Simultaneous Interactions of Ru Dye with Iodide Ions and Nitrogen-Containing Heterocycles in Dye-Sensitized Solar Cells

Iodide ions and nitrogen-containing heterocycles, including 4-t-butylpyridine, are fundamental in the electrolyte solution of dye-sensitized solar cells as a redox and a promotive additive, respectively. Although discrete interaction of iodide ions and of heterocycles with the sensitizing dye is known, their simultaneous interaction with the dye and its effect on dye-sensitized solar cell performance is still an open question. In this paper, we have studied simultaneous interactions of iodide ions and N-containing heterocycles with cationic Ru(2,2'-bipyridine-4,4'-carboxylic acid)(2)(NCS)(2) dye (N3) by means of density functional theory (DFT). The DFT results reveal that the dye interacted with both the iodide ions and the heterocycles at different sites simultaneously. The dye formed a S-I bond between the NCS ligand and I-, while concurrently forming two hydrogen bonds between the carboxyl group of its ligand and the heterocycles. However, the interaction of the heterocycles weakened the interaction of iodide ions, suppressing the regeneration of the oxidized dye, which was after electron injection to the TiO2 conduction band, by an I-/I-3(-) redox couple, and thereby reducing the short-circuit photocurrent density of the dye-sensitized solar cell.