Efficient and Bright Organic Radical Light-Emitting Diodes with Low Efficiency Roll-Off.
Organic radicals have been of interest due to their potential to replace nonradical-based organic emitters, especially for deep red/near-infrared (NIR) electroluminescence (EL), based on the spin-allowed doublet fluorescence. However, the performance of the radical-based EL devices is limited by low carrier mobility that causes a large efficiency roll-off at high current densities. Here, we report highly efficient and bright doublet EL devices by combining a thermally activated delayed fluorescence (TADF) host that supports both electron and hole transport and a tris(2,4,6-trichlorophenyl)methyl-based radical emitter. Steady-state and transient photophysical studies reveal the optical signatures of doublet luminescence mechanisms arising from both host and guest photoexcitation. The host system presented here allows balanced hole and electron currents, and we report a high maximum external quantum efficiency (EQE) of 17.4% at 707 nm peak emission with substantially improved efficiency roll-off: over 70% of the maximum EQE (12.2%) is recorded at 10 mA cm-2 , and even at 100 mA cm-2 , nearly 50% of the maximum EQE (8.4%) is maintained. Charge recombination is primarily at the radical guest sites, but there is evidence from magneto-EL for host recombination and later energy transfer to the radical guest. We consider this is an important step to the practical application of organic radicals to NIR light-emitting devices. This article is protected by copyright. All rights reserved.