Dye-sensitized Solar Cells

621 Pages·2010·38.59 MB·English

by  Kuppuswamy Kalyanasundaram

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
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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
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teaching and research works of the Ecole polytechnique fédérale de Lausanne.
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ISBN 978-2-940222-36-0 (EPFL Press)
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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.