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

Abstract

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). 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.

Mechanical breathing in organic electrochromics

“…The characteristic size in ground ITO electrode is in the micron scale, rendering a less dense mechanical interlock and less improved cyclic life. In addition to the surface roughness, SiO 2 NP can also change the physical properties of the ITO surface which benefits the interfacial adhesion 35,54 .…”

“…The growth of the bulk stress in the organic film as well as the interfacial stress between the soft polymer and the hard substrate can cause bending of the thin double layer, wrinkling of the film electrode, crack at the interface, and debonding of the thin film from its electron conduction network. Although tremendous efforts have been placed in synthesizing new materials and modifying the interfacial adhesion 4,17,[35][36][37][38][39][40] , the mechanistic understanding of the damage initiation and evolution in organic thin film electrochromics remains elusive. The rational design of OECDs of enhanced mechanical reliability requires careful analysis on the generation of mechanical strain, the growth of stresses, the translation of mechanical failure into the degradation of device performance, and then a guidance of design to identify key parameters to optimize in future experiments.…”

Advanced developments in nonstoichiometric tungsten oxides for electrochromic applications

“…Open and stretchable applications because of their flexible molecular frameworks interjunctions. 10,110,111 The metal oxides achieve ultrasmall particle sizes with large specific surface area and are expected to exhibit excellent EC functionality, possibly due to short ions diffusion length, easier electrolyte accessibility, excellent coloration efficiency, cyclic stability, and durability.…”

Responsive Smart Windows from Nanoparticle–Polymer Composites

“…[87,88] Various types of electrochromic materials have been used in smart window applications, including inorganic transition metal oxides (e.g., V 2 O 5 , WO 3 , IrO 2 , Nb 2 O 5 , MoO 3 ), [68,89,90] small molecules (e.g., viologens, [91,92] and Prussian blue [63,90,93] ), ITO nanocrystals (NCs), [4,62] electrochromic polymers [91,94] PDLCs, [71,72] and inorganic-organic hybrids. [87,95,96] For high performance windows, the active materials should exhibit strong coloration efficiency, high optical contrast between the transparent and opaque state, competitive photo-stability, and fast switching speed. [79,83] High contrast between the clear state and opaque state, and more functionality (e.g., reducing heat gain) are desired to extend their applications to office windows, and commercial building and residential house windows.…”