Configuration effect of novel bipolar triazole/carbazole-based host materials on the performance of phosphorescent OLED devices

Article abstract of DOI:10.1016/j.orgel.2012.06.025

A series of structurally isomeric carbazole/triazole (TAZ)-based bipolar host materials 1-4 were designed and synthesized. These new materials were found to exhibit wide energy gaps (E_g: 3.29-3.52 eV), high triplet energies (E_T: 2.56-2.76 eV), high thermal stability (T_d: 426-454 °C), high glass-transition temperatures (T_g: 116-156 °C) and excellent film-forming property. Green and blue emitting devices with fac-tris(2-phenylpyridine)iridium (Ir(ppy)₃) and iridium(III) bis(4,6-(di-fluorophenyl)pyridinato-N,C^2′)picolinate (FIrpic) as phosphorescent dopants have been fabricated. The measurements of turn-on voltages, efficiencies and luminance suggested that the practice of combining carbazole's high triplet energy and excellent hole-transporting ability with TAZ's electron-transporting ability at the molecular level was effectively translated into better performance at the device level. The molecular structure of compound 4 is well-correlated with its efficiencies, which (32.7 and 21.1 cd/A for green and blue devices, respectively) were the best among the four materials.

Full text of DOI:10.1016/j.orgel.2012.06.025

Organic Electronics 13 (2012) 2210–2219
Contents lists available at SciVerse ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
Configuration effect of novel bipolar triazole/carbazole-based host
materials on the performance of phosphorescent OLED devices
Jinyong Zhuang a,b, Wenming Su b, Wanfei Li b,c, , Yuyang Zhou b, Qi Shen a, , Ming Zhou b,c,
⇑
⇑
⇑
a Key Laboratory of Organic Synthesis of Jiangsu Province, Department of Chemistry and Chemical Engineering, Dushu Lake Campus, Soochow University,
Suzhou 215123, PR China
b Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, 398 Ruoshui Road, Suzhou Industrial Park, Suzhou, Jiangsu 215123, PR China
c SunaTech Inc., Suzhou Industrial Park, Suzhou, Jiangsu 215123, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of structurally isomeric carbazole/triazole (TAZ)-based bipolar host materials 1–4
were designed and synthesized. These new materials were found to exhibit wide energy
gaps (Eg: 3.29–3.52 eV), high triplet energies (ET: 2.56–2.76 eV), high thermal stability
(Td: 426–454 °C), high glass-transition temperatures (Tg: 116–156 °C) and excellent film-
Received 3 March 2012
Received in revised form 12 June 2012
Accepted 17 June 2012
Available online 2 July 2012
forming property. Green and blue emitting devices with fac-tris(2-phenylpyridine)iridium
0
(Ir(ppy)3) and iridium(III) bis(4,6-(di-fluorophenyl)pyridinato-N,C2 )picolinate (FIrpic) as
Keywords:
Bipolar
Host materials
Triazole
Carbazole
Phosphorescent
OLED
phosphorescent dopants have been fabricated. The measurements of turn-on voltages, effi-
ciencies and luminance suggested that the practice of combining carbazole’s high triplet
energy and excellent hole-transporting ability with TAZ’s electron-transporting ability at
the molecular level was effectively translated into better performance at the device level.
The molecular structure of compound 4 is well-correlated with its efficiencies, which (32.7
and 21.1 cd/A for green and blue devices, respectively) were the best among the four
materials.
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
and copper(I) [16,17] complexes have also received in-
creased attention because of their better device stability
Organic light emitting diodes (OLEDs) have attracted
much attention from both the academic and industrial com-
munities for their practical applications in displays and so-
lid state lighting [1–3]. Compared to fluorescent dopants,
the phosphorescent dopants have become a main stream
materials system due to their much higher internal quan-
tum efficiencies (close to 100%) as a result of harvesting
both singlet and triplet excitons [4–13]. Choosing dopant
materials is mainly focused on the cyclometalated irid-
ium(III) complexes. In recent years, platinum(II) [14,15]
and the lower cost, respectively. On the other hand, the
design of a proper host material matching well with its cor-
responding phosphorescent dopants is of crucial impor-
tance for OLED performance improvement [18]. To ensure
an exothermic energy transfer from the host to the dopant
and a confinement of triplet excitons on the dopant, a larger
HOMO–LUMO (HOMO – the highest occupied molecular
orbital; LUMO – the lowest unoccupied molecular orbital)
energy gap (Eg) and a higher triplet energy (ET) level of the
host materials than those of the phosphorescent dopants
are needed. Furthermore, keeping the balance of carrier
mobilities is very important to achieve high-efficiency
phosphorescent OLEDs [19]. Generally, the hole mobilities
of hole-transport materials such as 1,1-bis[(di-4-tolylami-
no)phenyl]cyclohexane (TAPC) or 4,40-bis[N-(1-naphthyl)-
N-phenylamino]biphenyl (NPD) [20,21] are about three
orders of magnitude higher than the electron mobilities of
⇑
Corresponding authors. Address: Suzhou Institute of Nano-Tech and
Nano-Bionics, Chinese Academy of Sciences, 398 Ruoshui Road, Suzhou
Industrial Park, Suzhou, Jiangsu 215123, PR China. Tel.: +86 512
62872558; fax: +86 512 62872181 (W. Li, M. Zhou), tel.: +86 512
65880306; fax: +86 512 65880305 (Q. Shen).
