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 HOMO and shallow LUMO levels along with sufficiently high energy S1 and T1 states that make them suitable hosts for yellow emitters in electroluminescent devices. Yellow-emitting thermally activated delayed fluorescence (TADF) organic light-emitting diodes (OLEDs) were 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 was compared to analogous devices using CBP and mCBP as host materials. The pyridine-containing host devices showed markedly improved external quantum efficiencies (EQE) and decreased roll-off. The 7 wt% tri-PXZ-TRZ-doped device exhibited 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 exhibited very low efficiency roll-off with an EQE of 16.0% at a luminance of 10,000 cd ^m-2.