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

摘要

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.