Novel electron-type host material for unilateral homogeneous phosphorescent organic light-emitting diodes with low efficiency roll-off

Article abstract of DOI:10.1039/c2jm33988a

Two novel simple electron-transport (ET) type host materials, 2,6-bis(3-(1-phenyl-1H-benzo[d]imidazol-2-yl)phenyl)benzene (MDBIP) and 2,6-bis(3-(1-phenyl-1H-benzo[d]imidazol-2-yl)phenyl)pyridine (MDBIPy) have been designed and synthesized. The two compounds exhibit high decomposition temperatures (T_d: 444 °C for MDBIP and 450 °C for MDBIPy) and a stable amorphous glassy state (T_g: 108 °C for MDBIP and 110 °C for MDBIPy). In the typical device ITO/MoO₃ (10 nm)/4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (NPB, 80 nm)/TCTA (5 nm)/Host: Ir(ppy)₃ (9 wt%, 20 nm)/TmPyPB (40 nm)/LiF (1 nm)/Al (100 nm), when MDBIPy was used as host material, the device B showed a maximum efficiency of 21.1%, 75.8 cd A^-1 and 83.6 lm W^-1. When MDBIPy was utilized as both the electron transporting and host layer, the unilateral homogeneous device C exhibited a maximum efficiency of 20.6%, 74.2 cd A^-1 and 71.8 lm W^-1. More interestingly, device C showed low efficiency roll-off relative to device B. When the brightness of the device is over 1000 cd m^-2, the current efficiency of device C is higher than device B. At a brightness of 5000 cd m^-2, the current efficiency of device C is only roll off 5.6%. These results demonstrate that utilizing electron-transport type host materials to fabricate unilateral homogeneous PhOLEDs is a promising way to simplify the device configuration and optimize the performance of OLED devices.

Full text of DOI:10.1039/c2jm33988a

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PAPER
Novel electron-type host material for unilateral homogeneous phosphorescent
organic light-emitting diodes with low efficiency roll-off†
a
a
a
a
a
a
b
*
*
Xiao Yang, Hong Huang, Biao Pan, Shaoqing Zhuang, Matthew P. Aldred, Lei Wang, Jiangshan Chen
and Dongge Mab
Received 20th June 2012, Accepted 16th September 2012
DOI: 10.1039/c2jm33988a
Two novel simple electron-transport (ET) type host materials, 2,6-bis(3-(1-phenyl-1H-benzo[d]
imidazol-2-yl)phenyl)benzene (MDBIP) and 2,6-bis(3-(1-phenyl-1H-benzo[d]imidazol-2-yl)phenyl)
pyridine (MDBIPy) have been designed and synthesized. The two compounds exhibit high
decomposition temperatures (Td: 444 ꢀC for MDBIP and 450 ꢀC for MDBIPy) and a stable amorphous
glassy state (Tg: 108 ꢀC for MDBIP and 110 ꢀC for MDBIPy). In the typical device ITO/MoO3 (10 nm)/
4,40-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (NPB, 80 nm)/TCTA (5 nm)/Host: Ir(ppy)3 (9 wt%,
20 nm)/TmPyPB (40 nm)/LiF (1 nm)/Al (100 nm), when MDBIPy was used as host material, the device
B showed a maximum efficiency of 21.1%, 75.8 cd Aꢁ1 and 83.6 lm Wꢁ1. When MDBIPy was utilized as
both the electron transporting and host layer, the unilateral homogeneous device C exhibited a
maximum efficiency of 20.6%, 74.2 cd Aꢁ1 and 71.8 lm Wꢁ1. More interestingly, device C showed low
efficiency roll-off relative to device B. When the brightness of the device is over 1000 cd mꢁ2, the current
efficiency of device C is higher than device B. At a brightness of 5000 cd mꢁ2, the current efficiency of
device C is only roll off 5.6%. These results demonstrate that utilizing electron-transport type host
materials to fabricate unilateral homogeneous PhOLEDs is a promising way to simplify the device
configuration and optimize the performance of OLED devices.
