Molecular Semiconductors: Photoelectrical Properties and Solar Cells PDF
Preview Molecular Semiconductors: Photoelectrical Properties and Solar Cells
1. Simon· 1.-1. Andre Molecular Semiconductors Photo electrical Properties and Solar Cells Editors: 1. M. Lehn, Ch. W. Rees With 166 Figures and 41 Tables Springer-¥erlag Berlin Heidelberg New York Tokyo Prof. Jacques Simon Ecole Superieure de Physique et Chimie Industrielles de la Ville de Paris lO, rue Vauquelin F-7523l Paris Cedex 05 Dr. Jean-Jacques Andre Centre de Recherches sur les Macromolecules (C.N.R.S.) 6, rue Boussingault F-67083 Strasbourg Cedex ISBN -13: 978-3-642-70014-9 e-ISBN -13: 978-3-642-70012-5 DOl: 10.1007/978-3-642-70012-5 Library of Congress Cataloging in PublIcatIOn Data Simon, J. (Jacques), 1947-Molecular semiconductors. Bibliography: p. Includes index. \. Semiconductors. 2. Molecular crystals. 3. Photoelectricity. 4. Solar batteries. I. Andre, J.-J. (Jean-Jacques), 1940-. II. Lehn, J.-M. (Jean-Marie). III. Rees, Charles W. (Charles Wayne). IV. Title. QC61\.S583 1984 537.6'22 84-20197 This work is subject to copyright. All rights are reserved, whether the whole or part of the material is concerned, specifically those of translation, reprinting, re-use of illustrations, broadcasting, reproduction by photocopying machine or similar means, and storage in data banks. Under § 54 of the German Copyright Law where copies are made for other than private use, a fee is payable to "VerwertungsgeseJ1schaft Wort", Munich. © by Springer-Verlag Berlin Heidelberg 1985 Softcover reprint of the hardcover 1st edition 1985 The use of registered names, trademarks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. 2152/3020-543210 A Elisabeth "Sans elle tout ce qui est ne sera it pas" (c. de Gaulle, d propos de Mme de Gaulle) A fa memo ire de Pierre Vitali A Madeleine De grands physiciens ont fort bien trouve pourquoi les lieux souterrains sont chauds en hiver et froids en ete. De plus grands physiciens ont trouve dep uis peu que cela n' etait pas. (Fontenelle) Soleil, je t' adore comme les souvages ... (Jean Cocteau) Preface During the past thirty years considerable efforts have been made to design the synthesis and the study of molecular semiconductors. Molecular semiconductors - and more generally molecular materials - involve interactions between individual subunits which can be separately synthesized. Organic and metallo-organic derivatives are the basis of most of the molecular materials. A survey of the literature on molecular semiconductors leaves one rather confused. It does seem to be very difficult to correlate the molecular structure of these semiconductors with their experimental electrical properties. For inorganic materials a simple definition delimits a fairly homogeneous family. If an inorganic material has a conductivity intermediate between that of an insulator « 10-12 n-1 cm-1) and that of a metal (> 103 n-1 cm-1), then it is a semiconductor and will exhibit the characteristic properties of this family, such as junction formation, photoconductivity, and the photovoltaic effect. For molecular compounds, such simplicity is certainly not the case. A huge number of molecular and macromolecular systems have been described which possess an intermediate conductivity. However, the various attempts which have been made to rationalize their properties have, more often than not, failed. Even very basic electrical properties such as the mechanism of the charge carrier formation or the nature and the density ofthe dopants are not known in detail. The study ofm olecular semiconductor junctions is very probably the most powerful approach to shed light on these problems. The physico-chemical characteristics of such devices depend on the molecular structure of the semiconductor and on the presence of minute amounts of impurities. Most of the fundamental transport parameters (mobility and nature of the charge carriers, density and distribution of the dopants, nature of the conduction mechanism, etc.) may be determined from junction studies in the dark or under illumination. In the latter case, we have the transformation of light into electricity, the photovoltaic effect. The term "solar cell" is often preferred to designate these devices. Although the terms "photovoltaic effect" and "solar cell" cover exactly the same type