Solar Cells: Types and Applications [1st ed. 2023] 9789819973323, 9789819973330, 9819973325

This book highlights developments in the field of solar cells. The chapters in this book address a wide range of topics

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Solar Cells: Types and Applications [1st ed. 2023]
 9789819973323, 9789819973330, 9819973325

Table of contents :
Preface
Acknowledgments
Contents
1 Introduction to Solar Cells
1.1 Introduction
1.1.1 N-Type Semiconductor
1.1.2 P-Type Semiconductor
1.2 Solar Cell as p–n Junction Diode
1.2.1 Construction
1.2.2 Working Mechanism
1.2.3 I-V Characteristics of a Solar Cell
1.2.4 I-V Characteristics of a Photovoltaic Array
1.2.5 Equivalent Circuit and Analysis of a Solar Cell as a Diode
1.3 Parameters of a Solar Cell
1.3.1 Short-Circuit Current (ISC)
1.3.2 Open-Circuit Voltage (VOC)
1.3.3 Fill Factor (FF)
1.3.4 Efficiency (Η)
1.3.5 Characteristic Resistance (RCH)
1.3.6 Series Resistance (RS)
1.3.7 Shunt Resistance (RSH)
1.3.8 Quantum Efficiency (QE)
1.3.9 Spectral Response (SR)
1.4 History of Solar Cells
1.5 How Do Solar Panels Work?
1.5.1 Photovoltaic (PV) Cells
1.5.2 The Solar Panels
1.5.3 Types of Solar Panel
1.6 The Solar Spectrum
1.7 Solar Cells Generations
1.7.1 First-Generation Solar Cells
1.7.2 Second-Generation Cells
1.7.3 Third-Generation Cells
1.7.4 Fourth-Generation Cells
1.8 Global Solar Power Market
1.9 Cost of Solar Energy
1.10 Factors Affecting the Cost of Solar Cell
1.11 Applications
1.12 Summary
1.13 Points to Remember
References
2 Silicon-Based Solar Cells
2.1 Introduction
2.2 Silicon Substrates
2.3 Processing Steps to Obtain High-Quality Si Substrates
2.3.1 Refining
2.3.2 Crystal Growth
2.3.3 Cutting and Polishing
2.4 Cleaning Steps to Use Si Substrates for Further Application
2.5 Solar Cell Processing Technologies
2.5.1 Texturization
2.5.2 Formation of p–n Junction
2.5.3 Doping by Diffusion
2.5.4 Ion Implantation
2.5.5 Edge Isolation
2.5.6 Anti-reflection Coating
2.5.7 Metallization
2.5.8 Testing and Sorting
2.6 Thin-Film PV Cells and Amorphous Silicon
2.7 Applications
2.7.1 Monocrystalline Silicon Solar Cells
2.7.2 Polycrystalline Silicon Solar Cells
2.7.3 Amorphous Silicon Solar Cells
2.8 Summary
2.9 Important Timelines
References
3 CIGS-Based Solar Cells
3.1 Introduction
3.2 c-Si versus CIGS Solar Cells
3.3 Optical Bandgap
3.3.1 Issues Related to High Bandgap CIGS Layer
3.3.2 Graded Bandgap CIGS Layer
3.4 Implications of Sodium Incorporation
3.5 CIGS Film Deposition Approaches
3.5.1 Co-evaporation Approach
3.5.2 Sequential Deposition Approach: Selenization/Sulfurization
3.5.3 Non-vacuum Deposition Approach
3.6 Buffer Layer and Transparent Conducting Oxide
3.7 Flexible CIGS Solar Cells
3.8 Factors Affecting the Performance of CIGS Solar Cells
3.9 Applications
3.10 Summary
3.11 Important Timelines
References
4 Organic Solar Cells
4.1 Introduction
4.2 Historical Background
4.3 Active Layer Materials
4.3.1 Electron Donor
4.3.2 Electron Acceptor
4.4 Fabrication Approaches
4.5 Basic Working Principles
4.5.1 Exciton Generation
4.5.2 Exciton Diffusion
4.5.3 Exciton Dissociation
4.5.4 Charge Transfer
4.5.5 Charge Collection
4.6 OSC Device Architectures
4.6.1 Single-Layer Architecture
4.6.2 Bilayer Heterojunction Architecture
4.6.3 Bulk Heterojunction Architecture
4.6.4 Tandem BHJ Architecture
4.7 Significant Parameters Affecting the Morphology of Photoactive Layer
4.7.1 Material Composition
4.7.2 Effect of Solvent
4.7.3 Solvent Annealing
4.7.4 Thermal Annealing
4.7.5 Additive
4.8 Applications
4.9 Summary
4.10 Important Timelines
References
5 Perovskite Solar Cells
5.1 Introduction
5.2 Historical Background
5.3 Structure of Perovskites
5.4 Development of Different Device Configurations
5.4.1 Liquid Electrolyte Dye-Sensitized Structure
5.4.2 Solid State Mesoscopic Structure
5.4.3 Meso-Superstructured Structure
5.4.4 Regular Structure
5.4.5 Planar n-i-p Heterojunction Structure
5.4.6 Planar p-i-n Heterojunction Structure
5.5 Fabrication Methodologies of Perovskites for PV Application
5.5.1 Perovskite Layer Fabrication Approaches
5.5.2 Large-Scale Manufacturing Techniques
5.6 Finding a Solution to PSCs’ Instability Dilemma for Practical Implementation
5.6.1 The Golden Triangle
5.6.2 Tackling Stability Concern
5.6.3 Accelerated Aging Studies and Mixed Stability Tests
5.6.4 The Actual Costs of Perovskite Solar Cells
5.7 Applications
5.8 Summary
5.9 Important Timelines
References
6 Organic–Inorganic Hybrid Solar Cells
6.1 Introduction
6.2 Basic Operating Principles and Device Architecture
6.3 Bulk Heterojunction OIH Solar Cells
6.3.1 Benefits of Bulk Heterojunction Configuration
6.3.2 Issues Responsible for Constrained Performance
6.3.3 Inverted-type Hybrid Bulk Heterojunction Solar Cells
6.4 Bilayer Heterojunction OIH Solar Cells
6.5 Materials
6.5.1 Ideal Properties of Photoactive Layer
6.5.2 Significant Material Groups
6.6 Performance Restrictions
6.6.1 Nanoparticle Surface Chemistry
6.6.2 Nanomorphology
6.7 Applications
6.8 Summary
6.9 Important Timelines
References
7 Solar Cell Modeling Parameters
7.1 Introduction
7.2 Equivalent Circuit Models
7.2.1 Single-Diode Model
7.2.2 Double-Diode Model
7.2.3 Single-Diode Model Versus Double-Diode Model
7.2.4 Models Other Than Single- and Double-Diode Model
7.3 Summary
References
8 Characterization Techniques
8.1 Introduction
8.2 External Quantum Efficiency
8.2.1 Apparatus for Measuring External Quantum Efficiency (EQE)
8.2.2 Calibration Process for Measuring External Quantum Efficiency (EQE) of a Solar Cell
8.2.3 Internal Quantum Efficiency of a Solar Cell from Reflectance Data
8.2.4 QE Measurement Data
8.2.5 Spectral Response
8.2.6 Solar Cell Current
8.3 Energy Conversion Efficiency
8.4 I–V Curve
8.4.1 I–V Curve of a Solar Cell
8.4.2 Solar Panel I–V Characteristic Curves
8.5 The Electrical Characteristics of a Photovoltaic Array
8.5.1 Solar Array Parameters
8.6 Illumination for I–V Curves
8.6.1 Illumination Sources
8.6.2 Deviations from Air Mass 1.5
8.7 I–V Curve Measurement Apparatus
8.7.1 Light Sources for Testing a Solar Cell
8.7.2 Temperature Control
8.8 Electrical Measurement
8.8.1 Calibration
8.8.2 Comparing Jsc from QE and IV Measurements
8.8.3 Spectral Mismatch
8.9 Summary
References
9 Future in Solar Cell Technology
9.1 Introduction
9.1.1 The Rising Significance of Solar Energy
9.1.2 Current State of Solar Cell Technology
9.1.3 The Future Horizon: Advancements and Possibilities
9.1.4 Addressing Challenges for a Sustainable Solar Future
9.1.5 The Road Ahead: An Integrated Energy Landscape
9.2 Material Benefits
9.3 Efficiency Drive
9.4 Band Together
9.5 Tricks of the Light
9.6 Light Trapping and Waveguiding
9.7 Spectral Shaping and Photon Upconversion
9.8 Defect Engineering for Charge Carrier Transport
9.9 Energy-Selective Contacts for Reduced Recombination Losses
9.10 Future Perspectives
9.11 Summary
9.12 Important Timelines
9.13 Points to Remember
References

Citation preview

Sandeep Arya Prerna Mahajan

Solar Cells Types and Applications

Solar Cells

Sandeep Arya · Prerna Mahajan

Solar Cells Types and Applications

Sandeep Arya Department of Physics University of Jammu Jammu and Kashmir, India

Prerna Mahajan Department of Physics University of Jammu Jammu and Kashmir, India

ISBN 978-981-99-7332-3 ISBN 978-981-99-7333-0 (eBook) https://doi.org/10.1007/978-981-99-7333-0 © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 This work is subject to copyright. All rights are solely and exclusively licensed by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Singapore Pte Ltd. The registered company address is: 152 Beach Road, #21-01/04 Gateway East, Singapore 189721, Singapore Paper in this product is recyclable.

Solar cells, a viable option to illuminate a cleaner and brighter future.

Dedicated to my parents, wife and children (Kavish and Kaiyra) Sandeep Arya

Preface

Solar cells convert sunlight directly into electricity through the photovoltaic effect. Solar cells, also known as photovoltaic cells, are semiconductor devices that generate electric current when exposed to light. Solar cells harness sunlight, a renewable and virtually limitless source of energy. Unlike fossil fuels, solar energy does not produce greenhouse gases, air pollutants, or contribute to climate change. This makes solar power a crucial tool in reducing carbon emissions and combating climate change. Solar cells can be installed on rooftops, open land, and in urban areas, bringing energy production closer to where it’s needed. This reduces transmission and distribution losses that occur with centralized power generation and long-distance transportation of electricity. This book is aimed at the students who are pursuing Engineering, Masters, and Research in the field of solar cell technology. We focus on basics of solar cells, their types, and applications. We have classified the solar cells in accordance with their generations and elaborated the key points and principles associated with it. This preface embarks on a journey that uncovers the science behind solar cells, from energy bands to electron flow, and explores their characterization techniques along with manifold applications across industries. From the synergy of material science to the optimization of efficiency, the pursuit of enhancing solar cell technology fuels researchers and engineers alike. As the world’s energy landscape shifts, we find ourselves at the precipice of a solar revolution, one that holds the promise of an energy-abundant future while respecting the delicate balance of our planet. In the chapters that follow, we delve into the principles that underpin solar cells, unravel their technological intricacies, and unveil the myriad ways in which they are reshaping how we harness energy and envision a sustainable tomorrow. In summary, Solar Cells: Types and Applications provides a comprehensive overview of the science, technology, applications, and implications of solar cells in our modern world. It serves as a valuable resource for researchers, students, professionals, and anyone

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interested in understanding the role of solar cells in shaping the future of energy production. Jammu and Kashmir, India

Sandeep Arya Prerna Mahajan

Acknowledgments

I acknowledge Mr. Aamir Ahmed, Dr. Anoop Singh, and Dr. Sonali Verma at the University of Jammu for their assistance rendered in ensuring the success of this book. I would also express my deep gratitude to all the faculty members, technical and non-technical staff whose encouragement has been valuable in completing this book. I sincerely acknowledge the rest of my colleagues for their reliable assistance in a pleasant working environment. I may miss the mark if I do not record a special and unending gratitude to my ever-loving and understanding parents, Sh. Narain Dass and Smt. Vidya Devi. I take this opportunity to express my deep gratitude, and I reached this stage only because of their support and dreams. I thank my beloved wife Dr. Arti, for her continuous support and love. Thank you, my younger sister and her husband (Mrs. Supriya and Mr. Shubam) for your love, prayers, and moral support. I would like to take a moment to express my deepest gratitude to the two incredible souls who have brought immeasurable joy, inspiration, and love into my life—my cherished children, Kavish Arya and Kaiyra Arya. I thank all my family members for their unconditional love, patience, prayers, and support of all kinds during the study. I expect to be pardoned if I have missed the name of anybody inadvertently who knowingly or unknowingly helped me during my investigations and final compilation. Sandeep Arya

First and foremost, I would like to express my sincere thanks to Almighty for His blessings. I am extremely grateful with the deepest sense of gratitude toward my mother, Smt. Pratibha Rani, for her love and constant support. I heartily pay tributes to the heavenly soul of my father, Sh. Vinod Kumar Dogra, for his blessings. I am highly indebted to my guide, Dr. Sandeep Arya, for all the encouragement that he granted me in accomplishing this work. This acknowledgement would not be complete without mentioning the painstaking efforts and support of my family. The love and moral support of my

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beloved husband, Mr. Deepak Gupta, enabled me to accomplish this task by reviving me through his care, sacrifice, and affection. I would like to express thanks to my darling and sweet little daughter, Pravika Gupta, for being a perennial source of excitement and exuberance at all times and being such a good girl always cheering me up. It is my great pleasure to thank my elder brother, Mr. Varun Dogra, and my bhabhi, Mrs. Smily Gupta, for their inspiration and regular motivation. I am also thankful to my mother-in-law, Smt. Swarn Gupta, father-in-law, Sh. O. P. Gupta, and my grandparents, Smt. Rajkumari and Sh. Krishan Lal Gupta, for their constant support. No words are adequate to express my indebtedness to my in-laws for taking a good care of my little child in my absence to accomplish this feat. Prerna Mahajan

Contents

1 Introduction to Solar Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1.1 N-Type Semiconductor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1.2 P-Type Semiconductor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 Solar Cell as p–n Junction Diode . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.1 Construction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.2 Working Mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.3 I-V Characteristics of a Solar Cell . . . . . . . . . . . . . . . . . . . . . 1.2.4 I-V Characteristics of a Photovoltaic Array . . . . . . . . . . . . . 1.2.5 Equivalent Circuit and Analysis of a Solar Cell as a Diode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3 Parameters of a Solar Cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.1 Short-Circuit Current (I sc ) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.2 Open-Circuit Voltage (V oc ) . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.3 Fill Factor (FF) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.4 Efficiency (H) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.5 Characteristic Resistance (RcH ) . . . . . . . . . . . . . . . . . . . . . . . 1.3.6 Series Resistance (RS ) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.7 Shunt Resistance (RSH ) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.8 Quantum Efficiency (QE) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.9 Spectral Response (SR) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.4 History of Solar Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.5 How Do Solar Panels Work? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.5.1 Photovoltaic (PV) Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.5.2 The Solar Panels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.5.3 Types of Solar Panel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.6 The Solar Spectrum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.7 Solar Cells Generations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.7.1 First-Generation Solar Cells . . . . . . . . . . . . . . . . . . . . . . . . . . 1.7.2 Second-Generation Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.7.3 Third-Generation Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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1.7.4 Fourth-Generation Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.8 Global Solar Power Market . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.9 Cost of Solar Energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.10 Factors Affecting the Cost of Solar Cell . . . . . . . . . . . . . . . . . . . . . . 1.11 Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.12 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.13 Points to Remember . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

22 23 25 27 30 31 32 33

2 Silicon-Based Solar Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Silicon Substrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3 Processing Steps to Obtain High-Quality Si Substrates . . . . . . . . . . 2.3.1 Refining . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.2 Crystal Growth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.3 Cutting and Polishing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4 Cleaning Steps to Use Si Substrates for Further Application . . . . . 2.5 Solar Cell Processing Technologies . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5.1 Texturization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5.2 Formation of p–n Junction . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5.3 Doping by Diffusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5.4 Ion Implantation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5.5 Edge Isolation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5.6 Anti-reflection Coating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5.7 Metallization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5.8 Testing and Sorting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.6 Thin-Film PV Cells and Amorphous Silicon . . . . . . . . . . . . . . . . . . . 2.7 Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.7.1 Monocrystalline Silicon Solar Cells . . . . . . . . . . . . . . . . . . . 2.7.2 Polycrystalline Silicon Solar Cells . . . . . . . . . . . . . . . . . . . . . 2.7.3 Amorphous Silicon Solar Cells . . . . . . . . . . . . . . . . . . . . . . . 2.8 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.9 Important Timelines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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3 CIGS-Based Solar Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 c-Si versus CIGS Solar Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3 Optical Bandgap . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.1 Issues Related to High Bandgap CIGS Layer . . . . . . . . . . . . 3.3.2 Graded Bandgap CIGS Layer . . . . . . . . . . . . . . . . . . . . . . . . . 3.4 Implications of Sodium Incorporation . . . . . . . . . . . . . . . . . . . . . . . . 3.5 CIGS Film Deposition Approaches . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5.1 Co-evaporation Approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5.2 Sequential Deposition Approach: Selenization/ Sulfurization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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3.5.3 Non-vacuum Deposition Approach . . . . . . . . . . . . . . . . . . . . 3.6 Buffer Layer and Transparent Conducting Oxide . . . . . . . . . . . . . . . 3.7 Flexible CIGS Solar Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.8 Factors Affecting the Performance of CIGS Solar Cells . . . . . . . . . 3.9 Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.10 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.11 Important Timelines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

89 90 91 93 94 95 95 97

4 Organic Solar Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2 Historical Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3 Active Layer Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.1 Electron Donor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.2 Electron Acceptor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4 Fabrication Approaches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5 Basic Working Principles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5.1 Exciton Generation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5.2 Exciton Diffusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5.3 Exciton Dissociation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5.4 Charge Transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5.5 Charge Collection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.6 OSC Device Architectures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.6.1 Single-Layer Architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.6.2 Bilayer Heterojunction Architecture . . . . . . . . . . . . . . . . . . . 4.6.3 Bulk Heterojunction Architecture . . . . . . . . . . . . . . . . . . . . . 4.6.4 Tandem BHJ Architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.7 Significant Parameters Affecting the Morphology of Photoactive Layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.7.1 Material Composition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.7.2 Effect of Solvent . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.7.3 Solvent Annealing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.7.4 Thermal Annealing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.7.5 Additive . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.8 Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.9 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.10 Important Timelines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

101 101 105 106 106 107 110 111 112 113 115 115 115 117 118 118 119 121

5 Perovskite Solar Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2 Historical Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3 Structure of Perovskites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4 Development of Different Device Configurations . . . . . . . . . . . . . . . 5.4.1 Liquid Electrolyte Dye-Sensitized Structure . . . . . . . . . . . . 5.4.2 Solid State Mesoscopic Structure . . . . . . . . . . . . . . . . . . . . . .

131 131 132 135 137 138 138

121 122 122 122 123 123 123 125 125 127

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5.4.3 Meso-Superstructured Structure . . . . . . . . . . . . . . . . . . . . . . . 5.4.4 Regular Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4.5 Planar n-i-p Heterojunction Structure . . . . . . . . . . . . . . . . . . 5.4.6 Planar p-i-n Heterojunction Structure . . . . . . . . . . . . . . . . . . 5.5 Fabrication Methodologies of Perovskites for PV Application . . . . 5.5.1 Perovskite Layer Fabrication Approaches . . . . . . . . . . . . . . . 5.5.2 Large-Scale Manufacturing Techniques . . . . . . . . . . . . . . . . 5.6 Finding a Solution to PSCs’ Instability Dilemma for Practical Implementation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.6.1 The Golden Triangle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.6.2 Tackling Stability Concern . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.6.3 Accelerated Aging Studies and Mixed Stability Tests . . . . . 5.6.4 The Actual Costs of Perovskite Solar Cells . . . . . . . . . . . . . 5.7 Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.8 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.9 Important Timelines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

140 141 142 143 143 144 151

6 Organic–Inorganic Hybrid Solar Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2 Basic Operating Principles and Device Architecture . . . . . . . . . . . . 6.3 Bulk Heterojunction OIH Solar Cells . . . . . . . . . . . . . . . . . . . . . . . . . 6.3.1 Benefits of Bulk Heterojunction Configuration . . . . . . . . . . 6.3.2 Issues Responsible for Constrained Performance . . . . . . . . 6.3.3 Inverted-type Hybrid Bulk Heterojunction Solar Cells . . . . 6.4 Bilayer Heterojunction OIH Solar Cells . . . . . . . . . . . . . . . . . . . . . . . 6.5 Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.5.1 Ideal Properties of Photoactive Layer . . . . . . . . . . . . . . . . . . 6.5.2 Significant Material Groups . . . . . . . . . . . . . . . . . . . . . . . . . . 6.6 Performance Restrictions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.6.1 Nanoparticle Surface Chemistry . . . . . . . . . . . . . . . . . . . . . . . 6.6.2 Nanomorphology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.7 Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.8 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.9 Important Timelines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

165 165 167 170 172 173 173 175 177 177 179 185 185 187 188 189 190 191

7 Solar Cell Modeling Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2 Equivalent Circuit Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2.1 Single-Diode Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2.2 Double-Diode Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2.3 Single-Diode Model Versus Double-Diode Model . . . . . . . 7.2.4 Models Other Than Single- and Double-Diode Model . . . . 7.3 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

197 197 199 200 203 206 206 207 208

154 155 155 156 157 158 159 160 161

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8 Characterization Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2 External Quantum Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2.1 Apparatus for Measuring External Quantum Efficiency (EQE) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2.2 Calibration Process for Measuring External Quantum Efficiency (EQE) of a Solar Cell . . . . . . . . . . . . . . 8.2.3 Internal Quantum Efficiency of a Solar Cell from Reflectance Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2.4 QE Measurement Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2.5 Spectral Response . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2.6 Solar Cell Current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.3 Energy Conversion Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.4 I–V Curve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.4.1 I–V Curve of a Solar Cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.4.2 Solar Panel I–V Characteristic Curves . . . . . . . . . . . . . . . . . 8.5 The Electrical Characteristics of a Photovoltaic Array . . . . . . . . . . 8.5.1 Solar Array Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.6 Illumination for I–V Curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.6.1 Illumination Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.6.2 Deviations from Air Mass 1.5 . . . . . . . . . . . . . . . . . . . . . . . . . 8.7 I–V Curve Measurement Apparatus . . . . . . . . . . . . . . . . . . . . . . . . . . 8.7.1 Light Sources for Testing a Solar Cell . . . . . . . . . . . . . . . . . . 8.7.2 Temperature Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.8 Electrical Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.8.1 Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.8.2 Comparing Jsc from QE and IV Measurements . . . . . . . . . . 8.8.3 Spectral Mismatch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.9 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

211 211 212

9 Future in Solar Cell Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.1.1 The Rising Significance of Solar Energy . . . . . . . . . . . . . . . 9.1.2 Current State of Solar Cell Technology . . . . . . . . . . . . . . . . . 9.1.3 The Future Horizon: Advancements and Possibilities . . . . . 9.1.4 Addressing Challenges for a Sustainable Solar Future . . . . 9.1.5 The Road Ahead: An Integrated Energy Landscape . . . . . . 9.2 Material Benefits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.3 Efficiency Drive . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.4 Band Together . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.5 Tricks of the Light . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.6 Light Trapping and Waveguiding . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.7 Spectral Shaping and Photon Upconversion . . . . . . . . . . . . . . . . . . . 9.8 Defect Engineering for Charge Carrier Transport . . . . . . . . . . . . . . .

237 237 237 238 239 239 239 240 241 243 244 246 247 248

214 216 217 218 220 221 222 223 223 224 225 225 226 227 227 228 228 229 230 230 231 232 233 233

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9.9

Energy-Selective Contacts for Reduced Recombination Losses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.10 Future Perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.11 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.12 Important Timelines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.13 Points to Remember . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

249 250 251 252 253 254

Chapter 1

Introduction to Solar Cells

1.1 Introduction Solar cells are the electrical devices that directly convert solar energy (sunlight) into electric energy. This conversion is based on the principle of photovoltaic effect in which DC voltage is generated due to flow of electric current between two layers of semiconducting materials (having opposite conductivities) upon exposure to the sunlight [1]. A solar cell is a type of photoelectric cell which consists of a p–n junction diode. Solar cells are also called photovoltaic (PV) cells. An intrinsic (pure or undoped) semiconducting material like silicon (Si) or germanium (Ge) does not contain any free charge carriers. They contain four electrons in their outermost shell and just act like resistors [2]. The conductivity of such intrinsic semiconductors can be improved by adding specific impurities within the crystal lattice of it. This process is called doping. Impurities + Intrinsic Semiconductor = Extrinsic Semiconductor

1.1.1 N-Type Semiconductor When a very low concentration of pentavalent impurity (atoms with 5 electrons in their outermost shells) is added to intrinsic semiconductor without modifying its crystal structure, an N-type semiconductor is formed. Arsenic (As) and antimony (Sb) are few examples of pentavalent impurities [3]. Figure 1.1 shows the alteration of silicon crystal with the addition of pentavalent impurity atom. On adding a pentavalent impurity into an intrinsic semiconductor, following changes take place: • The intrinsic semiconductor converts into an N-type semiconductor. © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 S. Arya and P. Mahajan, Solar Cells, https://doi.org/10.1007/978-981-99-7333-0_1

1

2

1 Introduction to Solar Cells

Fig. 1.1 Arrangement of atoms in N-type semiconductor

• The extra electrons of the impurity atom (pentavalent) do not indulge in the covalent bonding. • Since the pentavalent impurity atoms donate an extra electron to the semiconductor. Thus, they are termed donor atoms. • The donated electrons can freely move within the crystal structure. Thus, N-type semiconductors contain free electrons. • Electrically, an N-type semiconductor is neutral in nature. • To move an electron from valence band to conduction band in an N-type semiconductor, 0.7 eV of energy is required. • An N-type semiconductor is only able to conduct 0.005 eV of energy applied to it. • In an N-type semiconductor, electrons are the majority carriers and holes are the minority carriers. • The quantity of donor atoms added to the intrinsic semiconductor decides the majority charge carriers in its structure. • The electric current in an N-type semiconductor is conducted via the majority carriers (electrons).

1.1 Introduction

3

1.1.2 P-Type Semiconductor When a very low concentration of trivalent impurity (atoms with 3 electrons in their outermost shells) is added to an intrinsic semiconductor without modifying its crystal structure, it results into the formation of a P-type semiconductor [4]. The two most commonly used trivalent impurities are indium (In) and gallium (Ga). Figure 1.2 shows the alteration of silicon crystal with the addition of trivalent impurity atom. The following changes occur in an intrinsic semiconductor when a trivalent impurity is added into it: • A P-type semiconductor is formed. • A trivalent atom contains only three electrons and is one electron short which creates a hole in the semiconductor. • The hole in a P-type semiconductor is positively charged. • An electron from the neighboring atom can fill the hole as very low energy is required to break the covalent bond. • A new hole is created in the bonded region from where the electron is transferred to the hole. As a result, the movement of holes takes place within the semiconductor.

Fig. 1.2 Arrangement of atoms in P-type semiconductor

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1 Introduction to Solar Cells

• Typically, in a P-type silicon semiconductor, 1–106 of the trivalent impurity is doped into the material. As a result, the P-type silicon will have more number of holes as compared to electron–hole pairs in a silicon semiconductor. • At room temperature, the conductivity of a P-type semiconductor is almost similar to a pure semiconductor. • Electrically, a P-type semiconductor is neutral. • In a P-type semiconductor, holes are the majority carriers and electrons are the minority carriers. • A p-type semiconductor may conduct by only 0.05 eV of applied energy.

1.2 Solar Cell as p–n Junction Diode The function of a solar cell is basically similar to a p–n junction diode [5]. However, there is a big difference in their construction.

1.2.1 Construction The construction of a solar cell is very simple. A thin p-type semiconductor layer is deposited on top of a thick n-type layer. Electrodes from both the layers are developed for making contacts. A thin electrode on the top of the p-type semiconductor layer is formed. This electrode does not obstruct light to reach the thin p-type layer. Thus, a p–n junction is formed just below the p-type layer. Similarly, a current collecting electrode is formed at the bottom of the n-type layer. The whole assembly is then encapsulated inside a thin glass to protect the solar cell from any mechanical shock. Figure 1.3 shows the constructional details of basic p–n junction diode solar cell [6]. Electron flow Photon 0

V

Electron Photon Hole Front Electrical Contact

N-type P-type

Depletion layer Back Electrical Contact

electron-hole recombination Fig. 1.3 Solar cell as p–n junction diode

1.2 Solar Cell as p–n Junction Diode

5

1.2.2 Working Mechanism When light is incident on a solar cell, it can easily enter the p–n junction through the extremely thin N-type layer. The photons from the light contain sufficient energy to break the thermal equilibrium of the junction and thus create many electron– hole pairs in the depletion region. The electrons travel toward the n-type side of the junction, and holes travel toward the p-type side of the junction [7]. After crossing the junction, the electrons and holes cannot return to the depletion layer due to creation of a potential barrier at the junction. As the concentration of electrons and holes starts to increase on their respective sides, the p–n junction starts to behave as a battery cell. A small current flows through an external load connected across the junction. A normal solar cell produces 0.5 V voltage, has bluish black color, and is octagonal in shape. It is the building block of a solar panel and about 36–60 solar cells are arranged in 9–10 rows to form a single solar panel. A solar panel is 2.5–4 cm thick and by increasing the number of cells, the output wattage increases. For commercial purpose, about 72 solar cells are arranged in rows and columns.

1.2.3 I-V Characteristics of a Solar Cell Plotting current vs. voltage for a particular solar cell, array, or module is called its I-V characteristics. Using I-V characteristics, the efficiency and energy conversion ability of a solar cell is calculated. By knowing Pmax of a solar cell or panel, the performance and solar efficiency of the device can be determined [8]. The current produced in a solar cell is directly proportional to the intensity of radiation and is governed by the photoelectric effect, i.e., with an increase in the intensity, the current increases. However, an increase in the temperature of the solar cell reduces its voltage. The I-V characteristics of a solar cell are actually the graph plotted between the current and voltage of the solar cell at a particular temperature and intensity of radiation. I-V characteristic curves help in providing information regarding the operating conditions where a solar panel can perform to its optimum capacity known as maximum peak power point (MPP). Figure 1.4 shows the basic I-V characteristics of a solar cell. The I-V characteristics of silicon solar cell at room temperature are shown in above graph. Power delivered is equal to the product of current and voltage of the solar cell. For a specific intensity of radiation, the power curve as shown in Fig. 1.4 can be obtained by multiplying all voltages with corresponding currents from point to point, both for short-circuit and open-circuit condition. When the solar cell is in open-circuit condition (no load), the current will be minimum and the voltage will be maximum. This voltage is known as solar cell open-circuit voltage (V OC ). However, in short-circuit condition, the voltage will be minimum and the current will be maximum. This current is known as solar cell short-circuit current (I SC ). Thus, maximum voltage is available in a solar cell for open-circuit condition, and maximum current is available for short-circuit condition. However, it is important to note that

6

1 Introduction to Solar Cells

Fig. 1.4 I-V characteristics of a solar cell. Reproduced from [8] under common creative 3.0 License

no power generates in the cell under these two conditions. A solar cell generates maximum power at a point in between these two extremes known as maximum power point (MPP). At MPP, current (I MP ) and voltage (V MP ) are maximum in the solar cell. On an I-V curve, the MPP is located near the bend as shown in Fig. 1.4. Because the output voltage and current of a solar cell are both temperature dependent, the actual output power will vary with variations in ambient temperature.

1.2.4 I-V Characteristics of a Photovoltaic Array A photovoltaic (PV) array is built by interconnecting various solar cells together and I-V characteristics are then plotted to determine its efficiency and other parameters [9]. Figure 1.5 shows the I-V characteristics of a PV array. Now, there are two possible combinations in which the smaller panels can be interconnected, i.e., series and parallel. The voltage increases when the panels are connected in series and current increases when connected in parallel combination. However, the power generated in Watt (W) in both the combinations is still calculated using equation P = V × I. The position of MPP in series and parallel combinations is shown in Fig. 1.5.

1.2 Solar Cell as p–n Junction Diode

7

Fig. 1.5 I-V characteristics of a solar panel. Reproduced from [9] under common creative License

1.2.5 Equivalent Circuit and Analysis of a Solar Cell as a Diode The light shifts IV curve of a solar cell into 4th quadrant as shown in Fig. 1.6 [9]. Without illumination, the solar cell has the same characteristics as that of a normal p– n junction diode under forward bias condition. This current is known as dark current. However, when sunlight shines on the solar cell, the IV curve starts shifting to fourth quadrant thereby generating power and with increase in the intensity of sunlight, the shift toward fourth quadrant also increases. Illuminating a cell adds to the normal “dark” currents in the diode so that the diode law becomes: [ ( ) ] qV I = I0 e nkT − 1 − I L (1.1) where I L is the current generated due to sunlight. The above equation is only valid for the IV curve in the 4th quadrant. For the IV curve in the first quadrant, the equation becomes [ ( ) ] qV (1.2) I = I L − I0 e nkT − 1 For voltages < 100 mV, the value of exponential term is very large and at further lower voltages, I L term dominates. Hence, −1 term can be neglected in the equation, i.e.,

8

1 Introduction to Solar Cells

Fig. 1.6 I-V Curve of a solar cell: a without light; b when sunlight shines on the cell; c with greater light intensity; and d solar cell curve is flipped by convention. Reproduced from [9] under common creative 3.0 License

[ ( )] qV I = I L − I0 e nkT

(1.3)

Plotting the above equation gives the IV curve as shown in Fig. 1.4 with the relevant points on the curve.

1.3 Parameters of a Solar Cell Knowledge of following parameters is very important before understanding the I-V characteristics:

1.3.1 Short-Circuit Current (ISC ) The current that flows through a solar cell when there is no voltage across the cell is called short-circuit current [10, 11]. In other terms, when solar cell is in shortcircuit condition, the current that flows through the cell is called short-circuit current

1.3 Parameters of a Solar Cell

9

(I SC ). The creation and collection of light-generated carriers cause the flow of shortcircuit current in a solar cell. The light-generated current and short-circuit current for an ideal solar are identical. Therefore, the largest current that may be extracted from a solar cell is the short-circuit current. The short-circuit current depends on the following factors: • Solar cell area: The area of a solar cell strongly affects the short-circuit current. Hence, to remove this dependence, we mostly used short-circuit current density (J SC ) in place of short-circuit current. The unit of J SC is mA/cm2 . • Number of photons: I SC from a solar cell is directly proportional to the intensity of light. • Spectrum of the incident light: AM1.5 spectrum is considered as standard for solar cell measurements. While comparing solar cells made from same material, the most important parameters are surface passivation and diffusion length. For a solar cell with uniform generation and perfect passivation, J SC can be calculated as: ( ) JSC = qG L n + L p

(1.4)

where “G” is the rate of generation, and L n and L p are the diffusion length of electrons and holes, respectively. The relation between J SC and ISC is as follows: ISC = JSC × A

(1.5)

Thus, I SC is equal to J SC times the cell area.

1.3.2 Open-Circuit Voltage (VOC ) The maximum voltage available at zero current in a solar cell is called open-circuit voltage (V OC ). The V OC in a solar cell depends upon the amount of forward bias applied to the cell [7, 11–14]. Using current equal to zero in the solar cell equation, the equation for V OC is as follows: VOC =

( ) IL nkT ln +1 q I0

(1.6)

From the above equation, V OC depends directly on the temperature, i.e., with increase in temperature, V OC increases. However, in reality, the V OC decreases with an increase in temperature and this is due to the fact that increase in temperature also increases the value of I 0 . The increase in I 0 with temperature is more rapid and as a result, the V OC of a solar cell decreases with the temperature. There is another method of calculating V OC from the concentration of carriers, and the equation is given as follows:

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1 Introduction to Solar Cells

VOC =

) ( kT (N A + Δn)Δn ln q n i2

(1.7)

where “kT/q” is the thermal voltage, “N A ” is the doping concentration, “Δn” is the excess carrier concentration, and “ni ” is the intrinsic carrier concentration.

1.3.3 Fill Factor (FF) Fill factor (FF) determines the maximum power that can extracted from a solar cell. It depends upon V OC and I SC because at these two conditions, no power is generated from the solar cell [7, 11–14]. The fill factor (FF) is defined as the ratio of the maximum power from the solar cell to the product of V OC and I SC and is given as: FF =

PMP VOC × ISC

(1.8)

FF =

VMP × IMP VOC × ISC

(1.9)

Fill factor measures the “squareness” of solar cell and can also be calculated from the area of largest rectangle that can fit in the I-V curve of a solar cell. From Eq. 1.9, the cell with larger V MP will have higher value of FF.

1.3.4 Efficiency (H) Efficiency of a solar cell is the ratio of energy output provided by the solar cell to the energy input taken for that output. However, the efficiency does not depend only on the energy input and output. It depends upon the temperature of solar cell, spectrum of the light, and intensity of the radiation. Hence, before calculating the efficiency, these parameters must be controlled [7, 11–14]. For example, for terrestrial solar cells, the efficiency is calculated at 25 °C temperature under AM1.5 conditions. Mathematically, the efficiency (η) of a solar cell is the ratio of maximum power output to the power input: η=

Pmax Pin

(1.10)

where Pmax = VOC × ISC × FF. The input power for efficiency calculations is 1 kW/ m2 or 100 mW/cm2 . Thus, the input power for a 100 × 100 mm2 cell is 10 W and for a 156 × 156 mm2 cell is 24.3 W.

1.3 Parameters of a Solar Cell

11

Fig. 1.7 Characteristic resistance of a solar cell. Reproduced from [9] under common creative 3.0 License

1.3.5 Characteristic Resistance (RCH ) The output resistance of a solar cell at its MPP is called its characteristic resistance (RCH ). In other words, a solar cell operates at its MPP when its characteristic resistance (RCH ) is equal to the resistance of load (RL ) [7, 9–14]. In order to understand the mechanism of parasitic loss in a solar cell, RCH is an important parameter. The RCH is equal to the inverse of slope of an I-V curve of a solar cell and is shown in Fig. 1.7. For most cells, RCH is equal to: RCH =

VOC VMP ≈ IMP ISC

(1.11)

Here, RCH is expressed in Ω (ohms) when I MP or I SC is used, and it is expressed in Ωcm2 (ohm cm2 ) when J MP or J SC is used.

1.3.6 Series Resistance (RS ) One of the characteristics of a solar cell that can be reduced but not entirely removed is series resistance (RS ). It mostly reduces the FF of a solar cell [7, 13, 14]. However, the high value of series resistance can also decrease the value of I SC . The series resistance exists in a solar cell due to three main reasons: passage of current between base and emitter, resistance due to top and rear metal contacts, and resistance at

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1 Introduction to Solar Cells

Fig. 1.8 Solar cell with series resistance. Reproduced from [9] under common creative 3.0 License

contact between silicon and metal. Figure 1.8 shows the schematic of a solar cell with series resistance. The solar cell equation in presence of series resistance is: (

I = I L − I0 e

q(V +I R S ) nkT

)

(1.12)

where “I L ” is the light-generated current, “q” and “k” are constants, “V ” is the voltage across the cell, “RS ” is the series resistance of cell, “T ” is the temperature, and “η” is the ideality factor. The effect of RS on I-V characteristics of a solar cell at V OC is negligible, but it strongly affects the I-V curve at region near the V OC . Hence, calculating slope at V OC in the I-V curve is the simplest method.

1.3.7 Shunt Resistance (RSH ) Shunt resistance (RSH ) reduces the efficiency of a solar cell and causes significant power loss by providing an alternate path to the flow of current generated by light [7, 13, 14]. As a result, less current passes through the solar cell junction that reduces the output from the solar cell. Figure 1.9 represents the circuit diagram of a solar cell with shunt resistance. The solar cell equation with shunt resistance is given as: Fig. 1.9 Schematic of a solar cell with shunt resistance. Reproduced from [9] under common creative 3.0 License

1.3 Parameters of a Solar Cell

13 (

I = I L − I0 e

qV nkT

)



V RSH

(1.13)

where “I” represents the output current, “I L ” is the light-generated current, “q” and “k” are constants, “η” is the ideality factor, “V ” is the voltage across the cell, “T ” is the temperature, and “RSH ” is the shunt resistance.

1.3.8 Quantum Efficiency (QE) Quantum efficiency (QE) is an important parameter to investigate the performance of a solar cell. The quantum efficiency of a solar cell can be defined as “the ratio of number of charge carriers collected by a solar cell to the number of photons of particular energy incident on it.” QE of a solar cell can be unity or we can say that a solar cell behaves as an ideal one when all the charge carriers produced by all the photons (of particular energy or wavelength) are collected in a solar cell [9, 15]. It is important to note that if the energy of a photons is less than the bandgap of the material, the quantum efficiency will always be zero. For an ideal solar cell, the gold square line shown in Fig. 1.10 represents the quantum efficiency.

Fig. 1.10 Graphical representation of QE of a silicon solar cell. Reproduced from [9] under common creative 3.0 License

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1 Introduction to Solar Cells

The entire light incident on a solar cell is not used for generating charge carriers. Some of the light is transmitted through the cell, and some part is reflected. Considering such optical loss of the energy, the quantum efficiency is further classified as external and internal quantum efficiency. The efficiency calculated after considering such optical losses is called external quantum efficiency (EQE). The internal quantum efficiency (IQE) is the quantum efficiency calculated for the photons that are neither reflected nor transmitted by the solar cell. Mostly, IQE is calculated for the solar cell. However, the EQE can be obtained from the IQE curve after measuring the transmission and reflection of the cell.

