Amorphous silicon solar cells: Thermodynamic models for realistic performance characteristics

Amorphous silicon solar cells exhibit solar conversion efficiencies that are typically a factor of 2 - 5 lower than those of high-quality crystalline semiconductor solar cells. Using theoretical thermodynamic models which are not material specific, we analyse the physical basis for this considerable difference. For amorphous silicon solar cells, a simple thermodynamic model, which includes band-tail recombination as the essential recombination loss mechanism, can accurately predict realistic values for the key solar cell parameters (efficiency, open-circuit voltage, short-circuit current and fill factor), as well as their dependence on solar cell temperature. Our analysis neglects recombination losses due to dangling bond or other deep (midgap) non-band-tail states, and hence represents the limit to performance for a zero density of such states.