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Organic Photovoltaics
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OPTICAL ENGINEERING
Founding EditorBrian J. Thompson
University of RochesterRochester, New York
1. Electron and Ion Microscopy and Microanalysis: Principles andApplications, Lawrence E. Murr
2. Acousto-Optic Signal Processing: Theory and Implementation, edited by Nor man J. Berg and John N. Lee
3. Electro-Optic and Acousto-Optic Scanning and Deflection, Milton Gottlieb,Clive L. M. Ireland, and John Martin Ley
4. Single-Mode Fiber Optics: Principles and Applications, Luc B. Jeunhomme
5. Pulse Code Formats for Fiber Optical Data Communication: Basic Principles and Applications, David J. Morris
6. Optical Materials: An Introduction to Selection and Application, Solomon Musikant
7. Infrared Methods for Gaseous Measurements: Theory and Practice, edited by Joda Wormhoudt
8. Laser Beam Scanning: Opto-Mechanical Devices, Systems, and DataStorage Optics, edited by Gerald F. Marshall
9. Opto-Mechanical Systems Design, Paul R. Yoder, Jr.10. Optical Fiber Splices and Connectors: Theory and Methods,
Calvin M. Miller with Stephen C. Mettler and Ian A. White11. Laser Spectroscopy and Its Applications, edited by
Leon J. Radziemski, Richard W. Solarz, and Jeffrey A. Paisner12. Infrared Optoelectronics: Devices and Applications, William Nunley
and J. Scott Bechtel13. Integrated Optical Circuits and Components: Design and Applications,
edited by Lynn D. Hutcheson14. Handbook of Molecular Lasers, edited by Peter K. Cheo15. Handbook of Optical Fibers and Cables, Hiroshi Murata16. Acousto-Optics, Adrian Korpel17. Procedures in Applied Optics, John Strong18. Handbook of Solid-State Lasers, edited by Peter K. Cheo19. Optical Computing: Digital and Symbolic, edited by Raymond Arrathoon20. Laser Applications in Physical Chemistry, edited by D. K. Evans21. Laser-Induced Plasmas and Applications, edited by Leon J. Radziemski
and David A. Cremers22. Infrared Technology Fundamentals, Irving J. Spiro
and Monroe Schlessinger23. Single-Mode Fiber Optics: Principles and Applications, Second Edition,
Revised and Expanded, Luc B. Jeunhomme24. Image Analysis Applications, edited by Rangachar Kasturi
and Mohan M. Trivedi25. Photoconductivity: Art, Science, and Technology, N. V. Joshi
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26. Principles of Optical Circuit Engineering, Mark A. Mentzer27. Lens Design, Milton Laikin28. Optical Components, Systems, and Measurement Techniques,
Rajpal S. Sirohi and M. P. Kothiyal29. Electron and Ion Microscopy and Microanalysis: Principles
and Applications, Second Edition, Revised and Expanded, Lawrence E. Murr
30. Handbook of Infrared Optical Materials, edited by Paul Klocek31. Optical Scanning, edited by Gerald F. Marshall32. Polymers for Lightwave and Integrated Optics: Technology
and Applications, edited by Lawrence A. Hornak33. Electro-Optical Displays, edited by Mohammad A. Karim34. Mathematical Morphology in Image Processing,
edited by Edward R. Dougherty35. Opto-Mechanical Systems Design: Second Edition,
Revised and Expanded, Paul R. Yoder, Jr.36. Polarized Light: Fundamentals and Applications, Edward Collett37. Rare Earth Doped Fiber Lasers and Amplifiers,
edited by Michel J. F. Digonnet38. Speckle Metrology, edited by Rajpal S. Sirohi39. Organic Photoreceptors for Imaging Systems, Paul M. Borsenberger
and David S. Weiss40. Photonic Switching and Interconnects, edited by Abdellatif Marrakchi41. Design and Fabrication of Acousto-Optic Devices,
edited by Akis P. Goutzoulis and Dennis R. Pape42. Digital Image Processing Methods, edited by Edward R. Dougherty43. Visual Science and Engineering: Models and Applications,
edited by D. H. Kelly44. Handbook of Lens Design, Daniel Malacara and Zacarias Malacara45. Photonic Devices and Systems, edited by Robert G. Hunsberger46. Infrared Technology Fundamentals: Second Edition,
Revised and Expanded, edited by Monroe Schlessinger47. Spatial Light Modulator Technology: Materials, Devices,
and Applications, edited by Uzi Efron48. Lens Design: Second Edition, Revised and Expanded, Milton Laikin49. Thin Films for Optical Systems, edited by Francoise R. Flory50. Tunable Laser Applications, edited by F. J. Duarte51. Acousto-Optic Signal Processing: Theory and Implementation, Second
Edition, edited by Norman J. Berg and John M. Pellegrino52. Handbook of Nonlinear Optics, Richard L. Sutherland53. Handbook of Optical Fibers and Cables: Second Edition, Hiroshi Murata54. Optical Storage and Retrieval: Memory, Neural Networks, and Fractals,
edited by Francis T. S. Yu and Suganda Jutamulia55. Devices for Optoelectronics, Wallace B. Leigh56. Practical Design and Production of Optical Thin Films, Ronald R. Willey57. Acousto-Optics: Second Edition, Adrian Korpel58. Diffraction Gratings and Applications, Erwin G. Loewen
and Evgeny Popov59. Organic Photoreceptors for Xerography, Paul M. Borsenberger
and David S. Weiss
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60. Characterization Techniques and Tabulations for Organic Nonlinear OpticalMaterials, edited by Mark G. Kuzyk and Carl W. Dirk
61. Interferogram Analysis for Optical Testing, Daniel Malacara, Manuel Servin, and Zacarias Malacara
62. Computational Modeling of Vision: The Role of Combination, William R. Uttal, Ramakrishna Kakarala, Spiram Dayanand, Thomas Shepherd, Jagadeesh Kalki, Charles F. Lunskis, Jr., and Ning Liu
63. Microoptics Technology: Fabrication and Applications of Lens Arrays and Devices, Nicholas Borrelli
64. Visual Information Representation, Communication, and ImageProcessing, edited by Chang Wen Chen and Ya-Qin Zhang
65. Optical Methods of Measurement, Rajpal S. Sirohi and F. S. Chau66. Integrated Optical Circuits and Components: Design and Applications,
edited by Edmond J. Murphy67. Adaptive Optics Engineering Handbook, edited by Robert K. Tyson68. Entropy and Information Optics, Francis T. S. Yu69. Computational Methods for Electromagnetic and Optical Systems,
John M. Jarem and Partha P. Banerjee70. Laser Beam Shaping, Fred M. Dickey and Scott C. Holswade71. Rare-Earth-Doped Fiber Lasers and Amplifiers: Second Edition,
Revised and Expanded, edited by Michel J. F. Digonnet72. Lens Design: Third Edition, Revised and Expanded, Milton Laikin73. Handbook of Optical Engineering, edited by Daniel Malacara
and Brian J. Thompson74. Handbook of Imaging Materials: Second Edition, Revised and Expanded,
edited by Arthur S. Diamond and David S. Weiss75. Handbook of Image Quality: Characterization and Prediction,
Brian W. Keelan76. Fiber Optic Sensors, edited by Francis T. S. Yu and Shizhuo Yin77. Optical Switching/Networking and Computing for Multimedia Systems,
edited by Mohsen Guizani and Abdella Battou78. Image Recognition and Classification: Algorithms, Systems,
and Applications, edited by Bahram Javidi79. Practical Design and Production of Optical Thin Films: Second Edition,
Revised and Expanded, Ronald R. Willey80. Ultrafast Lasers: Technology and Applications,
edited by Martin E. Fermann, Almantas Galvanauskas, and Gregg Sucha81. Light Propagation in Periodic Media: Differential Theory and Design,
Michel Nevire and Evgeny Popov82. Handbook of Nonlinear Optics, Second Edition, Revised and Expanded,
Richard L. Sutherland83. Polarized Light: Second Edition, Revised and Expanded, Dennis Goldstein84. Optical Remote Sensing: Science and Technology, Walter Egan85. Handbook of Optical Design: Second Edition, Daniel Malacara
and Zacarias Malacara86. Nonlinear Optics: Theory, Numerical Modeling, and Applications,
Partha P. Banerjee87. Semiconductor and Metal Nanocrystals: Synthesis and Electronic
and Optical Properties, edited by Victor I. Klimov
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88. High-Performance Backbone Network Technology, edited by Naoaki Yamanaka
89. Semiconductor Laser Fundamentals, Toshiaki Suhara90. Handbook of Optical and Laser Scanning, edited by Gerald F. Marshall\91. Microoptics Technology: Second Edition, Nicholas F. Borrelli92. Organic Light-Emitting Diodes: Principles, Characteristics, and Processes,
Jan Kalinowski93. Micro-Optomechatronics, Hiroshi Hosaka, Yoshitada Katagiri,
Terunao Hirota, and Kiyoshi Itao94. Organic Electroluminescence, Zakya Kafafi and Hideyuki Murata95. Engineering Thin Films and Nanostructures with Ion Beams,
Emile Knystautas96. Interferogram Analysis for Optical Testing, Second Edition,
Daniel Malacara, Manuel Sercin, and Zacarias Malacara97. Laser Remote Sensing, Takashi Fujii and Tetsuo Fukuchi98. Passive Micro-Optical Alignment Methods, Robert A. Boudreau
and Sharon M. Doudreau99. Organic Photovoltaics: Mechanisms, Materials, and Devices,
edited by Sam-Shajing Sun and Niyazi Serdar Saracftci100. Handbook of Optical Interconnects, edited by Shigeru Kawai101. GMPLS Technologies, edited by Naoaki Yamanaka, Kohei Shiomoto,
and Eiji Oki102. Laser Beam Shaping Applications, edited by Fred M. Dickey,
Scott C. Holswade, and David Shealy
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Organic Photovoltaics
Mechanisms, Materials, and Devices
Sam-Shajing SunNorfolk State UniversityNorfolk, Virginia, U.S.A.