1.3.9 Spectral Response (SR) Conceptually, the spectral response (SR) is very similar to the QE. However, the spectral response is the ratio of current produced by the solar cell to the power incident on the cell [16]. The choice of using SR or QE depends upon the application. The SR uses power of light at each wavelength, whereas QE uses number of photons incident on cell. The SR can be calculated from QE using equation: SR =

qλ QE hc

(1.14)

1.4 History of Solar Cells In 1839, Alexandre-Edmond Becquerel, a French physicist, was the first person who defined the photovoltaic (PV) effect. He found that certain materials when illuminated with light produce small electric currents. In 1870, Heinrich Hertz first studied this effect in case of selenium (Se), and thus, “Se” became the first material used in solar cell technology [14, 17–20]. However, the efficiency of the Se-solar cells was very low, i.e., 1–2%. In 1940s and 50s, a major boom was observed in commercializing the solar cells due to the production of pure silicon crystals via Czochralski (CZ) process. It was the Bell Laboratories in 1954, which developed the silicon-based solar cell with 4% efficiency. The silicon solar cells received their major application with the famous US Space program and were used to power radio in US Vanguard Satellite. Since then, solar cells are used as vital components of the various space programs. These are used in all kind of satellites, i.e., defense, communication, research, etc. The computer industry, particularly the semiconductor technology, has contributed greatly to the development of solar cell technology. Both the solar cells and transistors are made from the same material, and they also work on the same physical mechanism. As a result, the advances in one field have eventually provided new information for the benefit of other technology. It was quite unfortunate that

1.4 History of Solar Cells

15

despite all these developments and technology, the solar cells were still found to be very expensive. However, in 1970s, the world oil crisis struck the world with surprise and companies started to shift toward making solar cell technology more affordable. Since then, the solar cell technology has been on the path of continuous development. Millions of dollars have been put into this technology and various promising materials with better efficiency have been introduced [21–26]. For example, polycrystalline cadmium telluride (CdTe), non-crystalline (amorphous) silicon, gallium arsenide (GaAs), copper indium diselenide, etc. are some of the materials that have been the part of discussion in recent years. Currently, the solar cells have reached 15–22% efficiency. An overview of the key milestones in the history of solar cells is as follows: • Discovery of the photovoltaic effect (1839): French physicist Alexandre-Edmond Becquerel first observed the photovoltaic effect, the principle behind solar cells, in 1839. He discovered that certain materials produced small electric currents when exposed to light [27]. • Selenium photovoltaic effect (1876): In 1876, British electrical engineer William Grylls Adams and his student Richard Evans Day observed that selenium produced electricity when exposed to light. This marked the first practical application of the photovoltaic effect [28]. • The first solar cell (1883): Charles Fritts, an American inventor, is credited with building the first true solar cell in 1883. He coated a thin layer of selenium with an extremely thin layer of gold to form a crude photovoltaic device [22]. • Albert Einstein’s explanation of the photoelectric effect (1905): Albert Einstein’s work on the photoelectric effect in 1905 provided further insight into the nature of light and its interaction with materials. His research laid the foundation for a better understanding of the photovoltaic effect [29]. • The first silicon solar cell (1954): In 1954, Bell Laboratories researchers, led by Daryl Chapin, Calvin Fuller, and Gerald Pearson, developed the first practical silicon-based solar cell [30]. This cell achieved an efficiency of around 6%, a substantial improvement compared to earlier versions. • Space applications and growth (1958–1970s): Solar cells found early applications in space exploration. In 1958, the Vanguard 1 satellite launched with a small array of solar cells. Subsequently, solar cells became a crucial component in powering satellites and space missions [31]. • Increase in efficiency and widespread adoption (1980s-2000s): Over the decades, advancements in material science and manufacturing processes led to significant improvements in solar cell efficiency and a reduction in production costs. This fueled the adoption of solar cells for various terrestrial applications, including residential and commercial solar panels [32]. • Thin-film solar cells and concentrated photovoltaics (CPV): In the 1970s, researchers began developing thin-film solar cells, which required less material and were more flexible than traditional silicon cells. Additionally, concentrated photovoltaic systems emerged, which used lenses or mirrors to focus sunlight onto small, highly efficient solar cells [33].

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1 Introduction to Solar Cells

• Third-generation solar cells: These solar cells have been developed in twentieth century and include organic solar cells, DSSC, and QD solar cells. These cells aim to overcome the limitations of traditional silicon-based cells and explore new materials and technologies to enhance efficiency [34]. • Current advancements: As of my last update in September 2021, solar cell technology continued to advance, with research focusing on improving efficiency, exploring new materials, and incorporating solar cells into various innovative applications like building-integrated photovoltaics (BIPV) and solar-powered vehicles [18]. The history of solar cells is a testament to human ingenuity and determination to harness the power of the sun for renewable energy generation. As technology continues to progress, it is likely that solar cells will play an increasingly vital role in meeting the world’s energy needs sustainably.

1.5 How Do Solar Panels Work? The sunlight fall on a solar panel mounted on the roof of a house, top of a street light, top of a car, etc. The solar cells in the panel convert light into electricity, and this electricity is then use to run vehicle, light street lamps, run TV, and water geysers. [9]. A simple solar panel used in day-to-day life is shown in Fig. 1.11. Fig. 1.11 Schematic of solar panel to generate electricity from the sunlight. Reproduced from [9] under common creative 3.0 License

1.5 How Do Solar Panels Work?

17

1.5.1 Photovoltaic (PV) Cells In the starting period of their development, the solar cells were primarily used to power calculators and satellites. One of the key advantages of the solar cells is that they can work even in a cloudy atmosphere. Different types of materials used for fabricating solar cells are already discussed in this chapter. However, their efficiency depends upon the purity of the material and various other factors such as temperature, moisture, etc.

1.5.2 The Solar Panels When various solar cells are connected together as module or array, they are commonly known as solar panels. A typical solar panel consists of two silicon layers where the atoms in top layer are unstable and when the light hits the top layer, the valence electrons come out of the atoms. The electrons then move to the bottom layer causing an electric current to flow. This electric current can travel through a load by making external connection at the end of both layers.

1.5.3 Types of Solar Panel The latest developments and several decades of hard work have led to the fabrication of various solar panels [35]. Table 1.1 presents various solar cell panels available along with their efficiency, advantages, and disadvantages. Table 1.1 Types of solar panels and their efficiencies Type of solar panel

Efficiency (%)

Advantages

Disadvantages

Monocrystalline solar panels (Mono-SI)

~ 20

High lifetime and efficiency, used for commercial applications

High cost

Polycrystalline solar panels (p-Si)

~ 15

Cost-effective

Low life and efficiency, sensitive to variation in temperature

Thin-Film: amorphous silicon solar panels (A-SI)

~ 7–10

Cost-effective, flexible

Short life span

Concentrated PV cell (CVP)

~ 41

High efficiency and performance

Cooling system required

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1 Introduction to Solar Cells

1.6 The Solar Spectrum Sun is the ultimate source of energy on earth, and various radiations emitted by the sun are called solar radiations [9]. A solar spectrum as shown in Fig. 1.12 is obtained when solar radiations are plotted in terms of wavelength (nanometers) and irradiance (Js−1 m−20 or Wm−2 nm−1 ). Some of the important points concluded from the solar spectrum are: • Below 750 nm, majority of the solar energy falls in the visible region. • For wavelengths less than 300 nm, O3 and O2 gases absorb most of the UV radiations. The O3 gas also absorbs some visible radiations. • When visible radiations fall on earth, 70% of these radiations reach the sea level. • Large portion of the visible light falling on earth reflects back or gets scattered by the clouds and particles in atmosphere. Thus, a large share of visible energy never reaches the earth. • Water vapors, O3 , and CO2 absorb infrared radiations in the large wavelength regions. The major absorbers of the IR radiations are carbon dioxide, water vapors, and ozone. The energy in solar irradiation comes in the form of electromagnetic waves of a wide spectrum [36]. The energy and wavelengths of the radiations are inversely proportional, i.e., shorter wavelengths have high energy and longer wavelengths have less energy.

Fig. 1.12 Solar spectrum along with various atmospheric absorbing these radiations in range of 240 nm to 2.5 μm wavelengths. Credit Nick84 [CC BY-SA 3.0(link is external)], via Wikimedia Commons

1.7 Solar Cells Generations

19

1.7 Solar Cells Generations The advances in solar cells are reported from time to time which are further divided into different generations as shown in Fig. 1.13. Reference of this figure: Justyna Pastuszak and Pawel W˛egierek, “Photovoltaic Cell Generations and Current Research Directions for Their Development,” Materials (Basel). 2022 Aug; 15(16): 5542. https://doi.org/10.3390/ma15165542.

Fig. 1.13 Different generations of solar cells

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1 Introduction to Solar Cells

1.7.1 First-Generation Solar Cells As the name suggests, the first-generation solar cells are the first commercially available solar cells. The fabrication technology of these solar cells is very advanced, and hence, they are still applicable in the world [36]. They include crystalline silicon and gallium arsenide (GaAs) wafer-based solar cells. Silicon is an indirect bandgap semiconductor (1.12 eV at 300 K), whereas GaAs is a direct bandgap semiconductor (1.43 eV at 300 K) [37]. Thus, GaAs possesses better optical properties than silicon. However, the crystalline silicon-based solar cells dominate the commercial market. The silicon solar cells are mono or polycrystalline in structure. In polycrystalline silicon cells, various silicon crystals are grouped together during the fabrication process while making a single solar cell. These are more economical and popular. Advantages of GaAs over c–Si • GaAs has better optical properties than Si due to its larger bandgap. • The material used in GaAs solar cells is much less than the Si solar cells. • As we know, the efficiency of a solar cell decreases with an increasing temperature. However, GaAs is immune to such temperature variations which make it a preferred choice for areas with hot climate. • GaAs can withstand harsh conditions without compromising its efficiency. • The mobility of electrons via GaAs is much higher than the Si. • The energy conversion efficiency of GaAs solar cells is higher than the crystalline Si cells. It is hard to imagine that despite such advantages over Si solar cells, the GaAs solar cells are not widely used for commercial applications [38]. Some of the reasons which can explain such situation are: the fabrication process of GaAs solar cells is very costly, GaAs has both rare earth materials and hence maintaining the continuous supply in market is difficult and expensive [5], and arsenic (in GaAs) is found to be the reason of various health disorders. It is carcinogenic, damages kidneys and lungs, causes skin irritation and infections, neurological disorders, etc. [6, 7]. On the other hand, Si is one of the abundant elements and is not known for such health disorders. Therefore, on larger scale GaAs solar cells are not preferred.

1.7.2 Second-Generation Cells Second-generation solar cells are based on thin-film technology and are cheaper than the first-generation cells. The thickness of these cells (approx 1 μm) is much lower than the wafer solar cells. Three main materials used in second-generation cells are: (a) Amorphous silicon (a-Si) (b) Cadmium telluride (CdTe) (c) Copper indium gallium diselenide (CIGS).

1.7 Solar Cells Generations

21

The amorphous silicon (a-Si) thin-film solar cells are made by coating doped Si on a substrate, and these cells have captured the market in last 20 years. The a-Si is prepared via a low-temperature process, which allows the application of various polymer and flexible substrates during fabrication [39]. Among the secondgeneration solar cells, the a-Si solar cells are the most developed. Cadmium telluride (CdTe) is a direct bandgap material with bandgap of 1.5 eV. Most of the solar radiations are around 1.5 eV. Hence, CdTe has good light absorption capacity which helps in attaining high efficiency [40]. Moreover, the preparation of CdTe thin films is cheaper, takes short time for recovery, and produces least carbon footprint. However, the only problem is the toxic nature of cadmium, which can be countered by recycling process. Copper indium gallium diselenide (CIGS) thin-film solar cell is fabricated by depositing copper, indium, gallium, and selenide on a substrate. The glass or plastic are mostly used as substrates. The absorption capacity of CIGS is highest among the second generation solar cells. However, for better efficiency, the thickness of the thin film should be much lower than the other semiconductor materials. The choice of substrates makes these solar cells cost-effective [41].

1.7.3 Third-Generation Cells The latest solar technology that aims at passing the Shockley–Queisser (SQ) limit of solar cells comes under the category of Third-generation solar cells [42]. These solar cells can achieve the maximum theoretical efficiency, i.e., 31–41%. Third-generation solar cells include: (a) (b) (c) (d)

Quantum dot solar cells Dye-sensitized solar cells Polymer-based solar cells Perovskite solar cell. • In quantum dot (QD) solar cells, a zero-dimensional nanomaterial called quantum dot is used as light absorbing material. The quantum dots of transition metals combined with a solution are pasted on a silicon substrate. When a photon of light falls on the top layer (QD layer), it creates a single electron–hole pair which results in the flow of current. In certain transition metal quantum dots, a single photon can create multiple electron–hole pairs [43]. • Dye-sensitized solar cell (DSSC) functions on the principle of artificial photosynthesis and was first used in 1990s. In these solar cells, a dye molecule is placed between the transparent electrodes [44]. When light passes through one electrode, it excites the electrons in the dye molecule, which then travel to the other electrode where they are collected and transferred to the external load. These solar cells are cost-effective, can work in different temperatures, and have achieved an efficiency of 13%.

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1 Introduction to Solar Cells

• Polymer solar cells are designed on a polymer or plastic substrate. Hence, one of the key features of these solar cells is their remarkable flexibility. The solar cells work on a combination of donor and receiver. Mostly, the polymer acts as a donor, whereas fullerene is used a receiver. Since a larger number of optically active polymers are available, the choice of polymers is very vast. These solar cells can be incorporated into textiles which paves way to a new application of solar cell technology [45]. • A recent innovation in the solar cell technology is the introduction of perovskite materials. These solar cells have attained the maximum efficiency of 31%. They can revolutionize the solar energy technology. Currently, these solar cells are confined to the labs due to their low stability.

1.7.4 Fourth-Generation Cells The fourth-generation or 4G solar cell technology is the future of solar energy harvesting. This technology aims at combining organic and inorganic materials for fabricating solar cells. These solar cells will benefit from the stability of inorganic materials and flexibility of the organic material, which will help in attaining higher efficiency. The technology is still in development stage, and commercialization had not yet been fully realized. Fourth-generation solar cells represent the next frontier in solar cell technology, aiming to further improve efficiency, reduce manufacturing costs, and overcome some of the limitations of previous generations [46]. Some of the potential fourth-generation solar cell technologies include: • Tandem Solar Cells: Tandem solar cells combine multiple layers of different semiconductor materials, each absorbing different portions of the solar spectrum. By utilizing a wider range of wavelengths, tandem cells can potentially achieve higher conversion efficiencies compared to single-junction cells [47]. • Intermediate Band Solar Cells: Intermediate band solar cells are designed to create an “intermediate” energy level within the bandgap of the semiconductor, allowing for more efficient absorption of lower-energy photons that are typically wasted in conventional solar cells [48]. • Nanowire Solar Cells: Nanowire solar cells utilize arrays of tiny semiconductor wires to increase the surface area available for light absorption. This approach can enhance light trapping and absorption, leading to potential efficiency improvements [49]. • Multi-Exciton-Generation Solar Cells (MEG): MEG solar cells aim to enhance efficiency by generating multiple electron–hole pairs (excitons) from a single absorbed photon, a phenomenon that is typically limited in traditional solar cells [50]. 4G solar cells use tin-doped indium oxide as transparent substrate. However, recent developments have introduced metal grid structures, metal nanowires, and

1.8 Global Solar Power Market

23

graphene as alternative substrates. Due to their large surface–volume ratio, nanomaterials enable usage of large polymer material. While these fourth-generation solar cell technologies hold great promise, they also face various challenges in terms of stability, scalability, and cost-effectiveness. As research continues and advancements are made in materials science and manufacturing processes, fourth-generation solar cells may become more viable for widespread commercial applications in the future. Solar cell technology has advanced greatly from wafers to the perovskite base solar cells. These advancements will no doubt play an important role in reducing carbon footprint and finally achieving the dream of a sustainable energy resource. Nanocrystal QD-based technology has a theoretical potential of turning more than 60% of the total solar spectrum into electricity. Furthermore, polymer-based flexible solar cells have opened up a realm of possibilities. The main issues with emerging technologies are instability and degradation over time. However, the ongoing research shows promise, and widespread commercialization of these latest solar cell modules might not be far off.

1.8 Global Solar Power Market In 2020, the global market value of solar cell technology was USD 170.55 billion. Due to outbreak of the COVID-19 pandemic, there was a negative impact on the market value of solar power. However, the market is expected to grow, and the value may reach USD 293.18 billion in 2028 [51, 52]. Solar energy is the most clean and abundant energy resource available on earth. The most developed countries in the world such as USA, Spain, China, and Germany have the largest solar energy resources. The policies of governments regarding the usage of renewable resources have also helped in the installation of solar projects in different countries. For example, China has made it mandatory for large-scale projects to be on solar FIT payment. Other countries are also promoting the development of the solar technology to harness solar energy and meet the increasing energy demand. In future, there will be a huge boom in the development of solar technology around the globe. Moreover, to meet the goals of sustainable development and environmental protection, various industries and factories are also relying on solar energy harvesting [52]. Asia pacific installed 67 GW of solar energy projects in 2019 which saw a major increase in the installation of solar projects across the world. The global solar technology is classified into two categories: photovoltaics (PV) and concentrated solar power (CSP). PV systems include mono- and poly-Si, thinfilms, and others, whereas CSP is divided into the power tower, linear Fresnel, and parabolic trough. Currently, PV systems dominate the global market as 120 GW PV project was installed in 2020. Among PV systems, mono-Si panels are mostly preferred due to their low-cost, high-efficiency, non-toxic characteristics, and high energy yield. When it comes to the global application, the solar technology is divided on the basis of utility, residential, and non-residential [52]. Figure 1.14 represents the global application of solar technology in the form of a pie chart. The utility occupies

24

1 Introduction to Solar Cells

Fig. 1.14 Pie chart representing the global application of solar cell technology

the major share followed by residential and non-residential applications [53]. Major projects such as one 60 GW project installed in 2020 and a 500 MW project signed by Ethiopian government and Masdar are some utility applications of the technology. On the other hand, installation of solar panels on rooftops is expected to increase the residential applications by a great margin by 2028. On the global scale, China is the leading all nations in solar energy production. Only in 2020, solar projects of 48 GW capacity were installed in China. Followed by China is the European Union (EU), USA, African, and Middle Eastern countries. In 2020, EU installed 18.2 GW projects, USA installed 19 GW project in North America, and Middle East installed 1.5 GW projects. In 2021, solar energy witnessed a 22% jump in its generation which was only passed by the wind energy among all renewable energy resources. Gradually, solar energy is becoming the cheapest source of electricity all around the world, and more nations are investing in solar technology [53]. Here are some important aspects of the global solar market: • Solar Capacity Installation: The total installed solar photovoltaic (PV) capacity had been steadily increasing across the globe. Many countries and regions had set ambitious targets for renewable energy adoption, with solar playing a crucial role in achieving those goals. • China’s Dominance: China had been the world’s largest solar market for several years, in terms of both solar panel manufacturing and installations. The Chinese government’s support and incentives for solar energy, as well as the country’s large-scale manufacturing capabilities, had contributed to China’s significant presence in the solar industry. • Rapid Growth in Other Regions: Apart from China, other regions were also experiencing substantial growth in their solar markets. Countries in Europe, such as

1.9 Cost of Solar Energy







• •

25

Italy, Germany, and Spain had been early adopters of solar energy and continued to invest in solar installations. The USA, India, and several countries in Southeast Asia were also witnessing impressive growth rates in their solar markets. Utility-Scale and Distributed Solar: The solar market saw a mix of utility-scale solar projects, which involved large solar farms generating electricity for the grid, and distributed solar installations, such as rooftop solar panels on homes and businesses. Both sectors contributed to the overall growth of the solar market. Declining Costs: The cost of solar panels and associated technologies had been steadily declining over the years. This reduction in costs, coupled with technological advancements, had made solar energy more economically viable and competitive with conventional energy sources in many regions. Energy Storage Integration: With the growing adoption of solar energy, energy storage solutions, such as batteries, had also gained importance. These storage systems helped mitigate the intermittent nature of solar power and allowed for better grid integration and energy management. Corporate and Industrial Adoption: Many corporations and industries had been actively embracing solar energy to meet their sustainability goals, reduce carbon emissions, and take advantage of cost savings from renewable energy sources. Government Policies and Incentives: Government support in the form of subsidies, tax incentives, feed-in tariffs, and renewable energy targets had played a crucial role in driving the growth of the solar market in various countries.

It is important to note that the global solar market is constantly evolving, and developments beyond September 2021 may have further shaped its landscape. Factors such as changes in government policies, advancements in solar technologies, and shifts in global energy dynamics can influence the growth and dynamics of the solar industry in the future. Figure 1.15 shows the scenario of the share of power capacity from various sources. Out of all the sources, solar energy is the one whose utilization is growing exponentially. This is also reflected by the fact that there are over 10 million jobs associated globally with the renewable energy industries, with solar photovoltaics being the largest renewable employer [13].

1.9 Cost of Solar Energy The cost of solar energy has been steadily declining over the years, making it one of the most cost-effective and competitive renewable energy sources. In the early 1980s, the cost of solar panels was around $20 per watt. By 2021, the average cost had dropped to less than $1 per watt, representing a substantial decrease in the price of solar panels over time. Swanson’s Law, similar to Moore’s Law in the semiconductor industry, describes the empirical relationship between the price of solar photovoltaic (PV) modules and cumulative shipped volume [55]. It states that for every doubling of cumulative shipped volume, the cost of solar PV modules decreases by around 20%. This observation has held relatively true for the solar industry and has been a

26

1 Introduction to Solar Cells

Fig. 1.15 Share of power capacity from various sources. Reproduced from [54] under CC BY 4.0. (World Energy Outlook 2022)

driving force behind cost reductions. Government support and incentives have played a vital role in reducing the cost of solar energy. Many countries have implemented policies like tax credits, grants, feed-in tariffs, and renewable energy mandates to encourage the adoption of solar power. These incentives help reduce the upfront costs for consumers and businesses, making solar energy more financially attractive. Continuous research and development in solar cell technology have led to higher efficiency and lower manufacturing costs. Advancements in materials, cell structures, and manufacturing processes have improved the overall performance of solar panels while reducing production expenses [56]. Moreover, the growth of the solar market and the increasing demand for solar panels have led to economies of scale. Larger production volumes enable manufacturers to negotiate better deals on raw materials and streamline production processes, resulting in cost savings. Innovations in solar cell design and materials have contributed to higher conversion efficiencies and lower costs. Thin-film solar technologies, such as cadmium telluride (CdTe) and copper indium gallium selenide (CIGS) solar cells, have emerged as alternatives to traditional crystalline silicon solar cells, offering cost advantages for specific applications. Silicon is a key component in most solar panels. As the solar industry has grown, the production of silicon has increased, leading to a decrease in its price. This reduction in silicon prices has contributed to the overall cost reduction of solar panels. Figure 1.16 shows the National Renewable Energy Laboratory (NREL) data regarding history and future projection in the cost of crystalline silicon modules. It is expected that due to inclusion of new roadmaps with advanced innovations, cost of silicon solar cells will further reduce in coming years. The increasing adoption of solar energy has driven competition among solar manufacturers and installers. This

1.10 Factors Affecting the Cost of Solar Cell

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Fig. 1.16 Roadmap to decrease the cost of silicon solar cells. Reproduced from [57] under common creative License

competition has led to price reductions and improved services as companies strive to attract customers [57]. The increasing adoption of solar energy has driven competition among solar manufacturers and installers. This competition has led to price reductions and improved services as companies strive to attract customers.

1.10 Factors Affecting the Cost of Solar Cell National Renewable Energy Laboratory (NREL) is an organization which monitors the cost and issues of the solar technology, and Figure 1.17 shows different stages for manufacturing of silicon solar cells [57]. The analysis of NREL is mostly based on the bottom-up cost models where the cost of various components from the base are analyzed along with their supply chain. The NREL analysis is based on: • Minimum sustainable prices (MSPs)

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Fig. 1.17 NREL analysis of manufacturing costs for silicon solar cells. Reproduced from [57] under common creative License

• Step-by-step cost of manufacturing • Pathways to reduce the cost of manufacturing process. In the bottom-up model, cost of equipment, energy, materials, labor, and facility used at each step of manufacturing is calculated and modeled. This helps to precisely estimate the cost of each step and possible drivers that affect the price. Complete production process of solar cells using bottom-up model is shown in Fig. 1.18. In spite of all the above examples, other key factors affecting the cost of solar energy are as follows [58–60]: • Technological Advancements: Advances in solar cell technology and manufacturing processes can lead to more efficient solar panels, reducing the cost per watt of electricity generated. • Solar Panel Efficiency: Higher efficiency solar panels can produce more electricity from the same amount of sunlight, thus reducing the overall system size and associated costs. • Economies of Scale: As the solar industry grows and more solar panels are produced, economies of scale come into play, leading to lower production costs. • Raw Material Prices: The cost of materials used in solar panels, such as silicon, aluminum, and other semiconductor materials, can impact the overall cost of solar energy. • Installation Costs: The cost of installing solar systems can vary based on factors such as labor costs, permitting requirements, and site-specific considerations. • Financing and Incentives: Government incentives, tax credits, grants, and rebates can significantly reduce the upfront costs of installing solar systems. Additionally, innovative financing models like solar leasing and power purchase agreements (PPAs) can make solar energy more accessible and affordable. • Regional Solar Potential: The amount of sunlight a region receives (solar irradiance) directly affects the energy output of solar panels and, consequently, the return on investment for solar installations.

1.10 Factors Affecting the Cost of Solar Cell

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Fig. 1.18 Complete production model of solar cells via bottom-up used by the NREL. Reproduced from [57] under common creative License

• Energy Storage Integration: The inclusion of energy storage solutions, such as batteries, can increase the overall cost of a solar energy system but can also provide additional value by enabling energy storage and grid balancing. • Local Regulations and Permitting: Permitting processes and local regulations can impact the overall cost and timeline of installing solar systems. • Grid Connection and Infrastructure: The cost of connecting a solar system to the electrical grid may vary depending on the distance to the grid connection point and the necessary infrastructure upgrades. • Competition in the Solar Market: The level of competition among solar panel manufacturers and solar installation companies can influence pricing and affordability. • Currency Exchange Rates: For internationally traded solar components, fluctuations in currency exchange rates can affect the cost of solar energy in different countries.

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• Maintenance and Operational Costs: While solar panels have low operational costs, periodic maintenance is essential for optimal performance, and the costs associated with maintenance can impact the overall economics of solar energy. As solar technology continues to evolve and global demand for renewable energy increases, these factors will continue to interact and shape the cost of solar energy. Additionally, the ongoing development of energy storage solutions and integration with other renewable energy sources may further influence the overall cost and competitiveness of solar energy systems.

1.11 Applications Solar cells have a wide range of applications across various sectors due to their ability to convert sunlight directly into electricity. These applications leverage the clean, renewable, and sustainable nature of solar energy. Here are some notable applications of solar cells: • Residential Solar Power: Solar panels installed on rooftops of homes generate electricity for household consumption. Excess energy can be fed back into the grid or stored for later use, reducing electricity bills and reliance on non-renewable energy sources. • Commercial Solar Power: Businesses and industries install solar panels on their premises to offset energy costs and reduce their carbon footprint. Solar power can contribute significantly to a company’s sustainability goals. • Utility-Scale Solar Power Plants: Large solar farms with extensive arrays of solar panels generate substantial amounts of electricity that can be supplied to local communities or integrated into the grid. • Remote Power Generation: Solar cells provide power to remote and off-grid locations where conventional electricity infrastructure is unavailable or impractical. Applications include remote monitoring stations, communication towers, and research outposts. • Spacecraft and Satellites: Solar cells are a primary power source for spacecraft and satellites, operating reliably in the vacuum of space to support various functions and scientific missions. • Transportation: Solar cells can be integrated into electric vehicles (EVs) to help charge batteries and extend the vehicle’s range. Solar panels on EVs capture sunlight while parked or in motion, contributing to their energy efficiency. • Agriculture and Irrigation: Solar-powered water pumps and irrigation systems offer a sustainable solution for watering crops and providing agricultural communities with access to reliable water sources. • Water Desalination: Solar-powered desalination plants use solar energy to purify seawater, addressing water scarcity in coastal regions by producing fresh drinking water.

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• Emergency and Disaster Relief : Portable solar kits provide essential power during emergencies and natural disasters, supporting communication, medical equipment, and lighting in affected areas. • Outdoor Lighting: Solar-powered streetlights, garden lights, and pathway illumination enhance safety and visibility in public spaces without requiring external power sources. • Educational Tools: Solar cells are used in educational settings to teach students about renewable energy concepts and the principles of photovoltaic technology. • Hybrid Energy Systems: Solar cells can be combined with other renewable energy sources like wind or hydropower, along with energy storage solutions, to create hybrid systems that ensure continuous power supply. • Developing Nations: Solar cells play a crucial role in providing electricity to remote and underserved communities in developing countries, improving quality of life and supporting economic development. • Green Architecture: Integrated solar panels in building design contribute to sustainable architecture, reducing a building’s energy consumption and carbon footprint. • Research and Innovation: Solar cell technology continues to advance, with ongoing research into more efficient materials, manufacturing techniques, and applications, such as flexible and transparent solar panels. The diverse applications of solar cells underscore their potential to reshape energy systems, drive environmental sustainability, and enhance resilience in various sectors worldwide.

1.12 Summary Solar cell is a device which converts solar energy into electrical energy without using any chemicals or moving parts. When large number of solar cells are arranged in a particular order (rows and columns), it results into the formation of a solar module or array. Solar panels are used to power satellites, electronic equipment, vehicles, calculators, lights, etc. Photovoltaics is a fast-growing market: 30% Compound Annual Growth Rate (CAGR) is witnessed in the last ten years. China is the leading producer of solar energy holding 75% of world share followed by Europe and USA or Canada each with 1% share. Among various solar cells, silicon wafers cover 95% of the total production and mono-Si covers 84% of the total share of silicon wafers.

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1.13 Points to Remember • Alexandre-Edmond Becquerel (1820–1891): The French physicist who first discovered the photovoltaic effect in 1839, laying the foundation for solar cell technology. • Charles Fritts (1850–1903): American inventor credited with creating the first true solar cell in 1883, using a thin layer of selenium coated with gold. • Albert Einstein (1879–1955): His work on the photoelectric effect in 1905 helped explain the interaction of light with materials and laid the theoretical groundwork for solar energy. • William Grylls Adams (1836–1915) and Richard Evans Day (1854–1938): British researchers who observed the photovoltaic effect in selenium in 1876. • Russell Ohl (1898–1987): American engineer who invented the first silicon solar cell in 1941 while working at Bell Laboratories. • Daryl Chapin (1906–1995), Calvin Fuller (1902–1994), and Gerald Pearson (1905–1987): The team of researchers at Bell Laboratories developed first Si solar cell in 1954. • Martin Green (1942-present): An Australian researcher known as the “father of photovoltaics” is famous for increasing the efficiency of Si solar cells. • Michael Grätzel (1944–present): A Swiss chemist who invented the dye-sensitized solar cell (DSSC) or Grätzel cell in the early 1990s, which uses organic dyes to capture sunlight. • Zhores Alferov (1930–2019), Akasaki Isamu (1929-present), and Nakamura Shuji (1954-present): These three physicists were awarded the Nobel Prize in Physics in 2000 for their work in developing blue LED technology, which later contributed to high-efficiency LEDs used in solar cells. • Henry Snaith (1974-present): A British physicist known for his pioneering work in perovskite solar cells, leading to remarkable efficiency improvements and the potential for low-cost solar technology. • Miroslav Carr (1950–present): Czech scientist and entrepreneur known for his work in solar photovoltaics and contributions to the development of thirdgeneration solar cells. • Jack West (1933–2017): American engineer and entrepreneur who played a key role in developing the copper indium gallium selenide (CIGS) thin-film solar cell technology. • Roof angle: To get maximum energy from the sunlight, solar panels need to be mounted at 22° and 75° inclination. However, the angle may vary depending upon the location.

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31. Torchynska, T.V., and G. Polupan. 2004. High efficiency solar cells for space applications. Superficies y vacío 17 (3): 21–25. 32. Sampaio, P.G.V., and M.O.A. González. 2017. Photovoltaic solar energy: Conceptual framework. Renewable and Sustainable Energy Reviews 74: 590–601. 33. Chopra, K.L., P.D. Paulson, and V. Dutta. 2004. Thin-film solar cells: An overview. Progress in Photovoltaics: Research and applications 12 (2–3): 69–92. 34. Ananthakumar, S., J.R. Kumar, and S.M. Babu. 2019. Third-generation solar cells: Concept, materials and performance-an overview. In Emerging nanostructured materials for energy and environmental science, 305–339. 35. Rathore, Neelam, Panwar Narayan Lal, Yettou Fatiha, and Gama Amor. 2019. A comprehensive review of different types of solar photovoltaic cells and their applications. International Journal of Ambient Energy 42 (10); 1200–1217. 36. Hahn, Y.B., T. Mahmoudi, and Y. Wang. 2023. Next-generation solar cells: Principles and materials. CRC Press. 37. Sze, S.M., and J.C. Irvin. 1968. Resistivity, mobility and impurity levels in GaAs, Ge, and Si at 300 K. Solid-State Electronics 11 (6): 599–602. 38. Alami, A.H., S. Alasad, H. Aljaghoub, M. Ayoub, A. Alashkar, A. Mdallal, and R. Hasan. 2023. First-generation photovoltaics: history and conventional manufacturing. In PV Technology and manufacturing, 7–19. Cham: Springer International Publishing. 39. Böer, K.W., and U.W. Pohl. 2023. Semiconductor physics. Springer Nature. 40. Bosio, A. 2023. CdTe-based photodetectors and solar cells. In Handbook of II-VI Semiconductor-based sensors and radiation detectors: Volume 2, photodetectors, 205–230. Cham: Springer International Publishing. 41. Alami, A.H., S. Alasad, H. Aljaghoub, M. Ayoub, A. Alashkar, A. Mdallal, and R. Hasan. 2023. Second-generation photovoltaics: thin-film technologies. In PV technology and manufacturing, 65–75. Cham: Springer International Publishing. 42. Zdyb, A. 2023. Third generation solar cells. Taylor & Francis. 43. Prashant V. Kamat. 2013. Quantum dot solar cells. The next big thing in photovoltaics. The Journal of Physical Chemistry Letters 4 (6): 908–918. 44. Hagfeldt, Anders, Gerrit Boschloo, Licheng Sun, Lars Kloo, and Henrik Pettersson. 2010. Dye-sensitized solar cells. Chemical Reviews 110 (11): 6595–6663. 45. Li, Gang, Rui Zhu, and Yang Yang. 2012. Polymer solar cells. Nature Photonics 6: 153–161. 46. Rehman, Fatima, Iqrar Hussain Syed, Saira Khanam, Sumbel Ijaz, Haris Mehmood, Muhammad Zubair, Yehia Massoud, and Muhammad Qasim Mehmood. 2023. Fourth generation solar cells: A review. Energy Advances. https://doi.org/10.1039/D3YA00179B 47. Shrivastav, N., J. Madan, and R. Pandey. 2023. A short study on recently developed tandem solar cells. Materials Today: Proceedings. 48. Ramiro, Iñigo., and Antonio Martí. 2021. Intermediate band solar cells: Present and future. Progress in Photovoltaics 29 (7): 705–713. 49. Zhang, Yunyan, and Huiyun Liu. 2019. Nanowires for high-efficiency, low-cost solar photovoltaics. Crystals 9 (2): 87. https://doi.org/10.3390/cryst9020087. 50. Lee, Jongwon, and Chi-Hyung. Ahn. 2023. Multiple exciton generation solar cells: Numerical approaches of quantum yield extraction and its limiting efficiencies. Energies 16 (2): 993. https://doi.org/10.3390/en16020993. 51. Alami, A.H., A.G. Olabi, A. Mdallal, A. Rezk, A. Radwan, S.M.A. Rahman, S.K. Shah, and M.A. Abdelkareem. 2023. Concentrating solar power (CSP) technologies: Status and analysis. International Journal of Thermofluids 18: 100340. 52. https://www.precedenceresearch.com/solar-photovoltaic-glass-market 53. https://www.grandviewresearch.com/industry 54. “Share of cumulative power capacity by technology, 2010–2027”. IEA.org. International Energy Agency (IEA). 5 December 2022. Archived from the original on 4 February 2023. Source states “Fossil fuel capacity from IEA (2022), World Energy Outlook 2022. IEA. Licence: CC BY 4.0.” 55. https://www.forbes.com/home-improvement/solar/cost-of-solar-panels/

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56. https://www.solar.com/learn/solar-panel-cost/ 57. https://www.nrel.gov/solar/market-research-analysis/solar-manufacturing-cost.html 58. Siti, M.S.Z., et al. 2019. Solar energy: Trends, advancements, and challenges. Energies 12 (4): 649. https://doi.org/10.3390/en12040649. 59. Nemet, G.F. 2019. Solar photovoltaics: Markets, economics and policies. Energy Policy 135: 111034. https://doi.org/10.1016/j.enpol.2019.111034. 60. Teske, S. et al. 2016. Energy [R]evolution 2015: The global energy transition. Greenpeace International, European Renewable Energy Council (EREC). [Report] Retrieved from: https:// www.greenpeace.org/international/publication/10984/energy-revolution-2015/

Chapter 2

Silicon-Based Solar Cells

2.1 Introduction A solar cell or photovoltaic cell is built of semiconductor material where the lowest lying band in a semiconductor, which is unoccupied, is known as the conduction band (CB), while the band where all valence electrons are found is known as the valence band (VB). The bandgap is the name for the space between these two bands where there are no energy levels. Today’s solar cells are largely built using silicon crystals [1–3]. According to Fig. 2.1a, silicon contains 14 electrons with 4 of them in the outermost shell. Its electronic configuration is [Ne] 3s2 3p2. When group 13 trivalent boron atoms are used to dope silicon, each dopant atom only shares 3 electrons with its neighbor Si atom since they lack an outermost shell electron than silicon. As seen in Fig. 2.1b, this will leave a hole or vacancy in the VB. This kind of semiconductor is known as a p-type semiconductor. Similar to this, when pentavalent arsenic (As) atom, which has five electrons in the valence shell and belongs to group 15, is doped into silicon, the extra electron of each As atom becomes a free electron as it is not chemically bonded with the Si atom. Figure 2.1c illustrates how these free electrons, which are unable to chemically attach to the nearby Si atoms, are free to roam about in the CB. This kind of semiconductor is referred to as an n-type semiconductor. Therefore, from this figure, we can say that an n-type semiconductor is an electron-rich semiconductor, while a p-type semiconductor is a hole-rich semiconductor. The n-type and p-type semiconductors are brought together to construct a solar cell. The excess holes and additional electrons in the p-type and n-type semiconductors respectively combine at the contact region. When these electrons and holes combine, positive ions remain in the n-type material due to the loss of electrons, while negative ions remain inside the p-type material due to the depletion of holes. An electric field is created between p- and n-type materials as a result of the accumulation of charges in the depletion zone that prevents any further pairing of electrons and holes. The negative charges of the p-type material oppose any attempts of n-type

© The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 S. Arya and P. Mahajan, Solar Cells, https://doi.org/10.1007/978-981-99-7333-0_2

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Fig. 2.1 Illustration of electronic structures of undoped and doped silicon

material electron to cross the barrier, while the positive charges of the n-type material would repel any attempts by holes. The electrons are kept on the n-side by this barrier, and the holes are kept on the p-side. A p–n junction is the name of this obstruction. An electron–hole pair is produced with a distinct potential energy if light hits this p–n junction with higher energy in comparison with silicon’s energy bandgap. This occurs when electrons get propelled to the CB from the VB. Owing to the presence of electric field at the junction, holes will migrate to the p-side while electrons to the n-side. If a wired connection is made between an external circuit and the solar cell, the electrons will go toward the junction from the n-type substance, then travel via the external circuit, and finally arrive at the p-side. Eventually, there will be recombination of electrons with the holes. By attaching metal contact to the bottom as well as top of the solar cell, electricity would be extracted for use outside the solar cell, for instance, to power an electrical device. Only a few volts, or around 1.5 W, of

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electricity are produced by each comparable cell. The energy generated by several such cells is combined in a solar panel to provide a practical quantity of electrical current and voltage. For both commercial and residential applications, a solar array is made by combining many solar panels. Many solar arrays are then attached to the inverter for converting direct current (DC), which is what solar cells and solar panels produce, to alternating current (AC). The solar panels can power a broad range of technologies, including domestic appliances, parking meters, streetlights, space stations, and calculators. These can also be combined with energy sources including natural gas, wind energy, and nuclear energy. Solar cells made of silicon with a single junction may convert light between 300 and 1100 nm. By stacking many such cells with various operating spectra in a multijunction structure, a wider spectrum for light harvesting may be attained. Therefore, compared to conventional single-junction silicon solar cells, multi-junction solar cells are capable of greater conversion efficiencies. Latitude, time of day, weather, and other variables all affect how much solar radiation is incident at any particular site. The most popular solar spectrum for studying and contrasting the performance of various photovoltaic (PV) cells under controlled circumstances is air mass (AM) 1.5. One sun, or AM1.5, is equivalent to 1000 W per square meter. The “1.5” indicates that the length of time it takes for sunlight to travel from atmosphere to reach the surface of earth is 1.5 times that length when the sun is overhead, i.e., the shortest length. AM0, on the other hand, is utilized for planetary purposes. The earth’s atmosphere receives roughly 342 W/m2 of solar energy on average. Out of this, the whole earth system, including the atmosphere itself, seas, and the land surfaces, absorbs about 240 W/m2 , [4, 5]. Our demands for a whole year can be met by the solar energy that our planet gets in a single day. For instance, the UK receives 1095 kWh/m2 of solar energy year or 3 kWh/m2 every day on average. We receive 37 times additional solar energy per meter square than the required energy each year, assuming that an average UK family utilizes roughly 30 kWh/m2 energy yearly. In fact, if we could correctly harness it, adequate energy falls over a very small area to satisfy the needs for a whole year. The question is how to efficiently capture all of this energy and transform it into useable energy. This means that we have a conversion problem, not an energy shortage, on our planet. The development of systems that efficiently gather, absorb, and convert energy from the sun into electricity is the key to efficiently converting solar energy into power.

2.2 Silicon Substrates For the generation of energy from sunlight, silicon has developed into a dominating technology that now makes up and over 80% of the market [6]. Silicon is a silverygray, glossy solid when it is pure. It exists as the second most plentiful element (approximately 27.5% by mass) in the crust of the earth after oxygen, and there is enough of it to meet the world’s terawatt-scale energy needs. The elemental form

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of silicon is not found in nature; rather, it is found as silicon dioxide or silica [7]. The most prevalent type of silica is quartz, which is widely distributed in sand. In addition to its availability, silicon continues to be the preferred material for photovoltaic applications. All of silicon’s naturally occurring forms are non-toxic. Silicon creates a tightly adherent protective coating (at the interface between the environment and silicon) which is chemically stable as well as extremely thin, called silicon oxide. Silicon is also non-corrosive. By lowering the recombination centers at the surface and serving as a passivation layer, this naturally occurring electrically insulating oxide enhances the performance of Si-based solar cell devices [8]. In addition, the oxide layer reduces the possibility of deterioration of Si wafer within the solar module. As a result, the stability and durability of the technology helps it in achieving traction in the photovoltaic industry. In contrast to materials with greater bandgaps, which have lower short-circuit current (I SC ) and higher open-circuit voltage (V OC ), a semiconductor with smaller bandgap absorbs more number of photons, producing lower V OC and higher I SC . A balance between a low energy gap material and a large energy gap material is required for optimal output power and efficiency. In case of single-junction solar cell, the best possible value of bandgap is close to 1.1 eV and the SQ limit is estimated around 30% for such Si solar cells having 1.1 eV bandgap [9]. The record solar cell efficiency in the laboratory is up to 25% for monocrystalline Si solar cells and around 20% for multi-crystalline Si solar cells. At the cell level, the greatest efficiency of the commercial Si solar cell is around 23%, while at the module level, it is around 18– 24% [10, 11]. The present challenges for photovoltaics field include reducing the gap between the efficiencies recorded in research and those attained in commercial production.

2.3 Processing Steps to Obtain High-Quality Si Substrates The process of creating silicon substrates, which are needed for the fabrication of semiconductor devices, involves multiple steps. Silica is utilized to create metallurgical grade silicon (MG-Si), which is subsequently refined and purified through a number of phases to create high-purity silicon which can be utilized in the solar cells. The silicon is first extracted from beach sand. Sand mining is only carried out on a few numbers of beaches throughout the globe. After being taken from the sand, silicon must be purified before being used. It is initially heated until it gets melted into a highly pure liquid that is at least 99.9999999% pure and free of any flaws. With the aid of popular manufacturing techniques like the floating zone (FZ) or Czochralski (Cz) process, it is then allowed to harden into a silicon rod, or ingot. After that, the ingots are split into smaller ingot blocks, which are subsequently fragmented into wafers. Finally, these wafers are cleaned and polished in preparation for use. In this section, each of these processes has been thoroughly explained.

