A weak electron transporting material with high triplet energy and thermal stability via a super twisted structure for high efficient blue electrophosphorescent devices

Article abstract of DOI:10.1039/c1jm13488d

A high triplet energy (E_T = 3.2 eV) electron transporting/hole blocking (ET/HB) material, 1,2,4,5-tetra(3-pyrid-3-yl-phenyl)benzene (TemPPB) with a super twisted structure and high thermal stability has been synthesized. An external quantum eff

Full text of DOI:10.1039/c1jm13488d

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PAPER
A weak electron transporting material with high triplet energy and thermal
stability via a super twisted structure for high efficient blue
electrophosphorescent devices†
Lixin Xiao, Boyuan Qi,a Xing Xing,a Lingling Zheng,a Sheng Kong,a Zhijian Chen,a Bo Qu,a Lipei Zhang,a
a
*
b
Ziwu Ji and Qihuang Gong
a
*
Received 22nd July 2011, Accepted 22nd September 2011
DOI: 10.1039/c1jm13488d
A high triplet energy (ET ¼ 3.2 eV) electron transporting/hole blocking (ET/HB) material, 1,2,4,5-tetra
(3-pyrid-3-yl-phenyl)benzene (TemPPB) with a super twisted structure and high thermal stability has
been synthesized. An external quantum efficiency (EQE) of 19.6% was achieved by using TemPPB as
the ET/HB material in a blue electrophosphorescent device, much higher than the EQE of 12.5% for the
device using the conventional ET material, 3-(4-biphenyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-
triazole (TAZ). In addition, the weak ET property of TemPPB resulting from its super twisted structure
can be enhanced via n-type doping with LiF. An EQE of 24.5% was achieved by combining n-type
doping and a double-emission layer. This shows an alternative way to design ET/HB materials with
high ET and improved thermal stability for blue electrophosphorescent devices.
C2 ]picolinate (FIrpic), and triplet excitons and/or holes cannot
0
Introduction
be confined within the emissive layer.
For both displays and solid state lighting, efficiency is still the
crucial factor in the application of organic light-emitting devices
(OLEDs). Thus the full use of singlet and triplet excitons, elec-
trophosphorescence, has attracted increasing attention since it
was first reported.1–3 The use of phosphorescent emitters is one of
the most important strategies to improve the efficiency of
OLEDs. Despite great achievements in red and green phospho-
rescent materials, improving efficiency for blue emission seems to
be the final obstacle for commercialization of full color displays
and white light application.
A high ET may be obtained if the p-conjugation system is
blocked. Two ways can be used to achieve this, one of which is to
introduce a central atom connection. By introducing phenyl or
boron as the central group, Kido and his co-workers developed
a series of ET/HB materials with high ET, e.g. 1,3,5-tri(p-pyrid-
3-yl-phenyl)benzene (TpPyPB),5 1,3,5-tri(m-pyrid-3-yl-phenyl)
benzene (TmPyPB),5 3,300,5,500-tetra(3-pyridyl)-1,10;30,100-terphenyl
(B3PyPB),6 and tris[3-(3-pyridyl)mesityl]borane (3TPYMB)7,8
and achieved external quantum efficiencies (EQEs) of over 20%
for blue PhOLEDs. Introducing a phosphine oxide (P]O) group
is also possible to weaken the molecular conjugation; derivatives
with high ET as effective bipolar hosts and ET/HB materials for
blue PhOLEDs have been reported.9–14
Generally, the hole mobility is much higher (about 1000 times)
than the electron mobility in organic semiconducting materials,4
and thus holes are much easier to transport than electrons in
phosphorescent OLEDs (PhOLEDs). Therefore, the develop-
ment of efficient electron transporting/hole blocking (ET/HB)
materials is crucial to improving the efficiency of PhOLEDs.
