ANOMALOUS TRANSITION IN CHARGE-TRANSPORT BEHAVIOR OF POLYSILANE

Anomalous transition of the dis pers ion parameter alpha was observed for the first time in poly(n-butyl-n-pentylsilane) (PBPS). Time-of-fli ght photocurrent (I) of PBPS was measured. The photocurrent transient was dispersive: I proportional to t(-(1-alpha)). The dispersion parame ter alpha dropped around the glass transition temperature T-g with inc reasing temperature, and became temperature-independent above T-g. The mobility was described within the framework of a formalism based on d isorder due to Bassler and coworkers. Both the diagonal disorder param eter sigma, the variance of the density of states, and the off-diagona l disorder parameter Sigma, the degree of positional disorder, decreas ed around T-g. Nevertheless the ratio of the off-diagonal disorder par ameter Sigma to the diagonal disorder parameter sigma was found to inc rease above the transition temperature. This anomalous transition is r ationalized by the assumption that the off-diagonal disorder dominates charge transport in PBPS above T-g.