Wet-process feasible novel carbazole-type molecular host for high efficiency phosphorescent organic light emitting diodes

Article abstract of DOI:10.1039/c4tc01423e

Wet-process organic light-emitting diodes (OLEDs) are crucial to realize cost-effective and large-area roll-to-roll fabrication of high quality displays and lightings. In this study, a wet-process feasible carbazole based host material, 2-[4-(carbazol-9-yl)butyloxy]-9-[4-(carbazol-9-yl)butyl]carbazole (6), is synthesized, and two other carbazole hosts, 2-[5-(carbazol-9-yl)pentyloxy]-9-[5-(carbazol-9-yl)pentyl]carbazole (7) and 2-[6-(carbazol-9-yl)hexyloxy]-9-[6-(carbazol-9-yl)hexyl]carbazole (8) are also synthesized for comparison. All the three host materials exhibit high triplet energy, and possess high solubility in common organic solvents at room temperature. On doping a green phosphorescent emitter fac tris(2-phenylpyridine)iridium (Ir(ppy)₃) into host 6, the device shows an efficacy of 51 lm W^-1 and a current efficiency of 52 cd A^-1 at 100 cd m^-2 or 30 lm W^-1 and 40.7 cd A^-1 at 1000 cd m^-2. The high efficiency may be attributed to the host possessing an effective host-to-guest energy transfer, the ability for excitons to generate on both host and guest, and excellent film integrity.

Full text of DOI:10.1039/c4tc01423e

Journal of
Materials Chemistry C
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PAPER
Wet-process feasible novel carbazole-type
molecular host for high efficiency phosphorescent
organic light emitting diodes†
Cite this: J. Mater. Chem. C, 2014, 2,
707
8
a
a
b
a
a
Jwo-Huei Jou,* Sudhir Kumar, Daiva Tavgeniene, Chih-Chia An, Po-Hsun Fang,
b
b
b
Ernestas Zaleckas, Juozas V. Grazulevicius and Saulius Grigalevicius*
Wet-process organic light-emitting diodes (OLEDs) are crucial to realize cost-effective and large-area roll-to-
roll fabrication of high quality displays and lightings. In this study, a wet-process feasible carbazole based host
material, 2-[4-(carbazol-9-yl)butyloxy]-9-[4-(carbazol-9-yl)butyl]carbazole (6), is synthesized, and two other
carbazole hosts, 2-[5-(carbazol-9-yl)pentyloxy]-9-[5-(carbazol-9-yl)pentyl]carbazole (7) and 2-[6-(carbazol-
9-yl)hexyloxy]-9-[6-(carbazol-9-yl)hexyl]carbazole (8) are also synthesized for comparison. All the three host
materials exhibit high triplet energy, and possess high solubility in common organic solvents at room
temperature. On doping a green phosphorescent emitter fac tris(2-phenylpyridine)iridium (Ir(ppy)
3
) into
ꢀ2
Received 2nd July 2014
Accepted 22nd August 2014
ꢀ1
ꢀ1
host 6, the device shows an efficacy of 51 lm W and a current efficiency of 52 cd A at 100 cd m or
ꢀ
1
ꢀ1
ꢀ2
3
0 lm W and 40.7 cd A at 1000 cd m . The high efficiency may be attributed to the host possessing
DOI: 10.1039/c4tc01423e
an effective host-to-guest energy transfer, the ability for excitons to generate on both host and guest, and
excellent film integrity.
www.rsc.org/MaterialsC
comparatively higher cost due to the huge wastage of organic
materials in the chamber itself. To make the resultant products
1
. Introduction
Organic light emitting diode (OLED) is a potential technology to highly cost-effective and large-area roll-to-roll fabrication,
1–3
realize high quality displays and solid state lightings. Nowa- solution-processable OLEDs with higher efficiencies are in
1
5–19
days, a wide range of OLED based portable display products and extreme demand.
4,5
some large size displays have already been in the market.
Recently, phosphorescent OLEDs have drawn considerable terparts,
attention because of their ability to harvest both singlet and phorescent OLEDs have rarely been reported.
triplet excitons simultaneously through intersystem crossing, and co-workers reported a maximum efficacy of 4.4 lm
Over the past years, compared with vapor-deposited coun-
2
0–23
wet-process feasible molecular hosts for phos-
1
2, 24–32
Kakimoto
ꢀ
1
6–9
approaching near 100% internal quantum efficiency.
