High-triplet-energy tri-carbazole derivatives as host materials for efficient solution-processed blue phosphorescent devices

Article abstract of DOI:10.1039/c0jm03365k

A novel series of solution-processible carbazole-based host materials, 3,6-bis(N-carbazolyl)-N-phenylcarbazole (BCC-36), 3,6-bis(3,6-di-tert-butyl-9- carbazolyl)-N-phenylcarbazole (BTCC-36), 2,7-bis(N-carbazolyl)-N-phenylcarbazole (BCC-27), and 2,7-bis(3,6-di-tert-butyl-9-carbazolyl)-N-phenylcarbazole (BTCC-27), is designed and synthesized. Owing to the highly twisted configuration, these hosts exhibit high triplet energy levels (2.90-3.02 eV) and high glass transition temperatures (147-210°C). They also exhibit appropriate HOMO energy levels (-5.21 - 5.36 eV), resulting in an improved hole-injection property. These novel compounds are employed to fabricate phosphorescent organic lighting-emitting diodes (OLEDs) as the host materials doped with the guests of iridium(iii) bis(4,6-difluorophenylpyridinato)- picolinate (FIrpic) and iridium(iii) bis(4′,6′- difluorophenylpyridinato)tetrakis(1-pyrazolyl)borate (FIr6) by spin coating. The best device performance of FIrpic based blue-emitting devices has a rather low turn-on voltage of 3.9 V, a maximum efficiency of 27.2 cd A^-1 (11.8 lm W^-1), and a maximum external quantum efficiency of 14.0%. Moreover, the best device performance of FIr6 based deep-blue-emitting devices exhibits a turn-on voltage of 4.9 V, a maximum efficiency of 11.5 cd A ^-1 (4.9 lm W^-1), and a maximum external quantum efficiency 6.8%. The performance data are outstanding for solution-processed blue phosphorescent OLEDs. The Royal Society of Chemistry 2011.

Full text of DOI:10.1039/c0jm03365k

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PAPER
High-triplet-energy tri-carbazole derivatives as host materials for efficient
solution-processed blue phosphorescent devices†
*
Wei Jiang, Lian Duan, Juan Qiao, Guifang Dong, Deqiang Zhang, Liduo Wang and Yong Qiu
Received 6th October 2010, Accepted 7th January 2011
DOI: 10.1039/c0jm03365k
A novel series of solution-processible carbazole-based host materials, 3,6-bis(N-carbazolyl)-N-
phenylcarbazole (BCC-36), 3,6-bis(3,6-di-tert-butyl-9-carbazolyl)-N-phenylcarbazole (BTCC-36),
2,7-bis(N-carbazolyl)-N-phenylcarbazole (BCC-27), and 2,7-bis(3,6-di-tert-butyl-9-carbazolyl)-N-
phenylcarbazole (BTCC-27), is designed and synthesized. Owing to the highly twisted configuration,
these hosts exhibit high triplet energy levels (2.90–3.02 eV) and high glass transition temperatures
(147–210 ꢀC). They also exhibit appropriate HOMO energy levels (ꢁ5.21–ꢁ5.36 eV), resulting in an
improved hole-injection property. These novel compounds are employed to fabricate phosphorescent
organic lighting-emitting diodes (OLEDs) as the host materials doped with the guests of iridium(III) bis
(4,6-difluorophenylpyridinato)-picolinate (FIrpic) and iridium(III) bis(40,60-difluorophenylpyridinato)
tetrakis(1-pyrazolyl)borate (FIr6) by spin coating. The best device performance of FIrpic based blue-
emitting devices has a rather low turn-on voltage of 3.9 V, a maximum efficiency of 27.2 cd Aꢁ1 (11.8 lm
Wꢁ1), and a maximum external quantum efficiency of 14.0%. Moreover, the best device performance of
FIr6 based deep-blue-emitting devices exhibits a turn-on voltage of 4.9 V, a maximum efficiency of 11.5
cd Aꢁ1 (4.9 lm Wꢁ1), and a maximum external quantum efficiency 6.8%. The performance data are
outstanding for solution-processed blue phosphorescent OLEDs.
fabricated by solution processing have been developed with
success,8–13 high performance solution-processed blue POLEDs
are still scarce, especially deep-blue POLEDs, owing to the lack
of appropriate host materials. For efficient solution-processed
blue POLEDs, a high triplet energy gap (>2.75 eV) is necessary,14
which prevents the reverse energy transfer from the dopant to the
host. In addition, suitable highest occupied/lowest unoccupied
molecular orbital (HOMO/LUMO) energy levels matching those
of the adjacent layers are required to reduce the operational
voltage. Moreover, good solubility and film formation ability of
the host materials are also important.
Introduction
Organic lighting-emitting diodes (OLEDs) have attracted a great
deal of attention as potential full-color flat-panel displays and
lighting sources owing to their ability to be driven by low volt-
ages, have high brightness, and are easily fabricated in large area
and thin film devices.1–5 The efficiencies of OLEDs have been
dramatically improved because of the development of efficient
phosphorescent molecules containing transition metals that can
harvest both singlet and triplet excitons for emission, providing
the opportunity to realize internal quantum efficiency close to
100%.6,7 Up to now, most highly efficient phosphorescent
OLEDs (POLEDs) have been fabricated by vacuum thermal
evaporation, which requires complex technological processes
and a large amount of organic materials are wasted, leading to
relatively high fabrication costs. Solution processes offer an
attractive alternative to vacuum deposition techniques, mainly
due to their better compatibility with low-cost production tech-
niques and large substrates. While red and green POLEDs
Recently, solution-processed blue POLEDs using small
organic molecules and conjugated polymers as host materials
have been reported.15–21 It is known that the chemical purity and
structural uniformity are very important factors that have
a dramatic influence on the performance of POLEDs. Impurities,
especially the metal catalyst used in polymer synthesis, could
result in exciton quenching and device failure. In contrast to their
polymer counterparts, small-molecule host materials are
advantageous as they can be easily synthesized and purified.
