Hopping in Exponential Band Tails

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Abstract
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.