan and co [18] reported a record-high PCE of 28.5% (certified 28.0%) in all-perovskite TSCs by using normal ICLs of C60 /SnOx /Au/PEDOT:PSS to connect both bottom WBG perovskite and top NGB perovskite subcells, forming a configuration of glass/ITO/NiO/SAM/WBG perovskite/C60 /SnO2 /Au/PEDOT:PSS/NBG perovskite/C60 /BCP/Cu (SAM: 2PACz and MeO-2PACz). The complex ICLs generally contain four or more layers deposited by different processes, which may become a burden for actual applications. To reduce the number of layers, Huang and Co. reported an ICL in all-perovskite TSCs consisting merely of a C60 layer and a SnO2−x (0 < x < 1) layer [14]. The SnO1.76 layer that can be formed by the incomplete oxidization of tin has ambipolar carrier transport property due to the presence of a large density of Sn2+ . The C60 /SnO1.76 ICL forms Ohmic contacts with both wide- and narrow-bandgap perovskite subcells with low contact resistivity. Thus, these ICL structures can achieve efficiencies of 24.4% and 22.2%, respectively, for small-area TSC (5.9 mm2 ) and large-area TSC (1.15 cm2 ).

8.2.3

Perovskite/Organic Tandems

Due to the large chemical composition and bandgap tunability of organic semiconductors, perovskite/organic TSCs have also been considered to a great potential for high-throughput and cost-effective production of flexible and lightweight next-generation thin-film photovoltaics. As shown in Figure 8.5a, perovskite/organic TSCs also contain an ICL to connect WBG perovskite and NBG organic subcells. In general, the structures of ICL can be composed of C60 /SnOx layers, charge recombination layer (CRL), and molybdenum oxide (MoOx ). For example, Hou and coauthors developed a CRL with 4-nm-thick sputtered IZO layer inserted between organic bathocuproine (BCP) and MoOx as an ICL structure to enhance electrical properties and transmittance in the near-infrared region [19]. Recently, a record efficiency of 25.22% (24.27% certified) was obtained in the

Metal NBG organic subcell

Top contact ETL

MoOx CRL c60/SnOx

WBG perovskite HTL IZO ZnO CdS

ICLs

CIGS

WBG perovskite subcell

Back contact Glass

TCO

(a)

(b)

Figure 8.5 Schematic of (a) perovskite/organic tandem solar cells and (b) perovskite/CIGS tandem solar cells.

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perovskite/organic TSCs by defect passivation of WBG perovskites and an efficient ICL structure of C60 /BCP/Au (∼0.5 nm)/MoOx (10 nm) [20]. It has also predicted that a theoretical upper limit of perovskite/organic monolithic 2T heterojunction TSCs is 46.1% by rational design ICL structures and suppression of device interface recombination [21].

8.2.4

Perovskite/CIGS Tandems

The first fabrication of perovskite/CIGS TSCs was done by mechanically-stacked a semitransparent perovskite top cell and CIGS as bottom cells to design 4T tandems, which resulted in a PCE of 18.6% [22]. Afterward, Shen and Co. reported a PCE of 23.9% in 4T perovskite/CIGS TSCs by using multi-cation perovskite compositions as an absorber in semitransparent perovskite top cells. Recently, a record PCE of 28.4% was obtained in 4T perovskite/CIGS TSCs by tuning bandgap of CIGS solar cells [23]. Compared to 4T configurations, monolithic 2T perovskite/CIGS TSCs still show lower efficiencies due to difficulty in solution processing when interconnecting the perovskite top cell on CIGS surface with nm size roughness. Nevertheless, monolithic 2T perovskite/CIGS TSCs have presented some advantages. They can be produced on flexible substrates and, similar to all thin-film technologies, show a significantly lower carbon footprint per kWh produced, thus being an efficient, flexible, sustainable, and lightweight solution. The basic structure of 2T perovskite/CIGS TSCs is shown in Figure 8.5b. The CRL of TCO (such as IZO) can connect CIGS bottom subcell and perovskite top subcell. To fully coverage the rough CIGS surface, Albrecht and Co. used SAMs (such as 2PACz and MeO-2PACz) to act as simple hole-selective contacts in tandems, leading to a record PCE of 24.2% [24].

8.3 Perovskite Alloy-Based Composites as Absorbers The bandgap of perovskite materials can be continuously adjusted via composition engineering. All the A-, B-, and X-sites have significant influences on the bandgap of perovskite materials. For example, compared to MAPbI3 (∼1.55 eV) and FAPbI3 (∼1.48 eV), CsPbI3 has an ideal bandgap of 1.73 eV, which is highly appropriate for the development of perovskite-based TSCs. However, it is difficult to fabricate stable CsPbI3 films under ambient conditions due to the phase transition from black metastable α-phase (cubic) to yellow and more stable non-perovskite δ-phase [25]. Thus, mixing both multi-cations (MA, FA, or Cs) and -anions (I, Br, or Cl) can yield stable perovskite films with bandgap tunability [26]. To develop efficient WBG perovskite top cells in TSCs, the optimal bandgap values of WBG perovskites should be a range of 1.67–1.75 eV [27]. Besides, substituting Pb with Sn will result in a remarkable reduction in bandgap (∼1.4 eV) that combines with WBG perovskites to fabricate efficient all-perovskite TSCs. In this section, A-, B-, and X-site alloy-based composites as light absorbers in perovskite-based TSCs (such as perovskite/Si, all-perovskite, and perovskite/organic tandems) are introduced. Besides, the

8.3 Perovskite Alloy-Based Composites as Absorbers

materials design of composite-based perovskites, film fabrication methods, and device performance in various TSCs are further presented.

8.3.1

A-Site Alloy-Based Composites

A-site alloying method can tune both bandgaps and optoelectronic properties of hybrid perovskites, as well as strengthen their structures, thus enabling highly efficient and stable perovskite photovoltaics. For example, Qin et al. [28] fabricated four WBG perovskites: FA0.48 MA0.37 Cs0.15 PbI2.23 Br0.77 (1.65 eV), FA0.57 MA0.43 PbI2.4 Br0.96 (1.69 eV), FA0.5 MA0.38 Cs0.12 PbI2.04 Br0.96 (1.69 eV), and FA0.51 MA0.38 Cs0.11 PbI1.85 Br1.15 (1.72 eV). They found that Cs alloying can promote crystal growth and suppress defect formation. As-fabricated perovskite/Si 2T TSCs with a 1.69 eV-bandgap perovskite absorber of FA0.5 MA0.38 Cs0.12 PbI2.04 Br0.96 composition delivered a PCE of 22.22%. Furthermore, Duong et al. [29] found that a smaller ionic size of Rubidium (Rb) addition into the triple-cation perovskites can enlarge the bandgap to 1.73 eV (FA0.75 MA0.15 Cs0.10 Pb(I0.66 Br0.33 )3 + 5% RbI), resulting in improved crystallinity, moisture resistance, photo-stability, high carrier mobility, and reduced defect migration and J-V hysteresis. They further used a 23.9% silicon cell to fabricate 4T perovskite/Si TSCs, leading to a PCE of 26.4% (10.4% from the silicon cell). Afterward, Wu et al. [30] also obtained a 1.62 eV-bandgap Rb doped Cs0.07 Rb0.03 FA0.765 MA0.135 Pb(I0.85 Br0.15 )3 with good uniformity and low pinhole density. By combining this WBG top cell with the homojunction c-Si bottom cell, they obtained 1 cm2 2T perovskite/Si TSCs with stabilized PCE of 22.5% and V oc of 1.75 V. All inorganic composite-based perovskites have been also explored to fabricate perovskite-based TSCs. Under light illumination, all inorganic perovskites (CsPbIx Br1−x ) have also undergone severe phase segregation, just like hybrid WBG perovskites due to the high content of Br [3]. Wang et al. [31] found that a smaller cation of Rb can be doped into the inorganic perovskite lattice to reduce light-induced phase segregation of all inorganic perovskites. Besides, they also observed that Rb doping can widen the bandgap of inorganic perovskites. Thus, such results also suggest that Rb-Cs composite-based all inorganic perovskites require a higher I content for the same bandgap than that of pure Cs-based counterparts. For example, a bandgap of 2.0 eV in the Rb0.15 Cs0.85 PbI1.75 Br1.25 perovskite has a lower Br content than the same bandgap of CsPbI1.4 Br1.6 . Furthermore, they found that the Rb-Cs composite-based all inorganic perovskites have a larger degree of lattice distortion than pure Cs-based counterparts, suggesting suppressed light-induced phase segregation. This is because Rb alloy can increase the energy barrier of halide ion migration in Rb-Cs composite-based all inorganic perovskites. Finally, they used the 2.0-eV Rb0.15 Cs0.85 PbI1.75 Br1.25 perovskites in all-perovskite triple-junction TSCs and achieved an efficiency of 24.3% (certified 23.29%). Such all-perovskite triple-junction TSCs also showed high operational stability at maximum power point under room temperature and air mass 1.5 global (AM 1.5G) one-sun illumination. These results indicate that Rb-Cs composite-based all inorganic perovskites offer a promising route to reduce light-induced phase segregation and enhance performance and stability of perovskite-based TSCs.

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8.3.2

X-Site Alloy-Based Composites

X-site alloy is the most widely used approach to achieve efficient single-junction and multi-junction solar cells. In general, X-site anions can be I, Br, Cl, or thiocyanate (SCN), and two or more components are required to obtain desirable bandgaps and structures of perovskites. The amount of Br in the X-site alloy-based perovskites has a significant impact on their bandgap range. For example, Noh et al. systematically studied the structural, optical, and electronic properties of Br-I alloy-based perovskite system MAPb(I1−x Brx )3 by changing the contents of Br [32]. It was found that the structure of MAPb(I1−x Brx )3 can be transformed from an unstable tetragonal phase (x = 0–0.13) to a stable cubic phase (x ≥ 0.2), which is ascribed to the reduction of lattice parameter with an increase in the amount of Br. Thus, the cubic phase MAPb(I1−x Brx )3 (x ≥ 0.2) based solar cells are more stable against high humidity when compared to tetragonal phase (x = 0–0.13) based devices. Besides, the bandgap of MAPb(I1−x Brx )3 can be changed from 1.58 eV (x = 0) to 2.28 eV (x = 1). Thus, adding more bromine (Br) leads to more stable perovskites due to the stronger bond in the lead (Pb)-Br bond compared to the Pb-iodine (I) bond, which is attributed to the smaller ionic radius of Br (1.96 Å) when compared to the ionic radius of I (2.20 Å), and the higher electronegativity of Br. However, light induces Br-I halide segregation that can lead to an inhomogeneous energy landscape within the Br-rich WBG-perovskites, forming I- and Br-rich regions with different bandgaps. Thus, such a Br-I mixed halide heterogeneity has led to low open-circuit voltage and operational stability of WBG PSCs and TSCs. Xu et al. [26] reported a method for incorporating large amounts of Cl into the lattice of WBG perovskites, building triple-halide perovskites (FAMACsIBrCl) with a bandgap of 1.67 eV. The photocarrier lifetime and charge-carrier mobility were remarkably improved in triple-halide perovskite films. It was also observed that a suppression of light-induced phase segregation in films even at 100-sun illumination intensity and less than 4% degradation in semitransparent top cells after 1000 hours of maximum power point operation at 60∘ C. Finally, they fabricated 27%-efficient monolithic 2T perovskite/Si (front polished) TSCs. To be compatible with the widely used solution-based perovskite deposition techniques, most 2T perovskite/Si TSCs are fabricated by using a front-side polished or low textured (3 μm) Si surface, Tan and Co. [33] developed a hybrid two-step deposition method consisting of sequential co-evaporation and spin-coating processes, as shown in Figure 8.6a. In detail, a conformal inorganic framework was first formed by co-evaporating PbI2 and CsBr on the pyramid-shaped textured silicon; then, an organic salt solution containing FAI and FABr with anion additives of a mixture of MACl and MASCN was sequentially spin-coated onto the inorganic framework, followed by pre-annealing at 90∘ C in N2 atmosphere to volatilize the solvent and then annealing at 150∘ C in air to obtain a homogeneous perovskite film. To compare

8.3 Perovskite Alloy-Based Composites as Absorbers Co-evaporation

Organic salt deposition

Inorganic frame

FAI FABr

Pre-anneal at 90°C in N2

MACI MASCN

Annealing at 150°C in air

CsBr

Pbl2

(a) Control

MACI

MASCN

MA(CI0.5SCN0.5)

(b)

Figure 8.6 (a) Schematic of the hybrid two-step deposition method, (b) top-view (up) and cross-sectional (down) SEM images of perovskite thin films without (control) and with different additives (MACl, MASCN, and MA(Cl0.5 SCN0.5 )) on textured silicon. Source: Reproduced with permission from [33]/John Wiley & Sons.

the effect of anion additives on perovskite film morphology and coverage on the fully textured Si surface, top-view and cross-view of SEM images are characterized, as shown in Figure 8.6b. Compared to other three perovskite films, perovskite films containing both MACl and MASCN additives can be homogeneously formed on the textured Si surface with relatively large grain sizes. The configuration of 2T fully textured monolithic perovskite/Si TSCs is shown in Figure 8.7a. Figure 8.8b shows the cross-sectional SEM image of the corresponding tandem and photo of actual tandem device with an aperture area of 1.05 cm2 . It can be clearly observed that the textured surface with pyramid sizes of 3–5 μm was well-covered by the conformally coated perovskite films and other functional layers. As-fabricated 2T monolithic perovskite/Si TSCs achieved a champion PCE of 28.9% (V oc = 1.85 V, J sc = 19.8 mA/cm2 , and FF = 78.9%) with a stabilized PCE of 28.6% (Figure 8.6d). As shown in Figure 8.6c, both perovskite and silicon subcell shows a well-matched integrated J sc of 20.1 mA/cm2 from the EQE spectra, which contributes to the pyramidal Si textured structures with a low reflection loss below 2% in the wavelength range from 380 to 1100 nm. Furthermore, the encapsulated TSCs also showed excellent operational stability, maintaining 80% for 2000 h MPP tracking under one-sun illumination in ambient air with a relative humidity of 30%–40%.

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Ag MgF2 IZO SnO2 C60 Perovskite NiO/SAMs ITO a-Si:H (i/n) c-Si (n)

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a-Si:H (p/i) ITO Ag (a)

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Figure 8.7 (a) Configuration of perovskite/Si tandem, (b) cross-sectional SEM images of perovskite/Si tandem with additive and photographs of actual tandem cell with front and back sides, (c) EQE spectra of a current-matched fully textured monolithic perovskite/Si tandem cell, and (d) MPP tracking of the tandems and PCE distributions of 20 individual tandem devices. Source: Reproduced with permission from [33]/John Wiley & Sons.

8.3.3

B-Site Alloy-Based Composites

As we discussed in Chapter 2, the ion radius of Sn2+ (1.18 Å) is very close to that of Pb2+ (1.19 Å), namely, APbX3 and ASnX3 perovskite structures have almost the same tolerance factors and ns2 electronic structures. The B-site Pb can be partially or completely substituted by Sn to obtain NBG Pb-Sn alloy composite-based perovskites (0.8–1.20 eV) [35]. Thus, NBG PSCs show great potential as bottom subcells to connect WBG perovskite top subcells for all-perovskite TSCs [36]. Early in 2017, Zhao and colleagues [37] used a B-site alloy strategy to fabricate (FASnI3 )0.6 (MAPbI3 )0.4 perovskite composites with a bandgap of 1.25 eV and spectral response up to 1030 nm. They fabricated a thicker layer of Pb-Sn alloy-based perovskite (thickness of ∼600 nm) that showed a long carrier lifetime over 250 ns. Then, as-fabricated 4T all-perovskite TSCs by combining 1.58 eV-perovskite FA0.3 MA0.7 PbI3 as the top subcell and 1.25 eV-(FASnI3 )0.6 (MAPbI3 )0.4 as the bottom subcell presented a PCE over 21%. After that, they further improved carrier lifetime (∼1000 ns) and thickness (∼1000 nm) of the (FASnI3 )0.6 (MAPbI3 )0.4 by using

8.4 Additives-Based Perovskite Composites as Absorbers

N2 quenching

Coating speed Blade

Ink + F6TCNNQ

Mix-SAM

PCE ~ 29.7% Figure 8.8 Schematic of blade-coated perovskite inks with F6TCNNQ additives on textured silicon bottom subcells. Source: Reproduced with permission from [34]/American Chemical Society.

guanidinium thiocyanate (GuaSCN) additives [38]. A high PCE of over 20% and J sc over 28 mA/cm2 for single-junction solar cells was achieved. Besides, they enabled 25%-efficient 4T and 23.1%-2T all-perovskite TSCs. Thus, high-quality Sn-Pb alloy composite-based perovskite thin films with greater than micrometer thickness are needed to permit sufficient light harvesting, as well as several micrometer-long carrier diffusion lengths to ensure effective charge-carrier transport and extraction. It is significant to obtain a high photocurrent density in Pb-Sn alloy compositebased perovskite subcells for highly all-perovskite TSCs, yet this is challenging owing to the short carrier diffusion length within Pb-Sn perovskites. In order to further improve photocurrent density and carrier diffusion length, Tan and Co. used ammonium cation (4-trifluoromethyl-phenylammonium) to passivate the surface of Pb-Sn alloy composite-based perovskite films [15]. As a result, such perovskite films showed an improved carrier diffusion length of over 5 μm and thickness of over 1.2 μm, yielding a high J sc of 33 mA/cm2 in single-junction solar cells with a PCE of 22.2%. More importantly, a certified efficiency of 26.4% was achieved in all-perovskite TSCs, which exceeded that of the best-performing single-junction perovskite solar cells for the first time.

8.4 Additives-Based Perovskite Composites as Absorbers As we discussed in Chapter 6, perovskite composites incorporating functional additives can significantly improve the structures and optoelectronic properties of perovskites, which enable highly efficient and stable single-junction PSCs. In this section, we will discuss the effects of additive-based perovskite composites as absorbers on the performance and stability of perovskite-based TSCs. The

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structures, crystal growth dynamics, and optical and electronic properties of WBGand NBG-perovskite composites in perovskite-based TSCs will be presented.

8.4.1

Additive-Based Wide-Bandgap Perovskite Composites

To minimize V oc loss in a WBG perovskite subcell is significant for obtaining a high V oc in the TSCs. Although the Br-I alloy-based WBG perovskites can obtain the optimum bandgap for tandems, the V oc deficit unfortunately increases with the bandgap. Besides, compared to pure iodide perovskites, Br-rich WBG perovskites present bulk deep trap density and have undergone more severe photoinduced halide segregation, which results in significant V oc losses and J sc mismatches in tandems [39]. Huang and colleagues [40] developed Br-I alloy-based WBG perovskite composites incorporating trimethylphenylammonium tribromide (TPABr3 ) additives (Cs0.1 FA0.2 MA0.7 Pb(I0.85 Br0.15 )3 -TPABr3 ). They found that such Br-I alloy-based WBG perovskite composites have a reduced bulk deep trap density, yielding an improved V oc of 1.25 V and 1.92 V in single-junction and TSCs, respectively. Furthermore, they prepared such Br-I alloy-based WBG perovskite composites on textured silicon by air-knife-assisted blade-coating method for the fabrication of double-side textured perovskite-silicon tandem cells. The textured silicon bottom cells can be fully covered with micrometer-thick perovskite composite-based films. The champion TSCs presented hysteresis-free J-V curves, with a PCE of 28.6% with a high V oc of 1.92 V, J sc of 19.0 mA/cm2 , and FF of 0.785. To further improve WBG perovskite crystallization quality in the textured silicon bottom cells, Subbiah et al. [34] introduced 2,2′ -(perfluoronaphthalene-2,6diylidene)dimalononitrile (F6-TCNNQ) as a molecular p-dopant in WBG perovskite precursor ink to fabricate high-quality micrometer-thick WBG perovskite films on textured silicon bottom cells by blade-coating method, as shown in Figure 8.8. Compared to control films, WBG perovskite composite films with F6-TCNNQ additives greatly influenced crystallization dynamics and perovskite grain morphology, leading to the formation of single vertical grains. Besides, they found that F6TCNNQ acts as a strong oxidizing dopant molecule and also can reduce the Pb0 /Pb2+ ratio in the perovskite and thus lower trap states. Thus, as-fabricated double-side textured perovskite/Si TSCs by the blade-coating method showed a champion efficiency of 29.7%.

