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

Abstract

cis-X2Bis(2,2′-bipyridyl-4,4′-dicarboxylate)ruthenium(II) complexes (X = Cl−, Br−, I−, CN−, and SCN−) were prepared and characterized with respct to their absorption, luminescence, and redox behavior. They act as efficient charge-transfer sensitizers for nanocrystalline TiO2 films (thickness 8–12 µm) 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. © 1993, American Chemical Society. All rights reserved.