Flat-Panel Displays: Poised To Take Over Large And Small Screens

Look around anywhere and you're bound to see flat-panel display technology at work—at the office, your doctor's office, bank and retail-store terminals, your home, your car, wherever. Most notable in this arena are large-screen (diagonals greater than 30 in.) consumer TVs, where liquid-crystal displays (LCDs) and plasma-panel displays (PDPs) are trying to compete with the venerable CRT. Rear-projection technologies using digital mirror devices (DMDs) and front-projection organic LED (OLED) technology are also mounting a challenge for large-screen dominance.

The CRT is still king in large-screen displays, mainly due to its high resolution, large grayscale range, and ability to show motion without any artifacts, all at an amazingly low price compared to flat-panel displays. Performance in non-CRT displays, particularly LCDs and PDPs, continues to improve while costs come down. Nevertheless, they still command prices of several thousand dollars, a price point too high for many mass-market consumer TV applications. They're seen mainly in high-end premium TVs.

One dark-horse candidate, the electroluminescent (EL) display flat panel, could figure more prominently in large-screen consumer TVs mainly due to its inherently low-cost manufacturing advantages. For instance, iFire Technology has demonstrated a "color by blue" technology that promises to cut costs even further for large-screen thick-film EL panels (Fig. 1). The company estimates that it will cost 30% to 40% less than other flat-panel display technologies.

The "color by blue" EL panel features performance on par with commercial TV products now available. Luminance is 600 cd/m² (greater than 300 cd/m² with contrast enhancement). It also boasts a 500:1 contrast ratio, a white-color temperature of 10,000 K, and a color gamut that meets 118% of EBU standards and 85% of NTSC standards. The new color technology uses a high-luminance inorganic blue phosphor, combined with special color-conversion materials that absorb the blue light and re-emit red or green light, to generate other colors.

Commercial volume production of 34-in. diagonal HDTV display modules is expected to begin in 2005. Production will be done by Dai Nippon Printing in Japan. In addition, iFire has a nonexclusive technology agreement with Sanyo to develop 34-in. diagonal EL displays.

EL displays aside, LCDs, PDPs, and rear-projection TVs will garner more and more of the total worldwide dollar market over the next few years, although their total numbers will grow more slowly. That's according to a study by iSuppli/Stanford Resources, presented by senior vice president David E. Mentley at the last SID Conference (Fig. 2). No matter what type of flat-panel technology is used, LCDs now capture about 80% of the total dollar volume spent on flat-panel displays worldwide. They dominate the scene in notebook and laptop computers, monitors, and small-screen TVs (less than 30-in. diagonal). Not surprisingly, most technological advancements are occurring in LCDs.

LCD'S CONTINUAL FACELIFT Consistently, LCDs show marked improvement in all performance aspects, including readability, response times, and driving characteristics, with low-cost manufacturing as a major objective. One example is a 30-in. thin-film transistor (TFT) LCD public-information display from Global Display Solutions that's sunlight-readable and viewable under all weather conditions.

Some semiconductor companies with large fabrication facilities, like Samsung, believe that they have the edge in reducing LCD manufacturing costs because they can amortize their production costs by using existing fabrication facilities. Last month, Samsung announced an 1870- by 2- by 200-mm glass substrate for its seventh-generation TFT LCD line, which will allow for the production of 32- and 40-in. diagonal TFT LCDs. Samsung has already demonstrated the largest diagonal LCD panel, a 54-in. prototype.

LG Philips, a joint venture between Netherlands-based Royal Philips Electronics and South Korea-based LG Electronics, has already introduced a 52-in. prototype active-matrix LCD color panel that it intends to mass-produce by year's end (Electronic Design, Jan. 20, 2003, p. 24). At the Society for Information Display Conference in May, both prototypes on display showed brilliant and clear color images (Fig. 3).

The LG Philips prototype uses Philips' super-in-plane switching technology to achieve 12-ms response times. It's another indication of the improvements in LCD switching times, which are now at 10 ms or less for smaller-size displays, making for fewer smearing effects as can be seen in the Sharp's AQUOS line of TVs with LCD panels.

Active-matrix LCDs (AMLCDs) continue to be the major technique used for portable and desktop displays due mainly to their low power consumption, light weight, and sharp image quality. Yet AMLCD backlighting lacks efficiency. No more than 10% of the backlighting output is emitted from the display's front to the viewer, because the polarizer and color filters absorb much of this light. Blocking effects caused by a limited aperture ratio for each AMLCD pixel is another reason.

