Elucidating the Key Role of Phosphorescent Sensitizers in High‐Performance, Long‐Lifetime Hyperphosphorescent Organic Light‐Emitting Devices Based on a Multi‐Resonance Thermally Activated Delayed Fluorescence Emitter

Hyperphosphorescent organic light‐emitting devices that employ phosphorescent sensitizers are essential for achieving low‐power consumption, long‐lifetime, and high‐resolution displays, yet the roles of sensitizers in device stability remain unclear. In this study, we investigated phosphor‐assisted thermally activated delayed fluorescence (TADF)‐sensitized fluorescence devices as an advanced hyperphosphorescent technology using the TADF host DMIC‐TRZ , the multi‐resonance TADF (MR‐TADF) emitter PhPXZ‐tCzBN , and three phosphorescent sensitizers: Ir(ppy) ₃ , Ir(ppy) ₂ (acac) , and Ir(mppy) ₃ . Although the sensitizers exhibited similar optical properties and enabled efficient energy transfer to the MR‐TADF emitter, the operational lifetimes differed greatly—by up to eightfold. The observed lifetime trend was consistent with that of the corresponding binary phosphorescent devices without MR‐TADF emitter. Most importantly, the results suggest that the carrier transport characteristics of each sensitizer, together with the asymmetric carrier mobility of the host material, are associated with differences in the emission zone distribution and operational stability. Sensitizers showing stronger hole‐trapping behavior are proposed to favor exciton formation closer to the hole‐transporting layer interface, whereas weaker trapping may allow a broader carrier distribution and contribute to improved operational stability. Increasing the Ir(ppy) ₃ concentration to 20 wt% further increased the operational lifetime up to LT ₉₅  = 4491 h at 1000 cd m ⁻² .