FUNDAMENTAL SCIENCES Chemistry DYE-SENSITIZED SOLAR CELLS Edited by K. Kalyanasundaram With contributions by: Michael Bertoz, Juan Bisquert, Filippo De Angelis, Hans Desilvestro, Francisco Fabregat-Santiago, Simona Fantacci, Anders Hagfeldt, Seigo Ito, Ke-jian Jiang, K. Kalyanasundaram, Prashant V. Kamat, Ladislav Kavan, Jacques-E. Moser, Md. K. Nazeeruddin, Laurence Peter, Henry J. Snaith, Gavin Tulloch, Sylvia Tulloch, Satoshi Uchida, Shozo Yanagida and Jun-ho Yum Forewords by: Michael Grätzel and Shozo Yanagida EPFL Press A Swiss academic publisher distributed by CRC Press CRC Press Taylor &. Francis Group Taylor and Francis Group, LLC 6000 Broken Sound Parkway, NW, Suite 300, Boca Raton, FL 33487 Distribution and Customer Service [email protected] www.crcpress.com Library of Congress Cataloging-in-PublicationD ata A catalog record for this book is available from the Library of Congress. This book is published under the editorial direction of Professor Hubert Girault (EPFL). The publisher, editor and authors of this book would like to thank the Swiss Federal Institute of Technology (EPFL) for its generous support towards the publication of this book and are grateful to the following industrial sponsors for their participation that helped make this project possible: Dyesol Group, Sefar A.G. and enerStore Consulting, Ltd. (PflfSHB is an imprint owned by Presses polytechniques et universitaires romandes, a Swiss academic publishing company whose main purpose is to publish the teaching and research works of the Ecole polytechnique fédérale de Lausanne. Presses polytechniques et universitaires romandes EPFL - Rolex Learning Center Post office box 119 CH-1015 Lausanne, Switzerland E-mail: [email protected] Phone: 021/693 21 30 Fax: 021/693 40 27 www.epflpress.org © 2010, First edition, EPFL Press, Lausanne (Switzerland) ISBN 978-2-940222-36-0 (EPFL Press) ISBN 978-1-4398-0866-5 (CRC Press) Printed in France All right reserved (including those of translation into other languages). No part of this book may be reproduced in any form - by photoprint, microfilm, or any other means - nor transmitted or translated into a machine language without written permission from the publisher. PREFACE The last century witnessed an incredible number of technological advances that have changed our lifestyle considerably. The extensive use and growing dependence on electrical and electronic equipment have increased the energy/power requirements on a global scale. With dwindling fossil-fuel reserves, there is an urgent need to find alternative energy resources to meet the growing demand. Alternate energy resources must be efficient, cost-effective and ecologically friendly. The harnessing of solar energy, in this context, becomes a very attractive proposition. The sunlight reaching the earth's surface every day far exceeds the annual demand. A moderately efficient solar cell array (with 8-10 % efficiency) covering a limited area of the earth's surface would be able to provide an enormous amount of electric power and thus help reduce greenhouse-gas emissions. Chemists have been interested for a long time in the harnessing of sunlight, either to drive useful chemical transformations or to convert the light directly into electrical energy. Two short publications in Nature by Honda, Graetzel and cowork- ers have had a dramatic impact on the focus of research for those chemists interested in photochemical and conversion and storage of solar energy (A. Fujishima and K. Honda, Nature, 238, 37 (1972) and B. O'Regan and M. Graetzel, Nature 353, 737 (1991)). The first publication demonstrated the possibility of the photo-decompo­ sition of water into its constituent elements through the irradiation of semiconduc­ tor electrodes such as Ti0 immersed in aqueous electrolyte. The second publication 2 described two important variants of this photo-electrochemical cell, specifically the use of high-surface-area mesoporous materials for the oxide substrate, and the appli­ cation of dye molecules to harvest the sunlight. Both these propositions have proven to be seminal to a new field of scientific research. Interest in the research and development of DSCs is now spread across numer­ ous academic and industrial laboratories. Over six thousand research publications have appeared in the primary scientific literature on the performance features, and the number of patents being filed in this