To mimic the performance of the tandem solar cells, ternary blend solar cells with a single active layer of P3HT:PCPDTBT:PC61BM were cast from chlorobenzene and thermally annealed. By varying blending ratio, thermal annealing time and P3HT molecular weight, the device performance was enhanced relative to the binary references. To understand this, the morphology of the active layer was studied using hard and soft x-ray scattering methods in concert with bright field and energy resolved transmission electron microscopies. We found that the phase separation of the amorphous PCPDTBT and P3HT guided the formation of P3HT fibrils, resulting in a unique multi-length-scale morphology. This morphology consisted of bundles of well-defined P3HT fibrils, forming a network, imbedded in an amorphous mixture of the PCBM, PCPDTBT and P3HT. The two polymers acted independently in their specific photoactive ranges, and the sensitization of PCPDTBT benefited the cascade charge transfer. This multi-length-scale morphology was linked to the improved device performance of P3HT: PCPDTBT:PC61BM and the photophysics of the active layer.