Degradation of Blue MR‐TADF OLEDs: Host‐dependent Effects of mCBP and MADN Revealed by Fragmentation Profiling

ABSTRACT Blue organic light‐emitting diodes (OLEDs) employing multi‐resonance thermally activated delayed fluorescence (MR‐TADF) emitters such as ν‐DABNA exhibit narrowband emission and high color purity. However, their intrinsically slow reverse intersystem crossing (RISC) leads to triplet accumulation and accelerates device degradation. In this study, we provide insight into degradation behavior by coupling host engineering with electrical and chemical diagnostics. MADN‐hosted devices exhibit minimal EQE roll‐off and 2.47‐fold longer lifetime than mCBP host counterparts, whereas mCBP yields a higher peak EQE but severe efficiency roll‐off and faster degradation. Impedance spectroscopy reveals a pronounced low‐frequency resistance increase exclusively in degraded mCBP devices, which is consistent with trap build‐up near the EML region. Laser desorption/ionization time‐of‐flight mass spectrometry (LDI‐TOF MS) provides direct chemical evidence that ν‐DABNA‐derived fragments nearly doubled, and carbazole‐derived fragments increased 7.6‐fold in mCBP devices, consistent with bond dissociation energy (BDE) predictions identifying weak bonds in the anionic and triplet states. Overall, these results suggest that host‐dependent differences in triplet energy alignment, BDE, and exciton management contribute to the observed degradation behavior in the MADN‐ and mCBP‐hosted blue MR‐TADF OLEDs. They further indicate that LDI‐TOF MS can serve as an effective tool for elucidating EML degradation chemistry at the molecular level.