A dicarbazole-triazine hybrid bipolar host material for highly efficient green phosphorescent OLEDs

Article abstract of DOI:10.1039/c2jm14686j

A novel bipolar host material 9-(4,6-diphenyl-1,3,5-triazin-2-yl)-9′- phenyl-3,3′-bicarbazole (CzT) comprising a dicarbazole donor linking to an electron deficient 1,3,5-triazine acceptor was synthesized and characterized. The molecular design of CzT allows the spatial separation of HOMO and LUMO on the donor and acceptor moieties, respectively, giving sufficient triplet energy (2.67 eV) together with promising physical properties and morphological stability (T_g = 134°C). This tailor-made material was used as a host for an efficient yellowish-green iridium complex, bis(o-tolylpyrimidinato- N,C²′)Ir(iii)acetylacetonate [(TPm)₂Ir(acac)]; the as-fabricated phosphorescent OLEDs (PHOLEDs) demonstrated high performance, with maximum efficiencies of 20.0% (75.7 cd A^-1 and 71.3 lm W ^-1) and 20.1% (76.3 cd A^-1 and 72.7 lm W^-1) achieved in trilayer and bilayer device architectures, respectively. The efficiency of the trilayer device can sustain ～20% over a wide brightness range from 10² to 10³ cd m^-2 due to the more effective confinement of emissive excitons within the emitting layer.

Full text of DOI:10.1039/c2jm14686j

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PAPER
A dicarbazole–triazine hybrid bipolar host material for highly efficient
green phosphorescent OLEDs
a
b
a
a
b
b
Chih-Hao Chang,* Ming-Cheng Kuo, Wei-Chieh Lin, Yu-Ting Chen, Ken-Tsung Wong,* Shu-Hua Chou,
b
c
c
d
Ejabul Mondal, Raymond C. Kwong, Sean Xia, Tetsuya Nakagawa and Chihaya Adachi
d
Received 21st September 2011, Accepted 13th December 2011
DOI: 10.1039/c2jm14686j
0
0
A novel bipolar host material 9-(4,6-diphenyl-1,3,5-triazin-2-yl)-9 -phenyl-3,3 -bicarbazole (CzT)
comprising a dicarbazole donor linking to an electron deficient 1,3,5-triazine acceptor was synthesized
and characterized. The molecular design of CzT allows the spatial separation of HOMO and LUMO on
the donor and acceptor moieties, respectively, giving sufficient triplet energy (2.67 eV) together with
ꢀ
promising physical properties and morphological stability (T ¼ 134 C). This tailor-made material was
g
2
0
used as a host for an efficient yellowish-green iridium complex, bis(o-tolylpyrimidinato-N,C )Ir(III)
acetylacetonate [(TPm)
2
Ir(acac)]; the as-fabricated phosphorescent OLEDs (PHOLEDs) demonstrated
ꢁ
1
ꢁ1
high performance, with maximum efficiencies of 20.0% (75.7 cd A and 71.3 lm W ) and 20.1%
ꢁ1
ꢁ1
(
76.3 cd A and 72.7 lm W ) achieved in trilayer and bilayer device architectures, respectively. The
2
3
ꢁ2
efficiency of the trilayer device can sustain ꢂ20% over a wide brightness range from 10 to 10 cd m
due to the more effective confinement of emissive excitons within the emitting layer.
Introduction
transport and charge balance are the two crucial factors for high-
6,7
efficiency OLEDs. One promising strategy for carrier balance
is to incorporate an electron-donating moiety (donor, D) and an
electron-withdrawing moiety (acceptor, A) into a single bipolar
Phosphorescent OLEDs (PHOLEDs) have become the subject of
intensive investigation in recent years because of their tremen-
1,2
dous potential in both display and lighting applications.
