Electrochromic Polymers May Mean Brighter Future for Flat-Panel Displays

The use of super-thin layers of inexpensive electrochromic polymers to generate sharp colors that can be quickly changed may lead to a less expensive way to develop flat-panel displays that are also brighter, clearer and more energy efficient.

Sandia National Laboratories researcher Alec Talin inspects a plasmonic array sample using a probe station microscope. Courtesy of Dino Vournas.

Researchers at the Center for Nanoscale Science and Technology demonstrated high contrast, fast monochromatic and full-color electrochromic switching using two different electrochromic polymers, polyaniline (PANI) and poly(2,2-dimethyl-3,4 propylenedioxythiophene) (PolyProDOT-Me₂). Both PANI and PolyProDOT-Me₂ polymers can be electrodeposited as conformal, extremely thin coatings on metal structures with well-controlled thicknesses.

The researchers significantly enhanced the interaction of light propagating as deep-subwavelength-confined surface plasmon polaritons using arrays of metallic nanoslits. They created plasmonic arrays of vertical nanoscale slits perpendicular to the direction of the incoming light. The slits were cut into a very thin aluminum track, coated with an electrochromic polymer. When light hit the aluminum nanoslits, it was converted into surface plasmon polaritons (SPPs) that contained frequencies of the visible spectrum that could travel along the dielectric interfaces of aluminum and the electrochromic polymer. The distance between the slits in each array (pitch) corresponded exactly to the wavelengths of red, green and blue light. The pitch determined which wavelength — red, blue or green — was transmitted down through the array, traveling along the interface between the thin polymer layer and the aluminum substrate.

Because the polymer was just nanometers thick, it required very little time to change its state of charge and therefore its optical absorption of colored light. Plasmonic electrochromic switchable configurations have the advantages of both fast switching speed and high optical contrast. The switchable configuration was able to retain the short temporal charge-diffusion characteristics of thin electrochromic films, while maintaining the high optical contrast associated with thicker electrochromic coatings.

By controlling the pitch of the nanoslit arrays, it was possible to achieve a full-color response with high contrast and fast switching speeds, while relying on just one electrochromic polymer. Because the light traveled a relatively long distance along the surface of the aluminum slits coated with the thin polymer, it saw a much thicker polymer layer. The material turned a desirable deep black when a tiny electric current sent across the top of the slit cut off the entering light, and did so in milliseconds. When the current was flicked off, light frequencies passed through the slits and instantly turned on the pixel. Because the carefully spaced slits let in light only at a particular frequency, a single kind of polymer coating served in a neutral capacity to deliver all three emanated colors.

“These very inexpensive, bright, low-energy micropixels can be turned on and off in milliseconds, making them fit candidates to provide improved viewing on future generations of screens and displays,” said researcher Alec Talin. “The nanoslits improve the optical contrast in a thin electrochromic layer from approximately 10 percent to over 80 percent.”

Electrochromic polymers by themselves are not a new invention. But Talin and his collaborators discovered a way to switch electrochromics on and off in the milliseconds required to create fast-moving images. Conventional electrochromic displays require thick polymer layers to obtain good contrast between bright and dark pixels. But thick layers also require long diffusion times for ions and electrons to change the polymer’s charge state, making them most useful for static information displays.

The work was supported at Sandia by Nanostructures for Electrical Energy Storage, an Energy Frontier Research Center of the Department of Energy, and was published in Nature Communications (doi: 10.1038/ncomms10479).