Layer-by-layer assembled electrochromic films for all-solid-state electrochromic devices

A layer-by-layer (LBL) assembled electrochromic (EC) film was investigated by using poly(aniline-N-butylsulfonate)s (PANBS) as an electrochromic anionic polymer, and acid-doped polyaniline (PAN) and vinylbenzyldimethyl-n-octadecylammonium salts (VBOD) as a polycation. The layer sequence of the EC films was varied to optimize EC properties by changing the order of layer deposition. Electrochemical and electrochromic properties of EC films were highly dependent on the layer sequence. In the EC film composed alternatively of the PANBS/PAN and PANBS/VBOD bilayers (L1), higher redox current density was observed than that of the EC film composed conductive PANBS/PAN layers. Furthermore, optical change (Delta OD) between the colored and the bleached state was largest in L1 among the EC films assembled in this study. This indicates that charge transport in L1 is more facile, reducing interlayer barrier (ILB) energy for charge transfer, possibly due to the flexible long alkyl chains of VBOD that may facilitate ion transport. An all-solid-state EC device based on an optimized LBL composition showed an EC response at 3 V within 1 s, with a stable memory effect.