DYE-SENSITIZED SOLAR CELLS 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 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-Publication Data 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. 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: ppur@epfl.ch 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. FIRST FOREWORD It is a pleasure and honor to contribute this Foreword to the special monograph cover- ing the latest progress in dye sensitized solar cells (DSCs), which have been invented in our laboratory and remain the focus of intense investigations on the international level. The DSC is a unique photovoltaic converter in as much as it is the only solar cell that mimics natural photosynthesis. Like green plants and algae it uses a molecu- lar absorber, the dye, to harvest sunlight and generate electric charges, achieving for the fi rst time successfully the separation of the two functions of light harvesting and charge-carrier transport. This opens up many new options for practical applications, such as the realization of transparent glass panels that generated electric power from the sun. The DSC also employs a radically new concept for solar cells introducing three-dimensional (bulk) interpenetrating network junctions instead of the custom- ary fl at p-n layer embodiment by conventional solar cells. Thus in several regards the DSC is a unique photovoltaic converter that uses a approach in the harvesting of solar light that is disruptive with regards to conventional semiconductor technology. The thirteen chapters written by leaders in this rapidly developing fi eld address the salient features of these novel type of solar cells, starting from the fundamental level to industrial applications. They provide a comprehensive picture of the latest advances of this technology and an in-depth analysis of the elementary process that lead to electric-power production from sunlight. Thus the reader obtains an excellent overview on the most relevant aspects of the operation of DSCs, starting from the quantum-mechanical modeling of light absorption and charge-carrier generation, to a detailed analysis of the electron-transfer and charge-carrier-conduction processes involved in the light-to-electric power-conversion process. Apart from these funda- mental aspects, the reader is shown how far the practical development of new photo- voltaic contender has progressed since it inception some 18 years ago. I am extremely grateful to the authors that have contributed to this book for providing such superb overviews, refl ecting the fascinating research results that this new fi eld has been creating. Michael Grätzel Lausanne, February 2010 SECOND FOREWORD It is a great privilege and honor to write a foreword to the monograph on the dye- sensitized solar cell (DSC). Firstly, I must mention the recent achievement in the laboratories of Professor Michael Grätzel of the EPFL; a new-dye-based DSC has given light-to-energy conversion in lab-size cells (≤ 1 cm2) of about 12 %. To my understanding, this feat has been achieved through molecule- and ion-interface sci- ence and electron-transfer science applied to the vicinity of the new dye molecules adsorbed on mesoscopic nano-crystalline TiO (nc-TiO ). 2 2 The design of DSCs – with their mesoscopic structures of dye-adsorbed nano crystalline-TiO fi lm infi ltrated with iodide/iodine electrolytes – was inspired 2 by photo synthesis. Since then, tremendous progress has been made thanks to the knowledge and wisdom of coordination chemistry, inorganic material chemistry and electrochemistry. These cells, once referred to as photosynthetic solar cells or dye- sensitized ceramic solar cells, have developed thanks to the innovation and enthusi- asm of research teams with the aim to fi nd an effi cient method for generating clean and renewable energy. The conviction is that solar energy will be able to supply all our energy needs, which, currently is on the order of 13 terawatts. Dye Sensitized Solar Cells provides a comprehensive overview, bringing together the fundamentals of DSCs from the materials, performance and mechanistic aspects; it is also an advanced level monograph that summarizes key advances and technical issues. As explained in this book, the principle of the DSC is quite different from that of crystalline-material-based solar cells. At the beginning, a few experts in the fi eld of semiconductor physics were very skeptical about the potential effi ciency of electron injection and charge separation at the interface between nano-crystalline TiO and dye 2 molecules. In addition, some experts in the fi eld of photocatalysis chemistry ques- tioned the long-term stability of DSCs under solar light irradiation. The monograph poses the foundation for the current paradigm in semiconductor photovoltaics, i.e., built-in electrical fi eld to separate the photo-generated electron and hole pair is not operative in DSCs. In this sense, the DSC may be regarded as a mesoscopic hetero- junction solar cell or as a mesoscopic electron-injection solar cell. The unfortunate observation of photo-degradation of dye molecules on nano-crystalline TiO may be 2 rationalized as the consequence of the inevitable photochemical reactions of the dye molecules with co-adsorbed oxygen and water molecules on nc-TiO . 2 The DSC invention and their progress in the laboratories of the EPFL can be admired by referring to a proverb which states that “Prosperity comes from the prepared mind”. The contributions of the leading DSC research teams can be also admired from the perspective of a phrase that I found at an Institute in Trivandrum viii Second Foreword (India): “Determination is a vital and essential factor in achieving success. The joy of success has encouraged many worthy researchers and engineers of DSCs”. Cadmium-telluride solar-cell modules, with a price of less than 1 $/watt, are now growing only in the solar power-plant market, but they have a limitation due to the producer liability issues arising from toxicity of materials. Readers of the mono- graph will come to realize that DSC modules, by achieving high conversion effi ciency and long-term stability, will lead to the creation of new jobs. Thus, investment in DSC research and in the development of functional materials for DSC modules continues, for example in such areas as cost-effective panchromatic dyes; transparent and highly conductive sheet materials; and mesoscopic metal oxides with electron-active mor- phology. It is not unrealistic to hope that scientists and engineers who work on DSC technology, through their mission to improve solar technology, will have a real impact on our world in very near future. Shozo Yanagida, Emeritus Professor Osaka University, January, 2010.