Suppressing Carrier Accumulation Through HOMO Level Adjustment of Pure Hydrocarbon Host Layers for Stable Hyperfluorescence OLEDs

Hyperfluorescence (HF) organic light‐emitting diodes (OLEDs) enable high external quantum efficiency (EQE) and excellent color purity. However, operational stability remains a critical limitation. In particular, the energetic misalignment between the host and adjacent layers can induce carrier accumulation, thereby accelerating device degradation. While host design has traditionally emphasized triplet energy and intrinsic stability, the role of host in governing electrical degradation remains insufficiently understood. Here, we report a series of pure hydrocarbon host materials designed as positional isomers by attaching an identical phenyl‐substituted tetraphenyl unit at different positions of the spirobifluorene backbone. This subtle structural modification enables systematic tuning of the host energy without significantly altering the triplet energy or photophysical properties. All HF devices exhibit high EQEs of 25%, yet their operational lifetimes differ significantly. Transient electroluminescence measurements reveal that devices with deeper host HOMO levels exhibit pronounced hole accumulation, whereas suppression of the transient spike correlates with the longer device lifetime. Further increasing the HOMO offset at the host/transport interface amplifies charge trapping and accelerates degradation, establishing a direct link between HOMO alignment and device instability. These results identify precise control of host HOMO adjustment as a critical design parameter for mitigating charge‐induced instability in blue HF‐OLEDs.