Low driving voltage, high quantum efficiency, high power efficiency, and little efficiency roll-off in red, green, and deep-blue phosphorescent organic light-emitting diodes using a high-triplet-energy hole transport material

Article abstract of DOI:10.1002/adma.201102045

Improved device performance in red, green, and deep-blue phosphorescent organic light-emitting diodes is achieved using a new high-triplet-energy hole transport material. The driving voltage is lowered and the power efficiency is improved by more than 70% using the new hole transport material. Copyright

Full text of DOI:10.1002/adma.201102045

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Low Driving Voltage, High Quantum Efficiency, High Power
Efficiency, and Little Efficiency Roll-Off in Red, Green, and
Deep-Blue Phosphorescent Organic Light-Emitting Diodes
Using a High-Triplet-Energy Hole Transport Material
Yong Joo Cho and Jun Yeob Lee*
Phosphorescent organic light-emitting diodes (PHOLEDs) have
been developed because of high quantum efficiency of phospho-
rescent emitting materials.[1–3] A theoretical quantum efficiency
over 20% has already been obtained in red, green, and blue colors
by synthesizing new emitting materials and charge-transport
materials in combination with optimized device architecture.[4–8]
Many research focused on developing host and dopant mate-
rials, but the development of charge transport materials is also
important to maximize the light-emitting properties of the
phosphorescent emitting materials. There are several require-
ments for the charge transport materials for PHOLEDs, which
include high triplet energy for triplet exciton blocking, highest
occupied molecular orbital (HOMO) or lowest unoccupied
molecular orbital (LUMO) levels for efficient hole or electron
injection, a charge confining HOMO or LUMO level, and high
carrier mobility. In particular, a high triplet energy over 2.80 eV
is required for the charge-transport materials for deep-blue
PHOLEDs because the triplet energy of the deep-blue emitter
is around 2.75 eV.[8,,9] Several hole and electron transport mate-
rials have been reported as high-triplet-energy charge-transport
materials. Typically, carbazole- or diphenylamine-type high-tri-
plet-energy hole transport materials have been used as the hole
transport materials for PHOLEDs,[10–12] while pyridine- or phos-
phine-oxide-type high-triplet-energy electron transport materials
have been widely used.[13–18] Recently, a diphenylphosphine-
oxide-type electron transport material was found to be effective
as the high-triplet-energy electron transport layer.[8]
In the case of the hole transport layer, N,N′-dicarbazolyl-3,5-
benzene (mCP) was universally used as the high-triplet-energy
hole transport material in PHOLEDs.[19,,20] It has a high triplet
energy of 2.90 eV for all red, green, and blue applications and
moderate hole transport properties in addition to the HOMO
level suitable for hole injection into the host material in the
emitting layer. However, the mCP suffers from poor thermal
stability and high driving voltage. Although 1,1-bis[(di-4-
tolylamino)phenyl]cyclohexane (TAPC) shows high triplet
energy and high hole mobility, the HOMO level of 5.5 eV is not
suitable for hole injection into the emitting layer.[7] Therefore,
a new hole transport material with a high triplet energy over
2.90 eV, thermal stability, high hole mobility, and HOMO level
for efficient hole injection is required.
In this work, a new high triplet energy hole transport mate-
rial, 3,5-di(9H-carbazol-9-yl)-N,N-diphenylaniline (DCDPA),
was synthesized as the universal hole transport material for
red, green, and deep-blue PHOLEDs. It was compared with
common mCP in terms of material properties and device per-
formances. It was demonstrated that the DCDPA improved
the driving voltage, power efficiency, quantum efficiency and
efficiency roll-off of red, green, and deep-blue PHOLEDs while
overcoming the poor thermal stability of mCP.
