© 2014 American Chemical Society. In this work, we investigated the effects of solvent(s)-polymer(s) interactions on the morphology of all-polymer bulk-heterojunction (BHJ) active layers cast from cosolutions. We demonstrate that altering the interactions between the solvent and both the donor and acceptor polymers in the cosolution prior to film-casting induces different solid-state morphological characteristics that subsequently leads to differences in the device performance of organic photovoltaics (OPV). Poly(3-hexylthiophene), P3HT, was codissolved poly[[N,N'-bis(2-octyldodecyl)-napthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5 ′-(2,2 ′-bithiophene)], P(NDI2OD-T2), or otherwise known as ActivInk N2200, in dichlorobenzene, chlorobenzene, and xylene. According to the qualitative interaction map we propose, all three solvents exhibit favorable interactions with P3HT. The extent of incompatibility these solvents exhibit with P(NDI2OD-T2), however, varies, with xylene as the worst solvent for P(NDI2OD-T2) among those examined. Polymer-polymer interactions in xylene are, thus, more favorable compared to P(NDI2OD-T2)-xylene interactions. Grazing-incidence wide-angle X-ray scattering measurements on the cast films suggest that this preferential affinity between the two polymers disrupts crystallization in the blends; P(NDI2OD-T2) crystallinity decreases and, concurrently, results in shorter P3HT coherence lengths. Significant mixing of the two polymers is also evidenced. OPVs comprising P3HT and P(NDI2OD-T2) active layers cast from xylene exhibit the best device characteristics compared to OPVs whose active layers are cast from di- or mono-chlorobenzene. We attribute the improved OPV performance for the xylene-cast active layer to the presence of a more intermixed network of nanocrystalline domains of the two polymers, which originates from the affinity of P3HT and P(NDI2OD-T2) in the parent cosolution.