OLEDs go organic

C&I Issue 1, 2013

For the first time, Japanese scientists have developed an efficient organic light-emitting diode (OLED) that is truly organic, as it does not contain any heavy metal atoms.

OLEDS emit light in response to an electric current and are used to create the digital displays for TVs, computer monitors and mobile phones. Applying an electric current causes electrons to travel across the OLED, with holes – the gaps where the electrons have vacated – travelling in the opposite direction. When the electrons and holes meet in the middle, they combine to form excitons, which quickly decay to emit visible light.

Or at least that is what happens to around 25% of the excitons generated in an OLED, which are known as singlet excitons. The other 75% are known as triplet excitons and they don’t normally decay to emit visible light, greatly limiting the efficiency of OLEDs.

Scientists have found that adding heavy metals such as iridium to OLEDs can encourage these triplet singlets to decay, producing OLEDs that generate much more light, but at extra expense. Now, Chihaya Adachi and his colleagues at Kyushu University have managed to design an OLED that promotes the decay of triplet excitons without needing costly heavy metals (Nature, 2012, 492, 234).

‘For commercialisation of OLEDs, there is a strong demand for realising high efficiency without using heavy metal complexes,’ Adachi says. ‘We returned to the basic idea of molecular design and succeeded in controlling the energy structure of organic molecules.’

In so doing, Adachi produced an OLED consisting of the organic molecule carbazolyl dicyanobenzene (CDCB) that could generate light from both singlet and triplet excitons. As a result, this OLED can convert over 90% of the energy in an applied electric current into light. Furthermore, by adding methyl and phenyl groups to CDCB, Adachi was able to produce OLEDs that emitted light of various different colours.

The group is now looking to investigate the physics of these OLEDs in more detail, and to develop them commercially.

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