Metamorphic GaAsP and InGaP Solar Cells on GaAs

We have investigated wide-bandgap, metamorphic GaAs1-xPx and InyGa1-yP solar cells on GaAs as potential subcell materials for future 4-6 junction devices. We identified and characterized morphological defects in tensile GaAs1-xPx 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 approximate to 10(6) cm(-2) for films with lattice mismatch <= 1.2%. Metamorphic InyGa1-yP solar cells were grown on these optimized GaAs1-xPx graded buffers with bandgaps (E-g) as high as 2.07 eV and open-circuit voltages (V-oc) 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 GaAs1-xPx itself could act as an efficient lower subcell in a multijunction device. GaAs0.66P0.34 single-junction solar cells with E-g = 1.83 eV were fabricated with V-oc = 1.28 V. Taken together, we have demonstrated that GaAs1-xPx graded buffers are an appropriate platform for low-TDD, metamorphic GaAs1-xPx and InyGa1-yP solar cells, covering a wide bandgap range.