Efficient GaAs/Ge/Si solar cells

Epitaxial GaAs layers with a reduced dislocation density have been grown on Ge-coated Si substrates by using a new technique involving multiple growth interrupts and thermal cycles. The open-circuit voltage of shallow-homojunction solar cells fabricated in these GaAs layers was found to increase with the number of interrupts and thermal cycles. Small-area cells with conversion efficiencies up to 14% (AM1) have been obtained. In addition, monolithic tandem cells composed of a GaAs top cell and a Si bottom cell that are connected by a thin epitaxial Ge layer have been fabricated.