Molecular Engineering of a Tetradentate Pt(II) Emitter Enables Deep‐Blue PhOLED with Ultra‐Low Efficiency Roll‐Off and Exceptional Brightness Stability

Deep‐blue phosphorescent OLEDs (PhOLEDs) with high efficiency and high color purity often suffer from serious efficiency roll‐off and brightness decay, which critically limit their practical application. Herein, we report a rigid and sterically hindered tetradentate Pt(II) emitter, PtKW2, designed via synergistic modulation of molecular rigidity, intermolecular stacking suppression, and triplet localized excited‐state ( 3 LE) character. By introducing a bulky 2,6‐diisopropylphenyl group, a ring‐locked framework, and a π‐conjugated extended benzofuro[3,2‐ c ]carbazole moiety, PtKW2‐doped film exhibits deep‐blue emission at 459.0 nm with an ultra‐narrow full width at half maximum (FWHM) of 18.0 nm, a high photoluminescence quantum yield ( Φ PL ) of 94%, and a short excited‐state lifetime ( τ ) of 4.4 µs. A PtKW2‐based bottom‐emitting PhOLED achieves deep‐blue electroluminescence at 467 nm with Commission Internationale de l’Éclairage (CIE) coordinates of (0.122, 0.167). Notably, a record‐low efficiency roll‐off of only 1.2% at 1000 cd m − 2 , a record‐high L 90 of 5091 cd m − 2 , and a record‐high J 90 of 20.4 mA cm − 2 are simultaneously achieved among reported Pt(II)‐based OLEDs with CIE y < 0.20. This study establishes an effective molecular engineering strategy for overcoming efficiency roll‐off and brightness degradation in high‐performance deep‐blue PhOLEDs.