Metamorphic GaAsP and InGaP Solar Cells on GaAs

We have investigated wide-bandgap, metamorphic GaAs1-xPxand 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-xPxgraded 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 ≈ 106cm-2for films with lattice mismatch ≤1.2%. Metamorphic InyGa1-yP solar cells were grown on these optimized GaAs1-xPxgraded 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 GaAs1-xPxitself could act as an efficient lower subcell in a multijunction device. GaAs0.66P0.34single-junction solar cells withEg= 1.83 eV were fabricated withVoc= 1.28 V. Taken together, we have demonstrated that GaAs1-xPxgraded buffers are an appropriate platform for low-TDD, metamorphic GaAs1-xPxand InyGa1-yP solar cells, covering a wide bandgap range.