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:xIL]-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 (sigma(25 degrees C)) of similar to 7.5 mS cm(-1) with an enhancement of similar to 369%. The partial replacement of succinonitrile by the IL eliminated the abrupt change in the slope of the log sigma 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 nu(C N), nu(CH2), nu(a,CC), nu(a,N-CH3), and nu(s,N-butyl) modes, indicating an interaction between succinonitrile and the IL. The area ratio A(CH2)/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 similar to 139 and similar to 122% increases in J(SC) and eta, respectively.