E-mail addresses: wfli2007@sinano.ac.cn (W. Li), qshen@suda.edu.cn
(Q. Shen), mzhou2007@sinano.ac.cn (M. Zhou).
1566-1199/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.orgel.2012.06.025

J. Zhuang et al. / Organic Electronics 13 (2012) 2210–2219
2211
electron-transport materials (ETMs) such as tris(8-quinoli-
nolato) aluminum (Alq3) and 2,9-dimethyl-4,7-diphenyl-
1,10-phenanthroline (BCP) [22]. One successful and effec-
tive approach to balancing the hole and electron mobilities
is to use bipolar host materials [23–25]. Recently many
groups have been working on bipolar host materials using
different electron-donating and electron-accepting units,
which greatly enriched the host materials [26–33]. In these
work, the devices based on the bipolar host materials dem-
onstrated better performance than those based on 4,40-N,N0-
dicarbazole-biphenyl (CBP) and 1,3-bis(9-carbazolyl)ben-
zene (mCP), which are the common references for green
and blue host materials.
Carbazole moiety, which possesses sufficiently high
triplet energy and hole mobility, has been widely used as
a structural unit for developing OLED host materials. Some
well-known examples are CBP [34], mCP [35], 3,5-di(N-
carbazolyl)tetraphenylsilane (SimCP) [36] and 4,40-N,N0-
dicarbazole-2,20-dimethyl-biphenyl (CDBP) [37]. 1,2,4-tria-
zole derivatives, on the other hand, have been proved to
have excellent electron-transporting and hole-blocking
abilities due to their highly electron-deficient TAZ moiety
[38–42]. A good example of its derivative is 3-phenyl-4-
(10-naphthyl)-5-phenyl-1,2,4-triazole, which has been
used as host for green emitters [43].
morphological, photophysical and electrochemical proper-
ties of the compound 1–4, as well as the performance of he
OLEDs hosted by the compounds have been investigated.
Devices using Ir(ppy)3 and FIrpic as the green and the blue
dopants have been fabricated and evaluated. The relation-
ship between the structures of these four isomers and
properties is also discussed.
2. Results and discussion
2.1. Synthesis and characterization
Four bipolar isomeric host materials (1–4) were synthe-
sized following the same Suzuki coupling reaction, starting
from dibromo-substituted 1,2,4-triazole and boronic acid
in this work. Their chemical structures and the synthetic
routes to them are shown in Scheme 1. Four precursors,
i.e., two dibromo-substituted 1,2,4-triazole and two 3-(or
4)-(9-phenyl-9H-carbazole) phenylboronic acid were pre-
pared according to the previously reported procedure
[44–46]. All compounds were purified with silica columns
using petroleum ether, ethyl acetate and dichloromethane
as eluants. Finally, these compounds were structurally con-
firmed by 1H NMR, 13C NMR, and mass spectrometry.
The efforts in searching better bipolar materials have
never been exhausted. In this article, we report the design
and synthesis of a series of novel bipolar host materials
containing carbazole and 1,2,4-triazole building blocks
(Scheme 1). We expect that varying coupling mode can
tune the photophysical and electrochemical properties,
and then modulate their energy gaps, energy levels and
triplet energies of the four isomers. We incorporated these
two functional moieties by different coupling modes or at
different positions of the bridging phenyl ring, and
anticipated that these host materials would have both
electron- and hole-transporting properties. The thermal,
2.2. Thermal and morphological analysis
The thermal properties of these materials were investi-
gated by thermogravimetric analysis (TGA) and differential
scanning calorimetry (DSC) (Fig. S1 and S2 in Supporting
information). Their decomposition temperatures (Td) are
in the range 426–454 °C, as defined by 5% weight loss.
The high Td values suggest that these compounds can en-
dure vacuum thermal sublimation at a reasonably high
temperature during OLED device fabrication. They were
also found to have very high glass-transition temperatures
(Tg) ranging from 116 to 156 °C, which are much higher
Scheme 1. Synthetic routes to 1–4 (reaction condition: reflux under argon with Pd(PPh3)4 and Na2CO3 in toluene/EtOH solvent).

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J. Zhuang et al. / Organic Electronics 13 (2012) 2210–2219
than those of the analogous carbazole-based host materials
such as CBP (Tg = 62 °C [2]) and mCP (Tg = 60 °C [2]). The
crystallization temperatures (Tc) and the melting points
(Tm) were also measured and are listed in Table 1. The rigid
triazole core may be the reason for the high Td and Tc val-
ues. As an exception, no obvious Tc or Tg signal was de-
tected for 1.
