We report single crystal InP-CdS solar cells having AM2 efficiencies of 15 percent and polycrystalline thin-film cells having AM2 efficiencies of 5.7 percent. Basic studies of the interface reveal that the thin-film efficiency is presently limited at least in part by the quality of the InP within the grains, and not exclusively by interface phenomena intrinsic to a polycrystalline cell. Preliminary accelerated life tests indicate a remarkable insensitivity of these unencapsulated cells to the ambient.