Benzimidazole-carbazole-based bipolar hosts for high efficiency blue and white electrophosphorescence applications

Article abstract of DOI:10.1039/c2jm31765f

A series of novel bipolar blue phosphorescent host materials mBICP, mBINCP and mBIPhCP have been designed and synthesized, which comprehensively outperform the widely used phosphorescent host, 1,3-di(9H-carbazol-9-yl)benzene (mCP). The thermal, photophysical and electrochemical properties of these host materials were finely tuned through linking different carbazole moieties to the benzimidazole. mBICP (T_g = 84 °C) and mBIPhCP (T_g = 103 °C) exhibit high morphological stabilities in comparison with mCP. Theoretical calculations show that the HOMO/LUMO orbitals of these materials are mainly dispersed on the electron donating and electron accepting groups, respectively. A blue PhOLED device fabricated using mBICP as the host exhibits a maximum external quantum efficiency (η_EQE,max) of 18.7% and a maximum power efficiency (η_P,max) of 33.6 lm W^-1. Interestingly, the external quantum efficiencies (η_EQE) are still as high as 17.1% at a high luminance of 1000 cd m^-2. Furthermore, the two-color, all-phosphor and single-emitting-layer white device hosted by mBICP achieved a maximum external quantum efficiency (η_EQE,max) of 20.5% corresponding to a maximum power efficiency (η_P,max) of 53.3 lm W^-1.

Full text of DOI:10.1039/c2jm31765f

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PAPER
Benzimidazole–carbazole-based bipolar hosts for high efficiency blue and
white electrophosphorescence applications
a
a
a
a
b
b
Hong Huang, Xiao Yang, Biao Pan, Lei Wang, Jiangshan Chen, Dongge Ma and Chuluo Yang
ac
*
*
*
Received 21st March 2012, Accepted 24th April 2012
DOI: 10.1039/c2jm31765f
A series of novel bipolar blue phosphorescent host materials mBICP, mBINCP and mBIPhCP have
been designed and synthesized, which comprehensively outperform the widely used phosphorescent
host, 1,3-di(9H-carbazol-9-yl)benzene (mCP). The thermal, photophysical and electrochemical
properties of these host materials were finely tuned through linking different carbazole moieties to the
benzimidazole. mBICP (Tg ¼ 84 ꢀC) and mBIPhCP (Tg ¼ 103 ꢀC) exhibit high morphological stabilities
in comparison with mCP. Theoretical calculations show that the HOMO/LUMO orbitals of these
materials are mainly dispersed on the electron donating and electron accepting groups, respectively. A
blue PhOLED device fabricated using mBICP as the host exhibits a maximum external quantum
efficiency (hEQE,max) of 18.7% and a maximum power efficiency (hP,max) of 33.6 lm Wꢁ1. Interestingly,
the external quantum efficiencies (hEQE) are still as high as 17.1% at a high luminance of 1000 cd mꢁ2
Furthermore, the two-color, all-phosphor and single-emitting-layer white device hosted by mBICP
achieved a maximum external quantum efficiency (hEQE,max) of 20.5% corresponding to a maximum
.
power efficiency (hP,max) of 53.3 lm Wꢁ1
.
et al.12 synthesized carbazole–diphenylphosphine hybrids
(mCPPO1), in which the introduction of PO and carbazole units
simultaneously endowed the material with bipolar charge
transport properties, and the external quantum efficiency of
bis((3,5-difluoro-4-cyanophenyl)pyridine) iridium picolinate
(FCNIrpic)-doped blue PhOLEDs was enhanced to 25.4%.
Cheng reported another carbazole–phosphine oxide hybrid
(BCPO) with a high Tg of 137 ꢀC and high ET of 3.01 eV.13 BCPO
was applied as a host for FIrpic, which exhibited a low turn-on
voltage of 2.8 V and a maximum external quantum efficiency of
23.5%. Recently, Gong et al.13 introduced a benzimidazole and
1. Introduction
Phosphorescent organic light-emitting diodes (PhOLEDs) have
attracted much attention because they can approach 100%
internal quantum efficiency by utilizing both singlet and triplet
excitons when they incorporate organometallic complexes as
triplet emitters.1–4 To date, red and green phosphorescent elec-
troluminescent devices with high efficiencies, long lifetimes, and
proper CIE coordinates have been well developed.5–7 However,
highly stable and efficient blue PhOLEDs are still rare. As the
host for blue OLED devices, the following aspects need to be
considered: (i) higher triplet energy than photon energies of blue
light ($2.7 eV), preventing a reverse energy transfer, (ii) HOMO/
LUMO levels match with the neighboring functional layers,
reducing charge carriers injection barrier into the emitting layer
and (iii) bipolar transporting characteristics, to increase the
electron and hole recombination ratio in the emitting layer.
