DISCOTIC LIQUID-CRYSTALS - A NEW CLASS OF FAST PHOTOCONDUCTORS

We showed for the first time that discotic liquid crystals are well su ited for a new class of fast photoconducting materials. Due to their s pontaneous orientation and their dynamical fluctuations in the mesopha se, they show exceptionally high mobilities of 1.10(-3) cm2/Vs, about two to three orders of magnitude higher than those obtained for conven tional amorphous polymers. Further on, the Gaussian transport (for hol es in the mesophase) is remarkable, which is characterized by the exis tence of a conduction band and the absence of trapping states. In cont rast, the charge carrier transport in amorphous photoconductors is gen erally trap-dominated which limits technical properties, leading to lo w carrier mobilities and undesirable memory effects. Measurements of t he transient photocurrents, using the time-of-flight technique, have b een carried out in the discotic liquid crystalline photoconductor hexa pentyloxytriphenylene. The carrier mobilities and the underlying trans port mechanisms both in the crystalline phase and in the mesophase are discussed in the framework of ideal intrinsic (Gaussian) transport, m ultiple shallow trapping, and deep trapping transport, respectively.