Two OELD (organic electroluminescence displays) manufacturers unveiled new developments in the display technology at the Electronic Display Exhibition (EDEX) 2002 that opened in the Tokyo Big Sight exhibition hall on Tuesday.
OELDs emit their own light, removing the need for a backlight and thus saving power. This makes them particularly suitable for mobile devices. They can also display moving images and offer good color reproduction, and are therefore expected to be a popular choice for next-generation displays, especially for 3G (third-generation) devices.
Japanese printer maker Seiko Epson Corp., based in Suwa, Nagano Prefecture, demonstrated 2.1-inch active-matrix 260,000 color OELD prototypes with a resolution of 130ppi (pixels per inch) at the exhibition. These are made of a new polymer material that has been developed by U.K.-based Cambridge Display Technology Ltd. Seiko Epson developed this polymer into a new type of OELD, using its own ink-jet printing technology, said Masahiro Uchida, an engineer in Seiko Epson's Displays Operations Division.
Polymer is used instead of the standard low-molecular material used in organic light-emitting diodes. Unlike low-molecular material, polymer can be dissolved in a solvent with color added, and then directly sprayed onto a polysilicon board base, Uchida said.
"This method means that material won't be wasted, because the polymer can be jetted to the part where it is needed," Uchida said. "There is no need for large equipment to produce larger OELDs with this method, either. It will allow OELDs to be produced at lower cost with lower power than the small-molecule type."
To put color on the low molecular type involves a more complicated and difficult vacuum deposition method.
Another developer demonstrating polymer ink-jet OELD prototypes was Toshiba Matsushita Display Technology Co. Ltd., a joint venture formed by Toshiba Corp. and Matsushita Electric Industrial Co. Ltd.'s display units. The company was officially established on April 1 this year.
The joint venture unveiled what it claims is the world's largest polymer type ink-jet XGA-W OELD prototype at EDEX. The 17-inch prototype has taken two years of research, said Jun Hanari, a senior specialist at OELD Technology Group at Toshiba Matsushita Display Technology's research and development (R&D) center.
Seiko Epson insists that it was first in the development of the polymer type ink-jet OELDs, having started work on it five years ago. It is planning to mass produce the product in 2004.
Toshiba Matsushita Display Technology hopes to mass produce the 2.2-inch OELD for cell phones and PDAs (personal digital assistants) by the end of this year, Hanari said.
Toshiba Matsushita Display Technology is planning to keep developing the low-molecular OELD as well, Hanari said. "The polymer type has potential for the future market, but as the low molecular product has been researched and developed longer, the color brightness is better and the polymer type still needs to have a better image quality," he said.
Japan's leading manufacturer of color active-matrix OELDs, Sanyo Electric Co. Ltd., takes a similar view. The company has been developing low-molecular OELDs and is ready to mass produce 2.2-inch products for mobile handsets this year, said Keita Nagao, a chief planner at Electronic Device Business Headquarters of Sanyo Electric.
Osaka, Japan-based Sanyo Electric has no plans to develop the polymer product at present, said a Sanyo engineer at EDEX. "There are several strict specifications that need to be met, such as heat-resistance, for OELDs used in mobile handsets, and the polymer type is still not able to meet them," he said.
Even the low molecular type still needs to be developed and improved, Nagao said. One of the highest hurdles in OELD technologies is life span. With an LCD (liquid crystal display), the life span depends on the life of its back light, which runs for about 10,000 hours. An OELD, on the other hand, only runs for a few hours, Nagao said.
That hurdle can be cleared by improving the material used. This will also bring brighter colors and faster moving-image responses, Nagao said. "In three to four years, our OELDs will be improved enough to be mainstream," he said.
Another OELD developer, Tohoku Pioneer Corp., agrees that low-molecular OELDs give better image quality, and has been developing these, Yoichi Munekata, an assistant manager at OEL Production Division of Tohoku Pioneer said.
Tohoku Pioneer is experienced in producing passive-matrix OELDs, which have achieved a longer life span than the active-matrix version, Munekata said. Although the display carries only four colors, the company was the first in the world to begin mass production of the passive-matrix type, in 1997, and its products were included in Motorola Inc.'s mobile handsets in 2000.
Passive-matrix OELDs still have potential uses in displays for consumer products such as MD (MiniDisc) players, wrist watches, DVD players and refrigerators, the company said, showing prototypes at EDEX.
Tohoku Pioneer, based in Yonezawa, Yamagata Prefecture, expects its passive-matrix OELDs to be build into mobile handsets in Japan in May this year, Munekata said.
Since Japanese handsets commonly carry full-color LCDs, four-colored passive OELDs will not be suitable for the main display. However, a clam-shell style phone in Japan recently carried another small display window on the back that can be used as a clock or a caller identification display while the phone is folded. Tohoku Pioneer's OELDs will be used in such windows, Munekata said.
The company plans to keep developing the passive-matrix type as well as developing the active-matrix color OELDs simultaneously, he said.
EDEX 2002 will continue until Thursday.