However, it is difficult for a small molecule host to meet the
requirements of both high triplet energy level and good solubility
in common organic solvents.
Key Lab of Organic Optoelectronics and Molecular Engineering of
Ministry of Education, Department of Chemistry, Tsinghua University,
Beijing, 100084, China. E-mail: qiuy@mail.tsinghua.edu.cn; Fax: +86 10
62795137; Tel: +86 10 62782197
The most common small molecule hosts for POLEDs are
carbazole derivatives, owing to their high triplet energy and
excellent hole transporting properties.22–26 Through the linking of
bulky and large-gap moieties into the 3,6-positions of
† Electronic supplementary information (ESI) available: Supporting
information. CCDC reference numbers 790490. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/c0jm03365k
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a carbazole, an unusual suppression of the electronic coupling
between molecules was achieved, and only a small reduction in
the triplet energy was observed, resulting in a family of host
materials with both high triplet energy levels and excellent film
formation ability suitable for blue POLEDs.27,28 Recently, 2,7-
substituted carbazole derivatives were reported as the promising
hole-transporting and emitting materials for small-molecule
OLEDs,29 however, blue POLEDs using 2,7-substituted carba-
zole derivatives as host materials have note been explored. In
this paper, we synthesized a series of tri-carbazole derivatives,
3,6-bis(N-carbazolyl)-N-phenylcarbazole (BCC-36), 3,6-bis(3,6-
di-tert-butyl-9-carbazolyl)-N-phenylcarbazole
(BTCC-36),
2,7-bis(N-carbazolyl)-N-phenylcarbazole (BCC-27), and 2,7-bis
(3,6-di-tert-butyl-9-carbazolyl)-N-phenylcarbazole (BTCC-27),
using a carbazole monomer as the rigid core with two carbazole
groups linked to the core part through the 3,6- and 2,7-positions
respectively. Additionally, tert-butyl groups are introduced into
the molecular structure to ensure good solubility and to form
high-quality films. As a result, the newly synthesized compounds
possess three important characteristics: I) high triplet energy
levels (2.90–3.02 eV) because of the nonconjugated linkage; II)
appropriate HOMO energy levels (ꢁ5.21–ꢁ5.36 eV), thereby
facilitating the transfer of holes from poly(3,4-ethyl-
enedioxythiophene):poly(styrene-4-sulfonate) (PEDOT:PSS) to
the emitting layer; III) the capability of forming stable amor-
phous thin films as a result of the highly twisted configuration of
the molecules. It is once reported that solution-processed deep-
blue POLEDs based on iridium cored dendrimers, achieved
a maximum efficiency of 5.4 cd Aꢁ1 (3.4 lm Wꢁ1), and a maximum
external quantum efficiency of 3.9%.30 However, up to now, no
report on solution-processed deep-blue OLEDs based on small-
molecule hosts has been found in the literature. In our work, with
the newly synthesised hosts and the blue phosphorescent emitter
FIrpic, the device performance reached a maximum efficiency
value of 27.2 cd Aꢁ1 (11.8 lm Wꢁ1, 14.0%). And when the deep-
blue phosphorescent emitter FIr6 was used, the deep-blue-emit-
ting devices reached a maximum efficiency of 11.5 cd Aꢁ1 (4.9 lm
Wꢁ1, 6.8%), which is much better than the iridium cored den-
drimer based devices.
Scheme 1 Synthetic routes toward BCC-36, BTCC-36, BCC-27 and
BTCC-27.
same route as BCC-36 and BTCC-36, by reacting intermediate
(3) with iodobenzene to form 2,7-dibromo-N-phenylcarbazole
(4), followed by a palladium-catalyzed aromatic C–N coupling
reaction of (4) with carbazole and 3,6-di-tert-butylcarbazole.
All compounds were purified using the silica column method
with further purification by sublimation, producing very pure
1
powders. H-NMR, mass spectrometry, and elemental analysis
were employed to confirm the chemical structures of above-
mentioned compounds as described in the experimental section
(see ESI†, Fig. S1, Fig. S2 and Fig. S3). The molecular structure
of BCC-27 was further determined by single-crystal X-ray crys-
tallographic analysis. As shown in Fig. 1, the dihedral angles of
benzene–carbazole and carbazole–carbazole were in the range of
50–70ꢀ, resulting in a rigid backbone with a highly twisted
structure. This result matches the 3D model of BCC-27 opti-
mized by the Gaussian 03, program which is to be discussed in
the following section.
