Polymer: fullerene blends have been widely studied as an inexpensive alternative to traditional silicon solar cells. Some polymer: fullerene blends, such as blends of poly(2,5-bis( 3-tetradecylthiophen-2-yl)thieno[3,2-b] thiophene (pBTTT) with phenyl-c71-butyric acid methyl ester (PC71BM), form bimolecular crystals due to fullerene intercalation between the polymer side chains. Here we present the determination of the eutectic pBTTT: PC71BM phase diagram using differential scanning calorimetry (DSC) and two-dimensional grazing incidence X-ray scattering (2D GIXS) with in-situ thermal annealing. The phase diagram explains why the most efficient pBTTT: PC71BM solar cells have 75-80 wt % PC71BM since these blends lie in the center of the only room-temperature phase region containing both electron-conducting (PC71BM) and hole-conducting (bimolecular crystal) phases. We show that intercalation can be suppressed in 50: 50 pBTTT: PC71BM blends by using rapid thermal annealing to heat the blends above the eutectic temperature, which forces PC71BM out of the bimolecular crystal, followed by quick cooling to kinetically trap the pure PC71BM phase. (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 49: 499-503, 2011