PHOTOELECTROCHEMISTRY AND PHOTOVOLTAICS OF LAYERED SEMICONDUCTORS

Physics and Chemistry of Materials

with Low-Dimensional Structures

VOLUME 14

Editor-in-Chief

F. LEVY, Institut de Physique Appliquee, EPFL, Departement de Physique, PHB-Ecublens, CH-IOI5 Lausanne, Switzerland

Honorary Editor

E. MOOSER, EPFL, Lausanne, Switzerland

International Advisory Board

J. V. ACRIVOS, San Jose State University, San Jose, Calif., US.A.

S. BARISIC, University of Zagreb, Department of Physics, Zagreb, Yugoslavia

J. G. BEDNORZ, IBM Forschungslaboratorium, Riischlikon, Switzerland

C. F. van BRUGGEN, University ofGroningen, Groningen, The Netherlands

R. GIRLANDA, Universita di Messina, Messina, Italy

D. HAARER, University of Bayreuth, Germany

A. J. HEEGER, University of California, Santa Barbara, Calif., US.A.

H. KAMIMURA, Dept. of Physics, University of Tokyo, Japan

W. Y. LIANG, Cavendish Laboratory, Cambridge, UK.

P. MONCEAU, CNRS, Grenoble, France

J. ROUXEL, CNRS, Nantes, France

M. SCHLUTER, AT&T, Murray Hill, N.J., US.A.

I. ZSCHOKKE, Universitiit Basel, Basel, Switzerland

The titles published ill this series are listed at the end of this volume.

PHOTOELECTROCHEMISTRY AND PHOTOVOLTAICS OFLAYERED SEMICONDUCTORS

Edited by

A. Aruchamy Department of Materials Science and Engineering, The University of Arizona, Tucson, AZ, U.S.A.

KLUWER ACADEMIC PUBLISHERS DORDRECHT / BOSTON / LONDON

Library of Congress Cataloging-in-Publication Data

Photoelectrochemlstry and photovoltalcs of layered semlconductors I

ed ited by A. Aruchamy. p. cm. -- (PhysIcs and chemistry of materlals wIth low

-dimensional structures i v. 14) ISBN 978-90-481-4111-1 (hb ac I d free paper) 1. Semiconductors--Optical properties. 2. Layer structure

(Solids) 3. Photochemistry. 4. Photoelectrlclty. 5. Photovoltalc effect. I. Aruchamy, A. II. Serles. OCSll.S.0SP52 1992 537.S'22S--dc20 91-42037

ISBN 978-90-481-4111-1 ISBN 978-94-015-1301-2 (eBook)

CIP

DOI 10.1007/978-94-015-1301-2

Published by Kluwer Academic Publishers, P.O. Box 17,3300 AA Dordrecht, The Netherlands.

Kluwer Academic Publishers incorporates the publishing programmes of D. Reidel, Martinus Nijhoff, Dr W. Junk and MTP Press.

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TABLE OF CONTENTS

Preface xi

E. BUCHER / Photovoltaic Properties of Solid State Junctions of Layered Semiconductors 1 1. Introduction 2

1.1. Introductory Remarks 2 1.2. Basic Physics of the Photovoltaeffect 4

1.2.1. How a solar cell works 4 1.2.2. Efficiency of a solar cell 6

2. Special Structural Aspects of Layered Semiconductors, related to Photo-voltaic Properties 9

3. Single Crystal Growth of Layered Compounds 13 3.1. III-VI Compounds 13 3.2. IV-VI Compounds 14 3.3. Transitionmetal Dichalcogenides 15 3.4. Ternary and Miscellaneous Compounds 18

4. Thin Film Preparation 18 5. Doping Experiments 20 6. Optical Properties 28 7. Transport Properties 40 8. Photovoltaic Properties 48

8.1. Photovoltaic and Device Properties of III-VI Compounds 49 8.1.1. GaSe 49 8.1.2. GaTe 53 8.1.3. InSe 53

8.2. IV-VI Compounds 55 8.3. Molybdenum Chalcogenides 56 8.4. Tungsten Chalcogenides 58 8.5. Ternary Compounds 60

9. Outlook 61 Acknowledgement 63 References 64

H. TRIBUTSCH / Electronic Structure, Coordination Photoelectrochemical Pathways and Quantum Energy Conversion by Layered Transition Metal Dichalcogenides 83 1. Introduction and Strategy 84 2. Electronic Structure and Photoelectrochemical Function 89

