High-efficiency deep-blue-phosphorescent organic light-emitting diodes using a phosphine oxide and a phosphine sulfide high-triplet-energy host material with bipolar charge-transport properties

Article abstract of DOI:10.1002/adma.200903321

Highly efficient deep-blue-phosphorescent organic light-emitting diodes have been developed using bipolar-type high-tripletenergy host materials with a carbazole core and two phosphine oxide (PPO21) or phosphine sulfide (PPS21) units. A high quantum effic

Full text of DOI:10.1002/adma.200903321

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High-Efficiency Deep-Blue-Phosphorescent Organic
Light-Emitting Diodes Using a Phosphine Oxide and a
Phosphine Sulfide High-Triplet-Energy Host Material
with Bipolar Charge-Transport Properties
By Soon Ok Jeon, Kyoung Soo Yook, Chul Woong Joo, and Jun Yeob Lee*
Blue-phosphorescent organic light-emitting diodes (PHOLEDs)
have been developed for more than 10 years towards use in
active-matrix-type organic light-emitting diodes. There has been
much improvement in quantum efficiency, lifetime, and color
purity although the device performances of the blue PHOLEDs
are not yet good enough for practical applications.
core structure. Phosphine oxide (PPO21) and phosphine sulfide
(PPS21) host materials with the carbazole core structure were
synthesized and evaluated as host materials in the deep-blue
PHOLEDs. A theoretical maximum quantum efficiency over 19%
with a deep-blue CIE coordinate of (0.14,0.16) was demonstrated
in the deep-blue PHOLEDs using the high-triplet-energy host
materials for the first time.
Most research into blue PHOLEDs was focused on the
development of new host and dopant materials.[1–14] The
best-known host material in the blue PHOLEDs is
N,N-dicarbazolyl-3,5-benzene (mCP).[1] It has good hole-transport
properties due to a carbazole unit in the backbone structure and a
wide triplet bandgap of 2.90 eV for efficient energy transfer.
However, its electron injection and transport properties are poor
because of the high energy of the lowest unoccupied molecular
orbital (LUMO) of 2.4 eV. Silicone-based wide-triplet-bandgap
host materials were also developed[2–5] and tetraaryl-based silane
materials have been used as host materials in blue PHOLEDs.[2,3]
However, the energy of the highest occupied molecular orbital
(HOMO) of the silane-based host materials is around 7.0 eV,
which is not suitable for hole injection. Therefore, it was difficult
to balance holes and electrons in the light-emitting layer. To
overcome the poor hole injection in the silane-based host
materials, silane compounds with a carbazole moiety in the
molecular structure were evaluated as triplet host materials in
blue PHOLEDs.[4,5] However, the carbazole-based host materials
showed strong hole-transport properties and bipolar transport
behavior was not observed. In addition, phosphine oxide-type
host materials were synthesized, but only sky-blue PHOLEDs
were reported due to the low triplet energy.[6,7] Our group also
reported phosphine oxide-type host materials with a carbazole
moiety in the backbone structure and high efficiency could be
obtained.[14]
The host materials synthesized in this work have
a
9-phenylcarbazole core structure with two phosphine oxide or
phosphine sulfide units. One diphenylphosphine oxide or sulfide
unit was attached to the 3-position of the carbazole unit to control
the HOMO level and the charge transport properties. The other
diphenylphosphine oxide or sulfide unit was connected to the
phenyl group of the 9-phenylcarbazole to manage the electro-
n-transport properties.
The host materials were synthesized by the coupling reaction
of the chlorodiphenylphosphine with 3-bromo-9-(4-bromophenyl)-
carbazole using n-butyllithium followed by oxidation and
sulfonation (Scheme 1). The product was purified by a column
chromatography and it was confirmed with 1H NMR spectro-
scopy, differential scanning calorimetry (DSC), high performance
liquid chromatography (HPLC) and mass spectrometry (MS). The
purity of the host materials was over 99% from HPLC. Physical
properties of the host materials are summarized in Table 1..
The two high-triplet-energy host materials showed high
glass-transition temperature (Tg) above 110 8C due to the two
rigid diphenylphosphine oxide or sulfide groups. The signifi-
cantly higher Tg of the PPS21 compared to PPO21 is due to large
atom size of the sulfur. The two diphenylphosphine oxide or
sulfide units also stabilized the amorphous morphology of the
host materials, and a smooth surface roughness less than 1 nm
was obtained from the evaporated film. The surface morphology
of the evaporated films was kept stable even after thermal
treatment at 80 8C for 1 h because of the rigid molecular structure
and corresponding high Tg.