area is growing exponentially (already more than 300 in 2009 for the DSC area alone). The overall solar-to-electrical conversion efficiency has surpassed 10 % for lab-size cells (under areas of 1 cm2) and 8 % for modules (25 - 100 cm2). In recognition of the pioneering contributions made by the Swiss group, the DSC is already referred to as Graetzel Cell. The secondary literature on DSCs (reviews) is rather limited, most often covering the work of specific research groups or conference presentations. The DSC is an important contemporary tech­ nology, and one that is rapidly evolving. This monograph presents a comprehensive introduction to this new emerging area. Indeed, the DSC is the outcome of the cross XV111 Dye-Sensitized Solar Cells fertilization of concepts used in photovoltaic solar cells and nanoscience, nanotech- nology and light-induced electron transfer reactions. Many features of DSC are unique and advantageous over the solar cells based on crystalline or amorphous silicon. Nearly all the components of the DSC are "tunable", including the semiconducting oxide substrate, the dyes, the electrolytes, the redox mediator and the counter electrode. This has opened great opportunities for chemists and material scientists. Transparency and multi-color design alone offer huge poten­ tial for the integration of DSCs as part of the building architecture. The book is organized broadly in two parts. The first half is an overview of the material choices and performance features of all key components of the DSC. The second half covers several experimental techniques that help decipher the functioning of the DSCs in more detail, as well as theoretical calculations that help understand the key parameters that characterize the performance of the solar cells in quantita­ tive terms. Nearly all the mechanistic studies to quantify parameters that control the overall performance of the solar cells are discussed. For completeness, the monograph includes chapters dealing with the scaling-up issues that must be faced to take lab-cell studies that are academic in nature to the commercialization of the technology in the form of large-area solar panels and numerous electronic gadgets. The book benefits from an excellent team of authors, all of whom are experts with long hands-on experience in various aspects of the DSC technology and have made seminal contributions to our understanding on how these solar cells operate. The book is suitable as a text for a one-semester advanced-level course for upper-level undergraduates and graduate students; it will also serve as a reference work for self- study for active researchers in the field. In view of the interdisciplinary nature of DSC science, the book should be of interest to those working in the fields of chemistry, physics, material science and engineering. It is a pleasant task to thank all the contributing authors who were kind enough to spare time from their busy schedule to write the chapters and thus share their exper­ tise with the scientific community at large. At a personal level, it has been a great privilege for me to be associated in photochemistry research with Prof. Dr. Michael Graetzel for nearly three decades, sharing both the excitement and agony during the long period as DSCs matured from one of academic curiosity to an important member of the family of "third generation solar cells", ready for commercialization in the near future. Special thanks also go to Dr. Fred Fenter of the EPFL Press for all his help in putting together this volume. K. Kalyanasundaram Lausanne, February 2010 CONTENTS PREFACE xvii 1 PHOTOCHEMICAL AND PHOTOELECTROCHEMICAL APPROACHES TO ENERGY CONVERSION 1 K. Kalyanasundaram 1.1 The sun as an abundant energy resource 1 1.2 Photochemical conversion and storage of solar energy (artificial photosynthesis) 2 1.3 Photographic sensitization 5 1.4 Photoelectrochemical conversion of solar energy 6 1.4.1 Photogalvanic cells 6 1.4.2 Generations of photovoltaic solar cells 7 1.4.3 Photoelectrochemical solar cells with liquid junctions . . .11 1.4.4 Photoredox reactions of colloidal semiconductors and particulates 14 1.5 Dye sensitization of semiconductors 16 1.5.1 Dye sensitization of bulk semiconductor electrodes 16 1.5.2 Dye-sensitized solar cells - an overview 17 1.5.3 Sequence of electron-transfer steps of a DSC 18 1.5.4 Key efficiency parameters of a DSC 19 1.5.5 Key components of the DSC 21 1.5.6 Quasi-solid state DSCs with spiro-OMeTAD 32 1.5.7 Improvement in efficiency through the nanostructuring of materials 33 1.5.8 Dye solar cells based on nanorods/nanotubes and nanowires 34 1.5.9 Sensitization using quantum dots 35 1.5.10 Semiconductor-sensitized ETA solar cells 36 1.5.11 DSCs based on/?