Enhanced electroluminescence and reduced efficiency roll-off in electrophosphorescent devices using a very high electron mobility material as emitter host and electron transporter

We demonstrate markedly improved electroluminescence (EL) intensity and reduced external quantum efficiency (EQE) roll-off of a device which presents two emitting layers of fac-tris-(2-phenylpyridine) iridium [Ir(ppy)₃] in 4,4′-bis(9-carbazolyl)-2,2′-biphenyl (CBP) and 1,3-bis[2-(2,2′-bipyridine-6-yl)-1,3,4-oxadiazo-5-yl]benzene (Bpy-OXD); 4,7-diphenyl-1,10-phenanthroline/Bpy-OXD layers were used as hole blocking and electron transport layers. The device exhibits a maximum EQE of 15.5% (corresponding to an efficiency of 56.8 cd A^-1) at a luminance of 600 cd m^-2, which is twice the value (8.2%) of the reference diode using Ir(ppy)₃ doped CBP as a single emitting layer; the device still maintained a 6.5% efficiency at a luminance of 70 450 cd m^-2. The mechanism of this marked improvement in EL intensity and decrease in the EQE roll-off was also explored in detail.