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Although multi‐resonance thermally activated delayed fluorescent (MR‐TADF) emitters simultaneously achieve high color purity and high efficiency in organic light‐emitting devices (OLEDs), MR‐TADF‐based OLEDs suffer from severe efficiency roll‐off and lifetime issues for practical application. To overcome these issues, a key solution is hyper OLED technology sensitized by TADF or phosphorescent emitters. In this study, a series of π‐extended phenoxazine‐based asymmetric MR‐TADF emitters for long lifetime, low‐power consumption, and narrow emission band green OLEDs is developed. As a sensitizer, a well‐known phosphorescent emitter is used, fac‐tris(2‐phenylpyridinato‐C2,N)iridium(III) [Ir(ppy)3]. Consequently, these MR‐TADF emitters achieve hyper OLEDs with a power efficiency of 150 lmW−1, an external quantum efficiency of 27%, and a long lifetime LT95 of over 3000 h at high brightness of 1000 cdm−2. These performances are among the best in scientific literature. [ABSTRACT FROM AUTHOR]

Although multi-resonance thermally activated delayed fluorescence (MR-TADF) emitters exhibit near-unity photoluminescence quantum yield (PLQY) and narrow-band emission, their aggregation-caused quenching (ACQ) and random molecular orientation in organic light-emitting devices (OLEDs) restrict the maximization of their full potential. In this study, we developed a series of novel phenylene-bridged MR-TADF emitters with varying numbers of carbazole units. These emitters exhibited sky-blue emission at approximately 480 nm with a near-unity PLQY, a small full width at half maximum of 26–31 nm, and horizontal molecular orientation of ∼ 80%. The chemical modifications with a phenylene bridge and varied carbazole units reduced the ACQ behavior at high concentration ratios and greatly enhanced the values from 62% to 80%. The corresponding OLED exhibited an external quantum efficiency of approximately 30%. [ABSTRACT FROM AUTHOR]