A wet- and dry-process feasible carbazole type host for highly efficient phosphorescent OLEDs

Article abstract of DOI:10.1039/c5tc02889b

A wet- and dry-process feasible host material is crucial to realize, respectively, low cost roll-to-roll fabrication of large area and high performance organic light-emitting diodes (OLEDs) with precise deposition of organic layers. We demonstrate in this study high efficiency phosphorescent OLED devices by employing a newly synthesized carbazole based host material 1,6-bis[3-(2-methoxy-3-pyridinyl)carbazol-9-yl]hexane (compound 5). Moreover, two other carbazole hosts 1,6-bis[3-(6-methoxy-3-pyridinyl)carbazol-9-yl]hexane (compound 4) and 3,6-di(2-methoxy-3-pyridinyl)-9-ethylcarbazole (compound 6) are also synthesized for comparison. By doping a typical green emitter fac tris(2-phenylpyridine)iridium (Ir(ppy)₃) in compound 5, for example, the resultant wet-processed device exhibits at 100 cd m^-2 a current efficiency of 27 cd A^-1 and a power efficiency of 16.1 lm W^-1. The dry-processed device shows a current efficiency of 61 cd A^-1 and a power efficiency of 62.8 lm W^-1. The high efficiency may be attributed to the host possessing an effective host-to-guest energy transfer, effective carrier injection balance, and the device architecture enabling excitons to generate on both the host and the guest.

Full text of DOI:10.1039/c5tc02889b

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Mater. Chem. C, 2015, DOI: 10.1039/C5TC02889B.
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DOI: 10.1039/C5TC02889B
Wet- and dry-process feasible carbazole type host for highly efficient
phosphorescent OLEDs
Jwo-Huei Joua,*, Snehasis Sahooa, Sudhir Kumara,b, Hui-Huan Yua, Po-Hsun Fanga,
Meenu Singha, Gintare Krucaitec, Dmytro Volyniukc, Juozas Vidas Grazuleviciusc,
Saulius Grigaleviciusc,*
aDepartment of Materials Science and Engineering, National Tsing-Hua University,
Hsin-Chu-30013, Taiwan, Republic of China.
bInstitute for Chemical and Bioengineering, ETH Zürich, Vladimir Prelog Web 1, 8093
Zürich, Switzerland.
cDepartment of Polymer Chemistry and Technology, Kaunas University of Technology,
Radvilenu plentas 19, LT50254, Kaunas, Lithuania.
*Corresponding
authors:
jjou@mx.nthu.edu.tw
(J.-H.
Jou)
and
saulius.grigalevicius@ktu.lt (S. Grigalevicius)
Abstract
A wet- and dry-process feasible host material is crucial to realize, respectively, low cost
roll-to-roll fabrication of large area and high performance organic light-emitting diodes
(OLEDs) with precise deposition of organic layers. We demonstrate in this study, high
efficiency phosphorescent OLED devices by employing a newly synthesized carbazole
based
host
material
1,6-bis[3-(2-methoxy-3-pyridinyl)carbazol-9-yl]hexane
(compound 5). Moreover, two other carbazole hosts 1,6-bis[3-(6-methoxy-3-
pyridinyl)carbazol-9-yl]hexane (compound 4) and 3,6-di(2-methoxy-3-pyridinyl)-9-
ethylcarbazole (compound 6) are also synthesized for comparison. By doping a typical
green emitter fac tris(2-phenylpyridine)iridium (Ir(ppy)3) in the compound 5, for
example, the resultant wet-processed device exhibits at 100 cd m-2 a current efficiency

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DOI: 10.1039/C5TC02889B
of 27 cd A-1 and a power efficiency of 16.1 lm W-1. The dry-processed device shows a
current efficiency of 61 cd A-1 and a power efficiency of 62.8 lm W-1. The high
efficiency may be attributed to the host possessing an effective host-to-guest energy
transfer, effective carrier injection balance, and the device architecture enabling
excitons to generate on both host and guest.
1. Introduction
Organic light-emitting diodes (OLEDs) have drawn enormous attention due to their
increasing applications in flat-panel displays and solid state lightings.1 A wide variety
of OLED based display devices have already emerged in the market from the last few
years, and lighting products are also developing rapidly.1-2 Nowadays, phosphorescent
materials become ideal for fabricating high-efficiency OLEDs, because they can
theoretically approach a nearly 100% internal quantum efficiency by harvesting singlet
and triplet excitons simultaneously through intersystem crossing.3-6 A suitable
phosphorescent host material can play an effective role in reducing both the
concentration quenching and triplet-triplet annihilation effects frequently occurring in
an undoped system.7 The host materials should have a few important properties, such
as an energy level matching with the neighboring layers to ensure a low injection barrier,
a good carrier mobility to realize a high recombination rate, a proper bipolarity to
maximize charge injection balance, an effective host-to-guest energy transfer,8-9 and a

