Ion diffusion and electrochromic performance of poly(4,4′,4″-tris[4-(2-bithienyl)phenyl]amine) based on ionic liquid as electrolyte

• Ionic liquids [BMIM]BF4, [BMIM]PF6 and conventional electrolyte TBAP were utilized for the fabrication of polymer films and EC devices.
• The improved ionic conductivity with the introduction of ionic liquid leads to ions more easily dope into the polymer film.
• The thickness of polymer film plays the critical role for the EC switching speed.
• The size of doping anion has essential influence on the electrochemical stability.

Ionic liquids 1-butyl-3-methyl-imidazolium tetrafluoroborate ([BMIM]BF4), 1-butyl-3-methylimidazoliumhexafluorophosphate ([BMIM]PF6) and conventional medium tetrabutylammonium perchlorate (TBAP) were utilized as electrolyte for the preparation of poly(4,4′,4′'-tris[4-(2-bithienyl)phenyl]amine) (PTBTPA) films and assembling of EC devices. FTIR spectra and EDX pattern demonstrate an effective doping of the anion into the PTBTPA film at the oxidation state. SEM images investigate the thickness of the polymer films and the morphologies of crimped structure at the oxidation state. The ion diffusion distance (the thickness of polymer film) plays a more critical role for the EC switching speed. According to the electrochemical impedance spectra, the improved ionic conductivity with ionic liquid [BMIM]BF4 as the electrolyte promote ions more easily dope into the polymer film which leads to the higher optical contrast. In addition, the size of doping anion during the redox process has essential influence on the electrochemical stability.