Donor Extension on Spiro‐Acridine Enables Highly Efficient TADF‐OLEDs with Relieved Efficiency Roll‐Off

Abstract Organic light emitting diodes based on thermally activated delayed fluorescent (TADF) emitters with both high external quantum efficiency (EQE) and low efficiency roll‐off are under urgent pursuing. Three TADF emitters based on fluorene‐spiro‐acridine derivatives as donors are developed. Composing of rigid donor and acceptor, all emitters exhibit high PLQYs of 96–99%. Direct conjugation attached on acridine offers an enhanced reverse intersystem crossing ( k RISC ) in IA‐TRZ, while the extension via spiro‐junction accelerates the radiative decay rate ( k r ) in 2S‐TRZ and IT‐TRZ. Remarkably, all emitters enable high electroluminescent performance with EQE of 35.6% for 2S‐TRZ, 36.1% for IT‐TRZ and 32.0% for IA‐TRZ, respectively, and very high luminance of up to 100 000 cd m −2 . Surprisingly, 2S‐TRZ and IT‐TRZ that hold larger k r and smaller k RISC than that of IA‐TRZ exhibit more relieved efficiency roll‐off, indicating the significance of k r . More importantly, IT‐TRZ with a moderate k r and k RISC among these emitters shows the lowest roll‐off which demonstrates that the comprehensive consideration of all photophysical processes is important to attain excellent devices. It also emphasizes the molecular modulation method through side group that can induce elaborate control on decay channels for finely optimizing emitters. This study can provide a useful perspective in designing practical emitters.