Studies of charge carrier transport in some organic molecular crystals

STUDIES OF

CHARGE CARRIER TRANSPORT

IN SOME ORGANIC MOLECULAR CRYSTALS

by

S. RAVINDRA NATH

Thesis submitted to the Indian Institute of Technology, Delhi

for the award of the degree of

DOCTOR OF PHILOSOPHY

OELNI

Department of Physics

INDIAN INSTITUTE OF TECHNOLOGY, DELHI

1984

CERTIFICATE

This is to certify that the thesis entitled

"Studies of Charge Carrier Transport in Some Organic

Molecular Crystals", being submitted by Mr. S. RAVINDRA NATH to the Indian Institute of Technology, Delhi, for the award of the degree of Doctor of Philosophy in Physics is a

record of bona fide research work carried out by him.

Mr. S. RAVINDRA NATH has worked under my guidance and supervision and has fulfilled the requirements for the submission of'this thesis.

The results obtained in this thesis have not been submitted, in part or full, to any other university or

institution for the award of any degree or diploma.

(Dr. S CC..MATHUR) Professor of Physics

Indian Institute of Technology, Delhi, New Delhi 110 016

ACKNOWLEDGEMENTS

I would like to acknowledge with gratitude the assistance and encouragement of very many people during the course of work, culminating in this manuscript.

First and foremost to my supervisor, Prof. S.C. Mathur, who has been a constant source of helpful understanding and

stimulating guidance throughout •the course of the work.

I would like to thank Dr. D.C. Dube for all the help and encouragement I have received from him.

I would also like to thank Prof. P.K.C. Filial, Head of the Physics Department and Dr. N.K. Bansal, Centre for Energy Studies, for providing me fellowship for part of the period, which enabled me to complete this work.

The present work could hardly have been possible without the assistance and cooperation on the part of a large number of colleagues and friends, especially, Dr. A.K. Batra, Dr. J.P.S. Chauhan, Dr. A.D. Sharma, Dr. Keya Dhar,

Mr. Rakesh Kumar, Mr. B.K. Sharma, Dr. C.P.S. Tomar,

Mr. G.D. Sharma, Dr. K.S. Reddy, Mr. A.R. Laxman and Mr. Rama Rao.

My thanks are also due to various staff at our Computer Centre, Workshop, I.D.D.C. and Glass Blowing Workshop.

It is a pleasure to thank Mr. S.L. Aneja for neat typing of the manuscript and Mr. R.N. Kapoor, Mr. Jagdish

V

VT

and. Mr. N.S. Gupta for neat tracing of the figures.

Finally I owe a great debt of gratitude to my parents for setting me on the path towards intellectual pursuits and to my brother and sister for their constant

encouragement.

(S. RAVINDRA NATH)

CONTENTS

CHAPTER Page

GENERAL INTRODUCTION AND SYNOPSIS .. 1-5 OF THE THESIS

II REVIEW OF PRESENT STATUS OF UNDERSTANDING.. 6-43 OF ELECTRONIC'PROCESSES IN ORGANIC SEMI-

CONDUCTORS WITH PARTICULAR REFERENCE TO ORGANIC MOLECULAR CRYSTALS

2.1

Introduction

.. 6

2.2

Aromatic hydrocarbons: Model

.. 7

compounds of organic semiconductors

2.3 Electronic states in organic .. 7 molecular crystals

2.4 Trapping states in organic .. 12 molecular crystals

2.5 General concepts of electrical .. 15 conduction

2.6 Charge carrier generation .. 18

2.7

Field dependent effects

..

²³

2.8 Charge transport mechanism in .. 31 organic semiconductors

III EXPERIMENTAL TECHNIQUES AND METHOD OF ..

44-71

THEORETICAL BAND STRUCTURE CALCULATIONS

3.1 Introduction .. 44

3.2 Purification methods .. 44

3.3

Crystal growth

..

⁵⁰

3.4

Sample preparation

.. 53 3.5

Measurement techniques

•.

⁵⁴

3.6

Method of band structure calculation..

62

III

IV

CHAPTER Page

IV RESULTS AND DISCUSSION . 72-160

4.1 Anthraquinone ., 73

4.2 Biphenyl .. 101

4.3

p-Terphenyl .. 118

4.4 Pyrene .. 131

4.5

Anthracene and naphthalene

.. 143 4.6 Summary of

results and conclusions

.. 154 4.7

Concluding remarks and scope

for

^.. 159

future work

REFERENCES .. 161-175