-type semiconductor 37 1.6 Conclusions 38 1.7 References 38 2 TITANIA IN DIVERSE FORMS AS SUBSTRATES 45 Ladislav Kavan 2.1 Titania: fundamentals 45 x Dye-Sensitized Solar Cells 2.2 Electrochemistry of titania: depletion regime 48 2.2.1 Photoelectrochemistry under band-gap excitation 49 2.2.2 In-situ FTIR spectroelectrochemistry in the depletion regime 52 2.2.3 Photoelectrochemistry under sub-band-gap excitation. . . .52 2.3 Electrochemistry of titania: accumulation regime 55 2.3.1 Capacitive processes 56 2.3.2 Li-insertion electrochemistry 57 2.3.3 Spectroelectrochemistry of titania in the accumulation regime 59 2.4 Titania photoanode for dye sensitized solar cells 60 2.4.1 Non-organized titania made by decomposition of Ti(IV) alkoxides 61 2.4.2 Electrochemical deposition of titania 62 2.4.3 Aerosol pyrolysis 63 2.4.4 Organized nanocrystalline titania 64 2.4.5 Single-crystal anatase electrode 71 2.4.6 Other methods of producing titania electrodes for DSC . .. 73 2.4.7 Multimodal structures 74 2.5 Conclusion 76 2.6 Acknowledgements 76 2.7 References 76 3 MOLECULAR ENGINEERING OF SENSITIZERS FOR CONVERSION OF SOLAR ENERGY INTO ELECTRICITY 83 Jun-ho Yum and Md. K. Nazeeruddin 3.1 Introduction 83 3.2 Ruthenium Sensitizers 84 3.2.1 Effect of protons carried by the sensitizers on the performance 85 3.2.2 Effect of cations in the ruthenium sensitizers on the performance 86 3.2.3 Device stability 88 3.2.4 Effect of alkyl chains in the sensitizer on the performance 89 3.2.5 Effect of the ^-conjugation bridge between carboxylic acid groups and the ruthenium chromophore 92 3.2.6 High Molar Extinction Coefficient Sensitizers 96 3.2.7 Tuning spectral response by thiocyanato ligands 99 3.2.8 Non-thiocyanato ruthenium complexes 101 3.3 Organic sensitizers 102 3.3.1 High efficiency organic sensitizers 102 3.3.2 Near-IR absorbing sensitizers 109 3.4 References 113 Dye-Sensitized Solar Cells xi 4 OPTIMIZATION OF REDOX MEDIATORS AND ELECTROLYTES . .117 Ke-jian Jiang and Shozo Yanagida* 4.1 Introduction 117 4.2 Charge transfer processes in DSCs 118 4.3 Electrolyte components and their roles in the DSCs 121 4.3.1 Organic solvents 121 4.3.2 Cations 121 4.3.3 Additives 123 4.3.4 Electron mediators 125 4.4 Ionic liquid, quasi-solid and solid electrolytes 128 4.4.1 Ionic liquid electrolyte 128 4.4.2 Active iodide molten salts 132 4.4.3 Nonactive iodide molten salts 135 4.4.4 Additives in ILEs 139 4.4.5 Quasi-solid electrolyte 139 4.5 Remarks and prospects 141 4.6 References 142 5 PHOTOSENSITIZATION OF Sn0 AND OTHER OXIDES 145 2 Prashant V. Kamat 5.1 Dependence of the Sensitization Efficiency on the Energy Difference 146 5.2 Coupled Semiconductor Systems 147 5.3 SnO -C -Ru(bpy)f+System 149 2 60 5.4 Probing the Interaction of an Excited State Sensitizer with the Redox Couple 151 5.5 Sensitization of Nanotube Arrays 153 5.6 Charge Separation of Organic Clusters at an Sn0 Electrode 2 Surface 154 5.7 Concluding Remarks 156 5.8 Acknowledgements 156 5.9 References 156 6 SOLID-STATE DYE-SENSITIZED SOLAR CELLS INCORPORATING MOLECULAR HOLE-TRANSPORTERS 163 Henry J. Snaith 6.1 Introduction 163 6.2 Spiro-OMeTAD-based solid-state dye-sensitized solar cell 165 6.3 The influence of additives upon the solar cell performance 166 6.4 Charge generation: Electron Transfer 168 6.5 Reductive quenching 171 6.6 Charge generation: Hole-transfer 171 6.7 Charge transport in molecular hole-transporters 174 6.8 Hole mobility in spiro-OMeTAD 175 xii Dye-Sensitized Solar Cells 6.9 Influence of charge density on the hole-mobility in molecular semiconductors 175 6.10 The influence of chemical p-doping upon conductivity and hole-mobility 177 6.11 The influence of ionic salts on conductivity and hole-mobility . .. 