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.