Abstract
In a solid-state dye-sensitized solar cell, a fast-ion conducting (σ(25°C) > 10(-4) S cm(-1)) solid redox mediator (SRM; electrolyte) helps in fast dye regeneration and back-electron transfer inhibition. In this work, we synthesized solid Co(2+/3+) redox mediators using a [(1 - x)succinonitrile: x poly(ethylene oxide)] matrix, LiX, Co(tris-2,2'-bipyridine)(3)(bis(trifluoromethyl) sulfonylimide)(2), and Co(tris-2,2'-bipyridine)(3)(bis(trifluoromethyl) sulfonylimide)(3) via the solution-cast method, and the results were compared with those of their acetonitrile-based liquid counterparts. The notation x is a weight fraction (=0, 0.5, and 1), and X represents an anion. The anion was either bis(trifluoromethyl) sulfonylimide [TFSI(-); ionic size, 0.79 nm] or trifluoromethanesulfonate [Triflate(-); ionic size, 0.44 nm]. The delocalized electrons and a low value of lattice energy for the anions made the lithium salts highly dissociable in the matrix. The electrolytes exhibited σ(25°C) ≈ 2.1 × 10(-3) (1.5 × 10(-3)), 7.2 × 10(-4) (3.1 × 10(-4)), and 9.7 × 10(-7) (6.3 × 10(-7)) S cm(-1) for x = 0, 0.5, and 1, respectively, with X = TFSI(-) (Triflate(-)) ions. The log σ-T(-1) plot portrayed a linear curve for x = 0 and 1, and a downward curve for x = 0.5. The electrical transport study showed σ(TFSI(-)) > σ(Triflate(-)), with lower activation energy for TFSI(-) ions. The anionic effect increased from x = 0 to 1. This effect was explained using conventional techniques, such as Fourier transform infrared spectroscopy (FT-IR), X-ray diffractometry (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), UV-visible spectroscopy (UV-vis), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA).