Tetracyclic π‐Extended Multi‐Resonant TADF Emitters for Green PSF‐OLEDs With 37.7% EQE and BT.2020‐Compatible Top‐Emitting Devices

In phosphorescence/TADF‐sensitized fluorescence (PSF/TSF) OLEDs, the use of multi‐resonance (MR) emitters as terminal emitters enables the simultaneous achievement of high color purity and high device efficiency. However, achieving both properties simultaneously remains a significant challenge. Herein, we developed a series of green MR‐TADF emitters, namely Ph‐DBF, Me‐NapBF , and Ph‐NapBF , by expanding the peripheral planarity of the B/N emitting unit through the incorporation of a tetracyclic naphtho[2,3‐b]benzofuran structure. This enhanced rigidity suppresses structural deviations in the excited state, resulting in narrow full‐widths at half maximum (FWHMs) of <23 nm and near‐unity photoluminescence quantum yields (PLQYs). Linear extension of π‐delocalization through aromatic rings bathochromically shifts the emission and strengthens the absorption bands, concurrently increasing the molar extinction coefficient (ε), a critical factor influencing the exciton‐harvesting efficiency of terminal emitters. The increased ε values of these emitters enabled bottom‐emitting PSF‐OLEDs with external quantum efficiencies up to 37.7% ( Ph‐NapBF ) and 35.5% ( Me‐NapBF ), maintaining 33.3% and 30.9% at 1000 cd m −2 , respectively. Furthermore, the corresponding top‐emitting OLEDs exhibited pure green emission with CIE(x,y) coordinates of (0.17, 0.79), meeting the BT.2020 green color standard, and achieved a high current efficiency of 277.2 cd A −1 .