Resonance Raman scattering of a dye-sensitized solar cell: Mechanism of thiocyanato ligand exchange

A method has been developed to study the mechanisms of the thiocyanato ligand exchange reactions between the redox couple I(-)/I(3)(-) and the ruthenium complex bis(tetrabutylammonium) cis-bis(thiocyanato)bis(2,2 ' -bipyridine-4-carboxylic acid, 4 ' -carboxylate)ruthenium(II), in a sealed dye-sensitized solar cell using resonance Raman scattering. Raman lines from the solar cell were assigned to vibrations from the different constituents. In a cell with only lithium iodide added to the solvent, the formation of triiodide was observed at 114 and 226 cm(-1). The amount of thiocyanato group coordinated to the dye, estimated from the CN stretching observed at 2105 cm(-1), decreased with the increased amount of iodine added or triiodide formed in the electrolyte. It was shown that the thiocyanato ligands formed a complex with iodine, I(2)SCN(-), indicated by a weak Raman line at 165 cm(-1). From UV-vis measurement, the dye after the SCN(-) loss is proposed to be a cis-bis(iodo)bis(2,2 ' -bipyridine-4-carboxylic acid, 4 ' -carboxylate)ruthenium(II) complex. It is suggested that the electrolyte additive 4-tert-butylpyridine suppresses the loss of the thiocyanato group from the dye.