Transparent photo-electrochromic capacitive windows with a Bi-dopant redox ionic liquids

A transparent self-powered photoelectrochromic capacitive window (PECW) was developed by combining a high color contrast electrochromic polymer, a transparent energy-harvesting layer, and a redox ionic liquid (RIL). The RILs were made of a 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO) moiety connected to an imidazolium cation via a pyrimidine bridge, which was counterbalanced with a bis(trifluoromethanesulfonyl)imide (TFSI) anion. The potential at the PECW electrodes was precisely controlled by the RILs to match the redox reaction for electrochromic switching and photovoltage generation that triggers autonomous photocoloration. The capacitance of the PECW was enhanced by using a bi-dopant system of RILs, which provided p- and n-doping by TFSI and the RIL cation. The PECW with a photosensitizing dye showed high transparency (>90 %) for allowing a high electrochromic color contrast (>85 %) with a long electrochromic cyclability over 4,000 cycles of the window. The optimized PECW recorded the highest photocoloration efficiency (65cm²W⁻¹min⁻¹) among known photo electrochromic windows. The photocharged energy in the PECW could be transferred to power other electronics, such as electrochromic windows or LEDs. With the new RILs, PECWs provide an integrated model that consists of electrochromic and energy-harvesting functions in one device. Thus, the working principle of the PECW can be widely applied to energy-saving smart windows and multifunctional electrochemical devices.