Based on these issues, a few of bipolar hosts8–11 capable of
hosting phosphors have been developed. For example, Lee
diphenylamine into
a tetraphenylsilicon skeleton to form
a bipolar host, a device with p-BISiTPA as a host shows excellent
performance, with external quantum efficiency as high as 16.1%
for blue, and 19.1% for WOLED. All of these efficient blue
bipolar hosts have great potential application in practical PhO-
LEDs. Nevertheless, efficient PhOLED host materials still
require further developments.
Generally, there are two methods for designing high triplet
energy bipolar hosts for blue PhOLEDs, one approach is to
apply extremely high triplet energy units to constitute the
molecules.14–16 The other way is to limit the extent of conjugation
between groups.17–19 Groups with high triplet energies usually
possess small steric volumes and the bipolar hosts built with them
often exhibit low thermal and bad morphological stabilities.20
Thus, in this study, the compromises between the two methods
were adopted to design blue bipolar host materials. Hole-trans-
porting carbazole groups21–23 and electron-transporting
aWuhan National Laboratory for Optoelectronics, School of
Optoelectronic Science and Engineering, Huazhong University of Science
and Technology, Wuhan 430074, P. R. China. E-mail: wanglei@mail.
hust.edu.cn
bState Key Laboratory of Polymer Physics and Chemistry, Changchun
Institute of Applied Chemistry, Graduate School of Chinese Academy of
Sciences, Chinese Academy of Sciences, Changchun 130022, P. R. China.
E-mail: jschen@ciac.jl.cn
cDepartment of Chemistry, Wuhan University, Wuhan 430072, P. R.
China. E-mail: clyang@whu.edu.cn
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benzimidazole groups,9,23–25 which all possess relatively high
triplet energies, were selected for the construction of the novel
bipolar compounds. As the carbazole units are linked to the
benzimidazole segment via the meta-position of the C2-phenyl,
the target molecules are highly nonplanar and have minimal
donor–acceptor interactions. As a result, all the new blue hosts
keep high triplet energies in a range of 2.7–3.0 eV, which enable
their applications as hosts for blue phosphorescent dopants. Blue
PhOLED hosts doped with iridium(III) bis[2-(40,60-difluoro-
phenyl)-pyridinato-N,C(20)]-picolinate (FIrpic) were fabricated
and these showed excellent performances. Meanwhile, the two-
color, all-phosphor and single-emitting-layer white device hosted
by these new materials were also fabricated, and achieved
a maximum external quantum efficiency (hEQE,max) of 20.5% and
2.3. Device fabrication and measurement
The EL devices were fabricated by vacuum deposition of the
materials at a base pressure of 5 ꢂ 10ꢁ6 Torr onto glass precoated
with a layer of indium tin oxide (ITO) with a sheet resistance of
20 U per square and transmissivity of 80% in blue light. Before
deposition of an organic layer, the clear ITO substrates were
treated with oxygen plasma for 5 min. The deposition rate of
organic compounds was 0.9–1.1 A sꢁ1. Finally, a cathode
ꢀ
composed of LiF (1 nm) and aluminium (100 nm) was sequen-
tially deposited onto the substrate in the vacuum of 10ꢁ5 Torr.
The L–V–J of the devices was measured with a Keithley 2400
Source meter and PR655. All measurements were carried out at
room temperature under ambient conditions.
a maximum power efficiency (hP,max) of 53.3 lm Wꢁ1
.
2.4. Synthesis
2. Experimental section
9-(3-(1-Phenyl-1H-benzo[d]imidazol-2-yl)phenyl)-9H-carbazole
(mBICP). A mixture of 2-(3-bromophenyl)-1-phenyl-1H-benzo
[d]imidazole (1.39 g, 4.00 mmol), carbazole (0.75 g, 4.50 mmol),
CuI (23 mg, 0.12 mmol), 18-crown-6 (32 mg, 0.12 mmol), K2CO3
(2.76 g, 20 mmol), and DMPU (2.0 mL) was refluxed under
nitrogen for 48 h. After cooling, the mixture was extracted with
CH2Cl2 and washed with the dilute HCl solution, and then the
organic layer was dried over anhydrous Na2SO4. After removal
of the solvent, the residue was purified by column chromato-
graphy on silica gel using CH2Cl2 as the eluent to give a white
powder. Yield: 88%. 1H NMR (400 MHz, CDCl3) d (ppm): 8.09–
8.07 (m, 2H), 7.92–7.90 (m, 2H), 7.65–7.57 (m, 6H), 7.41–7.23
(m, 9H), 7.01–6.99 (d, J ¼ 8.0 Hz, 2H). 13C NMR (100 MHz,
CDCl3) d [ppm]: 151.55, 142.93, 140.48, 137.61, 137.31, 136.81,
131.98, 130.30, 128.77, 128.55, 127.95, 127.54, 125.98, 123.73,
123.37, 123.27, 120.20, 120.05, 110.58, 109.55. MS (APCI): m/z
436.1 [M + H]+. Anal. calcd for C31H21N3 (%): C 85.34, H 4.93,
N 9.73; found: C 85.36, H 4.94, N 9.70.