An
W
by doping an Ir(ppy)
3
guest into triphenylamine/benz-
0
appropriate molecular host material is required to minimize imidazole hybrid as host, tris(2-methyl-3 -(1-phenyl-1H-benzi-
concentration quenching effects and triplet–triplet annihilation midazol-2-yl)biphenyl-4-yl)amine. A maximum efficacy of 7.3
3
1
ꢀ
1
10
in an undoped phosphorescent emitter A high-triplet energy lm
W
was realized by doping
a
bipolar host,
0
0
molecular host material is crucial because it can be used to tris(2,2 -dimethyl-4 -(1-phenyl-1H-benzimidazol-2-yl)biphenyl-
conne triplet excitons on the emitter and balance carrier 4-yl)amine with Ir(ppy)
3
2
3
guest. Kido and co-workers reported
ꢀ
1
ꢀ2
11–14
injection.
an efficacy of 11 lm W
at 100 cd m
by employing a
OLED devices can be fabricated by the thermal evaporation carbazole-type host, 1,3-bis(3-(3,6-di-n-butylcarbazol-9-yl)
and spin-coating deposition of organic molecular materials. phenyl)benzene, and tris(2-(4-tolyl)phenylpyridine)iridium(III)
3
3
Thermal evaporation appears to be a successful approach to as guest. Jou and Grigalevicius's groups reported bipolar
0
achieve high efficiency with small molecular hosts. However, it carbazole/phenylindole hybrid as host, 9,9 -bis[6-(carbazol-9-
0
ꢀ1
is limited due to numerous issues such as condition of high yl)hexyl][3,3 ]bicarbazole, and an efficacy of 16.4 lm W was
3
4
3
thermal stability of organic molecules, low throughput and realized by using Ir(ppy) as guest. Ma and Yang's groups
ꢀ
1
achieved an efficacy of 26 lm W by using a oxadiazole/tri-
phenylamine hybrid, 2-(3,5-bis(4 -(diphenylamino)phenyl)
0
a
35
Department of Materials Science and Engineering, National Tsing-Hua University,
phenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole,
as
host.
Hsin-Chu-30013, Taiwan, Republic Of China. E-mail: jjou@mx.nthu.edu.tw
Hence, synthesizing a wet-process feasible molecular host is
greatly considerable to realize low cost, large-area, high
throughput and high performance OLED based products.
b
Department of Polymer Chemistry and Technology, Kaunas University of Technology,
Radvilenu plentas 19, LT50254, Kaunas, Lithuania. E-mail: saulius.grigalevicius@ktu.lt
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c4tc01423e
1
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Scheme 1 Schematic illustration of the synthesis of the carbazole-type hosts, 6, 7, and 8.
Table 1 Photophysical and electrochemical characteristics of the novel
host 6, compared with those of the small molecular hosts 7 and 8
a
b
c
d
e
f
l
ab
l
em
E
T
E
S
HOMO
LUMO
[eV]
T
g
T
d
ꢁ ꢁ
Host [nm] [nm]
[eV] [eV] [eV]
[ C] [ C]
6
7
8
345
345
345
356, 368 2.95 3.54 5.48
1.95
1.95
1.96
64
58
20
361
345
338
368
369
2.95 3.54 5.48
2.96 3.52 5.47
a
b
c
Triplet-energy level at 77 K. Singlet energy-gap. HOMO values are
measured by the cyclic voltammetry (CV) method. The semi-oxidation
ox
potential (E1/2) could be 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
ox
energy of HOMO could be obtained from the EHOMO ¼ ꢀ(E1/2 + 4.8).
d
The energy of LUMO could be obtained by subtracting the optical
bandgap from the HOMO energy level, [ELUMO ¼ (EHOMO ꢀ E
g
)].
e
f
Glass transition temperature. Thermal decomposition temperature.
In this study, a high-efficiency, green phosphorescent OLED
fabricated by employing a wet-process feasible newly synthe-
sized carbazole-type host material, 2-[4-(carbazol-9-yl)butyloxy]-
9
-[4-(carbazol-9-yl)butyl]carbazole (6) that possesses high
triplet-energy and low carrier injection barrier is presented.
In the device doped with Ir(ppy) guest, the host 6 exhibits a
power efficiency of 51 lm W and a current efficiency of 52 cd
3
ꢀ
1
ꢀ2
ꢀ2
A
at 100 cd m . For comparison, two other hosts, 2-[5-(car-
bazol-9-yl)pentyloxy]-9-[5-(carbazol-9-yl)pentyl]carbazole (7) and
-[6-(carbazol-9-yl)hexyloxy]-9-[6-(carbazol-9-yl)hexyl]carbazole
8), were also studied. For host 7 containing device, its power
2
(
ꢀ
1
ꢀ1
ꢀ1
efficiency was 29.1 lm W (37.5 cd A ), while 23.3 lm W
ꢀ1
(35.9 cd A ) for the compound 8-containing counterpart.
