A series of block copolymers with fixed length of the semiconductor-block poly(3-butylthiophene) (P3BT) and varying length of the insulator-block polystyrene (PS) are synthesized. These copolymers are blended with phenyl-C61-butyric acid methyl ester (PCBM) for the bulk heterojunction photoactive layers. With appropriate insulator-block length and donoracceptor ratio, the power conversion efficiency increases by one order of magnitude compared with reference devices with pure P3BT/PCBM. PS blocks improve the miscibility of the active layer blends remarkably. The P3BT-b-PS crystallizes as nanorods with the P3BT core covered with the PS-block, which creates a nanoscale tunneling barrier between donor and acceptor leading to more efficient transportation of charge carriers in the semiconductors.