Bipyridyl oxadiazoles as efficient and durable electron-transporting and hole-blocking molecular materials

Article abstract of DOI:10.1039/b510720b

We have demonstrated new and efficient electron transporting (ET) and hole blocking (HB) materials for organic light-emitting devices (OLEDs). The bipyridyl-substituted oxadiazole derivatives can form a stable glassy state with glass transition temperatur

Full text of DOI:10.1039/b510720b

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Bipyridyl oxadiazoles as efficient and durable electron-transporting and
hole-blocking molecular materials
Musubu Ichikawa,*a Taro Kawaguchi,a Kana Kobayashi,a Tetsuzo Miki,b Kenji Furukawa,a Toshiki Koyamaa
and Yoshio Taniguchia
Received 27th July 2005, Accepted 18th October 2005
First published as an Advance Article on the web 4th November 2005
DOI: 10.1039/b510720b
We have demonstrated new and efficient electron transporting (ET) and hole blocking (HB)
materials for organic light-emitting devices (OLEDs). The bipyridyl-substituted oxadiazole
derivatives can form a stable glassy state with glass transition temperatures greater than 100 uC.
The bipyridyl-substituted oxadiazole derivatives have excellent ET, HB and electron accepting
ability and also exhibit electrical operation durability. Bipyridyl-substituted oxadiazoles are a
promising candidate for ET and HB materials for OLEDs.
wide band gap properties because oxadiazole restricts exten-
Introduction
sions of p-conjugation beyond the ring even if the molecule is
co-planar.4,16 Accordingly, OXDs are a promising ET and
Organic light-emitting devices (OLEDs) have received much
attention due to their potential applications to flat panel
HB material for OLEDs, but it has been widely accepted
displays. OLEDs are generally composed of functionally
that OXDs should acquire durability capacity for long-term
operations in OLEDs.10 Here, we show that bipyridyl-
divided organic multi-layers, e.g., hole transporting (HT),
emissive, and electron transporting (ET) layers, and so on.1,2
substituted oxadiazole compounds have efficient electron-
In the last decade, many kinds of amorphous molecular semi-
transporting and hole-blocking properties with high thermal
conductor materials,3,4 working as HT materials5–8 and ET
stability and practical operation durability.
materials,9–14 have been proposed, and HT molecular semi-
conducting materials have become practical due to their high
Experimental
charge carrier mobility and excellent operational durability.
On the other hand, there have been few reports of ET organic
semiconducting amorphous materials with high performance
(high-speed transportation of electrons, easy injection of elec-
trons from the cathode, and good operational durability).12
Tris(8-hydroxyquinolinato)aluminum(III) (Alq), which is the
historical material reported in the first bright OLEDs by Tang
et al. in 1987, is still conventionally used as an ET material
regardless of its slow electron mobility15 because it exhibits
high operational durability. However, as a reflection of the
imbalanced performance between ET and HT materials,
electron-feeding from the cathode into an emissive layer control
device characterizes OLEDs. Therefore, the development of
efficient ET organic materials is a challenge of high priority.
Efficient ET materials provide some advantages, such as
lowering the operating voltage and power consumption.
Moreover, if the ET materials have wide band gaps, in other
words, deeper highest occupied molecular orbitals (HOMO),
such ET materials can also work as hole blocking (HB)
materials. The complete confinement of a hole in an emissive
layer by the HB layer raises the quantum efficiencies of
electroluminescence (EL). Oxadiazole is a well-known electron
accepting component for building ET materials with high
electron mobility, and oxadiazole derivatives (OXDs) exhibit
Measurements
1H NMR spectra were recorded on a JEOL JNM-EX270
spectrometer (270 MHz). Elemental analysis was carried out
with a Yanaco MT-3 CHNcoder. Thermal analysis was
performed on a Seiko Instruments DSC-6200 at a heating
rate of 10 uC min21 for differential scanning calorimetry
(DSC) under nitrogen. X-Ray diffraction (XRD) patterns of
thin films were measured using Cu Ka radiation with 40 kV
and 150 mA. Ultraviolet (UV) and visible absorption spectra
and fluorescence spectra were recorded with a Shimadzu
UV-3150 spectrophotometer and a JASCO FP-750 spectro-
fluorometer, respectively. The HOMO level was measured
with a Riken Keiki AC-3 photoelectron emission spectrometer,
where the HOMO level was defined as being equal to the
ionization potential measured by photoelectron emission
spectroscopy. The lowest unoccupied molecular orbital
(LUMO) level was estimated from the HOMO level and the
band gap, which was determined by the absorption edge of the
UV and the visible absorption spectrum of the neat thin film.
