Novel carbazole/pyridine-based host material for solution-processed blue phosphorescent organic light-emitting devices

Article abstract of DOI:10.1016/j.dyepig.2011.08.009

3,6-Bis(3,5-di(pyridin-3-yl)phenyl)-9-phenyl-9H-carbazole, a novel host material for solution-processed blue phosphorescent organic light-emitting devices was synthesized by a Suzuki coupling reaction. The optical, electrochemical and thermal properties of this novel crabazole have been characterized. The compound exhibits a high glass-transition temperature (Tg = 161 °C) and high triplet energy (E_T = 2.76 eV). Additionally, atomic force microscopy measurements indicate that high-quality amorphous films of this novel compound can be prepared by spin-coating. Solution-processed blue phosphorescent organic light-emitting devices were obtained using the carbazole as the host material for the phosphorescence emitter iridium(III) bis(4,6-difluorophenylpyridinato)- picolinate and their electroluminescence properties were evaluated.

Full text of DOI:10.1016/j.dyepig.2011.08.009

Dyes and Pigments 92 (2012) 891e896
Contents lists available at SciVerse ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Novel carbazole/pyridine-based host material for solution-processed
blue phosphorescent organic light-emitting devices
*
Wei Jiang, Lian Duan, Juan Qiao, Guifang Dong, Deqiang Zhang, Liduo Wang, Yong Qiu
Key Lab of Organic Optoelectronics and Molecular Engineering of Ministry of Education, Department of Chemistry, Tsinghua University, Beijing 100084, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
3,6-Bis(3,5-di(pyridin-3-yl)phenyl)-9-phenyl-9H-carbazole, a novel host material for solution-processed
blue phosphorescent organic light-emitting devices was synthesized by a Suzuki coupling reaction. The
optical, electrochemical and thermal properties of this novel crabazole have been characterized. The
Received 27 May 2011
Received in revised form
9 August 2011
Accepted 10 August 2011
Available online 5 September 2011
compound exhibits
a
high glass-transition temperature (Tg
¼
161 ꢀC) and high triplet energy
(ET ¼ 2.76 eV). Additionally, atomic force microscopy measurements indicate that high-quality amor-
phous films of this novel compound can be prepared by spin-coating. Solution-processed blue phos-
phorescent organic light-emitting devices were obtained using the carbazole as the host material for the
phosphorescence emitter iridium(III) bis(4,6-difluorophenylpyridinato)- picolinate and their electrolu-
minescence properties were evaluated.
Keywords:
Host
Carbazole
Pyridine
Ó 2011 Elsevier Ltd. All rights reserved.
Solution-processed
Blue phosphorescent
High triplet energy
1. Introduction
because of the lower triplet energy gap (ET) of 2.53 eV relative
to those of blue triplet emitters such as iridium(III) bis(4,6-
Organic lighting-emitting diodes (OLEDs) have attracted a great
deal of attention due to their potential applications in full-color
flat-panel displays and lighting sources [1e3]. Recently, phospho-
rescent OLEDs have attract great interest because they can harvest
both singlet and triplet excitons for emission, theoretically yielding
100% internal quantum efficiency [4,5]. In phosphorescent OLEDs,
to reduce the quenching associated with the relatively long excited
state lifetimes of triplet emitters and tripletetriplet annihilation,
triplet emitters are normally used as emitting guests in a host
material. The fabrication of blue phosphorescent devices is still
challenging because the extent of conjugation and molecular size of
the host materials must be constrained to achieve a high triplet
energy. This requirement makes it difficult for the molecules
to acquire the morphological stability. Several carbazole-based
host materials are frequently used in phosphorescence devices
[6e12]. For example, 4,40-bis(9-carbazolyl)-2,20-biphenyl (CBP) is
commonly used as a host material, but not for blue triplet emitters
difluorophenylpyridinato)-picolinate (FIrpic) (ET ¼ 2.62 eV) [13].
One of the earliest hosts used for blue electrophosphorescence
devices is 1,3-bis(9-carbazolyl)benzene (mCP) (ET ¼ 2.90 eV) [9].