2.1. The Photoelectrochemical Dynamics of Layer-type Interface 89 2.1.1. Potential Dependence and Kinetics of Photocurrents 89

vi CONTENTS

2.1.2. Transition Metal Dichalcogenides with Reactive Surfaces Only 93

2.1.3. Water as an Electron Donor or Acceptor 96 2.1.4 Interfacial Coordination Chemistry as a Photoelectrochemical

Strategy 100 2.2. Advantages and Challenge for Quantum Energy Conversion 103

2.2.1. The Relation Between Dark-and Photocurrent 103 2.2.2. The Key to Solar Cell Performance 104

3. Layer Type Structure and Charge Carrier Dynamics 106 3.1. The Fate of Charge Carriers in Layered Structures 106 3.2. Laser Pulse and Photocurrent Transient Experiments 107 3.3. Microwave Conductivity Measurements 109

3.3.1. Time Resolved Microwave Conductivity Studies 109 3.3.2. Stationary Microwave Photoelectrochemical Studies 111

4. Discussion and Outlook 113 4.1. The Fate of Charge Carriers in Layer Type Compounds 113 4.2. Density of d-states, Oxidation State and Photoelectrochemical

Reactivity 114 4.3. Interfacial Coordination Chemistry as Key to Multi-electron Transfer

Catalysis 114 4.4. Strategies for Solar Cell Configurations with Transition Metal

Dichalcogenides 115 4.5. Outlook 116

Acknowledgement 116 References 116

F. DECKER, B. SCROSATI AND G. RAZZINI / Photoelectrochemical Solar Cells Based on Molybdenum and Tungsten Dichalcogenides 121 1. Introduction 122

1.1. Solid-State Properties of Molybdenum and Tungsten Dicha1-cogenides 122

1.2. Crystal Structure 122 1.3. Chemical Reactivity 124 1.4. Surface States as Recombination Centers 125 1.5. Dangling Bonds and Adsorption of Electrolytic Species 126

2. Interfaces and Photoelectrochemistry 127 2.1. Photovoltage and Redox Couples 127 2.2. Photocorrosion Processes 128 2.3. Passivation of Surface Defects 129 2.4. Surface Interactions 133 2.5. Iodine Layers 137

3. Photocatalytic Cells 137 3.1. Localized Photocatalysis in Iodide Electrolytes 137 3.2. Surface Decorations 142 3.3. Photodecomposition of Halogenic Acids 143

CONTENTS vii

4. Photoe1ectrochemical Cells for Solar Energy Conversion 147 4.1. Photoregenerative Solar Cells 147 4.2. Photoe1ectrochemical Cells with Po1ycrystalline Photoe1ectrodes 150

References 152

c. LEVY-CLEMENT AND R. TENNE / Modification of Surface Properties of Layered Compounds by Chemical and (Photo)electrochemical Processes 155 1. Introduction 156 2. The Mechanism of the (photo )electrochemical Corrosion 157

2.1. Influence of the electronic structure 158 2.2. Anisotropy with respect to the photocorrosion 159 2.3. Origin of the oxygen involved in photocorrosion 161 2.4. Shift of the band edges 164 2.5. Conclusion 165

3. Stabilization 165 3.1. Specific adsorption OfI3 166 3.2. Mediated electron transfer 166 3.3. Coating with conductive transparent films 167

3.3.1. Coating with an electronic conductive polymer 167 3.3.2. Formation of an interphase 168

4. Reduction of the Electrical Activity of Steps 170 4.1. Action of molecules and ions leading to blocking mechanisms 171

4.1.1. Binding of ligands 171 4.1.2. Semi-intercalation of large molecules 172 4.1.3. Formation of coordination compounds 174 4.1.4. (Ad)absorption of ions 174

4.2. Coating or electrodeposition of an organic polymer 176 4.2.1. Organic polymer painting 176 4.2.2. Electrochemically initiated insulating polymer 176

4.3. Photoetching 177 4.3.1. Mechanism of (photo )electrochemical etching 178 4.3.2. Engineering new surfaces 182

5. Effect of Photo(electrodeposition) of Catalysts for Production of Chemicals 183 5.1. Action of Noble Metal 184

5.1.1. Hz-Photoevolution on photocathodes 184 5.1.2. Clz and Brz generation on photoanodes 187

5.2. Noble metal dispersed in an electroactive confined polymer 188 5.3. Action of Heteropolyanions 188 5.4. Behaviour of surface-confined naphthoquinone derivative 189

Concluding Remarks 189 Acknowledgement 190 References 190

viii CONTENTS

W. JAEGERMANN / Surlace Studies of Layered Materials in Relation to Energy Converting Interlaces 195 1. Introduction 196 2. Crystal Structure of Layered Metal Chalcogenides 198 3. Electronic Structure of Layered Metal Chalcogenides 201 4. Interlaces 205

