TOKYO Eastman Kodak Co. and Sanyo Electric Co. Ltd. will demonstrate the largest-ever active-matrix, organic electroluminescent light-emitting diode (OLED) at the Society for Information Display 2000 International Symposium next week in Long Beach, Calif.
The 5.5-inch OLED is self-emitting, has full-color capability, and features a fast response time, low driving voltage and low power consumption. Those advantages give it the potential to become one of the most popular next-generation displays. "OLED displays are essentially superior to existing displays in various aspects. Our challenge is how to develop such a display," said Kiyoshi Yoneda, general manager of the LCD Division of Sanyo Semiconductor Co.
Active matrixes, which keep power consumption low and assure uniform brightness, are essential for large-sized panels. The development of the 5.5-inch panel proves that larger OLED panels are now possible. "Now we are confident that OLEDs can cover applications that need small panels for cellular phones to larger panels for PDAs and notebook PCs," said Yoneda.
In February 1999, Kodak and Sanyo announced that they were working together on the development of OLED displays, using Kodak's materials technology and Sanyo's low-temperature polysilicon LCD technology. The first result of their effort was a prototype of a 2.4-inch active-matrix, full-color OLED display. When the companies announced the 2.4-inch display last September, they called it the world's first active-type OLED.
The new 5.5-inch panel has a quarter-VGA resolution (240 x 320 pixels) with a brightness of 200 candela per square meter. It consumes 2 watts running at 10 volts. Yoneda claimed that the power consumption is lower than comparably-sized LCDs, which eat 2.5 W on average. The pixel transistors are optimized to maintain uniform brightness over the surface of the panel. The aperture ratio is about 50 percent, an improvement over the 30 percent ratio of the earlier 2.4-inch panel.
As photolithography technology is not applicable to the small-scale organic materials that the Kodak/Sanyo group uses, shadow-masking technology is used to form the RGB pixels. For the prototype, a 6-inch shadow mask is used to form the pixel layer, said Yoneda. But the 5.5-inch panel is the maximum size that can be produced with current shadow-masking technology.
Kodak and Sanyo have allied with Ulvac Co. Ltd. (Chigasaki, Japan) to produce larger OLED panels. Ulvac specializes in vacuum technology and is a supplier of plasma chemical vapor deposition and sputtering systems for LCD production. The three companies will develop production systems to form large-area RGB layers, using shadow-masking technology and a thin-film deposition system. They plan to complete the systems before the end of the year.
The improvement of the emitting efficiency of each color is another challenge for the companies. The efficiency is currently 1 candela per ampere for red, 10 cd/A for green and 2.5 cd/A for blue. "Though the panel can display full color with these efficiencies, if red's efficiency is improved to about 3 cd/A, the color reproduction will match that of CRTs," Yoneda said.
"Before volume production starts, we want to improve the emitting efficiency of red to further lower the power consumption, though it is already lower than that of LCDs," he said.
The two companies have drawn up a road map that calls for the development of a 10-inch panel some time in 2001. By the first quarter of 2002, Sanyo plans to start volume production of active OLEDs. Initially, they will produce smaller panels. The companies did not disclose information about the availability of samples.
"To compete with existing panels in cost, we need a deposition system with high productivity. We are working to prepare such a system. When Sanyo began volume production of low-temperature polysilicon LCDs, we did not regard it as important, so the production cost of LTPS LCDs at the beginning was high. This taught us a lesson," Yoneda said.