Table Of ContentSpringer Series in Solid-State Sciences
Editors: M. Cardona P. Fulde H.-J. Queisser
Principles of Magnetic Resonance 12 The Physics of Elementary Excitations
2nd Edition 2nd Printing By S. Nakajima
By C. P. Slichter
13 The Physics of Selenium and TeUurium
2 Introduction to Solid-State Theory Editors: E. Gerlach and P. Grosse
By O. Madelung
14 Magnetic Bubble Technology
3 Dynamical Scattering of X-Rays in By A.. H. Eschenfelder
Crystals By Z. G. Pinsker
15 Modern CrystaUography I··
4 Inelastic Electron Tunneling Spectroscopy
Symmetry of Crystals,
Editor: T. Wolfram
Methods of Structural Crystallography
5 Fundamentals of Crystal Growth I By B. K. Vainshtein
Macroscopic Equilibrium and Transport
Concepts. By F. Rosenberger 16 Organic Molecular Crystals
Their Electronic States
6 Magnetic flux Structures in
By E. Silinsh
Superconductors By R P. Huebener
17 The Tbeory of Magnetism I
7 Green's Functions in Quantum Physics
Ground State and Elementary Excitations
By E. N. Economou
By D. Mattis
8 Solitons and Condensed Matter Physics
Editors: A. R. Bishop and T. Schneider 18 Relaxation of Elementary Excitations
Editors: R. Kubo and E. Hanamura
9 Photoferroelectrics
By. V. M. Fridkin 19 Solitons, Mathematical Methods for
10 Phonon Dispersion Relations in Physicists
Insulators By H. Bilz and W. Kress By G. Eilenberger
11 Electron Transport in Compound 20 Theory of Nonlinear Lattices
Semiconductors By B. R Nag By M. Toda
E. A. Silinsh
Organic Molecular
Crystals
Their Electronic States
With 135 Figures
Springer-Verlag Berlin Heidelberg New York 1980
Professor Dr. Edgar A. Silinsh
Institute of Physics and Energetics,
Latvian SSR, Academy of Sciences,
Riga-6, USSR
Series Editors:
Professor Dr. Manuel Cardona
Professor Dr. Peter Fulde
Professor Dr. Hans-Joachim Queisser
Max-Planck-Institut fUr Festk5rperforschung, Heisenbergstrasse I
D-7000 Stuttgart 80, Fed. Rep. of Germany
Revised translation of the Russian edition:
Elektronnye sostoyaniya organicheskikh molekulyarnykh kristallov (Electronic States of
Organic Molecular Crystals)
by E. A. Silinsh
© by Zinatne, Riga (1978)
English translation by J. Eiduss in collaboration with the author.
ISBN -13: 978-3-642-81466-2 e-ISBN-13: 978-3-642-81464-8
DOT: 10.1007/978-3-642-81464-8
Library of Congress Cataloging in Publication Data. Silinsh. E. A. Organic molecular crystals. (Springer series in solid·
state sciences ; v. 16) Revised translation of the author's Elektronnye sostoyaniya organicheskikh molekulyarnykh
kristallov, 1978 ed. Bibliography: p. Includes index. L Molecular crystals. 2. Chemistry. Organic. 3. Crystals-Electric
properties. 1. Title. II. Series. QD92LS539313 1980 547 80·14967
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, reuse 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 the publisher, the amount of the fee to be determined by agreement with
the publisher.
© by Springer·Veriag Berlin Heidelberg 1980
Sol1cover reprint of the hardcover 1s t edition 1980
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.
Foreword
This book is based on the results of many years of experimental work by
the author and his colleagues, dealing with the electronic properties of
organic crystals. E. Silinsh has played a leading role in pOinting out the
importance of the polarization energy by an excess carrier, in determining
not only the character of the carrier mobility in organic crystals, but in
determining the band gap and the nature of the all-important trapping site
in these crystals. The one-electron model of electronic conductivity that
has been so successful in dealing with inorganic semiconductors is singular
ly unsuccessful in rationalizing the unusual physical properties of organic
crystals. A many-body theory is required, and the experimental manifestation
of this is the central role played by the crystal polarization enerqies in
transferring the results obtained with the isolated molecule, to the solid.
The careful studies of E. Silinsh in this field have shown tn detail how
this polarization energy develops around the excess carrier (and also the
hole-electron pair) sitting on a molecular site in the crystal.
As with all insulators, trapping sites playa dominant role in reducing
the magnitude of ~he current that can theoretically pass through the organic
crystal. It is usually the case that these trapping sites are energetically
distributed within the forbidden band of the crystal. For many years, an
exponential distribution has shown itself to be useful and reasonably
correct: However,' E. Sil i nsh has poi nted out the i nadequaci es of thi s as
sumption, and has demonstrated the greater validity of a Gaussian distribu
tion. Recent work in different parts of the world, particularly with
amorphous materials, has substantiated the point of view of E. Silinsh.
