Organic LEDs Based on Dendrimers

Collaborative work between researchers at the University of Oxford (Oxford, UK) and University of St. Andrews (St. Andrews, Scotland) has demonstrated extremely efficient phosphorescence from organic light-emitting diode (OLED) structures using dendrimer materials. The dendrimers comprise an organic or organometallic light-emitting core connected to surface groups known as dendrons. They effectively combine the characteristics of light-emitting polymers (LEPs) with those of OLEDs.

The LEDs were made at the Organic Semiconductor Centre at St. Andrews with support from the Scottish Higher Education Funding Council. The products will be commercialized by the spin-off company Opsys (Oxford, UK). Opsys CEO Michael Holmes claims that highly efficient single organic layer dendrimer devices could greatly reduce manufacturing costs. The most efficient conventional OLEDs are fabricated with three or more layers that are sequentially deposited by thermal evaporation. However, dendrimer-based OLEDs can be fabricated by solution processing, which is far simpler, and raises the possibility of printing displays by the use of ink jets. The OLED devices have a single layer of organic material between the electrodes, currently a wide bandgap biphenyl host material. The first iridium-cored dendrimer devices gave a peak power efficiency of 6.9 lm/W, with a brightness of ~1500 cd/m² and a current density of 5 mA/cm².

Dendrimers, unlike LEP materials, incorporate the best features of materials made of small molecules, said the University of Oxford's Paul Burn, who, with Professor Ifor Samuel of the University of St. Andrew, is leading the research. The dendrimers include highly efficient phosphorescent cores, yet can be processed in solution. This enables the molecular engineering of these materials to tailor their properties to the needs of a particular application.

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