8
host material. However, the molecular design strategy employed
Electro-phosphorescence can efficiently make full use of both the
singlet and triplet excited states, rendering the internal quantum
to incorporate the individual characteristics of D and A
components while simultaneously retaining a reasonably large
9–11
3–5
efficiency the ideal level of 100%. Apparently, the use of the
host–guest strategy with triplet emitters (guest) dispersed
homogeneously within a suitable organic matrix (host) gives
PHOLEDs the unity internal quantum efficiency due to the
suppression of triplet–triplet annihilation, which is detrimental
to the efficiency of triplet emitters. After years of research efforts,
both the efficiencies and lifetimes of PHOLEDs have improved
significantly. Particularly, the red emission of the commercially
available active matrix OLED display is now dominated by
phosphorescence. It is reasonable to envision the green phos-
phors will also take over from the green fluorescent emitters in
the near future. As such, there exists intense demand for the
development of new green host materials. In general, charge
triplet energy to prevent the reverse energy transfer effect
is
crucial. Some recent examples verified the promise of bipolar
host materials in PHOLEDs. For example, Yang et al. reported
that a triphenylamine/oxadiazole-based hybrid bipolar host
material (m-TPA-o-OXD) which served as a host of the green
emitter (ppy) Ir(acac) imparted an external quantum efficiency
2
ꢁ1 12
of 23.7%, and power efficiency of 101 lm W . Chou and Cheng
designed and synthesized a hybrid carbazole/phosphine oxide-
based bipolar host material (BCPO), exhibiting a high glass
ꢀ
transition temperature of 137 C and triplet energy (E
T
¼ 3.01
13
eV). Green PHOLED employing the BCPO host of the green
dopant Ir(ppy) was obtained with a relatively low turn-on
voltage of 2.1 V and maximum external quantum/current/power
3
ꢁ1
ꢁ1
efficiencies of 21.6%/83.4 cd A /87.5 lm W respectively. In
addition, Kido et al. developed a bipolar host material by
ꢀ
a
Department of Photonics Engineering, Yuan Ze University, Chung-Li,
32003, Taiwan. E-mail: chc@saturn.yzu.edu.tw; Fax: +886-3-4514281;
Tel: +886-3-4638800 ext. 7517
combining carbazole and pyridine, exhibiting high T (102 C)
g
1
4,15
and triplet energy (E ¼ 2.71 eV),
giving a green PHOLED
T
b
doped with Ir(ppy) with maximum external quantum and power
3
Department of Chemistry, National Taiwan University, Taipei, 10617,
Taiwan. E-mail: kenwong@ntu.edu.tw; Fax: +886-2-33661666; Tel:
ꢁ
1
efficiency of 26.9% and 102 lm W respectively. Furthermore,
Yang et al. utilized a Si atom as a saturated bridge to link tri-
phenylamine (D) and benzimidazole (A) moieties, giving a novel
bipolar host p-BISiTPA, in which the p-conjugation is efficiently
+
c
886-2-33661667
Universal Display Corporation, 375 Phillips Blvd., Ewing, NJ, 08618,
USA
d
Center for Organic Photonics and Electronics Research (OPERA),
Kyushu University, Motooka 744, Nishi, Fukuoka, 819-0395, Japan
16,17
confined on the individual components.
The green PHOLED
3
832 | J. Mater. Chem., 2012, 22, 3832–3838
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employing p-BISiTPA as the host and (ppy) Ir(acac) as the green
2
dopant exhibits maximum external quantum/power efficiencies
ꢁ1
of 22.7%/72.4 lm W . Besides, triazine was also adopted as the
acceptor component of bipolar hosts, for example, Adachi et al.
reported a bipolar host material (TRZ-3Cz) comprised of
18
triazine and carbazole moieties. By employing TRZ-3Cz as the
host and Ir(ppy) as the dopant, the green PHOLED was
obtained with maximum external quantum efficiency as high as
3
ꢁ1
1
0.2% and power efficiency of 14 lm W . Obviously, the device
performances employing triazine-embedded bipolar host mate-
rials largely lag behind as compared to those of other bipolar
systems. In addition, to the best of our knowledge, bipolar host
materials incorporating triazine as the electron deficient moiety
8,19
have not been much exploited for green PHOLEDs devices.
Furthermore, from the device aspect, the multiple-layer archi-
tecture possesses several merits such as energy-barrier reduction,
carrier balance, and exciton confinement. However, the inevi-
table demands of various functional materials may raise the
production cost, rendering the PHOLED-based display less
competitive as compared to other display technologies. There-
fore, it is pertinent to simplify the device configuration of OLEDs
Scheme 1 Synthetic pathways toward the bipolar host CzT.
2
-chloro-4,6-diphenyl-1,3,5-triazine (4) with 3-(9-phenyl-carba-
zol-3-yl)-9H-carbazole (3) in the presence of NaH in DMF at
room temperature. The chemical structures of CzT including
1
13
20,21
intermediates were characterized by H and C NMR spectros-
copy, mass spectroscopy, and elemental analysis (Experimental
section).
without compromising the performance.