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high triplet energy to avoid back energy transfer.10 In addition, a good host material is
expected to have a high glass transition temperature (Tg) to realize high thermal and
morphological stability of the device.11-13
OLED devices can be fabricated by utilizing both thermal evaporation and spin-
coating processes. It would be ideal if a host material possesses both wet- and dry-
process feasibilities so that it can realize low cost roll-to-roll fabrication for large area-
size devices and high brightness as well as long lifetime for high performance devices,
respectively.14-19
In the past years, a few wet- and dry-processable molecular host materials had
been reported. For example, Jou’s group reported a wet- and dry-process feasible EML
system containing a 4,4’-bis(carbazol-9-yl)biphenyl as host and a bis[5-methyl-8-trifl
uorome-thyl-5H-benzo(c)(1,5)naphthyridin-6-one] iridium (acetylacetonate) ((2-
CF3BNO)2 Ir(acac)) as green light emitting guest. The device showed, at 100 cd m-2, an
efficacy of 78.6 lm W-1 via spin-coating, while 65.4 lm W-1 via vapor deposition.17 They
further reported an efficacy of 47.1 lm W-1 via solution process, and 60.6 lm W-1 via
thermal evaporation process, by using a 4,4’,4”-tri(N-carbazolyl)triphenylamine
(TCTA) as host and a bis[5-methyl-5H-benzo[c][1,5] naphthyridin-6-one]iridium
(picolinate) (BNO) as green emitter.18 Lin’s group achieved an efficacy of 70 lm W-1
via solution process, and 21 lm W-1 via thermal evaporation process, by using a 4-4’-

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bis(carbazol-9-yl)biphenyl (CBP) as host and a bis[5-methyl-7-trifluoromethyl-5H-
benzo(c) (1,5) naphthyridin-6-one]iridium(picolinate) ((CF3BNO)2IrPLA) as green
emitter.19
In this study, we present a newly synthesized carbazole-type host material 1,6-
bis[3-(2-methoxy-3-pyridinyl)carbazol-9-yl]hexane (5) that possesses a high triplet
energy and is wet- and dry-process feasible. Two other carbazole group based hosts,
1,6-bis[3-(6-methoxy-3-pyridinyl)carbazol-9-yl]hexane (4) and 3,6-di(2-methoxy-3-
pyridinyl)-9-ethylcarbazole (6), were also synthesized for comparison. By spin coating,
at 100 cd m-2 for example, the host 5 containing device shows an efficacy of 16.1 lm
W-1 and a current efficiency of 27 cd A-1, while the host 4 and 6 containing devices
show an efficacy of 12 lm W-1 with a current efficiency of 21.4 cd A-1 and 2.4 lm W-1
with 3.4 cd A-1. By using dry-process, the host 5 composing device doped with an
Ir(ppy)3 green emitter shows an efficacy of 62.8 lm W-1 and current efficiency of 61 cd
A-1, while compound 4 and compound 6 containing devices exhibits an efficacy of 42.9
lm W-1 with 41.8 cd A-1 and 7.9 lm W-1 with 9.5 cd A-1, respectively.
2. Results and discussion
2.1. Synthesis
As shown in Scheme 1, the synthesis of the methoxypyridinyl substituted carbazole
based host materials, 4-6, were carried out by Suzuki coupling reaction. The host

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materials 4 and 5 were synthesized by coupling reaction of a 1,6-bis(3-iodocarbazol-9-
yl)hexane (3) with an excess of a 6-methoxy-3-pyridinylboronic acid and a 2-methoxy-
3-pyridinylboronic acid, respectively. Moreover, the host 6 was synthesized by
coupling reaction of a 3,6-di-iodo-9-ethylcarbazole (DECr)20 with an excess amount of
a 2-methoxy-3-pyridinylboronic acid. The key material, i.e. 3-iodo-9H-carbazole (2),
was synthesized from a commercially available 9H-carbazole using Tucker iodination
reaction.21 The twin-derivative 3 that contains two 3-iodocarbazol-9-yl fragments was
synthesized by the reaction of a 1,6-dibromohexane with an excess of the 3-iodo-
derivative of compound 2 under basic conditions by using the prior reaction
1
procedure.22 The resultant compounds (2-6) were characterized by using H NMR,
13CNMR spectroscopies and mass spectrometry. The 1H NMR and 13C NMR spectra of
compounds 4, 5 and 6 were presented in the Figure S1 and Figure S2. The data were
found in good agreement with the proposed structures.

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Scheme 1. Schematic illustration of the synthesis of the carbazole-type hosts 4, 5, and 6.
Table 1. Thermal, photophysical, and electrochemical characteristics of the novel host
5, compared with those of the other two molecular hosts 4 and 6.
a)
b)
c)
f)
g)
h)
i)
Host
Tg
Tm
Td
Hole
mobility
(cm2V-1s-1)
2×10-4
HOMOd) LUMOe)
Eg
λabs
[nm]
λPL
λPL
ET
[°C] [°C] [°C]
[eV]
5.60
5.24
5.71
[eV]
2.26
1.91
2.31
[eV]
3.34
3.33
3.40
[nm]
[nm]
[eV]
2.56
2.56
2.47
4
5
6
51
54
57
167
-
409
402
353
283
283
302
391,
375
391,
374
385
483,
455
483,
455
501
4×10-4
5×10-4
137
a)Glass transition temperature, b)melting temperature, c)decomposition temperature, d)HOMO values are
measured by the cyclic voltammetry (CV) method. The semi-oxidation potential (Eox1/2) was calculated
from (Ep1 + Ep2)/2 -0.48, where 0.48 is the correction value obtained by the oxidation system added to
ferrocenium/ferrocene (Fc+/Fc) as the internal standard, and then the energy of HOMO could be obtained
from the EHOMO = -(Eox1/2 + 5.1).23 e)The energy of LUMO could be obtained by subtracting the optical
bandgap from the HOMO energy level, [ELUMO = (EHOMO -Eg)], f)optical bandgap, g)PL spectra at room
temperature, h)PL spectra at 77K, and i)triplet energy of hosts.

