Metamorphic GaInP/GaInAs/Ge solar cells

High-efficiency, metamorphic multijunction cells have been fabricated by growing GaInP/GaInAs subcells that are lattice-mismatched to an active Ge substrate, resulting in GaInP/GaInAs/Ge 3-junction (3J) cells. The efficiency dependence of this 3J cell on lattice-constant of the top two cells and on sublattice ordering in the GaInP top cell is presented. A variety of composition-graded buffers have been explored through X-ray diffraction reciprocal space mapping to measure strain in the cell layers, and transmission electron microscopy to minimize misfit and threading dislocations. Quantum efficiency is measured for metamorphic 1.3-eV Ga_0.92In_0.08As (8%-ln GaInAs) cells and 1.75-eV Ga_0.43In_0.57P cells grown on a Ge substrate, as well as for the 3J cell based on 4%-in GaInAs. Three-junction Ga_0.43In_0.57P/Ga_0.92In _0.08As/Ge cells with 0.50% lattice-mismatch to the Ge substrate are measured to have AMO efficiency of 27.3% (0.1353 W/cm², 28°C), similar to high-efficiency, conventional GaInP/GaAs/Ge 3-junction cells based on the GaAs lattice constant