organic photovoltaics_e book

Organic Photovoltaics

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Copright 2005 by Taylor & Francis Group, LLCCopright 2005 by Taylor & Francis Group, LLC

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.

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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

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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


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


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


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,




Cyril Brochon,

Universite Louis Pasteur ECPM,

Strasbourg, France

Annick Burquel,


Mons, Belgium

Kevin M. Coakley,

Stanford University, Stanford,

CA, USA

Jerome Cornil,




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,


Atlanta, GA, USA

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Martin Drees,

Virginia Polytechnic Institute and


Hele`ne Dupin,


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,


Atlanta, GA, USA

Georges Hadziioannou,


Strasbourg, France

Randy Heflin,

Virginia Polytechnique Institute and


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,


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,


Mons, Belgium

Wendimagegn Mammo,


Gothenburg, Sweden

Seth R. Marder,


Atlanta, GA, USA

Michael D. McGehee,

Stanford University, Stanford,

CA, USA

Fanshun Meng,

East China University of Science &

Technology, Shanghai, China

Sun & Sariciftci / Organic Photovoltaics DK963X_prelims_i-xxii Final Proof page xvi 28.1.2005 4:09am

xvi Contributors


Britt Minch,

University of Arizona, Tucson, AZ, USA

Richard Mu,

Fisk University, Nashville, TN, USA

John Perlin,

Santa Barbara, CA, USA

Nils-Krister Persson,


Sweden

Erik Perzon,


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,


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,


Mons, Belgium

Sam-Shajing Sun,

Norfolk State University, Norfolk, VA,

USA

Mattias Svensson,


Gothenburg, Sweden

He Tian,

East China University of Science &

Technology, Shanghai, China

Akira Ueda,


Xiangjun Wang,


Sweden

Marvin H. Wu,


Wei Xia,

University of Arizona, Tucson,

AZ, USA

Seunghyup Yoo,


Atlanta, GA, USA

Fengling Zhang,


Sweden

Sun & Sariciftci / Organic Photovoltaics DK963X_prelims_i-xxii Final Proof page xvii 28.1.2005 4:09am

Contributors xvii


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


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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Contents xxi


Section 1General Overviews

Sun & Sariciftci / Organic Photovoltaics DK963X_c001 Final Proof page 1 22.1.2005 7:29am


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


3


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,


4 John Perlin


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].


The Story of Solar Cells 5


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].


6 John Perlin


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].


The Sto


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.


8 John Perlin


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].




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].


10 John Perlin


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




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].


12 John Perlin


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



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

14 John Perlin



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].




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

16 John Perlin



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.




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

Sun & Sariciftci / Organic Photovoltaics DK963X_c002 Final Proof page 19 25.1.2005 10:52pm

19Copright 2005 by Taylor & Francis Group, LLC

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

Sun & Sariciftci / Organic Photovoltaics DK963X_c002 Final Proof page 20 25.1.2005 10:52pm

20 Aloysius F. Hepp, Sheila G. Bailey, and Ryne P. Raffaelle

Copright 2005 by Taylor & Francis Group, LLC

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

organic photovoltaics_e book

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