Key Excitonic Processes Governing the Stability of Deep‐Blue Phosphor‐Sensitized Fluorescent Organic Light‐Emitting Diodes

Abstract Phosphor‐sensitized fluorescent (PSF) mechanisms offer a promising route toward efficient and stable deep‐blue organic light‐emitting diodes (OLEDs) by converting long‐lived triplets into short‐lived emissive singlets through energy transfer. However, the PSF architecture—combining a phosphorescent (PH) sensitizer with a multiple‐resonance (MR) thermally activated delayed fluorescent (TADF) emitter within a co‐host matrix—introduces intertwined excitonic processes that obscure the origins of degradation. Here, cryogenic photoluminescence spectroscopy, together with multichannel exciton‐kinetic modeling to disentangle these processes, is employed. This analysis reveals that degradation is primarily driven by dissociation of the MR emitters, triggered by high‐energy triplet accumulation. Further, it is shown that MR emitter stability is markedly improved when the activation energy for reverse intersystem crossing is increased and when Förster resonance energy transfer from the PH sensitizer to the MR emitter outcompetes Dexter energy transfer. Guided by these insights, a deep‐blue PSF OLED (CIE y ≤ 0.15) with an operational lifetime of T90 = 141 h at 1000 cd m −2 , far exceeding unoptimized devices (35 and 108 h) is demonstrated. This work provides the first quantitative identification of the excitonic processes governing PSF OLED stability and establishes molecular and device‐level design rules for long‐lifetime deep‐blue OLEDs.