2. Result and discussion
2
.1 Synthesis of the host materials
Carbazole-type host materials, 2-[4-(carbazol-9-yl)butyloxy]-9-[4-
carbazol-9-yl)buty]carbazole (6), 2-[5-(carbazol-9-yl)pentyloxy]-
-[5-(carbazol-9-yl)pentyl]carbazole (7) and 2-[6-(carbazol-9-yl)
(
9
hexyloxy]-9-[6-(carbazol-9-yl)hexyl]carbazole (8), were synthe-
sized using rather simple alkylation methods, as shown in
Scheme 1. The key starting materials, 9-(bromoalkyl)carbazoles
(
(
2–4) were prepared from commercially available 9H-carbazole
1) and an excess of corresponding dibromoalkane under basic
Fig. 1 Ultraviolet-visible and photoluminescence spectra of the novel
carbazole-type host materials, (a) 6, (b) 7, and (c) 8. All the data were
measured in tetrahydrofuran at room temperature.
conditions using tetra-n-butyl ammonium hydrogen sulphate
TBAHS) as a phase transfer catalyst. The compounds 2–4 were
(
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reacted with 2-hydroxycarbazole (5) under basic conditions to extensively higher than that of the green emitter, Ir(ppy)
3
, which
36,37
afford the carbazolyl containing derivatives 6–8 as host mate- exhibited a triplet-energy value of the 2.57 eV.
These host
1
rials. The newly synthesized derivatives were conrmed by H materials should enable the occurrence of effective energy
NMR spectroscopy and mass spectrometry (ESI Fig. S1–S3†). transfer from host-to-guest and exciton connement on guest,
12, 38–40
The data were found to be in good agreement with the proposed resulting in good device efficiency.
Hence, these values of
structures. The derivatives were soluble in common organic triplet energies are sufficiently higher to effectively conne the
solvents such as acetone, chloroform or THF at room triplet excitons on the guest and extensively prohibit back
temperature.
energy transfer to the hosts. Both photophysical and electro-
chemical properties of all the three hosts remain almost
unchanged by increasing the length of alkyl and alkyl ether
linkages.
2.2 Photophysical and electrochemical characteristics
Table 1 shows the photophysical properties of the three carba-
The electrochemical properties of the three carbazole-type
zole based host materials. Fig. 1 shows the ultraviolet-visible host molecules 6–8 were measured by cyclic voltammetry (ESI
UV-vis) and photoluminescence (PL) spectra of the materials 6– Fig. S4†). The highest occupied molecular orbital (HOMO)
dissolved in tetrahydrofuran (THF) at room temperature. The energy levels were estimated to be 5.48 eV, 5.48 eV, and 5.47 eV
(
8
singlet energy gaps were calculated from the intersection of UV- for 6, 7, and 8, respectively, using oxidation potential. The
vis absorption peaks, giving values of 3.54, 3.54 and 3.52 eV for lowest unoccupied molecular orbital (LUMO) energy levels of
compounds 6, 7 and 8, respectively.
the emitters were calculated to be 1.95 eV, 1.95 eV, and 1.96 eV
The experimentally determined triplet-energies were 2.95 eV, for 6, 7, and 8, respectively, from HOMO energy levels and
.95 eV, and 2.96 eV for the materials 6, 7 and 8, respectively,
2
which were calculated from the rst phosphorescent emission
peak of low temperature (77 K) PL spectra at 420 nm, 420 nm
and 419 nm, respectively, as shown in Fig. 2(a). These results
show that the triplet-energies of these three host materials are
Fig. 2 Photoluminescence (PL) spectra of the newly synthesized hosts
6
, 7, and 8 measured in tetrahydrofuran (a) at 77 K and (b) at room
temperature. The ultraviolet-visible (UV-vis) spectrum of the green
Ir(ppy) guest molecule is also shown. The comparatively large over-
3
lapping area between the 6 host PL and the green emitter UV-vis Fig. 3 Surface morphologies of the spin-coated films that comprise
spectra indicates a higher efficient host to guest energy transfer the pure hosts (a) 6, (b) 7, and (c) 8. All the films uniformities are
occurring in the 6-containing green OLED.
g
improved upon increasing the glass transition temperature (T ).
