Ionic liquid-inspired redox shuttles: Properties of a ferrocenylimidazolium salt as an efficient mediator for dye-sensitized solar cells

A ferrocenylimidazolium salt was found to be an efficient iodine/iodide-free redox mediator for a dye-sensitized solar cell (DSSC), giving rise to a photoconversion efficiency of 4% under optimized electrolyte conditions. The excellent redox mediation process was ascribed not only to the electrochemically reversible nature of the molecule with a large heterogeneous electrontransfer rate constant in the order of 10^-2 cm s^-1 at electrodes, but also to an exceptionally high solubility up to 2.7M to produce a large redox gradient in the mediation layer, thus allowing the increase of diffusion-limited current. The high solubility originating from the imidazolium group in the mediator molecule was reminiscent of ionic liquids, which are typically characterized by small absorption coefficients in a visible region. These properties provided a mass-transfer-based concept of molecular design for organic redox mediators in DSSCs.