Phenylcarbazole-dipyridyl triazole hybrid as bipolar host material for phosphorescent OLEDs

Article abstract of DOI:10.1039/c2jm15963e

Two bipolar host materials p-cbtz and m-cbtz, with both phenylcarbazole and 3,5-bis(2-pyridyl)-1,2,4-triazole moieties, were synthesized and tested for fabrication of various phosphorescent OLEDs. The sp³-hybridized nitrogen atom of the 1,2,4-triazole fragment disrupted the π-conjugation between the electron donating 9-phenylcarbazole and the electron accepting 3,5-bis(2-pyridyl)-1,2,4-triazole and, thus, exhibited promising properties such as bipolar charge transport feature (10^-5-10^-6 cm ² V^-1s^-1) and with relatively high triplet energy gaps (E_T = 2.82-2.75 eV). As a result, they were utilized as universal hosts for various phosphorescent OLEDs, showing high efficiencies and low efficiency roll-off. For example, the devices hosted by m-cbtz achieve maximum external quantum efficiencies (η_ext) of 8.8% for blue, 16.7% for green, 17.5% for yellow and 16.7% for red-emitting OLEDs. In addition, we also fabricated dual-emitter WOLEDs with a co-doped single emissive layer, exhibiting satisfactory device efficiencies (12.4%, 25 cd A^-1, 17 lm W^-1) and with highly stable chromaticity (CIEx = 0.26-0.27 and CIEy = 0.38) at an applied voltage of 8 to 11 V. These results demonstrated that the newly synthesized, phenylcarbazole-dipyridyl triazole host materials are advantageous for fabrication of various highly efficient phosphorescent OLEDs. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2jm15963e

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PAPER
Phenylcarbazole-dipyridyl triazole hybrid as bipolar host material for
phosphorescent OLEDs
a
Wen-Yi Hung, Guan-Min Tu, Shou-Wei Chen and Yun Chi
b
a
b
*
*
Received 17th November 2011, Accepted 6th January 2012
DOI: 10.1039/c2jm15963e
Two bipolar host materials p-cbtz and m-cbtz, with both phenylcarbazole and 3,5-bis(2-pyridyl)-1,2,4-
triazole moieties, were synthesized and tested for fabrication of various phosphorescent OLEDs. The
sp3-hybridized nitrogen atom of the 1,2,4-triazole fragment disrupted the p-conjugation between the
electron donating 9-phenylcarbazole and the electron accepting 3,5-bis(2-pyridyl)-1,2,4-triazole and,
thus, exhibited promising properties such as bipolar charge transport feature (10ꢀ5–10ꢀ6 cm2 Vꢀ1sꢀ1
)
and with relatively high triplet energy gaps (ET ¼ 2.82–2.75 eV). As a result, they were utilized as
universal hosts for various phosphorescent OLEDs, showing high efficiencies and low efficiency roll-
off. For example, the devices hosted by m-cbtz achieve maximum external quantum efficiencies (hext) of
8.8% for blue, 16.7% for green, 17.5% for yellow and 16.7% for red-emitting OLEDs. In addition, we
also fabricated dual-emitter WOLEDs with a co-doped single emissive layer, exhibiting satisfactory
device efficiencies (12.4%, 25 cd Aꢀ1, 17 lm Wꢀ1) and with highly stable chromaticity (CIEx ¼ 0.26–0.27
and CIEy ¼ 0.38) at an applied voltage of 8 to 11 V. These results demonstrated that the newly
synthesized, phenylcarbazole-dipyridyl triazole host materials are advantageous for fabrication of
various highly efficient phosphorescent OLEDs.
required to increase the electrons and holes recombination within
Introduction
the emissive layer.
Phosphorescent organic light-emitting diodes (PhOLEDs) have
high potential for fabrication of next generation full color
displays and solid-state lighting due to the low manufacturing
cost and excellent versatilities to adopt various applications. In
addition, in contrast to the typical fluorescent OLEDs, the
phosphorescent (Ph) OLEDs can reach to 100% of theoretical
internal quantum efficiency by harvesting both singlet and triplet
excitons.1 In order to maximize the actual efficiency and to
reduce concentration quenching (triplet–triplet annihilation) of
PhOLEDs, the host–guest strategy is implemented by dispersing
(or doping) the metal based phosphors (guests) homogeneously
into a suitable organic matrix (host) to suppress this detrimental
effect. The host material serves as the platform for holes and
electrons recombination to form excitons, as well as for both
singlet-to-singlet and triplet-to-triplet excitation energy transfer
from host to dopant. Hence, the triplet energies of the host
material should be higher than that of the phosphor to confine
the as-generated excitons within the emissive layer. In addition,
host materials with good carrier transporting properties are
To cope with this demand, recent research trends have shifted
to the development of host materials possessing bipolar prop-
erties.2 It is believed that, upon incorporating both the electron-
donating (D) and electron-accepting (A) moieties into the single
molecule, the resulting bipolar host materials are then able to
perform balanced injection/transportation/recombination of
charge carriers and, consequentially, to give OLEDs with higher
performance characteristics and lower efficiency roll-off.
