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SOLAR CELLS From Basics to Advanced Systems
McGraw-Hill Series in Electrical Engineering Consulting Editor Stephen W. Director, Carnegie-Mellon University
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Fitzgerald, Klngsley, and Umans: Electric Machinery Hu and White: Solar Cells: From Basics to Advanced Systems Odum and Odum: Energy Basis for Man and Nature
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SOLAR CELLS From Basics to Advanced Systems
Chenming Hu Richard M. White University of California, Berkeley
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SOLAR CELLS From Basics to Advanced Systems
Copyright © 1983 by McGraw-Hill, Inc. All rights reserved. Printed in the United States of America. Except as permitted under the United States Copyright Act of 1976, no part of this publication may be reproduced or distributed in any form or by any means, or stored in a data base or retrieval system, without the prior written permission of the publisher. 1234567890 HALHAL 89876543
ISBN
D-D7-D3Q7MS-fl
Library of Congress Cataloging in Publication Data Hu, Chenming. Solar cells.
(McGraw-Hill series in electrical engineering. Power, and energy)
Includes bibliographies and index. 1. Solar cells.
I. White, Richard M., date
II. Title. III. Series. TK2960.H8 1983
621.31'244
83-975
CONTENTS
Preface
Part 1 1 1.1 1.2 1.3
2 2.1
2.2 2.3 2.4 2.5
2.6
3 3.1 3.2
I*
Solar Cell Fundamentals Overview Box: How Much Oil?
5 6
How Solar Cell Systems Work Types of Cells How Much Power, and When? References Problems
7 8 10 '4 '5
Nature and Availability of Solar Energy
16
The Sun and the Sun-Earth Relative Motion
17
Box: The Sun as a Fusion Reactor Atmospheric Effects on Solar Radiation Solar Radiation Measurement and Instrumentation Geographical Distribution of Average Insolation
17 20 23 27 35
Effect of Collector Tilt Summary References Problems
36
36 37
Principles of Operation of Solar Cells
38
Elements of Solar Cell Operation Semiconductors
39 40
Vl CONTENTS
3.3 3.4 3.5 3.6
pn Junctions Short-Circuit Current
3.7
Efficiency
57
3.8
Factors Affecting the Conversion Efficiency
60
3.9
Summary
67
4 4.1 4.2 4.3 4.4 4.5
Part 2
5 5.1
Light Absorption and Carrier Generation Carrier Recombination
Box: Light Sensors: Photodiodes and Photoconductors
67
Problems
gg
Materials and Processing
69
Material Properties and Processing Techniques Box: Making an Integrated Circuit Conventional Silicon Cell Processing Processing Cadmium Sulfide Cells Environmental and Other Considerations
70 71 76 85 86
Summary
89
References Problems
90 90
Concentrators and Complete Photovoltaic Systems Concentration of Sunlight Solar Concentrators
5.4
Economics of Concentrator Photovoltaic Systems Concentrator Solar Cells Cooling and Collection of Thermal Energy
5.5
Summary
5.3
References Problems
6 6.1
Power Conditioning, Energy Storage, and Grid Connection
6.6
Maximum-Power-Point Tracking Principles of Maximum-Power-Point Trackers Stand-Alone Inverters Inverters Functioning with Power Grid Costs of Power Conditioners Issues of Energy Storage
6.7
Energy-Storage Technologies
6.2 6.3 6.4 6.5
57
References
Box: /-Number of a Camera Lens Box: The Equation of Time 5.2
42 46 4g 54
95
96 97 99 106 108
110 111 111 112
113 114
116 118
118 121
122 124
CONTENTS VH
6.8
Summary References Problems
126 126 127
Characteristics of Operating Cells and Systems
128
7.1
Characteristics of Commercially Available Cells
129
7.2
Types of Applications
7.3 7.4
Operational Photovoltaic Systems and Devices Summary References Problems
133 138 151 153 153
7
Economics of Photovoltaic Power
154
8.1
8
Some General Rules
155
8.2 8.3
Box: Ten Rules of Thumb Cost Analyses for Photovoltaic Power Utility Issues in Industrialized Countries
157 159 171
8.4
Issues in Developing Countries
173
8.5
Summary
174
References
174
Problems
174
Part 3
Solar Cell Improvements Advanced Cell Processing Techniques
179
9.1
Solidification and Thermal Activation
181
9.2
Forming Thin Self-Supporting Semiconductor Ribbons
9
and Sheets
185
9.3
Forming a Semiconductor Layer on a Substrate
190
9.4
Use of Ion Implantation, Lasers, and Electron Beams
193
9.5
Optical Coatings, Contacts, and Encapsulants
196
9.6
Continuous Cell Production in an Automated Factory
197
9.7
Summary
200
10
References
200
Problems
201
Thin-Film and Unconventional Cell Materials
202
10.1
Introduction
203
10.2
Amorphous Semiconductors
208
10.3
Miscellaneous Materials for Solar Cell Use
210
10.4
Prospects for Mass-Produced Thin-Film Cells Summary
213
References
221
Problems
221
10.5
219
Vili CONTENTS
Variations in Cell Structures
223
11.1
11
Review of Some Basic Structures
224
11.2
Unconventional Nonconccntrator Cells
225
11.3
Unconventional Concentrator Cells
227
11.4