Results and discussion
Thermal analysis
Synthesis and characterization
The thermal properties of the new compounds were investigated
by thermal gravimetric analyses (TGA) and differential scanning
The synthetic routes and chemical structures of BCC-36, BTCC-
36, BCC-27, and BTCC-27 are shown in Scheme 1. N-phenyl-
carbazole (1) was isolated with an 80% yield by a classic Ullmann
reaction of carbazole and iodobenzene in the presence of a cata-
lytic amount of copper(I) iodide and 1,10-phenanthroline in
DMF. Bromination of the carbazole derivative 1 with NBS in
CH2Cl2 at 0 ꢀC gave 3,6-dibromo-N-phenylcarbazole (2) with
a yield of 85%. The palladium-catalyzed aromatic C–N coupling
reaction of carbazole and 3,6-di-tert-butylcarbazole with inter-
mediate 2 led to BCC-36 and BTCC-36, respectively. 4, 40-
Dibromo-2-nitrobiphenyl was prepared in 90% yield by nitration
of 4, 40-dibromobiphenyl with concentrated nitric acid. A
reductive Cadogen ring-closure in the presence of triethyl phos-
phite then produced the 2,7-dibromocarbazole (3) in 50% yield.31
Compounds BCC-27 and BTCC-27 were synthesized via the
Fig. 1 ORTEP drawing (with 35% probability ellipsoids) of one of the
two BCC-27 molecules in the asymmetric unit.
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Fig. 2 TGA traces of BCC-36, BTCC-36, BCC-27, and BTCC-27
recorded at a heating rate of 10 ꢀC minꢁ1. Inset: DSC measurement
recorded at a heating rate of 10 ꢀC minꢁ1
.
calorimetry (DSC). As mentioned above, the introduction of
carbazole groups to the 3,6 and 2,7 positions of carbazole renders
the molecule in a non-planar structure. 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) in the range of 364–407 ꢀC and very
high glass-transition temperatures (Tg) ranging from 147–210 ꢀC
(Fig. 2), which are much higher than those of analogous carba-
zole-based host materials such as 4,40-N,N0-dicarbazole-biphenyl
(CBP) (62 ꢀC) and 1,3-bis(9-carbazolyl)benzene (mCP) (60 ꢀC).27
Noticeably, BTCC-36 and BTCC-27 exhibit much higher Td and
Tg values than BCC-36 and BCC-27, which is obviously related
to their increased molecular sizes by the introduction of tert-
butyl groups in their structures.
Fig. 3 AFM topographic images of BCC-36, BTCC-36, BCC-27,
BTCC-27 and mCP: left) each with 10 wt% FIrpic and 30 wt% OXD-7;
right) each doped with 10 wt% FIr6.
Furthermore, atomic force microscopy (AFM) was used to
investigate the morphology of films prepared from the
compounds doped with FIrpic, 1,3-bis[(4-tert-butylphenyl)-
1,3,4-oxadiazolyl]phenylene (OXD-7) and FIr6 through spin-
coating a 1,2-dichloroethane solution onto the PEDOT:PSS
layer. As we can see from Fig. 3, the surface of the compounds
doubly doped with 10 wt% FIrpic and 30 wt% OXD-7, are free of
pinholes and quite smooth, and the root-mean-square (RMS)
values range from 0.62 to 0.45 nm. For the hspin-coated, blue-
emitting film doped with 10 wt% FIr6, the RMS surface rough-
ness was between 0.48 and 0.34 nm. No phase separation or any
aggregated domains were observed. However, the films prepared
from mCP doped with FIrpic and FIr6 onto the PEDOT:PSS
layer exhibited a kind of aggregation, or phase segregation. The
results demonstrate that these new compounds can form uniform
amorphous films upon solution-processing, which is highly
important in improving the efficiency and lifetime of OLEDs.
accepting holes in OLEDs. In contrast, when the electrochemi-
cally active sites (3,6-positions of carbazole) are left unblocked,
the oxidation processes of BCC-36 and BCC-27 (inset of Fig. 4a
and c) are not reversible, with the oxidation potential gradually
shifting to lower potentials and the current increasing during
Electrochemical analysis
The electrochemical properties of the compounds were studied in
solution through cyclic voltammetry (CV) using tetrabuty-
lammonium hexafluorophosphate (TBAPF6) as the supporting
electrolyte and ferrocene as the internal standard (Fig. 4). During
the anodic scan in dichloromethane, BTCC-36 and BTCC-27
showed reversible oxidation behavior, which can be attributed to
the introduction of two tert-butyl groups at the 3,6-positions of
carbazole, suggesting that the compounds can be stable upon
Fig. 4 Oxidation part of the CV curves of (a) BCC-36, (b) BTCC-36, (c)
BCC-27 and (d) BTCC-27 in CH2Cl2 solutions (Inset: ten cycles).
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repeated CV scans.32 As revealed in the literature,28 it is impor-
tant to block the active sites of carbazole derivatives when the
compounds transport positive charge carriers in devices.
The HOMO energy levels were obtained according to the
following equation: HOMO ¼ ꢁ(4.8 + Eoox) eV,33 using ferro-
cene as an internal standard (ꢁ4.8 eV in a vacuum). These were
calculated to be ꢁ5.36, ꢁ5.30, ꢁ5.29 and ꢁ5.21 eV, for BCC-36,
BTCC-36, BCC-27, and BTCC-27 respectively, according to the
corresponding onset potentials of 1.20, 1.14, 1.13, and 1.05 V,
respectively. The HOMO energy levels are clearly raised and
approach the work function of PEDOT (ꢁ5.2 eV). In contrast,
the HOMO energy level of mCP was ꢁ5.91 eV according to the
corresponding onset potentials of 1.75 V (see ESI, Fig. S4†).
