Defect levels in the band gap of amorphous selenium

Energy locations in the band gap have been determined for the thermally accessible levels of the negative-U defect centers in amorphous selenium. Both the temperature dependence of the steady-state photocurrents, and an analysis of emission currents in the post-transit regime of a time-of-flight transient photoconductivity experiment on the same samples, agree on the presence of defect levels at (0.42 +/- 0.04) eV above the valence band mobility edge and (0.53 +/- 0.06) eV below the conduction band. Both measured current levels and the resolved energy positions of the defects are subject to the Poole-Frenkel effect. (C) 2004 Elsevier B.V. All rights reserved.