Light-emitting diodes based on conjugated polymers

CONJUGATED polymers are organic semiconductors, the semiconducting behaviour being associated with the π molecular orbitals delocalized along the polymer chain. Their main advantage over non-polymeric organic semiconductors is the possibility of processing the polymer to form useful and robust structures. The response of the system to electronic excitation is nonlinear—the injection of an electron and a hole on the conjugated chain can lead to a self-localized excited state which can then decay radiatively, suggesting the possibility of using these materials in electroluminescent devices. We demonstrate here that poly(p-phenylene vinylene), prepared by way of a solution-processable precursor, can be used as the active element in a large-area light-emitting diode. The combination of good structural properties of this polymer, its ease of fabrication, and light emission in the green–yellow part of the spectrum with reasonably high efficiency, suggest that the polymer can be used for the development of large-area light-emitting displays. By 1990, the development of solid-state light-emitting diodes (LEDs) had come a long way. Efficient LEDs based on inorganic semiconductors had already found widespread application. Molecular organic semiconductors were also coming to the fore - not only were they available in a range of colours but, unlike their inorganic counterparts, they could be readily made into flexible, large-area displays. But physicists were encountering problems with the long-term stability of the organic films. Then Jeremy Burroughes and colleagues produced the first polymer LED: moving from molecular to macromolecular materials solved the stability problem and meant that high-quality films could be made easily.