A Dual Electrochrome of Poly‐(3,4‐Ethylenedioxythiophene) Doped by N , N ′‐Bis(3‐sulfonatopropyl)‐4‐4′‐bipyridinium—Redox Chemistry and Electrochromism in Flexible Devices

An electrochromic zwitterionic viologen, N , N ′‐bis(3‐sulfonatopropyl)‐4‐4′‐bipyridinium, has been used for the first time for doping poly (3,4‐ethylenedioxythiopene) (PEDOT) films during electropolymerization. Slow and fast diffusional rates for the monomer at deposition potentials of +1.2 and +1.8 V, respectively yielded the viologen‐doped PEDOT films with granular morphology and with dendrite‐like shapes. The dual electrochrome formed at +1.8 V, showed enhanced coloration efficiency, larger electrochemical charge storage capacity, and superior redox activity in comparison to its analogue grown at +1.2 V, thus demonstrating the role of dendritic shapes in amplifying electrochromism. Flexible electrochromic devices fabricated with the viologen‐doped PEDOT film grown at +1.8 V and Prussian blue with an ionic liquid‐based gel electrolyte film showed reversible coloration between pale and dark purple with maximum coloration efficiency of 187 cm 2  C −1 at λ =693 nm. The diffusional impedance parameters and switching kinetics of the device showed the suitability of this dual electrochrome formed as a single layer for practical electrochromic cells.