The HOMO and LUMO levels of the host materials were
measured using cyclic voltametry (CV), and they are summarized
in Table 1. The HOMO/LUMO levels of the PPO21 and PPS21 are
mainly determined by the carbazole backbone structure, and the
diphenylphosphine oxide or sulfide groups shift the HOMO/
LUMO levels through the control of the electron density in the
carbazole core. The HOMO level of the PPO21 was 6.25 eV, which
corresponds to a change of 0.37 eV compared with that of the
phenylcarbazole moiety without any substituent, 5.88 eV.[15] The
LUMO level (2.68 eV) of the PPO21 was also shifted by 0.30 eV by
the diphenylphosphine oxide unit. The electron withdrawing
Although several classes of host materials have been
synthesized, no host materials could show
a theoretical
maximum quantum efficiency in the deep-blue PHOLED with
Commission International De L’Eclairage (CIE) color coordinate
(x þ y) values below 0.30. In this work, we synthesized
bipolar-type high-triplet-energy host materials with a carbazole
[*] Prof. J. Y. Lee, S. O. Jeon, K. S. Yook, C. W. Joo
Department of Polymer Science and Engineering
Dankook University
126, Jukjeon-dong, Suji-gu, Yongin, Gyeonggi, 448-701 (Korea)
E-mail: leej17@dankook.ac.kr
DOI: 10.1002/adma.200903321
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Br
N
Br
N
PPO21 and PPS21 was localized in the
carbazole backbone structure and the phos-
phine oxide and sulfide unit linked to the
carbazole unit also contributed to the HOMO.
Compared with the HOMO localized in the
carbazole core, the LUMO was mostly dis-
tributed over the diphenylphosphine oxide- and
sulfide-substituted phenyl unit attached to the
carbazole core structure. This indicates that the
HOMO level of the PPO21 and PPS21 is
mainly determined by the carbazole backbone
structure, while the LUMO level is mostly
affected by the diphenylphosphine oxide- or
sulfide-substituted phenyl group attached to
the carbazole core. In other words, the
diphenylphosphine oxide or sulfide group
linked to the carbazole unit controls the
HOMO energy level, while the second diphe-
nylphosphine oxide or sulfide group connected
to the phenyl group influences the LUMO
energy level of PPO21. In addition, the LUMO
distribution in the diphenylphosphine oxide- or
P
H
Br
Br
Toluene
N
NBS
n-BuLi
N
Br
1
2
P
3
P
P
P
S
O
Sulfur
CH Cl
H O
2
2
CH Cl
2
2
2
2
N
N
N
S
O
P
P
P
PPO21
3
PPS21
Scheme 1. Synthetic scheme of the PPO21 and PPS21.
character of the diphenylphosphine oxide shifted the HOMO/
LUMO levels far away from the vacuum level. This also indicates
that the electron-transport properties of the PPO21 can be
improved compared with those of the carbazole because of the
highly polar and electron-withdrawing phosphine oxide group.
Compared with the PPO21, the HOMO/LUMO-level shift in
PPS21 was less significant and HOMO/LUMO levels of 6.14 eV/
2.58 eV were obtained. The difference of the energy-level shift
depending on the pendant group attached to the carbazole core
structure can be explained by the polarity of the phosphine oxide
and phosphine sulfide groups. It was reported that the polarity of
the P ¼ X unit is increased in an order of O > S.[16] Therefore, the
highly polar P ¼ O group withdraws more electrons from the
carbazole core, resulting in the large energy-level shift. The
bandgap was not affected by the substituent of the carbazole core.
The triplet energy of the two materials was almost the same
because the triplet energy is determined by the carbazole core
(3.02 eV). The diphenylphosphine-based groups did not change
the triplet energy level of the carbazole core as the conjugation of
the carbazole core could not be extended further. The triplet
energy of the host materials was wider than that of the
sulfide-substituted phenyl group implies that the electron-
transport properties of the PPO21 and PPS21 can be improved
compared with those of the carbazole because of the highly polar
and electron-withdrawing phosphine oxide or sulfide group. In
particular, the LUMO mostly depends on the diphenylphospine
oxide- or sulfide-substituted phenyl group, without significant
contribution from the carbazole group and its strong hole-
transport properties, leading to an appropriate LUMO energy for
electron injection. It is expected that the PPO21 shows better
electron-transport properties than the PPS21 because the LUMO
orbital distribution is localized in the diphenylphosphine
oxide-substituted phenyl group.
