Steady-state and transient photoconductivity methods are used to investigate the electronic density of states in evaporated layers of amorphous selenium. From the temperature dependence of the steady-state photocurrent and, independently, from an analysis of the post-transit currents of time-of-flight transients, energy levels in the gap at 1.43 ± 0.02 eV and 0.40 ± 0.02 eV above the valence band have been determined for the occupied state of the negative-U centres. An absorption band around 1.50 eV is seen in the spectral distribution of the photocurrent. The distribution of tail states may-to first approximationbe described by a steep exponential with a characteristic width of ∼24 meV at the valence band and a more steeply declining functional of similar width at the conduction band.

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Steady-state photoconductivity measurements are carried out for bulk and thin-film amorphous selenium (a-Se) samples in the temperature range between 190 and 340 K. The temperature and light-intensity dependences of the photoconductivity reveal the presence of both mono-and bimolecular recombination regimes. The current activation energies measured in the two regions point to energy levels in the gap for the recombination centres at 0.36 ± 0.06 and 1.35 ± 0.10 eV above the valence band mobility edge. These values put a-Se in line with the other chalcogenide semiconducting glasses that exhibit negative-U behaviour.

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