Polymer/small molecule binaries, such as P3HT:PC61BM display eutectic phase behavior with optimum photovoltaic performance at slightly hypoeutectic compositions (with respect to polymer content). Such hypoeutectic blends readily solidify into a finely phase-separated matrix surrounding primary crystals of the small-molecular species. The resulting large interfacial area and component connectivity is a desirable microstructure for bulk-heterojunction solar cells.