Low-cost photovoltaic energy conversion using conjugated molecular materials has become increasingly feasible through the development of organic 'bulk heterojunction (BHJ)' structures(1-7), where efficient light-induced charge separation is enabled by a large-area donor - acceptor interface(2,3). The highest efficiencies have been achieved using blends of poly(3-hexylthiophene) (P3HT) and a fullerene derivative(8-12), but performance depends critically on the material properties and processing conditions. This variability is believed to be influenced by the self-organizing properties of P3HT, which means that both optical(13,14) and electronic(15,16) properties are sensitive to the molecular packing. However, the relationship between molecular nanostructure, optoelectronic properties of the blend material and device performance has not yet been demonstrated. Here we focus on the influence of polymer regioregularity (RR) on the molecular nanostructure, and hence on the resulting material properties and device performance. We find a strong influence of RR on solar-cell performance, which can be attributed to enhanced optical absorption and transport resulting from the organization of P3HT chains and domains. Further optimization of devices using the highest RR material resulted in a power conversion efficiency of 4.4%, even without optimization of electrodes(7).