We fabricated a yellow organic light-emitting diode (OLED) based on the star-shaped donor compound tri(9-hexylcarbazol-3-yl)amine, which provides formation of the interface exciplexes with the iridium(III) bis[4,6-difluorophenyl]-pyridinato-N,C2']picolinate (FIrpic). The exciplex emission is characterized by a broad band and provides a condition to realize the highly effective white OLED. It consists of a combination of the blue phosphorescent emission from the FIrpic complex and a broad efficient delayed fluorescence induced by thermal activation with additional direct phosphorescence from the triplet exciplex formed at the interface. The fabricated exciplex-type device exhibits a high brightness of 38 000 cd/m(2) and a high external quantum efficiency.
The starburst carbazole derivative
and phosphorescent bis-cyclometalated
iridium(III) complex (IC2) were used for the preparation
of multilayered “warm-white” organic light-emitting
diodes (OLEDs), the emission spectra of which are modulated by the
thickness of the phosphorescent layer. It was shown that the electroluminescence
spectra of the fabricated devices are more extended into the visible
region compared with the photoluminescence spectra of both component
materials. The observed extension of the electroluminescence spectra
can be assigned to the phosphorescent emission of the low-energy exciplex
formed at the interface of the emissive layers. The quantum-chemical
calculations performed by the DFT and (TD) DFT methods support the
formation of the low-energy triplet exciplex at the interface of the IC2 layer and the neighboring layer of the starshaped carbazole-based
compound, (4,4′,4″-tris[3-methylphenyl(phenyl)amino]
triphenylamine, tri(9-hexylcarbazol-3-yl)amine (THCA). Indeed, the triplet excited state of such bimolecular complex corresponds
to intermolecular charge transfer between IC2 and THCA. The experimentally observed electrophosphorescence of
these exciplexes is induced by strong spin–orbit coupling in
the THCA:IC2 complexes due to the Ir(III) heavy atom
effect. With dependence on the iridium(III)-complex film thickness
(5–9 nm), the CIE coordinates changed from (0.41, 0.41) to
(0.52, 0.47), corresponding to the warm white and orange color. The
brightness of the fabricated OLEDs at the 15 V bias was in the range
from 500 to 6000 cd/m2.
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