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2.2 Carrier mobility measurement
The hole mobility (μh) of the host materials (4-6) was characterized by the time-of-
flight (TOF) technique.24, 25 The μh was calculated by the formula of μh = d2/(V.tT),
where d is the layer thickness of the hosts (4-6), V the applied voltage, and tT the transit
time.26, 27 (see Fig. S3 for detail double logarithmic plots) The μh determined for
compounds 4-6 is shown as a function of the square root of electric field (E1/2) in Fig.
1. The μh of hosts 4-6 ranges from 5×10-4 to 2×10-4 cm2/(V·s), as shown in Table 1.
However, no detectable electron mobility (μe) can be observed for the newly
synthesized host materials 4-6 by the TOF method.
Figure 1. Hole mobilities of the hosts, 4, 5, and 6, as a function of square root of electric
field.
The compounds 4, 5, and 6 show hole mobility in the order 4<5<6. The reason
why compound 5 and compound 6 show a higher hole mobility than that of compound
4 counterpart may be attributed to the position of methoxy group on the pyridine unit.
The 6-methoxyl-3-pyridinyl substituted compound 4 with a para-linked methoxy group

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showed the lowest hole mobility because a large number of electrons are delocalized
near the N-atom. On the other hand, the 2-methoxyl-3-pyridinyl substituted compound
5 and compound 6 show high hole mobility due to the ortho-linked electron donating
methoxy group, resulting into the localization of electrons near the N-atom. Amongst,
compound 6 shows a highest hole mobility because of the extended conjugation in the
molecule, while the conjugation is disrupted in compound 5 due to the long alkyl chain
that connected the two carbazole units.28
2.3. Thermal characteristics
The thermal characteristics of compounds 4-6 were characterized by using differential
scanning calorimetry (DSC) and thermogravimetric analysis (TGA) techniques. As
shown in Table 1, the compound 4 shows a 5% weight loss decomposition temperature
(Td) of 409 oC, while 402 oC and 353 oC for the 5 and 6 counterparts, respectively. The
reason why the compounds 4 and 5 show higher thermal stability than that of the
compound 6 because of their higher molecular weight, which is resulted due to the twin-
carbazole molecular structures. The melting temperatures (Tm) of the hosts 4-6 range
from 137 oC to 167 oC, while the glass transition temperatures (Tg) range from 51 oC to
57 oC. The reason why single carbazole unit based compound 6 shows a slightly higher
o
Tg (57 C) than that of twin carbazole based counterparts because of their normal
hexane linkage.

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Figure 2. Ultraviolet-visible, photoluminescence and phosphorescence (PL at 77K)
spectra of the novel carbazole-type compounds, (a) 4, (b) 5, and (c) 6. All the data were
measured in tetrahydrofuran.
2.4. Photophysical and electrochemical characteristics
As shown in Fig. 2, the ultraviolet–visible (UV–Vis) and photoluminescence (PL)

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spectra of the compounds 4, 5, and 6 were measured in tetrahydrofuran (THF) at
room temperature. The optical band gaps were estimated from the edge of the
absorption spectra, giving values of 3.34, 3.33, and 3.40 for the compounds 4, 5,
and 6, respectively. The triplet-energies were calculated from the first triplet
emission peak of the low temperature PL spectrum measured in cryogenic medium
(liquid N2) at 77K. The compounds 4 and 5 showed a triplet-energy, 2.56 eV,
whereas, the compound 6 showed a slightly lower triplet energy of 2.47 eV. The
reason why host-6 shows a lower triplet energy may be attributed to its extended
conjugation in molecular structure, while in host-5 conjugation is interrupted by a
long aliphatic chain. As reported by Woon et al., the triplet energy would decrease
with the increase in effective conjugation length.29 Moreover, the host 4 and 5
shows a clear vibronic band at 355 nm due to the dimer structure. The reason why
a vibronic band does not appear in the low temperature (at 77K) PL spectrum of
host 6 is because of the absence of dimer.30
The electrochemical properties of the three carbazole-type host molecules 4–6
were measured by cyclic voltammetry (CV). The highest occupied molecular orbital
(HOMO) energy levels of the hosts were estimated to be 5.60 eV, 5.24 eV, and 5.71 eV
for the compounds 4, 5, and 6, respectively, using oxidation potential. The lowest
unoccupied molecular orbital (LUMO) energy levels were calculated to be 2.26 eV,
1.91 eV, and 2.31 eV for the compounds 4, 5, and 6, respectively, from HOMO energy
levels and optical energy band gaps, which were estimated from the edge of the
absorption spectra (Table 1).
2.5. Electroluminescence properties of devices

