THE TRANSITION FROM NONDISPERSIVE TO DISPERSIVE CHARGE-TRANSPORT IN VAPOR-DEPOSITED FILMS OF 1-PHENYL-3-P-D-ETHYLAMINO-STYRYL-5-P-DIETHYLPHENYLPYRAZOLINE (DEASP)

Hole mobilities have been measured for vapor deposited glassy films of DEASP as a function of: (i) temperature encompassing the glass transition temperature; (ii) electric field; and (iii) sample thickness. Time-of-flight photocurrent transients become dispersive below a temperature T(c) at which ln mu versus T-2 plots, mu being the operationally defined mobility, reveal a break. Both the decrease of T(c) with sample thickness and the slope of In mu (T < T(c)) versus T - 2 plots are in quantitative agreement with model predictions for charge transport in materials with energetic disorder yet devoid of polaron effects. The mu(T, E) behavior for T > T(g) is rationalized in terms of a temperature-induced increase in the quasi-static disorder.