Neat thin films prepared by vacuum deposition on quartz
glasses at a pressure of 6 6 1024 Pa or less were used as
samples for XRD, UV-visible absorption, fluorescence, and
photoelectron emission spectroscopy.
aDepartment of Functional Polymer Science, Faculty of Textile Science
and Technology, Shinshu University, 3-15-1 Tokita, Ueda,
Nagano 386-8567, Japan. E-mail: musubu@giptc.shinshu-u.ac.jp
bHodogaya Chemical Co., Ltd., Miyukigaoka 45, Tsukuba,
Ibaraki 305-0841, Japan
Materials
We synthesized new bipyridyl-substituted oxadiazole com-
pounds, 1,3-bis[2-(2,29-bipyridin-6-yl)-1,3,4-oxadiazol-5-yl]benzene
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Fig. 1 Chemical structures of new bipyridyl substituted oxadiazole
compounds and other materials used in this study.
(Bpy-OXD) and 2,6-bis[2-(2,29-bipyridin-6-yl)-1,3,4-oxadia-
zol-5-yl]pyridine (Bpy-OXDpy), whose chemical structures
are shown in Fig. 1, from the tetrazole intermediate via intra-
molecular ring formation17 as described below. They were
purified by silica gel column chromatography to give white
crystals, whose structures were confirmed by NMR spectro-
scopy and elemental analysis. Further purification was
carried out with temperature gradient sublimation in a flow
stream of 99.999% argon gas before use. Other chemicals
used in this study, whose structures are shown in Fig. 2a,
were from Nippon Steel Chemical (Alq, NPB and CuPc) and
H.W. Sands (OXD-7). These materials were used with no
further purification because they were sublimation grade.
Fig. 2 (a) Chemical structures of OLED materials used in this study
and device structures of fabricated OLEDs with Bpy-OXDs as ET
layers (b) and the reference device (c).
t,), 7.26 (2H, d). Elemental analysis: calc. for C29H17N9O2: C =
66.53, H = 3.27, N = 24.08. Found: C = 67.05, H = 3.47, N =
24.01%. nmax(KBr)/cm21 1584, 1541, 1463 and 1431. m/z (EI,
low resolution) 523 (M+ requires 523.15), 445 (M+ 2 pyridyl.
C24H13N8O2 requires 445.12) and others.
1,3-Bis[2-(2,29-bipyridin-6-yl)-1,3,4-oxadiazol-5-yl]benzene
(Bpy-OXD). 6-(2H-Tetrazol-5-yl)-2,29-bipyridine, 0.63 g in
10 ml dry pyridine, was stirred. Then, 0.29 g of isophthaloyl
dichloride was added slowly and refluxed at 115 uC for 6 hours.
After cooling, the reaction mixture was poured into 100 ml of
water. The white precipitate was isolated by filtration, washed
with water, and vacuum dried at 80 uC. Silica gel column
chromatography (CHCl3–MeOH = 20 : 1) gave 0.62 g of pure
white powder with an 81% yield. dH(270 MHz; CDCl3; Me4Si)
9.08 (1 H, s), 8.72 (2 H, br d, J 4.9), 8.65 (4 H, br d, J 8.1) 8.47
(2 H, br d, J 7.8), 8.35 (6 H, d, J 7.8), 8.05 (2 H, t, J 7.8), 7.86–
7.77 (3 H, m), 7.35 (2 H, dd, J 4.9). Elemental analysis: calc.
for C30H18N8O2: C = 68.96, H = 3.47, N = 21.44. Found: C =
69.27, H = 3.63, N = 21.30%. nmax(KBr)/cm21 1585, 1541,
1456 and 1427. m/z (EI, low resolution) 522 (M+ requires
522.16), 444 (M+2 pyridyl. C25H14N7O2 requires 444.12)
and others.