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ꢁ
ꢁ
optical energy band gaps, which were estimated from the of 361 C, corresponding to a 5% weight loss, while 345 C for 7
ꢁ
intersection of absorption and emission spectra (Table 1).
and 338 C for 8. The higher T and T characteristic of the host
g
d
6
facilitated relatively better lm integrity during the entire
41,42
2
.3 Thermal and morphological characteristics
As measured using differential scanning calorimetry (DSC), the
host molecule 6 showed a glass transition temperature (T ) of
4 C, 58 C for 7 and 20 C for 8 (Table 1). The slightly higher T
of the host 6 indicated a strong intermolecular interaction of
carbazole units due to short chain alkyl ether and alkyl junc-
tions. As investigated using thermogravimetric analysis (TGA),
fabrication process, especially during solvent removal.
Fig. 3 shows the atomic-force microscopy (AFM) images of
the compounds, 6, 7, and 8 containing lms by spin-coating.
The respective surface roughness values are 0.41, 0.57 and 0.75
g
ꢁ
ꢁ
ꢁ
6
g
nm, while the corresponding glass transition temperatures (T
are 65, 58 and 20 C. The surface roughness became smoother
g
)
ꢁ
as the T of the host material was increased. The relatively better
g

lm integrity may explain why compound 7 containing devices
d
the host 6 exhibited a thermal decomposition temperature (T )
Fig. 4 Schematic diagram of the energy-levels of the OLED device containing the presented host materials, 6, 7, and 8. Molecular structures of
the three studied hosts are also shown.
3 p c
Table 2 Effects of the different doping concentrations of Ir(ppy) on the operation voltage (OV), power efficiency (h ), current efficiency (h ),
external quantum efficiency (hext) and CIE coordinates of the novel host materials 6, 7, and 8-containing OLED devices
ꢀ2
@
100/1000 cd m
Wt%
ratio of dopant
Maximum luminance
ꢀ1
ꢀ1
ꢀ2
Host
OV [V]
h
p
[lm W
]
h
c
[cd A
]
hext [%]
1931 CIE coordinates
[cd m
]
6
5
4.7/6.0
4.0/5.2
3.6/4.7
3.5/4.5
3.2/4.3
3.0/3.9
6.9/8.5
6.4/8.1
4.0/5.5
3.7/4.9
3.4/4.4
3.1/4.1
3.5/4.6
4.6/6.0
4.3/5.6
3.8/4.9
3.5/5.1
3.7/5.3
22.7/14.1
33.8/21.5
36.3/23.2
44.6/26.9
50.8/30.3
37.9/28.3
25.4/14.3
27.7/16.2
32.1/18.5
39.5/24.2
38.0/25.2
42.5/29.1
25.2/15.9
22.3/13.0
31.1/16.2
38.1/23.3
39.0/21.4
30.4/18.7
34.0/26.7
43.3/35.2
41.8/34.5
49.8/38.7
51.8/40.7
36.7/34.5
55.6/38.8
56.3/41.8
40.8/32.4
46.7/38.1
40.8/35.1
42.2/37.5
28.4/23.1
32.4/25.1
41.9/28.8
46.0/35.9
43.9/34.6
36.1/31.1
9.5/7.4
(0.29, 0.63)/(0.29, 0.63)
(0.30, 0.63)/(0.30, 0.63)
(0.32, 0.62)/(0.32, 0.62)
(0.31, 0.63)/(0.31, 0.63)
(0.31, 0.63)/(0.31, 0.63)
(0.33, 0.62)/(0.32, 0.62)
(0.30, 0.63)/(0.30, 0.63)
(0.31, 0.63)/(0.31, 0.63)
(0.32, 0.62)/(0.32, 0.62)
(0.32, 0.62)/(0.32, 0.62)
(0.33, 0.62)/(0.33, 0.62)
(0.33, 0.62)/(0.33, 0.62)
(0.30, 0.63)/(0.30, 0.63)
(0.30, 0.63)/(0.30, 0.63)
(0.30, 0.63)/(0.30, 0.63)
(0.31, 0.63)/(0.31, 0.63)
(0.33, 0.62)/(0.32, 0.62)
(0.32, 0.62)/(0.32, 0.62)
7899
1
1
2
2
3
5
1
1
2
2
3
5
1
1
2
3
3
0
5
0
5
0
11.6/9.5
11.5/9.5
13.6/10.6
14.1/11.1
10.1/9.5
7.5/6.0
11 010
13 800
11 250
12 600
14 510
5761
8447
8954
12 750
15 730
17 810
6391
7767
9028
10 830
9252
7194
7
8
0
5
0
5
0
7.8/6.5
10.6/7.3
12.0/9.2
12.2/9.6
9.9/8.6
15.1/10.3
14.8/11.3
10.6/8.9
13/10.4
11.3/9.8
10.6/10.1
0
5
0
0
5
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showed better efficiency than the compound 8 containing
counterpart.
