Organic photovoltaic devices often show improved performances, if the active layer is made of a polymer blend. Due to the low miscibility of polymers the layer will phase separate and the lengthscale of the phase separation has a major influence on the device efficiency. We present a novel method to control the lengthscale of the phase separation, based on semiconducting polymer nanospheres (SPNs) forming the active layer. SPNs of M3EH-PPV (diameter 54nm) and CN-Ether-PPV (diameter 36nm) dispersed in water were produced by the miniemulsion process. Mono- and multilayers of these particles were fabricated by spincoating and photovoltaic devices utilizing these nanoparticles are shown to exhibit large external quantum efficiencies of up to 14%.