A photovoltaic effect was discovered and studied in the structures comprising thin films of perylenetetracarboxylic acid dianhydride (PTCDA) and oligo(phenylene-vinylene) (OPV) deposited in vacuum onto a silicon surface, as well as in the OPV/PTCDA/Si structure. A change in the surface potential under illumination in the visible spectral range was detected in situ using the electron probe technique. The spectra and the relaxation characteristics of the photoinduced potential were measured. The fast and slow components of the photopotential were revealed, which corresponded to the photoabsorption and relaxation processes in the silicon substrate (fast component) and in the organic film (slow component). These components have opposite signs, which accounts for the photopotential sign change on the passage from the short-wavelength (350-600 nm) to long-wavelength (700-905 nm) spectral regions.