However, the co-existence of these D and A groups may also lead
to an unwanted, intramolecular charge transfer, resulting in
severe reduction of the triplet energy of the host. To suppress the
intramolecular charge transfer, the electronic coupling between
D and A moieties must be reduced by a certain linker featured
with a twisted p-conformation, i.e. connecting the D and A units
through meta- and/or ortho-linkages instead of the para-position
on the hexagonal aromatic fragment.3 The incorporation of
saturated atomic linkages, such as sp3-hybridized atoms4 or non-
conjugated s-linkages,5 serve as an alternative to suppress direct
interactions between the D and A moieties.
From the material perspective, phenylcarbazole and/or tri-
phenylamine derivatives have been widely used as host materials
in PhOLEDs due to their sufficiently large triplet energies and
good hole-transporting ability. Thus, addition of electron-
transporting moieties into the above mentioned phenylcarbazole
or triphenylamine derivatives became a generalized strategy for
aInstitute of Optoelectronic Sciences, National Taiwan Ocean University,
Keelung, 202, Taiwan. E-mail: wenhung@mail.ntou.edu.tw; Fax: +886
2 24634360; Tel: +886 2 24622192 ext. 6718
bDepartment of Chemistry, National Tsing Hua University, Hsinchu, 300,
Taiwan. E-mail: ychi@mx.nthu.edu.tw; Fax: +886 3 5720864; Tel: +886
3 5712956
5410 | J. Mater. Chem., 2012, 22, 5410–5418
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making the demanded bipolar host materials. For example,
Sapochak et al. reported the incorporation of carbazole via
a non-conjugated N-linkage to the peripheral phenylene ring of
an n-type triphenylphosphine oxide (tPPO), giving the bipolar
host known as N-(4-diphenylphosphorylphenyl)carbazole
(MPO12).6 With FIrpic as the emitter, an MPO12-hosted OLED
exhibits a maximum external quantum efficiency of 7.1%. Guan
et al. reported the successful preparation of three carbazole/
oxadiazole derivatives, by linking the carbazole and oxadiazole
with the non-conjugated methylene spacer.5a The resulting
bipolar host materials t-CmOxa were then successfully used in
making red-emitting OLEDs with maximum external quantum
efficiency and power efficiency of 9.5% and 9.9 lm Wꢀ1. Lin et al.
reported a benzimidazole/carbazole hybrid with a biphenyl
bridge, which was then successfully used as a host for a single-
layer Ir(ppy)3 based OLED with maximum current and external
quantum efficiencies of 34.3 cd Aꢀ1 and 9.3%.7 Kim et al.
documented a series of bipolar hosts comprising 1,2,4-triazole
(TAZ) and 9-phenylcarbazole (Cz-Ph) for blue-emitting PhO-
LEDs.8 Yang et al. designed and synthesized a series of 1,2,4-
triazole-cored triphenylamine derivatives as bipolar host mate-
rials, the maximum external quantum efficiencies reached 16.4%
for red phosphorescence, and 14.2% for green-emitting
OLEDs.3d According to these literature reports,3d,8 we concluded
that, when the aryl functional groups connect to the 1,2,4-trizole
core, the severe twisting between 1,2,4-trizole and all aryl groups
has disrupted the p-conjugation, giving higher triplet energy and
then making the host materials more tolerable for PhOLEDs.
In this study we decided to circumvent the typical molecular
design for the 3,4,5-trisubstituted 1,2,4-triazole hybrids and
focus only on the 3,5-bis(2-pyridyl)-1H-1,2,4-triazole derived
bipolar hosts with either para- or meta-substituted 9-phenyl-
carbazole fragment located at the N-1 position, i.e. affording the
distinctive 1,3,5-trisubstituted 1,2,4-triazole core geometry. To
the best of our knowledge, although 3,4,5-trisubstituted 1,2,4-
triazoles (345tz) functionalities were widely used as the materials
for organic photoelectronic applications,3d,8,9 there is no prece-
dent for 1,3,5-trisubstituted 1,2,4-triazole (135tz) as a host of
emissive layer or even electron-transport materials (Scheme 1).
Naturally, we expect that different linking topologies of the 9-
phenylcarbazole and 3,5-bis(2-pyridyl)-1H-1,2,4-triazole would
allow fine-tuning of their photophysical properties, better
modulation of their triplet gaps, adequate HOMO/LUMO
separations and charge balance, and eventually making them the
universal design for hosts used in multicolor phosphorescent
OLEDs. The OLED devices fabricated using m-cbtz have ach-
ieved maximum external quantum efficiency of 8.8%, 16.7%,
17.5% and 16.7% for blue (FIrpic), green [(ppy)2Ir(acac)], yellow
[(Bt)2Ir(acac)], and red [Os(bpftz)2(PPhMe2)2], respectively.
Finally, mimicking the rapid developments in WOLED
research,10 two-emitter white OLEDs with co-doped single
emissive layer were also examined, exhibiting efficiency of 12.4%
and high chromatic stability (CIEx ¼ 0.26–0.27 and CIEy ¼ 0.38)
at an applied voltage of 8 to 11 V.
