LCDs Trump PDPs While LEDs, OLEDs Accrue Applications

Jan. 17, 2008
When it comes to displays, three technologies dominate the field—liquid crystal displays (LCDs), light-emitting diodes (LEDs), and organic LEDs (OLEDs). Plasma display panels (PDPs) and polymer LEDs (PLEDs)have some challenges ahead, though

When it comes to displays, three technologies dominate the field—liquid crystal displays (LCDs), light-emitting diodes (LEDs), and organic LEDs (OLEDs). Plasma display panels (PDPs) and polymer LEDs (PLEDs)have some challenges ahead, though success is possible. And, keep your eye on the active-matrix OLED (AMOLED) market.

LCDs have overtaken PDPs to claim the lion’s share of the flat-panel display market. OLEDs remain a dark-horse candidate for both flat-panel and discrete lighting applications. And, LEDs are making tremendous progress as white-light sources and in other applications like traffic lights and backlighting for flat-panel displays.

PDPs have enjoyed some market penetration with lower prices in applications that require diagonals of 34 in. or less. They suffer from much higher power-dissipation levels and lower resolution levels. However, studies show that efficacy and manufacturing improvements can make PDPs more competitive with LCDs.

LCD panels presently feature luminous efficacies of about 5 lm/W, while those for PDPs now feature 2 to 2.5 lm/W. Prototype PDPs with luminous efficacies of 5 lm/W have already been demonstrated, and some experts say that 10 lm/W is quite possible.

A comprehensive “apples-to-apples” analysis by DisplaySearch compared PDPs with diagonals of 50 to 52 in. to 52-in. LCDs. Results show that PDP costs would remain lower than those for LCDs and could be 29% lower at 5 lm/W and 59% lower at 10 lm/W (Fig. 1).

Meanwhile, researchers at the UCLA Henry Samueli School of Engineering and Applied Science have achieved up to18 lm/W from a red phosphorescent PLED to backlight PDP displays. They’re using a novel and simple combination of a polymer powder and a liquid mixture that’s added to a proprietary Canon material. So far, they’ve achieved luminous efficacies of 20 lm/W for white LED outputs and 30 lm/W for green emissions. Look for Canon to make these LED arrays available within two to three years.

LCDs Reign Supreme
LCD flat-panel displays are getting larger and less expensive. The largest is an LCD TV with a 108-in. diagonal from Sharp. Some experts believe the technology is close to reaching a finite limit on size. Yet there’s still more room for improvement. Last year, TPO Displays Corp. developed a process that resulted in the world’s thinnest daylight-readable module—just 0.95 mm thick.

The module is made on a low-temperature polysilicon (LTPS) process. TPO leveraged a cutting-edge chemical etching process that allows both bi-panes of the glass to be thinned to just 0.2 mm and enables the use of 0.25-mm ultra-thin light guides and very thin capacitors. The module is ideal for 2.0-in. QVGA (640 by 480 pixels) LCDs, exceeding the requirements for telecom, PDA, and digital still camera applications where the design footprint is a major consideration.

LCD panels of 15 in. or less now use LEDs instead of the tried-and-true cold-cathode fluorescent lamp (CCFL) for backlighting. That’s because LEDs cost less, they use materials that are more environmentally friendly, and they use less power. Also, they provide higher efficiency and longer lifetimes.

The increased brightness levels of white-light LEDs have made them feasable for use in the back- and edge-lighting of LCDs as well. Look for LEDs to backlight LCD panels greater than 15 in. In fact, many CCFL suppliers also offer LED backlighting options.

Unlike other flat-panel displays like LEDs or OLEDs, LCD technology isn’t self-emitting. It instead uses backlighting, which tends to increase power dissipation and limit the panel’s thinness. Although much progress has been made in manufacturing slimmer LCD panels, there’s a limit to how thin they can be since they need a backlighting source.

Yet this could change, as Sharp Corp. plans to develop a 1-in. thick LCD panel that will be integrated into mainstream displays produced at the company’s tenth-generation fab facility, scheduled to begin operation in March 2010. Sharp already has demonstrated a 52-in. prototype panel with a thickness of 20 to 29 mm. The company believes such a display will dissipate half the power of conventional LCD TVs and one-fourth the power of plasma-panel TVs.

An OLED in Your Future
Of all the flat-panel display technologies under investigation, none elicits more positive predictions than OLEDs for fixed and flexible displays, as well as for general lighting applications. Researchers are making advances in OLED efficiencies, light output levels, lifetimes, power dissipation, and manufacturing methods.

Konica Minolta has developed a white-light OLED with an efficiency of 70 lm/W at 1000 cd/m2, which makes it as efficient as an incandescent light bulb. Also, Novaled has used its PIN doping technology to achieve white-light OLED efficiency of 35 lm/W at 1000 cd/m2 with a lifetime of 1 million hours (Fig. 2). The company’s advanced green PIN phosphorescent OLEDs have record-setting lifetimes of 200,000 hours at 1000 cd/m2 and very low driving voltages.

Look for more advances in flexible OLED displays in the near future. Novaled has joined up with the large steel manufacturer ArcelerMittal to develop top-emitting OLEDs made on flexible steel substrates. The company has found a promising technique for extending OLED lifetimes by the use of very thin organic and inorganic multilayers of thin-film encapsulation (TFE) instead of conventional glass plates. This will enable flexible and rollable displays with a thickness in the micrometer range.

In cooperation with Universal Display Corp., which holds the original patents for phosphorescent OLED technology, LG Philips LCD has developed the first full-color flexible active-matrix OLED (AMOLED) using amorphous silicon.

“This display capitalizes on the strengths of OLED technology, and its image quality is really something to behold,” says Hyunhe Ha, executive vice president and head of the small and medium displays business unit at LG Philips LCD. “Although we are still in the initial stage with this technology, we expect to make some exciting advances in the near future.”

AMOLEDs have the advantage of enabling very thin TVs. In fact, Sony launched the industry’s first AMOLED TV last month. The XEL-1’s display is just 3 mm thin with an 11-in. diagonal screen.

LED technology not only is backlighting LCD panels, it’s also penetrating automotive, architectural, and general illumination applications, particularly white-light sources. Look for LED technology to shine brighter in the next few years, well beyond the mobile phones it mostly has been used in so far.

Recent announcements from LED manufacturers show very impressive results in efficacy and output lighting levels. There’s a conundrum here, though. Getting both high power outputs and high efficacy levels simultaneously is very challenging and will require greater improvements in LED designs, layouts, heat management, and manufacturing. Most LEDs today dissipate roughly three-fourths of their outputs as heat with the remainder as light. But dramatic improvements are under way.

Cree has demonstrated a white-light output of 1050 lm/W from a single power LED when driven by up to 4 A with an efficacy of 42 lm/W. Its latest XR-E series of power LED lamps delivers a cool white light output of up to 250 lm/W. These devices are being used in a wide range of indoor and outdoor locations as part of the company’s sponsored LED City Initiative, which now includes Raleigh, N.C., Ann Arbor, Mich., and Toronto, Ontario, Canada.

Osram Opto Semiconductor expects to introduce the Diamond Dragon single-chip surface-mount LED this month (Fig. 3). With an output of 250 lm/W, it will be available in whites ranging from color-corrected temperatures (CCTs) of 2700K through 6500K as well as all monochromatic colors.

Credit this advance to the LED’s low thermal resistance of 2.5 kΩ/W, provided by the surface-mount (SMT) package, which allows the heat produced in the chip to be efficiently removed from the LED package. According to the company, the LED is ideal for both indoor and outdoor general lighting applications.

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