Fundamentals of Solar Cell Design [1 ed.] 1119724708, 9781119724704

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Fundamentals of Solar Cell Design

Scrivener Publishing 100 Cummings Center, Suite 541J Beverly, MA 01915-6106 Publishers at Scrivener Martin Scrivener ([email protected]) Phillip Carmical ([email protected])

Fundamentals of Solar Cell Design

Edited by

Inamuddin, Mohd Imran Ahamed, Rajender Boddula, and Mashallah Rezakazemi

This edition first published 2021 by John Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 07030, USA and Scrivener Publishing LLC, 100 Cummings Center, Suite 541J, Beverly, MA 01915, USA © 2021 Scrivener Publishing LLC For more information about Scrivener publications please visit www.scrivenerpublishing.com. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, except as permitted by law. Advice on how to obtain permission to reuse material from this title is available at http://www.wiley.com/go/permissions. Wiley Global Headquarters 111 River Street, Hoboken, NJ 07030, USA For details of our global editorial offices, customer services, and more information about Wiley products visit us at www.wiley.com. Limit of Liability/Disclaimer of Warranty While the publisher and authors have used their best efforts in preparing this work, they make no rep­ resentations or warranties with respect to the accuracy or completeness of the contents of this work and specifically disclaim all warranties, including without limitation any implied warranties of merchant-­ ability or fitness for a particular purpose. No warranty may be created or extended by sales representa­ tives, written sales materials, or promotional statements for this work. The fact that an organization, website, or product is referred to in this work as a citation and/or potential source of further informa­ tion does not mean that the publisher and authors endorse the information or services the organiza­ tion, website, or product may provide or recommendations it may make. This work is sold with the understanding that the publisher is not engaged in rendering professional services. The advice and strategies contained herein may not be suitable for your situation. You should consult with a specialist where appropriate. Neither the publisher nor authors shall be liable for any loss of profit or any other commercial damages, including but not limited to special, incidental, consequential, or other damages. Further, readers should be aware that websites listed in this work may have changed or disappeared between when this work was written and when it is read. Library of Congress Cataloging-in-Publication Data ISBN 9781119724704 Cover image: Pixabay.com Cover design by Russell Richardson Set in size of 11pt and Minion Pro by Manila Typesetting Company, Makati, Philippines Printed in the USA 10 9 8 7 6 5 4 3 2 1

Contents Preface xv 1 Organic Solar Cells 1 Yadavalli Venkata Durga Nageswar and Vaidya Jayathirtha Rao 1.1 Introduction 1 1.2 Classification of Solar Cells 3 1.3 Solar Cell Structure 4 1.4 Photovoltaic Parameters or Terminology Used in BHJOSCs 5 1.4.1 Open-Circuit Voltage Voc 5 1.4.2 Short-Circuit Current Jsc 5 1.4.3 Incident-Photon-to-Current Efficiency (IPCE) 5 1.4.4 Power Conversion Efficiency ηp (PCE) 6 1.4.5 Fill Factor (FF) 6 1.5 Some Basic Design Principles/Thumb Rules Associated With Organic Materials Required for BHJOSCs 6 1.6 Recent Research Advances in Small-Molecule Acceptor and Polymer Donor Types 7 1.7 Recent Research Advances in All Small-Molecule Acceptor and Donor Types 30 1.8 Conclusion 47 Acknowledgement 48 References 48 2 Plasmonic Solar Cells T. Shiyani, S. K. Mahapatra and I. Banerjee 2.1 Introduction 2.1.1 Plasmonic Nanostructure 2.1.2 Classification of Plasmonic Nanostructures 2.2 Principles and Working Mechanism of Plasmonic Solar Cells

55 56 58 59 60 v

vi  Contents 2.2.1 Working Principle 2.2.2 Mechanism of Plasmonic Solar Cells 2.3 Important Optical Properties 2.3.1 Trapping of Light 2.3.2 Scattering and Absorption of Sunlight 2.3.3 Multiple Energy Levels 2.4 Advancements in Plasmonic Solar Cells 2.4.1 Direct Plasmonic Solar Cells 2.4.2 Plasmonic-Enhanced Solar Cell 2.4.3 Plasmonic Thin Film Solar Cells 2.4.4 Plasmonic Dye-Sensitized Solar Cells (PDSSCs) 2.4.5 Plasmonic Photoelectrochemical Cells 2.4.6 Plasmonic Quantum Dot (QD) Solar Cells 2.4.7 Plasmonic Perovskite Solar Cells 2.4.8 Plasmonic Hybrid Solar Cells 2.5 Conclusion and Future Aspects Acknowledgements References 3 Tandem Solar Cell Umesh Fegade List of Abbreviations 3.1 Introduction 3.2 Review of Organic Tandem Solar Cell 3.3 Review of Inorganic Tandem Solar Cell 3.4 Conclusion References 4 Thin-Film Solar Cells Gobinath Velu Kaliyannan, Raja Gunasekaran, Santhosh Sivaraj, Saravanakumar Jaganathan and Rajasekar Rathanasamy 4.1 Introduction 4.2 Why Thin-Film Solar Cells? 4.3 Amorphous Silicon 4.4 Cadmium Telluride 4.5 Copper Indium Diselenide Solar Cells 4.6 Comparison Between Flexible a-Si:H, CdTe, and CIGS Cells and Applications 4.7 Conclusion References

60 61 62 63 63 63 64 65 69 69 70 71 71 72 72 72 73 73 83 83 85 86 89 95 96 103

104 105 105 108 111 112 113 114

Contents  vii 5 Biohybrid Solar Cells Sapana Jadoun and Ufana Riaz Abbreviations 5.1 Introduction 5.2 Photovoltaics 5.3 Solar Cells 5.3.1 First-Generation 5.3.2 Second-Generation 5.3.3 Third-Generation 5.3.4 Fourth-Generation 5.4 Biohybrid Solar Cells 5.5 Role of Photosynthesis 5.6 Plant-Based Biohybrid Devices 5.6.1 PS I–Based Biohybrid Devices 5.6.2 PS II–Based Biohybrid Devices 5.7 Dye-Sensitized Solar Cells 5.8 Polymer and Semiconductors-Based Biohybrid Solar Cells 5.9 Conclusion References

117

6 Dye-Sensitized Solar Cells Santhosh Sivaraj, Gobinath Velu Kaliyannan, Mohankumar Anandraj, Moganapriya Chinnasamy and Rajasekar Rathanasamy 6.1 Introduction 6.2 Cell Architecture and Working Mechanism 6.3 Fabrication of Simple DSSC in Lab Scale 6.4 Electrodes 6.5 Counter Electrode 6.6 Blocking Layer 6.7 Electrolytes Used 6.7.1 Liquid-Based Electrolytes 6.7.1.1 Electrical Additives 6.7.1.2 Organic Solvents 6.7.1.3 Ionic Liquids 6.7.1.4 Iodide/Triiodide-Free Mediator and Redox Couples 6.7.2 Quasi-Solid-State Electrolytes 6.7.2.1 Thermoplastic-Based Polymer Electrolytes 6.7.2.2 Thermosetting Polymer Electrolytes

137

117 118 119 119 120 120 120 121 121 122 122 123 125 126 126 129 129

138 139 142 144 145 146 147 148 148 148 149 149 149 150 150

viii  Contents 6.7.3 Solid-State Transport Materials 6.7.3.1 Inorganic Hole Transport Materials 6.7.3.2 Organic Hole Transport Materials 6.7.3.3 Solid-State Ionic Conductors 6.8 Commonly Used Natural Dyes in DSSC 6.8.1 Chlorophyll 6.8.2 Flavonoids 6.8.3 Anthocyanins 6.8.4 Carotenoids 6.9 Calculations 6.9.1 Power Conversion Efficiency 6.9.2 Fill Factor 6.9.3 Open-Circuit Voltage 6.9.4 Short Circuit Current 6.9.5 Determination of Energy Gap of Electrode Material Adsorbed With Natural Dye 6.9.6 Absorption Coefficient 6.9.7 Dye Adsorption 6.10 Conclusion References

150 151 151 151 152 152 152 153 154 154 154 163 163 163

7 Characterization and Theoretical Modeling of Solar Cells Masoud Darvish Ganji, Mahyar Rezvani and Sepideh Tanreh 7.1 Introduction 7.2 Classification of SC 7.2.1 Inorganic Solar Cells 7.2.2 Organic Solar Cell 7.3 Working Principle of DSSC 7.4 Operation Principle of DSSC 7.5 Photovoltaic Parameters 7.6 Theoretical and Computational Methods 7.6.1 Density Functional Theory (DFT) 7.6.2 Basis Sets 7.6.3 TDDFT Method 7.6.4 Molecular Descriptors 7.6.5 Force Field Parameterization for MD Simulations 7.6.6 Excited States 7.6.7 UV-Vis Spectroscopy 7.6.8 Charge Transfer and Carrier Transport 7.6.9 Coarse-Grained (CG) Simulations

169

163 164 164 164 165

170 172 173 173 175 176 177 181 182 183 183 184 188 189 190 192 193

Contents  ix 7.6.10 Kinetic Monte Carlo (KMC) Modeling 7.6.11 Car-Parrinello Method 7.6.12 Solvent Effects 7.6.13 Global Reactivity Descriptors 7.7 Conclusion References

193 195 196 196 198 199

8 Efficient Performance Parameters for Solar Cells 217 Figen Balo and Lutfu S. Sua 8.1 Introduction 218 8.1.1 Potential, Production, and Climate of Ankara 225 8.2 Solar Radiation Intensity Calculation 225 8.2.1 Horizontal Superficies 225 8.2.1.1 On a Daily Basis Total Sun Irradiation 225 8.2.1.2 Daily Diffuse Sun Irradiation 227 8.2.1.3 Momentary Total Sun Irradiation 227 8.2.1.4 Direct and Diffuse Sun Radiation 228 8.2.2 On Inclined Superficies, Computing Sun Irradiation Intensity 228 8.2.2.1 Direct Momentary Sun Radiation 228 8.2.2.2 Diffuse Sun Radiation 228 8.2.2.3 Momentary Reflecting Radiation 229 8.2.2.4 Total Sun Radiation 229 8.3 Methodology 229 8.3.1 The Solar Radiation Assessments by Correlation Models With MATLAB Simulation Software 229 8.3.2 MATLAB Simulation Results and Findings 233 8.3.3 For Ankara Province, the Determinants of the Most Efficiency Solar Cell With AHP Methodology 233 8.4 Conclusions 238 References 240 9 Practices to Enhance Conversion Efficiencies in Solar Cell Andreea Irina Barzic 9.1 Introduction 9.2 Basics on Conversion Efficiency 9.3 Approaches for Improving Conversion Efficiencies in Solar Cells 9.4 Conclusion Acknowledgements References

247 247 249 253 264 264 265

x  Contents 10 Solar Cell Efficiency Energy Materials Zeeshan Abid, Faiza Wahad, Sughra Gulzar, Muhammad Faheem Ashiq, Muhammad Shahid Aslam, Munazza Shahid, Muhammad Altaf and Raja Shahid Ashraf 10.1 Introduction 10.2 Solar Cell Efficiency 10.3 Historical Development of Solar Cell Materials 10.4 Solar Cell Materials and Efficiencies 10.4.1 Crystalline Silicon 10.4.2 Silicon Thin-Film Alloys 10.4.3 III-V Semiconductors 10.4.4 Chalcogenide 10.4.4.1 Chalcopyrites 10.4.4.2 Cadmium Telluride (CdTe) 10.4.5 Organic Materials 10.4.6 Hybrid Organic-Inorganic Materials 10.4.6.1 Dye-Sensitized Solar Cell Materials 10.4.6.2 Perovskites 10.4.7 Quantum Dots 10.5 Conclusion and Prospects References

271

11 Analytical Tools for Solar Cell Mohamad Saufi Rosmi, Ong Suu Wan, Mohamad Azuwa Mohamed, Zul Adlan Mohd Hir and Wan Nur Aini Wan Mokhtar 11.1 Introduction 11.2 Transient Absorption Spectroscopy 11.2.1 Application of Transient Absorption Spectroscopy in Solar Cells 11.3 Electron Tomography  11.3.1 Application of Electron Tomography (ET) in Solar Cells 11.4 Conductive Atomic Force Microscopy (C-AFM) 11.4.1 Application of C-AFM in Solar Cells 11.5 Kelvin Probe Force Microscopy 11.5.1 Application of Scanning Kelvin Probe Force Microscopy for Solar Cells 11.6 Field Emission Scanning Electron Microscopy and Transmission Electron Microscopy

317

272 274 275 277 278 282 284 287 287 288 289 293 293 296 300 302 303

318 319 320 323 324 327 329 330 334 335

Contents  xi 11.6.1 Application of Field Emission Scanning Electron Microscopy and Transmission Electron Microscopy in Solar Cell 11.7 Conclusion References

338 340 340

12 Applications of Solar Cells Mohd Imran Ahamed and Naushad Anwar 12.1 Introduction 12.2 An Overview on Photovoltaic Cell 12.2.1 History 12.2.2 Working Principle of Solar Cell 12.2.3 First-Generation Photovoltaic Cells: Crystalline Silicon Form 12.2.4 Second-Generation Photovoltaic Cells: Thin-Film Solar Cells 12.2.5 Third-Generation Photovoltaic Cells 12.3 Applications of Solar Cells 12.3.1 Perovskite Solar Cell 12.3.2 Dye-Sensitized Solar Cell 12.3.3 Nanostructured Inorganic-Organic Heterojunction Solar Cells (NSIOHSCs) 12.3.4 Polymer Solar Cells 12.3.5 Quantum Dot Solar Cell (QDCs) 12.3.6 Organic Solar Cells 12.4 Conclusion and Summary References

345

13 Challenges of Stability in Perovskite Solar Cells Mutayyab Afreen, Jazib Ali and Muhammad Bilal 13.1 Introduction 13.2 Degradation Phenomena and Stability Measures in Perovskite 13.2.1 Thermal Stability 13.2.2 Structural and Chemical Stability 13.2.3 Oxygen and Moisture 13.2.4 Visible and UV Light Exposure 13.3 Stability-Interface Interplay 13.3.1 Chemical Reaction at the Interface 13.3.2 Degradation on the Top Electrode 13.3.3 Hysteresis Phenomenon in PSC Devices

371

345 348 348 348 351 352 353 354 354 355 356 357 358 360 362 362

371 373 373 375 376 378 379 379 380 381

xii  Contents 13.4 Effect of Selective Contacts on Stability 13.4.1 Electron-Transport Layers 13.4.2 Hole Transport Layers 13.4 Conclusion References

382 382 384 387 387

14 State-of-the-Art and Prospective of Solar Cells 393 Zahra Pezeshki and Abdelhalim Zekry Acronyms 393 14.1 Introduction 396 14.2 State-of-the-Art of Solar Cells 396 14.2.1 Production Volume 400 14.2.2 Cost Breakdown 400 14.2.3 Main Technologies 401 14.2.3.1 Si Solar Cell Arrays 401 14.2.3.2 DSSCs 403 14.2.3.3 Photoanodes 404 14.2.3.4 C/Si Heterojunctions 404 14.2.3.5 a-C/Si Heterojunctions 410 14.2.3.6 Non-Fullerene Acceptor Bulk Heterojunctions 410 14.2.3.7 a-Si 411 14.2.3.8 Perovskites 411 14.2.3.9 Metal-Halide–Based Perovskites 413 14.2.3.10 Sn-Based Perovskites 415 14.2.3.11 Heavily Doped Solar Cells 416 14.2.3.12 PV Building Substrates 416 14.2.3.13 Solar Tracking System 422 14.2.3.14 Solar Concentrators 425 14.2.3.15 Solar Power Satellite 426 14.2.3.16 Roof-Top Solar PV System 427 14.2.3.17 Short-Wavelength Solar-Blind Detectors 428 14.2.3.18 GCPVS 429 14.2.3.19 Microwave Heating in Si Solar Cell Fabrication 431 14.2.3.20 Refrigeration PV System 432 14.2.3.21 Solar Collectors and Receivers 433 14.2.3.22 Solar Drying System 435 14.2.3.23 Water Networks With Solar PV Energy 436

Contents  xiii 14.2.3.24 Wind and Solar Integrated to Smart Grid 14.2.3.25 Green Data Centers 14.3 Prospective of Solar Cells 14.4 Conclusion References

437 440 443 445 447

15 Semitransparent Perovskite Solar Cells Faiza Wahad, Zeeshan Abid, Sughra Gulzar, Muhammad Shahid Aslam, Saqib Rafique, Munazza Shahid, Muhammad Altaf and Raja Shahid Ashraf 15.1 Introduction 15.2 Device Architectures 15.2.1 Conventional n-i-p Device Structure 15.2.2 Inverted p-i-n Device Structure 15.3 Optical Assessment 15.3.1 Average Visible Transmittance 15.3.2 Corresponding Color Temperature 15.3.3 Color Rendering Index 15.3.4 Transparency Color Perception 15.3.5 Light Management 15.4 Materials 15.4.1 Photoactive Layer 15.4.2 Charge Transport Layers (ETL and HTL) 15.4.3 Transparent Electrode 15.5 Applications 15.5.1 Building-Integrated Photovoltaics 15.5.2 Tandem Devices 15.6 Conclusion References

461

16 Flexible Solar Cells Santosh Patil, Rushi Jani, Nisarg Purabiarao, Archan Desai, Ishan Desai and Kshitij Bhargava 16.1 Introduction 16.1.1 Need for Solar Energy Harnessing 16.1.2 Brief Overview of Generations of Solar Cells 16.1.3 Limitations of Solar Cells 16.1.4 What is Flexible Solar Cell (FSC)?

505

462 464 465 465 466 466 467 468 468 471 474 474 479 481 484 484 486 492 492

505 505 506 508 509

xiv  Contents 16.2 Materials for FSCs 16.2.1 Semiconductors 16.2.2 Substrates 16.2.3 Electrodes 16.2.4 Encapsulations 16.3 Thin-Film Deposition 16.3.1 R2R Processing 16.3.2 Chemical Bath Deposition 16.3.3 Chemical Vapor Deposition 16.3.4 Dip Coating 16.3.5 Spin Coating 16.3.6 Screen Printing 16.4 Characterizations for FSCs 16.4.1 Material Characterization 16.4.2 Device Characterization 16.5 Issues in FSCs 16.6 Performance Comparison of RSCs and FSCs 16.7 Applications of Flexible Solar Cell 16.8 Conclusion References

510 510 512 513 514 514 515 516 517 518 520 521 522 523 529 531 532 532 533 534

Index 537

Preface Solar cells are semiconductor devices that convert light photons into electricity in photovoltaic energy conversion and can help to overcome the global energy crisis. Solar cells have many applications including remote area power systems, earth-orbiting satellites, wristwatches, water pumping, photodetectors, and remote radiotelephones. Solar cell technology is economically feasible for commercial-scale power generation. While commercial solar cells exhibit good performance and stability, still researchers are looking at many ways to improve the performance and cost of solar cells via modulating the fundamental properties of semiconductors. Solar cell technology is the key to a clean energy future. Solar cells directly harvest energy from the sun’s light radiation into electricity are in an ever-­ growing demand for future global energy production. Solar cell–based energy harvesting has attracted worldwide attention for their notable features, such as cheap renewable technology, scalable, light-weight, flexibility, versatility, no greenhouse gas emission, environment, and economy friendly, and operational costs are quite low compared to other forms of power generation. Thus, solar cell technology is at the forefront of renewable energy technologies which are used in telecommunications, power plants and small devices to satellites. Aiming at large-scale implementation can be manipulated by various types used in solar cell design and exploration of new materials toward improving performance and reducing cost. Therefore, in-depth knowledge about solar cell design is fundamental for those who wish to apply this knowledge and understanding in industries and academics. This book provides a comprehensive overview on solar cells and explores the history to evolution and present scenarios of solar cell design, classification, properties, various semiconductor materials, thin films, wafer-scale, transparent solar cells, and so on. It also includes solar cells’ characterization analytical tools, theoretical modeling, practices to enhance conversion efficiencies, applications, and patents. This book is a unique reference

xv

xvi  Preface guide that can be used by faculty, students, researchers, engineers, device designers, and industrialists who are working and learning in the fields of semiconductors, chemistry, physics, electronics, light science, material science, flexible energy conversion, industrial, and renewable energy sectors. This book includes the 16 chapters and the summaries are given below. Chapter 1 highlights a variety of organic solar cells, documented in recent literature, developed to study solar cell efficiencies, with polymer donors and organic small molecule acceptors or as donors and acceptors. Chapter 2 discusses the plasmonic solar cells with a focus on the fundamental principle of solar cell, types and design of plasmonic metallic nanostructures and devices, novel properties of surface plasmon resonance, and energy conversion efficiency. The chapter explains about device mechanisms, solar cell design, and advancements in plasmonic solar cells to generate clean energy and solar fuels. Chapter 3 discusses the current problem of the energy crisis, depletion of conventional energy resource, and serious threat of global warming. The chapter discusses the Tandem solar cells’ developments in the last few years. Chapter 4 discusses solar cells based on three different thin films, e.g., amorphous silicon, cadmium telluride, and copper indium gallium selenide. Additionally, the structure of thin films and various coating techniques are discussed. Moreover, this chapter summarizes the modifications and performance improvement of thin-film solar cells. Chapter 5 comprises a brief discussion about the biohybrid solar cells, suitable substrate selection for fabrication, as well as the role of photosynthesis in biohybrid solar cells. The chapter discusses some biomimetic approaches borrowed from photosynthetic organisms and plants which can be implemented in biohybrid solar cells. Chapter 6 deals with various features of dye-sensitized solar cells (DSSCs). Here, the simple construction and working mechanism besides various components of DSSCs are elaborately discussed. Also, the various materials used for electrolytes and natural dyes are explained briefly along with performances of DSSCs. Chapter 7 addresses various computational methodologies from molecular mechanics to quantum mechanics for evaluation of compounds in terms of structural and electronic properties. The main objective is the assessment of photovoltaic parameters including absorption spectra, charge transfer, open-circuit voltage, peak current density, efficiency in light, and molecular descriptors toward the efficient performance of solar cells.

Preface  xvii Chapter 8 provides a solar radiation analysis of a region through the use of deterministic models following the specified climatic circumstances. The analysis includes a quantitative analysis for the selection of optimum solar system equipment. Chapter 9 describes the developments in photovoltaic materials and related devices. Brief description of solar cell generations and the factors that affect efficiency are reviewed. The prospects regarding practices to enhance the conversion efficiencies are shortly presented. Chapter 10 discusses the efficiencies and materials of conventional, modern, and emerging solar cell technologies. The use of inorganic, organic, and hybrid materials for a rational design of solar cells is discussed in detail. Additionally, the challenges faced by solar cell technologies and performance enhancement techniques are also discussed briefly. Chapter 11 highlights the latest and emerging characterization tools to study and investigate the properties and efficiency of solar cells. The emerging characterization tools discussed in this chapter are conductive atomic force microscopy, electron tomography, transient absorption spectroscopy, Kelvin probe microscopy, and surface morphology observation. Chapter 12 briefly describes the historic evolution, fundamental properties, and working principles of photovoltaic cells of various types. The discussion about the efficiency and applications of these solar cells helps the new researchers to develop new technologies and improve their work in the area of solar cell systems. Chapter 13 gives a detailed overview of the current efforts to enhance the stability of perovskite solar cell; moreover, the degradation causes and mechanisms are summarized. The strategies to improve device stability are portrayed in terms of structural effects, a photoactive layer, hole- and ­electron-transporting layers, electrode materials, and device encapsulation. Chapter 14 presents the progress of solar cells and their latest developments. The major goal is to show how they can be utilized for photovoltaic energy generation as a renewable energy source. This will help to identify the challenges and drawing prospects for the researchers in this field to further improve and develop solar cells and their applications. Chapter 15 discusses the design, materials, and applications of semitransparent perovskite solar cells. Different device architectures and the performance evaluation parameters are discussed in detail. The materials used in the photoactive layer, charge transport layers, and transparent electrodes are also presented in addition to the major applications and future scope of semitransparent perovskite solar cells.

xviii  Preface Chapter 16 presents an overview of flexible solar cell technology. The various aspects of this technology such as material requirements, and material and cell level characterization techniques and applications are discussed in detail. The chapter is primarily focused on developing an understanding of the current status and future challenges of flexible photovoltaic technology. Inamuddin Mohd Imran Ahamed Rajender Boddula Mashallah Rezakazemi June 2021

1 Organic Solar Cells Yadavalli Venkata Durga Nageswar1* and Vaidya Jayathirtha Rao2 1

CSIR - Indian Institute of Chemical Technology, Hyderabad, India 2 Hetero Research Foundation, TSIE, Balanagar, Hyderabad, India

Abstract

Limitations faced in using fullerene as an acceptor molecule in BHJOSCs directed research toward non-fullerene–based acceptors in BHJOSCs. Polymer donor and small-molecule acceptor combination is successfully explored to develop higher performance BHJOSCs. Various novel small acceptor organic materials are synthesized and fabricated as sBHJOSCs in combination with suitable polymer donors available. Performances of organic solar cells improved to over 17%, and further, it may cross even 20%. Simultaneously, researchers explored fullerene all small molecules for BHJOSCs. All small-molecule BHJOSCs do not use polymer donor due to certain limitations. Progress achieved from these investigations is remarkable and the efficiency displayed is around 14%. Both the research lines are found to be exceptional and will provide further improvement in the solar cell efficiency. Various examples discussed in this chapter deal with the recent research results reported in the literature on both the research domains. Keywords:  UV-visible absorption, device architecture, film morphology, ­non-fullerene blends, all small organic molecules, optical band-gap, photovoltaic parameters, photo conversion efficiency

1.1 Introduction Wind energy is renewable, land around turbine may be used for agriculture, and, further, newer technologies may provide better ways of

*Corresponding author: [email protected] Inamuddin, Mohd Imran Ahamed, Rajender Boddula, and Mashallah Rezakazemi (eds.) Fundamentals of Solar Cell Design, (1–54) © 2021 Scrivener Publishing LLC

1

2  Fundamentals of Solar Cell Design converting wind energy. There are certain limitations in the wind energy conversion like: unreliable wind source, low production of electricity, higher capital cost, environmental damages, and higher level of noise production. Coal energy is also in practice in some countries and they may be continued, but there are disadvantages like carbon-dioxide production, environmental pollution due to coal burn waste accumulation, and problems associated with coal mining. Fossil fuels are also in practice for generating electricity in some countries, because they are inexpensive, portable, and easily burnt. Limitations to fossil fuels are nonrenewable source, emissions due to burning, and global warming. Geothermal energy is a base load energy source, safer than fossil fuels, and globally sustainable. Disadvantages of this geothermal energy are contamination of unwanted trace elements, localized depletion of energy, and energy imbalance leading to geological instability. Hydrothermal energy for the production of electricity is also in practice in some countries, which has advantages like controlled way of electricity production, ease of water pooling in dams, and utilization of water released after electricity production for irrigation/agriculture. Other disadvantages are higher capital costs, ecological and environmental disturbance, possible wreckage of dams leading to flooding, and hostilities arising due to improper sharing of water. Nuclear energy is another form of energy that can be utilized for generating electricity and is practiced in some countries. It is relatively a clean energy, most concentrated energy and does not require big places/areas. The disadvantage is potential accidental hazards due to control deficiencies, management failures, and possible leaks (nuclear fission process). Energy from SUN is the most abundant form of renewable energy reaching the earth and is known as Solar Energy. Radiant light and heat emanating from SUN and reaching the earth can be utilized for various purposes. Solar energy is renewable, clean, and green and is a secured type of energy reaching earth 200,000 times more than the electrical energy generated in a day on the earth. Earth receives 174 peta Watts of sun radiation, in the form of 8% UV radiation, 46% visible light, and 46% infrared radiations, and is absorbed by earth atmosphere, oceans, and land mass. Because of the clean, green, and abundant renewable energy coming from sun is found to be more attractive for the researchers to work on the conversion of light energy in to electricity; presently, this has become a globally attractive and potential research domain to devise solar energy trapping units in to usable form of electricity, which can serve global energy requirements. Solar cell or photovoltaic (PV) cell is a device or unit which converts light

Organic Solar Cells  3 in to electricity and globally research scientists are making all out efforts to prepare an efficient solar cell with an excellent photo-conversion efficiency, such that it can be developed as viable technology for society. Silicon solar cells are already in the hands of citizens/public having ~26% efficiency with some limitations, and this situation placed research on organic solar cells the most demanding and desirable field.

1.2 Classification of Solar Cells The possible classification of solar cells is given in Figure 1.1. Among the many (Figure 1.1), organic solar cells attracted rigorous attention because of various advantages like simple preparation of organic solar materials, light weight (low density), low cost, flexibility of the PV modules, semitransparency, easy integration in to other products, low environmental impact, easy adoption of printing technology, and large area of fabrication.

SOLAR CELL

Organic Solar Cell

Inorganic Solar Cell

Perovskites

Thin Film

FullerenePolymer

Silicon Solar Cell

Dyesensitized

All Small Molecule

Crystalline

Thin Film

Polymer-Small Molecule FullereneSmallMolecule

Polycrystal (Popular)

Figure 1.1  Classification of solar cells.

Single Crystal

Compound Semiconductor

Thin Film (CIGS, CdTe)

Single Crystal (GaAs)

4  Fundamentals of Solar Cell Design Thin film solar cells are further put in to four categories. The two categories involving fullerenes have found limitations in due course of research investigations, although research was conducted on fullerene-based OSC over two decades. The other two categories, non-fullerene polymer smallmolecule and non-fullerene all small-molecule OSC, based on present scenario, are intensively investigated. Therefore, this chapter will be focused only on these two non-fullerene–based polymer-small-molecule and all small-molecule OSC.

1.3 Solar Cell Structure Fundamental steps occurring in a schematic representation of a typical solar cell device and its functioning are schematically provided in Figures 1.2 and 1.3. (a) Typical OSC devices based on donor-acceptor in bulk hetero-junction configuration, another way it is the sandwich of active organic blend material in between anode and cathode electrodes with light absorbing property. (b) Donor-acceptor hetero-junction solar cells with basic steps involved: 1) Photo-excitation of the donor-acceptor blend to generate an exciton/excited state [radicalanion/electron–radicalcation/ hole pair bound by ionic and radical (Coulomb) interactions]. 2) Exciton/ excited state diffusion to the donor-acceptor interface. Excitons/excited states that do not reach the inter-face, they recombine and do not contribute to the photocurrent (longer diffusion length, LD). 3) Dissociation of bound excitons at the donor-acceptor interface to form a geminate radical-anion (electron)–radical-cation (hole) pair [increased interfacial charge separation requires optimal energy offset between LUMO (lowest unoccupied molecular orbital) of the donor and LUMO of the acceptor material]. 4) Free charge carrier transport and collection at the external electrodes (require high charge-carrier mobility). (c) Fundamental processes

max. LD

(b) cathode

(c)

3 4

LUMO

1

2

cathode

anode 4

Figure 1.2  Typical solar cell.

– +

metal

LUMO acceptor

hν

active D–A layer anode

donor

(a)

HOMO HOMO

p

-

+

-

+ -

n PEDOT:PSS

glass/ITO

+

Organic Solar Cells  5

E N E R G Y

hV

O 1. Light Absorption

O

O 2. Exciton Diffusion

3. Charge Generation

O

O

4. Charge Transport

O 5. Charge Extraction

Figure 1.3  Possible events present in BHJOSCs.

(light illumination, exciton formation, charge separation, charge migration, and charge collection) of bulk-heterojunction solar cells (p = donor material, n = acceptor material).

1.4 Photovoltaic Parameters or Terminology Used in BHJOSCs 1.4.1 Open-Circuit Voltage Voc The voltage at which no current flows through a solar cell is called open circuit voltage Voc and it is the maximum voltage available from solar cell. Several studies have demonstrated a strong dependence of Voc on the energy difference ∆E between the HOMO (highest occupied molecular orbital) of donor material and LUMO of acceptor material of an organic solar cell.

1.4.2 Short-Circuit Current Jsc For V = 0, only the short-circuit current (Jsc) flows through the solar cell. Jsc represents the maximum current that could be obtained in a solar cell. This current depends on the number of absorbed photons, surface area of the photo active layer, device thickness, and charge transport properties of active material, which play important role.

1.4.3 Incident-Photon-to-Current Efficiency (IPCE) The incident-photon-to-current efficiency is defined as the ratio of the number of incident photons Nphoton and the number of photo induced charge carriers Ncharge which can be extracted out of the solar cell.

6  Fundamentals of Solar Cell Design

1.4.4 Power Conversion Efficiency ηp (PCE) It is a measure of the quality of the cell which provides evidence of how much power the cell will generate per incident photon. The efficiency ηp is the maximum electrical power Pmax per light input PL.

ηp = Pmax/PL= (JscVoc)FF 1.4.5 Fill Factor (FF) The FF, which determines the quality of solar cell can be obtained from the ratio of the maximum power output to the product of its Voc and Jsc and is always < 1.



FF = Pmax/PLJscVoc

1.5 Some Basic Design Principles/Thumb Rules Associated With Organic Materials Required for BHJOSCs The donor and acceptor molecules to be employed in BHJOSCs must have light absorption property matching the solar region, with high molar absorption coefficients and excellent width at half height of absorption spectrum. It would be best if absorption ranges of donor and acceptor materials are complementary. The HOMO/LUMO energy levels of molecules should be well matched for the possible photochemical electron transfer between acceptor and donor molecules. To obtain high open-circuit voltage (VOC), a deep HOMO level is to be adjusted, since maximum value of the VOC is determined by the energy difference between the HOMO level of the donor and LUMO level of the acceptor. Balanced charge (negative and positive) mobility of the organic blend materials is required for controlled charge transport. Good solubility of blend organic materials is essential for good film forming property and well mixing with acceptor materials to form nanoscale phase separation and ideal blend morphology. Suitable recognition points may be introduced in blend materials, such that they can involve in forming supra-molecular structures leading to well

Organic Solar Cells  7 organized film morphology, altered UV-visible absorption and providing good charge transport property. Blend materials, higher thermal stability, and their phase transition temperature data are to be tuned properly.

1.6 Recent Research Advances in Small-Molecule Acceptor and Polymer Donor Types Research on BHJ organic solar cells resulting from fullerene as acceptor is dealt at length for the past several years. During the course of investigations by many researchers, it was found that fullerene has limitations in using it as acceptor in solar cell devices. The shortcomings or limitations with properties of fullerene are like negligible light absorption in NIR and visible region, shape of the molecule, solubility, controlling the film texture or film morphology, stability of fullerene, and very limited tunability of energy levels of fullerene. These limitations lead the researchers to explore novel materials to initiate work on non-fullerene derived acceptors for BHJ organic solar cells to replace fullerene and take the system to improve efficiency of organic solar cells. The following part describes BHJOSCs involving non-fullerene acceptors with polymer donors. Dongxue Liu et al. reported two low band gap acceptor materials where in, linearly fused 6 five membered rings having four “Sulfur” atoms in the ring structure acted as donor central core moiety and the remaining part as acceptor at both sides (Figure 1.4) [1]. Thiophene and selenophene moieties are embedded as π-spacers to improve conjugation and also for intra-molecular charge transfer (ICT) leading to red shift in the absorption spectrum. But this thiophene or selenophene insertion did not affect the HOMO-LUMO values of the small acceptor molecules, 4TO-T-4F and 4TO-Se-4F. Two end groups with electron withdrawing nature, 2-(5,6-difluoro-3-oxo-2-2,3-dihydro-1H-inden-1-ylidene)malononitrile, helped in enhancing ICT. The optical band gaps estimated for 4TO-T-4F and 4TO-Se-4F were 1.3 and 1.27 eV, respectively. Optical, thermal, and electrochemical properties determined were found to be suitable as non-fullerene acceptors. Polymer PTB7-Th was used as donor, and the other two, 4TO-T-4F and 4TO-Se-4F, acted as non-fullerene smallmolecule acceptors in organic solar cell fabrications and the configuration of the cell was: IndiumTinOxide/ZnO/[Donor-PTB7-Th:Acceptor]blend/ MoO3/Ag. Solar cell performance was determined as 8.7% (PCE) for 4TO-T-4F with loss of energy of 0.55 eV and 7.4% (PCE) for 4TO-Se-4F with loss of energy of 0.57 eV.

8  Fundamentals of Solar Cell Design C6H13 F

F

3.85 eV

C6H13 S O

NC

EtHex O

S

S

NC

CN

S

S

S

CN

O

O

HexEt C6H13

4TO-T-4F

F F

4 T O T 4 F 5.3 eV

C6H13 C6H13 F

3.85 eV

F C6H13 S

NC NC

O

EtHex O

O

S

Se S

CN

Se

S HexEt

CN O

C6H13 4TO-Se-4F

F

F

4 T O Se 4 F 5.29 eV

C6H13

Figure 1.4  Vinylidine dicyano–difluoroindanone compounds.

Benzothiadiazole central ring fused with pyrrole and bithiophene rings on both sides (seven rings contiguously fused together) and carrying electron withdrawing end groups 2-(5,6-difluoro-3-oxo-22,3-dihydro-1H-inden-1-ylidene)malononitrileand2-(5,6-dichloro-3-oxo-2-2,3dihydro-1H-inden-1-ylidene)malononitrile was the main core designed and developed (Figure 1.5) by Yong Cui et al. for organic solar cells as low band gap organic materials [2]. Various techniques like UV-Vis-NIR absorption, fluorescence, thermal, electrochemical, AFM, TEM, and GIWAXS characterizations (morphology of films) as well as computational, X-ray diffraction (molecular packing), charge carrier mobility, TPV measurements, and highly sensitive EQE and EQEEL measurements were applied to generate the data. OPV fabrications were conducted using PBDB-TF as donor and BTP-4F and BTP-4Cl as non-fullerene acceptors. OPV cells were fabricated with an inverted configuration of Indium Tin Oxide/ZnO nano-particles/[donor polymer PBDB-TF:BTP-4X] blend/

Organic Solar Cells  9 N

S

N 4.02 eV S

S

R S NC

N

N

R1

R1

CN

O

CN

BTP-4F F

F N

S

N

S

4.12 eV

S

S NC

N

N

R1

R1

O

NC

5.65 eV

F

F

R

B T P 4 F

S

O

NC

R

R S CN

O

CN

BTP-4CI CI

CI

CI

CI

B T P 4 CI 5.68 eV

4.64 eV R1

F

R1 O

S

S S S

R1

S R2

S

S

PBDB-TF

O S

n

P B D B T F 5.45 eV

F

Figure 1.5  Fused benzo-thiadiazole–dicyanoindenone compounds.

MoO3/Al, where PBDB-TF polymer was selected as electron donor material. Photo-conversion efficiency determined from solar cell fabrications was 15.6% for BTP-4F and 16.5% for BTP-4Cl with 0.834 and 0.867 V, respectively. The high PCE was attributed to the chlorine effect by reducing the non-radiative energy (~0.26 eV) loss. Further, it is expected that fine

10  Fundamentals of Solar Cell Design tuning the low band gap material properties has a great potential to achieve higher conversion efficiencies. Bin Kan et al. described the synthesis of small-molecule donor NCBDT4Cl (ADA type) with ~1.40 eV optical band gap and used it for solar cell fabrication along with polymer (non-fullerene) acceptor PBDB-T-SF (Figure 1.6) [3]. The NCBDT-4Cl (ADA type) donor and acceptor PBDB-T-SF blend provided 300- to 900-nm range absorption with very good molar absorption co-efficient. Solution processed solar cells were constructed with the given cell configuration: ITO/polymer PEDOT-PSS/polymer PBDB-T-SF:NCBDT-4Cl/PDINO/Al—where in PDINO acted as electron transport layer. Fabricated device as casted (without any treatment) produced efficiency of PC 13.1%. The same device after annealing and solvent addition, gave a result of PCE 14.1% along with Voc of 0.85V, Jsc of 22.35 mA cm−2 and accompanied by loss of energy of 0.55 eV. Authors attributed C6H13 4.02 eV

C6H13

CI

C8H17 S

CN NC

O

S

CI

S S C8H17

O

NC

CI CI

F

C6H13

C6H13

NCBDT-4CI

R O

O

S S

S

RS

S R

S

S

5.60 eV

3.64 eV

SR

S

CN

N C B D T 4 CI

S

n

PBDB-T-SF F

Figure 1.6  Fused seven rings with dicyanoindenone end groups.

P B D B T S F 5.42 eV

Organic Solar Cells  11 the excellent display of 14.1% PC efficiency to the blend morphology as well as to the excellent charge transport property due to attached four chlorine atoms in NCBDT-4Cl (ADA type) donor at appropriate places. Contiguously fused five rings as central core moiety linked with thiophene spacer either side and further attached with electron withdrawing indenone group (IEIC and IEICO) small molecules were synthesized by Huifeng Yao et al. (Figure 1.7) as low band gap acceptor materials [4]. Light absorption for IEIC and IEICO extended in to NIR (near infrared) region 600 to 900 nm and the optical band gap determined to be 1.5 eV for IEIC and 1.34 eV for IEICO. The optical band gap difference between that of IEIC and IEICO was attributed to the “alkoxy group” present in IEICO. Device configuration developed after optimization was: ITO/PEDOT: PSS/PBDTTT-E-T:IEICO or IEIC/PFN-Br/Al. Polymer PEDOT:PSS and PFN-Br were used as anode and cathode interface layers. Morphology of the casted film was studied by AFM and TEM to get further insight of the device fabricated. The measured PV parameters were PCE for IEICO is 8.4  %; Voc = 0.82 V; and Jsc = 17.7 mA. Furthermore, the PCE was improved to 10.7% via tandem device fabrication. The loss energy was estimated to be 0.5 eV. PV parameters for IEIC were found to be inferior Hex

3.95 eV

Hex

NC

EtHex

O

CN

S

S S

S NC

O

HexEt

CN

Hex Hex

5.32 eV

IEIC Hex

3.90 eV

Hex

NC

OEtHex

O

S

S S NC

S O

HexEtO CN

Hex Hex

I E I C

CN

I E I C O 5.47 eV

IEICO

Figure 1.7  Fused ring linked with thiophene-dicyanoindenone.

12  Fundamentals of Solar Cell Design compared to IEICO. Authors claimed that the design leading to low band gap energy level of 1.34 eV, by introducing alkoxy group at suitable place of the small molecule helped to give higher efficiency. Huifeng Yao et al. synthesized linearly fused five rings core flanked by thiophene attached electron withdrawing end groups having efficient ICT, leading to an ultralow band gap acceptor material, IEICO-4F (Figure 1.8) [5]. The IEICO-4F optical band gap was found to be 1.24 eV, charge

Hex

HexEtO

F Hex

F

S

CN

S O

O NC

S CN

S

F Hex

OEtHex

EtHex

HexEtO

O

3.66 eV

S S

S

PB DT TT EF T

F

S S

S

n

5.24 eV

PBDTTT-EFT EtHex

S

N

S

N N J 5 2

S F

S HexEt

2.99 eV

HexDecyl

S

F

S

n J52

IE IC O 4F

F 5.44 eV

Hex

IEICO-4F

HexEt

4.19 eV

NC

5.21 eV

Figure 1.8  Fused ring coupled with fluorodicyanoindenone.

Organic Solar Cells  13 mobility calculated was 1.14 × 10−4 cm2 V−1 s−1 and the absorption of thin solid film extended in to NIR region with λmax of ~900 nm. PBDTTT-EFT (PTB-7-Th) and J52 (Figure 1.8) were employed as polymer donors for device fabrication based on their favorable absorption properties and also with suitable energy levels. Fabricated device parameters were: Voc = 0.739 V; Jsc = 22.8 mA; FF = 59.4%; and PCE = 10.0% using polymer donor PBDTTT-EFT (PTB-7-Th), J52 polymer donor exhibited a little smaller values compared to PBDTTT-EFT. It was advocated that ultralow band gap materials definitely have a role to play to improve the solar cell efficiencies. Huifeng Yao et al. came up with novel small molecules, ITCC and ITIC as non-fullerene acceptor materials for organic solar cells to determine PC efficiency (Figure 1.9) [6]. Linearly fused seven rings acted as donor, which was flanked on both sides with acceptors (thienyl fused indanone end groups) and it was designated as ADA type architecture. Thin films were subjected to GIWAXS investigations to understand whether there existed C5H11

C6H13

3.231 eV

S S

NC

O

NC

S

S

CN

O

CN

S C6H13

ITCC

I T C C

S 5.419 eV

C5H11 C6H13

3.341 eV

C6H13 NC

S

O

S

S NC

O

S

CN ITIC

CN

I T I C

C6H13 C6H13

Figure 1.9  Fused seven membered ring - thienodicyanoindenones.

5.453 eV

14  Fundamentals of Solar Cell Design π-π stacking to some extent and facilitated charge mobility in a better way than ITIC (which was reported earlier). Further, it indicated the role of thieno indanone and the superior design. Device architecture formulated was: ITO/PEDOT-PSS/ITCC or ITIC + PBDB-T/PFN-Br/Al. Solar cell PV parameters determined were: Voc = 1.01 V; Jsc = 15.9 mA; FF = 71%; PCE = 11.4% for ITCC and for ITIC: Voc = 0.93; Jsc = 17.0 mA; FF = 67%; PCE = 10.6%. Authors claim that investigations have high relevance, because of 11.4% PCE noted in non-fullerene solar cell device. The results are of promising nature and suggest that one can improve the PCE to higher levels with these non-fullerene–based solar cells. Yunlong Ma et al. synthesized two novel ladder type low band gap small molecules as non-fullerene acceptors for solar cell fabrication (Figure 1.10) [7]. Both the molecules, DTNIC6 and DTNIC8, were of ladder type because of linearly fused six rings and the two molecules differ in their alkyl groups attached. Both the molecules have strong absorption in 500- to 720-nm region. Film morphology was analyzed using hole and electron mobilities, AFM, TEM, and GIWACS information. PBDB-T polymer as donar and synthesized small molecules as acceptors were used in fabricating solar cell with device architecture as: ITO/TiO2:TOPD/PBDB-T:DTNIC8 or DTNIC6/MoO3/Ag. PV parameters generated were: for DTNIC6: Voc = 0.96 V; Jsc = 7.71 mA; FF = 45.6%; PCE = 3.39%; for DTNIC8: Voc = 0.96 V; Jsc = 12.92 mA; FF = 72.84%; PCE = 9.03%. DTNIC8 (which carried ethylhexyl alkyl chain) exhibited a high photo conversion efficiency compared to DTNIC6 which differed in alkyl chain structure from DTNIC8. The alkyl chain did not influence energy levels and light absorption properties, but exerted sizable effect on the solar cell efficiency. The alkyl chain group effect was believed to have control over film morphology. Yuze Lin et al. employed (Figure 1.11) medium band gap polymer donor—FTAZ (BG = 2.41 eV) with a non-fullerene low band gap acceptor—IDIC (BG = 1.6 eV) to make solar cells and to understand photo current efficiency [8]. FTAZ and IDIC have complementary absorption to cover 450- to 800-nm region and relatively have high electron and hole mobilities and well-matched energy levels. Single junction solar cell fabrication structure was: ITO/ZnO/FTAZ:IDIC/MoOx/Ag. Solar cell parameters observed were: Voc = 0.840 V; Jsc = 20.8 mA; FF = 71.8%; and PCE = 12.5%, Diiodooctane was used to tune the film morphology in these fabrications. The 12.5% PCE determined for non-fullerene solar cell was very high compared to FTAZ-PCBM blend, which showed only ~6%. Femtosecond transient absorption studies on the casted films indicated the formation of radical cation and radical anion (charged species) and their mobilities. Authors inferred that FTAZ-PCBM combination film provided

Organic Solar Cells  15 3.92 eV

Hex

O

NC

Hex

CN

S NC

S Hex

CN

O

Hex

DTNIC6

D T N I C 6 5.87 eV

3.93 eV D T N I C 8 5.91 eV

O

NC

EtHex

HexEt

CN

S NC

S HexEt

CN

EtHex

O

DTNIC8 2.92 eV

HexEt HexEt

S

S

S

P B D B T

O

O S

EtHex

S

S S

n

S EtHex

PBDB-T

5.33 eV

Figure 1.10  Fused six membered ring with dicyanoindenones.

only ~40% generation of charged species compared to non-fullerene FTAZ-IDIC film combination. Authors claim that non-fullerene blends have superiority over fullerene blends in achieving higher PCE values. Jie Zhang et al. prepared a small low band gap acceptor molecule-IFTIC (Figure 1.12) for evaluating its solar cell efficiency [9]. IFTIC carried fused bifluorene attached on both sides with thiophene as electron donating central core, with either side holding indenone moiety as electron acceptor. IFTIC showed absorption covering 450 to 700 nm and had suitable energy levels like 5.42 eV HOMO and 3.85 eV LUMO. PTB7-Th polymer was used as donor in these investigations for luminescence quenching (with IFTIC)

16  Fundamentals of Solar Cell Design C3H7

C6H13

C4H9

S

C6H13

2.92 eV

F T A Z

N N N

S

S

F

S F

C4H9 C5H11

5.33 eV

FTAZ

3.90 eV C6H13

C6H13

O

CN NC

S

S

CN

I D I C

NC

O C6H13

C6H13

IDIC

5.50 eV

Figure 1.11  Fused five membered ring with dicyanoindenone.

and as donor material. AFM and TEM techniques were used for monitoring morphology of the casted film. Fabricated device parameters were: Voc = 0.92 V; Jsc = 12.71 mA; FF = 54%; PCE = 6.33%. Authors claim that non-fullerene materials with relatively simple device structure and simple method of preparation of materials make this work attractive. Oh Kyu Kwon et al. developed a non-fullerene base material PV device which exhibited a good percent of photo current (Figure 1.13). PPDT2FBT, a very well ordered polymer, was used as donor and NIDCS-HO, a small acceptor molecule for building a solar cell device [10]. Interestingly, absorption spectra for polymer donor and small-molecule acceptor displayed complementary absorption, by covering 350- to 700-nm region. Non-fullerene–based conventional single solar cell device adopted structure was given as: ITO/PEDOT:PSS/active layer/Ca/Al. Blend film morphology was investigated systematically using charge mobility data, AFM, TEM, GIWACS, annealing temperature and other techniques. Device PV parameters determined were: Voc = 1.03 V; Jsc = 11.88 mA; FF = 63%;

Organic Solar Cells  17 3.85 eV C8H17 CN

C8H17

S

NC

O

CN

S

O C8H17

NC

C8H17

I F T I C

5.42 eV

IFTIC HexEt S

3.56 eV

COOEtHex

F

P T B 7 Th

S

S S

S

n

S EtHex

5.20 eV

PTB7-Th

3.27 eV

N O

D R 3 T D T C

S

S HexEt

S S

S

S

S S

C8H17

DR3TDTC

N O

S

S

S

S

C8H17

C8H17

EtHex

C8H17

4.93 eV

3.28 eV D R 3 T D T S

S

S N O

HexEt

S S

S S C8H17

C8H17

4.94 eV

Si

EtHex

S S

S

S

S

S C8H17

N O

C8H17

DR3TDTS

Figure 1.12  Bifluerene-dicyanoindenone.

PCE = 7.64%. Investigations indicated complementarity in absorption and suitably placed energy levels along with good film morphology can contribute to the solar cell efficiency. Sunsun Li et al. synthesized a series of novel methoxyl-modified dithieno[2,3-d:2ʹ,3ʹ-dʹ]-s-indaceno[1,2-b:5,6-bʹ]dithiophene-based (ITIC

18  Fundamentals of Solar Cell Design C2H5

C4H9

3.67 eV

F

O

F

P P D T 2 F B T

S

S

N O C2H5

S

N

n

C4H9

5.43 eV

PPDT2FBT

3.75 eV C6H13

O C4H9

N C2H5 O

O

O S

CN CN

S O

N O

C6H13

NIDCS-HO

C2H5 C4H9

N I D C S H O 5.79 eV

Figure 1.13  Dialkoxybenzene core–based molecule.

based) low band gap small-molecule acceptors, IT-OM-1, IT-OM-2, IT-OM-3, and IT-OM-4 (Figure 1.14), with A-D-A architecture, for the purpose of developing non-fullerene–based organic solar cells [11]. Position of “methoxy” substitutent on terminal group was systematically varied to understand the positional effect of substitution on optical, electrochemical, charge mobility, and more importantly molecular packing of these isomers. Donor molecule used in these investigations was PBDB-T polymer. PBDB-T donor blended with IT-OM acceptor film morphology was thoroughly investigated using AFM, GIWACS, TEM and charge mobility techniques. Devices were fabricated by adopting conventional cell configuration like: ITO/ZnO/active layer/MoO3/Al to evaluate PV parameters and in particular PCE. Additive 1,8-diodooctane was employed in these fabrications and also annealed at 150°C to make the film. The donoracceptor blend exhibited excellent UV-Visible absorption covering 350- to 800-nm region. Among the four isomers synthesized, IT-OM-2 demonstrated excellent PV parameters like: Voc = 0.93 V; Jsc = 17.53 mA; FF = 73%; PC efficiency of 11.9%. Furthermore, the PCE of >10% maintained when the thickness of the solar cell increased to 250 nm for IT-OM-2 blend system. Authors claim that it is possible to modulate intrinsic molecular

Organic Solar Cells  19 R3

C6H13

C6H13 R4

R2

NC

S

O

R1

3.76 eV

S

S

NC

O

S

CN

R4 C6H13

C6H13

I T R1 O M R2 1

CN

R3

5.50 eV

3.86 eV 3.80 eV 3.81 eV I T O M 2

I T O M 3

I T O M 4

5.49 eV 5.52 eV 5.49 eV

IT-OM-1 = R1 = OMe; R2=R3=R4=H IT-OM-2 = R2 = OMe; R1=R3=R4=H IT-OM-3 = R3 = OMe; R1=R2=R4=H IT-OM-4 = R4 = OMe; R1=R2=R3=H EtHex

HexEt

S

O

S

S S S

HexEt

S PBDB-T

S

2.92 eV EtHex O S

n

P B D B T 5.33 eV

Figure 1.14  Fused seven membered ring - methoxydicyanoindenones.

properties and also bulk film morphology to achieve excellent PCE in solar cells by designing molecules for fullerene free solar cells. Huanran Feng et al. described the synthesis of a new non-fullerene A-D-A–type low band gap acceptor small-molecule—FDNCTF (Figure 1.15) [12]. Linearly fused seven rings, three five-membered, two thiophene, and two benzene rings, acting as donor and with electron withdrawing end group (NINCN) flanked on either side of the molecule (FDNCTF) was designed. Properties of FDNCTF were (i) UV-visible absorption reaching near infrared with high molar absorption coefficient (~3 × 105 L mol−1), (ii) exhibited higher charge mobility, and (iii) more ordered arrangement of molecules in the film state, besides other properties. PBDB-T polymer having wide band gap acting as donor was employed for fabricating solar cells with FDNCTF. The solar cell devices were fabricated and the photo voltaic parameters were determined by adopting convention device architecture like ITO/PEDOT:PSS/PBDB-T:FDNCTF/PDINO/Al (PDINO is a cathode interlayer). FDNCTF as a small-molecule low band gap acceptor displayed impressive efficiency of 11.2%. PCE along with other parameters

20  Fundamentals of Solar Cell Design 3.73 eV C8H17

O NC

C8H17

O CN

S

S CN

NC

C8H17

C8H17

C8H17

C8H17

FDICTF

F D I C T F 5.42 eV

3.71 eV C8H17

O NC

C8H17

O

S

CN

S

CN C8H17

NC

C8H17 C8H17

C8H17

FDNCTF EtHex

HexEt

S

S

S

HexEt

S

EtHex

P B D B T

O

S

S

5.43 eV 2.92 eV

O

S

PBDB-T

F D N C T F

S

n

5.33 eV

Figure 1.15  Fused seven membered ring - dicyanoindenones.

like Voc = 0.93V, Jsc = 16.5 mA, and FF = 72.7%. For comparison purpose, they fabricated solar cell device with FDICTF as small-molecule low band gap acceptor with PBDB-T polymer as donor to understand structural aspects of small molecule on the solar cell efficiency. Interestingly, FDICTF with PBDB-T exhibited 10.06% showcasing the better design of FDNCTF in performance. FDNCTF and PBDB-T blend transient absorption studies indicated that charge mobility was very good as correlated with film morphology. These investigations highlighted that the molecular design with larger and conjugated electron withdrawing end groups improved

Organic Solar Cells  21 absorption, molecular packing in the film state which, in turn, improved the solar cell efficiency. Feng Liu et al. synthesized low band gap small molecule-ATT1as a non-fullerene acceptor, where dicyano-rhodanine group was attached on both sides of ATT1 (Figure 1.16) [13]. PTB7-Th polymer was selected as a donor in these studies. Solar cell device fabrication adopted a conventional procedure like ITO/PEDOT:PSS/PTB7-Th:ATT-1/PFN/Al. PV parameters determined were: Voc = 0.88V, Jsc = 16.18 mA, FF = 68%, and PCE = 9.78% at a film thickness of 100 nm, the efficiency was found to be increased when the film thickness increased to 130 nm—Voc = 0.87V, Jsc = 16.48 mA, FF = 70%, and PCE = 10.07%. But the efficiency decreased to 9.59% at a film thickness of 160 nm. Central core structure of the molecule C6H13

C6H13

COOC8H17 S

S S

S N

NC

N O

S O

3.63 eV

CN

NC

S

S

S

CN

C6H13

C6H13

COOC8H17

ATT-1

HexEt 3.56 eV S

P T B 7 Th

COOEtHex

F

S

S S S

S

n

EtHex 5.20 eV

A T T 1

PTB7-Th

Figure 1.16  Fused five membered ring - thienothiophene dicyanorhodanine.

5.50 eV

22  Fundamentals of Solar Cell Design ATT1 was more planar, had a high molar absorption coefficient, good charge transporting quality and facilitated film forming with good morphology. Authors believe that this design has capabilities to take forward. Yuvraj Patil et al. designed and synthesized two low band gap small non-fullerene acceptor molecules DPP7 and DPP8 (Figure 1.17) and investigated their solar cell parameters [14]. Diketopyrrolopyrrole and tetracyano-diene fragments acted as acceptor and carbazole moiety represents as donor part leading to D-A-D type architecture. Polymer was employed as donor material in the solar cell fabrications. The complimentary absorption of polymer-donor gives wide absorption for the blend. The configuration of the solar cell fabricated was: ITO/PEDOT:PSS/P: 3.56 eV C10H21 N

O

S

NC

CN

N

S O

N C10H21

NC

D P P 7

CN

5.20 eV

DPP7

3.56 eV C10H21 N S N

S

CN

N

N C10H21

DPP8

F

NC S

O

C8H17 Si

O

NC

CN

5.20 eV

3.23 eV

C8H17

N

N N N C6H13

N C6H13 Polymer

N

C8H17

P o l y m e r

S

S

S

Si

C8H17

D P P 8

n

5.09 eV

Figure 1.17  DPP-thiophene-tetracyanobutadiene-carbazole hybrid.

Organic Solar Cells  23 DPP7 or DPP8/PFN/Al. PV parameters obtained for DDP7 was 4.86% efficiency and for DPP8 was 7.19% efficiency. It was informed that the superior performance of DPP8 was due to its structure, which contributed mainly to the morphology of the film as well as to the betterment of the charge mobility. Jia Sun et al. synthesized two ultralow band gap small non-fullerene acceptor molecules, INPIC and INPIC-4F (Figure 1.18) [15]. These are interesting molecules in terms of design that there are contiguously nine rings fused together to form donor part of the molecule and flanked on either side by electron withdrawing groups like dicyano-indenone (INPIC) and dicyano-difluro-indenone, making them as A-D-A–type architecture. INPIC and INPIC-4F exhibited absorptions in 600- to 900-nm region, thereby complementing with the PBDB-T polymer acting as donor, and further the blend of donor-acceptor absorption encompassed 350 to 900 nm. Solar cell devices were fabricated according to the given configuration: ITO/ZnO/active layer/MoO3/Ag and the PV parameters were as follows: INSPIC-4F displayed impressive parameters Voc = 0.85V, Jsc = 21.6 X

C6H13 C8H17 NC

C6H13 O N S

N

O

CN

C8H17

EtHex

HexEt

S S S S HexEt

S

O

S

I N P I C

I N P I C -4 F

5.36 eV

INPIC = X = H INPIC-4F = X = F

X

3.94 eV

C6H13

C6H13 X

3.82 eV CN

S

S

S

NC

X

S PBDB-T

2.92 eV EtHex O S

n

P B D B T 5.33 eV

Figure 1.18  Fused nine membered ring - fluorodicyanoindenone.

5.42 eV

24  Fundamentals of Solar Cell Design mA, FF = 71.5%, PCE = 13.13%, whereas INSPIC showed only 4.31% efficiency. The difference between the two molecules INSPIC and INSPIC-4F was “fluoro” substitution and the same was reflected in solar cell efficiency improving from 4.31% to 13.13%. INSPIC-4F/PBDB-T blend morphology exhibited well defined texture, high charge mobility, and improved light absorption property, which were cited in favor of excellent efficiency shown by the INSPIC-4F. Zhuping Fei et al. synthesized low band gap non-fullerene acceptor small molecule, C8-ITIC (Figure 1.19) [16]. Acceptor molecule has seven contiguously fused rings flanked on either side by indacenodicyano electron withdrawing group and carries four n-octyl alkylchains. Authors employed two donors like (i) PBDB-T polymer and (ii) PFBDB-T polymer. Solar cell devices were fabricated by adopting given configuration: ITO/In2O2/ZnO/active layer/MoO3/Ag. Impressive PV parameters were obtained with conversion efficiency of 13.2% using C8-ITIC and PFBDB-T blend. The energy loss noted in the solar cell device is less than 0.56 eV. Non-fluorinated polymer PBDB-T with C8_ITIC blend recorded lower efficiency. Authors claim that polymer backbone selective fluorination is another important factor to achieve higher conversion efficiencies in organic solar cell devices. Jianfei Qu et al. synthesized four A-D-A–type non-fullerene small molecule [17] acceptors. Alkyl groups C2, C4, C6, and C8 were selected and 3.82 eV

Oct

O NC

Oct

S

CN

CN

S

S

Oct

O

C8-ITIC

EtHex

HexEt

S

O

S

S S S

HexEt

S PBDB-T

S

5.36 eV EtHex

3.29 eV EtHex P B D O B S n T

C 8 I T I C

NC

S

Oct

HexEt

S

O

S

S S S

5.33 eV HexEt

Figure 1.19  Fused seven membered ring - dicyanoindenone.

S

F PFBDB-T

S

3.46 eV EtHex P O S

B D B n T F 5.47 eV

Organic Solar Cells  25 were attached to the rhodanine end group having ring Nitrogen (Figure 1.20). Alkyl groups attached did not have much effect on their absorption properties. But these alkyl groups played a role on the film properties like, crystalinity, molecular packing, manifesting on the PV parameters. PBDB-T polymer was chosen as donor along with one of the acceptors synthesized as a blend material. PV measurements were determined with an inverted device structure: ITO/ZnO/active layer/MoO3/Ag. PBDB-T polymer donor with C6 small-molecule acceptor blend gave an excellent efficiency of 8.26% PCE. Furthermore, introducing thermal annealing with iodooctane solvent improved the efficiency to 9.29%. Other PV parameters observed were also good: Voc = 0.89V; Jsc = 15.80 mA/cm2; and FF = 58.12%. Investigations reported in this work indicated that effect of alkyl chain length has a pivotal role in tuning the PV parameters, in making suitable films. Kaili Wang et al. synthesized calamatic shaped A-D-A–type nonfullerene small-molecule acceptors, CPDT-4Cl and CPDT-4F (Figure 1.21), by varying chloro and fluoro substituents [18]. CPDT-4Cl and CPDT-4F exhibited absorptions extending in to 900-nm region. PBDB-T was employed as a polymer donor and its light absorption has complementarity with the two CPDT-4Cl and CPDT-4F acceptor molecules and the blend absorption of these (PBDB-T and CPDT-4Cl, and PBDB-T and CPDT-4F) cover 400- to 980-nm region. PV parameters were evaluated by adopting conventional device structure like: ITO/PEDOT-PSS/[PBDBT+Acceptor]/Phen-NaDPO/Ag having 9.47% efficiency for CPDT-4Cl and 9.26% efficiency for CPDT-4F, respectively. Other parameter like Jsc was found to be impressive like 21.3 mA/cm2 for [CPDT-4Cl + PBDB-T] blend and 20.1 mA/cm2 for [CPDT-4F + PBDB-T] blend. Authors express that the non-fullerene–type acceptors with NIR absorption have great scope to improve the organic solar cell efficiency. C6H13

NC NC

R N

S

O S

S N N S C6H13

3.70 to 3.38 eV 3.71 eV P C2 CN S B S D C4 N CN N B O N S R C6 T ITBTR-C2-R = Ethyl C8 ITBTR-C4-R = Butyl 5.28 to 5.18 eV C6H13

S C6H13

ITBTR-C6-R = Hexyl ITBTR-C8-R = Octyl

5.30 eV

Figure 1.20  Fused seven membered ring acceptors with variation in N-alkyl chain length.

26  Fundamentals of Solar Cell Design 3.93 eV 3.87 eV C X C P P D D T T CN 4CI 4CI

X

NC

Hex

Hex

CN S

S

S

S O

X

Hex

O

NC

Hex

5.29 eV 5.26 eV

X = CI = CPDT-4CI X = F = CPDT-4F

X

3.38 eV P B D B T 5.18 eV

Figure 1.21  Calamatic shaped non-fullerene small-molecule acceptors.

Eun Yi Ko et al. synthesized small acceptor molecules [19] containing dicyanovinylene (DCV2) and tricycanovinylene (TCV2) groups (Figure 1.22) as strong electron accepting moieties. Material properties were determined for IDT(DCV)2, IDT(TCV)2, and IDTT(TCV)2 (Figure 1.22) and PTB7-Th polymer used as donor to evaluate PV parameters with an inverted cell structure like ITO/Zno/PTB7-Th+Small Molecule/MoO3/ Ag. Reasonably good efficiency (2.8% to ~4%) was observed for all the C6H13

C6H13

3.65 eV C6H13 I D T CN (D S C NC V)2 IDT(DCV)2 5.81 eV

S

S

CN

CN NC

CN C6H13

C6H13 4.06 eV

C6H13 NC

NC

S

S

S

CN C6H13 IDTT(TCV)2

CN

C6H13

IDT(TCV)2

C6H13

S

CN CN

S

NC

C6H13

NC

4.18 eV

C6H13

C6H13

CN CN

I D T T (T C V)2

3.59 eV P T B 7 Th

5.20 eV 5.58 eV

Figure 1.22  Dicyano and tricyano vinylene–based non-fullerene small-molecule acceptors.

I D T (T C V)2 6.04 eV

Organic Solar Cells  27 prepared small acceptor molecules. Interestingly, these fabricated devices exhibited relatively high Jsc values as 11.02 to 11.98 mA/cm2. Thus, fabricated devices were stored in dark without encapsulation for about 1000 h and the device stability was monitored by recording absorption spectrum. The devices were found to be stable to oxygen, moisture, and carbondioxide for over a period of 1,000 hours indicating excellent shelf stability. Yamin Zhang et al. synthesized [20] non-fullerene acceptor smallmolecule F-2Cl by chlorination of parent molecule (Figure 1.23). PBDB-T polymer donor was difluorinated to make PM6 with changed HOMO and LUMO values. F-2Cl has absorption covering the range of 500 to 800 nm and PM6 has absorption covering the region of 400 to 680 nm with complementarity covering wide absorption range. The conventional solar device structure using F-2Cl as acceptor and PM6 as donor with a solar cell film thickness of 103 nm provided very good efficiency of 12.59% PCE with Voc of 0.94 V; Jsc of 17.96 mA/cm2; and FF of 77%. The total solar cell film thickness was changed to 600 nm by improving the active blend layer to get solar cell parameters like, Voc of 0.879 V; Jsc of 19.61 mA/cm2; FF of 58%; and solar cell efficiency of 10.05%. Authors mentioned that there are some changes leading to decrease the solar cell parameters, but the efficiency of 10.05% is remarkable considering the active layer blend thickness

CI

CI

CI

O

NC

O

C8H17

C8H17

C8H17

CN

S

S

CN

C8H17

3.86 eV

CI

CN

F 2 CI

5.50 eV

C8H17 C8H17

F-2CI F

EtHex

HexEt

S S S

HexEt

EtHex

O

S

S

3.65 eV

S

S PM6

P M 6

O S n

5.45 eV

F

Figure 1.23  Dichloro-dicyano-indocinyl based small-molecule acceptors.

