Materials scientists have developed improved charge transporting materials for use in polymeric light emitting diodes. 

Yong Cao's team from the South China University of Technology produced polyelectrolytes that can be used as hole transporting layers in light emitting devices. The polymers are only soluble in polar solvents, which Cao hopes could make device fabrication simpler.  

Organic light emitting diodes are usually layered structures made up of a hole transporting, an emitting and an electron transporting layer, which are sandwiched between two electrodes. To construct these devices successfully, the deposition of layers must not cause erosion of the adjoining layers. One method of achieving this is to use materials that are soluble in solvents in which the other layers are not. Most light emitting polymers are soluble in non-polar solvents, so Cao has investigated making materials for adjoining layers that are processable in polar solvents.  

Cao found that these triphenyamine-based materials have excellent hole transporting and electron blocking properties. They have a lower turn-on voltage and superior efficiency compared to the widely used hole-transport/electron-blocking material PEDOT - a polythiophene. 

'The data suggest that these new copolymers display outstanding hole-transport/electron-blocking characteristics,' said Christoph Weder, Professor of Macromolecular Science at Case Western Reserve University. 'The tailored solubility of the new triphenylamine-based copolymers in certain solvents is key to the fabrication of high-quality multilayer light-emitting diodes, because it prevents intermixing of the different layers. This makes these materials very useful as charge transport layers in multilayer light-emitting diodes.'

Cao's group are continuing to investigate new materials that are soluble in polar solvents. 'A great challenge in this field is to develop water/alcohol soluble charge transport layer materials which combine good device efficiency, long operating life, low operating voltage and ease in processing. Only the materials which meet all these requirements can find a place in flat-panel display technology,' said Cao.

Rebecca I Gillan