However, the low glass-transition temperatures (Tg) of CBP and
mCP leads to ready crystallization, especially when the dopant
concentration is low [14].
Concerning the fabrication of devices and displays, vapor
deposition requires vacuum processing, which is a relatively costly
process. In order to simplify the fabrication process of OLEDs and
reduce the cost of larger-area displays, much research effort have
been devoted to develop OLEDs with the emitting layer prepared by
solution-processing techniques [15e20]. A good host material for
a solution-processed blue phosphorescent OLED should have the
following features: high triplet energy, good charge carrier trans-
port properties, high solubility and film-forming ability. In our
previous paper, we have demonstrated solution-processed blue
phosphorescence OLEDs using a carbazole unit as the host with
a high-efficiency [11,18,21]. In this paper, we synthesized a novel
carbazole derivative 3,6-bis(3,5-di(pyridin-3-yl)phenyl)-9-phenyl-
9H-carbazole (BDPPC), using of a carbazole monomer as the rigid
core with two pyridine containing groups linked to the core
through the 3, 6 positions. Additionally, pyridine units are intro-
duced into the molecular structure to ensure good solubility to
* Corresponding author. Key Lab of Organic Optoelectronics and Molecular
Engineering of Ministry of Education, Department of Chemistry, Sipailou 2#,
Tsinghua University, Beijing 100084, PR China. Tel.: þ86 10 62779988; fax: þ86 10
62795137.
E-mail address: qiuy@mail.tsinghua.edu.cn (Y. Qiu).
form high-quality films. As a result, the newly synthesized
0143-7208/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2011.08.009

892
W. Jiang et al. / Dyes and Pigments 92 (2012) 891e896
compounds possess the following important characteristics: high
triplet energy levels (2.76 eV) because of the nonconjugated
linkage; the capability of forming stable amorphous thin films as
a result of the highly twisted configuration of the molecules and
good solubility in most common solvents. The application of this
new compound as a host material for solution-processed blue
phosphorescence devices has been demonstrated.
were added to 1,4-dioxane solution (50 mL) then, 2 M K2CO3 solution
(5 mL), which was dissolved in H2O, was added to the reaction
mixture. The mixture was heated under reflux for 24 h. After the
reaction finished, the reaction mixture was extracted with
dichloromethane and water. The organic layer was dried by anhy-
drous MgSO4 and filtered. The product was isolated by silica gel
column chromatography using petroleum ether/ethyl acetate (10:1)
eluent to afford a white solid (5.20 g, 65.0%). 1H NMR (600 MHz,
2. Experimental
CDCl3, TMS)
7.62e7.58 (m, 8H), 7.53 (t, J ¼ 6.9, 7.6 Hz, 1H), 7.48 (d, J ¼ 8.2 Hz, 2H),
7.34 (s, 2H); 13C-NMR(CDCl3,
): 144.79,141.51,135.37,130.22,128.10,
d
: 8.36 (d, J ¼ 1.4 Hz, 2H), 7.67 (t, J ¼ 8.2, 7.6 Hz, 2H),
2.1. General information
d
127.07, 126.59, 125.75, 125.68, 123.95, 119.14, 110.68; MS (MAL-
DIeTOF) [m/z]: calcd for C30H17Cl4N, 533.3; found, 533.5. Anal. Calcd.
for C30H17Cl4N: C, 67.57; H, 3.21; N 2.62. Found: C, 67.66; H, 3.32;
N 2.55.
All reactants and solvents, unless otherwise stated, were
purchased from commercial sources and used as received. 1H NMR
and 13C HMR spectra were measured on a Bruker ARX600 NMR
spectrometer with tetramethylsilane as the internal standard.
Elemental analysis was performed on an Elementar Vario EL CHN
elemental analyzer. Mass spectrometry was performed with
a Thermo Electron Corporation Finnigan LTQ mass spectrometer.
