Thin Film Solar Cells

Thin Film Solar Cells

Kasturi Lal Chopra Indian Institute of Technology New Delhi, India

and

Suhit Ranjan Das National Research Council Ottawa, Ontario, Canada

KAPARCHIEF

SPRINGER SCIENCE+BUSINESS MEDIA, LLC

Library of Congress Cataloging in Publication Data

Chopra, Kasturi L., 1933-Thin film solar cells.

Bibliography: p. Includes index. 1. Solar cells. 2. Thin films. I. Das, Suhit Ranjan, 1931-

TK2960.C48 1983 621.31 /244

(C) 1983 Springer Science+Business Media New York Originally published by Plenum Press, New York in 1983 Softcover reprint of the hardcover 1 st edition 1983

All rights reserved

. II. Title. 82-2126

No part of this book may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, microfilming, recording, or otherwise, without written permission from the Publisher

ISBN 978-1-4899-0420-1 ISBN 978-1-4899-0418-8 (eBook)DOI 10.1007/978-1-4899-0418-8

Dedicated to

OUR PARENTS who have been a source of inspiration to us

in our endeavor to explore the frontiers of science and technology

in the service of mankind

Preface

"You, 0 Sun, are the eye of the world You are the soul of all embodied beings You are the source of all creatures You are the discipline of all engaged in work"

- Translated from Mahabharata 3rd Century BC

Today, energy is the lifeline and status symbol of "civilized" societies. All nations have therefore embarked upon Research and Development pro-grams of varying magnitudes to explore and effectively utilize renewable sources of energy. Albeit a low-grade energy with large temporal and spatial variations, solar energy is abundant, cheap, clean, and renewable, and thus presents a very attractive alternative source. The direct conver-sion of solar energy to electricity (photovoltaic effect) via devices called solar cells has already become an established frontier area of science and technology. Born out of necessity for remote area applications, the first commercially manufactured solar cells - single-crystal silicon and thin film CdS/Cu2S - were available well over 20 years ago. Indeed, all space vehicles today are powered by silicon solar cells. But large-scale terrestrial applications of solar cells still await major breakthroughs in terms of discovering new and radical concepts in solar cell device structures, utilizing relatively more abundant, cheap, and even exotic materials, and inventing simpler and less energy intensive fabrication processes.

No doubt, this extraordinary challenge in R/D has led to a virtual explosion of activities in the field of photovoltaics in the last several years. Such new devices as barrier layer, inversion layer, cascade junction, tandem junction, spectrum splitting and shifting, and photoelectrochemical cells have emerged. Exotic materials of the type a-Si: H(F), a-SiC: H, Zn3PZ, CuInSe2, quaternaries and penternaries, and graded multicompo-nent semiconductors are very attractive options today. New materials technologies such as ribbon, LASS, TESS, RTR, spray pyrolysis, magne-

vii

viii Preface

tron sputtering, ARE, chemical solution growth, and screen printing are much talked about. The theoretical understanding of the photovoltaic process has advanced considerably as a result of the development of more refined and realistic models of various types of junctions and the associated solid state physics, particularly that applicable to polycrystalline materials. Nevertheless, the R/D explosion has not yet reached its culmination.

A characteristic feature of good R/D activity is turbulent evolution of the field through critical published literature, seminars, conferences, reviews, and books. If scientific attention is a measure of the importance of a field, photovoltaics is among the top few frontiers today which continue to be hotly debated in conferences and in reviews all over the world. In the area of published literature, H. J. Hovel's book Solar Cells (1975) has served as a useful textbook for several years. A recent textbook, Solar Cell Device Physics by S. J. Fonash (1981), has brought the subject of the physics of junctions up to date. D. Pulfrey's book Photovoltaic Power Generation (1978) contains basic theory of junctions and emphasizes systems aspects of photovoltaic conversion. Solar Energy Conversion: The Solar Cell by Richard C. Neville (1978) provides good coverage of the related solid state physics, device design, and systems. An indispensable handbook for the systems engineer is provided by H. Rauschenbach in his book Solar Cell Arrays (1980). However, none of these books has addressed the problem from the universally accepted point of view that viable devices for terrestrial applications must necessarily be Thin Film Solar Cells. Such aspects as thin film materials~ the associated preparation, measurement, and analysis techniques; and device technology are not discussed, or are treated perfunctorily at best in these books. The importance of these aspects and the fact that we in the Thin Film Laboratory have been engaged in very extensive R/D activities with thin film solar cells for over a decade have inspired us to undertake the job of writing a comprehensive book on the subject.

