Paper IS+SS-ThM6
Anchoring of N3 Dye Molecule on TiO₂(110) Surface and its Influence on Energy Level Alignments

Thursday, November 12, 2009, 9:40 am, Room C4

In dye-sensitized solar cells, the electronic coupling between the dye-sensitizer and TiO₂ determines the alignment of energy levels and the electron transfer dynamics. This electronic coupling is strongly modified by the way in which the dye molecules are adsorbed and anchored to TiO₂ surface, its relative orientations and geometrical structures. The present study is an attempt to understand how the dye molecules are bound to TiO₂ single crystal surface and their influence on the energy level alignments and electron transfer dynamics. Controlled adsorption of dye molecules (bis(4,4’-dicarboxy-2,2’-bipyridine)-bis(isothiocyanato)ruthenium(II) is known as N3) onto TiO₂ surface has been carried out to obtain desired anchoring and orientations by varying the concentration, hydrophobic substituents on dye and defect concentrations (in TiO₂). ToF-SIMS and PEEM mapping techniques were used to understand the distribution of dye molecules on TiO₂ surface. The XPS and UPS measurements were carried out to determine the structural and electronic properties including the stoichiometry of dye molecules on TiO₂ surface, the relative energy levels and band offsets in the absence and presence of UV light. The XPS results show that the atomic ratios of S to Ru and N to S are ~1 and ~5, respectively, which correspond to a non-stoichiometric composition of N3 molecule on TiO₂ surface. The theoretical atomic ratios of S to Ru and N to S in N3 molecules are 2 and 3, respectively. This suggests that an isothiocyanide group from N3 molecule is lost from the surface of TiO₂. The XPS from multilayers of N3 molecules on Si surface however show stoichiometric composition.