28  Fundamentals of Solar Cell Design of 600 nm. Authors explained that the morphology of the thick film (600 nm thickness) played key role in the observed efficiency. Yanbo Wang et al. synthesized [21] seven rings fused contiguously with either sides carrying halo-dicyanoindacenyl group (Figure 1.24) compounds F-H, F-F, F-Cl, and F-Br as non-fullerene small-molecule acceptors. PBDB-T polymer was used as donor in these investigations. A change in energy levels (HOMO and LUMO; Figure 1.24) of three compounds (F-F, F-Cl, and F-Br) carrying halogen was evident upon substituting hydrogen with halogen. All the molecules exhibited strong absorption in the region 550 to 700 nm. Solar cell device structure adopted was ITO/POEDOTPSS/PBDB-T + Acceptor/PDINDO/Al. The trend of efficiency, 9.59% for F-H, 10.85 for F-F, 11.47 for F-Cl, and 12.05 for F-Br, indicated that halogen substitution improved efficiency of fabricated solar cell. Tuning the light absorption, crystallinity of film and mobilities of non-fullerene acceptors may be contributing factors for improving the performance of fabricated solar cells. Reported investigations inform that design of halogenation strategy on non-fullerene small-molecule acceptors has a role to play in future research. Andrew Wadsworth et al. [22] synthesized two non-fullerene A-D-A– type small-molecule acceptors with contiguously fused rings (Donor) with attached BTD andrhodanine (O-IDTBR) or dicyanovinylelene (O-IDTBCN)groups (Figure 1.25) on both sides, to understand the role of end groups in tuning the organic solar cell photo voltaic parameters. Deeper lying energy levels are observed for O-IDTBCN carrying strong electron withdrawing dicyano vinylelene group. PTB7-Th low band gap polymer was employed as donor in solar cell fabrications. Overlay of absorption spectra of O-IDTBR, O-IDTBCN and PTB7-Th indicates that a coverage of 400- to 850-nm region. PTB7-Th blended with O-IDTBR or O-IDTBCN 3.38 eV

O

NC CN

3.79 eV

X

X

C8H17 C8H17

S C8H17

C8H17

O S

CN

F H

4.13 eV F F

4.04 eV 4.06 eV F CI

F Br

CN

C8H17 C8H17

X = H = F-H; X = F = F-F; X = CI = F-CI; X = Br = F-Br

Figure 1.24  Halo-dicyanoindacenyl derivatives.

P B D B T 5.18 eV

5.42 eV

5.72 eV

5.62 eV 5.62 eV

Organic Solar Cells  29 S

N

S

N

Octyl

S

Octyl

O S

S O N

N

Octyl

S

Octyl

N

N

S

O-IDTBR

S

3.38 eV 3.92 eV 4.18 eV

N

S

N

Octyl

NC

Octyl S

S NC

Octyl

CN O-IDTBCN

Octyl

N

S

N

CN

O I D T B C N

O I D T B R

P T B 7 T H 5.18 eV

5.56 eV

5.70 eV

Figure 1.25  Contiguously fused five rings having attached BTD and rhodanine or dicyanovinylelene groups.

were used for the fabrication of inverted OBHJS Cells with an architecture: ITO/ZnO/PTB7-Th+O-IDTBR or O-IDTBCN/MoO3/Ag. PCE recorded were found to be 9.5% for O-IDTBR and 10.5% for O-IDTBCN, furthermore other photo voltaic parameters were also improved. Improved charge separation and collection in [PTB7-Th + O-IDTBCN] blend was explained based on the average charge carrier mobility and lifetime data generated. Jianfei Qu et al. designed [23] A-D-A–type non-fullerene small-molecule acceptors, ITIC-2Br-γ and ITIC-2Br-m (Figure 1.26), attached with bromine on either side of dicyano-indocinyl group. ITIC-2Br-γ has bromine attached at specific position on the dicyano-indocene whereas the position of bromine attached in ITIC-2Br-m is not specified. ITIC-2Br-γ displayed higher absorption property compared to other bromo compound ITIC-2Br-m. Solid state crystal structure of ITIC-2Br-γ revealed that it induced stronger π-π interactions due to “O” -- “S” and “Br” – “S” proximity. PBDB-T-2F polymer was used as donor with ITIC-2Br-γ or ITIC-2Br-m for making blend material to determine photo voltaic

30  Fundamentals of Solar Cell Design C6H13

C6H13

Br O

NC S

S S

NC

O

S CN

ITIC-2Br -

NC

Br

S

O

3.56 eV

C6H13

C6H13

CN

S S

NC

Br

C6H13

C6H13

CN

S CN C6H13

C6H13 ITIC-2Br–m

O

Br

P B D B T 2 F 5.49 eV

3.90 eV 3.90 eV I T I C 2Br -

I T I C 2Br m

5.54 eV 5.53 eV

Figure 1.26  Fused seven membered ring with bromovinyldicyanoindenones.

parameters by fabricating cell with inverted configuration: ITO/ZnO/ PBDB-T-2F:acceptor/MoO3/Ag. PBDB-T-2F donor polymer blend with ITIC-2Br-γ acceptor provided very good conversion efficiency like 12.05%. The other combination PBDB-T-2F with ITIC-2Br-m showed lesser conversion like 10.88%. Authors advocate that position of bromine attachment changes the molecular moment influencing film morphology leading to better conversion numbers, indeed which is a “supra-molecular chemistry” concept.

1.7 Recent Research Advances in All Small-Molecule Acceptor and Donor Types Polymers acting as electron donor materials are used in solar cell fabrications. Indeed, the above section is completely on the same subject. In fact, these polymer donor materials are doing fine for BHJOSCs and the solar cell efficiency reached over 17%. Compared to polymers, smallmolecule donors and acceptors have some definitive advantages or unique merits like: clarity in chemical structure, defined molecular weight, easy

Organic Solar Cells  31 purification of small molecule, excellent batch to batch repeatability, easy to synthesize them, low cost preparation, tunable optical, thermal and electrochemical properties, and solubility character to an extent to control the film morphology, as well to establish role of chemical structure in solar cell device performance. These merits made many researchers to work on the functioning of all small-molecule BHJ organic solar cells. Present scenario reveals that all small-molecule BHJOSCs performed very well to show over 14% efficiency. The present part describes the achievements and progress associated with the all small-molecule BHJOSCs. Liyan Yang et al. reported a combination of non-fullerene wide band gap donor (DRTB-T) and non-fullerene low band gap acceptor (IC-C6IDT-IC) as shown in Figure 1.27, leading to a PCE of 9.08% [24]. Wide band gap donor DRTB-T was designed and synthesized, and material properties were evaluated. Absorption spectrum of donor DRTB-T and acceptor IC-C61DT-IC indicated their complementarity with a coverage of ~90% of the solar spectrum in the region of 300 to 900 nm. Solar cell device fabricated structure was: ITO/MoO3/DRTB-T:IC-C6IDT-IC/Al. Solar cell structure was optimized in terms of it thickness, film morphology, donor/

HexEt

HexEt

S

O

S

DRTB-T S EtHex

EtHex

3.91 eV Hex CN NC

Hex O

S S

O Hex

CN Hex

S

S

S EtHex

N S

S

S N

O

S

S

S S

S

S

S

3.34 eV

HexEt

NC

IC-C6IDT-IC

Figure 1.27  Trimeric BDT linked rhodanine.

IC C6 I D T IC 5.69 eV

D R T B T 5.51 eV

32  Fundamentals of Solar Cell Design acceptor ratio, and other aspects to get better efficiency. Indeed, it was reported 9.08% PCE with Voc of 0.98V, Jsc of 14.25 mA, and FF of 65%. Authors claim that the efficiency produced is the highest ever reported in non-fullerene all small-molecule solar cells. Ruimin Zhou et al. reported the synthesis of DTBDT based three small molecules [25] for the purpose of using them for all small BHJOSCs (ASM-OSC), where ZR1 acts as donor and IDIC-4Cl Y6 acts as an acceptor (Figure 1.28). Overlay of absorption spectra of donor and acceptors reveal that it encompasses 400- to 950-nm region, providing a big light absorption window. ITO/PEDOT-PSS/ZR1 + IDIC-4Cl or Y6 blend/ Al was the device structure adopted for measuring the PV parameters. ZR1 + Y6 combination blend exhibited excellent PCE of 14.34%, and further, it was certified PCE of 14.1%. It was interesting to note that the fabricated ASM-OSC device with [ZR1 + Y6] showed a high Jsc of 24.34 mA/ cm2. TEM and RSoXS data for [ZR1 + Y6] blend film forms a hierarchical morphology, which facilitates charge separate ratio and charge transport and hence the device exhibits over 14.34% efficiency. Authors found that the energy loss also got minimized in these devices due to better charge transport mechanism. EQEL measurements developed for these devices found to be at higher side and this higher EQEL number also indicates loss of energy may be of 0.24 eV. Present system provided a high PCE of 14.34%. All small-molecule BHJOSC further become highly functional with careful design to understand the morphology of the film and relations between small-molecule donor-acceptor interactions in forming the films. Haiyan Chen et al. developed [26] two liquid crystalline small-molecule donors BTR and BTR-Cl. BTR-Cl was prepared by chlorination (Figure 1.29) of core structure involving benzodithiophene attached with terthiophene and rhodanine end group. The chlorine was attached with thiophene moiety linked to central benzene ring. These new materials carrying alkyl chains acquires higher order liquid crystallinity and thereby providing a favorable film morphology leading to higher photo conversion efficiency of the given ASM-BHJOSC. BTR and BTR-Cl act as donor and Y6 as acceptor for fabrication of devices. The red shifted absorption in the film state, compared to the solution phase, is indicative of intermolecular interactions in the film state. Conventional device structure adopted was: ITO/ PEDOT-PSS/BTR or BTR-Cl + Y6 blend/Phen-NaDPO/Ag. The recorded efficiencies were 10.67% with Jsc 22.25 mA/cm2 for BTR and 13.6% with Jsc 24.17 mA/cm2 for BTR-Cl. The 13.6% efficiency was certified value also. The red shift in the absorption of film state, liquid crystalline property of the small molecules, charge mobilities coupled with GIWASX information

Organic Solar Cells  33 C4H9 C8H17

S

S

S

S

O

C4H9 S

N

C6H13

S

C6H13

C6H13 C8H17

S S

N

O

S

S

S

S

S

C6H13 ZR1

S

3.53 Cl

C8H17

CN

Cl

C8H17

NC

O

S S

O CI

CN

C8H17 CI

N

C8H17

S

C2H5

NC

F

I D I C 4 CI

Y 6

5.32

5.72

N S

N

S O

4.10

5.91

S

F

NC

IDIC-4CI

NC

Z R 1

4.10

C4H9

N C2H5

S O

CN CN

C4H9

Y6 F

F

Figure 1.28  Dicycanoindacenyl and rhodanine end group small-molecule donor and acceptors.

indicates the formation of a very good thin film morphology, which leads to higher efficiency. Haijun Bin et al. [27] developed two small-molecule (H21 and H22) donors based on benzodithiophene and alkylsilyl-thiophenyl moieties (Figure 1.30) for fabricating all small-molecule BHJOSC to evaluate PV

34  Fundamentals of Solar Cell Design C4H9

C6H13

S

C6H13 N

C6H13

O

S

S

S C6H13

S

S

O

S

S

S O

NC

C2H5 C4H9

NC

3.53

N

N

S C2H5 O C4H9

CN

B T R

CN 5.34

F

F

S

N C6H13 S

BTR-CI

S N

S

CI

C2H5

S

O

S

C6H13

S

C4H9

S

S

C6H13

S

S

C6H13

N

C2H5

S

S

N C6H13

C4H9 S

S

S

BTR

CI

C6H13 N

S

C6H13

C2H5

C6H13

O

C6H13

S

C4H9

C6H13 S

S

S

S

C2H5

S

Y6 F

F

3.70 B T R CI

4.10

Y 6

5.46 5.65

Figure 1.29  Benzodithiophene based (BTR and BTR-Cl) small-molecule donors.

parameters. IDIC is another small-molecule employed as acceptor in these investigations. H21 or H22 with IDIC as blend the absorption spectrum covers ~380- to780-nm region. Fabrication of solar cell was conducted to determine PV properties using a conventional configuration like: ITO/ PEDOT-PSS/H21 or H22 + IDIC blend/PDINO/Al and the blend annealed at 130 oC. H21 showed PCE of 7.62% and H22 exhibited higher efficiency at 10.29%. Silyl group and electron withdrawing end group of H21 and H22 played a role in enhancing the all small-molecule BHJOSC’s efficiency. Hole and electron charge transporting properties evaluated indicate that H22 has higher values compared to H21 and hence a higher %PCE was

Organic Solar Cells  35 Si

S

C6H13 N

C8H17

O

S

C8H17 S

S

O

S

S

S

S

S S

S S

S C8H17

C8H17

N C6H13 S

H21

Si

Si

HexEt O

C8H17

O

S

NC

S

C8H17

S

S

S

S

S

S S

O

S

C8H17

CN

O

EtHex

C8H17

H22

Si

C6H13 CN NC O

O S

C6H13 IDIC

3.59

H 2 1

H 2 2

5.38

5.39

3.90

C6H13

S

3.63

NC C6H13

I CN D I C 5.50

Figure 1.30  Small-molecule donors from benzodithiophene and alkylsilyl-thienyl–based conjugated side chains.

displayed by H22. Present investigations indicate that all small-molecule– based BHJOSCs have several advantages compared to the fullerene-based BHJOSCs. Beibei Oiu et al. [28] developed two benzodithiophene based small molecules (SM1 and SM2) as donors (Figure 1.31). SM1 has cyanoester as electron withdrawing end group and SM2 carries only ester as electron withdrawing end group. SM1 -- IDIC and SM2 -- IDIC blends exhibited light absorption covering ~350- to 780-nm region. All small-molecule BHJOSCs were fabricated by adopting a simple conventional device

36  Fundamentals of Solar Cell Design EtHex S

C8H17 C8H17 CN

HexEt O

S S

O

S

S

S

S S

O

O

CN

EtHex

C8H17

C8H17

EtHex S

C8H17

O

SM1

HexEt

O

S

S S

C8H17

S

S

S

S

S

S

S S

C8H17

O O

C8H17

SM2

HexEt

C6H13 CN NC

2.82 C6H13

O S

S O C6H13 IDIC

C6H13

2.70

3.90 I CN D NC I C 5.50

S M 1 5.24

S M 2

5.04

Figure 1.31  Benzodithiophene based small-molecule donors with electron withdrawing end groups-ester and cyanoester.

structure like: ITO/PEDOT:PSS/SM1 or SM2 + IDIC/PDINO/Al with thermal annealing at 115°C and the PV parameters were determined. SM1 as a donor molecule displayed higher power conversion efficiency (PCE) of 10.11% with Voc 0.905V, Jsc of 15.18 mA/cm2, and a FF of 73.55%, whereas the SM2 small molecule showed only 5.32% of PCE. The big difference in SM1 and SM2 molecules efficiencies was attributed to the cyano-ester electron withdrawing end group present in SM1. Morphology of the film was deduced using Photo-induced Force Microscopy (PiFM), a new technique. The charge moblities deduced indicate that SM1 has higher charge mobility compared to SM2. Huan Li et al. reported A-D-A–type [29] small molecule of benzodithiophene-based donor type (NDTSR) and used (Figure 1.32) two small-molecule acceptors (IDIC and ITIC) to fabricate All small-molecule BHJOSCs to evaluate PV properties and in particular the efficiencies. NDTSR

Organic Solar Cells  37 S EtHex

S

N

S

S

CN NC

O

NC

O

C6H13

C6H13

IDIC

S

I T I C

5.70

5.62

3.50 N D T S R 5.25

C6H13

C6H13

NC

S

O

O

S

CN ITIC

CN

S

S NC

3.93

I CN D I C

S

S

N

NDTSR

4.01

C6H13

S

C8H17

S HexEt S

C6H13

S

S

C8H17

O S

S

S

S

O

C8H17

S

C8H17

S

C6H13 C6H13

Figure 1.32  Trithieno BDT with rhodenone.

absorption is complimented by both IDIC and ITIC leading to the blend with an absorption encompassing ~350 to 780 nm. Solar cell was fabricated by adopting cell architecture as ITO/PEDOT-PSS/Active layer/Ca/Al. PV parameters were determined for NDTSR-ITIC blend, with PCE of 1.77% and NDTSR-IDIC blend, with PCE of 8.05%. Charge mobility was found to be higher for NDTSR-IDIC blend compared to NDTSR-ITIC blend. The variation in the PCE values between the two systems was attributed to the difference in the film morphology. Authors state that small-molecule donors or acceptors requires in improving the efficiency of solar cells. Yong Huo et al. in their research paper [30] discussed the synthesis of two small-molecule donors (DRBDT-TVT and DRBDT-STVT) (Figure 1.33). These two small molecules have BDT central core linked on either side with three thiophene units carrying rhodanine terminal groups, attached to central core DBT with two thiophenes linked with E

38  Fundamentals of Solar Cell Design S N

C8H17 S

S

S S

S

EtHex

S

S S DRBDT-STVT

S

EtHex

3.43 D R B D T T V T 5.14

O

S S C8H17

R

DRBDT-TVT

R=

S

S

S

O

R=

R

C8H17

S C8H17

N S

3.41 D 3.91 R B D I T D I S C T V T 5.11 5.65

Figure 1.33  BDT linearly linked trithiophene derivatives.

geometry double bond and differ in carrying S-alkyl group. IDIC small molecule was used as acceptor in these all small-molecule BHJOSC studies to generate PV parameters and to understand the role of structure in improving the efficiency of solar cell. Blend of IDIC with DRBDT-TVT or DRBDT-STVT displayed light absorption in ~350- to 780-nm region. Conventional device with architecture-ITO/PEDOT:PSS/DRBDT-TVT or DRBDT-STVT + IDIC/PDIN/Al was adopted to measure the solar cell parameters. Both the molecules exhibited decent efficiency like PCE of 6.63% for DRBDT-TVT and 6.51% for DRBDT-STVT. These results were compared with PC71BM acceptor based solar cell parameters to evaluate the advantages of all small-molecule BHJOSC. Yunchuang Wang et al. reported [31] the synthesis of three nonfullerene small acceptor molecules, IDIC8-Me, IDIC8-H, IDIC8-F (Figure 1.34), and a donor small molecule—DRCN5T. Indoceneodithiophene unit is the core with vinyledene dicyano moiety as end group in these three acceptor molecules. The three acceptor small molecules, IDIC8-Me, IDIC8-H, and IDIC8-F, differ in their group substitution leading to small changes in their HOMO-LUMO energy levels and also matched with donor small-molecule DRCN5T energy levels. Light absorption of blends prepared falls in to the region of ~350 to 750 nm. A conventional and simple solar cell architecture was adopted as: ITO/PEDOT:PSS/DRCN5T + IDIC8-Me or IDIC8-H or IDIC8-F/PDINO/Al, and the PV parameters were determined. PC Efficiency of 6.31% for IDIC8-Me, 8.00% for IDIC-H

Organic Solar Cells  39 3.72

C8H17

C8H17

CN

O

NC

S

S

O C8H17

Me

I D I C 8 CN M

Me

NC

C8H17

IDIC8-M

5.51 3.76

C8H17 CN

I D I C 8 H

C8H17 O

NC

S

S

CN

O C8H17

NC

C8H17

5.52

IDIC8-H

3.82 C8H17 CN

I D I C 8 F

F C8H17 O

NC

S

S

O C8H17

F

CN NC

C8H17

IDIC8-F

5.51 3.41

NC N O

NC

CN

CN

N C8H17

C8H17

S

O

S

S

S

S

S

S

D R C N 5 T

C8H17

C8H17 DRCN5T

Figure 1.34  Linearly linked pentathiophene with vinyldicyanoindenones.

5.22

40  Fundamentals of Solar Cell Design and 8.42% for IDIC8-F were observed. The fluorine substitution effected the change in the HOMO-LUMO energy levels compared to the methyl derivative synthesized and this could be rationale behind the 8.42% efficiency observed in these studies. Authors point out that linearly linked thiophene moiety with suitable end groups will improve the efficiency. Haijun Bin et al. described [32] the synthesis of two small-molecule donors (H11 and H12: Figure 1.35) with core structure BDTT flanked by thiophene-fluorobenzotriazole which was attached both sides with thiophene-vinylenecyanoester group as electron withdrawing group.

C8H17

C6H13 HexEt

O

S

F

S

F H11

N N N

S

S

F

S

NC

C6H13 S

N N N

O

C8H17

S

C8H17

O S

CN

F

C8H17

EtHex O

O EtHex

C6H13 HexEt

O

S F

S

NC

F H12 3.03

C6H13 S

N N N

O

C8H17

S

C8H17

S S

H 1 1

5.31

F

O EtHex

3.01

H 1 2

5.28

5.65

Figure 1.35  BDTT core linked benzotriazole derivatives.

O S

F

3.97 I D I C

N N N

S

EtHex O CN

Organic Solar Cells  41 IDIC was used as small-molecule acceptor. Light absorption spectrum of these donors and acceptors found to display complementarity and covered a wide range of absorption. Solar cells were fabricated with a conventional device structure of ITO/PEDOT-PSS/Blend of IDIC + H11 or H12/ PDINO/Al with and without annealing at 120°C. PCE observed for H11 is 9.73% and for H12 5.51%, under these fabrication conditions. Low lying HOMO energy level, higher charge mobility, and more orderly nature of film formation were the reasons informed for the higher efficiency found for the H11. Authors advocate that BDTT linked with benzotriazole moiety is new scaffold with decent efficiency of 9.73% and has the potential to improve the PCE involving further design of small molecules. Xiafei Cheng et al. synthesized [33] A-D-A–type conjugated four small donor molecules (Figure 1.36) differing in substitution: DRTT-T (alkylthiophene substituent); DRTT-R (ethylhexyl substituent); DRTT-OR (alkoxythiophene substituent); and DRTT (no substituent). Rhodanine group was attached to BDTT, terminally on either side with the central core thienothiophene moiety. Density Functional Theory informed that DRTT-OR and DRTT mould in to almost planar conformation, while DRTT-T and DRTT-R moulded in to twisted conformation due to the introduction of bulky substituents on TT units. These molecules were found to be soluble green solvents. F-2Cl was selected as small acceptor molecule to blend with the above four donor small molecules. Active blend prepared from F-2Cl and donor molecule provided light absorption covering ~350- to 780-nm region. Simple or conventional device architecture adopted for these molecules as ITO/PEDOT-PSS/Blend of F-2Cl + Donor/PDINDO/ Al, without annealing and with annealing at ~120°C to measure the photo voltaic parameters. DRTT-T exhibited decent efficiency like 9.37%, Voc = 0.95V; Jsc = 15.72 mA/cm2; FF = 62.8%, whereas DRTT-R displayed 10.45% efficiency, Voc = 1.00 V; Jsc = 16.82 mA/cm2; FF = 62.6% using THF solvent. Chloroform as solvent also afforded good results for the same compounds. The other two planar small-molecule donors, DRTT-OR and DRTT, showed satisfactory efficiencies. Twisted nature of the molecules, charge mobility, and film morphology factors are influential in displaying higher efficiencies. Xinxin Li et al. explained in their paper [34] the synthesis of A-D-A– type small donor molecule (P2TBR; Figure 1.37) for the purpose of fabricating all small-molecule BHJOSCs using IDIC as a small acceptor molecule. P2TBR was a non-fused p-dialkoxybenzene at center core with linearly attached thiophene and then BDTT and rhodanine terminal at both sides of center core. P2TBR and IDIC showed complementarity of absorption in solution phase absorption studies. P2TBR

42  Fundamentals of Solar Cell Design

S S

N

S

S

S

S

S

S

O

R

S

R

S

DRTT-R

R

S

S

S

S

3.36 S

S

O S

S

S

R

R

S

S

S

S

N S

R

O

D R T T R 5.40

R

R O

R

N

S

S

R = EthylHexyl group

R

O

O

S

S

S

S

S S

N

3.24 S

S

S

5.39

R

S

O

D R T T T

R R = EthylHexyl group

R

S

N S

DRTT-T

R

R

S

S

S

S

N

O S

S

S

S

3.35

R

R

R

D R T T OR 5.31

DRTT-OR

R = EthylHexyl group R

R S N

S S

S

S

S

O

3.23 S O S

S

S

S

S S

DRTT

CI

CI

CI

CN

5.29

CI

3.23

3.36

3.35

D R T T

D R T T OR

D R T T T

5.29

5.31

3.86 O

NC

S

R

R = EthylHexyl group

R

N

C8H17

C8H17

S C8H17

C8H17

F-2CI

O S

C8H17 C8H17

CN CN

D R T T

F 2 CI 5.50

Figure 1.36  Thienothiophene with BDTT Core linked with rhodanine.

3.24 D R T T R

5.39 5.40

Organic Solar Cells  43 EtHex EtHex N

S S

HexEt

S

O

S S

S S

O

S HexEt

O S

S

3.64

S

S EtHex

S O

N

HexEt

P2TBR

S

P 2 T B R 5.36

3.97 I D I C

5.6

Figure 1.37  Dialkoxybenzene linked BDTT with rhodanine end group.

and IDIC blend film, after SV annealing, displayed light absorption in the range ~350 to 750 nm. It was informed that solvent vapor annealing improved intermolecular interactions between donor and acceptor molecules. All small-molecule BHJOSC were fabricated using P2TBR donor and IDIC as acceptor blend, adopting a simple and conventional architecture like: ITO/PEDOT-PSS/P2TBR + IDIC ble3nd/ZnO/Al to determine PV parameters. Excellent efficiency of 11.5% was observed for the device fabricated (as given above) along with Voc = 0.94; Jsc = 17.5 mA/ cm2; and FF = 70.5. Authors claim that non-fused, linearly linked, smallmolecule donor with p-dialkoxybenzene core structure has potential to further achieve higher efficiencies. Zuojia Li et al. synthesized [35] two small acceptor molecules differing in their fluorine substitution (Figure 1.38), to understand the fluorine effect on the PV parameters of all small-molecule BHJOSCs. IDIC has five rings fused continuously with vinylilenedicyano indacene (IDIC) or vinylilenedicyano tetrafluoro indocene (IDIC-4F) end groups at both ends. DFDT(DPP)2 was selected as donor which contains four thiophene units linked linearly with 2-Fluorines attached to each thiophene in the middle of the molecule, both sides carrying diketopyrrolopyrrole with thiophene moiety. Acceptor and donor exhibited complementarity in their light absorption spectra. The donor-acceptor blend absorption starts at ~350 nm and ends at ~780 nm. Inverted solar cells were fabricated with configuration of: ITO/Zno/Blend of DFDT(DPP)2 + IDIC or DFDT(DPP)2 + IDC-4F/MoO3/Ag, to generate PV parameters. DFDT(DPP)2 + IDIC-4F Blend displayed PCE of 9.43% with Voc = 0.86 V; Jsc = 16.83 mA/cm2; FF = 65%. Authors inform that donor acceptor interaction in the blend leads to good crystallinity as well as improved morphology, and these are also the factors responsible for the improvement of efficiency.

44  Fundamentals of Solar Cell Design Alkyl

O

N

S

S

S

N

O

O

F

S

S

S

Alkyl

Alkyl N

O

N Alkyl

F DFDT(DPP)2

F

NC

CN

F

CN

CN 3.51 O

O S

S C6H13 C6H13

C6H13

C6H13

C6H13

C6H13

C6H13

C6H13

IDIC

S

IDIC-4F

S

O

D F D T (D P P)2 5.39

O

3.91

3.93

I D I C

I D I C 4 F

5.60

5.76

NC

NC CN

F

CN F

Figure 1.38  Tetrathiophene linked with DPP.

Huan Li et al. synthesized [36] a new A-D-A–type small donor NDTSR molecule (Figure 1.39). NDTSR has naphthalene fused with four thiophenes is the central core, and further, it is attached to three thiophene units on both sides and with rhodanine as terminal group at both ends. IDIC and ITIC (Figure 1.39) were taken as small-molecule acceptors. Solution phase light absorption for donor and acceptors indicate the complementarity in their light absorption. NDTSR with acceptor blend exhibited light absorption covering ~350- to 780-nm region. Conventional and simple configuration was adopted for fabricating the solar cell as: ITO/ PEDOT-PSS/Blend NDTSR + IDIC or NDTSR + ITIC/Ca/Al and the PV parameters were determined. The NDSTR + IDIC blend provided poor efficiency of 1.71%, whereas NDTSR + ITIC blend gave very good result by showing 8.05% efficiency. The big difference observed in using IDIC and

Organic Solar Cells  45 S

S EtHex

N

S

O C8H17

C8H17 S

S

S S

S

S

S

S S

S

C8H17

C8H17

O

S

N

HexEt

S

NDTSR

S

NC NC NC

O

CN

O

C6H13

S S

S C6H13

C6H13

C6H13

C6H13

C6H13

ITIC

S

C6H13

IDIC S O

S C6H13

O

3.5 N D T S R

3.93

4.01

I T I C

I D I C

5.62

5.70

CN

NC CN

CN

5.25

Figure 1.39  Fused NDTSR with rhodanine end group.

ITIC was attributed to the donor-acceptor molecular interactions leading to the formulation of ordered film, which could facilitate the charge ­mobility/migration effectively. Sachin Badgujar et al. prepared [37] two small molecules, one as donor—BDT3TR, and another as acceptor—O-IDTBR (Figure 1.40), for the purpose of studying their all small-molecule solar cell efficiency. Donor and acceptor were selected because of their complementarity in their light absorption spectrum. Blend of donor-BDT3TR and acceptor–OIDTBR absorption occurred in ~350- to ~650-nm region. Solar cell device

46  Fundamentals of Solar Cell Design

O N S

S EtHex HexEt

S

S

N O

S

N

S N

S

S

S

S

S

S

C8H17

EtHex HexEt S S

S

S

S

BDT3TR

S

S

S

S

S

HexEt

HexEt

HexEt

3.28

C8H17

O

S S C8H17 O-IDTBR

C8H17

N S

S

S N O

EtHex

EtHex

EtHex

S

N

N

S S

B D T 3 T R 5.10

3.89 O I D T B R 5.46

Figure 1.40  Linear BDTT linked trithiophene with rhodanine end group.

structure adopted was: ITO/PEDOT-PSS/Blend of BDT3TR + O-IDTBR/ ZnO/CPE/Al for measuring the PV parameters. An efficiency 6.96 % was recorded with other parameters as, Voc = 1.06 V; Jsc = 12.10 mA/cm2; FF = 55%. Authors mentioned that complimentary light absorption by donor and acceptor and high lying HOMO level of O-IDTBR could be the reasons behind the higher efficiency observed in these investigations. Further, they advocated that this was the first all small-molecule BHJOSCs. Jisu Hong et al. reported [38] the synthesis of three small-molecule acceptors (Figure 1.41). These were two naphthalenediimides linked to thiophene - NDICN-T: i) NDICN-T linked to bithiophene—NDICN-BT and ii) NDICN-T linked to (E)-1,2-di(thiophene-2-yl)ethane—NDICNTVT. The small-molecule donor employed was DTS-F. UV-visible light absorption and photoluminescence spectra were recorded for three acceptors and one donor and were found to have considerable overlapping. PV properties were obtained by fabricating conventional or simple solar cell architecture like: ITO/PEDOT-PSS/Blend of DTS-F with acceptor molecule/LiF/Al. The blend DTS-F with NDICN-TVT gave satisfactory efficiency of 3.01. Furthermore, to probe the solar cell fabrications and efficiency, authors evaluated film morphology, femtosecond transient absorption on films, and charge transport dynamics. These investigations inform that NDI (Naphthalene Diimide) derivatives can be probed as lead molecules in further studies.

Organic Solar Cells  47 EtHex O

N

O

EtHex O

CN

O

N

X

X= N

O

NC O

X=

= NDICN-T

S

S

O

EtHex

N

S

O

X=

EtHex

= NDICN-BT

S S

= NDICN-TVT

3.34 3.92

3.89

N D I C N T

N D I C N B T

N D I C N T V T

5.39

5.26

5.27

3.98

D T S F

Figure 1.41  NDIs linked with spacer.

1.8 Conclusion The chapter illustrates polymer donor and non-fullerene smallmolecule acceptor and all small-molecule (donor and acceptor) BHJSC’s performance. It is evident from the examples cited that non-fullerene–based BHJOSCs are becoming increasingly popular and in particular all small molecule–based BHJOSCs have just started their journey and will take off further. We can say that fullerene-based BHJOSCs are becoming outdated due to their limitations and solar cell efficiency. Several examples cited in this chapter provide over 17% efficiency achieved for polymer donor and non-fullerene small-molecule acceptor and over 14% efficiency cited for all small-molecule (donor and acceptor) BHJSCs. It becomes clear that a variety of designs have been developed to make small molecules in order to test them for solar cell efficiency. It is predicted that these small-molecule BHJOSCs are expected to display higher efficiency in near future by the use of proper design of molecules and solar cell fabrications. More interestingly, all small-molecule (acceptor and donor) materials have higher advantage than polymer donor and small-molecule acceptor type solar

48  Fundamentals of Solar Cell Design cells. All small-molecule (acceptor and donor) design and synthesis is less expensive, and higher purity can be achieved. Their optical, thermal, and electrochemical properties are tunable by proper design of molecules. Most of the examples discussed in this chapter are not clear with stereochemistry of the terminal double bond attaching to electron withdrawing group (like rhodanine or indanone), since the E and Z isomers differ in their properties. Particularly, their dipolemoment (vector) or polarizability (tensor) may have an impact on the molecular association in the formation of films. Hence, defining the stereochemistry of double bond will improve solar cell functioning. The film morphology can be controlled by applying supramolecular chemistry interactions, while designing the small molecules. The thermal stability and photo stability coupled with high PCE will place these BHJOSCs for the possible commercial applications in the near future.

Acknowledgement This chapter was authored purely out of academic interest to familiarize scholars about organic solar cell materials, particularly related to organic molecules. The examples covered in this review are chosen from recent literature appearing in different journals. The authors of this review are highly appreciative of the research articles published for their contributions in the area of organic solar cells. This chapter is only representative in nature and not intended to be exhaustive. Scholars are advised to go through original research publications for detailed information. The structures are also drawn briefly to give an idea about the products. The authors of this review further acknowledge the original contributors and publishers of the research articles cited here for their potential and interesting scientific work, with a larger interest in academic excellence and advancement. VJR thanks Dr. B. Parthasaradhy Reddy, Chairman Hetero Drugs, Pvt. Ltd., and Dr. K. Ratnakar Reddy, Director HR Foundation for their encouragement. VJR also thanks CSIR, New Delhi, for Emeritus Scientist Honor.

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Organic Solar Cells  49 2. Yong Cui, Huifeng Yao, Jianqi Zhang, Tao Zhang, Yuming Wang, Ling Hong, Kaihu Xian,BoweiXu, Shaoqing Zhang, Jing Peng, Zhixiang Wei,Feng Gao & Jianhui Hou; Over 16% efficiency organic photovoltaic cells enabled by a chlorinated acceptor with increased open-circuit voltages; Nature Communications, 15, 2515, 2019; https://doi.org/10.1038/ s41467-019-10351-5. 3. Bin Kan, HuanranFeng, Huifeng Yao, Meijia Chang, Xiangjian Wan, Chenxi Li, JianhuiHou & Yongsheng Chen; A chlorinated low-bandgap smallmolecule acceptor for organic solar cells with 14.1% efficiency and low energy loss; Science China Chemistry., 1-7, 2018. 4. Huifeng Yao, Yu Chen, Yunpeng Qin, Runnan Yu, Yong Cui, Bei Yang, Sunsun Li, Kai Zhang, and JianhuiHou; Design and Synthesis of a Low Bandgap Small MoleculeAcceptor for Efficient Polymer Solar Cells; Adv. Mater., 28, 8283–8287, 2016. 5. Huifeng Yao, Yong Cui, Runnan Yu, BoweiGao, Hao Zhang, and JianhuiHou; Design, Synthesis, and Photovoltaic Characterization of a Small Molecular Acceptor with an Ultra-Narrow Band Gap; Angew. Chem. Int. Ed., 56, 3045– 3049, 2017. 6. Huifeng Yao, Long Ye, Junxian Hou, Bomee Jang, Guangchao Han, Yong Cui,Gregory M. Su, Cheng Wang, Bowei Gao, Runnan Yu, Hao Zhang, Yuanping Yi,Han Young Woo, Harald Ade, and Jianhui Hou; Achieving Highly Efficient Nonfullerene Organic Solar Cells with Improved Intermolecular Interaction and Open-Circuit Voltage; Adv. Mater. 29, 1700254, 2017. 7. Yunlong Ma, Meiqi Zhang, Yu Yan, Jingming Xin, Tao Wang, Wei Ma, Changquan Tang, and Qingdong Zheng; Ladder-Type DithienonaphthaleneBased Small-Molecule Acceptors for Efficient Nonfullerene Organic Solar Cells; Chem. Mater., 29, 7942–7952, 2017. 8. Yuze Lin, Fuwen Zhao, Shyamal K. K. Prasad, Jing-De Chen, Wanzhu Cai,Qianqian Zhang, Kai Chen, Yang Wu, Wei Ma, Feng Gao, Jian-Xin Tang,Chunru Wang, Wei You, Justin M. Hodgkiss, and Xiaowei Zhan; Balanced Partnership between Donor and Acceptor Components in Nonfullerene Organic Solar Cells with >12% Efficiency; Adv. Mater., 30, 1706363, 2018. 9. Jie Zhang, Baofeng Zhao, Yuhua Mi, Hongli Liu, Zhaoqi Guo, Guojun Bie, Wei Wei, Chao Gao, Zhongwei An; A New Wide Band gap Small Molecular Acceptor Based on Indenofluorene Derivatives for Fullerene-Free Organic Solar Cells; Dyes & Pigments, 140, 261–268, 2017. 10. Oh Kyu Kwon, Mohammad AfsarUddin, Jung-Hwa Park, Sang Kyu Park, Thanh Luan Nguyen, Han Young Woo, and Soo Young Park; A High EfficiencyNonfullerene Organic Solar Cell with Optimized Crystalline Organizations; Adv. Mater., 28, 910–916, 2015.

50  Fundamentals of Solar Cell Design 11. Sunsun Li, Long Ye, Wenchao Zhao, Shaoqing Zhang, Harald Ade, and Jianhui Hou; Significant Influence of the Methoxyl Substitution Position on Optoelectronic Properties and Molecular Packing of Small-Molecule Electron Acceptors for Photovoltaic Cells; Adv. Energy Mater., 7, 1700183, 2017. 12. Huanran Feng, Nailiang Qiu, Xian Wang, Yunchuang Wang, Bin Kan, Xiangjian Wan, Mingtao Zhang, Andong Xia, Chenxi Li, Feng Liu, Hongtao Zhang, and Yongsheng Chen; An A‑D‑A Type Small-Molecule Electron Acceptor with End-Extended Conjugation for High Performance Organic Solar Cells; Chem. Mater., 29, 7908–7917, 2017. 13. Feng Liu, Zichun Zhou, Cheng Zhang, Thomas Vergote, Haijun Fan, Feng Liu, and Xiaozhang Zhu; A Thieno[3,4-b]thiophene-Based Non-Fullerene Electron Acceptorfor High-Performance Bulk-Heterojunction Organic Solar Cells; J. Am. Chem. Soc., 138, 15523–15526, 2016. 14. YuvrajPatil, Rajneesh Misraa, M. L. Keshtov, Ganesh D. Sharma; Small molecule carbazole based diketopyrrolopyrroles with tetracyanobutadieneacceptor unit as Non-Fullerene Acceptor for Bulk Heterojunction Organic Solar Cells; J. Mater. Chem. A, 5, 3311–3319, 2017. 15. Jia Sun, Xiaoling Ma, Zhuohan Zhang, Jiangsheng Yu, Jie Zhou, Xinxing Yin,Linqiang Yang, Renyong Geng, Rihong Zhu, Fujun Zhang, and Weihua Tang; Dithieno[3,2-b:2ʹ,3ʹ-d]pyrrol Fused Nonfullerene Acceptors Enabling Over 13% Efficiency for Organic Solar Cells; Adv. Mater., 30, 1707150, 2018. 16. Zhuping Fei, Flurin D. Eisner, Xuechen Jiao, Mohammed Azzouzi, Jason A. Röhr, Yang Han, Munazza Shahid, Anthony S. R. Chesman, Christopher D. Easton, Christopher R. McNeill, Thomas D. Anthopoulos, Jenny Nelson, and Martin Heeney; An Alkylated Indacenodithieno[3,2-b]thiophene-Based Nonfullerene Acceptor with High Crystallinity Exhibiting Single Junction Solar Cell Efficiencies Greater than 13% with Low Voltage Losses; Adv. Mater., 30, 1705209, 2018. 17. Jianfei Qu, Zhao Mu, Hanjian Lai, Hui Chen, Tao Liu, Shuai Zhang, Wei Chen, and Feng He; Alkyl Chain End Group Engineering of Small Molecule Acceptors for Non-Fullerene Organic Solar Cells; ACS Appl. Energy Mater., 1, 4724–4730, 2018. 18. Kaili Wang, Jie Lv, Tainan Duan, Zhefeng Li, Qianguang Yang, ⊥Jiehao Fu, Wei Meng, Tongle Xu, Zeyun Xiao, Zhipeng Kan, Kuan Sun, and Shirong Lu; Simple near-Infrared Nonfullerene Acceptors Enable Organic Solar Cells with >9% Efficiency; ACS Applied Energy Materials, Interfaces, 11, 6717–6723, 2019. 19. Eun Yi Ko, Gi Eun Park, Ji Hyung Lee, Hyung Jong Kim, Dae Hee Lee, Hyungju Ahn, Mohammad Afsar Uddin, Han Young Woo, Min Ju Cho, and Dong Hoon Choi; Excellent Long-Term Stability of Power Conversion Efficiency in Non-Fullerene-Based Polymer Solar Cells Bearing

Organic Solar Cells  51 Tricyanovinylene-Functionalized n-Type Small Molecules; ACS Applied Materials & Interfaces, 9, 8838–8847, 2017. 20. Yamin Zhang, Huanran Feng, Lingxian Meng, Yanbo Wang, Meijia Chang, Shitong Li, Ziqi Guo, Chenxi Li, Nan Zheng, Zengqi Xie, Xiangjian Wan, and Yongsheng Chen; High Performance Thick-Film Nonfullerene Organic Solar Cells with Efficiency over 10% and Active Layer Thickness of 600 nm; Adv. Energy Mater. 1902688, 2019. 21. Yanbo Wang, Yamin Zhang, Nailiang Qiu, Huanran Feng, Huanhuan Gao, Bin Kan, Yanfeng Ma, Chenxi Li, Xiangjian Wan, and Yongsheng Chen; A Halogenation Strategy for over 12% Efficiency Nonfullerene Organic Solar Cells; Adv. Energy Mater. 1702870, 2018. 22. Andrew Wadsworth, Helen Bristow, Zeinab Hamid, Maxime Babics, Nicola Gasparini, Colm W. Boyle, Weimin Zhang, Yifan Dong, Karl. J. Thorley, Marios Neophytou, Raja Shahid Ashraf, James R. Durrant, Derya Baran, and Iain McCulloch; End Group Tuning in Acceptor–Donor–Acceptor Nonfullerene Small Molecules for High Fill Factor Organic Solar Cells; Adv. Funct. Mater., 1808429, 2019. 23. Jianfei Qu, Duning Li, Huan Wang, Jiadong Zhou, Nan Zheng, Hanjian Lai, Tao Liu, Zengqi Xie, and Feng He; Bromination of the Small-Molecule Acceptor with Fixed Position for High-Performance Solar Cells; Chem. Mater., 31, 2019. 24. Liyan Yang, Shaoqing Zhang, Chang He, Jianqi Zhang, HuifengYao, Yang Yang, Yun Zhang, wenchaozhao, and JianhuiHou; A New Wide Band Gap Donor for Efficient FullerenefreeAll-small-molecule Organic Solar Cells; J. Am. Chem. Soc., 139, 1958–1956, 2017. 25. Ruimin Zhou, Zhaoyan Jiang, Chen Yang, Jianwei Yu, Jirui Feng, Muhammad Abdullah Adil,Dan Deng, Wenjun Zou, Jianqi Zhang, Kun Lu, Wei Ma, Feng Gao & Zhixiang Wei, All-small-molecule organic solar cells with over 14% efficiency by optimizing hierarchical morphologies; Nature Communications, 10, 5393, 2019. 26. Haiyan Chen, Dingqin Hu, Qianguang Yang, Jie Gao, Jiehao Fu, Ke Yang, Hao He, Shanshan Chen, Zhipeng Kan, Tainan Duan, Changduk Yang, Jianyong Ouyang, Zeyun Xiao, Kuan Sun, and Shirong Lu; All-SmallMolecule Organic Solar Cells with an Ordered Liquid Crystalline Donor; Joule, 3, 1–14, 2019. https://doi.org/10.1016/j.joule.2019.09.009 27. Haijun Bin, Jia Yao, Yankang Yang, Indunil Angunawela, Chenkai Sun, Liang Gao,Long Ye, Beibei Qiu, Lingwei Xue, Chenhui Zhu, Chunhe Yang, Zhi-Guo Zhang, Harald Ade, and Yongfang Li; High-Efficiency All-Small-Molecule Organic Solar Cells Based on an Organic Molecule Donor with AlkylsilylThienyl Conjugated Side Chains; Adv. Materials, 30, 1706361, 2018. https:// doi.org/10.1002/adma.201706361. 28. Beibei Qiu, Lingwei Xue,Yankang Yang, Haijun Bin, Yindong Zhang, Chunfeng Zhang, Min Xiao, Katherine Park, William Morrison, Zhi-Guo

52  Fundamentals of Solar Cell Design Zhang, and Yongfang Li; All-Small-Molecule Nonfullerene Organic Solar Cells with High Fill Factor and High Efficiency over 10%; Chem. Mater., 29, 7543−7553, 2017. 29. Huan Li, Yifan Zhao, Jin Fang, Xiangwei Zhu, Benzheng Xia, Kun Lu, Zhen Wang, Jianqi Zhang, Xuefeng Guo, and Zhixiang Wei; Improve the Performance of the All-Small-Molecule Nonfullerene Organic Solar Cells through Enhancing the Crystallinity of Acceptors; Adv. Energy Mater., 1702377, 2018. 30. Yong Huo, Cenqi Yan, Bin Kan, Xiao-Fei Liu, Li-Chuan Chen, Chen-Xia Hu, Tsz-Ki Lau, Xinhui Lu, Chun-Lin Sun, Xiangfeng Shao, Yongsheng Chen, Xiaowei Zhan, and Hao-Li Zhang; Medium Bandgap Small Molecule Donors Compatible with Both Fullerene and Non-fullerene Acceptors; ACS Appl. Mater. Interfaces, 10, 9587–9594, 2018. 31. Yunchuang Wang, Meijia Chang, Bin Kan, Xiangjian Wan, Chenxi Li, and Yongsheng Chen; All-Small-Molecule Organic Solar Cells Based on Pentathiophene Donor and Alkylated Indacenodithiophene-Based Acceptors with Efficiency over 8%; ACS Appl. Energy Mater., 1, 2150−2156, 2018. 32. Haijun Bin, Yankang Yang, Zhi-Guo Zhang, Long Ye, Masoud Ghasemi, Shanshan Chen, Yindong Zhang, Chunfeng Zhang, Chenkai Sun, Lingwei Xue, Changduk Yang, Harald Ade, and Yongfang Li; 9.73% Efficiency Nonfullerene All Organic Small Molecule Solar Cellswith AbsorptionComplementary Donor and Acceptor; J. Am. Chem. Soc., 139, 139, 5085−5094, 2017. 33. Xiafei Cheng, Miaomiao Li, Ziqi Guo, Jinde Yu, Guanghao Lu, Laju Bu, Long Ye, Harald Ade,Yongsheng Chen and Yanhou Geng; “Twisted” conjugated molecules as donor materials for efficient all-small-molecule organic solar cells processed with tetrahydrofuran; J. Mater. Chem. A, 7, 23008–23018, 2019. 34. Xinxin Li, Yan Wang, Qinglian Zhu, Xia Guo, Wei Ma, Xuemei Ou, Maojie Zhanga, Yongfang Li; A small molecule donor containing non-fused ring core for all-small-molecule organic solar cells with high efficiency over 11%; J. Mater. Chem. A, 7, 3682–3690, 2019. 35. Zuojia Li, Renping Liang, Jingwei Wang, Bing Na, and Hesheng Liu; SolutionProcessable All-Small-Molecule for High-Performance Nonfullerene Organic Solar Cells with High Crystallinity Acceptor; J. Phys. Chem. C, 123, 28021–28026, 2019. 36. Huan Li, Yifan Zhao, Jin Fang, Xiangwei Zhu, Benzheng Xia, Kun Lu, Zhen Wang, Jianqi Zhang, Xuefeng Guo, and Zhixiang Wei; Improve the Performance of the All-Small-Molecule Nonfullerene Organic Solar Cells through Enhancing the Crystallinity of Acceptors; Adv. Energy Mater., 1702377, 2018. 37. Sachin Badguja, Chang Eun Song, Sora Oh, Won Suk Shin, Sang-Jin Moon, Jong-Cheol Lee, In Hwan Jung, and Sang Kyu Lee; Highly Efficient and

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2 Plasmonic Solar Cells T. Shiyani1, S. K. Mahapatra2 and I. Banerjee1* School of Nanosciences, Central University of Gujarat, Gandhinagar, Gujarat, India 2 School of Basic and Applied Sciences, Central University of Punjab, Bathinda, India

1

Abstract

Photovoltaic (PV) cell is a fundamental solar energy conversion device that converts light energy into electric energy. The light absorption and charge recombination are main limiting factors on the efficiency of PV cell or solar cell. A limited efficiency of PV devices makes them less effective in market for clean energy production. Various tactics and methods are demonstrated to enhance the solar cell performance. Metallic nanoparticles have been utilized to fabricate solar cells because of its novel properties such as large surface to volume ratio and surface plasmon resonance (SPR). Plasmonic nanostructures can influence the absorption of light through scattering of surrounding molecules or particles. The plasmonic nanostructures can scatter or concentrate light at subwavelength scale for increasing absorption in active layer and hence enhancing the efficiency of PV devices. Therefore, the plasmonic nanostructures are promising candidates to develop high efficiency solar cells. We discuss about the fundamental mechanisms, ability to scale up the plasmonic with tailored optical properties, solar cell design, and recent advancements in plasmonic solar cells to generate clean energy and solar fuels. Keywords:  Plasmonic nanostructures, thin film, surface plasmon resonance, light scattering, solar cell

*Corresponding author: [email protected] Inamuddin, Mohd Imran Ahamed, Rajender Boddula, and Mashallah Rezakazemi (eds.) Fundamentals of Solar Cell Design, (55–82) © 2021 Scrivener Publishing LLC

55

56  Fundamentals of Solar Cell Design

2.1 Introduction Solar energy is cleanest energy source among other renewable energy. Earth receives annually about 1.7 × 105 TW energy on the surface of earth that is thousands times more than the total energy consumption of world, i.e., 12–13 TW [1]. Solar cell is a solar energy conversion device that works on photoelectric effect, which is schematically shown in Figure 2.1. The ejected electrons are called photoelectrons [2]. Photovoltaic (PV) cell converts light energy into electrical energy that can work on small and large-scale applications. Solar cell technology can make an important contribution to solve the energy problem on earth for the benefit of the society. The absorber material in solar cell absorbs the sunlight upon and then the electron-hole (e-h) pair is produced. The generated e-h pairs are needed to separate by completing a electric circuit to collect electrons as a flow of current. The cost of solar PV manufacturing is high due to expensive fabrication and toxic elements, which makes this technology less commercialized as compared to fossil fuels and nuclear energy [3, 4]. Schockey-Queisser limit suggests that the efficiency of solar energy to electric energy conversion for a single junction PV device can be achieved upto about 32%. The efficiency for cell consists of two layers, three layers, and a theoretical infinity layer can reach upto 42%, 49%, and 68%, respectively. An existed thin film solar cell technology with midgap infinite bands suggests a maximum efficiency of 77.2%. The single junction monocrystalline and polycrystalline silicon solar cells provide efficiencies about 20%–24% [5, 6]. Silicon solar cells are widely used to develop solar panels for generating electricity. The most of the price in this technology is

e-m radiation

Figure 2.1  Schematic representation of photoelectric effect. Einstein equation hf = ɸ + Ek, where h is plank constant (6.63 x 10-34 Js), f is the frequency of the incident light (Hz), ɸ (phi) is the work function (J), Ek is the maximum kinetic energy of the emitted electrons (J).

Plasmonic Solar Cells  57 because of silicon wafers and cell fabrication processes. Therefore, the advanced research is required in solar cell technology to find novel and cheap materials [7]. The thin film PV devices such as CIGS (~21%), GaAs (~37%), cadmium telluride (CdTe, ~19%), and CZTSSe (~13%) have been developed as alternative to silicon but low absorbance of near band gap light is a major problem [8, 9]. The usage of rare earth and toxic elements in CIGS, GaAs, and CdTe solar cells has made them less successful in market. Therefore, the novel design in device configuration is required to absorb maximum sunlight in solar cell. The various nanomaterials such as quantum dot, nanowire, thin film, and biomaterials can be used in combination to improve the performance of PV device. These combinations can enhance the light trapping on the surface of the PV cell. The biohybrid materials have been utilized to absorb wide range of sunlight for developing dye sensitized solar cells (DSSCs) as well as photoelectrochemical cells (PECs). They have achieved maximum efficiency of 30% at research scale [10, 11]. The multijunction solar cells have been demonstrated with maximum conversion efficiency of about 47%. The various intermediate layers have been explored in multi-junction solar cells enhancing the absorption of light and decreasing the recombination rate of e-h pair [12–14]. These solar cell technologies are summarized in Table 2.1. The major challenges for developing high efficient solar cells are increasing light absorption and minimizing recombination. The tuning in the Table 2.1  Various solar cell technologies and its efficiency. Solar cell technology

Maximum efficiency

Reference

First generation: Single junction Mono-crstalline and poly crystalline silicon, GaAs solar cells

~20–24% (silicon) ~ 38% (GaAs)

[7, 17, 90–92]

Second generation: Thin film solar cells such as CIGS, CdTe, CZTSSe, DSSCs

~ 21% (CIGS) ~ 19% (CdTe) ~ 13% (CZTSSe) ~ 18% (DSSCs, PECs)

[7, 17, 93–101]

Third generation: Hybrid DSSCs, PECs, quantum dot, perovskites, plasmonic solar cells

~ 30%

[7, 102–104]

Fourth generation: Multijunction solar cells with GaAs, InP and other intermediate layer

~ 47% (multijunction GaAs)

[7, 105–112]

58  Fundamentals of Solar Cell Design energy band gap can be useful to overcome these challenges. Recombination loss can be decreased by making thinner solar absorber in PV device. The PV absorber layers are generally thick to allow absorption with near wavelength and collection photocurrent. A major percentage of solar light spectrums are not absorbed in thin film solar cells, and hence, they are facing problem of low efficiency in comparison to traditional Si cells. Nanotechnology has the ability to make more efficient materials and PV devices. Plasmonic nanostructures can be helpful to reduce the thickness of PV absorbers while keeping their optical thickness constant. In plasmonic solar cells, the plasmon resonances in small metal nanoparticles are used to trap light. Then they transfer the energy to thin semiconducting layer to produce electricity through free charge carriers [15–17].

2.1.1 Plasmonic Nanostructure The specific wavelengths of light spectrum make the conductive electrons to oscillate collectively in the metal. This phenomenon is known as plasmonic resonance or surface plasmon resonance (SPR), which is schematically shown in Figure 2.2. Plasmon resonance of free electrons in metal nanoparticle is basis of the plasmonic effect. SPR wavelength is depending on the medium, size and shape of metal nanoparticles. SPR excitations increase more absorption and scattering of light compared to nonplasmonic nanoparticles [18, 19]. The optical properties can be tuned to absorb wide range of light spectrum of the electromagnetic radiation by tuning the thickness, size, shape, and composition of nanostructure. The reflecting color also can also be tuned by shifting the scattering and absorption, for example, solutions of gold and silver nanoparticles have ruby red and yellow color due to strong scattering, respectively [20, 21]. The resonance wavelength of plasmonic nanostructure is influenced by the refractive index (n) of material, for example, peak resonance of Ag nanoparticles (80 nm in size) in water (n = 1.33) is at 445 nm and in air is about 380 nm. e-cloud

Metal sphere

E-field e-cloud

Figure 2.2  Schematic representation of surface plasmon resonance.

Plasmonic Solar Cells  59 This theory was explored by Mie theory that can be used to analyze the scattering from spherical particle at any wavelength of light [22, 23]. The coupling effect in plasmonic nanoparticles is responsible for the shifting in absorption and color of particles. The scattering by the metal nanoparticles is more important than the absorption of light in plasmonic solar cells and hence plasmonic nanostructures have emerged as promising candidate to enhance the characteristics in solar cells, detectors, and sensors. The plasmonic nanoparticles of silver (Ag) and gold (Au) are mostly used at the top of the surface of PV device because of their SPR exist in the visible range. Al nanoparticles show SPR in UV range but they are unable increase the efficiency in plasmonic solar cells [24].

2.1.2 Classification of Plasmonic Nanostructures SPR are sensitive to the geometry of device, material, and light-matter interaction. The shapes of nanostructure can be classified as sphere, thin film, star, core-shell, disk, cavity, cage, etc. [25]. The plasmonic metal nanoparticles or thin films are used to get maximum absorption of light in absorbing layer. SPRs can be tuned to obtain desired frequency by using specific grating. The plasmonic nanostructures such as metal nanoparticle or thin film are mainly used in plasmonic solar cells that depend on the design and method. The most frequent and widely used design is the

Metal nanoparticles Semiconductor Substrate

Semiconductor Metal thin layer Substrate

Figure 2.3  The utilization of plasmonic nanoparticle (upper) and thin film (lower) for solar cells.

60  Fundamentals of Solar Cell Design deposition plasmonic nanoparticle at the top surface of PV device. The light is scattered by the SPR in all directions. This increases the overall absorption in solar cells. The core (metal)–shell (dielectric) designs are also used to absorb more light. The other design of plasmonic nanostructure is depositing a nano-sized thin film of metal on or below the semiconductor layer. The sunlight generates electric fields inside the absorber layer and electrons can be collected as a flow of current [26–28]. The schematic representation of plasmonic solar cell with nanoparticle and thin film configuration is shown in Figure 2.3.

2.2 Principles and Working Mechanism of Plasmonic Solar Cells 2.2.1 Working Principle A plasmonic nanostructure can be used as direct and indirect applications such as active PV material and plasmon modified active semiconductor, respectively [29]. The fundamental operational principle of plasmonic solar cell is scattering through SPR in metallic nanoparticles on semiconductor and surface plasmon polaritons at the metal/semiconductor interface. Solar cell performance mostly depends on photon energy and active PV layers. When a photon falls on the surface of semiconductor, the photons will pass through material (if photons have energy lower than the semiconductor band gap, Ephotons< Eg) and photons will reflect from the surface or they will be absorbed by the semiconductor layer (if photons have energy higher than the band gap of semiconductor, Ephotons > Eg). The unused sunlight produces heat in the solar cell through lattice vibrations and decreases the performance. An electron is excited into conduction band from the valance band due to the illumination of light and creates hole in semiconductor. Thus, absorbed photon generates e-h pair in the semiconductor. The electrons and holes have tendency to recombine due to their opposite charge, so they can be separated and the electrons can be collected as a flow of current. The fast collection of electrons would produce more electricity. Plasmonic nanostructures decrease the loss of incident photons by increasing optical path length in PV devices [30]. The very thin semiconducting layer absorbs less sunlight and hence the plasmonic nanostructures are used to enhance the absorption through scattering at resonance wavelengths in solar cell. The thinner conducting material will make quick collection of electrons but thin surface absorbs less photons. Hence, the thickness of photoactive material needs to be tuned for achieving maximum absorption

Plasmonic Solar Cells  61 of light, which will help to improve the efficiency of solar cell [31–33]. The overall absorption increases due to the surface plasmons in plasmonic solar cells. The plasmonic effect can be understood using polarizability of particles [34]. The polarizability α of the particle is given by Equation (2.1):

αx = (4πab2/3) [εp(ω) − εm]/[εm + Lx{εp(ω) − εm}]

(2.1)

where εp and εm correspond to dielectric function of metal nanoparticle and medium, respectively. Lx stands for depolarization factor and depends on shape of nanoparticle. The polarizability of spherical metal nanoparticle becomes maximum when resonance occurs. εp can be defined as Equation (2.2):

εp = 1 – [ωp/ω2 + lγω]

(2.2)

Plasmon frequency (ωp) is defined as Equation (2.3). It depends on free electrons in spherical particles. For example, gold, silver, copper, and aluminum show the resonance frequency in visible, UV, visible and UV, respectively. A gold show broader resonance peak than silver and it is highly stable where as silver is highly unstable [35]:

ωp = [Ne2/mε0]1/2

(2.3)

where N, e, me, and ε0 correspond to free electron density, charge of electron, electron’s effective mass, and dielectric constant in free space, respectively. A resonance frequency in free space can be given as Equation (2.4).

ωspr = √3ωp

(2.4)

2.2.2 Mechanism of Plasmonic Solar Cells Plasmonic nanostructures are used as light harvesting antennas in thin film solar cells. Plasmonic nanostructures such as metallic nanoparticles or thin films are used to trap or concentrate light. They can be introduced at top or back surface as antireflecting coating in solar cells. Various fabrication techniques can be used to deposit metal nanoparticles. A simple technique is the evaporation followed by heat treatment for making arrays of metal nanoparticles. The lithography is also used for making metal nanoparticles [36, 37].

62  Fundamentals of Solar Cell Design Metal nanoparticle Semiconductor Substrate

Semiconductor

(a) (b) Metal nanoparticle

Substrate

Semiconductor Substrate

(c) Metal nanoparticle

Figure 2.4  Approaches to integrate plasmonic nanoparticle in solar cell.

Plasmonic nanostructures can be used mainly in three manners. First, they can be positioned at the top surface as subwavelength scattering element in solar cell as shown in Figure 2.4a. The metallic nanoparticles dielectric particles are used to get scattering. Metallic nanoparticles are preferred because they can scatter more light than their geometry area. Second, a plasmonic nanostructure can be embedded at the interface or inside the absorber as shown in Figure 2.4b. The plasmonic resonance scatters the light inside the absorber layer and increases absorption at the interface. This design is more preferred in DSSC and single junction PV device. Third, the metallic nanostructure can be positioned as array of nanoparticle or very thin film at the back surface in solar cell as shown in Figure 2.4c. The light travels in a longer path and enhances the absorption. The plasmonic thin film does the task of light trapping and collection of charge carriers [38–40].

2.3 Important Optical Properties The plasmonic nanostructures are mainly used for increasing light absorption through scattering in plasmonic solar cells. The plasmonic nanostructures are affecting to optical properties such as light trapping, absorption, scattering, resonance wavelength range, and energy levels.

Plasmonic Solar Cells  63

2.3.1 Trapping of Light The metal nanoparticles are normally placed at a particular distance for trapping enough sunlight between nanoparticle and substrate in solar cell. The illuminated light intensity decreases with distance from the substrate in the case of embedded plasmonic nanoparticle. The top deposited plasmonic nanoparticle design is advantageous for illuminated light into the substrate. The more light escapes from device if there is no distance between the substrate and particle. The absorption is enhanced by folding light in absorber using plasmonic nanoparticles. The folding of light is mainly depending on the structural properties of metallic nanoparticles. The absorption is large in small nanoparticles because of increased near-field. However, very smaller metal nanoparticles go through ohmic loss. Hence, the surface plasmons can be utilized to improve the electric and optical behavior of solar cells. SPRs have ability to collect about 95% incident light where as conventional solar cell can collect about 30% of sunlight [41].

2.3.2 Scattering and Absorption of Sunlight The absorption and scattering of incident sunlight are main affecting parameters on the efficiency of solar cells. The utilization of plasmonic nanostructures at the top surface of the PV device improves the overall absorption through the scattering of light. For example, silicon does not absorb much light and hence more lights need to be scattered to increase the absorption. The deposited silver metal nanoparticles at the surface increases the absorption and scattering due to surface plasmons is about 10 times more than the nanoparticle. A broader plasmon resonance can be achieved in metal nanoparticles with a large scattering [42].

2.3.3 Multiple Energy Levels The multiple energy levels are possible by designing the stacks of solar cells on top of each cell with a different energy band gap to increase the absorption of light. The stacked structure of PV cell utilizes maximum sunlight and produces maximum energy conversion efficiency. The each cell should have same lattice parameters to decrease the energy losses [43]. The energy band gap values are summarized for various materials with their maximum absorption wavelength in Table 2.2. The band gap energy vs. λmax curve for different photoactive materials is shown in Figure 2.5.

64  Fundamentals of Solar Cell Design Table 2.2  Energy band gap of various absorber and metal nanoparticles. Material

Energy band gap (eV) at 300K

λmax (λ = hc/E = 1242/E) nm

Ge

0.66

1,881

Si

1.11

1,118

InP

1.27

977

CIGS

1.42

874

GaAs

1.43

868

CdTe

1.44

862

CZTS

1.5

828

CuO

1.7

730

CdSe

1.74

714

BiFeO3

2.09

594

Fe2O3

2.1

591

GaP

2.25

552

CdS

2.25

552

PbO

2.9

428

TiO2

3.2

388

SrTiO3

3.2

388

ZnO

3.4

365

SnO2

3.5

355

ZnS

3.6

345

SiO2

8.9

140

2.4 Advancements in Plasmonic Solar Cells Nanotechnology has huge impact on producing efficient energy conversion devices due to its ability to develop novel materials. There are many experimental reports that confirm the enhancement in light absorption and efficiency because of light scattering from metal nanoparticles in plasmonic solar cells. The plasmonic nanostructures utilize the incident

Plasmonic Solar Cells  65 10 Conduction Band

8

Energy gap, Eg

E (eV)

Valance Band

Insulators

6

4

Semiconductors

2

0

0

500

1000

1500

2000

Maximum Wavelength (nm) Figure 2.5  Bandgap energy vs. λmax curve for various material used in solar cells. The values of this curve that are mentioned correspond to Table 2.2.

photons with multiple energy values and decrease the loss of photon energies. Researchers are finding the best way to use maximum incident photons for more absorption and high efficiency. The plasmonic solar cells are classified mainly into two categories as direct plasmonic and plasmonicenhanced solar PV cells [44, 45]. The schematic representation of various plasmonic solar cells is shown in Figures 2.6a (direct plasmonic solar cell) and 2.6b and 2.6c (plasmonic-enhanced solar cells). The efficiency for various plasmonic solar cell technologies is given in Figure 2.7. The various plasmonic solar cells and its efficiencies are summarized in Table 2.3.

2.4.1 Direct Plasmonic Solar Cells The direct plasmonic solar cells utilize plasmonic nanoparticles as active light absorbers. The hot charge carriers in plasmonic nanoparticles can be produced by excitation of SPR. The hot electrons can be introduced into semiconductor for converting sunlight into electric energy and hot holes can also be injected into a p-type semiconductor. The separation of charge carriers makes possible to use plasmonic nanostructures directly as photo absorbers. The metallic nanoparticles such as Au and Ag offers plasmonic band in visible range that can act as light harvesting antenna. Yocefu Hattori and Jacinto Sa have demonstrated direct plasmonic solar

66  Fundamentals of Solar Cell Design Plasmonic nanoparticle

SiO2 Si SiO2

(b)

(a)

Plasmonic nanoparticle (c)

Transparent conductive electrode Hole transporting layer Plasmonic light absorbing layer

Photocathode

Electron transporting layer

Plasmonic photoanode

Transparent conductive electrode

Figure 2.6  The device configuration of (a) direct and (b) plasmonic enhanced plasmonic solar cell and (c) plasmonic DSSC.

Hybrid plasmonic Perovskites

Solar cell technology

Quantum dot PEC (H2) DSSC CZTS CdTe CIGS GaAs Silicon Hot carrier cell 0

10

20

Efficiency (%) Figure 2.7  Efficiency for various plasmonic solar cell technology.

30

40

Plasmonic Solar Cells  67 Table 2.3  Effect of various plasmonic nanostructures on solar cell technologies. Solar cell technology

Plasmonic nanostructure

Efficiency (%)

Reference

Hot carrier cell

Au@TiO2/ PEDOT

Direct plasmonic transparent active layer

0.2

[46] 2019

Au

Direct 24 plasmonic as active layer

[59] Wu, 2013

Ag with diameter 14-100 nm

At top surface

26

[27] Atwater HA, 2010 et al., 2010

Al

At top of surface

14.5

[53] 2013

Au

At top of surface

14

[115] Jacak, 2018

Ag

At top of surface

12.8

[115] Jacak, 2018

Ag

At top of surface

22.15

[94] 2019

Ag

At top surface

5.9

[51] 2008

Au

At top surface

4

[61] Londhe, 2016

Ag

At top surface

9.44

[115] Jacak, 2018

Au

At top surface

10.34

[115] Jacak, 2018

CdTe

Au

At top surface

9

[62] Kim, 2015

CZTS

Au

At back surface

9

[64] Omar, 2018

Silicon

GaAs

CIGS

Mechanism

(Continued)

68  Fundamentals of Solar Cell Design Table 2.3  Effect of various plasmonic nanostructures on solar cell technologies. (Continued) Solar cell technology

Plasmonic nanostructure

Efficiency (%)

Reference

DSSC

Ag

Embedded with TiO2

3.62

[85] 2017

Au (15 nm)

Embedded with TiO2

4.46

[73] 2014

PEC

Au@TiO2

Emdded with TiO2 for oxygen evolution

40 (H atom per electron)

[88] 2012

Quantum dot

Ag@PdS

At top surface

6.03

[114] Kawawaki, 2015

Au@PbS

At top surface

9.58

[81] Chen, 2018

CdS@GaAs

At top surface

18.9

[82] Lin, 2012

Ag

At top surface

13.46

[107] Aliaksandr, 2018

Ag@TiO2

Embedded with TiO2

16.3

[113] Saliba, 2015

Au@coreshell

Embedded with core-shell structure

19.42

[87] 2020

Au@ AuQD:organic

At top surface

13

[83] Phetsang, 2019

Ag@TiO2@Pa

Embedded with core-shell structure

20.2

[86] 2019

Au@ZnO:OEC

Embedded

10.5

[89] 2016

Perovskites

Hybrid plasmonic

Mechanism

Plasmonic Solar Cells  69 cell using gold surface plasmons [46]. Peafowl solar power company, a spinout company from Uppsala University, Sweden, has started to develop direct plasmonic transparent solar cells for commercial applications. The device configuration of this plasmonic solar cell is similar to traditional thin film PV cell. These plasmonic solar cells were manufactured inexpensively through a printing process at room temperature which can convert the light into electricity under very low light [47–49].

