Computational Studies of the Interaction between Ruthenium Dyes and X⁻ and X₂⁻, X = Br, I, At. Implications for Dye-Sensitized Solar Cells

Quantum chemistry in the form of relativistic density functional theory (DFT) combined with a continuum solvation model has been applied to study the interaction of two prototypical ruthenium dyes (N3 and its chlorinated form) and redox mediators X⁻ and X₂⁻, X = Br, I, At, with a view at the elementary reactions within a dye-sensitized solar cell (DSSC). Along the series Br, I, and At, increasing bond lengths of X₂, X₂⁻, and X₃⁻ are found, as well as an increasing reducing power of the X⁻/X₃⁻ redox couple. Inner-sphere seven-coordinate complexes between the dye and the redox species do not exist; however, the dyes form outer-sphere complexes with the X⁻ and X₂⁻ species. The thermodynamics of a recently proposed mechanism [J. Phys. Chem. C 2007, 111, 6561] involving a [dye⁺X⁻] intermediate are probed, and the existence of the intermediate and the elementary steps of the process are confirmed. The dye regeneration is thermodynamically more favorable for the N3 dye than its chlorinated counterpart. The regeneration of the neutral dye is favored for At, followed by the iodine and bromine systems (At > I > Br). This may be related to the observed superior performance in actual DSSCs of the iodide/triiodide redox couple over the alternative bromide/tribromide couple.