Effect of quaternary cations on the efficiency of quasi-solid-state dye-sensitized solar cells

Abstract

The electrolyte is one of the main components of Dye-Sensitized Solar Cells (DSSCs). In this study, a gel polymer electrolyte (GPE) was synthesized to maintain prolonged stability of the electrolyte and to enhance the performance of quasi-solid state DSSCs by using the mixed counter ion effect, utilizing four iodides which were LiI, CsI, tetra-hexyl-ammonium iodide (Hex₄NI) and 1-butyl-3-methylimidazolium iodide (BMII). Ethylene carbonate, propylene carbonate, polyethylene oxide, and 4-tertbutylpyridine were also included in the electrolyte. A series of electrolytes was investigated by varying the molar fraction of LiI and CsI. The investigation of the temperature dependence of conductivity revealed that these samples follow the Voggel-Tamman-Fulture behaviour. The sample in which LiI: CsI: Hex4NI: BMII molar ratio is 72:48:80:25 displayed a maximum conductivity of 12.69 mS cm-1 at the ambient temperature. A series of quasi-solid-state DSSCs was constructed by utilizing six-layered TiO2 photo-electrodes, and the photo-current conversion efficiencies of the cells were studied. DSSCs that contain only either CsI or LiI showed lower efficiencies of 3.68% and 3.48%, respectively. The four-salt system, which consisted of LiI: CsI: Hex₄NI: BMII in molar ratio 96:24:80:25, showed the highest cell efficiency of 6.36%, even though this was not the composition that showed the highest conductivity. This increase in solar cell efficiency can be attributed to the collective contribution of Hex4N⁺ , Cs⁺ , Li⁺, and BMI⁺ ions because larger cations like Hex₄N⁺ , Cs⁺ , and BMI⁺ increase open circuit voltage and smaller cations like Li⁺ improve charge transport and transfer in the system, increasing short circuit current density. This study suggests that LiI, CsI, Hex₄NI, and BMII is a novel iodide salt mixture suitable for fabricating highly efficient DSSCs.