One-dimensional hopping transport in a columnar discotic liquid-crystalline glass

Abstract

The charge carrier transport in liquid-crystalline triphenylenes is completely different from the transport in amorphous photoconductors. The high degree of order leads to quasi-one-dimensional transport. In this paper we are proposing a model for a field-assisted incoherent hopping process in one-dimensional systems based on Monte Carlo simulations. The numerically determined field and temperature dependences were compared with experimental results of a butyloxy-substituted triphenylene dimer. In this glass-forming material the mobility could be measured by a conventional time-of-flight technique in the very large temperature range between 128 and 410K. This dimer is, at room temperature, ordered in, a discotic hexagonal plastic phase with a very high hole mobility of 10⁻²cm²V⁻¹ s⁻¹. We have observed a transition from a non-activated to a thermally activated behaviour of the charge carrier mobility at –20°C, which we assign to a phase transition from the liquid-crystalline phase to the anisotropic glassy state. The temperature dependence in the lowtemperature state is in good agreement with our model with a calculated energy distribution of the hopping sites of 48meV. The field dependence is found to be much stronger than theoretically expected.