The morphology and stability of the host materials have
effects on the current efficiency and the lifetime of OLED
devices [47]. Using the atomic force microscopy (AFM)
operating in a tapping mode, we have observed the mor-
phology of films obtained via the vacuum deposition of
1–4 onto microscope slides. As shown in Fig. 1, the surfaces
of the as-prepared films are quite smooth and compact.
The uniform and smooth surface morphologies demon-
strated by all these bipolar materials suggest their good
manufacturability and potential utility in devices [48].
The root-mean-square (RMS) values of the films made of
these materials range from 1.19 to 1.48 nm, which are
much smaller than 9.45 nm of CBP (Table 1) film deposited
at the same growth rate. The obvious pin holes found on
the CBP film may be attributed to its low Tg and the conse-
quent crystallinity [29].
2.3. Photophysical properties
Fig. 2 shows the UV–Vis absorption and photolumines-
cence (PL) spectra of 1–4 in dicholoromethane (CH2Cl2) at
room temperature and the results are summarized in Ta-
ble 2. The absorption spectra of all the compounds exhibit
three absorption bands in the range of 230–350 nm. The
absorption around 292 nm for all the compounds have cer-
tain similarity to that of the unsubstituted carbazole
Fig. 2. Absorption spectra (4 ꢀ 10ꢁ5 M) (A) and PL spectra (1 ꢀ 10ꢁ5 M)
(B) of 1–4 in CH2Cl2 solutions at room temperature.
Table 2
Table 1
The effect of coupling position on the emission maxima.
Thermal properties and RMS data of 1–4 and CBP.
Compound
kmax (nm)
Compound
Tg (°C)
Tc (°C)
Tm (°C)
Td (°C)
RMS (nm)
1
2
3
4
–
–
375
309
291
273
–
453
454
452
426
–
1.19
1.42
1.38
1.48
9.45
156
116
137
62
254
155
218
–
para-
p
meta-
para-
p
meta-
p
1
2
3
4
408
411
385
357
CBP
p
p
p
p
p
monomer and thus can be attributed to the n–p⁄ transi-
tions [49]. The longer wavelength absorptions around
341 nm could be attributed to p–
p⁄ charge transition from
the electron-donating carbazole to the electron-accepting
triazole moiety. As shown in Fig. 2(A), the absorption
intensity around 341 nm decreased sharply from com-
pound 1–4, while the values of band gap increased on
the contrary. This may be attributed to the suppression
of intramolecular charge transfer by taking a meta coupling
mode, which strongly reduce the p–conjugation and widen
the band gap. Unlike the absorption spectra, symmetrical
and well-shaped emission spectra with the maximas rang-
ing from 357 to 411 nm, depending on the structure of the
conjugated systems (see Table 2), are observed for all these
Fig. 1. AFM topographic images of 1–4 and CBP (2 ꢀ 2
rate: 0.1 nm sꢁ1
lm). Film growth
.

J. Zhuang et al. / Organic Electronics 13 (2012) 2210–2219
2213
four compounds. Fig. 2(B) shows that the para-substituted
1 and 2 have very close emission wavelengths (408 and
411 nm, respectively), while the meta-substitution on the
3,4,5-triphenyl-4H-1,2,4-triazole moiety has shifted the
emission maxima to shorter wavelengths (blue shift) as
can be seen from 3 (385 nm) and 4 (357 nm). The meta
coupling mode has a higher twisted molecular configura-
tion than that of para mode, resulting in less intense intra-
molecular charge transfer, a blue-shifted emission and
higher triplet energy. As demonstrated in Table 2, while
the coupling position (meta or para) at 3,4,5-triphenyl-
4H-1,2,4-triazole has obvious effect on the emission
maxima, the coupling position at 9H-carbazol-9-yl-phenyl
does not show a consistent effect on the emission maxima.
These findings might be applicable to the design of molec-
ular structures to tune their emission properties so that the
host materials match the dopants’ absorbance in a better
manner [8].
A good host material should have a proper ET value,
which needs to be higher than that of the chosen phospho-
rescent guest emitter in the device design to prevent a re-
verse energy transfer from the guest to the host. To
determine the ET values of these materials, we have mea-
sured the phosphorescence emission (Fig. 3) of 1–4
(solid-state films deposited on quartz substrates) at 77 K.
The triplet energies of these compounds are calculated
based on the highest-energy vibronic sub-band of phos-
(2.76 eV) have much higher ET values than 1 (2.58 eV)
and 2 (2.56 eV), showing an inverse relationship between
the ET and the
p-conjugation degree of the carbazole and
1,2,4-triazole systems [30]. It is proved that the triplet
energies of these isomers can be adjusted by simply vary-
ing the meta coupling mode between 9H-carbazol-9-yl-
phenyl and 3,4,5-triphenyl-4H-1,2,4-triazole.
