A detailed theoretical analysis of an n−on−p graded band−gap AlxGa1−xAs−GaAs solar cell yields a maximum air mass zero power conversion efficiency of 17% compared to 9% for a similar GaAs cell. The analaysis includes surface and bulk minority carrier recombination, junction recombination current, spectrally varying surface reflection, and series resistance loss. The maximum efficiency is determined for a surface recombination velocity of 1×105 cm/sec and hole and electron diffusion lengths of 2.1 and 7.6 μm, respectively. The improved efficiency is primarily due to a built−in electric field, caused by the band−gap gradation, accelerating photogenerated holes toward the p−n junction. This field reduces the surface and bulk recombination of the holes, and thereby enhances their collection.