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Patterned polymers boost OLED efficiency

12 Jun 2008

The efficiency of organic light-emitting diodes (OLEDs) could be boosted by 40% thanks to a nanoimprinting lithography technique.

Nanomprinted low-refractive index layers could increase the light-emitting efficiency of organic light-emitting diodes (OLEDs) by 40% say researchers in Japan. They say that their technique could be especially useful for small or medium-sized devices.

"The main applications for this work are in flexible solid-state lighting (such as flexible lamps) with low mass production cost," Hideo Takezoe of the Tokyo Institute of Technology told nanotechweb.org. "The requirements for such devices are low-price organic materials, easy processes for mass production and high light extraction efficiency. Our work addresses all of these requirements."

OLEDs are promising for next-generation displays and lighting because they are simple to make. Although OLEDs are better than conventional inorganic LEDs in many ways, they do suffer from a low light-emitting efficiency of about 20%. This is because most of the light is trapped (due to total internal reflection) in the transparent substrate and the organic layers making up the devices.

Previous research on OLEDs has focused on using dielectric microstructures or roughening the surface to increase light-emitting efficiency. Now, Takezoe and co-workers have used an amorphous fluoropolymer as a substrate for OLEDs to increase their light-emitting efficiency by 40%.

The polymer has a low-refractive index of 1.34, which allows the light extraction from OLEDs to be increased compared with devices without such a layer. The material also allows high light transmittance over wavelengths of 200 nm to 2 µm. Moreover the polymer requires low curing temperatures, which means that it is possible to apply the polymer to organic substrates for making flexible devices that cannot endure high temperatures during processing.

The angle at which the light is extracted is 35°, which is an advantage for small or medium-sized OLEDs that are mainly viewed from the forward direction, say the researchers.

The team now plans to design a new grating to extract light at various wavelengths for making efficient white OLED lamps. It also hopes to fabricate white phosphorescent OLEDs by using simple nanoimprint lithography techniques and low-refractive index materials. "We are also looking for new materials with low resistance and high light transmittance to minimize waveguided light reflection/absorption under low-temperature processing conditions," added Takezoe.

The researchers reported their work in Appl. Phys. Lett.

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