Judicious Use of Chalcogens in Multiresonant Thermally Activated Delayed Fluorescent Emitters Leads to OLEDs with Efficiencies Exceeding 36% and Showing Mild Efficiency Roll‐Off

Abstract Strategies that produce OLEDs that simultaneously achieve outstanding electroluminescence efficiency, small efficiency roll‐off, good stability, and high color purity remain in strong demand. Herein, we report how judicious decoration of a benzochalcogenophene onto the BCzBN core leads to an acceleration of reverse intersystem crossing ( k RISC ) without sacrificing either color fidelity and photoluminescence quantum yield ( Φ PL ). Compared with BCzBN , the k RISC of BN‐S and BN‐Se are accelerated to 2.5 × 10 5 and 7.2 × 10 5 s −1 from 1.4 × 10 4 s −1 , each with near‐unity Φ PL s. BN‐S and BN‐Se show narrowband emission at 483 nm (FWHM of 22 nm) and 485 nm (FWHM of 22 nm). The corresponding non‐sensitized OLEDs using BN‐S and BN‐Se exhibited record‐breaking maximum external quantum efficiency (EQE max ) of 43.1 and 36.9%, without any external light extraction techniques. Furthermore, the BN‐Se ‐based OLED showed ultralow efficiency roll‐off, with EQEs remaining at 36.7 and 31.8% at 100 and 1000 cd m −2 , respectively. These emitters do not easily suffer from aggregation‐caused quenching. Even at 12 wt% doping, the EQE max of the OLEDs with BN‐S and BN‐Se were 36.7 and 36.4%, respectively, with mild efficiency roll‐off (EQEs at 1000 cd m −2 of 19.1 and 30.8%, respectively), and nearly unchanged electroluminescence spectra.