Electrochromic polymers are materials that change color upon an application of a voltage or current. In the Reynolds group, the materials are specifically conjugated polymers that switch from a colored film when in their charge-neutral state, to a colorless film when oxidized. What sets these materials apart from other families of electrochromic materials are their broad range of vibrant colors, as well as switching times on the order of seconds or less, and contrasts over 60% for most of our colors, all within an approximately 1 V window. Potential applications range from color-changing lenses and windows, to signage and displays, from aesthetic designs to smart heat management.
Upper left: demonstration of an electrochromic device switching from a colored to a colorless state upon application of a small voltage
Lower left: a segmented reflective device1 and an airbrush sprayed butterfly2
Right: an electrochromic window integrated with solar panels
In our group, a family of dioxythiophene-based electrochromic polymers have been synthesized that span the visible color palette in the neutral state, and switch to a colorless state when oxidized, with some examples shown above. This has been made possible through developing an understanding of the structure-property relationships that affect the color of the polymer.3,4,5Some strategies include fine-tuning the distribution of electron-poor and electron-rich units along the polymer backbone, or tuning the effective conjugation length by varying the steric bulk of the side chains. Using a small subset of building blocks, we are able to synthesize all polymers through direct-arylation polymerization, a route that has allowed our polymers to be scaled up to hundreds of grams. These materials are soluble in a range of non-chlorinated and environmentally friendly solvents, even in water with attachment of the proper side chains6,7allowing them to be solution-processed using various roll-to-roll processable techniques. .
We design our electrochromic polymers to be highly soluble in a variety of solvents at high concentrations making them applicable to a range of different coating and printing methods.8,9
In addition to processability, solubility allows electrochromic inks to be blended together in a predictable fashion the way we would expect paints to mix, allowing us to fine-tune coloration without the need to synthesize a new polymer for different hues. Blending also allows us to obtain films of broadly absorbing colors such as blacks and browns.
Left: cyan, magenta and yellow electrochromic solutions are blended together to give vibrant new colors10
Right: orange and blue solutions are mixed to give a reddish-brown hue. Prototype lenses are fabricated to demonstrate the switching of the brown in an actual device11
Blending various combinations of three of the polymers produces tunable black films, which transition through various intermediate hues before attaining a colorless oxidized state. By selecting polymers with similar oxidation potentials, we can even control the amount of coloration present at intermediate voltages for a more seamless and gradual color change.
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