Energy Transfer Loop Enables Thermally Activated Delayed Fluorescence with >20% EQE and Near-Zero Roll-Offs at 10⁴...

The bottleneck in efficiency stability at high luminance limits the development of thermally activated delayed fluorescence (TADF) diodes, due to the insufficient management of quenching factors, especially exciton-phonon coupling during reverse intersystem crossing (RISC) assistant with vibrational coupling. Herein, this challenge is overcome by a TADF sensitizer based "energy transfer loop" strategy. A dimethylacridine-phosphole oxide hybrid sensitizer named 24DDMACPPPO is constructed with vibration-free RISC and excited-state energy levels equal to a TADF emitter 4CzTPNBu. The former displaces the latter in RISC for phonon decoupling through reverse triplet and positive singlet energy transfer between them. As a result, co-doping 0 .1%wt. 24DDMACPPPO doubles the RISC rate constant and triples the singlet radiation rate of 4CzTPNBu, and simultaneously nearly halves non-radiation rate constants, giving rise to 70% and 90% increased photo- and electro-luminescence quantum efficiencies of 4CzTPNBu. More importantly, the energy transfer loop between 24DDMACPPPO and 4CzTPNBu combines their advantages in RISC, quenching suppression, and singlet radiation, giving rise to the record values of external quantum efficiency (≈23%wt.) and roll-off (2.6%) at 10⁴ cd m^-2.