Prototype tips Sony's hand on organic EL displays

Prototype tips Sony's hand on organic EL displays
TOKYO — Sony Corp. has developed an active-matrix 13-inch organic electroluminescent display prototype and is positioning it along with field-emission displays as a format for post-CRT displays.

"The final target of organic EL displays is a large-sized panel. Organic EL is a suitable format for the mid- to large-size displays necessary in the coming broadband era, and can possibly replace CRT TVs," said Suehiro Nakamura, president of Sony's Core Technology & Network Co. "[Flat] CRT TVs will continue to occupy an important position in the TV market, so we will continue to improve them. But at the same time, we will develop new displays which will eventually be replaced with CRT TVs. Organic EL and field-emission displays will be the two displays that have big potential in the TV market. Sony will continue to develop these two types of flat panels in parallel."

Sony has been working with Candescent Technologies Corp. since October 1988 on the development of field-emission displays.

LCD manufacturers with low-temperature polysilicon TFT technology are positioning organic EL displays as the next-generation flat-panel, because they can be built with TFT fabrication technology. Sony has been fostering low-temperature polysilicon TFT technologies in a production joint venture formed in 1997 with Toyoda Automatic Loom Works Ltd.

While other EL developers such as Sanyo Electric Co. Ltd. intend to start their EL businesses with small-sized panels for use in cellular phones, Sony's demonstration of the largest-ever active-matrix TFT OELD prototype makes clear its intention to offer mid- and large-sized displays in a volume market now dominated by CRT TVs.

"There is a hurdle which makes it difficult to develop large-sized organic EL displays," said Tetsuo Urabe, general manager of organic EL development at Sony's Core Technology & Network Co. To get fully controlled luminance on the entire screen, transistors on the large glass substrate have to be formed uniformly, which was difficult with conventional technology, he said. "Unevenness of transistors fabricated on a large screen is inevitable. If we try to increase the uniformity, the yield rate drops. We invented a structure in which even if transistors are of uneven characteristics, it won't affect the picture quality."

The solution developed by Sony engineers involves several technical developments.

Conventional EL displays have two transistors to drive each pixel, but if the transistors are not uniform across the entire screen the display's luminescence is uneven. Sony engineers designed each pixel of its latest EL display with four transistors, which helps keep the luminance uniform of an entire screen over 10 inches in size, Urabe said.

The group also developed a solid-state structure. An EL layer can deteriorate when exposed to moisture and oxygen, and conventional EL displays use metal caps and moisture-absorbents that add thickness to the panels while sealing the EL layer. In place of this metal cap and the moisture-absorbent, Sony employed a transparent seal that eliminates the moisture-absorbent and makes for an all solid-state panel that is thinner than the conventional structure.

The prototype measures 13 inches diagonally and has a resolution of 800 x 600 pixels. Brightness is 300 candela per square meter and the contrast ratio is 500:1. "It's already at the practical use level," said Urabe. Current lifetime and emission efficiency were not disclosed, but Sony engineers said they will continue work to extend the display's life and emission efficiency, with targets of 10,000 hours and 2 lumen/watt before the displays reach volume production, which is now planned for 2003.

Pilot production

Sanyo, a leading developer of EL technology, has also focused on active-matrix organic EL displays and plans to introduce products in 2002. "Organic EL display is a virgin market and we are going to develop the market first," said Sanyo president Yukinori Kuwano. Sanyo has allied with Eastman Kodak, its of EL materials, on organic EL display development, which yielded a 5.5-inch active-matrix prototype last year. The company plans to market active-matrix organic EL display in 2002 for mobile phones and portable equipment. In preparation, Sanyo will complete a pilot line in March. "Based on the result of the pilot line operation, we will plan further investment on the EL business," said Kuwano.

Sharp, Japan's top LCD supplier, is also eying organic EL displays. using its continuous grain (CG) silicon technology. Sharp is currently fabricating continuous grain silicon LCDs using a high-temperature polysilicon process. "Once low-temperature CG silicon technology is established, we can make EL displays just by forming an EL layer on the CG-silicon transistors," said Katsuhiko Machida, president of Sharp. "We'll pursue both LCD and organic EL display using CG silicon technology."