Los Alamos researcher presents bright idea

Contact: James Rickman, elvis@lanl.gov, (505) 665-9203

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LOS ALAMOS, N.M., Aug. 27, 2001 -- Researchers from the U.S. Department of Energy’s Los Alamos National Laboratory are exploring methods for creating more electrically efficient organic light-emitting diodes -- technology that could be used to create energy-efficient panels of light for use in buildings or homes.

Los Alamos researcher Darryl Smith today presented ideas related to solving organic LED power efficiency problems at the 222nd national meeting of the American Chemical Society in Chicago. Smith’s research focuses on the use of a chemical layer a mere molecule thick to help facilitate the flow of current from a power source into the self-assembled polymers that give organic LEDs their colorful glow.

Organic LEDs -- known as OLEDs -- currently are used in very thin, flat display screens, such as those in portable televisions, some vehicle dashboard readouts and in postage-stamp-sized data screens built into pilots’ helmet visors. Arrays of OLEDs just one millimeter thick can display full-color moving or stationary pictures and graphics at extremely high resolution. Because OLEDs emit their own light and can be incorporated into arrays on very thin, flexible materials, they also could be used to fashion large, extremely thin panels for light sources in buildings or elsewhere.

"Organic light-emitting diodes show great promise in a number of applications," Smith said. "Their biggest drawback, however, is that they’re not sufficiently efficient. Right now it takes a relatively large amount of electricity to make them work. If you can make OLEDs more energy efficient, then you potentially could create very bright, very long-lasting light sources that would be versatile, flexible and inexpensive to use."

A standard OLED consists of a transparent layer of an electrically conducting material, such as indium tin oxide, deposited on a transparent substrate. On top of this first conducting layer is another layer of organic polymer -- a chain of carbon-based molecules -- that emits light when excited by electrical current. A final conducting layer of material is deposited on top of the organic polymer layer. When voltage is applied to the two conducting layers, a current runs through the polymer layer and it emits photons, creating a light.

As simple as the process seems, it is not yet energy efficient. Because of the different chemical and physical properties of the conducting layers and the polymer layer, it often is difficult to create conditions that result in a smooth flow of current through the layers. As a result, more voltage must be applied to achieve the electric current required to generate photons from the organic polymer.

"Ideally what you want is to use the minimum amount of power to produce the light," Smith said.

Smith and Los Alamos colleague Ian Campbell have proven a method by which an intermediate chemical layer can be applied between a conducting layer and the polymer layer to achieve more efficient current flow. The process uses what nanotechnologists call a self-assembling monolayer to facilitate electrical flow between OLED layers.

The intermediate self-assembling monolayer comprises rows of molecules that line up in the same direction, like bristles on a brush, when applied to a substrate; inherent chemical properties of the molecule ensure proper alignment without external manipulation. By adding atoms at both ends of a monolayer molecule, the researchers developed a molecule that would anchor itself to the conducting layer while maintaining a distinct polarity across the entire molecule -- in essence giving the molecule charged ends like the poles of a bar magnet. The result is a molecular layer a few billionths of a meter thick that helps shuttle electrical charges between an OLED conducting layer and polymer layer.

The research still has a ways to progress. In order for OLEDs to be used as light sources, their efficiency probably must increase at least to the efficiency of existing fluorescent light bulbs. That means OLED efficiency must at least double based on comparisons of present OLEDs and fluorescent light bulbs.

Still, the gains in efficiency shown by use of a monolayer in OLED manufacturing has illuminated interest in the idea and feasibility of decorative panels of light that can be operated inexpensively by the consumer. With recent concerns over energy supplies and costs, the research is particularly relevant to the nation.

"I look at my role as providing a proof of concept for this type of technology," Smith said. "It will be up to industry to take these proofs of concept to create energy-saving devices that can be used by the public."

Los Alamos National Laboratory is operated by the University of California for the U.S. Department of Energy's National Nuclear Security Administration.

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