Results and discussion
Two substituted 9-phenylcarbazole compounds, for which the
halide substituent on the phenyl fragment is located at either the
para- or meta-position versus the unique nitrogen atom, were
synthesized using the Ullmann coupling reaction.11 After then,
the desired host materials p-cbtz and m-cbtz were synthesized by
a second C–N bond coupling between the aforementioned phe-
nylcarbazole and 3,5-bis(2-pyridyl)-1H-1,2,4-triazole in the
presence of CuI catalyst (Scheme 2). Both materials were
obtained as white powders in good yields and were purified by
both recrystallization and vacuum sublimation before spectral
characterization and device fabrication. In contrast, the phe-
nylene linker of p-cbtz and m-cbtz is located at the N-1 nitrogen
atom, rather than the N-4 nitrogen atom of the triazole fragment,
the latter is typical for triazole based host materials.3d,8,9 Such
a molecular arrangement is controlled by their synthetic
approach and, theoretically, could afford host materials with
different physical properties.
Their physical properties were investigated by thermogravi-
metric analysis (TGA) and differential scanning calorimetry
(DSC) in Table 1. In the TGA measurements, both p-cbtz and m-
cbtz exhibited high decomposition temperatures (Td, corre-
sponding to 5% weight loss) at 358 and 331 ꢁC, respectively. The
para-substituted p-cbtz appears to have a higher Tg value of
86 ꢁC than its congener with meta-substituted derivative (m-cbtz,
Tg ¼ 77 ꢁC), which can be rationalized by the greater disorder in
energy.12
Fig. 1 displays UV–Vis absorption and photoluminescence
(PL) spectra (recorded at room temperature) of p-cbtz and m-
cbtz in 2-MeTHF solution (10ꢀ5 M) and in the form of solid thin
film, as well as their phosphorescence spectra in 2-MeTHF
solution (10ꢀ5 M) at 77 K (see Table 1). Apparently, the intro-
duction of different linking topologies on 9-phenylcarbazole and
3,5-bis(2-pyridyl)-1,2,4-triazole only brings out limited
Scheme 1 Generalized structural drawing of 1,3,5-trisubstituted (135tz)
Scheme 2 Synthetic route to the host materials p-cbtz and m-cbtz;
and 3,4,5-trisubstituted 1,2,4-triazoles (345tz).
conditions: (i), CuI, K2CO3, DMF, reflux, 36 h.
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Table 1 Physical properties of p-cbtz and m-cbtz
lPL/nm
[sol./film] (mh/me)d/cm2 Vꢀ1 sꢀ1
(Tg/Tc/Tm)/ꢁC Td/ꢁC aE1/2OX/V (HOMO/LUMO/Eg)b/eV ETc/eV labs/nm [sol./film]
p-cbtz 86/—/215
m-cbtz 77/134/233
358
331
0.99
0.97
ꢀ5.69/ꢀ2.12/3.57
ꢀ5.68/ꢀ2.15/3.53
2.82
2.75
291, 323, 339/285, 330, 344 427/425
293, 325, 340/286, 329, 342 427/422
1.3 ꢂ 10ꢀ5/2.6 ꢂ 10ꢀ6
8.7 ꢂ 10ꢀ6/1.1 ꢂ 10ꢀ6
a
Oxidation potential was measured using cyclic voltammetry (CV) in CH2Cl2 solution. b HOMO was determined by AC-2 measurements, while LUMO
c
was calculated using the equation LUMO ¼ HOMO + Eg, where Eg was extracted from the absorption onset of solid film. ET was measured in 2-
d
MeTHF at 77 K with a sample concentration of 10ꢀ5 M and an excitation wavelength of 338 nm. m was determined from the TOF measurement
at E ¼ 9.0 ꢂ 105 V cmꢀ1
.
but the LUMO of p-cbtz (2.12 eV) is slightly higher than that of
m-cbtz (2.15 eV), which could be attributed to the distinctive
bonding mode of the phenylene linker.
In order to confirm this hypothesis that the appending of the
electron-donating 9-phenylcarbazole onto the electron-with-
drawing 3,5-bis(2-pyridyl)-1,2,4-triazole renders p-cbtz and m-
cbtz with promising bipolar transporting properties, we used
time-of-flight (TOF) techniques to measure the associated carrier
mobility.14 The device used for TOF measurement was prepared
through vacuum deposition of the tri-layer structure ITO glass/p-
cbtz (1.11 mm) or m-cbtz (1.51 mm)/Ag (150 nm), followed by
placing them inside a vacuum chamber for data measurement.
Fig. 2(a)–(d) display typical room-temperature TOF transients
for these materials under an applied electric field, for which we
observe dispersive transient photocurrents from both holes and
electrons. Using double-logarithmic representations [insets to
Fig. 2(a)–(d)], we extracted the carrier-transit times (tT), which
are required to determine the carrier mobilities, from the inter-
sections of the two asymptotes. We then calculated the mobility
using the formula m ¼ d2/VtT, where d is the sample thickness and
V is the applied voltage. Fig. 3 shows the carrier mobilities of p-
cbtz and m-cbtz plotted as a function of the square root of the
applied electric field, as well as the nearly universal Poole–
Frenkel relationship. The hole mobilities fell within the range
from 2.8 ꢂ 10ꢀ6 to 1.4 ꢂ 10ꢀ5 cm2 Vꢀ1 sꢀ1 for fields varying from
3.6 ꢂ 105 to 1 ꢂ 106 V cmꢀ1, which are higher than the electron
Fig. 1 Room-temperature UV-Vis absorption and PL spectra of p-cbtz
and m-cbtz in 2-MeTHF solution (10ꢀ5 M) and as neat film, as well as the
corresponding solution phosphorescence recorded at 77 K.
