Electrochromic conjugated polymers containing benzotriazole and thiophene performing sub-second response time and 916 cm2 C−1 superb coloration efficiency

In pursuit of high-performance electrochromic conjugated polymers (ECPs), three benzotriazole-thiophene ECPs employing benzotriazole as acceptor and different thiophene repeating units as donors, namely, PBTz-T, PBTz-BT, and PBTz-TT, were synthesized via direct (hetero)arylation polymerization (DHAP). Meanwhile, following the above ECPs model, we investigated the effect of corresponding conjugated chain length of monomers on their optical, electrochemical and electrochromic properties. The results showed that along with the increase of the numbers of thiophene in the repeating unit of ECPs, the electronic and optical band gaps decreased, the UV–Vis absorption and fluorescence spectra showed significant bathochromic shift, and the electrochemical cycle stability gradually improved. PBTz-T exhibited giant coloration efficiency (916 cm2 C−1 at 530 nm), which is one of the best results in the visible region. And higher coloration efficiency means less energy consumption, which holds great potential for various applications, including electrochromic back covers of smartphones. PBTz-BT displayed the highest transmittance (28% at 550 nm), and BTz-TT illustrated the fastest switching time (0.6 s/0.5 s for doping/dedoping process). Flexible electrochromic device (ECD) fabricated employed PBTz-T as functional layer gradually changed from red (reduction) to dark green (oxidation), and maintained good cycling stability.