IMPROVED ANALYSIS OF THE CONSTANT PHOTOCURRENT METHOD

A numerical model has been developed to simulate constant photocurrent method spectra. It takes into account the position of the Fermi energy and the full set of optical transitions between localized and extended states under subbandgap illumination, capture, emission and recombination processes. The comparison of simulated and measured spectra yields information about the density of localized gap states in amorphous silicon, that is the valence band tail, the integrated defect density, the defect distribution in energy and the charge state of the defect states. For n- and p-type hydrogenated amorphous silicon (a-Si:H) we achieved good agreement between simulation and experimental data. In the annealed state the defect absorption is dominated by a single defect peak which can be attributed to D- states in n-type material and D+ states in p-type a-Si:H. In undoped a-Si:H we observed more charged than neutral defect states, confirming the predictions of the defect-pool model. Furthermore, the simulations reveal that the defect chemical potential depends on the Fermi level as postulated by the defect-pool model. In the light-soaked state, undoped material shows an enhanced defect density. We observe an increase in the density of both neutral and charged defect states. The charged-to-neutral defect ratio does not change upon light soaking.