180 6.12 Current collection 181 6.13 Ti0 pore filling with molecular hole-transporters 187 2 6.14 Charge recombination: The influence of additives 192 6.15 Charge recombination: Ion solvation and immobilization 193 6.16 Charge recombination: Controlling the spatial separation of electrons and holes at the heterojunction 194 6.17 Enhancing light capture in solid-state DSCs 195 6.18 Alternative structures for mesoporous and nanostructured electrodes in solid-state DSCs 198 6.19 Outlook for hole-transporter based solid-state DSCs 203 6.20 References 203 7 PACKAGING, SCALE-UP AND COMMERCIALIZATION OF DYE SOLAR CELLS 207 Hans Desilvestro, Michael Bertoz*, Sylvia Tulloch and Gavin Tulloch 7.1 Introduction 207 7.2 From cells to panels 211 7.2.1 Definitions 211 7.2.2 Designs 211 7.2.3 Materials 214 7.2.4 Module performance - experiment vs. modeling 218 7.3 Long-term stability - the key to industrial success 224 7.3.1 Single cells 224 7.3.2 Modules 228 7.3.3 Panels 230 7.4 Scaling up to commercial production levels 231 7.4.1 Material costs and availability 231 7.4.2 Manufacturing 237 7.5 Commercial applications 240 7.6 Conclusions 245 7.7 Acknowledgements 246 7.8 References 246 8 HOW TO MAKE HIGH-EFFICIENCY DYE-SENSITIZED SOLAR CELLS 251 Seigo Ito 8.1 Introduction 251 8.2 Experimental considerations 252 8.2.1 Preparation of screen-printing pastes 252 8.2.2 Synthesis of Ru-dye 253 Dye-Sensitized Solar Cells xiii 8.2.3 Porous-Ti0 electrodes 254 2 8.2.4 Counter-Pt electrodes 258 8.2.5 DSC assembling 258 8.2.6 Measurements 260 8.3 Results and discussion 260 8.3.1 TiCl treatments 260 4 8.3.2 Effect of the light-scattering Ti0 layer 262 2 8.3.3 Thickness of the nanocrystalline Ti0 layer 263 2 8.3.4 Anti-reflecting film 263 8.3.5 ReproducibilityofDSCphotovoltaics 264 8.4 Conclusion 265 8.5 Acknowledgements 266 8.6 References 266 9 SCALE-UP AND PRODUCT-DEVELOPMENT STUDIES OF DYE-SENSITIZED SOLAR CELLS IN ASIA AND EUROPE 267 K. Kalyanasundaram, Seigo Ito, Shozo Yanagida and Satoshi Uchida 9.1 Introduction 267 9.2 Scaling up of laboratory cells to modules and panels 268 9.3 DSC development studies in various European laboratories . . . . 271 9.3.1 Energy Research Centre of the Netherlands (ECN) . . .. 271 9.3.2 Fraunhofer Institute for Solar Energy Systems (Fraunhofer ISE) 273 9.3.3 G24 Innovation 278 9.3.4 3GSolar, Israel 280 9.4 DSC development studies in various laboratories of Japan 281 9.4.1 Aisin Seiki Co. Ltd. and Toyota Central R&D Laboratories 281 9.4.2 Fujikura Ltd. (Japan) 287 9.4.3 Peccell Technologies, Inc. (Japan) 290 9.4.4 Sharp Co. Ltd. (Japan) 293 9.4.5 Sony Corporation Ltd. (Japan) 295 9.4.6 Shimane Institute for Industrial Technology (Japan). . . .296 9.4.7 TDK Co., Ltd. (Japan) 297 9.4.8 Eneos Co. Ltd. (Japan) 300 9.4.9 NGK Spark Plug Co., Ltd. (Japan) 301 9.4.10 Panasonic Denko Co. Ltd. (Japan) 303 9.4.11 Taiyo Yuden Co., Ltd. (Japan) 305 9.4.12 Dai Nippon Printing Company 306 9.4.13 Mitsubhishi Paper Mills and Sekisui Jushi Corporation 306 9.4.14 J-Power Co. Ltd. (Japan) 308 9.5 DSC Development Work in Korea and Taiwan 308 9.5.1 Korean Institute of Science and Technology (KIST). . . .308 9.5.2 Electronics and Telecommunications Research xiv Dye-Sensitized Solar Cells Institute(ETRI), Korea 311 9.5.3 Samsung SDI, Korea 311 9.5.4 Industrial Technology Research Institute of Taiwan (ITRI) 312 9.5.5 J Touch Taiwan 313 9.6 DSC development work in Australia and China 313 9.6.1 Dyesol, Australia 313 9.6.2 Institute of Plasma Physics, Chinese Academy of Sciences 317 9.7 Conclusion 318 9.8 Acknowledgement 319 9.9 References 319 10 CHARACTERIZATION AND MODELING OF DYE-SENSITIZED SOLAR CELLS: A TOOLBOX APPROACH 323 Anders Hagfeldt and Laurence Peter 10.1 Introduction 323 10.2 Theoretical background 324 10.2.1 Interfacial electron transfer processes in the DSC 324 10.2.2 Electron trapping in the DSC 328 10.2.3 Electron transport in the DSC 331 10.3 The toolbox 336 10.3.1 Determination of inj ection efficiency and electron diffusion length under steady-state conditions 336 10.3.2 Electrochemical and spectrolectrochemical techniques to study the energetics of the oxide/dye/electrolyte interface 342 10.3.3 Electrochemical measurements with thin layer cells. . . .354 10.3.4 Small-amplitude time-resolved methods 357 10.3.5 Methods based on frequency response analysis 362 10.3.6 Photovoltage decay 374 10.3.7 Determination of density of trapped electrons inDSCs 376 10.3.8 Measuring the internal electron quasi Fermi level in the DSC 383 10.3.9 Determining the electron diffusion length using IMVS and IMPS 386 10.3.10 Photoinduced absorption spectroscopy (PIA) 388 10.3.11 Conclusions 395 10.4 Acknowledgments 396 10.5 Appendix 1 Analytical IMPS solutions 396 10.6 Appendix 2 Numerical solutions of the continuity equation [10.115] 397 10.7 References 399