perturbation of the photophysical properties. A strong absorp-
tion peak at ca. 290 nm and two weak absorption peaks at ca. 325
and 340 nm are attributed to the p–p* and n–p* transitions of
the 9-phenylcarbazole. In addition, no charge transfer absorp-
tion band from the electron-donating 9-phenylcarbazole to the
electron-withdrawing 3,5-bis(2-pyridyl)-1,2,4-triazole can be
observed,8 indicating the disruption of the p-conjugation
between these parts owing to the connection at the sp3-hybridized
nitrogen atom. The emission spectra in solution and as neat film
are also similar, both giving peak patterns at ca. 422–427 nm.
The triplet energies of p-cbtz and m-cbtz were determined to be
2.82 and 2.75 eV by the highest energy 0–0 phosphorescence,
which are sufficiently higher than that of blue phosphor (FIrpic,
ET ¼ 2.65 eV)13 and all other long-wavelength phosphors,
implying effective confinement of the triplet excitons in the
emissive layer and blockage of back energy transfer from the
guest to the host.
The electrochemical properties of p-cbtz and m-cbtz were
investigated by cyclic voltammetry (CV). After that, we used
a photoemission spectrometer (Riken AC-2) to determine the
HOMO energy levels of p-cbtz and m-cbtz, while their LUMO
energy levels were estimated from the HOMO energy level and
the optical band gaps (Eg) using the equation LUMO ¼
HOMO + Eg. Table 1 also summarizes the HOMO and LUMO
energy levels. The HOMOs span a narrow range of 5.68–5.69 eV,
Fig. 2 Transient photocurrent signals for p-cbtz (1.11 mm thick) at E ¼
8.98 ꢂ 105 V cmꢀ1 and m-cbtz (1.51 mm thick) at E ¼ 1.06 ꢂ 106 V cmꢀ1
:
(a) (c) holes; (b) (d) electrons. Insets are the double logarithmic plots.
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the excitons within the emissive layer, owing to the greater triplet
energy gap than that of TAZ.24 For other devices, a 50 nm-thick
layer of 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene (TPBI)
was used as the ETL instead of DPPS/TAZ (in blue-emitting
OLEDs) for enhancement of electron injection. A cathode con-
sisting of a 0.5 nm-thick layer of LiF and a 100 nm-thick layer of
Al were deposited through a shadow mask.
Fig. 4 and 5 depict the current density–voltage–luminance (J–
V–L) characteristics, device efficiencies, and EL spectra of the
devices incorporating p-cbtz and m-cbtz host materials, respec-
tively. Table
2 summarizes the electroluminescence data
obtained. For all phosphorescent OLEDs, the higher efficiency
was achieved for the m-cbtz based devices. The blue emission
device B2 reveals a maximum brightness of 29500 cd mꢀ2 at 13.5
V (690 mA cmꢀ2) with the CIE coordinates of (0.16, 0.34). The
maximum external quantum (hext), current (hc), and power effi-
ciencies (hp) were 8.8%, 18 cd Aꢀ1, and 9.8 lm Wꢀ1, respectively.
Fig. 3 Electron and hole mobilities versus E1/2 for p-cbtz and m-cbtz.
mobilities: within the range from 9.2 ꢂ 10ꢀ7 to 3 ꢂ 10ꢀ6 cm2 Vꢀ1
sꢀ1 for fields varying from 6.3 ꢂ 105 to 1.1 ꢂ 106 V cmꢀ1. The
values of the carrier mobilities in both compounds at 9.0 ꢂ 105 V
cmꢀ1 are given in Table 1. Interestingly, the hole- and electron-
mobilities of p-cbtz are ca. two-times greater than those of m-
cbtz. This can be attributed to the fact that the meta-linkage may
possess a greater degree of twisted conformation versus the cor-
responding para-conformation and, consequently, larger
molecular disorders, which may impede the carrier hopping
among the adjacent molecules.15
To evaluate the practical utility of p-cbtz and m-cbtz as host
materials, we fabricated PhOLED devices with four phosphors,
0
namely:
[bis(4,6-difluorophenyl)-pyridinato-N,C2 ]iridium(III)
picolinate (FIrpic),16 bis(2-phenylpyridinato)iridium(III)(acety-
lacetonate) [(ppy)2Ir(acac)],17 bis(2-phenylbenzothiazolato)iri-
dium(III)(acetylacetonate) [(Bt)2Ir(acac)],18 and bis[3-(trifluoro-
methyl)-5-(4-tert-butylpyridyl)-1,2,4-triazolate]osmium(II)bis-
(dimethylphenylphosphine)
[Os(bpftz)2(PPhMe2)2,
OS1]19
(Scheme 3). To improve the hole injection from the anode,
poly(3,4-ethylene-dioxythiophene): poly(styrene sulfonic acid)
(PEDOT:PSS) was spun onto the precleaned ITO substrate to
form a 30 nm-thick polymer buffer layer. Two hole transport
layers (HTLs), which consisted of a 20 nm-thick layer of 4,40-bis
[N-(1-naphthyl)-N-phenyl]biphenyldiamine (NPB)20 and a 5 nm-
thick thick layer of 4,40,400-tri(N-carbazolyl)triphenylamine
(TCTA)21 were implemented. It is notable that TCTA with ET ¼
2.76 eV and HOMO/LUMO levels of 5.7/2.4 eV also served as
the triplet blocker to confine the excitons in the emissive layer.
