Xerographic properties of single- and double-layer photoreceptors based on amorphous selenium-tellurium alloys

Abstract

Single- and double-layer xerographic photoreceptors were fabricated by vacuum deposition of a-Se1-xTex alloy films on aluminium substrates. Double-layer photoreceptors comprised of a thin (a few micrometres) a-Se1-xTex alloy serving as a photogeneration layer (PGL) on a thick (50 to 60 μm) a-Se layer acting as a charge transport layer (TL). Xerographic dark discharge experiments on a-Se films indicated that the decay of the surface potential over the time scale of observation (∼ 500 sec) is essentially due to bulk thermal generation of holes, and their consequent sweep-out and depletion. Hole emission occurs from discrete midgap localized states and it is field-assisted. When a-Se is alloyed with tellurium the dark discharge becomes more rapid due to an increase in both the volume density and energy spread of the midgap hole emission centres with tellurium concentration. In double-layer photoreceptors, with a tellurium content over 3 wt%, although the PGL was only a few micrometres in thickness, the dark discharge was mainly due to the tellurium-rich PGL. The repetition of the xerographic cycle over many cycles leads to the saturation of the surface residual voltage which was used to determine the concentration Nt of deep hole traps. For a-Se, Nt (1.9 x 1013cm−3) was found to be comparable to the concentration of midgap- hole emission centres, which suggested that amphoteric neutral centres (possibly intimate valence alternation pairs) are involved in hole capture and emission. The saturated residual voltage Vr∞ in a-Se1-xTex alloy films increased with the tellurium content up to 6 to 8 wt% Te due to tellurium-introduced deep traps. In double layers, the saturated residual voltage was more than that expected from the properties of monolayers, due probably to the presence of deep hole traps at the PGL-TL interface. Xerographic time-of-flight (XTOF) experiments confirmed the previous results from the electroded TOF experiments that tellurium addition increases the density of shallow traps. Combining the TOF data with the saturated residual voltage measurements gives a hole capture coefficient Ct of ∼ 1.2 x 10−8 cm3 sec−1 for pure a-Se. Chlorine doping of a-Se resulted in a very rapid dark discharge and a fall in the saturated residual voltage.