From small dendrimers, mighty OLEDs grow

High efficiency organic light emitting diodes (OLEDs) using pioneering dendrimer materials have been developed by UK scientists.

High efficiency organic light emitting diodes (OLEDs) using pioneering dendrimer materials have been developed by scientists sponsored by display technology developer Opsys. The results have broad implications for the commercialisation of OLED technology in the flat panel display industry.

The Opsys sponsored teams, led by Dr. Paul Burn at the University of Oxford and by Professor Ifor Samuel at the University of St Andrews, have shown that dendrimers can be used to produce efficient solution processed OLED devices with just a single layer of organic material between the electrodes. In contrast, existing phosphorescent materials require multiple layers of deposition by more expensive evaporation techniques.

Dr Paul Burn of the University of Oxford, and a consultant to Opsys, explains, ‘Unlike light emitting polymers (LEPs), dendrimers can incorporate the best features of small molecule materials, such as highly efficient phosphorescent emitting cores, whilst also being solution processable. As such dendrimers are a promising alternative to both small molecule and LEP approaches to OLEDs.’

The only OLED materials other than dendrimers that can be deposited by solution processing methods are light emitting polymers (LEPs). However, LEPs are not as versatile, as the same chemical components of an LEP are responsible for both its light emitting features and its processing features. In dendrimers, the surface groups may be tuned independently of the emitting core, providing flexibility in the processing properties.

Opsys CEO Michael Holmes adds, ‘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.

‘Dendrimers have the potential to make long-term improvements in colour quality, power efficiency, lifetime and processing costs. These are necessary in order for OLEDs to meet expectations for their growth within the displays sector and to fulfil their transformational potential in general lighting, décor and signage applications.

‘Opsys is the exclusive licensee of patent applications covering this work. These additional patent applications will consolidate our position as the key intellectual property holder of light emitting dendrimer technology for OLEDs.’

Dendrimers: what the heck are they?

Light emitting dendrimers constitute a new but rapidly maturing materials technology in the field of OLEDs. They comprise an organic or organometallic light emitting core, which is connected to surface groups by branched organic dendrons. The dendritic structure controls core-core interactions and hence the photoluminescence and device properties of the materials. A key advantage of dendrimers is that the luminescent cores are kept well apart by the bulky dendrons, which minimises luminescence quenching.

The newly published research findings showed that a first generation iridium cored dendrimer doped into a wide bandgap biphenyl host displayed a peak power efficiency of 6.9 lm/W, measured at ~1500 cd/m2 brightness and a current density of 5 mA/cm2 – a remarkable efficiency for such a simple (single layer) device structure.

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