Moreover, a 25 nm-thick emissive layer consists of 10 wt.% of
phosphors doped into the host material. For the blue device,
electron transport layers (ETLs) consisted of a 5 nm-thick layer
of diphenyl-bis[4-(pyridin-3-yl)phenyl]silane (DPPS),22 and a 45
nm-thick layer of 3-(4-biphenylyl)-4-phenyl-5-(4-tert-butyl-
phenyl)-1,2,4-triazole (TAZ),23 for which DPPS (ET ¼ 2.7 eV,
HOMO/LUMO ¼ 6.5/2.5 eV) can be more effectively in blocking
Fig. 4 (a) Current density–voltage–luminance (J–V–L) characteristics,
(b) external quantum (hext) and power efficiencies (hP) as a function of
brightness, and (c) EL spectra for devices incorporating p-cbtz.
Scheme 3 Chemical structures of the four dopants used in this study.
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Both devices show the same main peak at 475 nm with a shoulder
peak at 500 nm, arising from the typical emission of the phosphor
FIrpic. These device performances are moderate among FIrpic-
based OLEDs presumably because the ETs of p-cbtz and m-cbtz
are not sufficiently high ensuring efficient energy transfer from
the host to the phosphor and triplet exciton confinement on the
phosphor. Another reason for deferred device performance is
that we chose TCTA as the exciton blocker to prevent exciton
diffusion to the hole transporting layer, because the ET of TCTA
is not large enough to effectively prevent diffusion of blue exci-
tons.25 However, this is a worthy sacrifice to acquire a compro-
mise among the ETs of RGB phosphors.
To further evaluate the suitability of p-cbtz and m-cbtz for low
energy triplet emitters, we fabricated green, yellow, and red-
emitting devices, for which the device performances are all better
than those obtained for blue OLEDs. Generally speaking, these
devices have similar architecture as the aforementioned blue
devices except for using TPBI as the ETL to facilitate electron
injection and transport. Fig. 4(a) and 5(a) depict the J–V plots
and other essential characteristics of the devices featuring TPBI
as ETL. They exhibited turn-on voltages of 2.5–3.5 V (defined as
the voltage at which EL became detectable) because of the
variation of HOMO energy level of dopants. The devices hosted
by p-cbtz exhibited significantly higher current density than
those with m-cbtz. We attribute the enhanced device current
when using p-cbtz as the host material, as revealed by its rela-
tively high carrier transporting properties. In green phospho-
rescent devices (device G1 and G2) using (ppy)2Ir(acac) as
dopant, hosts p-cbtz and m-cbtz have stronger influence on the
charge transport, such that the driving voltage is approximately
5.6 and 6.5 V at 1000 cd mꢀ2, respectively. Particularly, the device
G2 exhibited a relatively high brightness of 130800 cd mꢀ2 at
a current density of 1550 mA cmꢀ2 (13 V). In addition, the device
G2 incorporating m-cbtz host exhibited higher efficiencies
(16.7%, 61 cd Aꢀ1, and 62 lm Wꢀ1) relative to those of device G1
(15.1%, 55 cd Aꢀ1, and 43 lm Wꢀ1); these devices achieved better
performances for blue electrophosphorescence. The decreased
driving voltage of G1 can be attributed to high hole- and elec-
tron-mobility of p-cbtz. However, the efficiency of the device
based on p-cbtz is lower than that based on m-cbtz, which maybe
due to the inferior carrier balance that caused a greater degree of
triplet exciton quenching in the emissive layer. The same trend
can be observed for devices with different dopants.
Fig. 5 (a) Current density–voltage–luminance (J–V–L) characteristics,
(b) external quantum (hext) and power efficiencies (hP) as a function of
brightness, and (c) EL spectra for devices incorporating m-cbtz as host.
In comparison, lower efficiencies of 8%, 17.1 cd Aꢀ1, and 8.2 lm
Wꢀ1 were obtained for the p-cbtz based device B1. The EL
spectra of FIrpic-based devices are shown in Fig. 4(c) and 5(c).
