Charge-Carrier Transport in Liquid-Crystal Based Organic Photoconductors*1

Abstract

In this work we have used a one-dimensional model which incorporates energetic disorder to analyse the anomalous temperature dependence of the charge-carrier mobility in the liquid crystalline discotic twin HPAT2 and the broadening of the photocurrent decays. In order to render the main ideas as transparent as possible we have aimed at the simplest possible description which still includes the basic features. The model allows a closed-form expression for the Laplace transformed photocurrent. The temperature dependence of charge-carrier mobility can simply be calculated analytically if assuming a boxlike trap distribution. Assuming a temperature dependent trap parameter we could model the anomalous temperature dependence of charge-carrier mobilities. This temperature dependent peculiarity of the trap distribution is inferred from the temperature dependent molecular dynamics in the Dh phase, as concluded from X-ray measurements and NMR experiments. A `parallel connection' of different types of columns with different characteristic trap distributions could reproduce the broadened decay of photocurrent transients. The parallel connection was concluded from a homeotropic polydomain texture as seen in the polarizing microscope.