The DCDPA was designed to improve the glass transition
temperature (Tg) and hole transport properties of the common
mCP while keeping the high triplet energy of mCP. A diphe-
nylamine group was introduced in the molecular structure
to improve the hole transport properties of mCP. It has been
known that the diphenylamine group is better than the carba-
zole group in terms of hole transport properties due to strong
electron donating property of the diphenylamine group.[21,,22]
The introduction of the diphenylamine group can also
enhance the Tg of mCP because of the increased molecular
rigidity and molecular weight. In addition, the HOMO level
of the mCP can be shifted upward to the vacuum level for
efficient hole injection and the high triplet energy of the mCP
can be maintained because the diphenylamine was introduced
at meta position of the carbazole. The meta-substitution can
minimize the extension of the conjugation length and the tri-
plet energy of mCP can be maintained. The DCDPA was syn-
thesized by reaction of 1,3,5-tribromobenzene with two equiv-
alents 9H-carbazole followed by reaction with diphenylamine.
Synthetic method for the DCDPA is shown in Scheme 1. The
DCDPA was purified by recrystallization and column chroma-
tography, yielding pure DCDPA powder with a high purity of
over 99.0% from high-performance chromatography analysis.
The thermal properties of the DCDPA were measured by differ-
ential scanning calorimetry and the T of the DCDPA was observed
g
to be 109 °C. The T of DCDPA was increased by more than 40 °C
g
compared with that of mCP because of the diphenylamine unit.
The additional diphenylamine unit hindered the motion of the
molecule, leading to higher T in DCDPA than in mCP. The
Y. J. Cho, Prof. J. Y. Lee
g
Department of Polymer Science and Engineering
Dankook University 126
Jukjeon-dong, Suji-gu, Yongin, Gyeonggi, 448-701, Korea
E-mail: leej17@dankook.ac.kr
high T of the DCDPA improved the high-temperature morpho-
g
logical stability of the DCDPA. The vacuum-deposited mCP film
was crystallized even at 70 °C, but the DCDPA film morphology
was stable up to 100 °C. The additional diphenylamine group
improved the morphological stability at high temperature.
DOI: 10.1002/adma.201102045
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Br
N
H
N
Br
N
H
N
N
palladium acetate(II)
N
N
CuI
Br
Br
DCDPA
Scheme 1. Synthetic scheme of DCDPA.
Photophysical properties of the DCDPA were analyzed
using UV-vis and photoluminescence (PL) measurements.
Figure 1 shows the UV-vis, solution PL, and low temperature
PL spectra of the DCDPA. The DCDPA showed the absorp-
tion of the carbazole and diphenylamine units at 292 nm,
322 nm, and 338 nm. The bandgap was calculated from the
absorption edge of the UV-vis spectra and was 3.54 eV. Com-
pared with the mCP with a bandgap of 3.60 eV (Figure 1a),
the bandgap was reduced in the DCDPA because of the diphe-
nylamine unit in the molecular structure. This result agrees
with the trend of a simulated bandgap of mCP and DCDPA.
The simulated bandgap of DCDPA was 4.48 eV compared
with 4.64 eV of mCP. The simulated bandgap was larger than
the experimental bandgap. The solution PL measurement
was carried out in tetrahydrofuran and PL emission peak was
observed at 380 nm. Low temperature PL measurement was
performed to measure the triplet energy of the DCDPA hole
transport material and the triplet energy was 2.90 eV. The tri-
plet energy of the DCDPA was high enough for triplet exciton
blocking in red, green, and deep-blue PHOLEDs. In general,
the triplet energy of the deep-blue phosphorescent emitter is
between 2.70 eV and 2.75 eV.[8] Therefore, the DCDPA can
effectively suppress the triplet exciton quenching of red,
green, and deep-blue emitters by the hole transport layer.
The HOMO level of the DCDPA was measured using
cyclic voltametry (Figure S1, Supporting Information) and the
LUMO was calculated from the HOMO and bandgap obtained
from UV-vis spectrum. The HOMO level of the DCDPA was
5.88 eV from the onset of the oxidation peak and the LUMO
level was 2.34 eV. The HOMO level of DCDPA was shifted by
0.22 eV compared with 6.1 eV of the mCP[23] (5.90 eV by surface
analyzer)[24] by the strong electron-donating diphenylamine unit.
