Charge dynamics and recombination kinetics in columnar discotic liquid crystals

The time-dependent quasi-one-dimensional transport of electrons or holes along the molecular columns in columnar liquid crystals has been studied. Recent mobility measurements on the tryphenylene-based HAT-n materials have revealed Gaussian transits in the liquid crystalline phases but dispersion in the crystalline phases. In this work the factors governing the photoconductivity are rigorously analyzed using random-walk methods. Strong evidence for one dimensionality is found in the time- and electric-field-dependent photocurrent decays. The field begins to affect the decay for values of field energy per hop which are well below kT and comparable with trap concentration as predicted by theory. The trap concentration in the crystalline phase is governed by structural disorder. The traps however appear "shallow" and the long time current disagrees with the deep trap theory. In the liquid crystalline phase, these traps are annealed out by thermal fluctuations and, as consequence, no direct kinetic proof of one-dimensional kinetics is obtainable. This observation of quasi-one-dimensional transport of charge is in keeping with earlier findings for exciton motion in these materials.