Effects of Gaussian Disorder on Charge-Carrier Transport and Recombination in Organic Semiconductors

Reinder Coehoorn and Peter A. Bobbert

6.1 Introduction

Common fabrication technologies of organic semiconductor materials, such as high vacuum evaporation deposition of small molecule-based materials or solution processing of polymers, often lead to a disordered, amorphous or nearly amorphous structure. In such systems, with only a weak intermolecular interaction, injected electrons or holes reside on distinct molecular sites or (in the case of polymers) on distinct conjugated segments. The charge transport is due to hopping, that is, due to thermally assisted quantum mechanical tunneling in between the localized sites. Experimental evidence favoring this picture is the observation of an increase of the charge-carrier mobility with increasing temperature, as measured in unipolar devices in which the transport is due to a small concentration of charges injected from the electrodes. At high carrier concentrations, as obtained, for example, by strong molecular doping, a crossover to metallic transport can take place, characterized by a decrease of the conductivity with increasing temperature [1].

In this chapter, we discuss recent advances in our understanding of the charge-carrier transport in disordered organic semiconductors in the hopping regime. We focus on OLEDs and include a discussion of the effect of disorder on charge-carrier recombination and an application to single-layer ...

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