Fusion of Selenium‐Embedded Multi‐Resonance Units Toward Narrowband Emission and Fast Triplet‐Singlet Exciton Conversion

Abstract Developing multi‐resonance thermally activated fluorescence (MR‐TADF) emitters with both fast reverse intersystem crossing (RISC) rate and narrow emission bandwidth still remains a formidable challenge. Herein, a design strategy of fused MR skeleton containing heavy chalcogen (sulfur or selenium) for high‐performance MR‐TADF molecules is developed. Impressively, Se‐embedded emitter ( DSeBN ) shows extremely narrow full width at half maximum (FWHM) value of 16 nm and ultrafast RISC rate constant up to 2.0 × 10 6 s −1 . The organic light‐emitting diode (OLED) based on this emitter exhibits excellent performance parameters with extremely narrow FWHM of 17 nm and high external quantum efficiency (EQE) of 35.31%. Significantly, much suppressed efficiency roll‐off is achieved, in which the EQE still stayed at 32.47% and 25.05% at the luminance of 100 and 1000 cd m −2 , respectively. These results represent the state‐of‐the‐art device performance in terms of efficiency and FWHM, shedding new light on the development of practical MR‐TADF emitters.