White OLEDs Brighten
Organic light-emitting diodes (OLEDs)–which already make color displays in mobile phones and other devices brighter and more efficient–have taken a step closer to competing as sources of white light, too. In an OLED, electricity running through thin layers of organic materials causes them to emit photons. But only half the photons make it out of the materials, and three-fifths of those get scattered to the edges. Stephen Forrest, a University of Michigan electrical engineer, and graduate student Yiru Sun came up with a trick for making the diodes brighter: they use imprint lithography to stamp a hexagonal array of lenses, each a few micrometers in diameter, into a polymer substrate (left). The lenses direct light outward rather than sideways, boosting light output by 70 percent. “It’s a significant benefit,” says Vladimir Bulovi´c, co-head of MIT’s Laboratory of Organic Optics and Electronics. “There’s a lot of light in the OLED that never makes it out.” Eventually, energy-efficient sheets of glowing plastic could replace traditional light bulbs. Forrest says that with directive lenses and other improvements, OLEDs could reach an output of 100 lumens per watt in a couple of years, which would be better than the 90 lumens per watt of fluorescent bulbs. Manufacturing costs would then be the major remaining hurdle.
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