In this paper, we report the synthesis, characterization, and
PHOLED applications of a new bipolar host material, 9-(4,6-
0
0
We used differential scanning calorimetry (DSC) and ther-
mogravimetric analysis (TGA) to study the morphological
properties and thermal stabilities of CzT. Because of the
molecular configuration of CzT with adequate rigidity and steric
hindrance, we observed a well-defined glass transition tempera-
diphenyl-1,3,5-triazin-2-yl)-9 -phenyl-3,3 -bicarbazole
(CzT),
which combines the N-phenylcarbazole moiety as the electron
donor (D) and the 1,3,5-triazine moiety as the electron acceptor
(
A). The molecular design of CzT is based on the following
considerations: (1) carbazole is chemically stable itself and can be
easily modified at the 3-, 6-, or 9- position. In addition, a phenyl
group attached to the 9-position of carbazole can improve the
thermal and morphological stabilities. (2) The moderately high
oxidative potentials of carbazole-containing compounds make
them promising as hole-transport materials. Furthermore, the
high triplet energy level of carbazole makes carbazole-cored
materials efficient host materials for phosphors. (3) Since triazine
has an electron affinity larger than those of other typical elec-
tron-deficient heteroaromatic rings (e.g., pyridine, pyrimidine),
triazine derivatives are usually utilized as electron injection/
transport layers in OLEDs. The combination of the structural
features and properties of the carbazole and triazine units
endows CzT with a balanced transporting behavior for both hole
and electron, rendering CzT suitable as a bipolar host material.
When CzT was used as the host material doped with the green
ꢀ
ꢀ
ture (T
g
) of 134 C, a crystallization peak (T
ꢀ
c
¼ 249 C), and
a melting peak (Tm ¼ 267 C). The high decomposition
ꢀ
temperature (T ¼ 425 C, corresponding to 5% weight loss)
d
showed that CzT also exhibits good thermal stability. As a result,
CzT is capable of forming homogeneous and stable amorphous
films upon thermal evaporation.
Fig. 1 depicts the UV-Vis absorption and photoluminescence
spectra of CzT in dilute dioxane solution. Absorption peaks
appeared at 273 and 334 nm and the photoluminescence (PL)
spectrum of CzT exhibited a maximum peak at 526 nm. For
unambiguously determining the triplet energy of CzT, a 3 wt%
CzT:mCP film was prepared, and the delayed PL spectrum
(
Fig. 1) was captured at the time range between 1.2 and 8.30 ms
22
phosphorescent emitter (TPm) Ir(acac), a triple-layer device
2
gave maximum external quantum/current/power efficiencies of
ꢁ1
ꢁ1
2
0.0%/75.7 cd A /71.3 lm W , respectively. The promising
characteristics of CzT allow the device architecture further
simplified to a double-layer configuration, giving comparable
maximum external quantum/current/power efficiencies of 20.1%/
ꢁ1
ꢁ1
7
6.3 cd A /72.7 lm W , respectively.
Results and discussions
Scheme 1 displays the synthetic route towards CzT. The key
intermediate, 3-(9-phenyl-carbazol-3-yl)-9H-carbazole (3), was
synthesized in 74% yield through Suzuki coupling reaction of
Fig. 1 Room-temperature absorption (UV-Vis) and emission (PL) in
dioxane solution, and phosphorescence (Phos) of a 3 wt% CzT:mCP film
at 9 K.
23
3
-iodo-9H-carbazole (2) and the carbazole-based boronic ester
24
(
1), while CzT was obtained in 75% yield after treating
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 3832–3838 | 3833

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at 9 K, giving a triplet energy of 2.67 eV which is sufficient for
confining green phosphors. The phosphorescence spectrum of
CzT is blue-shifted as compared to the fluorescence. This result
indicates that the triplet state of CzT is localized on a specific
structural block, while the singlet state is originated from
25
a charge-transfer state, which can be confirmed by the solvent
polarity-dependent emission spectra shown below.
The PL quantum yield (PLQY) of CzT, determined by an
integrating sphere (Hamamatsu C9920) is 6% in dioxane. The
low PLQY implies that the efficient non-radiation processes
dominate the deactivation of the excited state. The observed
large Stokes shift indicates a possible conformational change
upon photoexcitation. Since CzT is composed of donor and
acceptor subunits, emission from a photoexcited intramolecular
charge transfer (ICT) can also account for the low PLQY and
evident Stokes shift. The solvent polarity-independent UV-vis
absorption spectra of CzT as shown in Fig. 2a indicate that there
is no strong charge transfer in the ground state. However, the PL
emission spectra of CzT show significant red-shifts from low
polarity to high polarity solvents along with the decrease of
emission intensity e.g. in cyclohexane blue emission (463 nm), to
green emission (548 nm) in CH Cl (Fig. 2b). This is the strong
Fig. 3 Frontier molecular orbitals HOMO (top) and LUMO (bottom)
of CzT calculated with DFT on a B3LYP/6-31G(d) level.
of HOMO and LUMO at hole and electron transporting moie-
ties, respectively, which is beneficial for efficient hole and elec-
tron-transporting properties (bipolar). Theoretical analysis
agrees with our observation on the solvent-polarity-dependent
PL from a polarized excited state due to the electron density
distribution shifting from dicarbazole to the triazine moiety upon
electronic excitation of CzT.
