We demonstrate for the first time the efficiency improvement of poly(3-hexylthiophene) (P3HT)-fullerene (C-60) bulk heterojunction photovoltaic cells by the introduction of trace concentrations of carbon black (CB) into the photoactive layer. As compared to control devices with only C-60, the addition of low concentration (12.5 ppm) of CB resulted in 35% improvement in short circuit current density (J(sc)) and 79% improvement in power conversion efficiency (PCE). CB served the dual function of enhancing electron transport as well as optimizing phase separation leading to high current density and PCE. The results indicate that CB is a promising additive for the performance enhancement of polymer photovoltaic cells and may work with diverse donor-acceptor systems. (C) 2014 Elsevier B.V. All rights reserved.