Highly efficient and stable pure-red phosphorescent organic light-emitting diodes based on heptacyclic bipolar hosts featuring an armor-like structure

Host material stability is essential to increase the lifetime for full-spectrum luminescence in efficient organic light-emitting diodes (OLEDs), especially red phosphorescent OLEDs (PhOLEDs). While the structural diversity of red emitters has developed rapidly in recent years, high-quality host materials are scarce and still rely heavily on traditional structures. Herein, a novel strategy is proposed by introducing heptacyclic heteroaromatics with an armor-like structure to design structurally non-traditional red hosts that are rigid enough to suppress conformational relaxation through reduced vibrations, thereby inhibiting non-radiative decay and improving stability. Accordingly, the red PhOLEDs based on the host named 2-([1,1′-biphenyl]-4-yl)-4-phenyl-1,3,5-triazine (Hept-TRZ) exhibit exceptional performance with a maximum current efficiency of 30.8 cd A −1 and the highest maximum external quantum efficiency (EQE) of 29.6% at 620 nm, as well as the high luminance of 65172 cd m −2 . In particular, with satisfactory morphological stability and well-balanced hole and electron transport properties, the PhOLED device retains 95% of the luminance (LT 95 ) even after 8777 hours of continuous operation at 1000 cd m −2 , firmly establishing the negligible efficiency roll-off and superior stability with the new heptacyclic host. This work validates the superiority of this molecular design strategy and provides an instructive strategy for designing effective hosts for highly efficient and stable red PhOLEDs. The armor-like heptacyclic framework enhances the rigidity and stability of host materials named Hept-TRZ. The luminescent devices based on Hept-TRZ as the red host achieve high efficiency and long lifetime of PhOLEDs.