The adsorption behavior of a ruthenium-based sensitizing dye to nanocrystalline TiO{sub 2}. Coverage effects on the external and internal sensitization quantum yields

The adsorption of the ruthenium-based dye molecules, cis-di(thiocyanato)bis(2,2{prime}-bipyridyl-4,4{prime}-dicarboxylate)ruthenium(II) (N3), to nanocrystalline TiO{sub 2} (anatase) was studied. Adsorption and desorption kinetics were measured. Effective adsorption isotherms and desorption isotherms were then obtained. A two-step dye adsorption mechanism is postulated where initial binding of N3 is with one carboxylate, with subsequent binding of two or more carboxylate groups. Dye (N3) coverage effects on the photon-to-current conversion efficiencies were investigated by measuring the photocurrent action spectra and the optical absorbance of nanocrystalline TiO{sub 2} films sensitized with various N3 coverages. The incident photon-to-current efficiency (IPCE) and the absorbed photon-to-current efficiency (APCE) showed abrupt increases at a coverage just above 0.3 monolayers. In order to explain the nonlinear increases in the IPCE and the APCE, the onset of a hole-hopping mechanism was proposed where at greater than 30% coverage hole transfer between adjacent N3 molecules becomes possible. This percolation of holes through the N3 network facilitates the regeneration of oxidized N3 molecules by the redox species (I{sup {minus}}) in the matrix of the nanoporous structure, resulting in the sudden increases in the IPCE and the APCE. Other mechanisms for this effect, including a role of N3 clusters in two-electron oxidation of I-, are also discussed.