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-/ I3- 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 obsd. at 114 and 226 cm-1. The amt. of thiocyanato group coordinated to the dye, estd. from the CN stretching obsd. at 2105 cm-1, decreased with the increased amt. of iodine added or triiodide formed in the electrolyte. It was shown that the thiocyanato ligands formed a complex with iodine, I2SCN-, 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.