Electrochromic polymers based on 3,5-di(9H-carbazol-9-yl)benzonitrile and bithiophene as anodically coloring films for high-contrast and rapid switching electrochromic devices

A 3,5-di(9H-carbazol-9-yl)benzonitrile (CzBN)-containing homopolymer (P(CzBN)) and five CzBN-containing 2,2′-bithiophene (BTP) copolymers, namely (P(CzBN-co-BTP)), P(CzBN-co-BTP2), P(CzBN-co-BTP3), P(CzBN-co-BTP4), and P(CzBN2-co-BTP), with various feeding molar ratios of CzBN and BTP were electrodeposited on transparent conductive indium tin oxide (ITO) electrodes, and their electrochromic performances were characterized. The P(CzBN-co-BTP4) film exhibited distinct electrochromic behaviors and displayed three different colors (brown, grayish-blue, and olive green) at different potentials. Electrochromic kinetic analyses of the anodic polymers revealed that P(CzBN-co-BTP) had a high transmittance change (ΔT%) (59.6% at 960 nm) and high coloration efficiency (η) (174.3 cm²/C at 960 nm) in three-electrode cells. Six dual-layer organic electrochromic devices (ECDs) were constructed using P(CzBN), P(CzBN-co-BTP), P(CzBN-co-BTP2), P(CzBN-co-BTP3), P(CzBN-co-BTP4), or P(CzBN2-co-BTP) as the anodically coloring layers and PEDOT as the cathodically coloring layer. The P(CzBN-co-BTP2)/PEDOT ECD had a high ΔT (43.7% at 627 nm) and a rapid response time (≤2.0 s), while the P(CzBN)/PEDOT ECD had a high η (616.7 cm²/C at 625 nm) and adequate optical memories. These electrochromic results suggest that cyano-containing polycarbazoles are promising high-contrast and rapid-response electrodes for ECD applications.