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 displaymore 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.
|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.
The industry has yet to find the optimal solution for manufacturing organic films. In Sony's XEL-1, a metal mask is placed immediately beneath the substrate, while organic materials vaporized from the melting pot are vapor-deposited onto it at RGB pixel location. But for substrates beyond a certain size, the metal mask will become distorted by its own weight or will emit heat from the melting pot, thus losing positional accuracy and decreasing aperture ratio and definition.
So Sony's engineering team sought out another technique for its 27-inch prototype. “The [laser-annealing] technique is also attractive for combining white organic EL with color filters,” Ishibashi said. Using a color filter can achieve ultrahigh density, because there is no mask required to pattern discrete red, green and blue subpixels. That lowers the cost of production, although the color purity decreases and power consumption rises.
The number of bad pixels won't change, regardless of panel size. “So we need to get rid of the causes of the bad pixel as best we can,” said Takatoshi Tsujimura, senior director of the OLED product division at Kodak. “To realize this, the combined use of white organic EL and a color filter is the best approach.”
According to Tsujimura, the luminescence efficiency improves twofold by installing white pixels to the conventional triad of RGB subpixels. “It's because no color filters are used for white pixels,” he said. “Thus, nothing prevents white light from coming through 100 percent. This is how high efficiency is achieved.”
Organic-EL panels are finding their way into applications other than displays, including solid-state lighting and solar cells. Kaneka, a diversified chemical manufacturer, last month said it would begin joint research with Osaka University on an OLED lighting device and an organic thin-film solar cell.
OLED's thinness, lightness and surface-emitting properties make it useful for illumination purposes. Thinness and light weight are particular advantages in automotive and aircraft applications.
“Organic ELs can already exhibit 50 lumens per watt; 100 lumens/W is expected to be realized in 2010,” said Gidas Sorin, CEO of German company Novaled. In contrast, the highest efficiency of a light bulb is approximately 20 lumens/W.If OLED technology was one cornerstone of the Display 2008 expo here, then e-paper was the other. Several companies demonstrated advances in this technology, which is already being used in cell phones, e-book readers, signage, labels and cell phones.
To manufacture its electrophoretic-display (EPD) e-paper, pioneer E Ink produces a “sheet” that comprises microencapsulated particles in a transparent liquid; a common electrode; and a release layer, which display makers or OEMs remove to laminate the sheet to a backplane. Inside the capsules are positively charged white and negatively charged black particles.
E Ink's “sheet” technology has already been adopted in more than 10 million Motorola cell phones. The same technology is also used in Kindle, the portable book reader Amazon.com launched last year in the U.S. market, as well as seven other e-book readers now in production.
E Ink also exhibited an e-paper device that accommodates a stylus pen input. It is built around a control IC—the S1D13521B, jointly developed by Seiko Epson and E Ink—along with E Ink's AM300 development kit and a built-in electromagnetic induction-type tablet made by Wacom.
Wacom specifically developed a very thin, light panel: At 0.35 mm and 40 grams, it's half as thick as and half the weight of its predecessor. To trim the weight, Wacom engineers used PET film in place of a glass substrate.
“We expect five to six companies to launch products based on the AM300 development kit this summer,” said Ryosuke Kuwata, vice president of E Ink.
Fujitsu, meanwhile, showed off e-paper technology with a multicolor cholesteric LCD. The panel, which does not use color filters, is constructed by combining three stacked display layers—one each for red, green and blue. The company exhibited an 8-inch, full-color PDA display panel capable of showing 1,024 x 768 pixels.
For its part, Bridgestone is working on a proprietary EPD technology called Quick Response Liquid Powder Display. QR-LPD is based on “electro-liquid powder,” which straddles liquids and conventional powdered solids. The electro-liquid powder, which flows like a particulate suspension, is extremely sensitive to electricity, thus ensuring fast responsiveness: Response time is as fast as 0.2 ms. The powder comprises positively charged black particles and negatively charged white ones.
Bridgestone exhibited an A3-size full-color panel (4,096 colors) using a color filter. The company said it has also developed a foldable display. n
Yoichiro Hata is managing editor of EE Times Japan , where this article first appeared. Additional reporting by David Lieberman , a freelance technology journalist based in Chapel Hill, N.C.