Red Organic Light‐Emitting Diode with External Quantum Efficiency beyond 20% Based on a Novel Thermally Activated Delayed Fluorescence Emitter

A novel thermally activated delayed fluorescence (TADF) emitter 12,15‐di(10H‐phenoxazin‐10‐yl)dibenzo[a,c]dipyrido[3,2‐h:2′,3′‐j]phenazine (DPXZ‐BPPZ) is developed for a highly efficient red organic light‐emitting diode (OLED). With rigid and planar constituent groups and evident steric hindrance between electron‐donor (D) and electron‐acceptor (A) segments, DPXZ‐BPPZ realizes extremely high rigidity to suppress the internal conversion process. Meanwhile, the highly twisted structure between D and A segments will also lead to an extremely small singlet–triplet energy split to DPXZ‐BPPZ. Therefore, DPXZ‐BPPZ successfully realizes an efficient fluorescent radiation transition and reverse intersystem crossing process, and possesses an extremely high photoluminescence quantum efficiency of 97.1 ± 1.1% under oxygen‐free conditions. The OLED based on DPXZ‐BPPZ shows red emission with a peak at 612 nm and a Commission Internationale de L'Eclairage (CIE) coordinate of (0.60, 0.40), and it achieves high maximum forward‐viewing efficiencies of 20.1 ± 0.2% (external quantum efficiency), 30.2 ± 0.6 cd A(−1) (current efficiency), and 30.9 ± 1.3 lm W(−1) (power efficiency). The prepared OLED has the best performance among the reported red TADF OLEDs. These results prove that DPXZ‐BPPZ is an ideal candidate for red TADF emitters, and the designing approach is valuable for highly efficient red TADF emitters.