We analyzed the degradation features by measuring the capacitance–voltage characteristics after electrically aging blue thermally activated delayed fluorescence (TADF) organic light-emitting diodes (OLEDs). The measurement was investigated in terms of the hole transfer layer (HTL) and electron transfer layer (ETL) structures. For the HTL, three different materials—N,N′–bis(naphthalen–1–yl)–N,N′–bis(phenyl)–benzidine (NPB), 4,4′,4″-tris(carbazol–9–yl)triphenylamine (TCTA), and

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... yl)benzene (mCP)—were used at the HTL/emission layer (EML) interface; the TCTA/EML interface had the highest stability among the interfaces. For the ETL, bis [2–(diphenylphosphino)phenyl] ether oxide (DPEPO) without further dopants was used as an exciton blocking layer (ExBL) to effectively confine the excitons at the EML. However, DPEPO has low stability and carrier mobility. Therefore, 0, 10, and 40 nm-thick ExBL devices were investigated; it was found that the 0 nm-thick ExBL device was the most stable. However, the 10 nm-thick ExBL is essential to confine the excitons at the EML, which ensures a high EL performance.