New host material for high-performance blue phosphorescent organic electroluminescent devices

Article abstract of DOI:10.1002/adma.201200972

A new carbazole derivative with two carbazole moieties on the C3 and C6 positions of carbazole and triphenylsilane directly linked to the N of carbazole is successfully used as a highly efficient blue phosphorescent host in an organic light-emitting diode (PhOLED). The blue PhOLED, with this new host, gives an extremely high external quantum efficiency. Copyright

Full text of DOI:10.1002/adma.201200972

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New Host Material for High-Performance Blue
Phosphorescent Organic Electroluminescent Devices
Jong-Kwan Bin, Nam-Sung Cho, and Jong-In Hong*
In recent years, a considerable amount of attention has been
paid toward the development of organic emissive materials
for potential applications in full-color flat-panel displays and
lighting sources. A number of red and green light-emitting
materials have already been identified for use in phosphores-
cent organic light-emitting diodes (PhOLEDs). PhOLEDs based
on these materials are characterized by high luminous effi-
ciency, reasonable color purity, and long lifetimes. However, the
development of blue phosphorescent OLEDs with high elec-
troluminescence (EL) efficiency remains a challenge because a
high bandgap energy and high triplet energy are required for
blue emitters. To avoid reverse energy transfer from the guest
to the host and to confine triplet excitons in the emitting layer,
the singlet and triplet levels of the host should be higher than
those of the phosphorescent dopants. This is still a rather chal-
lenging task for material researchers. Additionally, the large
triplet bandgap materials have to have high thermal, electro-
chemical, and morphological stability to improve operational
stability and maintain charge transport in devices; they must
also have sufficiently high singlet and triplet energy levels to
confine them both (e.g., ≥2.7 eV for Firpic) in the emitting
zone.[1–8] 1,3-Bis(carbazolyl-9-yl)benzene (mCP) has been widely
used as a host material in blue PhOLEDs owing to its highly
efficient hole transport properties and a wide triplet bandgap
of 2.90 eV for efficient energy transfer. However, its relatively
low thermal and morphological stability may hinder its appli-
cation as a host in a blue PhOLED (Tg = 65 °C). Silicon-based
wide triplet bandgap materials have been used for highly effi-
cient blue PhOLEDs. However, the highest occupied molecular
orbital (HOMO) level of UGHs is around 7.0 eV.4 Such a deep
HOMO level makes it difficult to balance holes and electrons in
the emitting zone. To improve the morphological stability, Chen
and Wang independently reported an mCP derivative with a
silane moiety, 3,5-di(N-carbazolyl)tetraphenylsilane (SimCP)
(Tg = 101 °C).[9,10] An OLED device fabricated using SimCP as a
host, exhibited a maximum power efficiency of 11.9 lm/W and
an external quantum efficiency of 14.4% when using the Firpic
dopant (iridium(III)bis[(4,6-di-fluorophenyl)-pyridinato-N,C2′]
picolinate). The higher performance of a SimCP-based device,
compared to an mCP-based device (12.3%, 9.3 lm/W), was
attributed to a hindering effect on the packing of the molecular
condensed phase that occurs because of the bulky and rigid
structure of SimCP, thus alleviating T1–T1 annihilation.[10]
In addition, a new electrochemically stable host material for
blue PhOLEDs, 9-(4-tert-butylphenyl)-3,6-bis(triphenylsilyl)-9H-
carbazole (CzSi), was synthesized by directly linking two triphe-
nylsilyl moieties to the C3 and C6 positions of carbazole, which
are electrochemically active sites.[11] The device using CzSi as
a host achieved a maximum external quantum efficiency of
15.7%, corresponding to a current efficiency of 30.6 cd/A and a
power efficiency of 26.7 lm/W.[11] Other than these, there have
been many other reports on the development of blue PhOLEDs,
thus demonstrating their significance. However, despite con-
stant efforts to accomplish high device efficiency, the develop-
ment of blue PhOLEDs with high electroluminance (EL) effi-
ciency remains a challenge because of difficulties in obtaining
materials with a wide triplet bandgap, the proper energy levels,
and mobility for balanced charge injection.[12,13]
Herein, we describe the design and synthesis of a new car-
bazole derivative that has two carbazole moieties on the C3 and
C6 positions of carbazole and a triphenylsilane moiety directly
linked to the N of the carbazole. This new carbazole derivative
is expected to have improved thermal properties, while main-
taining its advantageous electronic and optical characteristics.
