Up to 100% Formation Ratio of Doublet Exciton in Deep-Red Organic Light-Emitting Diodes Based on Neutral π-Radical

In a neutral pi-radical-based organic light-emitting diode (OLED), although the emission comes from the doublet excitons and their transition to the ground state is spin-allowed, the upper limit of internal quantum efficiency (IQE) is not clear, 50% or 100%? In this work, the deep-red OLEDs based on a neutral pi-radical were fabricated. Up to 100% doublet exciton formation ratio was obtained through rational designing device structure and host guest doping system. This indicates the IQE of neutral pi-radical-based OLEDs will reach 100% if the nonradiative pathways of radicals can be suppressed. The maximum external quantum efficiency of the optimized device is as high as 4.3%, which is among the highest values of deep-red/near-infrared OLEDs with nonphosphorescent materials as emitters. Our results also indicate that using partially reduced radical mixture as emitter may be a way to solve aggregation-caused quenching in radical-based OLEDs.