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Dendrimers boost OLED prospects

17 Jun 2002

Cage-like molecules promise lower manufacturing costs for organic LED makers.

High-efficiency organic LEDs (OLEDs) based on dendrimer materials have been developed by scientists sponsored by the UK start-up Opsys.

Dendrimers consist of an organic or organometallic light-emitting core connected to surface groups called dendrons. The surface groups can be tuned independently of the emitting core, providing flexibility in the processing properties.

Paul Burn, lead researcher of the Opsys-sponsored Oxford University team said: "Unlike light-emitting polymers (LEPs), dendrimers can incorporate the best features of small-molecule materials, such as highly-efficient phosphorescent cores, while also being solution-processable." This means dendrimers are a promising alternative to both small molecule and LEP approaches to OLEDs.

The Oxford researchers and their colleagues at the University of St Andrews, produced efficient solution-processed OLED devices with a single layer of organic material between the electrodes.

This first-generation iridium-cored dendrimer doped into a wide bandgap biphenyl host displayed a peak power efficiency of 6.9 lm/W, measured at about 1500 cd/m2 brightness and a current density of 5 mA/cm2. The researchers say that this is a remarkable efficiency for such a simple, single layer, device structure.

Opsys CEO Michael Holmes said: "Highly efficient single organic layer dendrimer devices could greatly reduce manufacturing costs. Currently, the most efficient OLEDs require three or more layers, which must be deposited sequentially by thermal evaporation. Dendrimer materials are set to change this as fabrication of OLEDs via solution-processable materials is simple and opens up exciting possibilities such as inkjet printing of displays."

Karl Heeks, director of system engineering at LEP developer Cambridge Display Technology, believes the work is an interesting development but is "unconvinced that Opsys's intellectual property rights (IPR) would fall outside CDT's IPR". CDT owns patents on large-molecule light-emitting materials and Heeks believes that "if the repeat units are large enough, they would fall inside CDT's IPR".

Holmes disagrees. He told Optics.org: "We are confident that our technology does not infringe on CDT's patents."

Author
Nadya Anscombe is editor of Optics.org and Opto & Laser Europe magazine.

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