electrons and holes, and decrease the drive voltage and improve
the device efficiency.3 Finally, the highest occupied molecular
orbital (HOMO)/lowest unoccupied molecular orbital (LUMO)
level should be matched with the adjacent layer, which will
reduce the interface energy barrier, and reduce the efficiency roll-
off, which arises from triplet–triplet annihilation (TTA) or triplet
exciton–polaron quenching (TPQ) at high current densities.4
In order to balance the electron–hole carriers, broaden the
recombination zone and reduce the efficiency roll-off at high
brightness, more researchers have turned their attention to the
development of bipolar host materials.5 For example, Chou and
Cheng designed and synthesized a green host material BCPO for
iridium(III) fac-tris(2-phenlypyridine) (Ir(ppy)3), which showed a
relatively low turn-on voltage of 2.1 V and a maximum external
quantum efficiency (EQE) of 21.6%.6 Kido et al. developed a new
bipolar host material based on carbazole and pyridine for green
OLEDs, which exhibited a maximum EQE and power efficiency
of 25.2% and 102 lm Wꢁ1, respectively.7 Yang et al. reported a
carbazole/oxadiazole hybrid molecule o-CzOXD8 for green and
red phosphorescent devices. The device exhibits a maximum
EQE as high as 20.2% for green and 18.5% for red electro-
phosphorescence. However, for most host materials, the electron
mobility is lower than that of holes, so it is also a very efficient
way to design and synthesize ET-type host materials to balance
the carriers and improve the device performance.9 Chiu et al.
reported an electron-transport type host OXD based on
1. Introduction
Phosphorescent organic light-emitting diodes (PhOLEDs) have
attracted much attention because they can achieve 100% internal
quantum efficiency by incorporating triplet and singlet emitters.1
Owing to these, PhOLEDs are the most noticeable candidates for
full-color flat-panel displays and solid-state lighting.2 For the
purpose of obtaining highly efficient PhOLEDs, the selection of a
suitable host material for the phosphorescent dopant is impor-
tant. A suitable host–dopant system could fundamentally affect
the device efficiency and the energy transfer from the host to the
dopant. In general, in order to achieve efficient electro-
phosphorescence, the following principles for the host material
should be followed: firstly, the triplet energy (ET) of the host
materials must be higher than that of the dopants. Secondly, the
host materials should have good charge-carrier transport prop-
erties, which can increase the recombination opportunities for
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
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c2jm33988a
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oxadiazole, which showed a maximum current efficiency of
17.2%, when doped with FIrpic.10 Lee et al. designed electron-
transport type host materials based on pyridine, which exhibited
a high external quantum efficiency of 13.3% in red phosphores-
cent organic light-emitting diodes.11 Leung et al. reported an
effective electron transport host BOBP, which exhibits a
maximum brightness of 43 000 cd mꢁ2 with an efficiency of 26 cd
Aꢁ1 in green phosphorescent organic light-emitting diodes.12
However, as we know, the procedure for fabricating these devices
is complex and the device performances still need further
improvement. ET-type materials, serving as the host layer and
electron transport layer (ETL) simultaneously, can be used to
construct simple unilateral homogeneous phosphorescent
OLED, which will reduce the interface energy barrier between
EML and ETL, broaden the exciton recombination zone and
improve the device performances. Wong et al. synthesized a new
ET-type host based on 1,3,5-triazine derivatives, 2,4,6-tris(bi-
phenyl-3-yl)-1,3,5-triazine (T2T), and in a unilateral homoge-
neous device, the maximum efficiencies reached up to 15.7%,
54 cd Aꢁ1, 56 lm Wꢁ1, respectively.13 Takizawa et al. fabricated a
green unilateral homogeneous device with TPBI as the host
material and ETL, but the maximum efficiencies are only 5.7%,
The glass transition temperature (Tg) was determined from the
second heating scan. Thermogravimetric analysis (TGA) was
undertaken with a PerkinElmer Instruments (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. Reduction
CVs of all compounds were performed in dichloromethane
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
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)15 with the 6-31 G(d) atomic
basis set. Then the electronic structures were calculated at
s-HCTHhyb/6-311++G(d, p) level.16 Molecular orbitals were
visualized using ADFview.
