New Exciplex‐Forming Co‐Host System and Thienothiadazole‐based Fluorescent Emitter for High‐Efficiency and Promising Stability Near‐Infrared OLED

Abstract Two new spirobifluorene (SF)‐centered donors (D) DPSF and DTSF are developed to blend with acceptor (A) CN‐T2T for exciplex formation. The transient photoluminescence characterizations of pristine donor and D:A blended films verify the donor aggregates emissions that lead to relatively low photoluminescence quantum yield (PLQY) of exciplex emission. However, the charge recombination at lower energy exciplex state leads the device with DPSF:CN‐T2T blend to give electroluminescence (EL) λ _max of 584 nm and external quantum efficiency (EQE) of 6.0%. The DPSF:CN‐T2T exciplex excitons are efficiently extracted by a D‐A‐D‐type fluorescence emitter TTDSF comprising thienothiadiazole core and SF peripherals via Förster energy transfer to give near‐infrared (NIR) emission (760 nm, PLQY 26%). After the electron‐transporting layer thickness optimization, the device with DPSF:CN‐T2T: 7 wt.% TTDSF as emitting layer affords EL λ _max 774 nm and EQE up to 5.3%, one of the best cases in NIR organic light‐emitting diodes (OLEDs). More importantly, the device stability has been examined to display an excellent lifetime (LT ₉₅ > 200 hours). This work manifests the judicious combination of an exciplex‐forming co‐host system and a designated NIR emitter equipping with a high quinoidal core and rigid peripheral SF groups that can realize an efficient NIR OLED.