2
2
signature of the polar excited state attributed to the photo-
induced charge transfer from the dicarbazole donor to the
triazine acceptor.
To understand the electronic structure of CzT, density func-
tion theory (DFT) calculations were performed at a B3LYP/6-
Fig. 4 reveals the bipolar electrochemical character of CzT, as
evidenced by using cyclic voltammetry (CV). The oxidation/
reduction potentials were measured in DMF solution (1.0 mM)
3
1G(d) level for the geometry optimization. Fig. 3 shows that the
HOMO orbital of CzT is mainly populated over the dicarbazole
and peripheral phenyl moiety, while the LUMO orbital is mainly
localized on electron-deficient triazine and its phenyl substitu-
tions. This result reveals that CzT has evident spatial separation
4 6
containing 0.1 M of n-Bu NPF as a supporting electrolyte at
ꢁ1
a scan rate of 100 mV s . A glassy carbon electrode and a plat-
inum wire were used as working and counter electrodes,
respectively. We observed one quasi-reversible oxidation poten-
tial at 1.21 V (onset) and reversible reduction potential at
ꢁ
1.51 V (vs. Ag/AgCl). As referred to the reversible oxidation
+
potential of ferrocene/ferrocenium (Fc/Fc ) redox couple, the
HOMO (from the onset oxidation potential) and LUMO (from
the half-potential) energy levels of CzT were estimated to
be ꢁ5.49 and ꢁ2.77 eV, respectively. The photophysical and
electrochemical data of CzT are summarized in Table 1.
The bipolar charge carrier transporting property of CzT was
verified by single carrier devices and compared to a well-known
0
carbazole-based bipolar material, 4,4 -bis(carbazol-9-yl)biphenyl
26,27
The hole and electron mobilities of CBP are 2 ꢃ 10ꢁ3
(
CBP).
Fig. 2 Absorption (a) and photoluminescence spectra (b) of 2 ꢃ 10ꢁ
CzT in different solvents.
5
M
Fig. 4 Cyclic voltammogram of CzT.
3
834 | J. Mater. Chem., 2012, 22, 3832–3838
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Table 1 Photophysical and electrochemical data of CzT at room temperature
ox
1/2 /V
red
1/2 /V
ꢀ
ꢀ
ꢀ
ꢀ
UV-Vis/nm
PL/nm
526
Q.Y. (%)
0.06
E
g,UV/eV
E
E
T
g
/ C
T
c
/ C
T
m
/ C
T
d
/ C
CzT
273, 300 (sh), 334
3.0
1.21
ꢁ1.51
134
249
267
425
ꢁ
4
2
ꢁ1 ꢁ1
and 3 ꢃ 10 cm V s , respectively, at the applied field of
structures of the OLED materials and the schematic structures of
the devices are shown in Fig. 6.
ꢁ1
0
.5 MV cm . The CBP-based single carrier devices were utilized
for probing the carrier transport behavior of CzT. The archi-
tecture of the hole-only devices was designed as: ITO/
PEDOT:PSS (ꢂ30 nm)/CBP or CzT (150 nm)/Au (20 nm)/Al
The electroluminescence (EL) characteristics of devices A are
shown in Fig. 7 and Table 2. The EL spectra of green devices
suggested that the emissions were exclusively derived from the
emitters, indicating that the emissive excitons are confined on the
(
150 nm), whereas the electron-only devices were composed of
ITO/Al (20 nm)/CBP or CzT (150 nm)/LiF (0.8 nm)/Al (150 nm).
