Metamorphic GaAsP and InGaP Solar Cells on GaAs

We have investigated wide-bandgap, metamorphic GaAs _1- x P x and In y Ga _1- y P solar cells on GaAs as potential subcell materials for future 4-6 junction devices. We identified and characterized morphological defects in tensile GaAs _1- x P x graded buffers that lead to a local reduction in carrier collection and a global increase in threading dislocation density (TDD). Through adjustments to the graded buffer structure, we minimized the formation of morphological defects and, hence, obtained TDDs ≈ 10 ⁶ cm ^-2 for films with lattice mismatch ≤1.2%. Metamorphic In y Ga _1- y P solar cells were grown on these optimized GaAs _1- x P x graded buffers with bandgaps ( Eg ) as high as 2.07 eV and open-circuit voltages ( Voc ) as large as 1.49 V. Such high bandgap materials will be necessary to serve as the top subcell in future 4-6 junction devices. We have also shown that the relaxed GaAs _1- x P x itself could act as an efficient lower subcell in a multijunction device. GaAs _0.66 P _0.34 single-junction solar cells with Eg = 1.83 eV were fabricated with V _oc = 1.28 V. Taken together, we have demonstrated that GaAs _1- x P x graded buffers are an appropriate platform for low-TDD, metamorphic GaAs _1- x P x and In y Ga _1- y P solar cells, covering a wide bandgap range.