Enhancing dye-sensitized solar cell performance; optimization of quaternary counterion-based gel polymer electrolyte without changing additives or net-ion composition

A series of novel gel polymer electrolytes (GPEs) was developed for quasi-solid-state dye-sensitized solar cells (DSSCs), to enhance their performance via mixed counterion effect. Here, LiI, CsI, tetrahexylammonium iodide (Hex₄NI), and 1-methyl-3-propylimidazolium iodide (MPII) were used as iodide salts for the preparation of this new GPE. The electrolyte series was investigated by varying the molar fractions of LiI and CsI, keeping the molar fractions of Hex₄NI and MPII constant. The molar composition of the iodide salts in electrolytes is MPII_0.25(Hex₄NI)_0.8CsI_(2-x)LiI_x, where x is the variable. The temperature dependence of conductivity showed Vogel-Tammann-Fulcher behavior. The sample with x = 0.72, where LiI to CsI to Hex₄NI to MPII molar ratio is 72:48:80:25, which gave 8.42 mS cm⁻¹ at 30 °C, displayed the maximum conductivity at all the temperatures. The dependence of the complex AC conductivity on frequency is examined in detail to study the impacts of dielectric polarization effects of the GPEs. Quasi-solid-state DSSCs were constructed by utilizing six-layered TiO₂ photoelectrodes, Pt counter electrode, and the novel GPE series. The three-salt electrolytes, containing LiI only and CsI only, containing DSSC showed efficiencies of 5.72% and 3.53% respectively. The four-salt system, which is composed of LiI to CsI to Hex₄NI to MPII with a molar ratio of 96:24:80:25, demonstrated the highest solar cell efficiency of 7.42%, due to the collective contribution of Hex₄N⁺, MPI⁺, Cs⁺, and Li⁺ ions in improving the charge transport in the electrolyte system. This study shows that DSSC performance can greatly be improved by optimizing counterion ratios without changing total ions in the electrolyte.