The I–V curves of the hole-only and electron-only devices are
shown in Fig. 5. Obviously, the experimental outcomes indicate
that the current densities of CzT-based devices were significantly
higher than those of CBP ones. Consequently, CzT possesses
superior carrier transport properties for both positive and
negative charges.
phosphorescent guests. Device A1 incorporated with Ir(ppy)
3
showed an external quantum efficiency of 14%. However, the
triplet energy of CzT may not be sufficiently high to completely
3
suppress the reverse energy transfer of Ir(ppy) back to the host,
leading to some energy loss. This assumption can be further
verified by using an emitter with a wider gap. Once the commonly
used sky-blue phosphor, iridium(III) bis[(4,6-di-fluorophenyl)-
2
0
4
Encouraged by the observed bipolar transport capability of
pyridinato-N,C ]picolinate (FIrpic), was adopted (device A2),
the peak external quantum efficiency decreased to about 10%.
The broad EL spectrum in device A2 indicates that the excitons
are possibly diffused back to the host. In a sharp contrast, device
13
CzT, green phosphorescent iridium complexes Ir(ppy)3 and
22
(
TPm) Ir(acac) were selected to combine with the bipolar host
2
CzT in the host–guest system. We fabricated trilayer devices
devices A) with the architecture as: ITO/TAPC (40 nm)/CzT
(
2
A3 incorporated yellowish-green (TPm) Ir(acac) which exhibited
doped with 8 wt% of iridium complex (20 nm)/TmPyPB (50 nm)/
LiF (0.5 nm)/Al (150 nm), where di-[4-(N,N-ditolyl-amino)-
phenyl] cyclohexane (TAPC) and 1,3,5-tri[(3-pyridyl)-phen-3-yl]
benzene (TmPyPB) were introduced as the hole-transport layer
28–30
(
HTL) and electron-transport layer (ETL), respectively.
Both
TAPC and TmPyPB possess a wide triplet energy gap for pre-
venting the exciton diffusion, high carrier mobility, and suitable
HOMO and LUMO levels matching the CzT host. The chemical
Fig. 5 Current–voltage (I–V) characteristics of the hole-only and elec-
Fig. 6 The structural drawing of materials used in OLEDs and the
tron-only devices.
schematic structures of devices.
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 3832–3838 | 3835

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Fig. 7 The electroluminescence characteristics of device A: (a) the normalized EL spectra, (b) current–voltage–luminance (I–V–L) characteristics, (c)
luminance efficiency/power efficiency versus luminance, and (d) external quantum efficiency versus luminance for the devices.
2
an EL with the CIE coordinate of (0.38, 0.59) at 10 cd m . The
ꢁ2
respectively. Apparently, this result clearly demonstrates that
PHOLEDs utilizing an appropriate bipolar host material are
beneficial for reducing efficiency roll-offs. It is noteworthy that
the luminance of device A3 reaches to a maximum of 153 009 cd
maximum external quantum/luminance/power efficiencies of
ꢁ1
ꢁ1
device A3 were up to 20%, 75.7 cd A , 71.3 lm W , respectively.
Without a specific optical structure tuning, the efficiencies imply
that a high internal quantum efficiency in device A3 is due to the
balanced charge flux and effective confinement of emissive
excitons within the emitting layer. The careful molecular design
imparts CzT with favorable characteristics to serve as an efficient
host material of yellowish-green phosphors.
ꢁ
2
m
at an operation voltage of 9.8 V.
We envisioned that the HOMO of CzT may be suitable for
direct hole injection, the conductive polymer, PEDOT:PSS,
consequently combined with ITO as the transparent anode.