These results reveal that the introduction of two carbazole
groups linked through the 3,6 and 2,7 positions of the carbazole
unit can lead to the reduction of the hole injection barrier,
thereby facilitating the injection of positive charge carriers. These
together with absorption spectra were then used to obtain the
LUMO energy levels (Table 1).
Fig. 5 Optimized geometries and calculated HOMO and LUMO density
maps for BCC-36, BTCC-36, BCC-27, and BTCC-27 according to DFT
calculations at the B3LYP/6–31* level.
Photophysical properties
Theoretical calculations
Fig. 6 exhibits the room-temperature absorption and photo-
luminescent (PL) spectra of the compounds in CH2Cl2 and all the
results are summarized in Table 1. The absorption spectra of all
the compounds exhibit similar patterns with three absorption
bands centered at 250–350 nm, which are nearly identical to
To gain insight into the relationship of the compounds at the
molecular level, the geometrical and electronic properties of the
compounds were studied using density functional theory (DFT)
calculations. The geometries of the new compounds in Fig. 5
show that the carbazole and phenyl units are significantly twisted
with the carbazole core, resulting in a non-planar structure in
each molecule. In the case of BCC-27, the dihedral angles of
benzene–carbazole and carbazole–carbazole were 57.6ꢀ, 57.8ꢀ
and 56.4ꢀ, respectively, which is close to the experimental values
of 59.1ꢀ, 60.5ꢀ and 66.5ꢀ from the single-crystal X-ray diffraction
analysis. These geometrical characteristics can effectively prevent
intermolecular interactions between p-systems and thus suppress
molecular recrystallization and limit the extent of conjugation
between the central core and the branches, which improves the
morphological stability of thin film and keeps the triplet energy
gap at a very high level as seen in these molecules. Calculated
HOMO and LUMO density maps of the compounds are also
included in Fig. 5. The LUMO levels of all the compounds are
localized predominantly on the carbazole core, while the HOMO
levels are distributed over the electron-rich tri-carbazole frag-
ment. The calculated values of the energy levels are in agreement
with the results measured by electrochemical CV.
Table 1 The thermal, electrochemical, and photophysical data of BCC-
36, BTCC-36, BCC-27, and BTCC-27
lmax,abs lem
lem
HOMOc/
Tg/Td (ꢀC) (nm)a (nm)a (nm)b ET(eV) LUMOd (eV)
BCC-36
BTCC-36 210/417
BCC-27 147/364
153/399
343
345
342
348
387
399
363
379
427
419
410
417
2.90
2.96
3.02
2.97
5.36/1.88
5.30/1.78
5.29/1.90
5.21/1.84
BTCC-27 201/389
a
b
Measured in CH2Cl2. Measured in 2-MeTHF glass at 77 K.
c
d
Deduced from cyclic voltammetry. Estimated from the optical band
gap together with HOMO values.
Fig. 6 Normalized absorption (a) and emission (b) spectra of BCC-36,
BTCC-36, BCC-27, and BTCC-27 in dilute dichloromethane solution.
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those of the unsubstituted carbazole monomer and thus can be
attributed to the n–p and p–p transitions.29 Upon UV excita-
tion, the PL spectra lost vibronic structure and the emission
peaks red-shifted to 363–399 nm with respect to the carbazole
monomer emission. Due to the steric effect of the two tert-butyl
groups at the 3,6-positions of the carbazole, the peaks of
absorption and emission of BTCC-36 and BTCC-27 are shifted
to longer wavelength, relative to BCC-36 and BCC-27.
Remarkably, the 2,7-substituted carbazole compounds have
a larger molar absorptivity than the 3,6-substituted compounds,
and show hypsochromic shift compared to the PL wavelengths of
3,6-substituted carbazole compounds. The highest energy 0–
0 phosphorescent emission was used to calculate the triplet
energy level, giving a value of 2.90 eV for BCC-36, 2.96 eV for
BTCC-36, 3.02 eV for BCC-27 and 2.97 eV for BTCC-27, higher
than the values of the commonly used triplet blue-emitter FIrpic
(2.62 eV) and FIr6 (2.72 eV).34,35 Using host materials that
possess high triplet energies is a provision for effective confine-
ment of the triplet excitons on the guest and, consequently,
prevention of back energy transfer between the host and dopant
molecules. Based on the results of the measurements, the newly
synthesized compounds with high triplet energy levels are
expected to serve as appropriate hosts for FIrpic and FIr6.