Niyazi Serdar SariciftciJohannes Kepler University of Linz
Linz, Austria
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Back cover illustration by Marcia Minwen Sun, 6-year-old daughter of the editor Dr. Sam-Shajing Sun.
Published in 2005 byCRC PressTaylor & Francis Group 6000 Broken Sound Parkway NW, Suite 300Boca Raton, FL 33487-2742
2005 by Taylor & Francis Group, LLCCRC Press is an imprint of Taylor & Francis Group
No claim to original U.S. Government worksPrinted in the United States of America on acid-free paper10 9 8 7 6 5 4 3 2 1
International Standard Book Number-10: 0-8247-5963-X (Hardcover) International Standard Book Number-13: 978-0-8247-5963-6 (Hardcover) Library of Congress Card Number 2004059378
This book contains information obtained from authentic and highly regarded sources. Reprinted material is quoted withpermission, and sources are indicated. A wide variety of references are listed. Reasonable efforts have been made to publishreliable data and information, but the author and the publisher cannot assume responsibility for the validity of all materialsor for the consequences of their use.
No part of this book may be reprinted, reproduced, transmitted, or utilized in any form by any electronic, mechanical, orother means, now known or hereafter invented, including photocopying, microfilming, and recording, or in any informationstorage or retrieval system, without written permission from the publishers.
For permission to photocopy or use material electronically from this work, please access www.copyright.com(http://www.copyright.com/) or contact the Copyright Clearance Center, Inc. (CCC) 222 Rosewood Drive, Danvers, MA01923, 978-750-8400. CCC is a not-for-profit organization that provides licenses and registration for a variety of users. Fororganizations that have been granted a photocopy license by the CCC, a separate system of payment has been arranged.
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Library of Congress Cataloging-in-Publication Data
Organic photovoltaics : mechanisms, materials, and devices / [edited by] Sam-Shajing Sun, Niyazi Serdar Sariciftci.
p. cm. -- (Optical engineering)Includes bibliographical references and index.ISBN 0-8247-5963-X (alk. paper)1. Photovoltaic cells. 2. Organic semiconductors. I. Sun, Sam-Shajing. II. Sariciftci, Niyazi Serdar.III. Title. IV. Series: Optical engineering (Marcel Dekker, Inc.)
TK8322.O73 2004
621.3815'42--dc22 2004059378
Visit the Taylor & Francis Web site at http://www.taylorandfrancis.com
and the CRC Press Web site at http://www.crcpress.com
Taylor & Francis Group is the Academic Division of T&F Informa plc.
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This book is dedicated to the 50th anniversary of the invention of silicon-based high
efficiency solar cells (Bell Labs), the 50th anniversary of the founding of the Inter-
national Solar Energy Society (ISES), and the 50th anniversary of annual meetings of
the International Society for Optical Engineering (SPIE).
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Foreword 1
Photovoltaic materials can be used to fabricate solar cells and photo detectors.
Inorganic photovoltaic devices were first demonstrated at the Bell Laboratories
more than 50 years ago. Today, silicon solar cells are big business. Their initial
applications were in earth satellites. A wider range of applications quickly emerged.
Because solar energy is perhaps the most obvious renewable energy source, large-
scale application of solar cell technology for the production of energy for our future
civilization is, and must be, a high priority a priority that becomes ever more
important as oil prices continue to increase and fossil fuel burning continues to
degrade the global environment. The silicon solar cell technology suffers, however,
from two serious disadvantages: The production cost is relatively high, and the rate
at which new solar cell area can be produced is limited by the basic high-temperature
processing of silicon.
Semiconducting polymers (and organic materials, more generally) provide an
alternative route to solar cell technology. Since these plastic materials can be pro-
cessed from solution and printed onto plastic substrates, they offer the promise of
being lightweight, flexible, and inexpensive. Moreover, web-based printing technol-
ogy is capable of generating large areas with rapid throughput. Although the overall
power conversion efficiency of current organic solar cell is relatively low compared to
that available from silicon technology, the efficiency can be improved through
systematic molecular engineering and the development of device architecture that is
optimally matched to the properties of these new photovoltaic materials. Organic
Photovoltaics: Mechanisms, Materials, and Devices, edited by Sun and Sariciftci, is a
very timely and comprehensive volume on the subject. It is certainly an important
starting point and reference for all those involved in research and education in this
subject and related fields.
Alan J. Heeger, Nobel Laureate
University of California at Santa Barbara
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Foreword 2
The first true inorganic solar cell had its roots in a two-phase process presently called
photoelectrochemistry. In 1839 Becquerel observed that a photovoltage resulted
from the action of light on an electrode in an electrolytic solution. In the 1870s it
was discovered that the solid material selenium demonstrated the same effect and by
the early 1900s selenium photovoltaic cells were widely used in photographic expos-
ure meters. By 1914 these cells were still less than 1% efficient. In 1954, Chapin
reported a solar conversion efficiency of 6% for a single-crystal silicon cell, marking
the beginning of modern-day photovoltaics. By 1958, small-area silicon solar cells
had reached an efficiency of 14% under terrestrial sunlight.
On March 17, 1958, the worlds first solar powered satellite Vanguard 1 was
launched. It carried two separate radios: the battery-powered transmitter operated
for 20 days and the solar cell powered transmitter operated until 1964, when it is
believed that the transmitter circuitry failed. Setting a record for satellite longevity at
that time, Vanguard 1 proved the merit of (inorganic) space solar cell power. Current
world-class space solar cells are based on multijunction GaAs and related materials
with efficiencies over 30%. But these cells are expensive and relatively brittle. Ter-
restrial power generation is dominated by silicon-based photovoltaics in three forms:
single crystal, polycrystalline, and thin-film. While these photovoltaics are much
cheaper than GaAs, turning essentially sand into pure silicon is very energy intensive.
Despite such shortcomings of inorganic solar cells, the worlds annual generating
capacity of solar cells in the past 15 years has increased tenfold up to nearly
600 MWp per annum.
Organic solar cells were only recently produced; the first organic solar cell
device was described by Tang in 1986. After less than two decades of research,
these cells are approaching a world-record efficiency of 3%. However, solar cell
technology does not fit neatly into an organic or inorganic paradigm. A hybrid
device employing dye-sensitized, nanocrystalline inorganic materials based on a
photoelectrochemical process was first developed by Gratzel in 1991. These cells
have achieved efficiencies surpassing the 10% limit. However, the predominant mass
in these systems consists of inorganic materials. And of course, research is quite
dynamic in the area of nanoparticle(-enhanced) solar cells.
As we enter into the third century of photovoltaics, Zweibel and Green in a
recent editorial observed that
. . . photovoltaics is poised to progress from the specialized applications of the
past to make a broader impact on the public consciousness, as well as on the well-
being of humanity . . . .
It is important to keep in mind the fact that solar cells are made to generate
electricity. Each solar power application results in its own unique set of challenges.
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These challenges can be addressed by a variety of technologies that overcome specific
issues involving available area, efficiency, reliability, and specific power at an optimal
cost. The particular application may be in aerospace, defense, utility, consumer, grid-
based, off-grid (housing), recreational, or industrial settings.
