TOKYO — LCDs and plasma screens may be the dominant choice for TVs today, and LCDs the displays of choice for almost every other application, but a pair of upstart technologies is vying to replace them. Organic LEDs, which have already made inroads in the portable display market, are threatening to move into the living room as a new TV display. Electronic paper, meanwhile, is carving out a share in portable devices, consumer electronics and electronic signs that demand particularly low power consumption and long battery life.
At the recent Display 2008 show here, E Ink, Fujitsu and Bridgestone independently exhibited advances of their e-paper displays, which are characteristically thin, light in weight and power stingy. For its part, Sony exhibited a prototype of a 0.3-mm-thick OLED display—more commonly known as an OEL (organic electroluminescent) display in Asia— which is a reduced-profile version of the 1.4-mm OLED the company designed into its OLED TV monitor, the XEL-1, launched in December. The company also demonstrated a 0.2-mm prototype, the thinnest OLED yet.
In addition, Sony showed a prototype 27-inch OLED TV monitor capable of displaying high-definition video images in a 1,920 x 1,080-pixel format.
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| Sony's prototype OLED, 0.2 mm thick, is the thinnest OLED demonstrated to date. It's capable of displaying a 320 x 220-pixel color image. |
Handheld devices, not TV sets, are the target for e-paper, most versions of which are bistable; that is, even if power is turned off, they retain the contents of a screen without the need for electrical refresh. A reflective display that requires neither the backlight of a transmissive LCD nor the light emission mechanisms of an OLED, e-paper is suitable for applications that demand a display with especially low power consumption.
Thinness is not the only strength of an OLED display, whose screen size can be as large as 30 inches on the diagonal. OLEDs offer many other features that suit them for TV receivers, including color reproduction that exceeds the NTSC specification and a quick response time, of several microseconds. Peak brightness is more than 1,000 candelas/meter2, and the contrast ratio is more than 10,000:1.
Samsung is already jumping on the OLED bandwagon. "We will produce 40- or 42-inch TVs with organic-electroluminescence displays by 2010," said Lee Woo Jong, a vice president who heads up marketing for Samsung SDI's mobile-display division. Samsung will start volume production of 14-inch OLED panels for notebook computers by the end of this year.
Chi Mei EL, a manufacturer of display panels based in Taiwan, has its eye on OLEDs too. The company will begin commercial production of 12.1-inch organic-EL displays for notebooks in the first half of 2009, with a plan to get into volume production for 32-inch panels for TV receivers in the latter half of next year, said vice president Park Sung Soo.
Despite the advances, problems remain in applying organic-EL technology to larger panels. OLED panels are produced by compiling thin-film transistors (TFTs) on a glass substrate, followed by a series of organic layers for injecting, transmitting and/or recombining electrons and holes. In producing large panels, it has proved impossible to reuse the same manufacturing techniques originally developed for making TFTs or emission layers for small OLED panels.
For instance, for its first OLED TV, the 11-inch XEL-1, Sony used low-temperature polysilicon technology as the TFT base and small-molecule material for its organic film. But for the company's 27-inch OLED prototype, Sony engineers used an advanced "microcrystal" silicon TFT technology, tapping into a technique the company calls laser-induced pattern-wise sublimation.
Yoshi Ishibashi, chief of Sony's processing technology department, explained that there are three production process methods for TFTs: laser annealing, direct deposition and solid-phase growth. "We think that there are no better choices than using laser annealing in the silicon crystallization process for large-size panels," said Ishibashi.
Although the electron mobility of microcrystal silicon is somewhat inferior to that of polycrystalline silicon, it has another important benefit, Ishibashi said: When it comes to manufacturing large substrates, microcrystal silicon's high in-plane uniformity is a major advantage.