Absorption spectra were recorded with a UVevis spectrophotom-
eter (Agilent 8453) and PL spectra were recorded with a fluo-
rospectrophotometer (Jobin Yvon, FluoroMax-3). TGA was recorded
with a Netzsch simultaneous thermal analyzer (STA) system (STA
409PC) under a dry nitrogen gas flow at a heating rate of
10 ꢀC minꢁ1. Glass-transition temperatures were recorded by DSC
at a heating rate of 10 ꢀC minꢁ1 with a thermal analysis instrument
(DSC 2910 modulated calorimeter). Cyclic voltammetry was per-
formed on a Princeton Applied Research potentiostat/galvanostat
model 283 voltammetric analyzer in CH2Cl2 solutions (10ꢁ3 M) at
a scan rate of 100 mV sꢁ1 with a platinum plate as the working
electrode, a silver wire as the pseudo-reference electrode, and
a platinum wire as the counter electrode. The supporting electro-
lyte was tetrabutylammonium hexafluorophosphate (0.1 M) and
ferrocene was selected as the internal standard. The solutions were
bubbled with argon for 10 min before measurements. The film
surface morphology was measured with AFM (Seiko Instruments,
SPA-400).
2.3.2. Synthesis of 3,6-bis(3,5-di(pyridin-3-yl)phenyl)-9-phenyl-
9H-carbazole (BDPPC)
A mixture of 3,6-bis(3,5-dichlorophenyl)-9-phenyl-9H-carba-
zole (0.5 mmol), 3-pyridineboronic acid (2.2 mmol), Pd2(dba)3
(0.05 mmol), PCy3 (0.05 mmol) and K3PO4 (2.2 mmol) were added
to 1,4-dioxane solution (50 mL). The mixture was refluxed under
nitrogen for 24 h. After the reaction finished, the reaction mixture
was extracted with dichloromethane and water. The organic layer
was dried by anhydrous MgSO4 and filtered. The product was iso-
lated by silica gel column chromatography using CH2Cl2/CH3OH
(10:1) eluent to afford a white solid (0.12 g, 31.0%). 1H NMR
(600 MHz, CDCl3, TMS)
d
: 9.01 (d, J ¼ 2.0 Hz, 6H), 8.67 (d, J ¼ 4.1 Hz,
6H), 8.53 (s, 3H), 8.05 (d, J ¼ 7.6 Hz, 6H), 7.97 (s, 6H), 7.80 (d,
J ¼ 9.6 Hz, 3H), 7.74 (s, 3H), 7.70e7.64 (m, 6H), 7.57e7.53 (m, 5H),
7.46e7.44 (m, 6H); 13C-NMR(CDCl3,
d): 148.97, 148.53, 143.80,
141.36, 139.53, 136.59, 134.79, 132.88, 130.21, 128.02, 127.13, 126.12,
125.98, 124.51, 124.13, 123.81, 119.25, 110.68; MS (MALDIeTOF)
[m/z]: calcd for C50H33N5, 703.8; found, 703.6. Anal. Calcd.
for C50H33N5: C, 85.32; H, 4.73; N 9.95. Found: C, 85.30; H, 4.68;
N 9.90.
3. Results and discussion
2.2. Device fabrication and performance measurements
3.1. Synthesis and characterization
In a general procedure, indiumetin oxide (ITO)-coated glass
substrates were pre-cleaned carefully and treated by UV ozone for
4 min. A 40 nm PEDOT:PSS aqueous solution was spin-coated onto
the ITO substrate and baked at 200 ꢀC for 10 min. The substrates
were then taken into a nitrogen glove box, where FIrpic-doped host
layer was spin-coated onto the PEDOT:PSS layer from 1,2-
dichloroethane solution and annealed at 80 ꢀC for 30 min. The
substrate was then transferred into an evaporation chamber, where
the TPBI was evaporated at an evaporation rate of 1e2 Å/s under
a pressure of 4 ꢂ 10ꢁ4 Pa and the Cs2CO3/Al bilayer cathode was
evaporated at evaporation rates of 0.2 and 10 Å/s for Cs2CO3 and Al,
respectively, under a pressure of 1 ꢂ10ꢁ3 Pa. The currentevoltagee
brightness characteristics of the devices were characterized
with Keithley 4200 semiconductor characterization system.