2.1. General information
All solvents and materials were used as received from commercial
1
suppliers without further purification. H NMR and 13C NMR
spectra were measured on a Bruker-AF301 AT 400 MHz spec-
trometer. Elemental analyses of carbon, hydrogen, and nitrogen
were performed on an Elementar (Vario Micro cube) analyzer.
Mass spectra were carried out on an Agilent (1100 LC/MSD
Trap) using ACPI ionization. UV-Vis absorption spectra were
recorded on a Shimadzu UV-VIS-NIR Spectrophotometer (UV-
3600). PL spectra were recorded on Edinburgh instruments
(FLSP920 spectrometers). Differential scanning calorimetry
(DSC) was performed on a PE Instruments DSC 2920 unit at
a heating rate of 10 ꢀC minꢁ1 from 30 to 250 ꢀC under nitrogen.
The glass transition temperature (Tg) was determined from the
second heating scan. Thermogravimetric analysis (TGA) was
undertaken with a PerkinElmer Instrument (Pyris1 TGA). The
thermal stability of the samples under a nitrogen atmosphere was
determined by measuring their weight loss while heating at a rate
of 10 ꢀC minꢁ1 from 30 to 700 ꢀC. Cyclic voltammetry
measurements were carried out in a conventional three-electrode
cell using a Pt button working electrode of 2 mm in diameter,
a platinum wire counter electrode, and an Ag/AgNO3 (0.1 M)
reference electrode on a computer-controlled EG&G Potentio-
stat/Galvanostat model 283 at room temperature. Reductions
CV of all compounds were performed in dichloromethane
5-(3-(1-Phenyl-1H-benzo[d]imidazol-2-yl)phenyl)-5H-pyrido
[4,3-b]indole (mBINCP). The compound
2 was prepared
according to the same procedure as the compound 1 but using
5H-pyrido[4,3-b]indole instead of carbazole. Yield: 85%. 1H
NMR (400 MHz, CDCl3) d [ppm]: 9.33 (s, 1H), 8.43–8.41 (d, J ¼
6.0 Hz, 1H), 8.17–8.15 (m, 1H), 7.96–7.90 (m, 2H), 7.68–7.56 (m,
6H), 7.42–7.26 (m, 7H), 7.10–7.08 (d, J ¼ 7.6 Hz, 1H), 6.85–6.84
(m, 1H). 13C NMR (100 MHz, CDCl3) d (ppm): 151.11, 145.43,
144.57, 142.96, 142.92, 140.50, 137.38, 136.83, 136.35, 132.34,
130.49, 130.37, 129.24, 128.89, 127.58, 127.51, 127.07, 123.86,
123.34, 121.73, 121.53, 120.64, 120.09, 110.59, 110.06, 104.79.
MS (APCI): m/z 437.1 [M + H]+. Anal. calcd for C30H20N4 (%):
C 82.55, H 4.62, N 12.84; found: C 82.52, H 4.63, N 12.85.
containing 0.1
M tetrabutylammoniumhexafluorophosphate
(Bu4NPF6) as the supporting electrolyte. The onset potential was
determined from the intersection of two tangents drawn at the
rising and background current of the cyclic voltammogram.
2.2. Computational details
9-Phenyl-3-(3-(1-phenyl-1H-benzo[d]imidazol-2-yl)phenyl)-9H-
carbazole (mBIPhCP). A mixture of 2-(3-bromophenyl)-
The geometrical and electronic properties were performed with
the Amsterdam Density Functional (ADF) 2009.01 program
package. The calculation was optimized by means of the B3LYP
(Becke three parameters hybrid functional with Lee–Yang–Per-
dew correlation functionals)26,27 with the 6-31G(d) atomic basis
set. Then the electronic structures were calculated at the s-
HCTHhyb/6–311++G(d, p) level.28 Molecular orbitals were
visualized using ADF view.
1-phenyl-1H-benzo[d]imidazole (1.04 g, 3.00 mmol), 9-phenyl-3-
(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9H-carbazole
(1.29 g, 3.50 mmol), Pd(PPh3)4 (173 mg, 0.15 mmol), 2 M
aqueous sodium carbonate solution (10 mL), and toluene (50
mL) and ethanol (25 mL) was stirred under reflux for 24 h. The
mixture was cooled to room temperature and saturated (sat.)
ammonium chloride solution (50 mL) was added, the mixture
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was extracted with CH2Cl2, after removal of the solvent, the
residue was purified by column chromatography on silica gel
using CH2Cl2 as the eluent to give a white powder. Yield: 92%.