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Fig. 3 illustrates the schematic energy-level diagram of green OLED devices containing
newly synthesized host materials 4, 5, and 6 and green emitter Ir(ppy)3.The devices
were composed of a 125 nm indium tin oxide (ITO) anode layer, followed by a 35 nm
poly(3,4-ethylene-dioxythiophene)-poly(styrenesulfonate)
(PEDOT:PSS)
hole
injection layer (HIL), Fig. 3 (a) a 20 nm single emissive layer (EML) with the green
emitter (Ir(ppy)3) doped in the host 5 via spin-coating deposition (wet-process) and
physical vapor deposition (dry-process), Fig. 3 (b) a 20 nm di-[4-(N,N-ditolyl-amino)-
phenyl]cyclohexane (TAPC) hole transporting layer (HTL) was also thermally
deposited between the PEDOT:PSS and EML of dry-processed device, a 32 nm 1,3,5-
tris(N-phenylbenzimidazol-2-yl)benzene (TPBi) electron transporting layer (ETL), a 1
nm lithium fluoride (LiF) layer, and a 100 nm aluminum (Al) cathode layer. Besides
the host 5, two other hosts, 4 and 6 were also studied herein.
Figure 3. Schematic diagrams of the energy levels of the OLED devices, containing a
green emitter Ir(ppy)3 in the three different hosts: 4, 5, and 6, using (a) wet- and dry-

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processes, and (b) dry-process with the incorporation of an hole transporting layer
(HTL) TAPC. Molecular structures of the typical green emitter and the three newly
synthesized hosts were also presented.
Figure 4. Comparison of a) power efficiency, b) current efficiency, c) current density,
and d) luminance of the devices with a 15 wt% ratio Ir(ppy)3 doped in the hosts 4, 5,
and 6.

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Table 2: Effects of doping concentrations of Ir(ppy)3 on the operation voltage (OV), power efficiency (ηp), current efficiency (ηc), external quantum
efficiency (ηext), CIE coordinates, and maximum luminance of the novel host materials, 4, 5, and 6 containing wet-processed OLED devices.
Host
Dopant
(wt%)
VON
(V)
OV
(V)
PE
(lm/W)
CE
(cd/A)
EQE
(%)
CIE coordinates
Max. Luminance
(cd/m2)
@ 10 cd/m2
4.6
@ 100/1000 cd/m2
6.2/ 15.0
10
12.5
15
5.5/ 7.4
3.5/ 6.4
1.7/ 4.2
(0.31, 0.62)/ (0.31, 0.62)
5313
5.0
5.0
5.0
5.6/ 6.9
5.6/ 6.9
5.6/ 6.7
9.0/ 7.9
12.0/ 9.5
9.1/ 9.2
16.0/ 17.4
21.4/ 21.0
16.2/ 19.7
4.4/ 4.8
5.9/ 5.8
4.5/ 5.4
(0.31, 0.63)/ (0.31, 0.63)
(0.31, 0.62)/ (0.31, 0.62)
(0.32, 0.62)/ (0.32, 0.62)
7024
7575
7733
4
5
6
17.5
10
12.5
15
4.4
4.5
4.5
4.6
4.0
3.7
3.7
5.2/ 6.7
4.9/ 6.2
5.3/ 6.7
4.7/ 6.1
4.7/ 6.3
4.4/ 5.7
4.4/ 5.7
13.1/ 10.3
14.2/ 13.5
16.1/ 13.5
11.3/ 13.4
3.4/ 8.1
21.5/ 21.9
22.6/ 26.6
27.0/ 28.7
17.3/ 26.0
5.2/ 16.2
6.4/ 21.0
3.4/ 17.6
6.0/ 6.1
6.3/ 7.4
7.5/ 8.0
4.7/ 7.1
1.5/ 4.6
1.8/ 5.8
0.9/ 4.8
(0.31, 0.62)/ (0.31, 0.62)
(0.31, 0.62)/ (0.31, 0.62)
(0.32, 0.62)/ (0.32, 0.62)
(0.31, 0.63)/ (0.31, 0.63)
(0.31, 0.62)/ (0.31, 0.62)
(0.31, 0.63)/ (0.31, 0.63)
(0.31, 0.63)/ (0.31, 0.63)
10340
11240
12000
12910
12990
17230
17260
17.5
10
12.5
15
4.5/ 11.6
2.4/ 9.6
17.5
3.7
4.3/ 5.5
3.1/ 11.9
4.4/ 20.9
1.2/ 5.8
(0.31, 0.62)/ (0.31, 0.62)
16990

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2.5.1 Wet-processed devices
Table 2 shows the electroluminescent characteristics of the host 5 based wet-processed
green OLED compared with those of the host 4 and 6 containing counterparts. As shown
in Fig. 4, the host 5-containing device exhibited a power efficiency of 16.1 lm W-1 with
a current efficiency of 27 cd A-1 at 100 cd m-2. For the host 4 containing device, its
power efficiency was 12 lm W-1 (21.4 cd A-1), while 2.4 lm W-1 (3.4 cd A-1) for the host
6 containing counterpart. The reason why the host 5 containing device exhibited highest
efficiencies amongst all the three studied hosts may be attributed to three efficiency
effective device architecture approaches, which are i) feasibility of effective exciton
generation on both host and guest, ii) high triplet energy, and iii) effective host-to-guest
energy transfer.
As shown in Fig. 3, the hole injection barrier for host 5 is 0.34 eV, which is
lower than that (0.36 eV) of the green Ir(ppy)3 emitter, enabling more holes to transport
to host 5. Whilst, the same architecture favors the injection of electrons into the guest
because there exists a -0.3 eV electron trap, in contrast to a 0.81 eV barrier to enter into
the host. These would hence lead excitons to generate on both host and guest and result
in high device efficiency.10, 31-33
Furthermore, the hosts 4 and 5 showed a triplet-energy of 2.57 eV, higher than
that of the green emitter, whose triplet-energy is 2.57 eV.34,35 This would allow the hosts

