Identifying the Molecular Origins of Green BN-TADF Material Degradation and Device Stability via in situ Raman Spectroscopy.

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