Although several traditional ET/HB compounds have been used
in blue electrophosphorescence, high efficiency cannot be ach-
ieved because their triplet energy (ET) is lower than that of the
blue emitter iridium(III) bis[(4,6-difluorophenyl)-pyridinato-N,
We previously prepared a weak ET material, diphenyl-bis[4-
(pyridin-3-yl)phenyl]silane (DPPS) by using a silicon atom as the
central group to weaken the conjugation,15 with effective HB
property and high ET with effective exciton blocking (EB)
property to achieve nearly a 100% internal quantum efficiency
(IQE) in a FIrpic-based device. Therefore, ET/HB materials with
higher ET and large enough ion potential (Ip) compared to those
of phosphorescent dyes are crucial to improving the efficiency of
blue PhOLEDs.16
aState Key Laboratory for Mesoscopic Physics and Department of Physics,
Peking University, Beijing, 100871, P. R. China. E-mail: lxxiao@pku.edu.
cn; qhgong@pku.edu.cn
bSchool of Physics, Shandong University, Jinan, 250100, P. R. China
† Electronic supplementary information (ESI) available: TOF
measurement and low temperature photoluminescence spectrum. See
DOI: 10.1039/c1jm13488d
Based on the results above, inserting a central atom, e.g. silicon
(ET ¼ 2.7–3.3 eV),10,16 boron (ET ¼ 3.0 eV)7,8 or P]O (ET ¼ 3.3
eV)10 to weaken the p-conjugation is an effective way to achieve
high ET. However, the central atom connection led to poorer
thermal stability, and hence further improvements are still
needed. If the central atom was replaced by a phenyl group as
19058 | J. Mater. Chem., 2011, 21, 19058–19062
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mentioned above, although this could improve the thermal
properties, it would be difficult to further increase the ET (2.6–2.8
eV)5,6 because of their near coplanarity originating from their
lower steric hindrance. For example, the ET of the 3,5-dipyr-
idylphenyl derivative of B3PyPB is 2.77 eV, although it shows
a glass-transition temperature (Tg) of 106 C (Table 1).6
ꢀ
In this paper, we report a second way to achieve high ET by
using a super twisted structure and meta-position17 linking to
block the p-conjugation. We synthesize an ET/HB material,
1,2,4,5-tetra(3-pyrid-3-yl-phenyl)benzene (TemPPB), which
exhibits a super twisted structure originating from steric
hindrance. This conformation suppresses the electron delocal-
ization and results in a wide Eg and high ET. The molecular
conformation of TemPPB optimized by Gaussian 09 (Hartree–
Fock method, at the 6-31G* level) is shown in Fig. 1. The
distance between the upper and the lower groups is calculated to
Fig. 1 Molecular conformation of TemPPB optimized by Gaussian 09
(Hartree–Fock method, at the 6-31G* level).
super twisted structure. The lowest unoccupied molecular orbital
(LUMO) level was estimated to be 3.0 eV, as measured by cyclic
voltammetry (CV) with a three-electrode configuration as shown
in Fig. S1.† The highest occupied molecular orbital (HOMO)
level of TemPPB could be calculated from its LUMO level and
Eg as 7.0 eV which is low enough to block holes escaping from the
emitting material (EM) layer. The low temperature phospho-
rescence spectrum was measured in a frozen solution (2-methyl
THF) in liquid N2 with a phosphorescence mode. The phos-
phorescence spectrum is shown in Fig. S2.† The triplet energy is
calculated from the onset of the phosphorescence spectrum as
B3PyPB reported by Kido.6 The onset of phosphorescence of
TemPPB is 390 nm, and accordingly the ET energy of TemPPB
was calculated to be around 3.2 eV which is higher than the 2.7
eV for 3-(4-biphenyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-tri-
azole (TAZ), a conventional ET material in PhOLEDs. The
electron mobility of TemPPB was 3 ꢁ 10ꢂ7 cm2 Vꢂ1 sꢂ1 measured
using the time-of-flight (TOF) method (Fig. S3†). The low elec-
tron mobility of TemPPB might be due to the super twisted
structure.
ꢀ
be 9.18 A, which indicates that the molecular conformation is far
from coplanar. The super twisted structure is beneficial to the
suppression of aggregation in the film state and to the Tg which is
97 ꢀC, much higher than the 50 ꢀC for DPPS (with Si as the
central atom). These may increase the stability of the device.