8.4.2

Additive-Based Narrow-Bandgap Perovskite Composites

In general, to obtain a narrow Eg below 1.2 eV in Pb-Sn alloy-based metal halide perovskites, it requires a Sn content of 50%–60% [35, 41]. However, Sn2+ can be easily oxidized to Sn4+ upon air exposure or even in an inert atmosphere with a trace amount of oxygen, thus it becomes a challenge to fabricate stable and high-quality NBG perovskite films and their related solar cells. Some methods have been explored to overcome this limitation, such as purifying SnF2 sources and adding antioxidant additives to precursor solutions. Lee et al. [42] developed SnF2 -pyrazine complex to facilitate the homogeneous dispersion of SnF2 into

8.4 Additives-Based Perovskite Composites as Absorbers

H+ O– O S O

H+ O– O S O

K+ O– O S O

HO

Re

HO O S O

OH

NH2

–O

OH (a)

PSA

APSA

KHQSA

(b)

Sn2+

SnCl2 aggregate

Spin-coating

CB

W/O FAI+Snl2 10% SnCl2 (additive)

add

itive Antioxidant outer layer

ITO/NiOx

With a

dditiv e

(c)

Figure 8.9 (a) Molecular structures of PSA, APSA, and KHQSA. (b) Schematic illustration of the interaction between the additive functional group and Sn2+ ion. (c) Schematic illustration of the preparation of the Sn-based perovskite film. Source: Reproduced with permission [43]. Copyright 2019, John Wiley & Sons.

perovskite film, prevent the unwanted Sn2+ oxidation and reduce defect densities in Sn-halide perovskites. Tai et al. [43] also introduced a kind of antioxidant additive incorporating a sulfonate functional group into the perovskite precursor solution to prevent Sn2+ oxidation. The molecular structures of antioxidant additives are shown in Figure 8.9a. Besides, the sulfonate group (SO3 − ) could interact with Sn2+ in solution via coordination interactions and electrostatic attraction, as illustrated in Figure 8.9b. They also found that KHQSA contained two hydroxyl (−OH) groups and had higher antioxidant activity. Interestingly, compared to the pristine perovskite film with large SnCl2 aggregates, Sn perovskite-based composite films incorporating additives have a uniform surface morphology with the formation of an antioxidant outer layer, i.e. SnCl2 -additive complex layer, as shown in Figure 8.9c. Such a SnCl2 -additive complex layer can greatly enhance the oxidation stability of the perovskite film. Based on previous work, Tan and Co. [13] reported a strategy to reduce the formation of Sn4+ by adding metallic tin into NBG Pb-Sn perovskite precursor. Besides, they also found that the presence of Sn4+ in the precursor solution induced the formation of Sn vacancies. However, Sn vacancies can be suppressed in NBG Pb-Sn perovskite films when incorporating metallic tin in a precursor solution. As a result, 24.8%-efficient monolithic all-perovskite TSCs were achieved. Further, they used co-additives incorporating metallic tin and formamidine sulfinic acid (FSA) as antioxidant reactants that were added into Sn-Pb perovskite precursor solutions to prevent Sn2+ oxidize to Sn4+ . Furthermore, because FSA is less volatile than

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8 Composite-Based Perovskite Materials in Tandem Solar Cells

DMSO, the introduction of FSA into the precursor solution could result in delayed and more uniform crystallization. As-fabricated all-perovskite TSCs achieved a PCE of 25.6%, and showed better operational stability over 500 hours under one-sun illumination in ambient conditions [44].

8.4.3

2D-3D-Based Wide-Bandgap Perovskite Composites

Br-rich WBG perovskite films have been encountered with more defects, leading to large V oc loss and stability issues. However, WBG PSCs with high V oc but low V oc loss should be well developed to obtain high PCEs of TSCs. From Chapter 6 (see detail in Section 6.4), it can be found that the content of the 2D perovskites should be carefully controlled due to their high bandgaps and electrically insulating properties. To fabricate highly stable perovskite-based TSCs, the development of 2D-3D composite-based heterostructures is a general method. Yang and Co. [45] introduced larger organic cation 2-(4-fluorophenyl)ethylamine hydroiodide (F-PEAI) into WBG perovskite precursors to obtain composite-based 2D-3D perovskite heterostructures for all-perovskite TSCs. Such a larger organic cation has a high activation energy of ion migration, thus suppressing the halide phase segregation. Besides, compared to pristine WBG 3D-based single-junction solar cells (V oc = 1.23 V), solar cells based on composite-based 2D-3D perovskite heterostructures showed improved V oc from 1.23 to 1.35 V, resulting in an improved PCE from 16.0% to 19.4%. As-fabricated all-perovskite TSCs also showed a high PCE of 27% and better operational stability than that of control-based tandems. To further develop stable MA-free 2D-3D composite-based heterostructures, Tan and Co. developed a generic 3D to 2D conversion strategy to induce a preferential growth of wider dimensionality (n ≥ 2) atop WBG perovskite layers [46]. As-formed 2D-3D perovskite heterostructures allowed for easier charge extraction and transportation, decreased non-radiative interfacial recombination, and prevented photo-induced halide segregation. Moreover, as-fabricated all-perovskite TSCs exhibited a stabilized PCE of 28.1% and retained 90% of their initial efficiency after 855 h under continuous one-sun illumination. Another interesting results have been obtained with composite-based 2D-3D perovskite heterostructures at the buried interface, which was demonstrated in a perovskite/Si TSC in Figure 8.10. By changing the incident light in PL spectra measurements, low n values corresponding to 2D perovskites were observed in the composite-based 2D-3D perovskite heterostructure (Figure 8.10a,b). The XRD patterns further confirmed the presence of 2D perovskites (Figure 8.10c, left). By varying the incident angles, it was observed that most 2D perovskites are located at the buried interface of 3D perovskites (Figure 8.10c, right), which can significantly passivate defects located at the buried interface of WBG 3D perovskites. Figure 8.10d shows the schematic of 2D perovskites formed at the buried interface of 3D perovskites. Furthermore, the perovskite/Si TSC were fabricated based on such composite-based 2D-3D perovskite heterostructures, as shown in Figure 8.10e. Compared to pristine WBG 3D perovskites, the perovskite/Si tandem

(a)

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Figure 8.10 (a,b) PL spectra with various incident lights for (a) control and (b) 2D/3D perovskite films, (c) GIXRD spectra of control and 2D/3D perovskite films, (d) schematic of 2D perovskites formed at buried interface of 3D perovskites, (e) perovskite/Si tandem configuration, and (f) J-V curves.

8 Composite-Based Perovskite Materials in Tandem Solar Cells

incorporating the composite-based 2D-3D perovskite heterostructures showed improved performance (Figure 8.10f).

8.4.4

2D-3D-Based Narrow-Bandgap Perovskite Composites

As discussed in Section 8.3.3, NBG Pb-Sn perovskites are attractive for the fabrication of efficient all-perovskite TSCs. The Sn2+ oxidation in Pb-Sn perovskites has been widely accepted as a cause of high trap densities and a negative impact on solar cell device stability. Beyond that, Li et al. [47] found the presence of phase segregation in NBG Pb-Sn perovskites, which is also detrimental to the performance and stability of Pb-Sn perovskite-based devices. To solve this issue, they developed vertically aligned 2D-3D Pb-Sn mixed heterostructures by one-step vacuum-assisted blade-coating 4-fluorophenethylamine (FPEAI)-modified (MAPbI3 )0.75 (FASnI3 )0.25 perovskite precursors. As shown in the grazing incidence wide-angle X-ray scattering (GIWAXS) images and PL spectra in Figure 8.11a–c, the (FPEAI)-modified Pb-Sn perovskite films have vertically aligned perovskite crystal growth and formation of 2D perovskites in GBs. Figure 8.11d shows the schematic of Pb-Sn mixed perovskite crystals w/o FPEAI (green) and FPEAI (red) treatment. Such vertically

2.5

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Figure 8.11 (a,b) GIWAXS spectra of Pb-Sn mixed perovskite films (a) w/o FPEAI and (b) FPEAI treatment, (c) normalized PL spectra of Pb-Sn mixed perovskite films w/o FPEAI (green) and FPEAI (red) treatment, (d) schematic illustration of Pb-Sn mixed perovskite crystals w/o FPEAI (green) and FPEAI (red) treatment. Source: Adapted from [47].

8.5 Composite-Based Interconnection Layers (ICLs)

aligned 2D-3D composite-based crystals in combination with phase-pure Pb-Sn perovskite contribute to the remarkably enhanced carrier extraction and spectral response. Finally, (FPEAI)-modified Pb-Sn PSCs showed excellent photovoltaic performance and air stability. Compared to WBG 2D-3D perovskites that are widely used in perovskite-based tandems, the use of NBG 2D-3D Pb-Sn perovskites in tandems has been less reported. However, there is a great potential to develop highly efficient and stable all-perovskite TSCs based on NBG 2D-3D composite-based Pb-Sn perovskites in the future.

8.5 Composite-Based Interconnection Layers (ICLs) The introduction of an ICL in monolithic tandems is an additional key parameter that has a strong influence on the overall performance of TSCs. The ICL should present good electrical, optical, and robust interconnections between the subcells. Besides, a robust interconnection of the ICL is greatly needed in tandems to ensure the chemical stability of the front cell, preventing the destruction by strong polar solvents, such as dimethylformamide (DMF) and dimethyl sulfoxide (DMSO). To enhance the performance and stability of perovskite-based TSCs, the development of composite-based ICLs has been considered an efficient method for robust interconnection of both bottom and top subcells.

8.5.1 Composite-Based Interconnection Layers (ICLs) in Perovskite/Si Tandems As we discussed in Section 8.2, the ICL plays an important role in efficiently connecting both bottom and top subcells for high-efficiency monolithic tandem devices. As widely reported, SAMs are highly favorable as the ICL for efficient perovskite/Si TSCs due to their negligible optical- and electrical-loss ability. However, the inhomogeneity of SAMs results in defects at the interface between SAMs and TCO. Such uncompacted and inhomogeneous SAMs on the TCO will result in undesirable current leakage and severe non-radiative recombination, which can induce large V oc and FF losses in perovskite/Si TSCs. Wang and Co. [9] systematically studied three types of ICLs in perovskite/Si TSCs, i.e. NiOx , MeO-2PACz (2([2-(3,6-dimethoxy-9H-carbazol-9-yl)ethyl]phosphonic acid)), and composite-based NiOx /MeO-2PACz. They carried out conductive atomic force microscopy (C-AFM) images of the Si/ITO, Si/ITO/NiOx , Si/ITO/MeO-2PACz, and Si/ITO/NiOx /MeO-2PACz films to explore the homogeneity of such ICLs. It could be observed that the C-AFM image of ITO films covered on SHJ displayed an almost white image, suggesting a cluster of high current regions and good conductivity (Figure 8.12a). The Si/ITO/NiOx film also showed lots of holes with high current due to the high energy of the sputtering technology, indicating the continuous NiOx on the ITO surface (Figure 8.12b). Even though the MeO-2PACz exhibited better uniformity than the sputtered NiOx , MeO-2PACz still presented many high current areas (Figure 8.12c). When they prepared the MeO-2PACz

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8 Composite-Based Perovskite Materials in Tandem Solar Cells

C-AFM current

301.0 nA

200.0 pA

–1.5 μA

–100.0 pA

C-AFM current

200.0 nm

(a)

C-AFM current

(c)

200.0 nm

(b)

200.0 pA

200.0 pA

–1.00.0 pA

–100.0 pA

C-AFM current

200.0 nm

200.0 nm

(d)

Figure 8.12 (a–d) Conductive atomic force microscope (C-AFM) images of (a) Si/ITO substrate, (b) Si/ITO/NiOx film, (c) Si/ITO/MeO-2PACz film, and (d) Si/ITO/NiOx /MeO-2PACz film. Source: Reproduced with permission from [9]/Elsevier.

on a NiOx film, a more uniform MeO-2PACz film with better coverage with low current was obtained (Figure 8.12d). Such results indicate that composite-based NiOx /MeO–2PACz as ICLs can significantly reduce the current leakage-induced non-radiative recombination loss. Furthermore, monolithic 2T perovskite/Si TSCs were fabricated. The complete structure of TSCs is shown in Figure 8.13a: Ag/ITO/heterojunction Cz–silicon/ITO/NiOx /MeO–2PACz/Perovskite/PEAI/C60 /SnO2 /Ag. Figure 8.13b displays the J-V curves for three ICL-based tandem devices. Compared with a single layer of ICL NiOx or MeO-2PACz, the perovskite/Si TSCs based on the NiOx /MeO-2PACz composite-based ICLs showed substantial improved V OC of 1.87 V and FF of 81.8%, leading to a PCE of 28.47%. They also found that there is a significant gap in the FF values in TSCs using a single SAM layer or composite-based ICLs, that is, an average FF of 79.62% (single SAM layer) and 81.45% (NiOx /SAM composite-based ICLs). The NiOx /MeO-2PACz ICL-based tandems showed well-matched J sc values in the both top and bottom subcells (Figure 8.13c). Figure 8.13d depicts the composite-based ICLs fabricated on silicon solar cells and briefly describes the interaction between MeO-2PACz and NiOx and the ITO layer, showing well anchored on the surface of NiOx and an appropriate energy level band

mA/cm2

mA/cm2

(a)

(b)

mA/cm2

(c)

(d)

Figure 8.13 Performance of hybrid ICLs on Perovskite/Si tandem solar cells: (a) Device structure of monolithic Perovskite/Si tandem solar cells. (b) J-V curves of champion tandem devices based on three types of ICLs (inset is the photograph of a tandem with front and back sides). (c) EQE curves of champion tandem device with hybrid ICLs NiOx /MeO-2PACz. (d) The roles of hybrid interconnecting layer in perovskite/Si tandem solar cells. Source: Reproduced with permission from [9]/Elsevier.

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diagram. By XPS spectra analysis, they found that MeO-2PACz could be anchored on both the surface of NiOx and ITO, but MeO-2PACz had a robust connection with the surface of NiOx . Compared to the single-ICL MeO-2PACz-based perovskite/Si TSCs, perovskite/Si TSCs based on the composite-based NiOx /MeO–2PACz ICLs showed better operational stability while retaining 77% of the initial efficiency over 620 h.

8.5.2 Composite-Based Interconnection Layers (ICLs) in All Perovskite Tandems The formation of monolithic all-perovskite TSCs involves the connection of an NBG Sn-Pb perovskite back subcell and a WBG Br-I perovskite front cell. Thus, an interdiffusion barrier and no chemical reaction of interface contacts are required to ensure the chemical stability of the back cell against solvents for solution-processed tandem [48]. In general, the ICLs require highly compact, efficient interface contacts and appropriate charge transport. The SAMs, such as 2-carbazol-9-ylethylphosphonic acid (2PACz), MeO-2PACz and [4-(3,6-dimethyl-9H-carbazol-9-yl) butyl] phosphonic acid (Me-4PACz), have applied to the most advanced monolithic 2T perovskite/Si TSCs due to their advantages of scalable processing films, negligible optical and electrical loss, and appropriate energy level diagrams [49, 50]. Li and Co. [11] deposited composite-based SAMs incorporating 2PACz and MeO-2PACz on ITO/NiOx NPs as ICLs to improve interface contact, facilitate hole extraction, and reduce interface recombination. Due to the significantly different molecular dipole moments of 2PACz and MeO-2PACz (2.0 debye versus 0.2 debye), [51] such composite-based SAMs could tune the energy-level alignment between NiOx and WBG perovskite. The structure of both SAMs is shown in Figure 8.14b. They found that composite-based SAMs with a molar ratio of 3 : 1 (2PACz to MeO-2PACz) enabled the best PV performance of flexible PSCs. When such composite-based SAMs were selected as ICLs in flexible all-perovskite TSCs, an efficiency of 24.7% (certified 24.4%) was obtained. Furthermore, molecule-bridged interfaces allowed flexible all-perovskite TSCs to maintain their original performance after 10,000 cycles of bending at a radius of 15 mm. All-perovskite TSCs typically have an NBG perovskite back subcell that is deposited on the top of a WBG perovskite front subcell, as shown in Figure 8.4. Thus, the NBG perovskite back subcell is easily exposed to air, which can oxidize the NBG perovskites and finally reduce the performance of all all-perovskite TSCs. Wang et al. [52] demonstrated a substrate-configured all-perovskite TSC by reversing the processing order, that is, depositing back NBG perovskite subcell first on a substrate and then depositing the front WBG perovskite subcell on the top of back subcell, which can bury oxidizable NBG perovskite deep in the device stack. Due to the change in incident light direction, the substrate configuration widens the choice of flexible substrates, allowing the use of opaque materials such as metal foils and metal-coated polymer foils. The device structure of substrate-configured all-perovskite TSCs is shown in Figure 8.14. They also used composite-based SAMs (2PACz and MeO-2PACz) to fabricate rigid Cu electrode-based all-perovskite TSCs,

8.5 Composite-Based Interconnection Layers (ICLs)

(b)

mA/cm2

(a)

(c)

(d)

Figure 8.14 (a) Device structure and corresponding cross-sectional SEM image of all-perovskite tandem solar cells, (b) composite-based SAMs as an ICL, (c) J-V curves of rig and champion device, (d) device structure of all-perovskite flexible tandem solar cells. Source: [52]/Springer Nature/CC BY 4.0.

showing the best PCE over 25% with a minor hysteresis between reverse and forward scans, as shown in Figure 8.14c. Compared to the common configuration used for all tandems, substrate-configurated tandems showed excellent stability and retained 100% of its initial efficiency after 600 hours of operation. Furthermore, they fabricated flexible TSCs on Cu-coated PEN substrates (Figure 8.14d), leading to a champion PCE of 24.1% under reverse scan.

8.5.3 Composite-Based Interconnection Layers (ICLs) in Perovskite/Organic Tandems In Section 8.5.2, it can be found that the compact and thick ICLs (but result in parasitic light loss) in all-solution processed all-perovskite tandems are necessary to avoid the destruction of front WBG perovskite subcells when using non-orthogonal solvents. However, orthogonal-solvent fabricated WBG perovskite cells and NBG organic cells are ideal subcells for perovskite/organic tandems. Because orthogonal solvents that are suitable for the fabrication of organic cells cannot destroy the underlying front WBG perovskite cell [53]. To improve charge transport and ohmic contact between the subcells, composite-based ICLs have been explored in perovskite/organic tandems. Wang et al. [54] developed polymer/MoO3 /Ag/PFN-Br as composite-based ICLs to fabricate perovskite/organic TSC with a device structure of glass/ITO/ZnO/CsPbI2 Br/polymer/MoO3 /Ag/PFN-Br/PM6 :Y6 -BO/MoO3 /Ag, as

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(a) –2.40

electrons

Ag MoO3

PM6

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D18

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–4.3

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ZnO

ITO

CsPbl2Br

–4.31 –4.66

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polyTPD

–2.77

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172

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–5.47 –5.3

–5.3 –5.68

holes

–6.08 holes

–6.36

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(b)

Figure 8.15 (a) Schematic of perovskite/organic tandem with effective composite-based ICLs and corresponding cross-sectional SEM image. (b) Energy-level diagram of inorganic perovskite/organic tandem. Source: Reproduced with permission from [54]/American Chemical Society.

shown in Figure 8.15a. Here, polymer can be PBDB-T-Si, D18, and poly-TPD. Besides, they found that polyTPD/MoO3 /Ag/PFN-Br as ICLs show minimal additional parasitic absorption and sufficient hole mobility and quasi-Ohmic contact. The PBDB-T-Si or D18 in the ICLs exhibits similar parasitic absorption, leading to a decrease in both J sc of the overall performance in perovskite/organic TSCs. Figure 8.15b shows the energy levels and work mechanism of the tandem device. Finally, the perovskite/organic tandems based on such composite-based polyTPD/MoO3 /Ag/PFN-Br ICLs showed an efficiency of 21.1%. Hou and Co. [19] found that based on Ag ICL, perovskite/organic TSC leads to high optical loss, further reducing the current in the organic bottom cell. However, they used a 4-nm-thick sputtered IZO layer inserted between organic BCP and molybdenum oxide (MoO3 ) to form composite-based ICLs, which significantly enhanced electrical properties and transmittance in the near-infrared region. They

8.6 Composite-Based Charge Transport Layers

further found that 4 nm IZO sputtered on BCP exhibited large and uniform grains with a high surface coverage, contributing to shorter recombination lifetime and more effective recombination. Therefore, a maximum efficiency of 23.60% was achieved in perovskite/organic TSCs.

8.6 Composite-Based Charge Transport Layers As is well known, charge transport layers (i.e. electron and hole transport layers) play an important role in transporting electrons and holes for high-performance solar cells, respectively. As discussed in Chapter 5, the use of composite-based charge transport layers can substantially improve the performance and stability of single-junction solar cells. To further improve the performance and stability of perovskite-based tandems, composite-based charge transport layers have also been widely explored.