Therefore, a bilayer device (device B) without the TAPC was
fabricated as: ITO/PEDOT:PSS (ꢂ30 nm)/CzT doped with 8 wt
In addition, in view of the energy-level alignments, the HTL
and ETL could effectively reduce the excessive carrier leakage
and improve the charge carriers recombination zone over the
whole emitting layer. Furthermore, the bipolar property of CzT
with suitable energy levels also helps to facilitate the charge
carriers injections from the neighbouring layers, resulting in
a low turn-on voltage of 2.6 V. Upon increasing the practical
2
% of (TPm) Ir(acac) (20 nm)/TmPyPB (50 nm)/LiF (0.5 nm)/Al
(150 nm) (Fig. 6). The EL characteristics of devices B are also
summarized in Table 2. The EL spectra and CIE coordinate of
device B indicate that the emissive excitons are effectively
confined within the emitting layer. In the absence of TAPC,
device B exhibited maximum efficiencies of up to 20.1%, 76.3 cd
2
ꢁ2
3
ꢁ2
ꢁ1
ꢁ1
2
brightness to 10 cd m (and 10 cd m ) the external quantum/
A , 72.7 lm W , respectively. At a practical brightness of 10 cd
ꢁ2
3
(and 10 cd m ), the efficiencies remain high at 19.8%
ꢁ2
luminance/power efficiencies of device A3 remain 19.8% (19.7%),
m
ꢁ
1
ꢁ1
ꢁ1
ꢁ1
ꢁ1
ꢁ1
ꢁ1
7
4.6 cd A (74.5 cd A ) and 63.6 lm W (48.0 lm W ),
Table 2 EL characteristics of green and red phosphorescent OLEDs
EQE (%) LE/cd A
(17.2%), 75.1 cd A (65.3 cd A ), and 65.5 lm W (45.4 lm
ꢁ
1
ꢁ1
PE/lm W
V
on
CIEx,y
CIEx,y
a
b
c
a
b
c
a
b
c
d
b
c
Device
Emitter
Trilayer
A1
A2
A3
B
Ir(ppy)
FIrpic
3
14.3
9.9
20.0
20.1
14.3
9.6
19.8
19.8
13.8
8.3
19.7
17.2
48.0
27.4
75.7
76.3
47.9
26.5
74.6
75.1
46.6
23.1
74.5
65.3
47.2
30.5
71.3
72.7
39.0
21.9
63.6
65.5
29.6
14.5
48.0
45.4
2.9
2.8
2.6
2.8
0.374, 0.584
0.258, 0.473
0.382, 0.594
0.369, 0.599
0.371, 0.585
0.258, 0.473
0.380, 0.599
0.364, 0.612
(TPm)
(TPm)
2
Ir(acac)
Ir(acac)
Bilayer
2
a
b
Maximum efficiencies. Measured at a brightness of 100 cd m
ꢁ2
c
ꢁ2
d
ꢁ2
.
Recorded at 1000 cd m
.
Turn-on voltage measured at 1 cd m .
3
836 | J. Mater. Chem., 2012, 22, 3832–3838
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ꢁ
1
0
0
W ), respectively. Notable differences in the device character-
istics between device A3 and B are the maximum luminance and
current density. Device A3 configuring a trilayer architecture
incorporates with TAPC to give cascade energy levels for smooth
hole injection and a high triplet energy for preventing the emis-
Synthesis of 9-(4,6-diphenyl-1,3,5-triazin-2-yl)-9 -phenyl-3,3 -
bicarbazole (CzT)
A solution of sodium hydride (60% in oil, 120 mg, 3.0 mmol) and
3
-(9-phenyl-carbazol-3-yl)-9H-carbazole (816 mg, 2.0 mmol) in
DMF (40 ml) was stirred at room temperature for 1 h under
argon atmosphere. 2-Chloro-4,6-diphenyl-1,3,5-triazine (448 mg,
31,32
sive excitons quenching by PEDOT:PSS,
leading to better
device performance and stability especially under the high
brightness condition.
1
.67 mmol) was added to the solution at room temperature, then
refluxed overnight. The mixture was poured into water and the
precipitate was collected by filtration and washed with water,
methanol and DCM to get yellow solid CzT (800 mg, 75%). Mp:
ꢀ
1
Conclusions
267 C (DSC); H NMR (C
D Cl
2 2
4
, 400 MHz) d 9.18 (d, J ¼ 8.8
Hz, 1H), 9.15 (d, J ¼ 8.4 Hz, 1H), 8.76 (dd, J ¼ 7.8 Hz, J ¼ 1.6
Hz, 4H), 8.51 (s, 1H), 8.36 (s, 1H), 8.29 (d, J ¼ 7.6 Hz, 1H), 8.18
In conclusion, a new dicarbazole–triazine hybrid bipolar host
material CzT has been synthesized and characterized. CzT
ꢀ
(
d, J ¼ 7.2 Hz, 1H), 7.98 (d, J ¼ 8.4 Hz, 1H), 7.84 (dd, J ¼ 8.4 Hz,
exhibits a high glass transition temperature (T
g
) of 134 C and
J ¼ 1.6 Hz, 1H), 7.73–7.64 (m, 11H), 7.56–7.47 (m, 5H), 7.40–
ꢀ
a high decomposition temperature (T ¼ 425 C). Theoretical
13
d
7
1
1
1
1
2 2 4
.35 (m, 1H); C NMR (C D Cl , 100 MHz) d 172.2, 164.8,
analysis (DFT) reveals that CzT has evident spatial separation of
HOMO and LUMO at hole and electron transporting moieties,
respectively, which is consistent with the observation on the
solvent-polarity-dependent PL from a polarized excited state.