The HOMO level of DCDPA was suitable for hole injection into
the host material with the HOMO level range from 5.9 eV to
6.2 eV. The LUMO level was 2.34 eV, which was suitable for elec-
tron blocking from the emitting layer. The LUMO level of the
DCDPA was more suitable for electron blocking than that of
the mCP (2.50 eV). Therefore, the DCDPA can be effective as the
hole transport material for PHOLEDs because of HOMO level
for good hole injection, the LUMO level for electron blocking,
and the high triplet energy for exciton blocking. The shift of the
HOMO and LUMO level of the DCDPA compared with those
of mCP can be explained by the molecular orbital distribution
of DCDPA in Figure 1. Density functional theory calculations
for DCDPA were carried out using a suite of the Gaussian 09
computer program[25] to study the orbital distribution of the
DCDPA hole transport material. The nonlocal density func-
tional of Becke’s 3-parameters employing Lee–Yang–Parr func-
tional (B3LYP) with 6-31G∗ basis sets were used for the calcu-
lation. The HOMO was localized on the diphenylamine group
and carbazole, while the LUMO was localized on the phenyl
group attached to the diphenylamine group. The diphenylamine
group affected the HOMO and LUMO distribution of DCDPA,
shifting the energy levels close to the vacuum level due to strong
electron donating properties of the aromatic amine group.
The hole transport properties of DCDPA were compared
with those of mCP by fabricating hole-only devices. Hole-only
device data for DCDPA and mCP are shown in Figure 2. The
device structure of hole-only device was indium tin oxide
(ITO; 50 nm)/N,N′-diphenyl-N,N′-bis-[4-(phenyl-m-tolyl-amino)-
phenyl]-biphenyl-4,4′-diamine (DNTPD, 60 nm)/N,N′-di(1-
naphthyl)-N,N′-diphenylbenzidine(NPB, 5 nm)/mCP or DCDPA
Figure 1. UV-vis absorption and PL spectra (a) of DCDPA and molecular orbital simulation results (b) for DCDPA.
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The DCDPA is better than mCP in terms of both hole injection
and hole transport. The HOMO level of DCDPA is more suit-
able for hole injection than that of mCP due to low energy
barrier for hole injection from the hole injection layer to the
DCDPA in the hole-only device. The hole transport properties
are also improved in the DCDPA because of the diphenylamine
unit, which has strong electron donating character. Therefore,
the DCDPA showed high hole current density and can improve
the driving voltage and power efficiency of PHOLEDs
Red, green, and deep-blue PHOLEDs were fabricated to
compare DCDPA and mCP as the exciton blocking hole trans-
port materials. An iridium(III) bis-(2-phenylquinoline) acety-
lacetonate (Ir(pq)2acac) doped 9,9′-spirobi[fluorene]-2-yl(9,9′-
spirobi[fluorene]-7-yl)methanone (BSFM) layer was used as
the red-emitting layer, while iridium(III) tris(2-phenylpyridine)
(Ir(ppy)3)-doped BSFM was used as the green-emitting layer.
Deep-blue-emitting layer was iridium(III) bis((3,5-difluoro-4-cy-
anophenyl)pyridine) picolinate (FCNIrpic)-doped 9-(3-(9H-car-
bazole-9-yl)phenyl-3,6-bis(diphenylphosphoryl)-9H-carbazole
(CPBDC). The energy level diagram and triplet energy of the
materials are shown in Figure S2 (Supporting Information).
All HOMO levels were measured by cyclic voltametry. Figure 3
3000
2500
2000
1500
1000
500
mCP
DCDPA
0
0
2
4
6
8
Vo l t a g e ( V )
Figure 2. Hole current density of hole-only devices of mCP and DCDPA.