8.6.1 Composite-Based Hole Transport Layers in Tandems Similar to single-junction inverted PSCs, the hole transport layer also impacts the crystallization dynamics of WBG perovskites and hole transportation in the perovskite-based TSCs. Based on theoretical calculations and experimental results, Br-rich species preferentially nucleate first, which significantly affects the dynamics of WBG perovskite formation. Besides, a high Br content can cause faster crystallization of the WBG perovskite film, which creates defects associated with structural and composition nonuniformity, especially with the halides, including vacancies, interstitials, and antisites [55]. Thus, it is significant to design the hole transport layer to improve both WBG perovskite film quality and halide homogeneity during the crystal growth toward high-performance and phase-stable perovskite-based TSCs. To solve such issues, Wang and Co. [56] developed composite-based hole transport layers of NiOx /PTAA/Al2 O3 -methoxy-substituted phenyl ethyl ammonium (x-MeOPEA+ ). They found that the x-MeOPEA+ ligands in composite-based hole transport layers not only contribute to the crystal growth with vertical orientation but also promote halide homogenization and defect passivation near the buried perovskite/hole-transport-layer interface, as well as reduce trap-mediated recombination. By ToF-SIMS measurements, it can be observed that the presence of 3-MeOPEA+ ligands on the top of mp-Al2 O3 NPs. Furthermore, they found that 3-MeOPEA+ ligands can assist in the formation of homogeneous Br-I mixed halide WBG perovskites, as well as the 3D render overlay images of the distribution of I and Br (Figure 8.16b). The ToF-SIMS depth profile further showed that the intensity of Br and I remained almost constant throughout the entire WBG perovskite layer during sputtering time from 0 to 500 seconds. (Figure 8.16d). However, the control film (without 3-MeO-PEACl) presented heterogeneous I-Br distribution, as shown in the 3D render overlay image (Figure 8.16a), and the ToF-SIMS depth profile with obvious I-rich and Br-rich distributions (Figure 8.16c). By PL mapping results, it further can be observed that control WBG perovskite film with composition

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Figure 8.16 (a,b) ToF-SIMS 3D render overlay image of I-Br distribution and (c and d) corresponding depth profiles, and (e and f) PL mapping images of the control (w/o 3-MeO-PEACl) and target (ITO/NiOx /PTAA/Al2 O3 /3-MeO-PEACl/WBG-perovskites) films. Source: Reproduced with permission from [56]/American Chemical Society.

heterogeneity (Figure 8.16e), but the target film incorporated MeOPEA+ ligands showed a more uniform distribution in PL emission (∼725 nm) (Figure 8.16f), indicating the formation of homogeneous and low-defect films. Moreover, the phase segregation can be substantially migrated in the target film than that of the control film with severe phase segregation when both films were treated with continuous one-sun illumination. To further confirm the possibility of chemical interaction between 3-MeOPEA+ ligands and mp-Al2 O3 and WBG perovskite molecules, the density functional theory (DFT) calculation was carried out (Figure 8.17a,b). By calculating the absorption energy (Eabs ), they found that the methoxy (−MeO) and ammonium (−NH+ )

8.6 Composite-Based Charge Transport Layers

(b)

mA/cm2

(a)

mA/cm2

(c)

(d)

Figure 8.17 (a,b) DFT simulation of chemical interactions at the mp-Al2 O3 /WBG-perovskite heterointerface of (a) mp-Al2 O3 …WBG-perovskites, and (b) mp-Al2 O3 …MeO-NH4 + …WBGperovskites (E abs = −8.02747 eV). (c) Schematic of the monolithic perovskite/silicon tandem solar cell architecture. (d) J-V curves (inset is a photograph of the actual tandem cell). Source: Reproduced with permission from [56]/American Chemical Society.

groups were favorably absorbed on mp-Al2 O3 (−3.49 eV) and WBG perovskites (−3.22 eV), respectively. They further carried out DFT simulation of chemical interactions at the heterointerface of mp-Al2 O3 …WBG-perovskites, mp-Al2 O3 … NH4 + -MeO…WBG-perovskites, and mp-Al2 O3 …MeO-NH4 + …WBG-perovskites. Compared to the heterointerface of mp-Al2 O3 …NH4 + -MeO…WBG-perovskites (−4.34 eV), the heterointerface absorption energy of mp-Al2 O3 …MeO-NH4 + …WBGperovskites (−8.03 eV) was more negative, indicating that methoxy groups prefer to interact with mp-Al2 O3 by chemical bonding, and thus ammonium groups on the other side can passivate WBG-perovskite buried interfaces. Besides, the heterointerface absorption energy based on 3-MeOPEA+ ligands (−8.03 eV) was more negative than that based on 2-MeOPEA+ (−7.48 eV) or 4-MeOPEA+ (−7.25 eV) ligands, indicating 3-MeOPEA+ ligands can significantly improve heterointerface contacts. Finally, they further applied such composite-based hole transport layers of NiOx /PTAA/Al2 O3 -x-MeOPEA+ to fabricate 2T perovskite/Si TSCs, and the

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T100 = 600 h

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Figure 8.18 Operational stability of monolithic perovskite/silicon tandem solar cell at MPP tracking under one-sun illumination in ambient air conditions. Source: Reproduced with permission from [57]. Copyright 2024, American Chemical Society.

tandem configuration is shown in Figure 8.17c. The champion perovskite/Si tandem cell with a designated illumination area (da) of 1.159 cm2 achieved a PCE of 24.29% with a V oc of 1.902 V, J sc of 17.10 mA/cm2 and FF of 75.11%, and a small J-V hysteresis behavior (Figure 8.17d). The operational stability of encapsulated monolithic tandems was further tested by using the MPP tracking method in ambient air conditions, as shown in Figure 8.18. The target tandem device showed excellent operational stability and retained 100% of its initial PCE during 600 hours of MPP tracking under one-sun (AM1.5G) illumination. Furthermore, T90 lifetime measurement (the time as a function of PCEs decreases to 90% of its initial value) of the target tandem devices is more than 1000 hours under continuous operation.

8.6.2 Composite-Based Electron Transport Layers in Tandems The design of highly efficient electron transport materials (ETMs) is important to improve the performance and operational stability of perovskite-based TSCs. To further improve ETL-based interface contacts and reduce energy-level mismatches, Sun et al. [57] developed hybrid fullerenes (HF) incorporating fullerene (C60), phenyl C61 butyric acid methyl ester (PCBM), and indene-C60 bisadduct (ICBA) as composite-based ETLs for all-perovskite TSCs. Figure 8.19a shows the energy mismatch and Fermi level pinning between perovskites with C60, PCBM, ICBA, and HF. It can be observed that there is a smallest energy mismatch of 0.02 eV between perovskites and PCBM, and between perovskites and HF, leading to efficient electron transport. However, compared to the Fermi level between perovskites and HF (0.36 eV), a higher Fermi level of 0.39 eV was obtained between perovskites and PCBM, which resulted in a decrease in V oc . After optimization of the content ratio (C60:PCBM:ICBA), both WBG and NBG single-junction PSCs based on such composite-based ETLs showed an improved PCE of 19.0% and 20.6% with a high V oc of 1.321 V and 0.842 V, respectively. Such improved solar cell performance is attributed to improved conductivity, energy level alignment, and reduced interfacial non-radiative recombination. They further fabricated all-perovskite TSCs using such

8.6 Composite-Based Charge Transport Layers

0.53 eV

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Figure 8.19 (a) Band diagrams of the ETL/perovskite contacts. (b) The cross-sectional SEM image of an all-perovskite tandem, and schematic diagram of the structure of the series-connected all-perovskite tandem solar module. Source: Reproduced with permission from [57]/John Wiley & Sons.

composite-based ETLs in both WBG and NBG perovskite subcells, a configuration of glass/ITO/NiOx /SAMs/WBG perovskite/HF/ALD-SnO2 /Au/PEDOT:PSS/NBG perovskite/HF/ALD-SnO2 /Cu, as shown in Figure 8.19b (left). A champion PCE of 27.4% was achieved in monolithic all-perovskite TSCs with an aperture area of 0.049 cm2 . Finally, all-perovskite tandem modules (6 cm × 6 cm) were fabricated, as shown in Figure 8.19b (right). As-fabricated all-perovskite tandem modules with an aperture area of 20.25 cm2 exhibited a champion PCE of 23.3% under reverse scan, with a V oc of 16.97 V, a J sc of 1.79 mA/cm2 , and a FF of 76.5%. To improve charge extraction and reduce non-radiative recombination losses at perovskite/ETL interface in perovskite/Si TSCs, Mariotti et al. [7] developed composite-based ETLs incorporating C60 and PI. The presence of PI in the ETLs can create a more favorable band alignment between the perovskite and C60 by forming a positive dipole. By using such composite-based ETLs, a high V oc of 1.28 eV (91.5% of the detailed-balance limit) with a low V oc deficit of 400 mV was obtained in single-junction solar cells, yielding a PCE of 21.5% in single-junction solar cells. Besides, perovskite/Si (front polished) tandems based on C60 and PI showed a record V oc up to 2.0 V, leading to a record certified PCE of 32.5%. The

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Figure 8.20 (a) Schematic view of the perovskite/SHJ monolithic tandem cell with MoOx and MgF2 as buffer layers; (b,c) J-V curves of tandem cells with an aperture area of 0.25 cm2 and 1.43 cm2 . Source: Reproduced with permission [62]. Copyright 2018, John Wiley & Sons.

perovskite/Si tandems under an N2 -filled glovebox also showed better stability over 3000 hours, with 99.6% of their initial efficiency retained.

8.7 Composite-Based Interfacial Layers in Tandems 8.7.1 Composite-Based Buffer Layers As is well known, the interface buffer layer (such as BCP, SnO2 , MoOx , MgF2 , and LiF) plays an important role in ensuring robust interface contacts and antireflection in both single- and multi-junction solar cells. For example, BCP and SnO2 are commonly acted as interface buffer layers in single-junction PSCs. The BCP buffer layer with a WBG, high-electron-affinity, and hole-blocking properties can significantly improve ohmic contact and reduce interfacial recombination and charge accumulation at interfaces for further enhancement of performance and long-term stability [58, 59]. Wang et al. [60] used a compact SnO2 buffer layer to resist moisture and top electrode silver diffusion into 3D perovskite layer. They further found that SnO2 also can be an electron-transport bridge to enhance electron transfer from PCBM to Ag due to their similar values of the conduction band of PCBM (−4.2 eV) and ALD-SnO2 (−4.3 eV) and work function of Ag (−4.3 eV) [61].

8.7 Composite-Based Interfacial Layers in Tandems

In the perovskite-based tandems, MoOx , MgF2 , and LiF buffer layers have been widely used for interface contact improvement and antireflection. Sahli et al. used MoOx /IZO/MgF2 as composite-based buffer layers to reduce reflection and parasitic absorption losses in the perovskite/Si TSCs, as shown in Figure 8.20a [62]. Thus, as-fabricated monolithic perovskite/Si TSCs obtained an efficiency of up to 22.7% (stabilized efficiency of 22.0%) on an area of 0.25 cm2 and 21.7% (stabilized efficiency of 21.2%) on a larger scale of 1.43 cm2 , as shown in Figure 8.20b,c. Jang et al. [63] developed ZnO NPs/AZO (doped zinc oxide) as composite-based buffer layers to facilitate electron transfer and reduce perovskite degradation in perovskite/CuInSe2 (CISe) TSCs. The ZnO NPs layer also can protect the underlying PSC during AZO sputtering process. The perovskite/CuInSe2 (CISe) TSCs demonstrated a PCE of 11% with a negligible hysteresis. Such work indicates the great prospect of ZnO NPs/AZO in perovskite/CISe TSCs to improve device performance and stability.

8.7.2 Composite-Based Passivation Layer Surface or interface passivation of semiconductor films is a common method to mitigate film defects, such a method has especially been widely used in single-junction PSCs. In Chapter 6, we discussed the use of composite-based passivation layers for fabricating highly stable single-junction PSCs. In perovskite-based TSCs, the use of composite-based passivation layers is also an important strategy to improve device performance and stability. Liu et al. [64] used large alkylammonium passivators to reduce surface defects of WBG perovskites and improve the energy diagram between WBG perovskite and C60 ETL for highly efficient and stable perovskite/Si tandems. They showed an effective sequential interface engineering (SIE) strategy to form composite-based passivation layers by using ethylenediamine diiodide (EDAI2 ) and sequential 4-fluoro-phenethylammonium chloride (4F-PEACl) additives. The EDAI2 can narrow the conduction band offset (CBO) between the perovskite and C60 layers, and the 4F-PEACl can act as a positive dipole layer, thus reducing recombination losses. As shown in Figure 8.21a, WBG perovskite films treated with both EDAI2 and 4F-PEACl together result in improved performance, with a CBO of 0.23 eV and ionization energy of 6.59 eV than that of control (0.52 eV and 6.04 eV), EDAI2 (0.22 eV and 6.20 eV), and 4F-PEACl (0.34 eV and 6.60 eV), respectively, indicating suppressed non-radiative charge recombination and charge extraction [7, 65]. The WBG perovskite solar cell (bandgap of 1.67 eV) achieved a champion PCE of 21.8% with an impressive V oc of 1.262 V. Furthermore, they further fabricated monolithic perovskite/Si TSCs based on such composite-based passivation layers, and the configuration shown in Figure 8.21b (left). The cross-sectional SEM images of the front (top) and backside (bottom) of a tandem are shown in Figure 8.21b (middle). Compared to control tandems (average PCE of 26.5%), the perovskite/Si tandems that are based on composite-based passivation layers exhibit better performance and excellent reproducibility (Figure 8.21b, right). Such results highlight the potential of composite-based passivation layers in enhancing the performance of perovskite-based TSCs.

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Figure 8.21 (a) Energy level alignment of various perovskite films and C60 layer. (b) Illustration of monolithic perovskite/Si tandem solar cells, cross-sectional SEM images with the front (top) and backside (bottom) of tandem solar cells, and distribution statistics of PCE for the tandem devices. Source: Reproduced with permission from [64]/John Wiley & Sons.

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56 Zhang, X., Ma, Q., Wang, Y. et al. (2024). Ligand homogenized Br-I wide-bandgap perovskites for efficient NiOx -based inverted semitransparent and tandem solar cells. ACS Nano 18: 15991–16001. 57 Sun, H., Xiao, K., Gao, H. et al. (2024). Scalable solution-processed hybrid electron transport layers for efficient all-perovskite tandem solar modules. Adv. Mater. 36: 2308706. 58 Shibayama, N., Kanda, H., Kim, T.W. et al. (2019). Design of BCP buffer layer for inverted perovskite solar cells using ideal factor. APL Mater. 7: 031117. 59 Chen, C., Zhang, S., Wu, S. et al. (2017). Effect of BCP buffer layer on eliminating charge accumulation for high performance of inverted perovskite solar cells. RSC Adv. 7: 35819–35826. 60 Wang, Y., Ju, H., Mahmoudi, T. et al. (2021). Cation-size mismatch and interface stabilization for efficient NiOx -based inverted perovskite solar cells with 21.9% efficiency. Nano Energy 88: 106285. 61 Brinkmann, K.O., Zhao, J., Pourdavoud, N. et al. (2017). Suppressed decomposition of organometal halide perovskites by impermeable electron-extraction layers in inverted solar cells. Nat. Commun. 8: 13938. 62 Sahli, F., Kamino, B.A., Werner, J. et al. (2018). Improved optics in monolithic perovskite/silicon tandem solar cells with a nanocrystalline silicon recombination junction. Adv. Energy Mater. 8: 1701609. 63 Jang, Y.H., Lee, J.M., Seo, J.W. et al. (2017). Monolithic tandem solar cells comprising electrodeposited CuInSe2 and perovskite solar cells with a nanoparticulate ZnO buffer layer. J. Mater. Chem. A 5: 19439–19446. 64 Liu, Z., Li, H., Chu, Z. et al. (2024). Reducing perovskite/C60 Interface losses via sequential Interface engineering for efficient perovskite/silicon tandem solar cell. Adv. Mater. 36: 2308370. 65 Chen, H., Maxwell, A., Li, C. et al. (2023). Regulating surface potential maximizes voltage in all-perovskite tandems. Nature 613: 676.

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9 Issues for Commercialization of Perovskite Solar Cells 9.1 Introduction to The Current Status of Perovskite Solar Cells Perovskite solar cells (PSCs) have made remarkable achievements during the past few decades benefiting from the deeper understanding of the film formation process and increasingly mature fabrication techniques. With the rapidly boosting efficiency and stability of PSCs, the transformation of small-area cell fabrication innovations into modules is becoming increasingly significant toward their commercialization. Thus, industrialization and entering commercialization are undoubtedly the next steps. Although small-area PSCs enable a power conversion efficiency (PCE) over 26% (∼0.07 cm2 ) [1–3], the efficiency record drops slightly to 25% when scaling up to 1 cm2 [4]. The key challenge is to maintain high efficiency and low batch-to-batch variation during upscaling. This is because the fabrication techniques used for PSCs cannot be directly transferred to perovskite solar modules (PSMs). Modules are divided into five clusters based on their area (square centimeters): minimodule (14,000 cm2 ) [3]. It is worth noting that further expansion of the area will result in a more severe loss of efficiency. For example, a certified record PCE of 22.4% was achieved in a mini-module with a designated illumination area (da) of 26.2 cm2 ; when scaling up to 215.53 cm2 (da), the efficiency record drops to 20.6%; when further expanding the area to 1027.1 cm2 (da), the efficiency of such a small module drops to 19.2% [4]. Besides, large-area modules with all functional layers fabricated by scalable methods are also required and remain challenging. The second challenge for commercializing modules is stability. Apart from the intrinsic instability of perovskite material itself attributed to the ionic nature properties, perovskite-based devices can deteriorate more easily due to external environmental factors, including moisture, oxygen, heat, light, radiation, etc. Thus both internal and external encapsulation are necessary to boost the operational stability of PSMs. Besides, effective encapsulation also prevents potential lead (Pb) leakage [5–7]. In general, a photovoltaic (PV) product must be able to withstand different operational environments under global application scenarios, especially when faced with harsh conditions like extreme high/low temperatures

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and light soaking. Therefore, the PV community has relied on the International Electrotechnical Commission (IEC) standard to assess the operational stability of any commercialized solar cells. To achieve actual commercialization, PSMs should pass the operational stability test according to the IEC standard. The third challenge is to consider which technical routes are suitable for different perovskite-based PV products. Currently, most of the early-stage technology startups or manufacturers have their focus on the large-area single-junction PSMs, such as Microquanta Semiconductor, GCL, and Panasonic Co. Ltd. Perovskite-based tandems with higher efficiencies are also considered promising commercial technologies, such as perovskite/silicon and all-perovskite tandems developed by Longi and Renshuo Co. Ltd., respectively. Other perovskite-based functional products for multi-scenario applications have also been developed by Saule Technologies and Mellow Energy Co. Ltd., such as flexible and wearable devices, and Internet of Things (IoT)-based indoor PVs. Based on such various perovskite-based technologies, it is easy to discover the huge commercialization prospects for perovskite-based PVs. However, there is still a long way to go toward commercialization. In this chapter, we focus on the current state of module stability, upscaling, market prospects, and other related commercialization issues.

9.2 Solutions to Stability Issues The structure vulnerability of hybrid perovskites and their related multi-interface contacts in stacked solar cell configurations have limited the operational stability of the device. Boosting the intrinsic structure stability of hybrid perovskites and the interface stability of devices by additive and interface engineering has been widely explored, respectively, as we discussed in Chapters 3–8. It should be noted that the evaluation standards of PSMs are essential for both academic and industrial purposes. To achieve practical applications, further encapsulation of PV products is highly desirable to enhance environmental stability and prevent the Pb leakage caused by rainwater and physical damage. Thus, the encapsulation materials and methods are of vital importance for the lifetime of PSMs.

9.2.1

Evaluation Standards

PV products must be able to withstand diverse operational environments under global application scenarios, especially under harsh conditions, such as extremely high (85∘ C)/low (−40∘ C) temperatures and light soaking (ultraviolet light) [8]. To evaluate the long-term stability of solar modules, the IEC test, the so-called IEC 61215 norm, is widely used for silicon solar products [9]. Figure 9.1 and Table 9.1 display the simplified flowcharts and protocols for testing the stability of modules using the IEC 61215 norm [10]. As shown in Figure 9.1, light, temperature, and humidity have a significant impact on the operational stability of solar modules under outdoor working conditions. Table 9.1 briefly summarizes the important parameters of the perovskite-based module stability test based on the IEC 61215

9.2 Solutions to Stability Issues

Figure 9.1 Flowchart of PV module stability test for IEC 61215 standard. Source: [10]/Springer Nature/CC BY 4.0.

standard. The test mainly contains 1-sun illumination (1000 ± 100 W/m2 ), UV-light (15 kWh/m2 of UV irradiation between 280 and 400 nm), hot-spot (1000 W/m2 irradiance), outdoor conditions (MPP tracking at 800 kWh/m2 of irradiation), damp heat (85∘ C, 85% RH, 1000 h), thermal cycling (−40∘ C to 85∘ C, 10 circles), humidity freeze (−40∘ C to 85∘ C, 85% RH), hail impact (ice balls with a diameter of 25 mm and a speed of 23.0 m/s), and mechanical load (at least 2400 Pa).