26.4 cd Aꢁ1, 33.2 lm Wꢁ1 14 Therefore, it is still a great challenge
.
to design new ET-type host materials that can be used as the host
layer and ET layer, simultaneously.
In this paper, we report two new ET type host materials,
MDBIP and MDBIPy, based on benzimidazole, in which the
electron affinity and HOMO/LUMO were modified by intro-
ducing different linking spacers, such as benzene and pyridine,
respectively. The HOMO/LUMO orbitals of the compound
MDBIPy are almost completely separate and exhibit high elec-
tron mobility. When MDBIPy was used as the host material for
the doped emitter material Ir(ppy)3, the device B exhibited
maximum external quantum/current/power efficiencies of 21.1%,
75.8 cd Aꢁ1 and 83.6 lm Wꢁ1, respectively. Meanwhile, the
device C with MDBIPy as ETL and host showed a maximum
EQE, current efficiency and power efficiency of 20.6%, 74.2 cd
Aꢁ1 and 71.8 lm Wꢁ1, respectively. More interestingly, the
current efficiency of device C at a brightness of 1000 and 5000
cd mꢁ2 exhibits efficiency roll-off of only 1.1% and 5.6%,
respectively.
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 pre-
coated with a layer of indium tin oxide (ITO) with a sheet
resistance of 20 U per square. 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 aluminum
ꢀ
(100 nm) was sequentially 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.
2.4. Synthesis
Compound 3,300-bis(1-phenyl-1H-benzo[d]imidazol-2-yl)-1,10:30,
100-terphenyl (MDBIP). A mixture of the compound 1-phenyl-
2-(3-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)-1H-
benzo[d]imidazole (2.2 mmol), 1,3-dibromobenzene (1.0 mmol),
Pd(PPh3)4 (0.05 mmol), K2CO3 (2.0 M aqueous solution) (25 ml),
toluene (50 ml) and ethanol (25 ml) was stirred at 90 ꢀC for 24 h.
After it was cooled to room temperature, the dichloromethane
was added to the reaction mixture. The organic phase was
separated and washed with brine before drying over anhydrous
MgSO4. The solvent was evaporated off, and the solid residues
were purified by column chromatography on silica gel with
petroleum ether to afford the final compound as a white solid.
Yield: 80%. 1H NMR (400 MHz, CDCl3) d (ppm): 7.93–7.92 (d,
J ¼ 8.0 Hz, 2H), 7.76 (s, 2H), 7.68–7.66 (d, J ¼ 7.6Hz, 2H), 7.58–
7.52 (m, 6H), 7.47–7.44 (m, 4H), 7.42–7.34 (m, 7H), 7.33–7.27
(m, 7H). 13C NMR (100 MHz, CDCl3) d (ppm): 152.16, 140.96,
2. Experimental section
2.1. General information
All solvents and materials were used as received from commercial
suppliers without further purification. The synthetic routes of the
1
MDBIP and MDBIPy are outlined in Scheme 1. H NMR and
13C NMR spectra were measured on a Bruker-AF301 AT 400
MHz spectrometer. 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 ꢁa1PE Instruments DSC 2920 unit at a
ꢀ
ꢀ
heating rate of 10 C min from 30 to 250 C under nitrogen.
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Scheme 1 The synthetic routes toward the bipolar hosts MDBIP and MDBIPy.