2.4.2 Plasmonic-Enhanced Solar Cell They are considered as simple plasmonic solar cells. The crystalline or amorphous silicon and thin film solar cells can be developed using SPR of metal nanoparticles. The plasmonic nanostructure can act as light harvesting antennas to enhance optical path length of photon through scattering that resulting in enhanced absorption and generation of e-h pairs in semiconductors [50]. For example, deposition of Au, Ag, or Cu metal nanoparticle array in Si/SiO2 device increases 20 times more photocurrent in plasmonic solar cell. The various solar cell technologies such as conventional silicon, GaAs, CIGS, CdTe, and CZTS have been demonstrated using plasmonic nanostructures with improved efficiency. By making stacks of thin film PV cell, the multi-wavelength light can be absorbed in plasmonic solar cells through enhanced scattering [51–53].

2.4.3 Plasmonic Thin Film Solar Cells Plasmonic thin film designs in solar cells improve the absorption using SPR. This enhancement is mainly because of the manipulation of light for getting desired circulation, absorption, and scattering. The very thin photo active layer upto 100 nm has been proved theoretically. They can use cheaper substrate than silicon such as plastic, steel, or glass [54]. The arrays of metal nanostructures at top of GaAs solar cell have been demonstrated and achieved enhanced performance in near-IR region [55]. The silver metal nanoparticle based plasmonic GaAs solar cells have shown 8% increment in short circuit current and achieved 5.9% efficiency. The plasmonic nanoparticles have the potential to make thinner PV layers with enhanced performance. Integrating plasmonic nanostructures with silicon solar cells can significantly enhance the absorption in visible as well as near-IR spectrum. Green et al. have demonstrated the enhanced absorption in silicon thin film PV cells using Ag plasmonic nanoparticles [52]. Yang et al. have demonstrated monocrystalline silicon solar cells with effect of plasmon resonance on

70  Fundamentals of Solar Cell Design silicon nanowire decorated with Ag metal nanoparticles [53]. Yue et al. have demonstrated 15% enhancement in absorption by using plasmonic core-shell (the dielectric as core and metal as shell) insulators for a-Si solar cells [54]. Atwater et al. have demonstrated amorphous Si PV device with about 26% efficiency by usage of plasmonic nanostructure [59]. This device configuration has improved the electrical properties of the PV cells. Zhang et al. have fabricated plasmonic solar cell with about 18.2% efficiency using advanced light trapping approach in a proper device design, which has reduced the wafer thickness without loss in energy conversion [53]. Derkacs et al. have demonstrated 8.3% energy conversion efficiency with gold nanoparticles on thin film silicon [57]. Stuart and Hall et al. have achieved enhanced photocurrent by a factor of 18- at 800-nm wavelength in 165 nm thick Si/insulator with silver metal nanoparticles at the top surface of PV cell [56]. Schaadt et al. have shown the improvements in silicon solar cells by 80% at 500-nm wavelength using gold nanoparticles at top of surface [58]. Pillai et al. have shown the increment in photocurrent by 19%–33% using Ag plasmonic nanoparticles [52]. Singh and Verma et al. have used fabricated silicon plasmonic solar cell using copper plasmonic nanoparticles [65]. The mc-Si PV devices also have been shown using Al metal nanoparticles as antireflection coating (ARC) on top surface. This configuration has achieved the solar cell parameters such as 34.0 mA/ cm2 short circuit current (Isc) and efficiency of 14.5% [66]. Lian et al. have reported on the loss of hot electrons [59]. There is a need of quality research to develop plasmonic silicon solar cells for commercial applications. CIGS and CdTe PV cells have fabricated using Ag and Au plasmonic nanoparticles and achieved conversion efficiencies of about 10.34% and 9%, respectively [68, 69]. The research interest in CZTS has been increased due to its low cost, non-toxic and earth abundant elements. These results could open a new way of integrating plasmonic nanostructure with existing PV devices for higher efficiency. A layer of Au plasmonic nanostructure in CZTS has shown enhancement in light trapping and efficiency. CZTS solar cells with integration of plasmonic nanostructures as ARC coatings are promising to decrease reflection and for increasing absorption for increasing its efficiency at commercial scale applications [70, 71].

2.4.4 Plasmonic Dye Sensitized Solar Cells (PDSSCs) PDSSC with plasmonic Au nanoparticles embedded in TiO2 has shown 26% efficiency [73]. There are interesting mechanisms that are responsible for enhancing conversion of metal oxide semiconductor (MOS)–based solar cells using metal nanostructures. These mechanisms include the

Plasmonic Solar Cells  71 increment in photocurrent by local electric field enhancement and amplification of the electric field near semiconductor surfaces by surface plasmons [74]. The plasmonic DSSC with hematite coated on Au nanopillars and N719 dye has shown enhanced absorption in visible range and efficiency of 10.2% [75]. Chander et al. have developed plasmonic DSSC with Au nanoparticle embedded with TiO2 thin film with enhanced absorption in visible range and energy conversion efficiency of 4.46% [73].

2.4.5 Plasmonic Photoelectrochemical Cells The plasmonic electrodes can enhance the performance of PEC devices for electricity generation as well as water splitting by using more light. The plasmonic electrodes have more advantages than doped or dye sensitized electrodes. The PEC devices have lower efficiency upto 1%–5% [77]. The plasmonic TiO2 cell deposited with gold plasmonic nanoparticles at top surface has shown light to energy conversion efficiency of about 0.01%– 0.02% [78]. The photosensitization of plasmonic nanoparticles Lee et al. have made plasmonic photoanode using gold nanorod ceiled with semiconducting TiO2 for solar to hydrogen (H2) generation and achieved efficiency about 40% under visible light [88] Plasmonic Au nanoparticles act as a photosensitizer in PEC devices. This technology is still at laboratory scale. The device configuration and modifications in photoactive layers are needed to improve the efficiency in PEC cells [80].

2.4.6 Plasmonic Quantum Dot (QD) Solar Cells The plasmonic quantum dot solar cells use less material and hence they can offer more cheap solar energy conversion. There is an optimum coupling between electron excitations and plasmons under illumination of light to get best possible performance of PV device that depends on geometry and device design [81]. The high internal quantum efficiency about close to 100% has been achieved in CdSe quantum dot based photodetectorss but an are capable of reaching very high internal quantum efficiencies of close to 100%, but they have external quantum efficiency about only 1% [83]. Lin et al. have demonstrated GaAs PV cell with colloidal CdS quantum dot and achieved efficiency of 18.9% [82]. The plasmonic solar cell made up of PdS QD and Ag nanocubes has enhanced conversion efficiency from 4.45% to 6.03%. The efficiency of 8.09% was achieved with Au plasmonic Au NPs under light with broader wavelengths [84–86]. More studies and efforts are required to make this technology viable at commercial applications.

72  Fundamentals of Solar Cell Design

2.4.7 Plasmonic Perovskite Solar Cells Organic and inorganic perovskite nanostructures have shown promising prediction in solar cells due their high quantum efficiency and charge transport properties. The plasmonic perovskite solar cell using core-shell structure of TiO2 and Ag nanoparticles has shown efficiency of 16.3% [87–89]. The perovskite solar cell prepared using silver plasmonic nanoparticles at the back side of perovskite layer has shown enhanced photocurrents [90–91]. The perovskite solar cell with Au nanoparticles as fixed core and dielectric SiO2 as tunable shell configuration has shown conversion efficiency of 19.42% [92–94]. This technology has promising future to develop next generation high efficiency solar cells to generate clean energy.

2.4.8 Plasmonic Hybrid Solar Cells Plasmonic hybrid nanostructures offer more absorption of light in multijunction PV cells. The hybrid plasmonic cells include the earth abundant photoactive layers and cheap plasmonic metal nanoparticles as scattering elements. The organic or inorganic materials can be used as photoactive layer to increase the coupling of sunlight into photoactive material for enhancing light absorption. The hybrid plasmonic solar cell with Ag nanoparticles in TiO2/benzoic-acid-fullerene bishell (Ag@TiO2@Pa) has shown conversion efficiency of 20.2%. The enhanced light absorption and carrier extraction of devices are responsible for better performance of PV device [88]. Zhang et al. demonstrated a hybrid plasmonic cell by incorporating silver plasmonic nanoparticles at back side in silicon solar cell and achieved 69% enhancement in photocurrents with light. The plasmonic organic solar cell using Au quantum dots and localized surface plasmons has demonstrated an enhancement in efficiency upto 13% [89].

2.5 Conclusion and Future Aspects Nanoscience and nanotechnology provide the potential to enhance energy efficiency through materials engineering by tuning its optical and electrical properties. The very thin and optically thick absorbers can make the revolution for larger light absorption and hence high efficiency in plasmonic solar cells. Plasmoinc effect of metallic nanostructures is important in nanophotonics that permit a confinement and manipulation of sunlight at nanoscale. Solar cell technology should be more cheap, eco-friendly, and highly efficient to generate clean electricity. Plasmonic nanostructures

Plasmonic Solar Cells  73 such as metal nanoparticle (Ag, Au, Al, and Cu) or thin film (ultra metallic film) can serve as light harvesting antennas in solar energy conversion devices. Plasmonic nanostructures can be can be used directly as photoactive layer or integrated in integrating with existed solar cell technology to increase the absorption and efficiency. There are main three ways to use plasmonic nanostructures in PV device as the top of the surface, integration with photoactive layer, or at the back side of PV cell. Plasmonic PV cells provide enhanced PV properties with various mechanisms such as more trapping of sunlight, injection of hot electrons, enhancement in local electromagnetic (e-m) field, and light scattering by local surface plasmons. They have been achieved efficiency from lower to high about 33.5%. There are still many challenges to develop plasmonic solar cell technology at commercial applications. The properties such as absorption, work function, and plasmonic resonance frequency in metal nanostructures as well as semiconductor energy band gap are important parameters to get better performance in plasmonic solar cells. The interface between plasmonic nanostructure and photoactive material decides the charge and energy transfer mechanism. Tuning all the optical and electrical properties and interface engineering are challenging task and thus it required sophisticated fabrication and characterization tools. A better understanding on the surface plasmon effects would make this technology more efficient for developing next generation solar cells as well as clean fuels.

Acknowledgements Authors are thankful to CUG Gandhinagar for providing Non-NET fellowship to TS. Authors are also thankful to Dr. Mukesh Ranjan and Dr. J. H. Markna for fundamental discussion on plasmonic solar cells.

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Plasmonic Solar Cells  79 of Plasmonic Gold Nanobipyramids and Nanospheres. Advanced Energy Materials, 8:8, 1701194, 2018. 82. Chien-Chung Lin, Hsin-Chu Chen, Yu Lin Tsai, Hau-Vei Han, Huai-Shiang Shih, Yi-An Chang, Hao-Chung Kuo, and Peichen Yu, Highly efficient CdS-quantum-dot-sensitized GaAs solar cells. Optic Express. 20:S2, pp. A319-A326, 2012. 83. Sopit Phetsang et al., Investigation of a gold quantum dot/plasmonic gold nanoparticle system for improvement of organic solar cells. Nanoscale Adv., 1, 792–798, 2019. 84. Lin, S. J., Lee, K. C., Wu, J. L., and Wu, J. Y., Enhanced performance of dye-sensitized solar cells via plasmonic sandwiched structure. Applied Physics Letters, 99(4), 2011. 85. S. Saravanan, R. Kato, M. Balamurugan, S. Kaushik, T. Soga, Efficiency improvement in dye sensitized solar cells by the plasmonic effect of green synthesized silver nanoparticles. Journal of Science: Advanced Materials and Devices 2, 418e424, 2017. 86. Kai Yao, Hongjie Zhong, Zhiliang Liu, Zhiliang Liu, Plasmonic Metal Nanoparticles with Core–Bishell Structure for High-Performance Organic and Perovskite Solar Cells. ACS Nano 2019, 13, 5, 5397–5409. 87. Xun Cui, Yihuang Chen, Meng Zhang, Yeu Wei Harn, Jiabin Qi, Likun Gao, Zhong Lin Wang, Jinsong Huang, Yingkui Yang, Zhiqun Lin, Tailoring carrier dynamics in perovskite solar cells via precise dimension and architecture control and interfacial positioning of plasmonic nanoparticles. Energy Environ. Sci., 2020. https://doi.org/10.1039/C9EE03937F. 88. Joun Lee, Syed Mubeen, Xiulei Ji, Galen D. Stucky, and Martin Moskovits, Plasmonic Photoanodes for Solar Water Splitting with Visible Light. Nano Lett., 12, 5014−5019, 2012. 89. Wallace C. H. Choy and Xingang Ren, Plasmon-Electrical Effects on Organic Solar Cells by Incorporation of Metal Nanostructures. IEEE Journal of Selected Topics in Quantum Electronics, 22:1, 2016. 90. Stephanie Essig et al., Raising the one-sun conversion efficiency of III–V/Si solar cells to 32.8% for two junctions and 35.9% for three junctions. Nature Energy, 2, 17144, 2017. 91. Romain Cariou et al., III–V-on-silicon solar cells reaching 33% photoconversion efficiency in two-terminal configuration. Nature Energy, 2018. https:// doi.org/10.1038/s41560-018-0125-0. 92. Kunta Yoshikawa et al., Silicon heterojunction solar cell with interdigitated back contacts for a photoconversion efficiency over 26%. Nature Energy, 2, 17032, 2017. 93. Hung-Ling Chen et al., A 19.9%-efficient ultrathin solar cell based on a 205-nm-thick GaAs absorber and a silver nanostructured back mirror. Nature Energy, 2019. https://doi.org/10.1038/s41560-019-0434-y. 94. Sheng-Qing Zhu, Bin Bian, Yun-Feng Zhu, Jun Yang, Dan Zhang and Lang Feng, Enhancement in Power Conversion Efficiency of GaAs Solar Cells by

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Plasmonic Solar Cells  81 109. Jin-Hui Zhong et al., Nonlinear plasmon-exciton coupling enhances sum-frequency generation from a hybrid metal/semiconductor nanostructure. Nature Communications, 11:1464, 2020. | https://doi.org/10.1038/ s41467-020-15232-w. 110. F. Pelayo, Garcıa de Arquer, Agustın Mihi, Dominik Kufer, and Gerasimos Konstantatos, Photoelectric Energy Conversion of Plasmon-Generated Hot Carriers in Metal Insulator Semiconductor Structures. ACS Nano, 7:4, 3581– 3588, 2013. 111. Renxing Lin et al., Monolithic all-perovskite tandem solar cells with 24.8% efficiency exploiting comproportionation to suppress Sn(ii) oxidation in precursor ink. Nature Energy, 4, 864–873, 2019. 112. Jared S. Price et al., High-concentration planar microtracking photovoltaic system exceeding 30% efficiency. Nature Energy, 2, 17113, 2017. 113. Michael Saliba, Wei Zhang, Victor M. Burlakov, Samuel D. Stranks, Yao Sun, James M. Ball, Michael B. Johnston, Alain Goriely, Ulrich Wiesner, Henry J. Snaith, Plasmonic‐Induced Photon Recycling in Metal Halide Perovskite Solar Cells, Advanced Functional Materials, 25(31), 5038–5046, 2015. https:// doi.org/10.1002/adfm.201500669 114. Tokuhisa Kawawaki, Haibin Wang, Takaya Kubo, Koichiro Saito, Jotaro Nakazaki, Hiroshi Segawa, and Tetsu Tatsuma, Efficiency Enhancement of PbS Quantum Dot/ZnO Nanowire Bulk-Heterojunction Solar Cells by Plasmonic Silver Nanocubes, ACS Nano, 9, 4, 4165–4172, 2015. https://doi. org/10.1021/acsnano.5b00321 115. Janusz E. Jacak and Witold A. Jacak, Plasmonic Enhancement of Solar Cells Efficiency: Material Dependence in Semiconductor Metallic Surface NanoModification, 2018.

3 Tandem Solar Cell Umesh Fegade

*

Bhusawal Arts Science and P. O. Nahata Commerce College, Bhusawal, Maharashtra, India

Abstract

The world is facing the several problems but the energy crisis is the major concern for scientist community and intellectual. Energy production using conventional resources produces high amount of greenhouse gases which increases the temperature of earth as a results the polar ice melts. From last few decades, the renewable energy sources are used for to reduce the use of conventional resource. The sunlight is the biggest available source of renewable and scientist is keep try to produce electricity with high efficiency. The tandem solar cell is the third generation of solar cell. The tandem solar cell has two, three, and four junction and efficiency reached upto 32.8%, 44.4%, and 46.0%, respectively. In the present paper, we review the paper of tandem solar including its subtypes organic tandem solar, inorganic tandem solar, and hybrid tandem solar cell. Keywords:  Conventional resources, greenhouse gases, renewable energy, tandem solar cell

List of Abbreviations CO2 Carbon dioxide PV Photovoltaic VOC Open-circuit voltage FF Fill factor OPV Organic photovoltaic GO Graphene oxide PEIE Polyethylenimine Email: [email protected]

*

Inamuddin, Mohd Imran Ahamed, Rajender Boddula, and Mashallah Rezakazemi (eds.) Fundamentals of Solar Cell Design, (83–102) © 2021 Scrivener Publishing LLC

83

84  Fundamentals of Solar Cell Design ITO Indium tin oxide PCE Power conversion efficiency LBIC Light beam induced current EQE External quantum efficiency Cd Cadmium S Sulfur Ga Gallium Ag Silver P Phosphorus Si Silicon NIR Near infrared ZnO Zinc oxide In Indium CIGS Copper indium gallium diselenide MA Methylamonnium CQD Colloidal quantum dot DMD Dielectric-metal-dielectric Tin(IV) oxide SnO2 PSC Perovskite solar cell TSC Tandem solar cell OTSC Organic tandem solar cell ITSC Inorganic tandem solar cell HTSC Hybrid tandem solar cell PSEHTT:ICBA Poly[(4,40-bis(3-ethylhexyl)dithieno [3,2-b: 0030-d]silole)-2,6-diyl-alt-(2,5-(3-(2-ethylhexyl) t h i oph e n - 2 - y l ) t h i a z ol o [ 5 , 4 - d ] t h i a z ol e ] : indene-C60 bisadduct PSBTBT:PC70BM Poly[(4,40-bis(2-ethylhexyl)dithieno[3,2-b:20,30d]silole)-2,6-diyl-alt-(2,1,3-benzothiadiazole)4,7-diyl]:[6,6]-phenyl-C70 butyric acid methyl ester JSC Short circuit current density Au-doped SLGNRs Au-doped single layer graphene nanoribbons OHJs Organic heterojunctions CGLs Charge generation layers HAT-CN 1,4,5,8,9,12-hexaazatriphenylene-hexacarbonitrile m-MTDATA 4,4′,4″-tris(N-3-methylphenyl-N-phenylamino) triphenylamine MPE Maximum Power efficiency PEDOT:PSS Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate)

Tandem Solar Cell  85 CFPTSC Colorful flexible polymer tandem solar cells LBIC Light beam induced current EQE External quantum efficiency I-V Current-voltage +H3N−C6H12−NH3+ Dicationic hexane-1,6-diammonium) C3H7−NH3+ Monocationic n-propylammonium CIGS Copper indium gallium diselenide Methyl ammonium leads triiodide CH3NH3PbI3 CGS Copper gallium diselenide FAPbX3 Formamidinium lead halide ICO Cerium-doped indium oxide RF Radio frequency RT Room temperature ST-PSC Semi-transparent perovskite solar cell P/SHJ Perovskite/silicon-heterojunction SJSC Single-junction solar cells OCVP Open-circuit photo-voltage SCPCD Short-circuit photo-current density EL Electroluminescence TPSC Tandem polymer solar cells MCE Maximum current efficiency EQE External quantum efficiency

3.1 Introduction The increasing population and its energy consumption are demanding huge energy and create the energy crisis. To fulfil the energy need, increase in power production using conventional resources produces the greenhouse gases which disrupt the climate change [1]. Currently, 80% of energy is prepared by non-renewable resources worldwide. The conversion of resources to energy generate massive CO2 emissions, which increases the temperature of the earth and melts the polar ice as result rise in sea level [2]. To understand the energy crisis and problem created by greenhouse gases, the scientist is taking much interest in alternative renewable energy sources. The sunlight is the huge source of renewable energy and scientist is doing enormous research on photovoltaic (PV) technology. Today, scientist reaches up to third generation of solar cell, namely, tandem solar cell (TSC). The world-wide PV market has more than doubled in 2010, and the market nurtures again by almost 30% in 2011 [3].

86  Fundamentals of Solar Cell Design Tandem cell construction presents a lane toward superior module efficiencies over single-junction design owing to the capability to divide the solar spectrum into multiple bands [4]. The simplest structure where two absorbers are stacked is the so called tandem. The stacked layer presents in tandem of different bandgap materials. When the sunlight falls on the tandem cell, the high energy light absorb by the material has high bandgap and the low energy light is secondly absorb with the lower bandgap [5]. In tandem devices, the intermediary layer is the critical processing steps, building an ohmic contact among the two sub-cells [6]. The TSC has two, three and four junction and efficiency reached upto 32.8%, 44.4%, and 46.0%, respectively. The multi junction device modelling evaluation is near approaches of the scientists. There are various types of TSC that can be differentiating on the basis of the material used in the cell. The organic tandem solar cell (OTSC) [7–12] is most suitable and economic but it has low efficiency upto 15%. The inorganic tandem solar cell (ITSC) [13–17] has very expensive and high efficiency upto 46% and used in space applications. The hybrid tandem solar cell (HTSC) [18–24] is the third type and Perovskite tandem has already proven to be quite efficient (17%) and low cost, mostly because of cheap materials that are being used. 

3.2 Review of Organic Tandem Solar Cell Ning Li et al. [3] developed single-junction OPV based on GEN-2:[60] PCBM achieve a PCE of 6.63%, and maximum PCE of 10% is expected for a TSC incorporating an OPV12:[60]PCBM-based at ­bottom  and pDPP5T-2:[70]PCBM-based a top [25]. Abd. Rashid bin Mohd Yusoff et al. developed and characterized PSEHTT:ICBA and PSBTBT:PC70BM as the active layers and the TSC showed a high VOC 1.62V, a moderate JSC 8.23 mAcm2, FF 62.98%, leading to the PCE of 8.40% [26]. Lingxian Meng et al. reported the performance of TSC on 0.05 to 1.12 sun intensities. The PCEs above 15% intensity varies within 4.97 to 112.68 mW/cm2 and PCE of 17.87% at intensity 25.99 mW/cm2 [27]. Muath Bani Salim et al. reported 26 single-cell OPV devices to form the TOPV front and back cells using PBDTS-TDZ:ITIC in the front cells and PTB7-Th:O6T4F:PC71BM gives 18.6% efficiency [28]. Lijian Zuo et al. demonstrated two mirror-like electrodes, ultrathin Ag capped with a dielectric TeO2 layer, and a thick Ag electrode. As a result, a top-­illuminated ITO-free SCTOPV show 7.4% and ITO-based 7.5% [29]. Abd. Rashid bin Mohd Yusoff et al. reported that Au-doped SLGNR electrodes are implemented in TSC, and PCE achieved is 8.48% comparable to generally used

Tandem Solar Cell  87 transparent electrode [30]. Hengda Sun et al. designed OHJ CGL that is composed of HAT-CN/m-MTDATA, which shows that MCE and EQE are achieved as 201cd/A and 54.5%, respectively (Figure 3.1) [31]. Bangwu Luo et al. reported the use of PEDOT:PSS as the top transparent electrode in TSC. The fabricated CFPTSC with polymer electrodes displays PCE values from 7.23% to 8.34% and yielded color of the cells [32]. Miaomiao Li et al. reported a solution-processed TSC, based on DR3TSBDT and DPPEZnP-TBO, which propose proficient and shows the power conversion efficiency of 12.50% [33]. Thue T. Larsen-Olsen et al. reported opto-electronically probe individual junctions and carrier 1000

Current Density (mA/cm2)

100 10 1 0.1 0.01

HAT-CN/m-MTDATA m-MTDATA only equation fit linear fit

1E-3 1E-4 1E-5 1E-6 -3

-2

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Figure 3.1  (a) J-V plot A and B. (b) Temperature-current density graph. (Reprinted with the permission from reference [31].)

88  Fundamentals of Solar Cell Design transport across interfaces in fully printed and coated TPSC. Inherent limitations to the accuracy of EQE and LBIC measurements on non-ideal TSC are described through the use of a small-signal electrical model Figures 3.2 and 3.3 [34].

(a)

35 30

Front illumination (-> Flextrode / 1st AL / 2nd AL) Back illumination (-> pv410 / 2nd AL / 1st AL)

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300 400 500 600 700 800 900 Wavelength (nm) 405nm laser 637nm laser

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W/g

rid

er Silvtrode c e l e

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6 nm

0

er Silv

grid

Figure 3.2  (a) Non-biased EQE spectra and (b) LBIC images using two different laser wavelengths. (c) Schematic structure tandem device. (Reprinted with the permission from reference [34].)

T2

No bias

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0.8 V + red led bias

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MH306:PCBM MH301:PCBM

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Figure 3.3  (a) LBIC with and without voltage and light biasing. The 405-nm LBIC probe laser and 660-nm bias. (b) Schematic structure of tandem PSC. (Reprinted with the permission from reference [34].)

Tandem Solar Cell  89

3.3 Review of Inorganic Tandem Solar Cell Z. Deng et al. developed the GaxIn1xP top sub-cell of a GaxIn1xP/GaAs double-junction TSC is exposed the EL image, 93 lm at forward bias of 2.75 V, which is 30 times more than unbiased GaxIn1xP single layer [35]. M. Elbar et al. reported PV double junction CGS/CIGS TSC, based on CGS and CIGS structures as top and bottom cells, respectively. The performance of single CGS and CIGS solar cells is with fixed thicknesses at 0.26 and 3.5 lm, respectively, and the PCE of 18.92% and 20.32% respectively [36]. Li Wang et al. fabricated GaAsP/SiGe TSC on Si, which shows 40% conversion efficiency (Figure 3.4) and the three-terminal efficiency of 20.6% under 1× illumination [37]. Zekun Ren et al. investigated GaAs/GaAs/Si triple-junction structural design in which GaAs and Si form a non-ideal bandgap combination. GaAs/ GaAs/Si has attained 33.0% efficiency and harvested efficiencies between 31.4% and 32.1% [38]. Lukas Kranz et al. fabricated NIR-transparent PSC which enables PCE up to 12.1% but at CIGS enabled a device with 19.5% highest efficiency reported for a polycrystalline thin film [39]. Maoqing Yao et al. reported nanowire on-Si TSC on addition of the GaAs nanowire top and the Si bottom with a VOC of 0.956 V and an efficiency of 11.4% [40]. Zhe Liu et al. demonstrated the framework on the example of a 21.3% efficient stacked four-terminal GaAs/Si TSC. The short-circuit current density in the GaAs/Si TSC achieves 37.8 mA/cm2 with experimentally possible parameters [41]. R. Lachaume et al. reported that over 30% efficiency could be achieved for the tandem with only ~7 μm of epi-SiGe and ZnO/Ag back metallization. In the latter ideal scenario, the highest efficiency achievable

Top contact ARC Window

GaAs..79P.21 Top cell

Middle contact

BSF

Buffer + TJ + Nucleation Si.18Ge.82 Bottom cell Si1-xGex graded buffer Si substrate Back contact

Figure 3.4  Fabricated 3-TGaAsP on SiGe/Si device. (Reprinted with the permission from reference [37].)

90  Fundamentals of Solar Cell Design

ITO

aAs p+ G s i GaA n Ga

As

n+ Ga

As

BCB ask

SiN m

p+ Si N Si

n+ Si

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ntact ck co Al ba

Figure 3.5  Schematic of GaAs nanowire-on-Si tandem solar cell. (Reprinted with the permission from reference [40].)

is ~37% with 1.1-μm-thick Al0.15Ga0.85 as top cell and epi-Si0.73Ge0.27 as bottom cell less than ~30-μm thickness [42]. Shizhao Fan et al. reported 1.7 eV/1.1 eV GaAs0.75P0.25/Si TSC having efficiency of 20.0% and 16.5% efficient GaAs0.75P0.25 single-junction top cell on Si and a 7.78% efficient GaAs0.75P0.25 filtered Si bottom cell (Figure 3.6) [43]. Manuel Schnabel et al. demonstrated tandem cell architecture with three terminals by combining GaInP and Si sub-cells to attain a GaInP/Si tandem cell with a two-terminal efficiency of 26.4 ± 1.0%. Three terminals show the efficiency of 0.9 ± 0.2% and two terminals show the efficiency of 27.3 ± 1.0% [44]. Nicolas Cavassilas et al. investigated, through a multiscale approach, a TSC based on a van der Waals heterostructure composed of two monolayers of transition metal dichalcogenides and predict that a PCE of 30.7% [45]. David M. Fabian et al. reported very efficient SJSC containing the dicationic material as a photoactive layer displayed an OCVP at 400 mV and SCPCD of ∼30 μA/cm2 (Figures 3.7 and 3.8) [46]. Colin D. Bailie et al. established semi-transparent cell onto CIGS and poor class multicrystalline Si to attain solid-state polycrystalline TSC efficiency above 25% [47]. Miguel Anaya and coauthors proposed a new tandem structural design in which both top and bottom cells are made of absorbers and devices shows the efficiencies at 35% [48]. Jiadong Qian et al. reported different degradation rates of perovskite cells and silicon cells in a tandem solar module degradation. PCE of 28.7% and 27.6% enable the economic viability of two- and four-terminal modules [49]. Alexander J. Bett et al. developed semi-transparent PSE in the regular n-i-p structure,

Tandem Solar Cell  91 AuGe/Ni metal grid

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GaAs (n+) contact (50 nm) TiO2/SiO2 ARC AllnP (n+) window (20 nm) GaAsP (n+/p) emitter/base (50 nm/1150 nm)

Growth direction

GalnP (p+) BSF (25 nm) AlGaAsP (p+)/GaAs (n+) TJ (50 nm/5 nm) GaAsyP1-y (n) compositionally graded buffer (4140 nm) GaP (n) nucleation (46 nm) Epitaxial Si (UID) buffer (200 nm) c-Si (p) (224 um) a-Si:H (i) (6 nm) a-Si:H (p+) (11 nm) ITO (150 nm) Ag (200 nm)

(b) Energy (eV)

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0.2

1.2

1.4 2 3 4 5 6.0 Depth (µm)

(c)

6.

7

229

230

230.6 230.8

(d)

100

16 Current density (mA/cm2)

EQE or (1-R) (%)

80

60 GaAsP 40

Si

(1-R)

20

0 300

450

600 750 900 Wavelength (nm)

1050

1200

12

8

Jsc = 15.94 mA/cm2 Voc = 1.629 V FF = 77.0% Efficiency = 19.99%

4

0 0.0

1 cm

0.4

0.8 1.2 Voltage (V)

1.6

Figure 3.6  (a) Schematic of the GaAsP/Si 2J cell. (b) Energy band diagram. (c) NRELmeasured EQE absorptance (1-R). (d) LIV 2J cell. The inset of panel (d): image of 0.138 cm2 2J cell. (Reprinted with the permission from reference [43].)

92  Fundamentals of Solar Cell Design

Figure 3.7  Crystal structure of (HDA)3CuBr8. (Reprinted with the permission from reference [46].) (a)

(b) (HDA)3CuxBr8 (PA)6CuxBr8

Normalized Abs. @ 500 nm

Normalized Absorbance

1.0 0.8 0.6 0.4 0.2 0.0 400

500

600

700

Wavelength (nm)

800

1.0 0.8 0.6 0.4

(HDA)3CuxBr8 (HDA)3CuxBr7Cl (HDA)3CuxBr6Cl2 (PA)6CuxBr8

0.2 0.0 0

4

8

12

Time (days)

Figure 3.8  (a) Absorption of thin films. (b) Normalized absorbance with respect of time. (Reprinted with the permission from reference [46].)

which is presented with ITO directly sputtered on the hole conducting material Spiro-OMeTAD and showed efficiency of 14.8% [50]. Abd. Rashid bin Mohd Yusoff et al. developed three photosensitive materials with a bandgap ranging from 1.3 to 1.82 eV. PCE obtained are of 10.39% and 11.83% on double-junction and triple-junction cell,

Tandem Solar Cell  93 Illumination from glass side

MgF2 ITO Spiro-OMeTAD Perovskite PCBM:PMMA

Mesoporous TiOx TiOx FTO Glass

Current Density [mA/cm2]

20 Au

16

Illumination from ITO side

12 8 4 0 0.0

0.2

0.4

0.6

0.8

1.0

1.2

Voltage [V]

Figure 3.9  Schematic arrangement of the cell and its I-V characterstics. (Reprinted with the permission from reference in CC licenses [50].)

respectively [51]. Mi-Hee Jung et al. reported flexible solar cell by combining nanopaper and perovskite in which bad gap is controlled by addition of Br ion into the CH3NH3PbI3. The MCE and IPCE were achieved with 6.37% and 40% at CH3NH3PbI3 perovskite, respectively [52]. Maximilian T. Hörantner et al. introduced a perovskite-on-silicon TSCs and determine that the ideal bandgap for a perovskite “top-cell” is 1.65eV, which has 32% efficiency. Furthermore, it shows that TSCs are yielding 30% energy output than single junction silicon (Figure 3.10) [53]. Marko Jost et al. presented monolithic perovskite/CIGSe TSC with a perovskite top cell. The performance is improved by the polymer PTAA at the NiOx/perovskite (b)

40

CIGSe

30 20 10 0 0.0

25

Perovskite

20

J [mAcm-2]

J [mAcm-2]

(a)

AM1.5G Reduced light intensity

0.2

0.4

V [V]

15 10

PTAA NiOx NiOx/PTAA

5

0.6

0 0.0

0.2

0.4

0.6

V [V]

0.8

1.0

Figure 3.10  J-V characteristics (a) J-V of CIGSe single-junction. (b) Perovskite single junction reference cell with different HTLs. (Reprinted with the permission from reference [54].)

1.2

94  Fundamentals of Solar Cell Design interface. This hole transport bilayer facilitates a 21.6% PCE at ∼0.8 cm2 area [54]. Yuqian Ai et al. fabricated sulfide-passivated ETL which improves the electron collection efficiency. Based on this S-SnO2 ETL, the PCE of the PSC is significantly promoted from 18.67% to 20.03% [55]. Afsal Manekkathodi et al. reported four-terminal (4T) tandem which provides a PCE exceeding 20%, for a perovskite-CQD TSC. The highest-performing front semi-transparent perovskite solar cells exhibit a PCE of 18% [56]. Philipp Loper and coauthors presented a four-terminal tandem consisting of a CH3NH3PbI3 top and a c-Si heterojunction bottom which has efficiency at 13.4% [57]. Yuhei Ogomi et al. reported Sn/Pb halide–based PSC and it shows best performance using CH3NH3Sn0.5Pb0.5I3 perovskite and 4.18% efficiency with VOC 0.42V, FF 0.50, and SCC 20.04 mA/cm2 Figures 3.11 to 3.13 [58]. Jialong Duan et al. fabricated FAPbX3 perovskite nanocrystals, highmelting-point ligands to modify the perovskite/carbon interface in allinorganic CsPbBr3 PSC, which are efficient PV device, and elevated PCE up to 8.55% is achieved [59]. Shichong et al. reported ICO transparent electrode with high mobility of 51.6 cm2/Vs, a low resistivity of 5.74 × 10−4 Ωcm and transmittance of 83.5%. The SHJ two-terminal TSC gets 8.06% step up in PCE from 18.85% to 20.37% [60]. Jian Liu et al. reported a two-terminal perovskite (PVSK)–organic HTSC and PFN/doped MoO3/ MoO3 structure as interconnection layer (ICL). The HTSC attains VOC of 1.58V and FF of 0.68 [61].

Au/Ag P3HT

CH3NH3SnxPb(1-x)I3 Perovskite

Porous TiO2 Compact layer (TiO2) FTO Glass

Figure 3.11  Structure of CH3NH3SnxPb(1 − x)I3 PSC. (Reprinted with the permission from reference [58].)

Tandem Solar Cell  95 20.0

CH3NH3PbI3 CH3NH3Sn0.3Pb0.7I3 CH3NH3Sn0.5Pb0.5I3 CH3NH3Sn0.7Pb0.3I3 CH3NH3Sn0.9Pb0.1I3 CH3NH3SnI-

Current density (mA/cm2)

15.0 10.0 5.0 0.0 -5.0 -10.0 -0.2

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

Voltage (V)

Figure 3.12  I-V curves for CH3NH3SnxPb(1 − x)I3 PSC. (Reprinted with the permission from reference [58].) 0.7 CH3NH3PbI3

0.6

CH3NH3Sn0.3Pb0.7I3 CH3NH3Sn0.5Pb0.5I3

IPCE

0.5

CH3NH3Sn0.7Pb0.3I3

0.4 0.3 0.2 0.1 0.0 300

400

500

600

700

800

900

1000

1100

Wavelength (nm)

Figure 3.13  IPCE curves for CH3NH3SnxPb(1 − x)I3. (Reprinted with the permission from reference [58].)

3.4 Conclusion In summary, this chapter discussed the TSC, its multiple junction system, and its different types such as OTSC, ITSC, and HTSC. We review their last few year back research paper and concluded that the tandem solar cell has two, three, and four junction and efficiency reached upto 32.8%, 44.4%, and 46.0%, respectively. The OTSC is economic but low efficiency upto 15%. The inorganic tandem solar is very expensive and high efficiency upto 46%.

96  Fundamentals of Solar Cell Design The HTSC is the third type, and Perovskite tandem has already proven to be quite efficient (17%) and low cost. The third-generation HTSC is the future hope of the scientists and intellectual, because it is economic and, day by day, its efficiency improved by scientists.

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4 Thin-Film Solar Cells Gobinath Velu Kaliyannan1, Raja Gunasekaran2, Santhosh Sivaraj3, Saravanakumar Jaganathan4 and Rajasekar Rathanasamy3* Department of Mechatronics Engineering, Kongu Engineering College, Erode, Tamil Nadu, India 2 Department of Mechanical Engineering, Velalar College of Engineering and Technology, Erode, Tamil Nadu, India 3 Department of Mechanical Engineering, Kongu Engineering College, Erode, Tamil Nadu, India 4 Department of Engineering, Faculty of Science and Engineering, University of Hull, Hull, United Kingdom 1

Abstract

For the past few decades, thin-film solar cells are considered as a potential alternative for space and earth photovoltaics. Due to minimal usage of material, thin-film solar cells are favorable and also it provides a broad variety of design and production options. A number of substrates, such as fluid or solid, metal, or insulator, can be used for the deposition of various materials including absorber, buffer, contact, and reflector. Copper indium gallium selenide (CIGS), amorphous silicon (a-Si), and cadmium telluride (CdTe) are the three main technologies for thin-film solar cells. The deposition techniques such as PVD, CVD, ECD, plasma-based, and synthetic are utilized. This mobility allows it possible to customize and design layers in order to enhance the efficiency of the system. The thin-film product manufacturing is complicated and involves careful control over the entire process series for the devices that are large needed for practical applications. Research and development of these cells in modern, exotic, and basic technologies and different materials, but easy manufacturing methods need to be followed in a concentrated manner. To achieve high-efficiency devices for wide range of applications proper knowledge on deposition processes of thin film should be must. The commercial success of different cells and technologies can be evaluated by the simple manufacturing and cost per watt. It is expected that cheap and moderately effective thinfilm solar cells would receive a commercial place in the market. *Corresponding author: [email protected] Inamuddin, Mohd Imran Ahamed, Rajender Boddula, and Mashallah Rezakazemi (eds.) Fundamentals of Solar Cell Design, (103–116) © 2021 Scrivener Publishing LLC

103

104  Fundamentals of Solar Cell Design Keywords:  Thin-film solar cells (TFSC), amorphous silicon (a-Si), cadmium telluride (CdTe), copper indium diselenide (CIS)

4.1 Introduction The utilization of solar power for the electric power production has proved to be one of the valuable approaches to solve the global energy demand. A solar cell is the device which converts sun’s solar energy into electric energy. The solar cells should be cost effective and highly reliable to dominate conventional renewable energy. Research has been going on various solar technologies to increase durability, cost effectiveness, and performance with tremendous improvement, like wafer, thin film, and organic. Si solar cells are very effective from laboratory to commercial success and constitutes about 90% of the world’s photovoltaic market [1]. The minimum material usage and increase in efficiency of energy conversion determine the cost effectiveness. Wafer technology is successful in increasing the efficiency on the other hand thin film can satisfy minimum use of materials. These both goals must be achieved simultaneously to increase the electricity production at low cost and solar cells in to commercial market [2]. The three thin-film solar cells such as copper indium gallium selenide (CIGS), amorphous silicon (a-Si), and cadmium telluride (CdTe) are commercially used. Direct bandgap between the three materials are common, that permits the usage of thin-film materials [3] and possess a minimum coefficient of temperature. Despite wafer film technology have high temperature coefficient and the low intensity of light will not affect their performance. These technologies are integrated to form building integrated photovoltaics (BIPVs). The application of a-Si solar cells is especially in electronic gadgets such as watches and calculators. Thin-film material possesses significantly less absorption coefficient than that of its crystalline components. The requirement of silicon when using a-Si is lesser than that of CdTe and CIGS and also less toxic. The commercial application of CdTe is limited due to the use of cadmium which is harmful to the environment. As seen from 1980s, a-Si proved to be commercial and becomes a consistent power source for clocks, watches, and calculators. Even though CIGS and CdTe are latest technologies, they are very better than α-Si while considering energy conversion efficiency.

Thin-Film Solar Cells  105

4.2 Why Thin-Film Solar Cells? Thin-film technology has developed into a significant field of study worldwide. In recent times, the development of new materials and different coatings resulted in immense growth of thin-film technologies which have profoundly altered both condensed solid state physics and daily life. Thin-film technologies are well established to produce integrated circuits in all electronic components such as solid-state devices and also microelectronic integrated circuits. Multi-layer configuration helps in realizing selective optical filter, electronic component, solid state lasers, LCDs, and solar cells. The optical, electrical, and mechanical surface nature of materials and products for different application such as optical coatings, magnetic film, and data storage and hard surface coatings are changed by using various thin layer technologies. In order to develop new products utilized in current or in specific application including space and bio-medical application, thin films technologies are to be developed by both solid-state and optical researchers. Thin film for photovoltaic modules is recognized as the second PV generation, which are deposited on large substrates area directly, such as glass foils or panels. The manufacturing cost of photovoltaic is less due to minimum quantity of material and less handling costs so it is well suited for whole integrated processing. Thin-film solar cells offer improved device performance because of the wide variety of choices in terms of device design, fabrication methods and a large variety of substrates. The various substrates such as metal or insulator, flexible, or rigid can be used for the deposition of different layers such as buffer, contact, absorber, and reflector. Various techniques such as LPCVD and plasma-based CVD, sputtering, and sublimation for the deposition of the materials are used. Solar energy systems are feasible and competitive widely not only on the basis of cost alone, the less toxicity and the availability of materials also contributes. For the long-term material availability, the research and development of new materials and process will be able to significantly increase efficiency at low cost.

4.3 Amorphous Silicon Amorphous silicon (a-Si) possess direct bandgap which makes it possible to absorb large fraction of sunlight for few micrometers of a thin layer

106  Fundamentals of Solar Cell Design [1, 2]. Abnormal electrical behavior and short minority carrier diffusion lengths are caused by the dangling bonds and short orders in amorphous material. Dangling bond density can be reduced to considerable magnitude by hydrogen passivation over a-Si which can be denoted as a-Si:H. Due to this hydrogenation, Staebler-Wronski light degradation effect is produced which reduces the photoconductivity of the hydrogenated a-Si. The optical absorption spectrum of hydrogenated a-Si:H is transparent up to 1.7 eV and is very absorptive starting from 2 eV. These beginning properties have led to a tremendous use of a-Si in solar panel industries and research institutes. The main advantage of using a-Si:H is shorter energy payback time In the year 1970, the hydrogenated a-Si was produced using glass material at minimum temperature less than 600°C. Plasma-enhanced chemical vapor deposition method at 200°C used for thin-film photovoltaic solar cells [4]. This methodology provides many outstanding properties in producing photovoltaic electric power at less cost and increase in optical absorption coefficient. This helps to enable very thin absorbing thicknesses about 300 nm. The deposition at minimum temperature of 200°C of silicon diode of large area substrates that are more flexible and monolithic which interconnects all the cells. The global PV market share of hydrogenated a-Si is low because of the moderate efficiency of only 6% of single junction photovoltaic module of large area [5]. The main reason behind this low average efficiency is because of the “Staebler-Wronski effect” [6], which induces light degradation of the module efficiency at initial level. There are many upcoming researches in this area that are continuing to find solutions to reduce this effect. It is seen that the highest average efficiency for a hydrogenated a-Si solar cell of single junction is about 9.5% produced by the University of Neuchatel in the year 2003 [7]. Due to the lack of spatial uniformity in transparent conductive oxide layer and in the silicon film, the manufacturing becomes difficult while using substrates larger than 1 m2 [5]. Due to the high scope of hydrogenated a-Si solar cell, there must be significant improvement, and so, large hydrogenated a-Si solar cell of single junction modules with stable average efficiency of 7% must be introduced as soon as possible. In this regard, p-i-n structure imposed on a superstrate of glass is produced by a company Kaneka [8]. They used thin and stable hydrogenated a-Si cells possessing high light absorbing properties. This light trapping is enabled by the cell deposited with a textured transparent conductive oxide layer and a reflector in the back surface consisting of a two layer stack of silver and aluminium doped zinc oxide (Zno:Al ) [8, 9]. Figure 4.1 shows the structure of a p-i-n hydrogenated a-Si solar cell imposed on a soda lime glass superstrate. The transparent conductive

Thin-Film Solar Cells  107

Sunlight

Glass (soda lime) Front TCO (SnO2 or ZnO:AI) p+ (a-SiC:H or µc-Si:H) i (a-Si:H) n+ (a-Si:H or µc-Si:H) Rear TCO (ZnO:AI) Rear contact (Ag)

Figure 4.1  Schematic of a state-of-art p-i-n a-Si:H solar cell on a glass superstrate [24].

oxide layer at the front should possess a maximum optical transmission and a minimum sheet resistance of 10 Ω/sq and for apparent wavelengths. The transparent conductive oxide having thickness of approximately 1 μm is used to attain this low sheet resistance [10]. Figure 4.2 shows the method of interconnection of nearby hydrogenated a-Si solar cells. This method depends on two principal conditions such that electrically non-conductive should be used as supporting material and the sheet resistance of individual layers of the solar cells should be high about N105 Ω/sq.

Sunlight

Scribe 3 (rear electrode & Si)) Scribe 2 (Si)

Glass

Scribe 1 (front TCO) Font TCO a-Si:H solar cell Rear TCO Rear metal/reflector Cell n+2

Cell n+1

Cell n

Figure 4.2  Series connection of a-Si:H solar cells on a TCO-coated glass superstrate [24].

108  Fundamentals of Solar Cell Design This assures that while depositing transparent conductive oxide (TCO) over the side region of walls, the cells are marginally shunted. The initial step in the solar cell process is the deposition of the transparent conductive oxide layer over that scribe 1 (first of parallel scribe) is deposited. After that the three layers of semiconductor are deposited to make a solar cell. Then, the secondary set of scribe that intrudes within the layers of semiconductor deposited and so that the transparent conductive oxide layer is exposed locally. This process is followed by the rear electrode deposition which is a transparent conductive oxide plus metal. Lastly, parallel scribe of the third set impends within the rear electrode of metal and transparent conductive oxide and through the semiconductor layers. This enables the path of the current flow and arranges all the solar cells on glass panel as series connection. The three sets of parallel scribes are deposited by means of lasers or mechanical tools. The hydrogenated a-Si photovoltaic technology provides advantages on the large scale development by the deposition of silicon for LCD (liquid crystal displays). The LCD manufacturers are also offering all the services such as design, fully installed setup and also ready to operate for hydrogenated a-Si photovoltaic module. Several plasma-enhanced chemical vapor deposition machines with specifications such as in-line, batch-type, and cluster tool for glass of size 1.4 m2 are currently available in market. The latest PECVD machines give a maximum output of about 10MWp/year of single junction hydrogenated a-Si photovoltaic modules.

4.4 Cadmium Telluride Most of the thin-film solar cell technology uses CdTe as one of the primary material. CdTe offers high absorption coefficient because of its direct bandgap and it can be produced easily because of the availability of many manufacturing methods. This material is a stable compound as same as that of CIGS. When surface recombination is limited, high efficiency solar cells can be produced by using thin-film CdTe which makes it acceptable. In 1972, the first reported thin-film graded gap CdTe and cadmium sulfide (CdS) p-n heterojunction solar cell was developed by Bonnet and Rabnehorst with efficiency of 6%. Figure 4.3 shows the cell that is produced by a three step process of deposition of vapor phase CdTe at high temperature and CdS evaporation of high vacuum [11]. In the study of Jsc. Bonnet and Rabnehorst cordially encountered that pure molybdenum substrate experiences maximum series resistance and less FF. This also led to many

Thin-Film Solar Cells  109 Light

Light Glass

CdS film In2O3 film

lead wire

CdTe film

Cu2Te

Electrode

Figure 4.3  Schematic cross section of a ceramic thin film CdTe solar cell [2].

problems including the back contact between molybdenum and cadmium telluride. The transport of the photo-generated carriers is enhanced because of the graded gap feature of CdTe. The high efficiency CdTe solar cells can produced by the activation treatment of cadmium chloride [12]. Different fabrication methods are established due to the continuous interest in the field of CdTe technology. In the work done by Bonnet, vacuum deposition and vapor growth methods are used but screen-printing methods are used in 1976 by Nakayama et al. [13] in CdTe solar cells. The CdS film and the indium oxide (In2O3) films are coated on front layer of the glass which acts as a substrate. The use of CdS limits both of the surface recombination and series resistance at the layer of n-CdTe and it also acts as a non-rectifying electrical junction to the CdTe layer. This helps to increase the efficiency maximum of about 8.1% CdTe solar cell [14]. Further exploration by Bube and others [15] of CdS and CdTe heterojunction solar cell lead to the increase in efficiency about 8.4% The n-CdS, n-CdTe, and p-CdTe junctions are created by the process of diffusion of donors from the n-CdS into p-CdTe during the formation of junction in this structure. In this process, the CdS film acts as a contact to the CdTe homojunction. It is seen that the effects of the recombination velocity the interface are reduced by increasing the effective acceptor doping concentration of the CdTe to 1,017 cm−3. Crystal silicon solar cells are costly compared to CdS/CdTe solar cells. Eastman Kodak Company invented different fabrication techniques for the cells to achieve high

110  Fundamentals of Solar Cell Design conversion efficiency of CdS/CdTe solar cells [16, 17]. These techniques produce an average conversion efficiency up to 8.9%. A high efficient device can be produced by the deposition of semiconductor layers and controlling in an oxygenated environment helps to incorporate oxygen atoms within the semiconductor layers. Later on the same oxygen inducing procedure on the layers of semiconductor obtained maximum stable efficiency up to 10.5%. CdTe’s p-type character can be magnified by the oxygen and it assures the device’s shallow junction behavior. Integration of tellurium within the metal contact and p-type CdTe layer recorded a 10% output efficiency by Tyan and others at Eastman Kodak Company in 1982 [18]. The exterior part of the CdTe associated with the layer of tellurium is lacking in cadmium and the CdTe grain boundaries are perfect. It is seen that the ultimate cell properties are repeatedly controlled by junction preparation technique. While the heterojunctions are produced by vacuum evaporation technique, homojunctions are produced by chemical vapor deposition. Werthen and others [19] stated that open-circuit voltage can be significantly enhanced while using low-doped CdTe in 1983. The recombination at the interface can be greatly reduced by the use of low doped CdTe that lead to depletion region recombination of depletion region. Werthen and others stated that low doping is a feasible way for all CdTe solar cells. In 1984, Kuribayashi and others [20, 21] discovered that the enhancement of p-type character and the device shallow junction behavior of the CdTe lead to increase the efficiency up to 12.8%. It is achieved by imparting oxygen and carbon electrodes during process of the CdS and CdTe layer deposition. Light Glass CdS film CdTe film

Ag+In electrode Ag electrode C electrode

Figure 4.4  Cross section of the CdS/CdTe solar cell [2].

Thin-Film Solar Cells  111 During heat treatment applicable level of copper (50–100 ppm) should be in the carbon paste so that the carbon diffuses into CdTe layer will lead to the formation of CdTe layer p+ type. Contact resistance between the carbon electrode and CdTe can also be reduced as shown in Figure 4.4.

4.5 Copper Indium Diselenide Solar Cells Fabrication of copper indium diselenide solar cells modules are done by the configuration method that is from rear to front. It will eliminate the transparent supporting material, which makes the choice of substrate more flexible. Soda lime glass is considered as the one of the most commonly used substrate because of its less cost and availability. Another major reason is the diffusion of Na atoms from the substrate in to CIS layers leads to increase in the concentration of doping in the absorbing layer during fabrication process. The initial step in the process is to clean the glass substrate before depositing and scribing thin molybdenum film. Then, the polycrystalline CIS absorber film comprises of five elements such as Cu, In, Ga, Se, and S is deposited. Copper indium diselenide possesses the bandgap of 1.0 eV, and this can be improved while replacing indium by gallium. As seen, the absorbing layer in CdTe is generally p-type doped. Deposition of CIS film can be done by direct or indirect method. In direct method, film was deposited by thermal evaporation, whereas in the indirect method, a semiconductor compound is formed by placing many layers one by one. Figure 4.5 shows the soda lime glass substrate and the structure of copper indium diselenide. The top layer of the glass consists of molybdenum,

n-type ZnO : AI i-ZnO n-type CdS p-type Cu(In Ga)Se2 Molybdenum Soda Lime Glass

Figure 4.5  The structure of CIGS solar cell [2].

112  Fundamentals of Solar Cell Design which serves as the contact for the p-type CIGS. The formation of main junction between p-type CIGS and n-type cadmium selenide acts as the buffer layer. A layer of ZnO is placed on the upper side of the cadmium selenide, which serves as the front contact by forming n-type aluminium zinc oxide layer. The semiconductors of these solar cells are very small about 1.2 to 4.04 µm when compared to the crystalline silicon which is 17 to 200 µm. The process control systems of copper indium diselenide materials have more requirements and the system is considered as complex than CdTe. The photo-generated carriers are separated by the hetero-junction established on n-type CdS in CdTe solar cells. The uniform coating of the copper indium diselenide absorber is ensured by the deposition of CdS film of very thin about 50nm in a chemical bath. After that the semiconductor layers are scribed over that. Next step is the sputtering of zinc oxide (ZnO) layer over CdS, then the transparent conductive layer of aluminium doped zinc oxide of thickness 1μm is deposited and scribed. The copper indium selenide technology produces high efficiency of about 19.9% by smaller solar cell [22]. Anyhow commercialisation of this methodology becomes very tedious. The modules that are available commercially are efficient up to 11–13% [23]. The uniform properties throughout the film cannot be achieved along substrates of large area using maximum output equipment due to presence of five elements in CIS absorber layer system which is considered as the main issue of the copper indium diselenide technology. The other known issues are the use of indium which is a scarce element and the use of cadmium. This influences the cost of the PV modules.

4.6 Comparison Between Flexible a-Si:H, CdTe, and CIGS Cells and Applications The production of a-Si:H and CdTe solar cells are cost effective and easier than CIGS solar cells. The stability of a-Si:H cells is lesser than that of CIGS and CdTe cells. Both CIGS and CdTe solar cells produce high efficiency. But currently, built-in substrate configuration of CIGS flexible foils CIGS cells shows high efficiency than CdTe cells. This proves the better structure for flexible photovoltaic solar cells. Demonstration of upgrading CIGS cells is carried on with some limitation of the types of substrates. The industrial scalability of CdTe solar cells can be improved by its stoichiometry

Thin-Film Solar Cells  113 property. Superstrate configuration is well suited for this technology which gives efficiency of about 22%, whereas substrate configuration has efficiency of 13.6%. A flexible substrate that should be transparent has a limited choice of best superstrate, i.e., ultra-thin glass substrates. The hydrogenated a-Si of long single module is much used to cover for long roof at low cost and is easily scalable. A wide variety of substrates can be used which makes it more feasible. Flexible modules do not need mounting racks for installation; it can be fixed by glue on the metal and automobile roofs. Hence, the applications of flexible modules are not limited as listed below. Flexible modules of 60 W are used in power camping equipment in automobiles such as headlamps and wipers. Flexible modules find application in night vision goggles, recharging field communication radios, GPS systems, mobile phones, and laptops. Flexible cells are mounted on the top and backside of the portable devices to generate power of approximately 1 W which can be used for energizing portable devices, such as calculators and mobile phone. Flexible panels are fixed on the top of the vehicle roof that is used to charge battery.

4.7 Conclusion This chapter compared the boons and banes of three different thin films such as a-Si, CdTe, and CIGS. CdTe and CIGS films are very successful in comparison with the crystalline silicon film because of its high efficiency. As the hike in module efficiency of CdTe and CIGS thin-film technologies, it led to increase in market share in the future, as the continuous reduction of production cost than the crystalline silicon. The reliability of all the technologies depends on the temperature and availability of materials and also determines the demand of the photovoltaic solar cells in future. CdTe being a material of direct band serves as the primary candidate of several thin-film technologies as it requires thickness of 1 to 2 µm for achieving higher efficiencies. Despite an issue in cadmium disposal remains always, thin-film solar cells cost $1/Watt in the year 2009. In the previous decade, solar cells hold the global record in achieving high thin-film efficiencies. The consumer electronics industries are always dominated by a-Si solar cell which has been peak since its introduction in 1980’s. A-Si solar cells are not so economical for terrestrial applications due to very minimum efficiency. The system cost including inverter, battery, power electronics, wiring, and other auxiliaries is very high when compared to the cost of the solar cells. The cost of electricity from CdTe and CIGS solar cells

114  Fundamentals of Solar Cell Design and even crystalline silicon are less when compared to the a-Si due to the high system costs. This could be one of the serious reasons for the extinct of the commercial usage of the a-Si technology. The elimination of recombination losses and by solving issues related to availability of materials leads to the potential growth of CIGS technology. Major issues are related to increase in efficiency, reduction of material usage, and manufacturing cost. Crystalline silicon technologies are preferred because of the low manufacturing cost and this causes the decline of the thin-film technologies. CdTe has the highest share in global market followed by CIGS and a-Si. The multi-junction technology and the micro-silicon mixes will increase the future scope of α-Si technology. The upcoming thin-film technologies like copper zinc tin sulfide (CZTS) and calcium titanium oxide called perovskites which uses earth’s abundant materials and easy processing lead to rising applications of photovoltaics with high reliability and stability. Additionally, quantum dot photovoltaics are much encouraging in producing multi-junction solar cells with high efficiency by fabricating in normal temperature and air stable operation. Anyhow, surface chemistry understanding and the disorder of mid gap state remains still. Despite the decrease in share of solar cells in world market, high temperatures and diffused light conditions will definitely make thin-film solar cells as an emerging sources in forthcoming era.

References 1. Chopra, K.L., P.D. Paulson, and V. Dutta, Thin-film solar cells: an overview. Prog. Photovolt. Res. Appl., 12(23): p. 69–92, 2004. 2. Lee, T.D. and A.U. Ebong, A review of thin film solar cell technologies and chal­lenges. Renew. Sust. Energ. Rev. 70: p. 1286–1297, 2017. 3. Shah, A., et al., Photovoltaic technology: the case for thin-film solar cells. science, 285(5428): p. 692–698, 1999. 4. Kuwano, Y., et al., A new integrated type amorphous Si solar cell. Jpn. J. Appl. Phys., 20(S2): p. 213, 1981. 5. Lechner, P. and H. Schade, Photovoltaic thin‐film technology based on hydrogenated amorphous silicon. Prog. Photovolt. Res. Appl., 10(2): p. 85–97, 2002. 6. Staebler, D. and C. Wronski, Reversible conductivity changes in discharge‐ produced amorphous Si. Appl. Phys. Lett., 31(4): p. 292–294, 1977. 7. Meier, J., et al., Potential of amorphous and microcrystalline silicon solar cells. Thin Solid Films, 451: p. 518–524, 2004.

Thin-Film Solar Cells  115 8. Tawada, Y., H. Yamagishi, and K. Yamamoto, Mass productions of thin film silicon PV modules. Sol. Energy Mater Sol. Cells, 78(1–4): p. 647–662, 2003. 9. Yamamoto, K., et al., Large area thin film Si module. Sol. Energy Mater Sol. Cells, 74(1-4): p. 449–455, 2002. 10. Song, D., et al., Investigation of lateral parameter variations of Al-doped zinc oxide films prepared on glass substrates by rf magnetron sputtering. Sol. Energy Mater Sol. Cells, 73(1): p. 1–20, 2002. 11. Bonnet, D. and H. Rabenhorst. New results on the development of a thinfilm p-CdTe-n-CdS heterojunction solar cell. in Photovoltaic Specialists Conference, 9 th, Silver Spring, Md. 1972. 12. Fiducia, T.A., et al., 3D distributions of chlorine and sulphur impurities in a thin-film cadmium telluride solar cell. MRS Advances, 3(56): p. 3287–3292, 2018. 13. Nakayama, N., et al., Ceramic thin film CdTe solar cell. Jpn. J. Appl. Phys, 15(11): p. 2281, 1976. 14. Herrmann, D., et al. CIGS module manufacturing with high deposition rates and efficiencies. in 2014 IEEE 40th Photovoltaic Specialist Conference (PVSC). IEEE, 2014. 15. Mitchell, K.W., A.L. Fahrenbruch, and R.H. Bube, Evaluation of the CdS/ CdTe heterojunction solar cell. J. Appl. Phys., 48(10): p. 4365–4371, 1977. 16. Tyan, Y.-S., Polycrystalline thin film CdS/CdTe photovoltaic cell., Google Patents, 1980. 17. Tyan, Y.-S., Semiconductor devices having improved low-resistance contacts to p-type CdTe, and method of preparation., Google Patents, 1982. 18. Tyan, Y.-S. and E. Perez-Albuerne. Efficient thin-film CdS/CdTe solar cells. in 16th Photovoltaic Specialists Conference. 1982. 19. Werthen, J.G., et al., Surface preparation effects on efficient indium‐tin‐ oxide‐CdTe and CdS‐CdTe heterojunction solar cells. J. Appl. Phys., 54(5): p. 2750–2756, 1983. 20. Kuribayashi, K., et al., Preparation of low resistance contact electrode in screen printed CdS/CdTe solar cell. Jpn. J. Appl. Phys, 22(12R): p. 1828, 1983. 21. Matsumoto, H., et al., Screen-printed CdS/CdTe solar cell of 12.8% efficiency for an active area of 0.78 cm2. Solar cells, 11(4): p. 367–373, 1984. 22. Green, M.A. and K. Emery, Research short communication solar cell efficiency tables (version 31). Progress in Photovoltaics: Res Appl, 2008. 23. Powalla, M. and D. Bonnet, Thin-film solar cells based on the polycrystalline compound semiconductors CIS and CdTe. Adv. Optoelectron, 2007. 2007. 24. Aberle, A.G., Thin-film solar cells. Thin solid films, 517(17): p. 4706–4710, 2009.

5 Biohybrid Solar Cells Sapana Jadoun1,2* and Ufana Riaz2† Faculty of Chemical Sciences, Department of Analytical and Inorganic Chemistry, University of Concepción, Concepción, Chile 2 Material Research Laboratory, Department of Chemistry, Jamia Millia Islamia, New Delhi, India 1

Abstract

For the last some decades, numerous works have been reported on biohybrid electrodes using some biocomponents of photosynthetic apparatus for the conversion of sunlight to produce electricity. Up to now, these works are limited to laboratory scale the future of these could be solar cells efficiently which will be a brand new method of using photosynthetic protein complexes as light-harvesting devices. The main challenge is the appropriate selection of species for specific protein complexes for building low-cost and highly efficient device. The present chapter consists of a brief discussion about the solar cells, their generation and efficiencies, the role of photosynthesis in biohybrid solar cells, as well as it includes the substrate used for fabrication of biohybrid solar cells. Keywords:  Photosystem I and II, semiconductors, reaction center, photosynthesis, solar cell, light-harvesting

Abbreviations AgCl Silver chloride CdTe Cadmium telluride DSSCs Dyes sensitized solar cells NADPH Nicotinamide adenine dinucleotide phosphate PANI Polyaniline *Corresponding author: [email protected] † Corresponding author: [email protected] Inamuddin, Mohd Imran Ahamed, Rajender Boddula, and Mashallah Rezakazemi (eds.) Fundamentals of Solar Cell Design, (117–136) © 2021 Scrivener Publishing LLC

117

118  Fundamentals of Solar Cell Design PPDs Polyphenylenediamine PSCs Perovskite solar cells PS I Photosystem I PSII Photosystem II PV Photovoltaic QDs Quantum dots TCO Transparent conductive oxide 3D Three-dimensional

5.1 Introduction Over time, photovoltaic devices such as solar cells have experienced theatrical variations, fabricating an enormous range of tactics for light-toenergy conversion [1]. Solar cells of first two generations offer 15%–22% efficiency, while third-generation solar cells provide about 23.2% efficiency in devices for lab, and they include perovskite solar cells (PSCs), organic solar cells, and quantum dots (QDs) solar cells [2, 3]. Inorganic, organic and biological light responsive substance represents a noteworthy landmark in progress of solar cell [4]. In photovoltaic devices, excitons are formed by an appropriate light source, but formerly, they can recombine and are detached by a charge transfer or an electric field to a nearby acceptor or donor [5]. This field pushes the electrons toward the one electrode and holes toward the other electrode and forms an electron-hole pair which can be denoted by “special pair” [6, 7]. So, these carriers after reaching the electrodes offer a voltage which could be used in several applications extending from photodetectors to solar cells [8]. Recently, for solar energy conversion, some biohybrid solar cells have been fabricated via biomimetic approaches borrowed from photosynthetic organisms and plants due to some processes of plants during photosynthesis, like dynamic self-repair, quantum effects, and light-harvesting property which makes them a suitable candidate for man-made photovoltaic devices [9, 10]. Nature itself evoluted some photosynthetic processes, which is a fully optimized technology allowing the translation of solar energy as well as acclimatization of some of the organic substrates from billions of years of evolution [11]. The researcher paid a lot of attention to biological material in the last few decades such as PS I (Photosystem I), PS II (Photosystem II), and Rhodopsins, as the principle photoactive component to construct biohybrid solar energy conversion devices with high efficiency. For the making of fuels of interest, use of the two significant protein centers involved in oxygenic photosynthesis, PSI and PSII, a semiartificial Z-scheme impersonator method could be ideally exploited [12].

Biohybrid Solar Cells  119 Devices based on conducting polymers, such as polyaniline [13] and its derivatives like PPDs (polyphenylenediamines), are now in trends [14, 15]. They are well known for their Л-conjugated backbone. These have been castoff for numerous implementation, such as supercapacitors, light-­emitting diodes, sensors, and solar cells lithium-ion batteries. They revealed large visible light absorption strength and mechanical conformity which results in solution processability and sizable density of photogenerated carriers makes it useful in thin and transparent film processing [16]. In 2000, Alan J. Heeger, Hideki Shirakawa, and Alan MacDiarmid were the winner of Nobel prize for development of conducting polymers [17]. G. Veerappan et al. [18] explained the importance of conducting polymers for their use as an electrocatalyst along with conducting substrates in dye sensitized solar cells because of their several needed properties, like electrocatalytic property, low cost, enhanced conductivity, transparency, great environmental stability, and ease of their electrochemical and chemical preparation under mild aqueous conditions. Previously, conducting polymers have been successfully utilized for biosensors [19], bioimaging [20], and in photovoltaic devices [21] revealed its biocompatibility nature along with its non-toxic nature.

5.2 Photovoltaics Photovoltaic states to solar energy consumption via conversion of light into electricity by those substrates exhibit photovoltaic effect [22]. Alexandre Becqerel reported firstly about the photovoltaic effect. They fabricated a cell via two platinum electrodes which were placed in the acid solution having AgCl, generated electricity after light exposure [23, 24]. Afterthought some researchers studied about semiconductor theory and crystal growth, and then, in 1946, fabrication of first silicon-based solar cell was done by Bell Laboratories [25, 26]. While energy crises during 1970, there was a need to reduce fossil fuel consumption; hence, the researchers were in keen to develop more cheap solar cells [27]. This chapter comprises solar cells, their generation, photosynthesis, components of photosynthetic apparatus, their fabrication etc.

