Pathways to High-Efficiency GaAs Solar Cells on Low-Cost Substrates

Development of approx. 20% efficient GaAs solar cells on sub-mm grain-size, optical-grade poly-Ge substrates is reported. The techniques developed in the growth of GaAs layers on the various crystalline orientations of a poly-Ge substrate and for the amelioration of the detrimental effects of grain-boundaries in GaAs solar cells are relevant to the development of high-efficiency GaAs solar cellson other low-cost substrates. The development of Ge thin-films on glass for the nucleation and growth of GaAs active layers is described. The minority-carrier lifetimes measured in thin-film GaAs-AlGaAs double-hetero (DH) structures grown on Ge-on-glass templates are rather low in the range of 60 psec. However, molybdenum appears to be a better alternative to Ge-on-glass for the growth of GaAsthin-films with improved minority-carrier lifetimes (approx. 170 psec) in GaAs DH structures on molybdenum. Initial results also suggest that Se doping for the GaAs layers lead to longer lifetimes, consistent with our current understanding of high-performance p+-n GaAs solar cells with Se-doping for the n-base on poly-Ge substrates.