Additives are known to improve the performance of organic photovoltaic devices based on mixtures of 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 [6,6]-phenyl C61-butyric acid methyl ester (PCBM). The evolution of the morphology during the evaporation of the mixed solvent, which comprises additive and chlorobenzene (CB), is investigated by in-situ grazing incidence X-ray scattering, providing insight into the key role the additive plays in developing a multi-length-scale morphology. Provided the additive has a higher vapor pressure and a selective solubility for PCBM, as the host solvent (CB) evaporates, the mixture of the primary solvent and additive becomes less favorable for the PCPDTBT, while completely solubilizing the PCBM. During this process, the PCPDTBT first crystallizes into fibrils and then the PCBM, along with the remaining PCPDTBT, is deposited, forming a phase-separated morphology comprising domains of pure, crystalline PCPDTBT fibrils and another domain that is a PCBM-rich mixture with amorphous PCPDTBT. X-ray/neutron scattering and diffraction methods, in combination with UV–vis absorption spectroscopy and transmission electron microscopy, are used to determine the crystallinity and phase separation of the resultant PCPDTBT/PCBM thin films processed with or without additives. Additional thermal annealing is carried out and found to change the packing of the PCPDTBT. The two factors, degree of crystallinity and degree of phase separation, control the multi-length-scale morphology of the thin films and significantly influence device performance.