Energy level alignment at titanium oxide-dye interfaces: Implications for electron injection and light harvesting

We have performed density functional theory calculations to describe the changes in the electronic structure of oligothiophene derivatives equipped with carboxylic acid anchoring groups upon chemical grafting on TiO<inf>2</inf> clusters. The adsorption promotes a partial pinning effect for the lowest unoccupied molecular orbital of the dye, i.e., a LUMO energy level alignment with respect to the TiO<inf>2</inf> conduction band edge (CBE) irrespective of the oligothiophene length, which we ascribe to a strong hybridization between the dye discrete (LUMO) level and the cluster conduction band (CB). This is borne out by the fact that no pinning is observed, when decoupling the conjugated segment from the metal oxide substrate, e.g., by introducing a phenylene (PTn-Ph) or vinylene phenylene (PTn-Vi-Ph) moiety in a meta configuration or when there is a large energy mismatch between the dye frontier levels and the oxide conduction band, as is the case for oligothiophene-S,S-dioxides (with LUMO levels deep below the oxide CBE). Implications for electron injection into the titania and the optical absorption properties of the oxide-dye hybrids are discussed.