Table 2 EL performances of devices
Imax/mA
cmꢀ2
hc/cd
Aꢀ1
hp/lm
Wꢀ1
Dopant (10 wt.%)
Vonb/V
Lmax/cd mꢀ2
hext/%
hext at 103 cd mꢀ2c/%
B1a
G1
Y1
R1
W1
B2
G2
Y2
R2
W2
FIrpic
(ppy)2Ir(acac)
(Bt)2Ir(acac)
Os(bpftz)2(PPhMe2)2, OS1
FIrpic + 0.3% OS1
FIrpic
(ppy)2Ir(acac)
(Bt)2Ir(acac)
Os(bpftz)2(PPhMe2)2, OS1
FIrpic + 0.3% OS1
3.5
3.0
3.0
3.5
3.5
3.5
2.5
3.0
3.5
3.5
15500 (12.5 V)
97100 (11 V)
430
1210
1080
1090
640
8
15.1
16
16.5
9.1
8.8
16.7
17.5
16.7
12.4
17.1
55
40.4
20.3
17.9
18
61
43
18.2
25
8.2
43
27.7
18.8
11
9.8
62
38
8.0 (7.5V)
14.9 (5.6V)
15.8 (5.5V)
14.4 (6.5V)
9.0 (6.7V)
8.8 (7.5V)
15.3 (6.2V)
16.8 (6.8V)
13.5 (7.9V)
12.1 (7.3V)
78000 (10.5 V)
29100 (10.5 V)
21200 (11.5 V)
29500 (13.5 V)
130800 (13 V)
106000 (14 V)
36700 (13 V)
690
1550
1700
1050
1000
12.5
17
51000 (13.5 V)
a
The notation 1 and 2 indicates the devices fabricated with host materials p-cbtz and m-cbtz, respectively. b Turn-on voltage at which emission became
c
detectable. The values of driving voltage and hext of device at 1000 cd mꢀ2 are depicted in parentheses.
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Next, the yellow-emitting OLEDs were fabricated using
(Bt)2Ir(acac) as dopant. Excellent performance of yellow elec-
trophosphorescence can be achieved for devices Y1 and Y2
(Table 2). Device Y2 hosted by m-cbtz displays substantially
higher efficiencies (17.5%, 43 cd Aꢀ1, and 38 ml Wꢀ1) than device
Y1 hosted by m-cbtz (16%, 40.4 cd Aꢀ1, and 27.7 ml Wꢀ1).
Remarkably, device Y2 shows rather low efficiency roll-off at
a brightness of 1000 cd mꢀ2, while the recorded hext is still
maintained as high as 16.8%. These values are comparable to
those of the best yellow phosphorescent OLEDs in the litera-
ture,26 which may serve as the potential emitting element for
generating white emission in combination with a complementary
blue phosphor.27 Finally, we demonstrated red-emitting OLEDs
using the emitter Os1 (devices R1 and R2). Both devices also
exhibit high performance. For instance, the device R1 exhibited
a maximum brightness of 29100 cd mꢀ2 at 10.5 V and respectable
EL efficiencies (16.5%, 20.3 cd Aꢀ1, and 18.8 lm Wꢀ1) with the
CIE coordinates of (0.63, 0.34) while the device R2 exhibited
a maximum brightness of 36700 cd mꢀ2 at 13 V and very good EL
efficiencies (16.7%, 18.2 cd Aꢀ1, and 12.5 lm Wꢀ1) with the CIE
coordinates of (0.64, 0.35). The devices displayed relatively pure
red emission and there is no residual emission from the host and/
or adjacent layers in Fig. 4(c) and 5(c), indicating that the elec-
troluminescence is solely originated from the Os(II) dopant and
with completed energy transfer from host to dopant. More
importantly, the excellent performances, disregarding their
emission hue, were obtained from the essentially identical
architecture and incorporation of different phosphors, which
make them very attractive for commercial applications of full-
color displays.
Fig. 6 (a) J–V–L characteristics, (b) plots of EL efficiency versus
brightness for white devices under different hosts.
In addition to the evaluation for multi-color phosphors, we
also fabricated two-emitter WOLEDs (device W1 and W2) by
utilization of 10 wt.% FIrpic and 0.3 wt.% Os1 co-doped into
either p-cbtz or m-cbtz to form a single emissive layer. The
proportion of Os1 required to produce balanced white emis-
sion in these devices is relatively low because the red emission
occurred from the combined effects of efficient energy transfer
from the blue phosphor and exciton formation by direct
charge trapping on Os1 dopant.28 Fig. 6 shows the J–V–L
characteristics and curves of external quantum efficiency and
power efficiency versus brightness. Device W1 with p-cbtz as
host achieved a turn-on voltage of 3.5 V and a maximum
brightness of 21200 cd mꢀ2 (640 mA cmꢀ2) at 11.5 V. The
maximum values were 9.1%, 17.9 cd Aꢀ1 and 11 lm Wꢀ1 for
hext, hc, and hp, respectively. In comparison, device W2 using
m-cbtz as the host exhibited higher performance with even
better efficiencies (hext, hc, and hp) of 12.4%, 25 cd Aꢀ1, and 17
lm Wꢀ1
, respectively. Both devices W1 and W2 exhibit
a significantly reduced efficiency roll-off: hext is still as high as
9% for W1, and 12.1% for W2 at 1000 cd mꢀ2. Moreover, the
WOLEDs showed excellent chromatic stability as shown in
Fig. 7. When the voltage increases from 8 to 11 V, the
Commission Internationale de L’Eclairage (CIE) coordinates
varied only slightly from (0.29, 0.38) to (0.27, 0.37) for device
W1, and from (0.27, 0.38) to (0.26, 0.38) for device W2. This
could be attributed to the use of the bipolar host which may
result in balanced charge fluxes and a broad distribution of
recombination regions within the emissive layer. As a result,
the recombination ratio in emissive layer was maintained
Fig. 7 The normalized EL spectra of white OLEDs employing (a) p-cbtz
and (b) m-cbtz as host at various voltages.