1H NMR (400 MHz, CDCl3) d (ppm): 8.17–8.15 (d, J ¼ 7.6 Hz,
1H), 8.06 (s, 1H), 7.96–7.94 (d, J ¼ 7.6 Hz, 1H), 7.86 (s, 1H),
7.71–7.69 (d, J ¼ 7.6 Hz, 1H), 7.66–7.56 (m, 8H), 7.50–7.29 (m,
12H). 13C NMR (100 MHz, CDCl3) d [ppm]: 152.47, 143.06,
141.84, 141.36, 140.48, 137.59, 137.36, 137.29, 132.52, 130.25,
130.05, 129.96, 128.90, 128.63, 128.39, 128.31, 127.72, 127.68,
127.59, 127.09, 126.21, 125.33, 123.85, 123.43, 123.08, 120.36,
120.15, 119.93, 118.85, 110.50, 109.99, 109.97. MS (APCI): m/z
512.3 [M + H]+. Anal. calcd for C37H25N3 (%): C 86.83, H 4.94,
N 8.23; found: C 86.85, H 4.90, N 8.25.
copper(I)-catalyzed C–N coupling at meta-position in good
yields (88% and 85%, respectively), and for the compound
mBIPhCP, the carbazole moiety was linked by the Pd-catalyzed
Suzuki coupling reaction in a very good yield (92%). All the
target materials were further purified by repeated temperature-
gradient vacuum sublimation. The chemical structures of the
compounds mBICP, mBINCP and mBIPhCP were fully char-
acterized by 1H NMR and 13C NMR spectroscopies, mass
spectrometry and elemental analysis.
3.2. Thermal properties
The thermal properties of compounds mBICP, mBINCP and
mBIPhCP were determined by thermogravimetric analysis
(TGA) and differential scanning calorimetry (DSC) (Fig. 1). Key
thermal data of these compounds are listed in Table 1. The
compounds mBICP and mBIPhCP exhibit relatively high glass
transition temperatures (Tg) at 84 ꢀC and 103 ꢀC in the DSC
heating cycle, respectively, which are significantly higher than the
Tg of widely used 4,40-N,N0-carbazole-biphenyl (CBP: 62 ꢀC) and
1,3-bis(9-carbazolyl)benzene (mCP: 60 ꢀC).31 The compound
mBINCP exhibits no discernible Tg in the heating cycle even after
rapid cooling. Compounds mBICP, mBINCP and mBIPhCP
exhibit good thermal stability with decomposition temperatures
(Td, 5% weight loss) at 359, 337, and 395 ꢀC, respectively. All of
these indicate that the introduction of the benzimidazole moiety
improves the morphological stability of host materials. The
relatively high Tg and Td values make these compounds avoid
phase separation upon heating and have the potential to be
fabricated into high performance devices by vacuum thermal
evaporation technology.
3. Results and discussion
3.1. Synthesis and characterization
The bipolar molecules were synthesized by the Ullman & Suzuki
Coupling reaction in one step. The synthetic routes and chemical
structures of compounds, mBICP, mBINCP and mBIPhCP are
depicted in Scheme 1.
The intermediate 2-(3-bromophenyl)-1-phenyl-1H-benzo[d]
imidazole, 5H-pyrido[4,3-b]indole and 9-phenyl-3-(4,4,5,5-tetra-
methyl-1,3,2-dioxaborolan-2-yl)-9H-carbazole were synthesized
according to the literature.29,30 For the compounds mBICP and
mBINCP, the electron-donating carbazole moiety was intro-
duced into the withdrawing benzimidazole segment through the
3.3. Photophysical properties
Fig. 2 presents the absorption spectra and photoluminescence
(PL) spectra of compounds mBICP, mBINCP and mBIPhCP in
toluene (ca. 2 ꢂ 10ꢁ5 M) and as the vacuum-evaporated film on
a quartz substrate. The related photophysical properties are
summarized in Table 1. The absorption maxima of compounds
mBICP, mBINCP and mBIPhCP in dilute toluene solution were
observed at 292, 282 and 295 nm, respectively, which are assigned
to the carbazole-centered n–p* transition. For the compounds
mBICP and mBIPhCP, the absorption peak in the thin film is
similar to that in dilute solution, which means that no significant
intermolecular interactions occur in the ground state. But for
Scheme 1 Synthesis routes of the compounds mBICP, mBINCP and
mBIPhCP.
Fig. 1 (a) DSC traces of the compounds mBICP, mBINCP and mBIPhCP recorded at a heating rate of 10 ꢀC minꢁ1. (b) TGA thermo-grams of the
compounds mBICP, mBINCP and mBIPhCP recorded at a heating rate of 10 ꢀC minꢁ1
.