140.92, 137.29, 137.16, 130.37, 130.03, 129.11, 128.81, 128.77,
128.53, 128.41, 127.59, 126.31, 125.88, 123.50, 123.11, 119.93,
119.90, 110.51. MS (APCI): calcd for C44H30N4: 614.2, found,
615.4 (M+1)+. Anal. calcd. C, 85.97, H, 4.92, N, 9.11, found C,
85.94, H, 4.85, N, 9.15%.
heating cycle, respectively, which are significantly higher than the
Tg of widely used 4,4-N0,N0-carbazole-biphenyl (CBP: 62 C).18
ꢀ
In addition, the compounds MDBIP and MDBIPy also exhibit
good thermal stability with decomposition temperatures (Td, 5%
weight loss) at 444 and 450 ꢀC for MDBIP and MDBIPy,
respectively. The relatively high Tg and Td values make these
compounds avoid phase separation upon heating and they have
the potential for fabrication into high performance devices by
vacuum thermal evaporation technology.
Compound
2,6-bis(3-(1-phenyl-1H-benzo[d]imidazol-2-yl)
phenyl)pyridine (MDBIPy). The compound MDBIPy was
prepared according to the same procedure as the compound
MDBIP but using 2,6-dibromopyridine to instead 1,3-dibromo-
benzene. Yield: 84%. 1H NMR (400 MHz, CDCl3) d (ppm): 8.28–
8.27 (m, 2H), 8.15–8.13 (d, J ¼ 8.0 Hz, 2H), 7.94–7.92 (d, J ¼ 8.0
Hz, 2H), 7.94–7.92 (m, 1H), 7.72–7.70 (d, J ¼ 8.0 Hz, 2H), 7.52–
7.42 (m, 10H), 7.39–7.28 (m, 10H). 13C NMR (100 MHz, CDCl3)
d (ppm): 155.95, 152.27, 142.98, 139.43, 137.50, 137.26, 137.07,
130.26, 129.99, 129.95, 128.75, 128.61, 128.26, 128.05, 127.52,
123.48, 123.11, 119.90, 118.93, 110.54. MS (APCI): calcd for
C43H29N5: 615.2, found, 616.4 (M+1)+. Anal. calcd. C, 83.88, H,
4.75, N, 11.37, found C, 83.68, H, 4.72, N, 11.42%.
Since efficient film-forming properties of light emitting mate-
rials are crucial for the performance of devices, the surface
morphologies of vacuum-deposited thin films of MDBIP and
MDBIPy were studied by atomic force microscopy (AFM). As
shown in Fig. 2, the solid film samples exhibit a root-mean-
square (RMS) roughness of 0.647 and 0.369 nm for MDBIP and
MDBIPy, respectively. The relatively small RMS of MDBIPy in
the thin films makes it desirable for high performance OLEDs.
3.3. Photophysical properties
Fig. 3(a) shows the UV-Vis absorption and photoluminescence
spectra of MDBIP and MDBIPy in dilute toluene solution. The
two compounds exhibit similar absorption. The maximum
absorption at around 292 nm may be attributed to the p–p*
transition from benzimidazole. The maximum PL emission peaks
of MDBIP and MDBIPy are 352 nm and 351 nm, respectively.
The two compounds show similar absorption and emission
spectra which can be ascribed to their similar spatial structure.
The phosphorescence spectra of MDBIP and MDBIPy in
2-methyltetrafuran solution at 77 K show the highest energy
peaks at 478 and 484 nm, respectively (Fig. 3(b)), which corre-
spond to triplet energy levels at 2.60 and 2.56 eV. Generally,
green phosphorescent emitters such as Ir(ppy)3 possess triplet
energy levels at about 2.43 eV, indicating that both MDBIP
and MDBIPy can act as appropriate host materials for green
phosphorescent emitters.
3. Results and discussion
3.1. Synthesis and characterization
Scheme 1 illustrates the synthetic routes for the target molecules.
The key intermediate benzimidazole boronic ester (BIB) was
synthesized according to the literature.17 The target host mate-
rials MDBIP and MDBIPy were facilely synthesized by the
typical Suzuki coupling reaction of BIB with 1,3-dibromo-
benzene or 2,5-dibromopyridine in high yield (>80%). All the
target materials were further purified by repeated temperature-
gradient vacuum sublimation before testing. The two target
compounds were fully characterized by 1H NMR and 13C NMR
spectroscopies, mass spectrometry and elemental analysis.
