Researchers at the University of Toronto, in Ontario, have increased the speed of their new color-changing material tenfold. The material, which uses photonic crystals, reflects bright, intense light of any color from red to blue, switching color based on the voltage applied to it. The technology could enable brighter, flexible color displays for electronic readers and billboards.
“To get color changes that go across from UV all the way to near infrared–it’s the only material on the planet that can do it,” says chemistry professor Geoffrey Ozin, who led the new work. “All I’m doing here is with one material tuning the voltage.”
Reading devices such as the Amazon Kindle, the Sony Reader, and Plastic Logic’s new reader use a black-and-white e-paper from Boston’s E Ink. E-paper reflects light instead of emitting it, which makes it less power hungry and easier to read in bright sunlight. Displays using a color version of E Ink’s technology are expected to reach the market in the next few years, but their pixels will be divided into three subpixels, with red, green, and blue filters. Light from the subpixels is mixed in varying intensities to produce different colors. “That means you just have one-third of the [pixel] area that displays red,” says Jacques Angele, cofounder of the French e-paper company Nemoptic. “So you reduce brightness by a factor not far from three.”
The key advantage of the new technology is that the photonic crystal making each pixel can be tuned to emit different colors. “In principle, they should be able to get good brightness more similar to printed paper, compared to current e-paper technology,” Angele says. Increasing the speed with which the material changes color moves it one step closer to practical applications.
The Toronto researchers reported the new version of the material in an online Angewandte Chemie paper. In addition to changing colors more rapidly, the material also covers a much wider color spectrum.
Opalux, the Toronto-based startup commercializing the technology, is already using the new material to make color-changing displays. The display is currently made on glass but could easily be made on flexible substrates, says Andre Arsenault, coauthor of the paper and cofounder of Opalux.
A photonic crystal is any nanostructure with a regular pattern that influences the motion of photons. By changing the structure slightly, you can change the color of light that the crystal reflects. Previously, the Canadian researchers made photonic crystals using stacks of hundreds of silica nanospheres embedded in a polymer. They sandwiched these stacks along with an electrolyte–a material that conducts ions–between two transparent electrodes coated on glass. When different voltages are applied, the electrolyte goes in and out of the polymer, which swells and shrinks, altering the distance between the nanospheres. This changes the wavelength of the reflected light.
The crucial change in the new material is that it does not contain silica. The researchers dissolve the silica nanospheres using an acid solution. This leaves behind a porous, weblike polymer structure, which now acts as the photonic crystal. The researchers fill the pores with electrolyte and sandwich the material between electrodes.
The electrolyte is now in direct contact with a much larger portion of the polymer’s surface area, so it goes in and out of the polymer faster and more evenly, speeding up the color change and increasing the range of colors possible. “When the active polymer is filled with silica spheres, there’s no void space available for [electrolyte] to go in and out,” Arsenault says. “So to get to the bottom of the structure, [it has] to diffuse from the top all the way down, which can be a long way to go.”
The new material has caught up with the speed of E Ink’s display. The photonic-crystal pixels can switch color in about a tenth of a second, according to Arsenault. By contrast, says Angele, E Ink’s pixels take about a fifth of a second. (But Angele adds that Nemoptic’s displays–which use a material called nematic liquid crystals–switch color in a hundredth of a second.)
Angele says that one drawback of the photonic-crystal approach could be that it depends on the flow of electrolyte in response to electricity. This electrochemical cycle is similar to that used in rechargeable batteries. “So it might face the same issues of rechargeable batteries, where efficiency decreases after enough cycles,” Angele says. To create a practical display, the Toronto researchers will have to make sure that the device can endure thousands of cycles. Precisely controlling the amount of electrolyte that infuses the polymer to get a specific color might also be a challenge, Angele adds.
There are other hurdles to overcome. The pixels change easily from shorter-wavelength colors to longer ones–from blue to green to red–but switching color in the reverse direction is slower. The pixels also need to have more color contrast. The researchers hope to make the material better by adding nanoparticles to the polymer.
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