We report on the performance of green phosphorescent organic
light-emitting diodes (OLEDs) based on the well-known
device structure of a hole-transport layer, an emissive layer with host 4,4'-di(carbazol-9-yl)-biphenyl [CBP] and the
green phosphor emitter
fac tris(2-phenylpyridinato-N,C2,) iridium [Ir(ppy)3], a hole-blocking layer of 2,9-dimethyl-4,7-
diphenyl-1,10-phenanthroline [BCP] and and
tris-(8-hydroxyquinolinato-N,O) aluminum [Alq3] as an
electron-transport
layer. Using spin-coated hole-injection/transport layers with increasing ionization potentials and decreasing hole
mobilities, external quantum efficiencies of up to 18.1% at 100 cd/m2 were measured in such devices. Furthermore, by
removing the electron-transport layer of Alq3 and increasing the thickness of BCP, devices with efficiencies of 21.2%
and 72 cd/A at 100 cd/m2 were obtained. Achieving such high efficiencies with a simplified hybrid structure in which
one layer is processed from solution and only two other organic layers are deposited from the vapor phase is desirable
for the fabrication of low-cost OLEDs.
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