Electroluminescent properties
To assess the utility of those compounds as host materials in
OLEDs, the solution-processed electrophosphorescent devices
were using a blue emitter, FIrpic, as the dopant. Devices of type
A with the configuration ITO/PEDOT:PSS/Host:OXD-7(30 wt
%):FIrpic(10 wt%)/TPBI/Cs2CO3/Al have been fabricated, where
the conducting polymer PEDOT:PSS was used as the hole-
injection layer; the electron-transporting material OXD-7 was
mixed into the host materials to facilitate electron transport in
the emitting layer,36–38 FIrpic with an optimized concentration of
10 wt% was used as the emitter; TPBI was used as the hole- and
exciton-confining layer; and Cs2CO3 was used as the electron-
injection layer. Fig. 7 presents the current density–voltages–
luminance (J–V–L) characteristics and curves of efficiency versus
current density of the devices of type A. According to the J–V
characteristic curves of the devices of type A, the introduction of
two tert-butyl groups in the molecules leads to a decrease in
current density, with an increase of turn-on voltage (corre-
sponding to 1 cd mꢁ2) of 4.0 V, 4.3 V, 3.9 V and 4.2 V for BCC-
36, BTCC-36, BCC-27, and BTCC-27, respectively, which are
much lower than previously reported values based on solution-
processed blue electrophosphorescent devices.19,20,21 The low
turn-on voltage indicates an efficient and balanced charge
injection, transport, and combination, most likely due to the
HOMOs energy levels of these hosts being better matched with
the PEDOT:PSS and thereby facilitating the hole injection of the
device. The BCC-36 and BCC-27 based devices show the
maximum luminance (Lmax) of 18 600 cd mꢁ2 (13.5 V) and
19 200 cd mꢁ2 (14.0 V), the maximum efficiency (hc,max, hp,max) of
19.7 cd Aꢁ1, 8.5 lm Wꢁ1 and 18.5 cd Aꢁ1, 7.6 lm Wꢁ1, and
a maximum external quantum efficiency (hext,max) of 10.3% and
9.7%, respectively. The performances of the BTCC-36
and BTCC-27 based devices are much better with Lmax of 27 300
cd mꢁ2 (13.7 V) and 27 200 cd mꢁ2 (14.5 V), the maximum
Fig. 7 (a) I–V–L characteristics and (b) luminance efficiency versus
current density plots of the BCC-36, BTCC-36, BCC-27, and BTCC-27
for device of type A. (inset: EL spectra for A devices).
efficiency (hc,max, hp,max) of 27.2 cd Aꢁ1, 11.8 lm Wꢁ1 and 21.7 cd
Aꢁ1, 9.3 lm Wꢁ1, and a maximum external quantum efficiency
(hext,max) of 14.0% and 11.1%, respectively. The performances of
the devices are far superior to those of the corresponding mCP-
based devices (Lmax of 8900 cd mꢁ2, hc,max of 6.2 cd Aꢁ1 and
hext,max of 3.1%) in our previous report.20 Table 2 summarizes the
performance of the devices of type A. As shown in Table 2, the
devices based on compounds with tert-butyl groups, BTCC-36
and BTCC-27, show a better performance than BCC-36 and
BCC-27 based devices. This result is also consistent with the view
that the introduction of tert-butyl groups into the host materials
can reduce the intermolecular interactions and suppress triplet–
triplet annihilation in the devices.39
The electroluminescence (EL) spectra of devices are almost
identical with the Commission Internationale de L’Eclairage
(CIE) coordinates of (0.15, 0.32), corresponding to the emission
of FIrpic, which was comparable to the literature value reported
by Kido’s group,40 with CIE coordinates of (0.16, 0.32). When
the driving voltage was increased to 12 V, the CIE coordinates of
the emission color remained almost unchanged. Furthermore, no
additional emission coming from the host materials was
observed, indicative of efficient energy transfer from the host to
FIrpic. The obtained results are among the best for solution
processed blue POLEDs based on small molecular host mate-
rials. It was reported that the low-lying HOMO level of the host
materials is expected to give negative influence on the device
performance.41 However, there are also some reports of high
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Table 2 Summary of the characteristics of FIrpic and FIr6 doped blue phosphorescent devices
b
c
)
Device
A
Host
Von (V)a
Lmax (cd mꢁ2
)
hc,max (cd Aꢁ1
hp,max (%)d
hext,max (%)e
CIE (x, y)f
BCC-36
BTCC-36
BCC-27
BTCC-27
BCC-36
BTCC-36
BCC-27
BTCC-27
mCP
4.0
4.3
3.9
4.2
4.9
5.0
5.0
5.1
6.8
18600 (13.5 V)
27300 (13.7 V)
19200 (14.0 V)
27200 (14.5 V)
11200 (13.4 V)
14500 (13.9 V)
12300 (13.4 V)
13700 (13.5 V)
9500 (15.5 V)
19.7
27.2
18.5
21.7
7.4
11.5
6.8
9.8
3.6
8.5
11.8
7.6
9.3
3.1
4.9
2.8
4.3
0.9
10.3
14.0
9.7
11.1
4.2
6.8
4.1
5.8
2.0
(0.15, 0.32)
(0.15, 0.33)
(0.15, 0.32)
(0.15, 0.34)
(0.16, 0.25)
(0.15, 0.24)
(0.15, 0.24)
(0.16, 0.24)
(0.15, 0.24)
B
a
b
Recorded at 1 cd mꢁ2
Measured at 8 V.
.
Maximum luminance. c Maximum current efficiency. d Maximum powder efficiency. e Maximum external quantum efficiency.
f
efficiency blue POLEDs based on the host with low-lying
HOMO levels.19,42 The high efficiencies of the blue PhOLEDs
were attributed to the lower barrier injection of holes and
balanced recombination thereafter. Therefore, we attribute the
high performance to the excellent film forming ability and the
high triplet energy of host materials and balanced charge
recombination in the devices.
It is noteworthy that all the host materials possessed high
triplet energy levels (>2.90 eV), which are suitable to serve as
appropriate hosts for deep-blue phosphorescent emitters such as
FIr6. Up to now, there has been no report on solution-processed
deep-blue POLEDs with small-molecule hosts in the literature.
