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General design considerations for making optimal use of new photovoltaic materials

The search for ever higher conversion efficiencies for photovoltaic devices is leading to the investigation of a number of novel materials combinations for the realization of multi-junction solar cells. We have used the detailed balance approach to analyze the optimum use of band gaps in a multi-junction device of up to 6 sub-cells. The results suggest that as the number of sub-cells increases the importance of the bottom most band gap becomes less critical, assuming the optimum band gap combination for that value can be obtained. Given this greater freedom in choice, the potential for the use of silicon as an active substrate is investigated along with a cell thinning `current sharing' approach to improve current mismatch in the device. Results show a more robust design space of multi-junctions with active silicon substrates when the current sharing approach is used, with performances close to the optimum for a completely free choice of band gaps.