The electroluminescent spectra were collected with a Photo
Research PR705 Spectrophotometer. All measurements of the
devices were carried out in ambient atmosphere without further
encapsulations.
Scheme 1 illustrates the synthetic procedures for BDPPC.
Initially, the starting 3,6-dibromo-9-phenylcarbazole was synthe-
sized according to methods previously described [18]. 3,6-bis(3,5-
dichlorophenyl)-9-phenyl-9H-carbazole was prepared from 3,6-
dibromo-9-phenylcarbazole and 3,5-dichlorophenylboronic acid
catalyzed by Pd(PPh3)4 in 1,4-dioxane. Then the Suzuki cross-
coupling reactions of 3,6-bis(3,5-dichlorophenyl)-9-phenyl-9H-
carbazole with the 3-pyridineboronic acid led to BDPPC in
the presence of Pd2(dba)3/PCy3 (yield: 31.0%). The target compound
was purified by silica column chromatography and recrystalliza-
tion. 1H NMR, 13C-NMR, mass spectrometry, and elemental
analysis were employed to confirm the chemical structures of
above-mentioned compounds as described in the experimental
section.
3.2. Thermal analysis
Thermal properties of BDPPC were investigated by thermal
gravimetric analyses and differential scanning calorimetry. The
introduction of pyridine containing groups to 3- and 6- positions of
carbazole renders the molecule non-planar. Such a molecular
configuration is strongly beneficial to the thermal stability, as
indicated by the high decomposition temperature (Td, corre-
sponding to 5% weight loss) of 510 ꢀC and very high Tg of 161 ꢀC
2.3. Materials
2.3.1. Synthesis of 3,6-bis(3,5-dichlorophenyl)-9-phenyl-9H-
carbazole
A mixture of 3,6-dibromo-9-phenylcarbazole (1.5 mmol), 3,5-
dichlorophenylboronic acid (3.2 mmol), and Pd(PPh3)4 (0.06 mmol)

W. Jiang et al. / Dyes and Pigments 92 (2012) 891e896
893
N
Br
Br
N
Cl
Pd(PPh
)
3 4
+
K CO
2
3
Cl
Cl
B(OH)
Cl
2
N
Cl
N
Pd (dba) /PCy
2
3
3
+
Cl
K PO
3
4
B(OH)
N
N
N
2
N
BDPPC
Scheme 1. Synthetic routes toward BDPPC.
(Fig. 1), which are much higher than those of analogous carbazole-
based host materials such as CBP (62 ꢀC) and mCP (60 ꢀC). The
morphologies of BDPPC were characterized by atomic force
microscopy (AFM) using standard tapping mode. As can be seen
from Fig. 2, the surface of neat BDPPC film is pinhole-free and quite
smooth, and the root-mean-square (RMS) value is 0.32 nm. The
surface morphology of the spin-coated films was found to be stable
after thermal treatment at 100 ꢀC for 6 h. These results demonstrate
that BDPPC is capable of forming amorphous films through solu-
tion-processing.
to the FIrpic would be efficient. Fig. 3(a) also depicts the phos-
phorescence spectrum of BDPPC measured from CH2Cl2 at 77 K.
The triplet energy of BDPPC, measured from the low-temperature
photoluminescence spectra, was 2.76 eV, higher than reported for
the common triplet blue-emitter FIrpic. Using host materials that
possess high triplet energy is a provision for effective confine-
ment of the triplet excitons on the guest and, consequently, for
prevention of back energy transfer between the host and dopant
molecules. To further confirm their exciton confinement property,
the transient photoluminescence decay of 5wt% FIrpic doped into
BDPPC and CBP was measured (Fig. 3(b)). The CBP: FIrpic film
exhibits an extremely long decay component, which can attrib-
uted to the exothermic back energy transfer from FIrpic to CBP
[23], while the BDPPC: FIrpic film clearly exhibits a mono-
3.3. Photophysical properties
Fig. 3(a) depicts the UVevis absorption and photoluminescent
(PL) spectra of BDPPC in CH2Cl2. The absorption peak at around
exponential decay curve and a relative lifetime of 1.3
ms, indi-
cating that the triplet energy transfer from FIrpic to BDPPC was
completely suppressed [24].
