Engineering the Electrochromism of Layer-by-Layer Assembled Films
Dean M. DeLongchamp, P T. Hammond
This work applies the processing technique of layer-by-layer (LBL) assembly to the creation and development of new electrochemically-active composites. Layer-by-layer assembly is a type of assisted self-assembly that can create macromolecular complex films with a fine degree of control. The classical LBL process involves alternating exposure of an ionized substrate to dilute aqueous solutions of polycations and polyanions. With each exposure, a polyion layer is deposited and surface ionization is reversed, allowing a subsequent layer of opposite charge to be deposited. Smooth and uniform composite films of any thickness and composition can be created to meet a wide variety of applications.Here we investigate the capability of LBL assembly to alter and enhance the properties of electrochromic films by varying molecular blending. The chromophores for this investigation were appropriated from all corners of the materials spectrum, including discrete redox polymers, conjugated polymers, soft colloidal suspensions, and inorganic nanoparticle dispersions.Some results of interest:The combination of a discrete electrochromic polyviologen with a hydrophilic counterpolyacid known for fast ionic conduction dramatically accelerated the switching of the composite. Furthermore, polyviologen radical cation dimerization was controlled by modulating counterpolyanion hydrophobicity, an effect that shifted its colored hue.Changing counterpolymer acidity modulated the electrochromic color intensity of the conducting polymer polyaniline by shifting acidity-dependent equilibrium between two polyaniline forms within the film.LBL assembled films containing the conducting polymer colloid poly(3,4-ethylene dioxythiophene) (PEDOT):poly(styrene sulfonate) featured an unusual variation in film morphology and deposition character that influenced electrochemical resistivity, leading to a non-monotonic switching speed variation with increased film thickness.LBL films containing metal hexacyanoferrate nanocrystals of the Prussian blue family were constructed that displayed fast and deep electrochromic coloration; synthetic nanocrystal variation led to several different inorganic/polymer composite films that could potentially be considered as elements in a full-color switchable CMYK (Cyan, Magenta, Yellow, Black) display.Then followed the successful fabrication of dual electrochrome electrodes. The capability of combining two electrochromophores into a single film led to two strategies: enhanced contrast and multihued coloration. The LBL combination of polyviologen and the PEDOT colloid resulted in a film with superior contrast that spanned a large range of the visible spectrum owing to the additive contributions of the two polymers. The LBL combination of polyaniline and Prussian blue into the same electrode resulted in highly tunable multihued coloration, switching reversibly between clear, green, and blue. These two dual electrochromes exhibited very different operation: the polyviologen/PEDOT system displayed strong electrochemical interactions resulting from a suspected charge-trapping mechanism, while the polyaniline/Prussian blue system exhibited no observable interactions at all. The wide-ranging possibilities offered by dual electrochrome design strategies form a singularly unique contribution that LBL assembly can add to the field of electrochromics fabrication and the general fabrication of electroactive coatings.