Defect levels and charge carrier photogeneration in amorphous selenium layers

Time-of-flight post-transit data that suggested positions in the band gap at ∼ E _v+0.4 eV and ∼ E _c −0.55 eV for the thermally accessible levels of the intrinsic negative-U centers of a-Se have been re-examined. The introduction into the Onsager formalism of a distribution of initial carrier separations in the photogeneration process, together with random internal field fluctuations, and of the ensuing processes of bimolecular recombination and deep trapping lead to a good description of experimentally observed photogeneration efficiency in a-Se.