3.2. Thermal properties and morphological properties
The thermal properties of compounds MDBIP and MDBIPy
were determined by thermogravimetric analysis (TGA) and
differential scanning calorimetry (DSC) (Fig. 1). Because the two
compounds have similar chemical structures and molecular
weights, they also show similar thermal properties. The detailed
thermal data of these compounds are listed in Table 1. The
compounds MDBIP and MDBIPy exhibit relativꢀ ely high glass
3.4. Electrochemical properties
Cyclic voltammetry (CV) was performed to investigate the elec-
trochemical properties of MDBIP and MDBIPy (Fig. 4). The
compounds exhibit irreversible oxidation due to the instability of
their radical cations.19 HOMO and LUMO energy levels of the
two compounds were calculated from the onset potentials for
oxidation together with their absorption spectra (Table 1). The
ꢀ
transition temperatures (Tg) of 108 C, and 110 C in the DSC
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Fig. 1 The TGA thermograms of the compounds MDBIP (left) and MDBIPy (right) recorded at a heating rate of 10 ꢀC minꢁ1. Inset: the DSC traces of
the compounds MDBIP and MDBIPy recorded at a heating rate of 10 ꢀC minꢁ1
.
HOMO/LUMO levels of the MDBIP and MDBIPy are 5.90/2.36
and 5.89/2.19 eV, respectively. This can reduce the hole injection
barrier from the hole transporting layer (HTL) to MDBIP and
MDBIPy, compared with commercial host materials such as
TPBI (6.3 eV).20
3.5. Theoretical calculations
To understand the electronic structures of MDBIP and
MDBIPy, density functional theory (DFT) calculations were
performed at a B3LYP/6-31 G(d) level for the geometry opti-
mization. As shown in Fig. 5, both the HOMO and LUMO
Fig. 2 AFM topographic images of the MDBIP and MDBIPy.
LiF were employed as the hole injection layer (HIL) and electron
injection layer (EIL), respectively. NPB and TmPyPB act as the
hole and electron transport layers (HTL/ETL), respectively. A
pure thin TCTA layer with a high triplet energy (ET ¼ 2.85 eV)22
was used as the exciton blocking layer (EBL) to prevent exciton
diffusion and improve the efficiency. The green phosphorescent
dopant Ir(ppy)3 was co-evaporated with the host materials to
examine the performance of the new electron-type host materials
MDBIP (device A) and MDBIPy (device B).
orbitals of MDBIP are mainly dispersed on the benzimidazole
units, whereas for the compound MDBIPy, the LUMO is
dispersed on the benzimidazole, and the HOMO is located on the
linking spacer 2,6-diphenylpyridine. Since separation between
the HOMO and LUMO levels is preferable for the prevention of
reverse energy transfer,17,21 MDBIPy may be more suitable as a
host material for Ir(ppy)3 as compared with MDBIP. The
calculated HOMO/LUMO values are 5.65/2.41 for MDBIP,
and 5.56/2.55 for MDBIPy, which correlate well with the
observations from cyclic voltammetry (see Table 1). By the way,
because of the separation of the HOMO and LUMO, MDBIPy
may possess intramolecular charge transfer (CT) emission.
The PL spectra of MDBIPy in THF solution exhibit a longer
wavelength PL peak, which also verified these features (see
Fig. S1†).
The current density (J)–voltage (V)–brightness (B) character-
istics, and the efficiency curves of devices A and B are shown in
Fig. 7(a) and (b), respectively. The EL data of the devices are
summarized in Table 2. Both device A and B exhibit low turn-on
voltages (2.8 V for device A, 2.7 V for device B), which could be
attributed to their matching HOMO levels with TCTA (5.8 eV).
