DISCOTIC LIQUID-CRYSTALS - A NEW CLASS OF FAST PHOTOCONDUCTORS

We showed for the first time that discotic liquid crystals are well suited for a new class of fast photoconducting materials. Due to their spontaneous orientation and their dynamical fluctuations in the mesophase, they show exceptionally high mobilities of 1.10(-3) cm2/Vs, about two to three orders of magnitude higher than those obtained for conventional amorphous polymers. Further on, the Gaussian transport (for holes in the mesophase) is remarkable, which is characterized by the existence of a conduction band and the absence of trapping states. In contrast, the charge carrier transport in amorphous photoconductors is generally trap-dominated which limits technical properties, leading to low carrier mobilities and undesirable memory effects. Measurements of the transient photocurrents, using the time-of-flight technique, have been carried out in the discotic liquid crystalline photoconductor hexapentyloxytriphenylene. The carrier mobilities and the underlying transport mechanisms both in the crystalline phase and in the mesophase are discussed in the framework of ideal intrinsic (Gaussian) transport, multiple shallow trapping, and deep trapping transport, respectively.