Bis-imidazolium iodide organic ionic plastic crystals and their applications to solid state dye-sensitized solar cells

Novel dicationic bis-imidazolium iodide salts have been discovered as new organic ionic plastic crystalline materials. Most of them show multiple solid-solid phase transitions below their melting temperatures. Their phase transition temperatures are dependent on the imidazolium cation structure. According to Timmermans' definition of plastic crystals, bis-imidazolium iodide salts with either n-heptyl or n-octyl side arms can be classified as “true plastic crystals” because of their low ΔS_f values (<20 J mol⁻¹ K⁻¹). The bis-imidazolium iodide salts are stable up to 260 °C under thermal gravimetric analysis. The ionic conductivities, investigated using dielectric relaxation spectroscopy, follow the Arrhenius temperature dependence with discontinuities and changes in slopes at the observed solid–solid phase transition temperature. Dye-sensitized solar cells (DSSCs) fabricated by a whole solid-state electrolyte consisting of n-hexyl side-armed bis-imidazolium iodide (BII-6) show a 4.93% power conversion efficiency (η): a remarkable result for the solid-state electrolyte system compared to that obtained using an liquid electrolyte with 1-butyl-3-methylimidazolium iodide (η = 8.00%) as a reference composition.