LAUSR

The understanding of the external enhancement effects from host matrixes on thermally activated delayed fluorescence (TADF) emitters is crucial but incomprehensive at present. Herein, a series of phosphine oxide (PO) acceptors mDBSOSPO (m = 2, 3, and 4, corresponding to PO substitution position) and 4,4'‐bis(9‐carbazolyl)‐2,2'‐dimethylbiphenyl (CDBP) as donor is used to construct CDBP:mDBSOSPO exciplex matrixes with typical TADF behaviors. After doped with a conventional yellow TADF emitter 4CzTPNBu, the exciplex matrixes dramatically elevate the reverse intersystem crossing (RISC) efficiencies up to 99%, effectively reduce triplet nonradiative rate constant, and tenfold increase singlet radiative/nonradiative ratio beyond 30 in the case of CDBP:2DBSOSPO:3% 4CzTPNBu. The time‐resolved photoluminescence and electroluminescence (EL) spectra demonstrate that in contrast to single‐molecular hosts, besides the additional RISC channel for TADF facilitation, the exciplexes support the charge preseparation for the step‐by‐step charge transfer‐based energy transfer featuring effective quenching suppression. These external enhancement effects of the exciplex matrixes lead to the state‐of‐the‐art EL performances of their yellow TADF diodes, including the recording power and quantum efficiencies of 114.9 lm W−1 and 30.3% to date.