To test the applicability of the hosts for deep-blue emitters, we
fabricated devices of type B with a configuration of ITO/
PEDOT:PSS/Host:FIr6 (10 wt%)/TAZ/Cs2CO3/Al. The 3-(4-
biphenyl-yl)-4-phenyl-5(4-tert-butylphenyl)-1,2,4-triazole (TAZ)
was used as the electron-transporting and hole-blocking layer.43
The devices performances are presented in Fig. 8 with the key
parameters summarized in Table 2. As shown in Table 2, the
present devices show turn-on voltages of ꢂ5.0 V, which are better
than those of conventional deep-blue POLEDs using vacuum
deposition technology.41,44 The devices of compounds with tert-
butyl groups, BTCC-36 and BTCC-27, show better performance
than the BCC-36 and BCC-27 devices. For instance, the BCC-36
and BCC-27 devices display a hc,max and hp,max of 7.4 cd Aꢁ1, 3.1
lm Wꢁ1 and 6.8 cd Aꢁ1, 2.8 lm Wꢁ1, and a hext,max of 4.2% and
4.1%, respectively, while the BTCC-36 and BTCC-27 devices
show much better with a hc,max and hp,max of 11.5 cd Aꢁ1, 4.9 lm
Wꢁ1 and 9.8 cd Aꢁ1, 4.3 lm Wꢁ1, and a hext,max of 6.8% and 5.8%,
respectively. All devices exhibit a similar maximum luminescence
wavelength at round 460 nm, which originates from the triplet
emission of FIr6. The relative intensity of the shoulder at 487 nm
is varied, which is attributed to an optical effect of different
recombination zones.45 Furthermore, no additional emission
coming from host materials was observed, indicative of efficient
energy transfer from the hosts to FIr6. As shown in Fig. 8, the
CIE coordinates are (0.16, 0.25), (0.15, 0.24), (0.15, 0.24), and
(0.16, 0.24) for BCC-36, BTCC-36, BCC-27, and BTCC-27
devices, respectively, which was comparable to the literature
value reported by Thompson’s group,35 with the CIE coordinates
of (0.16, 0.28). For comparison, we prepared a device employing
mCP as the host material in the same device architecture, and
found the corresponding turn-on voltage to be 6.8 V and
maximum efficiency to be 3.0 cd Aꢁ1, 0.9 lm Wꢁ1 (Table 2). The
Fig. 8 (a) I–V–L characteristics and (b) luminance efficiency versus
current density plots of the BCC-36, BTCC-36, BCC-27, and BTCC-27
for devices of type B. (inset: EL spectra for devices of type B).
BCC-36, BTCC-36, BCC-27, and BTCC-27 based devices show
significantly improved performance, in the turn-on voltage,
brightness and luminance efficiency, as compared to those of
mCP. Moreover, the comprehensive performance is higher than
the iridium cored dendrimers based solution-processed deep-blue
POLEDs (5.0 V, 5.4 cd Aꢁ1, 3.4 lm Wꢁ1, 3.9%).30
Experimental
General information
All reactants and solvents, unless otherwise stated, were
purchased from commercial sources and used as received.
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J. Mater. Chem., 2011, 21, 4918–4926 | 4923

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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 Ele-
mentar Vario EL CHN elemental analyzer. Mass spectrometry
was performed with a Thermo Electron Corporation Finnigan
LTQ mass spectrometer. Absorption spectra were recorded with
a UV-vis spectrophotometer (Agilent 8453) and PL spectra were
recorded with a fluorospectrophotometer (Jobin Yvon, Fluo-
roMax-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. The glass-
transition temperature was recorded by DSC at a heating rate of
10 ꢀC minꢁ1 with a thermal analysis instrument (DSC 2910
modulated calorimeter). Cyclic voltammetry was performed 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 elec-
trolyte 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 were
taken. The film surface morphology was measured with AFM
(Seiko Instruments, SPA-400). The photographs of solutions
with host and dopant materials are provided in ESI†, Fig. S6.
for 6 min. A 40 nm PEDOT:PSS (Baytron P VP CH 8000)
aqueous solution was spin coated onto the ITO substrate and
ꢀ
baked at 210 C for 10 min to remove the residual water. The
substrates were then taken into a nitrogen glove box, where the
emitting layers were 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 30 nm TPBI (TAZ) was evaporated at an
evaporation rate of 1–2 A sꢁ1 under a pressure of 4 ꢃ 10ꢁ4 Pa and
ꢀ
the Cs2CO3/Al bilayer cathode was evaporated at evaporation
rates of 0.2 and 10 A sꢁ1 for Cs2CO3 and Al, respectively, under
ꢀ
a pressure of 1 ꢃ 10ꢁ3 Pa. The active area of the device was
9 mm2. The current–voltage–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 atmo-
sphere without further encapsulation.
Materials
Compounds
N-phenylcarbazole,47
3,6-dibromo-N-phenyl-
carbazole,47 4,40-dibromo-2-nitrobiphenyl,31 2,7-dibromocarba-
zole31 and 2,7-dibromo-N-phenylcarbazole29 were prepared
according to published procedures. 3,6-Bis(N-carbazolyl)-N-
phenylcarbazole (BCC-36), 3,6-bis(3,6-di-tert-butyl-9-carba-
zolyl)-N-phenylcarbazole (BTCC-36), 2,7-bis(N-carbazolyl)-N-
phenylcarbazole (BCC-27), and 2,7-bis(3,6-di-tert-butyl-9-car-
bazolyl)-N-phenylcarbazole (BTCC-27) were synthesized by the
same procedure: A mixture of 3,6-dibromo-9-phenylcarbazole
(or 2,7-dibromo-9-phenylcarbazole) (1.5 mmol), carbazole (or
3,6-di-tert-butylcarbazole) (3.2 mmol), Pd2(dba)3 (0.06 mmol),
tri-tert-butylphosphine (0.06 mmol), t-BuONa (4.5 mmol) and
toluene (50 mL) was heated at 110 ꢀC for 24 h. After cooling, the
mixture was treated with water and extracted with diethyl ether.
The organic extract was dried over MgSO4 with removal of the
volatiles. The residue was purified by column chromatography
on silica gel using ethyl acetate–n-hexane as the eluent.
X-Ray structural analysis
Colorless crystals of BCC-27 suitable for X-ray diffraction
studies were grown by vacuum sublimation at 330 ꢀC. Single-
crystal X-ray-diffraction data were obtained using a Bruker
APEX charge-coupled device (CCD) diffractometer. The
measurements were performed at 273 K using graphite-mono-
ꢀ
chromatized Mo-Ka radiation (l ¼ 0.71073 A). Direct phase
determination yielded the positions of all non-hydrogen atoms.
All non-hydrogen atoms were subjected to anisotropic refine-
ment. All hydrogen atoms were generated theoretically and rode
on their parent atoms in the final refinement. Crystal data for
BCC-27: C42H27N3, M ¼ 573.67, monoclinic, P21/c, a ¼ 26.259
3
ꢀ
ꢀ
ꢀ
ꢀ
(5) A, b ¼ 14.225(3) A, c ¼ 16.316(3) A, V ¼ 6076(2) A , Z ¼ 8,
33684 reflections measured, 10680 unique (Rint ¼ 0.094) which
were used in all calculations. The final wR(F2) was 0.135 (all
data). The crystallographic data of BCC-27 has been desposited
to the Cambridge Crystallographic Data centre with deposition
number CCDC-790490. This data can be obtained free of charge
from the Cambridge Crystallographic Data centre via www.ccdc.
cam.ac.uk/data_request/cif.
N-Phenylcarbazole (1). The mixture of iodobenzene (8.16 g, 40
mmol), carbazole (5.60 g, 35 mmol), copper(I) iodide (0.06 g, 3
mmol), 1,10-phenanthroline (0.06 g, 3 mmol), K2CO3 (4.90 g,
35 mmol), and dried DMF 100 mL was refluxed under an argon
atmosphere for 24 h. After cooling to room temperature, the
solvent was removed under vacuum and the residue was
extracted with dichloromethane. The product was then obtained
by column chromatography on silica gel with petroleum ether/
ethyl acetate (20 : 1) as the eluent, to yield a white solid (6.80 g,
80.0%). 1H-NMR (600 MHz, CDCl3, d): 8.18 (d, J ¼ 8.2 Hz, 2H),
7.62–7.57 (m, 4H), 7.49–7.41 (m, 5H), 7.32 (t, J ¼ 8.2 Hz, 2H).
Quantum chemical calculations
The geometrical and electronic properties of BCC-36, BTCC-36,
BCC-27 and BTCC-27 were performed with the Gaussian 03
program package.46 The calculation was optimized at the
B3LYP/6-31G(d) level of theory. The molecular orbitals were
visualized using Gaussview.
3,6-Dibromo-N-phenylcarbazole (2). N-Phenylcarbazole (1)
(6.80 g, 28 mmol) was dissolved in CH2Cl2 (300 mL) and lined
with aluminium foil. A solution of N-bromosuccinimide (10.68 g,
ꢀ
60 mmol) in CH2Cl2 (100 mL) was added slowly at 0 C. The
mixture was stirred for another 12 h, then quenched with water
and extracted with ether. The organic extract was dried over
MgSO4 with removal of the volatiles. The residue was recrys-
tallised with ethanol to give a white solid (9.54 g, 50.0%).
Device fabrication and performance measurements
In a general procedure, indium-tin oxide (ITO)-coated glass
substrates were pre-cleaned carefully and treated by UV ozone
4924 | J. Mater. Chem., 2011, 21, 4918–4926
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1H-NMR (600 MHz, CDCl3, d): 8.19 (d, J ¼ 8.2 Hz, 2H), 7.62 (t,
J ¼ 7.6 Hz, 2H), 7.50–7.42(m, 5H), 7.32 (t, J ¼ 8.0 Hz, 2H).
130.3, 128.1, 127.0, 123.7, 123.3, 122.1, 121.4, 119.8, 116.3, 109.2,
108.7, 34.8, 32.1, 31.7. MS (MALDI-TOF) [m/z]: calcd for
C58H59N3, 797.5; found, 797.6. Anal. calcd for C58H59N3: C,
87.28; H, 7.45; N 5.26. Found: C, 87.18; H, 7.33; N 5.42.
