There is little investigation into the impact of molecular conformation on device efficiency and degradation of boron‐nitrogen thermally activated delayed fluorescence emitters (BN‐TADF). Herein, three highly‐efficient green BN‐TADF emitters have been designed to unveil the impact of peripheral phenyl groups on device efficiencies and lifetimes. Compared to BN‐PhOH with the lowest EQEmax of 19 %, BN‐PhOCH3 and BN‐PhN(CH3)2 have achieved strongly enhanced EQEmax of 25.6 % and 24.1 %, respectively. Importantly, the device lifetimes (LT50) are dramatically improved from 1.7 h of BN‐PhOH to 4.4 h of BN‐PhOCH3 and 7.7 h of BN‐PhN(CH3)2 without encapsulation. According to in situ Raman spectroscopy and simulations, BN‐PhN(CH3)2 of less conformation change after aging exhibits the best photostability. It is proposed that the torsion angle change between the BN core and the peripheral phenyl group results in BN‐TADF degradation. This knowledge means precisely tuning peripheral groups of BN‐TADF can achieve both higher device efficiencies and longer lifetimes.

The design of blue emitters is still desired for high performance organic light-emitting diodes (OLEDs). In this work, two blue emitters with the fused triphenylamine (FTPA) moiety, PIAN-FTPA and PI-FTPA,...