Tuesday, November 21, 2006

Efficiency Jump for White OLEDs

Microscale lenses and better materials move OLEDs closer to lighting our world.
By Neil Savage

In an advance that could hasten the day when energy-efficient glowing plastic sheets replace traditional lightbulbs, a method for printing microscopic lenses nearly doubles the amount of photons coming out of the materials, called organic light-emitting diodes, or OLEDs.

Stephen Forrest, an electrical engineer and vice president of research at the University of Michigan, says his technology increases the light output of the thin, flexible OLEDs by 70 percent. "They just create local curvature that allows light to pass through," he explains.

This means that OLEDs, which are currently used for superbright color displays in a number of applications, are getting closer to being competitive as white-light sources too. "It's a significant benefit, because the one big challenge in OLEDs is coming up with ways to get light out of them," says Vladimir Bulovic, head of MIT's Laboratory of Organic Optics and Electronics. "There's a lot of light in the OLED that never makes it out."

The benefits could be substantial. Sandia National Laboratory projects that if half of all lighting is solid-state by 2025--that is, made up of OLEDs and their technological cousin, LEDs made from inorganic semiconducting materials--it will cut worldwide power use by 120 gigawatts. That would save $100 billion a year and reduce the carbon dioxide emitted by electrical plants by 350 megatons a year. And OLEDs would offer more variety in lighting design, since they would take the form of flexible sheets.

But while LEDs are taking over a number of applications, from traffic lights to high-end architectural applications, getting enough light out of OLEDs to make them practical remains tricky. When electricity runs through the thin layers of organic polymers that make up the OLEDs, it causes the material to emit photons. The problem is that only about half of the photons ever reach the surface of the device, and the majority of those that do make it that far get turned back at the last instant. That's because the glass or plastic substrate on which the layers of the OLED are deposited has a high index of refraction, but the open air into which the photons are traveling has a low index. When they hit the glass/air interface, about three-fifths of the photons get scattered to the edges of the glass and never reach an observer's eye.

Researchers have tried several methods to send those photons in the desired direction, including inscribing gratings into the OLED and coating the surface with a silica gel that has a low index of refraction. Unfortunately, most of those methods caused a blurring effect or changed the color of the light when viewed at different angles. Researchers also tried larger lenses, but that required aligning the lenses with the OLED, a step that adds to the cost and complexity of manufacture.

Instead, Forrest uses microlenses, tiny hemispheres of polymer a few micrometers in diameter that direct the light forward from the OLED. He uses imprint lithography, essentially stamping a hexagonal array of lenses into a liquid polymer. Once it has hardened, the polymer layers making up the OLED can be deposited on top of the lenses. The ones he has made aren't perfect, Forrest says, but can be improved by a company that decides to optimize the manufacturing process.

With the lenses, described last month in a paper in the Journal of Applied Physics, Forrest is getting OLEDs to an external quantum efficiency--the percent of photons generated within the OLED that actually make it all the way out--of about 32 percent, up from previous highs of around 18 percent. The more important challenge, he says, is increasing the internal quantum efficiency--the percent of electrons that are turned into photons--so that there are more photons to get out. Right now that's at about 60 to 70 percent, but there's no theoretical reason why it can't make it to 100 percent.

Forrest says OLEDs could reach a light output of 100 lumens per watt within a couple of years, which would be far better than the 50 to 75 lumens per watt of fluorescent bulbs. (OLEDs have already far surpassed the 15 lumens per watt of incandescents.) The Department of Energy, which funds research into new forms of lighting, has a goal of 150 lumens per watt in 10 to 15 years. Even though they're brighter, OLEDs will have to become a lot cheaper to compete with existing lightbulbs.

Janice Mahon, vice president of technology commercialization at Universal Display Corp., which licenses Forrest's technology, says it's possible there will be some "entry-level" white-lighting OLEDs on the market in the next two years or so. Those might be small-area OLEDs used as architectural accents or in emergency signs. OLEDs for general illumination--large wall panels to light up a room, say--won't likely be available for more than five years, and probably for more than ten, she says. "It's anybody's guess."

Forrest isn't only working on the substrates. He recently improved the materials that make up the OLED layers. Typically, OLEDs have used a mix of phosphorescent materials that shine red, green, or blue, with the colors combining to make white light. But because of the differences in their wavelengths, a blue photon contains a lot more energy than a red one does, and thus takes more energy to create, with the result that the blue phosphor isn't as efficient as the others. The blue phosphor also breaks down more quickly, leading the color of the light to grow more yellow as the OLED ages. Changing power levels can also affect the color of the light.

So Forrest replaced the blue phosphor with a material that produces blue photons through fluorescence, a process that requires higher-energy electrons than phosphorescence. Forrest designed the layers so the fluorescent material, which is more efficient and more stable than the blue-phosphor material, was nearest the cathode and could capture higher-energy photons, then pass lower-energy ones to the other layers, where they'd create green and red light. Not only does his design make more-efficient use of power, but it also maintains its color when the power levels are decreased, leading to an OLED with adjustable brightness but stable color.
Original Article
Copyright Technology Review 2006.