Electronic and Molecular Surface Structure of a Polyene−Diphenylaniline Dye Adsorbed from Solution onto Nanoporous TiO₂

The surface electronic and molecular structure of a new organic chromophore useful for dye-sensitized nanostructured solar cells has been investigated by means of electron spectroscopy. Initially the use of a simple molecular system containing the polyene−diphenylaniline chromophore in a solar cell device was verified. The electronic and molecular surface structure of the functional dye-sensitized interface was then investigated in detail by a combination of core level spectroscopy, valence level spectroscopy, X-ray absorption spectroscopy, and resonant photoemission spectroscopy. The results indicate a dominating orientation of the molecule at the surface, having the diphenylaniline moiety pointing out from the surface. Valence level spectroscopy, X-ray absorption spectroscopy, and resonant photoemission spectroscopy were used to experimentally delineate the frontier electronic structure of the molecule, and the experimental spectra were analyzed against theoretical spectra, based on density functional theory. Together the investigation gives insight into energy matching of the molecular electronic states with respect to the TiO 2 substrate as well as the localization of the frontier electronic states and the direction of the charge-transfer absorption process with regards to the TiO 2 surface.