Organic photovoltaics will most likely provide solutions in applications where
price or large area challenges, or both, dominate, such as consumer or recreational
products. Inorganic materials will most likely predominate in aerospace and defense
applications: satellites, non-terrestrial surface power, and planetary exploration.
However, as efficiencies, environmental durability, and reliability improve for or-
ganic photovoltaics, other applications such as off-grid solar villages and utility-scale
power generation may well be within reach. Technology developments based upon
the work described in this monograph will likely revolutionize the way we exploit the
suns energy to generate electricity on the Earth and beyond.
Aloysius F. Hepp and Sheila G. Bailey
Photovoltaic and Space Environments Branch
NASA Glenn Research Center
Cleveland, Ohio, U.S.A.
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viii Foreword
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Preface
Energy and environment have become two of the most critical subjects of wide
concern nowadays, and these two topics are also correlated to each other. An
estimated 80% or more of todays world energy supplies are from the burning of
fossil fuels such as coal, gas, or oil. However, carbon dioxide and toxic gases released
from fossil fuel burning contribute significantly to environmental degradation, such
as global warming, acid rains, smog, etc. In addition, fossil fuel deposits on Earth is
not unlimited. Due to todays increased demands for energy supplies coupled with
increased concerns of environmental pollution, alternative renewable, environmen-
tally friendly as well as sustainable energy sources become desirable.
Sunlight is an unlimited (renewable and thus sustainable), clean (non-pollut-
ing), and readily available energy source, which can be exploited even at remote
sites where the generation and distribution of electric power present a challenge.
Today, any crude oil supply crisis or environmental degradation concern resulting
from fossil fuel burning has prompted both people and government to consider
solar energy resource more seriously. The technique of converting sunlight directly
into electric power by means of photovoltaic (PV) materials has already been
widely used in spacecraft power supply systems, and is increasingly extended for
terrestrial applications to supply autonomous customers (portable apparatus,
houses, automatic meteo stations, etc.) with electric power. According to U.S.
DOE EIA, NREL U.S. PV Industry Technology Roadmap 1999 Workshop and
Strategies Unlimited, photovoltaics is becoming a billion dollar per annum indus-
try and is expected to grow at a rate of 15 to 20% per year over the next few
decades. Nevertheless, one major challenge for large-scale application of the
photovoltaic technique at present is the high cost of the commercially available
inorganic semiconductor-based solar cells. In contrast, recently developed organic
and polymeric conjugated semiconducting materials appear very promising for
photovoltaic applications due to several reasons:
1. Ultrafast optoelectronic response and charge carrier generation at organic
donoracceptor interface (this makes organic photovoltaic materials also
attractive for developing potential fast photo detectors)
2. Continuous tunability of optical (energy) band gaps of materials via mo-
lecular design, synthesis, and processing
3. Possibility of lightweight, flexible shape, versatile device fabrication
schemes, and low cost on large-scale industrial production
4. Integrability of plastic devices into other products such as textiles, packaging
systems, consumption goods, etc.
While there exists a number of books covering general concepts of photovoltaic
and inorganic photovoltaic materials and devices, there are very few comprehensive
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and dedicated books covering the recent fast-developing organic and polymeric
photovoltaic materials and devices. It is therefore the mission of this book to present
an overview and brief summary of the current status of organic and polymeric
photovoltaic materials, devices, concepts, and ideas. This book is well suited for
people who are involved in the research, development and education in the areas of
photovoltaics, photo detectors, organic optoelectronic materials and devices, etc.
This book would also be helpful for all those who are interested in the issues related
to renewable and clean energy, particularly solar energy technologies.
Sam-Shajing Sun, Ph.D.
Niyazi Serdar Sariciftci, Ph.D.
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x Preface
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Acknowledgments
I would like to acknowledge the following people for their important or special
contributions/roles to this book project:
(1) Taisuke Soda at CRC Press for his enthusiasm, patience, and professional
assistance during the entire project period. I also thank Robert Sims,
Helena Redshaw at CRC Press, Balaji Krishnasamy at SPI-Kolam, and
others for their professional and efficient assistance.
(2) All authors who contributed to this book for their high quality and timely
contributions.
(3) Professor Serdar Sariciftci at Linz Organic Solar Cell Institute in Austria.
Professor Sariciftci accepted my invitation to serve as a co-editor of this
important book. He was very helpful and always quickly responded to my
queries and concerns during the project.
(4) Drs. Aloysius Hepp and Sheila Bailey at NASA Glenn Research Center.
Both Drs. Hepp and Bailey assisted our polymer photovoltaic research
projects during the past several years. I remember when Dr. Hepp encour-
aged me during my first polymer photovoltaic project in 1999; he drove all
the way from Cleveland, Ohio, to my office at Norfolk, Virginia, and
showed me some of the great potential applications of organic and polymer
photovoltaic materials in the future. It was those sponsored research efforts
that eventually lead me to initiate this book project. Drs. Hepp and Bailey
also advocated and contributed to this book with their valuable review
chapter and a Foreward.
(5) Dr. Charles Lee at Air Force Office of Scientific Research. Dr. Lee also
advocated and effectively assisted our photovoltaic polymer research and
educational projects.
(6) Dr. Zakya Kafafi at Naval Research Laboratory. Dr. Kafafi shared with
me her valuable insights into editing other books and also contributed (with
her coauthor) an excellent review chapter to this book.
(7) Professor Alan Heeger at UC Santa Barbara. Professor Heeger constantly
showed his strong interest and advocacy to organic and polymer related
photovoltaic projects. He also contributed an important Foreward to this
book.
(8) Dr. Aleksandra Djurisic at the University of Hong Kong. Many chapters of
this book were reviewed and commented by Dr. Djurisic. Many authors
expressed their appreciation for the excellent reviews and suggestions, and
most of those credits should go to Dr. Djurisic.
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(9) Last but not the least, my parents (D. X. Sun & W. R. Sha) who are taking
care of my little daughter (Marcia Minwen Sun), and my wife (Li Sun) who
allows me to work overtime so often.
Sam-Shajing Sun, Ph.D.
Norfolk State University
Norfolk, Virginia, USA
August 2004
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xii Acknowledgments
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Editors
Sam-Shajing Sun , Ph.D., EditorDr. Sun obtained his B.S. in physical chemistry from
Peking (Beijing) University in 1984, his M.S. degree
in inorganic/analytical chemistry from California
State University at Northridge in 1991, and his
Ph.D. in polymer/materials chemistry from Univer-
sity of Southern California in 1996. Dr. Suns Ph.D.
dissertation (under the direction of Professor Larry
R. Dalton) was titled Design, Synthesis, and Char-
acterization of Novel Organic Photonic Materials. After a postdoctoral experience at the
Loker Hydrocarbon Institute, Dr. Sun joined faculty team at Norfolk State University in 1998.
Since then, Dr. Sun has won a number of US government research/educational grant awards in
the area of optoelectronic polymers. He also founded/co-founded, and is currently leading a
couple of research/educational centers on advanced optoelectronic and nano materials. Dr. Sun
teaches organic and polymer chemistry in both undergraduate and graduate programs in
chemistry and materials sciences. Dr. Suns expertise and main research interests are in the
design, synthesis, processing, characterization, and modeling of novel polymeric solid-state
materials and thin film devices for optoelectronic applications. Dr. Sun is particularly inter-
ested in developing self-assembled macromolecules (SAM) that can efficiently convert Sun
light into electricity.
Niyazi Serdar Sariciftci, Ph.D., Co-editorDr. Sariciftci obtained his M.S. in physics at Univer-
sity of Vienna in 1986, and his doctorate in physics at
University of Vienna in 1989. Dr. Sariciftcis Ph.D.
dissertation (under the direction of Prof. Dr. Hans
KUZMANY and Prof. Dr. Adolf NECKEL) was
titled Spectroscopic investigations on the electro-
chemically induced metal to insulator transitions in
polyaniline with specialization on in situ Optical,
Raman and FTIR spectroscopy during doping pro-
cesses. From 1989-1991, Dr. Sariciftci joined Physics
Institute of University of Stuttgart, Fed. Rep. of Germany, in the project Molecular Elec-
tronics SFB 329 of Deutsche Forschungsgemeinschaft, c/o Prof. Michael MEHRING,
with specialization on electron spin resonance (ESR), electron nuclear double resonance
(ENDOR) and photoinduced electron transfer on supramolecular structures. From 1992-
1996, Dr. Sariciftci was a senior research associate at the Institute for Polymers & Organic
Solids at the University of California, Santa Barbara (directed by Prof. Alan J. Heeger, Nobel
Laureate 2000 for Chemistry) working in the fields of photoinduced optical, magnetic reson-
ance and transport phenomena in conducting polymers. In 1996, Dr. Sariciftci was appointed
as the Ordinarius Professor (Chair) of the Institute for Physical Chemistry at the Johannes
Kepler University in Linz/Austria. In 2000, Dr. Sariciftci founded the Linz Institute for
Organic Solar Cells (Linzer Institut fur organische Solarzellen or LIOS).
