A ruthenium complex with superhigh light-harvesting capacity for dye-sensitized solar cells.

A dye-sensitized solar cell (DSSC) using Ru complexes as a photosensitizer was first reported by O Regan and Gr"tzel in 1991. The low-cost, easy preparation make DSSC one of the most promising photovoltaic cells for conversion of sunlight to electricity. Numerous sensitizers have been prepared, and their performance has been tested. A conversion efficiency of up to 11% was achieved by using cis-di(thiocyanato)bis(2,2’-bipyridyl-4,4’-dicarboxylate)ruthenium(II) (N3) as a photosensitizer. However, the conversion efficiency of DSSCs is still lower than that of the silicon-based photovoltaic cells. To obtain a high conversion efficiency, optimization of the short-circuit photocurrent (Isc) and open-circuit potential (Voc) of the cell is essential. The value of Voc depends on the edge of conduction band in TiO2 and the redox potential of I / I3 , otherwise Isc is related to the interaction between TiO2 and the sensitizer as well as the absorption coefficient of the sensitizer. The conduction band of TiO2 was known to have a Nernstian dependence on pH. Thus, the molecular engineering of the ruthenium complexes for achieving the highest efficiency was attempted to increase the molar absorption coefficient and reduce the number of protons on the complexes. 4,4’-Dicarboxylic acid-2,2’-bipyridine (dcbpy) has been considered as the best anchoring ligand in Ru sensitizers. Finding new metal-complex sensitizers with higher conversion efficiency was achieved by modifying one of the anchoring ligands. Replacement of one of the dcbpy anchoring ligands with a highly conjugated ancillary ligand represents a molecular engineering approach for increasing the absorption coefficient and therefore the photocurrent density of the sensitizers as reported by Gr"tzel and coworkers. Herein, we report a new ruthenium photosensitizer CYC-B1 in which one of the dcbpy ligands in N3 was replaced with abtpy, a bipyridine ligand substituted with alkyl bithiophene groups. CYC-B1 has the highest absorption coefficient among the Ru-based photosensitizers used in DSSCs, and its power-conversion efficiency is 10% higher than that ofN3 under the same cell fabrication and measuring procedures carried out in our laboratory. CYC-B1 was prepared in a typical one-pot synthesis, and its structure (Scheme 1) was identified from NMR