Effect of co-adsorption dye on the electrode interface (Ru complex/TiO2) of dye-sensitized solar cells

The co-adsorption interface structure of isothiocyanate (R-N=C=S) in N719 dye was investigated using a system of N719 alone and an N719 + D131 co-adsorption system. The sulfur core level (S 1s) and sulfur K absorption edge (S K-edge) were examined in detail using X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS), respectively. The S 1s XPS spectra revealed that the binding energies were shifted approximately 9 eV higher in N719 alone because of interactions between the R-N=C=S of N719 and nanocrystalline TiO2. However, this strong interaction disappeared in the N719 + D131 co-adsorption system. Comparing the S K-edge NEXAFS spectra against the case of N719 alone revealed that the resonance adsorption peak at 2483 eV, which was attributed to an interaction between sulfur and the substrate, did not appear in the N719 + D131 co-adsorption system. This peak was observed under oblique incidence, but was almost indiscernible under normal incidence. These results indicate that the interface structure of sulfur atoms that strongly interacts with nanocrystalline TiO2 substrate changes to become non-interacting in the N719 + D131 co-adsorption system. We conclude that the co-adsorption dye has the unique property of inhibiting strong interactions between the S atom in the R-N=C=S group of the N719 dye and the nanocrystalline TiO2 surface