Electronic Design

LCDs, LEDs, OLEDs, And EPDs Light The Way

Display technologies continue to march forward, with LCDs, LED displays, organic LEDs (OLEDs), and electrophoretic displays (EPDs) leading the charge. LEDs and OLEDs may even overtake LCDs for some applications, while EPDs carve out a niche in electronic-ink displays for portable and flexible electronic products. Nonetheless, LCD applications are flourishing and becoming more diverse, requiring equally diverse design challenges to meet different performance requirements.

NEC Electronics America foresees important industrial and medical applications that require innovative approaches to satisfy various demands. Topping the list are the needs for portability, low power consumption, ruggedness, energy efficiency, environmental friendliness, and the ability to operate in variable lighting conditions both indoors and outdoors.

LCDs are still used in all kinds of products, large and small, despite a downward revision of market size as price erosions affect profits. For example, LCD 14-, 15.4-, 7-, 19-, 22-, 32-, and 42-in. panels are reportedly being sold at cost and sometimes at a loss. Despite this, LCD TV shipments are expected to rise 20% this year, up from 18.9% last year.

POWERING DOWN • Two major trends permeating the display industry are lower power consumption and greener LCD panels. For instance, NEC’s use of LED backlighting offers distinct advantages over cold-cathode fluorescent lamps (CCFLs). It addresses the needs for low-voltage and low-power operation, as well as low electromagneticinterference (EMI) levels. Also, it eliminates the need for an inverter; reduces thickness and weight; increases reliability; and lowers the risk of damage due to vibration and shock.

For indoor and outdoor environments, NEC incorporates two proprietary transflective LCD technologies: super-reflective and super-transmissive natural light technology (SR-NLT and ST-NLT, respectively). SR-NLT is based on transflective (semi-transmissive) technology with both transmissive and reflective characteristics, allowing users to change backlight modes on and off in response to changes in the outdoor lighting environment. ST-NLT is transmissive and can produce high-contrast images even in bright outdoor light.

According to NXP Semiconductors, adaptive smart-LED backlight dimming for LCD screens can be used to calculate the brightness level for each LED to satisfy different lighting requirements. This saves power and increases contrast ratio.

LCDs are now finding use in outdoor displays. Targeting these applications, Samsung’s 70-in. diagonal “super bright” LCD digital signage panel boasts a 1500-nit output, which the company calls the brightest such panel to date. The key is the use of local dimming of backlighting LEDs that continually adjust picture brightness in very precise increments.

By doing so, the technology doesn’t have to brighten areas that don’t require it. It increases the panel’s dynamic contrast ratio up to 200,000:1, too. Separately, Samsung says it has also developed the first “blue phase” LCD panel, which achieves 240-Hz driving speed for high-speed video.

Increasing an LCD’s efficiency is another way of cutting power consumption. A new design from Fairchild Semiconductor reduces the traditional four layers of printed-circuit boards (PCBs) used in a thin-film-transistor (TFT) LCD down to two, using highly integrated Fairchild FAN5069 and n-channel MOSFETs. The result is 90% efficiency.

Researchers at Scotland’s University of Strathclyde developed a reflective bi-stable LCD that can maintain static images in full daylight without using an external power supply. The scientists are looking to license this technology for solarpower display advertisements, mobile phones, PDAs, ATMs, electronic paper, smart cards, and disposable displays.

Thinner LCDs are also in the offing. Philips Research Labs developed a prototype lightweight (10-kg), 32-in. LCD TV that’s just 8 mm thick—about 20% the thickness of the slimmest commercially available panels. The key is a 1-mm thick light-guide plate that distributes light from highpower LED backlights.

Two competitive approaches to LCDs, using shutters and LED light sources to boost performance and lower cost, are being tried as well. Both exploit MEMS technology and existing TFT fabrication techniques.

One such approach is Pixtronix’s digital MEMS micro-shutter technology to produce the PerfectLight display, which uses a TFT backplane. The company says it consumes 75% less power than a traditional LCD panel while delivering exceptional image quality that’s 105% the National Television System Committee (NTSC) color gamut with 24-bit depth, a 1000:1 contrast ratio, and a 170° viewing angle.

A time-multiplexed optical shutter (TMOS) technology being developed by Uni-pixel also utilizes MEMS technology and LED light sources. It promises to deliver brighter, thinner, and less expensive LCDs via a light guide with TFTs and a special micro-optical MEMS layer (called the Opcuity active layer) to boost brightness and efficiency levels and reduce the display’s thickness (Fig. 1).

LEDS FINDING NEW APPS • Another hot area for LEDs involves energy savings, with designers using discrete LEDs in red, orange, blue, and green as well as high-power white sources for greater operating efficiencies and lower power consumption. According to ElectroniCast Consultants, LEDs used in residential and commercial/government exterior lighting represented an 86% share of worldwide consumption of solid-state illumination last year. Although that will drop to 67% this year, it will increase in value to $631.68 million in 2012.

Ruediger Mueller, CEO of Osram Opto Semiconductors, estimates that 19% of worldwide electricity goes toward lighting and that LEDs can help reduce light energy consumption by 30%. That figure can rise to 50% by adding more intelligence to the LED.