295 nm could be assigned to the ne
zole moiety, while the longer-wavelength absorption at around
325 nm can be attributed to
* transitions from the carbazole
p
* transitions of the carba-
pep
3.4. Electrochemical analysis
moiety to the pyridine moiety [22]. The PL emission peak
at 392 nm for BDPPC was significantly overlapped with the
metal-to-ligand charge-transfer absorption peak of FIrpic, indi-
cating that the Förster energy transfer from the host materials
The electrochemical properties of BDPPC were studied in
solution using cyclic voltammetry (CV) using tetrabutylammonium
hexafluorophosphate(TBAPF6) as the supporting electrolyte
and ferrocene as the internal standard. As shown in Fig. 4, during
the anodic scan in dichloromethane, an oxidation peak is observed
at 1.54 V, which was ascribed to the oxidation of the carbazole unit
of BDPPC. No reduction waves were detected. On the basis of the
onset potentials for oxidation, we estimated the HOMO energy
of BDPPC to be ꢁ5.55 eV, with regard to ferrocene (ꢁ4.8 eV
below vacuum) [25]. The highest occupied molecular orbital
(HOMO) energy level of BDPPC is approaching the work function of
4-ethylenedioxythiophene):poly(styrene-4-sulfonate) (PEDOT:PSS,
ꢁ5.2 eV), thereby facilitating the injection of holes to the emitting
layer [20]. Through subtraction of the optical energy gap from the
HOMO energy level, we calculated the energy level of the lowest
unoccupied molecular orbital (LUMO) to be ꢁ2.02 eV. To gain more
insight into the electronic structure of BDPPC, density functional
theory (DFT) calculations were performed at the B3LYP/6-31G(d)
level for the geometry optimization. As shown in Fig. 4, the
HOMO level of BDPPC is mainly located at the electron-donating
carbazole fragments, whereas the LUMO level has a contribution
from the electron-accepting pyridine units. The calculated values of
HOMO and LUMO energy levels are in agreement with the elec-
trochemical experimental results.
Fig. 1. TGA traces of BDPPC recorded at a heating rꢁat1e of 10 ꢀC minꢁ1. Inset: DSC
measurement recorded at a heating rate of 10 ꢀC min
.

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W. Jiang et al. / Dyes and Pigments 92 (2012) 891e896
Fig. 2. AFM topographic images of (a) neat BDPPC and (b) neat BDPPC film thermal treated at 100 ꢀC for 6 h.
3.5. Electroluminescent properties
brightness of 1100 cd mꢁ2. As revealed in Fig. 5(b), the maximum
luminance efficiencies of this single-layer blue-emitting device
were 0.2 cd Aꢁ1, and the corresponding external quantum effi-
To assess the utility of this solution-processable compound as
a host material in OLEDs, we fabricated single-layer blue electro-
phosphorescent devices with BDPPC as the host and 10wt% FIrpic as
the dopant. The single-layer device with the configuration ITO/
PEDOT:PSS/Host:FIrpic(10wt%)/Cs2CO3/Al has been fabricated by
spin-coating. Fig. 5(a) presents the current densityevoltagee
luminance (IeVeL) curve of BDPPC-based device, which was turned
on at 4.5 V (corresponding to 1 cd mꢁ2) and had a maximum
ciencies and power efficiencies were 0.11% and 0.14 lm Wꢁ1
.
