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 ob served in the different temperature and phase regions of the liquid-cr ystalline (LC) photoconductor hexapentyloxytriphenylene (HPT). Transie nt photocurrents and carrier mobilities for various temperatures, elec tric fields, and sample histories were examined. The ideal intrinsic G aussian transport, observed for holes in the mesophase, puts HPT into a new class of high mobility materials with both hole mobilities on th e order of 1.10(-3)cm(2)/Vs and a steplike current decay. These featur es result from the fact that there is obviously neither a positional d isorder nor an energetic disorder, which would impair efficient charge transport. This is probably achieved by special molecular packing mec hanisms in combination with the dynamical processes of the LC phase an d due to favourable properties of the HPT molecule, which has a rigid and conjugated core, a symmetric structure, and no permanent dipole mo ment.