Peripherally Heavy‐Atom‐Decorated Strategy Towards High‐Performance Pure Green Electroluminescence with External Quantum Efficiency over 40 %

Abstract Heavy‐atom integration into thermally activated delayed fluorescence (TADF) molecule could significantly promote the reverse intersystem crossing (RISC) process. However, simultaneously achieving high efficiency, small roll‐off, narrowband emission and good operational lifetime remains a big challenge for the corresponding organic light‐emitting diodes (OLEDs). Herein, we report a pure green multi‐resonance TADF molecule BN‐STO by introducing a peripheral heavy atom selenium onto the parent BN‐Cz molecule. The organic light‐emitting diode device based on BN‐STO exhibited state‐of‐the‐art performance with a maximum external quantum efficiency (EQE) of 40.1 %, power efficiency (PE) of 176.9 lm W −1 , well‐suppressed efficiency roll‐off and pure green gamut. This work reveals a feasible strategy to reach a balance between fast RISC process and narrow full width at half maximum (FWHM) of MR‐TADF by heavy atom effect.