As shown in Fig. 7(a), the drive voltage of device A is remarkably
higher the device B, such as at a current density of 40 mA cmꢁ2
the drive voltages of devices A and B are 9.3 and 7.8 V, respec-
tively. Device B hosted by MDBIPy exhibits a maximum EQE of
21.1%, a maximum current efficiency of 75.8 cd Aꢁ1 and a power
efficiency of 83.6 lm Wꢁ1, which are significantly higher than
those for MDBIP (17.8%, 63.7 cd Aꢁ1, 65.8 lm Wꢁ1). The rela-
tively low drive voltage and high performance of device B
compared to device A may be due to the separation of the
3.6. Electroluminescence
In order to investigate the utility of MDBIP and MDBIPy as
host materials for the green phosphor Ir(ppy)3, green PhOLEDs
with the structure of ITO/MoO3 (10 nm)/NPB (80 nm)/TCTA
(5 nm)/Host: Ir(ppy)3 (9 wt%, 20 nm)/TmPyPB (40 nm)/LiF
(1 nm)/Al (100 nm) were fabricated. The relative HOMO/LUMO
energy levels of the materials are illustrated in Fig. 6. MoO3 and
Table 1 The photophysical and electrochemical properties of MDBIP and MDBIPya
Abs (nm)
PL (nm)
Eg (eV)
HOMO/LUMOexp (eV)
HOMO/LUMOcal (eV)
ET (eV)
Tg/Td (ꢀC)
MDBIP
MDBIPy
292
292
352
352
3.71
3.68
5.90/2.19
5.89/2.21
5.65/2.41
5.56/2.55
2.60
2.56
108/444
110/450
a
HOMO/LUMOexp: measured from the CV; HOMO/LUMOcal: calculated from the DFT; Eg: calculated from the onset of the absorption spectrum.
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Fig. 3 (a) Room-temperature absorption (UV-Vis) and emission (PL) of MDBIP and MDBIPy in toluene solution; (b) phosphorescence (Phos) of
MDBIP and MDBIPy in 2-methyltetrafuran at 77 K.
HOMO/LUMO levels (see Fig. 5) and better ET properties of
MDBIPy, and the efficient electron injection from TmPyPB to
the emissive layer can be ascribed to tunneling effects.
In order to further evaluate the electron transport character of
MDBIP and MDBIPy, and the reasons that the device based on
MDBIPy exhibits better performance, the electron mobility of
the MDBIP and MDBIPy were investigated by the time-of-flight
(TOF) transient-photocurrent technique. The TOF device was
fabricated with the structure of ITO/Organic (2.4 mm)/Al
(100 nm). The samples were illuminated at 355 nm through the
ITO electrode from an optical parametric oscillator (OPO)
pumped by a Q-switched Nd:YAG laser. The transient current
was measured using a digital storage oscilloscope (DPO7104;
bandwidth: 1 GHz). The electron mobility (m) was calculated
Fig. 4 The cyclic voltammetry curves of MDBIP and MDBIPy.
Working electrode: Pt button; reference electrode: Ag/Ag+. Oxidation
from the values of the transit time (Tt), the sample thickness (D)
and the applied voltage (V) by using the following equation:23
CV was performed in dichloromethane containing 0.1 M n-Bu4NPF6 as
the supporting electrolyte at a scan rate of 100 mV sꢁ1
.
m ¼ D2/(VTt)
As shown in Fig. 8, the material MDBIPy showed a relatively
higher electron mobility in different electric field intensities
compared to MDBIP. MDBIPy exhibits a high electron mobility
about 1.5 ꢂ 10ꢁ4 cm2 Vꢁ1
s
ꢁ1 at an electric field of 8.3 ꢂ 105 V
cmꢁ1, which is comparable and even higher than those of typical
electron transport materials, such as Alq3 24 and OXD-7.25 The
relatively high electron mobility of MDBIPy can be ascribed to
the introduction of the pyridine moiety to the molecule. As we
know, pyridine is an electron-deficient moiety, which will be
beneficial for accepting electrons from the anode.26 All the results
from the electron mobility could explain why device B has
excellent performances compared to device A and imply that
MDBIPy has the potential to be used as the ETL in PhOLEDs.