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2.3.1 Refining Silicon dioxide (SiO2 ) or silica is thermally reduced in the presence of carbon to produce silicon. The accessibility of silicon of the greatest purity possible is a crucial need for developing crystalline silicon (c-Si) starting from the raw (unprocessed) materials [12]. In the single crystals, the existing imperfections or flaws might reduce the solar cell efficiency due to charge carrier’s recombination. There are three categories of silicon, each with a different degree of impurity: (a) solar grade silicon, (b) semiconductor grade silicon, and (c) metallurgical grade silicon. Equation (2.1) describes how to recover MG-Si from silica in the presence of carbon. This method is a quartzite reduction process, and it takes place at roughly 1800 °C in an electrode arc furnace. Δ

SiO2 + C −→ Si + CO2

(2.1)

MG-Si is only 98% pure; impurities include transition metals, alkaline earth metals, carbon, as well as high levels of phosphorus (P) and boron (B). Impurities enhance the rate of charge carrier recombination by creating defect states inside the bandgap. MG-Si is no longer appropriate for use in electronic applications since this has an impact on its electronic characteristics. Annual production of MG-Si exceeds 2 million tons, with the majority of its utilization going to metallurgical industries. The demand for this silicon is least driven by the semiconductor and PV sectors. On a silicon-content basis, ferrosilicon makes up around 65% of the silicon production worldwide exclusive of the USA [13]. Additionally, for semiconductor and solar grade silicon, the metallurgical grade silicon is utilized as a raw material. As demonstrated in Eq. (2.2), pulverized MGSi reacts with anhydrous hydrochloric acid to create trichlorosilane. This reaction occurs in a fluidized bed reactor (FBR). Si + 3HCl → SiHCl3 + H2

(2.2)

Through a number of distillations, trichlorosilane is separated from impurity chlorides such BCl3 , AlCl3 , and FeCl3 . In order to get highly pure silicon, a reaction finally takes place between pure trichlorosilane and hydrogen, as stated in Eq. (2.3). SiHCl3 + H2 → Si + 3HCl

(2.3)

In the 1950s, Siemens developed this method known as Siemens deposition reactor (SDR) method which is today the norm for electronics grade silicon. Two thin polysilicon rods with diameter of about 5 mm are used in the process, which occurs within a gigantic SDR vacuum chamber. Si is then deposited over the rods that develop highly pure polysilicon rods with diameter of up to 300 mm of columnar silicon grains. When compared to alternative technologies, such as the upgraded metallurgical grade (UMG) and FBR, the SDR process is the most costly since it consumes a lot of

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energy. But of all methods, it yields the purest silicon [14]. Due to the lower manufacturing costs and turnaround time, the UMG approach is growing in popularity for PV applications, but it is not applicable to the semiconductor sector. However, large-grained and highly pure single-crystalline substrates (grain size: > 100 mm) or multi-crystalline substrates (grain size: 1–100 mm) are needed to produce silicon solar cells of satisfactory performance. Amorphous silicon (a-Si) substrates (grain size: E gf > E gm (where E gf : front surface bandgap, E gm : middle surface bandgap, and E gr : rear surface bandgap) because the bandgap is wider on each side (caused by the higher Ga at %) but narrower at the notch. In order to enhance electronic features like lower recombination losses, etc., a

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Fig. 3.3 Schematic diagram of graded bandgap CIGS absorber incorporating different profilings: a double, b normal, c no grading. Here, E gf , E gm , and E gr represent the bandgaps for front, middle, and rear surfaces, respectively

graded bandgap composition is applied. V OC is improved by front grading and J SC is increased by back grading, whereas reductions in surface/bulk recombination at the rear contact interface are because of BSF, which is formed by Ga grading. As a result of slow Ga atom diffusion over the expanding surface, the graded conduction band profile is obtained. Band bending at the CIGS/CdS interface is responsible for the conduction band grading offset seen on the front side. If there is a high concentration of Ga in the CIGS layer, the notch will become more pronounced, and there will be less collection of minority carriers from the absorber. To optimize the conduction band profile and bandgap for maximum V OC and efficiency, the concentration of Ga must be carefully tuned.

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3.4 Implications of Sodium Incorporation When the growth of the absorber over the SLG substrate takes place at ~ 550°C, the sodium from SLG substrate gets diffused toward the CIGS layer via Mo layer as a result of thermal activation process. Na in CIGS absorber helps in the passivation of grain boundaries and surfaces. Including a small amount of sodium (0.1%) to the CIGS layer boosts cell performance. To achieve low-temperature Na doping on flexible substrates, a 30 nm NaF layer is deposited before the CIGS layer is deposited. The maximum temperature that flexible (polyimide) substrates can sustain is 450°C. It is worth noting that when the temperature of the CIGS growth process is high, cell efficiency is also high. Since Na diffuses across the Mo layer, its characteristics have a major impact on the device efficiency. While Na diffusion does increase cell efficiency in the lab, it also causes a non-uniform distribution of Na over the device’s sides, which degrades cell performance. Precursors containing Na, such as NaF, are placed on Mo-coated SLG substrate for preventing inhomogeneous diffusion [15, 16]. A diffusion barrier layer often made of Cr, SiO2 , or SiN is placed between Mo and SLG to prevent Na from diffusing from SLG to the CIGS film. Following are some of the positive results seen by including Na at a lower concentration in CIGS solar cells [17–20]: (i) Na (about 0.1 at % concentration) is integrated into the CIGS film-forming NaSex bond with selenium. This incorporation of Se prevents V Se donor generation. (ii) Na inclusion enhances the morphology of the film, concentration, and conductivity of the holes through the formation of acceptor-type NaIn defect. It also aids in grain boundary passivation and boosts V OC , FF, and overall device efficiency. Furthermore, Na inclusion diminishes charge compensation and forms NaCu defects that passivate shallow donor-type InCu anti-sites. One reason why Na-containing CIGS films have better conductivity is because there are less V Se donors to compensate for any losses in conductivity. (iii) The non-radiative centers at the grain boundaries of CIGS crystals are passivated by Na diffusion within the CIGS crystal grains. (iv) There must be Cu-poor CIGS film for achieving minimal recombination at the interface. Copper gets hidden underneath the surface oxide layer because Na integration pulls oxygen toward the surface of the chalcogenides material, inducing the creation of oxides of gallium and indium. As a result of Na incorporation, the CIGS surface has less Cu, and there is less interfacial recombination. Lower consideration has been given to the potentially disastrous implications of greater Na concentrations. The inclusion of Na influences the interdiffusion kinetics of the existing elements and causes a different graded bandgap profile and defect development, both of which are undesirable effects. Na has been incorporated into the CIGS layer in a number of different ways to achieve desirable electronic properties. Na has been employed in its elemental

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form as well as in compounds such Na2 Se, Na2 S, and NaF layers [21–23]. However, SLG substrate is often incorporated as the Na source. Some other techniques are described as well such as evaporating a thin NaF precursor film (a) before, (b) after, or (c) during CIGS deposition [24–26]. The case (b) is followed by post-deposition vacuum annealing, and it stands out as the strategy that yields the best outcomes. The addition of Na during CIGS growth has been associated to a decrease in J SC [27, 28]. Polymer layers or metals may be used as a substrate because Na integration into the CIGS layer can be managed by the precursor film.

3.5 CIGS Film Deposition Approaches Good-quality CIGS absorber films are developed at temperatures in the range of 450–600°C. Though many different techniques for deposition exist, the methods that exhibit widespread popularity for both small-scale (laboratory) and large-scale (industrial) manufacturings are only few. Co-evaporation, successive selenization/ sulfurization, and non-vacuum approaches (primarily involving particle deposition through printing appropriate inks over substrate followed by subsequent annealing) are three primary categories into which these deposition processes fall. Several other methods besides those mentioned above have also been explored for depositing CIGS absorber layer. Pulsed laser deposition (PLD), electron beam deposition (EBD), metal organic chemical vapor deposition (MOCVD), and molecular beam epitaxy (MBE) are all examples of such processes. Table 3.1 lays forth the pros and cons of a variety of such methods. The decisive factor for choosing between various deposition processes might vary between laboratory scale production and industrial manufacturing. In a laboratory setting, optimizing CIGS film composition and cell performance is of utmost importance. However, in addition to efficiency, factors such as cost, process tolerance, repeatability, and throughput are crucial for industrial production.

3.5.1 Co-evaporation Approach The co-evaporation method has been utilized for the fabrication of various CIGS solar cells. The deposition of thin layers of elements takes place at varying temperatures and deposition rates in vacuum throughout the various development phases, which may be used to classify the various growth processes. The elemental flux is controlled by atomic absorption spectroscopy, and the inline detection of the composition of CIGS film is controlled by X-ray fluorescence. Figure 3.4 depicts many examples of co-evaporation processes. Growth processes are commonly referred by the following terms [29–31], depending on the total number of stages they undergo:

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Table 3.1 Advantages and drawbacks of the different growth processes utilized in the formation of CIGS films Technique

Advantages

Drawbacks

Co-evaporation

Highly explored method for small-scale fabrication

Keeping several sources under tight control at once is challenging, leading to inconsistencies in stoichiometry, poor repeatability, and uneven coverage

Selenization/ sulfurization

Large-area deposition process

Back contact adhesion issues, production complications, and the presence of toxic gases

Electrodeposition

Process is performed at ambient temperature and at a low cost

It is not easy to optimize a process

Screen printing

Low scrap rates, extensive packaging, and rapid output

Drying phase segregation and inhomogeneities arise, no ability to control the Ga profile

Spray coating

Inexpensive, scalable, and high throughput

Process involves wastage of precursors

Spin coating

Consistent films (small-scale), inexpensive equipment, and simple procedures

The lack of homogeneity across a large area, the waste of resources, and the incompatibility of roll stock

Doctor’s blade

Low scrap rates, roll-to-roll compatibility, and has improved stoichiometric control

The sluggish rate of solvent evaporation causes accumulation

MBE

This technique is ideal for fundamental High efficiency not found, research (like phase segregation, defect explored only in case of studies), and ultra-high vacuum small-area deposition deposition ensures negligible contamination

MOCVD

Growth rate is higher than MBE and it Industrial applications are not is practical for fundamental research possible, and large-area deposition is not documented. This method is not as rapid as MBE

EBD

High stoichiometry and purity of the film

No reports of large-area deposition; incompatible with commercial production

PLD

Films with the intended composition may successfully grow, good film stoichiometry allows for the avoidance of the CuSe binary phase

Because large-area stoichiometry has not been established, this method is unsuitable for use on a wide scale

Inkjet printing

Maskless patterning in inkjet printing reduces the number of steps required for processing, roll-to-roll compatibility

Low performance

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Fig. 3.4 Graphic representation of various co-evaporation approaches

(i) Single-stage process is Cu deficient (lack of Cu) and each element has just one evaporation rate throughout the process. (ii) The Boeing process is a two-step deposition method that uses Cu-rich and Cupoor growth (by lowering Cu flux) in the first and second stages, respectively. In this case, the substrate temperature is increased between the depositions of the first and second layers. (iii) In the three-stage procedure, firstly, In and Ga are deposited, then Cu (second stage), and finally again In and Ga (third stage). The three-step method also makes use of a graded Ga/In profile. It means that this approach is a mishmash of Cu-rich, Cu-poor as well as graded Ga/In profile. (iv) By adjusting the In/Ga/Cu fluxes, a graded absorber may be produced. In the co-evaporation approach, a steady Se flow is also delivered. The three-stage CIGS approach is attuned in order and has shown promise for the mass production of highly efficient CIGS thin-film modules, exhibiting a verified efficiency of 19.3%.

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CIGS solar cells are able to attain high efficiency when a Cu-poor absorber, where Cu/(In + Ga) < 1 is employed. However, when a Cu-rich absorber is employed, where Cu/(In + Ga) > 1, the CIGS material produced exhibits superior electrical characteristics, viz. transport properties, bulk recombination, and defect density. By using a Cu-poor absorber, recombination at the interface between the CIGS and CdS layers gets drastically minimized, leading to high conversion efficiency. Due of increased interface recombination, Cu-excess CIGS cells perform poorly. Cell performance is also diminished when Cu is present in excess because it is incorporated as highly conductive Cu2 Se at the surface. Low FF and cell efficiency are caused by reduced shunt resistance attributable to Cux Se segregation to the grain boundaries. The current is low in Cu-rich devices because of tunneling-enhanced recombination [32]. When CIGS solar cells are produced in vacuum, it presents a number of difficulties and drawbacks. The vacuum procedure is expensive and inefficient. The flux distribution of evaporation sources is a cosine. Since this was the case, it became impossible to keep large areas of the film consistent in compositional terms and to maintain abrupt compositional transitions. It is necessary for the source of evaporation to be positioned in a top-down arrangement for supporting large-area substrates and to uniformly heat them over 550°C [33]. Due to its low reactivity, there should be excess of selenium to prevent the loss of selenium from the CIGS layer and subsequent condensation of its vapors across the sidewalls. The condensation of Se vapor is a significant barrier to recycling.

3.5.2 Sequential Deposition Approach: Selenization/ Sulfurization Sequential deposition involves sputtering Cu, In, and Ga onto a substrate before annealing it in chalcogen environment, i.e., selenium or sulfur environment to create a layer of CIGSSe and/or CIGS, respectively. Sputtering is well-suited for both mass manufacturing and large-area deposition in the first phase. As a second step, a selenization/sulfurization process is applied to the stack of precursor metals (Cu/ In/Ga) during rapid thermal processing (RTP) or reactive annealing, creating chalcopyrite material (CIGS, CIGSSe). Hydrogen selenide (H2 Se) and hydrogen sulfide (H2 S), which are hydride gases and/or elemental (Se or S), are employed as sources of chalcogen. Elements or hydride gases are used to selenize or sulfurize the metal stack according to a set stoichiometry. The disadvantage of these hydride gases is that they are venomous. The benefits, on the other hand, are that these gases are simple to regulate and have highly reactive nature. By means of stacking Cu/In/Ga precursor films with a top evaporated Se film, harmful gases may be eliminated. Here, selenization of stacked layers is performed using RTP technique in a Se or inert environment.

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In the sulfurization and/or selenization procedures, through the incorporation of sulfur or selenium or both, at high temperatures (500°C), the reactions among metal precursors occur to generate compounds like Cu(In1−x Gax )(Se1−x Sx )2 , Cu(In1−x Gax )Se2 , or Cu(In1−x Gax )S2 . Since the buffer layer (CdS) includes sulfur, the latter enhances the contact with CdS. To complete this process, the reactions take place in a RTP (infrared lamp) furnace or resistance heated furnace. The secondstage process necessitates the utilization of a well-optimized time–temperature reactant profile. The best efficiency achieved using a two-stage selenization method was 19.2% [34].

3.5.3 Non-vacuum Deposition Approach Non-vacuum solution techniques have emerged as an easy, low-cost alternative because of the high production cost and complexity of vacuum procedures. The third method of CIGS production is the non-vacuum technique, which avoids the need of the vacuum evaporation step altogether. The non-vacuum method consists of two stages: low-temperature precursor deposition and high-temperature annealing in a chalcogen environment. When compared to vacuum-based methods, the main benefits of non-vacuum methods are as follows: (i) (ii) (iii) (iv) (v) (vi) (vii)

Roll-to-roll compatibility. Higher output. Lower energy contribution. Improved stoichiometric regulation. Lower capital outlay. Better material consumption. Lower material expenditure.

Non-vacuum methods include chemical bath deposition (CBD), electroless deposition, electrodeposition, inkjet printing, screen printing, spin coating, and spray coating. Non-vacuum methods make it more difficult to create films with a graded chemical composition and result in inhomogeneous, less dense films as well as contamination. Based on the deposition technique and the degree of mixing of the precursor materials, non-vacuum techniques for the deposition of CIGS films fall into the subsequent three broad categories: (a) electrochemical approach, (b) particulate approach, and (c) solution-based approach. A single-step or multiple steps can be used to perform electrodeposition. For the purpose of single-step electrodeposition, all the elements are deposited concurrently, and for decreasing the individual deposition potentials simultaneously or in close proximity, complexing agents are employed. After annealing, the formation of the final layer is the most important stage. One-step electrodeposition makes process optimization challenging. CIGS film deposition using electrodeposition method has resulted in efficiencies of 15.4% and 13.8% on stainless-steel and glass substrates, respectively [35, 36]. Most deposition processes rely on particulate methods like

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spin coating, spray coating, and screen printing. Material is milled into granular chunks (particles) followed by deposition over the substrate to get the final film. Laser ablation, laser pyrolysis, solution precipitation, etc., are few of the ways used to create particulate materials like metal selenides, metals, metal oxides, and CIGS. Doctor’s blade, screen-printing, or spin-coating processes are used to create the absorber film making use of particulate material. The obtained film is annealed at a high temperature in chalcogen environment for forming the final CIGS absorber film. Sintering of CIGS particles is done at very high temperatures. The usage of metal particles has been restricted because of oxidation and handling concerns [37]. Slurries of mixtures comprising selenide particles or metal oxides have been observed to provide the best outcomes. Compounds based on hydrazine, as well as organometallics and other materials, have been employed in solution-based processes, with the greatest efficiency recorded being 15.2% [38]. Due to its toxicity and reactivity, however, hydrazine has never been used outside of a laboratory. Nanosolar [39] claimed the greatest efficiency to be 17.1% on flexible foil substrate using non-vacuum approach. Solution processing enables low-cost, largescale manufacturing with little material waste. High minority carrier lifetime, long diffusion length, high collection efficiency, good absorber quality, and CIGS energy gap of 1.16 eV, all contributed to the excellent performance of solution processed CIGS devices. However, the cell’s limited efficiency is a result of its poor FF and high series resistance. The series resistance is approximately three times more in comparison to the one obtained in a vacuum deposition approach.

3.6 Buffer Layer and Transparent Conducting Oxide Deposition methods and buffer layers are explored extensively throughout the years, with varied results. Using CBD technique, the deposition of an improved CdS buffer layer is carried out above the absorber CIGS film. The buffer layer is typically prepared from alkaline solution (pH > 9) containing sulfur and cadmium sources along with a complexing agent. For cadmium source, cadmium salt like CdI2 , CdSO4 , etc., is employed. For sulfur source, thiourea is utilized and the hydrolysis of thiourea is regulated by the complexing agent ammonia (via pH) which provides the ligand for the cadmium ions. NaH3 is present in CBD solutions; hence, etching of the CIGS surface also removes Na-containing surface along with oxides prior to CdS nucleation [40]. With a decreased thickness of 40–80 nm, the CdS layer serves more as a buffer than an emitter. The best option is still CdS, although it has risks due to the toxicity of the Cd component. Zinc sulfide and indium sulfide are two possibilities for Cd-free buffer layers in CIGS solar cells, which have a greater bandgap than CdS and boost the solar cell’s blue response, and consequently, it is J SC . The CBD Zn(S,O,OH) buffer layer-based solar cell exhibited a high efficiency of 19.7%, according to Solar Frontier [41]. As per the reports by Hariskos et al. [42] and Kobayashi et al. [43], Zn(O,S) buffer layer-based solar cells have been shown to have a high efficiency (>18%).

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Different Cd-free buffers have been investigated as heterojunction partners including Zn1−x Mgx O, Znx Sn1−x Oy , etc. [44]. (Zn,Mg)O/Zn(O,S,OH) buffer layer has been claimed to have highest device efficiency of 22.8% [45]. Higher surface photovoltage is created when CdS is utilized; hence, in Cd-free CIGS solar cells, the V OC may be lower in comparison to CIGS/CdS solar cells [46]. If a buffer film made of Zn1−x Mgx O is used, the conduction band offset may be adjusted by altering the concentration of Mg. A deeper knowledge of reduced defect density at the interface of buffer layers and band alignment is necessary for Cd-free buffer layers. To allow light to reach the absorber, where the generation of charge carriers take place, a front contact layer composed of a TCO (such as ZnO) is placed atop the CdS film. The TCO should be n-type and should have an appropriate bandgap (>3.3 eV), and it acts like a window layer. To get the photogenerated current from the TCO layer to the contacts without substantial losses, we need very high sideways’ conduction alongside the TCO layer. In a ZnO bilayer, the layer of intrinsic ZnO is often very thin (100 nm), while the layer of n-type-doped ZnO is typically quite thick (300 nm). To prevent Al from diffusing inside the absorber layer, Al-doped ZnO (ZnO:Al) must be isolated from the CdS by a layer of intrinsic ZnO. The TCO layer is typically made from a highly conductive film of ZnO:Ga or ZnO:Al. It is interesting to note that the Zn2+ in ZnO:Al film is replaced by doped Al3+ , resulting in an increase in free charge carriers. As a result, the ZnO:Al layer exhibits lower resistivity in comparison to the intrinsic ZnO layer. TCOs are selected on the basis of desirable properties, including compatibility after further processing, moisture stability, transparency, and conductivity. Moisture absorption in a ZnO:Al film is improved by the presence of Al. Wet conditions are harmful for the ZnO:Al layer, and its resistivity dramatically degrades when exposed to moisture, increasing by an order of magnitude after exposure [47]. Therefore, the primary difficulty with using ZnO:Al in CIGS solar cells is its long-term stability.

3.7 Flexible CIGS Solar Cells Flexible photovoltaic devices enable new opportunities for integrated solar technology. In order to bring down production expenses, roll-to-roll manufacturing and flexible substrates are used. With the help of polyimide substrates, flexible CIGS solar cells have brought remarkable growth in recent years. The monolithic interconnection of polyimide-based cell structures has been achieved through a three-step laser scribing procedure. In contrast to conventional solar panels, flexible solar cells may be formed over a variety of thin and light weight surfaces, including curved and uneven surfaces which can be rolled up for minimizing storage space. There are three main categories for these substrates: (i) metallic foils like Cu, Al, Mo, Ti, stainless-steel, (ii) ceramic (Zirconia), and (iii) polymer (25 µm polyimide) sheets. It has been reported that solar cells on polyimide substrate attained 20.4% efficiency which suggests that the performance of solar cells fabricated on flexible substrates may one day be the same as that on rigid substrates [48].

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Though, it is the case of only small-area cells, the efficiency of processing on both flexible and rigid small-area substrates at low (375–450°C) and high (>550°C) temperatures, respectively, is quite similar. High-grade absorbers are produced by growing a layer of CIGS over polyimide or metal foils at 450 or 550°C. Furthermore, due to the electrical insulating properties of polymer substrates, monolithic integration of solar cells may be accomplished by direct contact. By placing a dielectric barrier layer on metal substrates, monolithic integration is feasible. The barrier layer acts as a diffusion barrier to prevent substrate contaminants from penetrating the Mo interfaces and as an electrical insulator between the Mo and substrate. Manufacturing on a big scale with a high throughput and yield along in addition to high efficiency is essential for low-cost production. For commercialization, one of the crucial requirements is long-term stability. Although flexible solar cells have shown promise, there are still numerous problems [49–51] to be resolved and are mentioned below: (i) There is a dearth of flexible substrates with desirable characteristics, such as those that are physically and chemically stable and have adequate mechanical traits. (ii) Specific polymer and metal films (like copper and aluminum) are employed as flexible substrates. Such metals can be heated and processed as they can tolerate very high temperatures. Despite this, they are rather dense, have a high thermal expansion coefficient, and are somewhat rough. As per different manufacturers, there are vastly different values for the coefficient of thermal expansion. (iii) Impurities may be found in almost all metals (including steel). Diffusion of impurities from metallic foils to the absorber is detrimental to the performance of device and should be avoided. The reactivity of Se with Al and Cu is a concern; nevertheless, CuInSe2 may be advantageously synthesized from a Cu substrate. (iv) Polymer substrates are not suitable for processing at higher temperatures (550–600°C) because they are smoother and less dense than metals. Highefficiency cells often need to be processed at higher temperatures. Substrates made of the polymer polyimide may tolerate brief exposure to temperatures as high as 450°C. Poor absorber quality and performance result from processing at low temperatures (400°C) when employing polymer substrates. (v) Na doping in a CIGS absorber requires an additional layer (such as NaF). (vi) The use of ceramic substrates as flexible substrates is another option. However, they break easily, making mass manufacture difficult. Compared to ordinary glass substrates, the flexible substrates of ceramic, polymer, and metal films have around 1–2 orders of magnitude lesser thicknesses, typically 25–400 µm. (vii) By carefully adjusting the (Ga)/(Ga + In) ratio, graded bandgap CIGS absorbers are obtained using a high-temperature technique. There is no confirmation of this attainment at a low-temperature process. (viii) Since a wet chemical approach is utilized to deposit a CdS layer, its development is incompatible with roll-to-roll manufacturing.

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(ix) When a polymer substrate undergoes a temperature fluctuation during rollto-roll production inside a multi-chambered vacuum system, the substrate’s volume increases.

3.8 Factors Affecting the Performance of CIGS Solar Cells The existence of defects in the CIGS absorber and their interactions with CdS and Mo, surface morphology, grain size, stoichiometry, etc., are some of the fundamentals that might influence the performance of CIGS solar cells. Smaller surface area at the CIGS/CdS contact means less interfacial recombination as well as lower saturation current when the CIGS film has a roughness of less than 50 nm. One drawback of a film that is smoother is that it reflects more light. Bandgap engineering may boost the performance of CIGS solar cells. Different solar cell performance parameters can also be improved due to the notch structure of CIGS layer. J SC is enhanced because free electrons are swept away from the neutral region of CIGS toward the p–n junction by the quasi-electric field generated as the energy of the conduction band progressively rises toward the back contact in the notch structure. Energy lost due to thermalization and recombination at the p–n junction may be curtailed with the aid of front grading. However, the defect density in CIGS rises with rising Ga content; hence, high-Ga CIGS is of lesser quality than low-Ga CIGS. The usual thickness of CIGS film is reported to be 1.6 µm [52], to operate at a high efficiency. Efficiency drops dramatically beyond 1 µm in thickness owing to decrease in FF, J SC , and V OC . Optical losses and shunting are all linked to deterioration. As discussed previously, a broad bandgap window layer is needed to increase J SC by increasing the transmission of light toward the absorber, allowing for additional photogeneration taking place. As a result of larger bandgap of Zn(O,S) in comparison to CdS, it is utilized as a window layer in place of CdS. V OC is also enhanced by increasing the amount of Ga present in the space charge area [53]. Ga segregation toward the back of CIGS film has been reported to cause a low V OC in the two-step selenization process [54]. By increasing the CIGS bandgap close to the surface through a sulfurization step, was able to boost V OC . A post-deposition under potassium fluoride atmosphere was also conducted to enhance V OC alongside the two-step selenization procedure. The window layer thickness and other TCO properties can be optimized to increase J SC . Enhancing the thickness of the CdS buffer layer has a negative impact on cell performance (such as a decrease in V OC ). Series resistance, contact resistance, and parasitic leakage current of the cell must be lowered to improve FF. The parasitic leakage current may be minimized by enhancing density and crystallinity of the CIGS absorber film [55]. Electronic inhomogeneity is a critical feature that limits the performance of CIGS solar cells.

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3.9 Applications Copper indium gallium selenide (CIGS)-based solar cells are a type of thinfilm photovoltaic technology that offers several advantages, including flexibility, lightweight design, and potential for low-cost manufacturing. Here are some applications of CIGS-based solar cells along with examples: • Building-Integrated Photovoltaics (BIPV): CIGS solar cells can be integrated into building materials such as windows, facades, and roofing materials, allowing structures to generate solar power while maintaining esthetic appeal. This integration helps to reduce energy consumption and reliance on traditional power sources. • Portable and Flexible Solar Chargers: CIGS solar cells can be incorporated into flexible and lightweight solar chargers for portable devices like smartphones, tablets, and laptops. These chargers are convenient for outdoor enthusiasts, travelers, and emergency situations. • Agricultural Applications: CIGS solar panels can be used to power irrigation systems, water pumps, and other agricultural equipment. These applications help farmers to achieve more efficient and sustainable water and energy management. • Off-Grid Power Solutions: CIGS-based solar cells are suitable for providing power in remote or off-grid areas where traditional energy infrastructure is lacking. They can be used to power remote telecommunications equipment, lighting, and monitoring systems. • Consumer Electronics: CIGS solar cells can be integrated into wearable devices, smartwatches, and other consumer electronics, providing a supplementary power source and extending battery life. • Transportation: CIGS solar panels can be integrated into the surfaces of vehicles such as electric cars, buses, and bicycles. They can help to charge auxiliary systems and contribute to overall energy efficiency. • Space Applications: CIGS-based solar cells have potential applications in space missions due to their lightweight and flexible nature. They could be used on satellites, space probes, and other spacecraft. • Military and Defense: CIGS solar cells can be integrated into military equipment, including tents and shelters, to provide power in remote or tactical situations where traditional power sources might be limited. • Emergency and Disaster Relief: CIGS solar panels can be quickly deployed in disaster-stricken areas to provide immediate power for lighting, communication, and medical equipment during relief efforts. • Educational and Research Projects: CIGS-based solar cells are used in educational settings and research projects to study thin-film photovoltaic technology, materials science, and energy conversion efficiency. • Hybrid Energy Systems: CIGS solar panels can be combined with other renewable energy sources like wind or energy storage systems to create hybrid energy solutions for more reliable and consistent power generation.

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95

• Developing Countries: CIGS-based solar cells could play a role in providing clean energy solutions in regions with limited access to traditional power sources, contributing to improved living conditions and economic development. CIGS-based solar cells have the potential to provide versatile and efficient solutions in various applications due to their unique characteristics and benefits. As the technology continues to advance, we may see even more diverse applications emerge.

3.10 Summary The characteristics of the materials and the numerous growth procedures utilized in the production of CIGS solar cells are covered in this chapter. Ga/(In + Ga) ratio of 0.26 and Cu/(In + Ga) ratio of 0.88–0.92 are optimal ratios to form highly efficient CIGS-based solar cells. The film defects become more prevalent when Ga/(In + Ga) ratio is greater than 0.3. With a low S/(S + Se) ratio (1 μm), high electron mobility (800 cm2 /Vs), and ambipolar charge transport behavior [6–11]. The exciton type is the primary distinction between organic and inorganic absorbers. While the inorganic layer exhibits Wannier-type exciton, the absorber layer involving organic materials fundamentally exhibits Frenkel-type exciton. Wannier-type exciton is exhibited by the perovskites employed in PSCs. As a result, the produced charge carriers in perovskites act similarly to how they do in inorganic material. By regulating the size and form of the component particles during production, the behavior of the fabricated materials is in fact customizable.

5.2 Historical Background The perovskite was initially a mineral, calcium titanium oxide (CaTiO3 ) that Gustav Rose identified in the Russian Ural Mountains during his visit in 1839. The substance was termed “Perovskite” in honor of a Russian mineralogist, Count Lev Alekseevich Perovski, who conducted more study on it [12]. The optoelectronic characteristics of the organic–inorganic perovskites were studied by Mitzi et al. in the 1990s [13].

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Fig. 5.1 Comparative performance of Si and perovskite-based solar cells in terms of PCE, cost, and lifetime

They said that the compound demonstrated high exciton characteristics and added that it may find use in transistors, solar cells, and LEDs. Kojima et al. [14] discovered the PV production phenomena in this material initially. Perovskite was initially employed in solar cells in 2009 when it was used as a liquid sensitizer in a dye-sensitized arrangement. The device achieved an efficiency of 3.2% and 3.81% using methylammonium (MA: CH3 NH3 )-based liquid sensitizers MAPbBr3 and MAPbI3 , respectively. Due to the liquid electrolyte, the device was very unstable and only functioned for a short period of time. Im et al. [15] used comparable dye-sensitized theories and nanocrystalline quantum dots. The device demonstrated increased PCE from 3.8 to 6.54%, nevertheless, crumpled after operating for few minutes on account of MAPbI3 quantum dots that get dissolved in the redox electrolyte solution. This configuration acknowledged little consideration because of the instability that was brought on by the liquid electrolytes. In 2012, Kim et al. [16] created an all-solid-state PSC to solve the issue brought on by the use of liquid electrolytes. The device achieved a 9.7% efficiency using the Spiro-OMeTAD as a hole transport layer (HTL). This led to a significant revolution in the history of PSCs since it augmented the efficiency while also demonstrated significantly extending lifespan for 500 h stability test without encapsulation. Burschka et al. [17] in 2013 constructed the cell in a planar layout and used a two-step sequential deposition procedure to achieve an efficiency of 15%. Instead of treating both of them simultaneously, they used the sequential deposition procedure, depositing the PbI2 layer first and then the MAI layer. This arrangement produced a

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thick and homogeneous perovskite layer, which resulted in a high J SC (20 mA/cm2 ). Im et al. [18] produced PSCs with MAPbI3 absorber in 2014 by employing two-step solution processing. Controlling the MAPbI3 cuboid’s size allowed them to achieve a high performance by enabling improved charge transfer and optimal light harvesting. It was claimed that the size of cuboids was influenced by MAI concentration as well as the time for which PbI2 was exposed to the solution ahead of being spin coated. This research team achieved an enhanced PSC with 17.01% efficiency. Li-doped mesoporous titania layer (m-TiO2 ) offered higher electronic characteristics as well as quicker transfer of electrons because of reduced trap states, according to Giordano et al. [19], which resulted in 19.3% efficiency. Less than 0.3% of this device’s hysteresis loss was noticeable. In 2015, Yang et al. [20] showed how to produce formamidinium (FA: CH(NH2 )2 )-based FAPbI3 films of higher quality with homogeneous, dense, and substantial microstructures having (1, 1, 1) crystallographic orientation. They achieved PCE of 20.2% by using FAPbI3 as opposed to MAPbI3 . Vacuum flash solution processing was used by Li et al. [21] in 2016 to create a perovskite layer with crystalline behavior and homogeneous morphology. They achieved PCE of 20.5% using FA0.81 MA0.15 PbI2.51 Br0.45 as the photoactive perovskite absorber. A novel method for creating perovskite films was described in 2016 by Bi et al. [22], where they also used a polymer to get enhanced electronic properties. They achieved PCE of 21.6% and used poly (methyl methacrylate) (PMMA) to enhance the growth and nucleation processes. Yang et al. [23] employed various cations, including FA and mixed-halide anion as the photoactive absorber. As organic cation solution was mixed with iodide solution, they were able to lower the concentration of deep-level defect states. The resulting efficiency was 22.1% for small scale and 19.7% in a 1 cm2 cell. In 2018, there was a breakthrough when the research group in Chinese Academy of Sciences achieved 23.3% record PCE. Additionally, perovskite-silicon tandem design surpassed the highest possible efficiency of single-junction silicon solar cells by achieving a PCE of 28% [24]. By customizing the quality of the perovskite layer delivering little recombination loss, a high V OC was achieved and the PCE of singlejunction PSC was increased to 23.7% [25]. Yoo et al. [26] in 2021 suggested the significance of charge carrier management in enhancing the photovoltaic performance and overcoming the theoretical limit for PSCs. A CBD-processed pinhole-free SnO2 film was developed as ETL with low defect density and beneficial band alignment demonstrating exceptional optoelectronic features. This minimized the recombination of charge carriers at ETL–perovskite interface leading to 25.2% PCE [26]. Currently, Min et al. [27] have achieved a single-junction PSC efficiency of 25.8% (certified 25.5%). They described the creation of a 2 nm thick FASnClx interlayer between perovskite layer and Cl-bonded SnO2 ETL coated with Cl-containing FAPbI3 solution. Though the precise crystal structure was not examined, it was believed to be functioning as a coherent interlayer between the SnO2 electrode and the perovskite layer, allowing for fast charge collection and low charge recombination. About 25.65% and 25.83% efficiencies were obtained in forward and reverse scan modes, respectively, thanks to the implementation of a coherent interlayer. As a consequence,

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this finding may help guide the creation of novel approaches to the production and design of PSCs that are both efficient and stable. It is significant to note that FA is included as a cation in most of the PSCs and is also thought to have an ideal red-shifted energy gap and has more stability than MA. On the other hand, it has a relatively larger size resulting in lattice distortion. Perovskite solar cells are also being stabilized and given a longer lifespan in addition to the substantial research being done to boost efficiency. A highly stable MAPbI3 perovskite with 3D/2D junction exhibiting 13% PCE and a year of stability was created by Gracini et al. [28]. Polytetrafluoroethylene, a hydrophobic polymer, was applied on top of the PSC by Hwang et al. [29]. They found low degradation for more than 30 days under ambient air conditions. From all of these studies, it can be inferred that the PSC technology is now undergoing one of the fastest growth rates.

5.3 Structure of Perovskites Perovskite materials have a crystal structure that is similar to that of calcium titanate (CaTiO3 ), and their chemical composition conforms to the standard formula ABX3 (where X is generally halogen, carbon, oxygen, or nitrogen) [30]. There are two different kinds of halide perovskites: (i) perovskites based on alkali halides and (ii) perovskites based on organic–inorganic halides. The perovskite that is formed from alkali halides is made up of monovalent alkali cations (A: Li+ , Na+ , K+ , Rb+ ,Cs+ , etc.); divalent cations (B: Sn2+ , Pb2+ , Ge2+ , etc.); and halogen anions (X: F− , Cl− , Br− , and I− ). Perovskite that is based on organic–inorganic halides contains an organic monovalent cation (A: CH3 NH3 + , CH3 CH2 NH3 + , or NH2 CHNH2 + ) [31]. Here, “A” ion in the crystal coordinate system has the coordinates (0, 0, 0) and “B” has the coordinates (0.5, 0.5, 0.5) and are both cations, where ionic radii of “A” being typically more in comparison with “B” and “X” stands for an anion with the coordinates (0.5, 0.5, 0). The cations A and B create the cuboctahedral and octahedral geometries, by coordinating with 12 and 6 “X” anions, respectively. Researchers are able to employ this halide-based perovskite in the area of solar cells because it has desirable electrical, optical, and magnetic characteristics. The ideal structure of a perovskite has the greatest symmetry and has a cubic structure, as illustrated in Fig. 5.2. A resides in the interstices of the structure, whereas the BX6 octahedral network, which fills the corner, is very important to the process of determining the transport characteristics, phase transition, and bandgap of the material [32]. The perovskite formation propensity may be shown by means of the Goldschmidt tolerance factor (t), which is evaluated from Eq. (5.1) RA + RX t=√ 2(R B + R X )

(5.1)

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Fig. 5.2 Ideal structure of perovskite: a octahedral and b cuboctahedral geometry

In this equation, RA , RB , and RX represent the ionic radii of cations A as well as B and anion X, respectively. For perovskite to maintain its stable crystalline structure, t must lie somewhere in the range 0.88–1.1. It is often assumed that perovskite will be stable if t is found to fall within the range that has been defined; nevertheless, it has been observed that even if t is found to fall within the range of 0.8–0.9, perovskite will be unstable [33]. In the process of perovskite production, an extra element called the octahedral factor (µ) is taken into consideration. This factor is represented by Eq. (5.2). µ=

RB RX

(5.2)

µ is used as a tool for analyzing the perovskite structure’s susceptibility to deformation as well as its stability. The perovskite may be stabilized with an octahedral factor that falls anywhere between 0.45 and 0.89 [34]. Methylammonium lead trihalide (MAPbX3 ) is by far the most prevalent kind of absorber material used for PSC. When the ionic radii of the halide anion grow from chlorine to bromine to iodine, the parameters of the unit cell increase as well, going from 5.68 to 5.92 to 6.27 angstroms (Å). More spherical form and bigger size of MA, on the other hand, cause a distortion in the network that, in turn, causes phase change if the temperature drops below a certain point. The orthorhombic structure is seen for temperatures below 160 K, the tetragonal structure is found for temperatures between 162.2 and 327.4 K, and the cubic structure is detected for temperatures over 327.4 K.

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MAPbI3 exhibits around 1.55 eV energy gap and is a direct bandgap material. MAPbBr3 , on the other hand, possesses comparably broader energy gap (2.3 eV) with absorption onset at 800 nm. The bandgap of MAPbX3 typically falls between the ranges of 1.5 and 2.3 eV. The material FAPbI3 has a bandgap that is considerably shorter (1.48 eV), which indicates extraction of more current on being used as an absorber; however, FAPbI3 is less stable. Variation may also be seen in the structural characteristics of MAPbX3 depending on the kind of halogen atom that was employed. By utilizing bromide ions of a smaller size instead of iodide ions, the lattice constant may be lowered, leading to a transition into a cubic phase that is more stable. This is also noticeable when the crystal structures of MAPbBr3 and MAPbI3 are examined, which crystallize in the cubic and tetragonal forms, respectively [35, 36].

5.4 Development of Different Device Configurations Since the inception of PSC with a dye-sensitized layout, ample of development and research has indeed been carried out in an effort to improve the device’s effectiveness and stability. The development of PSCs was mostly driven by improvements in device structure, which was one of the performance-determining aspects in the process. Initially, perovskite had been employed in the solar cell as a liquid electrolyte in a dyesensitized structure during 2009. Regrettably, this arrangement was not effectively utilized because of the unstable nature and low efficiency of the device (3.8%), which was triggered by the existence of the liquid electrolyte. Kim et al. eventually replaced the liquid electrolyte and incorporated a solid hole transport layer to create first solidstate PSC that boosted the PCE to ~10%. In addition to this accomplishment, the device was able to operate successfully for 500 h [16]. The PSCs essentially consist of the following five components: TCO, an electron transport layer (ETL), an absorber layer, a hole transport layer (HTL), and a cathode that is made of a metal. When it comes to determining the overall performance of a PSC, the transport layer is an extremely important factor. The HTL is responsible for blocking electrons in as well as collecting holes from the photoactive layer and transporting them to the cathode. The highest occupied molecular orbital (HOMO) of a material must be substantially higher compared to the perovskite layer for it to be able to act as a HTL. NiO, CuO, Cu2 O, CuI, PEDOT: PSS, PTAA, Spiro-OMeTAD, P3HT, PTB7, etc. are the commonly employed HTLs. The ETL is responsible for the blocking of holes and the collection of electrons from the photoactive absorber, as well as their transportation to the anode. It is required that the lowest unoccupied molecular orbital (LUMO) and HOMO energies of a material be greater than those of the photoactive layer for it to be appropriate for operation as an ETL [37, 38]. In order for the ETL to be able to let all the photons pass through it so that the absorber layer can get the most out of them, it should have high transmittance in the UV–Vis region. ZnO, SiO2 , SnO2 , TiO2 , etc. are the commonly employed ETLs. Transport layers are required to display features such as resistance to environmental deteriorating effects, non-toxicity, and strong thermal stability. As of present now, the efficiency of the

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PSC has been increased all the way up to 25.8% thanks to the massive amount of effort that has been done to enhance the functionality of the device. In this section, we will discuss the advancement of different configurations of the device as well as its performance specifications.

5.4.1 Liquid Electrolyte Dye-Sensitized Structure A liquid electrolyte, a perovskite sensitizer, a TCO substrate, nanoporous TiO2 , and a metal counter electrode are constituted in dye-sensitized solar cell (DSSC) structure as illustrated in Fig. 5.3a. The first DSSC that employed MAPbBr3 as a sensitizer for TiO2 produced PSCs with 2.2% PCE [39]. A PCE of 3.8% was attained when MAPbI3 was employed as the sensitizer [14]. The iodide absorber’s reduced bandgap and larger absorption spectrum resulted in an improved J SC . The prospect of hybrid organic–inorganic perovskites for the PV applications was clearly revealed in this study. Unfortunately, owing to low efficiency and poor device stability caused by the presence of liquid electrolyte, this device was never employed again. The optimal preparation of TiO2 nanoparticles and MAPbI3 absorber led to an efficiency of 6.5% [15]. This PSC showed a high absorption coefficient of the MAPbI3 perovskite, one of the most desired characteristics of materials to be employed for PV applications, and almost twice the PCE achieved before. Since liquid electrolyte DSSCs were very unstable (80% reduction in PCE in 10 min), research on them was discontinued because no appropriate liquid electrolyte was discovered in which the absorber remained stable. It was noted that the free energy gain from a chemical process may have contributed to a battery effect in these cells, which might enhance output power.

