Bipyridine‐Containing Host Materials for High Performance Yellow Thermally Activated Delayed Fluorescence‐Based Organic Light Emitting Diodes with Very Low Efficiency Roll‐Off

Two bipolar host materials 3-CBPy and 4-mCBPy are reported. These hosts are structural analogs of the common host materials CBP and mCBP wherein the phenyl rings have been replaced with pyridines. The two materials possess deep highest occupied molecular orbital (HOMO) and shallow lowest unoccupied molecular orbital (LUMO) levels along with sufficiently high energy S-1 and T-1 states that make them suitable hosts for yellow emitters in electroluminescent devices. Yellow-emitting thermally activated delayed fluorescence organic light-emitting diodes are fabricated using 2,4,6-tris (4-(10H-phenoxazin-10-yl)phenyl)-1,3,5-triazine (tri-PXZ-TRZ) as the dopant emitter with either 3-CBPy or 4-mCBPy employed as the host. Their device performance is compared to analogous devices using CBP and mCBP as host materials. The pyridine-containing host devices show markedly improved external quantum efficiencies (EQE) and decreased roll-off. The 7 wt% tri-PXZ-TRZ-doped device exhibits very low turn-on voltage (2.5 V for both 3-CBPy and 4-mCBPy) along with maximum external quantum efficiencies (EQE(max)) reaching 15.6% (for 3-CBPy) and 19.4% (for 4-mCBPy). The device using 4-mCBPy also exhibits very low efficiency roll-off with an EQE of 16.0% at a luminance of 10 000 cd m(-2).