Considering the excellent electron transport ability of
MDBIPy, we further fabricated a unilateral homogeneous device
by using MDBIPy acting as the ETL and the host, simulta-
neously. The device configuration is as follows: ITO/MoO3
(10 nm)/NPB (80 nm)/TCTA (5 nm)/MDBIPy: Ir(ppy)3 (9 wt%,
20 nm)/MDBIPy (40 nm)/LiF (1 nm)/Al (100 nm) (device C). The
J–V–B characteristics and the efficiency curves of device C are
also plotted in Fig. 7(a) and (b), respectively. It was found that
the drive voltages of device C are slightly higher than device B
with TmPyPB as the ETL. Device C exhibits a maximum EQE of
20.6%, a maximum current efficiency of 74.2 cd Aꢁ1 and a power
efficiency of 71.8 lm Wꢁ1, which are almost the same as those of
Fig. 5 Frontier molecular orbitals HOMO (left) and LUMO (right) of
MDBIP and MDBIPy, calculated with DFT on a B3LYP/6-31 G(d) level.
Fig. 6 Energy level diagram of the phosphorescent dyes and the device
structures.
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Fig. 7 (a) The current density (J)–voltage (V)–brightness (B) characteristics curves of devices A–C. (b) The external quantum efficiency (EQE) and
luminous efficiency curves of the devices A–C. (c) The EL spectra for devices A, B and C at the brightness of 100 cd mꢁ2
.
device B. Interestingly, device C showed low efficiency roll-off
relative to device B. Such as, when the brightness of device C
reaches 1000 cd mꢁ2, the EQE of the device still remains at up to
20.3%, which is even higher than device B. At a much higher
brightness of 5000 cd mꢁ2, the device C still exhibited an effi-
ciency as high as 19.4% and 69.7 cd Aꢁ1. The efficiency roll-off of
device C at 1000 and 5000 cd mꢁ2 is only 1.1% and 5.6%,
respectively, but the efficiency roll off of device B reached 2.7%
and 11.2% at the same brightness. When MDBIPy was used as
the host layer and ETL layer, simultaneously, the interface
energy barrier was facilitated and the exciton recombination
region was broadened, which will reduce the exciton–exciton and
exciton-dipolar annihilation, and lower the roll-off of device C at
high brightness. As shown in Fig. 7(c), it can be found that the
Fig. 8 The electron mobility of MDBIP and MDBIPy films plotted as a
EL spectra of the three devices exhibit a typical green emission
from Ir(ppy)3, with Commission International de l’Eclairage
(CIE) coordinates at around (0.29, 0.64), which indicates the
complete energy transfer from host to phosphor Ir(ppy)3. The
detailed device characteristics are shown in Table 2. Above all,
these results demonstrate that utilizing an electron-type host
material to fabricate unilateral homogeneous PhOLEDs is an
efficient way to simplify the device structure and optimize the
performance of devices.
function of the square root of the electric field.
4. Conclusion
In summary, two new electron-transporting type host materials,
MDBIP and MDBIPy, based on benzimidazole have been
synthesized and characterized. MDBIP and MDBIPy exhibit a
ꢀ
high decomposition temperature (Td ¼ 444 and 450 C) and a
Table 2 The electroluminescence properties of the devicesa
Device
Host/ETL
Von (V)
hc (cd Aꢁ1
b
)
hp (lm Wꢁ1
b
)
EQEb
CIE (x, y)c
A
B
C
MDBIP/TmPyPB
MDBIPy/TmPyPB
MDBIPy/MDBIPy
2.8
2.7
2.8
63.7, 60.0, 54.3
75.8, 73.3, 67.8
74.2, 73.5, 69.7
65.8, 32.7, 22.3
83.6, 49.8, 36.2
71.8, 46.1, 34.8
17.8, 16.7, 15.2
21.1, 20.5, 18.6
20.6, 20.3, 19.3
(0.28, 0.64)
(0.29, 0.64)
(0.29, 0.64)
a
Von: turn-on voltage; hc: current efficiency; hp: power efficiency; CIE (x, y): Commission International de I’Eclairage coordinates. b Order of measured
value: maximum, values at 1000 and 5000 cd mꢁ2
.
