Researchers have found that semiconducting molecules with unpaired electrons, called as ‘radicals’ can be used to fabricate highly efficient organic-light-emitting diodes (OLEDs), manipulating their quantum mechanical ‘spin’ property to surpass efficiency boundaries for traditional, non-radical materials.
Radicals are typically known for their high chemical reactivity and often damaging effects, from human health to the ozone layer. At present, radical-based OLEDs could form the foundation for next-generation displays and lighting technologies.
Reporting in Nature, the team from the University of Cambridge and Jilin University describe how stabilized radicals form electronic states called ‘doublets’, owing to the spin character being either ‘up’ or ‘down’.
Sending electricity through these radical-based OLEDs causes the formation of bright-doublet excited states which discharge deep-red light with near-100% efficiency. For traditional compounds (i.e. non-radicals without an unpaired electron), quantum-mechanical-spin considerations direct that charge injection forms 25% bright-‘singlet’ and 75% dark-‘triplet’ states in OLED operation. Radicals pose a sophisticated solution to this major spin problem which has bothered researchers ever since the development of OLEDs in the 1980s.
On the face of it, radicals in OLEDs shouldn't really work, which makes our results so surprising. The radicals themselves are unusually emissive, and they operate in the OLEDs with unusual physics.
Dr Emrys Evans, Lead Co-Author, Professor Sir Richard Friend's Group, Cavendish Laboratory.
When isolated in a host matrix and stimulated with a laser, the radicals, atypically, have close to unity efficiency for light emission. The extremely emissive behavior was translated to highly emissive LEDs, but with another surprise: in the devices, the electrical current injects electrons into the unpaired electron energy level of the radical, and draws electrons out of a lower-lying level, and another portion of the molecule, to develop bright-doublet excited states.
In the future, efficient blue- and green-light radical-based diodes could be made with further materials innovation. The scientists are working on manipulating radicals beyond lighting applications, and expect radicals to influence other branches of organic electronics research.
Professor Feng Li from Jilin University is a visitor at the Cavendish Laboratory and corresponding author for the work. He said: “The collaboration between the universities and research groups has been instrumental to the success of this work. In future, I hope that we can demonstrate more radical-based solutions for organic electronics.”