Charge-Transfer Sensitizers (X = C1-, Br-, I-, CN-, and SCN-) on Nanocrystalline Ti02 Electrodes

TLDR: In this paper, the performance of bis(thiocyanato)bis(2,2'-bipyridyl-4,4'-dicarboxylate)ruthenium(II) type redox sensitizers was investigated.

Abstract: cis-X~Bis(2,2'-bipyridyl-4,4'-dicarboxylate)ruthenium(II) complexes (X = C1-, 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 Ti02 films (thickness 8-1 2 pm) of very high internal surface area (roughness factor ca. lOOO), 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 Ti02 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/cmZ 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-terr-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. a broad spectral range in the~isible.~ A subsequent time-resolved luminescence study by Eichberger and Willig3m showed this effect to be due to ultrafast (7 < 7 ps) electron injection from the excited state of the complex into the conduction band of the oxide occurring with a quantum yield near 100%. These important observations warrant further studies of the behavior of the relatively unexplored class of bis(2,2'-bipyridyl)ruthenium(II)- type redox sensitizers. Apart from their chemical stability and ease of interfacial charge exchange with semiconducting solids, the attractive feature of these complexes is their large visible light harvesting capacity which is superior to that of the widely studied tris(bipyridy1) Ru(I1) analogues, making them a judicious choice for solar energy conversion devices.