High‐Efficiency and Narrowband OLEDs from Blue to Yellow with Ternary Boron/Nitrogen‐Based Polycyclic Heteroaromatic Emitters

Abstract The development of boron/nitrogen‐based polycyclic heteroaromatic emitters with multiple‐resonance thermally activated delayed fluorescence (MR‐TADF) property can efficiently promote the advancement of high‐efficiency organic light‐emitting diodes (OLEDs) with narrowband emission. Herein, a simple strategy to achieve four ternary boron/nitrogen‐based polycyclic heteroaromatic emitters (SBON, SBSN, DBON, and DBSN) from pure blue (463 nm) to yellow (553 nm) via tuning the coordination between B/N and heteroatom (O or S) is demonstrated, aiming to increase charge transfer delocalization of the polycyclic heteroaromatic emitters and adjust photo‐physical properties. This strategy endows the four emitters with full width at half maximum (FWHM) of 24, 27, 20, and 28 nm, respectively. Additionally, double‐boron type green and yellow narrowband emitters (DBON and DBSN) possess 98% photoluminescence efficiencies in doped films. Besides, considerable rate constants of reverse intersystem crossing (RISC) are achieved because of the small singlet–triplet excited state energy gap and large spin–orbit coupling values. Consequently, the OLEDs covering from blue to yellow based on these emitters show the maximum external quantum efficiency of 13.7%, 17.6%, 26.7%, and 21.8%, respectively, with low‐efficiency roll‐off. These results provide a feasible design strategy to construct boron/nitrogen‐based polycyclic heteroaromatic MR‐TADF emitters for efficient OLEDs with color‐tuning electroluminescence.