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Contributors
Maher Al-lbrahim,
Thuringian Institute for Textile and
Plastics Research, Rudolstadt, Germany
Gehan A.J. Amaratunga,
Cambridge University, Cambridge, UK
Mats R. Andersson,
Chalmers University of Technology,
Gothenburg, Sweden
Neal R. Armstrong,
University of Arizona, Tucson,
AZ, USA
Sheila G. Bailey,
National Aeronautics and Space
Administration Glenn Research Center,
Cleveland, OH, USA
Stephen Barlow,
Georgia Institute of Technology,
Atlanta, GA, USA
David Beljonne,
University of Mons-Hainaut,
Mons, Belgium, and Georgia Institute
of Technology, Atlanta, GA, USA
Robert E. Blankenship,
Arizona State University, Tempe,
AZ, USA
Carl E. Bonner,
Norfolk State University, Norfolk, VA,
USA
Jean-Luc Bredas,
University of Mons-Hainaut,
Mons, Belgium, and Georgia Institute
of Technology, Atlanta, GA, USA
Cyril Brochon,
Universite Louis Pasteur ECPM,
Strasbourg, France
Annick Burquel,
University of Mons-Hainaut,
Mons, Belgium
Kevin M. Coakley,
Stanford University, Stanford,
CA, USA
Jerome Cornil,
University of Mons-Hainaut,
Mons, Belgium, and Georgia Institute
of Technology, Atlanta, GA, USA
Liming Dai,
University of Dayton, Dayton,
OH, USA
Richey M. Davis,
Virginia Polytechnique Institute and
State University, Blacksburg, VA, USA
Aleksandra Djurisic,
University of Hong Kong,
Hong Kong, China
Benoit Domercq,
Georgia Institute of Technology,
Atlanta, GA, USA
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Martin Drees,
Virginia Polytechnic Institute and
State University, Blacksburg, VA, USA
Hele`ne Dupin,
University of Mons-Hainaut,
Mons, Belgium
Abay Gadisa,
Linkoping University,
Linkoping, Sweden
Yongli Gao,
University of Rochester, Rochester, NY,
USA
Brian A. Gregg,
National Renewable Energy
Laboratory, Golden, CO, USA
Joshua A. Haddock,
Georgia Institute of Technology,
Atlanta, GA, USA
Georges Hadziioannou,
Universite Louis Pasteur ECPM,
Strasbourg, France
Randy Heflin,
Virginia Polytechnique Institute and
State University, Blacksburg, VA, USA
Aloysius F. Hepp,
National Aeronautics and
Space Administration Glenn Research
Center, Cleveland, OH, USA
Masahiro Hiramoto,
Osaka University, Osaka, Japan
Harald Hoppe,
Johannes Kepler University of Linz,
Linz, Austria
Olle Inganas,
Linkoping University, Linkoping,
Sweden
Michael H.-C. Jin
Ohio Aerospace Institute, Brook Park,
OH, USA and
National Aeronautics and Space
Administration Glenn Research Center,
Cleveland, OH, USA
Zakya H. Kafafi,
Naval Research Laboratory,
Washington, D.C., USA
Bernard Kippelen,
Georgia Institute of Technology,
Atlanta, GA, USA
Chung Yin Kwong,
University of Hong Kong,
Hong Kong, China
Emmanuel Kymakis,
Cambridge University, Cambridge, UK.
Current Address: Technological
Education Institute, Crete, Greece
Paul A. Lane,
Naval Research Laboratory,
Washington, D.C., USA
Vincent Lemaur,
University of Mons-Hainaut,
Mons, Belgium
Wendimagegn Mammo,
Chalmers University of Technology,
Gothenburg, Sweden
Seth R. Marder,
Georgia Institute of Technology,
Atlanta, GA, USA
Michael D. McGehee,
Stanford University, Stanford,
CA, USA
Fanshun Meng,
East China University of Science &
Technology, Shanghai, China
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xvi Contributors
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Britt Minch,
University of Arizona, Tucson, AZ, USA
Richard Mu,
Fisk University, Nashville, TN, USA
John Perlin,
Santa Barbara, CA, USA
Nils-Krister Persson,
Linkoping University, Linkoping,
Sweden
Erik Perzon,
Linkoping University, Linkoping,
Sweden
Ryne P. Raffaelle,
Rochester Institute of Technology,
Rochester, NY, USA
L.S. Roman,
Federal University of Parana,
Curitiba-PR, Brazil
Niyazi Serdar Sariciftci,
Johannes Kepler University of Linz,
Linz, Austria
Rachel A. Segalman,
Universite Louis Pasteur ECPM,
Strasbourg, France
Steffi Sensfuss,
Thuringian Institute for Textile and
Plastics Research, Rudolstadt,
Germany
Venkataramanan Seshadri,
University of Connecticut, Storrs,
CT, USA
Gregory A. Sotzing,
University of Connecticut, Storrs, CT,
USA
Michelle C. Steel,
University of Mons-Hainaut,
Mons, Belgium
Sam-Shajing Sun,
Norfolk State University, Norfolk, VA,
USA
Mattias Svensson,
Chalmers University of Technology,
Gothenburg, Sweden
He Tian,
East China University of Science &
Technology, Shanghai, China
Akira Ueda,
Fisk University, Nashville, TN, USA
Xiangjun Wang,
Linkoping University, Linkoping,
Sweden
Marvin H. Wu,
Fisk University, Nashville, TN, USA
Wei Xia,
University of Arizona, Tucson,
AZ, USA
Seunghyup Yoo,
Georgia Institute of Technology,
Atlanta, GA, USA
Fengling Zhang,
Linkoping University, Linkoping,
Sweden
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Contents
Section 1: General Overviews
Chapter 1 The Story of Solar Cells
John Perlin
Chapter 2 Inorganic Photovoltaic Materials and Devices: Past,
Present, and Future
Aloysius F. Hepp, Sheila G. Bailey, and Ryne P. Raffaelle
Chapter 3 Natural Organic Photosynthetic Solar Energy Transduction
Robert E. Blankenship
Chapter 4 Solid-State Organic Photovoltaics: A Review of Molecular
and Polymeric Devices
Paul A. Lane and Zakya H. Kafafi
Section 2: Mechanisms and Modeling
Chapter 5 Simulations of Optical Processes in Organic
Photovoltaic Devices
Nils-Krister Persson and Olle Inganas
Chapter 6 Coulomb Forces in Excitonic Solar Cells
Brian A. Gregg
Chapter 7 Electronic Structure of Organic Photovoltaic Materials:
Modeling of Exciton-Dissociation and Charge-Recombination
Processes
Jerome Cornil, Vincent Lemaur, Michelle C. Steel, Hele`ne Dupin,
Annick Burquel, David Beljonne, and Jean-Luc Bredas
Chapter 8 Optimization of Organic Solar Cells in Both Space and
EnergyTime Domains
Sam-Shajing Sun and Carl E. Bonner
Section 3: Materials and Devices
Chapter 9 Bulk Heterojunction Solar Cells
Harald Hoppe and Niyazi Serdar Sariciftci
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Chapter 10 Organic Solar Cells Incorporating a pin Junction and a pn
Homojunction
Masahiro Hiramoto
Chapter 11 Liquid-Crystal Approaches to Organic Photovoltaics
Bernard Kippelen, Seunghyup Yoo, Joshua A. Haddock, Benoit Domercq, Stephen
Barlow, Britt Minch, Wei Xia, Seth R. Marder, and Neal R. Armstrong
Chapter 12 Photovoltaic Cells Based on Nanoporous Titania Films Filled
with Conjugated Polymers
Kevin M. Coakley and Michael D. McGehee
Chapter 13 Solar Cells Based on Cyanine and Polymethine Dyes
He Tian and Fanshun Meng
Chapter 14 Semiconductor Quantum Dot Based Nanocomposite
Solar Cells
Marvin H. Wu, Akira Ueda, and Richard Mu
Chapter 15 Solar Cells Based on Composites of Donor Conjugated
Polymers and Carbon Nanotubes
Emmanuel Kymakis and Gehan A. J. Amaratunga
Chapter 16 Photovoltaic Devices Based on Polythiophene/C60L. S. Roman
Chapter 17 Alternating Fluorene CopolymerFullerene
Blend Solar Cells
Olle Inganas, Fengling Zhang, Xiangjun Wang, Abay Gadisa, Nils-Krister
Persson, Mattias Svensson, Erik Perzon, Wendimagegn Mammo, and
Mats R. Andersson
Chapter 18 Solar Cells Based on Diblock Copolymers: A PPV
Donor Block and a Fullerene Derivatized Acceptor Block
Rachel A. Segalman, Cyril Brochon, and Georges Hadziioannou
Chapter 19 Interface Electronic Structure and
Organic Photovoltaic Devices
Yongli Gao
Chapter 20 The Influence of the Electrode Choice on the Performance
of Organic Solar Cells
Aleksandra B. Djurisic and Chung Yin Kwong
Chapter 21 Conducting and Transparent Polymer Electrodes
Fengling Zhang and Olle Inganas
Chapter 22 Progress in Optically Transparent Conducting Polymers
Venkataramanan Seshadri and Gregory A. Sotzing
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xx Contents
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Chapter 23 Optoelectronic Properties of Conjugated Polymer/Fullerene
Binary Pairs with Variety of LUMO Level Differences
Steffi Sensfuss and Maher Al-Ibrahim
Chapter 24 PolymerFullerene Concentration Gradient Photovoltaic
Devices by Thermally Controlled Interdiffusion
Martin Drees, Richey M. Davis, and Randy Heflin
Chapter 25 Vertically Aligned Carbon Nanotubes for Organic Photovoltaic
Devices
Michael H.-C. Jin and Liming Dai
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Section 1General Overviews
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1The Story of Solar Cells
John PerlinSanta Barbara, CA, USA
Contents
1.1. The First Solid-State Solar Cell
1.2. The Discovery of the Silicon Solar Cell
1.3. The First Practical Application of Silicon Solar Cells
1.4. Terrestrial Applications
1.5. The Future of Photovoltaics
References
Abstract This article tracks the development of early inorganic materials to
directly convert sunlight into electricity. It describes the technologies as well as
their applications. The role of individuals in developing solar cells and their many
usages is also emphasized.