The lack of effective donor–acceptor conjugation makes CzT
suitable for yellowish-green phosphors. The bipolar behavior of
CzT has been confirmed by CV and hole/electron only devices,
giving the chance to keep charge carriers balance in the emitting
layer. In addition, by employing CzT as the bipolar host for the
41.1, 140.0, 139.3, 137.8, 137.3, 137.0, 135.9, 133.1, 132.75,
29.9, 129.0, 128.8, 127.4, 127.2, 127.0, 126.7, 126.5, 126.4, 126.2,
25.6, 123.7, 123.4, 123.2, 120.4, 120,2, 120.1, 119.7, 118.7, 117.8,
17.7, 110.1, 109.9; MS (m/z, FAB+) 639 (44.69); HRMS (m/z,
FAB+) calcd for C H N 639.2423, found 639.2416; anal. calcd
5
4
5
29
C, 84.48; H, 4.57; N, 10.95, found C, 84.31; H, 4.15; N, 10.93.
Photophysics measurements
green phosphor complex, (Tmp)
2
Ir(acac), the PHOLEDs
Samples were purged with nitrogen and recorded in dilute
dioxane solution at RT. UV-Vis absorption spectra were recor-
ded using a Hitachi U2800A spectrophotometer. PL spectra were
recorded using a Hitachi F9500 fluorescence spectrophotometer.
Solution PLQY was determined using a calibrated integrating
sphere system (Hamamatsu C9920). The PL spectrum of a 3 wt%
CzT:mCP film at 9 K was measured using a streak camera system
demonstrate high performance, with maximum efficiencies of
ꢁ1
ꢁ
1
ꢁ1
2
0.0% (75.7 cd A and 71.3 lm W ) and 20.1% (76.3 cd A and
ꢁ
1
7
2.7 lm W ) achieved in trilayer and bilayer device architectures,
respectively. The trilayer device shows low efficiency roll-off over
2
3
ꢁ2
a wide brightness range of 10 to 10 cd m because the high
triplet energy of TAPC can prevent the emissive excitons
quenching by PEDOT:PSS. Our results clearly demonstrate that
utilizing an appropriate bipolar host material for PHOLEDs is
beneficial for enhancing the performance and stability of devices.
(
(
(
C4334, Hamamatsu Co.) equipped with
a
cryostat
GASESCRT-006-2000, IWATANI Co.) A nitrogen gas laser
MNL200, LASERTECHNIK BERLIN) with an excitation
wavelength of 337 nm was used.
Cyclic voltammetry
Experimental
The oxidation/reduction potentials were measured by cyclic
voltammetry (CV) in DMF solution (1.0 mM) containing 0.1 M
Synthesis of 3-(9-phenyl-carbazol-3-yl)-9H-carbazole 3
A mixture of 3-iodo-9H-carbazole (0.88 g, 3.0 mmol), Pd(PPh3)
4
of tetra-n-butylammonium hexafluorophosphate (TBAPF ) as
6
(
165 mg, 0.15 mmol), 9-phenyl-3-(4,4,5,5-tetramethyl-1,3,2-
ꢁ1
a supporting electrolyte at a scan rate of 100 mV s . A glassy
dioxaborolan-2-yl)-9H-carbazole (1.29 g, 3.5 mmol) and K PO
3
4
carbon electrode and a platinum wire were used as working and
+
counter electrodes, respectively. Ferrocene/ferrocenium (Fc/Fc )
ꢀ
(
1.8 g, 8.5 mmol) in dioxane (10 ml) was heated at 85 C with
vigorous stirring for 48 h under argon atmosphere. The mixture
was poured into water and extracted with DCM. The organic
redox couple in DMF/TBAPF6 occurs at E ¼ +0.52 V for
o
oxidation and reduction vs. Ag/AgCl (sat’d). All potentials were
recorded versus Ag/AgCl (sat’d) as a reference electrode.
