Photoelectrochemistry of Composite Semiconductor Thin Films. Photosensitization of SnO₂/CdS Coupled Nanocrystallites with a Ruthenium Polypyridyl Complex

The charge injection from an excited Ru(II) complex (viz., (Ru(bpy)2(L))2+, where L = 4-(2,2'-bipyrid-4-yl)phenyl diphosphonate) into SnO2 and SnO2/CdS films is investigated to elucidate the beneficial effect of composite semiconductor systems. The electrons injected from excited Ru(II) into CdS are transferred quickly into SnO2 nanoclusters, thus promoting the charge separation. Photoelectrochemical measurements suggest improved photon to photocurrent charge carrier generation efficiency in the SnO2/CdS composite system. The formation of the oxidation product, Ru(III), and the recovery of the sensitizer are investigated using transient absorption spectroscopy. The rate constant for the back electron transfer in the SnO2/CdS/Ru(II) composite system is slower by a factor of 2−3 than the one observed in the SnO2/Ru(II) system. The beneficial aspects of composite semiconductor systems in photochemical solar cells are presented.