Dye sensitization of nanocrystalline titanium dioxide with osmium and ruthenium polypyridyl complexes

A series of osmium polypyridyl complexes having various ground-state reduction potentials has been synthesized and used to sensitize nanoporous titanium dioxide electrodes to solar illumination. The spectral response and current vs potential properties of electrodes modified with these dyes have been compared with the behavior of their ruthenium analogues. The trends can be explained by the differences in absorption spectra and ground-state redox potentials. The osmium, complexes appear to be promising candidates for further optimization in operating photoelectrochemical cells for solar energy conversion applications. Of the materials studied, all complexes having ground-state redox potentials in methanol more positive than ∼0.4 V vs aqueous SCE were able to sustain oxidation of I^-/I₃^- with a high steady-state quantum yield. For electrodes with very low dye coverages, the open-circuit voltage was mainly determined by the rate of reduction of I₂, whereas for high dye coverages, the open-circuit voltage depended on the nature of the complex and on the dye loading level.