Effect of Cations on the Interactions of Ru Dye and Iodides in Dye-Sensitized Solar Cells: A Density Functional Theory Study

A density functional theory (DFT) study was performed to elucidate the effect of counter cations [H+, Li+, 1,2-dimethyl-3-propylimidazolium (DMPI+), and tetrabutylammonium (TBA(+))] on the interactions between Ru(II)-polypyridyl dye (N719) and iodide anions. The counter cations saturated the negative charge of the terminal carboxylic group. Symmetric bidentate binding with the oxygen atoms in the carboxylate group for the N719 ligand was observed with Li+. DMPI+ and TBA(+) formed two hydrogen bonds between each of the O atoms in the carboxylate group for the dye ligand and the H atoms of the alkyl groups. Cations drastically influenced the interaction between the dye and iodide ions via the S atom in the NCS ligand. For both monoiodide and diiodide ions, the interaction strength increased in the order TBA(+) < DMPI+ < Li+ < H+, which corresponds to the short-circuit photocurrent density of a dye-sensitized solar cell. The results suggest that the stronger the interaction of oxidized dye with the iodide ions, the easier the dye regeneration after electron injection into the TiO2 conduction band proceeds by a I-/I-3(-) redox couple, leading to a higher photocurrent in the cell.