Energy levels and charge state of intrinsic defects in amorphous selenium

Analysis of transient photoconductivity (TPC) measurements in terms of multiple-trapping transport kinetics provides strong evidence for the presence of two sets of ‘shallow’ traps. One set of electrically neutral hole and electron traps lies ∼0.17 eV and ∼0.28 eV from their respective band edges and has attempt-to-escape frequencies of the order of 109–1010 Hz, while a second set of charged states is located 0.43 eV, respectively 0.50 eV into the gap (all values ±0.02 eV) with attempt frequencies above 1011 Hz. Standard TPC on a-Se gap cells reveals the hole traps on the valence band side of the gap, but time-of-flight experiments on sandwich cells are required for the electron traps. The large value of their attempt frequency confirms the charged nature of the deeper traps.