(30 nm)/Al (100 nm). The hole current density of the DCDPA
was much higher than that of the mCP over the entire voltage
range. The hole current density is generally determined by the
energy barrier for hole injection and hole transport properties.
(a)
(b)
30
60
50
40
30
20
10
0
100000
10000
1000
100
mCP/ R
DCDPA/ R
mCP/ G
DCDPA/ G
mCP/ B
DCDPA/ B
25
20
15
10
mCP/ R
mCP/ G
mCP/ B
DCDPA/ R
DCDPA/ G
DCDPA/ B
10
5
1
0.1
0
0.01
0
2000
4000
6000
8000
10000
0
2
4
6
8
Luminance(cd/m2)
Vo l t a g e ( V )
(c)
(d)
90
80
70
60
50
40
30
20
10
0
mCP/R
DCDPA/R
mCP/G
DCDPA/G
mCP/B
DCDPA/B
mCP/ R
mCP/ G
mCP/ B
DCDPA/ R
DCDPA/ G
DCDPA/ B
0
5000
10000
15000
20000
25000
400
500
600
700
800
Luminance(cd/m2)
Wavelength (nm)
Figure 3. Current density–voltage–luminance (a), quantum efficiency–luminance (b), power efficiency–luminance (c), and electroluminescence
(d) data for the red, green, and deep-blue PHOLEDs with DCDPA.
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T a b le 1 . Device performances of DCDPA and mCP devices.
Driving voltagea)
[V]
Maximum quantum efficiency
[%]
Quantum efficiencya)
[%]
Power efficiencya)
[lm W−1
16.4
8.3
]
Red-DCDPA
Red-mCP
5.9
7.7
5.7
7.4
6.9
8.9
19.2
18.5
25.3
25.3
25.8
23.6
17.1
11.3
22.9
18.4
24.2
17.8
Green-DCDPA
Green-mCP
Blue-DCDPA
Blue-mCP
44.5
27.3
17.8
10.8
a)Data measured at 1000 cd m−2
.
shows the current density–voltage and luminance–voltage curves
of the red, green, and blue PHOLEDs. The current density of
the red, green, and deep-blue PHOLEDs was greatly improved
by using DCDPA instead of mCP and the driving voltage at
the same current density was reduced by about 2.0 V. The sig-
nificant increase of the current density is caused by the high
hole current density, as shown in Figure 2. The increased hole
current density contributed to the improvement of the driving
voltage at the same current density. Although the energy bar-
rier for hole injection between the DCDPA and the emitting
layer was higher than that between the mCP and emitting layer,
the good hole transport properties and the low energy barrier
for hole injection between the hole injection layer and DCDPA
increased the current density of the DCDPA devices. The lumi-
nance was also greatly increased by using the DCDPA. The
driving voltage at 1000 cd m−2 was reduced by ≈1.5 V to 2.0 V
depending on the color. The driving voltage of the red PHOLED
was reduced from 7.7 V to 5.9 V and that of the green PHOLED
was lowered from 7.4 V to 5.7 V. The driving voltage of deep-
blue PHOLED was changed from 8.9 V to 6.9 V. Therefore, the
driving voltage was considerably improved by the DCDPA hole-
transport-type exciton blocking layer.
The quantum efficiency–luminance and power efficiency–
luminance curves of the red, green, and blue PHOLEDs are
also shown in Figure 3. The maximum quantum efficien-
cies of the of the red, green, and blue PHOLEDs with mCP
hole-transport-type exciton blocking layer were 18.5%, 25.3%,
and 23.6%, respectively. The maximum quantum efficiencies
of the red, green, and blue PHOLEDs were similar, irrespec-
tive of the hole transport material (Table 1). However, the
quantum efficiency at 1000 cd m−2 was greatly enhanced by
using the DCDPA instead of mCP. The quantum efficiencies
of mCP devices at 1000 cd m−2 were 11.3%, 18.4%, and 17.8%
for red, green, and blue PHOLEDs, while those of DCDPA
devices were 17.1%, 22.9%, and 24.2% under the same con-
ditions. Therefore, the quantum efficiency at high luminance
was improved by the DCDPA hole transport layer. The effi-
ciency roll-off of the DCDPA device was less than 10% up to
1000 cd m−2 compared with about 30% for the mCP device.
