Electronic structure of the indium tin oxide/nanocrystalline anatase ( TiO 2 ) /ruthenium-dye interfaces in dye-sensitized solar cells

The electronic structure of two interfaces commonly found in dye-sensitized photovoltaic cells based on nanocrystalline anatase TiO 2 ("Grätzel cells") was investigated using photoemission spectroscopy (PES). X-ray photoemission spectroscopy (XPS) and ultraviolet photoemission spectroscopy (UPS) measurements were carried out on the indium tin oxide ( ITO ) / TiO 2 and the TiO 2 / cis -bis(isothiocyanato)bis( 2 , 2 ′ -bipyridyl- 4 , 4 ′ -dicarboxylato)-ruthenium(II)bis-tetrabutyl ammonium dye ("N719" or "Ruthenium 535-bisTBA") interfaces. Both contacts were investigated using a multistep deposition procedure where the entire structure was prepared in vacuum using electrospray deposition. In between deposition steps the surface was characterized with XPS and UPS resulting in a series of spectra, allowing the determination of the orbital and band lineup at the interfaces. The results of these efforts confirm previous PES measurements on TiO 2 /dye contacts prepared under ambient conditions, suggesting that ambient contamination might not have significant influence on the electronic structure at the dye/ TiO 2 interface. The results also demonstrate that there may be a significant barrier for electron injection at the sputtered ITO / TiO 2 interface and that this interface should be viewed as a semiconductor heterojunction rather than as metal-semiconductor (Schottky) contact.