PHOTOCONDUCTIVITY IN DISCOTIC LIQUID-CRYSTALS - A NEW CLASS OF HIGH-MOBILITY MATERIALS

Using a time-of-flight technique, different transport mechanisms, deep trapping, multiple shallow trapping and Gaussian transport, can be observed in the different temperature and phase regions of the liquid-crystalline (LC) photoconductor hexapentyloxytriphenylene (HPT). Transient photocurrents and carrier mobilities for various temperatures, electric fields, and sample histories were examined. The ideal intrinsic Gaussian transport, observed for holes in the mesophase, puts HPT into a new class of high mobility materials with both hole mobilities on the order of 1.10(-3)cm(2)/Vs and a steplike current decay. These features result from the fact that there is obviously neither a positional disorder nor an energetic disorder, which would impair efficient charge transport. This is probably achieved by special molecular packing mechanisms in combination with the dynamical processes of the LC phase and due to favourable properties of the HPT molecule, which has a rigid and conjugated core, a symmetric structure, and no permanent dipole moment.