Engineering the electrochromism of layer-by-layer assembled
Dean M. DeLongchamp, Advisor Paula T. Hammond (MIT)
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:
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.
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.
Dean M. DeLongchamp
Polymers Division, Electronic Materials Group
National Institute of Standards and Technology
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Building 224 Room A325
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