Phenazine‐Based Thermally Activated Delayed Fluorescence Red Emitter With Negligible Efficiency Roll‐Off in Organic Light‐Emitting Diode

Red emitters with thermally activated delayed fluorescence (TADF) often suffer from low electroluminescence efficiency and poor device stability, primarily due to their small energy gap, fast nonradiative decay, and aggregation‐caused quenching. Although such emitters exhibit an efficient reverse intersystem crossing (RISC) process, their long excited‐state lifetimes and low radiative transition rates (k _r ) can lead to severe efficiency roll‐off in devices. In this study, we designed and synthesized two highly twisted donor–acceptor‐donor (D‐A‐D) type red TADF emitters ( DPhOz‐BPPZ and DPhTz‐BPPZ ) using dibenzo[a, c]dipyrido[3,2‐h:2′, 3′‐j]phenazine ( BPPZ ) as the electron acceptor. These compounds show emission peaks at around 660 nm with short photoluminescence lifetimes (<4 μs) and high k _RISC rates (5.21 × 10 ⁴ s ⁻¹ and 4.19 × 10 ⁴ s ⁻¹ ), in which the DPhOz‐BPPZ doped film (10.17 wt% in CBP) achieves a higher k _RISC rate of 1.68 × 10 ⁶ s ⁻¹ . Notably, DPhOz‐BPPZ demonstrates a high thermal stability and a typical TADF character. When it was applied as a dopant in the emitting layer, a red organic light‐emitting diode (OLED) was obtained with a high maximum external quantum efficiency (EQE _max ) of 10.8% and a significantly negligible efficiency roll‐off of only 0.19% at 100 cd m ⁻² , manifesting an excellent device stability among all reported phenazine‐based red TADF OLEDs.