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 (V_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.