2.4. Molecular simulation
Computer simulation of 1–4 was carried out to study
the HOMO/LUMO levels of the host materials at the molec-
ular scale and the results are shown in Figs. 4 and 5. The
geometric and electronic properties of the compounds
were obtained using the Gaussian 03 program package.
The calculation was optimized by means of the B3LYP
(Becke’s three parameter hybrid functionals with Lee–
Yang–Perdew correlation functionals) with the 6-31G(d)
basis set. Molecular orbitals were visualized using
Gaussview.
Fig. 4 shows the spatial distributions of the calculated
LUMO and HOMO energy levels of 1–4. The LUMO levels
of all the compounds are mainly located at the electron-
deficient 1,2,4-triazole core and at the phenyl rings linked
to it. The LUMO orbitals show almost symmetrical distri-
bution. As for the HOMO, compound 1 and 3 show sym-
metrical distribution both on the two carbazole moieties,
while 2 and 4 exhibit localized cloud distribution over
the single carbazole moiety caused by the different cou-
pling modes between 9-phenyl-9H-carbazole and triazole
(para and meta). The compounds 1 and 3 are para-substi-
tuted (9H-carbazol-9-yl)phenyl derivatives, while 2 and 4
are meta-substituted (9H-carbazol-9-yl)phenyl derivatives.
Both 1 and 3 show a symmetrical HOMO orbital distribu-
tion, while 2 and 4 exhibit complete separation.
phorescence spectra. Compound
3 (2.70 eV) and 4
Usually, holes and electrons in OLEDs are transferred
through the individual HOMO and LUMO levels [50]. The
complete localization of HOMO and LUMO means the
HOMO to LUMO transition becomes a typical charge trans-
fer transition, and is essential to the efficient hole and elec-
tron transport and to the prevention of reverse energy
transfer as well [51]. As illustrated in Fig. 4, compounds 2
and 4 exhibited complete separation of LUMO and HOMO
of their respective hole- and electron-transporting process,
whereas 1 and 3 exhibit lesser segregation of LUMO and
HOMO. Fig. 5 shows theoretical and experimental energy
levels of 1–4. Although the calculated energy levels were
higher than the ones determined experimentally (see Sec-
tion 2.5), the discrepancy between theoretical and experi-
mental values remain almost the same for all these
compounds. The intense location of electron cloud over
the TAZ core and carbazole of 4 and its effect on charge
transport have been reflected at the device level.
2.5. Electrochemical properties
The electrochemical properties of the compounds were
studied in N,N-dimethylformamide (DMF) through the cyc-
lic voltammetry (CV) at a scan rate of 100 mV sꢁ1 (shown
in Supporting information, Fig. S3). Each of these four
compounds displayed an irreversible oxidation wave.
Fig. 3. The phosphorescence spectra of 1–4 at 77 K.

2214
J. Zhuang et al. / Organic Electronics 13 (2012) 2210–2219
Fig. 4. HOMO and LUMO orbital distributions and energy levels obtained from molecular simulation of 1–4.
Table 3
Photophysical and electrochemical data of 1–4.
Compound
kab (nm)a
Eg
EHOMO
(eV)
ELUMO
ET
(eV)b
(eV)c
(eV)d
1
2
3
4
242/293/341
241/292/340
241/294/341
240/293/328
3.29
3.43
3.47
3.52
5.61
5.62
5.62
5.61
2.32
2.19
2.15
2.09
2.58
2.56
2.70
2.76
a
b
c
Absorption maximas.
Eg was calculated from the absorption edge of UV–Vis absorption.
LUMO level was calculated from energy gap and HOMO level.
ET was calculated from highest-energy vibronic sub-band of phos-
d
phorescence spectra at 77 K.
result in lower LUMO levels. As shown in Table 3, the cou-
pling mode between 9-phenyl-9H-carbazole and triazole
hardly affects the HOMO levels but the LUMO levels. This
explains that compound 1 possesses the lowest LUMO lev-
els among these compounds. The HOMO levels of com-
pounds 1–4 are well matched with those of 4,40-bis[N-(1-
naphthyl)-N-phenyl-aminobiphenyl (NPB, 5.4 eV) [53]
and TAPC (5.5 eV) [21], two common hole-transporting
materials. Compared to the triazole-containing electron-
transporting materials, e.g., 3,5-bis(4-tert-butylphenyl)-4-
phenyl-4H-1,2,4-triazole (HOMO/LUMO: 6.4/2.8 eV) [42]
Fig. 5. Theoretical (red) and experimental (black, determined by cyclic
voltammetry and UV–Vis) energy levels of 1–4. (For interpretation of the
references to colour in this figure legend, the reader is referred to the web
version of this article.)
No reduction peak was observed from these compounds
within the entire electrochemical window of DMF.