5.4.2 Solid State Mesoscopic Structure By employing a solid-state HTL known as Spiro-OMeTAD, Kim et al. [16] were able to effectively fabricate the first solid-state PSC and surmounted the instability concerns with liquid electrolyte. By virtue of its thin m-TiO2 layer, which is identical to the one utilized in liquid electrolyte DSSCs, however, the thickness has been decreased from 3 to 0.6 μm, and this device architecture has been given the moniker “mesoscopic structure.” Under this device setup, a portion of the HTL comes into direct contact with the perovskite sensitizer by penetrating through the m-TiO2 layer’s pores. The m-TiO2 is covered by a thick capping layer made of the residual HTL material, eliminating shunt between the ETL and the back contact. In mesoscopic structure, the working principle is different from that of traditional DSSCs. The solid-state Spiro-OMeTAD permits holes to travel more effectively than the electrolyte, which has poor charge carrier mobility. With a PCE of 9.7%, V OC of 0.89 V, J SC of 17.6 mA/cm2 , and FF of 62%, the champion device performed effectively. The HOMO level of the Spiro-OMeTAD (−5.22 eV) is better aligned with

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Fig. 5.3 Schematics of different configurations of perovskite solar cell: a liquid electrolyte DSSC structure, b solid-state mesoscopic structure, c meso-superstructured structure, d regular structure, e planar n-i-p heterojunction structure, and f planar p-i-n heterojunction structure

the valance band of MAPbI3 (−5.43 eV), facilitating more effective hole capture from the photoactive perovskite layer and broadening the hole Fermi energy splitting under illumination. As a result, when solid HTL was used in place of liquid electrolyte, it drastically enhanced the V OC from 0.71 to 0.89 V. Along with the PCE enhancement, the device stability also underwent a significant improvement. Some devices notably displayed improved PCEs after operating for 500 h. The TCO (FTO or ITO), hole blocking layer, mesoporous TiO2 , perovskite absorber, HTL, and metal electrode are the main components of cells with mesoscopic structures as demonstrated in Fig. 5.3b. The perovskite layer’s morphology (roughness, uniformity, grain size, surface coverage, etc.) has a substantial impact on device performance. A two-step deposition technique was used to produce MAPbI3 films with improved morphology. The in situ synthesis of MAPbI3 was triggered by dipping a TiO2 /PbI2 composite film into a 2-propanol solution of MAI. This method of fabrication of solar cell produced a PCE of 15.0% and increased repeatability [17]. By using solvent engineering, Jeon et al. [40] increased the density and homogeneity of the perovskite layers, resulting in a certified 16.2% PCE and better stability for MAPb(I1−x Brx )3 -based cells where x = 0.1–0.15. To create MAPbI3 cuboids with regulated size and to obtain 17% PCE, Im et al. [18] designed a two-step spin-coating technique. In a further advancement, Jeon et al. [41] used MAPbBr3 to stabilize the

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crystal structure of FAPbI3 having smaller bandgap than MAPbI3 and achieved a validated 17.9% PCE. Subsequently, Yang et al. [20] used FAPbI3 cells made using an intramolecular exchange technique to reach a verified 20.1% PCE. It is worth noting that these PCEs were attainable in small-area (1 cm2 ) cells, and that it is often unclear if hysteresis was taken into account while reporting the performance parameters of PV cells [42]. For perovskite solar cells to enter the market, it is important to address not only their high efficiency, but also their low price and reliable performance. To defray the high cost of Spiro-OMeTAD, many HTLs were created. Now the most effective organic HTL is PTAA, although it requires dopants such as Li-TFSI or TBP. Because of these dopants, the device’s stability suffers. Some organic HTLs that do not use dopants performed well. The efficiency of solar cells using the pure tetrathiafulvalene derivative TTF-1 as the HTL was 11.0% [43]. PCE for solar cells made with a P3HT/ graphdiyne composite HTL was 14.6% [44]. There are inorganic HTMs available now. With CuSCN and CuI HTLs in lieu of Spiro-OMeTAD, the devices produced PCEs of 12.4% and 6.0%, respectively [45].

5.4.3 Meso-Superstructured Structure By employing insulating Al2 O3 in place of m-TiO2 layer, Lee et al. [46] created the meso-superstructured structure, which is similar to the mesoscopic arrangement. The construction of this configuration is essentially identical to that of the mesoscopic structure; however, very thin continuous absorber layer made of perovskite materials was developed on top of the porous metal oxide as illustrated in Fig. 5.3c. PCE exceeding 10% was produced by utilizing MAPbI3–x Clx , a mixed-halide absorber. It is unclear whether or not the stoichiometric quantities of Cl exist in these films. Although the formula MAPbI3–x Clx will be used throughout this study, it is probable that only trace amounts of Cl are really present, as is indicated in the literature by the term MAPbI3 (Cl). The control device fabricated with m-TiO2 was only able to show a PCE of 7.6%, whereas the device with the porous Al2 O3 layer was able to attain a PCE of 10.9%. Intriguingly, certain devices with the Al2 O3 layer were able to attain a V OC value of 1.1 V, which is much higher. The longer diffusion length and low nonradiative recombination of the charge carriers in the mixed-halide perovskites are indicated by the low V OC deficit. Both optical and charge transport studies validated these remarkable optoelectronic features, which are very advantageous for the PV application. In a crucial aspect, meso-superstructured devices have different charge transport mechanism in comparison to mesoscopic cells. Results showed that both holes and electrons may be transported in the perovskite; therefore, a sensitized cell is unnecessary as induction of photoexcited electrons into Al2 O3 is also impossible. This is because the conduction band of Al2 O3 is higher as compared to MAPbI3-x Clx . Additionally, the DSSC employing mesoporous Al2 O3 and N719 dye had shown poor performance, which was an indication of the electrically insulating properties

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of Al2 O3 . As a result, the purpose of the porous Al2 O3 layer in this arrangement is limited to acting as a meso-scale scaffold for the perovskite layer. In perovskites, both holes and electrons have longer carrier diffusion length; as a result, electrons must be transported to the compact-TiO2 (c-TiO2 ) layer via the extremely thin perovskite absorber film [8]. These favorable electric characteristics showed that PSCs might be produced without a mesoporous layer. High PCE devices were made possible because of improvements in compositional control, interface passivation, deposition methods, and knowledge of the perovskite materials.

5.4.4 Regular Structure Researchers decided to dedicate more time to this field after discovering promising findings in meso-superstructured cells using a metal oxide scaffold fabricated on top of a perovskite layer. Thicker perovskite films might be employed in place of extremely thin perovskite absorber film in PSCs after the features of hybrid perovskites, such as longer diffusion lengths and low trap density, were disclosed. The perovskite film thickness was then raised so that longer-wavelength photons would be better absorbed, and shunting between the electrodes could be prevented. A device was developed that matched the meso-superstructure, however, had a thicker layer of perovskite as illustrated in Fig. 5.3d. Since this structure became so popular globally, it was given the name “Regular Structure” to reflect its widespread acceptance among academicians. This arrangement, with its pillared perovskite structure, was originally suggested by Heo et al. [47] in 2013. In this structure, MAPbI3 had filled the m-TiO2 pores completely, generating separate MAPbI3 pillars. Thin layers of PTAA were used as HTL to cover these pillars, with Au serving as the top electrode. The PCE of the device was 12%; however, the device’s functionality was constrained by the subpar quality of its surface morphology. The improvements in perovskite film deposition processes, which allowed thicker layer of perovskite placed over a thinner m-TiO2 film, drove the regular structure’s continued advancement. Additionally, Burschka et al. [17] modified this structure by deploying a thicker absorber layer over a thinner m-TiO2 film and by utilizing an additional 50 nm thin capping layer of perovskite atop 300 nm m-TiO2 film by means of two-step deposition technique. As a result, 15% PCE was achieved. Later, Im et al. [18] employed the same method while utilizing a 150 nm capping layer of perovskite over m-TiO2 layer of thickness 100 nm. The increased thickness of capping layer led to improved performance (PCE = 17%). Giordano et al. [19] proposed another modification wherein the TiO2 film was doped with lithium and a 300 nm capping layer was used resulting in 19.3% PCE. To improve the PSC’s functionality, Zhang et al. [48] improved the film production method by employing an external electric field-assisted annealing procedure instead of the standard annealing method. Optimized conditions under 2.5 v/μm electric field resulted in a rise in PCE from 15.33% to 17.26% for inverted PSC and from 16.77% to 19.18% for

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conventional PSC. This improved the carrier extraction and led to higher quality of the film.

5.4.5 Planar n-i-p Heterojunction Structure The construction of a cell is classified as either n-i-p or p-i-n, depending on the order in which different layers in the device are stacked, beginning with the layer that light first strikes. The planar n-i-p heterojunction structure evolved when the regular structure’s mesoporous oxide layer became thinner and was ultimately eliminated. The structure is similar to that of inorganic thin-film solar cells, with components metal anode, a p-type HTL, a perovskite layer, an n-type ETL, and TCO cathode, as illustrated in Fig. 5.3e. Photoexcited electrons and holes possess considerably longer lifetime to migrate toward the interfaces with the charge selective layers because of their low exciton binding energies of charge carriers and longer diffusion lengths. Lee et al. [46] developed the first planar n-i-p heterojunction PSCs utilizing MAPbI3–x Clx in 2012, however, only attained 1.8% PCE, most likely because of insufficient film covering, which caused shunting in the device. Efforts have been made to enhance the efficiency by modifying processing parameters such as thickness of different layers and the annealing temperatures yielding 4.5% and 11.4% efficiencies in 2013 and 2014, respectively [49, 50]. Despite the fact that the earliest n-i-p planar PSCs appeared to be made of smooth and homogeneous films, the absorber layer exhibited high surface defect density and porosity. Liu et al. [51] established a dual-source co-evaporation technique using MAI and PbI2 to form a consistent and smooth MAPbI3–x Clx perovskite film atop c-TiO2 film. The best device exhibited a PCE of 15%. Because of advancements in film deposition methods, today’s stateof-the-art planar perovskite solar cells made via solution-based deposition processes have more than 20% PCEs. Furthermore, these devices demonstrated consistent performance over hundreds of hours of operation. Kim et al. [52] achieved the highest efficiency in this design to date in 2022. They utilized a thin layer of polyacrylic acid-stabilized SnO2 quantum dots (paa-QD-SnO2 ) by substituting the frequently employed m-TiO2 ETL. This improved the light capture and significantly reduced non-radiative recombination at the perovskite–ETL interface. The introduction of paa-QD-SnO2 as an ETL resulted in 25.7% PCE (certified 25.4%) with active surface of 0.08 cm2 . About 20.6%, 21.7%, and 23.3% efficiencies were achieved with PSCs having areas 64 cm2 , 20 cm2 , and 1 cm2 , respectively. In addition, the devices displayed great operational stability, allowing the PSCs to be scaled up to larger areas. Because of these improvements in efficiency and stability, the planar n-i-p device configuration is a good contender for commercialized PSC production.

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5.4.6 Planar p-i-n Heterojunction Structure The relative positioning of the charge transport layers distinguishes the p-i-n configuration from the n-i-p configuration. The HTL is positioned on atop the TCO substrate in the p-i-n structure. The planar p-i-n heterojunction design, formerly employed in organic solar cells, was accepted as a substitute by the PSC industry as the planar ni-p and regular architectures advanced. The planar p-i-n heterojunction arrangement is frequently described as the “inverted” configuration because its HTL and ETL are arranged in the reverse manner from the standard arrangement. The TCO anode, the p-type HTL, the perovskite layer, the n-type ETL, and the metal cathode make up the p-i-n structure as illustrated in Fig. 5.3f. According to Jeng et al. [53], the first p-i-n perovskite solar cell with the structure ITO/ PEDOT: PSS/MAPbI3 /C60 /BCP/Al produced a PCE of 1.6% in 2013. They defined bathocuproine (BCP) as the ETL, PEDOT: PSS as the HTL, and MAPbI3 / C60 as a planar heterojunction. When PC61 BM was employed in place of C60 , the efficiency increased from 1.6% to 3.9%. The higher efficiency was due to both more effective electron insertion into the Al cathode and a higher-energy offset between the HOMO of donor MAPbI3 and the LUMO of acceptor PC61 BM. PSC efficiency in the p-i-n planar structure has advanced quickly. Although the earlier reports established the typical arrangement of the p-i-n planar structure, improvements in perovskite manufacturing processes were responsible for the continued development of the pi-n planar PSCs. Modern PSCs with the p-i-n structure showed both high PCEs as well as improved device stability after being exposed to the ambient environment for days. In 2022, Li et al. [54] accomplished the best efficiency to date in inverted planer configuration. Surface sulfidation of Pb-rich perovskite films allowed for the formation of a stable perovskite heterojunction. At the perovskite interface, the generated Pb-S bonds raised the Fermi level and caused an additional BSF for electron retrieval. The resultant p-i-n devices showed a high V OC (1.19 V) with PCE of 24.3%. Perovskite heterojunctions may be stabilized by the strong Pb-S bonds, which may also reinforce underneath perovskite structures with a similar crystal lattice. The devices maintained more than 90% of the original PCE after 2200 h of operation. Due to their low hysteresis effect, simplicity in construction, adaptability, and stability, inverted PSCs have received a lot of research attention.

5.5 Fabrication Methodologies of Perovskites for PV Application The viability of PSCs is shown by their quick ascent in performance from 3.8 PCE to 25.8% PCE in only a decade. Modifying device design, using innovative carrier transport materials, enhancing the absorber layer, and other measures were all taken to increase efficiency. Employing FA and MA together and using mixed halides are

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examples of how alterations to the anionic and cationic constituents may enhance the absorber layer. Additionally, performance booster elements like Cs and Rb have also been utilized to boost the quality of perovskite film. The structural as well as optoelectronic characteristics of the absorber layer rely not only on its organic and inorganic constituents, but also on the ambient variables like oxygen, moisture and temperature, film production process, annealing temperature, and additional solvent. Fabricating an effective PSC often requires precise regulation of the crystallographic phase, grain structure, and stoichiometry of the perovskite layer. Since film deposition methodology has a significant impact on these characteristics, researchers have performed numerous studies to set up a wide range of fabrication methods. The different film fabrication methods as well as PSC manufacturing procedures employed for mass production are discussed here.

5.5.1 Perovskite Layer Fabrication Approaches The quality of the perovskite layer is the most crucial component that decides the efficacy of the PSCs. Until now, a number of techniques have been developed to produce a film with enhanced perovskite quality. Film features such as phase purity, crystallinity, homogeneity, and morphology are directly impacted by processing methods, which in turn affect the performance of the devices. Perovskite activation energies (56.6–97.3 kJ/mol) are substantially lower than silicon (280–470 kJ/ mol). As a result, perovskites are anticipated to be easily synthesizable by many low-temperature methods which are mentioned below. The manufacturing process should give total film coverage over the substrate because partial film coverage might lead to the formation of shunt routes as ETL and HTL come in direct contacts. Additionally, the incident light may not be absorbed entirely if the photoactive absorber layer is not homogeneously covered. In this section, we will go through the different approaches for depositing perovskite films.

5.5.1.1

Single-Step Deposition

The ease of implementation and inexpensive financial investment necessary for the single-step deposition approach has contributed to its widespread adoption. Here, the spin coating of the precursor solution is typically done above the TiO2 scaffold. For the purpose of forming a precursor solution, inorganic and organic solutes are mixed together in a polar aprotic solvent like N-methyl-2-pyrrolidone (NMP), γbutyrolactone (GBL), dimethyl sulfoxide (DMSO), dimethylformamide (DMF), etc. Each of the above solvents has a high boiling point at normal temperature, although their vapor pressure is relatively low. Perovskite film is produced due to the strong ionic interactions between the solvents. During the spinning process, the formation of a homogeneous film occurs as a consequence of convective self-assembly as well as the evaporation of the solvent. Annealing is carried out at different temperatures

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Fig. 5.4 Schematics of single-step deposition approach

(80–150 °C) subsequent to the spin coating of precursor solution [40, 41]. A basic spin coating, on the other hand, can never generate a layer that is homogeneous and spread consistently across a large region. The homogeneity of the perovskite layer is impacted by the thickness of the TiO2 layer [50]. Despite the fact that it requires minimal processing stages and makes the operation simpler to carry out, however, single-step deposition leads to inhomogeneities and pinholes in the film caused by slow crystallization. Figure 5.4 illustrates a diagrammatic depiction of the single-step deposition technique.

5.5.1.2

Two-Step Sequential Deposition

As an outcome of the single-step deposition procedure, the final perovskite layer has uneven thickness and low surface coverage. A two-stage solution deposition approach was established to circumvent this problem. Firstly, PbI2 layer is deposited via spin coating onto a layer of nanoporous TiO2 kept at 70 °C. Subsequently, the reaction with the MAI solution transformed the layer into a perovskite. In this method, MAI is incorporated by either of the two ways: (i) the solution of MAI is spin coated over PbI2 film, and (ii) PbI2 layer is dip coated over MAI solution. In case of spin coating, both the spinning speed and the spinning duration matter greatly for the quality of the generated perovskite layer. However, the concentration as well as duration of the dipping process is a significant factor. The concentration of MAI affects the surface morphology, and it was found that smaller grains were produced as concentration increased [18]. There are limitations to this process, despite the fact that the film deposited is superior to that of the single-step deposition. The major issue is caused by the correlation between the roughness of the surface and the size of the grains. The roughness of a surface is directly proportional to the grain size. The diffusion length and carrier lifetime decrease with decreasing grain size in a smooth film, whereas

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Fig. 5.5 Schematics of two-step deposition approach

the surface recombination loss and leakage current increase with increasing grain size and roughness of a perovskite film. Thus, it is necessary to establish a suitable combination. Furthermore, in case of just partial coverage of PbI2 , predominantly in a planar configuration, the leftover PbI2 decreases light absorptivity as well as impedes carrier transportation, thus diminishing the performance of the PSCs. Such issues are addressed by solvent engineering. Figure 5.5 illustrates a diagrammatic depiction of the two-step deposition method.

5.5.1.3

Vapor-Assisted Solution Processing

Perovskite film is deposited using an innovative low-temperature vapor-assisted technique, and this approach was essentially a slight modification of the sequential two-step deposition method. In order to include MAI, a vapor deposition approach was used on top of spincoated PbI2 seed layer. MAI vapor was produced in an inert atmosphere at a temperature of 150 °C. The manufactured film was homogenous throughout and had complete coverage with grain sizes in the microrange. The most important part of this process is the growth of the film, which is accomplished by the as-deposited film of PbI2 in situ interacting with MAI vapor. This technique is conceptually distinct from either the existing solution procedure or the vacuum deposition approach, since it does not involve the simultaneous deposition of organic and inorganic species. The

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Fig. 5.6 Schematics of vapor-assisted solution approach

kinetic reactivity of MAI and the thermodynamic stability of perovskite are used in this process. The film had a very smooth appearance while undergoing complete precursor change. The problem of delamination, which may occur in two-step processes when washing from isopropyl alcohol, was circumvented by this procedure [55]. Figure 5.6 illustrates a diagrammatic depiction of the vapor-assisted solution approach.

5.5.1.4

Thermal Evaporation

In the thermal evaporation method, the organic and inorganic components are covaporized as illustrated in Fig. 5.7. Via concurrent evaporation of precursor salts of PbI2 and MAI from different sources at a pressure of 10−5 bar and their deposition in a 1:4 ratio, Liu et al. [51] created MAPbI3–x Clx films using this approach. The sensors were placed above ceramic crucible, and the process takes place inside a glove box containing nitrogen atmosphere. The vapor deposition process produces extraordinarily uniform films with crystalline characteristics in contrast to spin coating, which produces irregular film coverage with μm-sized crystalline platelets. This process makes it difficult to manage MAI deposition rate; however, this may be evaded via sequential deposition approach, where PbI2 layer is firstly coated followed by the deposition of MAI vapor. Two PSCs were fabricated via each of these techniques—solution processing and vapor evaporation, the one fabricated via latter method demonstrated improved efficiency.

5.5.1.5

Lewis Base Adduct Method of Lead (II) Iodide

The Lewis base adduct approach was used for the production of MAPbI3 film where PbI2 was dissolved in DMF with equimolar MAI and DMSO. Figure 5.8 illustrates a diagrammatic depiction of the Lewis base adduct method of lead (II) iodide. The interaction between the Lewis acid PbI2 and the Lewis base DMSO and/or iodide

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Fig. 5.7 Schematics of thermal evaporation method

results in the formation of adduct of PbI2 .DMSO and MAI.PbI2 .DMSO. The formation of a transparent adduct layer was achieved when the DMF solution was spin coated that contains PbI2 , DMSO, and MAI in an equal proportion. Diethyl ether is injected during the spinning process in order to ensure that all traces of DMF are eliminated, and the 1:1:1 adduct film is successfully produced. In addition, heating the film for one minute at 65 °C results in the removal of volatile DMSO from the adduct, which causes the film to change color to a dark brown. Using this adduct method, MAPbI3 film was produced that had strong charge carrier extraction, a slow recombination rate. This technology yields higher carrier mobility than the solution processing technique. The PCE of the device that was manufactured using this procedure was 19.7% [56].

Fig. 5.8 Schematics of Lewis base adduct method

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Fig. 5.9 Schematics of rapid deposition crystallization method

5.5.1.6

Rapid Deposition Crystallization

Subsequent to the deposition of perovskite atop the substrate, an anti-solvent is dropped within a very short amount of time in this method. The perovskite layer then has anti-solvent added to it, which speeds up the growth rate of the film and increases the number of nucleation sites. This results in a consistent film that has bigger-sized grain crystals. Common anti-solvents include acetonitrile, toluene, ethylene glycol, methanol, xylene, benzene, ethanol, benzonitrile, and chlorobenzene. Figure 5.9 illustrates a diagrammatic depiction of the rapid deposition crystallization approach.

5.5.1.7

Pulsed Laser Deposition

Inverted PSC was created by Liang et al. [57] in 2015 utilizing a pulsed laser deposition method demonstrated in Fig. 5.10. Firstly, PLD was used to deposit the PbI2 layer, then a straightforward spin-coating technique to produce the MAI layer. The method is simple to use, accurate, and it is feasible to perfectly control the thickness of the deposited layer. Since it is asymmetrical, material may be mass transferred stoichiometrically from target to substrate. The pressure, beam energy density, substrate temperature, and pulse repetition rate are the variables that need to be managed in PLD process. It was powdered in isopropanol, baked at 100 °C, compressed on a pallet, and then sintered at 120 °C in nitrogen atmosphere for eight hours. The spincoating method was used to deposit the MAI layer. The manufactured film had higher grain size and was compact, crystalline, homogeneous, and continuous.

5.5.1.8

Electrospray-Assisted Deposition

Using this method, Kavadiya et al. [58] created a PSC that was stable and resistant to moisture in a humid environment. This method uses a straightforward spin-coating

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Fig. 5.10 Schematics of pulsed laser deposition method

process to deposit a PbI2 film atop TiO2 -coated FTO substrate. Additionally, the MAI electrosprays by pumping at a higher voltage via a capillary needle and a monodispersed charge droplet is produced in Taylor cone-jet mode as illustrated in Fig. 5.11. This electrospraying deposition approach allows one to manage the film formation response, resulting in a stable layer with a moisture-resistant nature under ambient conditions and a consistent, smooth surface morphology. It is a flexible method that provides a precise control of the material to be deposited. Additionally, unlike solution-processed methods, material loss is prevented in this method. While the film created using this approach has an exceedingly smooth surface that is insoluble in water, the film created using the solution-processed approach has a hydrophilic surface that has a low contact angle with the droplet of solution. The stability of the electrosprayed film was tested by spraying a water droplet on it and then heating it to 100 °C. As a result, PbI2 degrades, but eventually acquires the crystalline property by changing color from yellow to black and finally to dark brown. Before and after spraying, the practically similar XRD results in the films demonstrated the presence of a self-healing process.

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Fig. 5.11 Schematics of electrospray-assisted deposition method

5.5.2 Large-Scale Manufacturing Techniques The notion of the PSCs being successfully commercialized was sparked by the remarkable electrical characteristics and ease of production of the perovskite materials. Spin coating has been extensively employed to this day in laboratories for research objectives. However, the spin-coating method cannot be incorporated to create large-area PSCs. In order to produce PSCs on a wide scale, many approaches are being investigated; some of these are listed below.

5.5.2.1

Inkjet Printing

In this method, the technology used is derived from that used in graphics and newspaper printing. The traditional printing process has been transformed into a digital one in which the intended print patterns are managed by computers. Due to the noncontact nature of the technique, a specific substrate material or form is not necessary. This technology has many advantages over conventional deposition methods,

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including cheap outlay, resourceful material consumption, mask-free on-demand patterning, scalability, design flexibility, and high-resolution printing. Wei et al. [59] employed inkjet printing technology for the first time in 2014 to fabricate a planar PSC. This approach allowed for precise as well as controllable patterning of the carbon electrode and moreover improved the interface between MAPbI3 and carbon electrode through immediate chemical transformation. To reduce interfacial charge recombination, a mixed ink was formulated using carbon and MAI to convert PbI2 to MAPbI3 . This resulted in a more robust interpenetrating contact between carbon electrodes and MAPbI3 as compared to the one generated using bare carbon ink. Consequently, a very impressive 11.60% PCE was attained. Hashmi et al. [60] explored the stability of PSC that had been inkjet printed with significant ultraviolet exposure in the open air. By using basic epoxy glue to encapsulate the PSCs, an amazingly strong stability was achieved under 1.5 sun UV irradiation in air for more than 1000 h. Using this technique, a PSC employing triple cation (Cs, MA, and FA) was reported by Mathies et al. [61] that achieved a steady performance. They employed inkjet printing method to deposit the active layer using 10% Cs in MA and FA with Pb(I/Br)2 , and the PSC showed stability against moisture and heat. Hence, it can be concluded that this approach has high production scalability and is proficient of delivering stable as well as long-lasting PSCs.

5.5.2.2

Drop Casting

It is possible to efficiently create PSC modules when perovskite precursors are drop casted onto screen-printed scaffolds. Because of its cheap price, ease of use, and high output rate, the method is suitable for mass manufacturing. This technique is carried out when perovskite precursor solution is simply poured over the substrate and then it is heated for eliminating the solvent. In this procedure, no extra fancy tools are required at all. In 2014, Mei et al. [62] pioneered the use of drop casting in PSC fabrication, wherein the perovskite film was deposited directly above a double-scaffold layer of m-ZrO2 /m-TiO2 , eliminating the need for HTL. In order to create a perovskite film with better pore filling and a lower defect concentration, 5-aminovaleric acid (5-AVA) was used as an additive in the precursor solution. They fabricated a 49 cm2 perovskite panel consisting of ten linked cells in series. TiO2 , ZrO2 , and carbon were screen-printed onto the scaffold, and then ((5-AVA)x (MA)1−x PbI3 ) was drop cast onto the carbon side. After 1000 h of operation, the PCE was still 10.4%. The perovskite layer was subsequently fabricated by Niu et al. [63] through a solutionbased hot casting technique. When using a planar design, they were able to get a uniform layer with no pinholes and 18% device efficiency. While this process does improve PCE over traditional spin coating, it still falls well short of what can be achieved with the latter. An automobile assembly line should be built for a steady output of vehicles.

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5.5.2.3

153

Doctor Blade Coating

Doctor blade coating approach is cheap, easy to implement, productive, compatible with roll-to-roll fabrication, and is therefore suitable for industrial-scale manufacturing of PSCs. In this process, a predetermined volume of perovskite precursor solution is poured over the substrate, and then a glass blade is slid over the surface in a linear motion at a high speed. During doctor blade deposition, keeping the substrate at a high temperature is recommended for a film that is smooth, uniform, and pinhole-free. Higher substrate temperature aids in solvent evaporation, nucleation, and crystal growth. Airflow across the substrate aids in the quick evaporation of the solvent. By adjusting the distance between the blade and the substrate, the concentration of the precursor solution, and the blade’s speed of movement, the film thickness and crystallization may be finely tuned. ITO/HTL/MAPbI3 /PC60 BM/C60 /BCP/Al was the first device design utilized by Deng et al. [64] to construct a PSC by doctor blade deposition. This device attained a PCE of 15.1%, and it was noted that the film’s grain was both significant and continuous. In comparison with standard spin-coating methods, this one uses far less precursor solution, which is one of its most striking features. The doctor blade method was subsequently applied to construct colorful PSC with perovskite photonics nanostructure, and the resulting devices achieved efficiency near to the optimal PSC [65]. By using carbon paste as the electrode and Al-doped ZnO nanostructures as the ETL, Shirazi et al. [66] fabricated an HTL-free PSC. In this instance, a doctor blade was used to cut the carbon electrode before it was annealed at 100 °C. Using this method, many people have worked to create PSCs with huge surface areas and optimal PCE.

5.5.2.4

Slot-Die Coating

The slot die and doctor blade coating method described earlier are very similar, but the former includes an ink reservoir attached to the blade, allowing it to evenly distribute the precursor solution throughout the substrate. Comparatively speaking, the layer achieved with this method is of higher quality than with the doctor blade method; nevertheless, slot-die coating requires a much greater quantity of precursor solution. The coating tip’s location can be precisely controlled thanks to the 3D printer attached with slot-die coater employed by Vak et al. [67]. The MAPbI3 perovskite layer was produced by two-step sequential deposition where a pinhole-free and homogeneous PbI2 layer was created by employing the slot-die coating approach followed by gas quenching. Deposition of the MAI layer, the second stage in perovskite layer manufacturing, was typically carried out by spin coating or dip coating, although this layer can also be fabricated by slot-die coating approach. HTL, ETL, perovskite, and mesoporous layers were all synthesized using this method [68]. With this method and laser patterning, Giacomo et al. [69] fabricated a 12.5 cm by 13.5 cm panel with a PCE of 10%.

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Spray Coating

Spray coating method has often been employed for fabricating organic solar cells and oxide thin films; however, fabricating perovskite thin films is also possible via this method. It has the same experimental setup as inkjet printing. Perovskite precursor solution is introduced through the quartz nozzle that connects the atomizing and deposition systems. The perovskite solution is broken down into very small droplets by an atomizer, which is then guided into the substrate by a low-pressure gas stream; the solvent is then evaporated, leaving behind a perovskite coating. This technique is categorized into the following categories based on how the droplets are dispersed across the substrate: (i) spraying using ultrasonic vibrations (ultrasonic spraying); (ii) spraying via swift gas flow (pneumatic spraying); and (iii) spraying by means of electrostatic repulsion force (electrospraying). Ultrasonic spraying has been utilized for producing perovskite and ETL layers. Barrows et al. [70] originally employed this technique in 2014 to achieve PCE of 11% in single-step deposition of MAPbI3-x Clx in a planar PSC. The spray coating process of depositing a perovskite layer allows for its quality to be managed by monitoring the properties of the precursor liquid droplets. The effectiveness of the deposited film is partially dependent on the surface tension of the material. Due to the high surface tension, the perovskite precursor solution tends to accumulate as a spherical cap on top of the substrate, reducing its wettability. In order to increase the solvent evaporation rate during deposition, the substrate should be maintained at a high temperature.

5.6 Finding a Solution to PSCs’ Instability Dilemma for Practical Implementation Besides other criteria like low toxicity, short energy payback time, etc., high stability (long lifespan), high power conversion efficiency, and low cost are the three most important ones to consider while bringing photovoltaic technology from the laboratory to commercial production. Organic–inorganic metal halide perovskites are among the most capable materials showing high efficiencies in solar cell devices, drawing significant interest from basic research. However, their practical applicability is still uncertain, due to the notoriously short device operating period. In this section, we have explored the problem of PSCs’ instability and suggested standardized techniques for characterizing devices so that they may compete with silicon in the industrial sector.

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Fig. 5.12 Golden triangle of solar cells

5.6.1 The Golden Triangle The technological viability for industrialization of photovoltaic technology is evaluated based on their efficiency, stability (or lifetime), and costs, generally known as the “golden triangle” as illustrated in Fig. 5.12. Silicon solar technology now holds more than 90% of the marketed PV industry due to its attractive package of 21% module efficiency, 25-year lifespan, and 0.3 $ W−1 cost which is approaching grid parity. Comparatively, PSCs show potential due to their high efficiency (>25%) as well as affordable production cost (expected to approach half of the c-Si). However, PSCs have serious issues with stability. To yet, the longest recorded PSC lifespan is about one year, which is much less than the 25 years anticipated from marketed PV technology. As a result, it is evident that PSC’s short lifespan is the primary factor preventing its widespread commercialization.

5.6.2 Tackling Stability Concern Many variables, both intrinsic (inside the PSC itself) and extrinsic (within the surrounding environment), determine the PSC’s lifetime. Encapsulation may solve issues caused by the environment, including oxygen and moisture, but the most

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significant concerns stem from the interfaces between the charge transport layers and the perovskite as well as the inherent instability of the bulk perovskite material. Thermal instability, hygroscopicity, and ion migration are the three primary intrinsic characteristics that contribute to perovskite instability. Environmental variables contribute to hygroscopicity, which may be mitigated by encapsulation. Composition adjustment, such as the use of FA cations, may raise the decomposition barrier/energy, hence mitigating the thermal instability. Finally, A site alkali doping and replacement as well as multiple dimensional perovskite engineering and organic molecular additives are now used to address the ion migration problem. Considering the high ionic mobility and higher external electric field exerted across different layers of PSC during current density–voltage measurements, ion migration is practically inevitable in all halide perovskites, and it is much more severe at the interfaces, defect sites, and the grain boundaries. However, we think the ion migration might be prevented via passivation of grain boundaries, by improving the quality of the sample (by decreasing the grain boundaries), and, most ideally, raising the ion migration barrier through engineering the packing density of the crystal lattice by ion substitution [71]. Because of their proximity to the photosensitive perovskite film, charge transport layers have the additional responsibility of shielding the perovskite layer from natural environment effects like dampness and heavy metal ions from the electrodes. Due to its high hygrpscopicity, propensity to crystallize, and fragility to both heat and moisture, the most often engaged hole transport layer Spiro-OMeTAD must be altered. Though it has been proven that using strong metal oxide, carbon, and other inorganic materials may effectively boost device stability, the PCE in such devices has yet to be tuned. Perovskite solar cells have a maximum lifespan of 10,000 h, which is equivalent to about one year; however, their power conversion efficiency was just 12% [72]. If we want a minimum efficiency of around 20%, the longest period of stability under illumination is just 1000 h. To compare with silicon technology, we may aim for an efficiency of about 20% and a lifespan of 15 years. There is no physical principle that prohibits PSCs from achieving both high efficiency and great stability, but the devices are not yet optimized for both. Due to the huge drive along with continual research efforts in the domain of PSCs, we think it is just a matter of time to cope up with silicon. The era of appreciating technical and gradual advances has arrived. Together, these little advancements will drive key metrics to their absolute limits; therefore, we are receptive to all potential approaches.

5.6.3 Accelerated Aging Studies and Mixed Stability Tests Researchers are becoming more interested in studying stability; however, it is tough to compare the results of lifetime tests done in various laboratories since they have all been conducted under distinct, non-standard circumstances. Here, we propose “AM 1.5 light soaking, maximum power point tracing, encapsulated or inert gas

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protected device, 25 °C temperature,” as the standard testing condition for emulating the realistic situation. Subsequent to light soaking, device temperature may rise, whereas in case of standard testing condition, constant temperature is maintained by the utilization of a heat sink. Perovskite solar cell laboratory tests only meet a subset of standard test criteria. For instance, it may be asserted that stability is achieved at a certain relative humidity or ambient atmosphere. We wish to note out that the air stability is not a major worry with realistic solar panels since they are always encapsulated to safeguard the module from mechanical damage, rain, and dust. Actually, the accelerated aging testing has to include testing under very humid conditions. To approach the industry standards like IEC 61,646 and 61,215 for thin film and crystalline solar cells, respectively, it is recommended that humid heat conditions like “85 °C and 85% relative humidity for 1000 h operating lifetime” be applied [73]. This may be done in tandem with the standard operating procedure for perovskite solar cell accelerated aging testing. It takes three months and two years to finish a 2000-hour and a 20,000-hour real-time testing, respectively. Instead of waiting two years to report the findings of real-time testing, accelerated aging tests should be conducted. Testing in the dry nitrogen atmosphere in an oven or on a hot plate at 85 °C, without or with light soaking, is a simpler approach to perform acceleration test than the traditional encapsulation method, which introduces uncertainty due to the operation itself. Meanwhile, the OLED sector makes the encapsulation method easily accessible, allowing for more rigorous and systematic testing. In accordance with established norms in the manufacturing sector, the device’s performance should be evaluated over time in relation to increasing environmental factors such as humidity, temperature, and light exposure. The present objectives are: first, to establish a link between the two lifetimes for PSCs, i.e., the accelerated aging and the realistic one; second, attainment of a lifetime of about 1000 h with 80% preservation of its original PCE at a relative humidity of 85% and a temperature of 85 °C. We think it is crucial to study stability in order to put perovskite technology into practical use.

5.6.4 The Actual Costs of Perovskite Solar Cells The levelized cost of energy (LCOE) is a useful performance metric that may provide some insight into the relative economic charisma of solar technology, while there is still room for improvement. The LCOE is the net present worth of the unit-cost (in US cents per kilowatt hour) of power throughout the useful life of a power producing equipment, for instance, a solar plant, and is calculated by dividing the total cost by the total amount of electricity produced. The advantage of LCOE is that it considers all three of the golden triangle’s critical factors. The LCOE is reduced in direct proportion to the rise in total power production that occurs when the PCE of the modules is raised.

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If the lifespan of the PV module is increased, the LCOE may be eliminated. There is almost inverse proportional dependency of expected LCOE on the lifetime of fabricated perovskite solar cell device. Since LCOE depends on PCE and lifetime in a similar way and since efficiency has already reached the near saturation stage, the community should move its attention to the stability research. According to Meng et al. [71], when the LCOE reaches 5.50 US cents per kilowatt hour for perovskite photovoltaics, it will be competitive with 21% efficient market-leading c-Si solar cell. For PSC having 19% PCE and at least 100 cm2 module size), it was suggested that a lifespan of 15 years is the threshold based on the LCOE calculation, which will be a significant increase from the present state of the technology.

5.7 Applications Perovskite solar cells are a rapidly advancing class of photovoltaic technology that holds promise for high efficiency, low cost, and versatile applications. They are made using perovskite materials, which are relatively inexpensive and can be easily processed. Here are some applications of perovskite solar cells along with examples: • Residential and Commercial Rooftop Installations: Perovskite solar cells can be used on rooftops of residential and commercial buildings to generate electricity from sunlight. Their potential for lower manufacturing costs and ease of integration make them attractive for broader adoption. • Building-Integrated Photovoltaics (BIPV): Perovskite solar cells can be integrated into building materials such as windows, facades, and roofing materials, enabling buildings to generate power while maintaining esthetic appeal. This can contribute to sustainable and energy-efficient architecture. • Portable Electronics and Wearables: Perovskite solar cells can be incorporated into portable devices like smartphones, smartwatches, and fitness trackers to provide supplementary power for extended battery life. • Flexible and Lightweight Solar Devices: Perovskite solar cells are flexible and can be applied to curved surfaces, making them suitable for applications like flexible solar panels and rollable solar chargers. • Emerging Economies and Developing Countries: Perovskite solar cells’ potential for lower manufacturing costs and ease of deployment could make them a viable option for expanding access to clean energy in regions with limited energy infrastructure. • Space Applications: Perovskite solar cells may find applications in space missions due to their lightweight and efficient energy conversion properties. They could be used on satellites, space probes, and other spacecraft. • Hybrid Systems: Perovskite solar cells can be combined with other solar technologies or energy storage systems to create hybrid energy solutions that enhance reliability and energy production.

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• Agricultural and Rural Applications: Perovskite solar cells could power irrigation systems, water pumps, and remote agricultural equipment in rural and off-grid areas. • Consumer Electronics Charging Stations: Perovskite-based charging stations could be set up in public spaces to provide convenient charging for smartphones and other devices. • Transportation: Perovskite solar cells can be integrated into electric vehicles (EVs) to supplement the vehicle’s power and potentially extend its range. • Educational and Research Applications: Perovskite solar cells can be used in educational settings to teach students about emerging solar technologies and renewable energy concepts. • Emergency and Disaster Relief: Perovskite solar cells can provide power during emergency and disaster situations, such as powering lighting, communication, and medical equipment in remote or affected areas. • Off-Grid Power Solutions: Perovskite solar cells could be used in off-grid and remote applications, providing power for lighting, communication, and smallscale electricity needs. • Art and Design Installations: Artists and designers can incorporate perovskite solar cells into installations, sculptures, and art pieces that interact with sunlight and generate visual effects. As perovskite solar cell technology continues to evolve and improve, it has the potential to revolutionize the solar energy landscape and find applications in a wide range of sectors, from residential and commercial to industrial and innovative artistic projects.

5.8 Summary In conclusion, solution-processed PSCs have advanced significantly, becoming a leading candidate among future PV technologies. Many different device layouts and material combinations have been used successfully in the development of highefficiency PSCs. In this chapter, we examined the history of devices and how their architectures and film deposition processes have progressed to an incredible extent. To overcome obstacles including long-term stability, further study of these materials and devices is required. More promising findings are anticipated in the future, and PSCs will continue to attract attention from the PV sector. Perovskite solar technology has shown superior processability and high efficiency compared to traditional photovoltaics, while the stability problem seems to be the last technical obstacle for the commercialization of PSCs. High efficiency and a longer lifespan along with concerted research efforts on material development and device design is the ultimate requirement. Moreover, parallel investigations are also required to protect them from degradations. Meanwhile, we encourage the community to think about establishing standardized methodologies to describe perovskite solar modules

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and provide valid comparisons across outcomes, from which the performance parameters might be contextualized toward future commercial applications.

