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.