2.4 Electroluminescent characteristics of devices
Fig. 4 illustrates the schematic energy-level diagram of green
OLED devices and the molecular structures of the newly
synthesized host materials, 6, 7, and 8. The devices are
composed of a 125 nm indium tin oxide (ITO) anode layer, a 35
nm poly(3,4-ethylene-dioxythiophene)-poly(styrenesulfonate)
(PEDOT:PSS) hole injection layer (HIL), a 20 nm single emissive
layer (EML) with the Ir(ppy) emitter doped in compound 6 host
3
via spin-coating, a 32 nm 1,3,5-tris(N-phenyl-benzimidazol-2-yl)
benzene (TPBi) electron transporting layer (ETL), a 1 nm
lithium uoride (LiF) layer, and a 100 nm aluminum (Al)
cathode layer. In addition, the host 6, two other compounds, 7
and 8, were also studied for comparison.
Table 2 shows the electroluminescent characteristics of the
host 6 containing green OLED compared with that of the 7 and
8
-containing counterparts. As shown in Fig. 5, host 6 containing
ꢀ1
device exhibited a power efficiency of 30.3 lm W
current efficiency of 40.7 cd A at 1000 cd m , which is the
highest among all the three host containing devices. For host 7
containing device, its power efficiency was 29.1 lm W (37.5 cd
A
with a
ꢀ
1
ꢀ2
ꢀ
1
ꢀ1
ꢀ1
ꢀ1
), while 23.3 lm W (35.9 cd A ) for the host 8 containing
counterpart.
As illustrated in Fig. 4, the architectures of the three hosts, 6,
7, and 8, containing device signicantly favors the injection of
holes to the hosts because they exhibit a hole injection barrier of
around 0.58 eV, which is 0.12 eV lower than that of the hole to
the guest (0.7 eV). While, the same architectures favor the
3
injection of electron into the Ir(ppy) guest because there exists
a ꢀ0.2 eV electron trap, while there is an around 0.75 eV barrier
for electron to enter into the hosts. Hence, these would lead
excitons to generate on both host and guest and result in high
19
device efficiency.
Compound 6 containing device showed a highest efficiency.
EL properties, including power efficiency, current efficiency and
EQE, were different for the devices containing the three
compounds 6, 7 and 8 because of the difference in the effec-
tiveness of host-to-guest energy transfer. As shown in Fig. 2(b),
the overlapping area between the absorption spectrum of guest
(
UV-vis of Ir(ppy)
the largest for compound 6, indicating a comparatively higher
host-to-guest energy transfer. Furthermore, compound
3
) and the emission spectrum of host (PL) was
6
showed an additional emission peaking at the lower wavelength
site (356 nm), which would trigger the higher energy emission
of the guest. This explains why the resultant electroluminescent
(
EL) spectrum of the green Ir(ppy)
blue-shied as compared with those into compounds 7 and 8 as
host, as shown in Fig. 6. There is only guest, Ir(ppy) , emission
3
doped into compound 6 was
3
Fig. 5 Comparison of (a) power efficiency, (b) current efficiency, (c)
current density, and (d) luminance of the devices with the 25 wt% ratio
Ir(ppy) doped in the host materials, 6, 7 and 8.
3
without any emission from hosts. This shows that the energy
was effectively transferred from the host to the guest.
As shown in Fig. 7, the device efficiency extensively depends
on the doping concentration of the green emitter. Taking the
host 6-based device for example, the power efficiency at 1000 cd
ꢀ2
ꢀ1
m
increased from 14.1 to 30.3 lm
W
as doping
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Fig. 6 Host effect on the electroluminescence (EL) spectra of the
devices containing three different host molecules, 6, 7 and 8, with 25 wt%
3
ratio green emitter, Ir(ppy) . The EL emission spectrum becomes slightly
broader as the host material is changed from 6 to 7 or 8, while the spectra
ꢀ
2
remain unchanged even at a higher luminance, 1000 cd m
.
concentration was increased from 5 to 25 wt%. The resulting
device showed the highest efficiency among all the studied
concentrations as 25 wt% guest was doped into the host 6.
However, as the concentration increased to 30 wt%, the power
efficiency started to decrease. This may be attributed to the
concentration-quenching efficiency roll-off, resulting from the
self-segregation of the emitter at high concentrations.
