Bulk heterojunction (BHJ) solar cells are fabricated using active material blends of poly(3-hexylthiophene) (P3HT) donor, indene-C-60 bisadduct (ICBA) acceptor, and an all-conjugated random copolymer (RCP) additive. By optimizing RCP loading, power conversion efficiencies (PCEs) up to 20% higher than those of a binary P3HT: ICBA mixture are achieved. The improved device characteristics are rationalized in terms of the differences between the photoactive thin film morphologies. Energy-filtered transmission electron microscopy reveals that incorporation of the RCP improves the degree of structural order of the BHJ fibrillar network and increases the extent of microphase separation between P3HT and ICBA. Additionally, a combination of atomic force microscopy and X-ray photoelectron spectroscopy analysis indicates segregation of the RCP at the free interface, leading to a shift in the surface potentials measured by Kelvin probe force microscopy. These changes, both in the bulk morphology and in the interfacial composition/energetics, are correlated to improved carrier collection efficiency due to a reduction of non-geminate recombination, which is measured by charge extraction of photogenerated carriers by linearly increasing voltage.