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 semiconductor electrodes of the type OTE/SnO2/TiO2 have been prepared, and their photosensitization with a ruthenium polypyridyl complex, Ru(II), has been carried out (OTE is an optically transparent electrode). Improved photoresponse, i.e., higher incident photon to current conversion efficiency (IPCE), higher 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 compared to those for simple OTE/SnO2/Ru(II) and OTE/TiO2/Ru(II) ones emphasize the potential of a coupled electrode in bringing about an efficient charge 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 components in the coupled system for vectorial electron transfer to ameliorate charge separation. The results of the variation of IPCE in the coupled OTE/SnO2/TiO2/Ru(II) system, where IPCE initially increases but later decreases as the thickness of the TiO2 film further increases, suggest an interplay between the forces of charge separation and charge recombination. The increase of IPCE is, of course, due to the better charge separation ability of the coupled system while the decrease points to the increased charge recombination losses. Similar arguments have also been put forth to explain the behavior of the back-electron-transfer rate with the thickness of TiO2 film in the coupled system.