Electronic Design

LCD, LED, OLED Advances Fuel Flat-Panel Market

Aggressive pursuit. That sums up the development climate for display technologies in myriad application categories. Many of these segments involve flat panels in diagonal sizes from less than an inch to several feet.

Optimistic market figures support the trend. Many analysts foresee multi-billion-dollar global display markets within the next few years. In TV displays alone, market research firm iSuppli Corp. predicts global LCD flat-panel TV set sales of 17.1 million units in 2009, up from 2004's 8.9 million units.

Much of the growth will occur in liquid-crystal displays (LCDs), light-emitting diode (LED) displays, organic LEDs (OLEDs), and plasma panel displays (PDPs). Surface-emitting displays (SEDs), flat-panel CRTs, and electroluminescent (EL) displays are expected to make some noise, too.

Anticipating an increase in sales of large-size flat-panel LCD TVs (greater than 35 to 40 in. in diagonal), LCD manufacturers have invested billions of dollars in next-generation fabrication facilities and processes. But the results of that investment have yet to materialize. Suppliers of PDPs and microdisplays for rear-projection TVs also are struggling to stay competitive.

Despite the underwhelming market, technological advances rage on. Last year, Samsung announced an 82-in. diagonal LCD TV screen. According to Samsung, this thin-film transistor (TFT) full-color LCD TV offers full high-definition image quality as well as the best available contrast ratio, viewing angle, and color saturation (Fig. 1). The company anticipates producing two such display panels from a single substrate.

The basic problem with widescreen flat-panel TVs is that they cost more than the venerable CRT, which continues to advance technologicially. Though LCDs still dominate the large-screen flat-panel TV market, 96% of them serve in markets that require a diagonal size of 40 in. or less (Fig. 2).

On another front, there's the need to increase image brightness and quality levels, as well as decrease power-consumption levels?particulary for battery-powered portable consumer products. One way to achieve this is via advanced pixel-rendering techniques (see ?A Balance Of Power,? Drill Deeper 11696).

Cell phones constitute the largest market for LCDs?by a wide margin. Consumers demand higher resolution, greater color quality, and lower power consumption. Cell phones with LCD diagonals of 2.2 in. and QCIF+ (176 by 220 pixels) or QVGA (240 by 320 pixels) formats are fairly common. They're expected to move to 2.4-in. diagonals and VGA (480 by 640 pixels) resolution. Some experts don't think mobile phone displays will move to diagonals greater than 2.4 in., at least in the near term.

Plasma Panels Evolve
PDPs continue to progress, despite the fact that some display experts warned of their demise at the hands of LCD technology. Right now, PDPs take top honors for the largest commercial flat-panel TV displays, checking in with diagonals of 82 in. Samsung has developed a record-setting PDP with a 120-in. diagonal (Fig. 3).

Like CRTs, PDPs are emissive displays containing phosphors that provide color. PDPs consist of tiny cells sandwiched between two plates of glass that also house a layer of magnesium oxide, the drive electronics, and phosphors. Applying a potential across the glass plates stimulates the cells to release ultraviolet photons that consequently excite color-specific phosphors (Fig. 4).

Plasma-panel technology can battle the competition in improved luminous efficiency levels (presently at about only 1 to 2 lm/W). It also offers lower power dissipation levels and higher levels of resolution.

The Advanced PDP Development Center has developed a 43-in. prototype PDP with luminous efficiency of 3.5 lm/W and an 11-in. PDP with 5.7 lm/W. The goal is to reach 5 lm/W by refining gas-discharge mechanisms, fluorescent materials, and drive circuitry. The center estimates that the 5-lm/W figure can lower the power consumption of a 40-in. PDP from 390 W to 100 W.

Matsushita (Panasonic) has developed a protoype 50-in. PDP with 1080-line progressive resolution. This is the smallest PDP that delivers more than 2 million pixels (1920 by 1080 pixels). It offers the same brightness levels of commercially available Panasonic PDPs, which feature 1366 by 768 pixels of resolution.