5.3 Solar Cells The solar cell is a device that directly converts sunlight into electricity by the photovoltaic effect [28]. Each of the solar cells can be referred to as the name of the semiconducting material they are fabricated of and the absorption

120  Fundamentals of Solar Cell Design capacity of sunlight [29]. Solar cells have been designed for various purposes such as some are designed for use in space while others have been fabricated to handle sunlight which reaches on the surface of the surface [30]. These can be developed by one monolayer or multilayer of light-absorbing material [31]. Generally, solar cells have categorized into four generations.

5.3.1 First-Generation These are made up of crystalline silicon which is traditionally chief photovoltaic technology including substrates like monocrystalline silicon and polysilicon [32]. Panels of these include silicon solar cells and made up of a single silicon crystal or cut from a silicon block of many crystals [33]. They are referred to as traditional, conventional, or water-based cells. These comprise single-crystal, large-area, and monolayer p-n junction diode, as well as these are able of producing energy in the form of electricity from sources of light by the sunlight wavelength. First-generation solar cells holding for more than 86% of the global solar cell market [27, 34, 35].

5.3.2 Second-Generation Second-generation solar cells are thin-film solar cells including CdTe (cadmium telluride), amorphous silicon, copper indium selenide/sulfide, micro-crystalline silicon, and polycrystalline silicon [36]. These solar cells are cheap to produce compared to conventional silicon solar cells due to the requirement of the lessened amounts of materials for creation [37]. These are built on semiconductors epitaxial deposits on lattice-matched wafers which can be divided into two classes, i.e., terrestrial and space. Terrestrial cells consist of low-cost processes and low AMO efficiencies too in production. As compared to terrestrial cells, space cells revealed high cost per watt as well as higher AMO efficiencies too [38–40]. This technology provides reduced mass and hence permits fitting panels on flexible or light materials in textiles, as well as in building-integrated photovoltaics, efficacy scale photovoltaic power stations, and the small stand-alone power system. This generation consist about 90% of the space market and consist of very few sections of the terrestrial photovoltaic market [41–43].

5.3.3 Third-Generation Third-generation solar cells comprise many thin-film technologies frequently defined as developing photovoltaics. These generation solar cells are independent on a conventional p-n junction for distinction photogenerated charge carriers; in this way, these are different from earlier

Biohybrid Solar Cells  121 semiconductor devices. In these, organic materials and inorganic materials are used often. Efficiencies of these were found low (45% AMO production efficiency), as well as constancy of absorber materials was also found too short for some commercial applications [44–46].

5.3.4 Fourth-Generation In fourth-generation solar cells, composite photovoltaic technology was adapted in which a multispectrum layer can be generated by mixing nanoparticles and polymers. These layers stacked together to create a cheaper and more efficient multispectrum solar cell. In this technology, first layer works as passing various types of light, while another layer is for the light that passes, and the last layer works for a cell as the IR spectrum layer, combinedly convert heat to electricity [47–50].

5.4 Biohybrid Solar Cells Firstly, in Vanderbilt University, biohybrid solar cells were reported by some researchers [51]. They suggested that biohybrid solar cells are a combination of organic matter and inorganic matter and a new type of renewable energy. They used PS I in place of organic matter which is a light sensitive protein complex placed in the membrane of thylakoid to O

R

hν P700

e + P700

* P700

p-Si e-

Figure 5.1  Fabrication of multilayered bio-hybrid solar cells. Reprinted from Ref. [52].

122  Fundamentals of Solar Cell Design refashion the ordinary procedure of photosynthesis to attain superior proficiency in translation of solar energy. Spinach was used by the source of PS I. It was done by isolating thylakoid membranes, afterthought purified to separate the PS I from thylakoid membranes and the results revealed 1,000 times enhanced electrical current over previous made solar cells and this was first ever made biohybrid solar cell. They have also researched the PS II complex proteins for the next biohybrid solar cells. In biohybrid solar cells, photonic energy can be gathered by PS I and later converted into chemical energy which generates a current through the cell. The cell has all the same inorganic material excepting the introduced and gathered PS I complex for many days on the gold layer. Then, the PS I was visible as a green thin film that enhances the energy conversion. Fabrication of multilayered biohybrid cell is presented in Figure 5.1 [53].

5.5 Role of Photosynthesis From the evolutionary history of life, the primary source of energy on Earth has been always remaining photosynthesis [54]. Photosynthesis is said to be always a sustainable process for converting light energy to chemical energy which can be stored in fuel cells. For the progress of unique solar energy conversion systems, photosynthesis provides an extraordinary effectual platform as well as provides new research tactics for developing biohybrid inspired materials, devices, etc., to imitate the proficiency attained in nature [55]. When a couple of excitation of two reaction centers, i.e., PSI and PSII, occurs, the photosynthetic apparatus operation starts. An electron transfer to the ferredoxin unit starts with photoexcitation of PSI and then NADPH production starts that leads to the activation of the Calvin cycle for CO2 to sugar fixation. On the other hand, electron transfer to quinone starts via the photoexcitation of PSII resulting in the stimulation of reduction of plastoquinone-9 which regenerates PSI via a chain of electron transfer processes. Oxidation of the Mn4O5 complex takes place by the oxidized species produced in PSII and catalyzes the oxygen development though renewing the PSII center. Charge separation by induction of light afterthought transfer of electron by photosynthesis results in the energy storage adduct.

5.6 Plant-Based Biohybrid Devices The progress of these were founded near 1980s via efficacious immobilization of photosynthetic apparatuses on electrodes [56]. After that, in 1983,

Biohybrid Solar Cells  123 Anode

Cathode

hυ

Antenna

Electron donor e-

Reaction center

eElectron acceptor

e-

e-

e-

Load

e-

Figure 5.2  Schematic of a general biohydrid solar cell. Reprinted with pemission of Ref. [62].

some researchers have given the first example of semiartificial photosynthetic device via successful immobilization of PS I and PS II on platinum [57]. For working of photosynthetic reaction centers on platinum oxide, some new immobilization methods were developed by Katz and coworkers for electron transportation from protein to electrode. Later, they immobilized these reactions centers on pyrolytic carbon too [58]. In the thylakoid membrane of cyanobacteria, green algae, and plants, the initial process of oxygenic photosynthesis occurs and various protein complexes cooperate in this phenomenon [59]. From these protein complexes, some of them (PSI and PSII) are capable of accomplishing light-induced charge separation which changes light energy to chemical energy [60]. Hence, successive eras of research have been devoted to developing biohybrid electrodes and artificial photosynthetic devices as active biohybrid solar energy alteration platforms [61]. A general scheme of operation of photosynthetic apparatus–based biohybrid solar cell has been given in Figure 5.2 [62].

5.6.1 PS I–Based Biohybrid Devices For direct photocurrent generation, previously protein films and bioelectrodes based on PS I were accumulated against metallic surface

124  Fundamentals of Solar Cell Design in various arrangements such as monolayer or multilayer which acts as a light-sensitive stuff [63–65]. PS I–based devices provide advantages such as its simple processing, vast natural abundant materials, low cost, ease of extraction, robustness, and scalability. PS I reveals outstanding contribution to the solar energy conversion by electron transfer processes in proteins as it is proteins that enable the natural photosynthesis, supercomplex of a reaction center, and light-harvesting complexes [66]. The function of PS I is to provide a suitable electron transfer pathway across the membrane by catalyzing the transfer of electrons from reduced cytochrome C6/plastocyanin present in the lumen to ferredoxin present in the stroma [67]. PS I has a large antenna that harvests light by some photosynthetic pigments by absorbing separate wavelengths and then transference of energy to (P700) chlorophylls and charge distincts here. Transfer of electrons having elavated energy through the A0 (Chla), A1 (phylloquinone), Fx, FA, and FB (Fe4S4 clusters) which are principal electron acceptors to ferredoxin takes place. Afterthought, excited electron travels to produce ATP and redox equivalents through cyclic/noncyclic phosphorylation passage and reached to carbohydrate production. This sequence is accomplished through the again reduction of P700+• via cytochrome C6. Besides, internal quantum efficiency for PS I is close to unity. PS I consists of 12 (in cyanobacteria) or 13 (in plant systems) subunits proteins; PS I of Synechococcus elongatus contains 22 carotenoids, 96 chlorophyll a molecule, 3 [4Fe4S] clusters, and 2 phylloquinone. PS I exists as a trimer in the innate membrane with a 1,068-kDa molecular mass for the whole complex [68]. Due to these reasons, recently, PS I derived photovoltaics devices that have been extensively researched as bio-inspired solar-energy conversion systems in replacements of traditional solar boulevards [69]. PS I–based devices offer many advantages over semiconductor photodiodes or dye-sensitized solar cells (DSSCs) such as its low cost, simple processing, readily abundant material, and scalability. Ciesielski et al. [53] reported the fabrication of stand-alone biohybrid photoelectrochemical cells based on PS I and discussed the incorporation of PSI into the electrochemical system for converting light energy into electrical energy. To produce a photocatalytic effect, a thick PSI complexes multilayer accumulated on the surface of the cathode which created ~2 μA/cm2 photocurrent densities at moderate light intensities. They also described the connection of voltage and current production of the cells as well as interactions of electrochemical mediators and PSI and revealed the device performance is restricted through the electrolyte by diffusional transport of the electrochemical mediators. Some unique

Biohybrid Solar Cells  125 features of this device were outstanding stability in ambient conditions up to a minimum of 280 days.

5.6.2 PS II–Based Biohybrid Devices Yehezkeli et al. [70] fabricated the photobiofuel cells by using an electrically wired carbon nanotubes/bilirubin oxidase improved cathode and electrically contacted PS II functionalized photoanodes. The cells produced electricity in aqueous solutions upon the irradiation of biohybrid electrodes. Lightning of PS II functionalized photoanodes occurred the oxidation of H2O to O2 along with electron transfer to the cathode and then O2 is reduced to H2O again (Figure 5.3). V e–

e– R hν

e–

e–

e–

S

S

BOD PS-II

Au

O2 BOD

PS-II

e–

H2O O

P680* / P680* –0.58 V

–. Pheo / Pheo . –0.42 V pMBQ/(Ox) Q– /Q pMBQ/(Red) A A –0.08 V 0.194 V

hν

O2/H2O 0.92 V

N H

H N O

(BOD) 0.644 V O2/H2O 0.92 V

P680+/ P680 1.25 V

Figure 5.3  Schematic presentation of the poly(mercapto-p-benzoquinone)/photosystem II/bilirubin oxidase/carbon nanotubes photoelectrochemical cell. Reprinted with pemission of Ref. [70].

126  Fundamentals of Solar Cell Design

5.7 Dye-Sensitized Solar Cells DSSCs are a recently emerging technology for energy harvesting because of low cost, earth-abundant conditions, highly efficient under ambient conditions, and integration into a thin film format along with absorption in the visible light region [71–73]. The fabrication of DSSCs comprises the insertion of a very large surface area semiconductor layer which enhances the solar cell efficiency [74–76]. These solar cells comprise two glass electrodes in which one side is layered by transparent conductive oxide (TCO) and in transparent conductive oxide substance most commonly used are indium tin oxide (ITO) and SnO2: F (FTO), fluorine-doped tin oxide. In the cell, photoanode is created by a semiconductor layer that s deposited onto transparent conductive oxide-glass. These semiconductors in nanoparticles size embedded to arrange a 3D structure, and on the surface of this layer, dye molecules are attached. Hence, the dye molecules are used as a sensitizer in DSSCs. When dye molecules get excited, it results in a transfer of an electron into the conduction band of the semiconductor material. Iodide/triiodide solution are highest used electrolyte for dye sensitized solar cells [74, 77–80]. Some sensitizers used in DSSCs are porphyrins [81– 83], indolines [84, 85], coumarins [86, 87], conjugated polymers [88–90], etc. Though, these sensitizers have many disadvantages such as high cost and complex synthesis. Thus, the use of photosynthetic pigment-protein complexes as sensitizer is exhilarating to exploration for attaining high efficiencies via light-induced charge separation [91].

5.8 Polymer and Semiconductors-Based Biohybrid Solar Cells Literature suggested that semiconductors having valence and conduction bands were found more suitable for charge transfer to or from proteins as compared with metals [92]. However, extensive work has been reported for development on reaction center–originated biophotovoltaics highly focused on metallic electrodes [93]. Charge recombination or special pair at the surface of metallic electrode outcomes in a cost of energy [94]. Generally, the function of a working electrode is an electron acceptor or donor. But in semiconductors, they can be used in biophotovoltaic systems using proteins as selective charge transfer because of semiconductor’s energy band structures [95]. Despite the probable advantages of using semiconducting electrodes, only very few devices were designed with

Biohybrid Solar Cells  127 semiconductors and most of the biophotovoltaic devices have been fabricated using metallic electrodes [96]. Gizzie et al. [97] first time synthesized the PANI-PSI supercomplex onto the gold electrode via electropolymerization method in an aqueous solution of PS I and aniline. They revealed the entrapment of biomolecular supercomplex in a conducting polymer network (Figure 5.4). In composite films, charge separation occurred via illumination of light at the P700 site and electron passed from here to conductive polyaniline and thereafter gold anode. This process is shown in Figure 5.5. The films were prepared in various concentrations of PSI and the best-prepared film was found at 0.1 μM PSI concentration in PANI solution with the efficiency of about 0.005% and photocurrent density of 5.7 μA cm−2. Yaghoubi et al. [98] reported biohybrid self-assembled materials as a photoactive electrode with the blend of zinc oxide with monolayers of photosynthetic reaction centers via cytochrome c linker. They characterized these via photoemission spectroscopy and electrochemical analysis and reported that these electrodes can offer a suitable electronic path for transport of electron from zinc oxide’s conduction band via protein network to reaction center, finally to the counter electrode through redox mediator. Gizzie et al. [99] fabricated solid-state solar cells by PSI-Polyaniline/ TiO2 composites by electropolymerization technique. They deposited aniline directly on TiO2 anode in the presence of PSI. This was an outstanding biohybrid photoactive composite for effectual charge separation and transport of these charge to electrodes from protein. This method opened a new way of sustainable solar energy conversion via artificial synthesis. Some PSI films were studied on gold electrodes for attaining light

PAni-PSI Composite Film NH2 NH2 H2N

NH2

NH2 H2N

NH2 NH2 NH2

H2N

NH2 NH2

Gold Electrode

Electrochemical Polymerization

Gold Electrode

Figure 5.4  Electrochemical synthesis of PANI-PSI solid films. Reprinted with pemission of Ref. [97].

128  Fundamentals of Solar Cell Design V (vs. NHE)

eV (vs. vacuum) -3

-1.5

P700*

e-

-1.0 FB-

-4

-0.5

NaAsc NaAsc+

0.0 0.5

h+

-5

eh+

e-

ee-

Gold

P700 PSI

1.0

PAni

1.5

-6

Figure 5.5  Potential energy diagram demonstrating the energy alignment of the PANIPSI composite. Reprinted with pemission of Ref. [97].

responsive behavior and electrochemical behavior under parched situations [100, 101]. Gordiichuk et al. [102] reported biohybrid solar cells fabrication by accumulating a cyanobacterial PSI monolayer onto a TiO2 substrate as well as they assisted the device with MoO3/Al cathode and a hole conducting layer of polytriarylamine (Figure 5.6).

AI MoO3 PTAA PSI

PSI

P700 PSI

PSI

TiOx ITO

FB

Glass

Figure 5.6  The structure of the fabricated device. Reprinted with pemision of Ref. [102].

Biohybrid Solar Cells  129

5.9 Conclusion In the nutshell, research on biohybrid solar cells is a new area to explore in recent years due to their promising properties and better efficiency as compared to the conventional p-n junction solar cells with inadequate conversion efficiency. Photosynthetic proteins with nature designed architecture are widely used in biohybrid solar cells as this is evolving as a new class of photovoltaic materials with a property to do the primary light-driven steps of photosynthesis with a quantum efficiency of ≈100%. Therefore, the use of various engineered materials with natural proteins such as PSI and PSII appears to be promising for developing biohybrid solar cells.

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6 Dye-Sensitized Solar Cells Santhosh Sivaraj1, Gobinath Velu Kaliyannan2, Mohankumar Anandraj1, Moganapriya Chinnasamy1 and Rajasekar Rathanasamy1* Department of Mechanical Engineering, Kongu Engineering College, Perundurai Erode, TamilNadu, India 2 Department of Mechatronics Engineering, Kongu Engineering College, Perundurai Erode, TamilNadu, India 1

Abstract

Solar energy is naturally green, clean, and renewable. Instead of traditional nonrenewable energy, solar energy is the most promising choice. Photovoltaic or solar cell is a device that turns solar energy efficiently into usable electric energy [1]. Now, modern solar cells are most likely rigid silicon solar cells with greater stability and longer working life. But the dye-sensitized solar cell (DSSC) is a peculiar solar cell whose energy conversion mechanism is analogous with the photosynthesis of plants. The DSSCs are thin-film solar cells, which were also referred as Gratzel solar cell. DSSCs do not require purest material as any of its components, but conventional silicon solar cells require purest form of materials for their better functionality. In large-scale fabrication, DSSCs can be synthesized through rollto-roll printing process [2]. In natural dye-based DSSCs, extraction of photosensitizers was not a tedious process and was extracted by means of organic solvent. Sensitizers were extracted sources that are easily extractable using the organic solvent, abundant, non-toxic, and bio-degradable. The essential parts of DSSC are electrode, counter electrode, electrolyte, and dyes used. In this chapter, various materials used for counter electrode, electrolyte, and natural dyes were discussed in detail. The DSSCs can be fabricated as flexible one. Hence, they can be installed over any curvatures and also be employed for the following applications: clothes, bags, automobile body, roof tops, etc. Keywords:  Solar cells, DSSC, electrolytes, counter electrodes, natural dyes

*Corresponding author: [email protected] Inamuddin, Mohd Imran Ahamed, Rajender Boddula, and Mashallah Rezakazemi (eds.) Fundamentals of Solar Cell Design, (137–168) © 2021 Scrivener Publishing LLC

137

138  Fundamentals of Solar Cell Design

6.1 Introduction Solar energy is the one of the majorly obtainable renewable energy source among all other energy sources. The utilization of renewable energy source might come into existence in order to obtain the energy free from hydro carbons. Hence, there is a need for conversion of solar energy into useful electrical energy and thermal energy. As the currently adopted energy sources were depleting at quicker rate, solar energy source might be the better replacement in place of fossil fuel energy sources. The cost of petrol and diesel increases gradually day by day which is also a reason for adapting cheap, clean, and pollution-free renewable energy. Some of the other renewable energy sources were geo-thermal energy, tidal energy, wind energy, hydel energy, etc. [3]. Due to global modernization, the electrical energy has become one of the primary needs in the human life. Electrical energy is obtained from the sun light by means of solar cells, while the obtained useful thermal energy from sunlight was achieved through solar heat collectors. The solar cells work on the principle of photoelectric effect. Photoelectrochemical solar cells (PECSCs) indulge in direct conversion of solar energy into electrical energy. The generated electrical energy can be utilized for many household electrical appliances. The main difference between the PECSC and photogalvanic solar cell (PGSC) is the type of electrodes used. In PECSC, the semiconductor electrodes were used, while PGSC holds metal electrodes for electron transfer. The solar collectors work under the principle of black body radiation. Solar cells are electronic devices which helps us to utilize solar light and convert it into useful electrical energy [4]. The solar cell was divided into four major categories based on their advancement, materials used and efficiency as first-generation, second-generation, third-generation, and fourth-generation solar cells. Among them, silicon-based solar cells were extensively used, but other generation solar cells were also engineered in order to attain certain specific properties (thin-film flexibility, transparency, long-term stability, etc.). As per National Renewable Energy Laboratory (NREL) efficiency chart, the DSSC achieved maximum power conversion efficiency (PCE) of 12.6%. Natural dye-based DSSC dye sensitizer can be easily extractable from natural resources such as leaves, flower, and fruit. The main advantages of DSSCs are mainly low production cost, power conversion even at lower light intensity, flexible nature, quicker energy payback, better sustainability, and easier waste management [5]. The main limitation of DSSCs is liquid electrolyte highly sensitive to heat. Fabrication of DSSCs does not require clean air-conditioned laboratory along with skilled technicians; rather, they can be synthesized under actual room atmospheric conditions.

Dye-Sensitized Solar Cells  139 Some of the notable parts of DSSCs are photoanode, electrolytes, counter electrodes, and seals. Photoanode is a semiconductor of wider energy band gap usually coated over the transparent conductive oxide (TCO). They were sensitized through adsorbed natural dyes in presence of visible light. Electrolyte helps us to close the electrical circuit by interconnecting the counter electrode and anode. The counter electrode is a TCO-coated glass substrate facilitating the generated electron transfer. Seals are used to affix the entire setup filled with natural dye. In DSSC, the sunlight passes through the transparent glass and then incident over the incorporated natural dye. Then, the electrons get excited from valence band to the conduction band. After flowing into the external circuit, the electrons were transmitted into the electrolyte followed by charge carrier transport into the dye. The separation of charge carriers (electrons) occurs in between the electrolyte and the semiconductor dye used. At present, lots of research studies were going on in improving the cell stability and enhancing performance of electrolyte which would make them commercialize in the near future. The transportation of generated electrons and holes might occur in two phases: electrolyte and TiO2 electrode. This is the main reason for lower recombination rate of electron-hole pairs in the DSSC. The DSSCs have achieved enhanced PCE than other photovoltaic cells even in diffuse light and at ambient temperature upto 50°C. Based on their transparency and flexible nature, they can be installed over window shadings and doors especially for room lighting purposes.

6.2 Cell Architecture and Working Mechanism DSSCs were one of the PECSCs whose operation in generating the electrical energy entirely varies as compared to other solar cells. This might also mimic the natural photosynthesis process and hence also known as artificial photosynthesizing cells. Some of the essential components of DSSCs are as follows: a) Electrode b) Counter electrode c) Dye as natural sensitizer d) Electrolyte e) Affixing components a. Electrode It is usually made up of semiconductor materials with much wider band gap. It absorbs UV light and visible light as they were sensitized with the

140  Fundamentals of Solar Cell Design dye adsorbed with their external surface [8, 9]. Some of the materials which perform the function of anode in DSSCs are TiO2, ZnO, SnO2, etc., with which the sensitizers were adsorbed. b.  Counter Electrode It is a conductive surface which acts as cathode. Some of the materials used as counter electrode are platinum, carbon coatings, graphite, etc. [6–8]. As platinum is quite costlier, hence carbon coting was preferred. c. Dye As Natural Sensitizer Natural dyes extracted from plants or artificially synthesized dyes mimicking natural dyes were used as photosensitizer. These dyes were adsorbed with the electrode surface, which traps the sunlight and supports electrode to generate electrical energy. The artificial dye commonly used in DSSCs is N719 dye [5, 6]. d. Electrolyte The iodide and tri-iodide (I–/I3–) redox couple was quite common element in DSSCs and taking up the role of electrolytes. Due to the sunlight incidence, the electrolyte gets reduced and excitation of electrons from valence band to conduction band [9, 11, 14]. e. Affixing Components Binder clips were most commonly used for affixing the entire DSSC setup comprising electrode adsorbed with photosensitizers, counter electrode, and electrolyte solution at their interface. Binder clips were used only when fabricated in lab scale [21]. DSSC’s working principle was quite different from the working mechanism of silicon solar cells. Their working is very similar with the photosynthesis process in plants. As like chlorophyll in plants, an adsorbed layer with the photoanode surface absorbs the incident light and initiates the process of power generation in DSSCs. DSSCs that comprise wider band gap semiconducting photoanode, electrolyte, counter electrode, and dye used were represented in Figure 6.1. The photoanode, which is adsorbed with photosensitizer, absorbs maximum in visible spectrum and UV light in certain extent [22]. The semiconductor nanoparticles have a wide area for adsorbing the dye molecules on the semiconductor surface and results in the ample quantities of light absorption by photoanode [1, 2]. Such semi-conductor nanoparticles, however, must be sintered together so that electrical interaction between the particles is available and the electrical conduction is possible through external circuit. The porous semiconductor

Dye-Sensitized Solar Cells  141 e-

D* Ef ΔE

Voc

–

+

Er I-

D

e-

I3e-

TiO2 Nanoparticles

Catalyst FTO Glass

FTO Glass e-

e-

Cyanidin-3-glucoside

Figure 6.1  DSSC working mechanism [7].

layer which is a conductive glass substrate, which is externally attached to the cathode, is affixed together with the binder clips. Finally, the cathode is also a conductive glass substrate coated with a catalyst like platinum (Pt) over it. In presence of sunlight, DSSCs absorb photons whose energy might equal to the energy difference between its lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) [13]. Electrons jump from the nominal ground state of the dye to its excited state known as dye photo-excitation. Furthermore, the electron is injected into the conduction band of photoanode in the excited state of the dye. The electrons after being mobilized into CB of photoanode were transported by means of diffusion through the photoanode in order to reach the conducting layer of FTO glass. Now, the iodide ion in the redox couple lends electron to the oxidized dye adhered with anode; henceforth, the molecules of dye were regenerated. The reduction of oxidized group of the electrolyte especially at the cathode was performed to obtain iodide ions [16]. The above processes are carried out in a loop and, as a result, generated electron flows by means of externally connected circuit as long as there is an incident of light over the solar cell. The efficiency of DSSC gets reduced because of the occurrence of multiple recombination reactions at various cell interfaces.

142  Fundamentals of Solar Cell Design The two significant recombination reactions are the recombination of the photoanode CB electron and the FTO electron with the triiodide ion of the electrolyte. The abovementioned recombination reactions undergo at very slower rate. For instance, triiodide reduction at cathode occurs faster than its recombination with photoanode electron. The recombination at the photoanode/electrolyte interface was sluggish than the electron diffusion through the photoanode for better cell functioning. Electron injection into photoanode CB from the excited dye is much more quick process and electron transport through the photoanode film which might be responsible for PCE determination. Electron diffusion through the photoanode depends on phase composition, level of crystalline nature, and intermittent connection between the particles. As a result, current density in the cell becomes higher. The power conversion methodology of DSSC was exactly mimicking the natural process of photosynthesis. The sunlight was trapped and then converted into useful starch using the chlorophyll pigments in plants by means of photosynthesis. By using the chlorophyll for light traption in DSSC, the following series of actions occur. In the first excitonic transition, red is absorbed, and hence, the electrons were excited from HOMO to zeroth LUMO [18]. In case of second excitonic transition, the blue color is absorbed and the ground state electrons were excited from HOMO to LUMO1. Here, the chlorophyll absorbs the sunlight at a wavelength of 430 nm. The enhanced carrying of energy was mainly identified with blue light when compared with the red light.

6.3 Fabrication of Simple DSSC in Lab Scale Initially, two conductive glass substrates for the fabrication of DSSC in laboratory scale were taken. The preparation of DSSC does not require any special atmospheric conditions instead can be fabricated in normal ambient atmospheric conditions. Before starting the fabrication process, the hands should be properly gloved and all lab equipment should be made clean. Now, with the use of disinfectant ethanol, the conductive substrates were cleaned with the cotton buds. Then, titanium dioxide particles were synthesized chemically under controlled atmospheric conditions. The glass substrate 1 was now dipped into the prepared TiO2 suspension with one side that is covered with the transparent adhering tapes. The side with uncovered tapes was adhered with TiO2 particles. For the purpose of effective bonding, the stained TiO2 glass substrate is annealed for about 10 minutes. Followed by this, the photosensitizers

Dye-Sensitized Solar Cells  143 (natural or artificial dye) were extracted or prepared using a ceramic dish and mechanical ceramic grinder. Subsequently, the prepared dye is taken in a beaker. Then, the TiO2 adhered conductive glass substrate was dip coated with the prepared dye [23]. The dye particles were gets adsorbed with the TiO2 surface. Subsequently, the glass substrate 2 was simply shown over the burning candle; hence, the surface facing the fire might be coated with the carbon and other side is made free [26, 32]. Instead

Cleaning of conductive glass substrate with ethanol

Glass substrate II

Glass substrate I Preparation of TiO2 suspension Coating of prepared TiO2 suspension

Carbon coating on the conductive side of the glass substrate (counter electrode)

Annealing of TiO2 coated glass for 10 minutes (Conducting electrode)

Staining TiO2 with the natural dye

Joining of two glass substrates with the help of binder clips

Application of Electrolyte in the interface of the glass substrates

Measurement of voltage and current for the fabricated solar cell

PCE ESTIMATION

Figure 6.2  Preparation of simple DSSC in lab scale at room temperature.

144  Fundamentals of Solar Cell Design of carbon coating, platinum can be used as a counter electrode but only drawback is not cost effective. Both the glass substrates were adjoined together by keeping uncoated surfaces externally with the help of binder clips. At this instant, any of the glass surfaces were drilled few holes at small extent. Through these holes, the electrolytic solution was injected using medical syringe. The electrolyte was injected especially at the interfacial layer of two glass substrates. The excess electrolytic solution which coming out of glass substrates were wiped off with the cotton. Power supply is taken out from the external surfaces of the glasses. The fabricated solar cell is then subjected to current-voltage measurement under closed and open atmospheric conditions using Keithley 2450 source meter [24]. As a result, the PCE of the prepared DSSC was calculated. Thus, a simple and cost-effective DSSC was prepared in laboratory scale which also does not require any skilled labor. The entire fabrication process was represented in the following flow chart (Figure 6.2).

6.4 Electrodes As for a classic DSSC working electrode, it can be considered to be created by combining three different materials: a conductive electrode (CE) with greater transparency, adsorbed dye layer, and the porous semiconductor. Usually used CEs are based on micrometric films, adhered with flexible glass substrates, of doped metal oxide semiconductors. The most commonly used electrode for DSSC is fluorine-doped tin oxide (FTO), because of its electrical conductivity and optical transparency. The graphene-based electrodes have been explored in place of FTO, which leads to better compatibility. The TiO2 photoanode is most commonly used anode in DSSCs. As compared to other metal oxides such as ZnO and SnO2, TiO2 still offers the better performance in photovoltaic devices. It is also available at abundant, which were characterized by using wellknown low-value artificial synthesis procedure. TiO2 nanostructures such as nanotubes, nanorods, or nano-opals were investigated to obtain improved charge mobility. Along with the TiO2 electrode adsorbed with dye had been engineered with a few reinforcements as nanoparticles which might improve usual PCE of solar cells as in Figure 6.3. The DSSC architecture shows the diagrammatic representation of gold nanoparticles incorporated with the TiO2 electrode [35]. In DSSCs, the double layered mesoporous TiO2 films as electrode reported maximum light transmittance.

Dye-Sensitized Solar Cells  145

hν

Au Au

Ce3+

Au Au FTO

TiO2

Au Au

Water–Based Electrolyte

Pt

ITO

Au Au Au Au

Ce4+

Figure 6.3  Gold nanoparticles incorporated at outer surface of photoanode [9].

6.5 Counter Electrode In DSSCs, numerous substances, which include carbon-based materials, and conducting polymers were used as CEs in place of Pt. Fabrication of DSSC at low price with higher overall performance is most challenging one [3]. Cost of dye-sensitized sun cells depends particularly on the counter electrode most commonly platinum is used, which is extraordinarily expensive. Various counter electrodes used in DSSCs are represented in the flow chart (Figure 6.4). Among numerous non-platinum counter electrode catalysts, carbon-based substances [5, 6, 10] are basically the great alternative which are low value and available in abundance. Conducting polymers were used as CE because of their easier fabrication and available at lower cost. Carbon-based materials have been extensively used as CE substances because of their enhanced electrocatalytic activity. Due to the similar structural and physical property, metal oxides, carbides, nirides, selenides [8, 37], etc., were used as effective substitution of Pt to act as CE. Due to the stronger bonding between metal ions, metal chalcogenides exhibit enhanced electron conductivity. From various studies, the overall performance was influenced by the effect of charge carrier transport ability of CE materials in DSSC. The regular arrangement of metallic

146  Fundamentals of Solar Cell Design Counter electrodes for DSSC

Conductive Polymers

Carbon based materials

Polyaniline

Graphite

Polypyrrole

Carbon nanotubes

Polythiophene

Carbon black

Poly (3, 4 phenylene dioxythiophene Poly-(3,4(ethylenedioxy) pyrrole)

Metal Selenides

Carbon nanofibres

Iron Selenide

Metals

Platinum

Cobalt Selenide Molybdenum Selenide Nickel Selenide Bismuth Selenide

Graphene Hydrogel

Copper Selenide

Figure 6.4  Various counter electrodes used in DSSCs.

selenide nanostructures over graphene [5] leads to enhanced electron transport, improved stability and shorter electron diffusion length. These have effectively been used for the supercapacitors, DSSCs and lithium-ion batteries, because of their advanced electrical conductivity, rich electrochemical redox reactions, higher chemical stability and lesser electron diffusion length. Also, the optimal stacking of graphene nanosheets results in enhancing the PCE of DSSC.

6.6 Blocking Layer The working performance of DSSC was further enhanced with the help of blocking layer (BL). BL reduces the rate of carrier recombination which leads to improved photocurrent generation, fill factor, and Voc. Furthermore, cell stability test has undergone for the determination of the dye molecule aging in open atmospheric condition [38]. Though lots of advancements were done in DSSCs, the only drawback is their stability which degrades over longer time. Charge carrier traption and charge

Dye-Sensitized Solar Cells  147 carrier leakage occur in DSSC mainly due to impurities into electrode. In order to overcome this, minimal recombination at photoanode-electrolyte interface obtained through BL may considered as most promising option. A minimal diffusion current mainly because of recombination leads to increase in Voc. The natural dye stability of DSSC rely on chemical and physical stability. Physical stability can be adapted through effective sealing of cell to resist escaping of liquid electrolyte. Chemical stability was mainly linked with irreversible reaction between hydroxyl group and the redox electrolyte. Ion depletion might lead to degradation of DSSC performance especially from the electrolyte. Thus, BL improves PCE of DSSCs.

6.7 Electrolytes Used During the DSSC operation, electrolyte is mainly responsible for hole transport and dye regeneration. Liquid electrolytes are the most investigated systems in DSSC field [28, 29, 32]. They basically comprise three main elements: a redox mediator, a solvent, and various additives. The various electrolyte materials used until now for DSSCs are represented in following flowchart (Figure 6.5). The electrolyte solvent must hold some properties as follows: • low melting point • higher boiling point • better photochemical stability

Electrolytes used in Dye Sensitised Solar Cells

Liquid based Electrolytes Electrical Additives

Organic based Solvents Ionic natured Liquids Iodide/Triiodide as a free mediator & Redox couples

Quasi-Solid State based Electrolytes

Solid state transport materials

Thermoplastic based Polymer electrolytes

Inorganic hole transport materials

Thermosetting Polymer electrolytes

Organic hole transport materials

Figure 6.5  Various electrolytes used in DSSCs.

Ionic conductors

148  Fundamentals of Solar Cell Design • high dielectric constant • low viscosity • high diffusion coefficients

6.7.1 Liquid-Based Electrolytes In DSSCs, liquid electrolyte comprising of solvents in organic nature and simple iodide/triiodide redox pair promote the rate of photo-energy conversion process. Liquid electrolytes have some essential characteristics, such as easy and quicker synthesis, better conductivity, lower resistance to flow, and better wetting at intermediate layers [36]. Today, liquid electrolytes are most commonly used. With the use of liquid-based electrolytes, the PCE of DSSC reaches 7.1% to 7.9%.

6.7.1.1 Electrical Additives Electrical additives especially in liquid electrolytes were adopted for obtaining optimal performance in the DSSCs. Some of the properties that were improved after the addition of electrical additives in DSSCs are as follows: • Recombination kinetics • Redox couple potential • Photovoltaic parameters of DSSCs The peculiar feature of the liquid electrolyte has resulted in enhanced efficiency and was utilized in conventional DSSCs. The behavior of electrolytes mainly was influenced by solvent, additives, and the interactions within them and interactions between photosensitive dyes, electrodes, and electrolytes.

6.7.1.2 Organic Solvents Organic-based solvent is a fundamental component in liquid electrolyte, which gives effective space for dissolution and diffusion of ionic conductor. Some of the basic criteria for the solvents in DSSCs are as follows: • • • •

low viscosity absorption of light is very poor poor solubility to sealant materials low toxicity and low cost.

Dye-Sensitized Solar Cells  149

6.7.1.3 Ionic Liquids Ionic liquid (IL) is a crystalline salt in liquid form; ILs or molten salts (liquid electrolytes) are composed of ions. Based on the melting point temperature, the optimal properties of ILs and molten salts were determined. Molten salts must possess higher melting temperature and higher resistance to flow, while the ILs must possess lesser resistance to flow and relatively lower melting point [9]. The latter has the capacity to flow freely at room temperature and hence known as room-temperature ILs. They also can act as solvents and organic-based salts. ILs were comprised of cations and anions. The cations are generally heavy, phosphonium or ammonium salts, weaker chemical bonding, and lesser charge densities. The anions can be categorized into the complex anions and pseudo halide ions.

6.7.1.4 Iodide/Triiodide-Free Mediator and Redox Couples The development of newer redox mediators might furthermore have influence in PCE of DSSC [18, 21]. From various studies, the interhalogen redox systems along with various electrolytes which were used in connection with ruthenium-based dyes in the DSSCs might achieve 6.4% PCE. In the working mechanism of DSSC, photosensitizer molecules at oxidized state were regenerated by the couple at reduced stage. At counter electrode, oxidized group of the couple gets diffused. The redox couple should possess asymmetrical behavior, which might tend to faster donation of electron [23]. Henceforth, this effect confirms the effective dye regeneration. Subsequently, the electron acceptance should occur at very slower rate; thus, there will be reduction in recombination losses. The iodide/triiodide couple holds adequate redox potential and later leads to much slower dye regeneration and electron recombination. Also, it possesses lower light absorption and better solubility in addition to improved stability [24]. Due to those features which were seemed to be unique, the iodide/triiodide couple was commonly used as electrolyte.

6.7.2 Quasi-Solid-State Electrolytes Quasi-solid state electrolytes are those which possess intermittent properties among liquid-based and solid-based electrolytes. They are of two categories, namely: thermosetting and thermoplastic polymer electrolytes.

150  Fundamentals of Solar Cell Design

6.7.2.1 Thermoplastic-Based Polymer Electrolytes The essential constituents of polymer electrolytes are oligomer, inorganic salt, organic-based solvent, and inorganic salts. The major functions of polymer were to act as framework or matrix to gel, absorb, solidify, and intermingle with liquid electrolyte. The solvent shortens the process of crystal formation and lowers electrolyte temperature, because it was commonly noticed in the junction of polymeric sequence [33]. The mixture of liquid-based electrolyte along with matrix made of polymer, which possesses heterogenous nature, gets converted to homogeneous system which was dilute and highly viscous in nature. In the process of gelation, because of weaker bonding among the solvents and the polymer based matrices, gelly natured electrolyte was attained by the adsorption and gelation. The bonding mainly depends on the temperature.

6.7.2.2 Thermosetting Polymer Electrolytes Thermosetting polymer electrolyte (TPE) is a kind of polymer gel electrolyte. The electrolytes were attained by means of covalent cross-linking of organic molecule which, in turn, leads to the formation of 3D polymeric network. These polymeric electrolytes have no reversible effect on temperature; hence, they were named as TPE. In the physical appearance, the TPEs were resembled as solid state electrolytes [4]. But some of the electrolytes which lie in this category were in liquid form and they were named as quasi-solid state electrolytes. The major variation among the thermoplastic polymer electrolytes and TPEs differ only with the type of boding occurred between each molecule. However, the TPEs have chemical cross-linking while thermoplastic polymer electrolytes possess physical cross-linking between each molecule. As TPEs have better stability in physical, chemical, and thermal aspects than thermoplastic polymer electrolytes, TPEs were mostly preferred for obtaining higher PCE along with long-term effective working life.

6.7.3 Solid-State Transport Materials The stability was the major issue for all quasi-solid state electrolytes because they contain thermally unstable solvents. The solvent might get exudate even after longer storage and prolonged air exposure. Due to this reason, all solid state materials responsible for charge carrier transport possess enhanced properties than quasi-solid state and liquid state electrolytes [28] mainly in large-scale fabrication of DSSCs. Major studies reveal that many

Dye-Sensitized Solar Cells  151 researches were being undergone for effective usage of solid charge carrier transport materials in place of quasi-solid and liquid-based electrolytes. The materials responsible for solid charge carrier transport include organic and inorganic hole transport materials (HTMs) and ionic conductors.

6.7.3.1 Inorganic Hole Transport Materials As per the optimal working mechanism of DSSC, the mostly preferred electron and hole transport layers were TiO2 electrode and iodide/triodide redox couple. The I−/I3− redox couple was used in place of p-type semiconductor. In general, all hole transport layers were semiconductor in nature, because charge carriers were by electrons or positive holes only [2]. In DSSCs, the hole transport layers hold some salts and hence mostly favor compensation of charges locally. Some of the noticeable properties of HTMs in DSSCs were as follows: • It must be capable to transmit holes from sensitized dyes which were followed by transfer of electrons from dye. • It should possess better hole mobility, as poor mobility of holes might be responsible for degraded DSSC performance. • Hole transport layer must be transparent and should be resistant to degradation at the time of deposition.

6.7.3.2 Organic Hole Transport Materials In general, organic HTMs are cheap and easy to synthesis. Organic HTMs were soluble easily in organic-based solvents. They can be fabricated simply using spin coating method [1]. They are categorized into polymeric HTM and molecular HTM. They are used in the following real-time applications: LEDs, thin film–based transistor, organic-based solar, cell etc.

6.7.3.3 Solid-State Ionic Conductors For easier solidification and better conductivity in comparison with organic HTMs, polymer-based electrolytes were used for easier construction of solid-state DSSCs [19]. The quasi-solid IL electrolytes, composite polymer electrolytes, and TPE possess much similar mechanism, functions, and structure as solid-state polymer electrolytes which were employed in DSSCs. Polyelectrolytes are single-ion conductors. They are more stable than other electrolytes.

152  Fundamentals of Solar Cell Design

6.8 Commonly Used Natural Dyes in DSSC 6.8.1 Chlorophyll Chlorophyll is a green pigment which makes green plants to prepare starch with the help of sunlight through the process of photosynthesis. Dye is the most essential component responsible photovoltaic performance of DSSC. The dye must have certain properties such as follows: • Stronger sunlight traption in the visible spectrum • Able to incorporate electrons into the electrode surface Chlorophyll can be used as photosensitizer in DSSC which was extracted from various parts of plants such as leaves [25], petals, sepals [22], and trunk. It is capable of absorbing red, blue, and violet lights and reflects green light, which makes plants to appear in green color. The absorption peaks for chlorophyll appears at 420- and 660-nm wavelength [16]. This is main reason for utilizing them as photosensitizer in the visible range. From studies, the maximum PCE achieved for DSSC with chlorophyll as photosensitizer is approximately maximum of 1%.

6.8.2 Flavonoids Flavonoids are extracted from various plants and were categorized based on their chemical structure as follows: • • • • • •

Chalcones Anthocyanins Catechins Isoflavones Flavones Flavanones

Degree of pigmentation in flavonoids was based on pH value and intraand inter-molecular interactions. Flavonoids possess carbon network of C6-C3-C6. Depending on the positions of aromatic ring bonded chemically with benzo pyranomoiety, flavanols were categorized into iso-flavonoids (3-benzo-pyrans), flavonoids (2-phenylbenzopyrans), and neo-flavonoids (4-bezopyrans) [35]. Flavonoids possess a structure with fifteen carbon atoms bonded with two phenyl rings which furthermore connected to three carbon bridges. Not all flavonoids were capable of absorbing visible light.

Dye-Sensitized Solar Cells  153 In general, flavonoid molecules have looser electrons; hence, less energy is needed for electron jump from HOMO to LUMO will be lesser. The colors of flavonoids that appear physically were those which were absorbed and reflected. These pigments adsorbed with the photoanode and displaced the hydroxy ions from photoanode surface which then combines with protons of flavonoids. The color appearance of flavonoid pigments depends on levels of oxidation in phenyl ring and was influenced by absorptivity and reflectivity of incident light by sensitizer molecules.

6.8.3 Anthocyanins Anthocyanins are present naturally in many plant species which were easily soluble in water [7]. It is one of the flavonoid compounds which usually seen in plenty of flower, fruits, and leaves mainly accountable for violet, red, and blue colors. They were also been available in noticeable amount in some other parts of plants such as roots, shoots, tubers, and seeds. Anthocyanins facilitate the easier adsorption of hydroxyl and carboxyl groups of chlorophyll to the photoanode surface [10, 13]. This might improve easier electron transfer to the CB of photoanode especially from the anthocyanin molecule. They were categorized based on two major factors depicted in following flowchart (Figure 6.6). They are polyhydroxyl derivatives based phenyl benzo-pyrylium salts. Thus, they were also known as flavylium salts. In general, they were constructed using three six-­membered rings contain oxygen molecule and carbon was also bonded with other rings. Hydroxyl and carboxyl groups influence the type of appearing color. Blue appearance is due to more hydroxyl group, while red color is due to more methoxy groups. They can be used as pH indicators as they respond

Anthocyanins

Based on the chemical forms Quinonoidal base Flavylium cation

Based on the number of sugar molecules with the structure Mono-Sides

Pseudo-base

Bio-Sides

Chalcone

Trio-Sides

Figure 6.6  Classification of anthocyanins.

154  Fundamentals of Solar Cell Design in accordance with the pH of surrounding medium. From various studies, the sensitizers for DSSC were extracted from mulberry, eggplant [20], grape, orange, pear, etc.

6.8.4 Carotenoids Carotenoids are more than 600 variants which were easily found in chloroplasts and chromoplasts in plants and also been found in some of the bacteria and fungi. They were formed naturally by 8-isoprene molecules [20]. Carotenoids that possess 40 carbon atoms have ability to harvest solar light and have some influential functions such as facilitating easier absorption of light for photosynthesis and give ultimate protection against the damage of chlorophyll in presence of sunlight. They were not prepared by animal species, and hence, they were obtained only through their regular diet and may involve in metabolism process. There are classified into two broad categories: xanthophylls and carotenes. Some of the types of carotenoids present in human are as follows: • • • • • •

Beta carotene Alpha carotene Beta cryptoxanthin Astaxanthin Zeaxanthin Lutein

Among them, zeaxanthin, astaxanthin, and lutein prevent blue light and UV light which might damage the human eye [35]. Carotenoid in fruits and flowers makes to appear in orange, red, and yellow colors. Photovoltaic performance of DSSCs with various materials used as essential parts were consolidate and tabulated in Table 6.1.

6.9 Calculations 6.9.1 Power Conversion Efficiency PCE is the ratio of generated energy from solar cell to input solar energy [27]. On behalf of light reflection, cell performance was influenced by luminosity, intensity of light, and cell temperature.

Pmax = Voc × Isc × FF

(6.1)

Betalains and

5

6

N-719 dye

4

anthocyanins

N-719 dye

cotinifolia leaf

Euphorbia

flower and

chiovenda

Acanthus sennii

3

2

Chlorophyll

1

pigments

Dye used

S. no

TiO2

TiO2

TiO2

TiO2

TiO2

Electrode used

nanocomposite

Co/Se and Ni/Se

clusters

platinum metal

shell charcoal

activated coconut

nanosheets

on graphene

dispersed

nanoparticles

selenide

cobalt-nickel

FTO glass)

(PEDOT-coated

graphite

used

Counter electrode

I-/I3-redox electrolyte

I-/I3-redox electrolyte

I-/I3-redox electrolyte

electrolyte

Polymer gel

Electrolytes used

6.43 & 5.23

2.06

7.85

9.42

0.15 & 0.136

Max. PCE (%)

0.62

0.683

0.47–0.6

0.49

Max. fill factor

Table 6.1  Various materials used as essential parts of DSSCs with their photovoltaic performance.

0.316–0.419

0.65

0.75

0.475–0.507

0.44

Voc (V)

8.8

19.49

18.33

Reference

[8]

[7]

[6]

[5]

[4]

[3]

(Continued)

0.352–0.642

cm−2)

Jsc (mA

Dye-Sensitized Solar Cells  155

Rhoeo spathacea

7

20% Basella alba

10

11

N3 dye

9

mixture

Tree Flower

and Flame

Pawpaw Leaf

Dye from

dye)

dubius (Red

Amaranthus

and 80%

(green dye)

Anthocyanin dye

8

dye

Dye used

S. no

TiO2

TiO2

TiO2

dispersed in

with carbon

nano sized Cu

TiO2

nanoparticles

TiO2 adsorbed Au

Electrode used

Pt

carbon coating

graphite

containing

plastic sheets

conductive

Pt

used

Counter electrode

I-/I3-redox electrolyte

system

of Ce4+/3+

Aqueous electrolyte

Electrolytes used

0.27

0.847

2.8 & 23

1.49

Max. PCE (%)

0.69

0.515

0.55–0.70.

Max. fill factor

0.518

0.385

0.76 to 0.81

Voc (V)

Table 6.1  Various materials used as essential parts of DSSCs with their photovoltaic performance. (Continued)

0.744

6.1–7.4

cm−2)

Jsc (mA

(Continued)

[13]

[12]

[11]

[10]

[9]

Reference

156  Fundamentals of Solar Cell Design

Male flowers

13

17

16

15

14

C106 dye

12

LEG4

(Solaronix)

N719 dye

Achiote seeds

Norbixin from

Bixin, annato,

ipomoea

Spinach and

cylindrica

Luffa

Dye used

S. no

mesoporous TiO2

TiO2

TiO2 & ZnO2

TiO2

TiO2

TiO2

double layered

Electrode used

thick

withh 100 nm

dioxythiophene

4-phenylene

Poly (3,

TiC and VC−MC

Pt

Pt

Pt

Pt

used

Counter electrode

electrolyte

Cobalt-based

I− & T2/T−

Redox couples I3−/

Z988 electrolyte

Electrolytes used

5.97

6.52 &7.63

0.37 (max)

0.131& 0.278

0.13

9

Max. PCE (%)

0.64

0.67 & 0.72

0.59

0.51 & 0563

0.60

0.762

Max. fill factor

0.85

0.808

0.778 &

0.57

0.5 & 0.54

0.52

0.694

Voc (V)

Table 6.1  Various materials used as essential parts of DSSCs with their photovoltaic performance. (Continued)

10.96

13.11

12.50 &

1.1

0.44

8.702

cm−2)

Jsc (mA

(Continued)

[19]

[18]

[17]

[16]

[15]

[14]

Reference

Dye-Sensitized Solar Cells  157

22

21

20

19

fructoside)

redox couple

Eclipta alba

nigrum and

Iodine (I-) and triiodide (I3-) as a

Pt

from Solanum

Dye extracted

TiO2

redox couple

(malvidin-3-

pigment

Iodine (I-) and triiodide (I3-) as a

Pt

anthocyanin

TiO2

redox couple

blue pea.

Polyphenolic

triiodide (I3-) as a

Iodine (I-) and

cabbage and

Dyes from red

Pt

redox couple

and table

rose)

triiodide (I3-) as a

Iodine (I-) and

(rose petals

TiO2

ammonium

peel

Pt

dm−3tetrabutyl

and eggplant

TiO2

0.5 mol

Cynoglossum,

Anthocyanin

containing

iodide

ethylenecarbonate

1:4 volratio of

Saffron,

Pt

Electrolytes used

Cristata,

TiO2

used

Counter electrode

acenonitrile-

Dyes from

18

Electrode used

Celosia

Dye used

S. no

0.77 & 0.60

0.55

0.73 & 067

0.81 & 0.67

2.61

tandem

Cristata

Celosia

Cynoglossum/

Max. PCE (%)

0.36 & 0.34

0.33

0.35 & 0.36

0.52

Max. fill factor

0.43

1.12

Voc (V)

Table 6.1  Various materials used as essential parts of DSSCs with their photovoltaic performance. (Continued)

4.46 & 4.04

4.06

4.38 & 4.16

4.57 & 4.23

4.49

cm−2)

Jsc (mA

(Continued)

[24]

[23]

[22]

[21]

[20]

Reference

158  Fundamentals of Solar Cell Design

25

24

tion of both

cosensitiza-

of both and

1:1 mixture

lum fruit,

odontophyl-

Canarium

Ixora flower,

dye

yellow & red

40%) of

(60% &

mixture

and optimal

thusdubius)

TiO2

Pt um iodide

Tetrabutylammoni-

a redox couple

(Amaran-

red spinach

Iodine (I-) and triiodide (I3-) as

Carbon coating

cuma longa),

Turmeric (Cur-

TiO2

Iodine (I-) and redox couple

Pt

Electrolytes used

pigment-

TiO2

used

Counter electrode

triiodide (I3-) as a

Basella alba

23

Electrode used

rubra spinach

Dye used

S. no

& 1.55

0.96, 0.59, 1.13,

& 1.078

0.378, 0.134,

0.70

Max. PCE (%)

0.46

0.62, 0.44,0. 47, &

0.481, 0.449 & 0.507

0.35

Max. fill factor

0.343

0.384, &

0.351,

0.385,

ture)

( opt. mix-

0.4993

0.48

Voc (V)

Table 6.1  Various materials used as essential parts of DSSCs with their photovoltaic performance. (Continued)

6.26, & 9.8

6.26,

2.45,

mixture)

4.264 (opt.

4.35

cm−2)

Jsc (mA

(Continued)

[27]

[26]

[25]

Reference

Dye-Sensitized Solar Cells  159

Pt

& 0.7025 0.5373

without sug-

thocyanin)

sugar (an-

without

with and

chinensis

ar and Rosa

0.8017, &

0.7509,

05597,

13.58

10.19 &

3.3

0.54

cm−2)

Jsc (mA

0.5526,

0.5839,

0.817

0.828 &

0.49

0.51

Voc (V)

0.5433,

0.664, & 0.6938

0.6775, .7045,

0.65

0.62

Max. fill factor

with and

0.29, & 0.27

0.22,

(carotenoid)

Pt 0.22,

TiO2

6 & 7.90

1.1

0.21

Max. PCE (%)

ria japonica

Dye from Ker-

29

HSs/P25

I3-)

extract

P25 and TiO2-

electrolyte (I-/

heartwood

N719 dye

(I-/I3 liquid

Liquid electrolyte

Sappan

Caesalpinia

and Rhein)

rgonidin

28

27

couples

Leucopelar-

iodine as redox

Anhydrous lithium

mpferol,

Pt

Electrolytes used

fistula (Kae-

TiO2

used

Counter electrode

iodide and

Dye from

26

Electrode used

Cassia

Dye used

S. no

Table 6.1  Various materials used as essential parts of DSSCs with their photovoltaic performance. (Continued)

(Continued)

[31]

[30]

[29]

[28]

Reference

160  Fundamentals of Solar Cell Design

Chlorophyll

32

mixture

fruit, &

mulberry

extract from

anthocyanin

ate leaf,

pomegran-

extract from

Ruthenium dye

31

solvents

methanol

ethanol and

Pt

Pt

TiO2

TiO2

PEG, & PAN

6P, BE, PMMA,

& 0.722

0.597, 0.548,

& 0.53

0.555,

056,

1.89

2.05 &

& 0.70 0.72, &

0.196 0.52, 0.53, & 0.49

0.53, 0.58,

0.192, &

0.61

0.51, 0.53,

0.40, 0.37, 0.48,

0.132,

0.49, & 0.39

cond.)

cm−2)

Jsc (mA

0.42, 0.40, 0.44,

0.082, 0.085,

(Dry

ethylene glycol

ing Acetone,

0.568

and iodine with

TiO2 film

0.5, &

0.525,

crystalline

0.51, 0.29, & 0.44

extracted us-

carbon coating

longa L.)

annealed pure 0.16

Voc (V)

0.10, 0.12, &

Max. fill factor

Max. PCE (%)

sium iodide

Electrolytes used Mixture of Potas-

used

Counter electrode

nano-

Dye from

30

Electrode used

Curcuma

Dye used

S. no

Table 6.1  Various materials used as essential parts of DSSCs with their photovoltaic performance. (Continued)

(Continued)

[34]

[33]

[32]

Reference

Dye-Sensitized Solar Cells  161

Iodine (I-) and

treated TiO2

TiO2 & TiCl4

Betacyanin

36

pigments

TiO2

N719

henna

cobalt telluride

graphite

& Pt

couple

I-/I3- as a redox

0.612.

0.477 &

8.59 & 8.19

0.157

mallow, &

0.229,

0.134, 0.215, &

a redox couple

triiodide (I3-) as

strawberry,

spinach,

red cabbage,

Dye from beet,

lum

odontophyl-

Canarium

from

derivatives)

din (cyanidin

anthocyani-

TiO2

0.39, 0.28, 0.53,

0.662 & 0.639

0.50

0.37, 0.55, &

0.37,

0.784

0.785 &

0.47

0.6, &

0.465,

0.46,

0.461,

16.35

16.53,

0.66

0.69, &

1.33,

0.55,

0.76, 0.97,

22.91

gonidin) and

&

idin (pelar-

6.57,

3.81,

10.33,

3.54,

cm−2)

Jsc (mA

9.74,

0.109,0.13,

Voc (V)

anthocyan-

Pt

0.603

0.484, 0.546, &

0.593, 0.376,0.529,

Max. fill factor

(maritimein),

nanocrystalline

5.98

35

34

1.43, &

ture, aurone

0.68, 099,

Max. PCE (%)

acidified mix-

Pt

Electrolytes used 0.60, 0.87,

TiO2

Mixture of

33

used

Counter electrode

flavanoids,

Electrode used

Dye used

S. no

Table 6.1  Various materials used as essential parts of DSSCs with their photovoltaic performance. (Continued)

[38]

[37]

[36]

[35]

Reference

162  Fundamentals of Solar Cell Design

Dye-Sensitized Solar Cells  163



η=

Voc   ×   I sc   ×  FF Pin

(6.2)

where Pmax = maximum power generated Voc = Open-circuit voltage Isc = Short circuit current FF = Fill factor η = Power conversion efficiency Pin = Input solar power

6.9.2 Fill Factor The fill factor (FF) is a performance indicator for solar cell. This is calculated by the ratio of generated maximum power (Pm) to the product of Voc and the short circuit current (ISC).

6.9.3 Open-Circuit Voltage Voc is voltage under zero load in an electrical system. It is the difference of electrical potential between any of two terminals in absence of external load.

6.9.4 Short Circuit Current ISC is the current through the solar cell at zero voltage condition. For an ideal solar cell, ISC and the generated solar current are equal. Hence, the maximum current obtained from photo cell is ISC.

6.9.5 Determination of Energy Gap of Electrode Material Adsorbed With Natural Dye The energy of the incident photons from sunlight will be directly proportional to the corresponding frequencies of the incident photons. This, in turn, depicts the relationship between the frequencies and energy of the black body. Energy band gap of electrode is determined by using



E = h ×ν =

hC λ

(6.3)

164  Fundamentals of Solar Cell Design where h = Planck’s constant v = Light intensity C = Velocity of incident light λ = wavelength of incident light Sun is a perfect black body. A black body need not be physically black. A black body is one which emits the radiation at varying frequencies and possesses poor absorptivity.

6.9.6 Absorption Coefficient The absorption coefficient is the degree of incident light intensity which passes through intermittent layers of solar cell.



Absorption Coefficient =

4π k λ

(6.4)

where K = Boltzmann Constant

6.9.7 Dye Adsorption Dye adsorption is the measure of amount of dye adsorbed with the electrode surface.



Dye Adsorption =

Co − C   × 100% Co

(6.5)

where Co = Initial Concentration of extracted dye C = Concentration after certain period

6.10 Conclusion Among all other renewable green energies, solar energy is the major energy source with which the entire future will rely on [39]. As far the sun is present, the plants and other living organisms will survive. Hence, the dyes for

Dye-Sensitized Solar Cells  165 the DSSC were abundant, readily available at very cheaper price. Major drawback is long-term stability of DSSC. Thus, the DSSCs with various essential materials also with different working roles have been discussed. The continual research has undertaken by the researchers for the synthesis and fabrication of organic energy–based solar cells. Among which, the DSSC was most prominent one. In recent years, DSSCs have gained much more attention than newly engineered solar cells mainly due to their lesser weight, easier fabrication, very less production, and overall cost than other solar cells, recyclable upto certain extent, easier tunable optical properties by reinforcing foreign particles, acceptable transparency but deliver lesser PCE than silicon solar cells [40]. Also, these do not require any specific operating and fabricating conditions and can be fabricated easily even by layman. Thus, DSSCs are the simplest solar cell as compared to other generation solar cells. In cost-wise aspect, DSSC with higher PCE and longterm stability might be the best alternative for conventional solar cells in near future.

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166  Fundamentals of Solar Cell Design 9. Lai, W.H., et al., Commercial and natural dyes as photosensitizers for a waterbased dye-sensitized solar cell loaded with gold nanoparticles. J. Photochem. Photobiol. A Chem., 195(2-3): p. 307–313, 2008. 10. Kohn, S., et al., Commercially available teas as possible dyes for dye-­ sensitized solar cells. Optik, 185: p. 178–182, 2019. 11. Kang, H.-Y. and H.P. Wang, Cu@ C dispersed TiO2 for dye-sensitized solar cell photoanodes. Appl. Energy, 100: p. 144–147, 2012. 12. Kabir, F., et al., Development of dye-sensitized solar cell based on combination of natural dyes extracted from Malabar spinach and red spinach. Results Phys., 14: p. 102474, 2019. 13. Kimpa, M.I., et al., Photoelectric characterization of dye sensitized solar cells using natural dye from pawpaw leaf and flame tree flower as sensitizers. 2012. 14. Babkair, S.S., et al., Dye sensitized solar cells based on double-layered titanium dioxide and their evaluation in tropical hot desert climate of Saudi Arabia. Superlattices Microstruct., 133: p. 106206, 2019. 15. Maurya, I.C., P. Srivastava, and L. Bahadur, Dye-sensitized solar cell using extract from petals of male flowers Luffa cylindrica L. as a natural sensitizer. Opt. Mater., 52: p. 150–156, 2016. 16. Chang, H., et al., Dye-sensitized solar cell using natural dyes extracted from spinach and ipomoea. J. Alloys Compd., 495(2): p. 606–610, 2010. 17. Gómez-Ortíz, N., et al., Dye-sensitized solar cells with natural dyes extracted from achiote seeds. Sol. Energy Mater. Sol. Cells, 94(1): p. 40–44, 2010. 18. Wu, M., et al., Economical Pt-free catalysts for counter electrodes of dyesensitized solar cells. J. Am. Chem. Soc., 134(7): p. 3419–3428, 2012. 19. Zhang, J., et al., Electrochemically polymerized poly (3, 4-phenylenedioxythiophene) as efficient and transparent counter electrode for dye sensitized solar cells. Electrochim. Acta, 300: p. 482–488, 2019. 20. Hosseinnezhad, M., S. Rouhani, and K. Gharanjig, Extraction and application of natural pigments for fabrication of green dye-sensitized solar cells. Opto-Electron. Rev., 26(2): p. 165–171, 2018. 21. Gokilamani, N., et al., Dye-sensitized solar cells with natural dyes extracted from rose petals. J. Mater. Sci.: Mater. Electron., 24(9): p. 3394–3402, 2013. 22. Gokilamani, N., et al., Utilization of natural anthocyanin pigments as photosensitizers for dye-sensitized solar cells. J. Solgel Sci. Technol., 66(2): p. 212–219, 2013. 23. Gokilamani, N., et al., Grape pigment (malvidin-3-fructoside) as natural sensitizer for dye-sensitized solar cells. Mater. Renew. Sust. Energy, 3(3): p. 33, 2014. 24. Gokilamani, N., et al., Solanum nigrum and Eclipta alba leaf pigments for dye sensitized solar cell applications. J. Solgel Sci. Technol., 69(1): p. 17–20, 2014. 25. Gokilamani, N., et al., Basella alba rubra spinach pigment-sensitized TiO 2 thin film-based solar cells. Appl. Nanosci., 5(3): p. 297–303, 2015.

Dye-Sensitized Solar Cells  167 26. Kabir, F., et al., Improvement of efficiency of dye sensitized solar cells by optimizing the combination ratio of natural red and yellow dyes. Optik, 179: p. 252–258, 2019. 27. Kumara, N., et al., Layered co-sensitization for enhancement of conversion efficiency of natural dye sensitized solar cells. J. Alloys Compd., 581: p. 186– 191, 2013. 28. Maurya, I.C., et al., Natural dye extract from Cassia fistula and its application in dye-sensitized solar cell: Experimental and density functional theory studies. Opt. Mater., 90: p. 273–280, 2019. 29. Ananth, S., et al., Performance of Caesalpinia sappan heartwood extract as photo sensitizer for dye sensitized solar cells. Spectrochim. Acta A Mol. Biomol. Spectrosc., 137: p. 345–350, 2015. 30. Ye, X.-Y., et al., Non-aqueous preparation of anatase TiO2 hollow microspheres for efficient dye-sensitized solar cells. Adv. Powder Technol., 30(10): p. 2408–2415, 2019. 31. Hemalatha, K., et al., Performance of Kerria japonica and Rosa chinensis flower dyes as sensitizers for dye-sensitized solar cells. Spectrochim. Acta A Mol. Biomol. Spectrosc., 96: p. 305–309, 2012. 32. Ruhane, T., et al., Photo current enhancement of natural dye sensitized solar cell by optimizing dye extraction and its loading period. Optik, 149: p. 174– 183, 2017. 33. Kim, M.-R., et al., Photovoltaic properties and preparations of dye-sensitized solar cells using solid-state polymer electrolytes. Mol. Cryst. Liq. Cryst., 444(1): p. 233–239, 2006. 34. Chang, H. and Y.-J. Lo, Pomegranate leaves and mulberry fruit as natural sensitizers for dye-sensitized solar cells. Sol. Energy, 84(10): p. 1833–1837, 2010. 35. Lim, A., et al., Potential natural sensitizers extracted from the skin of Canarium odontophyllum fruits for dye-sensitized solar cells. Spectrochim. Acta A Mol. Biomol. Spectrosc., 138: p. 596–602, 2015. 36. Torchani, A., et al., Sensitized solar cells based on natural dyes. Curr. Appl. Phys., 15(3): p. 307–312, 2015. 37. Dong, J., et al., Spin-coated cobalt telluride counter electrodes for highly efficient dye-sensitized solar cells. Mater. Res. Bull., 115: p. 65–69, 2019. 38. Kabir, F. and S.N. Sakib, Various impacts of blocking layer on the cell stability in natural dye based dye-synthesized solar cell. Optik, 180: p. 684–690, 2019. 39. Narayan, M.R., Dye sensitized solar cells based on natural photosensitizers. Renew. Sust. Energ. Rev., 16(1): p. 208–215, 2012. 40. Andualem, A. and S. Demiss, Review on dye-sensitized solar cells (DSSCs). Edelweiss Appli. Sci. Tech., 2: p. 145–150, 2018.

7 Characterization and Theoretical Modeling of Solar Cells Masoud Darvish Ganji1*, Mahyar Rezvani2 and Sepideh Tanreh3 Department of Chemistry, Qaemshahr Branch, Islamic Azad University, Qaemshahr, Iran 2 Young Researchers and Elite Club, Central Tehran Branch, Islamic Azad University, Tehran, Iran 3 Department of Nanochemistry, Faculty of Pharmaceutical Chemistry, Pharmaceutical Sciences Branch, Islamic Azad University (IAUPS), Tehran, Iran 1

Abstract

Solar cell (SC) has been a favoring answer to the need for direct conversion of solar energy to electrical one. Thanks to their inexpensive manufacturing cost, simple production process, and controllable optical characteristics, the dye-­ sensitized SCs (DSSC) have been proposed as a replacement to the conventional SCs. Recently, the advances in the nanoscience and nanotechnology, particularly its applications in the energy treatment, have introduced the possibility of going beyond our present options by developing technologies that are more effective, environmentally comprehensive, and cost effective. On the other hand, we are in a sensational era where computers are becoming more powerful and widely available. This has contributed to the development of powerful quantum methods that are aiding chemists in their pursuit to produce answers to the energy challenge. In this chapter, presenting the latest developments in SC efficiency, we focus on the computational power limitations and the challenges to address for achieving practical nanostructured DSSCs. The final objective is to achieve improved SCs by assessing molecular and electronic structures, polarizability, hyperpolarizability, atomic charges, absorption spectra, molecular descriptor parameters, etc., by using a variety of quantum chemical calculations including the density functional theory (DFT) and time-dependent DFT.

*Corresponding author: [email protected] Inamuddin, Mohd Imran Ahamed, Rajender Boddula, and Mashallah Rezakazemi (eds.) Fundamentals of Solar Cell Design, (169–216) © 2021 Scrivener Publishing LLC

169

170  Fundamentals of Solar Cell Design Keywords:  Solar cells, quantum chemistry, DFT, TD-DFT, modeling, molecular properties

7.1 Introduction According to Smalley, a Nobel laureate, for the coming 50 years, environmental problems and energy-related issues will define the most significant concerns to challenge with [1]. The ever-increasing demand for energy in its simplest form, i.e., fossil fuels, and too low prices of oil have ended up depleting oil reserves around the world [2]. On the other hand, the extent to which the environment is being contaminated by and the great amount of greenhouse gases generated upon combustion of the fuels are two key concerns raised by the present environmental policies [3]. These have led scientific community toward renewable sources of energy to protect the global environment and living organisms. A widely popular approach to the generation of energy from renewable sources is represented by solar energy systems by which sunlight is directly converted to electrical energy [4, 5]. Thanks to its abundance and continuous energy supply, the sun is known as the renewable energy source of choice. Many scholars have reported on the application of solar energy in every-day life because of not only its cleanness, but also apparent properties, e.g., color and transparency [6–9]. Among the wide spectrum of solar energy conversion devices developed so far, silicon-based devices have exhibited the highest conversion efficiency. The high efficiency and technological superiority of the silicon-based solar devices have set the stage for the devices to dominate over a major portion of the solar system market [10]. In the meantime, complex manufacturing process of the silicon-based solar devices has added to their final price significantly, thereby limiting their applications [11]. Fortunately, developed recently, thin-film organic solar cells (OSCs) provide such great features as flexibility, lightweight, and inexpensiveness, as compared to conventional silicon-based SCs [12]. As a variant of the thin-film SCs, DSSCs are fed by visible light as input and generate electricity through a sensitization process. Operating while causing no environmental problem, these systems have an organic dye used as sensitizer. The DSSC has been offered as a good replacement for the conventional SCs thanks to its benefits in terms of production cost, efficiency, and ease of production process. The features of DSSCs set the stage for increased research interest in this technology, as reflected in the increased number of related contributions in the past decade.

