The 64 x 64-pixel, passive-matrix reflective cholesteric LCD shown on the show floor was developed to offer ultrathin, lightweight, curved and flexible displays for portable devices such as PDAs, mobile audio systems, mobile phones and wearable labels and badges, said Peter Slikkerveer, principle scientist at Philips Research.

A number of companies including Philips have been investigating other flexible display technologies like organic polymer LED, woven electroluminescent displays. But this is the first time that anyone has demonstrated an LCD — typically made of rigid glass — on bendable, twistable plastic polymer substrates, Slikkerveer said.

The main technical challenges of making a passive-matrix LCD truly flexible was "getting the correct, homogeneous distance between the upper and lower substrate layers" and maintaining the spacing during flexing of the display panel, Slikkerveer said. Typically, the margin of error allowed for spacing between the two substrates is "less than 0.1 micron," he said, "more like .05 micron."

Other challenges included "mechanical redesign of the entire manufacturing process." The redesign was necessary because the company needed to reduce the temperature of all process steps in order to match the limits set by the plastic substrates. "All the steps in the manufacturing process needed to be reconsidered" in order to maintain the integrity of all layers, including the conductive layers during flexing, Slikkerveer said.

Philips Research scientists chose the passive-matrix reflective cholesteric LCD because it does not depend on polarized light and therefore has the wide viewing angle required for curved display surfaces. Under the natural bistable effect of cholesteric LCDs, pixels remain in their current state until re-addressed.

The display technology, which refreshes very slowly, could be suitable for slow-scanning applications like e-books, said Slikkerveer. The minimum of lamination layers required simplifies the construction process, including temperature and pressure.

The demonstrated cholesteric LCD featured black and white and gray-scale capabilities, a 64 x 64 passive matrix and bistable pixels even during flexing. The total foil thickness is 250 microns and the minimum radius of curvature is 2 centimeters.

Commercial possibilities

Philips Research has already built a workshop inside its lab to develop new processes for the flexible LCD. Although many technical problems concerning its manufacture have been solved, it may take three to four years before the technology can be turned into a commercial product, said Slikkerveer. "Flexible LCDs require you to change your factory and develop new processes . . . We need to make sure this technology becomes a standard, supported by other companies so that system companies will have a second source for their flexible LCDs," he said.