The role of solvent and morphology on miscibility of methanofullerene and poly(3-hexylthiophene)

► We examine miscibility of P3HT with a fullerene molecule PCBM. ► Amorphous polymer morphology enhances interdiffusion. ► Annealed bilayer is subjected to large phase-separation if PCBM is cast on dry P3HT. ► Phase-separated PCBM clusters extend throughout the bilayer film.