When material 8 was employed as the host, the resultant
ꢀ
1
ꢀ1
device exhibited a power efficiency of 15.9 lm W (23.1 cd A
)
ꢀ2
at 1000 cd m with a 5 wt% doping concentration of Ir(ppy)
3
guest. The efficiency becomes higher as the dopant concentra-
tion increased from 5 to 20 wt%. At 20 wt%, for example, the
ꢀ
1
ꢀ1
power efficiency is 23.3 lm W (35.9 cd A ). As the dopant
concentration increased further to 35 wt%, the efficacy reduces
ꢀ
1
ꢀ1
to 18.7 lm W (31.1 cd A ). Host 7 containing device showed
ꢀ
1
ꢀ1
the best efficacy of 29.1 lm W (35.9 cd A ) with a 30 wt%
doping concentration of Ir(ppy) . Compared to that of the
counterpart 8, the higher efficiency exhibited by the host 7
3
containing device may result mainly from the fact that material Fig. 7 Doping concentration (weight ratio with host) effects on the (a)
ꢁ
power efficiency, (b) current efficiency, (c) current density, and (d)
luminance results of the 6 host based green OLED device.
8
6
showed a poor lm integrity due to much lower T of 20 C.
g
Furthermore, we investigated the effect of all the three hosts,
, 7 and 8 devices operational stability, t , based on solution-
5
0
process feasible emissive layer. The operational lifetime of all
the devices was investigated without encapsulation. As
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ꢀ
2
measured at an initial brightness of 800 cd m , the resultant to form a hole-injection layer (HIL) on a pre-cleaned ITO anode.
devices have shown the operational stability of 27, 12 and 5.4 Before depositing the following emissive layer, the solution was
min, respectively, for compounds 6, 7 and 8 (ESI Fig. S5†).
prepared by dissolving the Ir(ppy) guest in three different novel
3
host molecules, 6, 7 and 8, in tetrahydrofuran at room-
temperature for 0.5 h with stirring. The resulting solution was
then spin-coated at 2500 rpm for 20 s under nitrogen. There-
aer, the electron-transporting layer TPBi, the electron injec-
tion layer LiF, and the cathode Al, were deposited by thermal
evaporation in a vacuum chamber at the vacuum level of less
3
. Conclusion
To conclude, in this report we demonstrated a molecular
carbazole-based host 6 with wet-process feasibility. By employ-
ing this host, green phosphorescent OLED devices with higher
efficiencies have been fabricated. The resulting device shows at
ꢀ
6
than 5 ꢂ 10 Torr.
ꢀ2
ꢀ1
The luminance, CIE chromatic coordinates, and electrolu-
minescent spectrum of the resultant green OLEDs were
measured by using Photo Research PR-655 spectrascan. Keith-
ley 2400 electrometer was used to measure the current–voltage
100 cd m , for example, a power efficiency of 51 lm W and a
ꢀ1
current efficiency of 52 cd A for a simple double layer green
phosphorescent OLED device. High efficiency may be attributed
to the host possessing an effective host-to-guest energy transfer.
The facile synthesis, excellent solubility in common organic
solvents, excellent lm integrity and very high triplet energy
come together to ensure that compound 6 can be a promising
host material for the low cost and large-area roll-to-roll fabri-
cation of energy-efficient phosphorescent OLEDs.
(
I–V) characteristics. The emission area of the devices was 25
2
mm , and only the luminance in the forward direction was
measured.
Acknowledgements
This work was nancially supported by National Science
Council through the grant numbers of 100-2119-M-007-011-
MY3 and 103-2923-E-007-003-MY3, Ministry of Economic Affairs
4
. Experimental methods
4.1 Materials characteristics and measurements
All the precursor compounds required for the synthesis were through the grant number MEA 102-EC-17-A-07-S1-181 and by
purchased from commercial sources and used without any Research Council of Lithuania (project TAPLLT1/14).
purication. Column chromatography purications were per-
formed with silica gel (70–230 mesh) as a stationary phase in a
1
column 50 cm long and 5 cm in diameter. H NMR spectra were References
recorded using a Varian Unity Inova (300 MHz) apparatus. Mass
1
2
F. So, J. Kido and P. Burrows, MRS Bull., 2008, 33, 663.
B. W. D'Andrade and S. R. Forrest, Adv. Mater., 2004, 16,
spectra of the compounds were obtained on a Waters ZQ 2000
spectrometer in the positive ion mode. EL spectra were recorded
in tetrahydrofuran at room temperature in quartz cuvettes
using a Fluorolog III photoluminescence spectrometer. UV-vis
spectra were recorded in toluene at room temperature using a
UV-vis 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 tetrabu-
1
585.
