Ruthenium sensitizer with a thienylvinylbipyridyl ligand for dye-sensitized solar cells.

A new ruthenium bipyridyl complex, coded as YX360, incorporating a conjugated thienylvinylbipyridyl ligand, cis-Ru(dtbpy)(dcbpy)(NCS)(2) [dtbpy = 4,4'-di(thienylvinyl)-2,2'-bipyridyl; dcbpy = 4,4'-dicarboxy-2,2'-bipyridyl], has been synthesized and studied as a dye in dye-sensitized solar cells (DSCs). The new dye is compared to its precursor N719, which is one of the best ruthenium-based sensitizers known so far. In the dye YX360 the lowest metal-to-ligand charge-transfer (MLCT) band is red-shifted by 10 nm and the molar extinction coefficient is dramatically increased as compared to N719. The reason can largely be attributed to the introduction of the extended π-conjugation unit to the ruthenium complex. Correspondingly, the incident photon-to-current conversion efficiency (IPCE) spectra of solar cells containing the dye YX360 show relatively higher values in the plateau region and a wider absorption spectrum relative to those of the dye N719. The effect is most pronounced for thinner TiO(2) films, for which comparable overall conversion efficiencies were obtained. However, as the TiO(2) film thickness is increased, DSCs containing N719 show superior conversion efficiencies. Although YX360 typically renders better short-circuit currents, the open-circuit voltage is suppressed because of larger electron recombination losses at the TiO(2)/dye/electrolyte interface. The results highlight that an extended aromatic ligand system in a sensitizing dye on the one hand improved light absorption, but on the other hand more efficiently loses photoelectrons through a recombination pathway via the dye to the electrolyte.