High efficiency GaAs-on-Si solar cells with high V/sub oc/ using graded GeSi buffers

Single junction AlGaAs/GaAs and InGaP/GaAs solar cells and test structures have been grown by molecular beam epitaxy (MBE) and metalorganic chemical vapor deposition (MOCVD), respectively, on Si wafers coated with compositionally-graded GeSi buffers. The combination of controlled strain relaxation within the GeSi buffer and monolayer-scale control of the Ill-V layer nucleation is shown to reproducibly generate minority carrier lifetimes exceeding 10 nanoseconds within GaAs overlayers. The III-V layers are free of long-range antiphase domain disorder, with threading dislocation densities in the high-10/sup 5/ cm/sup -2/ range, consistent with the low residual dislocation density in the Ge cap of the graded buffer structure. Single junction GaAs cells grown by both MBE and MOCVD on the Ge/GeSi/Si substrates demonstrated high V/sub oc/ values for GaAs cells grown on Si. Record V/sub oc/ values for MOCVD-grown single junction InGaP/GaAs cells exceeded 980 mV (AMO) with fill factors of 0.79. Additionally, external quantum efficiency data indicates no degradation in carrier collection from GaAs homoepitaxial cells for current single-junction cell designs grown by MBE. Based on these results, cell efficiencies in excess of 18.5% under AM0 conditions should be attainable with cell designs demonstrating state of the art J/sub sc/ values. Such cell performance demonstrates the potential and viability of graded GeSi buffers for the development of Ill-V cells on Si wafers.