Efficient and low potential operative host/guest concentration graded bilayer polymer electrophosphorescence devices

This study investigates enhanced electrophosphorescence and its mechanism in poly(N-vinyl carbazole) (PVK): N,N′-diphenyl-N,N′-bis(3- methylphenyl)-[1,1-biphenyl]-4,4′-diamine (TPD)/2-(4-biphenylyl)-5-(4- tert-butylphenyl)-1,3,4-oxadiazole (PBD): fac-tris(2-phenylpyridine)iridium [Ir(ppy) ₃] concentration graded bilayer electroluminescence devices. The two layers are partially intermixed at the bilayer interface because the upper layer (composed of Ir(ppy) ₃ and PBD) was spun cast from a solvent that slightly swells the bottom layer (composed of PVK and TPD). Moreover, PBD in the upper layer can act as an efficient electron transport layer as well as a hole blocking layer, resulting in greatly enhanced electronhole recombination. An indium tin oxide (ITO)/3,4- polyethylenedioxythiophenepolystyrenesulfonate (PEDOT)/[PVK:TPD/Ir(ppy) ₃:PBD] bilayer/LiF/Al device showed dramatically decreased turn-on and driving voltages, enhanced luminescence efficiency, and narrower emission spectra compared to those of conventional ITO/PEDOT:PSS/[PVK:TPD:Ir(ppy) ₃:PBD] blend/LiF/Al devices.