2.19 eV InGaP solar cells on GaP substrates

We have grown, via molecular beam epitaxy (MBE), the first metamorphic In_0.26Ga_0.74P solar cells with a 2.19 eV direct bandgap on GaP to serve as the top cell in a multi-junction device. Calculations show that the incorporation of a 2.0-2.2 eV top cell into future 4-6 junction cells could enable efficiencies as high as 60%. GaAs_xP_1-x graded buffers enabled a moderate threading dislocation density of 6×10⁶ cm^-2 in the In_0.26Ga_0.74P solar cells. Open circuit voltages (V_oc) as high as 1.42 V were observed under approximate AM1.5G illumination. Little work has been reported on the MBE growth of such high-bandgap In_yGa_1-yP, and we believe that this V_oc can be improved through systematic optimization of growth conditions. Although these devices were not optimized for current collection, we obtain an efficiency of 3.13%, surpassing that of the best GaP solar cells. Finally, as this composition is near the direct-indirect crossover point, we analyzed the low-energy cutoff of the external quantum efficiency spectrum and infer that our In_0.26Ga_0.74P cells are still in the direct regime.