Sensitization of TiO2 by polypyridine dyes -: Role of the electron donor

Dye-sensitized photoelectrochemical cells (DSSC) are characterized by electrochemical impedance spectroscopy (EIS) and Raman spectroscopy during their polarization. Cells realized with a dye recently synthesized in one of our laboratories, containing two terpyridyl (terpy) ligands, are compared with cells using commercial dyes (Ru535 and Ru620) containing isothiocyanates and either bipyridyl (bpy) or terpy ligands. Here, two points are emphasized, first, the role of the functional group (carboxylate or phosphonate) which ensures the linkage to TiO2 and, second, the role of the redox couple (I-/I-3(-)) present in the electrolyte which can react with the dye D to give unwanted intermediate species. Two species, each of them giving a characteristic Raman band in the low wavenumber range, are characterized by Raman spectroscopy. The first of these species is triiodide; the nature of the second one, which directly implies the oxidized form of dye, D+, is discussed. During the DSSC functioning, EIS allows one to discriminate three potential ranges, the photocurrent plateau, the recombination range, and the direct current range when the voltage decreases from anodic to cathodic. The second intermediate exists only in the photocurrent plateau, while I-3(-) exists also in the recombination range. These results do not depend on the nature (bpy or terpy) of the ligand. (C) 2003 The Electrochemical Society.