In dye sensitized solar cells (DSSCs) light is absorbed by molecules (dyes), which are attached to the surface of wide-bandgap semiconductors. For the paradigmatic case of organic molecules on TiO_0 surfaces, we are investigating the interactions of the dyes with the surface, the electronic structure and excited electronic states. Such information, obtained from calculations based on density-functional theory, can help to better understand the stability of DSSCs and the efficiency of the electron transfer, which is at the heart of the functioning of such solar cells.