Dependence of open circuit voltage of amorphous silicon solar cells on thickness and doping level of the p-layer

We have focussed on the thickness and the boron-doping concentration of the p-layer of amorphous silicon solar cells and systematically obtained data for open circuit voltage (VOC) and the built-in potential to reveal the mechanism causing a high VOC. A highly doped p-layer gives a higher built in potential in the entire thickness range, but VOC is limited by the carrier recombination caused by the doping-induced defects. A low-doped p-layer causes a higher VOC in a sufficiently thick film because less carrier recombination occurs due to the low-doped p-layer. The light induced defects are not negligible compared with the initial defects in the low-doped p-layer and thus more carrier recombination occurs. Moreover, some of the acceptors are compensated by light-induced defects. The highly doped p-layer, however, does not cause much VOC degradation because the light -induced defects are negligible compared with large number of doping-induced defects and acceptors. The experimental data show that the midgap defects induced by doping or light-soaking near the p/i interface cause the VOC limitation.