High-efficiency Ga0.75In0.25As/GaAs concentrator solar cells (Eg=1.15 eV, area=0.32 cm2) have been fabricated and tested at 21.4% efficiency under 380 AM2.4 sun. The cell performance parameters of open-circuit voltage, fill factor, and efficiency are presented as functions of concentration up to 987 sun. The spectral response, and current-voltage characteristics under concentration, of cells grown by organometallic vapor phase epitaxy on graded and ungraded lattice-constant buffer layers are compared. It is shown that a graded lattice-constant buffer layer is necessary for high-efficiency performance. In general, Ga0.75In0.25As cells have higher efficiencies and open circuit voltages than Si cells (Eg=1.11 eV) for concentrations in excess of about 200 sun.

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