The non-planar molecular structure of a large gap and the non-
conjugated triphenylsilane end-capping group would prevent
close packing of the molecules in the solid state and enable the
formation of smooth and pinhole-free thin films.[14]
Scheme 1 (in the Supporting Information (SI)) shows the
synthetic route to 9’-triphenylsilanyl-9′H-[9,3′,6′,9″]tercarbazole
(SitCz). Specifically, the N-unsubstituted carbazole (CzG2on)[15]
is treated with 2.5 M n-BuLi at −78 °C to give a lithiated inter-
mediate, which is subsequently quenched with chlorotriphenyl-
silane to give the desired SitCz in 76% yield after column chro-
matography. The structure was confirmed by 1H NMR spectros-
copy, 13C NMR spectroscopy, and mass spectrometry (MS).
The thermal behavior of SitCz was evaluated by means of
(a) differential scanning calorimetry (DSC) and (b) thermo-
gravimetric analysis (TGA) under a nitrogen atmosphere. A
5% weight loss was observed at 402 °C (see SI, Figure S1(b)).
The melting point (Tm) of SitCz was found to be 346 °C and the
glass transition temperature (Tg) was 168 °C (SI, Figure S1). In
general in OLEDs, amorphous thin films that possess high Tg
values are less vulnerable to heat. This results in more stable
device performances owing to the retention of the film mor-
phology during the operation of the devices.[16] SitCz is there-
fore more thermally stable than mCP or SimCP. To confirm the
morphological stability against thermal stress, we also carried
J.-K. Bin, Prof. J.-I. Hong
Department of Chemistry
College of Natural Sciences
Seoul National University
Seoul 151-747, Korea
E-mail: jihong@snu.ac.kr
Dr. N.-S. Cho
LG Display R&D Center
1007 Deogeun-ri, Wollong-myeon, Paju-si, Gyeonggi-do 413-811, Korea
DOI: 10.1002/adma.201200972
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DOI: 10.1002/adma.201200972
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out an atomic force microscopy (AFM) study of a vacuum-
deposited SitCz film (see SI, Figure S1(c)). These data indicate
that SitCz is stable enough to endure the high temperature at
which vacuum vapor deposition is carried out.
of TCz1.[17] In general, a higher triplet energy of the host mate-
rial would facilitate energy transfer from host to dopant in the
host-guest system. Thus, we would expect that SitCz would pro-
vide effective confinement of the triplet excitons on the guest
((Firpic); ET = 2.62 eV) and provide the consequential preven-
tion of back energy transfer from the dopant (Firpic) to the host
(SitCz) material. From these results, we postulate that SitCz
can be considered a promising host for a blue phosphorescent
OLED.[18]
To determine the optimized geometry and the frontier
molecular orbital levels, density functional theory (DFT) cal-
culations were carried out at the B3LYP/6-31G∗ using Spartan
‘08. As shown in SI, Figure S2, neither the HOMO nor the
lowest unoccupied molecular orbital (LUMO) is localized on
the triphenylsilane benzene rings, suggesting that the introduc-
tion of the silyl group on CzG2on would not result in a sig-
nificant change in the HOMO or LUMO levels. The triphenyl-
silane group therefore serves as an effective spacer and blocks
the π-conjugation of the carbazole core from extending to the
peripheral substitution. The calculated HOMO and LUMO
levels of SitCz were −5.11 and -1.04 eV, respectively.
The electrochemical properties of SitCz were characterized
by cyclic voltammetry. The oxidation scans were carried out in a
0.1 M solution of tetraethylammonium tetrafluoroborate in
anhydrous dichloromethane. A platinum disk and an Ag/AgCl
disk were used as the working and reference electrodes, respec-
tively. The platinum disk was used as the counter electrode.
Based on the HOMO energy of the ferrocene/ferrocenium redox
system (-4.8 eV), we were able to calculate the HOMO energy
values of SitCz, which was equal to -5.54 eV (see SI, Figure S3).
