High-contrast electrochromism from layer-by-layer polymer films

Layer-by-layer (LBL) assembly is the ideal processing technique to combine two electrochromic polymers into a single thin film composite with fine control over morphology and composition. Here we present a very-high-contrast electrochromic composite developed from the LBL assembly of two readily available cathodically coloring electrochromic polymers: poly(hexyl viologen) (PXV) and the poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:SPS) colloid. Characterization of assembly reveals that film thickness increases exponentially with increasing layer-pair number, seemingly due to the globular nature of the PEDOT:SPS colloid. Elemental analysis confirmed the presence of both polymers in the final film and allowed the determination of a bulk composition profile. In detailed electrochemical and spectral investigations, the "dual electrochrome" was compared to single-electrochrome LBL films, confirming that both polymers contribute to electrochromic switching. Due to the unusual film architecture and complex interactions between PXV and PEDOT, a charge-trapping system is formed within the layers, resulting in a type of electrochemical rectification. The performance of PXV/PEDOT:SPS as a material for electrochromic applications was extremely competitive, with color-change response times ranging from 1 to 4 s and high contrast between a transmissive oxidized state and a deep purple/blue reduced state. The LBL approach was used to optimize the thickness of the polymer film, and a 45-layer pair PXV/PEDOT:SPS film was found to exhibit a transmittance change of 82.1% at 525 nm, one of the highest reported contrasts in polymer electrochromics. These results demonstrate the power of the LBL processing technique to create new electroactive materials with both unusual properties and unequalled potential for application. Future directions are illuminated for the LBL development of multiply colored, fast switching, and high contrast electrochromic materials from cheap and readily available electrochromic polyions.