Theoretical Modeling of SCs  171 In a DSSC, the sensitizer absorbs incident light and induces an electron transfer to a wide bandgap (WBG) semiconductor. TiO2 is commonly adopted as the WBG semiconductor in the DSSC. In addition, photo-­ oxidized dye molecules are regenerated by using an electrolyte. A DSSC is composed of two main components, namely, a working electrode which plays as a sensitizer and a counter electrode such as platinum deposited conducting glass. The two components are arranged into a sandwich structure with an iodide/triiodide (I−/I3−) redox electrolyte. In this system, upon absorbing a photon, the immobilized sensitizer elevates to an excited state, thereby transmitting the electron to the conduction band of TiO2 surface. Next, upon an electron donation stage by the I−/I3− redox system, the reduction of oxidized dye was happen. The donated electron arrives at the back contact, by going across the semiconductor network, before being dispatched to the counter electrode through the exterior load. Triiodide reduction at the counter electrode contributes to iodide regeneration, thereby closing the circuit. Being capable of achieving conversion efficiencies beyond 10%, this simple system has gained a great deal of attention from several research groups [13–18]. The highest conversion efficiency reached by a DSSC (~12%) has been that of an advanced DSSC equipped with a ruthenium sensitizer [19–21]. It was known that Ruthenium complexes absorb light well in the visible (Vis) and near-ultraviolet (UV) area and exhibit large oxidation potentials in the excited state that way inject electron into conduction band of the TiO2 [22]. Nevertheless, limited availability and toxicity of ruthenium, as a heavy metal, have limited its application in photovoltaic (PV) devices. Therefore, inexpensive yet high-performance organic dyes or metal complexes are highly needed. Much efforts have been reported on the formulation of novel effective sensitizers for SCs. In this respect, a great deal of emphasis has been placed on organic sensitizers with either no metal ingredient or inexpensive metal complexes because of such advantages as low cost, easy synthesis, capability for modification and structural modulation, superior basic characteristics like Frontier Molecular Orbital (FMO) and absorption profiles (e.g., large molar absorption coefficients), and acceptable stability [23–26]. Theoretical and experimental studies have examined various organic dyes as sensitizer in DSSC; examples include coumarin [27], indoline [28], oligoene [29], thiophene [30], triarylamine [31], perylene [32], cyanine [33], hemicyanine [34], porphyrine derivations [35–37], and metalloporphyrine [38]. Shedding further light to electronic and structural characteristics of solar systems, computational chemistry and computer-assisted simulations

172  Fundamentals of Solar Cell Design can greatly contribute to the development of SCs by offering an inexpensive approach to the research and development for enhanced PV materials. This chapter begins with discussions on three computational methods, namely, (I) quantum mechanics (QM), molecular simulations such as (II) molecular dynamics (MD), and (III) Kinetic Monte Carlo (KMC) methods. Subsequently, the methodologies are combined to come with a stepwise process for examining thin-film phases in terms of morphology at molecular scale, with a discussion presented on impacts of such combination on relevant parameters. More specifically, the focus was on electron donor-acceptor systems because of their significant contributions to the efficacy of SCs. The present research can be utilized as a preliminary guide for undertaking simulations to study the molecular packing.

7.2 Classification of SC Generally, SCs have been classified under three classes, namely, organic, inorganic, and hybrid organic-inorganic. Figure 7.1 presents a detailed general classification of SCs.

Solar Cells

Non-Organic Solar cells Silicon based Solar cells

Organic Solar cells

Dye Sensitized Solar cells

Mono and Muliticrystaline Si Thin Film (a-Si)

Plastic Solar cells Monolayer Solar cells Bilalayer Solar cells Inverted Solar cells

Non-Silicon based Solar cells Tandem Solar cells Thin Film (CdTe, CIS, CIGS) Bulk Hetrojunction solar cells

Figure 7.1  Classification of solar cells.

Hybrid Solar cells

Perovskite Solar cells

Theoretical Modeling of SCs  173

7.2.1 Inorganic Solar Cells Inorganic or so-called classic SCs are made from pure silicon and generally categorized into silicon-based and non-silicon–based variants [39]. Being either mono- or poly-crystalline, the silicon-based SCs are significantly more popular because of their higher efficiency and wider market availability. Mono-crystalline silicon-based SCs have been acknowledged as the first-generation SCs. Despite their marginally higher calculated efficiency, mono-crystalline cells exhibit the same level of practical performance as that produced by poly-crystalline cells. Examples of real application of these cells include satellites, streetlights, space shuttles, military equipment, and industrial applications [40]. In other words, crystalline silicon cells are generally more durable than the non-silicon–based SCs [41]. The main challenge with a mono-crystalline SC is its high production cost in terms of energy demand for synthesizing defect-free single crystals [42]. Known as second generation SCs, thin-film SCs come with lighter weight and higher cost effectiveness, although still underperform conventional single-crystalline SCs [43, 44]. Such cells are generally fabricated by ­single- or multiple-layer deposition of thin films of silicon on a transparent, conductive support (e.g., steel, glass, or plastic). In the case of amorphous silicon, SCs have been widely applied in digital calculators and outdoor lights, with their usage in larger applications following an increasing trend, even though their efficiency is still inferior to single SCs. Examples of actually marketed amorphous SCs include cadmium telluride (CdTe), copper indium gallium diselenide (CIGS), and silicon-based thin-film SCs [46]. Numerous scholars have tried to enhance efficiency of CdTe and CIGS, ending up with efficiencies as high as about 21%, so as to outperform currently dominant cells in PV panels, i.e., silicon-based SCs [45].

7.2.2 Organic Solar Cell Generally referred to as third generation PV cells, OSCs have been broadly classified into different classes, including the DSSCs, plastic-based and polymer-based OSCs. The DSSC was first introduced in 1972 when chlorophyll was used to photo-sensitize incident light, thereby directly converting the light into a flow of electrons which were then transferred to a zinc oxide (ZnO) semi-conductive electrode [46]. The early chlorophyll and ZnO-based ­single-crystalline device ended up exhibiting efficiencies below 1%. The poor efficiency was attributed to inefficient light absorption on the relatively

174  Fundamentals of Solar Cell Design smooth chlorophyll surface. Later on, the PCE was enhanced remarkably upon the development of semi-conductive WBG TiOx electrodes which provided greater roughness. By 1991, efficiency of then-introduced DSSC was enhanced to 7% [47]. Finally, by 2015, making multiple modifications to dye and electrodes of the DSSC system improved its performance to 14.7% [48]. Depending on the production process through which the final cell is fabricated, SCs based on polymer as well as plastic have been categorized as follows: i. Monolayer SCs ii. Bilayer SCs iii. BHJ SCs iv. Inverted SCs v. Tandem SCs A pure OSC can provide power a conversion efficacy (PCE) as high as about 17.4%, but in commercial applications, PCEs with efficiencies above 3% are yet to be achieved. Given higher PCE of inorganic SCs, many researchers have attempted to combine inorganic and OSCs into so-called hybrid SCs to achieve enhanced power conversion efficiency [49]. Being still under development stage, hybrid SCs can be further enhanced in terms of PCE by improving their design and combinations of the organic and inorganic constituents. Perovskite SCs are among the best candidates to replace the inorganic SCs [50]; this is because of higher potential for generating PCEs comparable to conventional inorganic SCs. Composed of calcium titanium oxide crystals, the Perovskite was first observed in Ural Mountains. The new Perovskite group materials have exhibited a wide spectrum of applications in thermoelectric, piezoelectric, semiconducting, and superconducting materials [51]. Despite its environmental impacts, lead has been increasingly respected in recent research works on the lead-Perovskite SCs because of its high efficiency. The fact that computational approaches can be used to treat various materials similarly and the presence of systematic relationships between the materials and their PV performance properties have turned the computational methods into powerful tools for modeling molecular systems and materials at high accuracy. In this respect, the DFT and TDDFT have imposed large contributions into understanding the factors affecting the SC efficiency and the interaction between different cell components for definition of the resulted device properties.

Theoretical Modeling of SCs  175 In this chapter, we place a particular emphasis on the DSSC technology owing to it its high efficiency coupled with easy and low-cost production process, which together make it a good candidate for commercialization. Accordingly, in the following sections, we limit the scope of our theoretical discussion to the DSSC.

7.3 Working Principle of DSSC Figure 7.2 represents a DSSC and its operation mechanism. The DSSC works based on a charge separation process which goes through the following stages [52]. (i) Firstly, solar energy is absorbed by a photo-anode. This ends up exciting electrons from ground state to the excited one in the dye. (ii) Thanks to the energy level difference between the ground and excited states, electrons from the latter state are introduced to the semiconductor conduction band, upon which the dye turns oxidized. (iii) Being in touch with the dye, the electrolyte releases electrons into the dye in attempt to restore it to the ground state. Subsequently, it moves, through a diffusion process, toward the cathode hosting the reduction process, where Conducting glass TiO2 Injection

E vs NHE (V)

-0.5

Dye

Electrolyte

S* Maximum voltage

0

hv 0.5 1.0

Cathode

Red

Ox Mediator Interception Diffusion S°⁄ S+

Figure 7.2  Configuration of a DSSC. Here, the S°, S*, and S+ denote, respectively, the ground, excited, and oxidized states, for the sensitizer, while the Red/Ox denotes the redox mediator [56].

176  Fundamentals of Solar Cell Design initial state of the electrolyte is restored upon receiving electrons from the external circuit. (iv) The cycle is completed by not only the mentioned forward charge transfer processes, but also backward ones which decrease DSSC efficiency dramatically. These processes go through the following stages: a.  Electrons originated from the semiconductor move toward the oxidized dye. b. The introduced electrons into the semiconductor are recombined into the electrolyte (dark current). c. The excited-dye electrons are transferred to the dye in the ground state. The following measures can be taken to attenuate adverse effects of the s-called “backward transfer process”: (i) Maintaining a quantum yield for transferring charge to semiconductor [53]. (ii) Keeping the photosensitizer’s LUMO and HOMO further negative and positive than the semiconductor’s conduction band and the electrolyte’s redox potential, respectively [54]. (iii) Keeping the rate at which electrons are introduced into the semiconductor higher than the rate of excited-dye electron decay to the ground state [55].

7.4 Operation Principle of DSSC The efficiency of early DSSCs was too low, primarily because of too light absorption capacity of the then-used mono-layer dye (i.e., 1% of the incident light). Introduced in 1991, TiO2 electrodes of nano porosity offered roughness factors of an order of 1,000, thereby significantly increasing the light absorption efficiency, so that SCs of efficiencies as high as 7.1%–7.9% were developed [57]. This development served as a key driver for further research on SCs, leading to the emergence of advanced SCs with efficiencies as high of 11.2% [58]. Figure 7.2 shows the mechanism of operation for a DSSC, which can be decomposed into the following stages [59]: (i) An electron is excited via a particular cycle.

Theoretical Modeling of SCs  177 (ii) The electron is injected into TiO2 followed by iodine reduction at the opposite electrode, finally ending up with the electron going through TiO2 to the electrode. (iii) The electron is subjected to external work. (iv) The electrolyte hosts a diffusion process. (v) The oxidized dye undergoes a regeneration process. Lately, researchers have largely regarded the TiO2 because of its advantages for energy generation and environmental protection applications [60]. It has been widely used in DSSCs thanks to its mesoporous nano-­ crystalline structure which can provide large surface areas for absorbing dye molecules. Subsequently, one may excite the absorbed dye molecules by means of solar radiation to come with electron-hole pairs and proceed to have the dye molecules removed and transferred across the lattice of TiO2 [61]. For such a cell, the efficiency is controlled mainly by the dye’s absorption spectrum and anchorage to TiO2 surface [62]. Given the significant role played by the dye in absorbing the visible light and transforming the photon’s energy to electricity, great deals of attempt have been made to find effective sensitizer dyes [63]. In this respect, researchers have manufactured various sensitizers based on metal complexes and organic dyes; examples of these include porphyrins [64] and platinum complex [65]. Thanks to their higher efficiency and longer durability, Ru-based complexes have been widely applied as sensitizers.

7.5 Photovoltaic Parameters A DSSC has its PV parameters, including η (power conversion efficiency), Jsc/Voc close/open-current density, interface charge resistance, FF (fill factor), and IPCE (incident photon-to-current efficiency) controlled by the morphology of the adopted semiconductor, the spectroscopic characteristics of the dye, the electrical performance of the electrolyte, and the count of protons on the anchoring groups of the sensitizer. The parameter η can be determined by the following equation:



η=

J sc Voc  FF Pin

(7.1)

By definition, the FF is calculated by dividing the maximum DSSC power (Pmax) by the product of Voc and Jsc:

178  Fundamentals of Solar Cell Design



FF =

Pmax J sc Voc

(7.2)

The FF is a measure of the electrical and electrochemical energies that are wasted as the SC operates. Accordingly, a higher FF is favorable from the efficiency point of view as the SC output power is closer to the theoretical peak value. The Jsc denotes the peak current density that a device can produce at a voltage of zero. This value has contributions to the optical as well as electronic properties of the respective dye. One can obtain the Jsc value of a SCs at the standard condition sunlight by taking integral of the IPCE spectrum. With the light absorption spectrum taking a major part in the value of Jsc, the Jsc can be evaluated as follows:

Jsc = ∫ IPCE (λ). e. Φph, AM1.5G(λ)dλ

(7.3)

In which e denotes the primary charge while “φph, AM1.5G” refers to the AM-1.5G photon flux (here is equal to 100 mW/cm). By definition, the IPCE is the external-circuit photocurrent density under monochromatic cell illumination divided by the proton flux at which the SC is stroke. That is, the IPCE can be written as below:

Jsc = ∫ LHE (λ). Φinj(λ). Φreg. ηcc(λ)

(7.4)

In Equation (7.5), the LHE(λ) refers to the efficiency in light collecting while φinj and φreg are respectively, the quantum yields of the excited-dye electron injection and dye regeneration, and ηcc refers to the electron collection efficiency. Accordingly, the following equation gives the LHE:



LHE = 1 − 10−f = 1 – 10−A

(7.5)

DSSC performance evaluation has been generally based on the FF, energy conversion efficiency (η), Voc, and short-circuit current density (Jsc). Researchers have examined the roles of various parts (see Table 7.1), ending up suggesting different useful dyes to be used as photo-sensitizers for DSSCs [66–74]. Achieving conversion efficiencies as high as 7.1%, DSSCs based on rhenium were introduced in 1991 [75]. Later on, Nazeeruddin et al. [76] adopted a Ru-based dye (N3 dye) as sensitizer (N3-cis-di (thiocyanato) bis

Theoretical Modeling of SCs  179 Table 7.1  A list of different Ru-based dye sensitizers with their photoelectrochemical characteristics. Dye

Jsc (mA/cm2)

Voc (mV)

FF

η (%)

Ref.

N3

15.62

740

0.717

8.23

[88]

RC-62

17.78

707

0.728

9.10

N719

15.60

673

0.694

7.29

[89]

Black dye

23.49

683

0.721

11.57

[90]

Z907

14.6

720

0.693

7.30

[91]

K8

18

640

0.750

8.64

[92]

SJW-E1

21.6

660

0.620

9.02

[93]

CYC-B11

18.3

704

0.730

9.40

[94]

K73

17.22

740

0.694

9.00

[95]

C343

14

60

0.71

6.00

[96]

HRS-1

20.05

680

0.690

9.50

[97]

A597

11.83

778

0.780

7.25

[98]

(2, 2-bipyridine-4, 4-dicarboxylate) ruthenium) for a DSSC and ended up with a conversion efficiency of 10.3% in 1993. In 2001 and then in 2006, Nazeeruddin et al. extended their research on DSSC and studied “black dye”, a then-novel Ru-based dye [77] and N719 dye obtained conversion yields of 10.4% and 11.2%, respectively [78]. As another class of Ru-based dyes, polynuclear bipyridyl Ru dyes are designed to bear a different count of metal centers, as compared to the former two classes [79]. The dye is believed to exhibit such light absorption properties because of not only transition metal-to-ligand charge transfer (MLCT), but also HOMO and LUMO states. Investigations have shown that one can efficiently enhance the dye structure by modifying the acid groups protonation, so as to produce N719 dye. Being similar to N3 in structure, N719 dye has TBA+ (tetrabutyl ammonium) rather than H+ on the pair of carboxyl groups; as of present, the N719 is the most promising dye for DSSC [80, 81]. According to common practice, the lower the absorption coefficient, the thicker should be the metal semiconductor oxide electrodes before

180  Fundamentals of Solar Cell Design sufficient dye molecules can be absorbed [80, 82]. From an electron transfer point of view, such an increase in metal semiconductor oxide electrode thickness may end up decreasing the Jsc and Voc [79]. The metal-free dyes are superior over Ru-based complexes thanks to their enhanced value of molar extinction coefficient, conveniently modifiable absorption energy, and lower cost than Ru, non-toxicity, simple synthesis process, and stability at high temperature and/or under prolonged illumination [83]. Depending on the molecular structure, metal-free dyes have been classified into three main classes: donors, linkers and acceptors. Also known as D-π-A sensitizer, a linker is usually a π-conjugated system that bridges an donor (D) group to an receptor (A) one [84, 85]. Structural modifications to these dyes contribute to enhanced conversion efficiency [86]. Such modifications may be in the form of boosting the π-conjugation of the linkers and/or strengthening electron donation/reception capacity of the donor and receptor groups, respectively. Porphyin dyes or simply porphyins are composed of four modified pyrrole subunits bridged by CH groups (=CH-). Chemical structure of a porphyin dye complex indicates an extended π-conjugated system of high extinction coefficient where the incident lights in the visible region is absorbed intensively [80, 87]. Among others, porphyins are known to experience long excited state life (>1 ns), high electron injection rate (fs range), electron recombination in milliseconds and tunable redox potentials [87]. Table 7.1 reports details of the performance of different dyes. One of the main parameters affecting the PV character of a cell is the used substrate. Application of the TiO2 nanowires into DSSCs has been well respected. In this regard, an innovative hydrothermal growth-based approach has been proposed for manufacturing TiO2 nanowire arrays based on FTO glass substrates [99]. When incorporated into a standard DSSC, such TiO2 nanowires provided significantly higher conversion efficiencies (~5%) than those achieved with ZnO nanowires (1.2%–1.5%) of up to 40 µm in length [99]. Anatase TiO2 spheres exhibit an overall efficiency (η) of 3.5% (significantly lower than those of nanoparticle (7.37%), nanofiber (8.15%), and ellipsoid TiO2 spheres-based (7.93%) DSSCs) at the current density, open circuit voltage and FF values of 17.94%, 803 mV, and 0.65%, respectively [100]. The enhanced η was reportedly obtained by improved dye loading, light scattering capability, charge transfer rate, and electron lifetime [101, 102]. TiO2 can also be used to achieve the lowermost charge recombination through enhancing the resistance to charge transfer resistance at the TCO/electrolyte boundary [103, 104].

Theoretical Modeling of SCs  181 Table 7.2  Calculated efficacy of cells with various counter electrodes [109]. Counter electrode

Jsc (mA/cm2)

Voc (mV)

FF

η (%)

Pt

15.3

670

0.70

7.2

Pt-loaded MWCNT

17.2

690

0.71

8.6

MWCNT

15.1

655

0.68

6.7

Significantly affecting the PV property of a SC, the counter electrode can be promisingly made from platinum (Pt) to reduce the I3−, thanks to the transparency, good catalytic activity, and high exchange current density of the Pt. It is worth noting that, Pt is a naturally rare and precious metal that is prone to erosion once reacted with I3− content of the electrolyte, upon which reaction PtI4 is formed [104]. Therefore, researchers have searched for alternatives to the Pt, including carbon, graphene and conductive polymers [106]. Among others, carbon has been regarded the most thanks to its low price and good conductivity, corrosion resistance, heat resistance, and electro-catalytic activity toward the reduction of I3− [107]. Josef et al. used sheets of dry-spun multi-walled carbon nanotubes (MWNT) with graphene flakes (Gr-F) instead of the Pt catalyst and could hence improve the catalytic performance and power conversion yield (7.55%), as compared to the MWNT (6.62%) or graphene alone (4.65%) [108]. Most recently, Arman et al. achieved the most efficient PV cell to date (e.g., the power conversion value of 8.6%) [109]. Performance details of various counter electrodes are listed in Table 7.2.

7.6 Theoretical and Computational Methods The system complexity acts as source for most of challenges dealt when trying to understand nanostructured materials theoretically and computationally. Despite the fact that accurate computational methods are required to address the complexity of inter-atomic interactions occurring in a nanosystem, most of computational methods lose their accuracy when applied on such nano-sized systems. The case is even more challenging when it comes to the systems made up of hundreds or thousands of atoms with numerous relevant geometrical arrangements, even with computational methods which tend to establish a balance between computational cost

182  Fundamentals of Solar Cell Design and accuracy. The excited states’ properties can further add to the complexity of such a problem; in this respect, precise prediction of such properties for small to medium molecules via computational approaches is yet to be well addressed in the theoretical chemistry. During the past 20 years, the computational chemistry has grown into a very powerful tool for high-accuracy modeling of molecular systems and materials with the purpose of adjusting the balance between the device efficiency and its chemical structure. More specifically, the DFT/TDDFT has been widely employed to explore electronic absorption properties of isolated dye molecules or those adsorbed on semiconductor surfaces, so as to distinguish between different types of dye based on the absorption spectrum and favorable electron transfer from the photo-excited dye to the semiconductor. The DFT is the most extensively applied computational technique in DSSC research. It has been employed to analyze numerous dye and electrolyte structures, so as to keep synthetic efforts guided and focused properly. However, application of DFT is yet to go beyond the study of individual DSSC components. DSSCs may not be further improved unless the effects of concurrently changing main cell components are more thoroughly understood. As of now, the only way via which such comprehensive computational studies can be undertaken is to employ parameterized analytical force fields-based classical MD simulations. Usefulness of the force fieldbased approaches is more evident when it comes to the investigation of the influence of dye coverage on dye packing, semiconductor substrate, dye-dye interactions, and electrolyte accessibility. For instance, MD simulations have been used to model atomic force microscopy (AFM) images of Z907-sensitized TiO2 surfaces as a function of coverage [110]. To accomplish further information about electronic properties of dyes, the DFT has been utilized to optimize ground-state geometries of the complexes. Furthermore, the TDDFT method has been adopted to evaluate UV-Vis absorption spectra of different complexes, which had been then employed to investigate the UV-Vis absorption-related transitions in the complexes. The TDDFT results have been generally indicative of strong absorption in the visible range. One can use such information to design photosensitizers in DSSCs.

7.6.1 Density Functional Theory (DFT) DFT-based ground-state theoretical tools have thoroughly evolved theoretical and computational chemistry and material science. Back in 1964, Hohenberg and Kohn proposed their so-called Hohenberg-Kohn

Theoretical Modeling of SCs  183 theorems; according to these theorems, the sole single-particle electron density is capable of describing any and all properties of a physical system involving interacting electrons. In DFT, electron interactions are expressed by exchange-correlation (XC) functional, encompassing all the exchange and correlation interactions among the electrons. Accordingly, one ends up with the Kohn-Sham (KS) equations which are single-electronic and give, as solution, a group of molecular orbitals to be used to build the electron density. In practice, however, XC functional is approximated, with the quality of the approximation and the basis set employed to expand the density determining the accuracy of the calculations. As of current, even non-specialists are able to easily adopt the DFT for medium-sized systems because of high accuracy of the current XC functionals [hybrid ones in particular, e.g., renowned B3LYP) [111] which includes a certain amount of Hartree-Fock (HF) exchange] and their reasonable computational cost along with increased processing power of computers and the accessibility of quantum chemistry methods (ADF [112], Gaussian [113], Dalton [114], and Orca [115, 116]) or solid-state physics (Quantum Espresso [117], VASP [118], Siesta [119–122], DFTB+ [123], and Openmx [124, 125]).

7.6.2 Basis Sets As mentioned previously, electron density is made up of KS orbitals which are then expanded on a finite basis set to come with a practical solution for the KS equations. Generally, there are two groups of basis sets: (i) localized basis functions, centered on the nuclei, and, (ii) delocalized functions, typically plane waves (PWs), confined within a given volume. The two groups of basis sets have been addressed using the computer codes based on solid-state physics and quantum chemistry, respectively. A localized basis set refers to a set of choices for a small to medium-sized molecular system with a highly heterogeneous electron density commonly centered around the nuclei. At the other end of the spectrum, being inherently delocalized and periodic, the PWs can well address the electron density of solids and periodic systems and are generally limited to non-hybrid XC functionals. Although hybrid functionals can be efficiently applied in localized basis functions, these basis sets are known to be associated with heavier computational costs.

7.6.3 TDDFT Method As an extension to DFT, the TDDFT is mainly aimed at studying molecular properties related to such time-dependent domains as electric or magnetic

184  Fundamentals of Solar Cell Design potential fields. Being a ground-state theory for analysis at equilibrium, the DFT is inadequate for studying such properties as excitation energies or photo-absorption spectra. In this case, the main advantage of DFT, i.e., application of the density variable, which is generally simpler than the wave function, was focused to develop the TDDFT. Generally speaking, the TDDFT is achieved by reformulating the time-dependent QM and taking the density, rather than wave function, as the principle variable [126]. The TDDFT has been widely utilized to compute excited-state properties and optical spectra using the linear response theory [126–131]. But before that, the time-dependent potential shall be so weak to avoid complete destruction of the ground-state structure of the system. Such conditions are satisfied with weakly perturbed molecular arrangements under the effect of rather weak long-wavelength optical fields (e.g., the UV-Vis range) [131].

7.6.4 Molecular Descriptors In order to determine active materials, the most basic properties to consider are those related to the material electronic structure. Such properties are usually obtained using QM analyses. The QM results are known to include wave functions of the related molecular orbitals (e.g., HOMO and LUMO) along with values of electron affinity (EA), excited-state energy, and ionization potential (IP). The significance of these characteristics relies on the fact that, as demonstrated in the literature, the donor IP (also known as donor “HOMO energy”)–acceptor EA (“LUMO energy”) energy gap is related to the VOC of the cell [132]. The charge-transfer (CT) state energy exhibited similar correlation but at higher precision [133]; the energy refers to an excited level at donor-acceptor Interface where an electron is admitted by the acceptor molecule while the adjacent donor molecule has a hole [134]. Energy level of the ultimate charge-separated (CS) state can be approximated as the donor IP-acceptor EA gap, provided the donor hosts the hole while the acceptor has the electron residing on that, with the two domains being not Coulombically bound [134]. Moreover, since a SC’s efficiency is tightly linked to the flow of energy through the materials of which the device is made up (the electrodes, the pristine hole and electron transport phases, and the mixed donor-acceptor medium), it will be of great help to acquire a molecular insight of the whole energetic approach. In general, QM methods are categorized under three categories, including (1) highlevel (correlated) wave function–based methods, (2) DFT, and (3) semiempirical methods, named in the descending order of anticipated accuracy. The first class of QM methods encompasses post-HF methodologies [e.g., order-n Moller-Plesset theory (MPn) and configuration interaction (CI)];

Theoretical Modeling of SCs  185 such approaches typically exhibit high accuracy as those account for exchanges exactly and evaluate electron correlations in an extensive manner. DFT accounts for exchange and correlation effects by approximating those as a function of electron density; when it comes to XC effects, accuracy of modern DFT functionals is of the same order as that of the highlevel wave function methods, but at significantly lower computational cost. Significantly simplifying the approach via which electron-electron interactions are evaluated, semi-empirical methods are even more favorable in terms of computational cost; these methods account for the correlation effects using empirical parameters and tend to recover some level of accuracy. As far as computational requirements are concerned, the abovementioned classes of QM methods exhibit an ascending order of processing speed. Combining computational accuracy and cost optimally, DFT is by far the QM method of choice for investigating electronic structural characteristics of a PCS composed of some hundreds of atoms. Despite its popularity, B3LYP functional is associated with critical drawbacks that inhibit its application for describing such PCSs; this is particularly the case when it comes to OSCs. Such B3LYP functional density approach are known to over-delocalize the electron density, which ends up over-stabilizing planar conformations and thus overestimating torsion potential barriers in an EPCS [135–138]. Caused by electronic self-interactions, this delocalization error (DE) limits the application of B3LYP (and relevant functionals) for a CT state evaluation. Based on the above reasoning, long-range corrected (LC) functionals (e.g., ωB97XD15) are strongly recommended for investigating the systems with lengthened π-conjugation directions. Application of the LC functional tend to reduced DE by, as an example, “IP-tuning” the range-­ separation parameter ω in such a way to match the HOMO energy value the system to the corresponding IP: an exact functional-matched property [139]. In a similar approach, one may also optimize the ω-value by “gap-tuning”, so as to further parameterize the LUMO energy to the EA. As the need for analyzing more systems emerged, semi-empirical methods began to represent the method of choice, by which the task could be rendered feasible computationally. The group of techniques for which the intermediate neglect of differential overlap (INDO) method and relevant terms (i.e., Zerner’s intermediate INDO, ZINDO, ZINDO/spectra, ZINDO/S) are adopted is known to be of use for π-conjugated systems. In particular, as compared to DFT-derived methods, these methods work properly when the focus is on the electronic couplings between neighboring molecules [140–142], and evidence has confirmed their capabilities for

186  Fundamentals of Solar Cell Design computing the so-called charge recombination and exciton dissociation rates for the prototype systems of donor-acceptor scheme [143, 144]. Irrespective of the chosen technique, a molecular system cannot be evaluated through the QM unless the coordinates of its molecular system is well known. The coordinates can be manually produced using either default or specific bond angles, lengths as well as dihedrals or the same values measured upon experimentally investigating the crystal structure, if possible. Upon optimizing the system geometry, the system geometry at equilibrium can be estimated. The optimized geometry is employed to evaluate the molecular system in terms of electronic structure. Continuing with this chapter, explanations are given on the application of the QM results for an in vacuo single molecule for parameterizing the force field as a prerequisite for MD studies. Focusing specifically on a donor-acceptor molecular complex, one should characterize the CT states that give rise to the development of disparate holes and electrons in OSCs [145]. By enhancing the charge carrier mobility to improve the FF, one can further lower the VOC by making carriers encounter one another. Despite accurate results of high-level theoretical approaches (see, for example, coupled-cluster (CC) and/or CI for CT state energies), such methods come with dramatically higher computational cost as the system size increases (notice that, the fullerene derivative which serves as the most common electron acceptor is comprised of more than 60 carbon atoms). Based on related literature, CT state energies have been well approximated also using some other methods [146–149]. Among others, the TDDFT techniques that are basis on LC functionals have been preferred given good performance of those in the analysis of CT-state properties; majority of quantum chemical software packages offer such techniques as a standard feature [137]. Subsequently, extracted from the excited states, natural transition orbitals (NTOs) can be employed to characterize the nature of the CT states [150]. Typically speaking, a CT state has the hole-NTO and electron-NTO residing on the donor and acceptor, respectively [151]. Often times, strength of the hole-electron Coulombic interaction is measured by evaluating the hole-NTO and electron-NTO in terms of overlapping [152, 153]. CT state energies are significantly associated with molecular packing configuration, as represented by donor-acceptor inter-molecular spacing and relative orientations in the molecular complex [151, 154]. An important point regarding the EPCSs is to avoid simply considering HOMO and LUMO wave functions to evaluate (even preliminarily) the minimum intra-molecular

Theoretical Modeling of SCs  187 or inter-molecular excitation; in such cases, it is mandatory to take NTOs into account before a reliable conclusion can be drawn [155]. The gap of energy between the disparate fragments and the corresponding molecular complex provides a measure of interaction energy. For example, the following formula gives the interaction energy between donor and acceptor systems:



E = EDA − (ED + EA)

(7.6)

In which EA, ED, and EDA refer to total amounts of energy of the disparate acceptor and donor molecules and the complex, respectively. Given that molecular packing-resulted stabilization energy in PCSs is managed by van der Waals (vdW) interactions which are weak in nature, one shall employ a density functional by which dispersion interactions can be accounted for accurately in the course of interaction energy evaluations. The recommended methodology for this purpose is LC functional, ωB97XD, wherein the dispersion interactions are treated by empirical corrections. Given the significance of attenuating the basis set superposition errors (BSSEs), one may adopt the Boys and Bernardi’s counterpoise correction method [156]. Failure to apply such a correction may end up with artificial over-stabilization of the binding energy as any molecule across the complex would otherwise have access to excessive basis functions of the neighboring molecules, so that it imposes a variable effect depending on the basis set quality and the inter-molecular spacing. Accordingly, a high-accuracy binding energy cannot be obtained unless inconsistent BSSEs are eliminated. In a previous research, a group of metalloporphyrin (usually abbreviated as MP where M may be either of Co, Zn, and Fe) molecules as they interacted with silicon carbide nanotubes (SiCNTs) were examined by devising the DFT techniques [121]. The results showed that, out of the studied MPs, the FeP molecule exhibited the best energy performance (binding energy = −2.10 eV) as it was adsorbed, in a zigzag-oriented fashion, on the silicon site. The DFT calculations further indicated some spin-polarized density of state (DOS) spectra for the Fe-BNNT system, with a less prominent spin-polarization of the DOS spectra for other MPSiCNT complexes. We further showed that the FeP-SiCNT complex was worth being largely regarded in the SC technology. In an attempt to go beyond interaction energy calculations, one may undertake energy decomposition analysis by using symmetry-adapted perturbation theory (SAPT) to decompose different types of interactions

188  Fundamentals of Solar Cell Design into physically significant constituents: induction, electrostatic, dispersion, and exchange-repulsion constituents with contributions into total interaction energy [158]. The SAPT methodologies have been classified under a number of classes. Among others, the SAPT0 is not only the most simple one but also associated with the lowest computational cost [159, 160]. Implemented in the PSI4 package [161], the SAPT0 can analyze systems comprising of more than 200 carbon-like atoms at a reasonable level of accuracy. Notice that the BSSE may no more affect the interaction energies once energy decomposition analysis is used to determine the energies.

7.6.5 Force Field Parameterization for MD Simulations In this section, beginning with an approach toward parameterization of a well-established force field to take into consideration molecular properties of the considered material, and investigation of molecular packing configurations among organic materials, the one developed by Jorgensen et al. seems to be the most appropriate method for this purpose [162], as indicated by its large potential for estimating bulk-phase properties including compressibility, heat capacity, and density for various small organic molecules. In the meantime, with this force field, atomic partial charges and bond lengths and angles are determined empirically and then generalized. The lower-level HF approach or DFT evaluations at B3LYP method or Perdew–Burke-Ernzerhof (PBE) level are commonly utilized to describe dihedrals. Relying on such features, the all-atom optimized potentials for liquid simulations (OPLS-AA) functions is herein recommended to serve as an initial model and keep updating the parameters, as many of them as possible, by advanced DFT methods, as detailed below [162]. In principle, the basic requirement for a force field to serve as an initial model is that the post-parameterization force field can simulate the relevant condensed-phase properties, such as density. The optimal-­geometry (using long-range-corrected DTF analysis) values of bond angle and length can be used to update the parameters of the OPLS-AA for PCSs. For convenience, it is normal to keep constant bond angles and lengths at the respective DFT level to undertake constrained geometry optimization with the aim of retaining or deriving stock values of the force constants; subsequently, curve fitting is used to come with a potential. Following a similar approach, fixed-dihedral angles geometry optimization is required before new dihedral parameters can be derived. When it comes to inter-ring or inter-monomer bonds-related dihedrals, which are known to determine the conformation and, in turn, the extent of conjugation with respect to the π-conjugated backbones, it is strongly advised to obtain new parameters.

Theoretical Modeling of SCs  189 Let us reemphasize that an LC functional (e.g., ωB97XD) shall be used for conducting such geometry-constrained optimizations, so as to come with precise torsion barriers and local minima, which are entirely neglected with B3LYP [163]. Finally, one may determine the dihedral parameters upon sampling the dihedral angles via a similar approach at the level of force field, with the parameters tuned for reconstructing the torsion profile of the LC-DFT. Commonly, such a procedure is conducted by undertaking molecular mechanics (MM) computations based on the constrained-­ optimized geometries for different DFT-derived dihedral angles; this method suffers, however, from a couple drawbacks. Firstly, the hereby determined dihedral parameters refer specifically to the absolute-zero temperature DFT torsion, while the user is most likely willing to undertake analysis at higher temperatures (e.g., room temperature and melting point). Secondly, this method tends to neglect multiple possible configurations at a given dihedral angle as it considers a single configuration for each dihedral angle. One approach toward addressing these drawbacks is to undertake in vacuo simulation of one polymer chain or molecule based on MD principles over a temperature interval, e.g., 300–600 K, to evaluate the dihedral parameters. Finally, a molecule is known to exhibit a distinctive electron density distribution that cannot be demonstrated unless partial atomic charges are updated. Given sensitivity of the charges obtained from charge analysis such as Mulliken population to the utilized basis set, it is advisable to determine charges by simulating the molecular electrostatic potential, as is basically performed by ESP [164, 165] and RESP [166, 167] methods, among others. For a polymer, minor modifications to the charges on the terminal monomers are required to guarantee neutrality of total charge along different chains. Analogously, there might be a need for modifying (i.e., averaging) the charges to consider symmetry conditions.

7.6.6 Excited States Advantages of the TDDFT in terms of accuracy (comparable to correlated ab initio methods for expressing the excited states) and reasonable computational cost considering the system dimensions have spread its popularity much widely [168]. However, TDDFT is still limited to single-excited states while many problems related to current XC functionals involve long-range CT excitations wherein the starting orbital of a given transition exhibits no significant overlapping with the arriving orbital. This drawback has not hindered successful application of the TDDFT for the investigation of transition metal centers-contained organic molecules [169].

190  Fundamentals of Solar Cell Design Moreover, being based on the ground state geometry, the TDDFT can be used to reproduce absorption spectra, and no other non-equilibrium solvation model is capable of including the solvent response to the excitation process. More recently, the TDDFT has been able to compute excited state geometries [170, 171], thereby smoothing the road toward emission spectra and excited state dynamics calculations. Add to that the recent reports on efficient procedures for evaluating dense spectra [172]. By definition, an excited state is the one with one or more electrons at a higher level of energy as compared to that at the ground state. In order to determine excited states, one may devise the standard DFT to examine virtual and occupied states and perform the HF ground-state computations. Accordingly, the gap between occupied and unoccupied energy levels gives a measure of the excitation energy. However, more accurate information on such excited states can be obtained by going beyond HF and DFT. As an extension to DFT, the TDDFT is capable of handling electronically excited states and calculating such physical properties as absorption spectra easily. A common approach to the extraction of the excited states is to apply an external time-variant electric field to the system, where the electric field represents a small perturbation thereof as far as the linear response theory is concerned. Such perturbation can be in the form of, for example, light or atomic motions. Accordingly, one can evaluate the excitation energies without dealing with any and all the excited-state wave functions but rather simply referring to poles of the response function. Corresponding excitation energies and associated oscillator strengths (transition probability) can be addressed iteratively in a variational form. For the physical processes which normally cannot be understood unless a knowledge of excited states is available, the TDDFT can provide a good initial approximation; an example of such processes is photoexcitation. However, given quite poor nature of almost any virtual orbital, enough care shall be taken when adopting this methodology and it should be kept in mind that it can be employed for the lowest excitations only. According to the literature, the DFT provides a promising tool for predicting geometries of such complexes at equilibrium [172] and respective absorption spectra using the TDDFT with hybrid functionals [173].

7.6.7 UV-Vis Spectroscopy The most appropriate technique to represent the presence of chromophores in a molecule is electronic absorption spectroscopy, which is also known as ultraviolet-visible (UV-Vis) spectroscopy. Chromophores are the πelectrons or lone pair electrons in a molecule that exhibit tendency toward

Theoretical Modeling of SCs  191 absorbing the light in the UV-Vis range (200 to 800 nm). Consequently, the conjugated π-electrons in a molecule comprise the major structural component determined by this UV-Vis spectroscopic method. The UV-Visregion absorption property characterizes a class of molecules called “dyes”. Functional or elementary group of the compound responsible for the absorbance is called chromophore [174–177]. Despite the vibrational transitions caused by molecule-infrared (IR) light interactions, which can be captured by IR spectroscopy, the higher energy radiation in the UV-Vis region further cause some electronic transitions in many molecules. That is, upon absorbing the UV-Vis light, the molecules are provided with enough energy to set their electrons jump to a higher-energy orbital from a lower-energy one. Accordingly, when performed in this region, the absorption spectroscopy is also known as “electronic spectroscopy” or “UV-Vis spectroscopy”. The computational study of the electronic (UV-Vis) spectrum of a molecule involves obtaining its excited state properties, namely, the oscillator strength and the excitation energy. There are several quantum-­ chemical methods for such a purpose. Examples of these methods include random-phase approximation (RPA), configuration interaction singles ­ (CIS), approaches based on coupled-cluster (CC-based methods), the TDDFT, and a more recently proposed multi reference CI DFT method (DFT/MRCI). Thanks to its easy procedure, efficient computational cost and reasonable accuracy, the TDDFT is now the most popular theory for excited-state modeling of medium- to large-sized molecules [178, 179–182]. Today’s popularity of the TDDFT was anticipated in an important review stipulating “most probably, the excited-state computations will be performed in the future by the framework of the TDDFT approach, which is becoming more and more popular because of its simplicity and apparent blackbox behavior” [183]. Thus, calculations of the UV-Vis absorption spectra and excited-state characteristics of dye sensitizers have been dominantly performed by the TDDFT method in several theoretical research works [184, 185]. According to the literature, out of the large pool of the semi-empirical methods proposed so far, the ZINDO/S represents a key approach that has been frequently devised to address the analysis of electronic spectra. It has been tailored for a handful of atoms (e.g., transition metals) because of its high effectiveness coupled with inexpensiveness [186–188]. Many scholars have compared the ZINDO/S, as a semi-empirical approach, to the corresponding experimental data, indicating their good agreement [189–191]. Despite the similar outstanding agreements of the findings of the TDDFT

192  Fundamentals of Solar Cell Design and ZINDO/S with the corresponding actually measured transition energy, the ZINDO/S method has returned evidently superior results for higher excited states [192].

7.6.8 Charge Transfer and Carrier Transport Calculation of the so-called carrier transport has always been a challenge to researchers who were seeking to model nano SCs. A widely accepted assumption in this respect is the longer mean-free-path for a bulk free carrier, as compared to the nano-system dimension, ruling out the applicability of the semiclassical drift diffusion equation. The discrete nature of the Eigen state energies further rules out the applicability of even the Boltzmann equation [193]. Here, a more basic question raises if the Fermi’s golden rule (FGR) can be applied to express the state-to-state transition (e.g., upon phonon absorption). The FGR [194] (also known as the “golden rule of time-dependent perturbation theory”) is a formula focused on the transition rates. Here, the “time-dependent perturbation theory” is applied to the studied system as a state-to-state transition occurs as an element of a continuum of states. Heat-induced fluctuations and atomic repositions in some systems lead to rapid (within several sub-picoseconds) changes in the electron Eigen energy [195]. Since such changes may occur at so high rate exceeding the average transition time, the applicability of the FGR, wherein the state energy is presumed to be transferred throughout the transition period, becomes somewhat doubtful. There may be also cases where multiple phonon effects render significant if the coupling between the phonon and electron of surface is strong enough, complicating the computations required for evaluating the simple models for charge transfer as well as carrier transport [196]. A workaround is to compute constants of the electron-phonon coupling in each and any phonon mode. Even if the coupling constants are considered, the irrelevant phonon-supported hopping transport may be investigated, covering the multiple phonon transitions for deep state capturing and nonradiative collapse. Trying to address the obstacles ahead of calculating the charge transfer, both relevant and irrelevant electron transfers can take significant parts, with the conformational changes in the nanomaterial being able to impose major adiabatic-state alterations between the irrelevant hopping movements. A direct approach to the simulation of such concerns is to transform to the time domain [197–200], where the electron wave functions are constructed on the ground of the time-dependent (TD) Schrodinger’s equation, with the MD schemes describing the nuclei’s movements. This somehow mimics the procedure of real-time TDDFT

Theoretical Modeling of SCs  193 evaluation [201]. The primary distinctions include the consideration of the nuclei’s movements and the larger time scale (picoseconds or longer rather than femtosecond in many real-time TDDFT procedures) of procedure introduced in this section, so that the electron transition occurs. The state collapse (hopping) to an adiabatic state represents another significant distinction of the time-dependent method compared to the TDDFT [202]. Representing the quantum de-phasing (i.e., “incoherent movement”), this process refers to, as an instance, the phonon-assisted hopping. A time-­ domain simulation may render extremely expensive should the considered time span goes widely long, necessitating some approximations in many cases. In this respect, one may decouple the TD-Schrodinger’s equation by using the MD of the nuclei. Despite its good performance for the electron state, this approximation is not adequately localized and rather intensively liked with one or two phonon modes. That is, the atomic movements have nothing significantly linked with the existence of the carriers. On the basis of this approximation, classical models [e.g., the valence force field (VFF)] can be developed to address the MD of the nuclei. As a ball-andstick model, the VFF has been proposed to take the mechanical energy arrangements for bond angle bending and bond stretching as a basis for introducing the elastic energy.

7.6.9 Coarse-Grained (CG) Simulations Coarse-grained (CG) simulations were performed according to the same procedure as that adopted in the previously described atomistic MD simulations, except that CG particles moving under the effect of CG potentials were considered in this case. Now, it is possible to work on enlarged versions [in terms of time (hundreds of nanoseconds) and length (tens of nanometers)] of the systems that were subjected to the parameterization, so as to study such nano-scale behaviors as phase separation of donor and acceptor domains [203–206], in-solution self-assembly of polymer nanostructures [207–209], and short-range polymer crystallization [210]. Another application of CG simulations is to estimate mechanical characteristics of organic semiconductors [211].

7.6.10 Kinetic Monte Carlo (KMC) Modeling KMC modeling constitutes a basis for dynamically evaluating a morphology in terms of the charge carriers and excitons. The simulated morphology here may refer to a Cartesian lattice or atomic-scale morphology (e.g., the morphology obtained upon practicing the reverse-mapping on

194  Fundamentals of Solar Cell Design the CG nano-scale morphology). As far as the latter is concerned, the related mechanisms in a SC system exhibit direct contributions back from the morphology and other effective factors, including the chemical functionalities. In a KMC simulation, the studied electronic species are subjected to a series of operations (e.g., recombination and hopping among neighboring molecular sites). Each of such processes is associated with a certain rate (R)-controlled waiting time that further receives some contributions from a random value X ranging from 0 to 1. Accordingly, the shortest-waiting time event is chosen as the behavior for the particle.



τ=−

ln(X) R

(7.7)

Taking integral over a long series of events for numerous particles, the KMC simulation resembles the MD simulations in which the integration is performed over an extended series of time steps. Subsequently, average values of the desired parameters may be evaluated from the equilibrated/ stabilized particle trajectories. In this respect, the exciton dissociation, the exciton transport, the charge transport, charge recombination, and charge collection are some of the most significant processes engaged in an organic solar that can be simulated, although at various rates, with KMC simulations. Groves presented a detailed report on how to implement a KMC simulation to describe the mentioned processes [212]. The fact that a majority of existing works on the KMC simulation have addressed the charge transport and recombination in the bimolecular and geminate fashions by focusing on the morphological structure of the lattice [213–216] may not rule out the need for taking into account the molecular morphology in cases for which the electronic interaction rates are controlled by the exact molecular packing arrangement. A recent contribution of Jones et  al. is seemingly the first to the molecular morphology of the poly(3-­ hexylthiophene, as a π-conjugated polymer species, to the degree of mobility of the hole, with the electron mobility estimated via semi-empirical quantum-chemical calculations of the inter-molecular site charge transfer rates based on the semi-classic Marcus equation [217]. Another contribution presented a similar technique for evaluating the electron mobility in lattice systems [218]. Here, the morphology was specific to solely ordered or disordered systems rather than a long-range correlation-induced self-­ assembled (crystalline) system, such as that studied by Jones et al. [217]. Jones et al. explained the molecular mechanism under the effect of which

Theoretical Modeling of SCs  195 the hole mobility was higher in P3HT by focusing on the molecular weight and annealing temperature. To take a step forward, one must explore the solution-induced donor-acceptor systems in terms of the charge dissociation, recombination, and transport processes, so as to achieve more realistic blend morphologies.

7.6.11 Car-Parrinello Method When conducted according to the procedure proposed by Car and Parrinello [219], MD simulations on the basis of DFT provide a foundation on which basis scientists can study systems in terms of geometry and electronic structure and gain an insight into dynamics of chemical reactions [220]. Using such knowledge, researchers can see how thermal motion affects the considered properties [221] and determine optimum geometries of systems identified by a number of local minima across the potential energy surface [222]. As a classical MD technique, the CarParrinello (CP) method is grounded on an inter-atomic potential derived from DFT in an “on-the-fly” manner, i.e., for each and any nuclear configuration dealt as the system evolves. In many instances of standard application, a PW basis set is used in the CP method. In particular, one can achieve higher efficiency with CP method by working on “ultrasoft” ­pseudo-potentials. The results of this research show that, such a workaround may end up with more than 2 to 3 times saving at a comparable level of accuracy. It is worth noting that, generally speaking, in terms of accuracy, the geometries derived upon optimization using CP method resemble the geometries formulated using conventional quantum chemistry methods [220]. A previous report has elaborated on the way the Ru(bipy)3-(C60)2 triad has its electronic characteristics and structure affected by the temperature as well as non-local dispersion interactions, where the analysis was performed with the ab initio vdW-DF approaches [223]. The data which achieved through the DFT-based MD simulations showed the variability of the structural characteristics of the relevant molecules with the temperatures. The outcomes of the first-principles analyses on separated complex indicated the settlement of the HOMO/LUMO level on top of the fullerene spheroid. Nonetheless, a rise of temperature has been found to localize the LUMOs and HOMOs on the acceptor (fullerenes) and donor moieties (Ru(bipy)3), respectively. Based on the final result, the optimal distribution was found to be realized under ambient temperature (300 K), as shown in Figure 7.3 [223].

196  Fundamentals of Solar Cell Design HOMO

LUMO (a)

(b)

Figure 7.3  Calculated HOMO and LUMO states for (a) an isolated system and (b) a system constructed through DFT-based MD simulations at 300 K for Ru(bipy)3-(C60)2 triad. The negative and positive sites are marked in red and blue, respectively (isovalue = 0.02 a.u.) [223].

7.6.12 Solvent Effects Solvation effects shall be taken into account to directly relate the calculated properties to the corresponding experimental values [224]. Even though theoretical and modeling analyses for explicitly considering the solvent molecules are evidently the “exact” approach toward treating the solvation effects, such an approach considerably adds to the system dimensions, thereby increasing its computational cost. As such, this approach may address small/medium solutes. Accordingly, the solvation effects are usually taken into consideration care via continuum solvation models [225] where the respective solvent is defined as a structure-free dielectric media encapsulated in a cavity that is controlled by the molecular geometry. Among others, Polarizable-Continuum-Model (PCM) [226] and the Conductor-like Screening Model (COSMO) [231] are typically carried out continuum solvation models in computational software.

7.6.13 Global Reactivity Descriptors An important characteristic of a complex is the energy gap wherein its impact on the compound photocurrent is significant. Signifying the difference in energy between the HOMO and the LUMO states, this gap is where the electrons can freely jump between bands because of the significantly smaller band energies. The energy gap has been used as measure of the molecular reactivity. In this respect, the HOMO energy offers a good measure of the lowest electron ionization (EI) potential while the LUMO energy is a rather rough approximation of the EA of the molecule. The DFT

Theoretical Modeling of SCs  197 and ZINDO results have been successfully used to assess the bandgap, which agrees well with the experimental values [228–230]. The complexes for which the HOMO and LOMO energies are lower than the I−/I3− electrolyte redox potential (−4.8 eV) and higher than the TiO2 semiconductor conduction band (CB) (−4.0 eV) [231] can serve as excellent photosensitizers for reducing the bandgap energy by enhancing the   ­conjugation efficiency. Impacts of the metal species on the porphyrin ring, variations of the bridge site, and the porphyrine–ZnP duplicate on the HOMO energy have been studied previously [38]. As depicted in Figure 7.4, the complexes have their bandgaps reduced through the conjugation enhancement and the doubling of the porphyrine and ZnP portions. Figure 7.4 indicates that one can favorably achieve improved JSC by reducing the HOMO-LUMO gap energy to absorb a greater fraction of the energy emitted through the sunlight [232, 233]. A crucial topic in the theoretical chemistry is to assess the chemical reactivity of various compounds, as it controls a number of chemical properties (e.g., chemical hardness/softness and electronegativity/electrophilicity) that are essential for the material design. A common approach to the chemical reactivity assessment has been the ZINDO/S analysis. 0 L

-1

Energy (eV)

-2 -3 -4

L

L

L

L

L

L

4.299 eV

6.090 eV

4.131 eV

4.142 eV

3.736 eV

4.136 eV

H = HOMO L = LUMO

L

4.144 eV

3.869 eV

H

H

H

(b)

TiO2

-4.8

I-/I3*

(c)

(d)

H

H

H

H (a)

H

H

-7 -8

-4.0

5.947 eV

-5 -6

L

(e)

(f)

(g)

(h)

(i)

Dyes

Figure 7.4  The diagram of energy for C60-por-XP complex (X = metal) (a) X = Fe, (b) X = Ni, (c) X = Ti, (d) X = Co (Down state), (e) X = Co (Up state)s, (f) X = Zn. (g) C60-ZnPPor, (h) C60-por-ZnP double, and (i) C70-por-ZnP, TiO2, and electrolyte I/I3-.

198  Fundamentals of Solar Cell Design As an important characteristic of a chemical complex, the chemical hardness provides a metric of the complex resistance to the charge transport among the molecules [234, 235] (making a weaker chemical hardness usually favorable), with the chemical softness referring to the opposite concept [236]. The chemical hardness can be evaluated as follows:



η = ( ELUMO − EHOMO ) 2

(7.8)

As any decrease in the chemical hardness (η) adds to the conversion efficiency, a lower value of η favors the charge transfer, thereby increasing the JSC. Parr et al. [237] suggested that the electrophilicity (ω) can be seen as a global reactivity index that measures the required energy for stabilizing a molecular system. Accordingly, a higher value of ω would refer to a better ability to accept electron (i.e., charge) [238]. In this respect, a higher ­electron-accepting power is desired as it contributes to electron reception from the donor species. The abovementioned facts imply the desirability of the higher values of ω and ω+. The following expressions give the electrophilicity index: 2



ω = µ 2η



ω + = (I + 3A)2 16(I − A)

(7.9)



(7.10)

All by all, the chemical softness and electron-accepting power tend to enhance the JSC and, subsequently, the light conversion efficiency [239]. The chemical potential (μ) provides a measure of how strongly the electrons are tending to diverge out of an equilibrium state. Accordingly, a higher chemical potential implies a more unstable (i.e., reactive) molecular structure. Compiling what was mentioned in the above paragraphs, one may conclude that both lower chemical hardness (η) and higher chemical potential (μ) contribute to better electrophilicity.

7.7 Conclusion Recent developments in the SC technology were investigated with a special focus on the present status of the computational power and possible

Theoretical Modeling of SCs  199 future challenges faced by the nanostructured DSSCs. To this end, we reviewed various theoretical and computational approaches proposed for the assessment of the substances suggested for the DSSC in terms of structural and electronic characteristics. Modeling and simulations have definitely provided better understanding of the factors underlying the fabrication and performance of various SCs devices. In this respect, we addressed different theoretical approaches, including the DFT, TD-DFT, MD, KMC, atomistic and CG simulations, and the CP method, considering their aspects in terms of processable time and processing cost. We further elaborated on how these procedures could be merged to obtain a stepwise process for researching the molecular structures and its impact on related parameters. In the next step, we considered the exited states and UV-Vis spectroscopy and charge transfer analyses results to evaluate the solvent effect form both theoretical and computational points of view. We also evaluated the PV parameters including the PCE, Voc, Jsc, FF, IPCE, and LHE with a further discussion on the roles of the counter electrode and substrate on the PV parameters. As a primary purpose of computational chemistry, the reactivity assessment of chemical species was explained based on a handful of global reactivity descriptors, (e.g., η, ω, ω+, and μ), with the effects of these factors on the VOC, Jsc, and charge transfer further presented. Based on this literature review, we expect that, in near future, the relations between various substances and their PV efficacy characteristics can be explored using the computational and theoretical via straight-forward approaches. In summary, because of its high efficiency, easy production process, and low cost, the DSSC offers large potentials for commercialization in the solar energy market, so as to further expand the future scope of this industry.

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8 Efficient Performance Parameters for Solar Cells Figen Balo1* and Lutfu S. Sua2 Industrial Engineering Dept, Firat University, Elazig, Turkey School of Entrepreneurship and Business Administration, AUCA, Elazig, Turkey 1

2

Abstract

Both the number and the capacity of renewable energy applications have significantly increased as a consequence of enhanced awareness and strategies to meet growing requirements for energy. In latest years, therefore, all nations have worked to raise the sustainable energy share among other power generation techniques in order to guarantee energy independency. Energy security concerns, decreasing levels of conventional energy sources, and solar radiation leves are significant determinants the feasibility of installing sun energy technologies. Due to the absence of accessible meteorological information, various models were created to predict sun irradiation concentrations. All the same, these designs rely heavily on the features of climate and environment. For a particular climate region, this paper aims to determine the most proper models. This study is a part of country-wide solar radiation model development effort. In this research, to design the photovoltaic facility for maximal effectiveness under particular circumstances, a simulation software package is used to analyze the region under inquiry using the deterministic models of solar radiation in accordance with the specified climatic circumstances. According to the results obtained, the most suitable equipment for the Ankara province was determined by the AHP method among top 10 confirmed cell and modules worldwide. Keywords:  Solar energy, correlation models, panel efficiency, photovoltaic systems, data analysis

*Corresponding author: [email protected] Inamuddin, Mohd Imran Ahamed, Rajender Boddula, and Mashallah Rezakazemi (eds.) Fundamentals of Solar Cell Design, (217–246) © 2021 Scrivener Publishing LLC

217

218  Fundamentals of Solar Cell Design

8.1 Introduction Solar energy potential is increasing sharply as a result of the current energy policies of the nations. Considering the consumption of limited fossil fuel sources, the search for sustainable energy sources is of great importance more than ever now. Photovoltaic energy is increasing its share among other renewable energy sources to its relatively simple technology and decreasing costs. Many industrialized nations have already reached considerable amounts of installed solar energy plants. While investing in the photovoltaic systems, one of the most important steps is the initial research on the viability of such systems. Among various factors indicating the true potential of solar systems to be installed on a specific region is the climatic conditions. Although there are many indicators observed by meteorological stations throughout the world, there is a limited amount of measurement when it comes to sun irradiation which plays a major part in the potential of sun power. Figures 8.1 and 8.2 display the diagrams of overall sun irradiation and global net radiation, respectively. To make up for the lack of information on radiation values, many estimation models have been developed which are dependent on the climatic conditions. Scholars have begun to concentrate in recent years on the regional modelings of sun irradiation linked to solar farm plan. Many papers also indicated that the methodology of ANN (artificial neural networks) is more preferable than empirical modelings [3–5]. Shahaboddin and his coworkers utilized the extreme machine learning algorithm and ANN for Shiraz in Iran. Inputs are implemented for moisture, average temperature,

SPACE

Backscattered by air

Reflected by surface

INCOMING SOLAR RADIATION

Reflected by clouds ATMOSPHERE

Absorbed by water vapor and gases NET LONGWAVE RADIATION Absorbed by earth

OUTGOING LONGWAVE RADIATION

EARTH

Figure 8.1  Diagram of overall solar radiation [1].

Emitted by clouds Absorbed by clouds, water vapor, and gases

Emitted by water vapor and gases

Efficient Performance Parameters for SCs  219

–100

-50

0

50

CIRA

Figure 8.2  Global net radiation [2].

variation in temperature, and solar irradiation fraction length. The information concerning a period of 3 years is utilized for testing. The RMSE (error term) varied from 0.93 to 0.86 MJ/m2 day [6]. Kevin and co-­workers used a linear modelings for 41 places in China to predict worldwide sun irradiation. Depending on diverse set of elements, three places were divided into seven sun regions and nine heat areas, respectively. The ANN model was implemented with inputs such as average daily temperature, longitude, day number, altitude, latitude, and fraction of solar radiation length [7]. For 10 years, Li and his co-workers [8] utilized a nested modelings (sine and cosine features) for 79 locations in China with information. The error term ranged from 4.00% to 15.43%, while the RMSE varied between 1.83 and 1.03 MJ/m2 day. Janjai and co-workers worked on another model for 5 locations in Cambodia and 4 locations in Thailand. It is estimated that RMSE is 1.13 MJ/m2 day [9]. Mohammadi and co-workers [10] used an algorithm based on wavelet transforming and supporting vector machine for the Bandar Abass city in Iran. To train the models, data were utilized for a period of 10 years. The characteristic properties are used to separate in the range of the least and the highest ambient temperatures, solar irradiation length fraction, average ambient temperature, extraterrestrial global sun radiation, vapor pressure, and relative moisture. The RMSE varied from 1.79 to 1.81 MJ/m2 day, respectively. Qin and co-workers utilized the Marquardt-Levenberg algorithm with inputs such as daytime and nighttime area temperature differences, average area temperature, number of days of air pressure, vegetation index, and monthly rainfall. Seven-year

220  Fundamentals of Solar Cell Design data from 22 sites are used to train the ANN for Tibetan Plateau [11]. For Dezful in Iran, Behrang and co-workers researched the system of multilayer perceptrons and the functional network of radial bases. Inputs such as day number, average air temperature, evaporation, sun irradiation duration, wind velocity, and moisture were used by the six combinations of parameters. To develop the modelings, 1398 days have been utilized. The 214 days have been used for assay. The average total error in percentage rose between 22.88% and 5.21% [12]. Ozgoren and co-workers used the multi-non-linear regression ANN model for Turkey to acquire the most appropriate input layer features. To this end, they chose 10 features. To train artificial neural networks, optimization model by Levenberg-Marquardt is used [13]. Zang and co-workers [14] investigated the same modelings by diminishing two coefficients for thirty-five sites in China [15]. RMSE and the mean total error % ranged between 1.88 and 1.10 MJ/ m2 day for the 35 locations and from 16.22% to 4.33%, respectively. For Iseyin in Nigeria, Lanre and co-workers utilized ANN and the adaptive neuro fuzzy inference network. Inputs used were minimum and maximum temperature and length of sunlight. For testing, 6-year information was used for model training and 15-year information was utilized. The RMSE changed within the range of 1.09 and 1.76 MJ/ m2 day, during testing and training stages [16]. For 9 sites in China, Zhao and co-workers worked on linear modelings in which RMSE altered from 5.24 to 1.72 MJ/m2 day [17]. Senkal suggested an ANN modeling utilizing input superficies temperature, longitude, altitude, latitude, on the ground and two different superficies emissivity. Utilizing satellite data, recent three properties were identified. To train the ANNs, year-long information from 10 locations is utilized. The average error in analyzing and training was 0.32 and 0.16 MJ/m2 day, respectively [18]. Alvaro and co-workers utilized the satellite information in ANNs. It is noted that the output acquired is very nice [19]. Yadav and co-workers conducted the Waikato-Environment simulation program to succeed the most efficient entry properties for estimate. They obtained inputs as minimal and maximal temperature, average temperature, length of sunlight, and altitude, while the features of longitude and latitude with the lowest effect were recorded. The maximal mean absolute percentage error value was acquired by the artificial neural networks at 6.89% [20, 21]. In Liaoning city of China, Chen and co-workers searched five fraction modelings of solar irradiation length for three locations. Data has been used for 35 years from each site. Seventy percent of the information was evaluated to achieve empiric coefficient valuations. Thirty percent of the information was used for assay. For each of the stations, the empiric

Efficient Performance Parameters for SCs  221 coefficient valuations are observed. The RMSE altered from 2.73 to 1.98 MJ/m2 day for Chao Yang [22]. Li and co-workers evaluated eight fractional solar irradiation duration modelings in China for four stations for 11 years are utilized for calibration. Data were used for validation purposes for 4 years. For statistics, the RMSE is used as a determining factor. The modelings’ RMSE varied between 0.72 and 1.26 0.72 MJ/m2 day. The eight modeling RMSE altered from 0.7 to 1.33 MJ/m2 day [23]. To choose the proper data properties, Jiang and co-workers transferred Pearson correlation coefficients along with prior association rules. Parameters were selected for complete precipitation, average opaque sky cover, wind speed, mean temperature, minimal-maximal temperature, daylight temperature, heating-cooling degree days, relative moisture, and opaque sky cover [24]. To train the ANN, Mohamed used particle swarm optimization in Saudi Arabia. As inputs, following elements were used: the longitude, altitude, latitude, duration of solar irradiation, and the month. The forecast, all the same, was for global solar radiation on a monthly basis. Thirty-one locations’ data are utilized to assay ANNs. The average absolute percent error median value is 8.85% [25]. Katiyar and co-workers searched for four towns in India for quadratic, cubic, and linear modelings to estimate MJ/m2 day [26]. Senkal and co-workers researched the model of ANNs for twelve cities of Turkey. As inputs, the mean radiation beam, latitude, longitude, mean diffuse radiation, altitude was chosen to be significant. It is recommended to use the satellite-based methodology for forecasting the monthly average radiation. The mean error has altered from 2.75 to 2.32 MJ/m2 day [27]. Sun and co-workers assessed the impact of the auto-regressive moving average modelings to forecast sun radiation. They reported data from two sites in China during a period of more than 20 years [28]. Ayodele and co-workers applied a function in one year to the existing clearness index distribution. By using 7 years, daily sun radiation data was determined by the coefficient valuations. The efficacy of overall months is achieved with the exception of October. RMSE ranged from 0.221 MJ/ m2 day to 0.213 [29]. Fariba and colleagues studied the cloud based model and the Hargreaves model for Yazd in Iran. The 17-year data are utilized to get empiric coefficients. RMSE had improved between 0.71 and 1.12 MJ/m2 day [30]. In Spain, Gorka and colleagues collated three empirical temperature sourced modelings, ANNs, programming of gene expression, and the adaptive neuro-fuzzy inference method. 2,855 observations were used for testing. In addition, 44,420 investigations were utilized to assay the modelings. Such simulations used the five variations of the least and the highest air temperature, extraterrestrial irradiation, day number, and clear sky irradiation as data for the parameters. The optimized GEP’s

222  Fundamentals of Solar Cell Design RMSE altered between 3.31 and 3.49 MJ/m2 day. The RMSE of the respective configured adaptive neuro-fuzzy inference method shifted from 3.33 to 3.14 MJ/m2 day. The optimized ANNs’ RMSE for the other three input integrations altered from 2.93 to 2.97 MJ/m2 day [31]. Khorasanizadeh and his colleagues [32, 33] studied six templates for four provinces in Iran. The RMSE of the six modelings altered from 0.72 to 1.26 MJ/m2 day, and the average total error percent varied between 3.38% and 5.72%. Almorox and colleagues searched eight non-solar irradiance length modelings for seven sites in Spain that mainly depend on the highest and the lowest temperature. In some modelings, the average temperature characteristics, latitude, and the year’s day were concerned. Altitude RMSE of the eight models changed between 3.25 and 2.70 MJ/m2 day and the average total error percent ranged from 16.37% to 29.18% [34]. Amit and co-workers looked for a number of items that had utilized ANN in three reviews to measure solar radiation and forecast solar radiation on horizontal superficies. They stressed that artificial neural network modelings are more preferable than empirical ones [35]. For Gaize in Tibetan, three non-sun irradiance length modelings, two non-sun irradiance length fraction modelings, and three modified fractional sun irradiance length modelings were investigated by Liu and coworkers. In addition, 1,085 days were evaluated for calibration, while 701-day data were used for validation purposes. RMSE fluctuated between 3.13 and 1.68 MJ/m2 day. They argued that it was unnecessary to extract coefficient values for different seasons, respectively [36]. Bakirci examined 60 mathematical modelings developed for forecasting worldwide on a daily basis average solar radiation, in which many of the forecasts had solely the same equations with diverse constant regressive characteristics. Nevertheless, the many papers’ findings suggest that these constant criteria usually depend on the fields of inquiry [37]. Jamshid and coworkers studied one updated solar irradiation period fraction modelings and three fraction models of solar irradiation duration for two sites in Iran. They had used the vector regression support system. Of these, RMSE ranged between 3.70 and 2.14 MJ/m2 day. As kernel function data, the relative moisture, the lowest and the highest temperature, and period of sunlight selected [38]. Iranna and co-workers studied sixteen modelings of non-solar irradiation period to estimate average monthly clearness values. The precipitation, wind direction, altitude, relative moisture, longitude, and five other temperature sourced characteristic properties are used as inputs in evaluating inputs for 875 locations to test the modelings [39]. For seventeen Iranian provinces, Behrang and colleagues researched for eleven modelings using particle swarm optimization techniques [40]. Shamim and colleagues employed a fixed technique to estimate hourly solar irradiation.