9.2.2 Internal Encapsulation Internal encapsulation is an effective method to boost the structures of light absorbers or interface layers in stacked devices by selecting appropriate functional materials. As it is well known, solution-processed polycrystalline perovskite films present undesirable defects at their surfaces, buried interface, and grain boundaries (GBs) during the crystal growth, which significantly impact both device efficiency and stability. Internal encapsulation is indispensable to solve the intrinsic instability issues in perovskite-based devices, including ion migration, phase separation, surface and bulk chemical reaction, thermal decomposition, and hygroscopicity. The purpose of internal encapsulation, which involves GB encapsulation, surface/

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Table 9.1

Summary of tests of the IEC 61215 for perovskite solar modules.

Test

Conditions

Measurement of module operating temperature

Module operating near maximum power point

Performance at low irradiance Light soaking test

Total solar irradiance: 800 W/m2 Ambient temperature: 20∘ C Wind speed: 1 m/s Cell temperature: 25∘ C Irradiance: 200 W/m2 with IEC 60904-3 reference solar spectral distribution Cell temperature: 50 ± 10∘ C 1000 W/m2 with IEC 60904-9 reference solar spectral distribution At maximum power point tracking

UV precondition test

Thermal cycling test Damp heat test Humidity freeze test

15 kWh/m2 total UV irradiation in the wavelength range from 280 to 400 nm with 3% to 10% UV irradiance in the wavelength range from 280 to 320 nm at 60 ± 5∘ C 50 or 200 cycles from −40∘ C to 85∘ C with current as per technology-specific part up to 80∘ C 1000 h at +85∘ C, 85% RH 10 cycles from 85∘ C, 85% RH to −40∘ C with circuitry continuity monitoring

Hot-spot endurance test

Exposure to 1000 W/m2 irradiance in worst-case hot-spot condition

Outdoor exposure test

60 kWh/m2

Static mechanical load test

Three cycles of uniform load specified by the manufacturer, applied for 1 h to front and back surfaces in turn. Minimum test load: 2400 Pa

Hail test

25 mm diameter ice ball at 23.0 m/s, directed at 11 impact locations

Source: [10]/Springer Nature/CC BY 4.0.

interface encapsulation, single-layer or multilayer hydrophobic thin film encapsulation, etc., is to enhance moisture and oxygen stability, photostability, and thermal stability [11]. For example, Zhang et al. [12] used a fullerene derivative with hydrophobic property, bis-adduct 2,5-(dimethyl ester) C60 fulleropyrrolidine (bis-DMEC60) to selectively distribute it at GBs throughout the film to suppress ions migration and prevent moisture infiltration, which helps to improve the stability of the perovskite film. Meanwhile, the presence of two functional groups containing nitrogen and oxygen atoms in the bis-DMEC60 can passivate defects and increase charge transport at the GBs, thus suppressing carrier recombination. The as-fabricated mini-modules (6 cm × 6 cm) demonstrated an efficiency of 19.53% (certified 18.8%) and improved stability under continuous illumination. A PSM is formed by interconnecting a series of subcells with P1, P2, and P3 scribes, as shown in Figure 9.2. Except for the widely accepted vertical diffusion of halide species from perovskite layer to the top electrode (left), the interfacial halide-metal

9.2 Solutions to Stability Issues

X–

X–

X–

X–

X –, A+

X –, A+

Figure 9.2 Schematic illustration of the monolithic interconnection in a PSM and its degradation processes: (1) vertical diffusion; (2) lateral diffusion at P2 and P3 laser scribing lines; (3) barrier effects of the anti-diffusion layer in solar modules. Source: Reproduced with permission [13]. Copyright 2024, John Wiley & Sons.

electrode reaction at the P2-scribed regions between the interconnecting subcells also has a major impact on module stability (middle). Besides, such a halide-metal reaction or escape of perovskite components is irreversible. To overcome such issues, compact interfacial protection or inert material layers have been explored to inhibit the diffusion of volatile species in PSMs [13, 14]. To mitigate vertical diffusion of perovskite species, Gao et al. used a compact SnO2 layer deposited on the top of electron transport layer (ETL) by atomic layer deposition. They further developed an anti-diffusion layer of PbOx to suppress lateral diffusion of halides (right). The PbOx significantly prevented the halide-metal reaction at P2 area and against moisture at P3 area. The operational stability of PSMs was beyond 1100 h and 1400 h for maximum power point tracking and damp heat tracking in ambient air, respectively. Such an internal encapsulation strategy is a crucial step in achieving efficient and stable perovskite photovoltaic modules in the industry.

9.2.3 External Encapsulation External encapsulation is the mainstream method to solve the critical issue of moisture and oxygen intrusion, and heavy metal leakage (such as Pb and As). By using external encapsulation, the stability test of photovoltaic products should be able to pass the IEC 61215 standard. The external encapsulation generally contains two methods: UV-curable adhesive encapsulation and glass–glass vacuum laminated encapsulation [11]. The UV curing technology has been widely used in various optoelectronic devices due to its solvent-free processing, chemical inertness, good heat resistance, fast curing rate, high transparency, etc. The encapsulation glue can seal the components to enhance their resistance to moisture, shock, and dust, and accelerate heat dissipation, etc. The common commercially available encapsulation glues mainly include UV-curable adhesive, epoxy resin, and organic silicone.

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Figure 9.3 Schematic of both internal and external encapsulation of perovskite-based PVs. Source: [17]/Springer Nature/CC BY 4.0.

Glass–glass vacuum laminated encapsulation has been used to encapsulate silicon photovoltaics for reaching a lifetime of over 20 years. Currently, this encapsulation technology has also been explored for PSMs [15, 16]. In this technology, the encapsulation film bonds both the upper plate tempered glass and the solar cell. The commonly used encapsulation materials include polyolefin (POE), ethylene vinyl acetate (EVA), ethylene methyl acrylate (EMA), polyisobutylene (PIB), polyurethane (PU), and thermoplastic polyurethane (TPU) [11]. For example, Chen and Co. [17] developed an Al2 O3 (30 nm)/parylene (1 μm) bilayer barrier to encapsulate perovskite-based photovoltaics by glass–glass vacuum laminated encapsulation strategy. Furthermore, they used a low-cost and chemically inert Bi as an interfacial barrier to prevent the migration of halide ions and diffusion of Ag. Finally, the whole device was cast with epoxy resin to further enhance the water and oxygen barrier ability. As shown in Figure 9.3, they adopted both internal and external encapsulation for enhancing the operational stability of perovskite-based devices. Thus, as-fabricated devices showed excellent operational stability maintaining 90% and 93% of their initial efficiencies after continuous operation at 45∘ C for 5200 h and at 75∘ C for 1000 h under 1 sun equivalent white-light LED illumination, respectively.

9.3 Upscaling, Commercialization, and Challenges 9.3.1 Scalable Fabrication Methods Deposition methods. Scalable manufacturing of efficient PSMs is critical for commercialization, and the key factor for this process is the fabrication of pin-hole free and homogeneous large-area perovskite film, which requires precisely optimized

9.3 Upscaling, Commercialization, and Challenges

die

(a)

(b)

Figure 9.4 (a) Fabrication of perovskite solar modules by various deposition methods, and (b) advanced strategies for preparation of high-quality perovskite film.

processing techniques (Figure 9.4a). To date, the most explored technique for the highly efficient small-area PSCs is the spin coating method due to the merit of depositing high-quality perovskite films with well-defined thicknesses and different compositions, laying a solid foundation for efficient PSMs with a certificated PCE of 22.87% (active area, 24.63 cm2 ) [18]. However, this method is limited to the substrate size and waste of 90% of perovskite ink and is especially not applicable for the high throughput roll-to-roll (R2R) or sheet-to-sheet (S2S) manufacturing processes. Vacuum evaporation deposition method has shown the scalability for depositing perovskite films over 600 cm2 , while the high expenses of large-scale vacuum equipment will hinder the further commercialization of perovskite photovoltaics, leading to fewer reports of PSMs derived from this method [19]. From the levelized cost of energy (LCOE) perspective, the scalable deposition methods should be less expensive and highly compatible with high throughput production processes such as R2R and S2S. Therefore, blade coating, slot-die coating, inkjet printing, and spray coating are more promising alternative techniques for scalable fabrication of perovskite films. Among these techniques, blade-coating is the most studied one for preparing the large-area perovskite film, leading to the current high PCE of 20.4% PSMs (active area, 29.25 cm2 ) [20]. However, this method also has limitations on the substrate size because the film thickness is gradually decreased along the coating direction due to the non-continuous ink supply, thus it is difficult to achieve high efficiency when the active area exceeds 30 cm2 . Slot-die coating method works similar to blade coating but can overcome the thickness variation problem due to the slot-die head allowing continuous ink supply. Many reports with perovskite film over 200 cm2 derived from this method make it of great potential in industrial production. Inkjet printing is a noncontact manufacturing method with the merit of preparing patterned perovskite film, but the inkjet inks usually require slow drying to achieve good process stability and sufficient nozzle open times, which poses a great challenge in the film crystallization. Spray coating method has the advantage of coating over a large-area perovskite film in a short time, but the ink droplets ejected from the nozzle usually have different sizes and can be easily overlapped on the substrate, affecting the final film thickness and

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crystal quality. To summarize, the slot-die coating method will be the best candidate in the future scalable fabrication of PSMs. Strategies for high-quality perovskite films. Controlling nucleation and crystal growth is another key factor for high-quality large-area perovskite film. The perovskite ink preparation and the wet perovskite film drying are the critical stages for precisely regulating this nucleation and growth (Figure 9.4b). The regulation of ink preparation can be divided into three aspects: (i) solvent engineering, a method that combines solvents of different boiling points, surface energy, and donor number (DN ) to control the solvent evaporation and crystallization; (ii) composition engineering, a method that controls the composition of perovskite materials for regulating the band gap, stability, and crystallization of corresponding perovskite film; (iii) additive engineering, a way that additional materials are used to adjust the perovskite nucleation and growth during the film formation process. The methods used to rapidly dry the wet perovskite film to immediate supersaturation for accelerating heterogeneous nucleation are also very important and can be divided into antisolvent treatment, high temperature, gas quenching, and vacuum quenching. Among those additional treatments, the gas quenching method could be the most promising one due to the advantages of low equipment requirement, environmentally friendliness, no damage to the perovskite materials, and ease of assembly to the high throughput fabrication process.

9.3.2 Module Design and Process Fabrication process of modules. The PCE of perovskite solar devices is decreased while enlarging the photoactive area, which is mainly originated from the resistive loss related to transparent conductive oxide (TCO). To reduce such losses, solar modules have been designed by interconnecting many small subcells either in series (monolithic modules) or in parallel [21–24]. Similar to silicon-based PVs, the perovskite parallel solar modules can obtain a high photocurrent by accumulating photocurrent from the whole area (all subcells). However, parallel solar modules need a perfect photoactive area without any voids, because the existence of shunting paths will reduce module’s voltage, even probably to zero [25]. Thus, it is not popular to design parallel solar modules in thin film PVs. Thus, academic and manufacturing communities largely focus on the fabrication of perovskite monolithic solar modules. In comparison to small-area solar cell fabrication, module fabrication processes need to use extra laser or mechanical scribing steps to divide a large-area cell into desirable subcells by effective interconnections. Compared to the mechanical scribing method, the laser scribing process has been widely used to achieve high-quality and desirable line patterns, known as P1, P2, and P3 scribes. The module fabrication processes are shown in Figure 9.5a. Using the inverted wide-bandgap (WBG)-PSC as an example, the TCO is first removed by applying laser technology for obtaining the P1 lines. The aim is to separate the TCO film and define the individual subcells deposited over the scribed TCO film, whereas the goal of P2 scribing is to interconnect in series between two neighboring subcells by front metal electrodes. The P3

9.3 Upscaling, Commercialization, and Challenges

(a)

(b)

Figure 9.5 (a) Module fabrication processes; (b) module configuration, and structure cross-section view and related parameters, where TCO, HTL, and ETL are transparent conductive oxide, hole transport layer, and electron transport layer, respectively.

scribing process isolates the front electrode to separate neighboring subcells and achieve monolithic interconnections. Furthermore, some important parameters that determine its efficiency and stability should be noticed, including the safety area, dead area, active area, and aperture area, as shown in Figure 9.5b. The geometry fill factor (GFF) is a crucial parameter defined as the ratio of the active area to the aperture area (i.e. GFF = active area/aperture area), which can evaluate the performance of a module. P1, P2, and P3 scribing lines should have a suitable width and safety area to reduce the dead area and improve GFF for overcoming cell-to-module efficiency losses. Design of module structure. The design of the device structure is vitally important to further enlarge the area of modules without sacrificing their efficiencies. After laser scribing P1, P2, and P3 patterns, the module presents some important parameters that determine its efficiency and stability, i.e. safety area, dead area, active area, and aperture area (Figure 9.4b, right). It is widely accepted that resistance-induced power loss (pLoss ) decreases the efficiency of a module, which can be calculated using Eqs. (9.1–9.3) [25–28]: pLoss = pDead area + pTCO pDead area = pTCO =

WDead area WDead area + WActive area

3 WActive J 1 area × Rsh × MPP × 3 VMPP WActive area + WDead area

(9.1) (9.2) (9.3)

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where pDead area and pTCO represent the dead area loss and TCO resistive; W Dead area and W Active area are the width of both areas; Rsh , J MPP , and V MPP are the sheet resistance of TCO, current density, and voltage at maximum power point, respectively. Besides, the value of GFF remarkably affects aperture efficiency (𝜂 a ). The GFF and 𝜂 a can be expressed by Eqs. (9.4) and (9.5): Active area Aperture area Voc × Jsc 𝜂a = aperture area

GFF =

(9.4) (9.5)

According to the above equations, it is important to maximize the active area and minimize the dead area but the module should have enough safety areas. Dai et al. [26] simulate the relationship between the power loss (pLoss ) and active area for various dead areas (Figure 9.6a), further confirming the importance of the ratio of the active area and dead area. However, it is worthy to note that P2 and P3 scribes should be wide enough to guarantee low contact resistance and further avoid shunting between neighboring subcells. Wilkinson et al. [28] pointed out the importance of the design of the front electrode grid to increase the efficiency of large-area cells and modules (Figure 9.6b–d). The parameters including cell width (X), length (Y ), bus bar spacing (Sb ), bus bar width (W b ), finger spacing (Sf ), and finger width (W f ) remarkably influence the resistive loss of a module. For example, design A is widely used for small-area cells, but a higher efficiency for an area > 200 cm2 was achieved from design C. Assessment of module quality. Understanding the properties and defects of subcells and scribes in a module is of paramount importance to further improve the efficiency and stability of PSMs. Except for common current–voltage curves, other characterization methods, such as optical microscope, photoluminescence (PL), electroluminescence (EL), light-beam-induced current (LBIC) mapping, and lock-in thermography (LIT), have been widely used to investigate homogeneity or defects of modules [23, 29–31]. For example, the optical microscope is generally used to examine the quality and width of P1, P2, and P3 scribes; the PL and EL imaging provide important information about the distribution of local defects, charge recombination/transport obstacles related to impurities, and non-uniformity. The high defect areas appear darker in the PL and EL images. The LBIC mapping gives photocurrent generation and distribution of a module, which significantly reveals the homogeneity of perovskite layers. Besides, the degradation mechanism of modules can also be investigated by the LBIC mapping [32].

9.4 Status of Solar Modules Production 9.4.1 Module Efficiency Recent advances in the efficiency of modules. To date, n-i-p PSMs that were fabricated by spin-coated perovskite film could obtain a PCE of 22.87% with an active area of 24.63 cm2 , according to Nazeeruddin et al. [18]. They fabricated the

9.4 Status of Solar Modules Production

(a)

(b)

(c)

(d)

Figure 9.6 (a) The total power loss in a module as a function of the active-area width for various dead-area widths. Three cell designs: (b) A cell without metal grids, (c) B cell with parallel metal bus bars, and (d) C cell with parallel metal bus bars and fingers, where gold arrows indicate the direction of current flow through the front electrodes, X and Y represent cell width and length; S b , W b , S f , and W f are bus bar spacing, bus bar width, finger spacing, and finger width, respectively. Source: (a) [26]/American Physical Society/CC BY 4.0; (b–d) Reproduced with permission from [28]. Copyright 2018, John Wiley & Sons.

single-crystalline TiO2 rhombus-like nanoparticles as the electron transport layer, showing low lattice mismatch and high affinity with the perovskite layer, along with their high electron mobility and lower defect density. Seok et al. combined the gas-quenching-assisted blade coating method with solvent engineering using 2-methoxyethanol and N-cyclohexyl-2-pyrrolidone to obtain the high-quality perovskite film, exhibiting PCE of 20.4% in 29.25 cm2 n-i-p PSMs [20]. Huang et al. combined gas-quenching-assisted slot die coating method and additive engineering with N-methyl-2-pyrrolidone and KPF6 for the high-quality perovskite film, showing PCE of 20.42% and 19.54% with an active area of 17.1 and 65.0 cm2 ,

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respectively [21]. The progress on PSMs based on the large area film derived from inkjet printing, spray coating, and vacuum evaporation techniques is relatively slower, making them less attractive [33–36]. It needs to be mentioned that all the highly efficient n-i-p PSMs are based on doped Spiro-OMeTAD as the HTL, suffering from the instability issue and high expense, posing a great need for developing cheap dopant-free HTLs. On the other hand, p-i-n PSMs with copper as the top electrode have a great potential to obtain stable and efficient PSMs [37–40]. However, the PCE is relatively lower than that of the n-i-p PSMs, and the highest PCE is just 19.2% with an aperture area of 50.0 cm2 based on the blade-coating method [37]. Therefore, to improve the performance of p-i-n small-area PSCs close to that of the n-i-p PSCs, fundamental research is greatly needed to further increase the large-area p-i-n PSMs. Challenges for efficiency and stability improvement. Most reported PSMs still lag significantly in efficiency compared to small-area single PSCs with a PCE over 26%. Although most studies have focused on the fabrication of high-quality perovskite active layers by controlling the composition of perovskite inks for large-scale production, obtaining reproducible, homogeneous, and large-area perovskite films remains a challenge. Besides, another key issue is how to obtain all device layers by fully scalable fabrication methods, which is important for future commercialization. Similar to small-area cells, operational stability is also an intractable problem for PSMs. Except for the vulnerability of perovskite materials in outdoor conditions (such as humidity, temperature, and light), halide ions (such as I− ) from the perovskite layer can rapidly erode metal electrodes by direct contact in the P2 scribe channels [41]. Thus, appropriate top electrodes need to be explored and developed for highly efficient and stable PSMs. Furthermore, it is necessary to explore effective external and internal encapsulation strategies to hinder moisture and lead leakage as well as ion migration or diffusion. Finally, it is significant to establish a standardized characterization protocol for suitable valuation of the progress reported by research groups for boosting PSM development [42].

9.4.2

Market Prospect

The International Energy Agency reported that solar capacity expansion in 2023 increased to 286 GW, and this represents the main source of global renewable capacity growth [43]. The capacity will be continuously increasing in the future due to the growing environmental concerns, government incentives, and the decreasing cost of solar technology. To mitigate the global energy crisis, many countries, particularly in Europe, have proposed favorable policies on developing the renewable energy to strengthen energy security. PV technologies present the most effective way for direct conversion of sunlight into electricity without the formation of pollution and environmental problems. Perovskite PV technology has opened up new opportunities for solar manufacturers. Currently, most PSM start-ups have obtained financing through the primary market, making them a new favorite of the capital market. The production lines of PSM enterprises are progressing rapidly due to the advancement of processing techniques and the continuous investment

9.4 Status of Solar Modules Production

Figure 9.7

The 1 m × 2 m perovskite solar modules from GCL.

enthusiasm of industrial capital. For example, Chinese solar module maker CGL technology has fabricated large-area PSMs with an area of 1 m × 2 m, as shown in Figure 9.7. The PSMs also show an efficiency of over 19%. Furthermore, the CGL technology fabricated 4-T perovskite/silicon tandem solar modules with an area of 1 m × 2 m, yielding an efficiency of over 26% [44]. Besides, such single-junction PSMs and 4-T solar tandem modules have passed the TUV Rhineland IEC 61215 and the IEC 61739 certification tests. Another PSM manufacturer, Microquanta Co. Ltd., installed a 100 kW perovskite photovoltaic system in Zhejiang Province, China in 2022. The system is the first reported grid-connected perovskite solar system on the 100 kW scale, as shown in Figure 9.8. In 2023, Microquanta Co. Ltd. further installed a 260 kW perovskite-based PV power station over the water for fish farming applications, located in Qujiang District, Quzhou City, which is rich in water sources and farming [45]. In 2024, Microquanta reported that their PSMs had passed the operational stability test standard based on IEC 61215 and IEC 61730 norms. One of the other solar manufacturers has focused on commercializing perovskite/silicon or all-perovskite tandem modules. For example, LONGi Green Energy Technology announced a new world record efficiency of 30.1% for a commercial M6 size wafer-level perovskite/silicon tandem solar module at the 2024 Intersolar Europe event in Germany [46]. Surprisingly, Oxford PV fabricated large-scale and commercial-size perovskite/silicon tandem modules of 60-cell with an area of 1.7 square meters (1 m × 1.7 m) (Figure 9.9), leading to an efficiency record of 26.9%, which is the best efficiency so far [47]. Renshuo Co. Ltd. reported 24.5% efficient all-perovskite tandem modules with an aperture area of 20.25 cm2 by all blade-coating fabrication methods [48]. However, the commercialization progress of all-perovskite tandem modules is still slower than that of single-junction PSMs and perovskite/silicon tandem modules. This is because the stability of Sn-Pb alloy narrow-bandgap perovskites is poorer than that of regular bandgap

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Figure 9.8

Photo of 100 kW perovskite grid-connected solar farm from Micoquanta.

