LAUSR

Different to fluorescent and phosphorescent counterparts, white thermally activated delayed fluorescence (TADF) involves in multiple reverse intersystem crossing (RISC), leading to the correlation but competition between blue and other color components in both singlet and triplet allocations. Herein, three blue TADF emitters SSFAPO, DSFAPO, and TSFAPO, collectively named xSFAPO are developed, featuring a moderately electron‐withdrawing phosphine oxide (PO) acceptor respectively linked 1‐3 donors. Despite nearly identical blue emissions, photoluminescence quantum yields of xSFAPO are proportional to donor number. But, their RISC efficiencies are below 70%, markedly less than 85% of a conventional yellow TADF emitter 2,3,5,6‐tetrakis(3,6‐di‐(tert‐butyl)carbazol‐9‐yl)‐1,4‐dicyanobenzene (4CzTPNBu). Furthermore, sp3 hybrid configuration of PO enlarges steric hindrance of peripheral donor groups. So, Dexter energy transfer is impeded by increasing donor numbers. Among xSFAPO and 4CzTPNBu dually doped white‐emitting films, yellow emission from SSFAPO‐based film is the strongest, reflecting the predominance of fast Dexter energy transfer in triplet allocation. Therefore, SSFAPO endowed its warm‐white organic light‐emitting diodes (WOLEDs) with an external quantum efficiency of 25.1%, corresponding to 100% internal quantum efficiency, which are 1.25 and 1.60 folds of those of DSFAPO and TSFAPO‐based WOLEDs. These results suggest advantage complementarity of different components is crucial for developing white‐emitting systems with 100% exciton utilization.