This book, consisting of 12 chapters, begins with a scientific, technolog-ical, and economic justification of "why thin film solar cells?" in Chapter 1. This is followed by a detailed description of the electron transport and optical processes in monocrystalline, polycrystalline, and amorphous semiconductors, and in metal films in Chapter 2. Different types of electronic junctions and the associated physics are presented in Chapter 3. Chapter 4 discusses measurement techniques for the analysis of junctions. A comprehensive review of the major deposition techniques of interest to the field of thin film solar cells forms Chapter 5. The significant physical properties of thin film materials for solar cell applications are discussed in Chapter 6. Chapters 7 through 10 are devoted to a description of the fabrication and critical discussion/analysis of the performance of solar cells based on copper sulfide, polycrystalline silicon, new and emerging materi-

Preface ix

als, and amorphous silicon. An attractive alternative to an all solid state solar cell is a photoelectrochemical cell, which is the theme of Chapter II. The last chapter deals with novel and futuristic concepts that have been proposed and experimented with for obtaining high-efficiency solar cells. Finally, six appendixes deal with the solar spectrum, antireflection coat-ings, grid design, solar cell arrays, concentrators, and degradation and encapsulation of solar cells. In all, the book is illustrated with over 200 figures and contains 42 tables of accumulated data.

In a rapidly changing field, strong and prejudiced views, errors, and omissions are inevitable. This book is no exception and we take full responsibility for it. We do, however, appeal to our readers to be critical and to communicate its shortcomings to us. As emphasized in Chapter 1, an unambiguous choice of a viable thin film solar cell material has yet to emerge. And, thus, some or even a substantial part of what is presented here as technology may become obsolete very soon. Indeed, we would be very disappointed if this did not happen!

Despite the rapidly evolving nature of the field, this book represents the first major attempt to expose graduate students and RID scientists and engineers to a comprehensive treatment of many facets of materials, technologies, and solid state physics of thin film solar cell devices. We earnestly hope that this book, aside from serving as a text and research-cum-reference volume, will inspire the readers toward much awaited innovations in the field to make thin film solar cells viable enough to serve the energy-hungry societies of tomorrow.

Acknowledgments

No book of this size can be written without consulting and looting a vast number of published papers, reviews, and books. We are grateful to numerous authors and publishing companies for permitting us to repro-duce figures and data in tables. We thank all the members of the Thin Film Laboratory for their generous help and assistance in various forms in the preparation of the manuscript. In particular, we are grateful to R. C. Kainthla for his help in writing Chapter 11 and to D. K. Pandya, V. Dutta, I. J. Kaur, E. Shanthi (Iyer), Sarita Thakoor, P. K. Gupta, and Rajesh Mehta for collecting material for the appendixes. R. C. Budhani, Bharat Bhushan, S. Harshvardhan, S. Major, O. S. Panwar, T. V. Rao, Bodhraj, Madhu Banerjee, Satyendra Kumar, M. Rajeshwari, Jagriti Singh, K. Chidambaram, G. B. Reddy, Harminder Singh, and Mangal Singh assisted with figures, tables, and references. We are indebted to D. F. Williams and A. Banerjee for reading the manuscript and offering their critical com-ments.

x Preface

We gratefully acknowledge the support of Mr. N. S. Gupta for drawing the figures and of V. N. Sharma, S. D. Malik, H. S. Sawhney, Keshav Giri, Susan Farrell, Nicole Paquette, and Kim Burke for typing the manuscript. We thank the Indian Institute of Technology, New Delhi, for extending its facilities and providing financial support.

Finally, the book owes much to the exemplary patience and moral support of our neglected families, in particular our wives.

K. L. Chopra and S. R. Das

Contents

Chapter 1. Why Thin Film Solar Cells? 1

1.1. Introduction . . . . . 1 1.2. Solar Energy Conversion 3 1.3. Efficient Conversion 5 1.4. Materials Requirements for Efficient Conversion 7 1.5. What Are Thin Film Materials? . 7 1.0. Role of Thin Films in Solar Cells 10 1.7. Progress of Thin Film Solar Cells 14 IX Production of Thin Film Solar Cells . 15 1.9. Conclusions . . . . . . . . 17

Chapter 2. Basic Physical Processes in Solar Cell Materials 19

2.1. Introduction . . . . 19 2.2. Semiconductor Statistics 19 2.3. Transport Parameters. 31

2.3.1. Crystalline Semiconductors. 31 2.3.2. Polycrystalline Materials . 35 2.3.3. Heterogeneous Materials 36 2.3.4. Amorphous Materials. 39 2.3.5. Thin Films . . . 44 2.3.6. High-Field Effects 50

2.4. Optical Interactions. . 52 2.4.1. Reflectance and Transmittance at an Interface 53 2.4.2. Absorption Phenomena in Semiconductors 2.4.3. Carrier Generation and Recombination

