In an electrophotographic experiment, surface voltage is measured for a-Si1-xCx:H films of different thicknesses. It is observed that the thickness normalised dark decay rate (TNDDR) at longer times is smaller for thicker than for thinner specimens. This phenomenon has also been reported for other materials. In order to get a better understanding of the dark discharge process, the monte Carlo technique is used to model electrophotographic dark discharge in materials of which a-Si1-xCx:H is typical. The present study considers every carrier within the model after each increment of a short time step. This then allows bimolecular effects and the presence of space charge to be accounted for.
The study concentrates on two different discharge mechanisms and the effects they have on the TNDDR for varying specimen thicknesses. For the first of these, a negative charge is deposited on the surface of the sample and drifts through the bulk under the influence of the local internal field (Surface Injection model). In the second, electron/hole pairs are generated within the bulk with both carriers being mobile through the specimen (Bulk Generation model). The subsequent surface decay is due to the recombination of bulk generated holes and trapped surface electrons. Results from these models are compared with those found experimentally.