Poly(3-alkylthiophene)-based diblock copolymers with controllable block lengths were synthesized by combining the Grignard metathesis method, Ni-catalyzed quasi-living polymerization, and a subsequent azide-alkyne click reaction to introduce a fullerene functionality into the side chains of one of the blocks. The fiillerene-attached copolymers had good solubility (>30 g L-1 chlorobenzene) with high molecular weights (M-n > 20 000). The diblock copolymer films formed clear nanostructures with sizes of ca. 20 nm, driven by crystallization of the poly(3-hexylthiophene) block and aggregation of the fullerene groups, as observed in AFM phase images. The copolymer-based photovoltaic device showed a power conversion efficiency of 2.5%, with a much higher fill factor of 0.63 in comparison to the previously reported single component devices. These results indicate that rational material designs enable the construction of suitable donor-acceptor nanostructures for photovoltaic applications, without relying on the mixing of materials.