Photoluminescence Improves LCDs

Aaron J. Hand

Although electroluminescent, plasma and fluorescent technologies present certain advantages, liquid crystal displays (LCDs) continue to dominate the flat panel display industry. Liquid crystal's low power consumption, low-voltage operation and semiconductor compatibility make it the flat display of choice. But it also has its drawbacks: limits in brightness, efficiency and viewing angle.

ETH Zurich's photoluminescent film absorbs optical energy and emits it as linearly polarized light. Courtesy of the Polymer Technology Group, ETH Zurich.

LCDs -- particularly backlit color displays such as those used in laptop computers -- face efficiency problems because they use polarizers and color filters that absorb most of the backlighting, letting less than 10 percent of the light generated by the backlight through to the viewer. But scientists in ETH Zurich's Polymer Technology Group have developed a liquid crystal-based device that incorporates a photoluminescent polymer film, replacing the inefficient polarizer and color filter, and enhancing brightness.

The film is a blend of polyethylene and a luminescent, conjugated polymer that shines when backlit with an ultraviolet light. In these films, the polymer molecules are uniaxially aligned so that the films emit bright, polarized light.

This liquid crystal-based photoluminescent device also overcomes limitations associated with other designs, such as stability, depolarization effects or the large size and thickness of the luminescent layer.

While the institute's process uses materials that are similar to electroluminescent materials, the technology is different because it uses a conventional light source for excitation. Other research has made use of fluorescent materials, but the team led by Christoph Weder has taken this further, combining the fluorescent process with a polarized material.

"Other research teams try to make use of standard fluorescent materials, but they always have to combine their fluorescent color filters with standard polarizers," Weder said.

The group can apply its films on either the viewer side or the back side of a display. Applying the film on the back side causes a considerable increase in brightness and contrast, depending on the makeup of the photoluminescent layer. In one experiment, which was not optimized, the researchers measured their display at 30 and 4 cd m²² for the bright and dark states, compared with 17 and 7 cd m²² for a commercial liquid crystal display in similar conditions. The brightness can be enhanced considerably more by increasing the film's optical density. Applying the film on the viewer side of the device improved the viewing angle to more than 170°, but the brightness of the demonstrators declined.

The institute's technology is well suited to applications requiring bright monochrome displays, such as cellular phones or dashboards. The researchers will continue their work with the aim of producing high-resolution multicolor screens.