Ultraviolet photoelectron spectroscopy of nanocrystalline TiO2 films sensitized with (2,2′-bipyridyl)ruthenium(II) dyes for photovoltaic applications

Ultraviolet photoelectron spectroscopy (UPS) has been used to determine the highest occupied molecular orbital (HOMO) energy levels of a series of related 2,2′-bipyridyl-substituted ruthenium dyes utilized in the fabrication of photovoltaic devices. UPS analysis of nanocrystalline TiO2 thin films sensitized with a monolayer of cis-dithiocyanato-N,N-bis-(2,2′-bipyridyl-4,4′-dicarboxylic acid)-ruthenium(II), commonly referred to as “N3”, indicates that the HOMO is located 5.47 eV below the vacuum level. Combination of the UPS results with measurement of the bandgap of the dye by UV-Vis absorbance spectroscopy yields an estimated lowest unoccupied molecular orbital (LUMO) energy of 3.93 eV below the vacuum level. Similar analyses have been performed for the related dyes, (5-amino-1,10-phenanthroline)bis-(2,2′-bipyridyl-4,4′-dicarboxylic acid)-ruthenium(II), (1,10-phenanthroline)bis-(2,2′-bipyridyl-4,4′-dicarboxylic acid)-ruthenium(II), and (5-amino-1,10-phenanthroline)bis-(2,2′-bipyridine)-ruthenium(II); giving values of 3.35, 3.08, and 3.43 eV, respectively, for the positions of the LUMOs below the vacuum level. These results indicate that the LUMO of N3 has better overlap with the conduction band of TiO2 located 3.70 eV below the vacuum level, and this may at least partially be responsible for its outstanding photovoltaic performance.