Role of Iodide in Photoelectrochemical Solar Cells. Electron Transfer between Iodide Ions and Ruthenium Polypyridyl Complex Anchored on Nanocrystalline SiO₂ and SnO₂ Films

Nanostructured thin films of SiO 2 and SnO 2 cast on an optically transparent electrode were modified with ruthenium(II) polypyridyl complex, Ru(bpy) 2 (bpy-COOH) 2+, Ru(II). Kinetic and mechanistic details of interaction between iodide ions and excited Ru(II), as well as regeneration of the sensitizer by iodide ions, were elucidated using emission spectroscopy and laser flash photolysis. The reaction between excited Ru(II) and I - in SiO 2 films which occurred with a rate constant of 1.0 × 10 8 M -1 s -1 produced low yields of electron-transfer products. In the case of SnO 2 films modified with Ru(II), the charge injection was the dominant process with the production of oxidized sensitizer, Ru(III). The reaction between Ru(III) and I - controls the regeneration of sensitizer in a photochemical solar cells. The oxidation of I - by Ru(III) in SnO 2 films as confirmed from the production of I 2 •- occurred with a rate constant of 1.2 × 10 10 M -1 s -1 .