The host materials showed a main absorption peak of the
carbazole core at around 295 nm and 338 nm. As the diphenyl-
phosphine oxide and diphenylphosphine sulfide did not affect the
ultraviolet-visible (UV–vis) absorption of the host materials,
similar UV–vis absorption peaks were observed. A photolumi-
nescence (PL) emission peak of the host materials was observed
between 361 and 367 nm from emission of the carbazole core.[15]
The PL emission peak of the host materials was well overlapped
with the metal-to-ligand charge-transfer absorption peak of the
FCNIr around 420 nm, indicating efficient energy transfer from
the host materials to the FCNIr.
tris[(3,5-difluoro-4-cyanophenyl)pyridine]
iridium
(FCNIr)
dopant with a triplet bandgap of 2.80 eV,[8,9] and efficient energy
transfer from the host materials to the FCNIr is expected.
Molecular simulation of the PPO21 and PPS21 was carried out
to study the HOMO/LUMO levels of the host materials in
molecular scale and is shown in Figure 1. The HOMO of the
The phosphine oxide and phosphine sulfid-type materials are
suitable host materials for deep-blue PHOLEDs due to their wide
triplet bandgap over 3.00 eVand their properties as host materials
were evaluated.
Table 1. Physical properties of the PPO21 and PPS21.
Optical Analysis
Thermal Analysis
Tg [8C] Tm[a] [8C]
Electrical Analysis
Absortion [nm]
Emission [nm]
HOMO [eV]
LUMO [eV]
Bandgap [eV]
ET[b] [eV]
PPO21
PPS21
294, 338
296, 338
361
366
111
120
–
6.25
6.14
2.68
2.58
3.57
3.56
3.01
3.00
254
[a] melting temperature.[b] triplet energy.
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HOMO level and the hole mobility of PPS21 compared with
those of the PPO21. The HOMO energy level of the PPS21 is
0.11 eV higherꢁt5han that of the PPO21 and hole mobility of the
PPS21 (3ꢀ 10 cm2 Vꢁ1
s
ꢁ1) is higher than that of the PPO21
(9 ꢀ 10ꢁ6 cm2 Vꢁ1 sꢁ1), leading to a better hole injection. Similarly,
the electron-current density was high in the PPO21 due to the
appropriate LUMO level for better electron injection and high
electron mobility. The electron mobility of the PPO21 was
3ꢀ 10ꢁ6 cm2 Vꢁ1 sꢁ1 compared with 5 ꢀ 10ꢁ7 cm2 Vꢁ1 sꢁ1 of the
PPS21. There was no large difference of the hole- and
electron-current density in the PPO21, but the hole-current
density was much higher than the electron-current density in
PPS21. Therefore, it can be expected that holes and electrons can
be balanced in the PPO21. These results indicate that the PPO21
shows bipolar charge-transport properties.
The device performances of the PPO21 and PPS21 blue
PHOLEDs are shown in Figure 3. The current density was high in
the PPS21 device, while the luminance was high in the PPO21
device. Although the current density was high in the PPS21
device, due to efficient hole injection, the poor electron injection
in the PPS21 device decreased the luminance of the PPS21
device. Therefore, the quantum efficiency, which is determined
by the current density and the luminance was high in the PPO21
device, while the quantum efficiency was relatively low in the
PPS21 device. The holes and electrons were balanced in the
PPO21 device, while holes dominated the electrons in the PPS21
device, as shown in Figure 2. The quantum efficiency of the
PPO21 device was over 50% higher than that of the PPS21 device.
