An optimal nanoscale phase separation between the donor (generally, a conjugated polymer) and the acceptor (generally, a fullerene derivative) materials is one of the major requirements for obtaining high efficiency organic photovoltaic (OPV) device. Recent methods of controlling such nanostructure morphology in a bulkheterojunction (BHJ) OPV device involve addition of a small amount of solvent additive to the donor and acceptor solutions. The idea is to retain the acceptor materials into the solution for a longer period of time during the film solidification process, thus allowing the donor material to crystallize earlier. The ultimate morphology resulting from the solvent casting process of such multicomponent active layers involves a complex interplay of interactions between polymer/solvent, polymer/additive, fullerene/solvent, fullerene/additive, polymer/fullerene, and solvent/additive. In addition, multiple kinetic processes occur including solvent evaporation, phase separation, as well as polymer crystallization that lead to the final morphology of the active layer. Disentangling these different contributions is the key for optimization of the active layer morphology, and has been a primary emphasis of this dissertation. Accordingly, the major focus of this dissertation is twofold: to understand the parameters and interactions of solvent additives that govern the morphology evolution process of different low band-gap polymer/fullerene systems, as well as developing a laboratory-scale slot-die coating methodology, which not only mimics the large area roll-to-roll device fabrication process, but also plays an integral part on investigating the morphology evolution process of the polymer/fullerene blends. Two different low band-gap polymers (PDPPBT and PTB7) are investigated. Detail descriptions of the mechanisms leading to the final morphology are also provided
