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Micro lenses make OLEDS more efficient
15 October 2008, Opto-Electronics

Organic light emitting diodes (OLEDs) are optimistically being looked upon as the technology set to become disruptive in displays and lighting.

For this to happen however, there are challenges that need to be overcome, such as the efficiency and lifetime of OLEDs.

From the perspective of lighting applications, there are several initiatives being taken to make OLEDs more competitive with incandescent bulbs and fluorescent lights. A current problem with OLED-based lighting is that a major portion of white light is trapped within the device and only 20% of the light generated is released.

White light from OLEDs is created by applying a voltage across nanometre-thick layers of organic semiconductors that are sandwiched between a pair of electrodes. The applied voltage stimulates the material to emit light.

Suggesting improvements to the device design, researchers at the University of Michigan and Princeton University have demonstrated better efficiency by increasing the illumination released to 60%. Conventionally, the light is reflected internally along the surface of the semiconductor and not in the desired perpendicular direction. To overcome this problem, the researchers have introduced an organic grid along with micro lenses to direct the trapped light out of the OLED. The grids are used to refract the trapped light and guide it to the five micrometre thick dome-shaped lenses. The lenses emit the light in the vertical direction and help improve device efficiency.

This white OLED architecture is beneficial due to the fact that it can emit 70 lumens per watt of power, compared to the 15 lumens per watt of incandescent light bulbs. Despite the fact that fluorescent tubes still give out 90 lumens per watt, the quality of light from tube lights is harsh and could also make use of toxic substances such as mercury.

White OLEDS also have the advantage of being incorporated on to flexible substrates such as metal foils and plastic substrates. The constituent organic materials are carbon-based dyes akin to those in photographic prints and are economical.

The researchers aim to show further improvement by utilising OLEDs of higher efficiencies than those currently used in the project. The team remains optimistic about the scalable manufacturing of the technology and is convinced about the competitiveness of production costs.

The impact of adoption of such technologies in interior illumination could lead to a reduction in energy demand for lighting in buildings. While OLED technology would take a while to reach its disruptive potential, such developments could certainly accelerate the adoption of OLED lighting.

For more information contact Patrick Cairns, Frost & Sullivan, +27 (0)21 680 3274,,

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