Novel ruthenium sensitizers having different numbers of carboxyl groups for dye-sensitized solar cells: effects of the adsorption manner at the TiO₂ surface on the solar cell performance.

Two novel ruthenium sensitizers having multiple carboxyl groups ((TBA)[Ru{4'-(3,4-dicarboxyphenyl)-4,4″-dicarboxyterpyridine}(NCS)3] (TUS-21) and (TBA)[Ru{4'-(3-carboxyphenyl)-4,4″-dicarboxyterpyridine}(NCS)3] (TUS-37); TBA = tetrabutylammonium) have been synthesized as improved model sensitizers for the previously reported ruthenium sensitizer TUS-20 ((TBA)[Ru{4'-(3,4-dicarboxyphenyl)terpyridine}(NCS)3]). The absorption maxima of two MLCT bands and the absorption onsets of TUS-21 and TUS-37 were shifted to longer wavelengths of about 30 nm in comparison to those of TUS-20 by introducing a carboxyl group to the each terminal pyridine ring of the terpyridine ligand. TUS-21 and TUS-37 showed quite similar adsorption behaviors to the TiO2 surface, and this adsorption behavior was found to be different from that of TUS-20. ATR-IR measurements revealed that TUS-21 and TUS-37 bind to the TiO2 surface by using two carboxyl groups at the 3-position of the phenyl ring and at one of the terminal pyridine rings of the terpyridine ligand, while TUS-20 is reported to bind by using two carboxyl groups at the 3,4-dicarboxyphenyl unit. The dye-sensitized solar cell (DSC) with TUS-21 exhibited 10.2% conversion efficiency, which is much higher than that of the DSC with TUS-20 (7.5%), under AM 1.5 (100 mW/cm(2)) irradiation.