When it was employed in a blue PhOLED as the combined
ET/HB layer, an EQE of 19.6% was achieved which was
increased to 24.5% by combination with n-type doping and
a double-emission layer. This suggests an alternative way to
design ET/HB materials with high ET and improved thermal
stability for blue electrophosphorescent devices.
Results and discussion
The synthetic route of TemPPB is shown in Scheme 1. 3-bro-
mopyridine was reacted with 3-chlorophenyl boronic acid via
a Suzuki cross-coupling reaction18 to yield 1, followed by boro-
nation to afford 2 according to the literature,19 then reacted with
1,2,4,5-tetrabromobenzene via a second Suzuki cross-coupling
reaction. Finally a white powder of TemPPB was obtained and
purified via sublimation before further use. The absorption and
photoluminescence (PL) spectra of TemPPB, shown in Fig. 2,
were measured in both the film and in tetrahydrofuran (THF)
solution (1 ꢁ 10ꢂ5 M) at room temperature. The TemPPB film
was deposited under vacuum on a quartz substrate. The
absorption maximum of the TemPPB film (263 nm) is red-shifted
7 nm relative to the solution (256 nm). The peaks can be
attributed to the p–p* transition. The energy gap (Eg) of
TemPPB can be calculated from the absorption edge to be 4.0 eV,
which is wider than most previously reported ET materials. The
PL spectra show the peaks for the solution and film at 382 nm
and 384 nm, respectively. The slight difference indicates that
TemPPB tends not to aggregate in the solid state as a result of its
The thermal properties of TemPPB were measured by ther-
mogravimetric analysis (TGA) and differential scanning calo-
rimetry (DSC), which showed
a
thermal-decomposition
temperature (Td) at 400 ꢀC and a Tg at 97 ꢀC. As comparison, the
physical properties of materials including B3PyPB and DPPS are
summarized in Table 1.
To investigate the ET/HB property of TemPPB, multilayered
electrophosphorescent devices were fabricated. A glass substrate
precoated with an indium tin oxide (ITO) layer was treated as in
our previous report.23 A 20 nm layer of 4,40-bis[N-(p-tolyl)-N-
phenyl-amino]biphenyl (TPD) or 1,1-bis[(di-4-tolylamino)
phenyl]cyclohexane (TAPC) was spin coated as the hole inject-
ing/transporting layer. 4,40,400-tris-(N-carbazolyl)-triphenyl-
amine (TCTA) was used as the hole transporting/host layer,
Table 1 Physical properties of the materials used
Materials
Tg (ꢀC)
Ea (eV)
Ip (eV)
Eg (eV)
ET (eV)
Reference
TemPPB
B3PyPB
DPPS
TAZ
FIrpic
97
106
50
3.0
2.62
2.5
2.7
3.5
7.0
6.67
6.5
6.3
6.2
4.0
4.05
4.0
3.6
2.7
3.2
2.77
2.7
2.7
2.7
This work
6
16
20,21
22
70
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Scheme 1 Synthetic route for TemPPB.
Fig. 2 Absorption and PL spectra of TemPPB in THF (1 ꢁ 10ꢂ5 M) and
as a film.
Fig. 3 Current density and luminance–voltage and CE and PE–current
density of device A: ITO/TPD 20 nm/TCTA 20 nm/mCP: 8%FIrpic 20
nm/TemPPB 30 nm/LiF/Al (,-) and B: ITO/TPD 20 nm/TCTA 20 nm/
FIrpic as the blue phosphorescent dye, N,N0-dicarbazolyl-3,5-
benzene (mCP) as the host, TemPPB or TAZ (reference device)
as the ET layer, LiF as the electron injection layer and Al as the
cathode. The structures of the devices fabricated are shown in the
following.
mCP: 8%FIrpic 20 nm/TAZ 30 nm/LiF/Al (
).
effectively confined within the EM layer by TemPPB than TAZ.
In addition, since organic materials are normally better p-type
than n-type conductors, the mobilities of holes are expected to be
higher than those of electrons in the device. Because of the wider
Eg, i.e., the HOMO energy or Ip is deeper than that of TAZ, the
holes within the EM layer in device A can be more effectively
blocked than in device B.
