The structure evolution of all-polymer solar cells based on the blends of poly(3-hexylthiophene) (P3HT) and poly[(9,9-dioctyluorene)-2,7-diyl-alt-(4,7- bis(3-hexylthien-5-yl)-2,1,3-benzothiadiazole)-2′,2′′-diyl] (F8TBT) was investigated. The P3HT/F8TBT system exhibits crystallization-driven structure formation similar to the P3HT/phenyl-C₆₁- butyricacidmethylester (PCBM) blend despite the existence of a miscibility gap, which was determined for a blend containing regio-random P3HT. The lamellar crystallization of regio-regular P3HT was not perturbed by the addition of F8TBT. X-ray scattering studies indicate that F8TBT is segregated to the interlamellar amorphous phase, establishing a bulk heterojunction framework with the crystalline lamellae of P3HT. The excess F8TBT is accommodated at the film-substrate interface and at amorphous grain boundaries. The structural studies were correlated with the photovoltaic device performance of blend films that consisted of large P3HT spherulites. These device results emphasize the importance of a mesoscopic F8TBT network that separates the P3HT crystal domains. Our results suggest that the nanostructure formation in P3HT/F8TBT blends is determined by P3HT crystallization, resulting both in a 10 nm crystalline morphology and a F8TBT mesoscopic segregation network, both of which are beneficial for exciton dissociation. This journal is