Summary
235
References
236
Problems
236
Unconventional Cell Systems
237
12 12.1
Multiple-Cell Systems—Spectrum Splitting and
12.2
Cascade Cells Thermophotovoltaic (TPV) System
238 244
12.3
Photoelectrolytic Cell
247
12.4
Satellite Power System
251
12.5
Summary References Problems
254 255 255
Appendixes
257
1
Annotated Bibliography
257
2
Units and Relevant Numerical Quantities
259
3
Solar Spectrum—Air Mass 1.5
262
4
Abbreviations and Acronyms
264
5
Tabulation of Demonstrated Cell Efficiencies by Cell Type
268
6
Solar Cell Experiments
271
7
Computer Simulations of Photovoltaic Cells and Systems
276
8
Suppliers of Solar Cells
282
9
Overview of Some Operating Systems
285
Index
291
PREFACE
This book began as a set of notes for two University Extension courses presented by the authors for heterogeneous groups that included college teachers and students, solar cell researchers, utility company engineers, gov ernment agency staff, a lawyer, investment counselors, and two people inter ested in installing a solar cell system in their Nevada ranch house. Success of the courses with such a diverse audience suggested making the material available, considerably reworked and augmented, in book form. This book is intended both as a self-learning resource or professional reference and as a text for use in an undergraduate or first-year graduate course. A key feature of the book is modular organization. The twelve chapters, each containing problems and a reference list, may be read in almost any order, though they are grouped in three parts in order of increasing depth and complexity. The three parts discuss fundamentals, complete systems, and possibilities for improvements, respectively. In Part One, Solar Cell Fundamentals, Chapter 1 gives an overview of photovoltaic power generation and its appeals. After the nature and the geographical distribution of sunlight is examined in Chapter 2, principles of operation of solar cells are developed in Chapter 3, starting from a discussion of the properties of semiconductors. For this the reader needs no more back ground than introductory college courses in mathematics and physics, al though additional knowledge of semiconductors will be helpful. Chapter 4 concludes Part One with an examination of the materials and processing
methods used in making conventional pn-junction cells. Concentrators and Complete Photovoltaic Systems are the subject of Part Two. Concentrators (Chapter 5) and power conditioning and energy storage equipment (Chapter 6) may be used with solar cells for increased efficiency, convenience, and reduced cost. Examples of solar cell applications are given
ix
X PREFACE
in Chapter 7, and the economics of photovoltaic electricity are discussed in Chapter 8.
Imaginative new approaches for further reducing the cost of solar cells are discussed in Part Three, Solar Cell Improvements. Chapter 9 describes new techniques for manufacturing semiconductors, and Chapter 10 examines thin-film approaches. Novel cell structures and unconventional cell systems are summarized in Chapters 11 and 12, respectively. In addition, nine appendixes provide such information as an annotated bibliography, a list of solar cell suppliers, and a compilation of operational photovoltaic systems and their key design features. There is a table of recent laboratory cell efficiencies arranged by cell material and design. Several solar cell experiments that can be done with simple laboratory equipment are described in one appendix. Some readers may find useful the list of abbrevi ations and acronyms, and the discussion of computer simulations of solar cells and photovoltaic systems.
Perhaps no book can claim completeness in the coverage of a topic so interdisciplinary and fast-advancing; ours certainly does not. We have tried to stimulate thinking in addition to giving information, and hope to provide a foundation and framework upon which the reader can build further. While writing this book, we have enjoyed the support and counsel of many friends whom we wish to acknowledge. Alex Kugushev of Lifetime Learning Publications suggested to us the idea of writing the book. Professor Ned Birdsall and many other colleagues urged us on. Joan Stern was the able editor. Bettye Fuller typed the manuscript. Finally, our families' support was invaluable. Chenming Hu Richard M. White
CHAPTER TWO CHAPTER ONE
NATURE AND AVAILABILITY OF SOLAR
OVERVIEW
ENERGY
Components of a solar cell system,
Characteristics of solar energy and
types of cells that exist or are
ways of estimating, measuring, and
proposed, and global power needs
collecting it
PART
ONE SOLAR CELL FUNDAMENTALS
CHAPTER THREE
PRINCIPLES OF SOLAR
CELL OPERATION
Nature of semiconductors and the theory and efficiencies of solar cells