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invariant with increasing driving voltage, so as to keep the CIE
coordinates almost constant.
residue was dissolved in 150 mL of CH2Cl2 and the solution was
filtered. The filtrate was washed with deionized water three times,
dried over Na2SO4 and concentrated to dryness. The resulting
solid was purified by column chromatography, eluting with
a 1 : 1 mixture of CH2Cl2 and ethyl acetate. The colorless crystals
were crystallized from a mixture of CH2Cl2 and hexane (0.92 g,
1.98 mmol, 54.9%).
Conclusions
We have synthesized and characterized two bipolar host mate-
rials, for which the phenylcarbazole and 3,5-bis(2-pyridyl)-1H-
1,2,4-triazole fragments are linked with either para- or meta-
phenyl bridging units. Furthermore, the relationship between
their molecular structures and thermal, electrochemical and
photophysical properties was addressed, and the influence of the
host materials on the performances of phosphorescent OLEDs
has been studied. Apparently, the introduction of different
linking topologies (para- and meta-) only brings about limited
perturbation of the photophysical properties. But, the hole and
electron mobility of p-cbtz are ca. two-times higher than those of
m-cbtz, which is attributed to the meta-linkage with twisted
molecular conformation between phenylcarbazole and triazole,
for which this conformation significantly impedes the carrier
hopping and, in turn, gives reduced mobilities. For all OLEDs,
the higher efficiency was achieved for the m-cbtz based device
with maximum hext of 8.8%, 16.7%, 17.5% and 16.7% for blue
(FIrpic), green [(ppy)2Ir(acac)], yellow [(Bt)2Ir(acac)], and red
[Os(bpftz)2(PPhMe2)2, OS1], respectively. A two-emitter, all
phosphorescent WOLED hosted by m-cbtz in a single-emitting
layer gave a maximum hext of 12.4% and with high chromatic
stability. Thus, the present study presents an effective molecular
design for bipolar hosts of all type of PhOLEDs.
Spectral data of p-cbtz. MS (FAB): m/z ¼ 466 (M + 1)+, 465
1
(M+). H NMR (400 MHz, CDCl3): d 8.83 (d, J ¼ 4.4 Hz, 1H),
8.54 (d, J ¼ 4.8 Hz, 1H), 8.33 (d, J ¼ 8.0 Hz, 1H), 8.27 (d, J ¼ 8.0
Hz, 1H), 8.14 (d, J ¼ 8.0 Hz, 2H), 7.87 (t, J ¼ 8.0 Hz, 2H), 7.73
(d, J ¼ 6.4 Hz, 2H), 7.64 (d, J ¼ 6.8 Hz, 2H), 7.47–7.36 (m, 6H),
7.30 (t, J ¼ 7.2 Hz, 2H). 13C NMR (100 MHz, CDCl3): d 161.55,
153.83, 150.16, 149.38, 149.22, 147.20, 140.58, 137.97, 137.70,
136.97, 136.83, 127.34, 127.10, 126.12, 124.77, 124.69, 124.18,
123.57, 121.99, 120.40, 120.29, 109.65. Anal. Calcd for
C30H20N6: C, 77.57; H, 4.34; N, 18.09; found: C, 77.90; H, 4.47;
N, 18.36%.
Synthesis of 9-(3-iodophenyl)carbazole. Carbazole (1.0 g, 6.0
mmol) and 1,3-diiodobenzene (2.96 g, 9.0 mmol) were reacted
according to the general procedure documented for 9-(4-bro-
mophenyl)-9H-carbazole, giving 9-(3-iodophenyl)-9H-carbazole
as colorless solid (0.87 g, 2.1 mmol, 39%).
Spectral data of 9-(3-iodophenyl)carbazole. 1H NMR (400
MHz, CDCl3): d 8.12 (d, J ¼ 8.0 Hz, 2H), 7.92 (t, J ¼ 2.0 Hz, 1H),
7.79 (d, J ¼ 7.6 Hz, 1H), 7.55 (d, J ¼ 8.6 Hz, 1H), 7.43–7.37 (m,
4H), 7.34–7.26 (m, 3H). 13C NMR (100 MHz, CDCl3): d 140.54,
138.95, 136.42, 135.96, 131.23, 126.40, 126.09, 123.48, 120.36,
120.28, 109.58, 94.56.
Experimental
Synthesis
Synthesis of 9-(4-bromophenyl)carbazole. A mixture of carba-
zole (5.0 g, 30 mmol), 1,4-dibromobenzene (10.62 g, 45 mmol),
1,10-phenanthroline (1.62 g, 9 mmol), K2CO3 (12.4 g, 90 mmol)
and CuI (0.86 g, 5 mmol) was dissolved in DMSO (60 mL) under
Synthesis of m-cbtz. 9-(3-Iodobenzene)carbazole (1.2 g, 3.3
mmol) and 3,5-bis(2-pyridyl)-1H-1,2,4-triazole (0.6 g, 2.7 mmol)
were reacted according to the previous procedure, giving m-cbtz
as colorless solid (0.66 g, 1.42 mmol, 52.4%).
