In dye-sensitizd solar cells, a wide band-gap oxide semiconductor is activated by the adsorption of organic dye molecules whose highest occupied molecular orbital (HOMO) - lowest unoccupied molecular orbital (LUMO) energy gaps are in the useful portion of the solar spectrum. Moreover, effective charge transfer of photoexcited electrons from the molecule 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(1120) single crystal surfaces, we have compared the position of the HOMO and LUMO levels of the N3 Ruthenium based-dye with those of isonicotinic acid. Isonicotinic acid, a simpler form of the linker portion of the molecule that binds the N3-dye to the TiO2 surface, 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 the central ruthenium ion, to the LUMO delocalized on the dye linker to the surface. Comparison with experimental and theoretical values from the literature will be discussed.
Another class of dye molecules with the potential of great flexibility in structural and electronic configuration, is metalloporphyrins. As these molecules play an essential role in photosynthetic mechanisms, they are natural candidates for electron transfer mediator in DSSCs. Among the possible metalloprophyrins, the zinc tetraphenylporphyrins (ZnTPP) derivatives have been found to have similar electron injection and charge recombination properties as the important standard ruthenium dye N3 for DSSCs, as well as reasonable performances using TiO2 or ZnO as substrates. We are investigating the electronic structure, energy level alignment, and their changes with altered surface bonding geometries, using a selective functionalization with carboxylic anchoring groups of the meso-phenyl, of functionalized ZnTPP on single crystal TiO2 and ZnO surfaces. Recent results will be discussed.