2.5. Metal Films . . . 2.5.1. Granular Films. 2.5.2. Network Films . 2.5.3. Continuous Films

Chapter 3. Photo voltaic Behavior of Junctions

3.1. Introduction . . 3.2. Junctions in Dark

xi

57 02 67 68 68 69

71

71 72

xii Contents

3.2.1. Homojunctions. . 72 3.2.2. Heterojunctions. ~3 3.2.3. MS and CIS Junctions ~~

3.3. Effect of Illumination on Junction Behaviour lJ 1 3.3.1. Photovoltaic Parameters. . . . . . lJ2 3.3.2. Superposition Principle . . . . . . lJ6 3.3.3. Idealized Model for Efficiency Limit Predictions lJ6 3.3"+. Design and Optimization of Solar Cells 117

Chapter 4. Photo voltaic Measurements, Junction Analysis, and Material Characterization 153

4.1. Introduction . . . 153 4.2. Photovoltaic Measurements· 154

4.2.1. [- V Characteristics 154 4.2.2. Spectral Response 4.2.3. Optical Scanning

4.3. Junction Analysis 4.3.1. [- V Analysis 4.3.2. Capacitance Measurements· 4.3.3. DLTS Technique ....

4.4. Material Characterization 4.4.1. Structural Characterization· 4.4.2. Compositional Analysis. . 4.4.3. Optical Characterization· . 4.4.4. Electrical and Optoelectronic Characterization

Chapter 5. Thin Film Deposition Techniques

5.1. Introduction . . . . . 5.2. Physical Vapor Deposition

5.2.1. Vacuum Evaporation 5.2.2. Epitaxial Deposition 5.2.3. Sputtering Techniques

5.3. Chemical Deposition Techniques 5.3.1. Spray Pyrolysis Process . 5.3.2. Solution Growth Process. 5.3.3. Screen Printing. . . . 5.3.4. Chemical Vapor Deposition 5.3.5. Exchange Reactions 5.3.6. Electrodeposition 5.3.7. Anodization . . 5.3.8. Electrophoresis .

5.4. Liquid Deposition Techniques 5.4.1. Liquid Phase Epitaxy. 5.4.2. Melt Spinning Technique

5.5. Miscellaneous Techniques 5.5.1. Silicon (Si) 5.5.2. Cadmium Sulfide (CdS)

161 166 16~

168 170 176 17lJ 179 1~3

187 IlJO

195

195 1lJ6 1lJ6 202 204 211 211 221 233 240 257 25lJ 266 26~

26lJ 270 270 271 271 272

Chapter 6. Properties of Thin Films for Solar Cells

6.1. Introduction . . . 6.2. Semiconducting Films

6.2.1. Silicon (Si) 6.2.2. Cadmium Selenide (CdSe) 6.2.3. Cadmium Telluride (CdTe) . 6.2.4. Indium Phosphide (InP) . 6.2.5. Zinc Phosphide (Zn,Pz) . 6.2.6. Gallium Arsenide (GaAs) 6.2.7. Cadmium Sulfide (CdS) . 6.2.8. Copper Selenide (Cu,Se). 6.2.9. Copper Sulfide (Cu,S) 6.2.10. Copper Indium Selenide (CuInSe,).

6.3. Transparent Conducting Oxides 6.3.1. Cadmium Oxide (CdO) 6.3.2. Tin Oxide (SnO,) . . . 6.3.3. Indium Oxide (In,03)· . 6.3.4. Cadmium Stannate (Cd,Sn04 ) 6.3.5. Zinc Oxide (ZnO) ....

6.4. Transport Properties of Metal Films . 6.4.1. Electrical Properties 6.4.2. Optical Properties .

6.5. Dielectric Films. . . . 6.5.1. Electronic Properties 6.5.2. Optical Properties

Chapter 7. Cu2 S Based Solar Cells

7.1. Introduction . . . . 7.2. Fabrication Process

7.2.1. Device Geometry 7.2.2. Evaporated Cells 7.2.3. Sprayed Cells

7.3. Physical Model . . . 7.3.1. Microstructural. 7.3.2. Compositional Analysis

7.4. Photovoltaic Performance 7.4.1. I-V Characteristics 7.4.2. Spectral Response 7.4.3. Minority Carrier Diffusion Lengths. 7.4.4. Capacitance Measurements. . . . 7.4.5. Analysis of 1- V Characteristics. .

