Narrowband Green OLEDs Displaying High Efficiency at Ultra‐High Brightness Based on Multiple Energy Transfer Pathways

Multiple resonance thermally activated delayed fluorescence (MR‐TADF) organic light‐emitting diodes (OLEDs) featuring narrowband emission and high efficiency at ultra‐high luminance hold great potential for applications such as transparent displays and augmented reality (AR) technologies. However, long exciton lifetimes and inefficient reverse intersystem crossing (RISC) in MR‐TADF materials exacerbate triplet exciton accumulation and annihilation, leading to severe efficiency roll‐off at high brightness. This study demonstrates a breakthrough in mitigating the efficiency roll‐off of MR‐TADF OLEDs at ultra‐high brightness through innovative phosphor‐sensitized exciplex‐host systems. The engineered system leverages the RISC process in exciplex hosts synergistically combined with the exciton‐harvesting capability of phosphorescent sensitizers. These configurations preferentially facilitate the Förster energy transfer (FRET) process over multiple competing energy transfer pathways, thereby suppressing energy loss and maximizing radiative exciton utilization. Moreover, the balanced bipolar transport properties of the exciplex further broaden the exciton recombination zone and reduce exciton density. As a result, the optimized green OLED device achieves remarkable performance with a maximum luminance of 200 400 cd m −2 , along with a maximum external quantum efficiency (EQE) of 36.0%. Notably, it maintains exceptional EQEs of 30.4% and 27.7% at luminance of 5 000 and 10 000 cd m −2 , respectively, advancing the potential of MR‐TADF OLEDs toward ultra‐high luminance applications.