Ti oxide films were made by reactive DC magnetron sputtering onto electrically conducting glass substrates. The films were dye sensitized with an Ru complex, thereby yielding nanocrystalline solar cells. We investigated the microstructure of the films by X-ray diffraction, scanning electron microscopy, atomic force microscopy, and cyclic voltammetry on viologen-containing samples. The internal surface area was enhanced with increasing film thickness, and this property could be correlated with an enlarged photoelectric conversion efficiency. © 1999 Elsevier Science B.V. All rights reserved.