Covalent O-H Bonds as Electron Traps in Proton-Rich Rutile TiO2 Nanoparticles

The cation in the electrolyte of the dye-sensitized solar cell (DSSC) has a profound effect on electron trapping and transport behavior in TiO2 nanocrystalline film; this is one of the important factors that determines the overall efficiency of DSSCs. Here, we present a quantum mechanical investigation on the structures and energetics of proton-induced electron trap states and the thermodynamical barrier heights for the ambipolar diffusion of proton/electron pair using a large cluster model for the computations. Our calculations indicate that protons react with TiO2 to form covalent O-H bonds. This is in contrast to the reaction of Li+ with TiO2, in which case the alkali metal is more accurately described as a simple coordinating cation. The covalent O-H bonding leads both to deeper electron trap states and to significantly higher barriers for the diffusion of carriers. These results are qualitatively consistent with experimental observations, and they extend our understanding of the cation effect in DSSCs at an atomic level of detail.