Device A: ITO/TPD 20 nm/TCTA 20 nm/mCP: 8%FIrpic 20
nm/TemPPB 30 nm/LiF/Al
Device B: ITO/TPD 20 nm/TCTA 20 nm/mCP: 8%FIrpic 20
nm/TAZ 30 nm/LiF/Al
Device C: ITO/TPD 20 nm/TCTA 20 nm/mCP: 8%FIrpic 20
nm/TemPPB: 1%LiF 30 nm/LiF/Al
The super twisted structure results in not only a high ET, but
also low electron mobility for TemPPB because it is far from
coplanar. To enhance the ET property in a device, we doped
TemPPB with 1% LiF; the current density of the device increased
compared with the undoped ET layer (Fig. 4). Similar enhance-
ments in current density were also previously reported for tris(8-
hydroxyquinolato)aluminium (Alq3) upon doping with LiF25
and MnO.26 When the whole 30 nm ET layer was doped with 1%
LiF (device C), the current density was greatly enhanced (Fig. 4).
However, the efficiency of device C with doping was lower than
that of device A without doping. This might be attributed to
exciton quenching induced by doping, because the efficiency
Device D: ITO/TPD 20 nm/TCTA 20 nm/mCP: 8%FIrpic 20
nm/TemPPB 10 nm/TemPPB: 1%LiF 20 nm/LiF/Al
Device E: ITO/TAPC 20 nm/TCTA: 8%FIrpic 20 nm/mCP:
15%FIrpic 20 nm/TemPPB 10 nm/TemPPB: 1%LiF 20 nm/LiF/
Al
The current density and luminance–voltage characteristics of
devices A and B are shown in Fig. 3. The current density of device
A is slightly higher than that of the device B and the turn-on
voltages (at 1 cd mꢂ2) are 3.4 V and 4.3 V for devices A and B,
respectively, although the electron mobility of TemPPB is 3 ꢁ
10ꢂ7 cm2 Vꢂ1 sꢂ1, slightly lower than the ꢃ 10ꢂ6 cm2 Vꢂ1 sꢂ1 for
TAZ.6 This might be due to the pyridine component which
enhanced electron injection/transportation between the metal
electrode and organic layer.24
Moreover, device A achieves a maximum luminance of 19300
cd mꢂ2, a maximum current efficiency (CE) of 40.6 cd Aꢂ1 and
a maximum power efficiency (PE) of 29.2 lm Wꢂ1 corresponding
to an EQE of 19.6%, while device B shows a maximum luminance
of 16540 cd mꢂ2, a maximum CE of 24.1 cd Aꢂ1 and a maximum
PE of 16.1 lm Wꢂ1 corresponding to an EQE of 12.5% (Table 2).
The higher ET for TemPPB (3.2 eV compared to 2.7 eV for
TAZ) may be the major reason for the greatly enhanced
performance, because the triplet excitons are expected to be more
Table 2 Performance of the devices
Lmax
(cd mꢂ2
CEmax
(cd Aꢂ1
PEmax
(lm Wꢂ1
Device
Von (V)
)
)
)
EQE (%)
A
B
C
D
E
3.4
4.3
3.1
3.2
4.1
19300
16540
16000
17440
22680
40.6
24.1
19.9
33.0
52.2
29.2
16.1
19.6
32.4
38.5
19.6
12.5
8.0
11.9
24.5
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Fig. 5 Current density and luminance–voltage and CE and PE–current
density of device A: ITO/TPD 20 nm/TCTA 20 nm/mCP: 8%FIrpic 20
nm/TemPPB 30 nm/LiF/Al (,-) and E: ITO/TAPC 20 nm/TCTA: 8%
FIrpic 20 nm/mCP: 15%FIrpic 20 nm/TemPPB 10 nm/TemPPB: 1%LiF
Fig. 4 Current density and luminance–voltage and CE and PE–current
density of device A: ITO/TPD 20 nm/TCTA 20 nm/mCP: 8%FIrpic 20
nm/TemPPB 30 nm/LiF/Al (,-), C: ITO/TPD 20 nm/TCTA 20 nm/
mCP: 8%FIrpic 20 nm/TemPPB: 1%LiF 30 nm/LiF/Al (
TPD 20 nm/TCTA 20 nm/mCP: 8%FIrpic 20 nm/TemPPB 10 nm/
TemPPB: 1%LiF 20 nm/LiF/Al ( ).