5.9 Important Timelines • 1839—In the Ural Mountains of Russia, German researcher Gustav Rose found a novel calcium titanate-based mineral that was dubbed “perovskite” in honor of Russian mineralogist Lev von Perovski [12]. • 2009—Perovskite was initially used in solar cells as a liquid sensitizer in a dyesensitized arrangement. The device achieved an efficiency of 3.2% and 3.81% using perovskites MAPbBr3 and MAPbI3 , respectively [14]. • 2011—6.5% efficient perovskite quantum dot-sensitized solar cell was fabricated by Im et al. [15]. • 2012—Kim et al. created 9.7% efficient all-solid-state PSC using Spiro-OMeTAD as HTL to solve the issue brought on by the use of liquid electrolytes [16]. • 2013—A two-step deposition technique was used by Burschka et al. to produce MAPbI3 films. The fabricated PSC produced a PCE of 15.0% [17]. • 2014—Im et al. produced MAPbI3 cuboids-based solar cells and achieved an enhanced PSC with 17.01% efficiency [18]. • 2015—Yang et al. reported PCE of 20.2% by using FAPbI3 as opposed to MAPbI3 [20]. • 2016—Bi et al. used PMMA polymer to augment the growth and nucleation processes and achieved PCE of 21.6% [22]. • 2017-Yang et al. attained an efficiency of 22.1% for small scale and 19.7% in a 1 cm2 cell by employing a mixed-halide anion and various cations as an absorber layer to lower the concentration of deep-level defect states [23]. • 2018—There was a breakthrough when researchers in Chinese Academy of Sciences achieved the greatest efficiency of 23.3%. Additionally, perovskitesilicon tandem design surpassed the highest possible efficiency of single-junction silicon solar cells by achieving a PCE of 28% [24]. • 2021—A CBD-processed pinhole-free SnO2 film was developed as ETL with low defect density and beneficial band alignment by Yoo et al. This suppressed the recombination of charge carriers at the interface between perovskite and ETL leading to 25.2% PCE [26]. • 2021—UNIST has achieved a single-junction PSC efficiency of 25.8% (certified 25.5%) via the creation of a 2 nm thick FASnClx interlayer between perovskite layer and Cl-bonded SnO2 ETL coated with Cl-containing FAPbI3 solution [27].

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Chapter 6

Organic–Inorganic Hybrid Solar Cells

6.1 Introduction The high-power conversion efficiencies of first- and second-generation solar cells have drawn a lot of attention, but in order to meet the current demand, it will be difficult to overcome the high production costs and material availability issues associated with materials like indium [1]. Organic solar cells have benefits including cheap cost, flexibility, simple manufacturing, and scalability, which place this area amid applied science and engineering research. The power conversion efficiency of solution-processed organic solar cells has significantly increased. But there are still stability problems to be resolved because organic materials are so sensitive to oxygen and moisture. The concept of organic–inorganic hybrid (OIH) solar cells was developed with the intention of integrating the benefits associated with organic and inorganic semiconductors. Both organic and inorganic semiconductors are used in an OIH solar cell, which combines their benefits, including high solubility, good film formation, flexibility, and low cost of organic materials with those of inorganic semiconductors, including high charge carrier mobility and strong chemical stability. The donor and acceptor of organic solar cells must both be p- and n-type conjugated polymers, respectively. However, OIH solar cells also benefit from the incorporation of organic components, primarily in the form of conjugated polymers and inorganic nanoparticles. OIH solar cells perform all the processes necessary to generate the current from incident light, including light absorption, exciton formation and diffusion, exciton dissociation, carrier transportation, and carrier collection, just like any other silicon-based solar cell. Here, an inorganic electron acceptor material is used in place of the organic acceptor polymer, which could offer benefits while keeping costs low. Compared to their equivalents, some inorganic materials are more eco-friendly. Organic semiconductors can break down when exposed to light, which is a problem that can be fixed with inorganic semiconductors, which can also be used to generate charge carriers.

© The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 S. Arya and P. Mahajan, Solar Cells, https://doi.org/10.1007/978-981-99-7333-0_6

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The absorption characteristics of the nanoparticles can be altered by modifying the size and shape of inorganic acceptor materials, which creates a criterion for selecting the spectral window. A quicker speed of photo-induced charge carrier transfer and consequently, a higher charge transfer between donor and acceptor will be possible if the size can be decreased to that of quantum dots. It will be feasible to build a network of nanostructures that is well aligned, aiding in better electron routes and effective exciton dissociation. One of the key elements determining the device efficiency is the morphology of the photoactive layer and the nanoparticle surface chemistry. The utilization of inorganic electron acceptor material in the hybrid system can yield additional benefits, while simultaneously preserving its cost-effectiveness and ease of processing. Inorganic acceptor materials exhibit greater environmental stability in comparison to organic materials [2]. The incorporation of these materials into OPV devices has the potential to mitigate a significant limitation of the technology, namely the photo-induced degradation of the conjugated organic semiconductors. Subsequently, the generation of charge carriers through the absorption of excitons in the inorganic material can be accomplished [3, 4]. It is possible for the light absorption contribution of an inorganic acceptor to surpass that of PCBM in OPV devices [5, 6]. Moreover, the bandgap and absorption profile of inorganic nanoparticles are modified by quantum confinement, which is achieved through alterations in their size and shape [7]. The selection of the spectral window for the complementary absorption profile is made possible by this approach [8]. Inorganic quantum dots are recognized for their ability to facilitate rapid photo-induced transfer of charge carriers to organic semiconductors. The transfer rate has been determined to be in the picoseconds range [9]. Effective charge transfer between the donor and acceptor can be developed due to the faster transfer rate compared to other recombination mechanisms. Finally, it is possible to customize the physical dimensions of certain inorganic semiconductors, such as oxides, by means of synthesis techniques, resulting in the creation of vertically aligned nanostructures [10]. This phenomenon has the potential to result in device structures that facilitate effective excitonic separation and electron transportation pathways. These benefits can be achieved while preserving the solution processability, resulting in cost-effective and high-throughput device manufacturing. Despite the various theoretical benefits linked to the utilization of an inorganic electron acceptor, the operational efficiencies attained by hybrid solar cells are notably inferior to those of polymer: fullerene OPV devices. There are several factors accountable for this disparity. The paramount concerns appear to be associated with the surface chemistry of nanoparticles and the nanomorphology of the photoabsorber film. This chapter scrutinizes these two pivotal factors. The primary objective of this chapter is to provide an introduction to the notion of hybrid organic–inorganic solar cells and to investigate the inorganic materials that have the potential to yield highperformance OIH solar cells. The initial segment of the chapter presents the idea of OIH solar cells, expounding on the process of fabricating the device and its functional properties. An in-depth examination of the materials employed in the construction of these solid-state devices is presented. The study delves into the desirable attributes of donor and acceptor materials and provides a comprehensive inventory of previously

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investigated material combinations. Subsequent to the selection of materials, any constraints that may impede the performance of hybrid solar cells are analyzed. The text elucidates the contemporary comprehension of the commercial facets pertaining to this technology. This is achieved by emphasizing the preliminary inquiries that have surfaced in the recent past. Subsequently, it highlights significant domains of investigation that require further exploration in the forthcoming times.

6.2 Basic Operating Principles and Device Architecture Hybrid solar cells consist of blend films of inorganic semiconductors and conjugated polymers sandwiched between two metal electrodes as shown in Fig. 6.1. The use of inorganic semiconductors in the device design distinguishes organic solar cells from OIH solar cells. As a result, OIH solar cells’ operating principles are quite similar to those of organic solar cells and include the following basic steps [11]: (i) (ii) (iii) (iv) (v) (vi)

Absorption of photons; Generation of excitons within the active layer; Diffusion of excitons; Dissociation of excitons; Transport of charges to the appropriate electrodes; Collection of holes and electrons at the electrodes.

The photoactive layer of the bulk heterojunction hybrid solar cells consists of inorganic semiconductor nanoparticles and conjugated polymers. Conjugated polymers have been utilized to transport the holes (as donors), while inorganic semiconductors have been used extensively to transport the electrons (as acceptors). The nature of organic materials may either be a donor or an acceptor. Electron donors are defined as

Fig. 6.1 General structure of an OIH solar cell: a depiction for working principle of polymer/ nanoparticles and b energy level diagram and charge transfer process

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molecules with a low ionization potential that are simple to donate an electron. Electron acceptors are materials that readily receive an electron due to their high electron affinity. Intermolecular orbital overlap in the solid state determines whether they are effective electron or hole transporters. They may also function as both electron and hole transporters. Recently, it was shown that PCBM’s hole mobility is comparable to its electron mobility. An ideal acceptor should allow for effective electron transport, which is n-type, while an ideal donor should allow for efficient hole transport, or p-type. The terms “n-type” and “p-type” relate to semiconductors that are highly effective at conducting electrons and holes, respectively, in the context of organic materials. As a result, another way to describe organic semiconductors is as acceptor for the n-type and donor for the p-type. The donor transfers electrons to the acceptor in the nomenclature of organic and hybrid solar cells [12]. For instance, in the case of inorganic semiconductors, n-type silicon is produced by adding donor impurities. Organic and inorganic semiconductors have very distinct doping methods. As donor polymers, the majority of semiconducting polymers are hole conductors. They could also be conductors of electrons. Upon absorption of a photon within the donor material, the resultant outcome is the formation of an exciton. This exciton has the potential to undergo dissociation at a donor–acceptor interface. Upon segregation, the electron has been able to migrate to the acceptor at the interface. The electrons travel to the cathode in order to facilitate the process of charge accumulation. The hole generated in the donor material migrates across the polymer matrix and is collected at the anode. This process is depicted in Fig. 6.2a. The inorganic acceptor might also provide beneficial photocurrent. Upon absorption of light by the acceptor material, an exciton is generated, necessitating dissociation through the energy offset between the donor HOMO level and the acceptor valence band edge. Subsequently, the hole is transported to the donor at an interface and is transferred to the anode, whereas the electron remains in the acceptor and migrates to the cathode for charge collection. The aforementioned procedure is depicted in Fig. 6.2b. The HOMO and the LUMO of the inorganic nanoparticles and the conjugated polymers should be suitably selected for an appropriate charge transfer. For the effective charge transfer, The LUMO and the HOMO of the conjugated polymers should lie above the conduction and valence band edges of the inorganic nanoparticles, respectively. In this instance, the holes are moved from the valence band of the inorganic semiconductor to the HOMO level of the conjugated polymer, whereas the electrons are carried from the LUMO level of the conjugated polymer to the conduction band of the inorganic semiconductor. Organic materials undergo photoexcitations that result in bound electron–hole pairs, or excitons. Within their lifetime, excitons must be separated into free charge carriers. Otherwise, they could combine again, which is not the preferred course of action for the functioning of a solar cell. For the organic materials, the exciton diffusion length is between 5 and 10 nm. The excitons are separated into free charge carriers using a p–n junction. In the bulk heterojunction notion, the p–n junction is distributed throughout the bulk of the film by combining the p- and n-type semiconductors, allowing each exciton that reaches the junction to be dissociated into free charge carriers. The likelihood of exciton dissociation during

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Fig. 6.2 Diagrammatic illustration depicting charge transfer for: a photo generation in the electron donor and b photo generation in the electron acceptor

the lifespan of the excitons and within the length of exciton diffusion is increased by distributing the p–n junction throughout the film. This is one of the reasons why bulk heterojunction solar cells have higher short-circuit current densities and, consequently, power conversion efficiencies than bilayer heterojunction solar cells, which are made up of separate bilayer films of n- and p-type semiconductors sandwiched between two metal electrodes. As discussed before, conjugated polymer and inorganic semiconductor blend films are sandwiched between two metal electrodes. Conducting electrodes are utilized as substrates (for instance, glass or plastic coated with ITO). ITO serves as a transparent conducting electrode that permits light to travel through the cell. Commonly, an aqueous solution of PEDOT:PSS is deposited on the transparent conducting substrate. The ITO electrode’s surface quality is enhanced by the PEDOT:PSS layer, which also makes hole injection and extraction easier while lowering the likelihood of shorts. Additionally, by modifying the PEDOT layer chemically or electrochemically, this electrode’s work function may be altered [13]. On top of the PEDOT:PSS-coated

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ITO substrates, a photoactive layer made up of blends of conjugated polymer and inorganic nanoparticles is cast from solution. Al, Ag, Au, and other metals have been options for the second metal electrode. The metal selection should make sure that the metal and semiconductor make an ohmic contact.

6.3 Bulk Heterojunction OIH Solar Cells The bulk heterojunction idea developed in organic solar cell research was imitated in the early attempts to construct hybrid solar cells. By combining two organic semiconductors, one of which is an electron acceptor and the other of which is an electron donor, the bulk heterojunction idea for organic solar cells has been realized. The similar idea has been used in hybrid solar cells, which combine conjugated polymers with inorganic semiconductor nanoparticles. Diagrammatic illustration of bulk heterojunction OIH solar cell is given in Fig. 6.3. The CdSe nanocrystals, which exhibit absorption in the spectral region between 300 and 650 nm, were the first nanocrystals examined in hybrid solar cells [14]. Huynh et al. [15] reported one of the first investigations on hybrid bulk heterojunction solar cells utilizing CdSe nanoparticles. They showed that polymers and semiconductor nanorods might be used to create hybrid solar cells. The authors were able to modify the bandgap by controlling the nanorods length, which allowed effective electron transport across the device. This improved the overlap between the solar Fig. 6.3 Diagrammatic illustration of bulk heterojunction OIH solar cell

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emission spectrum and the nanorods’ absorption spectrum. Consequently, it attained a PCE of about 2%. The ability of the dispersion of the nanoparticles into the polymer matrix to provide a large interfacial area between two materials for a better charge transfer is the key to the effective hybrid bulk heterojunction solar cells employing nanocomposites. The nanoparticles have organic ligands adsorbed on their surface, which passivate the surface for stability and render them soluble. Although organic ligands are not required for nanoparticles, their presence has a significant impact on how well the particles disperse into the polymer matrix, making efficient dispersion of the particles very important. The effectiveness of binary solvents in assisting nanoparticle dispersion inside the polymer matrix has been established by Huynh et al. [16]. Additionally, it has been shown that the heat treatment expedites the elimination of the ligand and raises photocurrent, improving power conversion efficiency. Later research found that the hybrid solar cells’ PCE was affected by the CdSe morphology that was chosen, whether it was a nanoparticle, nanorod, or tetrapod. In comparison to hybrid solar cells made from nanorod/polymer blends, Sun et al. [17] showed that hybrid bulk heterojunction solar cells utilizing blends of branching CdSe nanoparticles and polymers had a superior photovoltaic performance. Under AM 1.5 illumination, they managed to attain a PCE of around 2%. They have demonstrated that the efficiency of electron extraction in devices using 3D CdSe tetrapods is higher than that in devices using 1D nanorods, and they have also noted that control of the nanoparticle shape in 3D CdSe tetrapods aids in controlling the morphology and effectiveness of devices containing nanoparticle/polymer blends. The choice of polymer is crucial for the overall performance of hybrid solar cells, in addition to the shape of the CdSe nanoparticles. In the hybrid bulk heterojunction devices, the usage of a low bandgap polymer poly[2,6-(4,4-bis-(2-ethylhexyl)-4Hcyclopenta [2,1-b;3,4-b' ]dithiophene)-alt-4,7(2,1,3-benzothiadiazole)] (PCPDTBT) and CdSe tetrapod mix resulted in a PCE of over 3%. For effective photon harvesting, the PCPDTBT, a low bandgap polymer with a wide absorption spectrum, proved useful. Therefore, using a greater overlap between the solar emission spectrum and the spectrum of polymer absorption led to higher performance [6]. CdS nanorods have also been employed as an inorganic semiconductor in hybrid solar cells. Under AM 1.5 illumination, devices containing MEH-PPV polymer and multi-armed CdS nanorods had a PCE of above 1%. The use of pyridine as a solvent in place of HDA was responsible for the increased efficacy. CdS nanocrystals’ solubility and dispersion in MEH-PPV film were both enhanced by the pyridine that was added to their surface during refluxing. Power conversion efficiency was increased as a consequence of an effective charge transfer and exciton dissociation [18]. The majority of research in the literature have focused on either the polymer choice or the morphology of the nanoparticles. The interface between the nanoparticles or nanorods and the polymer, however, is also a significant problem. It has been shown that CdS nanorods surface interface alteration may boost the effectiveness of hybrid bulk heterojunction devices. Aromatic acids were utilized as interface modifiers by Jiang et al. [19]. After the addition of an aromatic acid, they saw an improvement in efficiency. This increased performance was due to the CdS nanorods’

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decreased surface trap and defects, the rearrangement of the surface energy level through dipole creation and inhibition of the reverse charge transfer, and lastly the enhanced compatibility between the CdS nanorods and P3HT. Although CdSe and CdS nanoparticles have garnered a lot of attention for use in hybrid solar cells, the low power conversion efficiencies compared to organic and inorganic solar cells have prompted researchers to look into other inorganic semiconductor nanoparticles for use in hybrid solar cells. CuInS2 and CuInSe2 have also been explored as potential materials for hybrid solar cells. The photoconductivity and type of conductivity (n- or p-type) of CuInS2 , which has a high absorption coefficient (105 cm−1 ) may be modified by adjusting the stoichiometry [20]. On the other hand, CuInSe2 has a minimal bandgap and strong radiation stability. Although these research were the first to examine the creation and use of organic ligand-capped CuInS2 and CuInSe2 in hybrid solar cells, the PCE of these hybrid solar cells was somewhat constrained. Poor device performance may be attributed to morphology issues brought on by inorganic nanocrystals’ restricted dispersion, and conjugated polymers caused by the presence of organic ligands and high serial resistances. Despite extensive research into hybrid solar cells made up of mixtures of CdSe, CdS, CuInS2 , CuInSe2 , and conjugated polymers, poor device performance, hightemperature synthesis of inorganic nanoparticles, and the toxicity of Cd-containing materials have spurred the search for substitutes [21]. The precursor Ti(i-PrO)4 for TiO2 , which was introduced to the MDMO-PPV solution before spin casting, has been used as a substitute substance. This Ti(i-PrO)4 precursor was transformed into TiO2 in the presence of moisture and a subsequent high vacuum treatment. A PCE of 0.2% was found in these devices [22, 23]. High temperatures (>350 °C) are necessary for the crystallization of TiO2 , yet these temperatures may cause the polymer to distort since conjugated polymers cannot withstand high temperatures. The charge transfer in these hybrid devices is restricted by the TiO2 network’s crystallinity [24]. By using ZnO as a precursor, Beek et al. [25] have implemented a novel strategy. Through spin coating a solution containing an organozinc chemical and a conjugated polymer, followed by thermal annealing at a reasonable temperature, they created precursor-ZnO: polymer hybrid bulk heterojunction solar cells. Due to the formation of a crystalline ZnO network in the polymer phase, the PCE of the hybrid bulk heterojunction solar cells using the TiO2 : polymer hybrid solar cells’ amorphous precursor was greater. This idea led to hybrid solar cells with a PCE of more than 1%.

6.3.1 Benefits of Bulk Heterojunction Configuration The benefits of this idea may be summed up as follows [26]: (i) High absorption coefficients and high charge carrier mobilities are possible in inorganic semiconductors.

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(ii) By employing synthetic methods, the size quantization effect makes it simple to adjust the bandgap of inorganic materials. As a result, inorganic nanoparticles that absorb at various wavelengths may be found. (iii) By manipulating the n- and p-type doping levels of inorganic nanoparticles by synthetic means, the availability issue of acceptor materials, as is the case with organic solar cells, may be resolved. (iv) The fullerene-based acceptors, which have been extensively utilized in organic solar cells, were replaced with inorganic nanoparticles in the early research of hybrid solar cells. Fullerene synthesis requires a lot of energy and is challenging. In contrast, inorganic nanoparticle colloidal production is much simpler. Additionally, inorganic nanoparticles have a larger absorption range than fullerenes, allowing for the fabrication of thinner devices.

6.3.2 Issues Responsible for Constrained Performance The PCE of hybrid bulk heterojunction solar cells with different inorganic nanoparticle and semiconducting polymer combinations has been extensively studied; however, it is still insufficient. The following factors might be summed up as the cause of the restricted PCE [27]: (i) The synthesis of inorganic nanoparticles may necessitate high temperatures. (ii) The organic ligand surrounding the nanoparticles may prevent their dispersion in polymer matrix. (iii) The different synthesis routes may result in nanoparticles with different properties, affecting the reproducibility of the nanoparticles. (iv) The organic ligand itself is an insulator, blocking the electron transport between the particles. (v) The dangers of using toxic substances like Cd when creating inorganic nanoparticles. Despite the aforementioned issues, bulk heterojunction hybrid solar cells are a fascinating device idea that should be further researched in order to address these issues since there is a wide range of parameter space to choose from and only a tiny subset of conceivable combinations have been realized. A different approach to creating more effective hybrid bulk heterojunction solar cells may include using inorganic nanoparticles and quantum dots in the P3HT:PCBM blends [28].

6.3.3 Inverted-type Hybrid Bulk Heterojunction Solar Cells Because of its numerous benefits, including cheap cost, chemical stability, adaptability, organic material customization and versatility, bulk heterojunction-type solar cells have been the subject of a great deal of research over the last decade. The

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majority of published research has targeted increasing power conversion efficiency. However, stability is also a critical factor that must be carefully considered. Both internal and external factors contribute to organic solar cell deterioration [29]. Phase separation at the organic/cathode interface, phase separation at the semiconductor interface, interdiffusion at the interface, and morphological deterioration are all examples of intrinsic degradation. Oxygen and water both contribute to the extrinsic deterioration. Most extrinsic breakdown occurs in the presence of oxygen. Typically, a bulk heterojunction device consists of an ITO layer, a PEDOT:PSS layer, a photoactive layer (polymer:fullerene), and finally an Al layer. The rear metal contact Al is sensitive to air. However, the acidic nature of the PEDOT:PSS causes the active layer and the bottom electrode to deteriorate [30–32]. Non-corrosive metals such as Ag and Au are employed as replacements to Al to address these issues in traditional bulk heterojunction solar cells. This type of geometry is called inverted-type geometry. Several research teams have examined inverted-type solar cells. The inverted form has seen widespread application of TiO2 and ZnO as electron transport layers. TiO2 and ZnO films are advantageous as electron transport layers because of their high electron mobility, transparency in the area where the photoactive layer absorbs light, and amenability to practical synthesis. Inverted solar cells using TiO2 as electron transport layers had initial efficiencies between 3% and approximately 4% [33, 34]. Inverted-type solar cells’ performance may be impacted by a number of important variables. The structure of the electron transport films is a crucial factor. Several methods are described in the literature for synthesizing electron transport layers, most often for TiO2 and ZnO. The majority of these studies have focused on the sol-gel method of synthesis. Preparation techniques have a significant impact on thinfilm production, notably for TiO2 films. It has been shown that exciton dissociation and charge separation need a homogenous and well-defined TiO2 morphology [35]. Adding a buffer layer as a hole carrying layer between the photoactive layer and the Ag/Au electrode is another critical step toward optimizing inverted-type solar cells. It has been shown that inverted-type solar cells’ PCEs may be boosted with the use of buffer layers composed of materials like V2 O5 , MoO3 , NiO, and WO3 . They aid in preventing recombination at the interface, modifying the interface, and extracting holes from the active layer [36]. Inverted-type solar cells have also been investigated using CdS, ZnS, and In2 S3 as electron transport layers, in addition to metal oxides. PCEs between 0.3 and 3% have been attained by using these semiconductors as hole transport layers. The use of a polymer interlayer has led to a dramatic increase in the PCEs of inverted-type solar cells, from modest (0.3–3%) to ostentatious (nearly 10%) levels. A verified PCE of almost 10% was attained by Nam et al. [37] by including a neutral polymer interlayer of poly(2-ethyl-2-oxazoline) (PEOZ) between the ZnO layer and the photoactive layer. The dramatic drop in work function and the enhanced morphology after PEOZ layer addition were responsible for this massive improvement. Hybrid inverted-type solar cells have also made significant progress by the utilization of non-conjugated zwitterions. The free electron transport layers of metal oxide

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and inorganic semiconductors were made by combining zwitterions with polyethylene glycol (PEG). Using a variety of small molecule zwitterions, the device was able to attain PCE of about 8% [38]. Thanks to zwitterions, both the V OC and FF have been greatly enhanced. Recombination at the active layer/metal electrode interface was also inhibited by the homogeneous shape of zwitterion/PEG blends.

6.4 Bilayer Heterojunction OIH Solar Cells In order to create a bilayer heterojunction hybrid solar cell, an inorganic semiconductor and a conjugated polymer are cast as separate layers and then stacked on top of each other between two metal electrodes. The device structure of bilayer OIH solar cell is depicted in Fig. 6.4. Bilayer heterojunction solar cells have only a single p–n junction, which is defined inside the geometric interface between the p- and n-type semiconductors, in contrast to the multiple p–n junctions seen in bulk heterojunction hybrid solar cells. Only excitons with a lifetime short enough to reach this interface can be isolated. This means that exciton dissociation is primarily confined to a single interface which is one of the major drawbacks of bilayer heterojunction hybrid solar cells. Combining crystalline silicon with conducting polymers as the hole conducting layer is an alternative interesting methodology in bilayer heterojunction solar cells. The chemical and physical properties of thin films made from conducting polymers Fig. 6.4 Diagrammatic illustration of bilayer heterojunction OIH solar cell

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can be easily adjusted by tweaking the chemical structure of the polymer. The formation of a thin coating of conducting polymer on Si substrates, on the other hand, is simple and economical. One of the first materials examined to be applied as a hole transport layer in organic/Si heterojunction devices was polyaniline (PANI). Doped PANI is a material of interest because of its high hole-collecting efficiency and near-metallic conductivity. In the beginning, researchers examined PANI/Si heterojunctions that were deposited electrochemically. However, the rectification ratios of these devices were relatively low, and the films used were rather thick. In a study, PANI/Si heterojunctions were created by spin-coating PANI films on top of n-Si substrates, which led to increased rectification ratios. These devices were suggested for use as gas sensors because of their improved rectification ratio [39]. Wang and Schiff [40] explored how the open-circuit voltage of PANI/Si heterojunctions changed when exposed to light. In this investigation, a maximum V OC of 0.51 V was measured. However, when the results were extrapolated to films with a higher conductivity, they showed V OC of 0.7 V were feasible. P3HT has also proven to be a popular option as a conducting polymer. There is significant hole mobility in P3HT. However, the VB and CB of Si are chemically similar to those of P3HT, making charge transfer between the two materials more likely. As a low-cost and low-temperature alternative to conventional silicon solar cells, Si/P3HT heterojunction has been shown to be a practical option for photovoltaic applications. Avasthi et al. [41] discussed the critical parameters for effective solar cells on Si wafers. High photocurrent requires a significant offset between the CB of Si and the LUMO of the organic semiconductor, whereas a large offset between the VB of Si and the HOMO of the organic semiconductor is required. The authors have shown that Si/P3HT does in fact meet these requirements. As a result, their PCE ended up being greater than 10%. Surface photovoltage spectroscopy studies of the P3HT/ n-Si heterojunction have shown that the P3HT molecules interact strongly with the n-Si surface states. Band bending in the silicon substrate and the density of interface states of the P3HT/n-Si heterojunction are found to rise noticeably compared to bare Si. Charge separation and transport in P3HT layer is significantly slower than Si [42]. It has also been demonstrated that by treating the interface of P3HT/n-Si heterojunctions, the forward current density may be greatly increased by preventing the formation of an oxide layer. The ideality factor is also affected by the thickness of the oxide layer. Better performance, such as higher forward current density and lower ideality factor, is achieved with thinner oxide layers [43]. In organic/n-Si heterojunction devices, PEDOT:PSS has seen extensive application as a hole transport layer alongside PANI and P3HT. He et al. [44] investigated hybrid solar cells made by covering n-Si nanowires arrays with PEDOT. They managed a PCE of 9%. They showed that the overall PCE is affected by the thickness of the Si nanowires array. With a thickness of 0.9 µm, PCE of 9% was attained. They also noted that severe recombination due to increased nanowires aggregation limits solar cell performance with increasing nanowire thickness. The Si substrate has also been shown to play a role in photocurrent production.

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Another work using PEDOT:PSS/n-Si hybrid devices demonstrates the significance of Si surface termination conditions in the PCE. The efficiency of solar cells with hydrogen-terminated (H-Si) and oxide-terminated (SiOx -Si) Si surfaces was investigated by He et al. [45]. With a SiOx -Si substrate, they were able to achieve PCEs greater than 10%. The efficient charge separation is due to the band alignment and internal electric field at the junction interface. The BackPEDOT cell concept was established by Zielke et al. [46] to increase the performance of PEDOT/n-Si heterojunction devices that ultimately attained 17% PCE. On n-type crystalline silicon with a random-pyramid textured front, both frontand back-junction (BackPEDOT) cells were produced. Front junction solar cells were made by depositing the PEDOT:PSS layer on the front surface, while back junction solar cells were made by depositing the layer on the back surface. The authors find that the series resistance losses severely constrain the efficiency of this type of solar cell and added that their best BackPEDOT cell can achieve 21% efficiency by ignoring these losses.

6.5 Materials The total efficiency and consequently the success of this technology relies significantly on the materials chosen for usage in hybrid solar cells. An inorganic material with the true qualities may be employed as the electron acceptor in a hybrid solar cell, and those features are described in this section. It then provides a broad overview of the materials that have so far been studied, conducts in-depth evaluations of the materials in question with respect to the aforementioned desirable features, and discusses the benefits of each major material class.

6.5.1 Ideal Properties of Photoactive Layer 6.5.1.1

Donor Material

It’s critical to take electronic composition features and hole mobility into account when selecting a donor material. In relation to the acceptor material, the bandgap, HOMO, and LUMO levels are very significant. If certain design guidelines are followed while selecting the donor material, Scharber et al. [47] hypothesized that efficiencies surpassing 10% may be possible for an all organic device (using a PCBM acceptor). In relation to the vacuum level, they contend that the bandgap must be less than 1.74 eV and the LUMO level must be less than 3.92 eV. This will enable the use of a sizable amount of the solar spectrum across a relatively narrow bandgap while yet preserving a sufficient LUMO level offset to enable excitonic dissociation. Phenylene vinylene (PPV) was the donor material of choice in early OPV research. Researchers looked for donor materials that may offer improved performance as a

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result of the comparatively low PCEs of devices utilizing this polymer, which were principally brought on by limited hole mobility [48]. P3HT became the preferred polymer donor material since around 2002. When compared to PPV, P3HT has a number of benefits, including greater hole mobility, better environmental stability, and enhanced absorption. In-depth study has been conducted on the all-organic P3HT:PCBM system, and optimization has taken place for variables such as the solvent employed, the weight percentage ratio of donor and acceptor, light trapping methods, and annealing to enhance the nanomorphology. However, the system’s efficiency gains seem to have peaked, largely as a result of P3HT’s wide bandgap, which restricts the amount of solar spectrum that can be effectively gathered. Cyclopentadithiophene-based polymers have received a lot of interest recently in research because they may lead to narrow bandgap, highly absorbing donor materials. This low bandgap polymer has a substantially greater potential J SC than P3HT. The polymer PTB7 has also shown exceptional photovoltaic performance. With a 1.6 eV bandgap, this PTB family polymer possesses strong hole mobility, good solubility in organic solvents, and polymer alignment that encourages charge transfer [49]. These low bandgap polymers seem to have the potential to pave the way for future advancements in PCE for hybrid devices.

6.5.1.2

Acceptor Material

Fullerenes have several positive characteristics when paired with P3HT, such as good solubility, but they also have certain drawbacks [50]. Environmental stability and reduced absorption contribution are two of them. Some of these drawbacks may be mitigated by using an inorganic substance in place of the fullerene. The capacity to tune the bandgap via changes to the nanoparticle’s physical size is one of the key benefits of inorganic semiconductor nanoparticles. Due to the quantum confinement effect, significant bandgap variations have been seen for nanoparticles utilized in hybrid solar cells in comparison to the semiconductors’ bulk bandgap [51]. By changing the size of the nanoparticles to get them closer to the physical excitonic Bohr radius of the material, it is possible to create bandgaps that are significantly greater than the bulk bandgap. The device’s energy structure, which includes the electron affinity and ionization potential, may be modified using the quantum confinement effect in addition to the optical bandgap. To operate a hybrid photovoltaic system, the heterojunction’s energy structure is essential; hence, it is very desirable to be able to maximize this attribute. Particularly high quantum confinement effects are seen in semiconductors with low effective mass values. Physical limitations as well as electrical structure must both be carefully taken into account when selecting a material that could be able to do this. Realizing a trade-off based on raising both V OC and J SC is essential for maximizing the efficiency of hybrid solar cells. The bandgap of the material must be kept as small as possible in order to make use of more of the solar spectrum and guarantee excellent photon absorption and high J SC . However, as the V OC is reliant on the heterojunction’s diagonal bandgap, the V OC will be maximized if the acceptor material has a high

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lying conduction band edge. The ground state offset with regard to the donor material is another electronic criterion for an inorganic acceptor material. In order to allow excitonic dissociation when light is absorbed in the acceptor material, this offset level is necessary. In a hybrid device, this component is more crucial since the inorganic acceptor material will provide greater beneficial absorption. These three elements have been used by Xiang et al. [52] to depict the optimal electronic specifications of an inorganic acceptor for hybrid solar cells. The 1.5 eV bandgap for the inorganic acceptor reflects a trade-off between energy offset and absorption for high V OC . The HOMO level offset is adjusted at 0.3 eV in order to maximize V OC while still providing enough energy to overcome the excitonic binding energy. InSb quantum dots or quantum wires may have a highly desired electronic structure for hybrid solar cells employing P3HT as the donor material, according to a review of a range of narrow bandgap III-V semiconductors. It’s crucial to remember that this prediction only applies to a device that employs P3HT as the electron donor. When paired with a different material, band offset locations will change. Although this viewpoint offers a thorough understanding of the necessary electrical characteristics of a possible material, there are still several additional physical factors that may necessitate making a material compromise when choosing the acceptor material. The capacity to create a balance between the electron and hole mobilities, the quantity and cost of the inorganic material, the solubility in a common solvent with the donor material, the success of the nanomorphology of the donor/ acceptor phases, and others are some physical factors.

6.5.2 Significant Material Groups Several materials have been researched up to this date that serve as an electron acceptor in hybrid solar cells. CdSe, CdS, CdTe, Si, PbS, TiO2 , ZnO, and ZnS are some of these materials. Each of these materials has distinct electrical properties. Electronic heterojunction is produced when an interface between two different semiconductors forms. The three material characteristics of this heterojunction are: (i) bandgap, (ii) electron affinity, and (iii) ionization potential. Three different forms of heterojunctions may form: straddling gap (type I), staggered gap (type II), and broken gap (type III), depending on the characteristics of the materials involved. A type II heterojunction with cascading energy levels is required for a solar device to function properly. This is necessary to enable the transportation of holes to the anode and electrons to the cathode. When paired with polymeric donors, all inorganic acceptor materials often exhibit this electrical property. This implies that any material may be used to create a functioning photovoltaic device, but that the performance of the device is greatly influenced by the bandgap, electron affinity, and ionization energy of the material. The performance of the device will also be impacted by other physical factors, such as solubility in a common solvent. The four main material groups—cadmium compounds, silicon, metal oxide nanoparticles, and narrow bandgap nanoparticles—will be examined in this section.

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Cadmium Compounds

Cadmium sulfide (CdS) quantum dots combined with P3HT nano wires have produced high efficiencies in an OIH solar cell. Highly efficient P3HT:CdS hybrid solar cells were fabricated by Ren et al. [2] by combining a solvent-assisted chemical grafting and ligand exchange technique. P3HT nanowires and CdS were dissolved in various solvents for the chemical grafting procedure. Then, the two solutions were combined. The grafting procedure creates a hybrid film with enhanced contact between the donor and acceptor phases and a larger interfacial area. The CdS quantum dots underwent ligand exchange with ethanedithiol. As a result, the charge carrier transport in CdS was enhanced, and the interparticle distance was reduced. The high-lying CB edge of the CdS quantum dots allowed this device to reach a very high V OC of 1.1 V. This simple technology may be used as a general strategy to boost the performance of OIH solar cells because it permits precise regulation of the nanomorphology and increased interaction between the organic and inorganic material. In terms of visible-light absorption, adequate energy levels when linked with most conjugated polymers, and well-established production processes, cadmium selenide (CdSe) is an attractive potential material for OIH solar cells. It has been reported that the fullerene’s contribution to OPV’s photocurrent and absorption is more than the earlier expected [5]. Increased supplementary absorption may be achieved, however, by using an inorganic substance. The CdSe nanoparticles of the hybrid film absorb light in a helpful way. According to Dayal et al. [6], CdSe is responsible for 34% of the absorption in a PCPDTBT:CdSe system. This demonstrates that the inorganic acceptor material’s absorption contribution to the creation of photocurrent has a favorable effect on the device’s EQE profile. Charge separation and transport are facilitated by the electronic structure of CdSe, which forms a type II heterojunction with most conjugated polymers at the suitable energy level offsets. Bulk CdSe has a bandgap of around 1.74 eV, although it may be changed by adjusting the nanoparticle’s size [53]. CdSe nanoparticles with a mean diameter of 4.7 nm had a bandgap of 2 eV, according to research by Zhou et al. [4]. This is a more-than-ideal figure, but it still permits absorption up to 650 nm. Due to the comparatively high value of the conduction band edge, this material’s theoretical V OC has the potential to be fairly high. So far, the V OC values recorded for this acceptor material have been encouraging, with the highest value being 0.95 V [54]. This material’s wellestablished synthesis processes enable the creation of nanoparticles with complex morphologies. CdSe tetrapods distributed in a polymer matrix were synthesized by Dayal et al. [6]. These tetrapods have an average arm diameter of 5 nm and an arm length of 30–50 nm. Due to these dimensions, the nanomorphological structure may be enhanced, which in turn improves exciton dissociation and conductivity. Because of this structured arrangement, the electrical properties are enhanced. Although its toxicity is a significant issue, this material might serve as a model system through which hybrid solar cells are better understood.

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Silicon

The abundance, non-toxicity, and strong UV absorption of silicon make it a favorable material for hybrid solar cells. Moreover, the superior dielectric constant of silicon, in contrast to PCBM, inhibits retrograde transfer and enhances transportation from the interface. When combined with P3HT, silicon creates a robust type II heterojunction. The bandgap value of bulk silicon is 1.12 eV. The bandgap of silicon nanocrystals is increased due to quantum confinement. This phenomenon enhances the electronic properties of the material, rendering it more attractive. According to Liu et al. [55], the bandgap of silicon nanocrystals, which had an average size ranging from 3 to 5 nm, was estimated to be around 1.5 eV. This was determined by observing a variation in photoluminescence measurements in comparison to bulk silicon. Si nanocrystals exhibit an enhanced absorption profile owing to their relatively small bandgap. The enhancement observed is primarily confined to the UV region, owing to the relatively low absorption coefficient in comparison to P3HT. The electronic structure being favorable facilitates the attainment of a relatively high V OC value, as the upward shift of the conduction band edge resulting from quantum confinement is enabled. Recorded V OC values of up to 0.8 V surpassed those of the P3HT:PCBM blend. It was demonstrated that modifying the dimensions of silicon nanocrystals resulted in an alteration of the band structure, consequently affecting the overall performance. The research demonstrated that nanocrystals possessing diminutive physical dimensions, ranging from 3 to 5 nm, exhibited superior PCE owing to the enhancement of both V OC and J SC . An enhanced PCE of 1.47% was attained through the optimization of an annealing process. The optimal annealing parameters, determined to be 150 °C for a duration of 120 min, differ from the established parameters for P3HT:PCBM blends. Annealing leads to an augmentation in both J SC and fill factor. The observed phenomenon can be attributed to an augmentation in hole mobility, resulting in a device mobility that is more evenly distributed. A decrease in space charge accumulation and recombination occurs. The performance of these hybrid devices remains considerably low. The primary cause of this phenomenon can be attributed to the non-uniform nanomorphology, which arises from the agglomeration of silicon nanoparticles. Silicon nanowires represent a viable substitute for Si nanocrystals. The primary benefit of this methodology lies in the fact that the organized configuration of the nanowires can potentially enhance the efficacy of charge transportation. The formation of a silicon nanowire array through a wet etching method has been demonstrated by Huang et al. [56]. The present methodology is designed to address certain challenges commonly encountered in alternative fabrication methodologies, including CVD and laser ablation. The process of integrating Si nanowires onto a silicon wafer involved subjecting a P3HT:PCBM blend film to a temperature of 160 °C and exerting a downward force. Subsequently, a lateral force was employed to detach the silicon wafer from the P3HT:PCBM film. The findings indicate that the Si nanowires were responsible for absorption in the near-infrared (NIR) and visible regions. The widening of the spectral window of the device in question can be attributed to the comparatively reduced bandgap of silicon, as compared to the control P3HT:PCBM device. The enhanced

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mobility of charge carriers in silicon nanowires represents a supplementary benefit. The findings indicate a noteworthy augmentation in J SC as a result of the incorporation of Si nanowires which can be attributed to the combined effects of improved charge transport and enhanced absorption spectra. The preliminary findings indicate potential for the feasibility of silicon in hybrid photovoltaic cells, primarily owing to its abundance in nature and lack of toxicity.

6.5.2.3

Metal Oxide Nanoparticles

Hybrid solar cells have also investigated wide bandgap oxide semiconductors as inorganic acceptors. TiO2 , SnO2 , CeO2 , and ZnO are just few of the materials that have been studied. These semiconductors’ primary benefit lies in their potential to adopt oxide nanostructures with a vertical orientation. Physical dimensions of such ordered structures may be optimized to provide effective conduction routes and a large D-A interfacial area [57]. As a result, electron mobility is improved and a high dissociation yield is possible. TiO2 , with its large surface area semiconductor, is now the most thoroughly studied of these materials. This is because it has been widely employed in dye sensitized solar cells. However, TiO2 is not a good inorganic acceptor material because its electronic structure is undesirable. Oxide compounds are distinguished by a large bandgap, which results in a very little absorption contribution in the visible range. The highest achievable V OC is comparable to that of PCBM despite the huge bandgap because of the location of the conduction band edge, which creates a type II heterojunction with the suitable polymeric materials. Hybrid solar cells that make use of oxide materials often do not have particularly high V OC levels reported. This concept was proved by Mor et al. [58] for P3HT:PCBMinfiltrated TiO2 nanotubes. Both the P3HT:PCBM and P3HT:TiO2 interfaces are able to segregate charges, making this structure a double heterojunction. Effective charge production and well-organized geometry are credited with the stellar performance of the device. Electrospinning has been used to create vertical TiO2 nanowires in the anatase phase by Krishnamoorthy et al. [59]. Better electron transport and less charge carrier recombination make anatase phase TiO2 preferable to rutile phase TiO2 for optoelectronic applications. Because of the difficulty in tuning the crucial parameters of nanowire like diameter, height, and packing density, it has been argued that present synthetic techniques are insufficient for generating highly ordered nanowires. An improvement in photovoltaics might be made by the optimization of this electrospinning technology to create high throughput anatase nanowires with acceptable physical dimensions. ZnO has emerged as a material of attention in recent years. This is due to the fact that its electronic structure is very similar to that of TiO2 , but with some significant benefits. To begin, it typically exhibits greater electron mobility than TiO2 . In addition, there are several methods for synthesizing it. This makes it a promising material for mass-producing solar cells at cheap cost. The PCE of a photovoltaic device is dependent on numerous criteria in addition to the ordered structure of the active layer,

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making it very challenging to decide which synthesis process is best suited for solar applications despite the wide variety of methods available. If perfectly aligned nanostructures are desired, the hydrothermal growth process is the best suitable method. The existing methods of synthesizing ZnO are inadequate because of issues with repeatability and the lack of capacity to create structures with the necessary physical dimensions to optimize photovoltaic performance. A dense array of vertically oriented crystals is desirable because of the short excitonic diffusion length for polymeric donors, but no one has yet succeeded in physically replicating such a system, which would result in devices with large PCEs. ZnO’s wettability and stability are issues apart from their formation in the physical structure. Hole conducting polymers can penetrate the oxide matrix, which is what we call “wettability.” It is challenging to obtain complete polymer penetration in a highly packed structure with a large surface area, which is necessary for optimum photovoltaic performance. Another issue brought on by polymer infiltration is that the material arrangement is subpar in comparison to the polymer chain packing in a bulk active layer. As a result, the hole mobility in the flat BHJ device is reduced compared to that of polymers. In order to realize the performance gains associated with highly ordered bulk heterojunctions, it is essential to have a firm grasp on the polymer phase orientation inside the oxide nanoarray. Foong et al. [60] looked at the orientation and physical characteristics of regioregular P3HT that was contained in a TiO2 nanoarray. They hypothesized that P3HT would not be able to permeate the dense nanoarrays by gravity and capillary forces alone. Infiltration was achieved by annealing the system at 250 °C in a vacuum of 20 mTorr. The complete penetration of the polymer was demonstrated by SEM cross-sectional pictures. Both non-confined and nanoconfined P3HT showed evidence of edge-on orientation in XRD examination, while the peak intensity for nanoconfined P3HT was much lower. This is indicative of a more crystalline structure for the non-confined P3HT and a more disordered structure for the nanoconfined P3HT. Device performance was shown to be enhanced by TiO2 nanotube hybrids as compared to the bilayer benchmark device when the nanostructures were arranged in a vertical orientation, however this enhancement was only marginally connected with the increase in interfacial area. The orientation of the polymer, which controls the system’s hole mobility, has been hypothesized to affect the device’s ability to successfully carry charges. XRD analysis revealed that P3HT in nanoconfined systems was edge-on. This polymer orientation is thought to be the cause of the slight enhancement since it is detrimental to charge transfer. The potential of well-aligned bulk heterojunction devices may be unlocked by replacing P3HT with a polymer that, when nanoconfined, provides improved orientation and hole mobility. ZnO-based hybrid solar cells have been called into doubt for their durability [10]. The oxygen release mechanism of an oxide compound is harmful to the polymer material. As a result, the claimed performance of the device may drop precipitously. It has also been shown that exposure to ultraviolet light may significantly decrease the device’s output. An optical layer to reduce UV light in the active layer has been proposed by Shao et al. [61] as a way to increase the stability of a MEH-PPV:ZnO system. To do this, a layer of ZnO of different thicknesses was deposited between

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the active layer and the aluminum cathode. This change appears to make the device more stable when illuminated.

