Photovoltage profiling of hydrogenated amorphous Si solar cells

Abstract

We have used Surface Photovoltage Profiling (SPVP) to study hydrogenated amorphous Si (a-Si:H) p-i-n solar cells grown on glass/conductive-transparent oxide (CTO) or stainless-steel substrates. This technique involves the measurement by a Kelvin Probe of photoinduced changes in the surface potential with respect to a fixed back contact as tha cahh is layers of a-Si:H are consistent with these interface voltages. The junctions of the solar cell are analyzed in terms of their barrier heights (obtained by measuring Δφ at very high illumination levels at low temperature), their depletion widths (obtained from the photovoltage profiling), and their space-charge densities (estimated from published density-of-states curves). The principal reasons for poor cells were found to be: (1) poor i/n junction photovoltages, and (2) the presence of a low- or zero-field region in the i layer, caused primarily by high space-charge densities and correspondingly narrow depletion widths, leading to poor minority carrier current collection and fill factor. Similar low- or zero-field regions can also exist in cells of very good quality (efficiency ∼10%), but their harmful effects can be compensated for by higher diffusion lengths. Such zero-field regions may also play a pivotal role in the optically induced degradation of a-Si:H solar cells; we have found that in an unstable cell prolonged illumination can produce a fivefold increase in space-charge density above that before illumination, with a concommitant decrease in p/i junction depeletion width.