Electronic paper sees color

17 Jun 2002

Harnessing the light-reflecting properties of a unique polymer crystal could lead the way to color flexible displays.

Kenneth Marshall and colleagues at Rochester University, US, have developed colored flakes of a polymer cholesteric liquid-crystal (pCLC) that reflect visible to near-infrared wavelengths. Although pioneers of flexible displays have already developed black and white displays, Marshall believes that the pCLC flakes could form the basis of color paper displays.

The cholesteric liquid crystals comprise chiral molecules that align to form a helix structure, which selectively reflects light. Only light that has a wavelength equal to the helix "pitch" - the distance over which the helix rotates 360 degrees - will be reflected.

With this in mind, the researchers fabricated films of pCLC, which were then heated and fractured into micron-sized flakes. By altering the composition of the "parent" liquid-crystal films, Marshall and his team can create a range of flakes that each reflect a different color.

"Polymer films of different reflected colors can be physically blended together to make films and flakes of nearly any color," explained Marshall.

Although the researchers have yet to create an e-paper prototype, they have developed an electro-optic pCLC device. Flakes of the liquid crystal are suspended in a host fluid that is sandwiched between glass plates. Applying an electric field to the set-up makes the flakes "flip" and change from full reflection to no reflection with a loss of color.

"We have recorded the response times of individual flakes at 80 to 500 ms, " said Marshall. "This is well within the response times of conventional liquid crystal devices."

Eventually the team hopes to combine different colored flakes within a single device. "We could treat the flakes so that each color group responds to a different voltage or frequency level of the driving electric field," he said. "Individual groups of colors could then be made to switch on and off at will."

First, however, the researchers are trying to produce flakes with specific shapes and sizes. They believe that this is crucial to making devices that switch reproducibly.

"We also need to learn how to microencapsulate the flakes and the host fluid in a flexible polymer binder [rather than glass] to form a free-standing film," concluded Marshall.