A sterically hindered asymmetric D-A-D′ thermally activated delayed fluorescence emitter for highly efficient non-doped organic light-emitting diodes

Thermally activated delayed fluorescence (TADF) materials have opened a new chapter for high-efficiency and low-cost organic light-emitting diodes (OLEDs). Herein, we describe a novel and effective design strategy for TADF emitters which includes introducing a carbazole donor unit at the ortho -position, at which the donor and acceptor groups are spatially in close proximity to guarantee the existence of intramolecular electrostatic attraction and through-space charge transfer, leading to reduced structural vibrations, suppressed non-radiative decay and rapid radiative decay to avoid excited state energy loss. As a result, a green TADF emitter ( 2Cz-DPS ) showing high solid-state photoluminescence quantum efficiency (91.9%) and excellent OLED performance was produced. Theoretical simulations reveal that the non-adiabatic coupling accelerates the reverse intersystem crossing of 2Cz-DPS , resulting in a state-of-the-art non-doped OLED with an extremely high external quantum efficiency of 28.7%. An ortho -substituent design strategy promoting steric hindrance has afforded excellent AIE-TADF emitters, and high-efficiency TADF-OLEDs have been subsequently demonstrated.