The control of the bulk-heterojunction (BHJ) morphology in polymer/polymer blends remains a critical hurdle for optimizing all-polymer solar cells (all-PSCs). The relationship between donor/acceptor phase separation, domain size, and the resulting photovoltaic characteristics of PDFQx3T and P(NDI2OD-T2)-based all-PSCs was investigated. We varied the film-processing solvents (chloroform, chlorobenzene, o-dichlorobenzene, and p-xylene), thereby manipulating the phase separation of all-polymer blends with the domain size in the range of 30-300 nm. The different volatility and solubility of the solvents strongly influenced the aggregation of the polymers and the BHJ morphology of polymer blends. Domain sizes of all-polymer blends were closely correlated with the short-circuit current density (JSC) of the devices, while the open-circuit voltage (0.80 V) and fill factor (0.60) were unaffected. All-PSCs with the smallest domain size of ∼30 nm in the active layer (using chloroform), which is commensurate with the domain size of highly efficient polymer/fullerene solar cells, had the highest JSC and power conversion efficiency of 5.11% due to large interfacial areas and efficient exciton separation. Our results suggest that the BHJ morphology was not fully optimized for most of the previous high-performance all-PSC systems, and their photovoltaic performance can be further improved by fine-engineering the film morphology, i.e., domain size, domain purity, and polymer packing structure.