The HOMO energy levels were obtained according to
ox
the following equation: HOMO ¼ E
þ 4:80 (eV), E is re-
onset
ferred to Fc/Fc+ (4.80 eV in a vacuum) [52]. The calculated
HOMO energy levels of 1–4 are 5.61 eV, 5.62 eV, 5.62 eV,
and 5.61 eV according to the corresponding onset poten-
tials of 0.81 V, 0.82 V, 0.82 V and 0.81 V for 1–4, respec-
tively. The four compounds exhibit very small variations
of HOMO levels ranging from 5.61 to 5.62 eV, whereas
their LUMO levels vary in a larger range from 2.09 to
2.32 eV, depending on the coupling modes of electron-
donating moiety carbazole and the electron-accepting
moiety triazole. Detailed data are summarized in Table 3.
Generally a para molecular framework would give a more
and
3-phenyl-4-(10-naphthyl)-5-phenyl-1,2,4-triazole
(HOMO/LUMO: 6.6/2.6 eV) [43], the introduction of carba-
zole moieties raises both HOMO and LUMO levels of these
compounds and matches well with NPB and TAPC used for
the device fabrication (see Section 2.7) in this work. The
electrochemical properties of CBP, mCP, NPB, TAPC, 1,3,5-
tris(1-phenyl-1H-benzo[d]imidazol-2-yl)benzene (TPBI)
and BCP were also investigated under the same condition
(Fig. S4). The HOMO levels of CBP, mCP, NPB and TAPC
are 5.82, 5.63, 5.15 and 5.22 eV respectively. The LUMO
levels of TPBI and BCP were calculated from the reduction
onset potentials, which are 2.39 and 2.44 eV respectively.
extended p-conjugation than a meta orientation and, thus,

J. Zhuang et al. / Organic Electronics 13 (2012) 2210–2219
2215
Compared with the LUMO levels of compounds 1–4, the
barriers of electron transport from the electron-transport-
ing materials to the host materials are quite small, which is
beneficial for the OLED performance.
for bipolar materials. We have noticed that the phenycar-
bazole-dipyridyl triazole hybrids were reported by Hung
et al. [55] to have higher hole mobilities.
Among these compounds, 4 has the highest current
densities in both HOD and EOD, suggesting a better perfor-
mances that could be anticipated at a device level.
2.6. Charge transporting properties
To further investigate the charge transporting proper-
ties of compound 1–4, we fabricated single carrier devices.
We prepared hole-only devices (HOD), having the struc-
ture ITO/TAPC (8 nm)/materials to be tested (30 nm)/TAPC
(8 nm)/Al, to evaluate the hole transporting behavior of
compound 1–4. Having a high LUMO level, TAPC was used
to block electron from the cathode. We also fabricate
electron-only devices (EOD), with a structure of ITO/BCP
(8 nm)/materials to be tested (30 nm)/BCP (8 nm)/LiF
(1 nm)/Al, where BCP was used to block hole from the an-
ode for its low HOMO level. As shown in Fig. 6, although
the EOD have higher current densities than HOD, there is
no difference of an order of magnitude in the current den-
sities between EOD and HOD. This evidently proves the
bipolar transporting property of these materials.
2.7. Electroluminescent properties
To preliminarily assess the utility of these compounds
as host materials in OLED devices, phosphorescent OLEDs
were fabricated with Ir(ppy)3 and FIrpic as green and blue
phosphorescent emitters, respectively. Their structures are
ITO/NPB (40 nm)/host(1–4):Ir(ppy)3 (8 wt.%, 30 nm)/TPBI
(30 nm)/LiF (1 nm)/Al for green OLEDs and ITO/TAPC
(30 nm)/host(1–4):FIrpic (15 wt.%, 30 nm)/BCP (30 nm)/
LiF (1 nm)/Al for blue OLEDs. The devices incorporating
1–4 were fabricated by sequentially depositing organic
layers in one run onto indium tin oxide (ITO) glass sub-
strate in vacuum chamber (ꢂ4.5 ꢀ 10ꢁ4 Pa).
Figs. 7 and 8 presents the current density–voltages–
luminance (J–V–L) characteristics, the current efficiency,
power efficiency versus voltage and electroluminescence
(EL) spectra for the devices based on 1–4. Device
performance is summarized in Table 4. All the green emit-
ting devices exhibit turn-on voltages of 2.7–2.9 V, which
With the same carbazole as hole-transporting unit, the
phosphine-oxide-containing bipolar materials reported
by Shu co-workers [54] have also demonstrated higher
electron-transporting property. But this not a general rule
Fig. 6. Current density–voltage curves of HOD (solid) and EOD (open).
Fig. 7. J–V–L characteristics of green (A) and blue devices (B).

2216
J. Zhuang et al. / Organic Electronics 13 (2012) 2210–2219
Fig. 8. Current efficiency (A and B), power efficiency (C and D) versus voltage and EL spectra (E and F) for green and blue devices based on 1–4.
are lower than that (3.2 V) of devices with CBP as the host.
