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The phase diagrams for three conjugated polymer/fullerene blends of interest for polymer solar cells, namely semicrystalline poly(3-hexylthiophene) (P3HT):methanofullerene 6,6-phenyl C61-butyric acid methyl ester (PCBM), poly(2-methoxy-5-(3′,7′-dimethyloctyloxy)-p-phenylenevinylene) (MDMO-PPV):PCBM, and poly(2-methoxy-5-(2′-ethylhexyloxy)-p-phenylenevinylene) (MEH-PPV):PCBM, have been constructed based on X-ray scattering data and differential scanning calorimetery (DSC). Both melting point depression and glass transition temperature elevation were observed in the P3HT:PCBM blends as a function of increasing PCBM wt %. The PCBM solubility limit, i.e., the phase-separation point, was determined to be 30, 40, and 50 wt % PCBM for P3HT:PCBM, MDMO-PPV:PCBM, and MEH-PPV:PCBM mixtures, respectively. The phase behavior of the blends is directly correlated with electrical transport behavior determined by measuring field effect conduction in a transistor testbed. Specifically, below the solubility limit for PCBM in all three blends, only hole transport was observed, and above the solubility limit both hole and electron conduction were measured.
Kim et al. (Thu,) studied this question.