A popular fabrication technique for organic light emitting diodes (OLEDs) may be having a negative impact on performance, say European scientists.

Iridium complexes that contain two identical bidentate ligands and a third different ligand are a popular choice as OLED emitters owing to their excellent photophysical properties and their ability to be tuned for a range of colours.

Due to their structure, these complexes can display cis and trans isomerisation. For each complex, one isomer is normally more efficient and is chosen to be synthesised and used as the emitter. Impurities can significantly reduce device performance, so care is needed when depositing the sample.

This is often carried out using vacuum sublimation, where the compound is heated under vacuum causing it to vaporise before condensing onto a cooled surface. However, Etienne Baranoff and his group at the Swiss Federal Institute of Technology at Lausanne, have demonstrated that the high temperatures used in vacuum sublimation can result in formation of the other isomer.

The team investigated a potentially promising compound Ir(2-phenylpyridine)₂(2-carboxy-4-dimethylaminopyridine) known as N984 (a trans complex). Heating and depositing the compound onto devices results in formation of the cis isomer too, they found. This leads to a dramatic reduction in device performance, which they have attributed to the difference in highest occupied molecular orbitals between the two isomers. Baranoff believes that this discovery will be useful for scaling up and improving performance reproducibility.

'The longer a complex is heated and the higher the temperature, the more cis isomer will be formed during sublimation,' explains Baranoff, 'so for reproducibility, if the processing conditions are not kept exactly the same, the device will contain more or less isomer.'

'It raises awareness of a potentially important problem,' agrees Gareth Williams, an OLED expert at the University of Durham, UK. However, he also highlights that it has so far only been shown for one complex.

The researchers now plan to investigate other compounds as well as further exploring the isomer relationship.

Yuandi Li 

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