Current-voltage characteristics in organic semiconductor crystals: space charge vs. contact-limited carrier transport
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DD6.8.1
Current-voltage characteristics in organic semiconductor crystals: space charge vs. contact-limited carrier transport J. Reynaert*, V. I. Arkhipov, J. Genoe, G. Borghs, P. Heremans IMEC, Kapeldreef 75, B-3001 Leuven, Belgium * also with ESAT, Katholieke Universiteit Leuven, Leuven, Belgium ABSTRACT Numerous experimental studies, mostly based on the time-of flight (TOF) technique, showed that the conductivity in organic crystals can be analysed in terms of (trapcontrolled) band transport. However, recent comparative studies of TOF signals and space charge limited currents (SCLCs) in tetracene crystals revealed a striking difference in carrier mobilities estimated from TOF current transients and from SCLC curves. The analysis of the SCLC curves yielded the mobilities wildly varying within 6 orders of magnitude. Therefore, it is not always clear whether the measured current-voltage (IV) device characteristics are controlled by charge injection or by transport in the bulk. In this work, we formulate a model of dopant-assisted carrier injection across a metal/organics interface and use this model for the analysis of IV curves measured on a tetracene and perylene crystal. The model suggests the occurrence of an energetically disordered layer at the surface of an organic crystal. This might be either an amorphous phase of the same material or a crystalline layer with a high density of defects and/or impurities. Since, at variance with bulk properties, the surface of an organic crystal is poorly controlled and can be strongly modified upon the contact deposition, the model of injection-controlled IV characteristics can explain the striking difference between the TOF mobility and the apparent ‘SCLC mobility’ measured in tetracene crystals. In order to give more credence to the role of surface defects states in the dark charge transport, we compare IV characteristics measured on sandwich and coplanar structures. In the latter structure, surface states show a major contribution to the conductivity.
DD6.8.2
INTRODUCTION Single crystals of small molecule organic semiconductors form an interesting case of study as it is expected that their electrical properties are not subjected to grain boundaries or other defects due to morphological disorder, which are present in thin films of these same materials. Due to the structural order in well-grown crystals, the carrier transport is expected to happen through (trap-controlled) hopping of weakly localized carriers which is virtually equivalent to band transport [1, 2]. Therefore, it is interesting to derive the intrinsic properties of these organic materials such as carrier mobility or impurity (trap) concentration through their electrical characterization. A modest attempt is made in this introduction to give an overview of the optoelectrical methods that could eventually lead to a characterization of the electrical transport in an organic crystal.
TOF FET Organic crystal a
IV b
Figure 1: Opto-electronic methods for transport property characertization
The time-of-flight
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