of scientific interests, they must not be confused; pure scientists and industrialists must each have a language of their own. Junctions studies are, in any event, a very powerful and promising way of studying molecular semiconductors. Is the term "molecular semiconductor" a misnomer? It is certainly true that the mobilities normally measured are low. Would "doped insulator" be more appro. priate? The first chapter of the book will go over a few basic notions of solid state physics and try to solve this semantic problem. As far as it it possible a chemical interpretation of notions such as mobilities, Fermi levels, or band schemes will be given. Such interpretations will of course necessitate some approximations. It is hoped that what the chemist gains in intuitive understanding will more than com- pensate for what the physicist loses in precision. Of all the possible modes of conduc tion, emphasis will be placed on those most characteristic of molecular materials. In all cases the importance of the nature and the concentration of the impurities on the electrical properties will be emphasized. The relative contributions of structural defects and chemical impurities on the various trapping processes will be estimated. The second chapter is devoted to a general discussion of the various photoelectric phenomena present in molecular semiconductors. Basic photochemical properties of molecular compounds are first presented and the factors affecting the extent of energy transfer in molecular materials are briefly recalled. This then leads to a discussion of the various pathways for photogenerating charge carriers. After outlining the mech anisms of formation of semiconductor pin junctions and semiconductor/metal junctions, the photovoltaic properties of these devices will be discussed. At this point most of the scientific ideas underlying the operations of organic solar cells will have been hopefully mastered. A theme running through the first two chapters will be the distinction between two classes of molecular semiconductors: molecular crystals and polymers. The formers are available in highly purified form through the use of chromatography, distillation, sublimation, or crystallization. The mobility is limited by the usually smalln overlap. Macroscopic migration of charge then involves electron hopping from molecule to molecule. Problems thus arise in applying band theory. The second class, the polymers, provide a strong contrast. Once formed they cannot be purified to the standards of molecular crystals. The intrinsic mobility is now determined by the strong, covalent intrachain interactions and is thus large. A band theory then becomes less inappropriate. However, the impurities present in these systems may now completely dominate the mobility through trapping processes. Any particular case will show properties somewhere between the extremes of the pure molecular crystal and the impure polymer, and a clear understanding would necessitate a detailed knowledge ofiis chemical and physico-chemical properties. So rather than attempt a necessarily superficial coverage.of all known molecular semiconductors, a "typical" example of each of the two types will be discussed. Chapter 3 will discuss metall ophthalocyanines, "typical" molecular crystals. Chapter 4 focuses on polyacetylene as the "typical" polymer. In both cases the synthesis and physico-chemical properties of the two systems will be discussed first. A review is provided to the literature up to approximately July 1982. Once the materials have been defined their electrical dark properties can be discussed. Each chapter will end with a section devoted to the photoconduction processes and the photovoltaic effect. At the light of the previous considerations, an overview of all the main other molecular conductors or semi conductors is given in Chapter 5. To design and elaborate new molecular semiconductors will require an unusually close collaboration between chemists and physicists. It is hoped that this book will help the chemist to appreciate the importance of the physics involved, and will help the physicist appreciate the problems facing the chemist. By this means we hope to bridge the gulf between them. Acknowledgments Many people made possible the realization of this book, but we want to particularly acknowledge Professors