Paper EN-TuP3
Electronic Energy Level Alignment in Dye Sensitized Oxide Substrates

Tuesday, October 21, 2008, 6:30 pm, Room Hall D

In dye-sensitized solar cell applications, the HOMO-LUMO gap of dye molecules determines the useful portion of the solar spectrum, and charge transfer of photoexcited electrons to the substrate depends on the alignment of the LUMO to the substrate conduction band edge. We have used direct and inverse photoemission to measure the occupied and unoccupied electronic states of several dye-related molecules and determine their alignment with the band edges of single crystal and nanostructured TiO₂ and ZnO substrates. On well characterized rutile TiO₂(110) and wurtzite ZnO(11-20) single crystal surfaces, we have compared the position of the HOMO and LUMO levels of the N3 Ruthenium based-dye, of isonicotinic acid and catechol molecules. Isonicotinic acid, as a simpler form of the linker that binds the N3-dye on TiO₂, is found to have a very similar electronic structure as the N3-dye in the unoccupied states. This is consistent with the electronic transfer scheme where N3 excitation occurs from the HOMO localized on its Ruthenium center, to the LUMO delocalized on the dye linker to the surface. Catechol on TiO₂(110) however has no electronic states degenerated with the bottom of the conduction band edge. As a consequence the lower energy excitation process occurs via a HOMO to conduction band process. We have also measured the electronic structure of N3 adsorbed on more technologically relevant TiO₂ anatase nanoparticle and ZnO nanorod substrates.