Fabrication and measurements of OLEDs
OLEDs fabricated in this work had a configuration of ITO/
NPB (50 nm)/Alq (20 nm)/ET layer (30 nm)/Mg : Ag (10 : 1
by volume) alloy as shown in Fig. 2b (x = 30 nm).
N,N9-Dinaphthalen-1-yl-N,N9-diphenyl-biphenyl-4,49-diamine
(NPB) and Alq were used as the HT layer and emissive layer,
respectively. An indium-tin-oxide (ITO)-coated glass substrate
with a sheet resistance of 14 V square21 treated by O2-plasma
for 5 minutes in advance was used as the substrate and anode.
We also fabricated the ITO/NPB (50 nm)/Alq (40 nm)/Bpy-
OXD (10 nm)/Mg : Ag device (see Fig. 2b, x = 10 nm) to
investigate the ET property of Bpy-OXD and the ITO/NPB
(50 nm)/Alq (50 nm)/Mg : Ag (see Fig. 2c) as a reference. All
layers were sequentially deposited on the ITO-coated glass
substrate at a pressure of 6 6 1024 Pa or less. The emissive
area of the device was 2 6 2 mm2. The current density–applied
voltage–luminance characteristics of OLEDs were measured
using a Hewlett Packard HP4140B electrometer and a Topcon
BM-7 luminance colorimeter. Durability measurements of
OLEDs were performed at a constant current density of
25 mA cm22 with sealed devices, which were encapsulated
under a dry N2 gas (dew point of below 260 uC, O2 con-
centration of below 10 ppm) with a fresh desiccant.
2,6-Bis[2-(2,29-bipyridin-6-yl)-1,3,4-oxadiazol-5-yl]pyridine
(Bpy-OXDpy). 6-(2H-Tetrazol-5-yl)-2,29-bipyridine, 0.50 g in
10 ml dry pyridine, was stirred. Then, 0.26 g of 2,6-pyridine-
carbonyl-dichloride was added slowly and refluxed at 110 uC
for 9 hours. After cooling, the reaction mixture was poured
into 100 ml of water. The white precipitate was isolated by
filtration, washed with water, and vacuum dried at 80 uC.
Silica gel column chromatography (CHCl3–MeOH = 20 : 1)
gave 0.12 g of pure white powder with a 24% yield.
dH(270 MHz; CDCl3; Me4Si) 8.01–8.65 (13 H, m), 7.67 (2 H,
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Results and discussion
Thermal and electronic properties
Fig. 3 shows DSC curves of the new bipyridyl-substituted
oxadiazole derivatives (Bpy-OXDs). An endothermic peak due
to melting was observed at 267 uC in the first heating of Bpy-
OXD as shown in Fig. 3a. A baseline shift due to the glass
transition, an exothermic peak due to crystallization, another
endothermic peak, and the endothermic peak of melting were
observed in a second heating of Bpy-OXD. The endothermic
peak at 250 uC should be caused by a crystal phase transition.
On the other hand, an endothermic peak due to melting was
recorded at 235 uC in the first heating of Bpy-OXDpy, as
shown in Fig. 3b, and this was no longer detected in a second
heating. A baseline shift due to the glass transition only
appeared in a second heating. These DSC results confirmed
that Bpy-OXDs formed a stable glassy state with glass
transition temperatures (Tg) of 106 uC and 114 uC for Bpy-
OXD and Bpy-OXDpy, respectively. Since these Tg values
were comparable to that (94 uC) of NPB, a practical HT
material, the Bpy-OXDs are practical thermally stable
materials. XRD of Bpy-OXDs thin films also confirmed the
formation of a stable glassy state, showing only hollow curves
in the XRD patterns.
Fig. 4 UV and visible absorption and fluorescence spectra of 100 nm-
thick vacuum deposited Bpy-OXDs thin films. The black straight
arrow indicates the absorption edge used to determine the band gap of
Bpy-OXD.
HOMO levels of the materials (Bpy-OXD: 6.56 eV, Bpy-
OXDpy: 6.25 eV) were sufficiently deep to suppress hole-
leakage from the emissive layer to the cathode via the
Bpy-OXDs layer, and the hole confinement in the emissive
layer yielded high quantum efficiencies of EL. In addition,
the HOMO level of Bpy-OXD was 0.16 eV, smaller than that
of a typically used HB material, bathocuproine (6.40 eV).19
Bpy-OXDs can be utilized as efficient ET and HB materials
with high thermal stability.
