Dye Sensitization of Nanocrystalline Titanium Dioxide with Osmium and Ruthenium Polypyridyl Complexes

The goal of this work is to explore the idea of replacing the ruthenium metal center of polypyridine complexes with osmium to increase the overlap between the spectral response of the "Gratzel cell" and the solar spectrum. A series of osmium polypyridine dyes with various ground state reduction potentials has been synthesized, purified, characterized and utilized to sensitize nanoporous titanium dioxide electrodes to solar radiation. The dyes are ML'2(NCS)2, ML'2(CN)2, ML'3^2+ and ML2L'^2+ where M is Os(II) or Ru(II), L is bipyridine and L' is 4,4'-dicarboxy-2,2'-bipyridine. The visible absorption spectra of the Os complexes showed an additional absorption band at longer wavelengths than the Ru complexes. The spectral response and current-voltage properties of electrodes modified with the Os dyes have been compared with their Ru analogs. In general, the Os dye showed a similar maximum external quantum yield, an enhanced response at longer wavelengths, a higher photocurrent density and a similar open-circuit voltage than th Ru analog. The Os complexes thus appear to be promising sensitizers for energy conversion applications. The only discrepancy in this trend was observed for the OsL'2(NCS)2 complex, which showed a much lower maximum external quantum yield, lower photocurrent and lower open-circuit voltage than the Ru analog. Experiments suggest that the regeneration rate of Os(II)L'2(NCS)2 by iodide does not compete effectively with the regeneration rate of the dye by the electron in the TiO2. The ground-state redox potential of Os(II/III)L'2(NCS)2 (+0.42 V vs aq SCE in methanol) may represent a limit on how negative the redox potential of the dye can be to obtain efficient regeneration of the sensitizer by I-/13-. For electrodes with very low dye coverage, the open-circuit voltage was mainly determined by the reduction of I3-, whereas for high dye coverage, the open-circuit voltage also depended on the nature of the complex and on the dye loading level.