Keywords photovoltaics, solar cells, selenium, silicon, history
1.1. THE FIRST SOLID-STATE SOLAR CELL
The direct ancestor of solar cells currently in use originated in the last half of the 19th
century, with the construction of the worlds first seamless communication network
through transoceanic telegraph cables. While laying them under sea to permit in-
stantaneous communications between continents, engineers experimented with sel-
enium for detecting flaws in the wires as they were submerged. Early researchers
working with selenium discovered that the materials performance depended upon
the amount of sunlight falling on it. The influence of sunlight on selenium aroused
the interest of scientists throughout Europe, including William Grylls Adams and his
student Richard Evans Day. During one of their experiments with selenium, they
observed that light could cause a solid material to generate electricity, which was
something completely new.
But the science of the day, not yet sure whether atoms were real, could not explain
why selenium produced electricity when exposed to light. Therefore, most of the
scientists scoffed at Adams and Days work. It took the discovery and acceptance of
electrons and that light contains packets of energy called photons for the field
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of photovoltaics to gain credibility in the scientific community. By the mid-
1920s scientists theorized that when light hits materials like selenium, the
more powerful photons pack enough energy to knock poorly bound electrons
from their orbits. When wires are attached, the liberated electrons flow through
them as electricity. Many researchers envisioned the day when banks of selenium
solar cells would power factories and light homes. However, no one could
build selenium solar cells efficient enough to convert more than half a percent of the
suns energy into electricity, which was hardly sufficient to justify their use as a power
source.
1.2. THE DISCOVERY OF THE SILICON SOLAR CELL
An accidental discovery by scientists at Bell Laboratories in 1953 revolutionized
solar cell technology. Gerald Pearson and Calvin Fuller led the pioneering effort
that took the silicon transistor, now the principal electronic component used in all
electrical equipment, from theory to working device. Fuller had devised a way to
control the introduction of impurities necessary to transform silicon from a poor
to a superior conductor of electricity. He gave Pearson a piece of his intentionally
contaminated silicon. Among the experiments he did with the specially treated
silicon included exposing it to the sun while hooked to a device that measured
the electrical flow. To Pearsons surprise, he observed an electric output almost five
times greater than the best seleniumproduced. He immediately ran down the hall to tell
his good friend, Daryl Chapin, who had been trying to improve selenium to provide
small amounts of intermittent power for remote locations, Dont waste another
moment on selenium! and handed him the piece of silicon he had just tested. After
a year of painstaking trial-and-error research and development, Bell Laboratories
showed the world the first solar cells capable of producing useful amounts of power.
Reporting the event on page one, the New York Times stated that the work of Chapin,
Fuller, and Pearson may mark the beginning of a new era, leading eventually to the
realization of one of mankinds most cherished dreams the harnessing of the almost
limitless energy of the sun for the uses of civilization.
Few inventions in the history of Bell Laboratories evoked as much media
attention and public excitement as their unveiling of the silicon solar cell (Figure
1.1). Commercial success, however, failed to materialize due to the prohibitive costs
of solar cells. Desperate to find marketable products run by solar cells, manufacturers
used them to power novelty items such as toys and the newly developed transistor
radio. With solar cells powering nothing but playthings, one of the inventors of the
solar cell, Daryl Chapin could not hide his disappointment, wondering What to do
with our new baby?
1.3. THE FIRST PRACTICAL APPLICATION OF SILICON SOLARCELLS
Unknown to Chapin at that time, powerful backing of the silicon solar cell was
developing at the Pentagon. In 1955, the American government announced its
intention of launching a satellite. The prototype had silicon solar cells for its power
plant. Since power lines could not be strung out to space, satellites needed a reliable,
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long-lasting, autonomous power source. Solar cells proved to be the perfect answer
(Figure 1.2).
The launching of the Vanguard, the first satellite equipped with solar cells,
demonstrated their value (Figure 1.3). Preceding satellites, run by batteries, lost
power in a weeks time, rendering equipment worth millions of dollars useless. In
contrast, the solar-powered Vanguard continued to communicate with Earth for
many years, allowing the completion of many valuable experiments.
The success of the Vanguards solar power pack broke down the existing
prejudice at that time toward the use of solar cells in space. As the space race between
the Americans and Russians intensified, both adversaries urgently needed solar cells.
The demand opened a relatively large business for companies manufacturing them.
More importantly, for the first time in the history of solar power, the suns energy
proved indispensable to society; without the secure, reliable electricity provided by
photovoltaics, the vast majority of space applications so vital to our everyday lives
would never have been realized.
1.4. TERRESTRIAL APPLICATIONS
While prospects were looking up for solar cells in space in the 1960s and early 1970s,
their astronomical price kept them distant from Earth. In 1968, Dr. Elliot Berman
decided to quit his job as an industrial chemist to develop inexpensive solar cells to
bring the technology from space to Earth. Berman prophetically envisioned that with
a large drop in price, photovoltaics could play a significant role in supplying elec-
trical power to locations on Earth where it is difficult to run a power line. After 18
months of searching for venture capital, Exxon executives liked Bermans approach,
Figure 1.1. A public display of one of the first modules of the Bell silicon solar cell. Itappeared at the first international solar energy conference held in Tucson, Arizona [1].
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Figure 1.2. A technician attaches a nose cone embedded with two clusters of solar cells forthe first trip of solar cells into space. The subsequent launch proved the efficacy of solar cells to
power satellites [1].
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inviting him to join their laboratory in late 1969. Dismissing other solar scientists
obsession with efficiency, Berman concentrated on lowering costs by starting with
lower grade and therefore cheaper silicon and finishing up with less expensive
materials for packaging the modules. By decreasing the price from $200 per watt to
$20 per watt, solar cells could compete with power equipment needed to generate
electricity distant from utility poles.
Figure 1.3. Hoffman Electronics shows its success in installing solar cells on the Vanguard,the first satellite to use solar cells and the pioneer that paved their way as the primary power
source in space [1].
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The Sto
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The oil companies became the first major customers of solar modules. They had
both the need and the money. Oil rigs in the Gulf of Mexico had to have warning lights
and horns. Most of the oil rigs relied on huge flashlight-like batteries to run warning
lights and horns. The batteries needed maintenance and also had to be replaced
approximately every 9 months. The replacement required a large boat with an onboard
crane, or a helicopter. In contrast, a small skiff could transport the much lighter solar
module and accompanying rechargeable battery, resulting in tremendous savings. By
1980, photovoltaics had become the standard power source for warning lights and
horns on rigs in the Gulf of Mexico and throughout the world (Figure 1.4).
