Lighting up polymer LED technology.

Lighting up polymer LED technology.

Organic compounds could soon supplant semiconductors as the basis for the light emitting diodes (LEDs) that are increasingly forming the displays used in devices such as mobile phones and digital cameras. Organic LEDs (OLEDs) can produce cheap, lightweight displays that offer a high degree of resolution.

Small molecule OLEDs are highly efficient at generating light but are difficult to produce, whereas polymer LEDs (PLEDs) are simple to produce - via spin coating or inkjet printing - but are much less efficient. Now a European team of academic and industrial researchers has developed a highly efficient PLED, which may become the dominant LED technology.

PLEDs basically consist of five layers: a glass substrate, a transparent anode, a hole transporting layer, a light-emitting polymer, and a cathode. Electrons injected by the cathode travel to the light-emitting polymer, where they combine with holes injected by the anode to form excited states, known as excitons. These then decay to produce light. The problem is that two types of exciton can be produced: singlet and triplet. In normal light-emitting polymers, only singlets can be used to generate light, which limits the efficiency of the OLED. This can be overcome by doping with high-energy triplet emitters but current versions only work in small molecule OLEDs.

The researchers from Philips Research and TNO Industrial Technology, both based in the Netherlands, Covion Organic Semiconductors, Frankfurt, Germany, and the University of Durham, UK, decided to try to develop a light-emitting polymer that could be doped with a high-energy triplet emitter and still form part of a PLED. They based this light-emitting polymer on 9,9’-dialkyl-[3,3’]-bicarbazolyl, which previous research had indicated to have suitable attributes.

The researchers created nine different copolymers by combining the carbazole with compounds based on oxadiazole and fluorene. They then doped the compounds with an iridium triplet emitter. A carbazole-oxadiazole copolymer turned out to be most efficient, generating light from triplet excitons at levels similar to small molecule OLEDs. Expect this copolymer to turn up in your mobile phone in the near future.

Jon Evans