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OLEDs glow in efficient lighting quest

Posted: 02 Jul 2007 ?? ?Print Version ?Bookmark and Share

Keywords:efficient lighting quest? OLED research? OLED for mobile device TV?

OLEDs are staking out claims in mobile devices and TV displays, with the first OLED-based TVs expected to emerge this year. But the killer app for OLEDs may be as a high-efficiency white-light source for everyday illumination. A European research project has reported on a prototype that could bring the lighting application closer to realization, and similar programs are under way elsewhere.

Europe's three-year-old Organic LEDs for ICT and Lighting Applications (OLLA) project looks to develop high-brightness OLEDs for information and communication technology (ICT) and next-generation lighting. OLLA said in May that it had prototyped a white-OLED light source with an efficacy of 25lumens/W and a lifetime of more than 5,000hrs from an initial brightness of 1,000cd/m?.

"We doubled the efficiency and expanded the lifetime five times compared with a conventional, tungsten-filament bulb," said OLLA project manager Peter Visser. "In a few years' time, we expect OLED sources to be as efficient as a compact fluorescent source."

The OLLA project, which has just passed the halfway point of its funded duration, has a final efficacy target for OLED lighting of 50lumens/W, combined with a lifetime of 10,000hrs at 1,000cd/m? initial brightness.

The white-OLED light tile was designed at Philips Research Labs!a vocal OLED proponent!from a mixture of fluorescent and phosphorescent OLED materials within project partner Novaled AG's proprietary PIN (p-doped, intrinsic, n-doped) structure.

Last year, Philips and Novaled collaborated on a prototype device that achieved a color-rendering index (CRI) value of 88!competitive with triphosphor fluorescents!and an efficiency of 32lumens/W. (The CRI gauges a light source's ability to reproduce the true colors of objects illuminated by the source.) Total brightness was 1,000cd/m?. According to Novaled, the prototype exhibited a lifetime of more than 20,000hrs. Production devices would need to exhibit at least a 10,000hr lifetime, industry observers said.

Osram GmbH, one of largest lighting manufacturers in the world, is focusing on small-molecule technology for illumination applications. Its OLED elements have achieved lighting efficacy of 18lumens/W, compared with 12lumens/W for conventional bulbs.

Standards needed
In the absence of established benchmarks for OLEDs, the measured results are moving targets. "One of the problems in comparing the results of the different research groups around the globe is that there are no standards for OLED measurements," said Dietrich Bertram, manager of OLED development at Philips Lighting and technical coordinator of the OLLA project. "It makes an enormous difference when the measurement is done in an integrating sphere or when stacked layers are used. Therefore, the OLLA project decided to develop a white book on OLED lighting measurement, in order to support standardization of performance and measurement criteria."

Europe's OLLA project has shown OLED lighting sources can achieve potential efficiencies of 150 lumens/W.

Elsewhere, researchers at National Tsing Hua University in Taiwan are collaborating with Seattle-based Advanced Electroluminescent Sciences, an early-stage R&D company whose mission is to develop a solid-state lighting system based on white OLEDs. Researchers at Princeton University have also reported progress in white-emitting OLEDs.

The Princeton work was funded by the U.S. Department of Energy and Princeton spin-off Universal Display, and involved collaboration with the University of Southern California. The researchers modified the OLED structure to prolong the life of the blue-generating component, for which fast burnout has been a problem.

The researchers first made a standard OLED, placing four ultrathin organic layers on glass or transparent plastic. They made three of the layers serve as conduits for charges to reach a central "emissive" layer. When the oppositely charged molecules meet in the emissive layer, electrons jump from the negatively charged molecules to the positive ones and ultimately relax to their starting energy. The resultant emitted light can be tuned to cover a broad range of wavelengths.

The researchers note that previous OLEDs used phosphorescent blue, green and red dyes to generate light with greater energy efficiency than all-fluorescence-based devices. The researchers found that they could substitute a fluorescent dye for blue without sacrificing the OLED's properties.

Compact fluorescent bulb and fluorescent TL tube flank conventional incandescent bulb, top. The OLED's edge, with more tweaking, would be a long-lived, efficient white or tinted light source.

A leaner screen
Meanwhile, companies are beginning to deliver on OLED TVs and active-matrix (AM) OLEDs. Because OLEDs don't require a backlight, they consume less power than non-organic LEDs and can enable thinner screens than such other flat-panel technologies as LCDs and plasma. OLEDs can also be printed on flexible material to yield roll-up displays.

According to DisplaySearch, OLED display shipments rose 29 percent in 2006 to 72.1 million units, though revenue for the category declined 3 percent.

Sony Corp. said it will begin manufacturing and selling 11-inch OLED TVs later this year. Initial production levels will be 1,000 units per month, and the units will be manufactured by a joint venture between Sony and Toyota Industries Corp.

Toshiba Corp. and Matsushita Electric Industrial Co. Ltd, meanwhile, have formed their own joint venture!the Toshiba Matsushita Display Technology Co. Ltd!which will manufacture OLED panels for flat-screen TVs, with production of 20.8-inch screens beginning 2009.

Mitsubishi Heavy Industries Ltd said it plans to ship sample OLED panels this year and may begin mass production if demand is strong. And Konica Minolta Technology Center Inc. said it has teamed with General Electric Company for joint development of lighting products using OLED technology over the next three years.

Passive rival
Companies such as Samsung SDI Co. Ltd, LG Electronics Inc. and Chi Mei EL Corp. (CMEL) have embarked on full-scale production of AMOLEDs in 2007.

AMOLEDs consume less power than their passive-matrix counterparts, making them suitable for portable electronics where the display is larger than 2-3inches on the diagonal, according to Universal Display, which counts AMOLEDs in its portfolio of flat-panel intellectual property.

Universal Display recently signed an agreement with CMEL, a unit of TFT-LCD maker Chi Mei Optoelectronics. Universal will supply its phosphorescent OLED materials, manufactured by PPG Industries, to CMEL for use in the company's commercial AMOLED displays. Universal claims that its proprietary phosphorescent technology increases power efficiency by up to four times over conventional OLED technology.

With energy conservation front and center in the public consciousness, other researchers are looking to combine OLEDs with solar cells. Scientists at National Taiwan University have positioned solar cells behind OLEDs to provide a contrast superior to that achieved with polarizers while recycling energy that would otherwise be wasted.

The scientists reported that placing a solar cell in the back of the OLED absorbed the incident light and internal OLED emission, and then converted the light to electrical power for reuse as a photovoltaic device. The modest, 0.26 percent power-recycling efficiency of the experimental unit might be improved by using more-efficient OLEDs and solar cells, according to the scientists.

- Nicolas Mokhoff
EE Times

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