Oil and gas companies also need small amounts of electricity to protect well
casings and pipelines from corroding. Sending current into the ground electrochem-
ically destroys the corroding molecules that cause this problem. Yet many oil and gas
fields both in America and in other places of the world like the Middle East and
North Africa are in areas far away from power lines but have plenty of sunshine. In
such cases, solar modules have proven to be the most cost-effective way of providing
the needed current to keep pipes and casings corrosion-free.
Money spent on changing non-rechargeable batteries on the U.S. Coast
Guards buoys exceeded the buoys original cost. Hence, Lloyd Lomer, a lieutenant
commander in the Guard, felt that switching to photovoltaics as a power source for
buoys made economic sense. But his superiors, insulated from competition, balked at
the proposed change. Lomer continued his crusade, winning approval to put up a test
system in the most challenging of environments of Ketchikan, Alaska for solar
devices. The success of the photovoltaic-powered buoy in Alaska proved Lomers
point. Still, his boss refused to budge. Going to higher authorities in government,
Lomer eventually won approval to convert all buoys to photovoltaics. Almost every
coast guard service in the world has followed suit.
Solar pioneer Elliot Berman saw the railroads as another natural area for the
application of photovoltaics. One of his salesmen convinced the Southern Railway
(now Norfolk Southern) to try photovoltaics for powering a crossing signal at Rex,
Georgia. These seemingly fragile cells did not impress veteran railroad workers as
capable of powering much of anything. They therefore put in a utility-tied back up.
But a funny thing happened in Rex, Georgia that turned quite a few heads. That
winter the lines went down on several occasions due to heavy ice build up on the
wires. And the only electricity for miles around came from the solar array. As one
skeptic remarked, Rex, Georgia taught the Southern that solar worked!
With the success of the experiment at Rex, Georgia, the Southern decided to
put photovoltaics to work for the track circuitry, the railroads equivalent of air
traffic control, keeping trains at a reasonable distance from one another to prevent
head-on or back-ender collisions. The presence of a train changes the rate of flow of
electricity running through the track. A decoder translates that change to throw
signals and switches up and down the track to ensure safe passage for all trains in the
vicinity. At remote spots along the line, photovoltaics provided the needed electricity.
Other railroads followed the Southerns lead. In the old days, telegraph and
then telephone poles ran parallel to most of the railroad tracks. Messages sent
through these wires kept railroad stations abreast of matters paramount to the safe
and smooth functioning of a rail line. But by the mid-1970s, wireless communications
could do the same tasks. The poles then became a liability and maintenance
an expense. Railroads started to dismantle their poles. Whenever they found
their track circuitry devices too far away from utility lines, they began relying on
photovoltaics.
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The U.S. Army Signal Corps, which pioneered the use of solar power in space
by equipping the solar pack on the Vanguard, were the first to bring photovoltaics
down to Earth. In June 1960, the Corps sponsored the first transcontinental radio
broadcast generated by the suns energy to celebrate its 100th anniversary. The
Figure 1.4. Installing a solar panel on an oil rig [1].
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Corps, probably the strongest supporter of photovoltaics in the late 1950s and the
early 1960s, envisioned that solar broadcast would lead others to use photovoltaics to
help provide power to run radio and telephone networks in remote locations.
Fourteen years later GTE John Oades realized the Corps dream by putting up
the first photovoltaic-run microwave repeater in the rugged mountains above Monu-
ment Valley, Utah (Figure 1.5). By minimizing the power needs of microwave
repeater, Oades could power the repeater with a small photovoltaic panel instead,
saving all the expenses formerly attributed to them.
His invention allowed people living in towns in the rugged American west, hem-
med inbymountains, to enjoy the luxuryof long-distance phone service thatmost other
Americans took for granted. Prior to the solar-powered repeater, the cost incurred by
phone companies to bring in cable or lines was high, forcing people to drive for hours,
sometimes through blizzards on windy roads, just to make long-distance calls.
Australia had an even more daunting challenge to bring modern telecommu-
nication services to its rural customers. Though about the same size as the United
States, only 22 million people lived in Australia in the early 1970s, the time when its
Figure 1.5. John Oades, on left, stands on top of solar-powered repeater while inspecting theinstallation of his solar-powered microwave repeater [1].
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government mandated Telecom Australia to provide every citizen, no matter how
remotely situated, with the same radio, telephone, and television service as its urban
customers living in Sydney or Melbourne enjoyed.
Telecom Australia tried, but without success, traditional stand-alone power
systems like generators, wind machines, and non-rechargeable batteries to run autono-
mous telephone receivers and transmitters for its rural customers. Fortunately, by
1974, it had another option, relatively cheap solar cells manufactured by Elliot Ber-
mans firm, Solar Power Corporation. The first photovoltaic-run telephone was in-
stalled in ruralVictoria. Thenhundreds of others followed.By 1976, TelecomAustralia
judged photovoltaics as the preferred power source for remote telephones. In fact, the
solar-powered telephones proved to be so successful that engineers at Telecom Aus-
tralia felt ready to develop large photovoltaic-run telecommunicationnetworks linking
towns with colorful names like Devils Marbles, Tea Tree, and Bullocky Bone to
Australias national telephone and television service. Thanks to photovoltaics, people
in these and neighboring towns could dial long distance directly instead of having to
call the operator and shout into the phone to be understood. Also, they did not have to
wait for newsreels to be flown to their local station to view news already hours, if not
days old. Totally, 70 solar-powered microwave repeater networks were created by the
early 1980s, the longest spanning 1500 miles. The American and Australian successes
showed theworld in grand fashion that solar power worked andbenefited thousands of
people. In fact, by 1985, a consensus in the telecommunications field found photo-
voltaics to be the power system of choice for remote communications.
A few years earlier, in Mali, a country right below the Sahara, suffered, along
with its neighbors, from drought so devastating that thousands of people and
livestock dropped like flies. The Malian government knew it could not save its
population without the help of people like Father Verspieren, a French priest who
lived in Mali for decades and had successfully run several agricultural schools. The
government asked Verspieren to form a private company to tap the vast aquifers that
run underneath the Malian desert. Verspieren saw drilling as the easy part. His
challenge was pumping. No power lines ran nearby and generators lay idle for lack
of repairs or fuel. Then he heard about a water pump in Corsica that ran without
moving parts, without fuel, without a generating plant, just on energy from the sun.
Verspieren rushed to visit the pioneering installation. When he saw the photovoltaic
pump, he knew that only this technology could save the people of Mali. By the late
1970s, Father Verspieren dedicated Malis first photovoltaic-powered water pump
with these words, What joy, what hope we experience when we see that sun which
once dried up our pools now replenishes them with water! (Figure 1.6).
By 1980, Mali, one of the poorest countries in the world, had more photovol-
taic water pumps per capita in the world than any other country thanks to Verspie-
rens efforts. The priest demonstrated that success required the best equipment, a
highly skilled and well-equipped maintenance service, and financial participation by
consumers. Consumers invested their money for buying the equipment and ongoing
maintenance but more importantly, they came to regard the panels and pumps as
valued items, which they would help care for. Most successful photovoltaic water-
pump project in the developing world have followed Versperiens example. When
Father Verspieren initiated his photovoltaic water-pumping program, less than ten
photovoltaic pumps existed throughout the world. Now multitudes of them provide
water to people, livestock, and crops.
Despite the success of photovoltaic applications throughout the world in the
1970s and early 1980s, institutions responsible for rural electrification programs in
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developing countries did not consider installing solar electric panels to power villages
distant from urban areas. Working from offices either in the west or in large cities in
poorer countries, the developing countries only considered central power stations run
by nuclear energy, oil, or coal. Not having lived out in the bush, these experts
did not realize the gargantuan investment required to string wires from power plants
to the multitudes residing in small villages miles away. As a consequence, only largely
populated areas received electricity, leaving billions in the countryside without it. The
disparity in energy distribution helped cause migration to the cities in Africa, Asia,
and Latin America. As a consequence, megalopolises like Mexico City, Lagos, and
Mumbai faced the ensuing problems such as crime, diseases like acquired immuno-
deficiency syndrome (AIDS), pollution, and poverty. The vast majority of people,
still living in the countryside, do not have electricity. To have some modicum of
lighting, they have had to rely on ad hoc solutions like kerosene lamps. People buying
radios, tape recorders, and televisions also must purchase batteries. It has become
apparent that mimicking the western approach to electrification has not worked in
the developing world. Instead of trying to put up wires and poles, which no utility
in these regions can afford, bringing a 20- to 30-year supply of electricity contained in
solar modules to consumers by animal or by vehicle makes greater sense (Figure 1.7).