Figure 9.9 Perovskite/Si tandem module with 1.7 square meters (1 m × 1.7 m). Source: From [47]/PV magazine.

perovskites. Admittedly, the commercialization of all-perovskite tandem modules is still a long way to go and still poses big challenges. In addition, the development of perovskite IPVs is another promising direction, which increases energy utilization efficiency by offering supplemental power sources from ambient or artificial light to indoor electronics. It can be integrated into various devices and applications in IoT systems, including wireless sensors,

9.4 Status of Solar Modules Production

Figure 9.10 Indoor light harvesting WBG-perovskite cells to power wireless electronic devices connected by the Internet of Things (IoT) ecosystem.

smart home devices, and wearable electronics, providing a reliable and convenient source of power. Smart electronic devices connected by the IoT ecosystem play an important role in a variety of applications. A fundamental factor of such devices is to be powered in a safe, reliable, and sustainable way. Taking these into consideration, IPVs that convert indoor artificial illumination into electricity with a record power output of 151.24 μW/cm2 (U30, 1000 lx and 3000 K) have been considered for low-power-consumption IoT devices (∼1 mW) [49, 50]. Figure 9.10 shows a WBG-perovskite cell that harvests energy from ambient light and artificial illumination to power IoT-based smart electronics. Under LED illumination, a record i-PCE over 43% in the WBG-perovskite modules with a high GFF of 94% (aperture area > 10 cm2 ) has been achieved through the construction of twoand three-dimensional perovskite heterostructures and the optimization of device fabrication processes. [51] The development of IPVs involves designing high-performance lightweight, ultrathin, and flexible WBG-perovskite modules, which are more adaptable and could enable integration with sensors or other IoT devices in multi-scenario applications. By optimizing device configuration and laser ascribing processes, current flexible WBG-perovskite IPV modules under U30 (1000 lx) obtained a certified record i-PCE of 31.21% with a high GFF of 94% [52]. Higher indoor performance of flexible WBG-perovskite IPVs is preferred for future IoT-based electronics, such as for use in scenarios with short indoor illumination time or for designing desirable IPVs integrated electronic products.

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9.4.3

The Toxicity Issues of Lead in Modules

It is widely accepted that a real-world application of perovskite PV products due to a potential risk of Pb toxicity may threaten human health, including brain damage and kidney and liver dysfunction, especially in children [53, 54]. Thus, Pb toxicity issues should be considered. For example, most perovskite-based BIPVs or IoT-integrated IPVs are installed on/inside the buildings, which may have a big impact on daily-life environmental conditions when the PV products are cracked or damaged. Beyond that, large perovskite solar panels that are installed outside, such as in land power stations, rooftops, offshore seas, etc. have to endure hail, tornadoes, typhoons, and other harsh weather conditions. Pb immobilization and boost external encapsulation strategies are remarkably important for real commercialization applications. Jiang et al. [5] simulated a scenario where PSMs with various encapsulation methods were mechanically damaged by a hail impact (modified FM 44787 standard) and detected the Pb leakage rates. After PSMs were damaged, water dripping tests were performed. It was observed that PSMs with no encapsulation showed a high Pb concentration in the water, but PSMs with an epoxy resin encapsulation had a reduced Pb leakage. Another interesting work has been reported by Xiao et al. [6], who used Pb-adsorbing ionogels to prevent Pb leakage and withstand long-term operational stability tests. The PSMs encapsulated with such multifunctional ionogels have reduced Pb leakage to an undetectable level after the hail-damaged modules, and further passed the damp heat and thermal cycling accelerated stability tests according to IEC 61215 standard. They further used a low-cost mesoporous sulfonic acid-based Pb-adsorbing resin into perovskites as a scaffold in PSMs, which effectively immobilized Pb ions when the modules were exposed to rainwater. More importantly, the presence of insulating scaffolds in PSMs also did not sacrifice the device efficiency [7]. Both academic and industry communities require further exploration of Pb immobilization or encapsulation strategies for the real applications of perovskite PV products.

References 1 Chen, H., Liu, C., Xu, J. et al. (2024). Improved charge extraction in inverted perovskite solar cells with dual-site-binding ligands. Science 384: 189–193. 2 Zhou, J., Tan, L., Liu, Y. et al. (2024). Highly efficient and stable perovskite solar cells via a multifunctional hole transporting material. Joule 8: 1691–1706. 3 https://www.nrel.gov/pv/interactive-cell-efficiency.html . 4 Green, M.A., Dunlop, E.D., Yoshita, M. et al. (2024). Solar cell efficiency tables (version 64). Prog. Photovolt. Res. Appl. 32: 3–13. 5 Jiang, Y., Qiu, L., Juarez-Perez, E.J. et al. (2019). Reduction of lead leakage from damaged lead halide perovskite solar modules using self-healing polymer-based encapsulation. Nat. Energy 4: 585–593.

References

6 Xiao, X., Wang, M., Chen, S. et al. (2021). Lead-adsorbing ionogel-based encapsulation for impact-resistant, stable, and lead-safe perovskite modules. Sci. Adv. 7: eabi8249. 7 Chen, S., Deng, Y., Xiao, X. et al. (2021). Preventing lead leakage with built-in resin layers for sustainable perovskite solar cells. Nat. Sustain. 4: 636–643. 8 Osterwald, C.R. and McMahon, T.J. (2009). History of accelerated and qualification testing of terrestrial photovoltaic modules: a literature review. Prog. Photovolt. Res. Appl. 17: 11–33. 9 www.solarchoice.net.au/wp-content/uploads/2018-PV-Module-ReliabilityScorecard_DNV-GL.pdf. 10 Zhang, D., Li, D., Hu, Y. et al. (2022). Degradation pathways in perovskite solar cells and how to meet international standards. Communi. Mater. 3: 58. 11 Ma, S., Yuan, G., Zhang, Y. et al. (2022). Development of encapsulation strategies towards the commercialization of perovskite solar cells. Energy Environ. Sci. 15: 13–55. 12 Zhang, S., Li, M., Zeng, H. et al. (2022). Grain boundary and buried interface suturing enabled by fullerene derivatives for high-performance perovskite solar module. ACS Energy Lett. 7: 3958–3966. 13 Gao, Y., Liu, C., He, M. et al. (2024). Efficient and stable perovskite solar modules enabled by inhibited escape of volatile species. Adv. Mater. 36: 2309310. 14 Xiao, K., Lin, Y.-H., Zhang, M. et al. (2024). Scalable processing for realizing 21.7%-efficient all-perovskite tandem solar modules. Science 376: 762–767. 15 Fu, Z., Xu, M., Sheng, Y. et al. (2019). Encapsulation of printable mesoscopic perovskite solar cells enables high temperature and long-term outdoor stability. Adv. Funct. Mater. 29: 1809129. 16 Bush, K.A., Palmstrom, A.F., Yu, Z.J. et al. (2017). 23.6%-efficient monolithic perovskite/silicon tandem solar cells with improved stability. Nat. Energy 2: 1700. 17 Zhou, J., Liu, Z., Yu, P. et al. (2023). Modulation of perovskite degradation with multiple-barrier for light-heat stable perovskite solar cells. Nat. Commun. 14: 6120. 18 Ding, Y., Ding, B., Kanda, H. et al. (2022). Single-crystalline TiO2 nanoparticles for stable and efficient perovskite modules. Nat. Nanotechnol. 17: 598–605. 19 Turkevych, I., Kazaoui, S., Belich, N.A. et al. (2019). Strategic advantages of reactive polyiodide melts for scalable perovskite photovoltaics. Nat. Nanotechnol. 14: 57–63. 20 Yoo, J.W., Jang, J., Kim, U. et al. (2021). Efficient perovskite solar mini-modules fabricated via bar-coating using 2-methoxyethanol-based formamidinium lead tri-iodide precursor solution. Joule 5: 2420–2436. 21 Bu, T., Li, J., Li, H. et al. (2021). Lead halide–templated crystallization of methylamine-free perovskite for efficient photovoltaic modules. Science 372: 1327–1332. 22 Park, N.-G. and Zhu, K. (2020). Scalable fabrication and coating methods for perovskite solar cells and solar modules. Nat. Rev. Mater. 5: 333–350.

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23 Wang, Y., Ju, H., Mahmoudi, T. et al. (2021). Cation-size mismatch and interface stabilization for efficient NiOx -based inverted perovskite solar cells with 21.9% efficiency. Nano Energy 88: 106285. 24 Yang, Z., Zhang, W., and Wu, S. (2021). Slot-die coating large-area formamidinium-cesium perovskite film for efficient and stable parallel solar module. Sci. Adv. 7: eabg3749. 25 Zhu, P., Chen, C., Dai, J. et al. (2024). Toward the commercialization of perovskite solar modules. Adv. Mater. 36: 2307357. 26 Dai, X., Chen, S., Deng, Y. et al. (2022). Pathways to high efficiency perovskite monolithic solar modules. PRX Energy 1: 013004. 27 Palma, A.L., Matteocci, F., Agresti, A. et al. (2017). Laser-patterning engineering for perovskite solar modules with 95% aperture ratio. IEEE J. Photovolt. 7: 1674–1680. 28 Wilkinson, B., Chang, N.L., Green, M.A. et al. (2018). Scaling limits to large area perovskite solar cell efficiency. Prog. Photovolt. Res. Appl. 26: 659–674. 29 Ren, A., Lai, H., Hao, X. et al. (2020). Efficient perovskite solar modules with minimized nonradiative recombination and local carrier transport losses. Joule 4: 1263–1277. 30 Li, Z., Klein, T.R., Kim, D.H. et al. (2018). Scalable fabrication of perovskite solar cells. Nat. Rev. Mater. 3: 18017. 31 Nejand, B.A., Ritzer, D.B., Hu, H. et al. (2022). Scalable two-terminal all-perovskite tandem solar modules with a 19.1% efficiency. Nat. Energy 7: 620–630. 32 Song, Z., Watthage, S.C., Phillips, A.B. et al. (2015). Investigation of degradation mechanisms of perovskite-based photovoltaic devices using laser beam induced current mapping. Proc. SPIE 9561: 956107. 33 Abzieher, T., Moghadamzadeh, S., Schackmar, F. et al. (2019). Electron-beam-evaporated nickel oxide hole transport layers for perovskite-based photovoltaics. Adv. Energy Mater. 9: 1802995. 34 Li, J., Wang, H., Chin, X.Y. et al. (2020). Highly efficient thermally co-evaporated perovskite solar cells and mini-modules. Joule 4: 1035–1053. 35 Rolston, N., Scheideler, W.J., Flick, A.C. et al. (2020). Rapid open-air fabrication of perovskite solar modules. Joule 4: 2675–2692. 36 Yang, F., Jang, D., Dong, L.R. et al. (2021). Upscaling solution-processed perovskite photovoltaics. Adv. Energy Mater. 11: 2101973. 37 Chen, S., Dai, X., Xu, S. et al. (2021). Stabilizing perovskite-substrate interfaces for high-performance perovskite modules. Science 373: 902–907. 38 Wu, W.Q., Yang, Z., Rudd, P.N. et al. (2019). Bilateral alkylamine for suppressing charge recombination and improving stability in blade-coated perovskite solar cells. Sci. Adv. 5: eaav8925. 39 Deng, Y., Xu, S., Chen, S. et al. (2021). Defect compensation in formamidinium–caesium perovskites for highly efficient solar mini-modules with improved photostability. Nat. Energy 6: 633–641.

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40 Dai, X., Deng, Y., Van Brackle, C.H. et al. (2019). Scalable fabrication of efficient perovskite solar modules on flexible glass substrates. Adv. Energy Mater. 10: 1903108. 41 Bi, E., Tang, W., Chen, H. et al. (2019). Efficient perovskite solar cell modules with high stability enabled by iodide diffusion barriers. Joule 3: 2748–2760. 42 Zhang, L., Wang, Y., Meng, X. et al. (2024). The issues on the commercialization of perovskite solar cells. Mater. Futures 3: 022101. 43 IEA Renewable energy market update. Available at: https://iea.blob.core. windows.net/assets/67ff3040-dc78-4255-a3d4-b1e5b2be41c8/RenewableEnergy MarketUpdate_June2023.pdf accessed 2 2024. 44 https://www.pv-magazine.com/2024/06/20/gcl-says-perovskite-solar-modulepasses-silicon-degradation-tests. 45 https://www.perovskite-info.com/microquanta-announces-its-perovskite-waterfarming-pv-power-station-connected. 46 https://www.perovskite-info.com/longi-announces-new-world-record-efficiency301-commercial-m6-size-wafer-level. 47 https://www.pv-magazine.com/2024/06/19/oxford-pv-presents-26-9-cellefficiency-record-at-intersolar-europe-2024. 48 Gao, H., Xiao, K., Lin, R. et al. (2024). Homogeneous crystallization and buried interface passivation for perovskite tandem solar modules. Science 383: 855–859. 49 Ma, Q., Wang, Y., Liu, L. et al. (2024). One-step dual-additive passivated wide-bandgap perovskites to realize 44.72%-efficient indoor photovoltaics. Energy Environ. Sci. 17: 1637–1644. 50 Mathews, I., Kantareddy, S.N., Buonassisi, T., and Peters, I.M. (2019). Technology and market perspective for indoor photovoltaic cells. Joule 3: 1415–1426. 51 Ma, Q., Ma, M., Liu, L. et al. (2023). Wide-band-gap perovskite solar minimodules exceeding 43% efficiency under indoor light illumination. Device 1: 100174. 52 Zhang, C., He, M., Wu, S. et al. (2024). Occlusal architecture of the buried Interface enables record-efficiency flexible perovskite photovoltaic modules with enhanced in-plane bending mechanical endurance. Adv. Funct. Mater. 34: 2313910. 53 Li, J., Cao, H.-L., Jiao, W.-B. et al. (2020). Biological impact of lead from halide perovskites reveals the risk of introducing a safe threshold. Nat. Commun. 11: 310. 54 Zhang, H., Lee, J.-W., Nasti, G. et al. (2023). Lead immobilization for environmentally sustainable perovskite solar cells. Nature 617: 687–695.

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10 Characterization Methods for Composite-Based Perovskite Solar Cells 10.1 Composite-Based Perovskite Films Characterization 10.1.1 Growth Dynamics of Composite-Based Perovskites Crystal growth in materials such as halide perovskites is primarily governed by thermodynamic and kinetic factors. The growth rate per unit area of the crystal interface (u) is influenced by the reduction in free energy during the phase transition from liquid to solid. Kinetically, this growth rate (u) can be described by the equation: u = 𝜈a0 (1 − exp(−ΔG∕kT ))

(10.1)

where a0 is the distance across the α-β interface (approximately 1 atomic diameter), ΔG is the activation energy for diffusion, 𝜈 is the frequency factor given by 𝜈 = kT/(3πa0 3 𝜂) (where 𝜂 is the atomic mobility or viscosity, k is Boltzmann constant, and T is absolute temperature) [1]. To achieve high-quality organic–inorganic hybrid perovskite layers with minimal defects and disorder, it is crucial for crystal growth to proceed slowly. This slow growth allows the molecules assembling at the interface enough time to align in their energetically favored configuration, thereby minimizing the total Gibbs free energy [1]. In practical scenarios, the dynamics of crystal growth in halide perovskites is highly dependent on the chosen processing techniques. One-step method typically follows non-epitaxial growth models such as Volmer–Weber or Stranski–Krastanov mechanisms, where perovskite islands grow three-dimensionally to minimize surface free energy [2, 3]. Strong interactions at the precursor–perovskite interface often lead to lateral expansion of perovskite islands before they merge into adjacent single crystals, as evidenced by X-ray diffraction (XRD) and transmission electron microscopy (TEM) studies [4–6]. In contrast, the two-step method involves crystal growth at interfaces involving PbI2 solids, MAI/alcohol liquids, and atmospheric gases [7, 8]. This growth occurs at solid–liquid, solid–solid, or solid–gas interfaces, influenced significantly by the packing pattern of inorganic PbI2 solids. The dense packing of PbI2 precursor solids can hinder the migration of methylammonium (MA) ions toward the inner regions

10 Characterization Methods for Composite-Based Perovskite Solar Cells

of PbI2 , resulting in residual unreacted PbI2 phases [8, 9]. The two-step method is also associated with significant volume expansion. Introducing intermediates between precursors is an effective strategy to reduce diffusion rates, as growth rates are typically diffusion-controlled. Previous studies have demonstrated the efficacy of additives such as haloid acids, dimethyl sulfoxide, and 1,4-dioxane (DIO) in controlling crystal growth by modifying precursor interactions [10–12]. Future research could explore the introduction of organic molecules such as amino acids, amino alcohols, or polymers with functional groups to further manipulate nucleation and growth kinetics by altering the energy barriers for ionic building blocks. Liquid-phase TEM is utilized as a method to investigate the dynamic growth processes of materials. Wang et al. employed in situ liquid-phase TEM at 90∘ C to investigate the growth dynamics of perovskite films from precursor solutions. Figure 10.1a,b illustrates that perovskite films develop from individual nanocrystals nucleating on the substrate surface, with Figure 10.1a showing a higher density of nuclei compared to Figure 10.1b, possibly due to local variations in precursor concentration or substrate condition. The growth process involves nanocrystals

t – t0 = 1046.5 s

(a)

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Figure 10.1 Growth of methylammonium lead iodide perovskite films from a precursor solution using in situ TEM at 90∘ C. (a) High nucleation density leading to uniform film formation; (b) the process at lower nucleation density. SAED patterns confirm the tetragonal perovskite structure, with dashed circles indicating specific crystallographic planes. (b) Highlights nanocrystal coalescence (red arrows) and grain boundary angles (yellow and red dashed lines). (c) Area–time plots of four individual nanocrystals from (a) and (b). Source: Reproduced with permission from [13]/American Chemical Society.

10.1 Composite-Based Perovskite Films Characterization

growing and merging until they completely cover the substrate (Figure 10.1a: t − to = 1046.5–1051.5 s, Figure 10.1b: t − to = 1164–1196 s), after which the growth rate slows and stabilizes (Figure 10.1a: t − to = 1068.5 s, Figure 10.1b: t − to = 1229 s). This behavior corresponds to the Volmer–Weber growth model, where individual grains coalesce to form a continuous film. The study also reveals that smaller nanocrystals coalesce to form larger grains, which continue to grow until they form polycrystalline films, accompanied by reshaping of grain boundaries (GBs) to minimize energy (Figure 10.1b: t − to = 1229 s). Selected area electron diffraction (SAED) patterns confirm the crystalline nature of these films (Figure 10.1a,b), providing valuable insights into optimizing perovskite film synthesis for technological applications.