Efficient Performance Parameters for SCs  223 We used a meso-scale meteorological model for diverse atmospheric layers. They calculated the index value of the cloud cover with the air pressure and relative moisture by using available restrained data. By an empirical correlation, they developed transmission variable and clear sky irradiation to measure the real hourly solar irradiation. Applying irradiation transfer model was projected for clear sky radiation. Data was used for one year for research. The mean square root error was at 110.83 W/m2 [41]. Zhou and colleagues analyzed six fractional and three fractional solar irradiation duration modelings for 69 sites in China to estimate on a monthly basis mean solar radiation. In modified modelings the latitude and altitude is added as characteristics. The values on the equation are derived separately. The fraction of the sunlight period altered between 1.636 and 1.634. MJ/ m2 day [42]. Chelbi and his colleagues studied five empirical modelings for four sites in Tunisia [43]. Khorasanizadeh and co-workers measured three mean fractional solar irradiation length modelings, five average fractional solar irradiation duration modelings, and three non-solar irradiation duration modelings for six cities in Iran to forecast on the monthly basis average worldwide sun radiation. The relative moisture and temperature are introduced as characteristic properties in mean fractional modelings of sunshine period. The RMSE of all modelings improved between 0.82 and 0.47 MJ/m2 day relative to the solar irradiation period fraction modelings [44]. On Tibetan, Pan and coworkers researched the temperature-based exponential model for eleven aerological areas. The variation in temperature is utilized as data. To calibrate the software, information was applied for a period of 35 years. For research, data was applied for a period of 5 years. RMSE of the platform improved between 3.24 and 2.54 MJ/m2 day for all terminals [45]. For Isfahan in Iran, four solar irradiation period fraction modelings were provided with information during a period of more than 9 years by Kasra and coworkers. The data is tested through four years of data. Their mean root square error shifted between 1.18 and 1.1 MJ/m2 day [46]. Adaramola has searched six non-solar irradiation period modelings to forecast modelings and solar radiation. Relative moisture and ambient temperature are used in non-solar irradiation period modelings. The RMSE for the linear modeling varied between 4.78 and 8.25 MJ/m2 day [47]. To forecast mean values on the hourly basis for solar radiation, Janjai and co-workers defined a satellite-sourced model. According to hours, the relative RMSE ranged from 10.7% to 7.5% between 15:00 and 9:00 [48]. Wan Nik and colleagues studied six mathematical representations of the proportion of the on the hourly basis sun irradiation to the average radiation. The estimate had been made for monthly average hourly irradiation.

224  Fundamentals of Solar Cell Design From three Malaysian locations, the modelings were tested using data for 3 years. The relative root mean square error was found to range from 26.49% to 8.22% [49]. Park and colleagues looked for a linear empiric model for 22 locations in South Korea [50]. In Turkey, Kadir analyzed seven different fractional solar irradiation durations with information from 18 locations. He utilized modelings including quadratic, logarithmic, linear, and exponential formulas to predict the long-term mensural mean for solar irradiation. Implemented modelings’ output is defined with minor variations for the same locations [51]. Zeynab et al. compared two vector regression support modelings with fuzzy linear regression in [52], contrasting them with the adaptive neuro-fuzzy inference and ANN method [53]. Separately chosen as functions of two supporting vector regression modelings, polynomial and radial bases, statistics for 7 years were obtained from Iran’s only station for testing and training. As inputs, the extraterrestrial radiation, the day’s number, maximal-minimal temperature, real solar irradiation period, clear-sky sun irradiation, and maximum solar irradiation time were determined to be relevant. The RMSE of the recorded modelings were reported to be between 3.3 and 3.9 MJ/m2 day. In Turkey, Ahmet and his colleagues studied quadratic, cubic, and linear, empiric modelings for four provinces [54]. Antonio and colleagues planned a linear equation of correlation sunlight with the temperature change as daily and solar irradiation duration produce through utilizing the power balance between the soil layer’s adjacent layer and the atmosphere [55]. Fariba and his collaborators looked for 78 empirical modelings. They grouped them into four classifications: sun sourced, cloud sourced, meteorological characteristics sourced modelings, and others depend on temperature. They also brought a few modelings from each of the groups to Iran to create a model. A solar ray-based modeling with exponential expression defines the best performance [56]. In Saudi Arabia, El-Sebai and colleagues applied three average fractional solar irradiation duration modelings, three fractional solar irradiation duration modelings, and non-solar irradiation duration modelings to predict global sun irradiation on a monthly average. Temperature, relative moisture, and cloud cover were the characteristics classified into mean fractional modelings of solar irradiation period. The 9-year information was employed to define novel empirical coefficient values. For the nine modelings, the RMSE altered between 0.15 and 0.02 MJ/m2 day [57, 58]. Manzano and coworkers tested the linear Angstrom-Prescott model for 25 locations in Spain. Over 10 years of data were utilized for the calibration. The RMSE altered between 0.8 and 0.36 MJ/m2 day except for four places [59]. Lu and his co-workers utilized the ANNs modeling to predict regular radiation. The mean root square error values of modelings of

Efficient Performance Parameters for SCs  225 artificial neural networks are within 1.24–4.20 MJ/m2 day [60]. Hacer and coworkers investigated five fractional solar irradiation length modelings for seven locations in Turkey to predict average monthly radiation [61]. Shahaboddin and colleagues measured two solar irradiation fraction modelings, two average solar irradiation fraction modelings, and one non-solar irradiation period modeling for Shiraz in Iran. The RMSE of the five modelings improved from 1.55 MJ/m2 day to 1.3 [62]. For Shanghai in China, Yao and co-workers measured 89 monthly average radiation modelings. In Shanghai, many modelings are applied with the same mathematical terms, using diverse coefficients. They derived new convenient coefficients for five sun irradiation duration fraction modelings [63]. This research aims to reveal solar energy potential of Ankara, capital of Turkey, by determining the most appropriate solar radiation model and conducting the required analysis and simulation provided in the remainder of this paper. In addition to, the most proper solar cell to Ankara city is determined among worldwide top 10 confirmed cell and module. Within the framework of this study, it is aimed to raise awareness for designers interested in this subject.

8.1.1 Potential, Production, and Climate of Ankara The period of solar and land irradiation have significant value for solar related facilities. Therefore, extensive work on environment, solar energy efficiency, and existing facilities in Ankara needs to be undertaken. Turkey’s insolation map of and sun irradiation map of Ankara are displayed in Figures 8.3 and 8.4. Table 8.1 presents the radiation values (mean sun irradiation, irradiation function latitude, periodicity, and irradiation function phase shift values) in Ankara.

8.2 Solar Radiation Intensity Calculation 8.2.1 Horizontal Superficies 8.2.1.1 On a Daily Basis Total Sun Irradiation On a given day, total sun irradiation on a horizontal superficies can be computed by the below formula [64]:

2π (n + FKI ) I = I ort − FGI cos  365  

(8.1)

226  Fundamentals of Solar Cell Design 0

26

0

28 6

300

310

Erzurum

Ankara

320 7 340

Kayseri

Izmir

Diyarbakir

Konya 8

8

360

320

Antalya

36 0

310

5

5

3007 7

8

S E A Samsun

260

Istanbul

260 6

0

260

B L A C K 28

8 340

0

0

N

34

34

8

8

Sunshine Duration (hour perday)

MEDITERRANEAN SEA

Solar Radiation (cal/cm2 perday)

Scale Approximately 1:10 000.000

Figure 8.3  Turkey’s insolation map.

kWh/m2-year ÇAMLIDERE KIZILCAHAMAM

1400 - 1450

ÇUBUK NALLIHAN

BEYPAZARI

GÜDÜL

AYAȘ

1450 - 1500

KAZAN KALECIK KEÇIOREN AKYURT YENIMAHALLE SINCAN AL TINDAG ETIMESGUT MAMAK ÇANKAYA ELMADAĞ

ANKARA

1500 - 1550 1550 - 1600

GÖLBAȘI

1600 - 1650 BALA

POLATLI

1650 - 1700

KAYMANA

1700 - 1750

Figure 8.4  Ankara solar radiation map.

EVREN

1750 - 1800

ŞEREFLIKOÇHISAR

1800 - 2000

Efficient Performance Parameters for SCs  227 Table 8.1  The irradiation values. City

FKI

Latitude

FGI (MJ/m2.day)

Iort (MJ/m2.day)

Ankara

1.78

39.57

8.81

14.3

here FGI: irradiation function periodicity, FKI: irradiation function phase shift, n: days, Iort: yearly mean of daily total irradiation.

8.2.1.2 Daily Diffuse Sun Irradiation On a daily basis, total diffuse sun irradiation on a horizontal superficies can be defined utilizing formula (8.2) [65].

Iy = I (1 − B)2 (1 + 3B2)

(8.2)

where, Io: Out-of-atmosphere irradiation, B: Transparency index.

8.2.1.3 Momentary Total Sun Irradiation On a horizontal, momentary total sun irradiation superficies can be obtained utilizing formula (8.3) [66, 67].



Io =

24 I s (cos(e ).cos(d ).sin(ws ) + ws.sin(e ).sin(d )). f π

(8.3)

where; e: latitude angle; Is (W/m2): solar constant; f: solar constant correction factor; d: declination; ws: sunrise hour angle are computed using equations and tables. Irradiation is computed using formula as follows [65]:



 π Its = Ats cos  (t − 12)   t gi

(8.4)

228  Fundamentals of Solar Cell Design here; tgi: imaginary day length Ats: sun irradiation

8.2.1.4 Direct and Diffuse Sun Radiation Amount of momentary diffuse and direct sun irradiation on horizontal surfaces can be defined using Equations (8.5) and (8.6) [66, 67].



π  I ys = Ays cos  (t − 12) tg 

Ids = Its = Iys

(8.5)

(8.6)

here, Ays is function periodicity.

8.2.2 On Inclined Superficies, Computing Sun Irradiation Intensity 8.2.2.1 Direct Momentary Sun Radiation Direct momentary sun radiation on surfaces with various angles (30°, 60°, and 90°) is computed utilizing formulation as follows [67–69]:

Ibe = IbRb

Rb =

cosθ cosθ z

cos θz = sin d . sin e + cos d . cos e . cos w

(8.7)

(8.8) (8.9)

cos θ = sin d . sin (e − β) + cos d . cos (e − β) . cos w (8.10) 8.2.2.2 Diffuse Sun Radiation Diffuse radiation can be defined using the formulation as follows [67–69]:

Iye = RyIys

(8.11)

Efficient Performance Parameters for SCs  229 Ry for diffuse radiation (conversion factor) is computed using formulation as follows [25–28]:



Ry =

1 + cos(α ) 2

(8.12)

Ry parameter supplies the superficies’ slope. For vertical superficies (a = 90°), Ry is 0.5. Using this value, diffuse irradiation values for 30°, 60°, and 90° surfaces in Ankara can be computed.

8.2.2.3 Momentary Reflecting Radiation Reflecting radiation [66–70] is computed through the equation as follows:



I ya = Its p

1 +  cos(α ) 2

(8.13)

The reflecting rate to the environment is displayed with ρ and used with mean value of ρ = 0.2.

8.2.2.4 Total Sun Radiation Total radiation inclined surfaces is computed [66–70] using the equation as follows:

It = Ide + Iye + Iya

(8.14)

8.3 Methodology 8.3.1 The Solar Radiation Assessments by Correlation Models With MATLAB Simulation Software Figure 8.5 shows the values of (a) variation of current yearly average sun irradiation levels over a 24-hour cycle, (b) variation in current yearly diffuse sun irradiation values per hour, and (c) variation of the current annual direct solar radiation levels over the Ankara horizontal region for a 24-hour cycle. As shown in the figure, the momentary total sun irradiation’s maximum value on horizontal superficies in Ankara is 1.5075 W/m2 gauged on the year’s 355th day at 12:00 and the minimum value at 21:00. Figure 8.6 shows on a daily basis variations of

Momentary total radiation (It)

230  Fundamentals of Solar Cell Design

1.5 1 0.5 0 -0.5 -1 -1.5 400 350

300 250 200 150 100 Days (d)

50

0

10

5

0

15 Time (h)

20

25

Momentary diffuse radiation (Id)

(a)

1.5 1 0.5 0 -0.5 -1 400

350 300

250

200

Days (d)

150

100

50

0

0

5

10

15

20

25

Time (h)

Momentary direct radiation (Ib)

(b)

0.2 0.15 0.1 0.05 0 -0.05 -0.1 -0.15 -0.2 400

350

300

250

200 150 100 Days (d)

50

0

0

5

10

15

20

Time (h)

(c)

Figure 8.5  On horizontal superficies, annual sun irradiation values’ variation over a 24-hour.

25

Out of atmosphere radiation values

Solar constant correction factor

Sunrise angle

Annual values of declination angle

Momentary total radiation (It)

Efficient Performance Parameters for SCs  231 5.55 5.54 5.53 5.52 5.51 5.5 5.49 0

50

100

150

200 Days (d)

250

300

350

400

25 20 15 10 5 0 -5 -10 -15 -20 -25 0

50

100

150

200 Days (d)

250

300

350

400

115 110 105 100 95 90 85 80 75 70 65 0

50

100

150

200 Days (d)

250

300

350

400

1.05 1.04 1.03 1.02 1.01 1 0.99 0.98 0.97 0.96 0

50

100

150

200 Days (d)

250

300

350

400

x 105 3 2.5 2 1.5 1 0.5 0 -0.5 -1 -1.5 -2 0

50

100

150

200 Days (d)

250

300

350

400

Figure 8.6  Solar radiation on horizontal superficies. 

(a)

(b)

(c)

(d)

(e)

(Continued)

232  Fundamentals of Solar Cell Design 1

Function frequency

0.95 0.9 0.85 0.8 0.75 0.7 0.65 0

50

100

150

200 Days (d)

250

300

350

50

100

150

200 Days (d)

250

300

350

50

100

150

200 Days (d)

250

300

350

400

(f)

Daily diffuse radiation

5.55 5.54 5.53 5.52 5.51 5.5 5.49 0 5

400

(g)

x 10-5

Clarity index

0 -5 -10 -15 -20 0

Figure 8.6 (Continued)  Solar radiation on horizontal superficies.

a. b. c. d. e. f. g. h.

total daily sun irradiation sunrise declination angle sunrise angle (hourly) correction factor atmospheric irradiation function periodicity graph (Ays) diffuse sun radiation (Ats) horizontal superficies transparency index (B) in Ankara.

400

(h)

Efficient Performance Parameters for SCs  233 On the 355th day, the maximum function periodicity value for horizontal superficies is observed at Ankara as 0.74. On the 172nd calendar day, the peak sunrise angle, declination angle, and out-of-atmosphere irradiation levels are 110.01860, 23.44980, and 278010 W/m2, respectively. Whereas on the 80th day, the transparency index reaches its peak (0.0030), on 266th day its minimum level (−0.017) is seen. For solar correction element, minimum and maximum values consist of the 182nd and 365th days, respectively. Figure 8.7 offers measurements of momentary direct irradiation with 3 diverse angles (30°, 60°, and 90°) over a 24-hour time period. For all three angles, the highest values are obtained at 12:00, while the minimum ones are spotted at 03:00 on the same day (355th calendar day). Idbmax values are measured as 0.69, 0.632, and 0.4327 W/m2 for 30°, 60°, and 90°, respectively. Idbmin values are −1.1232, −0.8432, and −0.6132 W/m2 for the same angles. Figure 8.8 shows annual diffuse momentary irradiation levels for 3 angles (30°, 60°, and 90°). Maximum levels are computed as 0.158, 0.1523, and 0.1523 W/m2, respectively, on day 355. Annual values for 24-hour periods of momentary total sun irradiation are given in Figure 8.9. The maximum values for angles 30°, 60°, and 90° are computed as 0.0360, 0.1320, and 0.2522 W/m2, respectively, on the same day. Figure 8.10 supplies momentary total sun irradiation per annual angle and hours.

8.3.2 MATLAB Simulation Results and Findings Resulting from the above review, Ankara’s true potential can be computed using calculations of the solar characteristics given in Table 8.2. The sun irradiation values on both inclined and horizontal superficies are defined by utilizing MATLAB software program. According to the measured values, indicators show that the capacity for PV systems is in line with the expected levels. Comparing the expected values and the actual values is an integral step in designing photovoltaic technologies. Unit efficiency depends on different parameters. The use of acceptable radiation levels is of big significance for the plan of the optimal unit.

8.3.3 For Ankara Province, the Determinants of the Most Efficiency Solar Cell With AHP Methodology AHP approach is a significant quantitative technique that is utilized when making decisions, for various reasons. Capacity to consider several parameters simultaneously, being able to obtain the best solution, eliminating

Inclined surface momentary direct radiation (Ib)

234  Fundamentals of Solar Cell Design

1 0.5 0 -0.5 -1 400

350

300

250

200 150 100 Days (d)

50

0

0

Inclined surface momentary direct radiation (Ib)

(a)

0.8 0.6 0.4 0.2 0 -0.2 -0.4 -0.6 -0.8 400 350

10

300 250

200 150 100 Days (d)

50

0

0.6 0.4 0.2 0 -0.2 -0.4 -0.6 400 350 300

250

0

5

50

0

0

(c)

10

15 Time (h)

20

25

Direct momentary irradiation levels (60°)

200

150 100 Days (d)

25

20

Direct momentary irradiation levels (30°)

(b) Inclined surface momentary direct radiation (Ib)

5

15 Time (h)

5

10

15 Time (h)

20

25

Direct momentary irradiation levels (90°)

Figure 8.7  For 24-hour period, annual direct momentary irradiation values on inclined superficies.

unknown criteria when there is ambiguity, and the possibility of change in case of disputes are among the factors that contribute to the success of the process. The AHP approach allows the assessment of all options by taking into account many diverse criterions. It can be very difficult to do

Inclined surface momentary diffuse radiation (IbB)

Efficient Performance Parameters for SCs  235

0.2 0.15 0.1 0.05 0 -0.05 0.1 -0.15 -0.2 400

350 300 250 200 150 100 Days (d)

50

0

0

Inclined surface momentary diffuse radiation (IbB)

(a) 0.2 0.15 0.1 0.05 0 -0.05 0.1 -0.15 -0.2 400

350 300 250 200 150 100 Days (d)

50

0

0

Inclined surface momentary diffuse radiation (IbB)

(b) 0.2 0.15 0.1 0.05 0 -0.05 0.1 -0.15 -0.2 400

350 300 250 200 150 100 Days (d)

50

0

15 Time (h)

20

25

30° diffuse momentary irradiation

5

10

15 Time (h)

20

25

60° diffuse momentary irradiation

0

(c)

5

10

5

10

15 Time (h)

20

25

90° diffuse momentary irradiation

Figure 8.8  Yearly diffuse momentary irradiation values for inclined superficies.

this job manually and it takes considerable time. With AHP process, users can share their views flexibly and reach decisions in a more systematic way by using different requirements and objectives. It also offers a method of evaluating and using knowledge to re-assess the problems and to generate their solutions. The approach aids to avoid the confusion in cases of uncertainty and risk [71, 72]. AHP process consists of multiple stages. Initially, the problem’s description to be examined within the analysis is given, secondly a decision matrix for the selecting of alternatives and the determination of those parameters’

Inclined surface momentary radiation [hourly] (IrB)

236  Fundamentals of Solar Cell Design

0.05 0.04 0.03 0.02 0.01 0 -0.01 -0.02 -0.03 -0.04 400 350 300 250 200 150 100 Days (d)

50

0

0

Inclined surface momentary radiation [hourly] (IrB)

(a)

0.15 0.1 0.05 0 -0.05 -0.1 -0.15 400 350

300

50

0

0

(b)

15 Time (h)

20

25

Total momentary irradiation (30°)

250

200 150 Days (d) 100

Inclined surface momentary radiation [hourly] (IrB)

5

10

5

10

15 Time (h)

20

25

Momentary total irradiation (60°)

0.3 0.2 0.1 0 -0.1 -0.2 -0.3 400 350

300

250

200 150 100 Days (d)

50

0

0

(c)

5

10

15 Time (h)

20

25

Momentary total irradiation (90°)

Figure 8.9  Yearly total momentary irradiation values for inclined superficies.

relative priorities is defined. Then, these estimates, priority values percent are computed by comparing the options for each of the criteria, and using the resulting distribution, the best option is defined. For top 10 confirmed module and cells, the parameters and efficiencies are shown in Table 8.3. In this table, data are gauged under the worldwide AM 1.5 spectrum (103 W/ m2) at 25°C (IEC609043:2008, ASTM-G17303global) [73].

Momentary total radiation based on annual angle and hourly (It)

Efficient Performance Parameters for SCs  237

1.5 1 0.5 0 -0.5 -1 -1.5 400

350

300

250

200

150

Days (d)

100

50

0

0

5

10 Time (h)

15

20

25

Figure 8.10  Momentary total irradiation for yearly angle and hours.

Table 8.2  Sun irradiation attributes. Attributes

Ankara

Attributes

Total radiation

Imax (W/m2)

5.5454

Imin W/m2

5.4900

Declination angle

dmax

Sunrise hour angle

Ankara Mom. dir. Rad.

Idbmax (30°)

0.69

Idbmin (30°)

−1.1232

23.4498

Idbmax (60°)

0.632

dmin

−23.4498

Idbmin (60°)

−0.8432

wmax

110.0186

Idbmax (90°)

0.4327

wmin

69.9814

Idbmin (90°)

−0.6132

Ibmax (30°)

0.158

Mom. Dif. rad.

Out-ofIo(max) W/ m2 Atmosphere Radiation Io(min) W/ m2

278010 −176900

Ibmin (30°)

−0.214

Transp. Index Bmax

0.0030

Ibmax (60°)

0.1523

Bmin

−0.017

Ibmin (60°)

−0.2149

Iy(max) W/ m2

5.5445

Ibmax (90°)

0.1523

Iy(min) W/ m2

5.4900

Ibmin (90°)

−0.2149

Total diffuse radiation

(Continued)

238  Fundamentals of Solar Cell Design Table 8.2  Sun irradiation attributes. (Continued) Attributes

Ankara

Attributes

Function freq.

Ats(max)

0.999

Ats(min)

0.666

Mom. Tot. Rad.

It(max)

1.5075

It(min)

−1.5044

Mom. Dif. Rad.

(Ays)max

Mom. direct rad.

Ankara Mom. Irbmax (30°) reflecting I (30°) rbmin rad. Irbmax (60°)

−0.0425

0.94

Irbmin (60°)

−0.1571

(Ays)min

0.57

Irbmax (90°)

0.2522

Id(max)

0.9541

Irbmin (90°)

−0.3513

Id(min)

−1.165

Ib(max)

0.1413

Ib(min)

−0.2551

0.0360 0.1320

The weights according to AHP approach is displayed in Figure 8.11. It has been stated that the previous stages of the AHP approach include determining the priorities of parameters and rating alternatives based on those priorities. In this sense, the priorities of parameters obtained from the study are shown in Figure 8.12. The results indicate that “Efficiency” is the most important criterion and the “Fill Factor” criterion follows. Criterion “location” has the minimum weight among the others. The options are assessed based on the goals of parameters. Results are displayed in Figure 8.12. Figure 8.12 shows that the most efficiency material is “GaInAsP” when all the factors are considered together in relation with each other, followed by “GaInP”. In four out of the five parameters expressed in the final results, GaInAsP displays important dominance over the remaining options. It has also been found that “Organic” and “CZTS” are the alternatives with minimum scores.

8.4 Conclusions This chapter purposes to create a guide to choose the best panel by using real irradiation values of defined for optimum performance design of the PV systems. Sun irradiation values are measured to be used to build a photovoltaic device at appropriate efficiency levels. This study aimed to study a specific region with certain climatic characteristics and determine the

4.00

245.83

179.74

0.248

0.2504

Si

Si (multi-crystalline)

Si (large)

GaInAsP/GaInAs

GaInP

0.4798

0.512

0.2339(da)

0.4209

CdTe (thin‐film)

CIGSS (cd free)

CZTS (thin‐film)

CZTSS (thin‐film)

0.0407

0.0935

Organic (thin‐film)

Perovskite (thin‐film)

Cells (other)

0.4092

CIGS (thin‐film)

Cells (chalcogenide)

4.008

Area, cm2

Si (crystalline)

Cells (silicon)

Classification

(ap)

(ap)

(ap)

0.7306

(da)

(da)

(da)

(ap)

(ap)

(da)

(t)

(da)

(da)

Table 8.3  Top 10 confirmed module and cells.

35.21h

69.3

39.45f

31.69g

37.76f

16.31f

19.51d

42.5 e

40.55d

42.7 a

42.87d

Jsc, mA/cm2

22.7

12.1

12.6

11.0

± 0.8i

± 0.3k

± 0.3

± 0.2

± 0.5

± 0.3

21.4

22.0

± 1.4c

32.6

± 0.5

± 0.5

26.6

22.1

± 0.4

22.0

± 0.5

± 0.5

25.0

22.6

± 0.5c

25.8

Efficiency, %

1.144

0.8150

0.5134

21.74

e

0.7170

0.8872

0.7411

1.4932

2.024

0.7403

0.6717

0.706

0.7241

Voc, V

79.6

73.5

69.8

77.9

78.5

80.6

87.7

82.5

84.7

80.9

82.8

83.1

Fill factor, %

Efficient Performance Parameters for SCs  239

240  Fundamentals of Solar Cell Design Weights Efficiency Eff f iciency (%)

Area

V

I (A)

FF (%)

0.2439

0.4150

0.1435

0.1435

0.0540

Figure 8.11  The weights according to AHP approach. Scores 0.25

0.23

0.20 0.12 0.15

0.12

0.11

0.08

0.10

0.10

0.08

0.03

0.03 0.07

0.07

0.06 0.05

0.02

0.00

)

st

ry

(c

Si

(c

Si

s) ry

) rg (la Si

t)

ul

(m

Si

P

ln

Ga

sP

nA

l Ga

GS

CI

S GS

CI

Te Cd

S TS CZ

TS CZ

k vs ro e P

ite

c ni

ga Or

Figure 8.12  Scores according to resulting priorities.

solar capacity of the region under investigation. The selected city of Ankara which is the capital of Turkey carries the climatic characteristics of a wider region in the country. The study concludes with the investigation of the most efficient solar cell type with an AHP methodology.

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9 Practices to Enhance Conversion Efficiencies in Solar Cell Andreea Irina Barzic* “Petru Poni” Institute of Macromolecular Chemistry, Laboratory of Physical Chemistry of Polymers, Iasi, Romania

Abstract

The increasing need for energy imposes great accomplishments in the sector of renewable energy technologies that involve solar cells. The chapter presents the actual developments in photovoltaic materials and design of devices, emphasizing the importance of these aspects in conversion efficiency. A comparative presentation will be made concerning some of the advantages of all types of solar cell generations. The main factors that affect this parameter are reviewed. Issues of light harvesting in the solar cells are discussed, including how this is reflected in the device performance. The future prospects regarding practices to enhance the conversion efficiencies in solar cells are briefly presented. Keywords:  Materials, conversion efficiencies, surface texture, shielding layers, light scattering, solar cells

9.1 Introduction The crisis in energy sector on the entire globe imposes development of renewable energy technologies [1]. Sunlight is a natural energy resource, having the main advantage over the traditional power production systems that it can be directly transformed into electrical energy through solar cells. Such devices represent a green solution for cost effective generation of energy, without producing toxic materials [2]. The basic idea lies in photovoltaic (PV) effect, which is encountered in semiconducting materials that under the action of light photons, at a threshold energy, allow Email: [email protected]

*

Inamuddin, Mohd Imran Ahamed, Rajender Boddula, and Mashallah Rezakazemi (eds.) Fundamentals of Solar Cell Design, (247–270) © 2021 Scrivener Publishing LLC

247

248  Fundamentals of Solar Cell Design electrons to move thus producing electric current [3]. More explicitly, the solar cells exhibit an architecture relying on two adjacent semiconductor regions with distinct concentrations. When the p and n domains are united, holes travel from p zone, while electrons move from n zone leading to a diffusion current. The ions present in vicinity of the junction create an electric field in the contrary direction to the diffusion, which determines a drift current. In balanced conditions, these currents are balanced and, consequently, the overall net current is null. As the light reaches the solar cell, the energy of light quantum quasi-particles is taken by electrons which, in turn, are able to release themselves from nucleus links resulting holeelectron couple. Such carriers are powered by the electric field and moved in the bulk of p-n region. In the presence of an external load, at the solar cell terminals, an electric current and a potential difference can be recorded [3]. As a general rule, the working procedure of a solar cell involves the following principal features: • the absorption of radiations, producing electron-hole couples • the division of charge carriers of distinct sorts • the separate collection of generated carriers to an outside circuit. Given the relatively operating principle of the solar cells, the corresponding technologies have drawn an enormous attention of scientific and industrial communities, so they have known a truly astonishing evolution [4]. The investigations made up to date enabled classification of solar cells into four types, briefly presented as follows [4]: • first generation: refers mainly to devices containing monoor polycrystalline silicon or GaAs • second generation: is represented by amorphous silicon and microcrtystalline silicon, copper indium gallium selenide, and cadmium telluride/cadmium sulfide-based devices • third generation: used novel compounds like active quantum dots, nanocrystalline layers, tandem or stacked layered materials of inorganic III-IV compounds, organic polymers, and dyed-sensitized solar cells • fourth generation: is relatively new and involves “inorganics-­ in-organics”. It creates a balance between flexibility and low expenses of polymers combined with stability of inorganic

Enhanced Conversion Efficiency of Solar Cells  249 structures of nanometric dimensions (i.e., graphene and carbon nanotubes) The progress registered nowadays is culmination of the advances in the technology in the past decades reflected in around 30%–40% conversion efficiency [5]. The latter is mainly influenced by device architecture and constituent materials [5–10]. Literature [5, 9, 11, 12] discusses the performance of all categories of solar cells, outlining the benefits and disadvantages of certain of the four generations of PV devices. For several years, the PV industry was dominated by silicon-derived solar cells, but the new emerging solar technologies lead to more and more promising results, outperforming the first generation of solar cells. The main efforts are focused on finding novel high performance materials and design strategies of the devices that are reflected in a better conversion efficiency. The latter is strongly impacted by numerous factors [12–15]. A special consideration was ascribed to photon trapping strategies in PVs though advanced strategies, including the control of refraction properties and surface texturing of certain device layers [16, 17]. Proper light capturing by the PV device involves that lots of photons are collected from an impinging electro-magnetic wave with the aim to produce heat or charge carriers, excitons, or both [16, 17]. This can be accomplished via tricks in optics and photonics to improve the absorption of light in the device [16, 17]. The ultimate goal is to obtain a really black PV, but actual developments are still far from this objective. Considering the above concisely presented scientific background, the chapter attempts to present the actual state of art in solar cell technology by considering of the newest practices applied to improve the device conversion efficiency. A comparative discussion will be made regarding the performance of all four generations of solar cells, highlighting the advantages and future prospects. Particular attention will be given to the manner in which light can be efficiently harvested in the device and how this is reflected in the conversion performance.

9.2 Basics on Conversion Efficiency The conversion efficiency of a PV can be described as the quantity of radiation energy which could be turned into electricity by the device. The efficiency parameter is strongly impacted by numerous factors [12–15], such as:

250  Fundamentals of Solar Cell Design • • • • •

reflectance thermodynamic efficiency charge carrier separation efficiency charge carrier collection efficiency conduction efficiency

The reflectance of the outer layer of the solar cell can be controlled by selecting materials with low capacity of radiant energy. The cover of the solar cell must have adequate optical properties, not only in terms of absorption, but also concerning reflection and refraction. If these are properly controlled, the incident electromagnetic power is less reflected at an interface. The thermodynamic efficiency limit (denoted EL) is described by the absolute highest theoretically possible conversion efficiency of sunlight power to electrical energy [15]. Its highest value is appreciatively of 86%, representing the Chambadal-Novikov efficiency—an approximation linked to the Carnot limit, derived from the temperature of the photons generated by the Sun’s surface. There is a relation between thermodynamic efficiency and band gap. The “ultimate efficiency” was discussed by Shockley and Queisser [18] for common PV systems that contain only a single p-n region and consequently subject to a reduced EL value. Photons with suitable energy in regard to that of the absorber band gap are able to produce the desired output. In other words, if the incident photon energy does not exceed the level of band gap of the absorber material, no charges are produced so the energy is not transformed into productive yield and just makes heat upon absorption. Conversely, if light quasi-particles exhibit high energy, only the part of energy overcoming the band gap leads to useful output since it is absorbed, thus contributing to the kinetic energy of the charge carrier recombination. The surplus of kinetic energy is turned into warmth via quasi-particle interactions as the kinetic energy of the carriers is gradually lowered to equipoise speed. Because of these aspects, the solar energy cannot be transformed to electricity over a specific limit [15]. Based on the aforementioned aspects, it was revealed that a 95% ratio of work is attainable for pile of an infinite array of cells having band gaps with values covering all interval from infinite to null and a voltage in all the cells is near to the VOC value. Nevertheless, the 95% efficiency signifies that the electric power of the quantity of absorbed sunlight—the cells array produces radiation, which must be excluded from the initial light for estimation the quantity of warmth conveyed and the efficiency. The highest assumed efficiency foreseen is 86.8% for an array of a limitless network of cells, utilizing the light coming from sun. In the conditions in which the light is arriving only

Enhanced Conversion Efficiency of Solar Cells  251 from a certain zone of the sky the dimension of the sun, the EL is diminished up to appreciatively 69% [19]. Classical single-junction solar cells with an suitable band gap for the light wavelength range exhibit a utmost theoretical PV yield of 33.2%—known as the Shockley-Queisser limit [18]. PV devices having many band gap absorber layers enhance efficiency by imparting the light spectrum into little bins of thermodynamic EL is larger in all of them. Charge carrier separation is related to carrier motion and separation. For the latter, there are two causes, namely, the drift of carriers and diffusion of carriers. These two phenomena might work one against the other during PV operation. More specifically, an electron travelling through the junction from the p to the n region is being influenced by the electric field against the concentration gradient. A similar thing happens for a hole going in the contrary direction. The spatial collection efficiency means the fraction of photogenerated charge carriers at a certain point in the PV junction that redound to the photocurrent that comes out of PV. The conduction efficiency of the device is related to the thermal conductivity of semiconducting film and that of the surrounding optically black layer. Gaitho et al. [20] showed that at intermediate values of thermal conduction (around 312 K), the PV resulted power and consequently the efficiency is revealed to be big. More specifically, the stable efficiency reached the value of 11.8%, when the thermal conductivity was 1.072 ∙ 103 W/m K. A major issue is that all these factors are hard to be directly determined; therefore, they can be evaluated through the measurement of other parameters, namely: • • • •

quantum efficiency (QE) open-circuit voltage (VOC) maximum power point (MPP) fill factor (FF)

The QE can be defined by the amount of light quasi-particles which are turned into electric current in conditions of device functioning in short circuit regime. The “external” QE of a silicon PV system also considers the impact of optical losses. Reflectance losses are included in the QE parameter, as they impact “external QE”. The losses caused by recombination are represented by the QE, VOC, and FF. Resistive losses can be mainly quantified by the FF, and they also have an their share to the QE and VOC ratio [15]. Upon a phonon absorption by the solar cell, electron-hole couple is formed and, after reaching the junction, is contributing to the current of

252  Fundamentals of Solar Cell Design the PV device. If the carriers are not collected, they recombine and no electrical current is produced. The VOC is considered the utmost voltage within a PV device, which is occurring place at null current. The VOC parameter is attributed to the quantity of forward bias on the device owing to the bias of the PV active layer with the radiation induced electricity. The relation (9.1) illustrates the formula for the open-circuit voltage by considering the net current equal to zero:



VOC =

nkT  I L  ln  1 +   I0  q



(9.1)

where kT/q represents the thermal voltage, T is the temperature, n is ideality factor, I0 is the dark saturation current, and IL is the light produced current. An initial analysis of Equation (9.1) reveals that VOC  goes up linearly with increasing the temperature. In any case, this is not the case because I0 increases fast with temperature primarily owing to variation of the intrinsic carrier concentration. The effect of temperature is complicated and depends on the PV cell technology. The MPP is defined as the load for which the PV system could give highest electrical power at a known dose of irradiation. The MPP can be determined by enhancing the resistive load on light exposed cell constantly starting with a null value (a short circuit) to the biggest one (an open circuit); thus attaining the point that maximizes product between the voltage and current. The short-circuit current (ISC) within a device can be almost relative to the illumination, whereas the VOC could go down with around 10% with an 80% reduction of illumination. Solar cells of lower quality display a rapid decrease in voltage for gradual increase of current and might generate just 0.5 VOC at half ISC. The utilizable power output might sudden decrease beginning with 70% of the VOC × ISC magnitude to about 25%–50%. Providers mainly specify the PV “power” only as VOC × ISC product and do not provide the load curves which could significantly affect their true performance. The MPP parameter is changing as a function of incident illumination. For instance, the deposition of dust on solar cell panels is causing diminishment of the MPP parameter [21]. For devices sufficiently big to account for the additional cost, a MPP tracker analyzes the sudden power by constantly registering the voltage and current and utilizes such

Enhanced Conversion Efficiency of Solar Cells  253 data to dynamically adapt the load in such way that the utmost power is constantly conveyed, not considering the variations in illumination. The FF can be regarded as an essential parameter which helps understanding the behavior of PV systems. It is useful for discerning the quality of the device. The FF is described by the accessible power at the MPP imparted to the product of other two parameters, as shown in Equation (9.2):

FF =



MPP VOC ⋅ I SC

(9.2)

The FF parameter is highly impacted by the magnitude of the cell’s series, shunt resistances (Rsh), including diodes losses. Rising Rsh while reducing the series resistance (Rs) will determine the increase of the FF, which, in turn, is reflected in a bigger of the cell efficiency, getting the cell’s produced power near to the foreseen utmost value from theory [22]. Typical FF values are comprised in the interval of 50% to 82%. As an example, the FF of a classical silicon PV system was found around 80%.

9.3 Approaches for Improving Conversion Efficiencies in Solar Cells The conversion efficiency of the PV devices is the subject of many research papers [23–36]. Some of them are focused only on one kind of solar cells, while fewer are devoted to present the performance for all kinds of PV technologies, from classical to most modern ones. Based on available literature [23–36], the most relevant technical approaches employed for augmenting the efficiency of the solar cells are: • • • • • •

selection of adequate transparent conductor (TC) controlling radiation scattering from visible domain surface passivation radiative cooling anti-reflective (AR) coatings and textures properties of thin film materials

The operation of certain types of solar cells is significantly influenced by the optical features of the conductor layer. The latter is continuously

254  Fundamentals of Solar Cell Design illuminated and it must enable radiations to reach the surface of the active zone and to gather the formed carriers. Generally, layers displaying elevated optical transparency and good electrical transport features, like indium tin oxide, conducting macromolecular materials or conducting nanowire arrays can be useful for pursued goal. The balance among the optical and electrical properties; therefore, proper consistency of conducting nanoelements must be controlled to accomplish the desired efficiency. Granqvist [37] reviews the role of TCs in solar cells and shows that they can be used as “solar control”, current collectors, or “low-emittance” windows. TC properties like angular selectivity, spectral selectivity, and temporal variability must be carefully considered. The spectrally selective TC are represented by thin films formed from specific metals such as gold or titanium nitride or wide band gap semiconductors with considerable doping (commonly based on tin, indium, or zinc). Angular selective TCs, where the angular properties are the result of inclined columnar nanostructures (i.e., obliquely deposited Cr layers), are leading to higher efficiency. This is the results of nanostructures which are disposed so the incident flux exhibits an inclined angle to the layer surface normal. Fleischer et al. [38] also examined the effect of TC on efficiency of the PV device. They have investigated the restrictions imposed by single layer index matching materials for the glass/TC or TC/absorber, where the TC are quaternary oxides, like SnO2:S,F and absorber is Si:H. Using these materials, the efficiency increases up to 9.6%. Furthermore, it was proved that the resulted gain corresponding to TCO/absorber with small haze level is accounted by the AR features of the studied interface. The latter has the function of an effective medium having refraction properties in the middle of those characteristic for SnO2:F and a-Si:H. In their discussion, it was neglected the variations in the thickness of the absorbing layer for the scattered radiation. Jacobs and co-workers [39] attempt to re-define the requirements for TC materials used in solar cells. Metallization is a suitable alternative to solve the burden on a TC sheet resistance, leveling the competition among the majority of technologies, enabling other factors like material compatibility or degree of processing to take precedence. Whenever the possibility to introduce a metallic grid (thickness < 30 µm), the need for a TC shift significantly toward attaining ultra-high transparency (>95%). For sensitive cell architectures introduction of the wires may be highly useful to avoid large non-uniformities, and this is likely to be easier for flexible polymer layers than for glass. The demand related to sheet resistances (Rs) under 10 Ω sq−1 was shown to be important only when insertion of a metal grid becomes unfeasible. Moreover, performance of TC material with metal grid is much more higher regarding to the one of a sole coating satisfying

Enhanced Conversion Efficiency of Solar Cells  255 the 10 Ω sq−1 demand in the absence of the grid. It was revealed that the report of DC to optical conductivity is not an adequate predictor for PV performance that presents highly unphysical biases in the small and high Rs domains but also overstates the value of having small sheet resistance. So, it is important to prioritize transparency over Rs and to provide scalable approaches for inserting fine metal threads into front-side electrodes to enhance PV efficiency [39]. Many efforts have been devoted to the relation between light scattering phenomenon and solar cell efficiency [40–46]. In the classical dyesensitized PVs, titanium dioxide film on conductive glass substrate operates similar to an anode. Charge transport of TiO2 is enhanced under UV exposure and becomes very small in the dark. During dipping the semiconductor into the electrolyte, the latter is spread in such a manner that semiconductor/electrolyte system is produced at every nano-crystal. Under light exposure, electron-hole couple is generated, while the hole speed toward electrolyte must be quicker in comparison to the recombination of negative charges. Thus, an electrochemical variation is induced. Electron is carried via inter-related particles to the back contact. Electron and holes begin to move through diffusion in the semiconducting layer. A film of titanium dioxide particles having dimensions near to radiation wavelength in the optical spectral range has the role of scatterer by being blended within nanocrystalline layer to induce radiation scattering or making a diffusion coating on the nano- crystallites film to reflect the incoming radiation, with the goal to augment the moving route of photons inside the electrode layer. Thus, sometimes a light-scattering material is introduced in dye-sensitized PV since it enlarges the capacity to harvest radiation (particularly in the red part of the radiation spectrum) by reflecting the radiation travelling in the transparent conductive material. The schematic representation of a conventional photoelectrode involving the radiation scattering impact in dye-sensitized devices is depicted in Figure 9.1. There are certain factors that impact the light-scattering effect, such as [40–46]: • the thickness of the photoelectrode • the dimension of scattering (TiO2) particles in regard to the wavelength of optical radiation • thickness of the scattering layer • architecture of the hierarchical nanostructures Son et al. [40] analyzed the effect of structural differences in TiO2 on optical radiation scattering in dye-sensitized solar cell. They considered the thickness of both TiO2 and scattering layer on light harvesting. It was

256  Fundamentals of Solar Cell Design

Scattering layer

Transparent TiO2 layer

Transparent conductive layer

Incident visible radiation

Figure 9.1  Scheme for light-scattering effect in dye-sensitized solar cells.

demonstrated that as the light scattering layer becomes thicker leads to enhancement of the device output by augmenting the scattering ability in the long-wave zone and by extending the dye-loading. But, for a too much thick scattering layer negatively affects the electron travel in the TiO2 electrode as a consequence of the electron recombination in contrast to the improved light-scattering and dye-loading. Zhang et al. [41] proved that the use of hierarchical nanostructures like nanocrystallite aggregates have a double function: rendering large specific surface area, while producing radiation scattering. The employment of these nanostructures improves electron transport in the device. Depending on the architecture, the hierarchical nanostructures could generate adequate internal surface area for dye incorporation and they might determine very efficacious radiation scattering. In this way, it is possible to fabricate photoelectrode films with higher optical absorption comparatively to dispersed nanoparticles used in conventional devices. This enables diminishment of the thickness of the photoelectrode layer and as a result reduces probability of the charge recombination in PV device, allowing to enhance even more the conversion efficiency of already available dye-sensitized solar cells. Deepak et al. [42] discusses how light scattering enhances charge transport in the metal oxide and the importance absorption features of dyes in visible and IR zones of the solar spectral domain. Also, preparation of innovative materials for the scattering layer is discussed in regards with the device efficiency. Among the nanostructures as light scatterers, it is worth mentioning [42, 43]:

Enhanced Conversion Efficiency of Solar Cells  257 • spherical voids: carboxyl stabilized polystyrene spheres which are anchored onto the TiO2 surfaces; carbon spheres might be employed to produced pores in the TiO2 layer; • nanocomposites: nanowire-nanoparticle composites; nanofiber–nanoparticle composites. Light scattering layer based on nanostructures and using the concept of a double layer approach means that the separate scattering layer will lower the backscattering loss concomitantly with improving the efficiency. Deepak et al. [42] also reviews the main categories of compounds which can be utilized for radiation scattering in dye-sensitized devices where the scattering compound is inserted as an supplementary film or as component: large particles (TiO2, ZrO2), TiO2 nanotubes, TiO2 nanowires, TiO2 nanospindles, electrospun materials (TiO2 nanofibers, nest-shaped TiO2 nanostructures), nano-embossed hollow spherical TiO2, hexagonal TiO2 plates, TiO2 photonic crystals, cubic CeO2 nanoparticles, and surface adapted particles in scattering layer. Optical path length of incoming radiation might be enhanced by introducing large particles with the role of scattering film. Introduction of big particles in excess might also lower the internal surface area of the transparent TiO2 film. For proper carrier collection the optimum thickness of the TiO2 layer should be around 10–15 µm. If this values are exceeded, the path length of the negative charges and the redox particles in the electrolyte is larger. It results in enhancement of the series resistance of the film. In the device, the working electrode basically has 12- to 20-nm-sized TiO2 particles that are still non-absorbent in visible radiations and could increase the moving path of the radiation inside the photo-electrode. Furthermore, functional materials are good for improving capacity of scattering and high internal surface area as double function compounds: nanocrystalline spherical aggregates, core-shell materials, or 1D-3D nanostructure bilayer photo-anode. Sim et al. [44] reported the insertion of a scattering center and light distribution in its vicinity was analyzed via a numerical method. Starting from their simulations, it was revealed that it would be possible to change the quasi-particles distribution in the photo-anode to improve efficiency. Apart from inserting the scattering center, they have added the structure of the dome-shaped material to control the radiation spreading inside the photo-anode. The original 3D transparent electrode combined with the scattering center design enhanced the conversion efficiency of the device from 6.3% to 7.2%. Such data give useful insights regarding the methods to improve the already known limitations on dye-sensitized solar cell performance.

258  Fundamentals of Solar Cell Design Zaine et al. [46] also studied the impact of light scattering center on PV performance by having a positive influence on the long-wavelength reaction of the device. For such purpose, they have prepared adequate structure of SiO2 micro-core and TiO2 nanoshell material with the role of scattering center for the photo-electrode. Incorporation of 15% SiO2-TiO2 core-shell (STCS) into the superior coating of double film fabricated photoelectrode reflected in the biggest short circuit current. The better photocurrent resulted particularly in the interval of 400–650 nm. The augmentation is ascribed to the good radiation scattering of the STCS material that, in turn, enlarges the moving path of the light therefore expanding the percent of light quasi-particles collected by the sensitized dye molecules. The STCS has a great impact on lowering rate of negative charge recombination, thus making larger the electron lifetime. The surface passivation is another crucial aspect related to the PV efficiency [15, 47]. Considerable advances were performed to the front side of mass-produced PV systems; however, the aluminium back-surface was noted limit the obtaining of the desired efficiency [48]. The latter can be attained by making a passivated emitter and rear cells (PERCs). The passivation material consists of silica or aluminium oxide coating on which is deposited a silicon nitride film. The chemical deposition of a dielectric passivation film stack enables the achievement of a larger efficiency in silicon-based devices. This enabled a proper increase of PV performance for the Cz-Si wafer compound beginning with 17% to above 21% in 2010 [47] and the conversion efficiency for quasi-mono-Si to the outstanding result of 19.9%. Most employed passivation materials for PV systems are Al2O3 and SiO2. Nano-sized point contacts on aluminium oxide coating [49] and contacts on silicon oxide film [50] render the electrical linkage of the absorber material to the rear electrode. The point contacts on the aluminium oxide are fabricated by e-beam lithography technique, whereas the line contacts on the silicon oxide are formed via photolithography. In addition, the introduction of the passivation film leads to no modification the topography of the adjacent layers. Radiative cooling is related to the fact that an enhance in the PV temperature of around 1°C produces an efficiency reduction of appreciatively 0.45%. To overcome the inconvenient, a optically clear silica crystal film was used in design of solar panels. The transparent material can be viewed as a thermal black body that release IR radiation in environment meanwhile cooling the system up to 13°C [15, 51]. AR coatings and textures are important in fabrication of solar cells with good efficiency [52]. Antireflective layers might result in more destructive interference of incoming radiation. Consequently, all sunlight would be

Enhanced Conversion Efficiency of Solar Cells  259 reaching the PV device. In order to diminish reflection, the surface of the device is subjected to a texturizing procedure, thus reflected light hits the surface again. Introduction of a flat back surface besides the patterning the front surface enables keeping the light inside the PV, therefore making a bigger optical path. The texturizing surfaces can be generally produced by: • etching [52–55]: “random pyramid” texture, “inverted pyramid” texturing • lithography [56–58]: dicing saws or lasers. It is of paramount importance to mention a relatively recent scientific debate [59] that was focused on discussing whether random or periodic pattern is more adequate for light trapping in solar cells. No effective approach for accomplishment the pursued material surface topography for efficient light management in PV devices was formulated, meanwhile leaving place for novel ones. The influence of electrical parameters of the AR coatings was investigated by Swatowska et al. [60]. They have recorded current-voltage characteristics of multicrystalline silicon devices having as a-Si:C:H, a-Si:N:H, and TiOx as AR materials. It was noted that as the effective reflectivity coefficient is lower a higher efficiency and short circuit current were registered. Also, the refractive features and film thickness are important properties of the AR element. The a-Si:N:H layer has the most favorable impact on the device efficiency which is 14.25% that is close to that for titanium oxide layer, i.e., 14.00%. The result could be ascribed to the influence of many hydrogen bondings, formed in the structure of a-Si:N:H AR layer [60]. Therefore, a high quality reflective layer can improve the conversion efficiency of the PV system even by 30%. Semenova et al. [61] have prepared AR materials, based on porous silicon structures (PSSs) coated with nanodiamond (ND) films for silicon PVs, by electrochemical anodization. The silicon films having equally distanced pores effectively lower the reflection coefficient in regard to the monocrystalline silicon. The role of the ND on PSS is to stabilize the properties and avoid the degradation processes. The ND films made using the organic slurries produce almost no change on the reflection coefficient of the PSS. Covering solar cell with diamond-like films is optimal for increasing their resistance to ultraviolet radiation and protons. Thus, the ND layer has the function of both antireflection and protective material for silicon PVs. Selj and coworkers [62] propose to optimize the performance of solar cells by employing epitaxial silicon growth and customizing the

260  Fundamentals of Solar Cell Design antireflective features of the AR layer to correspond to the cell structure. The reduced AR thickness will move reflectance minima in the zone of lower wavelengths and produce higher fit of the absorption of the cells, in which the low wavelength domain is somehow more relevant in comparison to the long, poorly absorbed wavelengths. Baquedano and collaborators [63] attempted to enlarge the efficiency of PV devices by covering them with hydrophobic and hydrophilic AR nanostructured glasses by employing soft or colloidal lithography. The simple 1D and 2D nanopatterns (under the shape of wires and cones) were created and the effect of introducing disorder in the nanostructures on optical properties was studied. It was recorded a higher amount of transmitted light for ordered nanostructures of about 99% for wavelengths above 600 nm if ordered cones of nanodimensions are produced on both glass parts. The use of such glasses led to an increase of efficiency with an increment of 0.7% in comparison to the record of 26.6% for glass lacking nanostructures in 2017 [64]. Moreover, the wettability characteristics revealed that the specific disposal of the nanoelements enhanced the hydrophobic level of the glasses and thus meliorate their self-cleaning property. Conversely, the less ordered nanoelements conducted to better hydrophilic properties of prepared materials, enhancing their anti-fogging ability. The presented data indicate that by choosing the suitable morphology, the wettability will be tuned without reducing the performance of the PV. Uzum et al. [65] reported preparation multilayer AR materials based on ZrO2-doped polymer/spray-casted TiO2. Deposition of sole TiO2 layer on the a single part of crystalline silicon wafer led to a reflectance value around 5% recorded in range 300–1,100 nm. Reflectance was lowered to about 3% upon adding the spin-coated ZrO2 on the spray-casted TiO2 stratum. PV devices were accomplished using CZ-Si p-type support in the following situations: (1) in the absence of AR layer, (2) with TiO2-compact AR layer, and (3) in presence of ZrO2-doped polymer/TiO2 AR material [65]. The efficiency of the PV was correspondingly improved by a factor of 0.8%, namely, from 15.19% to 15.88%, as a result of insertion of AR multilayer. The short circuit current varied between 35.3 mA cm−2 and 37.2 mA cm−2 in regard to the device containing a single TiO2-derived AR material [65]. Moayedfar et al. [66] reviewed several types of AR layers based on composition and surface topography. Various double layer AR compounds were described, including MgF2/CeO2, SiO, CeO2, ZnS, MgF2/SiNx, mesoporous silica, Al2O3, TiO2, SiO2/ZnS, and MgF2/ZnS. Among the reported AR compounds, a 2D rectangular grating designed for light trapping on GaSb material displayed excellent performance.

Enhanced Conversion Efficiency of Solar Cells  261 A recent study of AR materials was done by Habubi et al. [67] proved that the PV efficiency can be enhanced by using nanostructured SnO2 doped with fluorine as AR layer. The material was placed on quartz support and sole crystal junction for silicon PV wafer via a chemical procedure relying on spray pyrolysis. The synthesized compound displays a polycrystalline nature and presents a prevalent preferred disposal along (110) plane. Upon doping average grain dimension increases together with gap energy from 3.62 to 3.75 eV. The latter is reflected in a variation efficiency from 4.86% to 5.46% and to 6.06% after inserting with sole of AR consisting in SnO2 and SnO2:F films. Another recent investigation in AR materials is that of Hossain et al. [68]. They developed an approach for the improving optical features of multilayer AR materials in PEDOT:PSS/c-Si heterojunction PV device by considering the optical interference transfer-matrix theory. MATLAB simulations were performed to evaluate optical reflectance magnitude for variable number of AR sheets on PEDOT:PSS placed on c-Si support. It was shown that the reflectance value is smaller than 4% for optical radiations domain for AR layer by alternately placing compounds with refractive index of precise values. Thus, such AR coating layers are recommended as the typical AR sheets for optical layers of the PV cells. On the other hand, the properties of thin film materials are essential for designing high performance solar cells with reduced costs. Also, adaptability to already available structures and frameworks in technology is very important [69]. Given the reduced thickness, such materials lack the optical absorption of bulk material PVs. Efforts to revise the issue have been made, particularly regarding the recombination occurring at the surface of thin sheets, which is the prevalent process taking place at nanometric level in thin-film PVs, and it is essential for the device performance. Regarding the materials for more efficient solar cell devices, a relatively novel trend is to use the concept of photon upconversion materials [70], which try to solution the issue of the losses related to light quasi-particles having sub-band-gap energy. This represents is an encouraging alternative to circumvent the mentioned drawback by turning the transmitted sub-band-gap quasi-particles into above-band-gap ones. There the device typically presents big QE. Late progress on several sorts of productive upconversion compounds includes: • rare-earth reinforced compounds • triplet-triplet annihilation compounds • quantum nanostructures

262  Fundamentals of Solar Cell Design The first category of upconversion layers is functioning better over 800 nm, whereas the triplet-triplet annihilation dye couples are more adequate for wavelengths below 700 nm. In addition, quantum nanostructures appear to be an innovation among the upconversion compounds, in which the dimension and form-generated quantum effects are utilized to adapt at the pursued wavelength domains [70]. Disregard the fresh advances, the enhancements in efficiency is still under 2%, too small comparatively to the theoretical assumptions of ~10%, as noted for PV containing Si, subjected to unconcentrated radiation [71]. This disagreement is attributed to some complication linked to photon upconversion compounds, which still have to be resolved [70]. Based on the available research, the scientific efforts are now directing toward flexible-wearable PVs for self-powered electronic devices [72]. Such systems represent a good alternative for green, inexpensive, and portable electricity sources with tremendous potential in telecommunication, transportation, and advanced sensors. Less addressed in literature is the problem of solar cell shielding layer which is mainly made by glass. The problem is that the cover glass is 90% of the entire weight of the device and owing to its rigidity is not adequate for flexible solar cells arrays. There are some transparent polymer materials used in fabrication of PVs, such as polyethylene terephthalate (PET) and polyethylene naphthalate (PEN) [73]. But, certain unwanted effects appear during raised temperature processing that destroys these flexible thermoplastic layers [73]. Utilization of thermostable polymers like polyimides in manufacturing solar cells is studied [74–78], but less attention is given to their role as shielding layers. Hulubei et al. [79] synthesized a series of polyimides containing thioether bridges as cover for solar cells in superstrate configuration. Molecular modeling indicated certain distinctions in the spatial molecular chain arrangement of prepared structures as a function of the aromatic or aliphatic feature of the dianhydrides moieties. Structural peculiarities of each polyimide were reflected in their optical properties. The thermo-gravimetric examinations demonstrated that the entirely aromatic structures are resistant up to 512°C, whereas the semialiphatic ones are thermally stable up to 335°C. For some of the obtained sulfur-based polyimides, the optical transparency is above 75% at 600 nm which is a good achievement, knowing that sulfur reduces this property. Apart from optical absorption losses, important measures have to be used for diminishment of reflection losses. In this regard, close analysis of the refractive index of the cover material must be done. The work of Hulubei et al. [79] is one of the few that account for the refractive index dispersion when addressing the issue of reflection losses. The sharp modification of

Enhanced Conversion Efficiency of Solar Cells  263 refraction properties at the interface of polyimide and adjoining TC layer must be optimized to get rid of reflection losses. In other words, light dispersion curves in each material should range in a similar manner without much discrepancy. At the polyimide interface with active layer from PV (i.e., ZnO or ZnO:Sn), the resulted optical losses are affected by the polymer structure. Figure 9.2 illustrates the manner in which reflection losses are taking place in a solar cells with superstrate configuration. In terms of optical performance, it was remarked that polyimides prepared from either cycloaliphatic semi-flexible dianhydride or aromatic rigid dianhidride containing sulfone group lead to the best balance of properties which correspond to the requirements for solar cells shielding layers [79]. Utilization of these polymer materials would be advantageous for solar cell efficiency since they allow a larger quantity of radiation to arrive to the active zone. Several reports [5, 9, 12, 28, 80] discuss the reported record for PV efficiency which is 47.1 registered for concentrator PVs having several junctions, designed at National Renewable Energy Laboratory from America. Almost similar performance is attained for concentrated solar cells, where the efficiency is around 40%. These devices are less expensive and have good thermal stability, but they require large space and installation time.

Reflection losses

Shielding layer

TC layer Active region

Metal contact

Figure 9.2  Schematic representation of reflection losses occurring in a solar cells with superstrate configuration.

264  Fundamentals of Solar Cell Design Such performance is followed by perovskites with an efficiency of 31%, which impose low expenses and are relatively fact to install. An intermediate efficiency around 10%–17% is noted for mono- and polycrystalline silicon, dye sensitized, CdTe, and CIGS solar cells. The lowest results were obtained for amorphous silicon and polymer-based PVs.

9.4 Conclusion Nowadays, the solar cells engineering is viewed as a part of the answer to the energy necessities and as an essential alternative of subsequent energy generation at worldwide level. Despite the remarkable developments in the area of solar cells, only seldom have been emphasized in mainstream science and engineering communities. In this chapter, the progress in varying types of materials and strategies to enhance the efficiency is summarized. The main parameters that affect the PV performance are described in order to understand by which means the conversion efficiency can be varied. Several light harvesting approaches have been proposed to avoid losses as a result of absorption, scattering, and reflection which rely on controlling material nano-topography, optical transparency, and refraction index dispersion. Even if approaches involving solar energy are relatively facile, there is still a great need to create an efficient and resistant material. Around 16% from solar radiation can be transformed into electric energy using a technology such as nanocrystal-derived solar cell. Polymer cells are affected in time during sunlight exposure, but such devices are still representing a feasible solution. Future studies should address this aspect by incorporating particles that enhance polymer reliability to prolonged sun exposure. In any case, the developments regarding the fourth generation of PVs will represent the next step in renewable energy area since they provide excellent performance levels which are truly competing with those of classical PVs working on Si, therefore opening a novel outlook for the solar energy technology.

Acknowledgements This chapter was prepared with financial support of Romanian National Authority for Scientific Research and Innovation, UEFISCDI, project PN-III-P1-1.1-TE-2019-1878 no. TE 83/1.09.2020.

Enhanced Conversion Efficiency of Solar Cells  265

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Enhanced Conversion Efficiency of Solar Cells  269 65. Uzum, A., Kuriyama, M., Kanda, H., Kimura, Y., Tanimoto, K., Fukui, H., Izumi, T., Harada, T., Ito, S., Sprayed and Spin-Coated Multilayer Antireflection Coating Films for Nonvacuum Processed Crystalline Silicon Solar Cells. International Journal of Photoenergy, 2017, 3436271, 2017. 66. Moayedfar, M., Assadi, M.K., Various types of anti-reflective coatings(arcs) based on the layer composition and surface topography: a review. Rev. Adv. Mater. Sci., 53, 187, 2018. 67. Habubi, N.F., Ismail, R.A., Mishjil, K.A. et al. Increasing the Silicon Solar Cell Efficiency with Nanostructured SnO2 Anti-reflecting Coating Films. Silicon, 11, 543 (2019). 68. Hossain, J., Mondal, B.K., Mostaque, S.K., Al Ahmed, S.R., Shirai, H., Optimization of multilayer anti-reflection coatings for efficient light management of PEDOT:PSS/c-Si heterojunction solar cells. Mater. Res. Express, 7, 015502, 2020. 69. Da, Y.,Yimin, X., Role of Surface Recombination in affecting the efficiency of nanostructured thin-film solar cells. Opt. Express, 21, A1065, 2013. 70. Shang, Y., Hao, S., Yang, C., Chen, G., Enhancing solar cell efficiency using photon upconversion materials. Nanomaterials (Basel), 5, 1782, 2015. 71. Trupke, T., Shalav, A., Richards, B.S., Würfel, P., Green, M.A., Efficiency enhancement of solar cells by luminescent up-conversion of sunlight. Sol. Energy Mater. Sol. Cells, 90, 3327, 2006. 72. Hasemi, S.A., Ramakrishna, S., Aberle, A.G., Recent progress in flexible– wearable solar cells for self-powered electronic devices. Energy Environ. Sci., 2020. 73. Kim, H.J., Kim, D.E., Effect of surface roughness of top cover layer on the efficiency of dye-sensitized solar cell. Solar Energy, 86, 2049, 2012. 74. Patel, D., Deshmukh, S.P., Polymer in Sustainable Energy. J. Miner. Mater. Charact. Eng., 11, 661, 2012. 75. Aydin, E., Sankir, N.D., Photovoltaic Performance and Impedance Spectroscopy Analysis of CuInS2Thin Film Solar Cells Deposited on Polyimide Foil via Spray Pyrolysis. Int. J. Electrochem. Sci., 12, 9626, 2017. 76. Niu, H., Wang, C., Bai, X., Huang, Y., New perylene polyimides containing p‐n diblocks for sensitization in TiO2 solar cells. Polym. Adv. Technol., 15, 701, 2004. 77. Dayneko, S., Tameev, A., Tedoradze, M., Martynov, I., Artemyev, M., Nabiev, I., Chistyakov, A., Hybrid heterostructures based on aromatic polyimide and semiconductor CdSe quantum dots for photovoltaic applications. Appl. Phys. Lett. 103, 063302, 2013. 78. Tiwari, A.N., Romeo, A., Baetzner, D., Zogg, H., lexible CdTe solar cells on polymer films. Prog. Photo.: Res. Appl., 9, 211, 2001. 79. Hulubei, C., Albu, R.M., Lisa, G., Nicolescu, A., Hamciuc, E., Hamciuc, C., Barzic, A.I., Antagonistic effects in structural design of sulfur-based

270  Fundamentals of Solar Cell Design polyimides as shielding layers for solar cells. Solar Energy Materials and Solar Cells, 193, 219, 2019. 80. Geisz, J. F., Steiner, M. A., Jain, N., Schulte, K. L., France, R. M., McMahon, W. E., Perl, E. E., Friedman, D. J., Building a Six-Junction Inverted Metamorphic Concentrator Solar Cell. IEEE Journal of Photovoltaics, 8, 626, 2018.

10 Solar Cell Efficiency Energy Materials Zeeshan Abid1, Faiza Wahad1, Sughra Gulzar1, Muhammad Faheem Ashiq1, Muhammad Shahid Aslam1, Munazza Shahid2, Muhammad Altaf1 and Raja Shahid Ashraf1* Department of Chemistry, Government College University Lahore, Lahore, Pakistan 2 Department of Chemistry, University of Management and Technology, Lahore, Pakistan 1

Abstract

Energy is vital for the sustainability of modern human society. The primary sources of energy, i.e., fossil fuels, are depleting at an alarming pace. Additionally, the excessive burning of fossil fuels has resulted in health disorders and environmental pollution. In this scenario, renewable energy resources, like solar energy, offer a promising alternative to fossil fuels. Solar energy can be collected by light-absorbing materials and converted into electrical energy using solar cells (SCs). For successful commercialization, SCs must deliver high power conversion efficiency (PCE) at affordable costs. The interdisciplinary area of SC development has drawn attention from diverse science and engineering research communities. Consequently, a myriad of SC materials was produced, followed by a dramatic increase in PCEs. We reviewed all the major classes of SC materials, including inorganic semiconductors, organic semiconductors, and organic-inorganic hybrid materials with notable PCE reports. Major challenges faced by material classes and remediation strategies are discussed briefly. At present, crystalline silicon SCs retain their dominance in the commercial market with a maximum PCE of 27.6%. On the other hand, perovskite SCs, cadmium telluride SCs, organic SCs, and dye-sensitized SCs have emerged as cheaper and flexible alternatives with maximum PCEs of 25.2%, 22.1%, 17.4%, and 12.3%, respectively. Emerging solar technologies have undoubtedly an enormous scope ahead as a mainstream source of energy.

*Corresponding author: [email protected] Inamuddin, Mohd Imran Ahamed, Rajender Boddula, and Mashallah Rezakazemi (eds.) Fundamentals of Solar Cell Design, (271–316) © 2021 Scrivener Publishing LLC

271

272  Fundamentals of Solar Cell Design Keywords:  Solar cells, power conversion efficiency, energy materials, inorganic semiconductors, organic semiconductors, organic-inorganic hybrids

10.1 Introduction All sectors of modern human society require a consistent supply of energy for proper functioning. However, the available energy resources, i.e., fossil fuels, have been depleting apace and may exhaust entirely within the next quarter-century. In addition to the finite supply, fossil fuels have led to global carbon imbalance, environmental pollution, and health disorders. Therefore, it is need of the hour to develop renewable energy resources to sustain the modern human society. Renewable energy is harnessed from nature like sunlight, wind, water, and biomass, etc. Unlike fossil fuels, renewable energy resources are abundant and environment-friendly. The use of renewable energies has recently increased. Still, several problems hinder their practical applications such as low energy density, a fluctuating supply of power, and inefficient collection and storage technologies. Solutions to these problems will not only ensure sustainable energy production from renewable energy resources but will also reduce the overdependence and consequences of fossil fuels [1–4]. Solar energy has been under the spotlight for many lucrative reasons. Firstly, solar energy is abundant and inexhaustible compared with our life span. The energy generated from forty minutes of sunlight irradiation on the Earth equals the amount of energy consumed annually worldwide. Secondly, solar energy is environmentally clean and has a low carbon footprint. If solar energy becomes the sole power generation source, the repercussion of fossil fuels, like global temperature rise and environmental pollution could be averted. Last but not least, solar energy can be collected from sunlight and directly converted into electric power by using a solar cell (SC) [5–7]. A SC comprises light-absorbing semiconductor materials sandwiched between the electrodes. When light falls upon a SC, it penetrates semiconductor materials and generates electric charges, which are transported to respective electrodes (Figure 10.1). Figure 10.2 illustrates a single SC, module, and panel. Strings of SCs are assembled to develop modules and further connected in series to design solar panels. An array of solar panels combined with electrical equipment is referred to as a photovoltaic (PV) system [8–10]. PV systems are idealized to have a long lifetime, automated operation, and easy maintenance. On the other side, solar power generation from

Solar Cell Efficiency Energy Materials  273 antireflection coating light front contact emitter

absorbing layer external load

rear contact

electron-hole pair

Figure 10.1  A general diagram of a SC displaying different layers responsible for capture and conversion of light.

