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 (σ_25°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 σ_25°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.