Metamorphic InGaP on GaAs and GaP for wide-bandgap photovoltaic junctions

Metamorphic triple-junction solar cells can currently attain efficiencies as high as 41.1%. Using additional junctions could lead to efficiencies above 50%, but would require the development of a wide-bandgap (2.0-2.2 eV) material to act as the top layer. In this work we demonstrate wide-bandgap In _yGa_1-yP single junction solar cells grown on GaAs _xP_1-x via solid source molecular beam epitaxy. Tensile GaAs_xP_1-x buffers grown on GaAs exhibited asymmetric strain relaxation along with formation of faceted trenches, 100-300 nm deep running parallel to the [0-11] direction. Using smaller grading steps and higher substrate temperatures we minimized the faceted trench density and achieved symmetric strain relaxation. In comparison, compressively-strained graded GaAs_xP_1-x buffers on GaP showed nearly-complete strain relaxation of the top layers and no evidence of trenches. We subsequently grew In_yGa_1-yP solar cells on the GaAs_xP _1-x buffers. Photoluminescence and transmission electron microscopy measurements gave no indication of CuPt ordering. Finally, we fabricated wide-bandgap In_yGa_1-yP solar cells and obtained V _oc as high as 1.42 V for In_0.39Ga_0.61P with E_g =2.0 eV. Preliminary device results indicate that MBE-grown In_yGa_1-yP layers are promising candidates for future use as the top junction of a multijunction solar cell.