10.1.2 Optical and Electrical Properties of Composite-Based Films Optical Properties. Understanding and characterizing the optical properties of semiconductor films are fundamental for their application in various optoelectronic devices. Several experimental techniques are employed to comprehensively analyze these properties. UV−Vis spectroscopy is a pivotal method that examines how semiconductor films absorb and transmit light across the UV and visible spectra. By measuring absorbance and transmittance, UV−Vis spectroscopy provides critical parameters such as bandgap energy, absorption coefficients, and optical constants. This information is crucial for assessing the transparency, light absorption efficiency, and overall optical quality of semiconductor materials, which are vital considerations in the development of photovoltaic devices, light-emitting diodes (LEDs), and other optical technologies. Ellipsometry, another powerful optical technique, offers nondestructive measurements of semiconductor films by analyzing changes in the polarization state of reflected light. It provides precise data on the refractive index, thickness, and surface roughness of films, offering insights into film composition and interface properties [14]. For instance, in Figure 10.2a, the incident beam and the normal to the surface define a plane known as the incident plane. The angle of incident light is denoted as θ. In the ellipsometry measurement, two parameters Psi (𝜓), and Delta (Δ), are derived from the expression of complex reflectance ratio (ρ): ρ ≡ tan(𝜓)eiΔ ≡ rp ∕rs ≡ (Erp ∕Eip )∕(Ers ∕Eis )

(10.2)

where 𝜓 and Δ are the amplitude ratio and phase difference of reflected p- to s-polarized light, respectively; r p and r s are the Fresnel reflection coefficients of p- and s-polarized light, respectively; Ep and Es are the electric fields for p- and s-polarized light, respectively. Ellipsometry aims to convert these measured quantities into sample properties like optical functions and film thickness. For a substrate, this transformation is simple: ψ characterizes the refractive index n, while Δ describes light absorption via the extinction coefficient k. However, for a film, 𝜓 and Δ cannot be directly translated into sample properties and require a more complex approach than straightforward algebraic solutions. To derive the

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10 Characterization Methods for Composite-Based Perovskite Solar Cells

Eip

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Figure 10.2 (a) Measurement principle of ellipsometry. (b) Schematic for SE data analysis. Source: Reproduced with permission from [15]/Elsevier.

properties of perovskites from 𝜓 and Δ, the measurement and analysis process involves four main steps, as depicted in Figure 10.2b [15]. PL spectroscopy involves exciting semiconductor films with light and analyzing the emitted photons. This technique reveals valuable information about the electronic band structure, defect states, and carrier dynamics within the material.

PL Intensity (Counts)

PL Intensity (Normalised)

Absorbance (Normalised)

10.1 Composite-Based Perovskite Films Characterization

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Figure 10.3 (a) Absorption (black) and photoluminescence emission (red) spectra of a MAPbI3 perovskite thin film. (b) PL decay of MAPbI3 perovskite thin film.

By studying the intensity, wavelength, and lifetime of PL emissions, researchers gain insights into the efficiency of radiative recombination processes and identify defects that can impact device performance. Absorption (black) and PL emission (red) spectra of a MAPbI3 perovskite thin film are shown in Figure 10.3a,b. PL spectroscopy is essential for optimizing the optical properties of semiconductor materials in optoelectronic device applications such as LEDs, lasers, photodetectors, photovoltaic cells, and sensors. Raman spectroscopy complements these techniques by probing the vibrational and rotational modes of molecules and crystals through light scattering. In semiconductor research, Raman spectroscopy provides valuable information about crystal structure, lattice vibrations, strain, and doping levels in films. Analysis of Raman spectra enables researchers to identify different phases of semiconductor materials, evaluate strain-induced shifts in phonon frequencies, and monitor the incorporation of dopants into the crystal lattice. This capability is particularly useful for characterizing semiconductor films and understanding their structural and optical properties in detail. Electrical Properties. In addition to optical characterization, understanding the electrical properties of semiconductor films is essential for the design and optimization of electronic devices. Various experimental techniques are employed to characterize these properties comprehensively. The Hall effect measurement is a powerful method used to determine the electrical conductivity, carrier concentration, and mobility of charge carriers in semiconductor films. By applying a magnetic field perpendicular to the current flow through the film, researchers can measure the Hall voltage generated by the Lorentz force acting on moving charge carriers. From these measurements, crucial parameters such as the type of majority carriers (n-type or p-type), carrier concentration, and mobility can be extracted, providing insights into the electrical performance and doping levels of semiconductor materials. The four-point probe method is widely used to measure the sheet resistance or resistivity of semiconductor films. This technique involves applying a known current through two outer probes and measuring the voltage drop across two inner probes. By accurately determining the electrical resistance of the film, researchers can assess its uniformity and conductivity. The four-point probe method is particularly valuable

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10 Characterization Methods for Composite-Based Perovskite Solar Cells

for measuring resistivity and sheet resistance in thin films and nanostructures, offering nondestructive evaluation of electrical properties. Capacitance–voltage (C–V) measurements are employed to characterize the doping density, carrier distribution, and junction capacitance in semiconductor films and devices. By applying a varying voltage across a metal–semiconductor junction and measuring the resulting capacitance, researchers can derive important parameters such as the doping concentration profile, depletion width, and flat-band voltage. C–V measurements play a crucial role in understanding the electrical behavior of semiconductor junctions, optimizing device performance, and evaluating the suitability of materials for specific applications. I–V measurements provide important insights into the electrical transport properties and performance of semiconductor films and devices. This technique involves applying a voltage bias across a semiconductor junction or device and measuring the resulting current. By analyzing key parameters such as the I–V characteristic curve, forward and reverse bias behavior, leakage current, and breakdown voltage, researchers can assess device functionality, evaluate contact resistance, and optimize the electrical performance of semiconductor materials in electronic and optoelectronic devices. Space-charge-limited current (SCLC) measurements are pivotal for probing the charge transport properties of semiconducting materials, particularly thin films and devices. SCLC measurements are performed with an electron-only device (EOD) or a hole-only device (HOD), in which only electrons or holes are injected into the semiconductor layer from electrodes, respectively. Figure 10.4a,b illustrates the structures of EOD and HOD, respectively. This method examines the I–V characteristics under conditions where charge carrier transport is constrained by the accumulation of space charge within the material. SCLC experiments typically involve applying a voltage across a semiconductor layer sandwiched between electrodes and measuring the resulting current (Figure 10.4c). A log–log plot of current density–voltage (J–V) from EOD or HOD exhibits three distinct regions,

ITO/ZnO

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Figure 10.4 Structures of electron-only device (a) and hole-only device (b), and (c) typical space-charge-limited current in carrier-only device.

10.1 Composite-Based Perovskite Films Characterization

ohmic region at low voltages where current increases linearly with voltage, the trap-filled-limit (TFL) SCLC region, and the trap-free SCLC region (Figure 10.3c). The trap-free SCLC region shows that the trap states are completely filled at sufficiently high bias voltage and no longer affect the current conduction. In the trap-free SCLC region, the current is dominated by charge carriers injected from the contacts, and the J–V characteristics follow the Mott–Gurney law [16, 17]: 9 V2 𝜀𝜇 (10.3) 8 L3 where V is the applied voltage, 𝜀 is the dielectric constant of active layer, L is the thickness of active layer, and 𝜇 is the mobility that can be calculated by Eq. (10.3). The TFL voltage (V tfl ) generally refers to the crossing point between the low-voltage region (i.e. ohmic region) and the TFL region. From the J–V plot, the TFL voltage and the trap density are related by [16, 18]: J=

qnt L2 (10.4) 2𝜀 where q is the elementary charge and nt is the trap density. nt can be estimated by using Eq. (10.4) with the value of V tfl . Vtfl =

10.1.3 Heterogeneity of Composite-Based Films Scanning Electron Microscopy (SEM). Measuring the heterogeneity of composite-based films involves assessing variations in composition, structure, and properties across the film’s surface or through its thickness. Several methods are commonly employed to quantify heterogeneity of composite films, each offering unique insights into the material’s characteristics. SEM provides high-resolution images that reveal the surface morphology and distribution of components within the composite film. By analyzing SEM images, researchers can identify phases, agglomerates, or clusters of different materials and assess their spatial distribution and size distribution. SEM coupled with elemental analysis (EDX) can also provide information about the elemental composition at specific locations on the film, offering detailed insights into compositional heterogeneity. High-Resolution Transmission Electron Microscopy (HR-TEM). HRTEM has been extensively and successfully used for analyzing crystal structures and lattice imperfections in various kinds of materials on an atomic resolution scale. It can be used for the characterization of point defects, stacking faults, dislocations, GBs, and surface structures. Thus, HR-TEM is also highly appropriate for analyzing the characteristics of composite-based perovskite films. For example, Wang et al. [19] explored the position of insulating polymer polymethyl methacrylate (PMMA) in composite-based perovskite films by using the HR-TEM technique. The composite-based perovskite film was first cut by a focused-ion-beam (FIB) technique to obtain a desirable sample for TEM imaging, as shown in Figure 10.5a. The areas 1 (near to surface) and 2 (buried interface), further detected by a high atomic resolution scale, clearly showed the PMMA and perovskite phases, as shown in Figure 10.5b,c. Moreover, the PMMA effectively occupies and passivates the GBs

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Figure 10.5 (a) Low-magnification TEM image, (b,c) high-resolution TEM images of area 1 (near to surface), and (d) area 2 (buried interface) for composite-based perovskite films incorporating PMMA additives, respectively. Source: Reproduced with permission from [19]/Elsevier.

and voids/pinholes at buried interfaces in polycrystalline perovskite films, clearly observed in area 2 (Figure 10.5d). Atomic Force Microscopy (AFM). AFM measures surface topography at the nanoscale by scanning a sharp probe over the film surface. AFM can detect variations in film thickness, surface roughness, and the presence of nanoparticles or aggregates. It is particularly useful for characterizing the roughness and mechanical properties of composite films and for mapping variations in surface features that indicate heterogeneity. Confocal Raman Microscopy maps the chemical composition and molecular structure of composite films with spatial resolution. By measuring Raman spectra at different points on the film, researchers can identify phases, crystallinity, and chemical interactions between components. This technique is valuable for understanding how different materials are distributed within the film and assessing local variations in composition. Conductive Atomic Force Microscopy (C-AFM). The aim of the C-AFM technique is to obtain a current image and corresponding current profile by scanning the sample surface when a voltage is applied between a conductive tip

10.1 Composite-Based Perovskite Films Characterization

and the sample. C-AFM not only images the morphology but also detects local electrical properties of the sample surface simultaneously at the microscopic scale, which is highly suited to probe the local effects of perovskite films and devices [20]. Ma et al. [21] systemically explored the surface morphology and electrical states of composite-based wide-bandgap (WBG) perovskite films incorporating oleylammonium iodide additives (OAmI) by the C-AFM technique. As shown in Figure 10.6a,d, the incorporation of a trace of additives significantly improved the crystallinity of WBG perovskites with fewer GBs than the control perovskite films. Besides, the composite-based WBG perovskite film had a lower potential fluctuation (Figure 10.6b,e) and current signal (Figure 10.6c,f) compared to the control perovskite film, indicating the passivation of film surface defects and leakage currents. The lower leakage currents in the composite-based WBG perovskite film could be attributed to improvements in film quality, suppression of halide vacancies, and ion migration. Besides, both high potential fluctuation and current signal in the control perovskite film were caused by the more active surface defects and halide vacancies, respectively, forming leakage channels that significantly affected the V oc of photovoltaic cells under weak illumination. X-Ray Photoelectron Spectroscopy (XPS). XPS analyzes the elemental composition and chemical state of a film. By irradiating the film with X-rays and measuring the energies of emitted electrons, XPS provides information about the chemical environment of different elements. This technique can detect surface segregation, oxidation states, and chemical bonding variations that contribute to heterogeneity in composite films. Transmission Electron Microscopy (TEM). TMS offers high-resolution imaging and EDX of thin sections of composite films, revealing detailed information about the internal structure, interfaces between phases, and the distribution of nanoparticles or crystallites within the film. TEM is particularly useful for studying nanoscale features and understanding the morphology of composite materials. Micro-computed Tomography (Micro-CT). Micro-CT provides three-dimensional (3D) images of composite films by scanning the sample with X-rays from multiple angles. It can visualize internal structures, porosity, and distribution of phases within the film, making it effective for nondestructive imaging of bulk samples and quantifying volumetric heterogeneity in composite materials. Electrical measurements, such as impedance spectroscopy or conductivity mapping, reveal variations in electrical properties across the film due to compositional differences. Optical techniques, such as UV–Vis spectroscopy or fluorescence microscopy, assess optical properties and uniformity in light absorption or emission, providing comprehensive characterization of composite films for various technological applications. Grazing-Incidence Wide-Angle X-Ray Scattering (GIWAXS). GIWAXS have been widely used in organic or organic–inorganic hybrid materials to understand several structure–property relationships, and film growth kinetic and dynamic features in these materials. [22] As shown in Figure 10.7, perovskite composition phases can be well detected. By increasing the contents of large-cation OAmI additives, PbI2 was reduced due to the formation of two-dimensional (2D) OAm2 PbI4

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Figure 10.6 (a,b) AFM topography, (c,d) potential images and the corresponding line profiles, and (e,f) conductive images and the corresponding line profiles for pristine 3D perovskite film and composite-based 2D–3D perovskite film. Reproduced with permission from [21]/Royal Society of Chemistry.

perovskites [21]. Besides, GIWAXS was also used to analyze the perovskite orientation with respect to the substrate in the thin films. Li et al. found that pristine Pb-Sn perovskite crystals had a random orientation of crystal grains within the polycrystalline film, confirmed by Debye–Scherrer rings at specific q values in GIWAXS image. However, the composite-based Pb-Sn perovskite film incorporated

10.1 Composite-Based Perovskite Films Characterization

1.6 2.0 1.0

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Figure 10.7 GIWAXS images of (a) 3D/2D heterointerface and (b) 2D–3D-F4TCNQ composite-based perovskite films. Source: Reproduced under a Creative Commons Attribution 4.0 International License [23]. Copyright 2023, Springer.

2-(4-fluorophenyl)ethylammonium iodide (FPEAI) additives showed sharp and discrete Bragg spots along the same q position from GIWAXS image, indicating highly oriented crystals perpendicularly aligned with the substrate [23]. Grazing Incidence X-Ray Diffraction (GIXRD). This technique has been successfully used to reveal lattice structure and residual stress in thin films of ZrO2 and TiN [24, 25]. As discussed in Chapter 4, the presence of residual stress can lead to perovskite lattice distortion, thus reducing solar cell efficiency and stability (see Section 4.1.6). The residual stress effects of composite-based perovskite films have been widely explored by the GIXRD method. Ma et al. [21] systemically explored the residual stress in both pristine 3D and 2D–3D composite-based perovskite films by the GIXRD method. As shown in Figure 10.8a,c, it can be observed that both crystallographic planes (i.e. 31.58∘ and 14.07∘ ) shift toward lower 2θ positions, indicating the presence of lattice distortion and tensile stress within the pristine 3D perovskite film. However, the composite-based 2D–3D film demonstrated almost the same 2θ position for the two crystallographic planes when ψ angles change from 0∘ to 40∘ , as shown in Figure 10.8b,d. Furthermore, linear fit curves for 2θ position as a function of sin2 ψ in the composite-based 2D–3D films (Figure 10.8f) had slower slopes than those of the pristine 3D perovskite films (Figure 10.8e), indicating efficient suppression of the residual stress and lattice distortion. By combining the techniques mentioned above, researchers can comprehensively analyze the heterogeneity of composite-based films, gaining insights into structure–property relationships and optimizing material design for specific applications in electronics, sensors, coatings, and biomedical devices. Each method offers unique advantages depending on the scale, resolution, and specific properties of interest for the composite material under study.

10.1.4 Chemical Interactions and Simulations 10.1.4.1 Chemical Interactions

Studying chemical interactions in composites involves a multifaceted approach combining experimental methods and computational simulations to probe how

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different components interact at the molecular level, influencing properties such as stability, reactivity, and performance. Experimentally, Fourier transform infrared spectroscopy (FTIR) analyzes chemical bonding and functional groups present in composites by measuring infrared absorption spectra. XPS provides insights into the elemental composition and chemical states of composite surfaces, detecting changes in oxidation states and surface chemistry resulting from interactions. Solid-state nuclear magnetic resonance (NMR) spectroscopy is employed to probe molecular structure and dynamics within composites, particularly revealing interactions involving hydrogen, carbon, and other nuclei. Thermal analysis techniques

10.1 Composite-Based Perovskite Films Characterization

like thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) assess thermal stability and phase transitions influenced by chemical interactions, offering data on decomposition temperatures and reaction kinetics. Electrochemical methods, such as cyclic voltammetry and electrochemical impedance spectroscopy (EIS), monitor changes in charge transfer and conductivity, reflecting electroactive composite interactions.

10.1.4.2

Simulations

Simulations play a crucial role in complementing experimental findings by providing detailed insights into molecular interactions within composites. Molecular dynamics (MD) simulations model the movement and interactions of atoms and molecules over time, predicting molecular structures, energetics, and dynamic behaviors influenced by chemical bonding and intermolecular forces. Density Functional Theory (DFT). DFT calculations utilize quantum mechanics to compute electronic structures, energies, and properties, predicting bonding energies, charge distributions, and electronic properties affected by interactions in composites. Monte Carlo simulations simulate statistical ensembles of configurations and interactions within composites, predicting equilibrium distributions and thermodynamic properties influenced by chemical interactions. Coarse-grained simulations simplify molecular representations to study large-scale behavior, exploring phase separation, self-assembly, and cooperative effects driven by chemical interactions. Integrating experimental data with simulation predictions enhances understanding of composite material behaviors, guiding their design and optimization across diverse applications in nanotechnology, catalysis, materials science, and biomedical engineering. Molecular Electrostatic Potential (MEP). MEP method is defined as the interaction energy between the charge distribution of a molecule and a unit positive charge. To understand the chemical interaction between WBG perovskites and additives, Ma et al. [21] simulated molecular interactions to determine the binding energies (ΔEb ) of perovskite-solvent adducts. As shown in Figure 10.9, they found that the binding energy (ΔEb ) of perovskite-OAm+ -DMF-DMSO-CHCl3 adducts was calculated to be −4.46 eV, which was more negative than that of the other three adducts, that is, perovskite-DMF-DMSO (−1.04 eV), perovskite-DMF-DMSO-CHCl3 (−2.19 eV), and perovskite-OAm + -DMF-DMSO (−3.83 eV). Such results indicated a higher affinity and improved interaction of the perovskite-OAm+ -DMF-DMSOCHCl3 adducts toward ITO/NiOx /PTAA/mp-Al2 O3 substrates, thus facilitating the formation of high-quality composite-based 2D–3D perovskite films on the substrates. Ab Initio Molecular Dynamics (AIMD). AIMD simulation permits chemical bond-breaking and forming events to occur and accounts for electronic polarization effects. AIMD has been successfully applied to a wide variety of important problems in physics and chemistry [26, 27]. For example, the AIMD simulation was used to study a dynamic and interaction process between OAm+ ligands and perovskite molecules [21]. As shown in Figure 10.10, the OAm+ ligands had a tendency to be

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ΔEb = –1.04 eV (a)

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Figure 10.9 Simulated molecular electrostatic potential (MEP) profiles of (a) perovskite-DMF-DMSO, (b) perovskite-DMF-DMSO-CHCl3 , (c) perovskite-OAmI-DMF-DMSO, and (d) perovskite-OAmI-DMF-DMSO-CHCl3 adducts (OAmI: oleylammonium iodide; DMF: dimethylformamide; DMSO:dimethylsulfoxide). Source: Reproduced with permission [21]. Copyright 2024, Royal Society of Chemistry.

adsorbed on the interfaces of 3D perovskites or A-site vacancies. The AIMD simulation concluded that OAm+ ligands could become important spacers to bridge 2D–3D heterostructures for suppressing halide migration and vacancy defects. Such simulation results also suggested that OAm+ ligands could effectively passivate defects presented at perovskites by the in situ-formed 2D–3D bulk heterostructures.

10.2 Devices Characterization 10.2.1 Carrier Mobility and Dynamics Studying carrier mobility and dynamics in solar cells is essential for optimizing their efficiency and performance across various experimental and computational approaches. Experimentally, techniques such as transient absorption spectroscopy (TAS) and time-resolved photoluminescence (TRPL) are employed. TAS tracks changes in absorption over time, revealing carrier lifetimes, diffusion lengths, and recombination rates. TRPL measures emitted photons post-photoexcitation, offering insights into carrier dynamics and recombination dynamics crucial for solar cell performance evaluation. Additionally, transient photocurrent and photovoltage

10.2 Devices Characterization

0 ps

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Figure 10.10 Ab initio molecular dynamics (AIMD) simulation of composite-based perovskite CsFAMAPbI3−x Brx -OAm+ interactions. Source: Reproduced with permission [33]. Copyright 2023, Elsevier.

measurements provide real-time data on carrier transport and recombination kinetics, helping determine carrier mobility and extraction efficiency. Kelvin probe force microscopy (KPFM) complements these methods by mapping surface potentials and charge distributions, aiding in understanding interface effects in solar cells. EIS assesses charge transport mechanisms and recombination processes, offering valuable information on carrier mobility and lifetime dynamics. Computational simulations play a pivotal role in understanding carrier behaviors within solar cells. Drift-diffusion models simulate carrier transport and recombination dynamics under varying conditions of doping, defect densities, and material properties, predicting carrier mobilities and spatial distributions within the device. Monte Carlo simulations track individual carrier trajectories through scattering events, predicting carrier mobility and thermalization processes influenced by material defects and interactions. Density functional theory (DFT) calculations

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provide insights into electronic structures, band alignments, and charge transport properties, offering predictions on carrier mobility influenced by material composition and defects. These simulations complement experimental findings, providing a comprehensive understanding of carrier dynamics crucial for optimizing solar cell design and performance. Device characterization techniques further validate and enhance understanding of carrier mobilities and dynamics in solar cells. I–V measurements under illumination and external zuantum efficiency (EQE) spectroscopy quantify device performance parameters such as fill factor, efficiency, and photon-to-electron conversion efficiency across different wavelengths. Hall effect measurements determine carrier concentrations and mobilities, evaluating doping effects and temperature dependencies on charge transport properties. By integrating these experimental, computational, and device characterization approaches, researchers can systematically explore and optimize solar cell materials and device architectures to achieve higher efficiency and reliability for sustainable energy applications.

