Low-mobility solar cells: A device physics primer with application to amorphous silicon

The properties of pin solar cells based on photogeneration of charge carriers into low-mobility materials were calculated for two models. Ideal p- and n-type electrode layers were assumed in both cases. The first, elementary case involves only band mobilities and direct electron-hole recombination. An analytical approximation indicates that the power in thick cells rises as the 1/4 power of the lower band mobility, which reflects the buildup of space-charge under illumination. The approximation agrees well with computer simulation. The second model includes exponential bandtail trapping, which is commonly invoked to account for very low hole drift mobilities in amorphous silicon and other amorphous semiconductors. The two models have similar qualitative behavior. Predictions for the solar conversion efficiency of amorphous silicon-based cells that are limited by valence bandtail trapping are presented. The predictions account adequately for the efficiencies of present a-Si:H cells in their "as-prepared" state (without light-soaking), and indicate the improvement that may be expected if hole drift mobilities (and valence bandtail widths) can be improved.