(a)

(b)

(c)

Figure 10.2  (a) Solar cell, (b) solar module, and (c) solar panel.

PV systems is more expensive compared with thermal, nuclear, and other renewable power generation resources. Moreover, solar energy is weather dependent and usually provides a small energy density. To address these demerits, researchers have focused on the advancement of PV technologies [11–14]. Several generations of SCs have been introduced, each with a unique set of materials and architectural design, to increase their affordability and power conversion efficiency (PCE) [15–21]. In this chapter, we will provide an account of prominent solar energy materials and their performance as well as efficiency optimization measures.

274  Fundamentals of Solar Cell Design

10.2 Solar Cell Efficiency Any SC requires four primary attributes to function correctly, i.e., (1) absorption of light, (2) generation of charge bearers, (3) transportation of charges to the respective electrode, and (4) charge extraction toward an external circuit. These functions govern the properties of a PV cell and its output power, and eventually, the PCE. PCE is the ratio of the maximum electric output to the input energy, and it depends on short-circuit current density (Jsc), open-circuit voltage (Voc), and fill factor (FF). Illuminated J-V characteristics is used to calculate these parameters at standard test conditions, i.e., 1000 W/m2 total irradiance, 1.5 spectrum air mass (AM), 25°C temperature. The relation for PCE calculation is given as

J sc × Voc × FF Pin

PCE =



The short-circuit current, Isc, is the maximum current provided by a SC when the net voltage across the device is zero. Isc is dependent on the photon flux incident and SC area, whereas Jsc, is independent of the SC area. Voc is the maximum voltage provided by a SC when the current across the cell is zero. The FF is the ratio of maximum electric output of a SC to the product of Jsc and Voc. Figure 10.3 shows the JV curve of a SC showing Jsc, Voc, and FF [9, 10, 17, 22].

JSC

illuminated J-V curve

current density

FF

rve r cu

put

out

e pow

voltage

Figure 10.3  J-V curve of a SC showing Jsc, Voc, and FF.

VOC

Solar Cell Efficiency Energy Materials  275 Some of the major limiting factors of PCE are low absorption range, thickness, shading or light reflectance, and morphological defects in the material structure. Therefore, it is desirable to tailor the SC materials for a high PCE by carefully manipulating their bandgap energy, spectral absorption, and light trapping capacity. We will discuss all the prominent SC materials with the efficiency enhancement concepts in Section 10.4.

10.3 Historical Development of Solar Cell Materials The human quest for light-absorbing tools and materials dates back to ancient times. Historical evidence suggests that Greeks and Romans used magnifying glasses and concentrating mirrors to make fire from the solar radiation as early as 7th century BCE. Centuries later, in 1767, Genevan physicist Horace-Bénédict produced the world’s first solar cooker comprising of multiple glass boxes insulated from inside and enclosed within each other to trap solar heat. Upon interaction with sunlight, the innermost glass box could achieve a temperature of 110°C, perfectly suitable for boiling and cooking purposes. In 1839, the French scientist Alexandre-Edmond Becquerel realized the phenomenon of PV effect through his experiments involving interaction between electrolytes and sunlight. In the following years, works of English electrical engineer, Willoughby Smith, and British natural philosopher, Professor William Grylls Adams, demonstrated photoconductivity of semiconductor materials. Their work eventually resulted in the development of the first solar module based on a gold-selenium junction with 1% PCE conceived by the American inventor Charles Edgar Fritts. The conception of the photoelectric effect in 1887 by the German physicist Heinrich Hertz and the theoretical works of Albert Einstein published in 1905 further rationalized the concept of harnessing and conversion of the sunlight. It was not until the 1920s that the largescale production of future energy materials began. In particular, Polish chemist Jan Czochralski introduced the method to produce high-grade crystalline materials, which became a crucial process in the development of crystalline silicon (c-Si) technology in the late 20th century. Years of 1953–1956 are marked as the birth of first-generation SC at the Bell laboratories USA with 6% PCE. During this period, the performance indicators of a SC were designed by American chemist Dan Trivich. Meanwhile, rapid progress in the studies on other potential energy materials such as cadmium sulfide (CdS), II-VI semiconductors, and organic semiconductor compounds was observed. Till the next decade, Si-based SCs remained the center of attention for many companies and laboratories

276  Fundamentals of Solar Cell Design supplying telecommunication satellites with energy. Unsurprisingly, the cost of the SCs was too high for a common and widespread application. Besides, the size, PCE, stability, and lifetime of SCs required thoughtful considerations. In 1970, the ongoing quest for Si alternatives resulted in III-V semiconductor, and gallium arsenide (GaAs)–based heterojunction SC by the Soviet physicist Zhores Alferov. In 1974, American physicist David E. Carlson and electrical engineer Christopher R. Wronski introduced the first thin-film SC (TFSC) using amorphous Si (a-Si) at RCA laboratories. Followed by the oil crisis in the late 1970s, the PV industry thrived from a space-oriented niche to a competitive energy production source. In 1980, a research group at the University of Delaware, USA, demonstrated first TFSC with 10% PCE, using junction of copper (Cu) and cadmium (Cd) sulfides i.e., Cu2S/CdS. While the new SC materials were continually developing, a large number of researches focused on improving the production methods of existing materials and improving their PCEs. In 1985, Australian scientists from the University of New South Wales reported the c-Si SCs with 20% PCE. In 1988, Michael Grätzel and Brian Table 10.1  Properties and features idealized for solar cell materials. Properties

Features

Availability

Convenient supply of material should be possible for industrial-scale production

Cost

Cost of the material should be competitive with alternative systems

Toxicity

Materials should be environmentally friendly or at least controllable if toxic

Stability

Materials should be resistive against degradation to allow a long lifetime and coverage of investment costs

Bandgap

Bandgap should be near 1.4 eV to maximize lightabsorption, convenient charge generation, and long diffusion length

Structure

Crystal lattice of thin films should have no or minimum morphological defects to reduce recombination losses

Production/Deposition

Production and deposition method should be available and cost-effective

Solar Cell Efficiency Energy Materials  277 O’Regan introduced SCs based on organic dyes as a semiconductor material. These so-called “dye-sensitized SCs” (DSSCs) offered much cheaper alternatives to the market-leading Si SCs. The trend of novel materials and PCE improvement went on, eventually achieving the 30% PCE milestone with the III-V semiconductor, Indium gallium phosphide (InGaP)/GaAs two-terminal concentrator SC, developed by the U.S. National Renewable Energy Laboratory (NREL) in 1994 [23–25]. From the beginning of the 21st century, PCE of existing inorganic and organic SC (OSC) materials have increased steadily. Researchers have synthesized novel materials and developed doping and coupling methods of existing materials to control bandgaps and acquire desired optoelectronic properties. Several years of solar materials development has equipped researchers with the tools and techniques to make rational choices when designing and synthesizing new materials. Table 10.1 summarizes some of the ideal features for SC materials. Emerging applications of SCs have served as the motivation for tailoring the optoelectronic characteristics of existing SC materials and the development of novel materials with discrete features.

10.4 Solar Cell Materials and Efficiencies In essence, SC materials are inorganic, organic, and hybrid semiconductors employed in different physical and chemical phases, forms, and sizes. These materials constitute three different generations of SCs, i.e., Si SCs, TFSCs, and emerging SCs. Figure 10.4 highlights the major categories of SCs materials. About a half-century ago, elemental Si set the foundation of Silicon Age. Since then, the modern world economy has been revolutionized with remarkable digital advances. Si exists in monocrystalline, polycrystalline, Inorganic Organic

Hybrid

Silicon Alloys

Polymers

Dyes/TiO2

III-V Semiconductors

Small Molecules

Perovskites

Silicon

Chalcogenides

Figure 10.4  Prominent inorganic, organic, and hybrid SC materials.

278  Fundamentals of Solar Cell Design and amorphous allotropes. The allotropic forms of Si differ in crystal packing and lattice structure. The c-Si SCs have dominated the market due to high performance and greater reliability compared with other PV technologies. Being the earliest kind in the industry, c-Si SCs are also called first-generation SCs [26, 27]. The subsequent generations of SCs use thin films instead of crystalline wafers. TFSC materials are inorganic and organic semiconductors deposited as very thin layers on substrates such as glass, steel, or a polymer foil. Film thickness and deposition parameters strongly influence the optoelectronic, morphological, and physicochemical properties of thin-films. TFSC materials were idealized to constitute lightweight, cost-effective, flexible, and high-performance SCs. While these materials have found potential smart applications, e.g., semi-transparent and building integrated PV solutions due to being lighter and flexible, their cost and performance still lag behind c-Si SCs, with a few exceptions. The mainstream TFSC materials include III-V semiconductors, chalcogenides, organic conjugating compounds, nanomaterials, and perovskites [28, 29]. We will now review prominent SC materials, their problems and challenges, efficiency enhancement concepts, and notable PCE reports from the literature.

10.4.1 Crystalline Silicon Crystalline Si (c-Si) has a bluish-grey appearance, a density of 2328 kg/ m−3, and a diamond-cubic crystal structure. Si atoms form a self-repeating and symmetric pattern in the crystal lattice. However, crystal pattern is not identical in all the planes, which makes c-Si an indirect bandgap material [30]. Hence, c-Si has a much lower absorption coefficient than direct bandgap materials, e.g., a-Si and GaAs. Because of the low absorption coefficient, c-Si wafers should be hundreds of microns think to absorb sufficient light. This requirement makes c-Si unsuitable for smart applications in addition to the rise in the processing and handling costs. Nevertheless, at present, c-Si SCs are prevailing SCs in the PV market [27]. From the perspective of spectral and bandgap utilization, c-Si can theoretically generate a maximum Jsc of 45 mA/cm2. The recombination rate of charge carriers in c-Si depends on the allotropic forms of Si. Mono c-Si, or single-crystalline Si, is made out of highest-grade Si and has a characteristic dark black external coloring. The entire crystal lattice of mono c-Si is a single, continuous, and homogeneous crystal with constant orientation and electronic properties. Poly c-Si, also known as multi-crystalline Si, comprises multiple small Si crystals or crystallites and has a visible metal flake effect when shaped into a SC. Since wafers of mono c-Si are free of

Solar Cell Efficiency Energy Materials  279 the grain boundary, their charge carriers have a longer life-time, and thus, they show much larger Voc values [31]. Figure 10.5 illustrates the apparent differences between mono c-Si and poly c-Si. Mono c-Si solar panels exhibit a high PCEs, great space-efficiency, and long lifetime as compared to the rival technologies. On the other side, mono c-Si SCs are the most expensive commercial SCs. The Czochralski Growth method produces large cylindrical ingots of mono c-Si, which are cut out from four edges to make Si wafers. Thus, wastage of the raw material leads to the price flare-up. A poly c-Si SC is produced from pouring melted Si into a square mold and cutting into a square-shaped wafer. The relatively straightforward production method of poly c-Si significantly reduces the raw material wastage and final manufacturing cost. However, the crystal structure in poly c-Si is less ordered compared with mono c-Si, and therefore it performs less efficiently and lives a shorter life [32–34]. The highest efficiency reported by a mono c-Si SC (with concentrator) and poly c-Si is 27.6% and 23.2%, respectively [35]. A conventional c-Si SC comprises a p-type wafer with a 1-µm-thin n-type wafer layer on the top. The n-type layer, also termed as the emitter layer, is kept thinner to allow higher charge generation and deeper diffusion length across the p-n junction (Figure 10.6). Several optimization techniques have developed to increase charge transport efficiency and decrease recombination losses. Covering the emitter layer with an insulating passivation layer is one example, which stabilizes the Si atoms in the emitter layer. Si oxides (SiOx) and Si nitride (SixNy) are commonly used as passivation layers and can be deposited through oxidation and plasma-enhanced chemical vapor deposition (PE-CVD), respectively [36]. Another approach is to increase the doping of the emitter layer to reduce minority charge carriers and surface recombination velocities. However, this approach requires careful consideration

(a)

(b)

(c)

(d)

Figure 10.5  (a) A monocrystalline Si lattice and (b) mono c-Si SC, (c) a polycrystalline Si lattice, and (d) poly c-Si SC.

280  Fundamentals of Solar Cell Design

front contact

antireflection coating n-type emitter

p-type layer

p-type wafer

rear contact

Figure 10.6  A general structure of a c-Si SC.

as heavy doping adversely affects the hole diffusion [37]. The front and rear contacts also require numerous physical and mechanic considerations. The front wafer surface is textured with discrete structures and covered with anti-reflective coatings to avoid the reflection losses, a concept termed as “Passivated Emitter Rear Locally” or PERL. To prevent the shading losses, the “Interdigitated Back Contact” (IBC) approach is used where both nand p-charge collectors are positioned backward to the SC [38–40]. Among other efficiency enhancement concepts, heterojunction with intrinsic thin layer (HIT) device structure holds great importance. Unlike the above-mentioned homojunction c-Si SC where both n- and players have a matching bandgap, HIT cells use two different junctions of Si, i.e., c-Si and hydrogenated a-Si (a-Si:H). In a typical device, an n-type c-Si layer is sandwiched between two intrinsic layers of a-Si:H, both of which are further connected to two additional layers of n- and p-type a-Si:H. The bandgap of a-Si:H is ~1.7 eV, which enables a wide spectral range of absorption and adds a good passivation layer to the c-Si wafer. Consequently, the charge carrier lifetime and Voc values are significantly enhanced. Further benefits of HIT SCs include its bifacial configuration. Since the conductivity of a-Si is weak, the charges are transported through a transparent conduction oxide (TCO) layer deposited, like indium tinoxide (ITO), on both front and back contacts. This architecture enables the cell to collect direct light from the front as well as the diffuse light reflecting its backside. Furthermore, the deposition of a-Si in a HIT cell is possible through PE-CVD with feasible processing conditions and low cost [41, 42]. Highest efficiency recorded by a Si heterojunction HIT is 26.7% [35]. Table 10.2 shows notable one sun cell, and module results for c-Si SCs reported in the literature.

Solar Cell Efficiency Energy Materials  281 Table 10.2  Notable performance results for c-SI SCs and modules. Types

PCE (%)

Area (cm2)

Voc (V)

Jsc (mA/ cm2)

(FF)

References

c-Si cell

26.7 ± 0.5

79.0 (da)

0.74

42.7

84.9

[43]

c-Si cell

26.3 ± 0.5

180.43 (da)

0.74

42.3

83.8

[44]

c-Si cell

26.1 ± 0.3

3.99 (da)

0.73

42.6

84.3

[45]

c-Si cell

26.0 ± 0.5

4.02 (da)

0.73

42.05

82.3

[46]

c-Si cell

25.8 ± 0.5

4.01 (da)

0.72

42.9

83.1

[43]

c-Si cell

25.7 ± 0.5

4.02 (da)

0.73

42.5

83.3

[43]

c-Si cell

25.6 ± 0.5

143.7 (da)

0.74

41.8

82.7

[47]

c-Si cell

25.3 ± 0.3

4.01 (da)

0.72

42.5

82.8

[44]

c-Si large cell

25.2 ± 0.5

153.48 (t)

0.74

41.3

82.7

[47]

c-Si large cell

25.1 ± 0.5

151.88 (ap)

0.74

40.8

83.5

[48]

c-Si cell

25.0 ± 0.5

4.0 (da)

0.71

42.7

82.8

[47]

c-Si module

24.4 ± 0.5

13177 (da)

79.5

5.04

80.1

[44]

c-Si module

23.8 ± 0.5

11562 (ap)

53.4

6.3

81.6

[47]

Si (DS wafer cell)

23.2 ± 0.3

247.79(t)

0.71

41.1

79.3

[46]

poly c-Si cell

22.3 ± 0.4

3.92 (ap)

0.67

41.1

80.5

[43]

poly c-Si cell

22.0 ± 0.4

245.83 (t)

0.67

40.6

80.9

[43]

poly c-Si cell

21.9 ± 0.4

4.0 (t)

0.67

40.8

79.7

[43]

poly c-Si cell

21.3 ± 0.4

242.7 (t)

0.67

39.8

80.0

[47] (Continued)

282  Fundamentals of Solar Cell Design Table 10.2  Notable performance results for c-SI SCs and modules. (Continued) Types

PCE (%)

Area (cm2)

Voc (V)

Jsc (mA/ cm2)

(FF)

References

poly c-Si cell

19.9 ± 0.4

15143 (ap)

78.9

4.80

79.5

[44]

poly c-Si module

19.5 ± 0.4

15349 (ap)

41.5

9.3

77.4

[47]

Abbrevations: ap, aperture area; da, designated illumination area; t, total area.

10.4.2 Silicon Thin-Film Alloys The c-Si performs great as a SC material but has a thick and inflexible device structure. Still, researchers managed to make TFSCs based on c-Si with a PCE of 21.2 [35, 49, 50]. However, a-Si is superior in its abundance, material simplicity, nontoxic nature, and convenient processing as a TFSC material. Unlike c-Si, the atomic arrangement of a-Si is disordered, showing a constant random network. Although the atoms are held together with a tetrahedral coordination structure, the valence electrons lack homogenous distribution and form dangling bonds. This distortion results in surface voids and volume deficiencies that can be passivated by hydrogen. The hydrogenated a-Si (a-Si:H) forms alloys with tetravalent elements like germanium (Ge) and carbon (C), and even with oxygen (O). Hydrogenated nanocrystalline (nc-Si:H) also forms alloys consisting of nano-sized grains embedded in the tissues of hydrogenated a-Si. Si alloys can be p-doped or n-doped with atoms of boron and phosphorus, respectively [51–53]. As a direct bandgap material, the bandgap of a-Si lies between 1.6 and 1.8 eV, which enables a higher absorption coefficient than c-Si. Therefore, a-Si is suitable for TFSC as the thin light-absorbing layer. A higher defect density limits the diffusion length of a-Si:H SC to ~100–300 nm. Therefore, a-Si:H cells are designed with p-i-n device structure, where a-Si:H layer is inserted as an intrinsic semiconductor between highly doped layers of p-type and n-type Si layers. This arrangement compensates for the poor conducting properties of a-Si. Figure 10.7 shows a general arrangement of layers in a Si alloy–based TFSC. A typical lab-scale device fabrication process starts with the deposition of ZnO:Al or ITO as the TCO layer via sputtering. The layer surface is treated with acid to impart surface texture, or otherwise, a pre-textured TCO layer is applied. Aluminum or silver strip is then deposited as the front contact, followed by the careful deposition of Si thin layers via the CVD method.

Solar Cell Efficiency Energy Materials  283 glass

TCO

back reflector (ZnO / SiOx)

Silicon p-i-n junction rear contact

Figure 10.7  A general structure of a Si thin-film SC.

Carefully choice of each layer is crucial to achieving a high PCE. For intrinsic layer, nc-Si:H offers a good result since it has a lower bandgap, and it offers better spectral utilization as compared to a-Si:H. High bandgap material, e.g., Si carbide(SiC) and SiOx are used for the p-layer, whereas SiOx or a-Si:H is employed as n-layer. For the SiC layer, silane (SiH4), and methane (CH4) are used as precursors, while SiOx layers can be deposited from the mixture of SiH4 with carbon dioxide (CO2). Diborane and phosphine are commonly used for p-doping and n-doping, respectively. The deposition of p-i-n junction is followed by covering the cell with a mask and, finally, a metallic back contact, often silver (Ag) [54, 55]. Apart from p-i-n junction, thin layers of a-Si:H and nc-Si:H are also combined in a p-n multi-junction architecture. In this tandem modular design, also known as micromorph, top a-Si:H layer absorbs in the visible region and transmits infrared light to bottom nc-Si:H layer. A multijunction Si TFSC with more than two junctions is also possible; however, the complementarity of materials is always a challenge [56, 57]. The major challenge of Si alloy–based SCs is degradation and efficiency reduction (up to 30%) of the cell within the six months of operation, a process called as the Staebler-Wronski effect [58]. Upon prolonged exposure to sunlight, the charge carriers have excessive recombination leading to the meta-stable defects in the Si alloy layers. This problem is encountered by making the cell surface nanotextured, which may prolong the average path length of light and allow light redistribution among different layers and junctions of the SC. Moreover, additional reflection layers with weak light-sensitivity, e.g., nc-SiOx:H are introduced between the junctions to redirect light toward the top junction [59]. Table 10.3 enlists promising reports on Si TFSCs reported in the literature.

284  Fundamentals of Solar Cell Design Table 10.3  Notable performance results for Si TFSCs. PCE (%)

Area (cm2)

Voc (V)

Jsc (mA/ cm2)

FF (%)

Reference

Si thin-film submodule

21.5

239 (ap)

0.687

38.5

80.3

[60]

Si thin-film submodule

21.4

240 (ap)

0.70

39

80

[60]

Si TFSC

14.3

1.1 (da)

1.9

10

74

[61]

Si TFSC

12.6

1 (da)

1.35

13.5

70.2

[62]

Si tandem

12.5

14321

280

0.9

70

[57]

Si thin-film minimodule

11.3

94 (ap)

0.50

30

72

[63]

Materials

10.4.3 III-V Semiconductors III-V semiconductors refer to the elements with three valence electrons, e.g., aluminum (Al), gallium (Ga), and indium (In), and five valence electrons, e.g., phosphorus (P) and arsenic (As). These elements have been combined in multiple forms to produce binary semiconductors like GaAs, GaP, InP, InAs, as well as ternary alloys and complex systems such as GaInAs, GaInP, AlGaInAs, and AlGaInP. The III-V solar materials, especially GaAs, have found a potential use in the satellite and space missions. Although the scarcity of Ga and toxicity of As has been a matter of concern, GaAs has drawn huge attention because of its record high PCE [64, 65]. GaAs single crystal, concentrator, and thin-film crystal single-junction SCs have achieved PCEs of 27.8%, 30.5%, and 29.1%, respectively [35]. Table 10.4 shows the highest efficiencies reported by III-V Semiconductors SCs. The crystal lattice of GaAs is like a diamond-cubic crystal structure with Ga and As atoms placed on alternating positions and a lattice constant of 565.315 pm. GaAs has a density of 5.3176 g/cm3 and a bandgap of 1.441 eV [66]. Due to a direct bandgap and high absorption coefficient, the GaAs layer thickness can be reduced multiple times compared with that of c-Si without affecting the absorption capacity. A single-junction GaAs SC structure is depicted in Figure 10.8. The record high PCE of GaAs is attributed to its multi-junction device structure. In a multi-junction SC, more than one bandgap is used to cover a large spectrum of light and minimize the energy losses. The GaAs wafer

Solar Cell Efficiency Energy Materials  285 top contact

Ohmic n-GaAs n-InGaP window p-GaAs emitter p-GaAs base metal back contact substrate

Figure 10.8  A general structure of a single-junction GaAs SC.

is placed between the higher bandgap semiconductor, i.e., GaInP with the bandgap of 1.86 eV on the top, and germanium (Ge) substrate on the bottom with the bandgap of 0.67 eV. This architecture allows the absorption of short wavelengths from the top by highest bandgap material and deeply penetrating longer wavelengths by the lowest bandgap material. Multi-junction SCs may include triple, quadruple, five, or even six junctions for improved spectral utilization. However, there is a risk of lattice mismatching in such systems turning the device into metamorphic multijunctions. Nevertheless, the proper profiling in the lattice constant through buffer layers can avoid the lattice mismatch. Keeping the unique bandgap-lattice constant combination is also a great challenge during device fabrication. Generally, III-V semiconductor layers are deposited through epitaxial growth technique, i.e., metal-organic chemical vapor deposition (MOCVD) using trimethylgallium, trimethylindium, trimethylaluminum, arsine gas, and phosphine gas as precursors. The crystalline growth process is both sensitive and expensive [67–69]. Due to the high-end applications of III-V SCs, the quality of III-V solar materials has not been compromised over the cost. However, there are some approaches to save the operational cost, such as the use of concentrator technology that concentrates a large amount of light onto small device areas. Although such approaches require additional services such as rapid cooling of devices to avoid or slow down the material degradation under elevated temperature, as well as guided operation to ensure the productive outcome [70].

286  Fundamentals of Solar Cell Design Table 10.4  Notable performance results for III-V semiconductor-based SCs. PCE (%)

Area (cm2)

Voc (V)

Jsc (mA/ cm2)

(FF)

References

InGaP/GaAs/ InGaAs multijunction cell

37.9 ± 1.2

1.047 (ap)

3.065

14.27

86.7

[44]

GaInP/GaAs/ GaInAs multi-junction

37.8 ± 1.4

0.998 (ap)

3.013

14.60

85.8

[71]

GaInP/GaAs monolithic multi-junction

32.8 ± 1.4

1.000 (ap)

2.568

14.56

87.7

[43]

GaInP/GaAs monolithic multi-junction

31.6 ± 1.5

0.999 (ap)

2.538

14.18

87.7

[44]

InGaP/GaAs/ InGaAs multi-junction

31.2 ± 1.2

968 (da)

23.95

1.506

83.6

[44]

GaAs TFSC

29.1 ± 0.6

0.998 (ap)

1.1272

29.78

86.7

[71]

GaAs TFSC

28.8 ± 0.9

0.9927 (ap)

1.122

29.68

86.5

[47]

GaAs thin-film module

25.1 ± 0.8

866.45 (ap)

11.08

2.303

85.3

[43]

GaAs thin-film module

24.8 ± 0.5

865.3 (ap)

11.07

2.288

84.7

[72]

GaAs thin-film module

24.1 ± 1.0

858.5 (ap)

10.89

2.255

84.2

[47]

GaInP cell

22.0 ± 0.3

0.2502 (ap)

1.4695

16.63

90.2

[48]

GaInP cell

21.4 ± 0.3

0.2504 (ap)

1.4932

16.31

87.7

[72]

GaAs multicrystalline cell

18.4 ± 0.5

4.011 (t)

0.994

23.2

79.7

[47]

Types

Solar Cell Efficiency Energy Materials  287

10.4.4 Chalcogenide Chalcogens are elements from group 16 of the periodic table such as sulfur (S), selenium (Se), and tellurium (Te). Compounds that consist of at least one chalcogen anion attached to one or more electropositive elements are classified as chalcogenides. This section does not include discussion on oxides and polonium (Po) and focuses on two mainstream chalcogenide TFSC materials, namely, chalcopyrite and cadmium tellurium (CdTe) [73].

10.4.4.1 Chalcopyrites Chalcopyrites are ternary semiconductor materials consisting of elements from the groups I, III, and VI of the periodic table. The crystal structure of I-III-VI semiconductors resembles the tetragonal crystal system of chalcopyrite mineral (CuFeS2). Copper indium diselenide (CuInSe2, CIS) and copper gallium diselenide (CuGaSe2, CGS) are two common examples of chalcopyrites which are mixed to produce larger systems such as [Cu(InxGa1-x)Se2, CIGS] (x = 0–1), or [Cu(InxGa1-x)(SeyS1-y)2, CIGSS] (y = 0–1). Bandgaps of CIS and CGS are 1.0 and 1.7 eV, respectively. In the CIGS mixture, the ratios of In:Ga (x) and Se:S (y) can be adjusted to acquire the desired bandgap from 1.0 to 1.7 eV. Being a direct bandgap semiconductor material, CIGS(S) shows a high absorption coefficient, which allows strong absorption of light and sufficient excitations at the film thickness of 1–2 µm. The improved film deposition techniques allow both high and low-temperature processing with the optimization of substrate types such as glass, steel, aluminum, and polymer foils [74, 75]. The best PCE reported by a CIGS SC is 23.4% [35]. In a typical CIGS SC, a thin layer of molybdenum (Mo) (~500 nm) is coated on a glass substrate as rear contact. This is followed by the deposition of p-type CIGS layer and comparatively thinner n-type CIGS layer on the top either by co-evaporation of precursors on a heated substrate or sputtering the precursors on a substrate at room temperature. The p-n junction is covered with a 50-nm thin buffer layer of CdS. Then, an intrinsic ZnO (i-ZnO) layer is deposited, followed by aluminum-doped ZnO (ZnO:Al). A minute quantity of sodium (Na) in CIGS layers reduces recombination and increases Voc. Na is added either during the deposition process or through using the soda-lime glass substrate as Na source. The bandgap of each layer is carefully tailored during industrial-scale production. The bandgap of the p-type CIGS layer is between 1.1 and 1.2 eV, whereas the n-type CdS buffer has a bandgap of 2.5 eV. The bandgap of ZnO is 3.2 eV, which helps minimize the absorption losses. Overall, the

288  Fundamentals of Solar Cell Design

n-ZnO:Al i-ZnO p-CdS buffer n-CIGS p-CIGS Mo rear contact glass substrate

Figure 10.9  A general structure of a CIGS SC.

deposition process is very complicated and challenging [76, 77]. Figure 10.9 shows a schematic structure of a CIGS SC. The possible scarcity of rare earth elements such as indium is becoming a matter of concern for CIGS SCs since it may limit its large-scale deployment. Therefore, some other chalcopyrite systems consisting of only abundant elements have also been investigated. Examples include iron-free kesterites such as Cu2ZnSnS4 (CZTS) and Cu2ZnSnSe (CZTSe), and their mixture Cu2ZnSn(SSe)4 (CZTSS). CZTS materials are considered to be less toxic but also less efficient than their CIGS competitors with the highest PCE of 12.6% [35, 78, 79]. CIGS SCs have demonstrated several recordhigh PCE values, as listed in Table 10.5.

10.4.4.2 Cadmium Telluride (CdTe) CdTe is an II-VI semiconductor material having a zincblende lattice structure and a bandgap of 1.44 eV. As a direct bandgap material, a few micrometers thick CdTe layer can absorb enough energy to allow electron excitations. CdTe-based SCs show a high PCE, lowest carbon footprint, and the shortest energy payback time [80]. At present, the highest PCE of CdTe stands at 22.1% [35]. In a CdTe heterojunction SC, transparent front contact, e.g., Cd-Snoxide alloy, is used as the glass superstrate. Then, the n-type CdS layer is deposited, followed by the p-type CdTe absorber layer by closed space sublimation method. Bandgaps of CdS and CdTe are 1.44 and 2.4 eV, respectively. The deposition of suitable metal back contact has been challenging

Solar Cell Efficiency Energy Materials  289 Table 10.5  Notable performance results for CIGS SCs. Area (cm2)

Voc (V) Jsc (mAcm2)

FF (%) Version

23.35 ± 0.5

1.043(da)

0.734

39.58

80.4

[48]

CIGS TFSC

22.9 ± 0.5

1.041 (da)

0.744

38.77

79.5

[45]

CIGS TFSC

22.60.5

0.4092(da)

0.7411

37.78

80.6

[44]

CIGS TFSC

22.3 ± 0.4

0.510 (da)

0.4219

39.38

78.2

[47]

CIGS cell

21.7 ± 0.5

1.044 (da)

0.718

40.70

74.3

[72]

CIGS cell

21.0 ± 0.6

0.9927 (ap)

0.757

35.70

77.6

[47]

CIGS (Cd free)

19.2 ± 0.5

841 (da)

48.0

0.456

73.7

[72]

CIGS (mini­ 18.7 ± 0.6 module)

15.892 (da)

0.701

35.29

75.6

[47]

CIGS cell (Cd free)

17.5 ± 0.5

808 (da)

47.6

0.408

72.8

[47]

CIGS large cell

15.7± 0.5

9703 (ap)

28.24

7.254

72.5

[47]

Types

PCE (%)

CIGS cell (Cd‐free)

since CdTe is not complimentary to many available materials. Stabilized antimony telluride layers combined with Mo have been successfully used as back contacts [81–84]. Figure 10.10 presents a general diagram of CdTe SC. CdTe PV technology has received a skeptical public opinion due to the toxicity of Cd and its environmental impact. Although the CdTe modules are recycled to avoid improper disposal, the recycling cost still exceeds the resale value. Besides, the tellurium being a rare and expensive metal has drawn concerns related to future scarcity and scalability limitations. Efforts are being made to combat the challenges [85]. Nevertheless, CdTebased SCs are among the highly efficient inorganic SCs, which are relatively cheap at the same time. Table 10.6 enlists some leading efficiency reports on CdTe-based TFSCs.

10.4.5 Organic Materials OSC materials are organic semiconductors such as conjugating polymers and organic small molecules. Organic semiconductors have delocalized

290  Fundamentals of Solar Cell Design

front glass n-doped SnO2 n-CdS buffer p-CdTe metal back contact

Figure 10.10  A general structure of a CdTe SC.

Table 10.6  Notable performance results for CdTe SCs. Materials

PCE (%)

Voc (V)

Jsc (mA/cm2)

FF (%)

Reference

CdTe

22.1

0.89

31.7

78.5

[86]

21.0

0.88

30.1

79.4

[87]

20.8

0.85

30.5

79.8

[88]

18.6

0.11

1.53

74.2

[89]

16.76

0.83

27.32

73.14

[90]

15.6

0.85

25.4

72

[91]

15.2

0.85

25.0

73.8

[92]

p-orbitals that allow electrons to flow throughout molecular crystals or amorphous thin films. The movement of electrons takes place between bonding and antibonding molecular orbitals, i.e., HOMO and LUMO, respectively. The distance between both orbitals is considered as bandgap of organic materials. Moreover, the concept of p-type and n-type inorganic materials is translated as “electron donors” and “electron acceptors” for organic semiconductors [93]. Electron donor organic materials have a low-ionization potential that results in the convenient release of electrons from HOMO orbital (valence band), leaving behind holes. In contrast, electron acceptors have a higher electron affinity, and they grab electrons readily to fill their LUMO orbital (conduction band). However, in contrast to inorganic semiconductors, the electron-hole pair does not diffuse

Solar Cell Efficiency Energy Materials  291 (a) Benz[b]anthracene

Pentacene

S

S

S S

S

S

DBP

a-Sexithiophene O

OH

HO

N N

2+

N

O

OH

Cu

N

HO

C101 Dye

Zn

N

N

N

N N

N Zinc phthalocyanine CN

Ru

N

N C

NCS

N

C N

NCS

N C

C N

TDCV-TPA

N

S

O O

CN N S N

S

S

N

N

HO

N

Copper(II) phthalocyanine

S

N

N

N

CH3(CH2)4CH2

S

N

N

DPSQ

CH3(CH2)4CH2

N

N

DPDCPB

C N

N

C N

OH

(b) H3C(H2C)6H2C

CH3 H3C

CH2(CH2)4CH3

CH2(CH2)4CH3 F

F

S S

S

S

n N NN CH2(CH2)6CH3

S S

J51 H3C(H2C)4H2C

S

C8H17O

OC8H17

S

P3HT n S

S

S

S

* n

N S TQ1

n

S

PQT-12

S

n

S

S

PBDB-T n

CH3

OCH3

CH3 N

CH3 O

O

S

S Si N N C8H17 C8H17 S PSiF-DBT

CH2(CH2)6CH3

CH3

CH3 H3C

RO

S S

S

n

R= * *

MEHPPV

CH3 CH3

Figure 10.11  (a) A few examples of commercially available donor materials (small molecules) in OSCs. (b) A few examples of commercially available donor materials (polymers) in OSCs.

292  Fundamentals of Solar Cell Design away quickly in the organic semiconductors. Instead, electron-hole pairs, or excitons, are present in the bound state with a lifetime of a few nanoseconds and diffusion length of roughly 10 nm [94, 95]. Therefore, considerable attention has been focused on carefully tailoring the bandgaps of materials for effective electron-hole partition and transport [96–98]. Some commercialized electron donors, i.e., both small molecules and polymers, are shown in Figures 10.11a and b, respectively. Figure 10.12 shows commercially available non-fullerene and fullerene acceptor compounds. Significant advancements in the design of OSC materials have rationalized their future potential and widespread application. Besides, OSCs are cheaper, solution-processable, and even printable onto flexible substrates. Like other TFSCs, OSCs may also be used in the smart and BIPV applications [99, 100]. However, certain problems of OSCs, including air sensitivity, instability, and short lifetime, require proper resolution for commercial success [101]. Organic compounds based on fullerene, an allotrope of carbon, gained tremendous popularity as an acceptor material. Their low-cost and convenient production, as well as flexibility, maintained a great interest in the OSC industry. However, the PCE efficiency of fullerene-based solar

O

60

O

70

OCH3

OCH3 [70]PCBM

[60]PCBM H3C O

N

H3C

S S

S

H3C

CH3

EH-IDTBR

O

S

N

S

CH3

H3C

S S

N

S O

ITIC H3C(H2C)4H2C

H3C(H2C)6H2C

N

CH2(CH2)6CH3 S N N O

S

N

FBR

S

N

CH3

S

CH3 H3C(H2C)4H2C CH2(CH2)4CH3 H3C(H2C)4H2C CH2(CH2)4CH3 O N O O N O

CH2(CH2)4CH3 H3C(H2C)4H2C N

S

O

S

S N H3C

N

H3C

CH3 N SN

S N

ICBA

CH3

O S N

SF-PDI N O O O N O N H3C(H2C)4H2C CH2(CH2)4CH3 CH2(CH2)4CH3 H3C(H2C)4H2C CH2(CH2)4CH3

Figure 10.12  A few examples of commercially available acceptor materials for OSCs.

Solar Cell Efficiency Energy Materials  293 could not compete other SCs. In addition, the amorphous nature and poor charge transport added to their limitations. Consequently, the research focus shifted to non-fullerene–based acceptors, which resulted in a vast library of much more effective acceptor materials [102, 103]. An OSC is constructed as a heterojunction cell with thin layers of acceptor and donor conjugated materials. The SC should be at minimum 100-nm thick allow exciton generation. However, the excitons can diffuse through the layers at a length of 10 nm only. A bulk heterojunction (BHJ) device structure is used to compensate for the difference. As illustrated in Figure 10.13, BHJ comprises the blend of electron-donor and electron-acceptor material where excitons can separate and diffuse into the layers more easily. The electrons are captured at a metal electrode, while the holes are collected at a TCO electrode such as ITO [104]. The highest PCE achieved by OSCs is 17.4% [104]. Table 10.7 shows notable reports on OSCs.

10.4.6 Hybrid Organic-Inorganic Materials Hybrid organic-inorganic materials combine the benefits of both inorganic and organic semiconductors to produce highly efficient SCs. Two major SCs based on hybrid materials are DSSC and perovskite SCs (PSCs) [126].

10.4.6.1 Dye-Sensitized Solar Cell Materials DSSC materials constitute a photo-electrochemical system with a combination of organic dyes, nanoparticles, and electrolytes. HOMO and LUMO levels of organic dyes, fermi level of TiO2, and redox potential of electrolyte ions greatly influence the PCE of DSSCs. Similar to OSCs, DSSCs are lowcost alternatives to expensive PV technologies. Applications of DSSC range from low-light indoor uses to heavy-duty outdoor services. However, the weather-dependency, low PCE and poor stability of electrolytes are limiting factors to their commercialization [127–129]. The best PCE demonstrated by DSSCs is 12.3% [35]. DSSC has a BHJ device structure with a dye-sensitizer such as ruthenium polypyridine acting as electron-donor and titanium dioxide (TiO2) nanoparticles serving as electron-acceptor in a blended form. An iodinebased electrolyte is filled between front contact and dye-sensitizer to connect both layers. On the other side, TiO2 is connected to TCO-based back contact. Platinum back contact is also used to facilitate the photoelectrochemical reactions in the cell. In a typical process, the light-excited electrons eject from the HOMO of dye-sensitizer to LUMO from where they

294  Fundamentals of Solar Cell Design

substrate

substrate

Cathode (ITO)

Cathode (ITO) Electron donor Electron acceptor Anode (Al, Mg, Ca)

(a)

Anode (Al, Mg, Ca)

(b)

Figure 10.13  A general structure of (a) multi-layer OSC and (b) BHJ OSC.

Table 10.7  Notable performance results for fullerene and non-fullerene OSCs. PCE (%)

Voc (V)

Jsc (mA/ cm2)

FF (%)

Reference

Spiro-OMeTAD

17.4

0.75

23.8

67.7

[105]

S1/Y6

16.4

0.88

25.4

74.0

[106]

PBDB-T-SF/Y6/ITCT

16.1

0.89

24.8

73.7

[107]

P2F-EHp/BTPTT-4F

16

0.811

26.7

74.1

[108]

PM6/Y6

15.4

0.84

25.0

73.1

[106]

ES1/Y6

15.4

0.87

24.9

71.3

[106]

PBDB-T-2Cl:IT-4F

14.4

0.865

21.79

77.0

[109]

J71/PM6/Br-ITIC

14.13

0.93

19.4

78.2

[110]

PBDB-T-SF/IT-4F/ ITCT

13.7

0.89

20.2

76.2

[107]

PhI/ffBT

13.3

0.90

19.40

75.5

[111]

PFBDB-T:C8-ITIC

13.21

0.945

19.65

72.1

[112]

ffPhI/ffBT

12.7

0.94

19.0

71.1

[111]

ZnP-TBO:6TIC

12.1

0.80

20.4

73.9

[113]

Donor/Acceptor Non-Fullerene OSCs

(Continued)

Solar Cell Efficiency Energy Materials  295 Table 10.7  Notable performance results for fullerene and non-fullerene OSCs. (Continued) Donor/Acceptor

PCE (%)

Voc (V)

Jsc (mA/ cm2)

FF (%)

Reference

PCE10:IDTBR:IDFBR

11.0

1.03

17.2

0.6

[114]

PffBT4T-2DT: IDTBR (1:1)

9.95

1.07

15.0

62

[115]

PTB7-Th/IEICO-4F

9.4

0.73

19.21

67.2

[116]

DBFI-EDOT:PSEHTT

8.9

0.929

13.8

62.9

[117]

DBFI-EDOT: PSEHTT:PBDTTFTTE

8.5

0.92

15.7

59.9

[117]

O-IDTBR:P3HT

6.30

0.72

13.9

0.603

[118]

P3HT:IDTBR:IDFBR

6.3

0.72

14.4

0.64

[114]

EH-IDTBR:P3HT

6.0

0.76

12.1

0.62

[118]

S4/Y6

5.8

0.93

12.8

49.1

[106]

P1

5.4

0.58

15

0.61

[119]

PTB7/PC61BM2

7.28

0.76

14.2

0.67

[120]

PffBT4T-2OD

7.3

0.77

13.5

70.3

[121]

PTB7/PC61BM1

7.33

0.75

15.5

63.3

[120]

TT4FIDTT/PC71BM

7.4

0.94

11.2

71.2

[122]

DT4FIDTT/PC71BM

9.15

0.92

13.49

55.3

[122]

PBTIBDTT-S/ PC71BM (1:2.0)

9.7

0.96

14.0

73.4

[123]

PBTIBDTT-S/ PC71BM (1:2.3)

10.1

0.94

14.8

72.7

[123]

MAPbI3:F3/PEG-[60

16.4

1.08

18.6

78.2

[124]

MAPbI3/PEG-[60]

18.4

1.1

21.1

79.3

[124]

P2-PC70BM

4.4

0.86

8.8

0.59

[125]

Fullerene OSCs

296  Fundamentals of Solar Cell Design

glass TCO Pt I-

Electrolyte

I3

TiO2 Dye TCO glass

Figure 10.14  A general structure of a DSSC.

are injected into TiO2 nanoparticles. While the electrons from TiO2 are transported to TCO-based back contact via diffusion, tri-iodide anions present in the electrolyte neutralize the positively charged dye molecules and are reduced into iodine anions on the counter electrode [130–132]. Figure 10.14 illustrates a general SC structure of DSSC. Electrolytes used in DSSC can freeze at low temperatures or experience thermal expansion under high temperatures. Moreover, high costs of dye and electrodes are also one of the major bottlenecks of DSSCs. Researchers have been focusing on developing stable electrolytes as well as discovering the efficient but cheaper alternatives to platinum [133]. Figure 10.15 shows the structure of commercialized dyes and electrolytes used in DSSC. Table 10.8 accounts for some leading reports on the performance of DSSCs.

10.4.6.2 Perovskites Perovskite materials are known by ABX3 formula, where A is larger cation and B being smaller cation, and X is halogen. For mixed halide perovskites, A cations are organic compounds e.g., CH3NH3+, CH3CH2NH3+, or NH2CH=NH2+), B cation is usually lead (Pb) for achieving efficient SCs, and halogens used are iodine (I), bromine (Br), and chlorine (Cl) [150]. A typical example of such compound is methylammonium lead triiodide (CH3NH3PbI3) [151]. Mixed halides have also gained popularity due to good stability and efficiency. Perovskite materials are promising SC materials with high solar absorption, easy fabrication method, convenient deposition, and weak non-radiative carrier recombination rate

Solar Cell Efficiency Energy Materials  297 Dyes O

O

OH

N SO

S

HO

O N

D102 Dye

S

Squarylium dye III

NCS H3C

O

N-3 Dye

O

H3C

OH

OO

K19 Dye

N

O O

N N Ru

O

S CH3

N

D149 Dye

N CH2(CH2)2CH3 O

Merocyanine 540

N S CH2(CH2)2CH3

N

O

N

-O

N -O

Electrolytes: O O F3C S N S CF3 O O CH3

O

OH

O

N

BMIM TFSI

O

NCS

-O

O

N

S

NCS

O

CH3 N+

N

N

HO

O NaO S O

N

OH

S

D131 dye

CH3(CH2)4CH2O

O

O

OH CN

N

N

CH3 N+ CH3

N

OCH2(CH2)4CH3

N

S HO

O

N

HO

D358 Dye S

NCS

Ru

N

O

CH2(CH2)9CH2 N

N

HO

N

O

OH

CH3 N N+ CH3

O O F3C S N S CF3 O O

PMIM NTF

NCS Ru NCS NCS

H3C

N+

CH3 CH3 CH3

3

N749 Black Dye O

N

+ CH3 N Cl

1-Ethyl-3-methyl-imidazolium chloride

CH3

Figure 10.15  Structures of organic dyes and electrolytes used in DSSC.

for the preparation of these materials. PSCs are popular for a high PCE, good optoelectronic properties, stability, and flexibility. Organometallic perovskite semiconductors have great PV applications including, low exciton binding energy [152], improved absorption [153], and charge diffusion as well as controllable energy levels [154, 155]. PSCs have found applications in building integrated devices, including windows and doors, and can be planted on rooftops. Further applications extend to portable devices such as watches and calculators, as they require low energy sources for working and can work easily even with low capacity batteries as backup power [156, 157].

298  Fundamentals of Solar Cell Design Table 10.8  Notable performance results for DSSCs. PCE (%)

Wavelength (nm)

Voc (V)

Jsc (mA/ cm2)

FF (%)

Reference

MAPbI3 +Au+ TiO2

14.92

Sunlight

0.99

21.6

70

[134]

YD2-o-C8

12.7

Sunlight

0.940

9.3

0.74

[135]

SGT-021/ HC-A4

12.3

Sunlight

1.83

10.3

65.0

[136]

SGT-121/ HC-A1

13.3

Sunlight

0.91

19.7

74.0

[136]

Z907

11.94

T5

1.05

6.15

64.3

[137]

Dye cell

11.9

538

0.74

22.5

71.2

[138]

C101

11.3

547

0.74

5.42

0.83

[139]

N719

11.2

540

0.84

17.73

0.74

[140]

AN11

11.3

LED

1.03

52.9

64.7

[137]

DSSCs

11.18

Sunlight

0.69

18.96

85.9

[141]

Dye mini module

10.7

541

0.75

20.2

69.9

[138]

C106

10.57

550

0.74

18.28

0.77

[142]

C104

10.53

553

0.76

17.87

0.77

[143]

N749

10.4

605

0.72

20.53

0.70

[144]

Ij-1

10.3

536

0.74

19.2

0.72

[145]

Z910

10.2

543

0.77

17.2

0.76

[146]

N3

10.0

534

0.72

18.2

0.73

[147]

N945

9.6

550

0.79

16.5

0.72

[148]

N719 (FS05)

8.16

Sun light

0.65

19.3

65.3

[149]

Materials

Solar Cell Efficiency Energy Materials  299 The device structure of PSC consists of perovskite absorber layer sandwiched between hole transport material (HTM) and electron transport material (ETM), as illustrated in Figure 10.16. Device classification is based on illumination of the transporting layer by light from different sides. If the light is projected from the ETM side, then it is named as a conventional n-i-p device, and if it is projected through the HTM side, then the device is called an inverted p-i-n device [158, 159]. The fabrication of PSC involves a glass substrate, transparent conductive oxide, charge transport layers, perovskite photo-absorber layer, and metal electrode. The general fabrication of PSC involves the deposition of transparent conductive material onto a glass substrate, followed by the coating of an electron transporting material. A perovskite absorber layer is deposited onto ETM which absorbs the photons thus producing electron-hole pair. Hole transporting layer and metal electrode are then deposited respectively. The electron-hole pair thus produced move to the respective electrodes to produce current [160]. Tandem PSCs have gained great importance as an emerging PV technology. In the fabrication of these devices, wide bandgap top cell and low bandgap bottom cell are used. The top cell captures high-energy photons and converts that energy into Voc with minimum losses. The remaining energy is passed on to the bottom cell and absorbed by it. In combination with perovskite, Si bottom cell is usually used as it gives high overall performance along with good optical absorption of photons. Apart from Si, perovskite in tandem with CIGS, has also found their applications. Perovskite/ Si monolithic tandem device has achieved a maximum theoretical PCE of approximately 30% [161].

metal contact HTM perovskite ETM ITO

Figure 10.16  A general structure of PSC.

300  Fundamentals of Solar Cell Design PSCs faced several problems and challenges, including uniform coating of components on the huge surface area, developing fabrication procedures, understanding device structure impact, and comprehending precursor chemistry to match with processing technique and stability of the device [162–164]. These factors are crucial for the scalable fabrication of all the device layers as well as stability and promising efficiency of devices. The use of conjugated polymers has shown to achieve a high PCE with great operational device stability [165]. Materials having strong optoelectronic properties and lower bandgap helped to achieve great performance as well as a reduction in the thickness of the photoactive layer [166]. Nucleation and crystal growth can be enhanced by using non-halide Pb sources such as lead acetate [167]. Besides, it can avert the environmental hazards of Pb metal. Synthesis of novel materials, lowering the processing costs, and reducing the environmental impact of PSC materials are the ongoing areas of interests for researchers worldwide [162]. PSCs have shown remarkable performance in both indoor and outdoor applications. Table 10.9 shows some remarkable results for PSC cells and modules reported in the literature.

10.4.7 Quantum Dots Quantum dots (QDs) are spherical semiconductor nanoparticles employed as a photoactive layer in the Quantum dot SCs (QDSCs). QDs are a fine example of spectral downconversion concept of SC efficiency enhancement, i.e., splitting of a high-energy photon into multiple low energy photons. Thanks to the so-called quantum confinement, QD semiconductors have increased coulombic interactions resulting in much larger and tunable bandgap as compared to their bulk form. As a result, QDs find a potential use where spectral and band utilization is practiced [170]. Conventional SCs usually lose a large amount of energy from a broad solar spectrum because of poor absorption in the infrared region. With QDs SCs, absorption in the near and far-infrared region can be achieved conveniently. In addition, processing methods of QDs have improved from expensive molecular beam epitaxy to the cheaper spin-coating methods. For industrial scale, spray-on or roll-printing systems could be used that significantly reduced the production cost of QDSCs [171, 172]. Device structure of QDSCs is similar to that of PSCs illustrated in Figure 10.17, except it has a layer of QDs instead of perovskite materials. The best efficiency QDSCs have shown so far is 16.6% [35]. QDs of toxic and carcinogenic heavy metals such as Pb and Cd have raised safety concerns. In response, several techniques have been

Solar Cell Efficiency Energy Materials  301 Table 10.9  Notable performance results of PSCs. Types

PCE %

Area cm2

Voc V

Jsc mAcm2

FF % References

Perovskite/ Si Tandem monolithic

28.0 ± 0.7

1.030 da

1.802

19.75

78.7

[48]

Perovskite/Si Tandem

28

0.2

0.98

10.6

71.5

[168]

Perovskite/Si Tandem

26.3

1.43

1.80

18.8

77.5

[169]

Perovskite/ Si Tandem monolithic

25.2 ± 0.8

1.088 da

1.793

19.02

73.8

[45]

Perovskite thin 25.2 ± film 0.8

0.0937 ap

1.1805

24.14

84.8

[46]

Perovskite/ Si Tandem monolithic

1.419 da

1.787

19.53

72.3

[45]

Perovskite thin 24.2 ± film 0.8

0.0955 ap

1.195

24.16

84

[48]

Perovskite thin 23.7 ± film 0.8

0.0739 ap

1.168

25.40

79.8

[71]

25.2 ± 0.7

Perovskite/ CIGS

22.4 ± 1.9

0.042 ap

1.774

17.3

73.1

[71]

Perovskite/ Si Tandem monolithic

23.6 ± 0.6

0.990 ap

1.651

18.09

79

[44]

Perovskite/ CIGS

23.3 ± 0.8

1.035 da

1.683

19.17

72.1

[46]

Perovskite thin 22.7 ± film 0.8

0.0935 ap

1.144

24.92

79.6

[43]

Perovskite thin 22.1 ± film 0.7

0.046 ap

1.105

24.97

80.3

[47

Perovskite thin 22.1 ± film 0.7

0.0946 ap

1.105

24.97

80.3

[44] (Continued)

302  Fundamentals of Solar Cell Design Table 10.9  Notable performance results of PSCs. (Continued) Jsc mAcm2

FF % References

1.0235 da 1.1932

1.64

83.6

[46]

20.9 ± 0.7

0.991 da

1.125

24.92

74.5

[43]

Perovskite cell

20.9 ± 0.7

277 da

1.070

20.66

78.1

[45]

Perovskite cell

19.7 ± 0.6

0.9917 da 1.104

24.67

72.3

[47]

Perovskite 17.3 ± minimodule 0.6

19.277 da 1.070

20.66

78.1

[46]

Perovskite 17.2 ± minimodule 0.6

17.28 da

1.070

20.66

78.1

[71]

Perovskite cell

802 da

57.3

0.3207

70.3

[46]

Perovskite 16.0 ± minimodule 0.4

16.29 ap

1.029

19.51

76.1

[72]

12.1 ± Perovskite minimodule 0.6

36.13 da

0.836

20.20

71.5

[44]

Perovskite submodule

11.7 ± 0.4

703 da

1.073

14.36

75.8

[71]

Perovskite

11.6 ± 0.4

802 da

23.79

0.577

68

[71]

Types

PCE %

Area cm2

Perovskite cell

21.6 ± 0.6

Perovskite cell

16.1 ± 0.5

Voc V

introduced to reduce the risk, including replacement of toxic QDs with non-toxic alternatives such as AgBiS2 or CuInSe2-x and encapsulation of toxic QDs in stable polymer shells [173].

10.5 Conclusion and Prospects In conclusion, SC efficiencies and energy materials have experienced a significant boost since the beginning of the 21st century. Thanks to the extensive investigations and conceptual advancements, SC material design and efficiency enhancement strategies have evolved tremendously. The prices

Solar Cell Efficiency Energy Materials  303 of conventional c-Si SCs have reduced, and many new technologies are making their way to the commercial market. TFSC materials have opened up new horizons for efficient production and effective utilization of energy. With transparent and semi-transparent SCs and building integrated PV solutions, the new era of modern technology has begun. Although the challenges regarding unaffordability, instability, and inefficiency of solar modules remain unresolved, a vigorous response and significant improvements have been observed in each area. Additionally, researchers are now equipped with a much more comprehensive range of tools and techniques to practice the theoretical knowledge. In the future, the computational design, bandgap tailoring, manufacture simplification, and device optimization processes will develop enough to gain better control of material properties, cost, and performance. High-efficiency concepts of multi-junction, concentrator, intermediate band, and hot-carrier SCs will more likely become a center of attention. Spectral conversion and multi-exciton generation concepts will be explored in-depth to find new directions toward the high-efficiency achievement goals. It is safe to say that if the current research trend continues, SC materials will improve further, and the PCEs of SCs will keep increasing steadily.

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312  Fundamentals of Solar Cell Design 127. Freitag, M.; Teuscher, J.; Saygili, Y.; Zhang, X.; Giordano, F.; Liska, P.; Hua, J.; Zakeeruddin, S. M.; Moser, J.-E.; Grätzel, M. J. N. P., Dye-sensitized solar cells for efficient power generation under ambient lighting. 11 (6), 372, 2017. 128. Mozaffari, S.; Nateghi, M. R.; Zarandi, M. B. J. R.; Reviews, S. E., An overview of the Challenges in the commercialization of dye sensitized solar cells. 71, 675–686, 2017. 129. Sharma, S.; Siwach, B.; Ghoshal, S.; Mohan, D. J. R.; Reviews, S. E., Dye sensitized solar cells: From genesis to recent drifts. 70, 529–537, 2017. 130. Sugathan, V.; John, E.; Sudhakar, K. J. R.; Reviews, S. E., Recent improvements in dye sensitized solar cells: A review. 52, 54–64, 2015. 131. Ye, M.; Wen, X.; Wang, M.; Iocozzia, J.; Zhang, N.; Lin, C.; Lin, Z. J. M. T., Recent advances in dye-sensitized solar cells: from photoanodes, sensitizers and electrolytes to counter electrodes. 18 (3), 155–162, 2015. 132. Kumavat, P. P.; Sonar, P.; Dalal, D. S. J. R.; Reviews, S. E., An overview on basics of organic and dye sensitized solar cells, their mechanism and recent improvements. 78, 1262–1287, 2017. 133. Gong, J.; Sumathy, K.; Qiao, Q.; Zhou, Z. J. R.; Reviews, S. E., Review on dye-sensitized solar cells (DSSCs): Advanced techniques and research trends. 68, 234–246, 2017. 134. Mali, S. S.; Shim, C. S.; Kim, H.; Patil, P. S.; Hong, C. K. J. N., In situ processed gold nanoparticle-embedded TiO 2 nanofibers enabling plasmonic perovskite solar cells to exceed 14% conversion efficiency. 8 (5), 2664–2677, 2016. 135. Yella, A.; Lee, H.-W.; Tsao, H. N.; Yi, C.; Chandiran, A. K.; Nazeeruddin, M. K.; Diau, E. W.-G.; Yeh, C.-Y.; Zakeeruddin, S. M.; Grätzel, M. J. s., Porphyrin-sensitized solar cells with cobalt (II/III)–based redox electrolyte exceed 12 percent efficiency. 334 (6056), 629–634, 2011. 136. Kang, S. H.; Jeong, M. J.; Eom, Y. K.; Choi, I. T.; Kwon, S. M.; Yoo, Y.; Kim, J.; Kwon, J.; Park, J. H.; Kim, H. K. J. A. E. M., Porphyrin Sensitizers with Donor Structural Engineering for Superior Performance Dye‐Sensitized Solar Cells and Tandem Solar Cells for Water Splitting Applications. 7 (7), 1602117, 2017. 137. Tsai, M.-C.; Wang, C.-L.; Chang, C.-W.; Hsu, C.-W.; Hsiao, Y.-H.; Liu, C.-L.; Wang, C.-C.; Lin, S.-Y.; Lin, C.-Y. J. J. o. M. C. A., A large, ultra-black, efficient and cost-effective dye-sensitized solar module approaching 12% overall efficiency under 1000 lux indoor light. 6 (5), 1995–2003, 2018. 138. Komiya, R.; Fukui, A.; Murofushi, N.; Koide, N.; Yamanaka, R.; Katayama, H. In Improvement of the conversion efficiency of a monolithic type dye-sensitized solar cell module, Technical Digest, 21st International Photovoltaic Science and Engineering Conference, 2011. 139. Gao, F.; Wang, Y.; Shi, D.; Zhang, J.; Wang, M.; Jing, X.; Humphry-Baker, R.; Wang, P.; Zakeeruddin, S. M.; Grätzel, M. J. J. o. t. A. C. S., Enhance the optical absorptivity of nanocrystalline TiO2 film with high molar extinction

Solar Cell Efficiency Energy Materials  313 coefficient ruthenium sensitizers for high performance dye-sensitized solar cells. 130 (32), 10720–10728, 2008. 140. Nazeeruddin, M. K.; De Angelis, F.; Fantacci, S.; Selloni, A.; Viscardi, G.; Liska, P.; Ito, S.; Takeru, B.; Grätzel, M. J. J. o. t. A. C. S., Combined experimental and DFT-TDDFT computational study of photoelectrochemical cell ruthenium sensitizers. 127 (48), 16835–16847, 2005. 141. Sun, K. C.; Sahito, I. A.; Noh, J. W.; Yeo, S. Y.; Im, J. N.; Yi, S. C.; Kim, Y. S.; Jeong, S. H. J. J. o. M. C. A., Highly efficient and durable dye-sensitized solar cells based on a wet-laid PET membrane electrolyte. 4 (2), 458–465, 2016. 142. Cao, Y.; Bai, Y.; Yu, Q.; Cheng, Y.; Liu, S.; Shi, D.; Gao, F.; Wang, P. J. T. J. o. P. C. C., Dye-sensitized solar cells with a high absorptivity ruthenium sensitizer featuring a 2-(hexylthio) thiophene conjugated bipyridine. 113 (15), 6290–6297, 2009. 143. Gao, F.; Wang, Y.; Zhang, J.; Shi, D.; Wang, M.; Humphry-Baker, R.; Wang, P.; Zakeeruddin, S. M.; Grätzel, M. J. C. c., A new heteroleptic ruthenium sensitizer enhances the absorptivity of mesoporous titania film for a high efficiency dye-sensitized solar cell. (23), 2635–2637, 2008. 144. Nazeeruddin, M. K.; Pechy, P.; Renouard, T.; Zakeeruddin, S. M.; HumphryBaker, R.; Comte, P.; Liska, P.; Cevey, L.; Costa, E.; Shklover, V. J. J. o. t. A. C. S., Engineering of efficient panchromatic sensitizers for nanocrystalline TiO2-based solar cells. 123 (8), 1613–1624, 2001. 145. Yum, J.-H.; Jung, I.; Baik, C.; Ko, J.; Nazeeruddin, M. K.; Grätzel, M. J. E.; Science, E., High efficient donor–acceptor ruthenium complex for dyesensitized solar cell applications. 2 (1), 100–102, 2009. 146. Wang, P.; Zakeeruddin, S. M.; Moser, J. E.; Humphry‐Baker, R.; Comte, P.; Aranyos, V.; Hagfeldt, A.; Nazeeruddin, M. K.; Grätzel, M. J. A. M., Stable new sensitizer with improved light harvesting for nanocrystalline dye‐ sensitized solar cells. 16 (20), 1806–1811, 2004. 147. Nazeeruddin, M. K.; Kay, A.; Rodicio, I.; Humphry-Baker, R.; Müller, E.; Liska, P.; Vlachopoulos, N.; Grätzel, M. J. J. o. t. A. C. S., Conversion of light to electricity by cis-X2bis (2, 2’-bipyridyl-4, 4’-dicarboxylate) ruthenium (II) charge-transfer sensitizers (X= Cl-, Br-, I-, CN-, and SCN-) on nanocrystalline titanium dioxide electrodes. 115 (14), 6382–6390, 1993. 148. Nazeeruddin, M. K.; Wang, Q.; Cevey, L.; Aranyos, V.; Liska, P.; Figgemeier, E.; Klein, C.; Hirata, N.; Koops, S.; Haque, S. A. J. I. c., DFT-INDO/S modeling of new high molar extinction coefficient charge-transfer sensitizers for solar cell applications. 45 (2), 787–797, 2006. 149. Su, R.; Ashraf, S.; El-Shafei, A. J. S. E., Structure-property relationships:“Double-tail versus double-flap” ruthenium complex structures for high efficiency dye-sensitized solar cells. 177, 724–736, 2019. 150. Green, M. A.; Ho-Baillie, A.; Snaith, H. J. J. N. p., The emergence of perovskite solar cells. 8 (7), 506, 2014. 151. Lai, W.-C.; Hsieh, W.-M.; Yu, H.-C.; Yang, S.-H.; Guo, T.-F.; Chen, P. J. O. E., Conversion efficiency enhancement of methylammonium lead triiodide

314  Fundamentals of Solar Cell Design perovskite solar cells converted from thermally deposited lead iodide via thin methylammonium iodide interlayer. 105713, 2020. 152. Stoumpos, C. C.; Malliakas, C. D.; Kanatzidis, M. G. J. I. c., Semiconducting tin and lead iodide perovskites with organic cations: phase transitions, high mobilities, and near-infrared photoluminescent properties. 52 (15), 9019– 9038, 2013. 153. Snaith, H. J. J. T. j. o. p. c. l., Perovskites: the emergence of a new era for lowcost, high-efficiency solar cells. 4 (21), 3623–3630, 2013. 154. Noh, J. H.; Im, S. H.; Heo, J. H.; Mandal, T. N.; Seok, S. I. J. N. l., Chemical management for colorful, efficient, and stable inorganic–organic hybrid nanostructured solar cells. 13 (4), 1764–1769, 2013. 155. Hao, F.; Stoumpos, C. C.; Chang, R. P.; Kanatzidis, M. G. J. J. o. t. A. C. S., Anomalous band gap behavior in mixed Sn and Pb perovskites enables broadening of absorption spectrum in solar cells. 136 (22), 8094–8099, 2014. 156. Di Giacomo, F.; Zardetto, V.; Lucarelli, G.; Cinà, L.; Di Carlo, A.; Creatore, M.; Brown, T. J. N. E., Mesoporous perovskite solar cells and the role of nanoscale compact layers for remarkable all-round high efficiency under both indoor and outdoor illumination. 30, 460–469, 2016. 157. Chen, C. Y.; Chang, J. H.; Chiang, K. M.; Lin, H. L.; Hsiao, S. Y.; Lin, H. W. J. A. F. M., Perovskite photovoltaics for dim‐light applications. 25 (45), 7064–7070, 2015. 158. Liu, T.; Chen, K.; Hu, Q.; Zhu, R.; Gong, Q. J. A. E. M., Inverted perovskite solar cells: progresses and perspectives. 6 (17), 1600457, 2016. 159. Kim, H.; Lim, K.-G.; Lee, T.-W. J. E.; Science, E., Planar heterojunction organometal halide perovskite solar cells: roles of interfacial layers. 9 (1), 12–30, 2016. 160. Wojciechowski, K.; Stranks, S. D.; Abate, A.; Sadoughi, G.; Sadhanala, A.; Kopidakis, N.; Rumbles, G.; Li, C.-Z.; Friend, R. H.; Jen, A. K.-Y. J. A. n., Heterojunction modification for highly efficient organic–inorganic perovskite solar cells. 8 (12), 12701–12709, 2014. 161. Almansouri, I.; Ho-Baillie, A.; Green, M. A. J. J. J. o. A. P., Ultimate efficiency limit of single-junction perovskite and dual-junction perovskite/silicon two-terminal devices. 54 (8S1), 08KD04, 2015. 162. Li, Z.; Klein, T. R.; Kim, D. H.; Yang, M.; Berry, J. J.; van Hest, M. F.; Zhu, K. J. N. R. M., Scalable fabrication of perovskite solar cells. 3 (4), 1–20, 2018. 163. Correa-Baena, J.-P.; Saliba, M.; Buonassisi, T.; Grätzel, M.; Abate, A.; Tress, W.; Hagfeldt, A. J. S., Promises and challenges of perovskite solar cells. 358 (6364), 739–744, 2017. 164. Ansari, M. I. H.; Qurashi, A.; Nazeeruddin, M. K. J. J. o. P.; Reviews, P. C. P., Frontiers, opportunities, and challenges in perovskite solar cells: A critical review. 35, 1–24, 2018. 165. Jung, E. H.; Jeon, N. J.; Park, E. Y.; Moon, C. S.; Shin, T. J.; Yang, T.-Y.; Noh, J. H.; Seo, J. J. N., Efficient, stable and scalable perovskite solar cells using poly (3-hexylthiophene). 567 (7749), 511–515, 2019.