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4 and 5 to enable an effective host-to-guest energy transfer and exciton confinement on
guest, resulting in high device efficiency.36-38 In contrast, the host 6 showed a slightly
lower triplet energy of 2.47 eV, which is not able to prevent the back energy transfer
from guest to host.10,33,39
Figure 5. The corresponding overlapping between the PL spectra of the hosts 4, 5, and
6 with the UV-vis spectrum of the Ir(ppy)3 dopant.
As shown in Fig. 5, all the three host materials can effectively transfer their
energy to the green emitter, as indicated by the large overlapping area between the PL
spectra of the hosts 4, 5, and 6 and the UV-vis spectrum of the guest. However, host 5
showed a PL emission peaking at a wavelength (376 nm) much lower than that of the
hosts 4 (391 nm) and 6 (385 nm), which would trigger the higher energy emission of
the guest, leading to a higher device efficiency.40

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Figure 6. Doping concentration effects on the a) power efficiency, b) current efficiency,
c) current density, and d) luminance results of the host 5 based green OLED devices.
As shown in Fig. 6, the device efficiencies significantly depend on the doping
concentration of the green emitter. Taking the host 5 based device for example, the
power efficiency at 100 cd m-2 increased from 13.1 to 16.1 lm w-1 as doping
concentration was increased from 10 to 15 wt%. However, as the concentration further
increased to 17.5%, the efficacy decreased to 11.3 lm W-1. This may be attributed to the
triplet-triplet annihilation as well as concentration-quenching, resulted from self-
aggregation of the emitter at high concentration.10,17,18 Moreover, as the doping

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concentration increases from 10 to 15 wt% the electroluminescence (EL) spectra exhibit
a stable emission peak at 516 nm. When the doping concentration is further increased
to 17.5 wt%, a bathochromic shift is observed with emission peaking at 520 nm.
However, as the concentration is increased to 17.5 wt%, the efficiencies start to drop.
This may be attributed to a concentration-quenching effect that can be confirmed by a
4 nm bathochromic shift in the EL spectrum of the 17.5 wt% Ir(ppy)3 doped host 5
based device, as shown in Fig.7.17,18,41
Figure 7. Effect of dopant concentration on the electroluminescence (EL) spectra of
the device containing host 5. The EL emission spectra becomes slightly narrower as the
dopant concentration was increased from 15 to 17.5 wt%, while the spectra remain
unchanged as the dopant concentration was changed from 10 to 15 wt%.

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Table 3. Effect of host materials on the turn-on voltage (VON), operation voltage (OV), power efficiency (PE), current efficiency (CE), external
quantum efficiency (EQE), CIE coordinates, and maximum luminance of the hosts (4, 5, and 6) doped with 10 wt% Ir(ppy)3 green emitter based
dry-processed OLED devices.
Host
ECL
VON
(V)
OV
(V)
PE
(lm/W)
CE
(cd/A)
EQE
(%)
CIE coordinates
Max. Luminance
(cd/m2)
@ 10 cd/m2
@ 100/1000 cd/m2
4
5
6
-
2.9
3.2/ 3.9
3.1/ 3.7
3.5/ 4.1
3.1/ 3.7
4.6/ 5.9
3.8/ 6.0
16.9/ 17.3
42.9/ 30.7
52.7/ 42.7
62.8/ 47.4
3.1/ 1.5
17.3/ 21.3
41.8/ 36.3
59.4/ 55.3
61/ 55.4
4.5/ 2.8
4.7/ 5.8
(0.32, 0.62)/ (0.32, 0.62)
(0.31, 0.62)/ (0.31, 0.62)
(0.31, 0.63)/ (0.3, 0.63)
(0.33, 0.61)/ (0.33, 0.61)
(0.23, 0.4)/ (0.22, 0.33)
(0.25, 0.47)/ (0.24, 0.41)
25810
26070
36810
47890
2181
TAPC
-
2.7
3.1
11.7/ 10.2
16.4/ 15.3
17.2/ 15.7
2.2/ 1.7
TAPC
-
2.6
4.0
TAPC
3.1
7.9/ 3.4
9.5/ 6.4
3.3/2.6
3964