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Table 1 Physical data of compounds mBICP, mBINCP and mBIPhCP
labs (nm)
lem,max (nm)
b
c
d
HOMO/LUMO exp (eV)
e
HOMO/LUMOcal (eV)
Compound
Solutiona
Film
Solutiona
Film
Tg (ꢀC)
Td (ꢀC)
ETf (eV)
mBICP
mBINCP
mBIPhCP
292
282
295
290
288
295
346, 363
337, 351
364, 380
389
378
390
84
No
103
359
337
395
ꢁ5.7/ꢁ2.3
ꢁ5.8/ꢁ2.4
ꢁ5.6/ꢁ2.4
ꢁ5.2/ꢁ2.1
ꢁ5.5/ꢁ2.1
ꢁ5.3/ꢁ2.0
3.0
2.8
2.7
Measured in toluene. b Obtained from DSC measurements. c Obtained from TGA measurements. d Determined from the onset of oxidation potentials
a
and the Eg ¼ HOMO ꢁ LUMO. e Values from DFT calculation. f Measured in 2-methyltetrahydrofuran at 77 K.
Fig. 2 (a) Absorption and PL-emission spectra of mBICP, mBINCP and mBIPhCP in toluene; (b) absorption and PL-emission spectra of mBICP,
mBINCP and mBIPhCP in thermally evaporated film (ca. 40 nm).
compound mBINCP, the maximum absorption is red shifted by
6 nm in the thin film, which may be caused by the strong electron
affinity of pyrido[4,3-b]indole. mBICP, mBINCP and mBIPhCP
show the main emission peaks at 363, 351, and 380 nm in toluene
solution, respectively. In the solid state, the main emission peaks
of them shifted to 389, 378, 390 nm, respectively. The optical
energy bandgaps of mBICP, mBINCP and mBIPhCP are 3.4,
3.4, and 3.2 eV, respectively, which are calculated from the
threshold of the absorption spectra in toluene solution. The
phosphorescence spectra were measured in a frozen 2-methyl-
tetrahydrofuran matrix at 77 K (Fig. 3), and the triplet energy
levels were estimated from the highest-energy vibronic sub-band
of the phosphorescence spectra. These novel compounds show
higher triplet energy levels (3.0 eV for mBICP, 2.8 eV for
mBINCP and 2.7 eV for mBIPhCP) than that of blue phosphor
iridium(III) bis[2-(40,60-difluorophenyl)-pyridinato-N,C(20)]pico-
linate (FIrpic, 2.65 eV), which implies that they could serve as
host materials for blue triplet emitters.
3.4. Electrochemical properties
The electrochemical properties of these compounds were probed
by cyclic voltammetry (CV) (Fig. 4). The compounds exhibit
reversible oxidation behaviors, which can be assigned to the
oxidation of the carbazole moiety. The energy of the highest
occupied molecular orbital (HOMO) determined from the onset
of the oxidation potentials is ꢁ5.6 to ꢁ5.8 eV. And the lowest
unoccupied molecular orbital (LUMO) energy levels were
calculated from the HOMO values and the energy gaps (Eg),
which are about ꢁ2.3 to ꢁ2.4 eV. The HOMO level of these
compounds is adjacent to that of the widely used hole transport
material NPB (ꢁ5.4 eV),31 which means the hole-injection
barriers from NPB to the compounds are small. The LUMO level
of mBICP (ꢁ2.3 eV) is slightly higher than that of mBIPhCP
(ꢁ2.4 eV), which could be attributed to the different linking
modes.
Fig. 4 CV curves of mBICP, mBINCP and mBIPhCP. Working elec-
trode: Pt button; reference electrode: Ag/Ag+. Oxidation CV was per-
Fig.
3
The phosphorescence spectra of mBICP, mBINCP and
formed in dichloromethane containing 0.1 M n-Bu4NPF6 as the
supporting electrolyte at a scan rate of 100 mV sꢁ1
mBIPhCP in a frozen 2-methyltetrahydrofuran matrix at 77 K.
.
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can be rationalized by the disruption of the p-conjugation
between the electron-donating and electron-accepting moieties.
The complete separation is preferable for efficient hole- and
electron-transporting properties and the prevention of reverse
energy transfer.32 The calculated HOMO/LUMO values are in
the range of ꢁ5.5 to ꢁ5.2/ꢁ2.1 to ꢁ2.0 eV, which correlates well
with the experimental data.
