The structural, energetic, and electronic properties of the TiO₂-electrolyte interface in dye-sensitized solar cells is studied by molecular dynamics simulations and electronic structure calculations. The investigation enlightens the mechanisms responsible for the recombination of photoelectrons with redox species in the electrolyte (back-reaction effect), taking into account the important influence of surface defects, the underlying solvent dynamics, and the presence of pyridine additives at the interface. The free-energy barrier for the adsorption of redox species at the TiO₂ surface is calculated. Electronic structure calculations of the TiO₂/redox/solvent system evidence the distinct recombination mechanisms for the different redox species. The study provides a deeper insight on the molecular processes taking place at the interface and should stimulate further theoretical and experimental investigations.