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2.5.2 Dry processed devices
The efficiency performance of the new synthesized host materials is further investigated
via vapor evaporation deposition. Table 3 shows the electroluminescent characteristics
of the thermal evaporation based OLED devices containing hosts 4, 5, and 6 doped with
a 10 wt% Ir(ppy)3 green emitter. The resultant host 5 containing device is also better
than that of the host 4 and 6 containing counterparts. At 100 cd m−2 for example, host
5 based device exhibits a power efficiency of 52.7 lm W-1 and a current efficiency of
59.4 cd A-1 with a maximum luminance of 36,810 cd m-2, respectively, which are over
200%, 240%, and 40% higher than that of host 4 containing counterpart. These results
are also significantly higher than that of spin-coated EML based devices. The reason
why the dry-processed devices showed better performance may be due to better film
uniformity and integrity, while pin-holes and rougher surface might easily present in
the resultant films from wet process especially after the evaporation of the solvent
applied.
The host 4 containing device showed higher device efficiencies than that of the
host 6. Taking 100 cd m-2 for example, host 6 based device exhibited a power efficiency
of 3.1 lm W-1 and a current efficiency of 4.5 cd A-1, which are nearly 450% and 280%,
respectively, lower than the host 4 based device. When host 6 was used as host, both
holes and electrons prefer to enter into the guest rather than into the host, because the
respective barrier for holes to emitter is 0.11 eV lower than that of the host. The Ir(ppy)3

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guest possesses an effective electron injection pathway, i.e., it shows a -0.3 eV electron
trap, in contrast to the host 6 that has an electron injection barrier of 0.39 eV. As a result,
the majority of the excitons would form on the Ir(ppy)3 guest, which could not trigger
the higher energy emission. Moreover, the reason why the host 6 based device exhibits
such poor efficiencies may be attributed to its low triplet energy.10,33,39
Figure 8. Effect of hole transporting layer, TAPC, on the a) luminance, b) power
efficiency, c) current efficiency, and d) EQE results of the devices with 10 wt% Ir(ppy)3
green emitter doped into the host 5.
The device efficiency was further enhanced as a 20 nm hole transporting layer,
TAPC, was deposited between the PEDOT:PSS layer and EML. As shown in Fig. 8,

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for the host 5 containing device, the resultant power efficiency, current efficiency, and
EQE were increased from 52.7 to 62.8 lm W-1, 59.4 to 61 cd A-1, and 16.4 to 17.2%,
respectively. By comparing the without TAPC (w/o TAPC) device, a narrow EL
spectrum is observed for the TAPC based device (w TAPC) counterpart, which may
result because of a small shift in the recombination zone. Moreover, the resultant device
also exhibited a significant enhancement in luminance, which increased from 36,810 to
47,890 cd/m2. For the host 4 containing device, the resultant power efficiency, current
efficiency, and EQE was increased from 16.9 to 42.9 lm W-1, 17.3 to 41.8 cd A-1, and
4.7 to 11.7%, respectively, while 3.1 to 7.9 lm W-1, 4.5 to 9.5 cd A-1, and 2.2 to 3.3%,
for the host 6-containing counterpart (Table 3). This enhancement may be attributed to
the fact that TAPC possesses an electron-modulating function.42,43 As shown in Fig.
3(b), the HOMO level of TAPC is 0.6 eV lower than that of PEDOT, which may block
a certain number of holes from entering the emissive-layer, balancing the carrier-
injection and improving the device efficiency.44,45
3. Conclusion
To conclude, in this report we demonstrate a new carbazole-based host 5 with both wet-
and dry-process feasibility. By employing this host, green phosphorescent OLED
devices with higher efficiencies have been fabricated. The device shows, at 100 cd m−2
for example, a power efficiency of 16.1 lm W−1 (27 cd A-1) by using spin-coating, while

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52.7 lm W-1 (59.4 cd A-1) using vapor-deposition. The high efficiency may be attributed
to feasibility of effective exciton generation on both host and guest, high triplet energy,
and effective host-to-guest energy transfer. The device efficiencies can be further
enhanced to 62.8 lm W-1 (61 cd A-1) by incorporating a TAPC hole transporting layer.
The resultant carbazole derivative may serve as one ideal host for fabricating energy-
efficient devices for solid state lighting and flat panel display applications via either
wet- or dry-processing.
4. Experimental
4.1 Synthesis
1,6-Bis[3-(6-methoxy-3-pyridinyl)carbazol-9-yl]hexane (4) 1.2 g (1.8 mmol) of 1,6-
bis(3-iodocarbazol-9-yl)hexane (3), 0.8 g (5.4 mmol) of 6-methoxy-3-pyridinylboronic
acid, 0.05 g (0.08 mmol) of PdCl2(PPh3)2 and 0.5 g (8.5 mmol) of powdered potassium
hydroxide were stirred in 20 ml of THF containing degassed water (2 ml) at 80 °C
under nitrogen for 24 h. After TLC control the reaction mixture was cooled and
quenched by the addition of ice water. The product was extracted by ethyl acetate. The
combined extract was dried over anhydrous Na2SO4. The crude product was purified
by silica gel column chromatography using the mixture of ethyl acetate and hexane (vol.
ratio 1:5) as an eluent. Yield: 0.79 g of yellow crystals.
MS (APCI+, 20 V): 631.3 ([M+H], 100 %). 1H NMR (300 MHz, CDCl3, , ppm): 8.38
(d, 2H, J=2.7Hz, Ar), 8.12 (d, 2H, J=1.5Hz, Ar), 8.03 (d, 2H, J=7.5Hz, Ar), 7.80 (dd,