Figure 1 shows the room-temperature UV-vis absorption and
PL spectra dissolved in dichloromethane and low-temperature
PL spectra of SitCz in a 2-methyltetrahydrofuran (2-MeTHF)
solution at 77 K. The absorption spectra of SitCz are nearly
similar to those of the unsubstituted carbazole monomer. The
bandgap energies (3.24 eV) were measured from the absorption
spectra, and the LUMO energies were estimated to be −2.30 eV
for SitCz (ELUMO (eV) = -(EHOMO - Egopt)). The phosphorescence
of SitCz was obtained in a frozen 2-methyltetrahydrofuran solu-
tion at 77 K. The highest energy phosphorescent emission peak
was located at about 414 nm, as the transition energy of T1 →
S0 is estimated to be as high as 3.00 eV, which is similar to that
We fabricated two types of multilayer structures: ITO/
HATCN (50 Å)/NPB (550 Å)/TAPC (100 Å)/mCP: 10 wt%
Firpic (250 Å)/TmPyPB (300 Å)/LiF (5 Å)/Al (1000 Å) (Device
I) and ITO/HATCN (50 Å)/NPB (550 Å)/TAPC (100 Å)/SitCz:
10 wt% Firpic (250 Å)/TmPyPB (300 Å)/LiF (5 Å)/Al (1000 Å)
(Device II). We used strongly electron-deficient and disk-shaped
depyrazino[2,3-f:2′,3′-h]quinoxaline-2,3,6,7,10,11-hexacarboni-
trile (HATCN) as the hole-injection layer (HIL).[19–22] We used
N,N′-bis(naphthalene-1-yl)-N,N’-bis(phenyl)-benzidine (NPB)
as the hole-transport layer (HTL) and di-[4-(N,N-ditoly-amino)-
phenyl]cyclohexane (TAPC) as the electron blocking layer.[23]
TAPC has a HOMO energy level of −5.5 eV and a LUMO energy
level of -2.0 eV.[24] Additionally, TAPC has a higher ET (2.87 eV)
than the typical blue phosphorescent guest material, Firpic.[18]
mCP or SitCz with a nearly optimal concentration (10 wt%)
of Firpic was used as the emitting layer. 1,3,5-tri[(3-pyridyl)-
phen-3-yl]benzene (TmPyPB) was used as the electron-transport
layers (ETL), and LiF as the electron-injection layer (EIL) at the
LiF/Al cathode interface. TmPyPB (HOMO = -6.68 eV, LUMO =
−2.73 eV, and T1 = 2.78 eV) has a wider triplet energy level than
the guest (Firpic) and thus effectively confines triplet excitons
inside the emitting zone. Additionally, TmPyPB has good elec-
tron injection and transport properties because of the three
pyridine-containing triphenylbezne; its electron mobility is
1.0 × 10−3 cm2/V·s at 6.4 × 105 V/cm. This high electron mobility
is necessary to obtain a high-efficiency blue PhOLED.[25,26]
Figure 2 shows the EL spectra of the 10% doped device driven
at 10 mA/cm2. Both devices, in which mCP and SitCz were used
as the blue phosphorescent hosts, gave similar EL spectra. The
1.2
UV
PL-298 K
PL-77 K
Device I
Device II
1.0
1.0
0.8
0.8
0.6
0.4
0.2
0.0
Si
N
0.6
N
N
0.4
0.2
0.0
SitCz
300
350
400
450
500
300
400
500
600
700
800
Wavelength (nm)
Wavelength (nm)
Figure 1. UV-vis absorption and PL spectra of SitCz. Room temperature
spectra were taken in dichloromethane and 77 K emission spectrum
(open circle) in 2-MeTHF frozen glass.
Figure 2. EL spectra of devices I and II along with the structures of
SitCz.
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T a b le 1 . Summary of the characteristics of devices I and II with mCP and SitCz as hosts.
Voltage
[V]
Current Efficiency
[cd/A]
Power Efficiency
[lm/W]
External Quantum
Efficiency [%]
CIE Coordinate
(x,y)
Brightness
[cd/m2]
Device I (mCP)
Device II (SitCz)
At 100 cd/m2/ At
1000 cd/m2
3.6a)/ 4.6/ 5.8
32.7/ 31.6
22.3/ 17.2
17.9/ 17.4
(0.14, 0.33)/
(0.14, 0.32)
4986 (at 10.2 V)
At 100 cd/m2/ At
1000 cd/m2
2.9a)/ 3.9/ 5.0
46.3/ 44.0
36.5/ 27.2
27.2/ 25.6
(0.13, 0.29)/
(0.13, 0.29)
10514 (at 8.4 V)
a)The applied voltage (Von) required for brightness of 1 cd/m2.
