Doped and non-doped organic light-emitting diodes based on a yellow carbazole emitter into a blue-emitting matrix

Abstract

A new carbazole derivative with a 3,3′-bicarbazyl core 6,6′-substituted by dicyanovinylene groups (6,6′-bis(1-(2,2′-dicyano)vinyl)- N, N′-dioctyl-3,3′-bicarbazyl; named (OcCz2CN) 2, was synthesized by carbonyl-methylene Knovenagel condensation, characterized and used as a component of multilayer organic light-emitting diodes (OLEDs). Due to its π-donor–acceptor type structure, (OcCz2CN) 2 was found to emit a yellow light at λ max = 590 nm (with the CIE coordinates x = 0.51; y = 0.47) and was used either as a dopant or as an ultrathin layer in a blue-emitting matrix of 4,4′-bis(2,2′-diphenylvinyl)-1,1′-biphenyl (DPVBi). DPVBi (OcCz2CN) 2-doped structure exhibited, at doping ratio of 1.4 weight %, a yellowish–green light with the CIE coordinates ( x = 0.31; y = 0.51), an electroluminescence efficiency η EL = 1.3 cd/A, an external quantum efficiency η ext = 0.4 % and a luminance L = 127 cd/m 2 (at 10 mA/cm 2) whereas for non-doped devices utilizing the carbazolic fluorophore as a thin neat layer, a warm white with CIE coordinates ( x = 0.40; y = 0.43), η EL = 2.0 cd/A, η ext = 0.7%, L = 197 cd/m 2 (at 10 mA/cm 2) and a color rendering index (CRI) of 74, were obtained. Electroluminescence performances of both the doped and non-doped devices were compared with those obtained with 5,6,11,12-tetraphenylnaphtacene (rubrene) taken as a reference of highly efficient yellow emitter.