4
extracts were washed with brine and dried over MgSO . The
solvent was removed by rotary evaporation, and the crude
residue was recrystallized in DCM to afford a white solid 3 (0.9 g,
1
4%). H NMR (DMSO-d
OLED device fabrication
7
6
, 400 MHz) d 11.27 (s, 1H, NH), 8.64
(
d, J ¼ 1.2 Hz, 1H), 8.52 (s, 1H), 8.37 (d, J ¼ 7.6 Hz, 1H), 8.22 (d,
J ¼ 7.6 Hz, 1H), 7.85–7.80 (m, 2H), 7.72–7.65 (m, 4H), 7.59–7.37
m, 7H), 7.30 (t, J ¼ 7.6 Hz, 1H), 7.17 (t, J ¼ 7.6 Hz, 1H);
NMR (DMSO-d , 100 MHz) d 141.0, 140.7, 139.5, 139.3, 137.4,
All purchased compounds were subjected to temperature-
gradient sublimation under high vacuum before use. OLEDs
were fabricated on the indium-tin-oxide (ITO)-coated glass
substrates (#15 U) with multiple organic layers sandwiched
between the transparent bottom ITO anode and the top metal
cathode. The organic and metal layers were deposited by thermal
13
C
(
6
1
1
1
34.3, 132.2, 130.6, 128.0, 127.0, 126.7, 126.0, 125.9, 125.4, 123.9,
23.6, 123.4, 123.1, 121.2, 120.8, 120.5, 119.0, 118.8, 118.6, 111.7,
11.5, 110.3, 110.1; MS (m/z, FAB+) 408 (16.87); HRMS (m/z,
ꢁ6
evaporation in a vacuum chamber with a base pressure of <10
FAB+) calcd for C30
H
20
N
2
408.1626, found 408.1627.
torr. The deposition rate of organic layers was kept at ꢂ0.1 nm
J. Mater. Chem., 2012, 22, 3832–3838 | 3837
This journal is ª The Royal Society of Chemistry 2012

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ꢁ
1
s . The conducting polymer, PEDOT:PSS (Clevos PVP
CH8000, H. C. Strack), was deposited by spin-coating. The
deposition system permitted the fabrication of a complete device
structure in a single pump-down without breaking vacuum. The
10 W.-Y. Hung, L.-C. Chi, W.-J. Chen, Y.-M. Chen, S.-H. Chou and
K.-T. Wong, J. Mater. Chem., 2010, 20, 10113.
1
1
1 L. Duan, J. Qiao, Y. Sun and Y. Qiu, Adv. Mater., 2011, 23, 1137.
2 Y. Tao, Q. Wang, C. Yang, C. Zhong, J. Qin and D. Ma, Adv. Funct.
Mater., 2010, 20, 2923.
2
active area of the device was 2 ꢃ 2 mm , as defined by the shadow
13 H.-H. Chou and C.-H. Cheng, Adv. Mater., 2010, 22, 2468.
1
4 S.-J. Su, H. Sasabe, T. Takeda and J. Kido, Chem. Mater., 2008, 20,
691.
5 S.-J. Su, C. Cai and J. Kido, Chem. Mater., 2011, 23, 274.
mask for cathode deposition. Current–voltage–luminance
1
(
I–V–L) characterization of the devices was measured using an
1
Agilent 4156C semiconductor parameter analyzer equipped with
a calibrated Si-photodiode. EL spectra of devices were collected
by using an Ocean Optics spectrometer.
16 S. Gong, Y. Chen, C. Yang, C. Zhong, J. Qin and D. Ma, Adv.
Mater., 2010, 22, 5370.
1
1
1
2
2
2
2
7 S. Gong, Y. Chen, J. Luo, C. Yang, C. Zhong, J. Qin and D. Ma, Adv.
Funct. Mater., 2011, 21, 1168.
8 K. S. Son, M. Yahiro, T. Imai, H. Yoshizaki and C. Adachi, Chem.
Mater., 2008, 20, 4439.
9 H.-F. Chen, S.-J. Yang, Z.-H. Tsai, W.-Y. Hung, T.-C. Wang and
K.-T. Wong, J. Mater. Chem., 2009, 19, 8112.
0 P. A. Lane, G. P. Kushto and Z. H. Kafafi, Appl. Phys. Lett., 2007, 90,
Acknowledgements
The authors gratefully acknowledge the financial support from
National Science Council of Taiwan (NSC 99-2221-E-155-035-
MY3, 98-2119-M-002-007-MY3) and Ministry of Economic
Affairs (100-EC-17-A-08-S1-042).
0
23511.
1 M.-T. Kao, W.-Y. Hung, Z.-H. Tsai, H.-W. You, H.-F. Chen, Y. Chi
and K.-T. Wong, J. Mater. Chem., 2011, 21, 1846.
2 W.-Y. Hung, T.-C. Wang, H.-C. Chiu, H.-F. Chen and K.-T. Wong,
Phys. Chem. Chem. Phys., 2010, 12, 10685.
3 S. H. Tucker, J. Chem. Soc., 1926, 1, 548.
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838 | J. Mater. Chem., 2012, 22, 3832–3838
This journal is ª The Royal Society of Chemistry 2012