Asymmetric Ternary Spiro Phosphine Oxide Hosts for Yellow Thermally Activated Delayed Fluorescence Diodes with External Quantum Efficiency Beyond 30%

Abstract Yellow thermally activated delayed fluorescence (TADF) organic light‐emitting diodes (OLEDs) are indispensable for lighting, navigation, caution, and so on; however, their development is far behind those of three‐elementary‐color and white analogs, partially because of the lack of host specificity. In this contribution, spiro[bithoxanthene] (ST) and spiro[thioxanthene‐xanthene] (STX) are used as spiro skeletons to form asymmetric ternary structures through introducing two electron‐donating tert ‐butyl carbazole (tBCz) and one electron‐withdrawing diphenyl phosphine oxide (DPPO) groups on different rings. The resulting hosts named STSPOtBCz2 and STXSPOtBCz2 not only inherit the steric hindrance of their spiro skeletons, but also achieve the ambipolar characteristics. Their suitable triplet energy levels (T 1 ) of ≈2.5 eV support efficient energy transfer to a conventional yellow TADF emitter 4CzTPNBu with the T 1 of ≈2.2 eV. It is shown that compared to STSPOtBCz2 with a symmetric core, the accumulated asymmetry of the spiro skeleton and the kind, number, and position of substitution groups in STXSPOtBCz2 further suppresses the emission quenching, resulting in the state‐of‐the‐art photoluminescent and electroluminescent quantum efficiencies of 93% and 30.6%, respectively. The maximum power efficiency beyond 90 lm W −1 further demonstrates the great potential of multi‐asymmetric host systems for energy‐saving practical application.