Abstract
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.