Researchers from the University of Cambridge and Jilin University discovered that radical-based OLEDs feature highly efficient emission - in fact they believe that this discovery could be an elegant solution to the problem of in-efficient OLED emission.
First-generation OLED emitters (fluorescent emitters) have a maximum internal quantum efficiency of 25% - as only a quarter of the electrons are in a singlet-state (that emit light) while 75% of the electrons are in a triplet-state. Current ways to achieve 100% IQE are either based on doping with heavy metals (phosphorescent emission) or either based on delayed fluorescence (TADF).
The new discovery can exploit the quantum mechanical spin property of electrons to increase the efficiency. The researchers describe how stabilized radicals form electronic states known as 'doublets'. The researchers managed to create a deep-red OLED emitter with near-100% IQE. The researchers say that this result was surprising, as they did not expect the radicals to emit light. However 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 behavior was translated to highly emissive LEDs, but with another twist: 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.