As for the current efficiency and power efficiency, com-
pound 3 and 4 based devices showed higher efficiencies
than that of CBP based device. All the devices in Fig. 8 exhib-
ited small roll-off at high current density, which can be
attributed to the bipolar nature of 1–4 [56]. Furthermore,
no additional emission coming from the host materials
was observed, indicative of an efficient energy transfer from
the host to the dopant, due to the higher ET values of these
compounds than that of Ir(ppy)3 (ET = 2.57 eV) [57]. In all
the host materials, the compound 3 and 4 outperformed
CBP in both the current efficiency and power efficiency.
Encouraged by the results obtained from the green
emitting devices, we further fabricated blue devices using
FIrpic as the dopant. Compound 4 showed the highest cur-
rent efficiency (21.1 cd Aꢁ1
)
and power efficiency
(18.7 lm Wꢁ1). However devices based on compound 1
and 2 exhibited extremely low luminances and low effi-
ciencies due to their lower ET values (2.58 and 2.56 eV
compared with 2.65 eV of the dopant FIrpic [57]), which
cannot confine excitons in the emitting layer effectively.
We further found that 1 and 2 showed two accompanying
EL peaks at around 440 nm and 580 nm, which are due to
the emission of hole-transporting material TAPC adjacent

J. Zhuang et al. / Organic Electronics 13 (2012) 2210–2219
2217
Table 4
EL data of the green and blue devices.
b
c
d
Compound
Dopant
Von (V)a
L (cd mꢁ1
)
gc (cd Aꢁ1
)
gp (lm Wꢁ1
)
Performance at 100 cd Aꢁ1
V (V)
gc (cd Aꢁ1
)
gp (lm Wꢁ1
)
1
2
3
4
Green
Blue
Green
Blue
Green
Blue
Green
Blue
2.7
4.4
2.7
5.0
2.9
3.2
2.9
3.2
3.5
–
16,796
517
24,544
532
23,771
6916
22,500
5827
11.7
2.2
13.0
1.5
28.9
17.7
32.7
21.1
18.3
7.5
11.1
1.2
11.2
1.3
21.8
13.6
22.1
18.7
8.7
3.9
8.6
3.9
9.3
5.1
4.6
4.0
5.4
5.4
–
10.8
1.1
12.9
1.8
15.0
16.0
28.0
15.4
3.8
9.2
8.6
11.1
9.3
8.6
10.8
21.9
8.9
2.4
–
CBP
mCP [34]
Green
Blue
19,420
9500
8.9
–
a
b
c
Turn-on voltage at 1 cd mꢁ2
Maximum luminance.
Maximum current efficiency.
Maximum power efficiency.
.
d
to the emitting layer [58,59]. No additional emissions were
observed for compound 3 and 4, which demonstrated a
completely energy transfer from the host materials to the
dopant due to their higher ET values (2.70 and 2.76 eV)
than that of FIrpic (ET = 2.65 eV). Both compound 3 and 4
showed better performance than mCP [35].
As anticipated from its triplet energy and the measure-
ments of single carrier devices, compound 4 demonstrated
the best overall performance (Table 4) for both green and
blue emissions.
Nichem Fine Technology Co. Ltd. All reactants and solvents,
unless otherwise stated, were purchased from commercial
sources and used as received. Dicholoromethane used in
photophysical experiments were of spectroscopic grade.
1H NMR and 13C NMR spectra were measured on a
400 MHz Varian NMR spectrometer in CDCl3 with trimeth-
ylsilane as an internal standard. Mass spectra were mea-
sured on
a Varian ProStar LC240 (America). UV–Vis
absorption and PL spectra were recorded on a Perkin-Ele-
mer Lambda 25 and a HITACHI F4600 fluorescence spectro-
photometer, respectively. Phosphorescence spectra were
measured with an Edinburgh Instruments FLS920 spec-
trometer. Thermogravimetric analysis (TGA) was under-
3. Conclusions
taken with
a
SII-EXSTAR 6000 TG/DTA 6200
A series of novel bipolar host materials containing
electron-transporting, hole-blocking 1,2,4-triazole and
hole-transporting carbazole moieties were designed and
synthesized. The theoretical and experimental results
demonstrated that the conjugation degree of these com-
pounds and the spatial distribution of HOMO–LUMO
energy levels were subjected to the coupling modes (para-
or meta-position) of both 1,2,4-triazole and carbazole moi-
eties. All these host materials exhibited high thermal
stability, good morphology, and bipolar properties, which
contributes to the performance improvement of the OLED
devices fabricated in these work. By taking a meta coupling
mode between 9H-carbazol-9-yl-phenyl and 3,4,5-tri-
phenyl-4H-1,2,4-triazole, compound 3 and 4 exhibited
excellent performances for both green and blue devices.