10.2.2 Trap Densities Studying trap densities in semiconductor films requires a multifaceted approach integrating experimental techniques, theoretical methods, and specialized measurements like SCLC for electron-only and HODs. Experimental Techniques. Deep level transient spectroscopy (DLTS) measures the capacitance transient response of semiconductor films after applying a voltage pulse, revealing deep-level defects (traps) within the bandgap. DLTS provides crucial information such as trap density, capture cross section, and activation energy, essential for understanding the impact of traps on semiconductor device performance. Thermal admittance spectroscopy (TAS) or thermally stimulated capacitance (TSC) techniques analyze changes in capacitance or conductance with temperature variations, identifying trap energy levels and distributions in the bandgap with high resolution. Deep level optical spectroscopy (DLOS) complements these techniques by utilizing light excitation to probe trap levels and optical properties, offering insights into both electrical and optical characteristics of traps in semiconductors. C–V and conductance–voltage (G–V) measurements assess interface trap densities near semiconductor–dielectric interfaces, providing quantitative data on trap distributions and their influence on semiconductor device behavior. Theoretical and Computational Approaches. Density of states (DOS) calculations, using DFT or empirical tight-binding models, predict trap densities and energy levels within semiconductor bandgaps. These calculations simulate electronic structures and defects, offering insights into trap formation mechanisms and distributions. Modeling trap filling and emission kinetics further enhance understanding by simulating carrier capture and emission processes and analyzing experimental transient responses to extract trap parameters such as density, energy level, and capture cross-sections. Numerical device simulations, such as TCAD simulations, integrate trap distributions into semiconductor device models, predicting how trap

–3 (a)

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10.2 Devices Characterization

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Figure 10.11 The defects passivation effect by calculated DOS for pristine 3D perovskite and composite-based perovskites incorporated PMMA additives: (a) Pb site, (b) I site, and (c) Pb-I antisite defects. Source: Reproduced with permission [19]. Copyright 2023, Elsevier.

densities affect carrier transport, recombination, and overall device characteristics. For instance, to understand the passivation effect of PMMA on perovskite films, Wang et al. calculated the DOS of pristine 3D perovskites and perovskites incorporating PMMA as shown in Figure 10.11. It could be observed that the trap state levels of the Pb-site were largely shallowed in the passivated perovskite films (Figure 10.11a), indicating undercoordinated Pb2+ cluster defects could be passivated by PMMA. To examine the Pb-I antisite defects, they further calculated the trap state levels of I-site and PbI3− frame, showing almost no trap sate levels in the composite-based perovskites (Figure 10.10b,c) compared with the pristine perovskites. They thought that such DOS results indicated the trap states presented at perovskites were remarkably passivated by polymeric PMMA. Space-Charge-Limited Current (SCLC) Method. The SCLC measurements are critical for estimating trap densities in electron-only and HODs. In EODs, SCLC determines electron mobility under varying electric fields, influenced by trap-assisted recombination. Similarly, HODs measure hole mobility affected by trap states in the semiconductor film (shown in Fig 10.4c). Analyzing SCLC data provides insights into trap densities and their impact on charge carrier mobility and recombination dynamics, essential for optimizing semiconductor materials and device structures. By combining experimental measurements with theoretical

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simulations and specialized SCLC techniques, researchers gain comprehensive insights into trap states in semiconductor films, facilitating advancements in material processing and device optimization for electronics, photonics, and renewable energy applications.

10.2.3 Stability Characterization Maximum Power Point Tracking (MPPT). The MPPT method is a technique used with variable power sources to maximize energy extraction when conditions change. For evaluating the efficiency of perovskite solar cells (PSCs) with strong hysteretic behavior, MPPT is the most trustworthy method. The MPPT method should be considered a more significant methodology for performance assessment in PSCs than the traditional J-V curve measurements. The MPPT algorithms provide an accurate assessment of device efficiency in real-world scenarios and provide insight into the short-term operational stability [28]. Time-of-Flight Secondary Ion Mass. Spectrometry (TOF-SIMS). The TOF-SIMS has been used to analyze perovskite photovoltaics made by a variety of methods, including cation uniformity (depth and lateral), changes in chemistry upon alternate processing, changes in chemistry upon degradation, and lateral distribution of passivating additives. For instance, using TOF-SIMS on multiple perovskite compositions, the information regarding halide perovskite formation as well as inhomogeneity critical to device performance can be extracted, providing one of the best proxies for understanding compositional changes resulting from degradation [29]. Encapsulation is crucial for ensuring the long-term stability and durability of solar cells against environmental factors such as moisture, oxygen, UV radiation, and mechanical stress. It acts as a protective barrier that shields solar cells from these elements, thereby minimizing degradation mechanisms that can reduce efficiency and lifespan. Effective encapsulation is essential for maintaining the integrity of semiconductor materials, interfaces, and electrical contacts within the device structure, thereby enhancing reliability and operational longevity. Despite these advancements, challenges persist in optimizing encapsulation for cost-effectiveness, scalability, and sustainability. Ongoing research focuses on developing eco-friendly encapsulation materials, improving manufacturing processes, and integrating smart encapsulation technologies for real-time monitoring of module performance and health. Continued innovation in encapsulation technologies remains critical for maximizing solar energy conversion efficiency and ensuring the reliability of solar cells in diverse environmental conditions [30–32].

References 1 Wang, K., Dong, Y., Congcong, C. et al. (2019). Recent progress in fundamental understanding of halide perovskite semiconductors. Prog. Mater. Sci. 106: 100580.

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19 Wang, Z., Liu, L., Wang, Y. et al. (2023). Green antisolvent-mediators stabilize perovskites for efficient NiOx -based inverted solar cells with Voc approaching 1.2 V. Chem. Eng. J. 457: 141204. 20 Si, H., Zhang, S., Ma, S. et al. (2020). Emerging conductive atomic force microscopy for MetalHalide perovskite materials and solar cells. Adv. Energy Mater. 10: 1903922. 21 Ma, Q., Wang, Y., Liu, L. et al. (2024). One-step dual-additive passivated wide-bandgap perovskites to realize 44.72%-efficient indoor photovoltaics. Energy Environ. Sci. 17: 1637–1644. 22 Steele, J.A., Solano, E., Hardy, D. et al. (2023). How to GIWAXS: grazing incidence wide angle X-RayScattering applied to metal halide perovskite thin films. Adv. Energy Mater. 13: 2300760. 23 Li, C., Pan, Y., Hu, J. et al. (2020). Vertically aligned 2D/3D Pb-Sn perovskites with enhanced charge extraction and suppressed phase segregation for efficient printable solar cells. ACS Energy Lett. 5: 1386–1395. 24 Stefenelli, M., Juraj, T., Angelika, R. et al. (2013). X-ray analysis of residual stress gradients in TiN coatings by a Laplace space approach and cross-sectional nanodiffraction: a critical comparison. J. Appl. Crystallogr. 46: 1378–1385. 25 Chen, Z., Prud’homme, N., Wang, B., and Ji, V. (2011). Residual stress gradient analysis with GIXRD on ZrO2 thin films deposited by MOCVD. Surf. Coat. Technol. 206: 405–410. 26 Iftimie, R., Minary, P., and Tuckerman, M.E. (2005). Ab initio molecular dynamics: concepts, recentdevelopments, and future trends. PNAS 102: 6654–6659. 27 Mosconi, E., Azpiroz, J.M., and Angelis, F.D. (2015). Ab initio molecular dynamics simulations of Methylammonium Lead iodide perovskite degradation by water. Chem. Mater. 27 (13): 4885–4892. 28 Juarez-Perez, E.J., Momblona, C., Casas, R., and Haro, M. (2024). Enhanced power-point tracking for high-hysteresis perovskite solar cells with a galvanostatic approach. Cell Rep. Phys. Sci. 5: 101885. 29 Harvey, S.P., Li, Z., Christians, J.A. et al. (2018). Probing perovskite inhomogeneity beyond the surface: TOF-SIMS analysis of halide perovskite photovoltaic devices. ACS Appl. Mater. Interfaces 10 (34): 28541–28552. 30 Rajput, P., Singh, D., Singh, K.Y. et al. (2024). A comprehensive review on reliability and degradation of PV modules based on failure modes and effect analysis. Int. J. Low-Carbon Technol. 19: 922–937. 31 Performance and Reliability of PV Systems, Solar Energy Industries Association (SEIA), https://seia.org/photovoltaics. (Accessed in October 2024) 32 Mittag, M., Haedrich, I., Neff, T., Hoffmann, S., Eitner, U., Wirth, H., TPEDGE: qualification of a gas-filled, encapsulation-free glass-glass photovoltaic module. In Proc. 30th Eur. Photovolt. Sol. Energy Conf. Exhib. 2015 33 Ma, Q., Ma, M., Liu, L. et al. (2023). Wide-band-gap perovskite solar minimodules exceeding 43% efficiency under indoor light illumination. Device 1: 100174.

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11 Perspectives and Future Work of Composites-Based Perovskite Solar Cells 11.1 Perspectives of Composites-Based Perovskite Solar Cells Since organolead halide perovskite nanocrystals were first introduced in a PV cell based on DSSC technology in 2009 [1], PSCs have shown great potential for the next-generation photovoltaic application due to an extraordinarily high PCE of over 26%. Over the last decade, researchers have made great progress toward increasing the efficiency and stability of PSCs. However, recent findings suggest that the most challenging issue for PSC commercialization is long-term stability under normal operating conditions. As described in Chapters 1 and 4, the stability issues are mostly related to materials in several aspects such as the design of device configuration, design of charge transport and/or interface materials, modification in electrode materials, and encapsulation procedures. To solve the instability problems of PSCs, robust new materials that can improve their chemical, electrical, optical, and thermal properties should be developed. It is worth noting that composite materials have advantages such as design flexibility, specialized chemical and physical properties, and resistance to a wide range of chemicals. Recently, the development of perovskite-based composites with composition engineering has been considered as an efficient strategy not only to stabilize the structures of perovskite but also to improve their chemical, optical, and electrical properties. Therefore, we recommend developing and optimizing the perovskite-based composite materials that may play a critical role in ensuring the long-term stability of PSCs as well as high power conversion efficiency. In addition to the long-term stability, manufacturing scalability with reproducibility also remains a challenge. A mass-production technology should be the most suitable for easy and low-cost fabrication for higher PCE and stability. There are numerous low-cost fabrication processes available, but the technique should have commercial viability for large-size cell fabrication. For example, spin-coating or drop-casting techniques used in most laboratories are not suitable for large-scale production, thus alternate easy and low-cost fabrication processes are needed to develop. To solve the aforementioned issues, many research groups around the world have conducted research on the development of new materials and fabrication techniques. More research is still needed for the practical use and commercialization

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of PSCs. Some of the key considerations and possible pathways for future research include: (1) Improvement of cell stability in extreme conditions such as heat accumulation due to high temperatures and prolonged illumination as well as moisture and oxygen. (2) Optimization of cell structure and interface engineering to improve charge separation/conduction and collection. (3) Numerical and/or AI-assisted design of functional perovskite-based composites as a function of dispersion phases, such as polymer, metal, and metal oxide nanoparticles; graphene and its derivatives; and/or their combinations. (4) Simulation technology to effectively study the molecular interactions in composites by molecular dynamics (MD) and promote the understanding of excitation behavior, electronic structures, charge distribution and dynamics, bonding energies, surface defects, and interactions between surface and functional molecules in composites by density functional theory (DFT). (5) Development of easy and large-scale fabrication and eco-friendly green production technology.

11.2 Future Work for Composites-Based Perovskite Solar Cells Although composite-based perovskite single-junction or tandem solar cells have made great progress in power-conversion-efficiency (PCE), some important work still needs to be further done for practical perovskite-based PV products. In this section, some future work will be discussed to achieve highly efficient, large-area, eco-friendly, and renewable perovskite solar modules (PSMs) for meeting commercial demands. It mainly focuses on fabricating functional layers in a module by scalable technology, dealing with toxicity and recycling of the whole device.

11.2.1 Scale-Up Processing Technology Key materials and processing challenges in scaling up perovskite-based PVs involve developing scalable deposition techniques for all device layer stacks, including the HTL, perovskite layer, ETL, and electrodes, as well as reliable schemes for module designs (i.e. laser-scribing process, interconnection, width, and number of sub-cells). In Chapter 9, most of the reported deposition techniques for large-area perovskite films are discussed, including blade coating, vacuum evaporation, sputtering, slot-die coating, roll-to-roll (R2R) coating, inkjet printing, etc. Thus further work should focus on the fabrication of all layers in a device without sacrificing quality and efficiency by those deposition techniques. Figure 11.1 shows a basic schematic of the entire R2R coating process for PSMs. All the layers can be deposited step-by-step on the Polyethylene Terephthalate (PET) substrate using the R2R coating method for fabricating flexible PSMs [2]. However, their efficiency (PCE ∼ 15%) still lags significantly behind that of fabrication by other deposition methods [3, 4].

Ink in

Module out

Figure 11.1

A schematic of R2R fabrication for large-scale PSMs. Source: [2]/Springer Nature/CC BY 4.0.

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Such an R2R coating method is promising for high-throughput large-area PSMs, but the design of composite-based material inks and modification of fabrication processes need to be further explored. For example, wet perovskite thin films are fabricated from inks made of precursor chemicals and then crystallized into the perovskite structure. Therefore, it is necessary to take into consideration the nucleation and crystal growth processes during perovskite printing. Hybrid deposition of large-area films is one of the most efficient ways to fabricate PSMs. For example, Tan and Co. [5] fabricated efficient all-perovskite tandem solar modules using scalable fabrication techniques combined with solution blade-coating and evaporation deposition methods. The HTLs (NiO/VNPB and PEDOT:PSS), NBG, and WBG perovskite layers (FA0.7 MA0.3 Pb0.5 Sn0.5 I3 and Cs0.35 FA0.65 PbI1.8 Br1.2 ) were fabricated by blade coating; ETLs (C60), top electrodes (Ag), and interconnecting layer (Au) were deposited by evaporation method; extra functional layer of SnO2 was deposited by atomic-layer-deposition (ALD). The fabricated all-perovskite tandem solar modules obtained a PCE of 21.7% with an aperture area of 20.25 cm2 . This work is a milestone in achieving a PCE of over 20% in all-perovskite tandem solar modules using fully scalable methods. However, compared to small-area tandems with a PCE over 30% [6], further work is needed to obtain high-performance large-area perovskite-based tandem solar modules by such fully scalable and easy fabrication methods.

11.2.2 Green Production Technology Solution-processed perovskite-based PVs have vital advantages in terms of cost and manufacturing, but this approach involves the dissolution of perovskite sources and charge transport materials in toxic solvents. For example, organic–inorganic halide perovskites are generally dissolved in toxic solvents, such as dimethylformamide (DMF), gamma-butyrolactone (GBL), and N-methyl-2-pyrrolidone (NMP); charge transport materials, such as PCBM, PTAA, and Spiro-OMeTAD, are dissolved in halogenated solvents such as toluene and chlorobenzene, which are highly toxic and detrimental to environment. The handling of large quantities of these solvents at an industrial scale would naturally cause serious environmental and health concerns. Vidal et al. [7] systemically perform a full life cycle analysis (LCA) to assess the human health toxicity and environmental impacts of solvents widely used for the deposition of organic–inorganic halide perovskites. Figure 11.2 shows the human health characterization factors expressed in disability-adjusted life years (DALYs) per kg of substance emitted for the scenario of emission to urban air. Except for the nonhazardous DMSO, most of the polar aprotic solvents have a much greater impact on the environment. Such results suggest that DMSO appears to be the most suitable solvent for mass production of perovskite PV products. However, it needs to further unlock the performance of PSCs fabricated using only DMSO solvent. DMSO solvent due to its high donor number benefits was generally used as a kind of additive in the perovskite precursor for the formation of a stable intermediate [8]. The inset from Figure 11.2 presents the LCA of perovskite PV manufacturing using solvents. Solvent transport to the perovskite factory, the removal from the thin film during drying,

11.2 Future Work for Composites-Based Perovskite Solar Cells

Figure 11.2 Human health characterization factors expressed in DALYs per kg of substance emitted for the scenario of emission to urban air. The insert is a schematic of full life cycle analysis of possible pathways for the production of perovskite PVs. Source: Reproduced with permission from [7]. Copyright 2020, Springer Nature.

solvent emission, treatment for energy recovery, and solvent recovery are illustrated. There are following four scenarios to be considered: (i) solvents during film drying are emitted directly to the environment; (ii) solvents are condensed, with only a small fraction being directly emitted; (iii) condensed solvents with high purity are reused; (iv) condensed solvents are further distilled and the fraction not distilled is incinerated. Note that solvent recovery is more environmentally friendly than incineration. Thus, researchers need to further explore and develop green solvents to fabricate high-quality and large-area perovskite films. For example, Zhao and Co. [9] reported eco-friendly biomass-derived green solvents with γ-valerolactone (GVL) and n-butyl acetate to dissolve formamidinium iodide, lead iodide, and methylamine hydrochloride, yielding to a high-quality and stable perovskite precursor. Such a perovskite precursor also allows for scaling up a mini-module with an efficiency of up to 20.23% with an aperture area of 12.25 cm2 . However, the fabrication of Spiro-OMeTAD HTL still involved toxic chlorobenzene solvent. The use of green solvents to fabricate all layers of a fully solution-processed device is promising but remains a challenge, which requires further exploration. To avoid the use of toxic solvents, the combination of vacuum-based deposition (evaporation and sputtering) and solution hybrid processes as mentioned in Section 11.2.1 may become one of the most effective ways to obtain high-quality and large-area perovskite PVs. Future works for the green production of perovskite PVs should focus on key material design, solvent selection, and fabrication methods to reduce human health and environmental risks.

229

Figure 11.3

A process of Pb components and glass/TCO substrates recycling from perovskite solar modules. Source: [12]/Springer Nature/CC BY 4.0.

References

11.2.3 Cyclic Utilization of Lead Components for Perovskite Precursors Over the past few decades, both industry and academia have mainly focused on improving and upscaling cell/module efficiency and stability of perovskite PVs. However, there is still a lack of concern about recycling PSMs. Effective recycling of spent PSMs will contribute to reducing the energy requirements and environmental consequences of their production and deployment, thus facilitating their sustainable development [10]. For instance, recycling the potential components of PSMs is a way to reduce resource use and create economic benefits from recycled materials, such as TCO-based substrates, back encapsulation glass, and lead-containing parts. Especially, the most highly efficient PSMs have used toxic lead-based metal halide perovskites, thus it is essential to develop recycling techniques or strategies for cyclic utilization of lead components as perovskite precursors. Bae et al. [11] conducted a systematic investigation to determine the toxicity potential of Pb-containing compounds when they are accidentally released from perovskite PVs. It was found that the order of less toxicity and cytotoxicity was to be Pb2+ > MAPbI3 > PbI2 = PbO. Thus, it is crucial to convert Pb to PbI2 = PbO compounds for further use as a perovskite precursor source. Chen et al. [12] reported efficient strategies to recycle toxic lead and valuable transparent conductors from PSMs. The recycling schematic processes of the potential components from PSMs are shown in Figure 11.3. A carboxylic acid cation-exchange resin acted as an adsorbent to recycle lead in decommissioned PSMs. By a short thermal treatment at high temperatures, the ITO glass substrates and back encapsulation glass could be separated and reused for module re-fabrication. However, it is a challenge to maintain the same or high efficiencies in PSMs when using both original and recycled materials. Thus, the use of recycling methods is significant, thus more collaboration from chemists, physicists, and engineers is required.

References 1 Kojima, A., Teshima, K., Shirai, Y., and Miyasaka, T. (2009). Organometal halide perovskites as visible-light sensitizers for photovoltaic cells. J. Am. Chem. Soc. 131: 6050. 2 Parvazian, E. and Watson, T. (2024). The roll-to-roll revolution to tackle the industrial leap for perovskite solar cells. Nat. Commun. 15: 3983. 3 Beynon, D., Parvazian, E., Hooper, K. et al. (2023). All-printed roll-to-roll perovskite photovoltaics enabled by solution-processed carbon electrode. Adv. Mater. 35: 2208561. 4 Weerasinghe, H.C., Macadam, N., Kim, J.-E. et al. (2024). The first demonstration of entirely roll-to-roll fabricated perovskite solar cell modules under ambient room conditions. Nat. Commun. 15: 1656. 5 Xiao, K., Lin, Y.-H., Zhang, M. et al. (2022). Scalable processing for realizing 21.7%-efficient all-perovskite tandem solar modules. Science 376: 762–767. 6 Green, M.A., Dunlop, E.D., Yoshita, M. et al. (2024). Solar cell efficiency tables (version 64). Prog. Photovolt. Res. Appl. 32: 3–13.

