DISPERSIVE ASPECTS OF THE HIGH-FIELD HOPPING MOBILITY OF MOLECULARLY DOPED SOLIDS WITH DIPOLAR DISORDER

The time-of-flight mobility of photoinjected charges in molecularly do ped polymers obeys a Poole-Frenkel law, mu proportional to exp(gamma r oot E), which is commonly viewed as arising from hopping transport amo ng sites with a large degree of energetic disorder. Recent theoretical investigations have focused on long-range correlations that character ize site energies when the dominant mechanism for energetic fluctuatio ns is the interaction of charge carriers with randomly-oriented perman ent dipoles of the dopant and host polymer. An exact calculation of th e steady-state drift velocity v(d) for a one-dimensional system with c orrelated dipolar disorder predicts a Poole-Frenkel law similar to tha t observed. In order to investigate another feature commonly observed in the high-field measurements, namely, the anomalous dispersion of th e current-time transients, we have performed an exact calculation of t he field-dependent diffusion constant D for the same dipolar disorder model. In the bulk limit we obtain an expression D = (KT/e)partial der ivative v(d)/partial derivative E that generalizes the normal Einstein relation and predicts a strongly field-dependent diffusion constant. (C) 1997 John Wiley & Sons, Inc.