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Sensitization Produces High-Efficiency Organic LEDs

Photonics Spectra
May 2000
Daniel S. Burgess

Organic LEDs promise much to the commercial display industry -- cool, low-voltage operation, smaller pixels, low cost -- but manufacturing difficulties and efficiency problems have prevented them from penetrating the market. A team of researchers from Princeton University and the University of Southern California in Los Angeles, however, has produced a high-efficiency organic LED, and its corporate sponsor hopes to bring the technology to limited flat panel display applications within the next six months.

The researchers suggest that the limitations of fluorescent dyes used in the LEDs can be circumvented with the use of phosphors. In their work, which was detailed in the Feb. 17 issue of Nature, they reported that they more than tripled the quantum efficiency of an organic LED based on a red fluorescent dye by pairing it with a phosphorescent sensitizer.

Although fluorescent dyes offer significant advantages over phosphorescent molecules, they are inefficient -- they can use only singlet excitons and so waste the energy in triplets. "Only one of four excited states ... can be captured from a conductive organic host molecule to emit light," explained Stephen R. Forrest, a professor at Princeton and co-author of the study. "In the best possible case, 25 percent quantum efficiency can be achieved."

But in 1998, the team, which is funded by Universal Display Corp. of Ewing, N.J., discovered that phosphorescent dopants capture both singlets and triplets. The problem, Forrest said, is that "there are far more fluorescent than phosphorescent molecules identified with characteristics suitable for displays."

The challenge was to develop a process that would combine the efficiency of the phosphors with the usefulness of fluorescent dyes. The team turned to Förster energy transfer -- a phenomenon predicted in 1959 by which phosphors transfer triplet excitons to the singlet state -- and the hybrid device displayed an energy transfer of nearly 80 percent from the phosphor to the dye.

Forrest said the method could lead to the production of flat panel displays for cell phones and for laptop and palm-size computers. "This work breaks a fundamental barrier to widespread use of [organic] LED displays," he said. But he noted that high-volume manufacturing processes must be developed before the LEDs can secure a slice of the $40 billion electronic display market.

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