Instead of having to wait for years to construct a centralized power plant and if ever
constructed, waiting for power lines to be connected to deliver electricity, it takes less
than a day to install an individual photovoltaic system.
Ironically, the French Atomic Energy Commission pioneered electrifying
remote homes in the outlying Tahitian Islands with photovoltaics. The Atomic
Figure 1.6. Young Malian boys watch a photovoltaic-driven pump fill their villages for-merly dry cistern with water [1].
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Energy Commission believed that electrifying Tahiti would mollify the ill feeling
in the region created by its testing of nuclear bombs in the South Pacific.
The Commission considered all stand-alone possibilities including generators,
wind machines, and biogas from the husks of coconut shells before choosing
Figure 1.7. In 1984, advertisements like this one appeared in major Sri Lankan newspapersto inform villagers living far away from utility lines that the Ceylon Electricity Board, the
national utility, offered them access to electricity from solar cells installed on-site [1].
The
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photovoltaics. In 1983, 20% of the worlds production of solar cells found its way to
French Polynesia. By 1987, half of all the homes on these islands received their
electricity from the sun.
Rural residents in Kenya have felt that if they have to wait for electricity from
the national utility, they will be old and gray. To have electricity instantly, many
Kenyans have bought photovoltaic units. The typical module ranges from 12 to
25 W, enough to charge a battery to run three low-wattage fluorescent lights and a
television for 3 h after dark. Electric lighting provided by photovoltaics does away
with the noxious fumes and threat of fire people had to contend with when lighting
their homes with kerosene lamps. Thanks to photovoltaics, children can now do their
lessons free from eye strain and tearing and from coming down with hacking coughs
they experienced when studying using kerosene lamps. Their performance in school
has soared with better lighting. Electric lighting also allows women to sew and weave
products, which bring in cash, at night. Newly gained economic power gives women
more say in matters such as contraception. In fact, solar electricity has proven more
effective in lowering fertility rates than birth control campaigns.
Electricity provided by photovoltaics allows people living in rural areas to
enjoy the amenities of urban areas without leaving their traditional homes for the
cities. The government of Mongolia, for example, believed that to improve the lot of
its nomadic citizens would require herding them into communities so that the
government could connect them to electricity generated by centralized power. The
nomads, however, balked. Photovoltaics allowed the nomadic Mongolians to take
part in the governments rural electrification program without giving up their trad-
itional lifestyle. Whenever they moved, the solar panels also came along, with yurt
and yaks and whatever else they valued.
With the growing popularity of photovoltaics in the developing world, solar
thievery has become common. Perhaps nothing better demonstrates the high value
people living in the developing countries place on photovoltaics than the drastic
increase in solar module thefts over the last few years. Before the market for photo-
voltaics took off in the developing world, farmers would build an adobe fence around
their photovoltaic panels to keep the livestock out. Now razor wire is used to cover
these enclosures to bar solar outlaws.
As the price of solar cells continues to drop, devices run by solar cells have
seeped into the suburban and urban landscapes of the developed world. For example,
construction crews have to excavate a location to place underground electrical
transmission lines every time; instead installing photovoltaics makes more economic
sense. Economics, for example, has guided highway departments throughout the
world to use solar cells for emergency call boxes along roads. In California alone,
there are more than 30,000 call boxes. The savings have been immediate. Anaheim,
California would have had to spend almost $11 million to connect its 11,000 call
boxes to the power grid. Choosing photovoltaics instead, the city had to pay only
$4.5 million. The city of Las Vegas found it cheaper to install photovoltaics to
illuminate its new bus shelters than dig up the adjacent street and sidewalk to place
new wires for the job.
In the mid-1970s and early 1980s, when governments of developing countries
began to fund photovoltaic projects, they copied the way electricity had been trad-
itionally produced and delivered, favoring the construction of large fields of photo-
voltaic panels far away from where the electricity would be used and delivering the
electricity by building miles of transmission lines. Others, however, have questioned
this approach. When Charles Fritts built the first selenium photovoltaic module in
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the 1880s and boldly predicted that it would soon compete with Thomas Edisons
generators, he envisioned each building to have its own power plant. Since the late
1980s, most people in the photovoltaic business have come around to Fritts view.
Instead of having to buy a huge amount of land, something unthinkable in densely
populated Europe and Japan, to place a field of panels, many people began to ask
why not turn each building into its own power station (Figure 1.8).
Using photovoltaics as building materials allows them to double as windows,
roofing, skylights, facades, or any type of covering needed on a home or building,
and makes sense since the owner gets both building material and electrical generator
rolled into one package. Having the electrical production situated where the electri-
city will be used offers many advantages like easing the burden on the electric
grid, eliminating losses that occur in transmission between power stations and end
users, helping prevent brownouts and blackouts, reducing vulnerability to terrorism,
and generally simplifying power production by eliminating many steps from extrac-
tion, processing, and transporting fuel to a power station and then sending that
power over great distances to electrify a home. Other advantages of photovoltaics
include:
Completely renewable nature
Environmentally benign construction
Absence of moving parts
Modularity to meet any need from milliwatts to gigawatts
Readily available and permanent fuel source
Figure 1.8. (Color figure follows page 358). Solar panels cover the rooftops of a Bremen,Germany housing complex [1].
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1.5. THE FUTURE OF PHOTOVOLTAICS
Though the photovoltaics industry has experienced a phenomenal annual 20%
growth rate over the last decade, it has just started to realize its potential. While
over a million households in India alone get their electricity from solar cells, more
than two billion still have no electrical service. The continuing revolution in telecom-
munications is bringing a greater emphasis on the use of photovoltaics. As
with electrical service, the expense of stringing telephone wires keeps most of the
developing world without communication services that people living in the more
developed countries take for granted. Photovoltaic-run satellites and cellular sites,
and a combination of the two, offer the only hope to bridge the digital divide.
Photovoltaics could allow everyone the freedom to dial up at or near home and of
course, hook up to the Internet.
Opportunities for photovoltaics in the developed world also continue to grow.
In the U.S. and Western Europe, thousands of permanent or vacation homes are too
distant for utility electric service. If people live in a vacation home that is more than
250 yards from a utility pole, paying the utility to string wires to their place costs
more than supplying their power needs with photovoltaics. Fourteen thousand Swiss
Alpine chalets and thousands of others from Finland to Spain to Colorado get their
electricity from solar energy.
Many campgrounds now prohibit recreational vehicle (RV) owners from
running their engines to power generators that run appliances inside the camp-
ground. The exhaust gases pollute, and the noise irritates other campers, especially
at night. Photovoltaic panels, mounted on the roofs of RVs, provides the electricity
needed without bothering others.
Restricting carbon dioxide emissions to help moderate global warming could
start money flowing from burning fossil fuels to photovoltaic projects elsewhere. The
damage wrought by the 19971998 El Nino gives us a taste of the harsher weather
expected as the Earth warms. The anticipated increase in natural disasters, brought
about by a more disastrous future climate, as well as the growing number of people
living in catastrophe-prone regions, make early warning systems essential.
The ultimate early warning device may consist of pilotless photovoltaic-
powered weather surveillance airplanes, the prototype of which is the Helios. The
Helios has flown higher than any other aircraft. Solar cells make up the entire top
of the aircraft, which consists of only a wing and propellers. Successors to the
Helios will have fuel cells on the underside of the wing. They will get their power
from the photovoltaic panels throughout the day, extracting hydrogen and
oxygen from the water discharged by the fuel cells the night before. When the
sun sets, the hydrogen and oxygen will power the fuel cells, generating enough
electricity at night to run the aircraft. Water discharged in the process will allow
the diurnal cycle to begin the next morning. The tandem use of solar cells and fuel
cells will allow the aircraft to stay aloft forever, far above the turbulence, watching
for and tracking hurricanes, and other potentially dangerous weather and natural
catastrophes.
Revolutionary lighting elements called light-emitting diodes (LEDs) produce
the same quality of illumination as their predecessors with only a fraction of energy.
LEDs therefore significantly reduce the amount of panels and batteries necessary for
running lights, making a photovoltaic system less costly and less cumbersome. They
have enabled photovoltaics to take over from gasoline generators the mobile warning
signs used on roadways to alert motorists about lane closures and other temporary
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problems that drivers should know about. The eventual replacement of household
lighting by LEDs will do the same for photovoltaics in homes.