UV-visible absorption and fluorescence spectra of neat thin
films of Bpy-OXDs are shown in Fig. 4. The absorption
maxima of both materials were 280 nm and the materials
emitted bright UV fluorescence. The band gaps of 3.64 eV for
Bpy-OXD and 3.58 eV for Bpy-OXDpy, which were deter-
mined by the absorption edge of the UV and visible absorption
spectrum of each neat thin film (see Fig. 4), were very wide.
These wide band gap characteristics resulted from the weak
p-conjugations of oxadiazole and, in addition, meta linkage in
all phenyl and pyridyl rings. LUMO levels of the compounds
(Bpy-OXD: 2.92 eV, Bpy-OXDpy: 2.67 eV) in a neat solid
were low enough to easily accept electrons from a cathode
metal such as Mg : Ag (work function: 3.7 eV). This nature
was caused by molecular-level hybridization of two electro-
philic moieties (pyridine and oxadiazole).18 In addition,
OLED performance
To evaluate the performance of Bpy-OXDs as ET materials,
we fabricated several OLEDs with Bpy-OXDs as ET layers.
Fig. 5 shows luminance vs. applied voltage and luminance vs.
current density characteristics of the Bpy-OXDs OLEDs, and
Table 1 summarizes the performance of the OLEDs. As
shown in Fig. 5a, the OLEDs with Bpy-OXD showed bright
luminescence at a lower voltage than the reference device.
Furthermore, the OLEDs with Bpy-OXD also exhibited higher
current efficiency (>6 cd A21) than the reference device, as
shown in Fig. 5b and Table 1. These tendencies indicated
the following: 1) an enhancement of the charge balance in the
emissive layer based on efficient feeding of electrons into
the emissive layer and 2) a reduction in hole-leakage caused
by efficient hole confinement by the deep HOMO level of
Bpy-OXD. In addition, as the thickness of the Bpy-OXD
layer increased, the turn-on voltage became smaller. This result
also suggested efficient ET in Bpy-OXD, because the HOMO
level of Bpy-OXD was so deep that holes were sufficiently
confined in the Alq emissive layer; that is, the recombination
zone was restricted to the Alq layer, and therefore, the thicker
Bpy-OXD device needed to transport electrons for a longer
distance than the thin Bpy-OXD (10 nm) device. Also, it
should be noted here that the reduction in turn-on voltage
observed for the 10 nm-thick Bpy-OXD device strongly
indicated the usefulness of Bpy-OXD as an electron injection
material.
Fig. 3 DSC curves of (a) Bpy-OXD and (b) Bpy-OXDpy in the first
(dashed line) and the second (solid line) heating.
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Fig. 6 Luminance vs. applied voltage characteristics of OLEDs of
30 nm-thick Bpy-OXD layer with different cathodes, whose structure
is ITO/NPB(50 nm)/Alq(20 nm)/Bpy-OXD(30 nm)/cathode (Mg : Ag
or LiF/Al). The LiF/Al cathode was composed of LiF(0.5 nm) and
Al(200 nm).
caused the slightly lower performance. On the other hand, each
Bpy-OXDs device exhibited better performance than another
reference device using OXD-7 (ref. 10), a representative
oxadiazole ET derivative at the same thickness. The LUMO
level of OXD-7 was also low (2.8 eV) enough to easily accept
electrons from the cathode. Hence, the better performance
of Bpy-OXDs compared to OXD-7 is probably due to it
containing no bulky substituents that lack electronic function,
like tert-butyl.
Fig. 6 shows the luminance–voltage characteristics of a
device with a 0.5 nm-thick LiF thin layer as an electron
injection buffer and an Al cathode, where the structure of
the organic multilayer is equal to that of the device with a
30 nm-thick Bpy-OXD layer using an Mg : Ag cathode. As
shown in Fig. 6, the LiF/Al device showed higher luminance
than the Mg : Ag cathode device; specifically, it attained a
luminance of 500 cd m22 at an applied voltage of only 3.4 V.