M. C. Gather, A. Kohnen and K. Meerholz, Adv. Mater., 2011,
3, 233.
3
2
4
5
S. R. Forrest, Nature, 2004, 428, 911.
S. Reineke, F. Lindner, G. Schwartz, N. Seidler, K. Walzer,
B. Lussem and K. Leo, Nature, 2009, 459, 234.
M. A. Baldo, D. F. O'Brien, Y. You, A. Shoustikov, S. Sibley,
M. E. Thompson and S. R. Forrest, Nature, 1998, 395, 151.
C. Adachi, M. A. Baldo and S. R. Forrest, Appl. Phys. Lett.,
6
7
8
2000, 77, 904.
J. P. Duan, P. P. Sun and C. H. Cheng, Adv. Mater., 2003, 15,
224.
tylammonium perchlorate (Bu
4 4
NClO ) as supporting electrolyte
ox
on a Chinstruments CH1604A potentiostat. The E1/2 values were
a
p
c
p
a
c
p
9 Q. Zhang, Q. Zhou, Y. Cheng, L. Wang, D. Ma and X. Jing,
determined as (E + E )/2, where E and E are the anodic and
p
Adv. Mater., 2004, 16, 432.
cathodic peak potentials, respectively. Differential scanning
calorimetry (DSC) measurements were carried out using a
Bruker Reex II thermosystem. The DSC curves were recorded
1
0 S.-J. Yeh, M.-F. Wu, C.-T. Chen, Y.-H. Song, Y. Chi, M.-H. Ho,
S.-F. Hsu and C.-H. Chen, Adv. Mater., 2005, 17, 285.
11 R. J. Holmes, S. R. Forrest, Y.-J. Tung, R. C. Kwong,
J. J. Brown, S. Garon and M. E. Thompson, Appl. Phys.
Lett., 2003, 82, 2422.
ꢁ
ꢀ1
in a nitrogen atmosphere at a heating rate of 10 C min
.
Thermogravimetric analysis (TGA) was performed under a
continuous nitrogen ow using a TGA Q50 apparatus at a
ꢁ
ꢀ1
12 S. Tokito, T. Iijima, Y. Suzuri, H. Kita, T. Tsuzuki and F. Sato,
Appl. Phys. Lett., 2003, 83, 569.
heating rate of 10 C min
.
1
3 S.-J. Su, H. Sasabe, T. Takeda and J. Kido, Chem. Mater., 2008,
4.2 Device fabrication and characterization
20, 1691.
Fig. 4 shows the schematic energy-level diagram of green OLEDs 14 S.-J. Su, C. Cai and J. Kido, Chem. Mater., 2011, 23, 274.
studied here. The fabrication process included rst spin- 15 J. S. Chen, C. S. Shi, Q. Fu, F. C. Zhao, Y. Hu, Y. L. Feng and
coating an aqueous solution of PEDOT:PSS at 4000 rpm for 20 s
D. G. Ma, J. Mater. Chem., 2012, 22, 5164.
This journal is © The Royal Society of Chemistry 2014
J. Mater. Chem. C, 2014, 2, 8707–8714 | 8713

View Article Online
Journal of Materials Chemistry C
Paper
16 T.-W. Lee, T. Noh, H.-W. Shin, O. Kwon, J.-J. Park, B.-K. Choi, 29 Y. Qian, F. Cao and W. Guo, Tetrahedron, 2013, 69, 4169.
M.-S. Kim, D. W. Shin and Y.-R. Kim, Adv. Mater., 2009, 19, 30 P. I. Shih, C. L. Chiang, A. K. Dixit, C. K. Chen, M. C. Yuan,
1
625.
R. Y. Lee, C. T. Chen, E. W. G. Diau and C. F. Shu, Org. Lett.,
2006, 8, 2799.
1
7 J.-H. Jou, M.-F. Hsu, W.-B. Wang, C.-L. Chin, Y.-C. Chung,
C.-T. Chen, J.-J. Shyue, S.-M. Shen, M.-H. Wu, W.-C. Chang, 31 Z. Ge, T. Hayakawa, S. Ando, M. Ueda, T. Akiike,
C.-P. Liu, S.-Z. Chen and H.-Y. Chen, Chem. Mater., 2009,
1, 2565.
H. Miyamoto, T. Kajita and M. Kakimoto, Chem. Mater.,
2008, 20, 2532.
