A Novel Ionic Liquid with Stable Radical As the Electrolyte for Hybrid Type Electrochromic Devices

An ionic liquid (IL), 1-butyl-3-{2-oxo-2-[(2,2,6,6-tetramethylpiperidin-1-oxyl-4-yl)amino]ethyl}-1H-imidazol-3-ium tetrafluoroborate (TILBF4), containing a stable radical, 2,2,6,6-tetramethyl-1-piperidinyl-oxy (TEMPO), was synthesized successfully. The TILBF4 shows two distinctive properties having the characteristics of ion-storage and supporting electrolyte materials. In this study, a hybrid electrochromic device (ECD) with a high coloration efficiency based on this TILBF4 and a poly(3,3-diethyl-3,4-dihydro-2H-thieno-[3,4-b][1,4]dioxepine) (PProDOT-Et2) thin film was proposed. The maximum absorbance of the PProDOT-Et2/TILBF4 ECD was noticed at 590 nm upon applying the cell potential between −1.0 and 1.0 V. The device exhibits a transmittance change (ΔT) of 62.2% at 590 nm when being switched in the first transition, with short switching times of 4.0 s for bleaching and 3.6 s for coloring. A relatively high coloration efficiency (η) of 983.0 cm2/C was achieved at 590 nm. Furthermore, the ECD maintained 98.0% of its initial ΔT after consecutive operation for 1000 cycles between −1.0 and 1.0 V.