Organic photovoltaics promise a number of key advantages over conventional silicon, namely: Ease of processing, low cost, physical flexibility and large area coverage. However, the solar power conversion efficiencies of pure polymer devices are poor. When nanocrystals are blended with a conducting polymer to create a bulk heterojunction structure the optical and electronic properties of both materials combine synergistically to enhance overall performance. We have investigated the dependence of efficiency on the polymer molecular weight, together with the role of nanocrystals in the photogeneration of charge carriers in bulk heterojunction solar cells. We found that a high molecular weight polymer resulted in the formation of small nanocrystals, and that nanocrystals act to enhance the natural spectral response of the polymer.