Dye-sensitized solar cells containing mesoporous TiO2 spheres as photoanodes and methyl sulfate anion based biionic liquid electrolytes

Abstract

Mesoporous TiO₂ spheres (hereafter, MSs) were synthesized and incorporated into the photoanode of a quasi-solid state dye-sensitized solar cell (QSS-DSSC) containing an air stable ionic liquid electrolyte, namely 1-propyl-3-methylimidazolium iodide (PMII) along with triethylmethylammonium methyl sulfate (TEMAMS) (65 : 35 = v/v), in order to improve its performance. The presence of large pores in the MS, as confirmed using a high resolution scanning electron microscope (HR-SEM) and a mercury porosimeter, facilitates the penetration of QSS-electrolytes into the thin film, whereas their high surface area (108.1 m² g⁻¹) helps for high dye loading. Further, the large particle size increases the scattering ability of incident light leading to an excellent increment in the number of photons. The performance of DSSCs containing bi-ionic liquid (bi-IL) based electrolytes, namely, PMII/TEMAMS and PMII/1-ethyl-3-methylimidazolium tetrafluoborate (EMIBF₄), with the addition of 0.2 M iodine, 0.4 M N-methylbenzimidazole (NMBI), and 0.15 M guanidiniumthiocyanate (GuSCN), was compared, where the DSSC based on PMII/TEMAMS bi-ILs demonstrates superior efficiency (6.18%) than that of PMII/EMIBF₄ based one (4.53%). The DSSC with PMII/TEMAMS shows extraordinary durability and unfailing stability for 1200 h even though it was stored in the dark at 50 °C. Further, the PMII/TEMAMS bi-IL electrolyte shows a gel-state at room temperature naturally without adding any gelators or polymers.