Conversion of Light into Electricity with Trinuclear Ruthenium Complexes Adsorbed on Textured TiO2 Films

A series of CN-bridged trinuclear Ru complexes of the general structure RuL₂(μ-(CN)Ru(CN)L₂′)₂] where L is 2,2′-bipyridine-4,4′-dicarboxylic acid and L′ is 2,2′-bipyridine ( 1 )2,2′-bipyridine-4,4′-dicarboxylic acid ( 2 ), 4,4′-dimethyl-2,2′-bipyridine ( 3 ), 4,4′-diphenyl-2,2′-bipyridine ( 4 ), 1,10-phenanthroline ( 5 ), and bathophenanthrolinedisulfonic acid ( 6 ) have been synthesized, and their spectral and electrochemical properties investigated. The two carboxylic functions on the 2,2′-bipyridine ligand L serve as interlocking groups through which the dye is attached at the surface of TiO₂ films having a specific surface texture. The role of these interlocking groups is to provide strong electronic coupling between the π* orbital of the 2,2′-bipyridine and the 3d-wave-function manifold of the conduction band of the TiO₂, allowing the charge injection to proceed at quantum yields close to 100 %. The charge injection and recombination dynamics have been studied with colloidal TiO₂, using laser photolysis technique in conjunction with time-resolved optical spectroscopy. Photocurrent action spectra obtained from photo-electrochemical experiments with these trinuclear complexes cover a very broad range in the visible, making them attractive candidates for solar light harvesting. Monochromatic incident photon-to-current conversion efficiencies are strikingly high exceeding 80% in some cases. Performance characteristics of regenerative cells operating with these trinuclear complexes and ethanolic triiodide/iodide redox electrolyte have been investigated. Optimal results were obtained with complex 1 which gave a fill factor of 75 % and a power conversion efficiency of 11.3% at 520 nm.