Liquid-liquid phase diagrams of binary polymer-solvent and fullerene-solvent mixtures were qualitatively predicted by using the Flory-Huggins (FH) lattice theory as a function of solvent, polymer, and chain length, in which the model system is a low bandgap polymer, poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b']dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT), and the fullerene derivatives [6,6]-phenyl C-61 butyric acid methyl ester (PC61BM) and[6,6]-phenyl C-71 butyric acid methyl ester (PC71BM). Herein, the FH interaction parameters (chi) for each binary system were estimated from the solubility parameter information, originating from the contact angle measurement leading to surface energy via the Newton-Raphson numerical method. Both polymer and fullerene solutions show an upper critical solution temperature (UCST) phase behavior based on the positive chi values, as known, "like dissolves like". However, if there is a crystallizable component in solution, the solid liquid phase equilibria (SLE) are present in addition to the liquid liquid phase equilibria (LLE). Then, based on this solution phase behavior, the phase diagrams of the PCPDTBT:PC61BM and PCPDTBT:PC71BM blends were constructed on the basis of thermal, optical, and morphological analyses, indicating, when the polymer composition is >60 wt %, i.e., the miscibility limit, the fullerene nanocrystals are phase-separated out from the binary polymer-fullerene mixtures. Finally, the glass transition temperature (T-g) elevation with increasing fullerene amounts was adequately described with the Gordon-Taylor and Fox models.