The better balanced materials properties of 4 make it the
best materials among these bipolar systems.
thermobalance (SII Nano Technology Inc., Tokyo, Japan)
under a nitrogen atmosphere by heating the samples from
25 to 700 °C at a heating rate of 10 °C minꢁ1. Differential
scanning calorimetry (DSC) was performed on a SII Exstar
6000 (DSC 6220) instrument. The samples were heated at
a heating rate of 10 °C minꢁ1 from 25 to 400 °C under ar-
gon. Cyclic voltammetry (CV) was carried out in a three-
electrode electrochemical configuration with a platinum
working electrode, a platinum auxiliary electrode and an
Ag wire pseudo-reference electrode. The potential was re-
ferred to the internal standard ferrocene/ferrocenium (Fc/
Fc+) redox couple. The electrochemical cell was loaded
with nitrogen-purged DMF and tetrabutylammonium
hexafluorophosphate (TBAPF6) supporting electrolyte in a
nitrogen-filled glove box (oxygen and water contents <
0.1 ppm) at room temperature. A PARSTAT 2263–2 Ad-
vanced Electrochemical System with PowerSUITE software
was used for electrochemical measurement and control.
The film surface morphology was measured with by the
atomic force microscopy using AFM Vecco Dimension
3100.
The results from both the green and the blue devices
suggest that, at the molecular level, the direct para-cou-
pling between triazole phenyl ring and carbazole should
be avoided or chosen with caution in designing the bipolar
materials from these systems.
4. Experimental section
4.2. Materials
4.1. General information
4.2.1. 9,90-(40,400-(4-Phenyl-4H-1,2,4-triazole-3,5-
diyl)bis(biphenyl-40,4-diyl))bis(9H-carbazole) (1)
Ir(ppy)3 and FIrpic were received from SunaTech Inc.,
NPB, TAPC, CBP, mCP, TPBI and BCP were purchased from
To a solution of 3,5-bis(4-bromophenyl)-4-phenyl-4H-
1,2,4-triazole (1.30 g, 2.90 mmol) and 3-(9H-carbazol-9-

2218
J. Zhuang et al. / Organic Electronics 13 (2012) 2210–2219
yl)phenylboronic acid (1.70 g, 5.80 mmol) in 65 mL of tol-
uene and 12 mL of ethanol were added to 25 mL of 2.0 M
aqueous Na2CO3 solution. The reaction mixture was then
purged with argon for ten minutes before adding tetra-
kis(triphenylphosphine)palladium(0) (0.37 g, 0.32 mmol).
After refluxing for 18 h under argon, the resulting mixture
was cooled to room temperature and then poured into
water and extracted with 150 mL (3 ꢀ 50 mL) dichloro-
methane. The combined organic phase was then washed
with 300 mL (3 ꢀ 100 mL) water and dried with anhydrous
Na2SO4. After removal of the solvent by rotary evaporation,
the residue was purified by silica gel column chromatogra-
phy to afford 1 as an white solid (1.91 g, 84.1%). 1H NMR
(400 MHz, CDCl3) d ppm: 8.18–8.14 (dt, J = 0.8, 8 Hz, 4H),
7.82–7.81 (t, J = 2.0 Hz, 2H), 7.57–7.55 (t, J = 2.4 Hz, 2H),
7.67–7.62 (m, 8H), 7.62–7.60 (t, J = 2.0 Hz, 3H), 7.59–7.58
(m, 1H), 7.57–7.55 (m, 1H), 7.55–7.53 (m, 1H), 7.48–7.46
(m, 1H), 7.46–7.43 (m, 5H), 7.43–7.42 (m, 2H), 7.41–7.40
(m, 1H), 7.33–7.31 (m, 2H), 7.31–7.29 (m, 3H), 7.29–7.28
(d, J = 1.8 Hz, 1H). 13C NMR (100.64 MHz, CDCl3) d ppm:
154.54, 141.30, 140.69, 138.90, 137.39, 135.28, 130.49,
130.20, 129.86, 129.26, 128.40, 127.92, 127.67, 127.34,
127.03, 126.00, 125.97, 123.44, 120.33, 120.05, 109.72.
MS (ESI, m/z) [(M+H)+]: Calcd. for C56H37N5: 779.3049;
found: 780.3110.
4.2.4. 9,90-(30,300-(4-Phenyl-4H-1,2,4-triazole-3,5-
diyl)bis(biphenyl-30,3-diyl))bis(9H-carbazole) (4)
Compound 4 was obtained as a white powder by the
same procedure as that for 1 (1.82 g, 79.6%). 1H NMR
(400 MHz, CDCl3) d ppm: 8.20–8.15 (d, J = 7.6 Hz, 4H),
7.68–7.57 (m, 6H), 7.53–7.46 (m, 6H), 7.43–7.38 (m, 6H),
7.35–7.30 (m, 7H), 7.30–7.28 (d, J = 0.8 Hz, 1H), 7.24–7.19
(s, 2H), 7.12–7.07 (d, J = 7.6 Hz, 2H), 6.90–6.82 (t,
J = 8.0 Hz, 2H), 6.42–6.30 (t, J = 7.6 Hz, 1H). 13C NMR
(100.64 MHz, CDCl3) d ppm: 154.43, 141.90, 140.91,
139.88, 138.21, 135.27, 130.31, 130.11, 129.60, 129.27,
128.29, 128.15, 127.68, 127.37, 127.28, 126.31, 125.95,
125.90, 125.74, 123.32, 120.34, 120.00, 109.78. MS (ESI,
m/z) [(M+H)+]: Calcd. for C56H37N5: 779.3049; found:
780.3110.
