High‐Efficiency TADF Dendritic Emitters Enabled by Synchronously Inhibiting Degenerated Triplet Excited States and Structural Relaxation Toward Solution‐Processed OLEDs with EQE Over 33%

ABSTRACT Manufacturing high‐performance solution‐processed organic light‐emitting diodes (OLEDs) employing thermally activated delayed fluorescence (TADF) dendritic emitters remains a formidable challenge due to the lack of efficient TADF dendrimers. Herein, a symmetry breaking strategy is adopted to construct an asymmetrical TADF dendrimer denoted as DMAC‐XT‐TCz . An in‐depth analysis of the photophysical properties combining with theoretical calculation expose that the asymmetrical architecture of target emitter switches degenerated triplet excited states to isolated counterpart, thereby effectively breaking the degeneracy of vibrational levels and boosting the spin flip of triplet excitons. Additionally, the nonradiative decay is also suppressed due to imbedding oxygen linkage to locking electron‐donating skeleton. Thus, near‐unity photoluminescence efficiency and excellent reverse intersystem crossing rate of 7.8 × 10 5 s −1 can be achieved for DMAC‐XT‐TCz . Impressively, the optimized solution‐processed OLEDs achieve an attractive external quantum efficiency of 33.7%, which is the highest value for TADF dendrimer‐based OLEDs. By using DMAC‐XT‐TCz as sensitizer, the solution‐processed narrowband OLEDs based on a multiple‐resonance TADF emitter also acquire record‐high device performances with current efficiency of 117.7 cd A −1 . This study highlights the significance of asymmetric architecture in designing high‐efficiency TADF dendrimer, and provides an effective strategy to boost solution‐processed narrowband OLEDs through adopting TADF dendrimer as sensitizer.