Self-Bleaching Behaviors in Black-to-Transmissive Electrochromic Polymer Thin Films

Polymer-based electrochromic smart windows are an emerging energy-saving technology. There are several technological hurdles in the development of organic electrochromics. In this article, the self-bleaching behaviors of a black electrochromic polymer (ECP-black) thin film were investigated. We found that the electrochemical break-in process led to a less dense morphology and the increased free volume facilitated ion permeation in the ECP-black thin films. The polarized interface between the polymer thin film and transparent indium-tin-oxide (ITO) electrode made charge transfer accessible, which caused the self-bleaching behaviors. Herein, we proposed two approaches to study and mitigate the self-bleaching phenomenon. First, a densely packed morphology was regenerated by increasing the cathodic polarization time under open-circuit conditions (<i>V</i>_off). The second involved the modification of the electrode (ITO) surface with a partial coverage of the octadecyltrichlorosilane layer. The combination of the two approaches rendered the ECP-black thin film capable of maintaining the colored state for up to 900 s. To extend the scope of our studies, self-bleaching of ECP-magenta and ECP-blue thin films were also tested and suppressed by using these two methods. Additionally, the cycling stability of the ECP-black has been improved from ∼600 cycles to up to 2300 cycles without a noticeable decay of optical contrast

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)-<i>co</i>-(4-benzoylphenyl methacrylate) (PTMA-<i>co</i>-BP) has been developed as the ion storage layer in electrochromic devices (ECDs). After photo-crosslinking, the dissolution of PTMA-<i>co</i>-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-<i>co</i>-BP increased because of the formation of a crosslinked network. By matching the redox capacity of the PTMA-<i>co</i>-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