GaAs solar cells grown on intentionally contaminated GaAs substrates

III-V solar cells have the best photovoltaic performance of any semiconductor materials, but deposition of these materials relies on high-quality single crystal GaAs wafers for their superior performance. The cost of a 6″ GaAs substrate approaches $100 at mid-to high-volume, making high-efficiency devices grown on these wafers only relevant in high-value, niche markets. We explored the effect of growing bulk GaAs crystal boules using the vertical-gradient freeze method with simulated lower purity input materials in order to reduce their cost. [1] We observe that intentional impurities added to the melt before the crystal growth occurs segregate to the top of the boule during crystal growth. This results in very low levels of contaminants incorporating in the majority of the boule. We then diced the contaminated boule into individual GaAs substrates and polished them to an epi-ready finish. Single junction GaAs solar cells grown on these contaminated wafers showed no performance degradation compared to growth on a high-purity control wafer (Figure 1). Both sample sets showed open circuit voltages of ∼ 1.09 V and efficiencies ∼ 27% under one sun conditions. Details of this experiment can be found in Ref 1. Current density vs voltage measurements in the dark (Figure 2) revealed no increase in J O2 recombination due to these added impurities, even at very low injection levels. These results suggest that lower purity Ga and As source materials can be used during crystal growth to reduce the cost of wafers.