6.5.2.4

Narrow Bandgap Nanoparticles

The primary objective of utilizing materials with intrinsically low bandgaps is to broaden the spectral range of solar cells to encompass longer wavelengths. The production of photocurrent within the NIR spectrum is advantageous due to its potential to mitigate the thermal impact associated with these wavelengths, while concurrently augmenting the J SC . The characteristic of inorganic nanoparticles with low bandgap holds significant potential when combined with polymers that exhibit absorption restricted to the visible region, such as P3HT. The utilization of Pb-based nanoparticles has gained popularity in research due to their capacity to absorb within the NIR spectrum. A low bandgap value of 0.41 eV has been reported for bulk PbS in the literature [62]. The valence band alignment of PbS presents a hindrance to the effective transport of holes to the polymer, thereby impeding the dissociation of excitons generated in the quantum dot. The resulting band structure can be classified as a type II heterojunction with weak characteristics. Moreover, the magnitude of the bandgap indicates that the highest achievable V OC value is relatively limited. The study conducted by Guchhait et al. [8] involved the incorporation of TiO2 nanorods into P3HT:PbS solar cells. The valence band edge position of TiO2 confers upon it a high degree of electron accepting capability. Devices featuring TiO2 were found to exhibit a significant rise in short-circuit current. The conversion of absorption in the NIR to photocurrent is not possible under energetically unfavorable conditions in the absence of TiO2 nanorods. However, the inclusion of TiO2 facilitates the dissociation of excitons, thereby enhancing the performance of the device. The study exhibited that altering the physical dimensions of the quantum dot enables the manipulation of its electronic structure. Observation of differences in photocurrent is possible by altering the diameter of the nanoparticle. A discernible optimal diameter for quantum dots has been established. The aforementioned phenomenon can be explained by two factors: firstly, the decrease in absorption observed for quantum dots with smaller diameters, and secondly, the relative position of the conduction band with respect to the conduction band edge of TiO2 . Attaining a type II heterojunction through the utilization of low bandgap inorganic electron acceptor is typically a challenging endeavor. Attaining a PCE may prove challenging even if the aforementioned goal is accomplished, due to the absorption that occurs in the acceptor. This absorption results in notable thermalization losses for photons within the visible spectrum. The occurrence of multiple exciton generation (MEG) is a potential avenue for improving the efficiency of hybrid solar cells that utilize inorganic acceptor nanoparticles with low bandgaps [63]. MEG is a phenomenon that involves the production of multiple electron–hole pairs through the absorption of a single, high-energy photon. The process of MEG in bulk semiconductors is notably inefficient due to the high threshold photon energy required, which is attributed to the need for crystal

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momentum conservation. Furthermore, it is imperative that the rate of production is comparable to the rate of dissipation through electron–phonon interactions [64]. The photon energies necessary for MEG exhibit limited applicability in the context of solar energy conversion. The efficacy of MEG in nanoparticles is significantly amplified as a result of the relaxation of the momentum conservation prerequisite. The MEG’s lower threshold requirement implies that the said phenomenon could be detected at photon energies that hold significance for the conversion of solar energy. The utilization of transient absorption spectroscopy by Beard et al. [65] enabled the investigation of MEG in silicon nanocrystals. The threshold energy for MEG was determined to be 2.4 ± 0.1 E g for silicon nanocrystals with a diameter of 9.5 nm. At a photon energy of 3.4 E g , a quantum yield of 2.6 has been observed. The pragmatic implementation of this phenomenon has the potential to result in photocurrent quantum yields exceeding 100%. Photocurrent improvements were reported by Semonin et al. [66] in solar cells containing lead selenide (PbSe) quantum dots. The observed improvements, as indicated by EQE analyses, were ascribed to the MEG phenomenon. The study reports that PbSe quantum dot solar cells attained peak EQE values of 114%, thus establishing the occurrence of MEG in quantum dots and its potential to result in EQE values exceeding 100%. The present report exhibits significant potential for improving the efficiency of hybrid solar cells through the utilization of low bandgap nanoparticles.

6.6 Performance Restrictions Using inorganic acceptor materials should give a variety of advantages, including improved absorption, conductivity, and device design, as was mentioned before. Therefore, why are the PCE values for state-of-the-art hybrid solar cells so much lower than those for advanced organic photovoltaic cells? To this point, the development of this technology has been delayed by a number of challenges, the majority of which are largely related to an increase in the density of trap states, issues with nanoparticle surface chemistry, and inadequate control over D-A nanomorphology. This section summarizes the chief restrictions exhibited by the utilization of inorganic acceptor materials, and features potential approaches for improvement.

6.6.1 Nanoparticle Surface Chemistry The electron mobility of inorganic materials inside a polymer matrix is often rather low, despite the fact that inorganic semiconductors have fundamentally superior conductivity. Size and shape may be manipulated, and particle stability can be induced in inorganic nanoparticles by bonding an organic ligand material to their surface during production [67]. The charge transfer at the D-A interface and the mobility of electrons in the acceptor phase are both slowed by the presence of

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this organic ligand, which is electrically insulating. Reduced device efficiency is a direct result of increased recombination, which decreases collected photocurrent. In order to reestablish the favorable charge carrier properties of the inorganic material, many efforts to improve the efficiency of hybrid solar cells have focused on eliminating this insulating layer. A common technique is achieved by exchanging the organic insulating ligand layer produced during synthesis for the shorter, more conductive pyridine molecules [16]. This is believed to boost efficiency by facilitating charge transfer and transmission. After the nanocrystals have undergone a ligand exchange, however, it might be challenging to create a solvent combination that keeps the polymer and nanocrystals stable. Further, the exposure of dangling bonds in the nanocrystals brought about by the exchange process might create additional unwanted trap sites, leading to increased recombination sites. There seems to be a need for some kind of surface modification; however, ligand exchange may not be the ideal way to go about it. To mitigate the effect of the insulating ligand, a unique strategy was presented. Hexanoic acid washing was used to treat CdSe nanoparticles instead of ligand exchange [68]. After reacting with hexanoic acid, a salt is formed, which is thought to remove the organic ligand from the nanoparticles’ surface. The immobilized ligand on the particle surface is replaced with an easily removable salt. The size of the nanoparticle and the space between them were both shrunk as a result of this process. This is believed to be the cause of the enhanced charge transfer and, by extension, the increased efficiency of the device. This fast and easy method may be used to other types of nanoparticle formations, such nanorods. This hexanoic acid washing process was used to modify the surface of CdSe nanorods. Device PCEs greater than 3% were achieved by combining PCPDTBT with large sized nanoparticles that had been treated with hexanoic acid [69]. The enhancement of charge transfer across the inorganic network was largely responsible for this performance. Instead of using ligands for passivation, inorganic films may be grown inside a polymer by in situ growth. Reynolds et al. [70] utilized in situ grown CdS contained inside precoated P3HT films to fabricate hybrid devices. This work provides a thorough analysis of the differences and similarities of hybrid systems that include P3HT and either (i) in situ grown CdS films or (ii) CdS quantum dots synthesized with different capping ligands. Improved charge transmission can be achieved thanks to the in situ generated linked network of nanoparticles. The polaron production and longevity of in situ grown CdS blends are much greater than those of oleic acid and hexyl amine capped quantum dots blends. Adding more CdS to the in situ grown mix also increases the stability of the produced charges. The device efficiency of in situ grown CdS blends was significantly improved due to a large rise in J SC and a concurrent improvement in V OC . This was ascribed to enhanced nanomorphological organization, which increased charge creation and charge collection at the electrode, and decreased charge carrier recombination. According to the results, nanomorphological limitations play a vital role in the generation of long-lived charges.

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6.6.2 Nanomorphology Due to its influence on the excitonic dissociation efficiency, the nanomorphological arrangement of donor and acceptor atoms inside the active layer is crucial in defining device PCE. This is critical due to the poor charge carrier mobility and short excitonic diffusion length of organic materials. Excitonic dissociation, which is associated with the D-A interfacial area, and charge carrier conductivity and collection, which is associated with the creation of percolated channels, are at odds with one another in a bulk heterojunction design. It seems that hybrid solar cells are often more difficult to manage in terms of morphology than OPV devices. Possible causes include the need for a more complicated solvent and the aggregation of inorganic nanoparticles into larger aggregates. The solvent utilized for hybrid solar cells is crucial since the nanocrystals and polymers have differing solubilities. In order to determine the best solvent for a MEH-PPV:CdS combination, Wang et al. [18] looked into using both pyridine and chlorobenzene. It was shown that the solvents had a significant effect on the dispersion of CdS nanoparticles in PPV. CdS nanoparticles are far less likely to aggregate and are distributed more uniformly throughout the film cast using pyridine solvent. There is a causal relationship between this and enhanced J SC and photoresponse. This suggests that the solvent makes a significant difference in how the nanocrystals are arranged inside the polymer. Due to the different solubilities of the donor and acceptor materials, the solvent utilized may not be a simple solvent but rather a complex combination. For hybrid solar cells, the solvent choice is even more critical, and there is still a lot of room for improvement in the optimization of new material combinations. The nanomorphology of the device might be enhanced by an annealing procedure performed after fabrication. Heat treatments may increase hole mobility in the donor material of P3HT by inducing crystallinity in the polymer and consequently creating more efficient conducting routes. This information can be used to hybrid solar cells to some extent. By manipulating the thermal annealing temperature, Liu et al. [55] improved the efficiency of P3HT:Si hybrid solar cells. Optimal conditions will differ from those of the frequently used P3HT:PCBM, hence effort toward optimizing such parameters will become vital as other material combinations are studied. The impact of thermal annealing on a hybrid device’s active layer was also studied by Kuo et al. [71]. The PDTTTPD polymer was combined with CdSe tetrapods to create a new compound. A considerable improvement in electrical performance was seen after thermal annealing at 130 °C for 20 min. The pyridine ligands on the surface of the CdSe tetrapods were assumed to have been removed during the annealing process, leading to an improvement in performance. This procedure was seen to have significant effects on the nanomorphology of the film. CdSe packing density was improved by removing surface pyridine, which decreased the distance between particles. It was hypothesized that enhanced electrical performance would result from the aggregation of CdSe due to increased electron transport. Growing vertically well-aligned nanostructures is another effective method for manipulating nanoscale features. The physical parameters of such structures may

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be controlled to facilitate efficient electron transit and a large D-A interfacial area. The II-VI semiconductor ZnO, which can be generated by a number of synthetic processes and has high electron mobilities, has lately become particularly interesting as an inorganic material to be employed as a vertically aligned structure. However, significant PCEs have not been seen with such a structure, despite the fact that it has the potential to lead to extremely efficient bulk heterojunctions with near optimum designs. The capacity to correctly infiltrate the polymer into the structure, together with the development of a high surface area, seems to be the primary constraint of these structures.

6.7 Applications Organic–inorganic hybrid solar cells combine organic materials, often polymers, with inorganic materials like semiconducting nanoparticles to create solar cells with unique properties and advantages. These hybrid solar cells aim to harness the benefits of both organic and inorganic materials to improve efficiency, stability, and costeffectiveness. Here are some applications of organic–inorganic hybrid solar cells along with examples: • Flexible and Lightweight Solar Panels: Hybrid solar cells can be fabricated on flexible substrates, allowing for the creation of lightweight and bendable solar panels. These panels are suitable for various applications where traditional rigid solar panels are not feasible. Example: Using a blend of organic polymers and quantum dots to create flexible solar panels that can be integrated into wearable devices, clothing, or rollable solar blankets for portable power generation. • Building-Integrated Photovoltaics (BIPV): Organic–inorganic hybrid solar cells can be integrated into building materials like windows, facades, and roofing materials, enabling energy-efficient buildings that generate electricity. Example: Incorporating transparent hybrid solar cells into windows or glass facades to generate electricity while maintaining transparency for natural lighting. • Low-Light and Indoor Applications: Hybrid solar cells often exhibit improved performance under low-light conditions and indoor lighting, making them suitable for applications in indoor environments or under cloudy skies. Example: Powering wireless sensors, IoT devices, or remote controls that operate indoors using ambient light. • Portable and Off-Grid Power Generation: The flexibility and lightweight nature of hybrid solar cells make them suitable for portable and off-grid power generation, such as camping, remote areas, and emergency situations. Example: Developing portable solar chargers that fold or roll up for easy transport and can charge electronic devices while on the go.

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• Hybrid Solar-Powered Vehicles: Hybrid solar cells can be integrated into the surfaces of vehicles, providing an additional power source to recharge batteries and improve energy efficiency. Example: Adding solar cells to the roof or body of electric vehicles (EVs) to extend their range and reduce dependency on grid charging. • Energy Harvesting for Wearable Electronics: Hybrid solar cells can be integrated into wearable devices, enabling them to harvest energy from ambient light and extend the battery life of the devices. Example: Creating smartwatches, fitness trackers, or health monitoring devices that use hybrid solar cells to recharge their batteries while worn. • Remote Sensing and Monitoring: Hybrid solar cells can power remote sensors and monitoring systems in locations where traditional power sources are unavailable or impractical. Example: Deploying environmental monitoring stations, wildlife tracking devices, or agricultural sensors in remote areas using solar energy for power. • Educational and DIY Kits: Hybrid solar cells can be used in educational kits and DIY projects to teach about renewable energy, electronics, and sustainable technologies. Example: Designing solar-powered educational kits that allow students to build their own solar-powered gadgets and learn about photovoltaics. These applications showcase the versatility of organic–inorganic hybrid solar cells in various sectors, ranging from energy production to technology integration and sustainability initiatives.

6.8 Summary This chapter gives an overview of the third generation of solar cells, i.e., organic– inorganic hybrid solar cells. Over the last ten years, there has been a lot of research done on hybrid solar cells made up of both organic and inorganic semiconductors. Bulk heterojunction hybrid configuration was the major approach used to create hybrid solar cells. Although the efficiencies of OIH solar cells are still inferior to that of first- and second-generation solar cells, widespread study has helped scientists understand their limits and how to get around them. This is in fact a significant step since both the dilemma and the solution are well defined. The commercialization of these solar cells is the primary objective. In addition to efficiency, lifespan stability and manufacturing costs are also important and must be considered equally. In less than 25 years, the modest levels of efficiency have been raised to ambitious levels, and we are now discussing a potential next step in commercialization. The field is seeing rapid growth and high levels of interest. On the other hand, there are many options available in the parameter space, which is straightforward and has a low processing cost.

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6.9 Important Timelines • 2002—Huynh et al. fabricated CdSe nanorod/P3HT hybrid solar cells [15]. • 2005—Watt et al. fabricated PbS nanocrystals/MEH-PPV hybrid solar cells with 0.7% efficiency [72]. • 2005—OIH solar cells with vertically aligned CdTe nanorods and P3OTconjugated polymer were constructed leading to > 1% PCE [73]. • 2005—Beek et al. fabricated 1.6% efficient ZnO nanoparticles/MDMO-PPV hybrid solar cells [74]. • 2005—Improved efficiency of 2.4% was attained when CdSe tetrapod structures were incorporated with new donor OC1 C10 -PPV [75]. • 2006—2.6% PCE was achieved by Wang et al. via blending red polyfluorene copolymer APFO-3 with CdSe-branched nanoparticles [54]. • 2008—TiO2 nanorod/P3HT hybrid solar cells with 0.98% efficiency were fabricated [76]. • 2009—Kwak et al. incorporated acceptor-CdS nanowires array with donor P3HT leading to 1.73% PCE [77]. • 2009—Olson et al. achieved 1.8% PCE using CdSe quantum dots in hybrid device [78]. • 2009—1.15% efficiency was attained in OIH solar cell using Si quantum dots [79]. • 2009—Efficiency increased to 1.93% with c–Si nanowires arrays [56]. • 2009—A new hybrid bulk heterojunction P3HT/ZnS-based OIH device was fabricated yielding 1.26% PCE [80]. • 2009—2% efficient cells were fabricated using ZnO acceptor by Oosterhout et al. [81]. • 2009—P3HT/TiO2 nanorod BHJ solar cells were fabricated with 2.2% efficiency [82]. • 2010—OIH solar cells with CdSe tetrapod and low bandgap PCPDTBT polymer were constructed by Dayal et al. leading to 3.19% PCE [6]. • 2010—Outstanding efficiency of 5.09% was attained when Si nanowire arrays were incorporated with donor PEDOT:PSS [83]. • 2011—4.1% PCE was achieved with CdS quantum dots/P3HT hybrid cells [2]. • 2011—OIH cells were fabricated with acceptor CdSe nanorods:quantum dots and donor PCPDTBT yielding > 3% efficiency [69]. • 2011—3.2% efficient cells were fabricated using CdTe tetrapod acceptor and new donor PSBTBT-NH2 [84]. • 2011—There was a breakthrough when 10.3% PCE was accomplished with the utilization of Si nanorods along with donor Spiro-OMeTAD [85]. • 2012—Syu et al. incorporated acceptor-Si nanowires array with donor PEDOT:PSS leading to 8.4% PCE [86]. • 2013—Si/PEDOT:PSS hybrid solar cells were fabricated with 11.48% PCE [87]. • 2014—2D CoS nanosheets were utilized for high-performance OIH cells yielding efficiency of 11.2% [88].

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• 2016—Arici et al. fabricated SWNT/Si hybrid solar cells with 17% PCE [89].

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Chapter 7

Solar Cell Modeling Parameters

7.1 Introduction The extraction of solar cell modeling parameters is an essential step in the development of accurate solar cell models. Accurate solar cell models are crucial for optimizing the design of solar cells and improving their efficiency, leading to more widespread adoption of solar energy as a clean and sustainable source of power [1]. A solar cell is a device that converts sunlight directly into electrical energy. The efficiency of a solar cell is determined by several factors, including the materials used in the cell, the design of the cell, and the operating conditions of the cell. To accurately model the performance of a solar cell, one of the key aspects is to determine various parameters that govern the cell’s behavior, i.e., short-circuit current, fill factor, open-circuit voltage, and dark current [2–4]. The process of parameter extraction involves measuring electrical characteristics of the solar cell and using this data to fit a mathematical model to the experimental data. The mathematical model then helps in predicting the behavior of the solar cell under different scenarios. Parameter extraction is a complex process that requires careful experimental design, data analysis, and model fitting. The accuracy of the extracted parameters is critical in developing an accurate model for the solar cell. There are several techniques for parameter extraction, and each technique has its advantages and disadvantages [5]. The choice of the technique depends on the specific requirements of the model and the experimental setup. One of the most common techniques for parameter extraction is the use of a solar simulator. A solar simulator is an instrument that mimics the properties of sunlight, allowing the electrical characteristics of the solar cell to be measured under controlled conditions. The solar simulator can be used to vary the light intensity and the temperature of the solar cell, allowing the electrical characteristics to be measured under a range of operating conditions [6]. Another technique for parameter extraction is the use of a dark current–voltage (I–V) curve which is obtained by measuring the relationship between current and

© The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 S. Arya and P. Mahajan, Solar Cells, https://doi.org/10.1007/978-981-99-7333-0_7

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voltage of the solar cell under dark conditions, i.e., when there is no light falling on the cell. The dark current–voltage curve is used to determine the ideality factor and reverse saturation current of the solar cell, which are critical parameters for accurately modeling the behavior of the cell. In addition to the dark current–voltage curve, other techniques can be used to extract solar cell modeling parameters. For example, the open-circuit voltage and short-circuit current can be measured directly, and the fill factor can be determined from the I–V curve. The parameters obtained from these measurements can then be used to fit a mathematical model to the experimental data. The accuracy of the parameter extraction process depends on several factors, including the accuracy of the measurement equipment, the precision of the experimental setup, and the quality of the data analysis. Advances in measurement equipment, data analysis tools, and mathematical modeling have made parameter extraction more accurate and efficient than ever before [7–10]. In recent years, there has been a growing interest in developing accurate models for organic solar cells. Organic solar cells are a promising alternative to traditional inorganic solar cells, as they offer several advantages, including low-cost manufacturing, lightweight, and flexibility. The development of accurate models for organic solar cells requires accurate parameter extraction techniques that take into account the unique properties of these cells. One of the challenges in parameter extraction for organic solar cells is the measurement of the exciton diffusion length. Excitons are electron–hole pairs that are generated when light is absorbed by the organic material in the solar cell. The excitons can travel a certain distance before they recombine, and this distance is known as the exciton diffusion length. The exciton diffusion length is a critical parameter for accurately modeling the behavior of organic solar cells, and several techniques have been developed to measure this parameter [11]. Measuring the exciton diffusion length is an essential step in the extraction of solar cell modeling parameters, as it provides critical information about the transport properties of charge carriers in the material. There are various techniques available to measure the exciton diffusion length, including: Time-Resolved Photoluminescence (TRPL) Spectroscopy: TRPL spectroscopy is a technique used to evaluate the excited-state dynamics in a material. In TRPL spectroscopy, a sample is excited with a short pulse of light, and the subsequent emission of light is monitored over time. By analyzing the decay of the emission, it is possible to determine the lifetime of the excited state and the diffusion length of the exciton [12]. The diffusion length can be determined by analyzing the decay of the emission signal as a function of the distance from the excitation spot. The decay of the emission signal is typically described by an exponential function, and the decay rate depends on the diffusion length of the exciton. By fitting the emission decay curve to a mathematical model, the diffusion length of the exciton can be determined. Transient Absorption (TA) Spectroscopy: TA spectroscopy is another nondestructive technique that provides information about the excited-state dynamics in a material. In TA spectroscopy, a sample is excited with a short pulse of light, and the subsequent absorption of light is monitored over time. By analyzing the change

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in absorption over time, it is possible to determine the lifetime of the excited state and the diffusion length of the exciton [13]. The diffusion length can be determined by analyzing the change in absorption as a function of the distance from the excitation spot. The change in absorption is typically described by an exponential function, and the decay rate depends on the diffusion length of the exciton. By fitting the change in absorption to a mathematical model, the diffusion length of the exciton can be determined. Surface Photovoltage (SPV) Spectroscopy: SPV spectroscopy is a non-destructive technique that measures the potential difference between the front and back surfaces of a solar cell under illumination. By analyzing the SPV signal as a function of the excitation spot position, it is possible to determine the diffusion length of the exciton. The SPV signal is typically described by an exponential function, and the decay rate depends on the diffusion length of the exciton. By fitting the SPV signal to a mathematical model, the diffusion length of the exciton can be determined [14]. Time-of-Flight (TOF) Mobility Measurements: TOF mobility measurements involve measuring the time it takes for charge carriers to travel a known distance in a material. By measuring the mobility of the charge carriers and their diffusion length, it is possible to calculate the exciton diffusion length [15]. Each of the above-mentioned technique has its advantages and disadvantages, and the choice of technique depends on the specific requirements of the experiment. Thus, the extraction of solar cell modeling parameters is essential in the development and optimization of solar cell technologies. It involves characterizing the properties of the materials used in the solar cell, such as their optical, electronic, and morphological properties, and using this information to create a mathematical model that can predict the device’s performance. The modeling parameters extracted from these measurements are critical in designing and optimizing solar cell devices. They provide critical information about the device’s efficiency, stability, and reliability, which are essential in developing commercial solar cell products. Additionally, the modeling parameters help researchers to understand the underlying physical and chemical processes that govern the operation of solar cells. By understanding the fundamental mechanisms involved in the conversion of light energy into electrical energy, researchers can develop new materials and architectures to improve the efficiency and stability of solar cells. Moreover, accurate modeling parameters can also help in the development of standards and protocols for the characterization and testing of solar cells. This can ensure that solar cell technologies are consistently evaluated and compared, which can lead to more reliable and efficient devices [16].

7.2 Equivalent Circuit Models Equivalent circuit models are mathematical models that describe the electrical behavior of a solar cell as a combination of idealized electrical components, such as resistors, capacitors, and diodes. The equivalent circuit model provides a simplified

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representation of the solar cell’s electrical behavior, which can be used to extract key modeling parameters, such as the device’s series resistance, shunt resistance, and recombination parameters. Equivalent circuit models are based on the principle of conservation of energy and charge. The model includes a set of electrical components, such as resistors and capacitors, which represent the various loss mechanisms in the solar cell. The components are connected in a series or parallel configuration, which reflects the device’s electrical behavior [17–20]. The equivalent circuit model typically consists of a combination of series and parallel resistances and diodes. The most commonly used equivalent circuit models for solar cells are the one-diode model and the two-diode model. The one-diode model assumes solar cell as a single diode, which represents the recombination losses in the device. The one-diode model includes a series resistance, which represents the internal resistance of the device. This model is simple and easy to implement, but it may not accurately capture all the loss mechanisms in the device. The twodiode model includes an additional diode, which represents the parasitic losses in the device. The two-diode model includes both a series resistance and a shunt resistance, which represent the internal and external resistances of the device, respectively. This model is more complex but can more accurately capture the loss mechanisms in the device in comparison with one-diode model. Other equivalent circuit models, such as the Lambert-Wolf model and the modified Lambert-Wolf model, have also been developed for specific types of solar cells, such as dye-sensitized solar cells and organic solar cells [21]. In short, equivalent circuit models are commonly used in the extraction of solar cell modeling parameters. These models provide a simplified representation of the electrical behavior of the device, which can be used to extract key modeling parameters, such as the device’s series resistance, shunt resistance, and recombination parameters. The one and two-diode models are the most commonly used models, but other models have been developed for specific types of solar cells.

7.2.1 Single-Diode Model The most commonly used equivalent circuit model for the extraction of solar cell modeling parameters is called single-diode model (SDM). The SDM assumes solar cell as a single diode, which represents the recombination losses in the device. The SDM includes a series resistance, which represents the internal resistance of the device. The SDM is simple and easy to implement, making it a popular choice for modeling and simulation of solar cell devices. The SDM is based on the ShockleyQueisser theory, which assumes that the solar cell can be modeled as a p–n junction diode and describes the maximum theoretical efficiency of a solar cell. The efficiency of the solar cell is limited by the amount of light that can be absorbed by the device and converted into electrical energy [22]. Thus, the single-diode model is a simple model used to represent the behavior of a diode in an electronic circuit. It assumes that the diode can be represented as

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a single nonlinear circuit element, with an ideal diode in parallel with a resistance and a current source. The ideal diode is a nonlinear element that conducts current in only one direction, from the anode to the cathode, when a sufficient forward voltage is applied across it. The resistance in the model represents the parasitic resistance of the diode, which results in voltage drop across the diode during its working state. The current source represents the minority carrier current that flows through the diode when it is in reverse bias. The single-diode model can be used to analyze the behavior of a diode in a circuit, such as its voltage–current characteristics, its power dissipation, and its efficiency [23]. It is commonly used in the design and analysis of electronic circuits that use diodes, such as rectifiers, voltage regulators, and oscillators. The model can be expressed mathematically using the following equation:  qV  I = IS e nkT − 1 − Ir where “I” is the diode current, “I s ” is the saturation current, “q” is the charge of an electron, “V ” is the voltage across diode, “n” is the ideality factor, “k” is the Boltzmann constant, “T ” is the temperature, and “I r ” is the reverse saturation current. It is important to note that the single-diode model is not always accurate in all circumstances because it is a simplified model to explain the complex behavior of a diode. More advanced models, such as the double and SPICE diode model, are available for more precise simulations of diode behavior in electronic circuits.

7.2.1.1

Derive an Expression for a Single-Diode Model

Single-diode model is a basic equivalent circuit model derived from physical principles (e.g., Gray, 2011) and used to explain the I–V curve of module, array, or a cell in various operating conditions. Figure 7.1 represents a single-diode model which is actually the circuit of a single solar cell. It consists of an ideal diode in parallel with a resistance and a current source [23]. To derive the expression for the single-diode model, we can start with the diode equation, which is a fundamental equation that describes the current–voltage relationship of a diode. The diode equation is given by above equation. The diode equation represents the behavior of a real diode, which consists of a p–n junction that conducts current in one direction when a sufficient forward voltage is applied and has a current flowing in opposite direction when the diode is in reverse bias. The single-diode model simplifies the diode equation by denoting the diode with a single nonlinear circuit element. This element consists of an ideal diode in parallel with a resistance and a current source. The ideal diode is a nonlinear element that conducts current in only one direction, from the anode to the cathode, when a sufficient forward voltage is applied across it. Using Kirchhoff’s current law, we can derive the expression for the single-diode model. The law states that the current leaving a node in a circuit is equal to the current entering that node. In case of the single-diode model, we can apply this law to the

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Fig. 7.1 Single-diode model as an equivalent circuit for a single solar cell. Reproduced from [23] under common creative License

node that connects the diode to the rest of the circuit. At this node, we have two currents: the current through the diode and the current through the resistance. These two currents can be calculated using diode equation and Ohm’s law, respectively. Using Kirchoff’s law for current I, the equation for this equivalent circuit is I = IL − ID − Ish where “I L ” denotes the light current in the cell and “I D ” and “I Sh ” represent current lost due to recombination and shunt resistances. The current I D is derived from the Shockley equation for an ideal diode  V +I RS  ID = I0 e nVT − 1 where “I 0 ” denotes the saturation current, “n” is the ideality factor of a diode (the value of n is 1 or 2 for a single cell), and “V T ” is the thermal voltage given by VT =

kTc q

where “k” is Boltzmann’s constant and “q” is the elementary charge. Using Ish = V +I RS , the complete equation for the single-diode model is given as Rsh  V +I RS  V + IR S I = I L − I0 e nVT − 1 − Rsh In case of a photovoltaic module (md) or array with “N s ” number of uniform cells having same irradiance and temperature connected in series, Imd = Icell and Vmd = Ns × Vcell . Therefore, the single-diode equation for a module or array becomes (Tian, 2012)  VM +IM NS RS  V +I N R M M S S IM = IL − I0 e n NS VT − 1 − NS Rsh

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where “I M ” is the module current and “V M ” is the voltage of module or array. It is important to mention that while implementing the parameters, proper care should be taken because the parameters can be applicable to both cells and modules or arrays. For implementing parameters in arrays or modules, single-diode equation is strictly followed. A modified ideality factor “a” obtained by combining N S , V T , and η is used in some operations (e.g., De Soto et al., 2006) a≡

NS nkTc q

Thus, a single-diode model as an equivalent circuit for a single solar cell has been described. In this section, the module models are examined because these are the basic models used in PV modeling software packages for analyzing the performance of modeling arrays [24].

7.2.2 Double-Diode Model The double-diode model is an electrical circuit model used to describe the behavior of a diode in a nonlinear manner [25]. It is often used to model the behavior of a bipolar junction transistor (BJT), which consists of two p–n junction diodes. The doublediode model consists of two diodes connected in parallel with opposite polarity, i.e., forward and reverse-biased. The forward-biased diode represents the base–emitter junction of the BJT, while the reverse-biased diode represents the base–collector junction. The following equation represents the double-diode model:  Ic = Is1

   Vbc Vbe exp − 1 − Is2 exp −1 Vt Vt

where I c is the collector current, I s1 and I s2 are the saturation currents of the two diodes, V be is the voltage across the base–emitter junction, V bc is the voltage across the base–collector junction, and V t is the thermal voltage. This equation takes into account the nonlinear characteristics of the diodes and allows for the modeling of the BJT’s output characteristics such as the collector current as a function of the base– emitter voltage and the base–collector voltage. The double-diode model is useful for designing and analyzing BJT circuits in electronic devices. In case of single-diode equation, for ideality factor n, a constant value is obtained. However, in reality, n is dependent upon the voltage across the device. Thus, when the voltage is high, the recombination taking place at the surface and in the bulk regions of the device dominates. Hence, the value of n is taken as 1. However, at low voltages, the recombination taking place in the junction dominates, and the value of

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Fig. 7.2 Circuit diagram of a double-diode model. Reproduced from Ref. [26] under common creative License

n is taken as 2. To model the recombination at junction, a diode is added in parallel to the first diode as shown in Fig. 7.2, and the value of n is set at 2. The double-diode model equation under illumination changes as   q(V + J RS ) J = JL − J01 exp −1 kT   q(V + J RS ) V + J RS − J02 exp −1 − 2kT Rshunt It is important to note that making practical measurements from the above equation are very difficult. A small variation in the intensity of light overcomes the effect of second diode. Hence, the double-diode equation is modified in the dark as   q(V − J RS ) J = J01 exp −1 kT   q(V − J RS ) V − J RS −1 + + J02 exp 2kT Rshunt To make the calculation easier, the -1 terms in the exponential of both equations are typically ignored. Thus, the double-diode equation in illumination and dark becomes Under illumination   q(V + J RS ) J = JL − J01 exp kT   q(V + J R S ) V + J RS − − J02 exp 2kT Rshunt

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In the dark   q(V − J RS ) J = J01 exp kT   q(V − J RS ) V − J RS + + J02 exp 2kT Rshunt

7.2.2.1

Limitations of the Double-Diode Model

The double-diode model fails in case of silicon devices and gives wrong results. This is due to the fact that in silicon devices, the recombination depends upon the carrier concentration. When the voltage applied is increased, the concentration of the carriers increases which abruptly changes the recombination at rear surfaces. In such cases (PERL solar cells), the single-diode model best predicts the behavior.

7.2.2.2

Measuring Ideality Factor

In order to calculate the ideality factor (n), the slope of light-I–V curve, Suns-Voc, and dark-I–V curves gives the value of n. The basic cell equation in dark is  qV  I = I0 e nkT − 1 where “I” is the diode current, “I 0 ” is the dark saturation current, “q” and “k” are both constants, “V ” is the voltage across the diode, “n” is the ideality factor, and “T ” is the temperature in Kelvin. The −1 term is ignored for potential greater than 50–100 mV, and the equation reduces to  qV  I = I0 e nkT Taking the log of both sides of the equation  ln(I ) = ln(I0 ) +

 qV V nkT

The slope of the graph between ln (I) and ln (V ) gives the value of q/nkT, whereas the intercept indicates ln (I 0 ). Since n depends upon V and can be plotted against V or formulated as a single value. It is important to note that the voltage range also needs to be mentioned while taking the single value of n. However, any variations in the value of n indicate that some strange recombination process is taking place in the cell. Thus, the ideality factor plays a key role while understanding the recombination

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mechanism in the solar cell. Moreover, the value of I 0 obtained from the equation is only valid when n is stable. Some of the major problems while calculating the value of n are • A large peak is obtained in the ideality factor curve at low voltages due to the Rshunt , and this error cannot be corrected. • Similarly, a large peak in dark-I–V curves is obtained at high voltages due to the series resistance. However, this can be averted by using Suns-Voc curve. • The noise in the ideality factor while performing Suns-Voc measurements is very common and causes various problems. The effect of noise can be reduced by curve fitting and then calculating the slope of curve. • The ideality factor strongly varies with the temperature, and this can cause problems if the temperature changes during measurements.

7.2.3 Single-Diode Model Versus Double-Diode Model The single-diode model and double-diode model are both used to describe the behavior of a diode or a bipolar junction transistor (BJT) in a nonlinear manner [27]. However, there are some differences between these models. In single-diode model, the BJT or diode is treated as a single diode in series with a current source. This model assumes that the diode current is mainly determined by the voltage across the diode, and the current source is used to account for the reverse saturation current. On the other hand, the double-diode model represents the BJT as two diodes connected in parallel with opposite polarity. The forward-biased diode represents the base–emitter junction of the BJT, while the reverse-biased diode represents the base–collector junction. This model takes into account the nonlinear behavior of both diodes and allows for a more accurate description of the BJT’s behavior. The main advantage of the double-diode model over the single-diode model is that it provides a more accurate description of the BJT’s output characteristics, such as the collector current as a function of the base–emitter voltage and the base–collector voltage. This is especially important for designing and analyzing BJT circuits in electronic devices. However, the double-diode model is complex and may not be necessary for some applications where a simplified model is sufficient. In general, the choice of which model to use depends on the specific requirements of the application and the level of accuracy needed.

7.2.4 Models Other Than Single- and Double-Diode Model There are several other models used to describe the behavior of diodes and transistors, including the Ebers-Moll model, the Gummel-Poon model, and the Charge Control Model (CCM). The Ebers-Moll model is a more complicated model as compared to

7.3 Summary

207

the double-diode model. This model describes the behavior of a BJT by two diodes and two current sources. This model also includes the effects of minority carrier transport in the base region of the BJT [28]. The Gummel-Poon model is a more advanced model than the Ebers-Moll model that describes the behavior of a BJT by considering both the DC and AC behavior of the transistor. This model includes several parameters, such as the base resistance, the base transport factor, and the emitter injection efficiency [29]. The Charge Control Model (CCM) is another model used to describe the behavior of a BJT. This model includes detailed information about the transport of charge carriers in the device, such as the minority carrier lifetime, the mobility, and the diffusion coefficient [30]. Overall, the choice of model depends on the level of accuracy required for a specific application. For example, the single-diode model or double-diode model may be sufficient for simple circuit designs or for educational purposes, while more advanced models such as the Gummel-Poon or CCM may be necessary for more complex circuit designs and analysis.

7.3 Summary In conclusion, solar cell modeling parameters serve as crucial tools in deciphering the intricate behavior and performance of solar cells. These parameters, encompassing factors such as efficiency, voltage, current, and material properties, provide a comprehensive framework for understanding the conversion of sunlight into electricity. By leveraging these parameters, engineers and researchers can not only design and optimize solar cells for peak efficiency but also diagnose issues, predict performance under diverse conditions, and facilitate comparative analyses of different technologies. As solar energy continues to play a pivotal role in sustainable power generation, the continued refinement and application of solar cell modeling parameters accelerate progress in technology development, education, and the transition toward a cleaner energy future. Points to Remember • Current–Voltage Characteristics: Solar cell models should accurately represent the current–voltage (I–V) characteristics of the device under various operating conditions. This includes the open-circuit voltage (V oc ), short-circuit current (I sc ), and the maximum power point (MPP). • Single-Diode Model: The single-diode model is a widely used equivalent circuit model for solar cells. It includes parameters like shunt resistance (Rsh ), series resistance (Rs ), diode ideality factor (n), and diode reverse saturation current (I o ) to explain the behavior of a solar cell. • Temperature Dependence: Solar cell performance is affected by temperature changes. Modeling should consider temperature-dependent parameters like the

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• • • • • • • • • •

7 Solar Cell Modeling Parameters

bandgap energy, short-circuit current temperature coefficient, and the open-circuit voltage temperature coefficient. Irradiance Dependency: Solar cell output is also influenced by the intensity of incident light (irradiance). The model should incorporate parameters to account for irradiance-dependent behavior. Quantum Efficiency: Solar cells have different quantum efficiencies for different wavelengths of light. Accurate modeling requires quantum efficiency data and its incorporation into the model. Recombination Mechanisms: Various recombination mechanisms affect solar cell performance. Modeling should consider carrier recombination through processes such as Shockley–Read–Hall (SRH) recombination and surface recombination. Doping Profile: The doping profile of the semiconductor material affects the cell’s electrical characteristics. Accurate modeling needs to account for the doping concentration and its variation within the device. Carrier Transport: The model should consider carrier transport mechanisms, including diffusion, drift, and generation-recombination processes. Optical Losses: Light absorption and reflection within the solar cell structure result in optical losses. Modeling should consider these losses to estimate the cell’s overall efficiency. Transient Behavior: Solar cell models may also need to account for transient behavior, such as response to changes in incident light intensity or electrical load. Non-ideal Effects: Some non-ideal effects can impact solar cell performance, such as series resistance losses, shunt paths, and surface recombination. These should be included in the model. Material Properties: Accurate material properties like the bandgap, refractive index, and mobility are critical for realistic solar cell modeling. Simulation Software: Utilize appropriate simulation software (e.g., TCAD tools like Silvaco, Sentaurus, or other industry standards software) to implement the model and analyze the solar cell’s behavior under different conditions.

References 1. Tamrakar, R., and A. Gupta. 2015. A Review: Extraction of solar cell modelling parameters. International Journal of Innovative Research in Electrical, Electronics, Instrumentation and Control Engineering 3 (1): 55–60. 2. Appelbaum, J., and A. Peled. 2014. Parameters extraction of solar cells—A comparative examination of three methods. Solar Energy Materials and Solar Cells 122: 164–173. 3. Rand, B.P., D.P. Burk, and S.R. Forrest. 2007. Offset energies at organic semiconductor heterojunctions and their influence on the open-circuit voltage of thin-film solar cells. Physical Review B 75 (11): 115327. 4. Boukortt, N.E.I., and B. Hadri. 2018. Simulation of electrical characteristics of PERC solar cells. Journal of Electronic Materials 47 (10): 5825–5832. 5. Oliva, D., M. Abd Elaziz, A.H. Elsheikh, and A.A. Ewees. 2019. A review on meta-heuristics methods for estimating parameters of solar cells. Journal of Power Sources 435: 126683.