A glance at the Table of Contents of this timely book reveals an unusual
ly pertinent selection of topics. Not only are charge carrier phenomena
discussed, but also the excitonic phenomena that are exhibited to an unusual
degree in these organic crystals. In all cases, the approach taken by the
VI
author is admirably selective, direct, clear, and positive. He has presented
his opinion as to what is correct, and in so dOing, has rendered a signifi
cant service to the reader who may not be familiar with this field, and who
wishes to begin with the best thinking of an outstanding worker in the field.
The theoretical treatment is presented as a basis for understanding the
experimental results, and the book is filled with excellent figures and
tables. These provide a rich source of experimental data. The results from
the author's laboratory, dealing with heterocyclic compounds, are not usual
ly treated elsewhere in such a coherent fashion.
The entire area of electronic conductivity in organic materials is ex
panding at an unbelievab~e rate. These compounds are being used as photo
conductors in electrophotography and as radiation detectors; in the form of
polymers, and charge-transfer crystals, it has been possible to develop
quasi-one-dimensional crystals with conductivities that approach metals.
The book by E. Silinsh provides an excellent starting point for those two
wish to learn the important elements that distinguish this group of materials
from the more conventional materials. It will also remain useful despite the
passage of time because the opinions of the author will retain their validi
ty and superb tutorial quality.
New York, June 1980 Martin Pope
New York University
Preface
In the broad spectrum of solid-state science a new band, adjacent
overlappin~
interdisciplinary areas, has recently emerged-the physics of the organic
solid state.
In this new promising field of research organic crystals of aromatic and
heterocyclic have become the main objects of scientific interest.
m~lecules
This is due to the fact that, in the case of aromatic and a number of het
erocyclic organic crystals, the investigator has at his disposal detailed
information about their molecular and crystal structure. Such knowledge is
an indispensable precondition for successful studies of electron and exciton
processes in the crystal. Secondly, the above-mentioned classes of organic
crystals open new promising avenues for practical applications. A number of
aromatic and, especially, heterocyclic molecular crystals exhibit high quan
tum efficiency of photoconductivity, interesting photodielectric and photo
magnetic properties, pronounced anisotropy of optical and electrical par
ameters, high electronic polarizability and other exotic characteristics.
Thirdly, a systematic study of simple organic molecular crystals is a neces
sary prerequisite for proper understanding of electronic processes in more
complex organic solids, including polymers and molecular biosystems.
At present the investigation of electronic states in organic molecular
crystals is regarded as a self-contained problem of solid-state physics.
This is partially caused by the fact that, for organic molecular crystals,
a number of traditional concepts of solid-state physics are not valid, e.g.,
band theory of one-electron approximation.
In molecular crystals, due to the weakness of Van der Waals interaction,
the main electronic characteristics of molecules undergo only slight changes
in the process of solid phase formation. The domination of molecular proper
ties over the crystalline ones leads to a marked tendency of localization
of charge carriers and excitons on individual molecules of the crystal. As
a result, a number of qualitatively new properties emerge in organic mole-
VIII
cular crystals, such as electronic polarization of the lattice by a localized
charge carrier, vibronic relaxation of molecules in the process of the for
mation of genuine ionic states, formation of local states of polarization
origin in the regions of structural defects of the crystal, and other phen
omena, unfamiliar to covalent and ionic crystals.
The main purpose of the book is to investigate the present status and
future trends in the field of energy structure studies in molecular crystals.
The basic emphasis is laid on stationary conduction levels of thermalized
charge carriers, as well as on local trapping states. Dynamic aspects of
charge carri er generation and transport are di scu ssed only fragmentarily, in
context with the description of stationary states of the crystal.
The phenomenological models and basic concepts of energy structure of
molecular crystals are based on investigations of the author and his col
laborators, as well as on data reported by other workers. Although more than
500 references appear in the course of the discussion, the book is not meant
to be a review. The results of other authors have been mainly used for inter
preting or confirming theoretical principles and phenomenological models,
or for iliustrating methods of investigation.
This book was stimulated by the desire to formulate a scientific basis
for experimental investigation of molecular crystal energy structure. Since
this new field is still being developed, we discuss both recognized physical
models and theories, as well as problematic, controversial questions which
have, in our view, considerable heuristic value in stimulating experimental
and theoretical research in organic solid-state physics.
The physics of the organic solid state occupies a borderline between a
number of different disciplines-molecular spectroscopy, quantum chemistry,
solid-stat~ physics and electronics, crystal chemistry, organic and physical
chemistry, and even molecular biology. Therefore the present book may be
interesting and useful for specialists working in different fields.
The present work could hardly have been possible without the assistance
aM cooperation on the part of a 1a rge number of colleagues, near and far.
The author is grateful to Springer-Verlag and, in particular, to Dr. H.
Lotsch for the opportunity of pub 1i shi ng hi s work in Engl ish
The author greatly appreciates the help of his collaborators Dr. L.F.
Taure, Dr. A.K. Gailis, D.R. Balode, I.S. Kaulach, V.A. Kolesnikov, I.J.
