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The most promising device structure for organic photovoltaic devices presented to date is the ‘‘bulk-heterojunction’’whereby a hole-conducting (electron-donating) conjugated polymer, such as poly (3-hexylthiophene)(P3HT), is blended with an electron-conducting (electron-accepting) smallmolecular compound, such as a fullerene derivative. The reported strong composition-and thermal-treatment dependence of the power conversion efficiency of such binaries suggests that phase behavior, processing conditions and the resulting microstructure play a dominant role in the performance of devices based on these systems. Here, we propose a simple rationale for selecting the optimum composition of such crystalline/crystalline polymer/small molecule blends. We find that these binary systems feature simple eutectic phase behavior, and that the optimum composition for device performance is slightly hypoeutectic …