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 EQE max of 19 %, BN-PhOCH 3 and BN-PhN(CH 3 ) 2 have achieved strongly enhanced EQE max of 25.6 % and 24.1 %, respectively. Importantly, the device lifetimes (LT 50 ) are dramatically improved from 1.7 h of BN-PhOH to 4.4 h of BN-PhOCH 3 and 7.7 h of BN-PhN(CH 3 ) 2 without encapsulation. According to in situ Raman spectroscopy and simulations, BN-PhN(CH 3 ) 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.