) and D: ITO/
20 nm/LiF/Al (
).
Conclusions
decreased with increasing thickness of the doped ET layer. When
only 20 nm of the whole ET layer (30 nm) was doped with 1% LiF
(device D), the current density decreased compared to that of
device C, but was still higher than that of device A without
doping. Moreover the PE of device D is also higher than that of
device A, which indicates that the quenching of the excitons was
effectively suppressed by inserting 10 nm of undoped TemPPB
between the LiF doped layer and the emitting layer. These results
indicate that the weak ET resulting from the super twisted
structure could be improved by n-type doping.
In conclusion, an ET/HB material, TemPPB, was synthesized
with a super twisted structure and meta-position linking to
suppress electron delocalization, leading to a wide Eg (4.0 eV)
and high ET (3.2 eV) originating from the steric hindrance. An
EQE of 19.6% was achieved by using TemPPB as the ET/HB
material for a blue electrophosphorescent device, much higher
than the EQE of 12.5% for the device using the conventional ET
material, TAZ. Moreover, the super twisted structure is benefi-
cial to the suppression of aggregation in the film state and the
thermal stability was also improved which may increase the
stability of the device. In addition, the weak ET property of
TemPPB resulting from the super twisted structure can be
enhanced via n-type doping with LiF. An EQE of 24.5% was
achieved by a combination of n-type doping and a double-
emission layer. This shows an alternative way to design ET/HB
materials with high ET and improved thermal stability for blue
electrophosphorescent devices.
To reduce the triplet–triplet annihilation, a double-emission
layer was chosen and the emission region could be enlarged to the
TCTA layer. In addition, to avoid the back energy transfer from
the TCTA layer to the hole transporting layer, TPD with low ET
(2.3 eV)27 was replaced with TAPC (ET 2.9 eV)27 (device E). As
seen in Fig. 5, device E shows a maximum luminance over 22680
cd mꢂ2, a maximum CE of 52.2 cd Aꢂ1 and a maximum PE of
38.5 lm Wꢂ1 corresponding to an EQE of 24.5% (Table 2). The
enhancement of efficiency can be attributed to the n-type doping
of LiF and high ET of TAPC. However the turn-on voltage is
increased to 4.1 V because the HOMO of TAPC is 5.5 eV, higher
than the 5.3 eV of TPD which is closer to the work function of
ITO. It is also responsible for the lower current density of device
E compared to device A, because the n-type doped device showed
higher current density than that of device A. The electrolumi-
nescence (EL) spectra of the devices are very similar and are
shown in Fig. S4.†
Experimental
General
The 1H NMR and 13C NMR spectra were recorded on a Bruker
DPX-400 spectrometer (400 MHz). The mass spectrum was
recorded on a Bruker Apex IV FTMS spectrometer. Elemental
analysis was measured on an Elementar Vario MICRO CUBE
(Germany). Ultraviolet-visible (UV-vis) absorption and PL
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spectra were obtained on an Agilent UV-vis Spectroscopy
System (8453E). The low temperature phosphorescence spectrum
measurement was carried out on a Hitachi Fluorescence Spec-
trometer (F-4500) under liquid nitrogen from a frozen solution
(2-methyl THF) with a phosphorescence mode. The electro-
chemical properties were measured in CH3CN solution con-
taining 0.1 M of tetrabutylammonium perchlorate as the
supporting electrolyte at a scanning rate of 0.1 V sꢂ1 by cyclic
voltammetry on a CHI 650 electrochemistry workstation with
a Pt disk working electrode, Pt wire counter electrode, and
a saturated calomel reference electrode (SCE). Thermal proper-
ties were determined with thermogravimetric analysis (TA
Instrument Q50) and differential scanning calorimetry (TA
Instrument Q100).
crystal microbalance. The device performances were measured
with a Keithley 2611 source meter and a spectrophotometer
(Photo Research 650). All the measurements were carried out in
ambient atmosphere at room temperature.
