Modified N,N′-dicarbazolyl-3,5-benzene as a high triplet energy host material for deep-blue phosphorescent organic light-emitting diodes

Article abstract of DOI:10.1002/chem.201101095

Deeply phosphorescent device: A high efficiency deep-blue phosphorescent organic light-emitting diode was developed using a high triplet energy host material with carbazole and diphenylphosphine oxide units. A high quantum efficiency of 22.4 % with a colo

Full text of DOI:10.1002/chem.201101095

COMMUNICATION
DOI: 10.1002/chem.201101095
Modified N,N’-Dicarbazolyl-3,5-benzene as a High Triplet Energy Host
Material for Deep-Blue Phosphorescent Organic Light-Emitting Diodes
Yong Joo Cho and Jun Yeob Lee*[a]
It is important to balance holes and electrons in the emit-
ting layer to obtain high quantum efficiency in phosphores-
cent organic light-emitting diodes (PHOLEDs). The hole
and electron balance can be improved by using bipolar host
materials that can transport both holes and electrons.[1–14]
Therefore, there has been much effort to develop bipolar
host materials for high efficiency PHOLEDs.
Various bipolar host materials have been synthesized for
applications in red-, green-, and blue PHOLEDs. Both hole-
and electron-transport units were introduced in the molecu-
lar structure to balance holes and electrons in the emitting
layer.[1–14] Aromatic amine and carbazole groups have typi-
cally been used as the hole-transport units, whereas oxadi-
PHOLEDs. It was designed as the host material with an iso-
lated HOMO and LUMO in the molecular structure. The
HOMO was the localized hole-transport carbazole unit,
whereas the LUMO was the localized electron-transport di-
phenylphosphine-oxide-modified phenyl unit. DCPPO was
evaluated as the host material for deep-blue PHOLEDs and
demonstrated a high quantum efficiency of 22.4% with a
color coordinate of (0.14,0.17).
DCPPO has a backbone structure of common mCP with a
diphenylphosphine oxide moiety attached to the mCP back-
bone. mCP has been known to have strong hole-transport
and poor electron-transport properties due to the carbazole
group. Therefore, the mCP was modified with a strong elec-
tron-transporting diphenylphosphine oxide group. The di-
phenylphosphine oxide was introduced in the phenyl core of
the mCP to isolate the HOMO and LUMO; the carbazole is
a hole-transport unit and the diphenylphosphine-oxide-
modified phenyl is an electron-transport unit. As the
HOMO and LUMO can be localized on the hole- and elec-
tron-transport units, it is easy to obtain bipolar charge-trans-
port properties. The introduction of the diphenylphosphine
oxide group may improve the electron-transport properties
of the DCPPO and charge balance in the emitting layer. In
addition, the stability of the material under continuous oper-
ation can also be improved.
ACHTUNGTRENNUNGazole, pyridine, and imidazole have been introduced as the
electron-transport units. In general, bipolar host materials
showed high quantum efficiency due to an improved charge
balance in the emitting layer.
The bipolar host materials have also been developed for
deep-blue PHOLEDs.[8,9,14] The bipolar host materials for
deep-blue PHOLEDs should possess high triplet energy as
well as bipolar charge-transport properties. Phenylcarba-
zole,[8,9] tetraphenylsilane,[11] and triazine[10] core structures
were combined with other charge-transport units to obtain
bipolar charge-transport properties. Conjugation of the mo-
lecular structure was minimized to obtain high triplet
energy, and both hole- and electron-transport units were in-
troduced for bipolar charge-transport properties. Several
high triplet energy host materials were found to be effective
as host materials for deep-blue PHOLEDs.[8,9,14] The separa-
tion of the highest occupied molecular orbital (HOMO) and
lowest unoccupied molecular orbital (LUMO) was not com-
plete in these materials; the development of bipolar host
materials with a separated HOMO and LUMO is required
to reduce both the driving voltage and the bandgap.
DCPPO could be effectively synthesized from the bromi-
nated mCP intermediate, which was prepared by treating
the starting material 1,3,5-tribromobenzene with 9H-carba-
zole (Scheme 1). The intermediate was then phosphorylated
In this work, a high triplet energy bipolar host material
based on the N,N’-dicarbazolyl-3,5-benzene (mCP) core,
(3,5-di(9H-carbazole-9-yl)phenyl)diphenylphosphine oxide
(DCPPO), was developed as the host material for deep-blue
[a] Y. J. Cho, Prof. J. Y. Lee