Attempts to solve these impediments employ field sequential color-illuminated LCDs and static illumination. A novel method proposed by IBM is to use a linear fluorescent lamp with a special collimating optical structure as well as discrete LEDs. IBM researchers in the U.S. and Japan have also proposed a pixel-level data-line multiplexing technique for amorphous-silicon AMLCDs. It allows for the reduction of data drivers by one-half or less, without the need for a large number of demultiplexing circuits, no increase in gate-driver circuits, or a special scaling graphics processor. Reports show that this improves an AMLCD's power consumption and costs compared with conventional methods now in use.

In a joint effort, Nitto Denko and 3M came up with polarization film for LCDs that offers significantly improved brightness and viewing angle, as well as less thickness. The film selectively reflects and recycles the backlight of LCD panels without compromising an LCD's viewing characteristics.

Most major LCD-panel manufacturers have been adding transreflective full-color panels to their wares. These work in either the transmissive mode (with backlighting) or the reflective mode (using incident ambient light). Still to be overcome, though, are the low light efficiencies of reflective operation.

Though the response time of most LCDs is acceptable, improvements continue here as well to make for clearer moving images. One example is the "feed-forward" driving technology Optrex developed for TFT LCDs. It's incorporated in an ASIC that can be added to any LCD module. The chip contains a frame memory and a lookup table. It accelerates the frame-to-frame process by calculating and applying the necessary voltage overshoot. Intra-gray response times have been cut down from the traditional 84 ms to less than 20 ms using this technique.

ZBD (Zenithal Bistable Devices) hopes to have an LCD with memory on the market soon. ZBD devised a method that uses a finely ridged grating on the inner glass surface of an LCD cell for its super-twisted-nematic (STN) liquid crystal. Such a device will allow the writing of an image to the display, which can then retain the image indefinitely when power is turned off.

THE SOG SOLUTION One key goal for LCD makers is the use of silicon-on-glass (SOG) technology, where the same glass plate that holds the display material also holds the drive and other supporting electronics made from amorphous silicon. Because many types of circuits can be integrated on such a glass panel, experts predict that 1-m² system-on-a-panel displays may not be too far off.

One such notable system comes from Toshiba Electronic Components. It includes silicon sensors for image capture on the display's front, allowing, say, a PDA to scan a business card and store content in its memory (Electronic Design, July 7, 2003, p. 30).

Work also continues on improving the characteristics of polysilicon and the TFTs that drive them, so they may be fabricated on a single plate of glass. Sharp made notable strides in this area with its continuous grain silicon, which the company calls "system LCD." Such silicon exhibits electron mobilities close to those of bulk silicon and several times those of polysilicon, as well as hundreds of times greater than those of amorphous silicon.

Presently, Sony is building AMLCD panels using low-temperature polysilicon with all of the drive electronics on the glass, including row and column drivers, dc-dc converters, chip selects, and a timing generator. Designed for cell phones, these 1.94-in. diagonal displays feature resolution of 128 by 160 pixels. Sony began sampling them last month and expects to start production runs next month.

Philips has pioneered making liquid-crystal-on-silicon (LCOS) technology for flat-panel displays. It will be available on its line of Cineos TVs late this summer in the U.S. and later on in Europe.

OLEDs COME ON STRONG The rising star in flat-panel displays is light-emitting organic LED (OLED) displays. A problem area for OLEDs has been the color blue, which typically has a very short lifetime. Recent developments pushed the luminance lifetime of the OLED's blue light-emitting layer up to about 11,000 hours, compared to 40,000 hours typical for green and 60,000 hours typical for red.

Because OLEDs essentially generate light, they don't require any backlighting, erasing backlighting issues and shortcomings that prevail with LCDs. OLEDs also have wider viewing angles, faster response times, and higher contrast ratios, and they dissipate less overall power and operate over wider temperature ranges. Right now they're found in smaller displays on consumer items like cell phones, digital cameras, and DVD players. But they're also vying for larger-size screens in monitors, outdoor signage, automotive applications, and possibly TV applications.

Joint work by IBM researchers in Switzerland and New York and Chei Mei Optoelectronics in Japan has already produced a 20-in. diagonal super-amorphous silicon OLED, the largest to date. The researchers say the amorphous silicon technology has no size limitations for fabricating TFT arrays on substrates up to 2 m, opening up possible applications for consumer TVs.

