MORPHOLOGY AND ADSORBATE DEPENDENCE OF IONIC TRANSPORT IN DYE-SENSITIZED MESOPOROUS TIO2 FILMS

By modeling the steady-state mass transport in a thin-layer cell confi guration involving an electrochemically inactive mesoporous colloidal film, the limiting currents of an iodine containing electrolyte can be predicted, as a function of the film porosity. The porosity of the la yer is then determined by best fit of the porosity parameter in the mo del, given the experimentally determined limiting currents. Satisfacto ry agreement is found with the porosity values obtained by BET measure ments. In photoelectrochemical cell applications such as dye sensitize d nanocrystalline photoelectrodes for solar energy conversion devices, the porosities of the dye loaded films of e.g. anatase TiO2 play an e ssential role with respect to their efficiency. In particular, the fin dings of this work demonstrate that the presence of adsorbed sensitize r cis-(SCN-)(2) bis(2,2'-bipyridyl- 4,4'-dicarboxylate)ruthenium(II) i n TiO2 colloidal films appears to decrease the original naked film por osity by up to 30%. This depends on the initial bare film porosity and the dye diameter, thus resulting in a considerably lower effective ma ss-transport limit within the coated films. The effective diffusion co efficient of triiodide in these porous electrodes was found to not dev iate significantly with respect to the free-stream values, suggesting an influence of mesoporosity on the transport mechanisms involved.