Low Bandgap Conjugated Polymers Based on a Nature-Inspired Bay-Annulated Indigo (BAI) Acceptor as Stable Electrochromic Materials

The donor–acceptor (D–A)
approach, which is to incorporate
alternating electron-rich (donor) and electron-deficient (acceptor)
units along the conjugated polymer mainchain, has become an effective
method to provide an informed search for high performance electrochromic
low bandgap polymers. Herein a potent electron acceptor, namely, a
much more soluble version of the nature-inspired bay-annulated indigo
(BAI), was employed in the synthesis of two solution-processable donor–acceptor
polymers for efficient electrochromic devices (ECDs). The devices
fabricated from spin-coated polymer thin films can switch reversibly
between deep blue and transmissive light green hues, with high optical contrasts
in the visible and near-infrared (NIR) regions, good coloration efficiency
and promising ambient stability. In particular, electrochromic devices
based on the copolymer containing a carbazole donor unit exhibit optical
contrasts of 41% and 59% in the visible and NIR regions, respectively,
and a long-term stability of more than 7500 cycles under ambient conditions
with limited reduction in optical contrasts. Such longer term ambient
stability underlines the great potential of BAI-derived electron acceptors
for the development of practical EC materials.