Color tuning for black-to-transmissive conjugated copolymer with excellent electrochromic properties via electrochemical copolymerization of two donor-acceptor type monomers

Copolymerization towards producing broadly absorbing black neutrality for flexible electrochromics was investigated. Neutral-state saturated green and dark blue donor-acceptor-donor (D-A-D) polymers consisting of electron-acceptor thieno[3,4-b]pyrazine and benzo[1,2,3]triazole based units respectively, were chosen as distributed segments, simultaneous with their tendency to switch to transmissive oxidation states. Variation of the feed ratio of two precursors in a mixture solution was used to regulate the energy and spectral absorption range of electrochemical copolymerization films. Co-polymer PEM-11 in 1:1 monomer feed ratio demonstrated a full range of visible absorption, and switched between an improved saturated black neutral state (L* = 43.68, a* = 4.98, b* = -13.13) and a light gray transmissive oxidation state (L* = 67.30, a* = -1.36, b* = 1.67) reversibly, where L* represents white to black balance, a* red to green and b* yellow to blue, with the optical contrast of 35% at 590 nm and switching speed as low as 0.6 s. Furthermore, the PEM-11 film exhibited stable n-doping, favorable cycle stability, and a lower electrochemical/optical band-gap (0.80 eV/1.21 eV) in comparison with the reported individual neutral black ECPs. The results suggested that the as-obtained copolymer could be a promising candidate for electrochromic windows, e-papers, or eyewear applications. (c) 2020 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).