3.6. Carrier-transport properties
To evaluate the bipolar characters of the compounds mBICP,
mBINCP and mBIPhCP, hole-only devices with configuration:
ITO/MoO3 (10 nm)/NPB (80 nm)/mBICP or mBINCP or
mBIPhCP (20 nm)/MoO3 (10 nm)/Al (100 nm) and an electron-
only device having the configuration: Al (100 nm)/mBICP or
mBINCP or mBIPhCP (20 nm)/Alq3 (80 nm)/LiF (1 nm)/Al (100
nm) were fabricated, respectively. MoO3 and Alq3 layers were
used to prevent hole- and electron-injection from the cathode
and anode, respectively. For the hole-only devices, most of the
electrons can be impeded and only holes can be injected from the
anode to the organic layer, which could be assigned to the large
energy injection barrier between the MoO3 (LUMO ¼ ꢁ2.3 eV)
and Al (ꢁ4.3 eV) layers. In the case of the electron-only devices,
the Alq3 layer, which has a low-lying HOMO level of ꢁ5.8 eV,
was used to prevent hole-injection from the Al anode (ꢁ4.3 eV)
to the organic layers. As shown in Fig. 6, all of these compounds
exhibit higher hole mobility than the electron mobility. Espe-
cially, the hole mobility of the compounds mBICP and
mBIPhCP are much higher than the electron mobility, which
could be attributed to the hole mobility of the carbazole that is
much greater than the electron mobility of the benzimidazole.
The difference between the compounds mBICP and mBIPhCP
could be determined by the different linking modes. For the
Fig. 5 The molecular-orbital surfaces of HOMO and LUMO of the
compounds mBICP, mBINCP and mBIPhCP calculated at the DFT//
B3LYP/6-31G level.
3.5. Quantum chemical calculations
Theoretical calculations on the electronic states of these
compounds were carried out at the DFT//B3LYP/6-31G level in
the Amsterdam Density Functional (ADF) 2009.01 program to
acquire a better understanding of the bipolar properties. As
shown in Fig. 5, the HOMO orbitals are mainly located on the
electron-donating carbazole moiety or 5H-pyrido[4,3-b]indole
moiety, while the LUMO orbitals are mainly dispersed in the
electron-accepting benzimidazole moiety. These new compounds
have almost complete separation of the HOMO and LUMO
orbitals at the hole- and electron-transporting moieties, which
Fig. 6 The current density versus voltage curves of the hole-only and electron-only devices for the compounds mBICP, mBINCP and mBIPhCP.
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compound mBINCP, the hole mobility is close to the electron
mobility, which indicates that the 5H-pyrido[4,3-b]indole
possesses both high hole mobility and high electron mobility.
This indicates that these new compounds are capable of trans-
porting both electrons and holes, and exhibit bipolar trans-
porting properties.
Device
A
hosted by the compound mBICP exhibits
a maximum current efficiency (hc,max) of 36.4 cd Aꢁ1, a maximum
power efficiency (hp,max) of 33.6 lm Wꢁ1 and a maximum external
quantum efficiency (hEQE,max) of 18.7%, and device B using
pyridine–carbazole mBINCP as the host exhibits a performance
with hc,max of 29.0 cd Aꢁ1, hp,max of 21.0 lm Wꢁ1, and hEQE,max of
13.8%. In comparison, device C with the FIrpic doped into the
host mBIPhCP as the emitting layer shows high performance
with hc,max of 33.5 cd Aꢁ1, hp,max of 30.3 lm Wꢁ1, and hEQE,max of
17.0%. Especially, these devices show a low external quantum
efficiency roll-off (Fig. 8b). For example, when the brightness
reached 1000 cd mꢁ2, the hEQE was still as high as 17.0% for
device A. The low efficiency roll-off can be attributed to the
bipolar transporting properties of the compounds, and the field-
assisted dissociation of Coulombically correlated electron hole
(e–h) pairs32,33 was blocked effectively. The Commission Inter-
national de I’Eclairage (CIE) coordinates vary very little for the
devices based on the compounds mBICP, mBINCP and
mBIPhCP, and are the same as the emission of the phospho-
rescent emitter FIrpic, suggesting that the triplet energy
completely transfers from the hosts to the guest (Fig. 8c). The EL
performances of the devices A–C coincide with the difference in
the hole- and electron-injection barriers and the ET of the hosts.3
In terms of the applicability of the host materials for all-
phosphor WOLEDs (devices D–F) with the configuration of
ITO/MoO3 (10 nm)/NPB (70 nm)/mCP (5 nm)/mBICP or
mBINCP or mBIPhCP: 5 wt% FIrpic: 0.5 wt% (fbi)2Ir(acac)
(20 nm)/TmPyPB (40 nm)/LiF (1 nm)/Al (100 nm) were fabri-
cated. FIrpic and (fbi)2Ir(acac) were co-doped into the bipolar
host mBICP or mBINCP or mBIPhCP as the single-emitting
layer. Fig. 9 shows the J–V–L characteristics and curves of
external quantum efficiency and power efficiency versus current
density. Device D with the compound mBICP as the host
achieves a maximum brightness of 48 473 cd mꢁ2 at 13.7 V, hc,max
of 55.2 cd Aꢁ1, hp,max of 53.8 lm Wꢁ1, and hEQE,max of 20.5%;
3.7. Electroluminescent devices
To evaluate these new compounds as potential host materials for
a triplet dopant, devices A–C with typical device architectures of
ITO/MoO3 (10 nm)/NPB (40 nm)/mCP (5 nm)/mBICP or
mBINCP or mBIPhCP: FIrpic (6 wt%, 20 nm)/TmPyPB (40 nm)/
LiF (1 nm)/Al (100 nm) were initially examined. NPB and
TmPyPB were used as the hole- and electron-transporting layers,
respectively; mCP as a hole-transporting material and also as an
exciton blocker (ET, 2.9 eV) to prevent diffusion of exciton to the
NPB layer; MoO3 and LiF served as hole- and electron-injecting
layers, respectively. FIrpic doped in mBICP or mBINCP or
mBIPhCP was used as the emitting layer. Fig. 7 depicts the
relative energy levels of the materials employed in the devices.
