Polymer PIN light-emitting diodes

1 Dec 2009.  NUS scientists at the Organic Nano Device Laboratory (ONDL) have demonstrated the first p-i-n polymer organic light-emitting diodes with high electroluminescence efficiency and built-in potential.  These devices employ a p-doped hole-injection layer together with an n-doped electron-injection layer that sandwich a light-emitting polymer (LEP) layer of the same material. The report published in Applied Physics Letters (Zhou et al, Applied Physics Letters 95 (2009) 213303.) illustrates an important principle of using the p-i-n device architecture to achieve high device efficiency.  This approach is moreover compatible also with the use of air-state cathodes.

Although the advantages of the p-i-n heterostructure have been recognised for some time already, it has proven difficult to implement in polymers, because the deposition of the subsequent layer tends to dissolve the uinderlying ones. Thus previously, organic p-i-n structures can only be made using evaporated molecules, in which the different doped and undoped layers are sequentially evaporated in high vacuum. 

Now these scientists have developed a novel photocrosslinking to stop the dissolution from occurring.  Together with the various possibilities afforded by chemical doping, it should open the way to manufacture high-efficiency polymer light-emitting displays and lighting panels based on solution-processing technologies.  “Solution-processing is particularly attractive because it is materials- and energy-efficient, up-scalable and compatible with high-volume production,” says Rachael Png. “The key challenge now is to stabilise the dopant profile in these doped polymer layers.”