Enhancing Electron Injection in Dye-Sensitized Solar Cells by Adopting W6+-Doped TiO2 Nanowires: A Theoretical Study

As a donor type dopant in titanium dioxide, W6+ was found to move the conduction band (CB) edge of TiO2 downward and also to influence the electron-injection process in dyesensitized solar cells (DSSCs). To investigate the electron-injection capabilities of DSSCs and to optimize their efficiency by W6+ doping, the geometry and electronic properties of both free and adsorbed TiO2 nanowires were investigated on the basis of extensive density functional theory calculations. A four-layer (TiO2)(12) nanowire with 12 possible doping sites was set up, and the effect of W6+ in different positions was analyzed. The results indicate that in the W6+-doped (TiO2)(12) systems, the Ti-OW (OW = oxygen atom that is con-nected to the W atom) bonds are longer than the corresponding Ti-O bonds in (TiO2)(12). The CB edge is significantly influenced by the doping position. The CB energy level moves upward gradually as doped W6+ moves deep into (TiO2)(12). For all adsorbed catechol/(TiO2)(12) systems with the W6+ dopant, the LUMO maps are distributed over the layer in which W6+ is doped. The W6+ doping position plays a crucial role in the electron-injection and electron-transport process in DSSCs. Therefore, different positions of W6+ doping in TiO2 would be a feasible strategy to control and improve semiconductor materials to obtain DSSCs with more efficient electron injection.