Host Engineering Mitigates Triplet Annihilation in a Dibenzothiophene‐Phenanthroimidazole‐Based HLCT Emitter for High‐Efficiency Deep‐Blue OLED

Deep‐blue organic light‐emitting diodes (OLEDs) based on hybridized local and charge‐transfer (HLCT) emitters offer a promising route to achieving high efficiency and color purity; however, their practical performance is often limited by exciton loss processes in the solid state. In this study, a dibenzothiophene‐phenanthroimidazole‐based HLCT emitter is investigated, DTPBuPI (1‐(4‐(tert‐butyl)phenyl)‐2‐(4‐(dibenzo[ b , d ]thiophen‐4‐yl)phenyl)‐1 H ‐phenanthro[9,10‐ d ]imidazole), which exhibits reverse charge transfer (holes on the formally assigned acceptor and electrons on the donor)—termed rHLCT. Triplet–triplet annihilation (TTA) as the dominant loss channel in the neat films via quantitative brightness–current density analysis is identified. The undoped device showed a superlinear slope of 1.142 ± 0.019 (95% confidence interval [CI]: 1.104–1.180), confirming active TTA. Host‐doping spatially isolated the emitter, mitigating TTA to a negligible level (linear slope: 1.025 ± 0.013, 95% CI: 0.999–1.051). Thus, the hot‐exciton pathway appeared operative, yielding a high EQE of 13.9% with deep‐blue CIE (0.1606, 0.0383)—among the highest for solution‐processed deep‐blue HLCT OLEDs with CIEy < 0.04. The exciton utilization efficiency of 87.6% exceeded the TTA limit of 62.5%, demonstrating activation of the hot‐exciton channel. This study provided a framework for diagnosing and suppressing TTA in HLCT emitters, suggesting that solid‐state exciton kinetics are as critical as the molecular design for high‐performance OLEDs.