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Successful commercialization of LED products for general illumination, however, will require a number of factors, including low cost, longer lifetimes, higher intensity levels, better luminous efficacy, and improved color quality. All of these issues are under investigation with promising results in the lab.

Cree Inc. achieved a record 161 lm/W of efficiency for white-power LEDs. An independent survey on the adoption, benefits, and deployment benefits of LEDs taken by Mindwave Research Inc. on behalf of Cree validates projected strong LED lighting applications. The survey also confirms the awareness of the energy-efficient benefits of LEDs and indicates that the adoption trend should continue through the end of the decade.

LEDs are competing with CCFLs, both of which are emerging in all types of energy-saving lighting applications. CCFLs have established a niche in mainstream applications, but their lifetimes are limited compared to LEDs. On the other hand, LEDs are still comparatively more costly to move into mainstream applications.

Expenses are changing as LED manufacturing methods improve and prices drop. However, it will take a few more years to reap mass benefits from LED displays. Many lighting experts predict that the breakthrough in LED general lighting will occur by 2012 or 2013.

High-brightness LEDs have already carved out a niche in lighting for signs. Industry experts expect LEDs to eclipse neon lighting this year and widen their share in the years to come. LEDs should also see expanded use in appliances and automobile headlights.

High heat levels are a sticking point for high-brightness LEDs, though. According to the U.S. Department of Energy (DoE), 75% to 80% of the energy used to drive LEDs is converted to heat, which can reduce light output and produce a color shift. A solution is out there via better materials and improving driver techniques, though.

One novel method for cooling LEDs comes from Nuventix Inc. The oscillation of a diaphragm and the pulling in of air produces synthetic jet cooling. The air is then expelled in a turbulent fashion, setting up a secondary flow (Fig. 2).

OLEDS ARE IMPROVING • OLEDs now offer higher efficiency and light output, lower operating voltages, longer lifetimes, and greater full-color light outputs. Developments in high-brightness white-light OLEDs continue, eagerly supported by the DoE, which hopes to lower overall lighting energy costs.

The agency has set an output level of about 150 lm/W and voltage operation around 2 V for white-light OLEDs to reach the 50% level of power-conversion efficiency by 2012. And according to NanoMarkets, the OLED lighting market will reach $4.5 billion by 2013.

Collaborative work funded by the DoE and Universal Display Corp. and developed at Princeton University and the University of Michigan has shown promising results for increasing white-light LED output efficiencies. University of Michigan researchers are embedding low-index grids into the OLED active organic layers to increase efficiency levels to 70 lm/W, compared with 15 lm/W for incandescent lamps.

Conversion efficiency levels near 100% can be achieved with monochromatic OLEDs. But this is more of a challenge for white light, which typically comprises red, green, and yellow light-emissive structures. Universal Display’s phosphorescent OLEDs (PHOLEDs) have achieved power efficacies of 102 lm/W at 1000 cd/m2, with operating lifetimes of 8000 hours to 50% of initial luminance. The phosphorescent material’s molecules allow conversion efficiencies of electrical current to light photons of close to 100%.

French startup MicroOLED and France’s CEA-Leti announced an OLED microdisplay they claim has the finest pixel pitch (more than 1.7 million sub-pixels, which is two to four times greater than other emissive technologies) and the lowest power consumption (four times more efficient) for a compact 0.38-in. wide video graphics array or WVGA microdisplay. It’s designed for camcorders, digital still-camera eyepieces, and video or interactive eyeglasses. The underlying technology marries the capabilities of CMOS processing and the flexibility of OLED capabilities.

ELECTRONIC INK • Electrophoretic displays (EPDs) are slowly emerging in more applications. Together with OLEDs, EPDs are being used as electronic-ink (e-ink) flexible displays on portables. EPDs are light, thin, and rugged. They also offer low power dissipation and design independence.

Market research firm iSuppli Corp. expects the flexible display market to reach $2.8 billion by 2013, a 35-fold increase from the paltry $80 million market for 2007. Prominent EPD applications include the Phosphor watch from Art Technology Ltd., a smart card from SmartDriver, electronic shelf labels from UPM, a Hitachi Mobile phone, and an automotive key fob from Delphi.

According to Philips spinoff iRex Technologies, it has come up with a third-generation e-ink display that’s the only technology to emulate all of the functions of paper. The iRex 1000 e-reader uses a 10.2-in. diagonal EPD from E Ink to show the entire printed page of letter-sized sheets with 1.25-in. margins (Fig. 3). The display is produced on a glass substrate.

Plastic Logic is planning to introduce an 8.5- by 11-in. e-reader this year. Like the iRex 1000, it uses reflective highcontrast gray-scale material from E Ink. However, this device is a read-only product, while the iRex unit is a read-write product. But the Plastic Logic device is lighter at 13 oz (versus 20 oz) with a larger diagonal at 10.7 in. It’s also thinner at 0.25 in. (versus 0.5 in.) and has integral memory (versus a memory card for the iRex 1000). Plastic Logic uses a plastic substrate in its technology.

E Ink is constantly improving its EPD materials. Most recently, it introduced the Vizplex next-generation material imaging film for segmented display cells. It is 20% brighter and has higher resolution than previous-generation materials to support smaller display segments and more detailed artwork.

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