This work reports on organic bulk heterojunction solar cells based on poly(3-hexylthiophene) (P3HT) blended with [6,6]-phenyl-C61 butyric acid methyl ester (PCBM) in a configuration with so-called interdigital nanoelectrodes, i.e., vertical electrodes on substrates structured in the submicrometer range. In this setup, both electrodes are in place prior to the deposition of the photoactive blend solution and therefore allow for the application of a voltage during drying of the blend. A strong correlation is observed between the photovoltaic performance of these devices and the voltage that is applied during film formation. Even the polarity of the solar cells can be controlled with this method. It is suggested that this is a consequence of a strong segregation of donor and acceptor phases at the electrode interfaces induced by the applied voltage. Further experiments on planar solar cell geometries, including a solvent-vapor treatment and the introduction of an additional layer of pure P3HT, as well as numerical simulations, are presented. All results obtained are consistent with the suggested hypothesis.