Intrinsic Oxygen Vacancy and Extrinsic Aluminum Dopant Interplay: A Route to the Restoration of Defective TiO2

Density functional theory (DFT) and DFT corrected for on-site Coulomb interactions (DFT+U) calculations are presented on aluminum doping in bulk TiO2 and the anatase (101) surface. Particular attention is paid to the mobility of oxygen vacancies throughout the doped TiO2 lattice, as a means by which charge compensation of trivalent dopants can occur. The effect that Al doping of TiO2 electrodes has in dye-sensitized solar cells is explained as a result of this mobility and charge compensation. Substitutional defects in which one Al3+ replaces one Ti4+ are found to introduce valence band holes, while intrinsic oxygen vacancies are found to introduce states in the band gap. Coupling two of these substitutional defects with an oxygen vacancy results in exothermic defect formation which maintain charge neutrality. Nudged elastic band calculations have been performed to investigate the formation of these clustered defects in the (101) surface by oxygen vacancy diffusion, with the resulting potential energy surface suggesting energetic gains with small diffusion barriers. Efficiency increases observed in dye sensitized solar cells as a result of aluminum doping of TiO2 electrodes are investigated by adsorbing the tetrahydroquinoline C2-1 chromophore on the defective surfaces. Adsorption on the clustered extrinsic Al3+ and intrinsic oxygen vacancy defects are found to behave as if adsorbed on a clean surface, with vacancy states not present, while adsorption on the oxygen vacancy results in a down shift of the dye localized states within the band gap and defect states being present below the conduction band edge. Aluminum doping therefore acts as a benign dopant for "cleaning" TiO2 through oxygen vacancy diffusion.