Control over the ratio of two blocks in a new class of all-conjugated diblock copolymers, poly(3-butylthiophene)-b-poly(3-hexylthiophene) (P3BHT), provides a facile approach to precisely tune the molecular organization and nanoscale morphology in polymer bulk heterojunction (BHJ) solar cells. In stark contrast to the power conversion efficiency, PCE, of 1.08% in poly(3-butylthiophene) (P3BT)/[6,6]-phenyl-C 71-butyric acid methyl ester (PC 71BM) and 3.54% in poly(3-hexylthiophene) (P3HT)/PC 71BM solar cells, an attractive, high PCE of 4.02% was achieved in a P3BHT21/PC 71BM BHJ device in which the molar ratio of P3BT:P3HT in P3BHT21 was 2:1. The ratio of P3BT and P3HT blocks was found to exert a noteworthy influence on the molecular organization of P3BHT, the film morphology of P3BHT/PC 71BM blend, and the final performance of P3BHT/PC 71BM photovoltaic devices. This enhanced performance reflected a synergy of finer phase separation of P3BHT21 and PC 71BM and the formation of respective percolation networks of electron donor P3BHT and electron acceptor PC 71BM. The P3HT block rendered the P3BHT chains with favorable chemical compatibility for the diffusion of PC 71BM molecules, allowing for finer phase separation between P3BHT crystalline domains and PC 71BM domains at the nanoscale and maximizing the interfacial area of P3BHT21/PC 71BM for improved charge generation. The P3BT block facilitated the self-assembly of P3BHT chains into sufficient interpenetrating pathways for efficient charge transport and collection. Moreover, a small crystalline domain with a size of 10.4 nm formed in the active layer that is comparable to the exciton diffusion length of most conjugated polymers (∼10 nm).