Acknowledgements
This work was financially supported by the National Basic
Research Program of China (grants 2009CB930500 and
2007CB307000), NSFC (grants 61177020, 10934001, 60907015
and 10821062), and Beijing Municipal Science and Technology
Project (Z101103050410002). The authors thank Dr Z. Chu and
Prof. D. Zou of the College of Chemistry and Molecular Engi-
neering, Peking University for the TOF measurement and Ms J.
You and Prof. P. Wang of the Technical Institute of Physics and
Chemistry, CAS, for the phosphorescence spectrum measurement.
Synthesis of 1,2,4,5-tetra(3-pyrid-3-yl-phenyl)benzene
(TemPPB)
References
The syntheses of 3-pyrid-3-yl chlorobenzene (1) and 3-(3-
(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)pyridine (2)
were performed following the literature.19 1,2,4,5-tetra-
bromobenzene (0.67 g, 1.70 mmol), 2 (2.48 g, 15.8 mmol) were
put into a three-neck flask, then toluene (60 mL), ethanol (20
mL) and 2 M aqueous potassium carbonate (20 mL) were added
to it. Under nitrogen atmosphere, the mixture was stirred at 90
ꢀC for 1 h. Then tetrakis(triphenylphosphine)palladium(0) (Pd
(PPh3)4) (0.16 g, 0.1 mmol) was added to the mixture, followed
by refluxing for another 24 h. After cooling down to room
temperature, the mixture was poured into water and then
extracted with chloroform three times. The combined organic
phase was collected and dried with MgSO4. The solvent was
removed under reduced pressure. The residue was purified by
silica-gel column chromatography with chloroform/methanol
(30 : 1) as the eluent. The product (after removing the solvents)
was obtained as a white solid in 82% yield (0.97 g). Finally
a further purification was carried out via sublimation before use.
1H NMR (CDCl3, TMS, d): 7.26–7.28 (m, 4H), 7.43–7.49 (m,
16H), 7.61–7.63 (m, 4H), 7.73 (s, 2H), 8.53–8.56 (m, 8H); 13C
NMR (CDCl3, TMS, d): 123.52, 125.77, 129.14, 129.20, 129.47,
132.60, 134.37, 136.35, 137.61, 139.67, 141.34, 148.04, 148.37.
MS (ESI, m/z): 691.3 (calcd for C50H34N4: 690.3). Anal. Calcd
for C50H34N4: C, 86.93; H, 4.96; N, 8.11. Found: C, 86.90; H,
4.97; N, 8.13.
1 S. Hoshino and H. Suzuki, Appl. Phys. Lett., 1996, 69, 224.
2 Y. Ma, H. Zhang, J. Shen and C. Che, Synth. Met., 1998, 94, 245.
3 M. A. Baldo, D. F. O’Brien, Y. You, A. Shoustikov, S. Sibley,
M. E. Thompson and S. R. Forrest, Nature, 1998, 395, 151.
4 H. Antoniadis, M. A. Abkowitz and B. R. Hsieh, Appl. Phys. Lett.,
1994, 65, 2030.
5 S.-J. Su, T. Chiba, T. Takeda and J. Kido, Adv. Mater., 2008, 20,
2125.
6 H. Sasabe, E. Gonmori, T. Chiba, Y.-J. Li, D. Tanaka, S.-J. Su,
T. Takeda, Y.-J. Pu, K. I. Nakayama and J. Kido, Chem. Mater.,
2008, 20, 5951.
7 D. Tanaka, Y. Agata, T. Takeda, S. Watanabe and J. Kido, Jpn. J.
Appl. Phys., 2007, 46, L117.
8 D. Tanaka, T. Takeda, T. Chiba, S. Watanabe and J. Kido, Chem.
Lett., 2007, 36, 262.
