The adsorption behavior of a rutheninm-based sensitizing dye to nanocrystalline TiO2 - Coverage effects on the external and internal sensitization quantum yields

The adsorption of the ruthenium-based dye molecules, cis-di(thiocyanato)bis (2,2 '-bipyridyl-4,4'-dicarboxylate)ruthenium(II) (N3), to nanocrystalline TiO2 (anatase) was studied. Adsorption and desorption kinetics were measure d. Effective adsorption isotherms and desorption isotherms were then obtain ed. A two-step dye adsorption mechanism is postulated where initial binding of N3 is with one carboxylate, with subsequent binding of two or more carb oxylate groups. Dye (N3) coverage effects on the photon-to-current conversi on efficiencies were investigated by measuring the photocurrent action spec tra and the optical absorbance of nanocrystalline TiO2 films sensitized wit h various N3 coverages. The incident photon-to-current efficiency (IPCE) an d 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 w as proposed where at greater than 30% coverage hole transfer between adjace nt N3 molecules becomes possible. This percolation of holes through the N3 network facilitates the regeneration of oxidized N3 molecules by the redox species (I-) in the matrix of the nanoporous structure, resulting in the su dden increases in the IPCE and the APCE. Other mechanisms fbr this effect, including a role of N3 clusters In two-electron oxidation of I-, are also d iscussed. (C) 1999 The Electrochemical Society. S0013-4651(99)03-078-5. All rights reserved.