Transition of pore-size dependence of ion diffusivity in dye-sensitized solar cells

• We altered the average pore diameters of TiO2 mesoporous thin films using TiCl4 post-treatments.
• The ion diffusivities corresponding to variable pore diameters were measured using DC polarization measurements.
• Two opposite diffusion behaviors were present under the same scheme with a sharp transition in between.
• Surface diffusion is found to be involved in the mass transfer process.

The pore-size dependence of liquid diffusivity in mesopores has been under much debate as two opposite views have been developed. It is especially meaningful in dye-sensitized solar cells (DSSCs) because the ion diffusivity in electrolyte is closely related to the cell performance. Using DC polarization measurements, pore-size dependence of ion diffusivity in DSSCs was investigated based on TiO2 thin films with variable average pore diameters, which were prepared from TiCl4 post-treatments. The respective valid ranges of pore-size dependent and independent regimes were obtained from the trend of normalized diffusivities, which were separated by the transition point located at 5–7 nm in between. In addition, we showed that DSSC fabrication processes, e.g., dye loading, TiCl4 post-treatment will not trigger the transition of diffusion behaviors. Furthermore, the unexpected drop of diffusivity after one TiCl4 treatment is attributed to the involvement of surface diffusion in untreated TiO2 matrix.