Lithium-coordinating ionic conductor for solid-state dye-sensitized solar cells

A new solid-state ionic conductor is synthesized by linking an ether group to the nitrogen-atom of 1,2-dimethylimidazole with an iodide counter anion, and the single crystal structure is determined using X-ray crystallographic analysis. Replacement of the butyl group in 1-butyl-2,3-dimethylimidazolium iodide with an ether group induces a significant improvement in conductivity. When the solid ionic conductor is mixed with LiI alone, conductivity enhancement is more remarkable for the ether-containing ionic conductor due to the lithium coordination to the ether oxygen, which is able to avoid the aggregation of lithium cations with iodides and hence improves the transport properties of Li⁺. Owing to the π-stacking of imidazolium rings for the ether-containing ionic conductor, the increment of ionic conductivity is also more significant upon further doping with I₂. The ether-containing ionic conductor mixed with LiI alone as the solid electrolyte can even make the solid-state dye-sensitized solar cells work. Further doping with iodine achieves power conversion efficiency of 7.1%, which is much higher than that (5.3%) for the alkyl analogue due to the positive shift of the conduction band edge of titanium dioxide and suppression of charge recombination caused by the ether group.