CONVERSION OF LIGHT TO ELECTRICITY BY CIS-X2BIS(2,2'-BIPYRIDYL-4,4'-DICARBOXYLATE)RUTHENIUM(II) CHARGE-TRANSFER SENSITIZERS (X = CL-, BR-, I-, CN-, AND SCN-) ON NANOCRYSTALLINE TIO2 ELECTRODES

cis-X2Bis(2,2'-bipyridyl-4,4'-dicarboxylate)ruthenium(II) complexes (X = Cl-, Br-, I-, CN-, and SCN-) were prepared and characterized with respect to their absorption, luminescence, and redox behavior. They act as efficient charge-transfer sensitizers for nanocrystalline TiO2 films (thickness 8-12 mum) of very high internal surface area (roughness factor ca. 1000), prepared by sintering of 15-30-nm colloidal titania particles on a conducting glass support. The performance of cis-di(thiocyanato)bis(2,2'-bipyridyl-4,4'-dicarboxylate)ruthenium(II) (1) was found to be outstanding and is unmatched by any other known sensitizer. Nanocrystalline TiO2 films coated with a monolayer of 1 harvest visible light very-efficiently, their absorption threshold being around 800 nm. Conversion of incident photons into electric current is nearly quantitative over a large spectral range. These films were incorporated in a thin-layer regenerative solar cell equipped with a light-reflecting counter electrode. Short-circuit photocurrents exceeding 17 mA/cm2 were obtained in simulated AM 1.5 sunlight using lithium iodide/triiodide in acetonitrile or acetonitrile/3-methyl-2-oxazolidinone mixtures as redox electrolyte. The open-circuit photovoltage was 0.38 V and increased to 0.72 V by treating the dye-covered film with 4-tert-butylpyridine. A solar-to-electric energy conversion efficiency of 10% was attained with this system. The effect of temperature on the power output and long-term stability of the dye was also investigated. For the first time, a device based on a simple molecular light absorber attains a conversion efficiency commensurate with that of conventional silicon-based photovoltaic cells.