DFT/TDDFT study of the adsorption of N3 and N719 dyes on ZnO(101̅0) surfaces.

ZnO has attracted a great deal of research as a potential replacement of TiO2 for dye-sensitized solar cells (DSSCs), owing to the unique combination of interesting electronic properties (i.e., high electron mobility) and structural richness. Here, we present a DFT/TDDFT study about the interaction of the prototypical N3 and N719 Ru(II) sensitizers on ZnO models to understand some of the atomistic details that are crucial to the dye/semiconductor interaction. We pay particular attention to the adsorption mode of the sensitizer and to the effect of the complexation on the electronic structure of the dye. The sensitizers are predicted to strongly interact with the ZnO surface. In particular, the interaction is strengthened when three dye carboxylic groups are involved in the adsorption. Moreover, if the anchoring group bears a proton, the adsorption is predicted to be dissociative. The charge density donation from the dye to the semiconductor raises the valence and conduction band edges of the latter, in such a way that the optical gap of ZnO widens. Proton transfer from the dye to the semiconductor balances the charge donation effect and restores the electronic levels of the noninteracting fragments. The impact of dye/semiconductor interaction on the adsorbed dye optical properties is then discussed.