7.5. Effect of Various Process and Material Parameters 7.5.1. Heat Treatments . 7.5.2. CU,S Stoichiometry 7.5.3. CdS Resistivity. . 7.5.4. Doping . . . . 7.5.5. Znx Cd l _ x S Alloy Composition

Contents xiii

275

275 275 275 283 288 290 293 294 295 309 310 317 321 325 326 327 329 329 333 333 337 343 344 347

349

349 351 351 353 354 355 355 358 361 361 366 368 369 373 375 375 378 379 380 381

xiv Contents

7.5.6. Microstructure . . . . 7.6. Band Diagram and Loss Mechanisms 7.7. Conclusions . . . . . . . . .

Chapter 8. Polycrystalline Thin Film Silicon Solar Cells

8.1. Introduction . . . . . . 8.2. Current Status of Bulk Silicon Solar Cells. 8.3. Fabrication Technology . . . . 8.4. Photovoltaic Performance

8.4.1. Photovoltaic Characteristics 8.4.2. Junction Analysis 8.4.3. Loss Mechanisms

8.5. Future Directions . .

Chapter 9. Emerging Solar Cells

9.1. Introduction . . . . 9.2. Gallium Arsenide (GaAs)

9.2.1. Fabrication Process 9.2.2. Photovoltaic Performance 9.2.3. Junction Analysis .

9.3. Cadmium Telluride (CdTe). . 9.3.1. Fabrication Process 9.3.2. Photovoltaic Performance 9.3.3. Junction Analysis .

9.4. CdSe, Cu2_ x Se, Znln2Se4 • 9.5. Zinc Phosphide (Zn,P2) • 9.6. Indium Phosphide (InP) .

9.6.1. Fabrication Process 9.6.2. Photovoltaic Performance

9.7. Copper Indium Selenide (CuInSe2 ) 9.7.1. Fabrication Process 9.7.2. Photovoltaic Performance

9.8. Copper Oxide (Cu20) . 9.9. Organic Semiconductors 9.10. Future Directions . .

Chapter 10. Amorphous Silicon Solar Cells

10.1. Introduction. . . . . . . 10.2. Deposition of a-Si:H. . . . . . . . . .

10.2.1. Glow Discharge Deposition Conditions . 10.2.2. rf Sputtering . . 10.2.3. Pyrolysis of Silane 10.2.4. Discharge Kinetics

384 386 390

391

391 392 393 394 394 396 400 402

403

403 403 404 405 406 407 408 409 410 411 413 413 415 415 417 418 419 422 422 426

427

427 428 429 430 431 431

10.2.5. Substrate Effects 10.3. Properties of a-Si:H Films

10.3.1. Structural Properties 10.3.2. Compositional Properties. 10.3.3. Optical Properties . 10.3.4. Electrical Properties

10.4. a-Si:H Solar Cells. . . . 10.4.1. Device Configuration 10.4.2. Photovoltaic Performance 10.4.3. Loss Mechanisms 10.4.4. Stability

IO.S. Recent Advances . . IO.S.1. New Materials IO.S.2. New Processes IO.S.3. Novel Structures.

lO.fl. Scope of Future Work .

Chapter 11. Photoelectrochemical Cells

11.1. Introduction. . . . . . . . . . . . . 11.2. Theoretical Considerations. . . . . . . .

11.2.1. The Semiconductor/Electrolyte Interface 11.2.2. Design of a Photoelectrochemical Cell 11.2.3. Photocurrent and Photovoltage 11.2.4. Determination of Flat-Band Potential . 11.2.S. Photodecomposition of Semiconductor Electrodes

11.3. Photoelectrolysis of Water. 11.3.1. Energetic Conditions 11.3.2. Experimental Results

11.4. Photoelectrochemical Cell . 11.4.1. Cadmium Chalcogenide PEC Cells 11.4.2. Gallium Arsenide PEC Cells . . 11.4.3. Layered Semiconductor PEC Cells 11.4.4. Other n -Type Materials for PEC Cells. 11.4.S. PEC Cells with p-Type Materials.

II.S. Concluding Remarks . . . .

Contents

Chapter 12. Novel Concepts in Design of High-Efficiency

xv

433 433 434 435 43fl 437 444 444 445 449 453 453 454 454 454 455

457

457 457 457 4fll 463 404 4flS 4flK 46K 470 473 473 479 474 4KO 4KO 4KO

Solar Cells 483

12.1. Introduction . . . . . 12.2. High-Intensity Effects 12.3. Conventional Cells - New Designs

12.3.1. Single Absorber Layer Multijunction Solar Cell . 12.3.2. Fluorescent Wavelength Shifting . . .

12.4. Ultrahigh-Efficiency Cascade Solar Cells 12.4.1. Optical Filter-Mirror and Multiple Cells 12.4.2. Integrated Tandem Junction Solar Cells

4K3 484 487 487 491 494 49S 499

xvi Contents

Appendix A. Spectral Distribution of Solar Radiation. 507

Appendix B. Antireflection Coatings. 515

Appendix C. Grid Design. . 521

Appendix D. Solar Cell Arrays 531

Appendix E. Concentrators . 543

Appendix F. Degradation and Encapsulation of Solar Cells 549

References 561

Index 601