2,7-Dibromocarbazole (3). A mixture of 4,40-dibromo-2-nitro-
biphenyl (10 g, 28 mmol) and triethyl phosphite (50 mL) was
refluxed under nitrogen for 24 h. The excess trithyl phosphite was
distilled off and the residue was purified by column chromato-
graphy on silica gel using ethyl acetate–n-hexane (20 : 1) as an
Conclusions
In summary, we have designed and synthesized a new series of
host materials BCC-36, BTCC-36, BCC-27, and BTCC-27 for
solution processed blue phosphorescent organic light-emitting
devices. By substitution at the 3, 6 and 2, 7 positions of carbazole
with the rigid carbazole units, the new compounds showed highly
twisted configurations, which effectively suppress molecular
recrystallization and limit the extent of conjugation, and
exhibited excellent film formation ability, high triplet energy level
and electrochemical stability. Utilizing these new compounds as
hosts, the solution-processed blue phosphorescence OLEDs with
FIrpic as a dopant show low turn-on voltage of ꢂ4.0 V,
a maximum current efficiency of 27.2 cd Aꢁ1, a maximum effi-
ciency of 11.8 lm Wꢁ1 and a maximum external quantum effi-
ciency 14.0%. This can be attributed to the high triplet energy
level, well matched energy levels with the PEDOT:PSS layer and
excellent film forming ability. Moreover, we fabricated solution-
processed deep-blue POLEDs with FIr6 as a dopant, the devices
show low turn-on voltage of ꢂ5.0 V, high efficiency of 11.5 cd
Aꢁ1, 4.9 lm Wꢁ1. The performance of the devices is far superior to
those of the corresponding mCP-based devices, which is
1
eluent to provide a white solid (4.55 g, 85.0%). H-NMR (600
MHz, CDCl3, d): 8.08 (s, 1H), 7.86 (d, J ¼ 8.2 Hz, 2H), 7.60 (d,
J ¼ 1.5 Hz, 2H), 7.32 (t, J ¼ 8.2 Hz, 2H).
2,7-Dibromo-N-phenylcarbazole (4). The mixture of iodo-
benzene (3.26 g, 16 mmol), 2,7-dibromocarbazole (3) (4.55 g, 14
mmol), copper(I) iodide (0.04 g, 2 mmol), 1,10-phenanthroline
(0.04 g, 2 mmol), K2CO3 (2.80 g, 20 mmol), and dried DMF
50 mL was refluxed under an argon atmosphere for 24 h. After
cooling to room temperature, the solvent was removed under
vacuum and the residue was extracted with dichloromethane.
The product was then obtained by column chromatography on
silica gel with petroleum ether/ethyl acetate (20 : 1) as the eluent,
to yield a white solid (4.38 g, 78.0%). 1H-NMR (600 MHz,
CDCl3, d): 7.94 (d, J ¼ 8.2 Hz, 2H), 7.65 (t, J ¼ 7.6 Hz, 2H),
7.54–7.47(m, 5H), 7.40 (t, J ¼ 8.2 Hz, 2H).
BCC-36. White solid. Yield 82%. 1H-NMR (600 MHz, CDCl3,
d): 8.29 (s, 2H), 8.17 (d, J ¼ 7.6 Hz, 4H), 7.75–7.72 (m, 4H), 7.66
(d, J ¼ 8.9 Hz, 2H), 7.61–7.58 (m, 3H), 7.40–7.39 (m, 8H), 7.29–
7.27 (m, 4H). 13C-NMR (600 MHz, CDCl3, d): 141.9, 140.8,
130.4, 130.3, 128.3, 127.3, 126.3, 126.0, 124.0, 123.2, 120.4, 119.8,
111.4, 109.8. MS (MALDI-TOF) [m/z]: calcd for C42H27N3,
573.2; found, 573.4. Anal. calcd for C42H27N3: C, 87.93; H, 4.74;
N 7.32. Found: C, 87.85; H, 4.70; N 7.37.
outstanding for
a solution-processed blue phosphorescent
OLED. Our work suggests that highly efficient solution pro-
cessed blue phosphorescence OLEDs can be achieved in the
design of such host materials with high triplet energy, excellent
film-forming ability and morphological property.
Acknowledgements
BTCC-36. White solid. Yield 77%. 1H-NMR (600 MHz,
CDCl3, d): 8.24 (d, J ¼ 1.4 Hz, 2H), 8.16 (d, J ¼ 2.0 Hz, 4H),
7.75–7.70 (m, 4H), 7.64 (d, J ¼ 8.2 Hz, 2H), 7.60–7.57 (m, 3H),
7.46 (d, J ¼ 2.0 Hz, 2H), 7.45 (d, J ¼ 1.4 Hz, 2H), 7.34 (d, J ¼ 8.2
Hz, 4H), 1.47 (s, 36H). 13C-NMR (600 MHz, CDCl3, d): 142.5,
140.4, 140.2, 130.2, 127.2, 126.0, 123.9, 123.6, 123.1, 119.3, 116.2,
111.1, 109.1, 34.7, 32.1, 31.6. MS (MALDI-TOF) [m/z]: calcd for
C58H59N3, 797.5; found, 797.6. Anal. calcd for C58H59N3: C,
87.28; H, 7.45; N 5.26. Found: C, 87.26; H, 7.42; N 5.35.
We thank the National Nature Science Foundation of china
(Grant No. 51073089) and the National Key Basic Research and
Development Program of China (Grant No. 2006CB806203,
2009CB623604) for support. We also thank Prof Ru. Ji. Wang
for helpful in X-ray structural analyses.
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