Quasi-gel-state ionic liquid electrolyte with alkyl-pyrazolium iodide for dye-sensitized solar cells

A series of non-volatile viscous ionic liquid, 1-alkyl-2-methylpyrazolium iodides (RPyI: R = C₃-C₇), were synthesized for the electrolyte of dye-sensitized solar cells (DSSC). Most of RPyI revealed extremely viscous quasi-gel-state electrolytes potentially preventing leakage and evaporation of electrolytes of the solar cells. A DSSC using the electrolyte composed of 1-hexyl-2-methylpyrazolium iodide (C₆PyI) with iodine exhibiting higher conversion efficiency of 3.8% (under 1 Sun) for 10% larger short-circuit photocurrent, J_SC, than that with a conventional ionic liquid, 1-hexyl-3-methyllimidazolium iodide (C₆ImI) with iodine, in spite of less favorable viscosity (2013 mPa s (C₆PyI/I₂) vs. 1439 mPa s (C₆ImI/I₂)) for the physical diffusion of charge carriers in the electrolyte. Furthermore, the quasi-gel-state electrolytes composed of a series of RPyI ionic liquids surprisingly exhibiting comparable J_SC to that with much less viscous C₆ImI/I₂. We discuss the results of quasi-gel-state RPyI ionic liquid electrolyte for DSSC based on the Grotthus-like electron exchange mechanism of iodide redox species in the highly viscous RPyI ionic liquid evaluated qualitatively by Raman spectroscopic observation of poly-iodide species.