4.1. Interlace States in Semiconductor Junctions 207 4.1.1. Intrinsic surface states 207 4.1.2. Extrinsic surlace states 210

4.2. Space Charge Layers 212 4.3. SemiconductorlMetal Interlaces 214 4.4. Semiconductor/Electrolyte Interlaces 218

4.4.1. Idealized Schottky Type Junctions 218 4.4.2. Fermi level pinning 220

4.5. Semiconductor/Semiconductor Interlaces 221 5. Surlace and Interlace Analysis 224

5.1. Problems of Surlace and Interlace Characterization 224 5.2. Survey of Surlace Analytical Techniques 226

5.2.1. Photoelectron Spectroscopy 228 5.2.2. Low Energy Electron Diffraction 233 5.2.3. Low Energy Ion Scattering Spectroscopy 234

6. Surlaces of Layered Semiconductors 236 6.1. Preparation of van der Waals Surlaces 236 6.2. Properties of van der Waals Surlaces 238 6.3. Preparation and Properties of Non van der Waals Surlaces 241

7. Layered Semiconductor/Adsorbate Interaction 243 7.1. Adsorption Properties of van der Waals Surlaces 243 7.2. Adsorption Properties of Non van der Waals Surlaces 244

8. Layered Semiconductor/Electrolyte Interaction 245 8.1. Ex-Situ Analysis of Emersed Electrodes 245 8.2. URV Model Experiments of Electrolyte Interlaces 249

9. Layered SemiconductorlMetal Interaction 259 9.1. Group 6 B Layered Semiconductors 261 9.2. Other Layered Semiconductors 270 9.3 General Aspects of Schottky Barrier Formation on Layered Semi-

conductors 274 10. Semiconductor/Semiconductor Interlaces 277 11. Conclusions and Final Remarks 280 Acknowledgements 281 References 282

CONTENTS ix

M. W. PETERSON AND A.J. NOZIK / Quantum Size Effects in Layered Semi-conductor Colloids 297 1. Introduction 298 2. Preparation of Colloids 299

2.1. Reaction of metals with anions 299 2.2. Simple dissolution of the bulk semiconductor 299 2.3. Dissolution of the bulk semiconductor by sonication 300 2.4. Dissolution of bulk semiconductor by Li intercalation 300

3. Quantized Semiconducting Colloids of Layered Materials 301 3.1. Layered metal iodides: PbI2• BiI3 and HgI2 301

3.1.1. PbIz and BiI3 301 3.1.2. HgI2 303

3.2. Metal chalcogenides 310 3.2.1. Bi2S3 and Sb2S3 310

3.3. The group VI chalcogenides 312 4. Conclusions 315 5. References 316

A. ARUCHAMY AND M.K. AGARWAL / Materials Aspects of Layered Semiconductors for Interfacial Photoconversion Devices 319 1. Introduction 320 2. Semiconducting Layered Compounds 321

2.1. Transition Metal Dichalcogenides 323 2.2. Non-transition Metal Layered Chalcogenides 324 2.3. Ternary Compounds 325

3. Materials Aspects of Layered Compounds for Solar Energy Conversion 325 4. Preparation and Characteristics of Layered Materials 326

4.1. Single Crystal Growth 326 4.1.1. Vapor Transport Methods 327

4.1.1.1. Chemical Vapor Transport 327 4.1.1.2. Sublimation 329

4.1.2. Melt Growth 330 4.1.3. Flux Growth 330

4.2. Thin Films 331 4.2.1. Sputtered Films 331 4.2.2. Thin Films by Evaporation of Mo and Se 334 4.2.3. Chalcogenization of Mo Foils and Films 334 4.2.4. Laser Methods 335 4.2.5. Plasma CVD 335 4.2.6. Thin Films by Vapor Transport 336 4.2.7. Electrochemical and Chemical Deposition of Layered Semi-

conductors 337 4.2.7.1. High Temperature Electrolysis 337 4.2.7.2. Electrodeposition from Aqueous Solutions 337 4.2.7.3. Chemical Bath Deposition of Thin Films 338

x CONTENTS

4.2.8. Chemical Vapor Deposition 4.3. Polycrystalline Electrodes from Powders 4.4. Layered Materials by Novel Methods

4.4.1. van der Waals Epitaxial Growth of Thin Films 4.4.2. Exfoliation of Layers Using Intercalation Reactions

5. Concluding Remarks Acknowledgement References

Index

338 338 339 339 342 342 343 343

349

PREFACE

This volume aims at bringing together the results of extensive research done during the last fifteen years on the interfacial photoelectronic properties of the inorganic layered semiconducting materials, mainly in relation to solar energy conversion. Significant contributions have been made both on the fundamental aspects of interface characteristics and on the suitability of the layered materials in photoelectrochemical (semiconductor/electrolyte junctions) and in solid state photovoltaic(Schottky and p-n junctions) cells. New insights into the physical and chemical characteristics of the contact surfaces have been gained and many new applications of these materials have been revealed.

