LIGHT-EMISSIVE DISPLAY MATERIALS

Because of Japan's large investment in LCDs and because of the expertise of the review team members, the team placed heavy emphasis on reviewing LCD technology; however, it did make some observations on materials development in electroluminescent (EL) and plasma display panels (PDP). Because these technologies have the potential to achieve full color, Japan has dedicated research efforts on materials for them. It is argued that current LCD technologies require backlights with driving power approaching that of EL and PDP; thus, these technologies are potentially competitive with advanced materials and better cell design and fabrication methods (Pleshko, 1991; Friedman, 1991).

EL Materials

An excellent overview of material development in Japan was provided by Professors H. Kobayashi and S. Tanaka of Tottori University, who have worked with a number of companies: Sharp, HEC Kansai, Ohi, Fuji Electric, Nippon Sheet Glass, Matsushita, Komatou, and Toso. In their opinion, the brightness of a blue phosphor must be improved by a factor of five or six before the EL technology can become viable. They believe that EL could support 1000 x 1000 pixels without serious cross-talk problems. Cross-talk minimization and gray scale are both areas that need work.

A DC EL power/thin-film hybrid display is being studied by Nippon Sheet Glass, which has a 640- x 480-pixel display with 16 levels of gray with pulse width modulation. Materials work at Tottori is focused on improving the performance of color, particularly blue and white phosphors, by incorporating lithium, potassium, and sodium as charge compensating materials. The following phosphor combinations are presently under study in the Tottori University laboratories:

Tottori researchers report reasonable progress in both blue and white phosphors, especially when using an ArS atmosphere anneal at 630 degrees centigrade. They have shown three colors using their white phosphor with both dye and interference filters, with the dye filters providing a larger viewing angle. There appear to be trade-offs between this way of achieving color and that from RGB phosphors.

The EL technology is a difficult technology, and commercialization is not easy in Japan because EL has lower status than other technologies. Thus, researchers at Tottori University chose to publish their results rather than patent discoveries, to encourage more interest from Japanese industries.

PDP Materials

In recent years there have been substantial advances in the design of the discharge cell structure to prevent phosphor degradation from ion bombardment, giving encouragement for full-color PDP displays. With these advances, color PDP does not appear to be material-limited, although improved discharge gas and electrode materials would be welcome. Using phosphors requires gases that are efficient UV emitters but are not visible light that can compete with light from excited phosphors. Coatings that protect phosphors and that do not strongly absorb in the UV are needed (Friedman, 1991). Studies of phosphor excitation are also needed to improve the efficiency of phosphors (Komatsu, 1991).

The team did not have the opportunity to visit Japanese laboratories working on PDP materials. Hiroshima University and Photonics Imaging have recently made color AC-PDPs, and a color PDP was demonstrated at the 1991 Japan Electronics Show.


Published: June 1992; WTEC Hyper-Librarian