Highly Transparent Crosslinkable Radical Copolymer Thin Film as the Ion Storage Layer in Organic Electrochromic Devices

A highly transparent crosslinkable thin film made of the radical polymer poly(2,2,6,6-tetramethyl-4-piperidinyloxy methacrylate)-co-(4-benzoylphenyl methacrylate) (PTMA-co-BP) has been developed as the ion storage layer in electrochromic devices (ECDs). After photo-crosslinking, the dissolution of PTMA-co-BP in electrolytes was mitigated, which results in an enhanced electrochemical stability compared with the homopolymer PTMA thin film. Moreover, the redox capacity of PTMA-co-BP increased because of the formation of a crosslinked network. By matching the redox capacity of the PTMA-co-BP thin film and bis(alkoxy)-substituted poly(propylenedioxythiophene), the ECD achieved an optical contrast of 72% in a small potential window of 2.55 V (i.e., switching between +1.2 and -1.35 V), and it was cycled up to 1800 cycles. The ECD showed an excellent optical memory as its transmittance decayed by less than 3% in both the colored and bleached states while operating for over 30 min under open-circuit conditions. Use of crosslinkable radical polymers as the transparent ion storage layer opens up a new venue for the fabrication of transmissive-mode organic ECDs.