Secondary Acceptor Optimization for Full‐Exciton Radiation: Toward Sky‐Blue Thermally Activated Delayed Fluorescence Diodes with External Quantum Efficiency of ≈30%

Abstract Efficient blue emitters are indispensable for organic light‐emitting diodes (OLEDs) with respect to display and lighting applications. Because of their high‐energy excited states, both radiation enhancement and non‐radiation suppression should be simultaneously optimized to realize 100% exciton utilization. Here, it is shown that the excited‐state characteristics of blue thermally activated delayed fluorescence emitters can be precisely controlled by a secondary acceptor having moderate electronic effects on increasing the singlet charge‐transfer component and preserving the triplet locally excited‐state component. In addition of planar configuration between the donor and the primary acceptor, the radiative transition improvement and non‐radiative transition suppression can be simultaneously achieved for “full‐exciton radiation”. A molecule using diphenylphosphine oxide as the secondary acceptor exhibits ≈ 100% photoluminescence quantum yield on the basis of its tenfold increased singlet radiative rate constant, fivefold decreased singlet and triplet non‐radiative rate constants, and ≈ 100% reverse intersystem crossing efficiency, which further endows ≈ 100% exciton utilization efficiency to its sky‐blue OLEDs.