A maximum quantum efficiency of 19.2% was achieved in the
PPO21 device while the quantum efficiency of the PPO21 device
was 15.2% at 100 cd mꢁ2 and 12.1% at 1000 cd mꢁ2 with a CIE
color coordinate of (0.14,0.16). The quantum efficiency of the
PPO21 device was higher than the theoretical maximum
quantum efficiency 19% and this value is better than any other
data reported in the literature.[8,10,11,14] The problem of the rapid
efficiency roll-off in the deep-blue device was greatly improved in
the deep-blue PHOLEDs with the PPO21 host, and the high
quantum efficiency was achieved even at high luminance. The
maximum current efficiency was 22.5 cd Aꢁ1 and the maximum
power efficiency was 19 lm Wꢁ1 in the PPO21 device. The
current efficiency and power efficiency of the PPO21 device were
18.1 cd Aꢁ1 and 9.0 lm Wꢁ1 at 100 cd mꢁ2. The balanced charge
injection originated from the HOMO/LUMO level management,
using the electron-withdrawing phosphine oxide group in
Figure 1. Molecular-simulation results of PPO21 and PPS21. HOMO and
LUMO distributions are shown in this figure.
A basic device configuration was: indium tin oxide (ITO;
150 nm)/N,N0-diphenyl-N,N0-bis-[4-(phenyl-m-tolyl-amino)-phenyl]-
biphenyl-4,40-diamine (DNTPD; 60 nm)/ N,N0-di(1-naphthyl)- N,N0-
diphenylbenzidine (NPB; 10 nm)/N,N0-dicarbazolyl-3,5-benzene
(mCP; 20nm)/host:FCNIr (30 nm; 15%)/4,7-diphenyl-1,10-
phenanthroline (Bphen; 25 nm)/LiF (1 nm)/Al (200 nm). The
doping concentration of the FCNIr was 15% and two host
materials were used as the high-triplet-energy host in the
emitting layer.
Hole-only and electron-only devices of the PPO21 and PPS21
were fabricated to compare the hole and electron density in the
emitting layer. Device structures of the hole-only and electron-
only devices were ITO/DNTPD (60 nm)/NPB (10 nm)/mCP
(20 nm)/host (30 nm)/Bphen (25 nm)/DNTPD
(10 nm)/Al and ITO/2,9-dimethyl-4,7-diphenyl-
1,10-phenanthroline (10 nm)/DNTPD (60 nm)/
NPB (10 nm)/mCP (20 nm)/host (30 nm)/
Bphen (25 nm)/LiF/Al, respectively. Figure 2
shows the current-density–voltage curves of the
PPO21 and PPS21 hole- and electron-only
devices. The hole-current density was high in
the PPS21, while the electron-current density
was high in the PPO21. This indicates that the
PPO21 shows better performance than PPS21
in terms of electron injection, while PPS21
performs better than the PPO21 in terms of
hole injection. The high hole-current density in
PPS21 can be explained by the energy of the
Figure 2. Current-density–voltage curves of the hole only (a) and electron only (b) devices of
PPO21 and PPS21.
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the deep-blue PHOLEDs. The vibrational peak was intensified in
the PPS21 device because of the difference of the recombination
zone.[17] The recombination zone is shifted from the hole-
transport-layer side to the electron-transport-layer side in the
PPS21 device, inducing an optical effect in the device
accompanied by an increase of the vibrational peak in the
PPS21 device.
In conclusion, a high-quantum-efficiency deep-blue PHOLED
with CIE coordinates of (0.14,0.16) was developed using novel
high-triplet-energy bipolar host materials with a carbazole and
two phosphine oxide or sulfide units. The phosphine oxide was
more effective than the phosphine sulfide for the electron
injection, and a quantum efficiency of 19.2% corresponding to
the maximum theoretical quantum efficiency of the PHOLEDs
was achieved in the deep-blue PHOLEDs with the PPO21
high-triplet-energy host material for the first time. The develop-
ment of the bipolar host material, PPO21, was critical to the high
quantum efficiency due to charge balance in the light-emitting
layer. 100% internal quantum efficiency was obtained and it is
expected that further development of bipolar host materials may
enable the development of a deep-blue PHOLEDs with an
external quantum efficiency over 20%.
Experimental
Synthesis of 3-Bromo-9-(4-bromophenyl)-9-carbazole (2): To a solution of
1 (1.00 g, 3.1 mmol; see Scheme 1)[18] in N,N-dimethylformamide, a
solution of N-bromosuccinimide (0.73 g, 3.1 mmol) was added dropwisely
at 0 8C. The mixture was then stirred overnight at room temperature. The
solution was diluted with water (20 mL) and extracted with dichloro-
methane. After solvent removal, the crude product was purified by
recrystallization with an ethyl alcohol and gave white solids.
