Molecular and electronic ground and excited structures of heteroleptic ruthenium polypyridyl dyes for nanocrystalline TiO2 solar cells

Abstract

Heteroleptic ruthenium complexes of the type cis-[Ru(H₂dcbpy)(L)(NCS)₂], where H₂dcbpy = 4,4′-dicarboxy-2,2′-bipyridine and L = 1,10-phenanthroline (phen) (1) or dipyrido[3,2-a∶2′,3′-c]-phenazine (dppz) (2), were synthesized and their photochemical properties were investigated. The complexes showed a broad and intense metal-to-ligand charge transfer (MLCT) transition band in the visible region. The complexes were anchored to nanocrystalline TiO₂ film electrodes, and the photovoltaic properties of the resulting dye-sensitized solar cells were characterized and compared with the properties of cells prepared with cis-(NBu₄)₂[Ru(Hdcbpy)₂(NCS)₂] (N719). The efficiency of the 2-sensitized solar cell was 20% lower than that of the 1-sensitized solar cell, but neither was as efficient as the N719-sensitized solar cell. The electronic structures of the complexes were investigated by means of a time-dependent density functional theory method in an effort to better understand their effectiveness in TiO₂-based photoelectrochemical cells. The calculation results indicated that the character of the MLCT transitions in the long wavelength region differed between 1 and2, although their energy levels are nearly the same in the protonated forms. It was suggested that the performance of the 2-sensitized solar cell could be improved by the introduction of electron-donating groups on the dppz ligand of 2.