Radicals — semiconducting molecules with unpaired electrons — can be used to fabricate highly efficient OLEDs, according to scientists at the University of Cambridge and Jilin University. Using a luminescent radical emitter, the researchers demonstrated an OLED with maximum external quantum efficiency of 27 percent, at a wavelength of 710 nm. According to the team, this is the highest reported value thus far for deep-red and IR LEDs.

The emission process of the radical-based OLEDs originates from a spin doublet, rather than a singlet or triplet exciton. Stabilized radicals form electronic states (known as doublets), on account of the spin character being either up or down. The efficiency limitations imposed by triplet excitons are circumvented for doublets. The researchers exploited this spin property to overcome efficiency limitations such as those found in traditional OLEDs.

When isolated in a host matrix and excited with a laser, the radicals showed near-unity efficiency for light emission. The electrical current injected electrons into the unpaired electron energy level of the radical, and pulled electrons out of a lower-lying level to form bright-doublet excited states.

This highly emissive behavior was translated to highly emissive LEDs. “The radicals themselves are unusually emissive, and they operate in the OLEDs with unusual physics,” said researcher Emrys Evans.

For traditional compounds (i.e., nonradicals without an unpaired electron), quantum-mechanical-spin considerations dictated that charge injection will form 25 percent bright-singlet and 75 percent dark-triplet states in OLED operation. The researchers believe that radicals could offer a solution to this fundamental spin problem in OLEDs.

The researchers are investigating ways to exploit radicals beyond lighting applications, and they expect radicals to affect other branches of organic electronics research. With further materials research, efficient blue- and green-light radical-based diodes could be developed, the researchers said, forming the basis for next-generation displays and lighting technologies.

The research was published in Nature (https://doi.org/10.1038/s41586-018-0695-9).