Fig. 8 shows the current density–voltage–brightness (J–V–L)
characteristics and efficiency versus current density curves for the
devices and all of the EL spectra of the devices. The EL data of
the devices are summarized in Table 2.
Fig. 7 Energy level diagram of HOMO and LUMO levels (relative to
these values are 23 030 cd mꢁ2 at 13.8 V, 36.7 cd Aꢁ1
,
vacuum level) for materials investigated in this work.
Fig. 8 (a) Current density–voltage–brightness (J–V–L) characteristics for devices A–C; (b) external quantum efficiency and power efficiency versus
current density curves for devices A–C and (c) the EL spectra of the devices A–C.
13228 | J. Mater. Chem., 2012, 22, 13223–13230
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View Article Online
Table 2 Electroluminescence properties of the devicesa
Lmax [cd mꢁ2
]
(V at Lmax, V)
Device
Host/guest
Von (v)
]
]
hEQE
CIE (x, y)c
b
hc [cd Aꢁ1
b
hp [lm Wꢁ1
b
A
B
C
D
E
F
mBICP/Ir–B
mBINCP/Ir–B
mBIPhCP/Ir–B
mBICP/Ir–B + Ir–O
mBINCP/Ir–B + Ir–O
mBIPhCP/Ir–B + Ir–O
3.2
3.7
3.3
3.1
3.7
3.4
21 550 (10.8)
9279 (11.2)
18 170 (11.2)
48 473 (13.7)
23 030 (13.8)
31 920 (13.8)
36.4, 35.1, 32.7
29.4, 28.3, 26.7
33.5, 31.6, 28.7
55.2, 44.7, 39.4
36.7, 34.1, 31.5
45.4, 39.7, 35.3
33.6, 24.0, 17.0
21.0, 16.9, 11.6
30.3, 20.5, 13.5
53.3, 27.2, 16.7
31.7, 19.9, 12.7
39.4, 19.8, 12.3
18.7, 18.2, 17.1
13.8, 13.4, 12.6
17.0, 16.0, 14.6
20.5, 14.3, 12.8
12.7, 11.8, 11.0
16.6, 13.9, 12.5
(0.14, 0.32)
(0.14, 0.32)
(0.14, 0.32)
(0.34, 0.45)
(0.42, 0.48)
(0.38, 0.47)
a
Abbreviations: Von: turn-on voltage; Lmax: maximum luminance; V: voltage; hc: current efficiency; hp: power efficiency; CIE [x, y]: Commission
International de I’Eclairage coordinates. b Order of measured value: maximum, then values at 100 and 1000 cd mꢁ2
c
. Measured at 8 V.
31.7 lm Wꢁ1, and 12.7% for device E with the compound
mBINCP as the host, and for device F with the compound
mBIPhCP as the host, the maximum brightness is 31 920 cd mꢁ2
at 13.8 V, 45.4 cd Aꢁ1, 39.4 lm Wꢁ1, and 16.6%. The efficiencies of
devices D and F are remarkably higher than those of the
comparable white device using the conventional host mCP
(hc,max of 42.6 cd Aꢁ1, hp,max of 24.3 lm Wꢁ1, and hEQE,max of
14.7%),34 and among the highest for single-emitting layer
WOLEDs reported till now.34 Device D based on mBICP has the
lowest turn-on voltage and the highest power efficiency among
all the three WOLED devices, which could be attributed to the
well-matched energy levels of the host mBICP compared to the
dopants FIrpic and (fbi)2Ir(acac). Additionally, the performance
of devices D–F exhibits a significant enhancement in comparison
with the previously reported results under the similar device
structures using the conventional host material of mCP,35 in
which the hEQE of the device based on mCP drops to only 6.3% at
the luminance of 10 000 cd mꢁ2. In this work, hEQE of devices D–
F at the luminance of 10 000 cd mꢁ2 is still as high as 10.2%, 9.3%
and 10.5%, respectively. The high efficiencies at high luminance
Fig. 9 (a) Current density–voltage–brightness characteristics for devices D–F. (b) Current efficiency and power efficiency versus current density curves
for devices D–F. (c) The normalized EL spectra of device D at various voltages. (d) The normalized EL spectra of device E at various voltages. (e) The
normalized EL spectra of device F at various voltages.
