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Numerical calculations have been made of the effect of grain size on the short-circuit current and the AM1 efficiency of polycrystalline thin-film GaAs and InP (2 µm thick) and silicon (25 µm thick) p-n junction solar cells. Junction solar cells are seen to be more efficient than Schottky-barrier cells, due to the higher dark current associated with Schottky diodes. GaAs shows the highest efficiency and both GaAs and InP attain 90 percent of their maximum efficiencies at a grain size of 10 µm, while silicon requires grain sizes of 200 µm to attain 90 percent of maximum efficiency. However, the deleterious effect of poor lifetimes and mobilities is less for silicon polycrystalline cells than for the direct-bandgap devices.