2
18 J.-H. Jou, C.-J. Li, S.-M. Shen, S.-H. Peng, Y.-L. Chen, 32 Z. Ge, T. Hayakawa, S. Ando, M. Ueda, T. Akiike,
Y.-C. Jou, J. H. Hong, C.-L. Chin, J.-J. Shyue, S.-P. Chen,
J.-Y. Li, P.-H. Wang and C.-C. Chen, J. Mater. Chem. C,
H. Miyamoto, T. Kajita and M. Kakimoto, Adv. Funct.
Mater., 2008, 18, 584.
2
013, 1, 4201.
33 N. Aizawa, Y.-J. Pu, H. Sasabe and J. Kido, Org. Electron.,
2012, 13, 2235.
1
9 J.-H. Jou, Y.-M. Yang, S.-Z. Chen, J.-R. Tseng, S.-H. Peng,
C.-Y. Hsieh, Y.-X. Lin, C.-L. Chin, J.-J. Shyue, S.-S. Sun, 34 D. Mazetyte, G. Krucaite, J. Grazulevicius, C.-I. Chiang,
C.-T. Chen, C.-W. Wang, C.-C. Chen, S.-H. Lai and
F.-C. Tung, Adv. Opt. Mater., 2013, 1, 657.
F.-C. Yang, J.-H. Jou and S. Grigalevicius, Opt. Mater., 2013,
35, 604.
2
0 F.-M. Hsu, C.-H. Chien, P.-I. Shih and C.-F. Shu, Chem. 35 M. Zhu, T. Ye, X. He, X. Cao, C. Zhong, D. Ma, J. Qin and
Mater., 2009, 21, 1017. C. Yang, J. Mater. Chem., 2011, 21, 9326.
1 Z.-Q. Gao, M. Luo, X.-H. Sun, H.-L. Tam, M.-S. Wong, 36 N. Li, S.-L. Lai, W. Liu, P. Wang, J. You, C.-S. Lee and Z. Liu, J.
2
B.-X. Mi, P.-F. Xia, K.-W. Cheah and C.-H. Chen, Adv.
Mater, 2009, 21, 688.
2 T. Zhang, Y. Liang, J. Cheng and J. Li, J. Mater. Chem. C, 2013,
Mater. Chem., 2011, 21, 12977.
37 X. Yang, D. Neher, D. Hertel and T. K. D ¨a ubler, Adv. Mater.,
2004, 16, 161–166.
2
2
2
2
2
1
, 757.
38 A. P. Kulkarni, C. J. Tonzola, A. Babel and S. A. Jenekhe,
Chem. Mater., 2004, 16, 4556; G. Hughes and M. R. Bryce,
J. Mater. Chem., 2005, 15, 94.
3 H. Sasabe, T. Chiba, S.-J. Su, Y.-J. Pu, K. Nakayama and
J. Kido, Chem. Commun., 2008, 5821.
4 L. Duan, L. Hou, T.-W. Lee, J. Qiao, D. Zhang, G. Dong, 39 Y. Kawamura, S. Yanagida and S. R. Forrest, J. Appl. Phys.,
L. Wang and Y. Qiu, J. Mater. Chem., 2010, 20, 6392. 2002, 92, 87.
5 L. Zeng, T. Y.-H. Lee, P. B. Merkel and S. H. Chen, J. Mater. 40 P. A. Vecchi, A. B. Padmaperuma, H. Qiao, L. S. Sapochak
Chem., 2009, 19, 8772. and P. E. Burrows, Org. Lett., 2006, 8, 4211.
6 B. W. D'Andrade, M. A. Baldo, C. Adachi, J. Brooks, 41 J.-H. Jou, W.-B. Wang, S.-Z. Chen, J.-J. Shyue, M.-F. Hsu,
M. E. Thompson and S. R. Forrest, Appl. Phys. Lett., 2001,
9, 1045.
C.-W. Lin, S.-M. Shen, C.-J. Wang, C.-P. Liu, C.-T. Chen,
M.-F. Wu and S.-W. Liu, J. Mater. Chem., 2010, 20, 8411.
7
27 C. Adachi, M. A. Baldo, M. E. Thompson and S. R. Forrest, J. 42 J.-H. Jou, W.-B. Wang, S.-M. Shen, S. Kumar, I.-M. Lai,
Appl. Phys., 2001, 90, 5048.
J.-J.
Shyue,
S.
Lengvinaite,
R.
Zostautiene,
28 M. Ikaia, S. Tokito, Y. Sakamoto, T. Suzuki and Y. Taga, Appl.
Phys. Lett., 2001, 79, 156.
J. V. Grazulevicius, S. Grigalevicius, S.-Z. Chen and
C.-C. Wu, J. Mater. Chem., 2011, 21, 9546.
8714 | J. Mater. Chem. C, 2014, 2, 8707–8714
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