Therefore, the LiF layer can work as an efficient electron
injection buffer for Bpy-OXD as well as Alq and other
conventional ET materials.20 The current efficiency of the
device, however, was lower than that of the corresponding
Mg : Ag device, as shown in Table 1, and this confirmed the
high ET efficiency in the Bpy-OXD layer; that is, overfeeding
of electrons into the emissive layer was caused by combining
an efficient ET material (Bpy-OXD) and effective electron
injection buffer material (LiF). It should be noted that using
Fig. 5 Luminance vs. applied voltage (a) and luminance vs. current
density (b) characteristics of OLEDs, whose structure is ITO/
NPB(50 nm)/Alq(50 2 x nm)/ET layer(x nm)/Mg : Ag(200 nm).
Symbols indicate the material and thickness of the ET layer with the
exception of the reference device.
Fig. 5 also shows the luminance vs. voltage and luminance
vs. current density characteristics of a device with a 30 nm-
thick Bpy-OXDpy layer. As indicated in the figures and
Table 1, the device showed similar performance to the
corresponding Bpy-OXD device. Therefore, the two bipyridyl
substituted oxadiazole compounds are potentially useful as
ET and HB materials and also as materials for the electron
injection layer in OLEDs. However, the luminance vs. voltage
characteristics of the Bpy-OXDpy device were slightly poorer
than those of the Bpy-OXD device. A higher LUMO level of
the Bpy-OXDpy (2.67 eV) than that of Bpy-OXD (2.92 eV)
Table 1 Summarized performance of the OLEDs with Bpy-OXDs as ET materials
Entry
Von/V
Lmaxb/cd m22
gc /cd A21
c
gpd/lm W21 at luminance of 1000 cd m22
Bpy-OXD (10 nm)
Bpy-OXD (30 nm)
Bpy-OXDpy (30 nm)
OXD-7 (30 nm)
Reference
3.1
2.9
2.7
4.6
5.6
2.6
24000
25900
26500
16900
18300
38200
at 12.5 V
at 9 V
at 10 V
at 12 V
at 15 V
at 7.6 V
6.4
6.2
6.7
3.5
4.8
3.6
b
2.0
2.8
2.5
1.3
1.0
2.5
at 8 V
at 5.5 V
at 6.5 V
at 7.7 V
at 10 V
at 3.8 V
Bpy-OXD (30 nm)/LiF/Al
a
Von is defined as the applied voltage for determining the luminance of 10 cd m22
.
Lmax is the maximum luminance, representing the
c
applied voltage for determining the luminance. gc is the maximum current efficiency. gp indicates the power efficiency at the luminance of
d
1000 cd m22, representing the applied voltage for determining the efficiency.
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greater than 100 uC and emitted bright UV fluorescence.
The band gaps of the compounds were very wide (>3.5 eV).
Moreover, the bipyridyl-substituted oxadiazole derivatives had
excellent ET, HB and electron-accepting abilities and also
exhibited good thermal stability and operation durability.
Bipyridyl-substituted oxadiazoles are a promising candidate
for ET and HB materials for OLEDs.
Acknowledgements
The authors thank Dr Y. Nakajima and Mr D. Yamashita
of Riken Keiki for measuring the ionization potential of
compounds with their special photoelectron spectroscopic
equipment (Riken Keiki AC-3). This work was supported by
the Cooperative Link for Unique Science and Technology for
Economy Revitalization (CLUSTER) of Japan’s Ministry of
Education, Culture, Sports, Science and Technology. It was
also supported by the Ministry’s 21st Century COE program
and the Ministry’s Grand-in-Aid (No. 16760009) to M.I.
Fig. 7 Luminescence intensity changes (normalized by the initial
luminance) of the Bpy-OXD device and reference devices with long-
term continuous operation (constant current of 25 mA cm22). The
solid lines are guides for the eye. Device structures are described in the
main text.
efficient hole injection and modifying the transportation will
probably lead to brighter and more efficient luminescence at a
low operating voltage.
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Bpy-OXD in OLEDs. Fig. 7 shows the luminance changes for
continuous operation at 25 mA cm22 of a Bpy-OXD device
and reference devices using the practical ET material Alq and a
representative oxadiazole ET material OXD-7. Their device
structures were: ITO/CuPc(20 nm)/NPB(40 nm)/Alq(20 nm)/
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We have demonstrated new efficient electron-transporting and
hole-blocking materials for OLEDs. The bipyridyl-substituted
oxadiazole derivatives formed a stable glassy state with a Tg
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