We report the phase behaviour of polymer/fullerene/solvent ternary mixtures and its consequence for the
morphology of the resulting composite thin films. We focus particularly on solutions of polystyrene (PS), C60
fullerene and toluene, which are examined by static and dynamic light scattering, and films obtained from
various solution ages and thermal annealing conditions, using atomic force and light microscopy.
Unexpectedly, the solution phase behaviour below the polymer overlap concentration, c*, is found to be
described by a simple excluded volume argument (occupied by the polymer chains) and the neat
C60/solvent miscibility. Scaling consistent with full exclusion is found when the miscibility of the fullerene
in the solvent is much lower than that of the polymer, giving way to partial exclusion with more soluble
fullerenes (phenyl-C61-butyric acid methyl ester, PCBM) and a less asymmetric solvent (chlorobenzene),
employed in photovoltaic devices. Spun cast and drop cast films were prepared from PS/C60/toluene
solutions across the phase diagram to yield an identical PS/C60 composition and film thickness, resulting
in qualitatively different morphologies in agreement with our measured solution phase boundaries. Our
findings are relevant to the solution processing of polymer/fullerene composites (including organic
photovoltaic devices), which generally require effective solubilisation of fullerene derivatives and polymer
pairs in this concentration range, and the design of well-defined thin film morphologies.