Synthesis of 3-(diphenylphosphino)-9-(4-(diphenylphosphino)phenyl)-
9-carbazole (3): Into a 100 mL two-neck flask was placed 2 (2.00 g,
4.98 mmol) in THF (30 mL). The reaction flask was cooled to ꢁ78 8C and
n-butyl lithium (10 M in hexane, 1.14 mL) was slowly added dropwise. The
whole solution was stirred at this temperature for 3 h, followed by the
addition of chlorodiphenylphophine (2.53 g, 11.4 mmol) under argon
atmosphere. The resulting mixture was gradually warmed to ambient
temperature, quenched with methanol (10 mL), and extracted with
dichloromethane. The combined organic layers were dried over magne-
sium sulfate, filtered, and evaporated under reduced pressure. The white
powdery product was obtained to 1.45 g (32%).
Figure 3. Current-density–luminance–voltage (a), quantum-efficiency–luminance
(b), and electroluminescence (c) curves of the deep-blue PHOLEDs with
PPO21 and PPS21 host materials.
Synthesis of 3-(diphenylphosphoryl)-9-(4-(diphenylphosphoryl)phenyl)-
9-carbazole (PPO21): A mixture of 3 (1.45 g, 2.37 mmol), dichloromethane
(50 mL), and hydrogen peroxide (10 mL) was stirred overnight at room
temperature. The organic layer was separated and washed with
dichloromethane and distilled water. The extract was dried in vacuum,
affording a white solid of PPO21 in 97% yield. Tg ¼ 111 8C. 1H NMR
(200 MHz, CDCl3, d): 8.59–8.53 (d, 1H), 8.11–7.97 (d, 3H), 7.93–7.71 (m,
10H), 7.67 (m, 3H), 7.60–7.45 (m, 13H), 7.36–7.26 (m, 2H). Fast-atom
bombardment (FAB) MS m/z: 643 [M þ 1]þ.
PPO21, and resulted in the high quantum efficiency. Therefore,
the PPO21 is a promising host material for the deep-blue
PHOLEDs. However, the lifetime of the PPO21 device was only a
few hours at 100 cd mꢁ2 and further development of the
charge-transport materials as well as the host and dopant
materials may increase the lifetime of the deep-blue PHOLEDs.
Further optimization of the PPO21 deep-blue devices will follow
to enhance the lifetime of the device using various high-
triplet-energy charge transport materials.
Electroluminescence spectra of the PPO21 and PPS21
deep-blue PHOLEDs are also shown in Figure 3. PPO21 and
PPS21 devices showed a strong peak at 452 nm assigned to FCNIr
emission in addition to a vibrational peak at 482 nm. The
vibration peak at 482 nm was suppressed in the PPO21 device and
it showed a CIE value of (0.14,0.16). In PPS21 device, the strong
vibrational peak at 482 nm degraded the color performances of
Synthesis of 3-(diphenylphosphorothioyl)-9-(4-(diphenylphosphorothioyl)-
phenyl)-9H-carbazole (PPS21): To a dichloromethane solution of 3 (1g,
1.63 mmol) was added sulfur (0.12g, 3.92 mmol) at room temperature for
6 h. After the addition of water, extraction with dichloromethane and
evaporation of the solvent gave a white powder of PPS21 97% yield.
1
Tg ¼ 120 8C. H NMR (200 MHz, CDCl3, d): 8.63–8.56 (d, 1H), 8.11–8.07
(d, 3H), 8.01–7.94 (m, 4H), 7.90–7.63 (m, 7H), 7.53–7.45 (m, 13H),
7.36–7.32 (m, 3H). FAB MS m/z: 675 [M þ 1]þ.
Measurements: 1H NMR spectra were obtained on a Varian 200
(200 MHz) spectrometer. PL spectra were recorded on a fluorescence
spectrophotometer (HITACHI, F-7000) and UV–vis spectra were obtained
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using a UV–vis spectrophotometer (Shimadzu, UV-2501PC). The DSC
measurements were carried out on a Mettler DSC 822 instrument under
nitrogen at a heating rate of 10 8C minꢁ1. Low- and high-resolution mass
spectra were recorded on a JEOL JMS-AX505WA spectrometer in FAB
mode. The energy levels of the materials were determined by cyclo
voltammetry (CV) and UV–vis absorption, and the device performances of
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