Operational Stability Analysis of Blue Thermally Activated Delayed Fluorescence Organic Light-Emitting Diodes Using the Capacitance-Voltage Method

We analyzed the degradation features by measuring capacitance–voltage characteristics after electrically aging blue thermally activated delayed fluorescence (TADF) organic light-emitting diodes (OLEDs). The measurement was investigated in terms of hole transfer layer (HTL) and electron (ETL) structures. For 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), 1,3–bis(carbazol–9–yl)benzene (mCP)—were used at HTL/emission (EML) interface; TCTA/EML interface had highest stability among interfaces. ETL, bis [2–(diphenylphosphino)phenyl] ether oxide (DPEPO) without further dopants as an exciton blocking (ExBL) to effectively confine excitons EML. However, DPEPO has low carrier mobility. Therefore, 0, 10, 40 nm-thick ExBL devices were investigated; it found that 0 device most stable. 10 is essential EML, which ensures a high EL performance.