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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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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