The performance of dye-sensitized solar cells (DSSCs)
depends significantly on the adsorption geometry of the dye on the
semiconductor surface. In turn, the stability and geometry of the adsorbed
molecules is influenced by the chemical environment at the electrolyte/
dye/TiO2 interface. To gain insight into the effect of the solvent on the
adsorption geometries and electronic properties of dye-sensitized TiO2
interfaces, we carried out first-principles calculations on organic dyes and
solvent (water or acetonitrile) molecules coadsorbed on the (101) surface
of anatase TiO2. Solvent molecules introduce important modifications on
the dye adsorption geometry with respect to the geometry calculated in
vacuo. In particular, the bonding distance of the dye from the Ti anchoring
atoms increases, the adsorption energy decreases, and the two C−O bonds
in the carboxylic moieties become more symmetric than in vacuo. Moreover, the adsorbed solvent induces the deprotonation of
the dye due to the changing the acid/base properties of the system. Analysis of the electronic structure for the dye-sensitized
TiO2 structures in the presence of coadsorbed solvent molecules shows an upward shift in the TiO2 conduction band of 0.2 to
0.5 eV (0.5 to 0.8 eV) in water (acetonitrile). A similar shift is calculated for a solvent monolayer on unsensitized TiO2. The
overall picture extracted from our calculations is consistent with an upshift of the conduction band in acetonitrile (2.04 eV vs
SCE) relative to water (0.82 eV vs SCE, pH 7), as reported in previous studies on TiO2 flatband potential (Redmond, G.;
Fitzmaurice, D. J. Phys. Chem. 1993, 97, 1426−1430) and suggests a relevant role of the solvent in determining the dye−
semiconductor interaction and electronic coupling