EL spectra of the mCP- and SitCz-based devices show identical
patterns to Firpic emission without any emission from the host
and/or adjacent layer. The EL results suggest that the complete
energy transfer from mCP or SitCz (host) to Firpic (dopant)
takes place upon electrical excitation, and charge carriers and
excitons are perfectly confined in the emitting zone. Generally,
Firpic is a light-blue emitter (about 450–500 nm) with a typical
maximum emission peak around 472 nm and a vibrational peak
at 500 nm, with a CIE (x,y) around (0.17, 0.34).[27,28,11] However,
the EL spectra of the Firpic-doped blue phosphorescent devices
I and II developed here show more blue-shifted emission with
a CIE (x,y) around (0.14, 0.33) and (0. 13, 0.29) at 100 cd/m2,
respectively (Table 1). In the mCP device, the strong vibrational
peak at 500 nm degraded the color performance of the Firpic-
doped device, whereas the vibrational peak at 500 nm was
relatively more suppressed in the SitCz device. The vibrational
peak was intensified in the mCP device because of the shifted
recombination zone from the HTL side to the ETL side.[29]
Figure 3 shows the EL performances of devices I and II, and
their EL efficiencies are summarized in Table 1. In particular,
device I shows a turn-on voltage of 3.6 V at a luminance of
1 cd/m2, a current efficiency of 32.7 cd/A, a power efficiency
of 22.3 lm/W, and an external quantum efficiency of 17.9%
at a corresponding brightness of 100 cd/m2 with CIE coordi-
nates of (0.14, 0.33). The performance using an mCP host for a
blue phosphorescent OLED is comparable with the previously
reported values. Device II, on the other hand, showed a better
performance with a very low turn-on voltage of 2.9 V, a current
efficiency of 46.3 cd/A, a power efficiency of 36.5 lm/W, and an
external quantum efficiency of 27.2% at a corresponding bright-
ness of 100 cd/m2 with CIE coordinates of (0.13, 0.29). The per-
formance is the highest values for oligomer carbazole derivatives
with Firpic-based devices.[17,30–32] To further verify the difference
between the EL performances of devices I and II, their bandgap
energies were examined. Figure 4 shows the HOMO and
LUMO energy levels of the organic materials used in each layer.
This energy diagram reveals that the energy barrier (0.60 eV)
between the HOMO of NPB (5.30 eV) and the HOMO of mCP
(5.90 eV) in device I is larger than the difference (0.24 eV)
between the HOMO of NPB and the HOMO of SitCz (5.54 eV)
in device II. On the other hand, the energy barrier (0.43 eV)
from the LUMO of TmPyPB (2.73 eV) to the LUMO of SitCz
(2.30 eV) is slightly larger than the barrier (0.33 eV) for electron
injection from the LUMO of TmPyPB to the LUMO of mCP.
Since mCP is characterized by a lower HOMO than SitCz, hole
transportation from the HTL to the 10% doped emission layer is
more difficult to achieve in device I. In addition, this difference
a)
Device I
Device II
12000
120
100
80
60
40
20
0
10000
8000
6000
4000
2000
0
0
2
4
6
8
10
12
Voltage (V)
0
2
4
6
8
10
Voltage (V)
50
40
30
20
10
0
(b)
50
Power Effi. for Device I
Quantum Effi. for Device I
Power Effi. for Device II
Quantum Effi. for Device II
40
30
20
10
0
0
20
40
60
80
100
120
140
Current Density (mA/cm2)
Figure 3. EL performance of devices I and II. (a) Luminance and voltage
characteristics of device I and II. Inset: Current density vs. voltage.
(b) Quantum efficiency-power efficiency-current density characteristics of
devices I and II.
can explain why the electron-hole recombination may be unbal-
anced in the emitting zone of device I. This unbalanced charge
recombination also shifts the blue light-emitting zone to a
region close to the ETL, and therefore, device I shows a poor
performance.
The maximum efficiency in these devices was achieved
with a current efficiency of 47.1 cd/A, a power efficiency
of 47.7 lm/W, an external quantum efficiency of 29%, and a
brightness of 10514 cd/m2. Moreover, when the brightness
reached from 100 cd/m2 to 1000 cd/m2, the performance of
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light-emitting devices are studied here, such
large gap host materials may also be of use
for green and red phosphorescent devices.
Further application of the large gap host
materials in green, red, and white light-
emitting devices is underway and the results
will be reported in due course.
2.18 2.00 2.40
2.30 2.73
N
N
6.00
Experimental Section
5.30 5.50
5.90
5.54 6.68
Synthesis of SitCz: CzG2on (2 g, 4.02 mmol) was
dissolved in 200 mL of anhydrous tetrahydrofuran
(THF) under a flow of N2. After cooling to −78 °C,
n-butyllithium (2.5 M, 4.42 mmol, 1.77 mL)
was added into the THF solution of CzG2on.
