ANOMALOUS TIME-INDEPENDENT DIFFUSION OF CHARGE-CARRIERS IN A RANDOM POTENTIAL UNDER A BIAS FIELD

We present Monte Carlo simulation results for time-independent drift and spatial spreading of a packet of charge carriers executing a random walk within a manifold of hopping sites under a bias field. The shape of the density-of-states profile is assumed to be Gaussian (width sigma). The ratio eD/mu-kT, equal to unity for ordinary diffusion, is found to increase dramatically with increasing disorder and bias field. For sigma/kT = 3.5 and fields of order 10(6) V cm-1, eD/mu-kT reaches values of order 10(3). The effect accounts for the anomalous spreading of time-of-flight profiles often seen in photocarrier transit time studies on various polymeric systems. The role of disorder in charge-carrier transport is emphasized.