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

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 × 10⁵ and 7.2 × 10⁵ s^-1 from 1.4 × 10⁴ s^-1, each with near-unity Φ_PLs. 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.