Electronic and ionic carrier transport in discotic liquid crystalline photoconductor

We have investigated the negative charge carrier transport in discotic columnar phases of triphenylene derivatives, 2,3,6,7,10,11-hexapentyloxytriphenylene (H5T) and its related derivatives such as H4T and H6T, by time-of-flight experiments. We observed a fast transit for negative charge carriers in the discotic hexagonal columnar phase (D-h) of H5T, which corresponded to a high mobility of 10(-3) cm(2) V-1 s(-1) comparable to its hole mobility, in addition to a slow transit corresponding to the mobility of 10(-5) cm(2) V-1 s(-1) reported previously. We observed two transits in H4T and H6T as well, in which the fast mobility was on the order of 10(-2) and 10(-4) cm(2) V-1 s(-1) in H4T and H6T, respectively, while the slow mobility was almost the same (3-4x10(-5) cm(2) V-1 s(-1)) in both H4T and H6T. Through dilution experiments with n-dodecane, we found that these fast and slow transits were attributable to the electronic and ionic conduction, respectively, and that the latter was caused by negatively ionized impurity molecules trapping photogenerated electrons. We discuss that there exist two distinct carrier transport channels for electronic and ionic charges in the intrinsic nature of discotic columnar phases.