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Polymer solar cells are a low-cost alternative to inorganic photovoltaics, but reaching commercially viable efficiencies is heavily dependent on understanding and control of the polymer blend morphology in the devices, no matter which polymer is used. On page 499 of this issue, Nichole Cates Miller and colleagues construct the eutectic phase diagram of blends of the polymer pBTTT and fullerene derivative PC71BM. The phase diagram explains why solar cells with 75-80 wt % fullerene perform best, since these blends have both electron- and hole-conducting phases at room temperature, ensuring clear charge-conduction pathways to the electrodes. In addition, the authors also show how to suppress intercalation of the fullerene between the polymer side chains using rapid thermal annealing. The cover is an optical microscopy image of the phase separation in a 1:1 pBTTT:PC71BM blend that has been annealed above the eutectic temperature.