Charge carrier transport in layers of discotic liquid crystals as studied by transient photocurrents

Transient photocurrent measurements were performed for a series of layers of discotic liquid crystals and of perylene derivative prepared in different ways with an aim to characterize the charge carrier transport in configurations corresponding to photovoltaic devices and field effect transistors. The course of the transient photocurrent was found to be strongly dependent on the layer preparation procedure: the non-dispersive transport, and well defined transit times, were found only for “squeezed” sandwich-type sample of triphenylene derivative in which homeotropic orientation of the discotic columns was induced assuring formation of continuous paths connecting the opposite electrodes. In all other cases dispersive transport of charge carriers was observed, due to the spatial and/or energy disorder of hopping states and fast trapping by localized states. From the asymmetry of the transient photocurrents for different polarity of the illuminated electrode it was concluded, that holes are more mobile in the hexabenzocoronene derivatives and in the triphenylene derivative, while electron transport dominates in the perylene derivative.