π-Extended Carbazole Derivatives as Host Materials for Highly Efficient and Long-Life Green Phosphorescent Organic Light-Emitting Diodes

High-performance organic light-emitting diodes (OLEDs) that use phosphorescent and/or thermally activated delayed fluorescence emitters are capable of realizing 100 % electron-to-photon conversion. The host materials in these OLEDs play crucial roles in determining OLED performance. Carbazole derivatives are frequently used as host materials, among which 3,3-bis(9H-carbazol-9-yl)biphenyl (mCBP) is often used for lifetime testing in scientific studies. In this study, the π conjugation of the carbazole unit was expanded to enhance OLED lifetime by designing and developing two benzothienocarbazole (BTCz)-based host materials, namely m1BTCBP and m4BTCBP. Among these host materials, m1BTCBP formed a highly efficient [Ir(ppy)₃ ]-based OLED with an operational luminescence half-life (LT₅₀ ) of over 300 h at an initial luminance of approximately 12000 cd m^-2 (current density: 25 mA cm^-2 ). The LT₅₀ value at 1000 cd cm^-2 was estimated to be about 23 000 h. This performance is clearly higher than that of mCBP-based OLEDs (LT₅₀ ≈8500 h).