Effect of the Ligand Structure on the Efficiency of Electron Injection from Excited Ru−Phenanthroline Complexes to Nanocrystalline TiO₂ Films

We previously reported that the number of anchoring carboxyl groups affects the solar cell performance of dye-sensitized nanocrystalline TiO₂ solar cells using Ru−phenanthroline complexes as the sensitizer. To understand the mechanism in detail, the electron injection efficiency of sensitizing dyes adsorbed on nanocrystalline TiO₂ films has been studied by transient absorption spectroscopy. We found that the efficiency of electron injection is strongly affected by the number of carboxyl groups of the sensitizing dye, i.e., the efficiency of cis-bis(4,7-dicarboxy-1,10-phenanthroline)dithiocyanato ruthenium(II) (with four carboxyl groups) adsorbed on nanocrystalline TiO₂ films (DCP2/TiO₂) being 4 times as large as that of cis-(4-monocarboxy-1,10-phenanthroline)(1,10-phenanthroline)dithiocyanato ruthenium(II) (with one carboxyl group) adsorbed on the same films (MCPP/TiO₂). From the analysis of the time profile of the transient absorption, we conclude that the low efficiency of MCPP/TiO₂ is due to the presence of inactive dyes on the TiO₂ surface, which are not effective for electron injection. By comparing the absorbed photon-to-current conversion efficiencies (APCE) of the solar cells using these films, the effect of the number of the carboxyl groups on the solar cell performance can be attributed to the effect on the injection efficiency of electrons. We have also studied the effect of substitution of the countercations of DCP2. Two protons of carboxyl groups were substituted by two tetrabutylammonium (TBA) ions. The APCE of the DCP2-TBA is higher than that of DCP2, although the relative efficiencies of electron injection are similar for DCP2-TBA and DCP2. This indicates that the rate of reaction of the conducting electrons in the TiO₂ film with the I^-/I₃^- redox mediators (dark current) is reduced by substituting the protons by TBA ions.