Generally, for single-layer devices, severe exciton quenching exists
at the cathode interface, resulting in degraded device performance
[26]. To improve the performance of the device, a thin TPBI
electron-transporting and exciton-confining layer was inserted
between the light-emitting layer and the cathode. A device with
configuration of ITO/PEDOT:PSS/Host:FIrpic(10wt%)/TPBI(30 nm)/
Cs2CO3/Al has been fabricated. The brightness of the devices with
a TPBI layer was significantly increased to 14,400 cd mꢁ2, compared
with that of single-layer device. After the insertion of the TPBI layer,
the maximum efficiency of the BDPPC-based device was largely
improved to 7.1 cd Aꢁ1, and the corresponding external quantum
efficiencies and power efficiencies were 3.6% and 5.2 lm Wꢁ1. As
shown in Fig. 5(c), the electroluminescence (EL) spectra of BDPPC-
based devices are almost identical with the CIE coordinates of (0.16,
0.37) and (0.15, 0.32) for single-layer and double-layer devices,
respectively, corresponding to the emission of FIrpic. Furthermore,
no additional emission coming from BDPPC was observed, indica-
tive of efficient energy transfer from the host to FIrpic.
Fig. 3. (a) Normalized absorption, emission and phosphorescence (77 K) spectra of
BDPPC and FIrpic in CH2Cl2; (b) Transient photoluminescence decay of BDPPC: 5wt%
Firpic and CBP: 5wt% FIrpic.
Fig. 4. The CV curves of BDPPC in CH2Cl2 solutions (10ꢁ3 M). Inset: optimized geom-
etries and calculated HOMO and LUMO density maps for BDPPC.

W. Jiang et al. / Dyes and Pigments 92 (2012) 891e896
895
Fig. 5. Comparison of device characteristics (a) luminanceevoltageecurrent density, (b) Luminance efficiencyecurrent density and (c) External quantum efficiencyecurrent
densityepower efficiency, inset: EL spectrum.
[6] Su SJ, Sasabe H, Takeda T, Kido J. Pyridine-containing bipolar host materials for
4. Conclusions
highly efficient blue phosphorescent OLEDs. Chemistry of Materials 2008;20:
1691e3.
In summary, we have designed and synthesized a novel host
material BDPPC for solution-processed blue phosphorescent
organic light-emitting devices that comprises of two pyridine
fragments linked to the C3 and C6 positions of the carbazole unit.
The non-coplanar conformation of BDPPC provides steric bulk,
resulting in a stable amorphous film, and a very high Tg. Utilizing
this new compound as host material, the solution-processed blue
phosphorescence OLEDs with FIrpic as a dopant show a maximum
current efficiency of 7.1 cd Aꢁ1. In order to obtain high-efficiency
solution-processed blue phosphorescence OLEDs, optimization of
the BDPPC-based devices require further study. Our results pave the
way toward reducing the complexity and cost of fabricating effi-
cient OLEDs for use in displays and lighting applications.
[7] Tao Y, Wang Q, Yang C, Wang Q, Zhang Z, Zou T, et al. A simple carbazole/
oxadiazole hybrid molecule: an excellent bipolar host for green and red
phosphorescent OLEDs. Angewandte Chemie-International Edition 2008;47:
8104e7.
[8] Gao ZQ, Luo M, Sun XH, Tam HL, Wong MS, Mi BX, et al. New host containing
bipolar carrier transport moiety for high-efficiency electrophosphorescence at
low voltages. Advanced Materials 2009;21:688e92.
[9] Adamovich V, Brooks J, Tamayo A, Alexander AM, Djurovich PI, D’Andrade BW,
et al. High efficiency single dopant white electrophosphorescent light emit-
ting diodes. New Journal of Chemistry 2002;26:1171e8.
[10] Burrows PE, Padmaperuma AB, Sapochak LS, Djurovich P, Thompson ME.
Ultraviolet electroluminescence and blue-green phosphorescence using an
organic diphosphine oxide charge transporting layer. Applied Physics Letters
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[11] Jiang W, Duan L, Qiao J, Wang LD, Zhang DQ, Qiu Y. Novel star-shaped host
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Acknowledgements
We thank the National Nature Science Foundation of china (No.
51073089) and the National Key Basic Research and Development
Program of China under Grant No. 2009CB623604 for support.
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