A simple rationale for selecting the optimum composition of binary organic photovoltaic crystalline/crystalline polymer/small molecule blends was proposed. A unique relationship between the composition dependence of photocurrent generation and microstructure, controlled by the phase behavior of the two crystalline components, was also demonstrated. The rate of solidification is found to be critical for the P3HT/PC61BM binary when processed both from solution and the melt. The devices display a maximum short circuit photocurrent, which is maximized for blend films after thermal treatment at 140°C for 45 min. The binary systems are also found to be of simple eutectic in nature, featuring different eutectic compositions and temperatures. Optical measurements of dielectric function indicate that the effect of blend composition on the Columbic binding energy for a polaron pair in an effective medium is small.