Excitonic Degradation Mechanisms in Phosphorescent and Thermally Activated Delayed Fluorescence Organic Light‐Emitting Diodes

Abstract In this study, the degradation mechanisms of triplet exciton harvesting organic light‐emitting diodes (OLEDs), namely, phosphorescent OLEDs and thermally activated delayed fluorescence (TADF) OLEDs, are investigated. Two common green emitters, fac‐tris(2‐phenylpyridine) iridium (III) (Ir(ppy) 3 ) and 1,2,3,5‐tetrakis(carbazol‐9‐yl)‐4,6‐dicyanobenzene (4CzIPN), are doped in an exciplex‐forming cohost, and their degradation processes are comprehensively evaluated using various analytical approaches. Triplet–triplet‐annihilation induces the formation of defects, such as charge traps and exciton quenchers, triggering luminance loss during the device aging process of Ir(ppy) 3 ‐based phosphorescent OLEDs. Electron trapping‐induced triplet‐polaron annihilation and narrow emission zone severely impair the device stability of 4CzIPN‐based TADF OLEDs, despite limited material degradation and charge trap formation.