Theoretical Study on the Interactions between Black Dye and Iodide in Dye-Sensitized Solar Cells

Interactions between a carboxylated terpyridyl complex of tris-thiocyanato Ru(II), so-called “black dye”, and iodide ions were studied by using a density functional theory (DFT). All the oxidized linkage isomers [Ru(Htcterpy)(NCS) 3 ] ( 1 ), [Ru(Htcterpy)(NCS) 2 (SCN)] ( 2 and 3 ), and [Ru(Htcterpy)(SCN) 3 ] ( 4 ), where tcterpy is 4,4′,4′′-tricarboxy-2,2′:6′,2′′-terpyridine, interacted with iodide ions forming a S–I bond via the equatorial NCS ligand for 1 and 2, and the N–I bond via the equatorial SCN for 3 and 4 . For both monoiodide and diiodide ions, the interaction strength increased in the order of 3 < 4 < 2 < 1 . A two-step interaction with two monoiodides via the equatorial NCS ligand occurred in only isomer 1 with a tetrabutylammonium (TBA), which formed hydrogen bonds with the S atom in one of the axial NCS ligands and with the O atom in the carboxyl group of the central pyridine in the terpydidine ligand. On the basis of the theoretical results, regeneration mechanisms for the oxidized black dye by the I – /I 3 – redox couple after electron injection into the TiO 2 conduction band in dye-sensitized solar cells, and the effects of the dye isomers and TBA counterion on DSSC performance were discussed.