Muzikante, V.A. Skudra and the Photoemission group of the Laboratory,
headed by Dr. A.I. Belkind,for providing new experimental data for the book,
as well as the fruitful collaboration with A.J. Jurgis in electronic polariz-
IX
ation and local state energy spectra calculations, and with Dr. S. Ne5purek
in the study of SCLC theory for Gaussian trap distribution.
For translation of the original Russian book, as well as of the revised
and newly added portions of text the author is much indebted to Dr. J. Eiduss.
All those colleagues who have helped with valuable comments in private
discussions, conferences and correspondence are gratefully acknowledged.
In addition to the acknowledgements expressed in the preface to the
original Russian edition of the book [Zinatne, Riga 1978], the author wishes
to thank in particular Professor E.L. Frankevich. Dr. M.V. Kurik, Professor
T.J. Lewis, I.B. Rips, Dr. J. Sworakowski, Dr. L.F. Taure, Dr. A.T. Vartanyan,
Dr. J.O. Williams for useful comments and suggestions during the preparatory
period of the manuscript of this English edition.
The author is much indebted to D.R. Balode, I.J. Muzikante, I.E. Rozlte,
V.A. Skudra and L.F. Taure for meticulous and patient technical assistance
at all stages of this work. This does not, however, free the author from
full responsibility for any shortcomings or possible errors in the text.
Riga, January 1980 Edgar A. SiZinsh
Acknowledgments
The author is indebted to the following publishers for permission to re
produce their figures in the text:
The Royal Society of London: Taylor and Francis Ltd: Fig. 3.27.
Fig. 3.17.
Blackwell Scientific Publications
The American Institute of Physics: Ltd.: Fig. 3.29.
Figs. 1.la,S,18; 4.4-6; 5.21.
Academic Press Inc.: Fig. 3.9a.
The Institute of Physics (London):
Akademie Verlag, Berlin: Figs. 2.8,
Figs. 5.29,41,42,46.
14; 3.1,2,16; 5.25-34,40,45,50.
The Chemical Society (London):
Wroclaw Technical University Press:
Figs. 1.14; 3.12,19,25; 5.24.
Figs. 3.23,24; 5.35,36,39,47,48.
The North-Holland Publishing Co.:
Gordon and Breach Science Publishers
Figs. 1.15; 3.14; 4.7-9,11; 5.43.
Ltd.: Figs. 3.8,28,31; 4.1-3;
Wiley-Interscience: Figs. 3.6,9b,11, 5.4.,6.
30; 5.lD. Pergamon Press Ltd., Oxford: Figs. 3.3,
McGraw-Hill Book Co.: Figs. 3.4,10. 7,13;5.29b,50.
Nauka, Moscow: Fiqs. 1.lb,4;2.1;5.5,12
We also wish to thank these authors for their kind consent to reproduce
the following figures:
J.M. Thomas, F.R.S.: Figs. 1.15; J. Sworakowski: Figs. 3.8; 5.4,6,40,
3.3,12,13,17-19,23-25,28; 4.7-9; 44.
5.24,41,42. '
L. Schein: Fig. 1.19.
A.S. Oavydov: Fig. 2.1.
H. Baessler: Figs. 5.29,43.
J.O. Williams: Figs. 3.27,29.
V.A. Lisovenko: Figs. 3.14; 4.10,11.
N. Karl: Fig. 5.50
A.I. Kitaigorodsky: Figs. 1.1b,4,17;5.5
Contents
1. Introduction: Characteristic Features of Organic Molecular Crystals 1
1.1 Interaction Forces in Molecular Crystals ............ ........... 2
1.2 The Atom-Atom Potential Method ................................. 5
1.3 Aromatic Hydrocarbons-Model Compounds of Organic Molecular
Crystals....................................................... 10
1.3.1 Anthracene ............................................... 10
Anthracene as Model Compound ...... ....................... 10
Molecular Structure...................................... 11
Basic Molecular Parameters............................... 13
Crystal Structure........................................ 13
Elastic and Optical Properties ............. .......... .... 16
Metastable Phases in Anthracene ...................... .... 16
1.3.2 Naphthalene ........................ ........ .......... .... 17
Molecular Structure...................................... 18
Basic Molecular Parameters ...- ..................- .......... 18
Crystal Structure........................................ 18
Elastic and Optical Properties .... ...... ...... ...... ..... 19
1.3.3 Hi~her Linear Polyacenes ..... ........ .................... 19
Tetracene and Pentacene .................................. 20
Hexacene ................................................. 22
1.3.4 Ot~er Model Aromatic Compounds ........................... 22
1.4 Specific Properties of Electronic States in a Molecular Crystal 25
1.5 Basic Characteristics of Electronic Conduction States in
Molecular Crystals............................................. 27
1. 5.1 Band Theory Approach ..................................... 29
1.5.2 Hopping Versus Band Model................................ 34
1. 5.3 Band-to-Hopping Transition............................... 36
1.5.4 Electronic Polarization and Charge Carrier Self-Energy... 41
1.5.5 Other Types of Interaction .............. ............ ..... 44