Measured at 100 cd mꢁ2
.
c
23134 | J. Mater. Chem., 2012, 22, 23129–23135
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View Article Online
S. Lee, Org. Electron., 2010, 11, 1624; (c) F.-M. Hsu, C.-H. Chien,
Y.-J. Hsieh, C.-H. Wu, C.-F. Shu, S.-W. Liu and C.-T. Chen, J.
Mater. Chem., 2009, 19, 8002; (d) X. Cai, A. B. Padmaperuma,
L. S. Sapochak, P. Vecchi and P. E. Burrows, Appl. Phys. Lett.,
2008, 92, 083308; (e) W.-Y. Wong, C.-L. Ho, Z.-Q. Gao, B.-X. Mi,
C.-H. Chen, K.-W. Cheah and Z. Lin, Angew. Chem., 2006, 118,
7964; (f) S. Gong, Y. Chen, C. Yang, C. Zhong, J. Qin and D. Ma,
Adv. Mater., 2010, 22, 5370.
6 H. H. Chou and C. H. Cheng, Adv. Mater., 2010, 22, 2468.
7 (a) S.-J. Su, C. Cai and J. Kido, Chem. Mater., 2011, 23, 274; (b)
H. Sasabe, S.-J. Su, T. Takeda and J. Kido, Chem. Mater., 2008,
20, 1691.
8 Y. Tao, Q. Wang, C. Yang, Z. Zhang, T. Zou, J. Qin and D. Ma,
Angew. Chem., Int. Ed., 2008, 47, 8104.
9 (a) J. C. Jeong, H. S. Kim and J. G. Jang, Mol. Cryst. Liq. Cryst.,
2010, 530, 103; (b) A. Takahashi, A. Endo and C. Adachi, Jpn. J.
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good stability of surface morphology. In addition, by employing
MDBIPy as the host for the green phosphor Ir(ppy)3, and
TmPyPB as the ETL, the PhOLEDs demonstrate excellent
performance, with a maximum efficiency of 21.1%, 75.8 cd Aꢁ1
and 83.6 lm Wꢁ1. More interestingly, when MDBIPy was used as
both the ETL and host material, device C showed a maximum
EQE, current efficiency and power efficiency of 20.6%, 74.2 cd
Aꢁ1 and 71.8 lm Wꢁ1, respectively. In particular, the current
efficiency of device C at brightnesses of 1000 and 5000 cd mꢁ2
roll-offs are only 1.1% and 5.6%, respectively. Our results clearly
demonstrate that utilizing an appropriate ET type host material
to construct simple and high performance PhOLEDs is possible.
10 T.-L. Chiu, P.-Y. Lee, J.-H. Lee, C.-H. Hsiao and M.-K. Leung, J.
Appl. Phys., 2012, 109, 084520.
Acknowledgements
11 S. O. Jeon and J. Y. Lee, Mater. Chem. Phys., 2011, 127, 300.
12 M.-K. Leung, C.-C. Yang, J.-H. Lee, H.-H. Tsai, C.-F. Lin,
C.-Y. Huang, Y.-O. Su and C.-F. Chiu, Org. Lett., 2007, 9, 235.
13 H.-F. Chen, S.-J. Yang, Z.-H. Tsai, W.-Y. Hung, T.-C. Wang and
K.-T. Wong, J. Mater. Chem., 2009, 19, 8112.
This research work was supported by the central allocation grant
from NSFC/China (21161160442, 51203056) and Wuhan Science
and Technology Bureau (no. 01010621227) and the Analytical
and Testing Centre at Huazhong University of Science and
Technology.
14 V. A. Montes, S.-Y. Takizawa and P. Anzenbacher, Jr, Chem. Mater.,
2009, 21, 2452.
15 (a) C. Lee, W. Yang and R. G. Parr, Phys. Rev. B, 1988, 37, 785; (b)
P. A. Serena, J. M. Soler and N. Garcıa, Phys. Rev. B, 1988, 37,
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