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6. Domínguez, C., I. Antón, and G. Sala. 2010. Multijunction solar cell model for translating I–V characteristics as a function of irradiance, spectrum, and cell temperature. Progress in Photovoltaics: Research and Applications 18 (4): 272–284. 7. Singh, G.K. 2013. Solar power generation by PV (photovoltaic) technology: A review. Energy 53: 1–13. 8. Chan, D.S.H., J.R. Phillips, and J.C.H. Phang. 1986. A comparative study of extraction methods for solar cell model parameters. Solid-State Electronics 29 (3): 329–337. 9. Jervase, J.A., H. Bourdoucen, and A. Al-Lawati. 2001. Solar cell parameter extraction using genetic algorithms. Measurement Science and Technology 12 (11): 1922. 10. Chen, Y., X. Wang, D. Li, R. Hong, and H. Shen. 2011. Parameters extraction from commercial solar cells I–V characteristics and shunt analysis. Applied Energy 88 (6): 2239–2244. 11. Menke, S.M., and R.J. Holmes. 2014. Exciton diffusion in organic photovoltaic cells. Energy and Environmental Science 7 (2): 499–512. 12. Cook, S., A. Furube, R. Katoh, and L. Han. 2009. Estimate of singlet diffusion lengths in PCBM films by time-resolved emission studies. Chemical Physics Letters 478 (1–3): 33–36. 13. Chang, Y.H., R. Carron, M. Ochoa, C. Bozal-Ginesta, A.N. Tiwari, J.R. Durrant, and L. Steier. 2021. Insights from transient absorption spectroscopy into electron dynamics along the GaGradient in Cu (In, Ga) Se2 solar cells. Advanced Energy Materials 11 (8): 2003446. 14. Daboczi, M., I. Hamilton, S. Xu, J. Luke, S. Limbu, J. Lee, M.A. McLachlan, K. Lee, J.R. Durrant, I.D. Baikie, and J.S. Kim. 2019. Origin of open-circuit voltage losses in perovskite solar cells investigated by surface photovoltage measurement. ACS Applied Materials and Interfaces 11 (50): 46808–46817. 15. Choulis, S.A., J. Nelson, Y. Kim, D. Poplavskyy, T. Kreouzis, J.R. Durrant, and D.D.C. Bradley. 2003. Investigation of transport properties in polymer/fullerene blends using time-of-flight photocurrent measurements. Applied Physics Letters 83 (18): 3812–3814. 16. Nayak, P.K., S. Mahesh, H.J. Snaith, and D. Cahen. 2019. Photovoltaic solar cell technologies: Analysing the state of the art. Nature Reviews Materials 4 (4): 269–285. 17. Rodrigues, E.M.G., Melicio, R., Mendes, V.M.F., and J.P. Catalao. 2011, April. Simulation of a solar cell considering single-diode equivalent circuit model. In International conference on renewable energies and power quality, vol. 1, no. 9, 13–15. Spain. 18. Han, L., N. Koide, Y. Chiba, and T. Mitate. 2004. Modeling of an equivalent circuit for dyesensitized solar cells. Applied Physics Letters 84 (13): 2433–2435. 19. Merten, J., J.M. Asensi, C. Voz, A.V. Shah, R. Platz, and J. Andreu. 1998. Improved equivalent circuit and analytical model for amorphous silicon solar cells and modules. IEEE Transactions on Electron Devices 45 (2): 423–429. 20. Yu, F., G. Huang, and C. Xu. 2020. An explicit method to extract fitting parameters in lumpedparameter equivalent circuit model of industrial solar cells. Renewable Energy 146: 2188–2198. 21. Jain, A., and A. Kapoor. 2005. A new approach to study organic solar cell using Lambert W-function. Solar Energy Materials and Solar Cells 86 (2): 197–205. 22. Rhouma, M.B., A. Gastli, L.B. Brahim, F. Touati, and M. Benammar. 2017. A simple method for extracting the parameters of the PV cell single-diode model. Renewable Energy 113: 885–894. 23. Sharma, H., Pal, N., Singh, Y., and P.K. Sadhu. 2015. Development and simulation of stand alone photovoltaic model using Matlab/Simulink. International Journal of Power Electronics and Drive Systems 6 (4). 24. Song, Z., K. Fang, X. Sun, Y. Liang, W. Lin, C. Xu, G. Huang, and F. Yu. 2021. An effective method to accurately extract the parameters of single diode model of solar cells. Nanomaterials 11 (10): 2615. 25. Ishaque, K., Z. Salam, and H. Taheri. 2011. Simple, fast and accurate two-diode model for photovoltaic modules. Solar Energy Materials and Solar Cells 95 (2): 586–594. 26. Honsberg, C.B., and S.G. Bowden. 2019. Photovoltaics education website. www.pveducati on.org. 27. Shannan, N.M.A.A., Yahaya, N.Z., and B. Singh. 2013, November. Single-diode model and two-diode model of PV modules: A comparison. In 2013 IEEE international conference on control system, computing and engineering, 210–214. IEEE.

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28. Goradia, C., Vaughn, J., and C.R. Baraona. 1980. Theoretical results on the tandem junction solar cell based on its Ebers-Moll transistor model. In 14th Photovoltaic specialists conference, 172–177. 29. Sischka, F. 1990. Gummel-Poon Bipolar model. Agilent Technologies, Munich 86. 30. Dey, B.K., Khan, I., Mandal, N., and A. Bhattacharjee. 2016, October. Mathematical modelling and characteristic analysis of Solar PV Cell. In 2016 IEEE 7th annual information technology, electronics and mobile communication conference (IEMCON), 1–5. IEEE.

Chapter 8

Characterization Techniques

8.1 Introduction The increasing demand for renewable energy in recent decades has resulted in the development of various materials for application in solar cell technology. For solar cell optimization, it is essential to understand its fundamental properties and behavior. Characterization techniques are used to measure the electrical and optical properties of a solar cell, providing valuable information for the optimization of its efficiency [1]. The characterization of a solar cell typically involves measuring its current–voltage (IV) curve, external quantum efficiency (EQE), capacitance–voltage (CV) curve, and transient photovoltage (TPV) response. These techniques provide information on the solar cell’s maximum power output, open-circuit voltage, short-circuit current, fill factor, spectral response, doping concentration, depletion region width, carrier lifetime, and mobility. • Current–Voltage (IV) Curve The IV curve of a solar cell provides information on its electrical performance [2]. It measures the relationship between the current flowing through the solar cell and the voltage across it. The curve is obtained by sweeping the voltage applied to the solar cell while measuring the resulting current. The IV curve is used to determine the maximum power output of the solar cell, fill factor, open-circuit voltage, and short-circuit current. • External Quantum Efficiency (EQE) The EQE measurement is used to determine the efficiency of a solar cell in converting photons of a particular wavelength into electrical current [3]. The EQE helps to determine the spectral response, which is useful in designing solar cells for specific applications, such as those requiring high efficiency in a particular wavelength range [3].

© The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 S. Arya and P. Mahajan, Solar Cells, https://doi.org/10.1007/978-981-99-7333-0_8

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• Capacitance–Voltage (CV) Curve The CV curve is used to determine the doping concentration and depletion region width of a solar cell. It measures the capacitance of the solar cell as a function of voltage. The width of depletion region can be obtained from the capacitance minimum, while the concentration of dopant can be obtained from the slope of CV curve. This information is important in optimizing the performance of a solar cell, as the doping concentration and depletion region width can affect its efficiency and stability [2]. • Transient Photovoltage (TPV) Response The TPV measurement is used to study the carrier lifetime and mobility of a solar cell. The voltage response of a solar cell is measured in response to a short light pulse. The lifetime and mobility are important parameters that affect the efficiency of the solar cell. The carrier lifetime is the time taken by the recombination of electron– hole pairs, while the carrier mobility is a measure of the ease with which the carriers move through the solar cell material. The efficiency can be improved by increasing the mobility and lifetime of charge carriers [2]. • Other Characterization Techniques In addition to the techniques discussed above, there are other characterization techniques used to study the properties of solar cells. These include impedance spectroscopy, which measures the electrical impedance of the solar cell with respect to frequency, providing information on its internal resistance and capacitance. Photoluminescence spectroscopy measures the luminescence emitted by the solar cell when it is illuminated, providing information on the carrier recombination and material quality [4]. Raman spectroscopy is used to study the vibrational modes of the solar cell material, providing information on its chemical composition and crystal structure [5]. In summary, researchers have developed a wide range of characterization techniques to better understand solar cell behavior. This information can be used to improve solar cell efficiency and to design better solar cell architectures.

8.2 External Quantum Efficiency External quantum efficiency (EQE) is a characterization technique used to determine the efficiency of a solar cell in converting photons of a specific wavelength into electrical current. EQE measurements provide valuable information on the spectral response of a solar cell. EQE is a critical parameter in designing and optimizing solar cells for specific applications, such as those that require high efficiency in a particular wavelength range [3]. • Principle of EQE Measurement

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EQE measurements are based on the principle of photoconductivity, which is the increase in electrical conductivity of a material due to the absorption of light. When a semiconductor material absorbs light, an electron–hole pair is created, and the electrons and holes are separated by the electric field in the material. The separated charges contribute to an increase in electrical conductivity, which can be measured as a current. The EQE is defined as the ratio of the number of electron–hole pairs generated by the absorbed photons to the number of incident photons. It is expressed as a percentage and is wavelength-dependent [3]. • EQE Measurement Setup The EQE measurement setup contains a solar cell, a reference detector, and a monochromatic light source. The monochromatic light source produces light of a specific wavelength, which is then directed onto the solar cell. The current produced in the solar cell is measured in terms of wavelength of the incident light, whereas the reference detector records the incident light intensity. The EQE curve is obtained by dividing the measured current by the number of incident photons per unit area per unit time and the energy per photon. The resulting EQE curve provides information on the spectral response of the solar cell, which is the efficiency of the cell in converting photons of different wavelengths into electrical current [2, 3]. • EQE Curve Interpretation The EQE curve provides information on the external efficiency of the solar cell, which is the efficiency of the cell in converting photons into electrical current that can be collected by external electrodes. The EQE curve can be used to determine the maximum achievable efficiency of the solar cell, as well as its limitations due to material and structural properties. The EQE curve can be used to identify regions of low efficiency in the solar cell and to optimize its performance for specific applications. EQE curve interpretation can be used to identify the spectral response of the solar cell. The spectral response depends on the quality of material, energy density, and concentration of the dopant. By analyzing the EQE curve, it is possible to identify the spectral range where the solar cell has the highest efficiency. • Factors Affecting EQE Several factors can affect the EQE of a solar cell. One of the primary factors is the property of material used for solar cell fabrication. The EQE curve depends on the absorption coefficient of the material, which determines the number of photons that can be absorbed by the solar cell. The absorption coefficient is affected by the material’s bandgap energy, doping density, and material quality. The energy bandgap helps in determining the spectral range of photons a solar cell is capable of absorbing, while the doping density affects the recombination rate of electron–hole pairs. The material quality affects the probability of photon absorption and carrier recombination [6]. Another factor that affects the EQE is the structure of solar cell. The structural properties include the thickness and geometry of the solar cell, as well as the design

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of the electrodes. The thickness of the solar cell affects the probability of photon absorption and carrier recombination, while the geometry affects the surface area of the solar cell exposed to light. The design of the electrodes affects the collection of the generated current and efficiency of solar cell.

8.2.1 Apparatus for Measuring External Quantum Efficiency (EQE) EQE is a measurement of the efficiency of a photovoltaic device in converting photons into electrons. EQE is a key parameter that is used for investigating the performance of a solar cell or any electric device that works on light. Figure 8.1 shows the setup used for the measurement of EQE. The developed setup can be used for both bare and encapsulated solar cells. Measuring EQE requires a specialized apparatus that can measure the electrical current generated by the absorbed light [7]. This subsection provides a detailed explanation of the apparatus used to measure EQE, including its components, operation, and limitations. • Apparatus Components The apparatus used to measure EQE consists of several components that work together to accurately measure the efficiency of the photovoltaic device. The main components include a light source, monochromator, reference cell, sample holder, amplifier, and data acquisition system. Light Source: A stable and precise light source is required to illuminate the device being measured. Calibrated halogen lamp is preferred as light source due to its broad spectrum. However, other light sources, such as lasers or light-emitting diodes (LEDs), can be used for devices that require monochromatic light. Monochromator: To produce narrow band of wavelengths, a monochromator is used after light source. This is important for accurately measuring the spectral response of the device being tested. The monochromator consists of a prism or

Fig. 8.1 Experimental setup for the EQE measurement of both bare and encapsulated solar cells. Reproduced from Ref. [7] with permission. Copyright 2015, AIP Publishing

8.2 External Quantum Efficiency

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diffraction grating that separates the incoming light into its constituent wavelengths. The desired wavelength is then selected by adjusting the angle of the prism or grating. Reference Cell: The reference cell is a calibrated photodiode that is used to measure the intensity of the light source at each wavelength. The number of photons falling on sample can be accurately measured using reference cell. Sample Holder: Sample holder is used to hold the device being measured. The holder should be designed to minimize any reflections or losses of light that might occur. The sample holder is typically cooled to minimize thermal effects on the measurement. Amplifier: An amplifier with high input impedance is used to amplify the signals from the solar cell. High input impedance helps in minimizing the loading effects. Data Acquisition System: The data acquisition system is used to capture and store the electrical signal generated by the device being measured. The system typically includes a computer with specialized software for data analysis. • Apparatus Operation The EQE measurement process involves several steps that are controlled by the software used to operate the apparatus. The following is a typical sequence of steps: Calibration: The apparatus must be calibrated before each measurement to ensure accurate results. This involves measuring the intensity of the light source at each wavelength using the reference cell. Baseline Measurement: A baseline measurement is taken to measure the electrical response of the device in the absence of light. This is necessary to determine the dark current, which is subtracted from the total current measured during the measurement. Illumination: The device being measured is illuminated with a narrow band of wavelengths selected by the monochromator. The intensity of the light is adjusted to ensure that the device is not saturated, which would result in inaccurate measurements. Current Measurement: The electrical current generated by the device is measured using the amplifier and recorded by the data acquisition system. The measurement is repeated at several wavelengths to obtain the spectral response of the device. EQE Calculation: The EQE is calculated using the following equation: EQE =

Jphoton × 100% q × Pin

where “Jphoton ” is the photocurrent generated by the device, “q” is the electron’s charge, and “Pin ” is the power of the incident light.

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8.2.2 Calibration Process for Measuring External Quantum Efficiency (EQE) of a Solar Cell To accurately measure EQE, a calibration process is required to ensure the reliability and repeatability of the measurement. This subsection will provide a detailed explanation of the calibration process for measuring EQE of a solar cell [2, 8]. Equipment Setup: Before starting the calibration process, the equipment setup must be prepared. The equipment needed for EQE measurement includes a monochromator, a calibrated silicon, a power meter, a sample holder, lock-in amplifier, and a reference cell. The monochromator provides narrowband light source of a particular wavelength. The calibrated silicon reference cell is used to provide a reference signal for the power meter, which measures the light intensity. The lock-in amplifier measures the current, whereas the sample holder helps in keeping solar cell at one place during measurement. Dark Current Measurement: The first step in the calibration process is to measure the dark. The current produced when no light is incident on a solar cell is called dark current. The dark current can be measured by covering the solar cell with a black mask and applying a reverse bias voltage to the solar cell. The dark current measurement is important because it provides a baseline current value that can be subtracted from the current generated by the solar cell under test. Spectral Response Calibration: The next step in the calibration process is to calibrate the spectral response of the equipment. This is done by using the calibrated silicon reference cell to measure the light intensity at each wavelength of the monochromator. The power meter is used to measure the light intensity, and the lockin amplifier is used to measure the current generated by the silicon reference cell. The ratio of the current to the light intensity is then calculated for each wavelength to obtain the spectral response of the equipment. This spectral response is used to correct the EQE measurement for the spectral distribution of the incident light. EQE Measurement: The final step in the calibration process is to measure the EQE of the solar cell. This is done by applying a known bias voltage to the solar cell and measuring the current generated by the solar cell in response to the monochromatic light source. The lock-in amplifier is used to measure the current, and the power meter is used to measure the light intensity. The EQE is then calculated as the ratio of the measured current to the number of incident photons per unit area per unit time. The spectral response of the equipment is then used to correct the EQE measurement for the spectral distribution of the incident light. In summary, the calibration process is an essential step in accurately measuring the EQE of a solar cell. The process involves equipment setup, dark current measurement, spectral response calibration, and EQE measurement. By following the calibration process, reliable and repeatable EQE measurements can be obtained, which are important for optimizing the performance of solar cells.

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8.2.3 Internal Quantum Efficiency of a Solar Cell from Reflectance Data Reflectance data can provide valuable information about the internal quantum efficiency (IQE) of a solar cell. IQE is a measure of the percentage of photons that are converted into charge carriers within the solar cell. By using reflectance data, the spectral distribution of the light absorbed by the solar cell can be determined, and the IQE can be calculated for each wavelength. In this article, we will discuss the steps involved in using reflectance data to determine the IQE of a solar cell [9]. Equipment Setup: To measure the reflectance of the solar cell, a spectrophotometer is required. Figure 8.2 illustrates experimental setup of a spectrophotometer. Spectrophotometer is a device used to measure reflected light from the solar cell. The setup also includes a sample holder, light source, and a monochromator. The light from source is passed through monochromator to obtain narrowband light, which then falls on a solar cell and the spectrophotometer detects the reflected light. Dark Current and Voltage Measurements: When no light is incident on solar cell, the current and voltage produced is called dark current and dark voltage. Measurement of dark voltage and dark current is done prior to reflectance measurement to set a baseline value for IQE measurement. Reflectance Measurement: The solar cell is illuminated with the monochromatic light source, and the reflected light is measured by the spectrophotometer. The reflectance is calculated as the ratio of the reflected light to the incident light. Absorption Coefficient Calculation: The absorption coefficient of the solar cell is the measure of the extent to which the solar cell absorbs the light. Using the reflectance data and optical constants, the absorption coefficient of a solar cell is determined. The optical constant of the material include refractive index and extinction coefficient, both of these vary with the wavelength of light. The absorption coefficient is calculated using the following formula: ( ) 1 1 α = ln R T

Fig. 8.2 A schematic of an experimental setup of a spectrophotometer. Reproduced from Ref. [10] under common creative License

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where “α” is the absorption coefficient, “R” is the reflectance, and “T ” is the transmittance, which is assumed to be zero for a solar cell. IQE Calculation: Internal quantum efficiency of a solar cell can be calculated from EQE and α. The EQE is a measure of the percentage of photons that are converted into electrical current by the solar cell. It can be measured using the same equipment and setup as the reflectance measurement, with the exception of a sample holder that allows for the measurement of the photocurrent generated by the solar cell. The EQE is calculated using the following formula: ( EQE =

) Iph hc Pin λ

where “I ph ” is the photocurrent generated by the solar cell, “Pin ” is the incident photon flux density, “h” is Planck’s constant, “c” is the speed of light, and “λ” is the wavelength of the incident light. The equation of IQE is given as: IQE =

1− R 1 − (1 − d)α

where “d” is the thickness of the solar cell and “α” is the absorption coefficient. The IQE helps in determining the efficiency of solar cell. Thus, the use of reflectance data to determine the internal quantum efficiency of a solar cell involves equipment setup, dark current and voltage measurements, reflectance measurement, absorption coefficient calculation, and IQE.

8.2.4 QE Measurement Data The QE measurement data of a solar cell provides important information about its performance and can be used to optimize its design. The QE measurement data is typically presented as a plot of the QE as a function of wavelength. The QE can be expressed as a percentage or a fraction and is defined as the ratio of the number of charge carriers generated by the solar cell to the number of incident photons [2]. The QE of a solar cell is unity, when all the photons at particular wavelength are absorbed and the corresponding minority carriers generated are collected. The QE of photons with energy less than the bandgap of material used in solar cell is equal to zero. Figure 8.3 represents QE curve of an ideal solar cell. For example, consider a solar cell with a QE of 80% at a wavelength of 500 nm. This means that for every 100 photons that are incident on the solar cell at a wavelength of 500 nm, 80 electrons are generated and contribute to the electrical current. The QE curve is square in shape for ideal solar cell. However, due to recombination effect in most solar cells, QE curve is reduced from the ideal square shape. In solar cells, the mechanism which hinders the collection probability also affects the QE of cell. This can be illustrated by the fact that the number of charge carriers produced

8.2 External Quantum Efficiency

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Fig. 8.3 Quantum efficiency curve of silicon solar cell. Reproduced from Ref. [2] under common creative License

near the surface of a solar cell are affected by the surface passivation. Near surface of the solar cell, the blue light is absorbed mostly and the recombination at surface will affect the QE of solar cell in blue region. Similarly, green light is absorbed mostly in the bulk of solar cell, the low diffusion length will in bulk will reduce collection probability which will ultimately affect the QE of solar cell in green region. The EQE of solar is the efficiency of solar cell obtained after considering loss of energy due to reflection and transmission. EQE is important while studying the performance of solar cell because some part of incident energy is reflected and transmitted by the cell, only the remaining part is used for energy generation. On the other hand, internal quantum efficiency (IQE) is the efficiency of the solar cell in producing charge carriers from energy that in neither reflected nor transmitted through the cell. Hence, study of transmission and reflection of energy by solar cell is important to find the EQE of solar cell, and EQE is then used to obtain the IQE of the solar cell [2]. During the analysis of QE measurement data, following instructions should be followed: Measurement setup: The QE measurement setup should be carefully calibrated to ensure accurate and reproducible results. The standard conditions, i.e., use of reference cell and calibrate source, should be maintained before taking measurements. Spectral response: The QE measurement should be performed over a range of wavelengths to determine the spectral response of the solar cell. This information can be used to optimize the design of the solar cell to maximize its performance. External quantum efficiency (EQE): The ratio of number of carriers produced by a solar cell to the number of photons incident on a solar cell is called its EQE. The

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EQE can be calculated using the following equation: EQE = (Jsc /q)/(Pinc /A) where “J sc ” is the short-circuit current density, “q” is the electron charge, “Pinc ” is the incident power, and “A” is the illuminated area. Internal quantum efficiency (IQE): The ratio of number of charge carriers generated to the number of photons absorbed by the solar cell is called Internal Quantum Efficiency of solar cell. IQE helps in determining the ability of the solar cell in converting absorbed photons into charge carriers. The IQE can be calculated using the following equation: IQE = Jph /(q × Pinc / h × c) where “J ph ” is the photo-generated current density, “q” is the electron charge, “Pinc ” is the incident power, “h” is Planck’s constant, and “c” is the speed of light. Bandgap energy: The bandgap energy is an important parameter that determines the maximum efficiency of a solar cell. The QE measurement data can be used to determine the bandgap energy of the solar cell and optimize its design. The bandgap energy can be determined by identifying the wavelength at which the QE drops to zero. Overall, analyzing QE measurement data is a critical step in optimizing the design and performance of a solar cell. Careful consideration of the measurement setup, spectral response, EQE, IQE, and bandgap energy is necessary to ensure accurate and meaningful results.

8.2.5 Spectral Response Spectral response is an important parameter to understand the performance of a solar cell. It is equal to the ratio of current produced to the power incident on a solar cell [2, 11]. The spectral response curve of a typical Si solar cell is shown in Fig. 8.4. The spectral response at the intermediate wavelengths is similar to an ideal solar cell. However, below 400 nm, the spectral response is low due to maximum light absorption by glass. The spectral response is again zero at higher wavelengths because at such wavelengths, photon has higher energy than material’s bandgap. As a result, power loss occurs in solar cells at higher and lower wavelengths. The wavelengths at which the solar cell behaves ideally are ideal wavelengths for energy generation.

8.2 External Quantum Efficiency

221

Fig. 8.4 Spectral response of a silicon solar cell under glass. Reproduced from Ref. [2] under common creative License

8.2.6 Solar Cell Current Solar cells are devices that convert light energy into electrical energy through the photovoltaic effect. When light is absorbed by the solar cell, it generates electrons that flow through a circuit, creating an electrical current [12, 13]. The amount of current generated depends on the intensity and wavelength of the light source. Steps to calculate solar cell current are given as: 1. Determine the spectral content of the light source: The spectral content of a light source refers to the distribution of wavelengths that it emits. Different light sources have different spectral contents. The spectral content of the light source can be measured using a spectrometer. In most solar cells, the manufacturer provides the detail about spectral content. 2. Calculate the irradiance of the light source: Irradiance of a light source refers to the amount of power per unit area that is incident on the solar cell. You can measure the irradiance of the light source using a photodiode or obtain the information from the manufacturer. 3. Determine the efficiency of the solar cell: The efficiency of the solar cell can be obtained from the manufacturer or calculated by measuring the electrical power output for a known incident light power. 4. Calculate the solar cell current: Once you have the spectral content, irradiance, and efficiency, you can calculate the solar cell current using the following

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equation: I = E × A × QE where “I” in Ampere is the current generated in solar cell, “E” is the irradiance of the light, “A” is the area of cell in m2 , and “QE” is the quantum efficiency.

8.3 Energy Conversion Efficiency Energy conversion efficiency is the percentage of solar energy incident on solar cell that is converted into charge carriers. It is a critical parameter that determines the performance solar cell [14]. Following factors are important for determining energy conversion efficiency of a solar cell: Absorption efficiency: The percentage of the incident solar radiation absorbed by the solar cell is called its absorption efficiency. It depends on the thickness, bandgap, and refractive index of the material. Carrier collection efficiency: The percentage of the absorbed solar energy converted into charge carriers (electrons and holes) by the solar cell is called carrier collection efficiency. It depends upon the concentration of doping and width of depletion layer. Electrical power conversion efficiency: This is the percentage of the converted charge carriers that are collected as electrical power at the solar cell terminals. The electrical power conversion efficiency is determined by the open-circuit voltage, short-circuit current, and fill factor of the solar cell. The overall energy conversion efficiency of a solar cell is the product of these three factors. Mathematically: η = η_ abs × η_ coll × η_ elec where “η” is the energy conversion efficiency, “η_abs” is the absorption efficiency, “η_coll” is the carrier collection efficiency, and “η_elec” is the electrical power conversion efficiency. Typically, the energy conversion efficiency of commercial solar cells ranges from 15 to 25%. However, ongoing work is focused on making solar cells more costeffective and competitive with traditional energy sources.

8.4 I–V Curve

223

8.4 I–V Curve I–V curve is the graph plotted between the current produced by the solar cell and the voltage applied across it [2]. The I–V curve is used to determine key parameters such as the short-circuit current (Isc), the open-circuit voltage (Voc), the maximum power point (MPP), and the fill factor (FF). In a typical I–V curve, the current is plotted on the y-axis, and the voltage is plotted on the x-axis. The curve shows a characteristic shape that depends on the solar cell’s material properties and design. At low voltages, the current is very small, and the curve is almost horizontal. The current starts to increase with increasing voltage and the I–V curve becomes vertical. Eventually, the curve reaches a peak, which corresponds to the maximum power point. At higher voltages, the current drops off rapidly, and the curve becomes almost horizontal again. The shape of the curve is governed by various parameters such as efficiency, the intensity and spectral distribution of the incident light, and the temperature of the solar cell. By analyzing the I–V curve, researchers and engineers can gain insights into how to optimize the design and operation of solar cells to achieve the best possible performance.

8.4.1 I–V Curve of a Solar Cell The I–V curve of a solar cell represents the relationship between the current and voltage output of the solar cell under various conditions of illumination and temperature. It is a graph that plots the current produced by the solar cell against the voltage applied to the cell. The I–V curve of a Si solar cell is shown in Fig. 8.5. Under light, the diode law modifies as: [ qV ] I = IL − I0 e nkT − 1 where “I L ” is light-generated current. The voltage from above equation can be given as: V =

) ( IL − I nkT ln q I0

The shape of the I–V curve depends on the efficiency, material, and operating conditions of a solar cell. At zero voltage, the current through the solar cell is zero, as there is no external load. The current produced by a solar cell increases with an increase in the voltage across the solar cell. This region is known as the “ohmic region” or “linear region” of the curve, where the current increases linearly with voltage. Increasing the voltage further, the current increases at a diminishing rate. The I–V curve reaches a peak, which represents the maximum power point (MPP)

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Fig. 8.5 Current–voltage (IV) cure of a solar cell. Reproduced from Ref. [2] under common creative License

of the solar cell. At this point, the voltage and current are at their optimal values, and the solar cell is producing its maximum power output. If the voltage across the solar cell is increased beyond the MPP, the current decreases rapidly, and the I–V curve drops off sharply. This region is known as the “saturation region” or “negative resistance region” of the curve. In open-circuit mode, the maximum voltage at which the current will be minimum is known as open-circuit voltage (V oc ). However, in short-circuit mode, the maximum current at minimum voltage is known as short-circuit current (I sc ). Thus, maximum voltage is available in a solar cell for open-circuit condition, and maximum current is available for short-circuit condition. However, it is important to note that no power generates in the cell under these two conditions [2]. A solar cell generates maximum power at a point in between these two extremes known as maximum power point (MPP). At MPP, current (I mp ) and voltage (V mp ) are maximum in the solar cell. On an I–V curve, the MPP is located near the bend. As we know that in solar cells, both current and voltage generated depend upon temperature. Therefore, the power generated will change with changing temperature.

8.4.2 Solar Panel I–V Characteristic Curves A photovoltaic (PV) array is built by interconnecting various solar cells together and I–V characteristics are then plotted to determine its efficiency and other parameters. Figure 8.6 shows the I–V characteristics of a PV array.

8.5 The Electrical Characteristics of a Photovoltaic Array

225

Fig. 8.6 Current–voltage (IV) cure of a solar panel. Reproduced from Ref. [2] under common creative License

Now, there are two possible combinations in which the smaller panels can be interconnected, i.e., series and parallel. The voltage increases when the panels are connected in series and current increases when connected in parallel combination. However, the power generated in Watt (W) in both the combinations is still calculated using equation P = V × I. The position of MPP in series and parallel combinations is shown in Figure.

8.5 The Electrical Characteristics of a Photovoltaic Array The electrical characteristics of a photovoltaic array are simply its I–V characteristics. In addition to this, the variation of output voltage and current with respect to temperature is also studied under these characteristics [16]. The solar array parameters are mostly provided by the manufacturer and are given as:

8.5.1 Solar Array Parameters • Open-circuit voltage (V OC )—When no external load is connected, the maximum voltage produced is called V OC . The value of VOC for photovoltaic array depends upon the number of solar panels connected together. • Short-circuit current (I SC )—When solar cell is short circuited externally, the maximum current produced is called short-circuit current (I SC ).

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• Maximum power point (MPP)—Maximum power generated by a PV array when an external load is connected to it is called maximum power point. Mathematically, MPP = I MP × V MP and is expressed in Watts (W) or peak Watts (W p ). • Fill factor (FF)—The parameter which measures the maximum power generated by a PV array is called its fill factor. Mathematically, FF is equal to the product of ISC and VOC . For an ideal PV array, the value of FF is unity. However, for most of the PV arrays the value of FF is between 0.7 and 0.8. • Percent efficiency—The percent efficiency of a PV array is the ratio of power generated by the PV array to the number of radiations incident on it. The percent efficiency of most PV arrays is in range of 10–12% [17]. The power rating of PV arrays is an important parameter while considering its commercial applications. Power rating is simply the current and voltage produced by the array at maximum power. The power ratings enable users to determine which array would provide them the necessary power supply for a particular application. Good power ratings means that a PV array is operating at its MPP, and to determine the MPP of an array, I–V characteristics play a very important role.

8.6 Illumination for I–V Curves Illumination is a critical factor that affects the I–V curves of solar cells. The I–V curve represents the electrical characteristics of the solar cell under different illumination conditions. The position and shape of an I–V curve vary with the spectral distribution of the incident radiations [22]. The I–V curve is typically measured under standardized test conditions (STC) used for illumination. The STC illumination is defined as 1000 W/m2 of sunlight with a spectral distribution corresponding to the AM1.5 standard. The AM1.5 standard spectrum is a standardized solar spectral irradiance model that represents average solar radiation spectrum at sea level with sun at a zenith angle of 48.2º on a clear day. However, the actual illumination conditions of the solar cell can vary widely depending on factors such as the time of day, weather conditions, and the angle and position of the sun. These variations can cause changes in the shape and position of the I–V curve. The effect of the illumination level on the I–V curve can be observed by measuring the I–V curve at different light intensities. As the illumination level increases, the short-circuit current (I sc ) of the solar cell increases linearly, while the open-circuit voltage (V oc ) remains relatively constant. This results in an increase in the output power of the solar cell.

8.6 Illumination for I–V Curves

227

Table 8.1 Classification of solar simulators as per IEC 60,904–9 Ed.2.0 Class

Temporal instability Long term (%)

Spectral match (%)

Short term (%)

Irradiance inhomogeneity (%)

A

0.5

2

0.75–1.25

B

2

5

0.6–1.4

5

C

10

10

0.4–2.0

10

2

8.6.1 Illumination Sources One of the important factors that affects the measurement of solar cell parameters is a steady light source. The intensity and spectrum of the light source should resemble sunlight. A simple solution is to use sun [1], but the weather and atmosphere may vary from place to place. Moreover, the spectrum of sun also varies throughout the day [23]. The light source should have the following features; • Spatial non-uniformity of the source < 1%. • Variation in total irradiance < 1%. • Error in spectral mismatch should be less than 1%. The above-mentioned features are important to obtain 2% accuracy. The testers used during measurement are selected according to their temporal instability, spectral match, and irradiance inhomogeneity. Solar simulators are classified into three classes, i.e., A, B, and C. Table 8.1 shows different classes along with their features. The xenon arc lamp having filters attached has the features almost similar to the AM1.5G spectrum. However, halogen lamp is used for simple testing due to better stability. The temperature of halogen lamp is selected such that it produces more infrared radiations than UV radiations. The UV and visible radiations are incident on solar cell, whereas infrared radiations are reflected via back of the bulb and not made incident on cell.

8.6.2 Deviations from Air Mass 1.5 The spectrum of most light sources varies from the standard AM1.5 spectrum. Using light source with different spectrum than AM1.5 causes error in Isc measurements [24]. Two approaches are mainly employed to correct these differences. • Cell Calibration A calibration cell having spectral response similar to the cell under test is used by most of the testers. The intensity of the light of tester is modified such that Isc of the cell matches with already calculated Isc at some external lab. However, small variation occurs which can be removed by further calibration [25].

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• Measure Spectral Response The variation in the standard spectrum and the spectrum of light source will result in decreasing the efficiency of solar [3, 26]. To compensate for such decrease in efficiency, the spectral response is measured for the solar cell.

8.7 I–V Curve Measurement Apparatus I–V measurements require a complete setup of apparatus [13]. The main components of an I–V measurement apparatus are: Solar Cell: The first and basic component is the solar cell. The cell is mounted at some particular elevation (depending upon location) such that light from source is incident on it. Light Source: The light source is selected such that its spectrum approximately resembles the standard solar spectrum. Mostly, xenon lamp is used as light source. Load Resistor: To vary the load and measure the current generated by a solar cell at various voltages, a load resistor is externally connected to the solar cell. Voltage Source: To vary the voltage across solar cell, a voltage source is connected in series with the solar cell. Data Acquisition System: To store, measure, and analyze the data obtained after the solar cell operation, a data acquisition system is connected to the solar cell. It includes current amplifier, multimeter, and a computer with installed specific software for data analysis. The I–V measurement starts by exposing the solar cell to the light source and setting load resistor to maximum resistance so that the cell will operate in opencircuit condition and voltage across cell is measured. In the next step, the value of load resistor is slowly decreased and corresponding values of voltage are recorded in the computer. Finally, the current and voltage measurements are plotted as I–V curve.

8.7.1 Light Sources for Testing a Solar Cell Light sources used for testing solar cells should closely match the spectral distribution of sunlight and be capable of providing the desired intensity levels [27]. Here are some common light sources used for testing solar cells: Xenon Lamp: Xenon lamps are a popular choice for solar cell testing since they have a continuous spectrum that is similar to sunlight. These lamps can also provide high intensity light, which is necessary for some types of testing. Halogen Lamp: Halogen lamps are also used for solar cell testing, as they provide a spectrum that is similar to sunlight. These lamps have a lower intensity compared to xenon lamps, but they are still sufficient for most testing applications.

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229

Light-Emitting Diodes (LEDs): LEDs can be used for solar cell testing since they can be made to emit light with a specific wavelength. However, since they emit light at specific wavelengths, they may not provide a spectral distribution that is a close match to sunlight. Solar Simulators: Solar simulators are specialized light sources that are designed to closely mimic the spectral distribution of sunlight. These simulators are often used for testing solar cells since they can provide a consistent and repeatable light source. The choice of light source will depend on the specific testing requirements, including the type of solar cell being tested and the desired level of accuracy.

8.7.2 Temperature Control Temperature control is important testing parameter of solar cells because the efficiency of solar cells varies with temperature. As the temperature increases, the efficiency of solar cells typically decreases, which can affect the accuracy of the measurements. Therefore, it is important to control the temperature during testing to ensure accurate and consistent results. One-sun illumination is quite intense so there has to be some mechanism to remove the excess heat [28]. Here are some methods for temperature control during solar cell testing: Ambient Temperature Control: The simplest method of temperature control is to perform testing at a controlled ambient temperature. The testing can be performed in a temperature-controlled room or chamber to maintain a constant temperature. This method is suitable for testing at moderate temperatures. Water Bath or Thermostatic Control: A water bath or a thermostatic control system can be used to control the temperature of the solar cell during testing. The solar cell is placed in a water bath or in a controlled environment that is maintained at a constant temperature. This method is suitable for testing at temperatures that are higher than ambient temperatures. Peltier Devices: Peltier devices can be used to actively cool or heat the solar cell during testing. Peltier devices use the Peltier effect to transfer heat from one side of the device to the other, allowing for active temperature control. This method allows to test solar cells at wider temperature ranges. Temperature Chambers: Temperature chambers are specialized equipment used during solar cell testing to control the temperature. These chambers can be programmed to maintain a specific temperature range and can be used for both heating and cooling. In summary, temperature control is an important aspect of solar cell testing, and the choice of temperature control method will depend on the specific testing requirements. The testing can be performed at a controlled ambient temperature, or more advanced methods, such as water baths, Peltier devices, or temperature chambers, can be used for more precise temperature control.

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8.8 Electrical Measurement Electrical measurement is a critical aspect of testing a solar cell because it provides information about the performance of the solar cell under different conditions. The most common electrical measurements include I–V curve, EQE, spectral response, fill factor, dark current, and series and shunt resistance.

8.8.1 Calibration Calibration of a solar cell is a critical process that involves adjusting the measurements of a solar cell to ensure that they are accurate and reliable. The calibration process is essential because the performance of a solar cell can vary due to various factors such as temperature, humidity, and light intensity [12]. Here is a detailed discussion of the calibration of a solar cell and the steps involved in it. Step 1: Setting up a Reference Solar Cell The first step in calibrating a solar cell is to set up a reference solar cell. A reference solar cell is calibrated and used as a standard for comparison. The reference solar cell should have known and stable characteristics, such as its efficiency, I–V curve, and spectral response. The reference solar cell should be traceable to a national or international standard. Step 2: Determining the Calibration Parameters The next step in the calibration process is to determine the calibration parameters. Calibration parameters are used to adjust the measurements and ensure their accuracy. These parameters depend on the type of measurement being performed and may include correction factors for temperature, humidity, and light intensity. Step 3: Measuring the Solar Cell The solar cell is then measured under controlled conditions, and the measurements are compared to those of the reference solar cell. The measurements should be repeated several times to ensure consistency and reliability. Step 4: Adjusting the Calibration Parameters If the measurements of the solar cell differ from those of the reference solar cell, the calibration parameters are adjusted until the measurements match. This may involve adjusting the zero offset, gain, or linearity of the measurement equipment. Step 5: Verifying the Calibration

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231

Once the calibration is complete, the accuracy of the measurements is verified by measuring the reference solar cell again and comparing the results to the original measurements. The verification should also be done under controlled conditions. Step 6: Documenting the Calibration Finally, the calibration process should be documented. The documentation should include the reference solar cell’s characteristics, the calibration parameters, the measured values, and the verification results. The documentation should also include the date and time of the calibration, the name of the technician who performed the calibration, and any other relevant information. Thus, the calibration of a solar cell is a critical process that involves setting up a reference solar cell, determining the calibration parameters, measuring the solar cell, adjusting the calibration parameters, verifying the calibration, and documenting the process. The calibration process ensures that the measurements of the solar cell are accurate and reliable, which is essential for evaluating the performance of the solar cell.

8.8.2 Comparing Jsc from QE and IV Measurements J sc is an important parameter which represents the maximum current a solar cell can produce when its terminals are short circuited and no external load is connected. The J sc value can be obtained through different measurement techniques, including external quantum efficiency (EQE) and current–voltage (IV) measurements. The J sc value can be obtained directly from the EQE spectrum as the integrated current density over all wavelengths. On the other hand, from IV curve, the J sc value can be obtained as the current density at zero voltage. In theory, the J sc values obtained from EQE and IV measurements should be the same, as they both represent the maximum current density that the solar cell can generate. However, in practice, there may be small discrepancies between the two values. Some factors that could contribute to differences between the J sc values obtained from EQE and IV measurements include: • Differences in the illumination spectrum used for the EQE measurement compared to the IV measurement. • Non-ideal behavior of the solar cell under certain operating conditions, such as nonlinear response or shunt resistance. • Measurement errors or uncertainties in one or both measurements. To determine the cause of any discrepancies, it may be necessary to perform additional measurements or simulations under different conditions. In general, EQE measurements tend to provide more accurate and reliable Jsc values, as they are less affected by measurement errors and can account for variations in the spectral

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response of the solar cell. However, IV measurements are still widely used as a quick and convenient way to characterize solar cell performance.

8.8.3 Spectral Mismatch The situation in a solar cell when the spectrum of incident light is different from the standard solar spectrum is commonly known as spectral mismatch. The spectral mismatch heavily effects the measurement of solar cell efficiency and result is serious errors [33]. Particularly, the spectral mismatch affects the EQE of a solar cell and may over or under estimate the value of EQE for a solar cell. This is due to the fact that solar cell may respond differently to the light source than the solar spectrum. Researchers have developed methods to correct this spectral mismatch in the solar cells so that accurate efficiency measurements are made for the solar cell in order to understand their performance.

8.8.3.1

How to Correct Spectral Mismatch

Spectral mismatch of a solar cell is corrected by applying certain correction factors while measuring the efficiency [34]. Different methods used for spectral mismatch correction are: External quantum efficiency (EQE) correction: A reference solar cell with known EQE and spectral response is taken in this method. The measured EQE values of the solar cell under testing are then divided by the EQE of the reference cell. This helps in correcting the errors due to spectral mismatch. Current–voltage (IV) correction: In this method, a solar simulator having parameters similar to a solar spectrum is used for correction. Using solar simulator, the I–V curves is plotted for solar cell. From the I–V curves, the MPP is obtained, which is then used to find the efficiency of the solar cell. Finally, the efficiency measured is multiplied by a correction factor, which compensates for the difference is the spectrum of simulator and solar spectrum. Spectral response correction: In this method, a monochromatic light source (resembling solar spectrum) is used to calculate the spectral response of solar cell. Then a correction factor is calculated from the spectral response, which accounts for spectral difference of incident light and standard solar spectrum [35]. Finally, the measured efficiency values at each wavelength are multiplied by the correction factor to remove the mismatch error. The choice of these methods for correcting spectral mismatch depends upon the final application of solar cell. However, these methods have helped researchers to accurately understand the performance and efficiency of solar cell.