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 13223–13230 | 13229

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6 H. Sasabe, T. Chiba, S. J. Su, Y. J. Pu, K. Nakayama and J. Kido,
Chem. Commun., 2008, 5821.
7 Y. Tao, Q. Wang, C. Yang, Q. Wang, Z. Zhang, T. Zou, J. Qin and
D. Ma, Angew. Chem., Int. Ed., 2008, 47, 8104.
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9 W.-Y. Hung, L.-C. Chi, W.-J. Chen, E. Mondal, S.-H. Chou,
K.-T. Wong and Y. Chi, J. Mater. Chem., 2011, 21, 19249.
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which may result in balanced charge fluxes and a broad distri-
bution of recombination regions within the emitting layer. In
addition, devices D–F show good color stability (Fig. 9c–e).
When the voltage increases from 8 to 12 V, the CIE coordinates
vary only slightly from (0.34, 0.45) to (0.34, 0.44) for device D,
from (0.42, 0.48) to (0.41, 0.48) for device E, and from (0.38,
0.47) to (0.37, 0.47) for device F. At the driving voltage of 8 V,
devices D, E and F exhibit color rendering indexes (CRI) of 60,
52.0 and 53.8, respectively. The relative low value may be
induced by the absence of red light.
13 S. Gong, Y. Chen, C. Yang, C. Zhong, J. Qin and D. Ma, Adv.
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4. Conclusion
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15 S. Gong, X. He, Y. Chen, Z. Jiang, C. Zhong, D. Ma, J. Qin and
C. Yang, J. Mater. Chem., 2012, 22, 2894.
16 S.-J. Su, C. Cai and J. Kido, J. Mater. Chem., 2012, 22, 3447.
17 H. H. Chou and C. H. Cheng, Adv. Mater., 2010, 22, 2468.
18 F.-M. Hsu, C.-H. Chien, C.-F. Shu, C.-H. Lai, C.-C. Hsieh,
K.-W. Wang and P.-T. Chou, Adv. Funct. Mater., 2009, 19, 2834.
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21 Y. Tao, Q. Wang, C. Yang, C. Zhong, K. Zhang, J. Qin and D. Ma,
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22 S. Shao, J. Ding, T. Ye, Z. Xie, L. Wang, X. Jing and F. Wang, Adv.
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23 L.-C. Chen, W.-Y. Hung, W.-J. Chen, Y.-M. Chen, S.-H. Chou and
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24 S.-Y. Takizawa, V. A. Montes and P. Anzenbacher, Chem. Mater.,
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25 Y.-M. Chen, W.-Y. Hung, H.-W. You, A. Chaskar, H.-C. Ting,
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In conclusion, a series of bipolar hosts based on benzimidazole–
carbazole aromatic cores were designed and synthesized. Highly
efficient blue and single-emitting layer white devices were
successfully fabricated by using the bipolar compounds as host
materials. The devices hosted by mBICP, mBINCP and
mBIPhCP obtain maximum external quantum efficiencies of
18.7%, 13.8% and 17.0% for blue electrophosphorescence,
respectively. The two-color, all-phosphor single-emitting layer
white devices hosted by mBICP acquire a maximum external
quantum efficiency of 20.5% corresponding to a maximum
power efficiency of 53.3 lm Wꢁ1, which are among the highest for
single-emitting layer white PhOLEDs reported. Also, for the
devices hosted by the compounds mBINCP and mBIPhCP, the
single-emitting layer all-phosphor white devices exhibit
a maximum external quantum efficiency of 12.7% and 16.6%,
and a maximum power efficiency of 31.7 lm Wꢁ1 and 39.4 lm
Wꢁ1, respectively. All of these qualities can be attributed to the
usage of the bipolar compounds as hosts.
Acknowledgements
This research work was supported by the central allocation grant
from NSFC/China (21161160442) and Wuhan Science and
Technology Bureau (no. 01010621227) and the Analytical
and Testing Centre at Huazhong University of Science and
Technology.
30 M. Debeaux, M. W. Thesen, D. Schneidenbach, H. Hopf, S. Janietz,
€
H. Kruger, A. Wedel., W. Kowalsky and H.-H. Johannes, Adv. Funct.
Mater., 2009, 19, 1.
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A.
J. V. Grazulevicius and C. P. Hsu, Adv. Mater., 2007, 19, 862.
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J.
Simokaitiene,
S.
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