Iodide-Free Ionic Liquid with Dual Redox Couples for Dye-Sensitized Solar Cells with High Open-Circuit Voltage

A novel ionic-liquid mediator, 1-butyl-3-{2-oxo-2-[(2,2,6,6-tetramethylpiperidin-4-yl)amino]ethyl}-1H-imidazol-3-ium selenocyanate (ITSeCN), has been successfully synthesized for dye-sensitized solar cells (DSSCs). ITSeCN possesses dual redox channels, imidazolium-functionalized 2,2,6,6-tetramethylpiperidine N-oxyl (TEMPO) and selenocyanate, which can serve as the cationic redox mediator and the anionic redox mediator, respectively. Therefore, ITSeCN has a favorable redox nature, which results in a more positive standard potential, larger diffusivity, and better kinetic heterogeneous rate constant than those of iodide. The DSSC with the ITSeCN electrolyte shows an efficiency of 8.38% with a high open-current voltage (V_OC) of 854.3 mV, and this V_OC value is about 150 mV higher than that for the iodide-based DSSC. Moreover, different electrocatalytic materials were employed to trigger the redox reaction of ITSeCN. The ITSeCN-based DSSC with the CoSe counter electrode achieved the best performance of 9.01%, which suggested that transition-metal compound-type materials would be suitable for our newly synthesized ITSeCN mediator.