We have explored the use of polymer / small molecule organic composites in the form of a polymer / perylene diimide heterojunction bilayer in order to combine the advantageous properties of both materials. Using the electron transporting perylene benzimidazole (PBI) and the hole conducting polymer poly[2,5-dimethoxy-1,4-phenylene-1,2-ethenylene-2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene-1,2-ethenylene (M3EH-PPV), we have achieved increased power conversion efficiencies for a planar device of up to 0.71% under 80 mW/cm2 white illumination. By varying the order of the photoactive layers, we have probed the mechanisms creating the photovoltage and found that the photovoltage is not determined by the difference in electrode work functions alone. In addition to the internal field, the interfacial chemical potential gradient, caused by exciton dissociation at the polymer / perylene diimide interface, appears to contribute to the photovoltage. We also discuss why, contrary to some expectations, the polymer / perylene diimide devices are more efficient than the analogous pure small molecule perylene diimide / phthalocyanine cells.