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2H, J1=2.4Hz, J2=8.7Hz Ar), 7.47 (dd, 2H, J1=1.95Hz, J2=8.4Hz Ar), 7.39-7.11 (m,
6H, Ar), 6.76(d, 2H, J=8.4Hz, Ar), 4.17 (t, 4H, J=6.9Hz, NCH2( CH2)4CH2N), 3.92(s,
6H, 2×OCH3), 1.78-1.68(m, 4H, NCH2CH2(CH2)2CH2CH2N), 1.35-1.31(m, 4H,
NCH2CH2(CH2)2CH2CH2N). 13C NMR (CDCl3, , ppm): 162.98, 144.78, 140.80,
139.80, 137.82, 131.10, 125.97, 124.67, 123.44, 122.75, 120.46, 119.05, 118.47,
110.72, 109.04, 108.79, 53.61, 42.93, 28.84, 27.05.
1,6-Bis[3-(2-methoxy-3-pyridinyl)carbazol-9-yl]hexane (5) 1.6 g (2.4 mmol) of 1,6-
bis(3-iodocarbazol-9-yl)hexane (3), 0.9
g
(6.01 mmol) of 2-methoxy-3-
pyridinylboronic acid, 0.07 g (0.10 mmol) of PdCl2(PPh3)2 and 0.7 g (12.5 mmol) of
powdered potassium hydroxide were stirred in 20 ml of THF containing degassed water
(2 ml) at 80 °C under nitrogen for 24 h. After TLC control the reaction mixture was
cooled and quenched by the addition of ice water. The product was extracted by ethyl
acetate. The combined extract was dried over anhydrous Na2SO4. The crude product
was purified by silica gel column chromatography using the mixture of ethyl acetate
and hexane (vol. ratio 1:5) as an eluent. Yield: 0.76 g of amorphous material.
1
MS (APCI+, 20 V): 631.3 ([M+H], 100 %). H NMR (300 MHz, CDCl3, , ppm):
8.27(d, 2H, J=1.8Hz, Ar), 8.17 (dd, 2H, J1=1.95Hz, J2=5.1Hz, Ar), 8.12 (d, 2H,
J=7.2Hz, Ar), 7.71(dd, 2H, J1=1.5Hz, J2=7.2Hz, Ar), 7.65 (dd, 2H, J1=1.65Hz,
J2=8.7Hz, Ar), 7.49-7.33 (m, 6H, Ar), 7.27-7.20 (m, 2H, Ar), 7.00 (dd, 2H, J1=1.95Hz,
J2=5.1Hz, Ar), 4.28 (t, 4H, J=7.35Hz, NCH2(CH2)4CH2N), 4.01 (s, 6H, 2×OCH3), 1.95-
1.82
(m,
4H,
NCH2CH2(CH2)2CH2CH2N),
1.43-1.39
(m,
4H,
NCH2CH2(CH2)2CH2CH2N). 13C NMR (CDCl3, , ppm): 161.05, 144.96, 104.74,
139.79, 138.74, 127.37, 127.01, 125.75, 125.55, 122.87, 121.08, 120.44, 118.94,
117.16, 108.69, 108.28, 53.55, 42.89, 28.85, 27.05.
3,6-Di(2-methoxy-3-pyridinyl)-9-ethylcarbazole (6) 2.0 g (4.47 mmol) of 3,6-diiodo-
9-ethylcarbazole (DECr), 1.5 g (9.8 mmol) of 2-methoxy-3-pyridinylboronic acid,
0.037 g (0.14 mmol) of PdCl2(PPh3)2 and 1 g (17.8 mmol) of powdered potassium

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hydroxide were stirred in 20 ml of THF containing degassed water (1.5 ml) at 80 °C
under nitrogen for 24 h. After TLC control the reaction mixture was cooled and
quenched by the addition of ice water. The product was extracted by ethyl acetate. The
combined extract was dried over anhydrous Na2SO4. The crude product was purified
by silica gel column chromatography using the mixture of ethyl acetate and hexane
(vol. ratio 1:5) as an eluent. Yield: 1.25 g of yellow crystals. M.p.: 137 ºC (DSC).
MS (APCI+, 20 V): 424.1 ([M+H], 100 %). 1H NMR (300 MHz, CDCl3, , ppm): 8.28
(d, 2H, J=0.9Hz, Ar), 8.16 (dd, 2H, J1=1.5Hz, J2=1.8Hz, Ar), 7,74 (d, 1H, J=1.5Hz,
Ar), 7.73-7.70(m, 2H, Ar),7.68 (d, 1H, J=1.2Hz, Ar), 7.47 (d, 2H, J=6.3Hz, Ar), 7.03-
6.99(m, 2H, Ar), 4.42(q, 2H, J=7.2Hz, NCH2CH3), 4.01(s, 6H, 2×OCH3), 1.48(tr, 3H,
J=7.2Hz, NCH2CH3). 13C NMR (CDCl3, , ppm): 161.08, 144.99, 139.77, 138.76,
127.56, 127.17, 125.56, 123.09, 121.25, 117.17, 108.26, 53.58, 37.74, 13.93.
4.2 Materials
9H-Carbazole (1), 6-methoxy-3-pyridinylboronic acid, 2-methoxy-3-pyridinylboronic
acid, 1,6-dibromohexane, tetra-N-butylammonium hydrogen sulphate (TBAHS), tetra-
n-butyl
ammonium
chloride,
potassium
iodide,
potassium
iodate,
bis(triphenylphosphine)palladium(II) dichloride (Pd(PPh3)2Cl2), Alq3, sodium
hydroxide and potassium hydroxide were purchased from Aldrich and used as received.
All the precursor compounds required for the synthesis were used as such without
further purification. 3-Iodo-9H-carbazole (2) was obtained by a procedure of Tucker.21
1,6-Bis[3-iodocarbazol-9-yl]hexane (3) was synthesized by using the prior

