A Flexible Display with the Blues

Hank Hogan

A group of researchers at the University of Central Florida in Orlando is not feeling blue, even though its displays are. By inducing a controllable blueshift in a thin film, it successfully fabricated a flexible display that can be wrapped around a pole and still work. Potential applications include rollable reflective displays that do not need power.

A three-color flexible cholesteric polymer film is shown before bending (a) and rolled up and attached to a cylindrical post holder (b). Various voltages applied across the 8-μm-thick film during the curing process created the characters. Reused with permission from Haiqing Xianyu, Applied Physics Letters, 89, 091124 (2006). © 2006, American Institute of Physics.

Optics professor Shin-Tson Wu, who led the group, said that the discovery occurred when the researchers were making films using mixtures of cholesteric reactive mesogens — photopolymerizable monomers with a liquid crystal phase at a certain temperature range. They were filling indium-tin-oxide glass cells with the mixture when, Wu recalled, they tried an experiment. “We applied an AC voltage to drive the cell and observed the blueshift.”

He added that, because the molecular configuration in reactive mesogen can be solidified by photopolymerization, they cured the cell while voltage was applied, so as to freeze the colors generated in the material.

In constructing the cells, the scientists coated the inner side of each glass substrate with a thin alignment layer and rubbed the layer in antiparallel directions. When cured with no voltage applied, the cell reflection was red. When cured with 25 V, it was green and at 45 V, blue. Besides the color change, the reflection band broadened and the peak reflectance decreased.

To understand their results, they investigated what the electric field was doing to the molecules, using both optical and scanning electron microscopes. They found that, above a threshold voltage, an undulated texture — a two-dimensional periodic structure — appeared. They attributed this to an electric-field-induced deformation. Measurements showed that this undulation shifted the transmission to the blue and broadened it, thus tying the molecular rearrangement to the optical changes.

Wu said that, since the original discovery, the group has come up with a more accurate description and better reflection model of the deformed structure. Even without that, though, it has built rollable films of different colors that it successfully has applied to curved shapes.

Reflective displays could be an application for the technology, but Wu noted that some problems must be overcome first. “We need to eliminate the defects in the cells that enable the formation of oily streaks when an electric field is applied.” Such streaks, he explained, eventually will make the cell nonreflective.

Applied Physics Letters, Aug. 28, 2006, Vol. 89, 091124.