Issue Date: July 14, 2008
The Bulb Is Flat
EVEN AS consumers rush to replace their incandescent light bulbs with compact fluorescents, lighting companies are developing a technology that could render all the bulbs on today's store shelves obsolete: solid-state lighting based on organic light-emitting diodes (OLEDs). Originally intended for colorful alphanumeric and computer displays, OLEDs promise lighting companies both energy savings and novel design.
Conventional incandescent light bulbs have dominated electric lighting since the 19th century. In these lights, an electric current passes through a tungsten filament. The resistance in the filament generates heat and emits light via the same phenomenon that makes molten metal glow. These bulbs waste energy because they emit infrared and ultraviolet light that humans can't see.
In fluorescent lights, the electric current excites mercury-containing vapor, which in turn generates UV light that induces fluorescence in the bulbs' coating materials. People are using these lights in growing numbers because they are many times more efficient and longer lasting than incandescent bulbs.
The materials in solid-state lighting—inorganic and organic light-emitting diodes—emit light when an electric field is applied to them. Conventional inorganic light-emitting diodes (ILEDs), used, for example, in traffic lights, are based on semiconducting materials such as red-yielding aluminum gallium arsenide. OLEDs are based on organic semiconducting small molecules, such as quinolines and quinacridones, or polymers, including polyfluorene and poly p-phenylene vinylene. OLEDs have found applications in, for example, MP3 players, whose designers are attracted to their vibrant colors and energy savings compared with back-lit liquid-crystal displays.
To make a mark in more general lighting applications, OLEDs need to generate white light. To do that, for instance, polymer materials that emit blue light can be combined with red and green chromophores to yield white. In small-molecule-based devices, different colors are stacked on top of each other to make white.
When asked about OLEDs' potential advantages, Peter Visser first mentions the freedom they would offer to designers. Visser is a scientist with Philips and the project manager for the OLLA project to develop OLEDs for lighting applications. The project is funded mostly by the European Union and includes the electronics firms Philips, Osram Opto Semiconductors, and Siemens, as well as the materials suppliers Merck KGaA and H. C. Starck.
"For the first time, we have made the light bulb flat and spread it out over a large area," Visser says. Current bulbs and ILEDs, in contrast, are point sources that are diffused with lamp shades, lenses, or translucent sheets of plastic or glass. OLED developers envision luminescent walls or ceilings and even windows that let natural light in during the day and emit artificial sunlight at night.
OLEDs also promise energy efficiency. OLLA's lights have reached a luminous efficacy—how much light a source emits versus a given amount of power—of about 50 lumens per W. This figure exceeds the 15 lm/W of the typical incandescent bulb and is already on par with compact fluorescents. Last month, Ewing, N.J.-based materials developer Universal Display demonstrated an OLED light with 100 lm/W of output, about the same as white ILED lights and the fluorescent tube lights that illuminate countless office buildings.
COMPANIES ARE also making gains in the lifetime of the units. Visser says units that are still under testing have already achieved 10,000 hours of lifetime. A Universal Display device has registered 200,000 hours at 30 lm/W. By contrast, incandescent bulbs last only about 1,000 hours and fluorescent lights last up to 20,000 hours.
The EU is sponsoring a follow-up project to OLLA called OLED100.EU. It involves Siemens, Osram, Evonik Industries, and other firms. Goals for the project include efficacy of 100 lm/W, a lifetime of 100,000 hours, and a lighting cost of 100 euros per square meter.
David Lieberman, a consultant with the Glen Allen, Va., market research firm NanoMarkets, says OLED developers have made substantial progress. "In the last two years, a number of companies have achieved the basic requirements for lighting," he says.
Lieberman expects that solid-state ILED and OLED lighting will eventually replace incandescent and even fluorescent bulbs. "Fluorescent lights will be dinosaurs, there is no question about that, even the long, traditional ones," he says. "They are long, fragile glass tubes with mercury in them." OLED and ILED lights will coexist, he says, with OLEDs taking many applications because of design flexibility.
NanoMarkets projects that the global market for OLEDs in general illumination will grow from $2.4 million this year to $1.4 billion by 2014. The first such lights to sell, he says, will be purchased for their novelty by people of means. For instance, in May, Osram and designer Ingo Maurer unveiled a table lamp based on OLEDs at a lighting show in the U.S. Despite such successes, it will be years before consumers can buy OLED panels at Home Depot. Companies have to scale up manufacturing and bring down costs before that happens.
Cost is the central focus of General Electric's OLED project. The company began OLED research about eight years ago, when work on OLED displays was already well advanced, to see whether the technology could work for lighting, recalls Anil Duggal, manager of GE's advanced technology program in organic electronics. "If you want the technology to work for lighting, you have to think differently," he says. "You have to realize it is a totally different cost structure."
By 2004, the company had finished a 24-sq-in prototype panel that put out 1,200 lm with the same efficacy of an incandescent bulb. "From then on, what we have been working on is figuring out how to make OLEDs more efficient and lower cost," he says.
The company is focusing on roll-to-roll processing, whereby the light-emitting materials are applied to a flexible substrate via printing and other coating technologies. Most other OLED technologies require chemical vapor deposition (CVD) of materials onto a glass substrate. Duggal and many others in the organic electronics industry believe roll-to-roll processing will offer lower manufacturing costs in the long run.
IN MARCH, GE demonstrated the first devices made through roll-to-roll technology. The company also has roll-compatible devices that put out 35 lm/W and last 5,000 hours. A big challenge the company faced was finding a flexible plastic substrate that was a good enough oxygen and water barrier to protect the sensitive emissive materials. Thus the company used a plasma technique to deposit a flexible glass coating on the plastic substrate.
Duggal acknowledges that it is tougher to get good performance out of wet-process systems than it is with CVD. In wet processing, all of the performance has to come from fewer organic layers than with CVD. Attempts to create multiple wet coatings are stymied by the solvents, which tend to take off the most recently applied coating.
According to OLLA's Visser, companies developing CVD-based OLEDs can also bring down their manufacturing costs via continuous manufacturing rather than batch processing. He adds that manufacturing can also be done on larger and larger substrates that are then cut into smaller units, just as has been done in the liquid-crystal display industry.
Janice Mahon, vice president of technology at Universal Display, believes that proponents of both CVD and coating technologies should be able to reduce costs enough to be competitive. "One reason why we have pursued materials that can be used in different manufacturing techniques is that we don't think there is one winning process," she says.
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