A method for the synthesis of well-defined rod-coil block copolymers consisting of P3HT donor and C60 acceptor chromophores (P3HT-b-P(SxAy)-C60) in a molecular architecture is reported for use in bulk-heterojunction (BHJ) solar cells. In thin films of the resulting block copolymer, reproducible self-assembly into well-defined “nanofibrils” is observed. This is the first example of a block copolymer containing a C60 derivative that shows exclusively a nanofibrilar structure. A substantial improvement in device performance is achieved when the block copolymer is used as a “surfactant” for controlling the interface morphology of the P3HT:PCBM donor-acceptor phase domains within the composite.