GaAsP solar cells on GaP/Si grown by molecular beam epitaxy

We demonstrate metamorphic 1.73 eV GaAs_0.72P_0.28 solar cells grown by molecular beam epitaxy on high-quality GaP/Si templates and compare them to cells co-grown on bulk GaP. Cascading such a cell with a 1.1 eV Si junction in the substrate could enable a theoretical efficiency of 37% under the AM1.5G spectrum. Electron beam-induced current studies of our cells reveal a threading dislocation density (TDD) of ~1×10⁷ cm^-2 for cells on GaP/Si, which is significantly lower than previous reports. We believe that the combination of a highly optimized GaP/Si starting substrate with a well-designed metamorphic buffer enables these relatively low TDDs. Open-circuit voltages as high as 1.10 V were obtained, leading to a bandgap-voltage offset of 0.63 V. This bandgap-voltage offset is also lower than in previous reports, in qualitative agreement with the observation of lower TDD. Direct comparison with cells on bulk GaP confirm the relation between TDD and bandgap-voltage offset, indicating that more investigations to further reduce TDD in GaAsP single-junction cells are required to fulfill the ultimate goal of dual-junction integration on Si.