Three-dimensional solid state solar cells, or 3D-cells, are based on nanostructured n-type TiO2 electrodes as known from dye-sensitized solar cells, and completed with a p-type solid state semiconductor absorber CuInS2 which is absorber and hole conductor in one. To study the electron transport in this specific type of nanostructured solar cells, they are subjected to intensity modulated photocurrent spectroscopy. The photocurrent response of the 3D-cell turns out to be four orders of magnitude faster than the response of nanostructured dye-sensitized solar cells. No voltage dependency is found for the time constants, so we conclude that there is no macroscopic electric field present in the 3D-cell and that diffusion is the major driving force for carrier transport in the 3D-cell. The fact that an increase in illumination intensity results in a decrease in time constant, supports this idea because it shows, that charging of the nanostructured TiO2 film due to trap filling enhances the diffusion. The estimated diffusion coefficient of electrons in the nanostructured TiO2 is in the order of 10(-8) m(2)/S for high illumination intensities. (c) 2004 Elsevier B.V. All rights reserved.