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(9 H ‐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 (LT 50 ) of over 300 h at an initial luminance of approximately 12000 cd m −2 (current density: 25 mA cm −2 ). The LT 50 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 (LT 50 ≈8500 h).