New 'radical' approach may lead to brighter LEDs

Semiconducting molecules with unpaired electrons, termed 'radicals', can be used to develop very efficient organic-light-emitting diodes (OLEDs), scientists say.

The new approach, described in the journal Nature, exploits the quantum mechanical 'spin' property to overcome efficiency limitations for traditional, non-radical materials.

Radicals are usually noted for their high chemical reactivity and often detrimental effects, from human health to the ozone layer.

Now radical-based OLEDs could form the basis for next-generation displays and lighting technologies.

Researchers from the University of Cambridge in the UK and Jilin University in China found that stabilised radicals form electronic states known as 'doublets', on account of the spin character being either 'up' or 'down.'

Running electricity through these radical-based OLEDs leads to the formation of bright-doublet excited states which emit deep-red light with near-100 per cent efficiency.

 

For traditional compounds (ie non-radicals without an unpaired electron), quantum-mechanical-spin considerations dictate that charge injection forms 25 per cent bright-'singlet' and 75 per cent dark-'triplet' states in OLED operation.

Radicals pose an elegant solution to this fundamental spin problem which has troubled researchers ever since the development of OLEDs from the 1980s.

"On the face of it, radicals in OLEDs shouldn't really work, which makes our results so surprising," said Emrys Evans from the Cavendish Laboratory.

"The radicals themselves are unusually emissive, and they operate in the OLEDs with unusual physics," Evans said.

When isolated in a host matrix and excited with a laser, the radicals, atypically, have close to unity efficiency for light emission.

The highly emissive behaviour was translated to highly emissive LEDs, but with another twist, researchers said.

In the devices, the electrical current injects electrons into the unpaired electron energy level of the radical, and pulls electrons out of a lower-lying level, and another portion of the molecule, to form bright-doublet excited states.

In future, efficient blue- and green-light radical-based diodes could appear with further innovation.

The researchers are working on exploiting radicals beyond lighting applications, and expect radicals to impact other branches of organic electronics research.