Monolithic two-terminal GaAs/Ge tandem space concentrator cells

GaAs/Ge two-junction tandems up to 23.4% efficient at 9 AM0 suns were made. This efficiency is a record for a monolithic, two-terminal GaAs/Ge tandem cell, and also exceeds the best efficiency reported (23%) for a single-junction GaAs concentrator at 10 AM0 suns. A GaAs top cell was epitaxially grown on GaAs and Ge wafers simultaneously; the GaAs growth forms a bottom cell in the Ge to make a two-junction tandem cell, while the GaAs top cell on the GaAs substrate is a single-junction experimental control. The best GaAs/Ge tandem was three percentage points more efficient than the best control GaAs/GaAs concentrator; the difference is due to the active Ge bottom cell.<
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