9 J. Ding, Q. Wang, L. Zhao, D. Ma, L. Wang, X. Jing and F. Wang, J.
Mater. Chem., 2010, 20, 8126.
10 S. O. Jeon, S. E. Jang, H. S. Son and J. Y. Lee, Adv. Mater., 2011, 23,
1436.
11 S. O. Jeon, K. S. Yook, C. W. Joo and J. Y. Lee, Adv. Mater., 2010,
22, 1872.
12 S. O. Jeon, K. S. Yook, C. W. Joo and J. Y. Lee, J. Mater. Chem.,
2009, 19, 5940.
ꢁ
ꢁ
13 M. Mamada, S. Ergun, C. Perez-Bolıvar and P. Anzenbacher, Jr,
Appl. Phys. Lett., 2011, 98, 073305.
14 A. L. V. Ruden, L. Cosimbescu, E. Polikarpov, P. K. Koech,
J. S. Swensen, L. Wang, J. T. Darsell and A. B. Padmaperuma,
Chem. Mater., 2010, 22, 5678.
15 L. Xiao, S.-J. Su, Y. Agata, H. Lan and J. Kido, Adv. Mater., 2009,
21, 1271.
16 L. Xiao, Z. Chen, B. Qu, J. Luo, S. Kong, Q. Gong and J. Kido, Adv.
Mater., 2011, 23, 926.
ꢁ
17 I. Avilov, P. Marsal, J.-L. Bredas and D. Beljonne, Adv. Mater., 2004,
16, 1624.
Device fabrication and characterization
18 N. Miyaura and A. Suzuki, Chem. Rev., 1995, 95, 2457.
19 S.-J. Su, D. Tanaka, Y. J. Li, H. Sasabe, T. Takeda and J. Kido, Org.
Lett., 2008, 10, 941.
20 C.-C. Wu, C.-W. Chen and T.-Y. Cho, Appl. Phys. Lett., 2003, 83, 611.
21 M.-H. Tsai, H.-W. Lin, H.-C. Su, T.-H. Ke, C.-C. Wu, F.-C. Fang,
Y.-L. Liao, K.-T. Wong and C.-I. Wu, Adv. Mater., 2006, 18, 1216.
22 S. Tokito, T. Iijima, Y. Suzuri, H. Kita, T. Tsuzuki and F. Sato, Appl.
Phys. Lett., 2003, 83, 569.
23 S. Kong, L. Xiao, Y. Liu, Z. Chen, B. Qu and Q. Gong, New J. Chem.,
2010, 34, 1994.
24 T. Oyamada, H. Yoshizaki, H. Sasabe and C. Adachi, Chem. Lett.,
2004, 33, 1034.
25 K. R. Choudhury, J.-h. Yoon and F. So, Adv. Mater., 2008, 20, 1456.
26 J. Luo, L. Xiao, Z. Chen and Q. Gong, Appl. Phys. Lett., 2008, 93,
133301.
27 J. Lee, N. Chopra, S.-H. Eom, Y. Zheng, J. Xue, F. So and J. Shi,
Appl. Phys. Lett., 2008, 93, 123306.
An ITO-coated substrate with a sheet resistance of 10 U sqꢂ1 was
used as the anode. It was cleaned with deionized water, acetone
and ethanol in sequence under ultrasonication and finally treated
with oxygen plasma. TPD or TAPC was dissolved in 1,2-
dichloroethane, filtered through a 0.22 mm filter and spin-coated
onto the surface of the ITO to act as a hole injecting/transporting
layer. After that, it was loaded into a vacuum chamber and the
other materials were all deposited by thermal evaporation under
high vacuum (10ꢂ4 Pa). An ultra thin layer of LiF (0.5 nm) and an
aluminium layer (100 nm) were evaporated in vacuum as the
cathode, the area of which was defined as the active area of the
devices by a shadow mask with 2 mm diameter opening. The
thickness of the evaporating layer was monitored with a quartz
19062 | J. Mater. Chem., 2011, 21, 19058–19062
This journal is ª The Royal Society of Chemistry 2011