Low bandgap polymer (LBG): fullerene mixtures are some of the most promising organic photovoltaic active layers. Unfortunately, there are no post-deposition treatments available to rationally improve the morphology and performance of as-cast LBG: fullerene OPV active layers, where thermal annealing usually fails. Therefore, there is a glaring need to develop post-deposition methods to guide the morphology of LBG: fullerene bulk heterojunctions towards targeted structures and performance. In this paper, the structural evolution of PCPDTBT:PCBM mixtures with solvent annealing (SA) is examined, focusing on the effect of solvent quality of the fullerene and polymer in the annealing vapor on morphological evolution and device performance. The results indicate that exposure of this active layer to the solvent vapor controls the ordering of PCPDTBT and PCBM phase separation very effectively, presumably by inducing component mobility as the solvent plasticizes the mixture. These results also unexpectedly indicate that solvent annealing in a selective solvent provides a method to invert the morphology of the LBG: fullerene mixture from a polymer aggregate dispersed in a polymer: fullerene matrix to fullerene aggregates dispersed in a polymer: fullerene matrix. The judicious choice of solvent vapor, therefore, provides a unique method to exquisitely control and optimize the morphology of LBG conjugated polymer/fullerene mixtures.