ꢁ
nitrogen. The mixture was heated to 140 C slowly and stirred
overnight. After cooling to room temperature, solvent was
removed by distillation at reduced pressure. Ethyl acetate
(250 mL) was added to the residue and then filtered. The filtrate
was washed with deionized water three times, dried over Na2SO4
and concentrated to dryness. The residue was then purified by
column chromatography, eluting with a 1 : 10 mixture of CH2Cl2
and hexane to give the product. The colorless crystals were
crystallized from a mixture of ethyl acetate and hexane (5.1 g,
15.8 mmol, 52.8%).
Spectral data of m-cbtz. MS (FAB): m/z ¼ 465 (M+). 1H NMR
(400 MHz, CDCl3): d 8.81 (s, 1H), 8.58 (d, J ¼ 4.4 Hz, 1H), 8.31–
8.26 (m, 2H), 8.09 (d, J ¼ 7.6 Hz, 2H), 7.89–7.84 (m, 2H), 7.73–
7.71 (m, 2H), 7.66–7.63 (m, 1H), 7.60 (s, 1H), 7.40–7.37 (m, 2H),
7.34–7.31 (m, 4H), 7.28–7.25 (m, 2H). 13C NMR (100 MHz,
CDCl3): d 161.53, 153.79, 150.09, 149.30, 149.24, 147.24, 140.59,
140, 46, 137.91, 137.04, 136.77, 130.33, 127.25, 125.93, 124.93,
124.82, 124.68, 124.65, 124.14, 123.41, 121.95, 120.30, 120.18,
109.58. Anal. Calcd for C30H20N6: C, 77.57; H, 4.34; N, 18.09;
found: C, 77.65; H, 4.47; N, 18.30%.
1
Spectral data of 9-(4-bromophenyl)carbazole. H NMR (400
MHz, CDCl3): d 8.15 (d, J ¼ 7.8 Hz, 2H), 7.73 (d, J ¼ 6.4 Hz,
2H), 7.46–7.33 (m, 6H), 7.30 (t, J ¼ 5.6 Hz, 2H). 13C NMR (100
MHz, CDCl3): d 63.55, 59.74, 56.06, 51.65, 49.04, 46.44, 43.82,
43.35, 43.18, 32.50.
General procedures
All reactions were performed under nitrogen atmosphere and
solvents were distilled from appropriate drying agents prior to
use. Commercially available reagents were used without further
purification unless otherwise stated. All synthetic reactions were
monitored using pre-coated TLC plates 0.20 mm with fluorescent
indicator UV254. UV-vis and emission spectra were measured on
Hitachi U-3900 and F-4500 FL spectrophotometers, respec-
tively. The experimental values of HOMO levels were determined
Synthesis of p-cbtz. A mixture of 9-(4-bromophenyl)carbazole
(1.58 g, 4.3 mmol), 3,5-bis(2-pyridyl)-1H-1,2,4-triazole (0.8 g, 3.6
mmol), K2CO3 (1.52 g, 10.8 mmol) and CuI (0.14 g, 0.8 mmol)
was dissolved in DMF (65 mL) under nitrogen. The mixture was
heated to reflux for 36 h. After cooling to room temperature,
solvent was removed by distillation at reduced pressure. The
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with a Riken AC-2 photoemission spectrometer (PES), and those
of LUMO levels were estimated by subtracting the optical energy
gap from the measured HOMO. Differential scanning calorim-
etry (DSC) was characterized with a TA DSC Q200 at a heating
rate of 10 ꢁC minꢀ1 from 20 to 250 ꢁC under nitrogen. The glass
transition temperature (Tg) was determined from the second
heating scan. Thermogravimetric analysis (TGA) was measured
with a Perkin-Elmer Pyris 1 TGA instrument. Mass spectra were
obtained on a JEOL SX-102A instrument operating in electron
simultaneously in a glove-box using a Keithley 6430 source meter
and a Keithley 6487 picoammeter equipped with a calibration Si-
photodiode. EL spectra were measured using a photodiode array
(OTO SD1200).
Acknowledgements
We are grateful for the financial support from the National
Science Council of Taiwan (NSC 100-2112-M-019-002-MY3 and
98-2113-M-007-019-MY3).
1
impact (EI) or fast atom bombardment (FAB) mode. H NMR
spectra were recorded on a Varian Mercury-400 or an INOVA-
500 instrument. Elemental analysis was carried out with a Her-
aeus CHN–O Rapid Elementary Analyzer. Cyclic voltammetry
was performed in CH2Cl2 solution on a CH1621A instrument
with a scan rate of 100 mV sꢀ1. Tetrabutylammonium hexa-
fluorophosphate (TBAPF6 0.1 M) was used as supporting elec-
trolyte. The three electrode cell was used with Pt wire as working
electrode, glassy carbon as counter electrode. An Ag/Ag+ elec-
trode was used as reference electrode with ferrocenium–ferrocene
(Fc+/Fc) as internal standard. 3,5-Bis(2-pyridyl)-1H-1,2,4-tri-
azole (Hbpytz) was synthesized according to the literature
procedure.29
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