References

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8.9 Summary In conclusion, solar cell characterization is an essential process that allows researchers and engineers to evaluate the performance of a solar cell under different operating conditions and to optimize its design for maximum efficiency. There are several characterization techniques available for analyzing the electrical, optical, and structural properties of a solar cell. The most commonly used techniques include current–voltage (IV) measurements, and external quantum efficiency (EQE) measurements. Each technique provides unique insights into the performance of a solar cell. Overall, the choice of characterization technique depends on the specific research question or engineering problem being addressed and the availability of appropriate instrumentation. By using a combination of characterization techniques, researchers and engineers can gain a comprehensive understanding of the performance of a solar cell and optimize its design for maximum efficiency and durability. Points to Remember Ideality factor: Ideality factor or “n-factor” is important for determining recombination process and quality of junction of a solar cell. The value of ideality factor is “1” for simple recombination and “2” for special recombination process. Dark current: It is the current through a photoelectric or photoconductive cell when an electromotive force is applied in the absence of light. Illumination current: Current produced in the solar cell when light is incident on it is called illumination current. Solar irradiance: Solar irradiance is the power per unit area (surface power density) received from the sun. Solar irradiance is measured in watts per square meter (W/m2 ) in SI units.

References 1. Würfel, P., and U. Würfel. 2016. Physics of solar cells: From basic principles to advanced concepts. John Wiley & Sons. 2. https://www.pveducation.org/. 3. Ananda, W., 2017, July. External quantum efficiency measurement of solar cell. In 2017 15th International conference on quality in research (QiR): International symposium on electrical and computer engineering, 450–456. IEEE. 4. Suresh, M.S. 1996. Measurement of solar cell parameters using impedance spectroscopy. Solar Energy Materials and Solar Cells 43 (1): 21–28. 5. Nickel, N.H., P. Lengsfeld, and I. Sieber. 2000. Raman spectroscopy of heavily doped polycrystalline silicon thin films. Physical Review B 61 (23): 15558. 6. Mackel, H., and A. Cuevas. 2001. Spectral response of the photoconductance: A new technique for solar cell characterization. 7. Leyre, S., Proost, K., Cappelle, J., Durinck, G., Hofkens, J., Deconinck, G., and P. Hanselaer. 2015. Experimental validation of adding-doubling modeling of solar cells including luminescent down-shifting layers. Journal of Renewable and Sustainable Energy 7 (4).

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8. Chan, K.S., M.X. Heng, D. Ananthanarayanan, K.B. Choi, and J.W. Ho. 2022. Application of non-contact quantum efficiency measurement for solar cell fabrication process insights. Solar Energy 233: 494–503. 9. Basore, P.A. 1993, May. Extended spectral analysis of internal quantum efficiency. In Conference record of the twenty third IEEE photovoltaic specialists conference-1993 (Cat. No. 93CH3283–9), 147–152. IEEE. 10. Alshehawy, A.M., D.E.A. Mansour, M. Ghali, M. Lehtonen, and M.M. Darwish. 2021. Photoluminescence spectroscopy measurements for effective condition assessment of transformer insulating oil. Processes 9 (5): 732. 11. Paxman, M., J. Nelson, B. Braun, J. Connolly, K.W.J. Barnham, C.T. Foxon, and J.S. Roberts. 1993. Modeling the spectral response of the quantum well solar cell. Journal of Applied Physics 74 (1): 614–621. 12. Matson, R.J., K.A. Emery, and R.E. Bird. 1984. Terrestrial solar spectra, solar simulation and solar cell short-circuit current calibration: A review. Solar cells 11 (2): 105–145. 13. Hamadani, B.H., and B. Dougherty. 2015. Solar cell characterization. In Semiconductor materials for solar photovoltaic cells, 229–245. Cham: Springer International Publishing. 14. Fahrenbruch, A., and R. Bube. 2012. Fundamentals of solar cells: Photovoltaic solar energy conversion. Elsevier. 15. Duran, E., Piliougine, M., Sidrach-de-Cardona, M., Galan, J., and J.M. Andujar. 2008, May. Different methods to obtain the I–V curve of PV modules: A review. In 2008 33rd IEEE Photovoltaic specialists conference, 1–6. IEEE. 16. King, D.L., Kratochvil, J.A., and W.E. Boyson. 2004. Photovoltaic array performance model, vol. 8, 1–19. United States. Department of Energy. 17. Jain, A., S. Sharma, and A. Kapoor. 2006. Solar cell array parameters using Lambert Wfunction. Solar Energy Materials and Solar Cells 90 (1): 25–31. 18. Stokes, E.D., and T.L. Chu. 1977. Diffusion lengths in solar cells from short-circuit current measurements. Applied Physics Letters 30 (8): 425–426. 19. Qi, B., and J. Wang. 2012. Open-circuit voltage in organic solar cells. Journal of Materials Chemistry 22 (46): 24315–24325. 20. Green, M.A. 1982. Accuracy of analytical expressions for solar cell fill factors. Solar cells 7 (3): 337–340. 21. Green, M.A., K. Emery, Y. Hishikawa, and W. Warta. 2010. Solar cell efficiency tables (version 35). Progress in photovoltaics: Research and applications 2 (18): 144–150. 22. Kerr, M.J., and A. Cuevas. 2004. Generalized analysis of the illumination intensity versus open-circuit voltage of solar cells. Solar Energy 76 (1–3): 263–267. 23. Li, Y., N.J. Grabham, S.P. Beeby, and M.J. Tudor. 2015. The effect of the type of illumination on the energy harvesting performance of solar cells. Solar Energy 111: 21–29. 24. Roumpakias, E., O. Zogou, and A. Stamatelos. 2015. Correlation of actual efficiency of photovoltaic panels with air mass. Renewable Energy 74: 70–77. 25. Metzdorf, J. 1987. Calibration of solar cells. 1: The differential spectral responsivity method. Applied Optics 26 (9), 1701–1708. 26. Hartman, J.S., and M.A. Lind. 1982. Spectral response measurements for solar cells. Solar Cells 7 (1–2): 147–157. 27. Esen, V., S¸ Sa˘glam, and B. Oral. 2017. Light sources of solar simulators for photovoltaic devices: A review. Renewable and Sustainable Energy Reviews 77: 1240–1250. 28. Singh, P., and N.M. Ravindra. 2012. Temperature dependence of solar cell performance—An analysis. Solar Energy Materials and Solar Cells 101: 36–45. 29. McMahon, T.J., Basso, T.S., and S.R. Rummel. 1996, May. Cell shunt resistance and photovoltaic module performance. In Conference record of the twenty fifth IEEE photovoltaic specialists conference-1996, 1291–1294. IEEE. 30. Hovinen, A. 1994. Fitting of the solar cell IV-curve to the two diode model. Physica Scripta 1994 (T54): 175. 31. Pysch, D., A. Mette, and S.W. Glunz. 2007. A review and comparison of different methods to determine the series resistance of solar cells. Solar Energy Materials and Solar Cells 91 (18): 1698–1706.

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32. Dhass, A.D., Natarajan, E., and L. Ponnusamy. 2012, December. Influence of shunt resistance on the performance of solar photovoltaic cell. In 2012 International conference on emerging trends in electrical engineering and energy management (ICETEEEM), 382–386. IEEE. 33. Seaman, C.H. 1982. Calibration of solar cells by the reference cell method—The spectral mismatch problem. Solar Energy 29 (4): 291–298. 34. Emery, K.A., and C.R. Osterwald. 1986. Solar cell efficiency measurements. Solar Cells 17 (2–3): 253–274. 35. Meusel, M., R. Adelhelm, F. Dimroth, A.W. Bett, and W. Warta. 2002. Spectral mismatch correction and spectrometric characterization of monolithic III–V multi-junction solar cells. Progress in Photovoltaics: Research and Applications 10 (4): 243–255.

Chapter 9

Future in Solar Cell Technology

9.1 Introduction As the world faces increasing challenges posed by climate change and energy demand, the quest for renewable and sustainable energy sources has gained paramount importance [1]. Among these, solar energy stands out as a powerful and inexhaustible resource, radiating an estimated 173,000 terawatts of energy continuously onto the Earth’s surface, several thousand times the world’s total energy use [2]. Solar cell technology, which converts sunlight directly into electricity, has made significant strides since its inception and holds the key to unlocking the full potential of solar energy [3]. The history of solar cells dates back to the nineteenth century, with early discoveries in the photoelectric effect, laying the groundwork for the subsequent development of photovoltaic (PV) cells. In the mid-twentieth century, researchers made significant breakthroughs with the first practical silicon solar cell, which eventually paved the way for solar power applications in space missions and remote locations. There are few challenges for the implementation of solar cell technology as shown in Fig. 9.1. Over the years, advancements in solar cell technology have led to increased efficiency, reduced costs, and wider adoption across the globe.

9.1.1 The Rising Significance of Solar Energy As the world transitions toward a low-carbon future, the significance of solar energy cannot be overstated. Fossil fuels, the primary energy source for decades, have contributed to greenhouse gas emissions, climate change, and environmental degradation. In contrast, solar energy is clean, renewable, and non-polluting, making it an ideal solution to mitigate the adverse impacts of traditional energy sources. Solar power offers numerous advantages, including its vast abundance, decentralization

© The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 S. Arya and P. Mahajan, Solar Cells, https://doi.org/10.1007/978-981-99-7333-0_9

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Fig. 9.1 Costs and challenges associated with solar power. Reproduced from Ref. [3] under common creative 3.0 License

potential, and its suitability for both small-scale and large-scale applications. Moreover, as technology continues to improve, solar energy is becoming increasingly costcompetitive with conventional energy sources, accelerating its global deployment [4].

9.1.2 Current State of Solar Cell Technology The current landscape of solar cell technology predominantly revolves around crystalline silicon solar cells, which account for the majority of the market share. Silicon solar cells come in two main forms: monocrystalline and polycrystalline. While these technologies have served us well, they do have limitations, including high production costs, the requirement of pure silicon, and energy-intensive manufacturing processes [5]. To overcome these challenges, researchers and engineers have been diligently working on emerging solar cell technologies, such as thin-film solar cells, perovskite solar cells, and organic photovoltaics. These technologies promise the potential for higher efficiency, lower manufacturing costs, and novel applications.

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9.1.3 The Future Horizon: Advancements and Possibilities The future of solar cell technology is teeming with exciting possibilities, driven by advancements in materials science, nanotechnology, and innovative concepts. Breakthroughs in next-generation solar cell materials, like perovskite and organic materials, have showcased remarkable efficiency improvements in laboratory settings and are poised to revolutionize the industry. Furthermore, the integration of nanotechnology in solar cell design holds promise for enhancing light absorption, charge transport, and minimizing energy loss. Nanostructured solar cells and nanowire arrays present innovative approaches to capture sunlight more efficiently and boost overall performance [6].

9.1.4 Addressing Challenges for a Sustainable Solar Future While solar energy and solar cell technology hold enormous potential, there are several challenges that need to be addressed to ensure a sustainable future. One of the key obstacles is the intermittency of solar power due to its dependency on daylight availability. Consequently, effective energy storage solutions, such as batteries and other forms of energy storage, are crucial to overcoming this hurdle and enabling solar energy to provide consistent power supply even during periods of low sunlight. Moreover, the environmental impact of solar cell manufacturing and the management of end-of-life solar panels warrant close attention. Research in recycling methods and sustainable manufacturing practices will play a vital role in minimizing the ecological footprint of solar technology [7].

9.1.5 The Road Ahead: An Integrated Energy Landscape The future of solar cell technology envisions an integrated energy landscape where solar power works in harmony with other renewable sources like wind, hydropower, and energy storage solutions. The combination of these technologies will lead to a reliable, resilient, and sustainable energy grid capable of meeting the ever-growing global energy demand. In this chapter, we explore the trajectory of solar cell technology, from its historical roots to the present-day advancements. We will delve into the emerging solar cell technologies and the potential they hold in reshaping the energy sector. Furthermore, we will discuss the critical challenges faced by the solar industry and the innovative solutions being developed to overcome them. The chapter aims to inspire readers and stakeholders to envision a future powered by solar energy, where clean, renewable, and abundant sunlight fuels our journey toward a sustainable tomorrow [8].

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9.2 Material Benefits The future of solar cell technology holds great promise and potential, offering numerous material benefits that can significantly impact various aspects of society and the environment [9]. Below are some of the material benefits that can be expected from advancements in solar cell technology: • Increased Energy Efficiency: Advanced solar cell technologies, such as multijunction solar cells and perovskite solar cells, have the potential to achieve higher energy conversion efficiencies compared to traditional silicon-based solar cells. Higher efficiency means that more electricity can be generated from the same amount of sunlight, leading to increased energy output and better utilization of available resources [10]. • Cost Reduction: As solar cell technologies improve and become more efficient, their manufacturing costs are expected to decrease. This cost reduction will make solar energy more economically competitive with conventional energy sources, attracting further investment and leading to more extensive deployment of solar power systems [11]. • Abundant and Renewable Energy Source: Solar energy is a virtually limitless resource. As long as the sun continues to shine, we can harness its energy to generate electricity. Unlike fossil fuels, which are finite and depleting, solar energy offers a sustainable and environmentally friendly solution for meeting our energy needs [12]. • Energy Independence and Security: Investing in solar energy can enhance a country’s energy independence. By generating electricity locally from solar sources, nations can reduce their reliance on imported fossil fuels, thereby bolstering energy security and reducing vulnerability to fluctuations in global energy markets [13]. • Reduced Greenhouse Gas Emissions: Solar energy is a clean and low-carbon source of electricity. Using solar power instead of fossil fuels significantly reduces greenhouse gas emissions, helping to combat climate change and decrease the overall carbon footprint of energy production [14]. • Job Creation and Economic Growth: The expansion of solar energy installations and the manufacturing of solar cells and associated technologies create job opportunities in various sectors, including research, engineering, installation, and maintenance. These jobs contribute to economic growth and stimulate local economies [15]. • Decentralization and Grid Resilience: Solar energy systems can be installed at various scales, from residential rooftops to large-scale solar farms. This decentralization of energy generation helps to enhance grid resilience by reducing strain on centralized power generation and distribution infrastructure [16]. • Eco-Friendly Manufacturing: With advancements in solar cell technology, there is a growing emphasis on eco-friendly manufacturing processes. The use of sustainable materials and environmentally responsible production methods can reduce the environmental impact of solar cell manufacturing [17].

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• Flexible and Lightweight Designs: Thin-film solar cells and other emerging technologies offer flexibility and lightweight characteristics, enabling innovative applications in areas such as wearable devices, portable electronics, and buildingintegrated photovoltaics (BIPV) [18]. • Energy Access in Remote Areas: Solar energy can provide a viable solution for electrification in remote and off-grid regions, where extending traditional power infrastructure might be challenging and costly. Solar-powered microgrids and standalone systems can bring electricity to underserved communities, improving their quality of life and opportunities for economic development [19]. • Advancements in Energy Storage: Solar cell technology is closely linked to energy storage solutions. Continued research in energy storage technologies, such as batteries and hydrogen storage, will complement solar energy systems, allowing for efficient energy utilization and grid stabilization [20]. • Technological Innovation and Global Competitiveness: Advancements in solar cell technology drive technological innovation and research in various related fields. By leading the way in solar energy research and development, countries and companies can enhance their global competitiveness and contribute to shaping the future of sustainable energy [21]. • Building Integration and Architectural Flexibility: The development of innovative solar cell materials and designs allows for the integration of solar panels into building facades, roofs, and windows. Building-integrated photovoltaics (BIPVs) enable aesthetically pleasing and energy-efficient architectural solutions [22]. • Long-Term Investment: Solar energy installations have long lifespans, typically lasting 25–30 years or more. Investing in solar technology ensures a stable and predictable energy supply, making it an attractive long-term investment for businesses and utilities [23]. • Responsible Land Use: Solar farms and installations can be established on marginal or unused land, without displacing agriculture or natural habitats. This responsible land use minimizes competition for land resources and fosters sustainable development [24]. The future of solar cell technology holds immense material benefits that extend beyond clean energy generation. The continued progress in solar energy technologies can lead to increased energy efficiency, cost reduction, job creation, and environmental preservation. Furthermore, solar energy’s scalability and versatility open up new possibilities for powering various applications and improving energy access, ultimately contributing to a more sustainable and prosperous future for all.

9.3 Efficiency Drive The efficiency drive in future solar cell technology is aimed at maximizing the energy conversion efficiency of solar cells to make them more competitive, cost-effective, and capable of meeting the increasing global energy demand sustainably. Achieving

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higher efficiency levels is a key objective in advancing solar cell technology, and various strategies and innovations are being pursued to realize this goal: • Advanced Materials: Researchers are exploring new materials with superior light absorption and charge carrier transport properties. Materials like perovskites, organic compounds, and quantum dots have shown great potential for achieving higher efficiency levels compared to traditional silicon-based solar cells [25]. • Tandem and Multi-junction Solar Cells: Tandem and multi-junction solar cells consist of multiple layers of semiconductor materials with varying bandgaps, each optimized to absorb different portions of the solar spectrum. This approach allows for the capture of a broader range of wavelengths, increasing overall efficiency [26]. • Nanostructures and Light Trapping: Integrating nanostructures and textured surfaces into solar cell designs helps to enhance light absorption within the cell. These structures can trap light and increase the optical path length, leading to improved energy conversion [27]. • Hot Carrier Solar Cells: Hot carrier solar cells aim to capture excess energy from hot charge carriers generated during solar absorption, which would otherwise be lost as heat. By harnessing these carriers before they cool, efficiency improvements can be achieved [28]. • Tuning Energetic Losses: Researchers are focusing on reducing energetic losses within solar cells caused by processes like recombination, reflection, and thermalization of charge carriers. Identifying and mitigating these losses can enhance overall energy conversion efficiency [29]. • Spectral Conversion: Spectral conversion technologies convert photons with energies below the cell’s bandgap into usable photons, extending the effective absorption range and increasing overall efficiency [30]. • Perovskite–Silicon Tandem Integration: Combining advanced materials like perovskites with traditional silicon cells in tandem configurations allows for efficient use of different parts of the solar spectrum, leveraging the strengths of both materials [31]. • Manufacturing Innovations: Novel manufacturing techniques, such as roll-toroll processing and 3D printing, enable scalable and cost-effective production of high-efficiency solar cells [32]. • Light Management and Optical Coatings: The use of advanced optical coatings and light management techniques helps to reduce reflection losses and enhance light absorption within the solar cell [33]. • Efficient Charge Carrier Transport: Improving charge carrier transport within the solar cell materials is critical for minimizing losses and maximizing energy conversion efficiency [34]. • Efficient Energy Storage Integration: Pairing solar cells with advanced energy storage technologies, such as batteries or hydrogen storage, allows for better utilization of generated energy, particularly during periods of low sunlight [35]. • Durability and Longevity: Ensuring the long-term stability and durability of solar cells is crucial for maintaining high efficiency over their operational lifetimes [36].

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The efficiency drive in future solar cell technology is essential for accelerating the widespread adoption of solar energy as a primary source of electricity generation. By continuously improving efficiency and reducing costs, solar power becomes increasingly competitive with conventional energy sources, driving us closer to a sustainable and carbon–neutral energy future. As research and development efforts progress, the materialization of more efficient and cost-effective solar cells holds great promise for addressing climate change, energy security, and environmental sustainability on a global scale.

9.4 Band Together In the future of solar cell technology, the imperative to “band together” becomes ever more crucial as researchers, scientists, policymakers, and industries unite to advance this renewable energy source. Collaborative efforts are key to overcoming the challenges faced by solar energy and achieving widespread adoption. Researchers from various disciplines and institutions come together to explore and innovate in the realm of solar cell technology [5]. Their collective goal is to discover and optimize novel materials and fabrication techniques that can push the boundaries of efficiency and cost-effectiveness. Collaboration between material scientists, physicists, chemists, and engineers enables the development of advanced materials like perovskites, organic compounds, and quantum dots, which hold the potential to revolutionize solar cell efficiency. Moreover, the “banding together” ethos extend to international cooperation, where research institutions and scientists from different countries collaborate on solar energy research and development projects. These global partnerships foster the exchange of knowledge, resources, and best practices, propelling the pace of innovation and promoting the equitable distribution of solar energy advancements across borders. Policymakers and governments also play a critical role in the collective effort to drive solar cell technology forward. By working together, they can create supportive policies and regulatory frameworks that incentivize the deployment of solar energy systems. Feed-in tariffs, tax credits, and renewable energy mandates are some examples of policy mechanisms that spur investment in solar technology and facilitate its integration into the energy market. Policymakers also engage in international collaborations to set emission reduction targets and tackle climate change, recognizing the crucial role of solar energy in mitigating greenhouse gas emissions [7]. The collaborative spirit extends beyond scientific research and policymaking to include industries and private enterprises. Companies join forces to invest in research and development, manufacturing innovation, and scaling up production. The formation of consortia and partnerships allows for the sharing of resources and expertise, leading to breakthroughs in solar cell technology. Collective efforts also focus on creating sustainable and environmentally responsible manufacturing processes that reduce the ecological footprint of solar cells. Collaboration within the solar energy industry ensures continuous advancements in solar cell technology. Market leaders and startups unite

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to tackle shared challenges, such as the development of efficient energy storage solutions, addressing intermittency, and improving system integration. By pooling resources, the industry can accelerate the commercialization of new technologies and reduce costs, making solar energy more accessible to a broader population. Educational institutions and training centers also play a pivotal role in the collective endeavor to advance solar cell technology. By fostering knowledge-sharing and skills development, they equip a new generation of researchers, engineers, and professionals with the expertise needed to drive innovation in the solar energy sector. This capacity-building across different regions of the world enhances the global readiness to embrace solar energy as a mainstream power source. Furthermore, nongovernmental organizations (NGOs) and advocacy groups contribute to the collective movement by raising awareness about the benefits of solar energy and advocating for policies that support its growth. They play a crucial role in driving public engagement, pushing for sustainable practices, and highlighting the urgency of transitioning to renewable energy sources for a cleaner and greener future [37]. In the future, the “banding together” ethos in solar cell technology extend beyond the technical aspects to embrace social responsibility and inclusivity. Ensuring access to solar energy for underserved communities and developing countries becomes a priority. Collaborative efforts aim to bring clean electricity to remote and offgrid regions, empowering communities and fostering economic development. Additionally, the “banding together” approach involves fostering diversity and inclusivity within the solar energy sector [38]. Embracing a wide range of perspectives, experiences, and talents enriches the research and development process, leading to more holistic and innovative solutions. Collaboration that transcends cultural and geographical boundaries enriches the global solar energy community and enables a harmonized approach to solving the world’s energy challenges. In summary, the future of solar cell technology relies on the collective efforts of researchers, policymakers, industries, educational institutions, advocacy groups, and communities coming together to drive innovation and deployment. By fostering collaboration, knowledge-sharing, and inclusivity, we can unlock the full potential of solar energy and make it a central pillar of our sustainable energy future. The “banding together” ethos serves as a powerful catalyst, accelerating the transition to a cleaner, more resilient, and equitable energy landscape for the benefit of current and future generations.

9.5 Tricks of the Light In the future of solar cell technology, “Tricks of the Light” represent a fascinating array of innovative techniques and strategies that harness the properties of light to optimize the performance of solar cells [3]. These cutting-edge approaches aim to enhance light absorption, charge carrier generation, and energy conversion, paving the way for more efficient and cost-effective solar energy solutions. Below are some of the key “Tricks of the Light” being explored:

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• Nanotechnology and Nanostructures: Nanotechnology enables precise control over materials at the nanoscale. Researchers are designing nanostructures with unique light-manipulating properties to enhance light absorption and increase the interaction between photons and solar cell materials. Nanostructured surfaces can trap and guide light, improving the efficiency of solar cells [39]. • Spectral Shaping: Spectral shaping involves tailoring the absorption spectrum of solar cell materials to match specific wavelengths of light. By selectively absorbing certain wavelengths while reflecting or transmitting others, solar cells can optimize their energy capture and conversion efficiency [39]. • Photon Upconversion: Photon upconversion is a process that converts lowenergy photons into higher-energy ones, which can be utilized more effectively by the solar cell. By expanding the range of light that can be absorbed, photon upconversion enhances the efficiency of solar energy conversion [41]. • Plasmonic Enhancement: Plasmonic nanoparticles can concentrate and manipulate light at the nanoscale. Placing plasmonic materials in or near solar cell structures enhances light absorption and charge carrier generation, leading to higher energy conversion efficiency [42]. • Light Trapping and Waveguiding: Light-trapping structures, such as textured surfaces and nanoantennas, are designed to increase the path length of light within the solar cell. This allows for more interactions between light and the active material, boosting photon absorption and improving overall efficiency [42]. • Tandem and Multi-junction Solar Cells: Tandem and multi-junction solar cells stack multiple layers of semiconductor materials with varying bandgaps. Each layer is optimized to absorb specific wavelengths of light, enabling more efficient utilization of the solar spectrum [26]. • Quantum Dots and Intermediate Band Materials: Quantum dots and intermediate band materials can generate multiple electron–hole pairs from a single high-energy photon. This “multiple exciton generation” phenomenon increases the efficiency of energy conversion [3]. • Thermal Photovoltaics: Thermal photovoltaics capture not only visible light but also the infrared radiation emitted by warm objects. These solar cells can generate electricity from both sunlight and waste heat, making them more versatile and efficient in certain applications [3]. • Energy-Selective Contacts: Energy-selective contacts allow only charge carriers with specific energy levels to pass through, reducing recombination losses and enhancing charge extraction efficiency [42]. • Bidirectional Light Harvesting: Bidirectional solar cells can absorb light from both the front and rear sides, capturing both direct sunlight and diffuse light. This capability maximizes energy capture under varying lighting conditions [42]. • Defect Engineering: Researchers are exploring ways to control and engineer defects in solar cell materials to improve charge carrier transport and reduce recombination losses, leading to higher efficiency [3]. • Optical Coatings: Advanced optical coatings minimize reflection losses and enhance light trapping within solar cells, increasing their overall performance [3].

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These “Tricks of the Light” exemplify the ingenuity and innovation driving the future of solar cell technology. As researchers continue to explore and implement these strategies, solar energy will become an increasingly viable and sustainable solution to meet the world’s growing energy needs. The optimization of light-matter interactions holds the key to unlocking the full potential of solar cells and accelerating the transition to a cleaner and more renewable energy future.

9.6 Light Trapping and Waveguiding Light trapping and waveguiding are two essential techniques employed in solar cell technology to enhance the efficiency of solar cells by increasing light absorption and guiding photons toward the active regions. These approaches are especially valuable in thin-film solar cells, where optimizing light-matter interactions is crucial for achieving higher energy conversion efficiencies [44]. Let us delve deeper into how light-trapping and waveguiding techniques work and their impact on improving solar cell efficiency: Light Trapping: Light trapping aims to increase the probability of light absorption within the solar cell by prolonging the path length of photons traveling through the active material. In traditional solar cells, a significant portion of incident sunlight can pass through or be reflected, limiting the absorption and conversion of solar energy. Light-trapping structures are designed to capture and confine photons, ensuring that they spend more time interacting with the semiconductor material [44]. Some common light-trapping techniques include: Textured Surfaces: Implementing microstructures or nanostructures on the surface of the solar cell enhances light capture by increasing the surface area and scattering incoming light, thus promoting multiple interactions within the material. Optical Coatings: Anti-reflective coatings are applied to the front surface of the solar cell to reduce reflection losses and allow more light to enter the cell. Plasmonic Nanoparticles: Plasmonic nanoparticles can concentrate and confine light at the nanoscale, boosting light absorption and charge carrier generation. Waveguiding: Waveguiding is a technique used to direct light efficiently within the solar cell, ensuring that a greater proportion of photons reaches the active regions where they can be absorbed. Waveguiding structures are designed to confine and guide light toward specific regions within the solar cell, thereby increasing the probability of photon absorption [44]. Key waveguiding techniques include: Photonic Crystals: Photonic crystals are periodic structures that create bandgaps, preventing certain wavelengths of light from propagating in specific directions. By engineering the bandgap properties, photonic crystals can redirect light toward the active layer of the solar cell. Grating Structures: Gratings consist of periodic structures that act as diffraction elements, redirecting light to specific locations within the solar cell for enhanced absorption.

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Both light-trapping and waveguiding techniques play critical roles in enhancing the efficiency of solar cells. By increasing the absorption of incident light, these approaches mitigate losses and improve the overall energy conversion efficiency. This is especially beneficial for thin-film solar cells, which have reduced material thickness and light absorption capacity compared to traditional silicon-based solar cells. The integration of light-trapping and waveguiding techniques with other advancements in solar cell technology, such as novel materials and tandem solar cell configurations, holds tremendous potential for further efficiency improvements. As researchers continue to explore and optimize these “tricks of the light,” solar cells are poised to become more efficient, cost-effective, and capable of meeting the increasing global energy demand while contributing to a sustainable and greener energy future.

9.7 Spectral Shaping and Photon Upconversion Spectral shaping and photon upconversion are two advanced techniques used to enhance the efficiency of solar cells by optimizing the utilization of different parts of the solar spectrum. These approaches aim to capture a broader range of wavelengths and convert low-energy photons into higher-energy ones, thereby increasing the overall energy conversion efficiency of the solar cell. By harnessing the potential of spectral shaping and photon upconversion, solar cells can achieve higher energy conversion efficiencies, paving the way for a cleaner and more sustainable energy future. Continued research and development in these areas promise to drive solar cell technology forward, making it an increasingly vital component of our global energy landscape [45]. Spectral Shaping: Spectral shaping involves engineering the optical properties of the solar cell to selectively absorb specific wavelengths of light while transmitting or reflecting others. This technique allows the solar cell to efficiently capture photons in the most relevant energy range for the semiconductor material used. Some common approaches to spectral shaping include: Spectrally Selective Coatings: Coatings or materials with tailored absorption characteristics are applied to the solar cell’s surface to selectively absorb certain wavelengths while minimizing losses due to the transmission or reflection of others [46]. Quantum Dots and Nanomaterials: Quantum dots and other nanomaterials can be engineered to have tunable bandgap energies, enabling them to absorb light at specific wavelengths and convert it into charge carriers. Spectral shaping is particularly valuable in tandem solar cell configurations, where multiple layers of semiconductors with varying bandgaps are stacked. By optimizing the absorption of different portions of the solar spectrum, tandem cells can achieve higher efficiency than single-junction solar cells.

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Photon Upconversion Photon upconversion is a process where lower-energy photons are converted into higher-energy photons, which can then be utilized more effectively by the solar cell. This technique allows the solar cell to capture photons with energies below the semiconductor material’s bandgap, which would otherwise be lost as heat. Photon upconversion involves the use of specific materials, often rare-earth ions or organic dyes, that can undergo photon absorption and energy transfer processes [47]. Some photon upconversion mechanisms include: Triplet–Triplet Annihilation: In this process, two low-energy photons are absorbed by a sensitizer molecule, promoting it to an excited state (triplet state). When a third photon is absorbed by the sensitizer, energy is transferred to an emitter molecule, promoting it to a higher-energy state and emitting a higher-energy photon [48]. Two-Photon Absorption: Some materials can absorb two low-energy photons simultaneously, resulting in the emission of a single higher-energy photon [49]. Photon upconversion is particularly useful for increasing the efficiency of silicon solar cells, which have a relatively high bandgap and cannot efficiently utilize lowenergy photons. By implementing spectral shaping and photon upconversion techniques, solar cells can maximize their energy conversion efficiency and improve their performance under varying lighting conditions. These advanced approaches hold great promise for the future of solar cell technology, contributing to a more efficient and sustainable energy landscape. As research in these areas continues, solar cells are poised to play an increasingly significant role in meeting global energy needs and mitigating the impacts of climate change.

9.8 Defect Engineering for Charge Carrier Transport Defect engineering is a powerful technique used to improve the charge carrier transport properties of solar cells, ultimately enhancing their overall efficiency. Defects in the crystal lattice of semiconductor materials can significantly impact the movement of charge carriers (electrons and holes) within the solar cell, leading to recombination losses and reduced energy conversion efficiency. Defect engineering aims to control and manipulate the presence and distribution of defects to minimize their detrimental effects and maximize charge carrier transport [50]. There are several approaches to defect engineering in solar cells: • Passivation of Defects: One of the primary strategies in defect engineering is passivating defects to minimize their impact on charge carrier recombination. By introducing specific chemical elements or compounds, such as hydrogen or oxygen, at defect sites, the recombination centers can be neutralized, allowing charge carriers to move more freely without being trapped [51].

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• Doping and Alloying: Introducing controlled amounts of impurities (doping) or mixing different semiconductor materials (alloying) can modify the electronic properties of the material, reducing the density of harmful defects and improving charge carrier mobility [52]. • Selective Area Doping: By selectively doping specific regions of the solar cell, it is possible to enhance charge carrier extraction and collection, leading to a reduction in recombination losses [52]. • Defect Elimination during Growth: Special growth techniques can be employed during the fabrication of the semiconductor material to minimize the incorporation of defects in the crystal lattice, thereby improving the charge carrier mobility [53]. • Defect Passivating Layers: Adding passivating layers on the surface of the solar cell can reduce the impact of surface defects and prevent surface recombination, which can be a major loss mechanism [51]. • Interface Engineering: Careful design of interfaces between different layers in tandem or multi-junction solar cells can mitigate the impact of defects on charge transport [52]. • Quantum Well Structures: The introduction of quantum well structures can create energy levels that capture and confine charge carriers, preventing their recombination and enhancing their collection [53]. • Defect Engineering in Perovskite Solar Cells: As perovskite solar cells are prone to defects, defect engineering is particularly crucial in this context. Strategies such as annealing, additives, and interface engineering are being explored to minimize defects and improve charge transport in perovskite materials [51]. By employing defect engineering techniques, researchers and engineers can significantly enhance the charge carrier transport properties of solar cells, leading to reduced recombination losses and improved energy conversion efficiency. As solar cell technology continues to advance, defect engineering plays a vital role in maximizing the potential of solar energy as a clean and sustainable power source for a greener and more environmentally friendly future.

9.9 Energy-Selective Contacts for Reduced Recombination Losses Energy-selective contacts are a promising technique used to reduce recombination losses and improve the efficiency of solar cells. Recombination refers to the process where charge carriers (electrons and holes) recombine and lose their energy as heat, rather than contributing to the generation of electricity. Energy-selective contacts are designed to selectively allow charge carriers with specific energy levels to pass through, while blocking or minimizing the passage of carriers with energies that are prone to recombination. This approach helps to maximize the collection of energetic charge carriers and minimize recombination losses, thereby enhancing the overall

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energy conversion efficiency of the solar cell [54]. There are two main types of energy-selective contacts: • Electron-Selective Contacts: Electron-selective contacts, also known as electron transport layers, allow electrons to pass through while hindering the movement of holes. These contacts are typically engineered with materials that have a high electron affinity, creating a favorable energy band alignment that facilitates the efficient extraction and collection of electrons generated within the solar cell [55]. • Hole-Selective Contacts: Hole-selective contacts, also known as hole transport layers, enable holes to move freely while impeding the flow of electrons. These contacts are designed with materials that possess a low electron affinity and a high hole affinity, allowing for effective hole extraction and reducing the chance of electron–hole recombination [42]. Energy-selective contacts are crucial components in many advanced solar cell architectures, such as perovskite, organic, and tandem solar cells. They play a significant role in improving charge carrier extraction and collection, which directly impacts the overall efficiency of the solar cell. By effectively controlling the movement of charge carriers, energy-selective contacts contribute to minimizing recombination losses and maximizing the conversion of solar energy into usable electricity. Researchers are continuously exploring new materials and engineering approaches to further optimize energy-selective contacts and push the boundaries of solar cell efficiency. As solar cell technology continues to progress, energy-selective contacts are expected to play a vital role in advancing the adoption of solar energy as a clean and sustainable power source for the future.

9.10 Future Perspectives To outpace current solar cells, a new design would need to be able to capture more light, transform light energy to electricity more efficiently, and/or be less expensive to build than current designs. Energy producers and consumers are more likely to adopt solar power if the energy it produces is equally or less expensive than other, often non-renewable, forms of electricity, so any improvement to current solar cell designs must bring down overall costs to become widely used. The first option, adding hardware that allows the solar cells to capture more light, does not actually require that we abandon current solar cell designs. Electronics can be installed with the solar cell that let the cell track the sun as it moves through the daytime sky. If the solar cell is always pointing at the sun, it will be hit by many more photons than if it was only pointing toward the sun around midday. Currently, designing electronics that can track the position of the sun accurately and consistently for several decades at a reasonable cost is an ongoing challenge, but innovation on this front continues. An alternative to making the solar cell itself move is to use mirrors to focus light on a smaller, and therefore, cheaper solar cell [56].

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Another route to improving the performance of solar cells is to target their efficiency, so they are better at converting energy in sunlight to electricity. Solar cells with more than one layer of light-capturing material can capture more photons than solar cells with only a single layer. Recently, lab-tested solar cells with four layers can capture 46% of the incoming light energy that hit them. These cells are still mostly too expensive and difficult to make for commercial use, but ongoing research may one day make implementing these superefficient cells possible. The alternative to improving the efficiency of solar cells is simply decreasing their cost. Even though processing silicon has become cheaper over the past few decades, it still contributes significantly to the cost of solar cell installation. By using thinner solar cells, material costs decrease. These “thin-film solar cells” use a layer of material to harvest light energy that is only 2–8 µm thick, only about 1% of what is used to make a traditional solar cell. Much like cells with multiple layers, thin-film solar cells are a bit tricky to manufacture, which limits their application, but research is ongoing [57]. In the immediate future, silicon solar cells are likely to continue to decrease in cost and be installed in large numbers. In the USA, these cost decreases are anticipated to increase the solar power produced by at least 700% by 2050. Meanwhile, research on alternative designs for more efficient and less expensive solar cells will continue. Years from now, we are likely to see alternatives to silicon appearing on our solar farms and rooftops, helping to provide clean and renewable sources of energy. These improvements have and will continue to be made possible by increasing bulk manufacturing of solar cells and new technologies that make the cells cheaper and more efficient.

9.11 Summary In conclusion, the future of solar cell technology is exceptionally promising, offering a myriad of advancements and innovations that can revolutionize the renewable energy landscape. Over the years, solar cells have evolved from basic silicon-based systems to encompass a diverse array of materials, manufacturing processes, and efficiency-enhancing techniques. The pursuit of higher energy conversion efficiencies, reduced production costs, and improved sustainability has driven researchers and engineers to explore novel materials, such as perovskites and organic compounds, as well as advanced concepts like tandem and multi-junction solar cells. Additionally, “tricks of the light,” including spectral shaping, photon upconversion, lighttrapping, waveguiding, and energy-selective contacts, are being harnessed to optimize light absorption and charge carrier generation, further boosting solar cell efficiency. Nanotechnology and defect engineering are playing pivotal roles in manipulating material properties, minimizing recombination losses, and enhancing charge carrier transport. Moreover, solar cell technology is increasingly being integrated with energy storage solutions, enabling a more stable and reliable energy supply, even during periods of low solar irradiance. As solar cells become more cost-effective and

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efficient, they will find widespread applications in residential, commercial, industrial, and utility-scale settings, transforming how we generate and consume electricity. The future of solar cell technology also involves environmental considerations, such as eco-friendly manufacturing practices, recycling, and sustainable end-of-life management, making solar energy an even more environmentally friendly option compared to conventional fossil fuels. While challenges in scaling up production, energy storage, and grid integration persist, the collaborative efforts of governments, industries, and academia worldwide are driving rapid advancements in solar cell technology. With continuous research, policy support, and technological innovations, solar cells are set to become an increasingly integral part of the global energy mix, powering a greener and more sustainable future for generations to come. As the world embraces solar energy and other renewable sources, the vision of a carbon-neutral and climate-resilient world becomes ever closer, making solar cell technology a key enabler in the journey toward a more sustainable and brighter tomorrow.

9.12 Important Timelines • 1839: French physicist Alexandre Edmond Becquerel observed the photovoltaic effect, discovering that certain materials produce an electric current when exposed to light. This marked the beginning of the understanding of solar energy conversion. • 1883: The first solar cell was built by Charles Fritts, an American inventor, using selenium coated with a thin layer of gold. While inefficient by current standards, this early cell set the groundwork for subsequent advances. • 1954: Bell Laboratories researchers Daryl Chapin, Calvin Fuller, and Gerald Pearson developed the first practical silicon solar cell, achieving 6% efficiency. This breakthrough led to the commercialization of solar cells for space applications. • 1970s: The oil crisis during the 1970s sparked increased interest in renewable energy sources, including solar cells, as a viable alternative to fossil fuels for power generation. • 1980s: Photovoltaic technology began to find applications beyond space missions. Solar cells were increasingly used for powering remote locations, telecommunications infrastructure, and off-grid applications. • 1990s: Research and development efforts have focused on improving solar cell efficiency and reducing manufacturing costs. Thin-film solar cell technologies, such as amorphous silicon and cadmium telluride, gained traction. • Early 2000s: Multi-junction solar cells, originally developed for space applications, started making their way into terrestrial use. These cells employ multiple semiconductor layers to capture a broader spectrum of sunlight, leading to higher efficiency.

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• 2009: Researchers achieved a significant milestone by developing perovskite solar cells with over 10% efficiency, showcasing the potential of this emerging material in future solar cell technology. • 2010s: Perovskite solar cells rapidly advanced, with efficiency reaching over 20%, rivaling traditional silicon solar cells. The focus shifted toward stability and commercial scalability. • 2017: Tandem solar cells, combining perovskite and silicon layers, achieved record-breaking efficiencies, opening up possibilities for more efficient and cost-effective solar energy conversion. • Present and Beyond: The current decade sees ongoing efforts to enhance the efficiency, stability, and reliability of solar cell technologies. Advancements in materials, manufacturing, and integration techniques continue to shape the future of solar cells, with an emphasis on building integration, smart grid applications, and sustainable energy solutions.

9.13 Points to Remember • Advancements in perovskite solar cells, with high efficiency potential and ease of manufacturing, are driving the next generation of solar technology. • Tandem and multi-junction solar cells stack multiple layers of semiconductors to maximize light absorption and boost energy conversion efficiency. • Transparent solar cells integrated into windows and surfaces enable buildingintegrated photovoltaics without obstructing the view. • Energy storage integration with solar cells ensures stable energy supply even during periods of low sunlight. • Nanotechnology and nanomaterials are utilized for precise control of material properties, improving solar cell performance. • Defect engineering techniques minimize recombination losses and enhance charge carrier transport in solar cell materials. • Photon upconversion enables efficient utilization of low-energy photons, enhancing solar cell efficiency. • AI and machine learning optimize solar cell design, manufacturing, and performance through data-driven analysis. • Bifacial solar cells capture sunlight from both sides, increasing electricity generation capabilities. • Perovskite–silicon tandem solar cells combine the strengths of both materials to achieve record-breaking efficiency levels. • Space-based solar power proposes harvesting solar energy in space and transmitting it to earth, offering a constant energy supply. • Solar-powered desalination and water treatment contribute to sustainable access to clean water using solar energy. • Solar paint and solar textiles enable solar energy harvesting in everyday objects and surfaces.

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• Advanced materials like quantum dots and organic compounds offer potential for higher efficiency and lower production costs. • Perovskite solar cells hold great promise but require ongoing research to address stability and scalability challenges. • Artificial intelligence and machine learning optimize solar energy systems for greater efficiency and cost-effectiveness. • Grid-scale energy storage solutions complement solar cells, ensuring renewable energy reliability and grid stability. • Perovskite solar cells integrated into windows could transform buildings into energy-generating structures. • Bifacial solar cells offer potential for increased power output and more versatile installation options.

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