The reduced efficiency roll-off can be explained by the effi-
materials and poor hole transport properties of mCP. How-
ever, the efficient hole transport properties of the DCDPA can
increase the hole density at high voltage, balancing holes and
electrons at high voltage and luminance. Therefore, the effi-
ciency roll-off was suppressed in the DCDPA device. In addi-
tion, the triplet energy of the DCDPA was 2.90 eV, which was
high enough for triplet exciton blocking in the red, green, and
deep-blue PHOLEDs, resulting in high quantum efficiency
in the DCDPA-based PHOLEDs. The LUMO level of 2.34 eV
of DCDPA also contributed to the high quantum efficiency
by electron blocking effect. The energy barrier for electron
blocking was high in DCDPA device compared with mCP
device. There was at least 0.21 eV energy barrier for electron
injection from the host to the DCDPA layer (Figure S2, Sup-
porting Information), which blocked the electron leakage to
the hole transport layer.
The power efficiency of DCDPA device was also improved
by the DCDPA device. The power efficiencies of the DCDPA
devices at 1000 cd m−2 were 16.4 lm W−1 , 44.5 lm W−1 ,and
17.8 lm W−1 compared with 8.3 lm W−1 , 27.3 lm W−1 , and
10.8 lm W−1 of mCP for red, green, and blue PHOLEDs.
The power efficiency was doubled in red PHOLEDs and was
enhanced by more than 70% in green and blue PHOLEDs. The
improved power efficiency of the DCDPA device originates
from the low driving voltage and high quantum efficiency. As
explained above, the lowered driving voltage and increased
quantum efficiency of the DCDPA device induced the improve-
ment of the power efficiency.
In conclusion, a novel high-triplet-energy hole transport
material, DCDPA, was effective to improve the driving voltage,
quantum efficiency at high luminance, power efficiency, and
efficiency roll-off of red, green, and deep-blue PHOLEDs. The
DCDPA could be universally used as the hole-transport-type
exciton-blocking layer in all red, green, and deep-blue PHOLEDs
because of the high triplet energy, good hole injection proper-
ties, and electron blocking properties. This approach can be
useful for further development of high-triplet-energy hole trans-
port materials for red, green, and deep-blue PHOLEDs.
cient hole transport properties of the DCDPA hole transport Experimental Section
material. The efficiency roll-off is induced by the unbalanced
Synthesis: Detailed synthetic procedure and general analysis of the
material are described in the Supporting Information.
Device Fabrication and Measurements: The basic device structure of
PHOLEDs was ITO(50 nm)/DNTPD (60 nm)/NPB (5 nm) or poly(3,4-eth
charge density at high driving voltage. The degree of increase
of electron density is higher than that of hole density at high
voltage because of electron transport properties of the host
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ylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) (60 nm)/mCP
or DCDPA (30 nm)/host:dopant (30 nm, 3%)/diphenylphosphine oxide-
4-(triphenylsilyl)phenyl (25 nm)/LiF (1 nm)/Al (200 nm). DNTPD/NPB
was the hole injection layer for red and green devices, while PEDOT:PSS
was the hole injection layer for blue devices. The red-emitting material
was Ir(pq)2acac-doped BSFM, while the green-emtting material was
Ir(ppy)3-doped BSFM. FCNIrpic-doped CPBDC was the blue-emitting
material. All devices were encapsulated with a glass lid and a CaO getter
after device fabrication. Hole-only devices with a device structure of
ITO/DNTPD/NPB/mCP or DCDPA/Au were fabricated to compare hole
current density of mCP and DCDPA. The device performances of the red,
green, and blue PHOLEDs were measured with a Keithley 2400 source
measurement unit and CS1000 spectroradiometer after encapsulation.
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