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Optoelectronics/Displays??

Researchers focus on efficient flat light source

Posted: 01 Jun 2006 ?? ?Print Version ?Bookmark and Share

Keywords:Princeton University? University of Southern California? OLED? John Walko?

Researchers at Princeton University and the University of Southern California are claiming a major breakthrough in OLEDs for use as a flat, energy-efficient, natural light source.

The method of jumping from single-color displays based on OLEDs to such a source, which could have a significant impact on the way homes and offices are lit, is published in the latest issue of Nature.

OLED program
The research is the latest fruit of a 13-year OLED program led by Mark Thompson, professor of chemistry at the USC College of Letters, Arts and Sciences, and Stephen Forrest, formerly of Princeton University and now VP for research at the University of Michigan.

"This process will enable us to get 100 percent efficiency out of a single, broad spectrum light source," Thompson said.

The work has been funded by the U.S. Department of Energy and Universal Display Corp., which holds exclusive licensing rights to the group's OLED inventions, and which is working to create a market for paper-thin, power-efficient lighting.

Thompson and Forrest previously invented efficient single-color displays now ready to enter the market in next-generation cellphones. But subsequent attempts by several groups to create white-light OLEDs fell short. The biggest problem has been the fast burnout time of the blue component, since blue is one of the primary colors needed to make white.

The Nature paper presents a quantum mechanical trick that solves this problem. First, the researchers followed their standard recipe for making an OLED: placing four ultrathin organic layers on glass or transparent plastic. Three of the layers serve as highways for charges to reach a central "emissive" layer.

When the oppositely-charged molecules meet in the emissive layer, electrons jump from the negatively-charged molecules to the positive ones, and ultimately relax to their starting energy. In the process, light is emitted, which can be tuned to cover a broad range of wavelengths.

Energy efficiency
Previous OLEDs used phosphorescent blue, green and red dyes to generate light with greater energy efficiency than all-fluorescence-based devices (phosphorescence and fluorescence, both expressions of energy that is released as excited electrons fall back into their regular orbit, differ mainly in the speed of their response).

Thompson and Forrest found that they could substitute a fluorescent dye for blue without sacrificing the superior properties of OLEDs.

They suggest the fluorescent dye should prolong the lifetime of the blue component and also that it uses 20 percent less energy.

"We're hoping this will lead to significantly longer device lifetimes in addition to higher efficiency," Thompson said.

According to Forrest, the device eventually could achieve three times the efficiency of standard incandescent light bulbs.

"With a future emphasis on manufacturing technology, this structure may provide an important, low-cost and efficient means that will replace incandescent lighting in many different applications," said Forrest.

The biggest hurdle to making these devices commercially, according to Thompson, does not concern the OLED itself, but the plastic layer to be used as a backing in economical large-area devices. All mass-produced plastics allow some humidity to pass through to the OLED, eventually degrading it.

"There's no plastic that's hermetic enough to make devices that will last a long period," Thompson said, while predicting that this problem can be solved.

- John Walko
EE Times




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