To bring photovoltaics to mainstream will require further reductions in their
cost. Many researchers believe that greater demand could do the trick because for
every doubling of production, the price drops 20%. Others believe that new methods
of producing silicon solar cells will drop the price significantly. Some researchers are
exploring the development of producing less expensive silicon feed stock. At present,
most photovoltaic material is made from silicon grown as large cylindrical single
crystals or cast in multiple-crystal blocks. Cutting cells only 300 or 400mm thick fromsuch bulky materials demands excessive cutting, and half of the very expensive
starting material ends up on the floor as dust.
New less costly and wasteful ways of manufacturing solar cells promise much
lower prices. A number of companies, for example, have begun producing cells directly
from molten silicon; the hardened material, only about 100mm thick, is then fittedinto modules. Other companies have developed processes to spray photovoltaic
material onto supporting material. All these new techniques have potential for
mass production.
There are skeptics, however, who believe that todays techniques will never
reach a low enough price for mass use. Some optimists see emerging nanotechnology
as the answer. Authors contributing to this book are working with organic com-
pounds that can absorb light and change it into electricity. They envision depositing
these compounds on film-like material, which would cost very little to produce, and
could be easily adhered to building surfaces. Commercialization is yet to begin.
In truth, the number of potentially inexpensive ways to make solar cells being
pursued is dazzling. When Bell Laboratories first unveiled the silicon solar cell, their
publicist made a bold prediction:
The ability of transistors to operate on very low power gives solar cells great
potential and it seems inevitable that the two Bell inventions will be closely linked
in many important future developments that will influence the art of living.
Already, the tandem use of transistors and solar cells for running satellites,
navigation aids, microwave repeaters, televisions, radios, and cassette players in the
developing world and a myriad of other devices has fulfilled the Bell prediction. It
takes no great leap of the imagination to expect the transistor and solar cell revolu-
tion to continue until it encompasses every electrical need from space to Earth.
REFERENCES
1. J. Perlin, From Space to Earth: The Story of Solar Electricity. Harvard University Press:
Cambridge, MA, 2002.
2. www.californiasolarcenter.org/history.html.
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2Inorganic Photovoltaic Materials andDevices: Past, Present, and Future
Aloysius F. Hepp and Sheila G. BaileyPhotovoltaic and Space Environments Branch, NASA Glenn Research Center,
Cleveland, OH, USA
Ryne P. RaffaelleDepartment of Physics and Microsystems Engineering, Rochester Institute of
Technology, Rochester, NY, USA
Contents
2.1. Introduction
2.1.1. Recent Aspects of Advanced Inorganic Materials
2.1.2. Focus: Advanced Materials and Processing
2.1.3. Increased Efficiency
2.1.4. Increased Specific Power
2.2. Overview of Specific Materials
2.2.1. High-Efficiency Silicon
2.2.2. Polycrystalline Silicon
2.2.3. Amorphous Silicon
2.2.4. Gallium Arsenide and Related IIIV Materials
2.2.5. Thin-Film Materials
2.3. Advanced Concepts
2.3.1. Multijunction IIIV Devices
2.3.2. Nanotechnology Specifically Quantum Dots
2.3.3. Advanced Processing for Low-Temperature Substrates
2.3.4. Concentrator Cells
2.3.5. Integrated Power Devices
2.4. Applications
2.4.1. Terrestrial
2.4.2. Aerospace
2.5. Summary and Conclusions
References
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Abstract This chapter describes recent aspects of advanced inorganic materials
used in photovoltaics or solar cell applications. Specific materials examined will be
high-efficiency silicon, gallium arsenide and related materials, and thin-film ma-
terials, particularly amorphous silicon and (polycrystalline) copper indium selen-
ide. Some of the advanced concepts discussed include multijunction IIIV (and
thin-film) devices, utilization of nanotechnology, specifically quantum dots, low-
temperature chemical processing, polymer substrates for lightweight and low-cost
solar arrays, concentrator cells, and integrated power devices. While many of
these technologies will eventually be used for utility and consumer applications,
their genesis can be traced back to challenging problems related to power gener-
ation for aerospace and defense applications. Because this overview of inorganic
materials is included in a monogram focused on organic photovoltaics, funda-
mental issues common to all solar cell devices (and arrays) will be addressed.
Keywords copper indium selenide, (poly)crystalline, gallium arsenide, high-
efficiency solar cells, inorganic photovoltaic materials, multijunction devices,
quantum dots, silicon solar cells, amorphous silicon, thin films
2.1. INTRODUCTION
2.1.1. Recent Aspects of Advanced Inorganic Materials
A variety of different cell types are currently under development for future utility,
consumer, military, and space solar power (SSP) applications. The vast majority of
these needs are still currently met by the use of single-crystal silicon [1]. However, a
variety of other materials and even solar cell types are vying for the opportunity to
replace silicon, if not for all applications at least in certain specific ones. Close
relatives to single-crystal silicon such as polycrystalline silicon and amorphous silicon
are receiving much attention [2]. More exotic approaches such as polycrystalline
thin-film CuInGaSe2 [3] or CdTe [4], dye-sensitized or titania solar cells [5], and
even nanomaterial-enhanced conjugated polymer cells [6] are also garnering much
interest. However when it comes to highest efficiency cells, multijuction IIIV devices
are currently the leader. Of course, it is expected that current triple-junction gallium
arsenide (GaAs) cell technology will give way to quadruple junctions or possible
nanostructured approaches. However, ordinary Ge substrates may well give way to
Ge on Si substrates. These have the advantages of lower cost, higher strength, and
lighter weight. In addition to traditional lattice-matched multijunction GaAs cells,
we may also see the emergence of lattice mismatched or polymorphic cells [7].
2.1.2. Focus: Advanced Materials and Processing
There is currently a tremendous amount of research directed towards a thin film
alternative to traditional crystalline cells. Thin-film cells are quite attractive due to
the fact that many of the proposed fabrication methods are inexpensive and lend
themselves well to mass production [8]. These cells can be made to be extremely
lightweight and flexible, especially if produced on polymeric substrates. Thin-film
cells have been investigated for quite some time now. Cu2SCdS cells were developed
as far back as the mid-1970s. However, even the best results to date have been
plagued by low efficiencies and poor stability. The inorganic materials that have
received the most attention are amorphous silicon, CdTe, and CuInGaSe2 [24].
Recently thin-film polymeric cells that incorporate inorganic components such as
CdSe or CuInS2 quantum dots have garnered much attention [6,9]. Many researchers
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believe that these materials will hold the key to inexpensive, easily deployed, large
area, highly specific power arrays. This is due in part to the possibility of roll-to-roll
processing using low-cost spray chemical deposition or direct-write approaches to
producing thin-film solar cells on inexpensive lightweight substrates with these
materials [3,8,10].
2.1.3. Increased Efficiency
Multijunction technology has provided the most dramatic increase in inorganic solar
cell technology. Many groups have reported laboratory efficiencies well in excess of
30% AM1.5. Industrial lot averages are already above a 28% threshold [11]. The
move towards a quadruple junction solar cell has been problematic due to lattice
constant constraints. However, there are several new programs looking at lattice
mismatch approaches to multijunction IIIV devices, which can give a better match
to the solar spectrum than what is available in a lattice-matched triple-junction cell.
In addition, there is now a considerable amount of attention paid to the use of
nanostructures (i.e., quantum dots, wires, and wells) to improve the efficiencies of
IIIV devices. Recent theoretical results and experimental advances have shown that
nanostructures may afford dramatic improvement in cell efficiency [12,13].
2.1.4. Increased Specific Power
When considering specific power, or the power per mass of a solar cell or array, it is
clear that crystalline technology will be severely limited by the mass of the substrate
on which it is made. In order to achieve the types of specific power required to meet
portable power needs of consumer or military markets, as well as many large-scale
space power needs, the development of efficient thin-film photovoltaics on polymeric
substrates. When considering array-specific power, it is important to note that a cell-
specific power considerably higher than the array-specific power will be necessary.
The difference between the cell- and array-specific power must be sufficient to make
up for the interconnects, diodes, and wiring harnesses. Roughly speaking, the cell
mass usually accounts for approximately half the mass of the total balance of the
systems [14,15].
In addition to improving the cell efficiency or making the cells lighter, gains in
array-specific power may be made by an increase in the operating voltage. Higher
array operating voltages can be used to reduce the conductor mass. A typical array
designed for 28 V operation at several kilowatts output, with the wiring harness
comprising ~10% of the total array mass, yields a specific mass of ~0.7 kg/kW. If