Efficiency enhancement due to the combined mixed cation effect and TiO₂ nanofiller effect in PEO and ionic liquid-based dye-sensitized solar cells

Dye-sensitized solar cells were fabricated with a polyethylene oxide (PEO)-based quasi-solid (gel) electrolyte consisting of the ionic liquid 1-hexil-3-methylimidazoliym iodide (HMII), and tetrapropyl ammonium iodide (Pr₄N⁺I⁻) as the two iodide salts with two dissimilar cations. Titanium dioxide powder (TiO₂) (P-25) was added to the polymer electrolyte to enhance the iodide ion conductivity further by the nanofiller effect. The electrolyte and the solar cells were characterized by ionic conductivity and impedance measurements, DC polarization test, and current-voltage measurements. The maximum ionic conductivity of 6.95 × 10⁻³ S cm⁻¹ at 30 °C was exhibited by the electrolyte composition with a 5 wt% TiO₂ nanofiller concentration, and the activation energy of this electrolyte was 0.181 eV. The solar cell with this conductivity-optimized gel electrolyte had the highest short-circuit photocurrent of 8.72 mA cm⁻² and the highest efficiency of 4.04% with an open-circuit voltage of 685.4 V and a fill factor of 67.6%. The addition of TiO₂ nanofiller to the PEO-based gel polymer electrolyte has increased the electron recombination lifetime in the TiO₂ photoanode from 2.23 ms for the TiO₂ nanofiller-free electrolyte to 33.84 ms for the filler-added electrolyte. The enhancement of the solar cell efficiency is attributed to the mixed cation effect and the diffusion-mediated ionic conductivity increase due to the presence of the TiO₂ nanofiller.