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.