Bright idea from Los Alamos

Researchers from the US Department of Energy’s Los Alamos National Laboratory are exploring methods for creating more electrically efficient organic light-emitting diodes. The technology that could be used to create energy-efficient panels of light for use in buildings or homes.

Los Alamos researcher Darryl Smith 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 – known as OLEDs – their colourful glow.

‘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, current runs through the polymer layer and it emits photons, creating a light.

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.

Consequently, more voltage must be applied to achieve the electric current required to generate photons from the organic polymer.

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 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 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.

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 double, based on comparisons of present OLEDs and fluorescent light bulbs.

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