A new mechanism for interpreting the effect of TiO₂ nanofillers in quasi-solid-state dye-sensitized solar cells

A new mechanism is proposed against the Grotthuss-type exchange reaction, to interpret the TiO₂ nanofiller effect in quasi-solid-state dye-sensitized solar cells. Generally, the inclusion of TiO₂ nanofillers in a polymer gel electrolyte causes an enhanced diffusion coefficient and a reduced charge transfer resistance at the electrolyte/counter-electrode interface, thereby improving the photovoltaic performance of the corresponding solar cell. Herein, liquid electrolytes are treated by TiO₂ nanoparticles, and the resultant electrolytes yield similar effects on both the electrolyte properties and cell performance. This result suggests a facilitated movement of the triiodide species; however, it cannot be elucidated by the Grotthuss-type mechanism, because of the absence of nanoparticles in such liquid electrolytes. The X-ray photoelectron spectroscopy analysis shows that the TiO₂ particles can adsorb iodide ions through their acidic surfaces. The adsorption of iodide ions leads to negatively charged surfaces, which further induces attraction to cations. As a result, cation concentrations in the electrolyte are reduced, and furthermore, the triiodide species can move more easily owing to the attenuated electrostatic interaction with cations. This mechanism is considered to be a dominant reason for the TiO₂ nanofiller effect in quasi-solid-state dye-sensitized solar cells.