  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 &#61552;-donor-acceptor type structure, (OcCz2CN)2
was found to emit a yellow light at &#61548;max=590 nm (with the CIE
coordinates x=0.51; y = 0.47) and was used either as a dopant or as an
ultra-thin 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 1.5 weight %, a yellowish-green light
with the CIE coordinates (x = 0.31; y = 0.51), an electroluminescence
efficiency &#61544;EL=1.3 cd/A, an external quantum efficiency &#61544;ext= 0.4
% and a luminance L= 127 cd/m2 (at 10 mA/cm2) 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), &#61544;EL= 2.0 cd/A, &#61544;ext= 0.7 %,
L = 197 cd/m2 (at 10 mA/cm2) 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.
