Invited paperInteraction between disulfide/thiolate mediators and ruthenium complex in dye-sensitized solar cells

•Interactions of disulfide/thiolate redox mediator with Z907 dye are studied.•One thiolate anion cannot regenerate Z907+ completely through one-step mechanism.•Two thiolate anions can regenerate Z907+ completely via two-step mechanism like I−.•Unfound Z907-disulfide complex supports the slower recombination process in DSSC.

The intermolecular interactions of thiolate anions and disulfide molecules as disulfide/thiolate (T2/T−, S2/S−) redox couples with a Ru bipyridyl complex (Z907) are investigated using density functional theory. T and S are 1-methyl-1H-tetrazole-5-thiolate and 2-methyl-5-trifluoromethyl-2H-1,2,4-triazole-3-thiolate, respectively. Similar to the typical reductant I−, the S atoms of thiolate anions interact with the oxidized Z907 (Z907+) via the terminal S atoms of the isothiocyanate (NCS) ligands. However, nearly 40% of the atomic charge and spin still remain after complexation. If a second thiolate anion, as well as I−, is considered, the atomic charge and spin are transferred completely. The distances of the formed SS bonds for the Z907+-two anions species correspond to isolated dithiolate radicals. These results suggest that Z907+ can be completely regenerated by two thiolate anions via a two-step mechanism rather than by only one thiolate anion through a one-step mechanism as previously suggested. Unlike the I2 molecule, the unfound Z907-disulfide complex structure supports the slower recombination process with the disulfide/thiolate electrolyte compared with the I3−/I− system in dye-sensitized solar cells.