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.