D-π-A dye attached on TiO₂(101) and TiO₂(001) surfaces: Electron transfer properties from ab initio calculations

The stability, electronic, and transfer properties of a photo-sensitizer donor-(π-bridge)-acceptor (D-π-A) zinc-porphyrin dye adsorbed on anatase TiO₂(101) and TiO₂(001) surfaces are studied by periodic density functional theory calculations, including the screened hybrid functional. We also study the dye adsorption on the TiO₂(101) surface with an oxygen vacancy. Our results show that the deprotonated dye attaches strongly on surface Ti_5c atoms forming both bidentate bridging and chelating linkage modes. The protonated dye is found stable on pristine TiO₂(101), forming the monodentate linkage mode. However, on TiO₂(001) and defective TiO₂(101), the monodentate mode stabilizes after a spontaneous dye deprotonation. The electronic band structure of the TiO₂-dye assemblies shows the dye lowest-unoccupied molecular-orbital resonant with TiO₂ conduction bands, exhibiting a strong hybridization, where the electron injection potentials are calculated around 1 eV for all systems under study. This high value, together with the favorable electronic coupling, suggest a fast electron injection from the dye into the semiconductor as experimentally reported. Moreover, the electronic properties of TiO₂-dye assemblies show similarities independent of the adsorption modes or the surface characteristic, suggesting that the high efficiency of anatase TiO₂-based dye sensitized solar cell resides largely in its favorable electron injection properties.