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7 Vidal, R., Alberola-Borràs, J.-A., Habisreutnger, S.N. et al. (2021). Assessing health and environmental impacts of solvents for producing perovskite solar cells. Nat. Sustain. 4: 277–285. 8 Park, N.-G. (2021). Green solvent for perovskite solar cell production. Nat. Sustain. 4: 192–193. 9 Miao, Y., Ren, M., Chen, Y. et al. (2023). Green solvent enabled scalable processing of perovskite solar cells with high efficiency. Nat. Sustain. 6: 1465–1473. 10 Tian, X., Stranks, S.D., and You, F. (2021). Life cycle assessment of recycling strategies for perovskite photovoltaic modules. Nat. Sustain. 4: 821–829. 11 Bae, S.-Y., Lee, S.Y., Kim, J.-W. et al. (2019). Hazard potential of perovskite solar cell technology for potential implementation of “safe-by-design” approach. Sci. Rep. 9: 4242. 12 Chen, B., Fei, C., Chen, S. et al. (2021). Recycling lead and transparent conductors from perovskite solar modules. Nat. Commun. 12: 5859.

233

Index a ab initio molecular dynamics (AIMD) 217–218 acidic additive 135–136 additives-based perovskite composites as absorbers additives-based wide-bandgap perovskite composites 162–164 2D-3D based narrow-bandgap perovskite composites 166–167 2D-3D based wide-bandgap perovskite composites 164–166 additives-based wide-bandgap perovskite composites 162–164 Ag electrodes based PSCs 56 alkali halide doping strategy 110 all-inorganic halide perovskites 10 alloy structure in A, B or X site 31–33 Arrhenius equation 15 ascorbic acid (AA) 136, 137 A-site alloying method 157 atomic force microscopy (AFM) 110, 111, 212 atomic-layer-deposition (ALD) 77, 228 Auger recombination 22, 151 average visible transmittance (AVT) 20, 21 versus PCE 20, 105

b Boltzmann constant 15, 205 Brønsted proton acceptors 56

B-site alloy-based composites 160–161 building integrated photovoltaics (BIPV) 20, 104, 200

c capacitance–voltage (C–V) measurements 210 carbon additives-based perovskite composites 142–146 carbon-based composites ETL materials 81–82 HTL materials 82–83 for interfacial layer 84–85 carbon, graphene, defined 105–110 carbon nanoparticles (CNPs) 106, 107, 120 carbon nanotube (CNT) films 28, 35 carbon QD based composites 120, 122 carbon quantum dots (CQDs) 82 cell-to-module (CTM) 117, 193 ceramic-matrix composites (CMCs) 27, 28 charge recombination layer (CRL) 155, 156 charge transport layers (CTLs) 2, 16, 18, 19, 46, 50, 52, 83, 105, 154, 173 CIGS tandems perovskite 156 coarse-grained simulations 217 composite-based charge transport layers 34–35, 173–178 composite-based electrodes 35–36 composite-based perovskite active layer (PAL) 96

234

Index

composite-based perovskite films characterization chemical interactions 215–217 devices characterization, carrier mobility and dynamics 218–220 electrical properties 209–211 four-point probe method 209–210 growth dynamics of 205–207 heterogeneity 211–215 optical properties 207–209 simulations 217–218 stability characterization 222 trap densities 220–222 experimental techniques 220 SCLC method 221–222 theoretical and computational approaches 220–221 composite-based perovskite solar cells (CPSCs) 30, 75 development strategy 2 fabrication strategy for 3–5 interface engineering 4 composite functional materials advantages of 30–31 classification of 28 development alloy structure in A, B or X site 31–33 charge transport layers 34–35 composite-based electrodes 35–36 composite perovskites 33–34 properties of 27–30 composite materials, defined 27 composite perovskites 33–34 composites-based buffer layers 178–179 composites-based charge transport layers 173 electron transport layers in tandems 176–178 hole transport layers in tandems 173–176 composites-based hole transport layers in tandems 173–176 composites-based interconnection layers 167

in all perovskite tandems 170–171 in perovskite/organic tandems 171–173 in perovskite/Si tandems 167–170 composites-based interfacial layers in tandems composites-based buffer layers 178–179 composites-based passivation layer 179–180 conduction band offset (CBO) 179 conductive atomic force microscopy (C-AFM) 167, 212–213 control crystallization by chemical interaction 124 crystalline silicon solar cells 1 C60 pyrrolidine tris-acid (CPTA) 72 current–voltage (I–V) measurements 210, 220 Czochralski (CZ) silicon 153

d deep level optical spectroscopy (DLOS) 220 deep level transient spectroscopy (DLTS) 220 density functional theory (DFT) 43, 57, 99, 100, 104, 174, 175, 217, 219, 220 density of states (DOS) 100, 220, 221 deposition methods 154, 158, 190, 191, 226, 228 device encapsulation, robust design 62–63 device heterointerface instability 52 efficiency loss induced by 58–60 heterointerface defects of perovskite/ETL 52–54 heterointerface defects of perovskite/HTL 54–56 interaction with metal electrodes 56–58 differential scanning calorimetry (DSC) 217 dimethylformamide (DMF) 14, 167, 217, 228

Index

dimethyl sulfoxide (DMSO) 14, 46, 99, 100, 164, 167, 206, 217, 228 Dion–Jacobson (DJ) 2D perovskites 11–12 disability adjusted life years (DALYs) 228 dye sensitized solar cells (DSSCs) 51

e electrochemical impedance spectroscopy (EIS) 79, 109, 217, 219 electron transport layer (ETL) 3, 18, 30, 34, 72 carbon-based composites 81–85 inorganic-based composites with metal and metal oxide 76–81 organic-based composites 71–76 in tandems 176–178 electron transport materials (ETMs) 52, 72, 84, 176 energy conversion efficiency improvement 22–23 energy gap 21–22 interface defects 22 external quantum efficiency (EQE) 144, 159, 220

f facilitate strain release, heterostructure interfaces 124, 126–127 flexible perovskite solar cells (F-PSCs) 19–20, 170 4-fluorophenethylamine (FPEAI) 166–167 4-fluoro-phenethylammonium chloride (4F-PEACl) 179 2-(4-fluorophenyl)ethylamine hydroiodide (F-PEAI) 164 2-(4-fluorophenyl)ethylammonium iodide (FPEAI) 215 focused-ion-beam (FIB) technique 211 formamidine sulfinic acid (FSA) 163–164 formamidinium iodide (FAI) 9, 46, 52, 137, 140, 154, 158, 229

Fourier transform infrared spectroscopy (FTIR) 110, 216 Frenkel defects 44, 45, 110 full width at half maximum (FWHM) 21, 96

g gallic acid (GA) 135 γ-valerolactone (GVL) 229 geometric filling factor (GFF) 117, 193, 194, 199 glass-glass vacuum laminated encapsulation 189–190 global electricity 1 grain boundaries (GBs) 2, 3, 13, 19, 44–46, 60, 93, 96, 99–101, 104, 110, 115, 118, 124, 140, 141, 166, 187, 188, 207, 211, 213 graphene oxide/polyaniline (G-PANI) nanocomposites 82 graphene quantum dots (GQDs) 33, 79, 81 grazing incidence wide-angle X-ray scattering (GIWAXS) 118, 124, 166, 213–215 grazing incidence X-ray diffraction (GIXRD) 126, 215 green production technology 228–229

h hailde perovskite thin films 49 Hall effect 209, 220 heat-induced perovskite degradation 44 heterogeneity of composite-based films 211–215 heterointerface defects of perovskite ETL 52–54 HTL 54–56 heterointerfaces instability, efficiency loss induced by 58–60 heterojunction solar cells 19 highest occupied molecular orbital (HOMO) 140 high resolution transmission electron microscopy (HR-TEM) 211–212

235

236

Index

Hoke effect 43 hole-conductor free (HCF) 17, 19, 103 hole/electron-transport-free simple structure 19 hole transport layer (HTL) 3, 34, 54, 83, 144 carbon-based composites 82 inorganic-based composites with metal and metal oxide 79 organic-based composites 72 hole transport materials (HTMs) 18, 19, 35, 54, 63, 72, 74, 75, 133, 134 hybrid perovskites and solar cells (PSC) low-dimensional perovskites 10–13 perovskite crystal growth 14–16 single-crystal perovskites 13–14 three-dimensional perovskites 7–10 working principles 16–17 hydrazine additive 134–135 hydrazinium iodide (HAI) 135 hypophosphorous acid (HPA) 135

device structures design 60, 62 perovskite composites development 60 interaction with metal electrodes 56–58 interconnection layer (ICL) 153–156, 167, 168, 172 interface defects 2, 22, 46, 54, 83, 115 interface engineering 3, 4, 22, 60, 72, 104, 105, 186, 226 interfacial layer, carbon-based composites 84–85 internal encapsulation 52, 102, 187–189, 196 internet of things (IoT) systems 198 ions migration of perovskites 50–51 isopropyl alcohol (IPA) 118

k Kelvin probe force microscopy (KPFM) 53, 219

l i IEC 61215 norm 186 indene-C60 bisadduct (ICBA) 176 inorganic additives-based perovskite composites 103 acidic additive 135–136 alkali halide addtives 110–112 carbon, grapheme 105–110 hydrazine additive 134–135 metal oxides 103–104 semitransparent PSCs with metal oxide-based composites 104–105 SnCl2 additive 134 SnF2 additive 133–134 inorganic-based composites with metal and metal oxide ETL materials 76–79 HTL materials 79, 81 inorganic composite-based perovskites 157 instability problems device encapsulation robust design 62–63

large-volume amines (LVAs) 142 lead-based perovskites 7–8 lead-tin mixed perovskites 8–9 levelized cost of energy (LCOE) 191 Lewis base additives 137, 140 Lewis electron acceptors 56 light-emitting diodes (LEDs) 13, 207, 209 light-harvesting material 1 liquid-phase transmission electron microscopy (TEM) 206 lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI) 76 low-dimensional (LD) perovskites 10 3D heterostructure perovskite composites 114 0D-3D composites 118–119 1D-3D composites 118 2D-3D composites 114–118 Dion–Jacobson (DJ) 2D perovskites 11–12 one-/zero-dimensional (1D/0D) perovskites 12–13

Index

Ruddlesden–Popper (RP) 2D perovskites 10–11

m MAPbI3 degradation 42, 53 materials instability device efficiency loss induced by 51–52 heat-induced perovskite degradation 44 ions migration 50–51 moisture-induced perovskite degradation 41–42 perovskite film surface/buried interfaces 46–48 photo-induced perovskite degradation 42–44 point defects induced perovskite degradation 44–45 strain-induced perovskite lattice distortion 48–50 matrix material-based composites 27 maximum power point tracking (MPPT) 60, 222 metal electrodes interaction 56–58 metal halide perovskites (MHPs) 16, 31–33, 110, 162, 231 metal-matrix composites (MMCs) 28 metal oxides 19, 72, 75, 79, 103–104 methylammonium chloride (MACl) 44, 79, 158, 159 methylammonium iodide (MAI) 9, 41, 44, 52, 56, 137, 205 micro-computed tomography (micro-CT) 213 moisture-induced perovskite degradation 41–42 molecular electrostatic potential (MEP) 217, 218 Monte Carlo simulations 217, 219 Mott–Gurney law 211 multi-cation based perovskite compositions 93 multi-junction solar cells 9, 158, 178 multiple single-junction cells 151

n n-i-p based traditional structure 18 nitrogen-doped graphene oxide (Nx GO) 144, 145 N, N-dimethylformamide (DMF) 99

o oleylammonium iodide (OAmI) 114, 115, 213 1D-3D composites 118 one-/zero-dimensional (1D/0D) perovskites 12–13 organic additives-based perovskite composites 93, 137 organic ammonium halides 93–96 organic small molecules 96–99 polymer based materials 99–102 organic ammonium halides 93–96 organic-based composites ETL materials 71–72 HTL materials 72–76 organic small molecules 96–99 oxygen-induced photodegradation 43

p Pb-based perovskites 7, 8, 119 2,2′ -(perfluoronaphthalene-2,6-diylidene)dimalononitrile (F6-TCNNQ) 162 perovskite alloy-based composites as absorbers 156 A-site alloy-based composites 157 B-site alloy-based composites 160–161 X-site alloy-based composites 158–160 perovskite-based devices device heterointerface instability 52 efficiency loss induced by 58–60 heterointerface defects of perovskite/ETL 52–54 heterointerface defects of perovskite/HTL 54–56 interaction with metal electrodes 56–58 instability problems 60

237

238

Index

perovskite-based devices (contd.) device encapsulation robust design 62–63 device structures design 60, 62 perovskite composites development 60 materials instability device efficiency loss induced by 51–52 heat-induced perovskite degradation 44 ions migration 50–51 moisture-induced perovskite degradation 41–42 perovskite film surface/buried interfaces 46–48 photo-induced perovskite degradation 42–44 point defects induced perovskite degradation 44–45 strain-induced perovskite lattice distortion 48–50 perovskite-based tandems all perovskite tandems 154–155 perovskite/CIGS tandems 156 perovskite/organic tandems 155–156 perovskite/Si tandems 152–154 perovskite composites development 60 perovskite crystal growth, dynamics of 14–16 perovskite crystal lattice 45 perovskite/ETL heterointerface defects 52–54 perovskite film surface/buried interfaces 46–48 perovskite/HTL heterointerface defects 54–56 perovskite materials 2, 7–16, 33, 41, 52, 156, 192, 196 perovskite/organic tandems 155–156 perovskite quantum dots (PQDs) 120–121 perovskite/Si tandems 152–154, 164, 167–170, 176, 179, 198 perovskite solar cells (PSCs) 1

configurations of flexible perovskite solar cells 19–20 hole/electron-transport-free simple structure 19 n-i-p based traditional structure 18 p-i-n based inverted structure 18 semitransparent perovskite solar cells 20 current status of 185–186 cyclic utilization of lead components 231 energy conversion efficiency improvement 22–23 energy gap 21–22 interface defects 22 fabrication process of modules 192–193 future research 226–231 green production technology 228–230 market prospect 196–199 module quality assessment 194 module structure design 193–194 perspectives of composites-based 225–226 power conversion efficiencies 74 scalable fabrication methods deposition methods 190–192 high-quality perovskite films 192 scale-up processing technology 226–228 solar modules production efficiency and stability improvement 196 recent advances in efficiency of modules 194–196 stability issues 186 evaluation standards 186–187 external encapsulation 189–190 internal encapsulation 187–189 toxicity issues of lead in modules 200 working principles 16–17 perovskite solar modules (PSMs) 185, 226 GCL 197 IEC 61215 test 188

Index

(6,6)-phenyl-C61-butyric acid methyl (PCBM) 71 phenylethylammonium iodide (PEAI) 114 photo-induced perovskite degradation 42–44 photoluminescence (PL) 11, 13, 43, 79, 119, 120, 194 photoluminescence spectroscopy 208 p-i-n based inverted structure 18 planar heterojunction (PHJ) 16, 71, 84 planar n-i-p perovskite solar cells 73 PMMA-based composite perovskite film 49 point defects induced perovskite degradation 44–45 poly(9-vinylcarbazole) (PVK) 49, 76 poly (ethylene-co-vinyl acetate) (EVA) 141–142 polycrystalline perovskite films 14, 45, 48, 60, 93, 99, 100, 187, 212 polycrystalline perovskites 13, 19 Polyethylene Terephthalate (PET) 226 polymeric Lewis base 99, 100 polymer-matrix composites (PMCs) 27, 28, 30 polymer polymethyl methacrylate (PMMA) 99, 211 polymethyl methacrylate (PMMA), HR-TEM techniqu 99 power conversion efficiency (PCE) 1, 35, 71, 133, 151, 226 lower 9 4-pyrene oxy butylamine (PYBA) 124

q quantum dot additives-based perovskite composites 119 carbon QD based composites 120, 122 perovskite QD based composites 120 quasi-Fermi levels (QFLs) 16, 120

r reduced film strain by composites-based perovskites

control crystallization by chemical interaction 124 facilitate strain release by heterostructure interfaces 124, 126–127 reduce lattice strain by compositional design 122 reduced graphene oxide (rGO) 83 reduce lattice strain by compositional design 122 robust design, device encapsulation 62–63 Ruddlesden–Popper (RP) 2D perovskites 10–11

s scalable fabrication methods deposition methods 190–192 high-quality perovskite films 192 scanning electron microscopy (SEM) 21, 42, 110, 159, 179, 211 Schottky defects 44, 45, 110 selected area electron diffraction (SAED) 207 self-assembled monolayers (SAMs) 79, 154, 156, 157, 167, 170 self-assembled organic nanocomposites (SAONs) 72 semitransparent perovskite solar cells 20 semitransparent PSCs with metal oxide-based composites 104–105 sequential interface engineering (SIE) strategy 179 Shockley–Quisser (S–Q) 9, 21, 116 silicon-based solar cells 1 silicon heterojunction (SHJ)-bottom cells 153 single-crystal perovskites 2, 13, 14, 45, 60 single-junction c-Si solar cells 151 single-junction solar cells 9, 21, 151, 161, 164, 173, 177 single-walled carbon nanotubes (SWNTs) 82 Si tandems perovskite 167–170

239

240

Index

slot-die coating method 191 SnCl2 additive 134 SnF2 additive 133–134 Sn-PSCs, structures and additives 138 Sn-PS solar cells 145 solar modules production efficiency and stability improvement 196 recent advances in efficiency of modules 194–196 solar spectrum 23, 151 solid-state nuclear magnetic resonance (NMR) spectroscopy 216 space-charge-limited current (SCLC) 210, 220–222 Spiro-OMeTAD oxidation process 76 strain-induced perovskite lattice distortion 48–50 Stranski–Krastanov mechanisms 205

t tandem solar cells (TSCs) 151 additives-based perovskite composites as absorbers 161–167 composites-based charge transport layers 173–178 composites-based interconnection layers 167–173 composites-based interfacial layers in tandems 178–180 perovskite alloy-based composites as absorbers 156–161 perovskite-based tandems 151 all perovskite tandems 154–155 perovskite/CIGS tandems 156 perovskite/organic tandems 155–156 perovskite/Si tandems 152–154 4-tert-butylpyridine (TBP) 76 4-(tert-butyl)pyridinium bis(trifluoromethanesulfonyl)imide (BPTFSI) 76 thermal admittance spectroscopy (TAS) 218, 220

thermally stimulated capacitance (TSC) techniques 155, 164, 170–172, 220 thermogravimetric analysis (TGA) 217 thin-film solar cells 1, 62 3D crystal perovskites 10 three-dimensional perovskites all inorganic perovskites 10 lead-based perovskites 7–8 lead-tin mixed perovskites 8–9 tin-based perovskites 9–10 time-of-flight secondary ion mass spectrometry (TOF-SIMS) 173, 222 time-resolved mass spectrometry technique 56 time-resolved photoluminescence (TRPL) 79, 218 tin-based perovskites 9–10 TiO2 -reduced graphene oxide (T/RGO) 81–82 transient absorption spectroscopy (TAS) 218, 220 transmission electron microscopy (TEM) 100, 205, 206, 211, 213 transparent conductive oxide (TCO) 19, 20, 152, 156, 167, 192, 194, 231 2,2,2-trifluoroethylamine hydrochloride (TFEACl) 135 1,1,1-trifluoroethyl ammonium iodide (FEAI) 96 trimethylamine (TMA) 140 triphenylamine (TPA) 74–75, 84 tris(2,4,6-trimethyl-3-(pyridine-3-yl)phenyl)borane (3TPYMB) 71 2D-3D based narrow-bandgap perovskite composites 166–167 2D-3D based wide-bandgap perovskite composites 164–166 2D-3D composites 114–118 2D perovskites Dion-Jacobson (DJ) 11–12 Ruddlesden–Popper (RP) 10–11

Index

v

x

van der Waals gap 10–12 Volmer–Weber growth model 207 Volmer–Weber mechanisms 205

X-ray diffraction (XRD) 42, 48, 96, 112, 120, 133, 140, 164, 205 X-ray photoelectron spectroscopy (XPS) 42, 98, 107, 110, 170, 213, 216 X-site alloy-based composites 156, 158–160

w wide-bandgap (WBG) absorber materials 152 perovskite modules 116, 199 perovskite precursors 115, 162, 164

z 0D-3D composites, 118–119

241