Electrical transport properties of [(1 − x)succinonitrile:xpoly(ethylene oxide)]-LiCF3SO3-Co[tris-(2,2′-bipyridine)]3(TFSI)2-Co[tris-(2,2′-bipyridine)]3(TFSI)3 solid redox mediators

Abstract

A solid redox mediator (solid electrolyte) with an electrical conductivity (σ25°C) greater than 10−4 S cm−1 is an essential requirement for a dye-sensitized solar cell in the harsh weather of Gulf countries. This paper reports the electrical properties of solid redox mediators prepared using highly dissociable ionic salts: Co[tris-(2,2′-bipyridine)]3(TFSI)2, Co[tris-(2,2′-bipyridine)]3(TFSI)3, and LiCF3SO3 as a source of Co2+, Co3+, and Li+ ions, respectively, in a solid matrix: [(1 − x)succinonitrile:xpoly(ethylene oxide)], where x = 0, 0.5, and 1 in weight fraction. In the presence of large size of cations (Co2+ and Co3+) and large-sized and weakly-coordinated anions (TFSI− and CF3SO3−), only the succinonitrile-poly(ethylene oxide) blend (x = 0.5) resulted in highly conductive amorphous regions with σ25°C of 4.7 × 10−4 S cm−1 for EO/Li+ = 108.4 and 3.1 × 10−4 S cm−1 for EO/Li+ = 216.8. These values are slightly lower than 1.5 × 10−3 S cm−1 for x = 0 and higher than 6.3 × 10−7 S cm−1 for x = 1. Only blend-based electrolytes exhibited a downward curve in the log σ-T−1 plot, a low value of pseudo-activation energy (0.06 eV), a high degree of transparency, and high thermal stability, making it useful for device applications.