Department of Polymer Science and Engineering
Dankook University, 126, Jukjeon-dong, Suji-gu
Yongin-si, Gyeonggi-do, 448-701 (Korea)
Fax : (+82)31-8005-3585
E-mail: leej17@dankook.ac.kr
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201101095.
Scheme 1. Synthesis of DCPPO.
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with chlorodiphenylphosphine, yielding DCPPO after oxida-
tion with hydrogen peroxide. A detailed synthetic procedure
is described in the Supporting Information.
A molecular simulation of the DCPPO was carried out
using density functional theory calculations to study the
HOMO and LUMO distribution. Figure 1 shows the
~
&
^
Figure 2. UV/Vis absorption ( ) and PL spectra ( =solution PL;
=
*
solid PL; =Low T PL) of DCPPO.
phenyl)pyridine) iridium picolinate (FCNIrpic) dopant is
2.74 eV and effective energy transfer from DCPPO host to
FCNIrpic dopant is expected. In addition, the DCPPO
showed a high glass transition temperature of 1078C com-
pared with 558C for the mCP.[15] The glass transition temper-
ature was greatly increased because of the diphenylphos-
phine oxide unit.
Figure 1. HOMO and LUMO distribution of DCPPO.
HOMO/LUMO distribution and simulated HOMO/LUMO
levels of DCPPO. The HOMO of the DCPPO was localized
on the carbazole group of DCPPO, whereas the LUMO was
dispersed over the phenyl unit connected to the carbazole
group. The carbazole is an electron-donating unit, which in-
duced the localization of the HOMO level. The localization
of the LUMO in the phenyl unit is due to the electron-with-
drawing diphenylphosphine oxide group, which makes the
phenyl unit electron-deficient. The HOMO was localized on
the hole-transport carbazole unit and the LUMO was local-
ized on the diphenylphosphine-oxide-modified phenyl unit.
Although a few bipolar host materials with carbazole and
diphenylphosphine oxide groups were reported, the LUMO
was distributed over the hole-transport carbazole units.[8,9,14]
However, the LUMO of the DCPPO was distributed only
over the diphenylphosphine-oxide-modified phenyl unit,
without any dispersion of the LUMO over the carbazole
unit. Therefore, the bipolar charge-transport properties can
be obtained in the DCPPO. The simulated HOMO and
LUMO levels of the DCPPO were 5.44 and 1.18 eV, respec-
tively.
The photophysical properties of the DCPPO were ana-
lyzed by using UV/Vis and PL measurements. Figure 2
shows the UV/Vis, solution PL, solid PL, and low tempera-
ture (T) PL spectra of DCPPO. The DCPPO showed the ab-
sorption of the carbazole unit above 300 nm and phenyl unit
connected to the carbazole below 300 nm. The absorption
edge of the UV/Vis spectrum was 347 nm, which corre-
sponded to the bandgap at 3.57 eV. The solution and solid
PL emissions of the DCPPO were observed at 391 nm and
there was little difference in the peak position between the
solution and solid PL. The triplet energy of the DCPPO was
2.99 eV, which could be calculated from the first phosphor-
escent emission peak of low temperature PL spectrum at
414 nm. The triplet energy of the DCPPO was high enough
to use the host material as a deep-blue phosphorescent
dopant. The triplet energy of the bis((3,5-difluoro-4-cyano-
The HOMO of the DCPPO was measured using cyclic
voltametry (CV) and was 6.13 eV from the onset of the oxi-
dation curve of CV (see the Supporting Information, Fig-
ure S1). The LUMO (2.56 eV) was calculated from the
HOMO and bandgap (3.57 eV). Compared with the
HOMO/LUMO levels of the common mCP host material,
the LUMO level was lowered due the electron-withdrawing
diphenylphosphine oxide group and the HOMO was similar.
Therefore, the DCPPO can show similar hole-injection
properties to mCP and better electron-injection properties
than mCP, indicating bipolar charge-transport properties.
An electron-only device of DCPPO was fabricated to con-
firm the electron-injection and -transport properties of
DCPPO and was compared with a common mCP host.
Figure 3 shows the hole- and electron current density of
DCPPO and mCP. The electron current density of the
DCPPO was higher than that of the mCP, indicating better
electron-injection and -transport properties of the DCPPO.
The diphenylphosphine oxide group improved the electron
current density of the DCPPO device. The hole-only device
&
Figure 3. Hole- and electron current density of DCPPO ( =electron;
~
^
*
=hole) and mCP ( =electron; =hole).
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Chem. Eur. J. 2011, 17, 11415 – 11418

