What are dendrimers?

Dendrimers are similar to hyper-branched polymers: their structures contain repeating branching units called dendrons that radiate from a central core.  At the end of the dendron branches are surface groups, which can be designed to meet a wide range of purposes.  Dendrimers have an extremely high concentration of functional groups for their molecular weight and volume.   A key benefit is the high degree of freedom to tailor the surface groups to optimise the materials to meet different processing and application requirements. In the field of organic light emitting diodes (OLEDs), 'dendronization' of small molecules confers on them the ability to be applied in liquid form.

How do they work?

A transparent substrate bearing a transparent anode material, usually indium tin oxide, is coated with a series of layers of hole transporting, light emitting and electron transporting materials.  Finally a metallic cathode is deposited and the device is encapsulated to exclude oxygen and moisture.  When current is passed between the electrodes the dendrimer materials in the light-emitting layer are excited and produce light.

Features and benefits of dendrimers

• Design versatility
  The dendritic structure allows independent modification of the core (light emission), branching groups (charge transport) and surface groups (processing properties).
  
• Efficiency
  Dendrimers can incorporate the best features of small molecule materials with the processability of polymer-based OLEDs. For example, dendrimers provide an effective route to producing highly efficient phosphorescent materials which are processable from solution.
  
• Process flexibility
  The surface groups may be tuned independently of the light emitting core. This makes it possible to adapt dendrimers to various processing systems without compromising the quality of light emission.
  
• Colour purity
  The core can be optimised for colour purity with little effect on the processing properties of the system. The luminescent cores are kept well apart by the bulky dendrons, minimising luminescence quenching.

By combining the benefits of polymer and dendrimer systems, CDT has found that systems can be further optimised and improved. For example, by preparing polymer-dendrimer hybrid materials, a highly conjugated network can be produced that can be matched to low work function cathodes and made to operate at low voltages. Furthermore, by virtue of the option to use a phosphorescent emitter at the core of the dendrimer, the efficiency of polymer-dendrimer hybrid systems can also be enhanced.