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

Abstract

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 (POD): 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, POD in the upper layer can act as an efficient electron transport layer as well as a hole blocking layer, resulting in greatly enhanced electron-hole recombination. An indium tin oxide (ITO)/3,4-polyethylenedioxythiophene-Polystyrenesulfonate (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. (C) 2011 Elsevier B.V. All rights reserved.