4.3. OLED fabrication and measurement
OLED devices were fabricated by sequentially deposit-
ing organic layers using thermal evaporation in one run
under high vacuum (ꢂ4.5 ꢀ 10ꢁ4 Pa) onto an indium tin
oxide (ITO) glass substrate. Prior to use, the substrate
was degreased with solvents and cleaned in a UV-ozone
chamber for 15 min before it was loaded into the evapora-
tion system. The active device area was 3 ꢀ 3 mm2. The
thickness of the deposited layer and the evaporation speed
of the individual materials were monitored with quartz
crystal microbalance monitors. All electrical testing and
optical measurements were performed under ambient
conditions without further encapsulation. The EL spectra
were measured with a Spectra Scan PR655. The current–
voltage (I–V) and luminance-voltage (L–V) relations were
characterized with a computer controlled Keithley 2400
Sourcemeter.
4.2.2. 9,90-(40,400-(4-Phenyl-4H-1,2,4-triazole-3,5-
diyl)bis(biphenyl-40,3-diyl))bis(9H-carbazole) (2)
Compound 2 was obtained as a white powder by the
same procedure as that for 1 (3.38 g, 89.0%). 1H NMR
(400 MHz, CDCl3) d ppm: 8.19–8.10 (dt, J = 1.2, 8.4 Hz,
4H), 7.78–7.72 (m, 2H), 7.67–7.64 (m, 4H), 7.60–7.50 (m,
10H), 7.48–7.46 (m, 1H), 7.46–7.45 (m, 1H), 7.44–7.43
(m, 1H), 7.43–7.37 (m, 8H), 7.32–7.30 (d, J = 2.4 Hz, 1H),
7.30–7.28 (t, J = 2.0 Hz, 2H), 7.28–7.26 (d, J = 2.4 Hz, 1H),
7.24–7.23 (m, 1H), 7.22–7.21 (m, 1H). 13C NMR
(100.64 MHz, CDCl3) d ppm: 154.44, 141.86, 141.10,
140.78, 138.28, 135.15, 130.37, 130.16, 129.86, 129.21,
127.81, 127.06, 126.38, 126.24, 126.00, 125.95, 125.62,
123.36, 120.33, 119.99, 109.65. MS (ESI, m/z) [(M+H)+]:
Calcd. for C56H37N5: 779.3049; found: 780.3122.
Acknowledgements
The authors thank the National Natural Science Founda-
tion of China under the projects (Nos. 20902066 and
11004218), the Program for Industry Support of Jiangsu
Province (BE2009050) and the Hundred Talents Program
of the Chinese Academy of Sciences for financial support.
Appendix A. Supplementary data
4.2.3. 9,90-(30,300-(4-Phenyl-4H-1,2,4-triazole-3,5-
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/
j.orgel.2012.06.025.
diyl)bis(biphenyl-4,30-diyl))bis(9H-carbazole) (3)
Compound 3 was obtained as a white powder by the
same procedure as that for 1 (1.92 g, 84.1%). 1H NMR
(400 MHz, CDCl3) d ppm: 8.17–8.13 (dt, J = 1.2, 7.6 Hz,
4H), 7.81–7.79 (t, J = 1.2 Hz, 2H), 7.71–7.70 (t, J = 1.2 Hz,
1H), 7.70–7.68 (t, J = 1.2 Hz, 1H), 7.61–7.58 (m, 10H),
7.58–7.57 (m, 1H), 7.54–7.53 (t, J = 1.2 Hz, 1H), 7.52–7.51
(t, J = 1.2 Hz, 1H), 7.48–7.41 (m, 10H), 7.36–7.33 (m, 2H),
7.32–7.31 (d, J = 2.4 Hz, 1H), 7.31–7.29 (t, J = 2.0 Hz, 2H),
7.29–7.27 (d, J = 2.0 Hz, 1H). 13C NMR (100.64 MHz, CDCl3)
d ppm: 154.68, 140.75, 140.38, 139.09, 137.26, 135.59,
130.32, 129.77, 129.16, 128.38, 128.30, 128.13, 127.88,
127.59, 127.48, 127.32, 125.97, 123.45, 120.34, 120.04,
109.74. MS (ESI, m/z) [(M+H)+]: Calcd. for C56H37N5:
779.3049; found: 780.3108.
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