Hopping in Exponential Band Tails

Hopping of photoexcited carriers directly between localized, band-tail states gives rise to a new regime of energy relaxation which is manifested at low temperatures: For times shorter than a characteristic "segregation time," carriers hop directly to lower-energy states; at longer times, thermal excitation---resulting in multiple trapping---is more important. Even in the multiple-trapping regime, however, the current is carried at a transport energy within the band tail. We present a simple description which includes both of these processes in a single, self-consistent model, and estimate the segregation time.