In most solar cells charge carriers are photogenerated in the bulk semiconductor and separated by the internal electric field. Cells based on highly ordered films of molecular semiconductors, however, may operate by a surface-sensitized carrier generation mechanism in which charge carriers are first generated by asymmetric exciton dissociation at the illuminated semiconductor/electrode interface. The characteristics of systems based on this mechanism have received little attention in the context of solar power conversion, although such excitonic; processes are commonly studied in the field of electrophotography. We introduce here the use of redox polymer films as electrical contacts to molecular semiconductors. These electrodes provide a means of probing and modulating the interfacial exciton dissociation and electron-transfer processes. The polarity and efficiency of the interfacial charge separation process, leading to a positive or negative photovoltage, can be controlled by varying the energy levels of the redox polymer contacts without imposing an internal field in the cell. Photovoltages up to ca. 1 V can be achieved in cells where there is little or no band bending in the dark.