l-M. Lehn, H. Benoit, and C. Wippler. Professors M. Schott and G. Weill are thanked for helpful and stimulating discussions. We very gratefully acknowledge the help provided by everyone working at Centre de Recherches sur les Macromolecules ofStrasbourg (C.R.M.). Thanks are due to Mr. Ph. Gramain for offering us the opportunity of developing this subject at C.R.M. Mr. M. Martin, Mr. C. Piechocki, and Miss D. Markovitsi are thanked for their collaboration. C.N.R.S., P.I.R.S.E.M., and ELF are acknowledged for their financial support. This whole book has been rewritten and often re-written by Mr. S. Abbott. Table of Contents List of Symbols . . . . . . . . . . . I Basic Notions of Solid State Physics. . . . . . . . . . . . 6 1.1 Dark Conductivity: Generalities. . . . . . . . . . . 6 I.2 Conduction in Metals: Drude and Sommerfeld Models . 8 1.3 Band Model of Conduction. . . . . . . . . . . . . 10 1.4 Limitations to Band Theory . . . . . . . . . . . . 16 1.5 Hopping and Tunneling Mechanisms of Charge Migration 16 1.6 Charge Carrier Trapping Processes 19 a Molecular Crystals 19 b Polymers .......... . 24 n Photoelectric Phenomena in Molecular Semiconductors . 28 11.1 Light Absorption . . . . . . . . . . . 29 11.2 Energy Migration in Molecular Materials. 32 a Mechanisms of Energy Migration 32 b Effect of Traps . . . . . . . . 37 11.3 Photogeneration of Charge Carriers 39 11.4 Semiconductor Junctions. . . . . 42 a p-n Junctions: Formation and Electrical Properties 42 b Schottky Junctions: Semiconductor-Metal Contacts 51 c Insulator-Metal Contacts. . . . . . . . . . 53 11.5 Photovoltaic Effect . . . . . . . . . . . . . . . 59 a Molecular Solar Cells: Classical Formulation ... 59 b Molecular Solar Cells: Localized States Formulation. 63 c Effect of Surface States . . . . . . . . . . . . . 66 d Characterization of Junctions by the Capacitance Method. 68 ill Metallophthalocyanines . . . . . . . . . . . . 73 III.l Syntheses and Physico-Chemical Properties . 74 a Syntheses . . . . . . . . 74 b Structure and Morphology . 80 c Spectroscopic Properties . . 86 d Photoelectron Spectroscopy 94 e Oxidation-Reduction Properties. 98 f Electron Spin Resonance Measurements (ESR) 101 XII III.2 Dark Electrical Properties . . . . . 103 a Energy Band Structures . . . . . .1 03 b Electrical Properties: Intrinsic Case 104 c Determination of the Trapping Levels 110 d Doping of PcM by 02 . . . . . . . 112 e Doping of PeM by other Doping Agents 116 III.3 Photovoltaic Effect and Solar Cells 122 a Photoelectrical Properties . . . 122 b Photovoltaic Effect: Generalities 126 c Junction in the Dark. . . . . . 127 d Junction Studies under Illumination . 137 e Effect of Doping on the Performances of Molecular Solar Cells 145 f Solar Energy Conversion Efficiencies of Molecular Solar Cells . 147 IV Poly acetylene . . . . . . . . . . . . . . . . . . . . . . . 150 IV.1 Synthesis and Physico-Chemical Properties of Poly acetylene . 151 a Synthesis. . . . . . . . . . . . . . . . . . . . . . 151 b Morphology . . . . . . . . . . . . . . . . . . . . 155 c Molecular Weight and Length of the Conjugated Sequences. 157 d Stability of Polyacetylene and Effect of 02 158 e Isomers . . . . . . 161 f Crystalline Structures . 164 IV.2 Theoretical Properties . . 166 a Origin of the Band Gap 166 b Band Structure . . . . 169 c Bond Length Alternation Defects in Polyenes: the Solitons 171 IV.3 Properties of Doped Polyacetylene . 173 a Dopants and Doping Processes 173 b Structural Features . 175 c Optical Properties. . . . . . 176 d Magnetic Properties. . . . . 176 IV.4 Transport Properties of Polyacetylene 179 a Conduction Mechanisms at Low Doping Levels. 181 b Semiconductor-Metal Transition . . . . . . . 183 c Metallic Domain . . . . . . . . . . . . . . 184 d Comparison of the Models with the Experimental Results in the Low Dopant Concentration Domain ................ 185 IV.5 Photoelectric Properties and Solar Cells . . . . . . . . . . . . . 188 a Luminescence and Photoconductive Properties of Cis- and Trans- Polyacetylene. . . . . . . . . . . . . . . 188 b Junctions Properties and Molecular Solar Cells . . . . . . . . . 192 V The Main Other Molecular Semiconductors. . 201 V.I Aromatic Hydrocarbons and Graphite. 201 V.2 Metallo-Organic Derivatives 207 V.3 Charge Transfer Systems. . . . . . . 212
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