Modified N,N’-Dicarbazolyl-3,5-benzene as a Host Material
COMMUNICATION
data of the DCPPO was also added and it can be shown
that the hole- and electron current density of the DCPPO is
similar, indicating bipolar charge-transport properties of the
DCPPO.
The device performances of deep-blue PHOLEDs with
the DCPPO host were investigated by changing the doping
concentration of the deep-blue-emitting FCNIrpic dopant.
The doping concentration of the DCPPO was changed from
1 to 15%. The chemical structure and energy level diagram
of the device are shown in Figure 4. A detailed fabrication
Figure 5. a) Current density–voltage curves and b) luminance–voltage
curves of DCPPO devices.
Figure 4. Chemical structure of materials and energy-level diagram of the
blue device.
process for the deep-blue PHOLEDs is described in the
Supporting Information. Figure 5 shows the current density–
voltage and luminance–voltage curves of deep-blue PHO-
LEDs with the DCPPO host. The current density was re-
duced according to the increase of the doping concentration
and increased above 10%. The decrease of the current den-
sity below 10% doping concentration is caused by the
charge-trapping effect of the FCNIrpic dopant, whereas the
increase of the current density above 10% is due to the
charge-hopping effect at a high doping concentration. The
luminance followed a similar relationship as the current
density, although the luminance at a 3% doping concentra-
tion was rather high due to a high recombination efficiency
of the device.
Figure 6. Quantum efficiency–luminance curves of DCPPO devices.
LEDs was 22.4% and the quantum efficiency at 1000 cdmÀ2
was 18.4%. This efficiency value is one of the best efficiency
values reported in the literature. The maximum current effi-
ciency was 27.1 cdAÀ1 and the current efficiency at
1000 cdmÀ2 was 22.2 cdAÀ1. The high quantum and current
efficiency of the DCPPO device can be explained by the ef-
ficient hole- and electron-injection from the charge-trans-
port layer to the emitting layer. The HOMO level of the
DCPPO was 6.13 eV, leading to only 0.03 eV HOMO level
difference with the mCP hole-transport layer. The little
energy barrier for hole injection improves the hole injection
from the mCP layer to the DCPPO-emitting layer. The elec-
tron injection from the TSPO1 layer to the DCPPO-emit-
ting layer is also efficient as there is no energy barrier for
electron injection. Therefore, both holes and electrons can
Figure 6 shows the quantum efficiency–luminance curves
of the DCPPO devices. The quantum efficiency of the deep-
blue PHOLEDs was optimized at 3% doping concentration.
The maximum quantum efficiency of the deep-blue PHO-
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J. Y. Lee et al.
be efficiently injected from the charge-transport layer to the
emitting layer, improving the quantum efficiency of the
DCPPO device.
useful for the future development of high triplet energy host
materials for deep-blue PHOLEDs.
The electroluminescence (EL) spectra of the DCPPO
device is shown in Figure 7. The DCPPO device with a 3%
doping concentration exhibited a peak maximum at 458 nm
Keywords: bipolar host
functional calculations · host–guest chemistry · high triplet
energy · luminescence
· cyclic voltammetry · density
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and shoulder peak at 477 nm. The peak maximum was
slightly redshifted to 460 nm at high doping concentration
due to intermolecular interaction. The color coordinate of
the DCPPO device was (0.14,0.17) at 1 and 3% doping con-
centrations, whereas at 5 and 10% doping concentrations it
was (0.14,0.18). The intermolecular interaction between
dopant materials induced the redshift of the color coordi-
nate at a high doping concentration.
In conclusion, a mCP-based material, DCPPO, was syn-
thesized as the high triplet energy bipolar host material for
deep-blue PHOLEDs. DCPPO showed a high triplet energy
of 2.99 eV and was effective as the host material for deep-
blue PHOLEDs. A high quantum efficiency of 22.4% with
a color coordinate of (0.14,0.17) was achieved in the deep-
blue PHOLEDs by using the DCPPO host material. This
approach for the design of bipolar host materials can be
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Received: April 11, 2011
Published online: August 29, 2011
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Chem. Eur. J. 2011, 17, 11415 – 11418