Investigation of polycrystalline Ga_xIn_1-xP for potential use as a solar cell absorber with tunable bandgap

There is ongoing interest in developing a stable, low-cost, 1.6-1.8 eV top-cell material that can be used for two-junction (tandem) solar cells, particularly in combination with a silicon bottom cell. In this work, polycrystalline GaInP is grown and characterized to explore its properties and use for this purpose. The film composition and deposition temperature are varied to determine their effects on grain size, morphology, and photoluminescence (PL) over a range of bandgaps from 1.35 to 1.7 eV. An Al-assisted post-deposition treatment for 1.7-eV polycrystalline GaInP results in a 90-fold increase in peak photoluminescence (PL) intensity, a 220-fold increase in integrated PL intensity, and increased time-resolved PL lifetime from <2 ns to 44 ns. The increase in PL intensity and lifetime is attributed to a reduction of nonradiative minority-carrier recombination at the top surface, and at grain boundaries near the surface, due to the formation of a higher-bandgap AlGaInP alloy. These materials provide a viable path toward increased minority-carrier concentration under illumination and improved recombination properties needed for high-efficiency tandem solar cells.