By using imprint lithography to stamp a hexagonal array of lenses into a polymer substrate (shown above) diodes are made brighter. Credit: Courtesy of Y. Sun, S.R. Forrest, Journal of Applied Physics 100, 073 106

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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