Understanding the Electrical Transport–Structure Relationship and Photovoltaic Properties of a [Succinonitrile–Ionic Liquid]–LiI–I2 Redox Electrolyte

The properties of succinonitrile-based electrolytes can be enhanced by the addition of an ionic liquid (IL). Here, we have reported the relationship between the electrical transport properties and the structure of a new [(1 - x )succinonitrile: x IL]-LiI-I 2 electrolyte, where the mole fraction ( x ) of the IL (1-butyl-3-methyl imidazolium iodide) was varied from 0 to 40%. Compositional variation revealed the optimum conducting electrolyte (OCE) at x = 10 mol %, possessing an electrical conductivity (σ 25°C ) of ∼7.5 mS cm -1 with an enhancement of ∼369%. The partial replacement of succinonitrile by the IL eliminated the abrupt change in the slope of the log σ vs T -1 plot at the melting temperature of the succinonitrile-LiI-I 2 system, showing the Vogel-Tamman-Fulcher-type behavior owing to molecular chain disorder. Raman spectroscopy showed the I 3 - concentration nearly twice the I 5 - concentration for the OCE. Vibrational spectroscopy exhibited red shifts in the ν C≡N , ν CH 2 , ν a,CC , ν a,N-CH 3 , and ν s,N-butyl modes, indicating an interaction between succinonitrile and the IL. The area ratio A CH 2 / A C≡N increased slightly for x = 10 mol % (OCE) and largely for x > 10 mol %, indicating an increase in the C-H bond length. These observations indicated that the interaction between succinonitrile and the IL was enhanced at x > 10 mol %, which decreased the electrical conductivity of these electrolytes. Owing to fast ion transport, an OCE-based dye-sensitized solar cell showed a 40-55% decrease in the charge-transfer and Warburg resistances, resulting in ∼139 and ∼122% increases in J SC and η, respectively.