Chlorotriphenylsilane (1.78 g, 6.03 mmol) was
added dropwise to the lithiated CzG2on at −78 °C
for 30 min and stirred for 2 h. The reaction mixture
was then allowed to warm to room temperature for
12 h. Finally, 50 mL of water was poured into the
mixture to quench the reaction, and the resulting
product was extracted with chloroform, purified by
silica gel column chromatography (10% chloroform
in n-hexane used as an eluent), and further purified
by recrystallization (yield: 76%).
TAPC
N
N
N
9.90
NC
N
CN
mCP
N
N
N
N
N
N
NC
CN
N
N
NC
CN
1H NMR (300 MHz, CDCl3, δ): 6.93 (d, 2H, J =
8.8 Hz), 7.21–7.39 (m, 14H), 7.50–7.60 (m, 9H),
7.82 (d, 6H, J = 6.8 Hz), 8.15 (d, 4H, J = 6.8 Hz),
8.23 (s, 2H); 13C NMR (75 MHz, CDCl3) d 144.51,
141.56, 136.11, 131.84, 131.04, 130.40, 128.65,
127.20, 125.77, 125.15, 123.14, 120.21, 119.62,
118.70, 116.11, 109.755. HRMS (FT MS) m/z: calcd.
N
HATCN
NPB
TmPyPB
Figure 4. Energy diagram and chemical structures of materials used for fabrication of devices
I and II.
for [C54H37N3Si + H]+: 756.2836, found [M + H]+: 756.2834.
device II was still higher than device I with a current efficiency
of 44 cd/A, a power efficiency of 27.2 lm/W, and an external
quantum efficiency of 25.6% at CIE coordinates of (0.13,
0.29). To the best of our knowledge, these performances are
the best among blue PhOLEDs, when using a Firpic dopant,
which have been reported. The improved performance can be
attributed to the charge balance of electron and hole fluxes in
the emitting zone because the internal quantum efficiency of
the EL device is directly related to the balance of electrons and
holes in the device.[33,34] Thus, we expect that host materials
that have a high triplet state (ET) with a triphenylsilane moiety
directly attached to the carbazole N would possess potential
for effective charge balance and morphological stability in the
emitting zone.
Device Fabrication: Depyrazino[2,3-f:2′,3′-h]quinoxaline-2,3,6,7,10,11-
hexacarbonitrile (HATCN), N,N′-bis(naphthalene-1-yl)-N,N′-bis(phenyl)-
benzidine (NPB), Di-[4-(N,N-ditoly-amino)-phenyl]cyclohexane (TAPC),
1,3-bis(carbazolyl-9-yl)benzene (mCP), and 1,3,5-tri[(3-pyridyl)-phen-
3-yl]benzene (TmPyPB) were purchased from Luminescence Technology
Corp. OLEDs were fabricated by vacuum deposition onto patterned
ITO glass that had been thoroughly cleaned and subsequently treated
with oxygen plasma. Blue phosphorescent OLEDs were sequentially
fabricated onto the ITO substrates through thermal evaporation of
organic layers (evaporation rate: 2 Å/s; base pressure: 3 × 10−6 Torr).
The EL spectra and CIE color coordinates were obtained using a
Spectrascan PR650 photometer, and the current–voltage–luminescence
(J–V–L) characteristics were measured using a Keithley 2400 source
unit.
In summary, we have designed and synthesized a new host
material (SitCz) that has two carbazole moieties on the C3
and C6 positions of carbazole and a triphenylsilane directly
linked to the N of the carbazole. The higher Tg value (168 °C)
of SitCz compared to mCP or SimCP represents the sig-
nificantly enhanced morphological and thermal stability of
SitCz. The device with the optimized structure, ITO/HATCN
(50 Å)/NPB (550 Å)/TAPC (100 Å)/SitCz:Firpic 10 wt%
(250 Å)/TmPyPB (300 Å)/LiF (5 Å)/Al (1000 Å), is charac-
terized by high performance with a current efficiency of
46.3 cd/A, a power efficiency of 36.5 lm/W, an external
quantum efficiency of 27.2%, and CIE coordinates of (0.13,
0.29) at 100 cd/m2. The performance is the best not only
among Firpic-doped blue PhOLEDs but also among all
the blue PhOLEDs reported to date. Although, mainly, blue
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
Acknowledgements
This study was supported by the Basic Science Research Program
through the National Research Foundation of Korea (NRF) grant funded
by the Ministry of Education, Science and Technology (MEST) of Korea
for the Center for Next-Generation Dye-sensitized Solar Cells (No. 2012-
0000591).
Received: March 8, 2012
Published online:
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