The electronic structure and photo-induced charge carriers transport have been studied in NN-dimethylperylene-3,4,9,10-dicarboximide (Me-PTCDI) films evaporated on ITO substrate using surface photovoltage (SPV) technique. For the Me-PTCDI film with the thickness of 456 nm, with changing the illumination geometry, the amplitudes as well as the phases of the SPV response bands at the wavelength where Me-PTCDI strongly absorbs significantly changed, while those of the SPV response bands at the wavelength where Me-PTCDI weakly absorbs hardly changed. It was explained based on the different absorption lengths of Me-PTCDI at different wavelengths. The experiment results suggest that the photo-induced charge carriers generated at the surface and the interface region are separated by the action of the built-in electric field and move in the opposite directions. From the dependence of the work function on the film thickness, it is inferred that the interface and surface are all depleted with the interface space charge region extending to be about 200 nm. In addition, it is found that an acceptor state at 0.1 eV above the valence band exists at the surface as well as at the interface. (c) 2005 Elsevier B.V. All rights reserved.