Kodak and Cambridge Display Technology are working on two fundamentally different technologies, small molecule and polymer approaches, respectively, and are licensing a host of companies to work with them for advanced OLEDs. Kodak has a manufacturing partnership with Sanyo to produce OLEDs. CDT, which holds the fundamental patents on conjugated polymer light emission and controls a large patent portfolio, licensed its expertise to and is working with a number of companies to produce advanced OLEDs. These include Covion Organic Semiconductors GmBH, Delta Optoelectronics, Dow Chemical, Dupont de Nemours, Osram Opto Semiconductors, Philips, Seiko Epson, Sumitomo Chemical, and Toshiba.

Dupont itself has OLED agreements with Universal Display Corp. (UDC) to make OLEDs using Kodak's small-molecule method, as well as Cambridge Display Technology's polymer approach. UDC is working on phosphorescent OLEDs, which it claims are more efficient than the fluorescent OLEDs others use. In fact, UDC says that it has demonstrated that 100% of the electrical input to one of their OLEDs is converted to light.

This networking arrangement of OLED companies is indicative of the industry's cooperative effort to advance OLED technology. The Flat-Panel Display Forum (DFF) in Germany, a consortium of dozens of companies, was formed to advance the manufacture of OLEDs. To better position themselves in what they see as a future large market, many OLED suppliers are developing their own brand names, such as NuVue for Kodak, Olight for Dupont, and Pictiva for Osram Opto Semiconductors.

OLEDs hold the potential to make affordable large-panel displays on low-cost flexible substrates. Ink-jet printing has already been demonstrated as a means of OLED production. Because of this, they can be made into very thin films or foils, thin enough to produce on a continuous reel-to-reel production facility. Pioneer Corp. has developed a 3-in. diagonal full-color OLED using a plastic substrate by modifying a conventional plastic film. The new display offers performance characteristics equal to those of OLEDs made on a glass plate.

Like other flat-panel display technologies, OLEDs can be operated in two modes. The passive-matrix mode is used to provide acceptable resolutions for lower-size displays that require low power consumption. The active-matrix mode comes into play when higher resolution is needed for larger displays that also possess higher power consumption.

One of the first successful consumer products to employ OLEDs is the EasyShare 633 digital camera from Kodak. The OLED is produced via a joint arrangement with Sanyo. The camera, presently available in Japan, uses Kodak's AM550L full-color 2.2-in. diagonal OLED module as a back display. It features QVGA resolution (521 by 218 pixels).

Cambridge Display Technology has an OLED for the Norelco Spectra electric razor from Philips. Moreover, Dupont has shown 2.1-in. (128 by 64 pixels) and 4-in. (160 by 160 pixels) OLEDs. Though these products are made on glass substrates, future versions will implement plastic substrates. Dupont has commercialized its line of OLEDs, calling them Olight, which can be found in a host of consumer products (Fig. 4).

FLAT IS IN No matter what flat-panel technology becomes pervasive, one thing is certain. A new era of flat displays will permeate every aspect of society over the next few years. As for the CRT, it will still dominate TVs for at least the next three to five years. After that, it's anyone's guess if it can overcome the onrush of flat-panel display technology and its quest for lower manufacturing costs. CRTs may wind up like the classic TV vacuum tubes that were shunted aside by better products like transistors and ICs. Flat-panel display technology is a fast-moving train that's not making any stops.

Need More Information?

Cambridge Display Technology www.cdtltd.co.uk Chei Mei Optoelectronics www.cmo.com.tw Covion Organic Semiconductors www.covion.com Dai Nippon Printing www.dnp.co.jp DFF (Flat-Panel Display Forum) www.displayforum.de Dow Chemical Co. www.dow.com Dupont de Nemours Co. www.dupont.com Eastman Kodak Co. www.kodak.com Global Display Solutions www.gds.com iSuppli/Stanford Resources www.stanfordresources.com iFire Technology www.ifire.com IBM Corp. www.ibm.com LG Philips www.lgphilips-displays.com NEC Corp. www.nec.com

Nitto Denko Corp. www.nitto.com Optrex Inc. www.optrex.com Osram Opto Semiconductors www.osram.com Philips Electronics N.V. www.philips.com Pioneer Corp. www.pioneer.com Samsung Corp. www.samsung.com Sanyo Corp. www.sanyo.com Seiko Epson Corp. www.epson.co.jp Sharp http://sharp-world.com Sony Corp. www.sony.com Sumitomo Chemical www.sumitomo-chem.co.jp Toshiba Electronic Components www.toshiba.co.jp/product/sc.htm ZBD www.zbddisplays.com