In particular, the basal plane surface of the layered materials shows low chemical reactivity and specific electronic behaviour with respect to isotropic solids. In electrochemical systems, the inert nature of these surfaces characterized by saturated chemical bonds has been recognized from studies on charge transfer reactions and catalysis. In addition, studies on the role of the d-band electronic transitions and the dynamics of the photogene rated charge carriers in the relative stability of the photoelectrodes of the transition metal dichalcogenides have deepened the understanding of the interfacial photoreactions. Transition metal layered compounds are also recognized as ideal model compounds for the studies Involving surfaces: photoreactions, adsorption phenomena and catalysis, scanning tunneling microscopy and spectroscopy and epitaxial growth of thin films. Recently, quantum size effects have been investigated in layered semiconductor colloids.

Several layered materials possess favourable semiconducting properties and have attracted attention as a new class of solar cell materials. Significant optical-to-electrical / chemical energy conversion efficiencies have been obtained in solid state photovoltaic and photoelectrochemical cells. Research into the electronic interfacial device characteristics of this class of materiais is rather recent and of growing interest. It opens up new opportunities for the application of the layered materials. The potential of this class of materials has not been fully explored yet but appears to be limited mainly by the availability of suitable materials. Attempts have been made to produce good quality crystals and thin films of the layered semiconductors for photoelectronic device uses. Several approaches including a novel extension of molecular beam epitaxy for the preparation of the layered materials are being actively pursued to produce high quality single crystals and thin films.

This volume consists of seven chapters devoted to various aspects of the layered semiconductors: interfacial characteristics, optical-to-electrical(chemical) conversion parameters, photocorrosion and surface modification, surface electronic properties, quantum size effects and materials preparation. The first chapter by Bucher provides a comprehensive review of the solid state properties and photovoltaic studies of the solid state junctions of the layered semiconductors. Extensive data have been compiled on the various materials discussed in this chapter. In Chapter two Tributsch gives a critical analysis of the photoelectrochemical properties of the transition metal dichalcogenides considering the various fundamental aspects of interfacial reactions related to the crystal and electronic structure, coordination chemistry and catalysis. The issues critical to solar energy conversion are discussed. In Chapter three Scrosati and co­workers describe the photoelectrochemical behaviours of the layered transition metal dichalcogenides in PEC solar cells and in electrolysis celis. In Chapter four Levy-Clement and Tenne describe the various studies detailing the photocorrosion behaviours and the attempts at stabilizing the layered semiconductors by surface modification techniques. Several surface characterization techniques have been used in these studies and new applications of photocorrosion reactions are described here. In Chapter five Jaegerrnann has given an account of the surface and interfacial electronic properties of layered semiconductors characterized by UHV spectroscopic techniques. The Implications of surface electronic properties to interfacial energy conversion are discussed. Preparation and characterization of the quantized colloids of layered materials are described by Peterson and Nozlk in Chapter six. The optical absorption behaviours of the quantized colloids are given. In the last chapter by Aruchamy and Agarwal, an overview is given on the various layered semiconductors studied in PEC systems and the methods employed to prepare Single crystals and thin films. The need for improved preparative techniques to produce high quality materials has been emphasized. The results of research discussed In these chapters reveal several areas of interest for the layered materials. Application of these materials in devices awaits further materials research.

The editor would like to thank Professor F. Levy, EPFL, Lausanne, for suggesting this work and providing the needed help. Thanks are also due to Professor H. Tributsch, HMI, Beriln and Professor A

Xl

xii PREFACE

Fujishima, University of Tokyo for their encouragement. The authors of this volume are gratefully acknowledged for their fruitful collaboration by contributing the various chapters inspite of several other committments and shift In the research Interests of some of the authors In recent years. Professor D. R. Uhlmann, Head of the Department of Materials Science and Engineering, University of Arizona is acknowledged for his kind support and interest in this work. Acknowledgements are also due to Drs. Janjaap Blom, the K1uwer Academic Publishers for the patience and timely suggestions for the preparation of the book.

Tucson, Arizona, USA October 1991

A. Aruchamy