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methodology.22
The indium tin oxide (ITO) coated glass substrates with sheet resistance of 15
□ and light transmittance greater than 84% were purchased from Luminescence
Ω
̸
Technology Corporation. The hole injection material poly(3,4-ethylene-
dioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) with purity of 99.9% was
purchased from Bayer Taiwan Company. The electron confining/hole transporting
materials di-[4-(N,N-ditolylamino)-phenyl]cyclohexane (TAPC), and an electron
transporting material 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene (TPBi) were
purchased from e-Ray optoelectronic co. ltd. A typical green emitter tris(2-phenyl-
pyridine) iridium (Ir(ppy)3 was purchased from Luminescence Technology Corporation.
Lithium fluoride (LiF) (purity 99.95%) was purchased from Strem Chemicals
Corporation. Aluminum (Al) ingots (99.999%) were purchased from Showa Chemical
Co. Ltd. All materials were used without further purification.
4.3 Characterizations and measurements
Column chromatography purifications were performed with silica gel (70-230 mesh) as
a stationary phase in a column with 50 cm long and 5 cm diameter. 1H NMR spectra
were recorded using a Varian Unity Inova (300 MHz) apparatus. Mass spectra of the
compounds were obtained on a Waters ZQ 2000 spectrometer in the positive ion mode.
The PL spectra were recorded in THF at room temperature in quartz cuvettes using a

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Fluorolog III photoluminescence spectrometer. UV-vis spectra were also recorded in
THF at room temperature using a UV-Visible spectrophotometer.
Cyclic voltammetry (CV) experiments were performed on an electrochemical
workstation using a three electrode assembly comprising glassy carbon working
electrode, a non-aqueous Ag/AgCl reference electrode and an auxiliary platinum
electrode. The experiments were performed at room temperature under nitrogen
atmosphere in dichloromethane using 0.1 M tetrabutylammonium perchlorate
(Bu4NClO4) as supporting electrolyte on a Chinstruments CH1604A potentiostat. The
ox
a
c
a
c
E1/2 values were determined as (Ep + Ep )/2, where Ep and Ep are the anodic and
cathodic peak potentials, respectively.
Differential scanning calorimetry (DSC) measurements were carried out using a Bruker
Reflex II thermosystem. Thermogravimetric analysis (TGA) was performed on a
Netzsch STA 409. TGA and DSC curves were recorded in a nitrogen atmosphere at a
heating rate of 10 oC min.-1.
4.4 Device fabrication and characterization
4.4.1 Wet-processed
Fig. 3 (a) shows the schematic energy-level diagram of green OLEDs studied herein.
The fabrication process included firstly spin-coating an aqueous solution of
PEDOT:PSS at 4,000 rpm for 20 s to form a hole-injection layer (HIL) on a pre-cleaned

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ITO anode. Before depositing the following emissive layer, the solution was prepared
by dissolving the Ir(ppy)3 guest in three different novel host materials, 4, 5 and 6, in
THF at room-temperature for 0.5 h with stirring. The resulting solution was then spin-
coated at 2,500 rpm for 20 s under nitrogen. Followed were the depositions of an
electron-transporting layer TPBi, an electron injection layer LiF, and a cathode Al, by
thermal evaporation in a vacuum chamber at the vacuum level of less than 5 × 10−6 Torr.
4.4.2 Dry-processed
In a dry-processed OLED devices (Fig. 3 (a) and (b)), a 32 nm HIL of PEDOT:PSS
was spin coated on a pre-cleaned ITO anode. Subsequently, an optional 20 nm hole
transporting layer (HTL) of TAPC, a 20 nm EML, a 35 nm ETL of TPBi, a 1 nm EIL
of LiF, and a 100 nm Al cathode, were deposited by thermal evaporation in a vacuum
chamber at the ultra-high vacuum condition. The dry-process EML source was prepared
by the solution pre-mixing46 of the Ir(ppy)3 guest in the three different host materials,
4, 5, and 6.
4.4.3 Characterization
The luminance, CIE chromatic coordinates, and electroluminescent spectrum of the
resultant green OLEDs were measured by using Photo Research PR-655 spectrascan.
Keithley 2400 electrometer was used to measure the current-voltage (I–V)
characteristics. The emission area of the devices was 25 mm2, and only the luminance

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in the forward direction was measured.
Acknowledgements
This work was financially supported by Ministry of Science and Technology through
the grant numbers of 104-2119-M-007 -012- and 103-2923-E-007 -003 -MY3, Ministry
of Economic Affairs through the grant number MEA 102-EC-17-A-07-S1-181 and by
Research Council of Lithuania through the grant number of TAPLLT1/2014.
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