Dependence of Open Circuit Voltage of Amorphous Silicon Solar Cells on Thickness and Doping Level of the p-Layer

Abstract

We have focused 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 lower density of the doping-induced defects. After light-soaking, significant Voc degradation occurs with 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 the 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.