In addition, Matsushita joined Hitachi and Pioneer to develop a PDP that runs on half the power used by current PDPs. This joint effort, funded by Japan's Ministry of Economy, Trade and Industry, is expected to reach the market within a couple of years.

Those Darling OLEDs
The LCD juggernaut hasn't slowed OLEDs, considered the ?darling? of future flat-panel displays. Originally conceived as small displays for portables, they're showing lots of potential for computers of all sizes and could wind up in TV sets. Their diversity is expanding, too. Potential applications range from high-definition TVs to replacements for incandescent lamps and even fluorescent lighting.

Samsung already has demonstrated a 40-in. diagonal amorphous-silicon-based OLED for emissive flat-panel TV applications. The prototype unit features WXGA resolution of 1280 by 800 pixels. Seiko Epson also has revealed a 40-in. OLED display.

Universal Display Corp., working with Korea's Hyung Hee University, proved that OLEDs can achieve very good resolution levels. Its 200-dot/in. active-matrix transparent OLED display uses Universal's phosphorescent OLED (PHOLED) technology. The display's transparency promises many potential uses for flat-panel see-through screens where preserving partial visibility or bi-directionality is important. Applications include medical, industrial, architectural, entertainment, military, homeland security, and emergency services.

OLEDs must overcome short lifetimes, though, particularly for certain colors like blue. Limited lifetimes hamper OLEDs, which have half-life ratings of 1000 hours or less.

Recently, Cambridge Display Technology achieved lifetimes for blue OLEDs of 80,000 hours. Also, the company reached a milestone in advanced lifetimes last year when it developed blue polymers for OLEDs with 100,000 hours of life from an initial luminance of 100 cd/m2 .

Using its proprietary PIN technology, Novaled has developed a red top-emitting OLED that achieves record power efficiency of 10 lm/W at brightness levels of 500 cd/m2. Its projected lifetime is 100,000 hours. The ultimate goal of OLED manufacturers is to produce display panels with lifetimes of about 50,000 hours, but so far that remains elusive.

High-efficiency white OLEDs, all the rage in OLED development, are becoming desirable alternatives to full-color active-matrix OLEDs. That's because they can be coupled with color filters to circumvent the shadow-mask problem for RGB pixellation experienced during production. They also can produce higher resolution.

White OLEDs are appealing for lighting applications as well. At General Electric's Global Research Labs, work proceeds on large-area white OLED panels that emit 1200 lm. In a joint effort, Philips and Novaled have reported a record power efficiency level of 25 lm/W at brightness levels of 1000 cd/m2 .

A Bright Future For LEDs
Breakneck technological and market advances fuel the LED arena. Technology leaps in materials, packaging, thermal management, and processing continue to push LED applications in new directions.

Among the latest is the use of LEDs for backlighting LCD TVs and display monitors. Another hot area is general lighting for homes and offices, an effort strongly supported by U.S. government funding. Here, white-light emitting LEDs are rapidly taking over areas traditionally served by incandescent and fluorescent lights.

Just how big is this LED general lighting market? It's big enough to warrant a projected market revenue of nearly $900 million by 2010, according to iSuppli Corp. Currently, the market stands between $150 million and $200 million.

Last year, an important technological breakthrough in LED lighting came by way of the Lighting Research Center of the Rensselaer Polytechnic Institute. Its scattered photon extraction (SPE) technique will speed up the progress of LED lighting and help the nation conserve energy.

With SPE, an LED can produce significantly more white light without requiring any more energy. The U.S. Department of Energy's Building Technology Program and the National Energy Technology Laboratory funded the RPI research effort.

Ultimately, quantum-dot technology may be used to get the most efficiency from LEDs for better lighting. That's the belief among researchers at Vanderbilt University. They've shown that a quantum-dot approach to generating tunable broad-spectrum white light simplifies solid-state lighting. A single nanocrystal was used to produce white light when it was irradiated with commercially available blue LEDs.
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