Photoelectrochemistry of composite semiconductor thin films. Photosensitization of the SnO2/TiO2 coupled system with a ruthenium polypyridyl complex

In an effort to suppress charge recombination in nanoporous dye sensitized photoelectrochemical (DSPE) solar cells, nanocrystalline coupled semiconduc tor electrodes of the type OTE/SnO2/TiO2 have been prepared, and their phot osensitization with a ruthenium polypyridyl complex, Ru(II), has been carri ed out (OTE is an optically transparent electrode). Improved photoresponse, i.e., higher incident photon to current conversion efficiency (IPCE), high er photovoltage, lower back-electron-transfer rate, k(r), and similar front - and back-face action spectra in the coupled OTE/SnO2/TiO2/Ru(II) system c ompared to those for simple OTE/SnO2/Ru(II) and OTE/TiO2/Ru(II) ones emphas ize the potential of a coupled electrode in bringing about an efficient cha rge separation in nanocrystalline DSPE cells. A negligible photocurrent in a reverse composite OTE/TiO2/SnO2/ Ru(II) system underscores the importance of the proper placement of the energy levels of individual semiconductor c omponents in the coupled system for vectorial electron transfer to ameliora te charge separation. The results of the variation of IPCE in the coupled O TE/SnO2/TiO2/Ru(II) system, where IPCE initially increases but later decrea ses as the thickness of the TiO2 film further increases, suggest an interpl ay between the forces of charge separation and charge recombination. The in crease of IPCE is, of course, due to the better charge separation ability o f the coupled system while the decrease points to the increased charge reco mbination losses. Similar arguments have also been put forth to explain the behavior of the back-electron-transfer rate with the thickness of TiO2 fil m in the coupled system.