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11 Analytical Tools for Solar Cell Mohamad Saufi Rosmi1, Ong Suu Wan1, Mohamad Azuwa Mohamed2*, Zul Adlan Mohd Hir3 and Wan Nur Aini Wan Mokhtar2 Department of Chemistry, Faculty of Science and Mathematics, Universiti Pendidikan Sultan Idris, Tanjung Malim, Perak, Malaysia 2 Department of Chemical Sciences, Faculty Science and Technology, Universiti Kebangsaan Malaysia, Bangi, Malaysia 3 Faculty of Applied Sciences, Universiti Teknologi MARA, Pahang, Malaysia 1

Abstract

The sunlight is a renewable energy that is environmentally friendly, sustainable, and totally inexhaustible. It is an alternative energy which can be used to replace the non-renewable energy such as petroleum and fossil fuel. The solar cell has a bright future to be employed as a method to utilized the sunlight and has been undergone a large number of improvements. Since the development of firstgeneration silicon-based solar cells, a lot of research has been done to enhance the cell efficiency including the development of thin films and dye-sensitized and organic solar cells. However, advancement of solar cells technology is facing the crucial issues of costing and efficiency. With respect to aforementioned issues, immense effort has been devoted to understand the fundamental mechanism and properties of solar cells. This chapter will focus on the emerging characterization tools used in understanding and investigating the solar cell. The main characterization will focus on conductive AFM, electron tomography, transient absorption spectroscopy, and Kelvin probe microscopy. Besides that, the usage of field emission scanning electron microscopy (FESEM) as well as transmission electron microscopy (TEM) in analyzing the morphology of solar cell structure is also discussed in this chapter. Keywords:  Solar cell, morphology, efficiency, spectroscopy, microscopy

*Corresponding author: [email protected] Inamuddin, Mohd Imran Ahamed, Rajender Boddula, and Mashallah Rezakazemi (eds.) Fundamentals of Solar Cell Design, (317–344) © 2021 Scrivener Publishing LLC

317

318  Fundamentals of Solar Cell Design

11.1 Introduction The earth receives a tremendous amount of sunlight every day. Sunlight which also known as solar energy is available at no cost, limitless resource, thus harvesting it is indeed a great idea. Solar energy receives extra attention due to the fact that it can contribute to the reduction of pollution compared to the energy generates from fossil fuels. The main issue to be addressed is the development of technology, which is economical, ecofriendly nature, as well as flexibility. Solar cells are a well-known method for utilizing solar energy to produce electricity. It were developed based on the finding discovered in 1839 by Edmond Becquerel where shining light on an electrode immersed in a conductive solution would create an electric current [1]. Since then, a lot of effort has been made to empower the solar cells technology including the in depth study about photovoltaic effect, study of materials which possess photovoltaic effect as well as the effect of photoelectric. Currently, various types of materials are being researched to develop solar cells with reasonable cost and good efficiency. Solar cells are categorized into first-, second-, and third-generation cells. The wafer-based solar cells from crystalline silicon such as polysilicon and nanocrystalline silicon are the first generation of solar cells. While for the second-generation solar cells, the technology of thin film has been explored including amorphous of silicon, CIGS, and CdTe which are available readily in the market. The third-generation solar cells often described as an emerging photovoltaic which consists of various thin-film technologies, among them are organic materials, organometallic compounds, and inorganic substances [2]. An extensive progress has been achieved to design various types of solar cells; however, the efficiency is still insufficient, while material’s stability is yet questionable to be commercialized. In order to develop the solar cells with better efficiency, it is crucial to have a thorough insight of its working mechanism. Thus, this chapter will focus on the latest and emerging characterization tools to study and investigate the properties and efficiency of solar cells. The emerging characterization tools discuss in this chapter are conductive AFM, electron tomography (ET), transient absorption spectroscopy (TAS), and Kelvin probe microscopy. Besides that, the utilization of FESEM and TEM in analyzing the surface morphology of solar cell structure is also discussed in this chapter.

Analytical Tools for Solar Cell  319

11.2 Transient Absorption Spectroscopy Transient spectroscopy is a great tool to investigate the ultrafast phenomena in situ in real time. In 1950, the flash photolysis technique has been developed by Norrish and Porter which lead to the exploration of research in microsecond time. Using this technique, short-lived transient species are able to detect directly. This advancement of knowledge has led them to receive the Nobel Prize in Chemistry 1967. Later, the technique was continuously developed in temporal resolution as the short pulse laser technology also has advanced. At present, we can explore the ultrafast phenomenon with a resolution of femtosecond. With the establishment of flash photolysis technique in which we are shortening laser pulse duration, the temporal resolution was successfully decreased by eight time [3]. TAS has attracted huge interest due to its ability to observe all steps involve in the device operation including absorption, diffusion and dissociation of exciton as well as the process to transport and collect the charge. TAS is a pump probe–based spectroscopic technique where the effect of laser induced photoexcitation to the absorption change is observed. TAS is capable to monitor the progress of reactions at ample time frame and hence is able to quantitatively elucidate the phenomena of charge generation and recombination. The deep understanding about the charge generation and recombination phenomena is essential to significantly enhance the efficiency of solar cell. In a nutshell, TAS is a promising tool to be used in investigating the excited states phenomena happen in hybrid solar cells (HSCs). In obtaining the data from TAS, the intensity of probe light that transmitted throughout the sample will be measured prior, I0, and after, I, the photoexcitation. The intensity ratio of I0 and I can be express as the absorbance change, ΔOD = log (I0/I). The photo-product will result in the positive signal of ΔOD while emission or photo bleaching as a result in the increment in the ground state will give negative signal of ΔOD. On the other hand, Lamberts-Beer Law expresses the absorbance change, ΔOD by the formula ΔOD = εcl, where εcl are molar concentration, molar absorption coefficient and an optical path length, respectively. Therefore, it is actually a big challenge to observe the absorbance in polymer solar cells (PSCs) as thin film used as a photoactive layer is very thin (average ~100 nm @ 10−5 cm) and it resulted in a very small value of absorbance. To encounter this issue, we need to consider 1/50,000 of the total optical

320  Fundamentals of Solar Cell Design probe signal’s change which is separated from numerous noise [3]. As for ultrafast phenomenon of a time scale 75

[189]

[189]

[189]

[189]

[188]

[187]

[186]

[185]

[185]

[185]

[185]

Ref.

(Continued)

18.9

20.4

13.6

14.8

12.5

13.4

14.2

14.3

13.7

16.8

17.2

AV T P C E % %

Table 15.3  Summary of different single-junction and tandem semitransparent perovskite solar cells with the device structure and optoelectronic properties. (Continued)

488  Fundamentals of Solar Cell Design

FAMACs

CsPbI2Br

(FA0.95PbI2.95)0.85(MAPbBr3)0.15

MAPbI3

MAPbI3

MAPbI3

CH3NH3PbI3

CH3NH3PbI3

FTO/ITO

ITO

ITO

ITO

ITO

ITO

ITO

ITO

ITO/ AZO

26.

27.

28.

29.

30.

31.

32.

33.

34.

CH3NH3PbI3

FAMACs

ITO

25.

PCBM

C60(CH2) (Ind)

PCBM

c-HATNA

c-HATNA

c-HATNA

PC61MB

TiO2

TiO2

TiO2

ZnO

CsPbI3−xBrx (CsPbIBr2)

FTO/ ITO

24.

ETL

Perovskite absorber layer

Semitransparent electrode (Top/ bottom)

Sr. no.

PEDOT:PSS

modified NiO

modified NiO

c-TCTA-BVP

c-TCTA-BVP

c-TCTA-BVP

PTAA

17.6

20.4

20.8

18.82

20.90

21.25

22.0

13.13

22.16

Spiro-OMeTAD P3HT

21.71

8.65

Jsc (mA cm2)

EH44

NiOx

HTL

0.89

1.13

1.05

1.01

1.05

1.09

1.21

1.30

1.10

1.09

1.01

Voc (V)

73

80

74

70

73

78

81

70

79

78

63

FF %

70

20

20

-

-

-

-

-

-

35

11.5

18.1

16.2

13.4

16.1

18.2

21.5

12.0

19.6

18.5

5.57

AV T P C E % %

[196]

[195]

[195]

[194]

[194]

[194]

[193]

[192]

[191]

[191]

[190]

Ref.

Table 15.3  Summary of different single-junction and tandem semitransparent perovskite solar cells with the device structure and optoelectronic properties. (Continued)

Semitransparent Perovskite Solar Cells  489

490  Fundamentals of Solar Cell Design with their device structure. Two subcells are arranged in series for 2T, their open-circuit voltage (Voc) is dependent on the sum, whereas the lowest subcell limits the photocurrent. However, the efficiency of 4T can be found by taking a sum of two independent subcells. ST-PSCs are emerging as an attractive candidate for tandem devices as a top cell with CIGS [103, 123, 147], c-Si [22, 23, 54, 164–167], or small bandgap PSC as bottom cell [79, 143, 168], because of the low fabrication cost and tunable bandgap of ST-PSCs. In selecting suitable bandgap ST-PSCs, it is also important to have outstanding PCE and NIR transmittance, so that highly efficient TSCs can be achieved for 4T and 2T both. With subcells having the same bandgap combination, 4T TSCs are less responsive than 2T TSCs. 4T TSCs with higher PCE can be achieved by using top ST-PSC with a bandgap of 1.8 eV [169]. Catchpole et al. achieved a tandem efficiency of 26.4% by using mechanically stacked ST-PSC, having perovskite photo absorber with bandgap value of 1.73 eV on silicon bottom cell [170]. As 2T TSCs have a current limitation, so according to the Shockley-Queisser model, the perfect bandgap value found for ST-PSCs is 1.75 eV [87]. Owing to the parasitic absorption of the photoactive layer in ST-PSCs, a reduction in bandgap value was made to 1.73 eV. This adjustment helped to coincide with current limitations in TSCs with a photoactive layer of thickness greater than 1,000 nm [171, 172]. It has been observed that the perovskite layer of 1,000 nm thickness has adverse electricity losses. Hoke effect is present in perovskite with bromine-rich composition, resulting in reduced current and increase in non-linear Voc with increased bandgap [88, 173]. This results in a further decrease in the bandgap of perovskite to about 1.65 eV for 2T TSCs [169]. Huang et al. used a perovskite photoactive layer with bandgap value of 1.64 eV, having the composition of Cs0.15(FA0.83MA0.17)0.85Pb(I0.8Br0.2)3, and two additives MACl and MAH2PO2 were added, to passivate grain boundaries and increasing perovskite grain size. Consequently, Voc of 1.15 eV was achieved, and matching current was obtained by the top cell and the bottom Si cell [24]. For TSCs, bandgap of top ST-PSCs was optimized by using four different structure combinations and finding out their bandgap energy levels. The flat structure of the cell in contact with antireflection (AR) film on the front yielded a bandgap of 1.69 eV. When back-side texture was integrated with AR film on the front side, it yielded a bandgap of 1.65 eV. Whereas backside texture in combination with LM foil on the front side and both side texture yielded bandgap of 1.66 each. In textured devices, the top cell has a lower bandgap than the flat structure in order to match the bottom cell current, as backside Si cell has more NIR light reflection.

Semitransparent Perovskite Solar Cells  491 In comparison with AR film, it is seen that front-side textured TSCs have a good photon collecting effect to meet the need of perovskite with 0.01 eV wider bandgap. In addition, it imparts a better response to diffuse light, which is important for open-air conditions and BIPV. Top cell ST-PSC having active layer composition of Cs0.05(MA0.17FA0.83)Pb1.1(I0.83Br0.17)3 achieved 25.5% PCE for the tandem device for the structure combination, back-side texture integrated with front side LM foil [25]. One of the critical demands to increase the PCE of tandem devices is reducing parasitic absorption. This can be achieved by using ST-PSC with an inverted structure, having a thin layer of ETL of PCBM/SnO2 underneath a TE. This approach replaces the thick layer (200 nm) of SpiroOMeTAD in a planner structure and promises high PCE TSCs [21–25]. For the 2T tandem configuration, it is notable that the top ST-PSC manufacturing process should be consistent with the bottom cell. In particular, to protect silicon heterojunction (SHJ) bottom cell, the processing temperature of the top cell ST-PSC must be under 200°C [142]. Moreover, processing mechanics of top cells must be compatible with the morphology demand of the bottom cell. Sahli et al. fabricated TSC, which was a fully textured device based on monolithic perovskite/Si structure. This cell resulted in decreased reflection losses by implicating a light-capturing approach and acquired PCE of 25.2% with Jsc of 19.5 mA/cm2. The conformal growth approach was used to process the top cell on fully textured Si-SC at the bottom [22].

e– h+

O2

DSA

H2

CIGS

Perovskite

Figure 15.15  General schematic and energy potential diagrams of the perovskite and CIGS tandem water splitting cell [197].

492  Fundamentals of Solar Cell Design In addition to the use in TSCs as top cell, ST-PSCs also find applications in water splitting devices (Figure 15.15). Luo et al. fabricated a tandem device for solar water splitting by using CIGS photocathode in tandem with a top cell having a semitransparent MAPbBr3 photoactive layer. This cell catalyzed solar-to-hydrogen conversion efficiency of greater than 6%. Theoretically, further optimization of photoactive layers gave PCE of more than 20% [197].

15.6 Conclusion Overall research progress on ST-PSC has governed interesting practices and techniques to improve the absorption of light-harvesting materials, selection of TEs, and fabrication of the device. Production of novel materials through bandgap engineering coupled with various device structures resulted in achieving high PCEs. However, in addition to delivering a good PCE and AVT, ST-PSC materials, i.e., organic-inorganic hybrids and TEs, should be air-stable, cost-effective, and easy to process. Modern film-­deposition processes, moisture-resistant capping, and encapsulation of ST-PSC may provide solutions to these challenges. For commercialization success, issues of scalability and toxicity need to be overcome. For this purpose, the use of abundant materials and the replacement of toxic metals such as Pb with safer metals is encouraged. With the continued efforts, ST-PSC will thrive more in the coming years and may become a dominant PV technology in the future.

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16 Flexible Solar Cells Santosh Patil, Rushi Jani, Nisarg Purabiarao, Archan Desai, Ishan Desai and Kshitij Bhargava* Department of Electrical and Computer Science Engineering, Institute of Infrastructure, Technology, Research And Management (IITRAM), Ahmedabad, Gujarat, India

Abstract

Thin-film solar cells have gone through extensive research in recent past and are expected to grow further due to rising demand of energy and global warming. Although the thin-film solar cells have traditionally been fabricated on rigid and flat substrates called as the rigid solar cells, these rigid solar cells are not suitable for their installation on curvilinear surfaces such as the outer body of vehicle, roof of train coach, human body, etc. But generally, the flexibility is achieved at the expense of degrading performance and stability of solar cells which has been the severe bottleneck for their commercialization. This chapter presents an overview of the flexible solar cell technology. The important aspects covered in this chapter are the requirement of flexible solar cells, semiconductor and substrate materials required for fabrication, popular techniques for material and cell characterization, issues, and applications. Keywords:  Flexible solar cells, semiconductors, flexible substrates, characterization techniques

16.1 Introduction 16.1.1 Need for Solar Energy Harnessing The solar energy refers to heat and light energy coming from the sun. The technology which converts sunlight into electricity is referred to as the *Corresponding author: [email protected] Inamuddin, Mohd Imran Ahamed, Rajender Boddula, and Mashallah Rezakazemi (eds.) Fundamentals of Solar Cell Design, (505–536) © 2021 Scrivener Publishing LLC

505

506  Fundamentals of Solar Cell Design photovoltaic technology, and the device which makes it feasible is referred to as photovoltaic cells or solar cells. As economics says, “human wants are unlimited but the resources are scarce”, the availability of non-renewable sources is decreasing day by day, and thereby, we have now shifted our focus toward the development of renewable sources of energy. As these sources are non-toxic for environment, it is thus drawing tremendous amount of researcher interests. Among the several available renewable resources the most popular ones are solar, wind, and hydro energy. The scarcity and economic aspect of fossil fuels combined with problem of greenhouse gas emissions causing global warming make the priority of utilizing renewable energy resources. In this context, the advancements of renewable sources of energy are extremely vital for the economic growth of nations in near future [1]. There are various ways of harnessing solar energy such as photovoltaic (PV) systems, solar-water heating, and solar thermal electricity. The PV systems work on the basic principle of photovoltaic effect, i.e., when a semiconducting material is exposed to sunlight, electrons and holes are created leading to current flowing through the load. Typically, the efficiency of a commercial solar cell is more than 15%. The typical installation of a PV module, which is an array of PV cells, is done south-facing in either fixed and inclined mode or assisted by a tracking device that swivels them to obtain maximum power conversion efficiency.

16.1.2 Brief Overview of Generations of Solar Cells The amount of solar radiation striking the earth surface over a 3-day period is equal to the amount of energy stored in all fossil fuels [2]. To harness such a great extent of energy available from the sun, a solar cells become highly lucrative electronic device. Solar cells are usually divided into three main generations described as follows (Figure 16.1) [3]. (i) First-Generation Solar Cells These are generally based on silicon and are currently the most efficient ones available commercially. Around 80% of the commercially available solar panels fall under this category. Apart from being the most efficient one, these solar cells are also long lasting compared with other non-­silicon– based solar cells. However, they experience loss in efficiency at elevated temperature. At present, there are four varieties of silicon solar cells used in the manufacturing of solar panels for commercial use. (ii) Second-Generation Solar Cells The second-generation solar cells are constructed from semiconductor layers of few micrometer thickness due to which they are usually known as

Flexible Solar Cells  507 Classification of PVsolar cells

1st Generation PV solar cells

2nd Generation PV solar cells

3rd Generation PV solar cells

Monocrystalline silicon cells

Amorphous solar cells

Organic PV cells

Polycrystalline silicon cells

Cadmium Telluride (CdTe)/ Cadmium

Perovskite Solar cells

Copper indium Gallium diselenide (CIGS)

Dye-sensitized PV cells

Polymer PV cells

Copper Zinc tin sulfide PV cells

Quantum dot PV cells

Figure 16.1  Hierarchy of generations of solar cells.

thin-film solar cells. The use of reduced thickness of materials and reduced cost of the manufacturing process permits the manufacturers to produce cheaper solar panels. The cadmium telluride (CdTe) and copper indium gallium selenide (CIGS) solar cells are some common examples of second-­ generation solar cell. In 2009, around 16.8% of the PV panels sold belonged to this generation. First solar, which is currently the prime manufacturer and distributer of PV panels, globally utilizes CdTe as the solar cell material. The vital merit of this generation solar cell is that these are low cost (3.0 eV and high conductivity resulting due to its processing at high temperature. The ITO-coated PET has been the most successful combination for FSCs. However, ITO electrode is quite brittle, often causing cracks within electrode layer, and is also expensive which makes it unsuitable for incorporation in FSCs. Ag-NWs

514  Fundamentals of Solar Cell Design is also a commonly used electrode in FSCs as it exhibits high transmittance, low sheet resistance, and excellent mechanical strength. Nanowires are generally thinner as compared to other electrodes and that is why it is used when higher order of flexibility is desired. However, Ag-NW–based FSCs shows 17% lesser efficiency than ITO-based flexible cells. CNT has attracted wide interest for flexible electrode manufacturing owing to its consistent transparency in the visible range of solar spectrum, lucrative electrical features, great flexibility, and processing ease. The distinct advantage of CNT-based transparent flexible electrode is its low optical absorption compared to some of its other counterparts described above. The maximum efficiency by CNT-based solar cells is 8.9%.

16.2.4 Encapsulations The encapsulation is used to protect the solar cells from oxygen and humidity from entering into the layers of the structure and thereby enhances its performance and reliability. There are few encapsulation sheets that are being used in FSCs such as ethyl vinyl acetate (EVA), poly vinyl butyral (PVB), thermoplastic polyurethane (PUB), and CH3NH3PbI3. EVA is one of the most popular encapsulation materials used in PV manufacturing. The presence of cross-linking agent inside EVA makes it a thermosetting material. The water vapor transmission rate (WTVR) of EVA is 40 gm−2 day−1. It is weather resistant and offers long term reliability under light exposure due to 91% transmission [26, 27] PVB is a thermoplastic material which is being used since 1980s. PVB is also a single layer material and has very good optical transparency, good heat resistance, good adhesion to solar, glass, and other plastic [28, 29].

16.3 Thin-Film Deposition The production of FSCs can be classified into six vital components: (i) (ii) (iii) (iv) (v) (vi)

manufacturing technology contact topology substrate transport substrate velocity substrate feed degree of integration

The manufacturing technology is also divided into three parts, viz., additive, subtractive, and structuring manufacturing technologies. In additive

Flexible Solar Cells  515 manufacturing technology, semiconductors are being formed by film or layer deposition techniques, whereas in subtractive technique, unwanted deposited part of the semiconductor would be removed. Structuring manufacturing includes wet-de-wetting, stamping, and bonding. The manufacturing of FSCs via printing technology is based on orientation of specific layers over a substrate. In deposition process, substrates like metal, paper, plastic foils, or glass allow to undergo through the different printing mechanisms. In this section, we basically focus on the different film deposition methods or additive manufacturing technologies utilized for the manufacturing of FSCs. The following are the popular processing methods: 1. 2. 3. 4. 5. 6. 7.

Roll-to-Roll Processing (R2R) Chemical Bath Deposition Chemical Vapor Deposition (CVD) Dip Coating Spin Coating Drop Casting Screen Printing

16.3.1 R2R Processing The R2R processing belongs to the family of popular manufacturing techniques FSCs (Figure 16.4) [30]. It is also called as reel-to-reel or web-fed printing in which substrate is transported through two moving rollers. In this technique, additive/subtractive processes are used for creating the structures of continuous arrangement. The different R2R techniques include sheet-to-sheet, sheet-over-shuttle, and roll-to-sheet. Much of these technologies potentially describe the R2R technology assessment, i.e., substrate-based manufacturing methods. The most challenging aspect of said Foil Lamination of High quality

Thin Film Growth

Raw Material

Finished Component Rollers

Plasma

Rollers

Sputtering Process Ar Gas

Figure 16.4  Schematic diagram for R2R processing.

Rollers

516  Fundamentals of Solar Cell Design technology is to precisely transport the substrate; therefore, web is held at a specific tension whenever it crosses the reel. The web tension controlling in a web-fed machine is important to attain high quality products as well to enhance the production efficiency of machines. The R2R technique presents several advantages over its counterparts such as high mass production rates, high throughput, and low cost. However, to set up such a system the initial capital investment is high but the payback period is very short. The comparatively quicker setup makes this technology easy to handle for laboratory scale experiments. Also, R2R technology is the continuous process, in which the material delivery and consumption is balanced. Applications The modern R2R processing is useful in powering the manufacturing technology of the FSCs as follows: (i) Flexible PVs - CIGS-based and other flexible PV modules (ii) Printed or flexible thin-film batteries - laminar lithium-­ ion, etc. (iii) In flexible electronics, e.g., super capacitors, RFIDs, organics LEDs, sensors, and displays (iv) Thick film sensor materials (v) Multilayer capacitors (vi) Fuel cells - laminar solid oxide fuel cells (SOFCs)

16.3.2 Chemical Bath Deposition This technique was invented in 1869 and later Bruckman employed this method for thin-film deposition of lead sulfide (PbS). This method is employed for large area manufacturing. Also, it has been widely used for the deposition of buffer layers in solar cells. The technique is also known as chemical solution method or solution growth method. Figure 16.5 shows the typical arrangement for chemical bath deposition (CBD) process. The typical CBD process involves a solution, prepared by chalcogenide, metal ions, and added base, in which a substrate is immersed. The hydrolysis of metal ions is controlled by adding a complex agent into the solution [31]. The process is dependent on fact that how gradually the chalcogenide ions being released into the alkaline solution, buffered with free metal ions at a low concentration, is taking place. The concentration of metal ions is controlled through the formation of complex agent. Usually, the temperature of solution maintained below 100°C. The pH and concentration of

Flexible Solar Cells  517 Rotating mechanism Thermometer Substrate

Stand

Open Container Field Coil

Precursor solution

Figure 16.5  Schematic diagram for CBD process.

the solution form the solid phase to ex-solve leading to the formation of film over the substrate generally without subsequent heat treatment. Such approaches have been used in past to generate sulfide, selenide and various other non-oxide films. Such coatings can be produced at temperatures below 100°C, thus allowing the coating of materials unable to tolerate high processing temperature like polymers. The growth of thin films is strongly dependent on conditions such as composition, duration of deposition and temperature of solution, and topographical and chemical composition of substrate. The major advantage of CBD is its simplicity as it requires a solution container and substrate mounting device. CBD results in stable, bonded, uniform, and films with good reproducibility. The drawback of this method is wastage of solution post every deposition. Applications Since the properties of oxides make them an attractive alternative for their use as transparent conductive oxide (TCO) in solar cells, sensors, thermal barrier coatings, catalysts, and “super hydrophobic” layers. The CBD technique can be successfully used to coat ZnO thin films on glass substrates thus making it useful for fabrication of FSCs.

16.3.3 Chemical Vapor Deposition CVD is used to deposit the high quality metallic or inter metallic, ceramic compounds including elements, metals, and their alloys and is often used

518  Fundamentals of Solar Cell Design Thermometer

Reversible Valve

Carrier Gas Source Gas

Vent

Metal Substrate with thin film growth

Field Coil

Figure 16.6  Typical experimental setup of CVD.

to produce thin films for microelectronics manufacturing. In CVD process, the wafer is exposed to several volatile precursors, which chemically reacts with surface of substrate to produce thin-film of desired thickness. The volatile by-products are removed by flowing a gas through inside the reaction chamber (Figure 16.6). The microelectronics industry widely uses CVD technique to deposit materials in different forms such as monocrystalline, amorphous, polycrystalline, and epitaxial. These materials include silicon (dioxide, carbide, nitride, and oxynitride), carbon (fiber, nanofibers, nanotubes, diamond, and graphene), fluorocarbons, filaments, tungsten, titanium nitride, and several other high dielectric constant elements. There are various types of CVD techniques, including metal-organic, laser, atmospheric pressure, photochemical, low pressure, chemical beam epitaxy, chemical vapor infiltration, plasma-assisted, and plasma-enhanced CVD [32]. Applications CVD is commonly used to deposit semiconductor thin films and enhance the surface layer property. It is useful for atomic layer deposition for depositing very thin film of material. In some of the CVD applications like gallium arsenide is deposited in integrated circuits (ICs) and photovoltaic devices, carbides and nitrides offers wear-resistance. The polymerization is the most versatile applications of the CVD, which allows for super-thin coatings.

16.3.4 Dip Coating Dip coating is also a thin-film deposition technique in which the substrate is immersed in a solution-processable semiconductor solution (coating solution) and then withdrawn gradually at a controlled speed. The concentration of solution, temperature, and speed of withdrawal are the critical parameters to control the thickness of deposited layer over the substrate as depicted in Figure 16.7. The viscosity and surface tension of solution are

Flexible Solar Cells  519

Thin Layer formation

Substrate

Coating solution

Dipping

Withdrawal

Evaporation

Figure 16.7  Schematic diagram of the dip-coating process.

also important parameters and it controls the uniformity of deposited film. Post withdrawal of the substrate is annealed to result in the formation of uniform thin film [33]. Advantages (i) Simple set up and procedure (ii) Specific thickness achieved by parametric control (iii) Suitable for coating the both sides of flat substrate surfaces at the same time with only nanometer range roughness (iv) Graded coating can be achieved by varying the substrate withdrawal speed (v) Adaptability of high-precision batch processing and largescale processing (vi) Process can be optimized for use with low-concentration solutions by reducing the withdrawal speed (vii) Less expensive setup than other techniques like R2R (viii) Small equipment size (typical dimensions~ 10 cm × 30 cm) (ix) Significant drying time is requiring for good film formation Disadvantages (i) The wet coated film can get contaminated during the drying phase and thus it is carried out inside the clean room

520  Fundamentals of Solar Cell Design (ii) Coating on curvilinear or flexible substrates is cumbersome (iii) Post-deposition heat treatment is required thus increasing the manufacturing cost and scaling complexity (iv) During transition of coating material from liquid to solid state creates cracking of films especially in thicker films (v) Sufficiency large reservoir of solution is required for immersing the substrate leading to wastage of solution Application Dip coating is popular for large scale manufacturing but in small batches. Also, it is highly compatible with R2R processing. At the same time, this technique is economical, convenient, and easily adaptable. Therefore, it is often used for research purposes where it can be employed to coat organic semiconductors, protein and protective layers.

16.3.5 Spin Coating This technique of layer deposition is carried out in three stages, viz., dispense, spinning, and evaporation (Figure 16.8). In first stage, the solution is dispensed onto a flat substrate which then starts rotating at a constant angular speed. In second stage, the centrifugal force creates shear force on the dispensed solution, thus causing it to distribute uniformly over the surface resulting a thin film coated on substrate. In final stage, the spinning stops and allows the coat to dry out with help of evaporation. The thickness of coated film is determined by the centrifugal force applied (proportional to angular speed), time of rotation, and concentration of solution. Despite these, some other parameters dispensed volume and solution viscosity also Nozzle Thin Film Growth

Dispense

Spinning

Evaporation

Figure 16.8  Schematic showing the different stages of spin coating technique.

Flexible Solar Cells  521 affects the quality of the deposition. The typical range of film thickness achieved by spin coating is 1~200 μm [34]. Advantages (i) Simple and easy to handle technique requiring little training and expertise (ii) Economical and excellent coating on flat substrates (iii) Wide range of thickness deposition is possible (1~200 μm) (iv) Films are produced quickly and of reasonably sufficient uniformity since the drying time is quite less due to the rotating substrate (v) Post-deposition annealing is not always required as airflow generated by rotation is enough to dry the film (vi) Among all the techniques of thin-film deposition using solution, this is most effective and cost effective one Disadvantages (i) Unsuitable for large-scale production or batch processing as it is effective for coating small substrates (ii) Difficult to coat with low concentration solution and large amount of solution wastage (iii) Deposition on curvilinear or flexible surfaces is impossible with this method (iv) Deposition of a film with gradient thickness is impossible Applications The spin coating technique is widely employed to manufacture the ICs. Also, the method is useful for processing thin photoresists on wafers fabricating the thin-film electronic devices such as solar cells and light-emitting diodes. Besides these, it is also used to coat the heat resistant dielectric insulating films.

16.3.6 Screen Printing Screen printing is a fast growing technology in printing industry [35]. About 90% silicon and 60% perovskite-based solar cells are manufactured using screen printing technology. The basic principle of screen printing involves a stencil which produces the image over the substrate (Figure 16.9). Further, the photographic emulsion paste is applied over the

522  Fundamentals of Solar Cell Design INK

Blade

Print

en re Sc

Base

Figure 16.9  Schematic resembling the screen printing technique.

substrate. The phosphorus screen paste is applied to form a n-type layer followed by tabbing of silicon and aluminum on rear sides of the substrate. Now, at the rear side, the aluminum paste reacts with silicon to create the back surface field. Depending on the type of materials used the factors such as stencil surface smoothness, screen angle, emulsion on mesh thickness, type of paste, mesh count, image details, squeegee pressure, and number of imprints the screen can last for have to be decided. Despite these, some other parameters are also monitored at the time of screen printing such as ink composition, solid content, viscosity, evaporation rate, dispersion, press setup, wire diameter, percent of open area, emulsion thickness, mesh tension, room temperature, humidity, air turbulence, cleanliness, substrate surface, and shelf life of ink and screens.

16.4 Characterizations for FSCs There are two major classifications of characterization of FSCs, viz., material characterization and cell characterization [36]. While the purpose of material characterization is to pre-analyze the suitability of materials to be used in FSC manufacturing, the cell characterization is used to determine the response of FSCs under solar irradiance. Apart from the challenges faced to accurately carry out characterizations, the major task is to perform the data analysis. The material characterization deals with various techniques useful for characterizing the materials to be used in the manufacturing of FSCs. Some popular techniques of materials characterization are scanning near-field optical microscopy, spectroscopic ellipsometry (SE), photoluminescence, electron spin resonance (ESR), scanning electron microscopy (SEM), etc. For cell characterization, the photocurrent of cell is measured as a function of solar irradiance spectra, monochromatic illumination, applied

Flexible Solar Cells  523 voltage, illumination position, and sample temperature. Moreover, the impedance measurement as a function of frequency or alternating voltage is also used. The most basic characterization of a cell is to measure its current-density/voltage (J/V) characteristics where the current is measured in response to variety of irradiance conditions, thereby yielding four critical performance metrics, viz., short-circuit current density (JSC), open-circuit voltage (VOC), fill factor (FF), and power conversion efficiency (η). Some other frequently used cell characterization techniques are electroluminescence (EL) and capacitance spectroscopy. A brief description of these is as follows.

16.4.1 Material Characterization (i) Semiconductor Characterization (a) Scanning Near-Field Optical Microscopy (SNOM) SNOM is used for the characterization or analysis of light trapping properties of a textured surface [37]. The thickness of absorber layer is generally optimized so as to absorb maximum number of photons falling over it and at the same time limit its thickness below diffusion length of carriers and make the solar cell fabrication cost effective. Besides this, the textured surfaces are commonly employed for light scattering which extends the path length of light inside absorber layer. SNOM is a type of scanning-probe microscopy technique and it works similar to atomic force microscopy (AFM) in the noncontact mode (Figure 16.10). There are two working modes of SNOM available, viz., with and without aperture. In SNOM with aperture, a metal coated tapered fiber tip used in SNOMs with aperture to achieve the required guidance for light. At rear end of the probe, an aperture is produced by forming small holes in the coating. The light modes outside the tip and the fiber modes coupled through the aperture. Such microscope with optical resolution beyond refraction limit is defined as the size of the aperture which is lower than the wavelength of light. SNOMs not having aperture do not include a fiber tip. The tip acts as an antenna and light is scattered with strong field of the tip. The probe scattering occurs significantly near the field at the time of interaction of the tip with the surface. SNOM without aperture gives superior resolutions as compared to SNOMs using an aperture. However, SNOM with aperture is more suited to studying the light propagation and light scattering in FSCs. In SNOM probe, fiber tip is coated with Al and tapered with aperture size to about 50~80 nm. Piezo controlling system is used for scanning the

524  Fundamentals of Solar Cell Design

SNOM tip

SNOM tip Scanning Direction

fiber

(b) Optic

(a)

Near Field Zone Sample

Dither Piezo

Turning Fork Distance Detection

Light Source

Figure 16.10  (a) Scheme of collection mode SNOM setup in transmission geometry. (b) Shear-force technique for distance controlling.

surface of the sample. Since the tip is glued to a quartz tuning fork, distance controlling is realized by shear-force technique. (b) Spectroscopic Ellipsometry Since the beginning of research in thin-film solar cells, the SE has been utilized as one of the most constructive and non-invasive optical technique for the characterization of materials and devices [38]. This technique is based on the concept of polarization interaction with the sample under investigation. The polarization interaction occurs in two ways, i.e., when the light beam is transmitted through the sample or when it is reflected obliquely from the sample surface. In polarization, the emerging beam has a purely elliptical polarization state. Numerous optical properties and layer thickness of the sample can be analyzed with the help of the ellipse. The most popular instrument used for the measurements of such polarization is called as ellipsometry. In this configuration, initially an unpolarized collimated light beam is allowed to pass through a polarizer and compensator for elliptical polarization and then it is reflected back from the sample and is further transmitted to an analyzer before entering the detector. Figure 16.11 shows the ellipsometry arrangement in which the standards of both stepwise wavelength scanning SE used in ex situ applications, and multichannel SE used in high speed real-time, mapping, and online-based checking applications.

Flexible Solar Cells  525 (c) Time-Resolved Photoluminescence (TRPL) In a photovoltaic material, the minority carrier lifetime is a critical property to assess the material quality, and traditionally, it has been estimated by TRPL for films like a-Si and CIGS [39]. A typical schematic of TRPL experimental setup is shown in Figure 16.12. Two-photon excitation (2PE) Light Source

Monochromator Collimator

Detector

ϕ Ploarizer Analyser Compensator

Surface or Sample

Figure 16.11  Generic spectroscopic ellipsometry (SE) configurations.

Laser Pulse

Spectrograph

Computer

Cryostat

Figure 16.12  Experimental setup of TRPL.

Streak Camera

CCD Camera

Delay Unit

526  Fundamentals of Solar Cell Design TRPL estimations of thin polycrystalline samples are used for comparative assessment due to high surface and interfacial recombination. One photon and two photon excitation TRPL estimations are accumulated using a mode locked femtosecond laser with an optical parametric amplifier of tunable wavelength. A time bound single-photon counting system is required for TRPL lifetime measurements. 2PE TRPL are applied to devices to extract the most uniform possible excitation of carriers throughout the device thickness. The excitation spot size is roughly 50–100 μm, which is sufficient to disregard the impact of horizontal diffusion of carriers from the first volume on the decay curve. (d) Electron-Spin Resonance ESR or electron paramagnetic resonance (EPR) is one of the only available experiments that gives structural information about such defects states [40]. The spin detection of defects act as the local test to precisely monitor the local magnetic field distribution in its region which is characterized by line position and line state of the comparative ESR range or its dynamics. ESR is suitable for evaluating the number of defects in a given sample volume with a sensitivity of 1010–11 spins at 9 GHz. This mode is especially interesting to material analysts to upgrade the quality of material. The principle benefits of ESR is that, practically, any sort of test geometry can be characterized non-destructively and provide the sample comfortably fits into the resonator of ESR spectrometer. Common sample volumes that can be contemplated are 4×4×10 mm3 and the sample state can be vapor, fluid or solid solution, powder, crystalline, and thin film. The conductivity of the sample must be limited enough together not to influence the properties of the ESR resonator. When contemplating a thin film or interfaces, the sensitivity of ESR is effortlessly reached. Figure 16.13 shows the schematic of ESR spectrometer. The right side figure indicates how field modulation yields the derivative type of the ESR absorption signal. The B1 distribution is shown for a rectangular cavity. (e) Scanning Electron Microscopy The SEM technique is applied for imaging the layers of thin-film solar cells. The technique provides magnification which helps in understanding layer thickness and surface roughness down to a resolution below 1 nm [41]. Additionally, many a times, SEMs are inclusive of energy dispersive X-ray (EDX) indicators, which are utilized for knowing the elemental composition of thin films. The potential outcomes of SEM analysis is much more than just imaging and compositional investigation. In SEM, the imaging is done utilizing the secondary electrons (SEs) and backscattered electrons

Flexible Solar Cells  527 (BSEs). The various dissimilarities in pictures gives insight into compositions, microstructures and surface potentials. The schematic diagram of SEM instrument is shown in Figure 16.14 demonstrates that energy of the incident electron beam E(B) is regulated by means of an accelerated voltage and the electron-beam current “IB” is regulated by the current through the fibers of thermionic electron gun or by gaps for microscope with field discharge gun. The electron-beam energy E(B) characterizes the penetration depth “R” into the sample material. Reference Arm Attenuator

Phase Shifter

Gunn Diode bridge 9.5 GHz

Attenuator

Magnetic Field

ϕ Lock in Amplifier

Cavity

Modulation Coil

Figure 16.13  Schematic diagram of ESR spectrometer experimental setup. Electron Gun

BSE

SE N(E)

Accelerating Anode BSE Detector

Low Loss BSEs

Focusing Magnet SE Detector Scanning Magnet Auger Electrons 0

50 eV

Secondary Beam Detector

Energy (E) E(B)

Figure 16.14  The SEM measurement setup.

Sample

528  Fundamentals of Solar Cell Design (ii) Flexible Substrate Characterization The characterization of flexible substrate materials is different from that of semiconductors [42, 43]. The flexible substrate materials are generally required to undergo tests such as residual stress, fracture toughness, adhesion measurement, and hardness and elastic modulus measurements. (a) Residual Stress Measurement In FSCs, it is utmost important to know the material life span with consistent performance. Residual stresses belong to internal stresses present inside a solid stationary body under the equilibrium and in the absence of external stress generating forces. These stresses could be inherent, thermal, or hygroscopic in nature. Residual stresses are generated in thin films during fabrication due to processes like PVD, CVD, and RF magnetron sputtering. (b) Fracture Toughness Measurement Fracture toughness refers to stress resistance of a material to get fractured under the presence of a flaw or the highest stress intensity that a material can withstand without getting fractured. For FSCs, the load versus depth indentation curve test and controlled buckling test are used. (c) Adhesion Measurements Adhesion is the state under which two surfaces are held together through interfacial forces. For effective operation of FSC, the reliability of interfacial film is crucial. Spontaneous delamination of film can happen during the lifetime of FSCs due to the residual stress induced crack enlargement. So, the quality of adhesion needs proper characterization before fabrication of reliable FSCs. Adhesion is measured by two methods, viz., adhesion strength measurement and interfacial toughness measurements. Mostly, the techniques employed for adhesion measurements on rigid substrates cannot be utilized on flexible substrates. The interfacial fracture toughness measurement tests are based on the introduction of a stable crack of known length at an interface and subsequently modeling its propagation. Practically, there is no universal technique for determination of interfacial fracture toughness since the mechanism of failure varies with experimental setup and coating systems. (d) Hardness and Elastic Modulus Measurement There are several types of hardness and elastic modulus measurements like indentation, surface wrinkling, and bubble method. The method depends on the loading/unloading curve during indentation and the area function

Flexible Solar Cells  529 characteristic to the type of indenter. In recent years, some more sensitive measurements have also been designed as the forces required to study the nanometer level deformation are of low magnitude. (e) Encapsulation Characterization The encapsulation materials are examined to assess the mechanical and thermal properties. Several techniques are employed such as X-ray topography (XRT), thermally stimulated technique (TCS), adhesion technique, and peel test. The XRT process is used to examine process induced defects and growth striation on thin film and is a non-destructive method which means that it does not destroy the solar module during characterization. Besides it, the TSCs are used to examine the thermal properties of encapsulating material. TSCs generally conducted at temperatures of −150 to +70°C. The peak maximum current and the area under the TSC current peak are used for the determination of glass transition temperature, activation energy and relaxation frequency.

16.4.2 Device Characterization The characterization of a solar cell addresses its power generation capability. The major objective of cell characterization is to develop, evaluate, and design a unique, more accurate PV cell that can operate as consistently as possible with maximum efficiency [44]. Generally, the manufacturers conduct a quick measurement on finished module to check several of its performance parameters, e.g., I-V characteristics, spectral response, and quantum efficiency measurements. (i) Measurement of Current-Voltage Curve The electrical characteristics of a solar cell are the current versus voltage (I-V) curve, series resistance, temperature coefficients, spectral response, and quantum efficiency. Solar cell parameters obtained from I-V characteristics include the short circuit current-density JSC, open circuit voltage VOC, FF, and power conversion efficiency of the cell (η). Especially, FF represents the internal losses that are visually conversed by how much the I-V characteristic curve deviates from a rectangular shape as shown in Figure 16.15. The solar cells are characterized under Standard Test Conditions (STC) with air mass 1.5 global (AM 1.5G) solar spectrum and the total irradiance of 1,000 W/m2 at 300K. Practically, it is difficult to obtain STC conditions in an outdoor atmosphere. Therefore, the majority of testing laboratories perform these electrical measurements under indoor simulated sunlight

530  Fundamentals of Solar Cell Design 5

VM

0

VOC

-5 FF = IMVM / ISCVOC

-10 I (mA) -15 -20 -25

IM

-30

ISC

-35 -40 -0.1

0.0

0.1

0.2

0.3 0.4 V (Volts)

0.5

0.6

0.7

0.8

Figure 16.15  I-V characteristic of solar cell.

W-Lamp

Semitrons mirror

Splitter

Mirror

Shutter Integrator Detector Reflector Xenon Lamp

Lens PV Cell Sample Holder

Figure 16.16  I-V characteristics measurement setup.

environment. Figure 16.16 shows the experimental setup to measure the I-V characteristics. (ii) Quantum Efficiency Measurement Practically, all the electron-hole pairs generated by sunlight incidence do not get collected at the respective electrodes due to the recombination of minority charge carriers and this limits the performance solar cells. Therefore, in order to characterize this obvious limitation, the external

Flexible Solar Cells  531 Beam Splitter Light Sources (Xe)

Bias Light Chopper Wheel

PC

Lock in Amplifier

Monochromator Device under test

Reference cell

Monitor Cell

Current to Voltage Converter

Figure 16.17  EQE measurement setup.

quantum efficiency (EQE) is measured (Figure 16.17). It is defined as the number of electrons collected per photon incident on the solar cell. (iii) Electroluminescence Measurement of Solar Cells EL is the emission of light due to the application of forward bias voltage across a p-n junction diode. The EL characterization is mainly used to yield information about the carrier collection probability and lifetime of carriers generated inside the absorber layer of a cell. The typical EL characterization is of two types, viz., spatially and spectrally resolved EL which are used to scan the entire PV module and to also visualize the microscopic defects present inside the solar cells. Figure 16.18 shows the two experimental setups for EL characterization.

16.5 Issues in FSCs The fabrication of FSCs is based on several varieties of semiconductor and substrate materials and obviously like any other technology these too are facing commercialization challenges [45]. Some of the important issues to be dealt with in the area of FSCs are as follows. 1. 2. 3. 4. 5. 6. 7. 8.

Material optimization High fabrication cost Efficiency Life time of cell High processing temperatures Careful substrate handling Stability Inferior robustness

532  Fundamentals of Solar Cell Design Gedetector

(a) Solar Cell Monochromator

Function Generator (V)

Lock In Amplifier CPU

(b)

PC (Control Panel)

CCD Cam Dark Room

Solar Cell Base

Power Supply

Figure 16.18  (a) Spectrally resolved. (b) Spatially resolved EL setup.

9. Toxicity and recyclability 10. Biodegradability

16.6 Performance Comparison of RSCs and FSCs There are various pros and cons of both RSCs and FSCs. One will find rigid PV modules being installed in majority of locations but with the technological advent of flexible electronics modules; now, it has become rather common to observe flexible being also used in those locations where surfaces are not flat and sunlight is available in plenty. Table 16.1 compares the RSCs and FSCs in terms of the performance and fabrication features.

16.7 Applications of Flexible Solar Cell FSCs have wide range applications due to its unique feature of flexibility which cannot be obtained with RSCs. Some peculiar examples of non-flat

Flexible Solar Cells  533 Table 16.1  Comparison chart of RSCs and FSCs. RSCs

FSCs

Thick and bulky

Thin and Light weight and thin

Not bendable

Bendable upto 30°

High installation cost

Low installation cost

More structural constraints

Lesser structural constraint

Cannot be installed temporarily

Can be temporarily installed

Higher power conversion efficiency

Lower power conversion efficiency

Sufficient heat dissipation

Poor heat dissipation

Difficult to transport

Easily transportable

Cost per unit energy generated is less

Higher cost per unit energy generated

Can withstand extreme weather conditions

Cannot withstand extreme weather conditions

Difficult to be recycled (less eco-friendly)

Easier to be recycled (more eco-friendly)

Must be installed as per sun direction

Can be installed independent of sun direction

surfaces of interest are train roof, or building curvatures, vehicles, human body, etc. Some characteristic applications of FSCs are as follows. 1. 2. 3. 4.

Flexible PV integrated clothing Flexible PV integrated medical implants Flexible PV integrated electric vehicles Flexible PV integrated wearables such as smart watches and health monitoring devices.

16.8 Conclusion The flexible solar modules offer various distinct applications in various forms of life which are not possible with rigid solar modules. Although the unique feature of flexibility seems to be quite lucrative from the

534  Fundamentals of Solar Cell Design commercialization point of view, there are several challenges associated with this technology such as inferior performance and lower stability. Therefore, plethora of research prospects lie in the area of FSC development.

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Index

1D, 393, 404 3D, 393, 415, 446 5-ammoniumvaleric acid (5-AVA), 385, 386 Absorption, 248–251, 256, 260–262, 264 Absorption coefficient, 164 Absorption spectroscopy, 190–191, 318–321 Advancements in plasmonic solar cells, 64 direct plasmonic solar cells, 65 plasmonic dye-sensitized solar cells (PDSSCs), 70 plasmonic hybrid solar cells, 72 plasmonic perovskite solar cells, 72 plasmonic photoelectrochemical cells, 71 plasmonic quantum dot (QD) solar cells, 71 plasmonic thin-film solar cells, 69 plasmonic-enhanced solar cells, 69 Affixing components, 140 Al, 393, 399, 417–422 All-atom optimized potentials for liquid simulations (OPLS-AA) functions, 188 AM 1.5 G lamination, 378, 385 AM1, 393, 401, 410 Amorphous silicon, 105 Analytical hierarchy process, 217, 233–235

Anthocyanins, 153 Applications of solar cells, 354–362 a-Si, 393, 411, 416, 421, 425 Atomic force microscopy (AFM), 523 Atomic-scale morphology, 193–194 AUOS-SM, 394, 426 Average visible transmittance (AVT), 466 B3LYP functional density approach, 185, 188, 189 Back scattered electron (BSE), 527 Backward transfer process, 176 Ball-and-stick model, 193 Basis set superposition errors (BSSEs), 187 Basis sets, 183 Bending radii, 512 BIM, 394, 428 Bio-degradability, 508, 513, 532 Biohybrid solar cell, 121 Biophotovoltaics, 126 Blocking layer, 146 Boltzmann equation, 192 C, 394, 404–409, 445 C/Si, 394, 404–409, 445 Cadmium sulfide (CdS), 275 Cadmium telluride (CdTe), 108, 173, 288–289, 372 Calcium titanium oxide crystals, 174 Capacitance effect, 381 Carbon nanotube (CNT), 513–514

537

538  Index Carbon, PV property of SC, 181 Carotenoids, 154 Car-Parrinello method, 195–196 Carrier transport, 192–193 Cartesian lattice, 193–194 CB, 394, 406, 412, 415 CC, 394, 404, 410, 412, 415, 416, 432 Cell architecture and working mechanism, 139 Cell efficiency, 317, 334 CES, 394, 396 CFPTSC, 87 Chalcogenide, 286 Chalcopyrites, 286–288 Characterization of flexible solar cell, device characterization, 529 material characterization, 523 Charge carrier, 248, 249, 250, 251 Charge separation, 320 Charge transfer (CT), 184, 186, 192–193 Charge-separated (CS) state, 184 Chemical bath deposition (or solution growth method), 516–517 Chemical hardness (η), 198 Chemical stability, PSCs, 375–376 Chemical vapor deposition, 517–518 Chlorophyll, 152, 173–174 Chromophores, 190–191 Classification of plasmonic nanostructures, 59 Classification of solar cells, 3 Close/open-current density (Jsc/Voc), 177–178 CNT, 394, 406, 407, 445 Coarse-grained (CG) simulations, 193 Color rendering index (CRI), 468 Commonly used natural dyes in DSSC, 152 Conductive polymers, PV property of SC, 181 Conductor-like screening model (COSMO), 196 Configuration interaction (CI), 184–185

Configuration interaction singles (CIS), 191 Conventional resources, 85 Conversion efficiency, 247, 249, 250, 253, 256, 258, 259, 264 COP, 394, 433 Copper indium diselenide solar cells, 111 Copper indium gallium diselenide (CIGS), 173 Copper indium gallium selenide/ sulfide (CIGS), 372 Copper zinc tin sulfide, 114 Corresponding color temperature (CCT), 467 Coumarin, 171 Counter electrode, 140, 145 Coupled-cluster (CC) state, 186 Crystal silicon solar cells, 109 Crystalline silicon (c-Si), 278–280 Crystalline silicon form, 352–353 CST, 394, 433–434 Current density/voltage characteristics, 523, 529–530 Curve fitting, 188 CVD, 394, 421, 446 Cyanine, 171 DC, 394, 396–398, 431, 432, 440–443, 447 Declination angle, 227 Degradation, in PSCs, 373–379 on top electrode, 380–381 oxygen and moisture, 376–378 structural and chemical stability, 375–376 thermal stability, 373–375 visible and UV light exposure, 378–379 Delocalization error (DE), 185 Density functional theory (DFT), 182–183 application, 182 functionals, 185

Index  539 TDDFT, 169, 174, 182, 183–184, 186, 189–190 ZINDO results, 196–197 Density of state (DOS) spectra, 187 Determination of energy gap of electrode material adsorbed with natural dye, 163 DFT. see Density functional theory (DFT) Dielectric mirror (DM), 472 Diffuse sun irradiation, 227 Dihedrals, inter-ring/inter-monomer bonds-related, 188–189 Dip coating, 518–520 DMA, 394, 429 DNEA, 394, 436 Donors, linkers and acceptors (D-π-A sensitizer), 180 Double-junction, 92 DSP, 394, 429 DSSCs. see Dye-sensitized SCs (DSSCs), 277, 293, 355–356, 372, 394, 403–404 Durability, 508–510 DWCNT, 394, 407–409 Dye adsorption, 164 Dye as natural sensitiser, 140 Dyes, defined, 191 Dye-sensitized SCs (DSSCs), 126, 169, 173 calculated efficacy of cells, 181 configuration, 175 DFT in (see Density functional theory (DFT)) dye and electrodes of, 174 features, 170 operation principle, 176–177 photosensitizers in, 182 PV parameters, 177–181 Ru-based dye sensitizers, 178–179, 179 TDDFT (see Time-dependent DFT (TDDFT))

TiO2 nanowires into, 180 WBG semiconductor in, 171 with ruthenium sensitizer, 171 working principle, 175–176 EA (electron affinity), 184, 185, 187, 196 EB, 394, 406, 411, 415 Efficiencies, 121 Eigen energy, 192 EIS, 394, 403 Electrical additives, 148 Electrodes, 139, 144, 513 Electroluminescence measurement, 531 Electrolyte, 140, 147 Electron affinity (EA), 184, 185, 187, 196 Electron collection efficiency, 178 Electron ionization (EI) potential, 196 Electron spin resonance (or electron paramagnetic resonance), 522, 526–527 Electron transport layer (ETL), 479–480, 510, 512 Electron transport material (ETM), 464–465 Electronic absorption spectroscopy, 190–191 Electron-spin resonance, 526 Electron-transport layers, PSCs, 382–384 Electrophilicity (ω), 198 Encapsulations, 514 Engineered materials, 129 EPN, 394, 441, 443 EPP, 394, 445 ESS, 394, 440 ETL, 394, 410–411, 415, 446 EV, 394, 401 Exchange-correlation (XC) functional, 183, 185, 189 Excited states, 184, 189–190

540  Index Excited-dye electrons, 176 Exciton, 321–325 FA, 394, 411, 413–414, 417–419 Fabrication of simple DSSC in lab scale, 142 Fermi level, 331–333 Fermi’s golden rule (FGR), 192 Ferredoxin, 122 Fill factor (FF), 163, 177–178, 251, 253 First-generation photovoltaic cells, 351–352 FL, 394, 424 Flavonoids, 152 FLC, 394, 422–424 Flexibility, 505, 508–509, 512–514 Flexible modules, 113 Flexible panels, 113 Flexible PV panel, 509–511 Flexible solar cell, 509 definition, 509 need of FSC, 509 Flexible substrate characterization, 528 adhesion measurements, 528 encapsulation characterization, 529 fracture toughness measurement, 528 hardness and elastic modulus measurement, 528–529 residual stress measurement, 528 Flexible substrates, 512 metal foils, 512 plastics, 513 Force field parameterization for MD simulations, 182, 188–189 Frontier Molecular Orbital (FMO), 171 Fullerene (C60), 378 Fullerene spheroid, 195 Function periodicity graph, 232–233 G, 394, 406, 408, 409 Gallium arsenide (GaAs), 276 Gap-tuning, 185 GCPVS, 394, 396, 401, 429–431

Generation, 120 Generation solar cells, 506 first-generation solar cells, 506 second-generation solar cells, 506 third-generation solar cells, 508 Generations of solar cells, 506–508 GG, 394, 401 Global reactivity descriptors, 196–198 GO, 394, 404, 410, 446 Golden rule of time-dependent perturbation theory, 192 Goldschmidt tolerance factor (t), 375 Graphene, PV property of SC, 181 GW, 394, 400, 402, 444 Hartree-Fock (HF) exchange, 183 Hemicyanine, 171 Heterojunction intrinsic thin layer (HIT), 280 HEV, 394, 401 Hohenberg-Kohn theorems, 182–183 Hole transport bilayer, 94 Hole transport layers (HTL), PSCs, 384–387, 479–480, 510, 512 Hole transport material (HTM), 464–465 HOMO energy, 176, 179, 184, 185, 186, 195–198 Hopping, 193 HPVS, 394, 396 HST, 394, 433 HTL, 394, 410–414, 415, 446 Hybrid functionals, 183 Hybrid SCs, 174 Hydrogenated a-Si (a-Si:H), 282, 283 Hysteresis phenomenon, in PSCs, 381–382 IAS, 394, 411 IEMS, 394, 442 IES, 394, 396 I-layer, 394, 406, 410 Incident photon-to-current efficiency (IPCE), 177–178

Index  541 Incoherent movement, 193 Indium gallium phosphide (InGaP), 277 Indium tin oxide (ITO), 280, 476–477, 482, 488–490 Indoline, 171 Inorganic hole transport materials, 151 Inorganic SCs, 173, 174 Interconection layer, 94 Interface charge resistance, 177–178 Intermediate neglect of differential overlap (INDO) method, 185 Iodide (I-) redox electrolyte, 171 Iodide/triiodide-free mediator and redox couples, 149 Ionic liquids, 149 Ionization potential (IP), 184, 185 IOS, 395, 404 Issues of flexible solar cell, 531 Kinetic Monte Carlo (KMC) modeling, 193–195 Kohn-Sham (KS) equations, 183 LC (long-range corrected) functionals, 185, 186, 187, 189 LCA, 395, 436, 446 LDR, 395, 429 LED, 395, 447 Light harvesting devices, 117 Light scattering, 255–258 Light sensitive stuff, 124 Limitations of solar cells, 508 Liquid-based electrolytes, 148 Long-range corrected (LC) functionals, 185, 186, 187, 189 LSC, 395, 425–427 Luminosity, 466 Luminous, 330 LUMO energy, 176, 179, 184, 185, 186, 195–198 MA, 395, 411, 413–414, 417, 419, 446 MAPbBr3 PSCs, 374, 375

MAPbCl3 PSCs, 375 MAPbI3 PSCs, annealing temperature, 374f structural and chemical stability, 375–376 thermal and photo stability, 383–384 thermal stability, 373–375 TiO2/MAPbI3/Al2O3/spiroMeOTAD composite, 385 with FTZO, 383 Maximum power point condition (MPPT), 378 Metal halide perovskites, CsPbI3, 375 CsPbX3, 375 CsSnBr3, 375–376 CsSnCl3, 375 CsSnI3, 375 MAPbBr3, 374, 375 MAPbCl3, 375 MAPbI3 (see MAPbI3 PSCs) oxygen and moisture, 376–378 visible and UV light exposure, 378–379 Metal semiconductor oxide electrodes, 179–180 Metal-free dyes, 180 Metallization, 89 Metalloporphyrin (MP), 171, 187 Metal-to-ligand charge transfer (MLCT), 179 MicoSem, 395, 404 Micropillar, 326 Module and cells, crystalline, 239 multi-crystalline, 239 silicon, 239 thin-film, 239 Moisture, stability in PSCs, 376–378 Molar extinction coefficient, 180 Molecular descriptors, 184–188 Molecular dynamic (MD) simulations, CP method, 195 DFT-based, 195

542  Index force field parameterization for, 182, 188–189 Molecular mechanics (MM) computations, 189 Mono-crystalline silicon-based SCs, 173 Mott-Schottky analysis, 381 MPPT, 395, 397–398, 440–443 MSSC, 378, 379 Multi-walled carbon nanotubes (MWNT), 181 N3-cis-di (thiocyanato) bis (2, 2-bipyridine-4, 4-dicarboxylate) ruthenium, 178, 179 N719 dye, 179 Nanostructured inorganic-organic heterojunction solar cells (NSIOHSCs), 356–357 Nanowire (NW), 481 National Renewable Energy Laboratory (NREL), 277 Natural transition orbitals (NTOs), 186–187 NFA-BOHJ, 395, 410, 411 NIR-transparent, 89 Non-ideal bandgap, 89 Non-renewable resources, 85 Non-silicon–based SCs, 173 NOx, 395, 396 NP, 395, 404, 409 NS, 395, 404 OHJ CGL, 87 Oligoene, 171 Open-circuit current, 163 Open-circuit voltage, 110, 251–252 Operation principle, of DSSC, 176–177 Optical properties, 62 multiple energy levels, 63 scattering and absorption of sunlight, 63 trapping of light, 63 Opto-electronically, 88

OPV, 86 Order-n Moller-Plesset theory (MPn), 184 Organic hole transport materials, 151 Organic solar cells (OSCs), 170, 173–175, 360–362 Organic solvents, 148 Oxygen, stability in PSCs, 376–378 Oxygenated, 110 P3HT matrix, 385 PANI-PS I, 127 Parameterization for MD simulations, force field, 182, 188–189 PCE, 393, 395, 404–411, 413, 416–420, 445 PCM, 395, 434 PCS, 395, 404, 426, 428 PDMS, 395, 474 PEI, 395, 407, 414 Perdew–Burke-Ernzerhof (PBE) level, 188 Perovskite, 507–508, 512, 521 Perovskite absorbers, 474 Perovskite solar cell (PSC), 174, 354–355, 462 Perovskite solar cells (PSCs), stability in, 371–387 chemical reaction at interface, 379–380 degradation on top electrode, 380–381 degradation phenomena and stability measures, 373–379 electron-transport layers, 382–384 HTL, 384–387 hysteresis phenomenon, 381–382 optoelectronic properties, 375–376 overview, 371–373 oxygen and moisture, 376–378 selective contacts on, 382–387 stability-interface interplay, 379–382 structural and chemical stability, 375–376

Index  543 thermal stability, 373–375 visible and UV light exposure, 378–379 Perovskites, 296–300 Perylene, 171 PHEV, 395, 401 Phonon-assisted hopping, 193 Photoactive layer, 474 Photo-bleaching phenomenon, 378 Photocurrent generation, 123 Photodetectors, 118 Photoexcitation, 122, 190, 319 Photogeneration, 320 Photon, 324 Photo-oxidation process, stability in PSCs, 376 Photo-oxidized dye molecules, 171 Photosynthetic apparatus, 117 Photosystem-I, 118 Photosystem-II, 118 Photovoltaic (PV), 272, 462 Photovoltaic (PV) parameters, DSSC, 177–181 Photovoltaic device, 238 Photovoltaic parameters, 5 Phyloquinone, 124 PI, 395, 422, 442–443 PIC, 395, 424 Planckian locus, 467 Plane waves (PWs), 183 Plasma-enhanced chemical vapor deposition, 106 Plasmonic nanostructure, 58 Platinum (Pt), complex, 177 PV property of SC, 181 PMMA, 395, 409 PMSG, 395, 441–442 Polar ice, 85 Polarizable-continuum-model (PCM), 196 Poly(3-hexylthiophene), 194 Polyaniline (PANI), 384 Polyanilines, 119

Poly-crystalline silicon-based SCs, 173 Polyimide, 262, 263 Polymer solar cells, 357–358 Polymethyl methacrylate (PMMA) coating, 385 Polynuclear bipyridyl Ru dyes, 179 Polyphenylenediamines, 119 Porphyin dyes, 180 Porphyrine derivations, 171 Porphyrins, 171, 177, 180, 197 Power conversion efficacy (PCE), 174 Power conversion efficiency (PCE), 154, 371, 372, 373, 462 MAPbI3 perovskites, 374 PSC with Spiro-OMETAD, 385 Power conversion efficiency (η), 177–178 Principles and working mechanism of plasmonic solar cells, 60 mechanism of plasmonic solar cells, 61 working principle, 60 Probe light, 319, 320 PS I, 124 PS II, 125 PSCs. see Perovskite solar cells (PSCs) PV, 393, 395–404, 405, 411–412, 415– 422, 425, 427–429, 431, 432–434, 436–437, 440–447 Quantum dot solar cells (QDSCs), 300, 358–360 Quantum dots (QDs), 300–302 Quantum efficiency, 251 Quantum efficiency measurement, 530–531 Quantum mechanics (QM) methods, 172, 184, 185, 186 Quasi-solid-state electrolytes, 149 Quenching, 323 Raman spectroscopy, 380 Random-phase approximation (RPA), 191

544  Index Recombination, 110 Reflection, 247, 249–251, 255, 258, 259 Reliability, 508–509, 514, 528 Renewable energy, 104, 247, 264 Renewable energy sources, 85 Rhenium (Ru), Ru-based dye sensitizers, 178–179, 179t Ru(bipy)3-(C60)2 triad, 195 Roll-to-roll processing (R2R), 515–516 Ruthenium sensitizer, 171 SAPVS, 395–396, 431 Scanning electron microscopy, 522, 526–527 Scanning near-field optical microscopy (SNOM), 522–524 Schrodinger’s equation, 192, 193 Screen printing, 109, 521–522 SCTOPV, 86 Secondary electrons, 526–527 Second-generation photovoltaic cells: thin-film solar cells, 352–353 Selective contacts on stability, PSCs, 382–387 electron-transport layers, 382–384 HTL, 384–387 SEM, 395, 405 Semiconductor, 110 Semiconductor characterization, 523 Sensitizer, D-π-A, 180 in DSSC, 171 organic dye as, 170, 177 photo-sensitizers, 176, 178, 182, 197 Ru-based complexes, 177, 178–179 ruthenium, 171 UV-Vis absorption spectra, 191 SHF, 395, 447 Shielding layer, 262, 263 Short-circuit current, 163 Si, 395–396, 400–401, 404–411, 416, 421, 425, 431–432, 445–446

Silicon carbide nanotubes (SiCNTs), 187 Silicon-based SCs, 170, 173 Single junction (SJ) solar cell, 486 Single-crystalline SCs, 173 Single-walled carbon nanotubes (SWCNTs), 385 SiNW, 395, 407–408 Small-molecule acceptor and donor types, 30 Small-molecule acceptor and polymer donor types, 7 SO2, 395–396 Soda lime glass, 111 Solar cell (SC), 272, 462 Solar cell efficiency, 274–275 Solar cell structure, 4 Solar cells (SCs), characterization and theoretical modeling (see Theoretical modeling, of SCs) classification, 172–175 DSSC (see Dye-sensitized SCs (DSSCs)) hybrid, 174 inorganic, 173, 174 organic, 173–175 OSCs, 170, 173, 174 perovskite, 174 PSCs, stability in (see Perovskite solar cells (PSCs)) sensitizers for, 171 Solar energy, 272, 462 Solar energy systems, 105 Solid-state ionic conductors, 151 Solid-state polycrystalline, 90 Solid-state transport materials, 150 Solvation effects, 196 Spectroscopic ellipsometry, 522, 524–525 Spin coating, 520–521 Spinach, 122 Spin-polarization of DOS spectra, 187

Index  545 Spiro-MeOTAD layers, 380, 381, 382, 384, 385 Spiro-OMeTAD HTL, 377, 385 Stability, in PSCs. see Perovskite solar cells (PSCs) Stability-interface interplay, PSCs, 379–382 chemical reaction at interface, 379–380 degradation on top electrode, 380–381 hysteresis phenomenon, 381–382 Staebler-Wronski effect, 106 Standard test conditions, 529 Structural stability, PSCs, 375–376 Sulfide-passivated, 94 Sun irradiation, 218 Sunrise angle, 231–233 Sustainable energy, 462 SWCNT, 395, 407–409 Symmetry-adapted perturbation theory (SAPT), 187–188 System geometry, optimizing, 186 Tandem solar cell (TSC), 486–492 TDDFT. see Time-dependent DFT (TDDFT) Tetrabutyl ammonium (TBA+), 179 Texture, 253, 258, 259 TFSA, 395, 408, 409 TFSM, 395, 421 Theoretical modeling, of SCs, 169–199 basis sets, 183 Car-Parrinello method, 195–196 CG simulations, 193 charge transfer and carrier transport, 192–193 classification, 172–175 computational methods, 181–198 DFT (see Density functional theory (DFT)) excited states, 189–190 force field parameterization for MD simulations, 188–189

global reactivity descriptors, 196–198 inorganic SCs, 173 KMC modeling, 193–195 molecular descriptors, 184–188 organic SCs, 173–175 overview, 170–172 peration principle, 176–177 PV parameters, 177–181 solvation effects, 196 TDDFT, 169, 174, 182, 183–184, 186, 189–190 UV-Vis spectroscopy, 190–192 working principle, 175–176 Thermal evaporation, 111 Thermal stability, PSCs, 373–375 Thermally stimulated technique (TCS), 529 Thermoplastic-based polymer electrolytes, 150 Thermosetting polymer electrolytes, 150 Thin-film materials, 104 Thin-film solar cell (TFSC), 103, 173, 276–278, 505, 507, 509 Thin-film technology, 105 Thiophene, 171 Third-generation photovoltaic cells, 353–354 Threshold, 326 Time-dependent DFT (TDDFT), 169, 174, 183–184 excited-state modeling, 189–190, 191 findings of, 191–192 on LC functionals, 186 real-time, 192–193 Schrodinger’s equation, 192, 193 UV-Vis absorption spectra, 182, 191–192 Time-dependent perturbation theory, 192 Time-resolved photoluminescence (TRPL), 522, 525–526 Titanium dioxide (TiO2),

546  Index as WBG semiconductor, 171, 174 conduction band (CB), 197 conduction band of, 171 electrodes of nanoporosity, 176 electron acceptor film of, 376 electron-conducting layer of, 378 mechanism of operation for DSSC, 176–177 nanowires into DSSCs, 180 TiO2/MAPbI3/Al2O3/spiroMeOTAD composite, 385 under UV exposure, 383 Z907-sensitized TiO2 surfaces, 182 Tomography, 323 TPSC, 88 Transparency index, 232–233 Transparent, 253–258, 262 Transparent conductive oxide (TCO), 108, 280, 481–484 Transparent electrode (TE), 481–484 Triarylamine, 171 Triiodide (I3-) redox electrolyte, 171 Triple-junction cell, 92 TW, 395, 401 TWh, 395, 444 Ultraviolet-visible (UV-Vis) spectroscopy, 190–192 UV, 395, 416, 429

UV light exposure, stability in PSCs, 378–379, 383 UV-Vis absorption, 182 VAC, 395, 433 Valence force field (VFF), 193 VAM, 395, 433–434 Van der Waals heterostructure, 90 VB, 395, 406, 415 Visible and UV light exposure, stability in PSCs, 378–379 Water degradation mechanism of PSCs, 376–378 Water vapor transmission rate (WTVR), 514 Wide bandgap (WBG) semiconductor, 171, 174 Work function (WF), 481–482 Working principle, of DSSC, 175–176 WTG, 395, 437, 440–442 X-ray topography (XRT), 529 Z907-sensitized TiO2 surfaces, 182 Zerner’s intermediate INDO (ZINDO), 185, 191–192, 197 Zinc oxide (ZnO), 112, 173, 180 Zinc oxide (ZnO) stability, 383

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