3,6-Diaryl substituted 9-alkylcarbazoles as hole transporting materials for various organic light emitting devices

Article abstract of DOI:10.1016/j.dyepig.2014.02.008

Phenyl, naphthyl or biphenyl disubstituted 9-alkylcarbazoles were synthesized by a multi-step synthetic rout. The materials were examined by various techniques including thermogravimetry, differential scanning calorimetry, and electron photoemission spect

Full text of DOI:10.1016/j.dyepig.2014.02.008

Dyes and Pigments 106 (2014) 1e6
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
3,6-Diaryl substituted 9-alkylcarbazoles as hole transporting materials
for various organic light emitting devices
,
G. Krucaite a, D. Tavgeniene a, J.V. Grazulevicius a, Y.C. Wang b, C.Y. Hsieh b, J.H. Jou b
,
*
,
G. Garsva c, S. Grigalevicius a
*
a Department of Organic Technology, Kaunas University of Technology, Radvilenu plentas 19, LT50254 Kaunas, Lithuania
b Department of Materials Science and Engineering, National Tsing-Hua University, No. 101, Kaungfu Rd., Hsin-Chu 30013, Taiwan, ROC
c Vilnius University, Kaunas Faculty, Department of Informatics, Muitines St 8, LT44280 Kaunas, Lithuania
a r t i c l e i n f o
a b s t r a c t
Article history:
Phenyl, naphthyl or biphenyl disubstituted 9-alkylcarbazoles were synthesized by a multi-step synthetic
rout. The materials were examined by various techniques including thermogravimetry, differential
scanning calorimetry, and electron photoemission spectroscopy. These derivatives were tested as hole
transporting materials in OLEDs with Alq3 as the emitter. Some of the devices exhibited promising
overall performance with a turn-on voltage of 2.5 V, a maximal current efficiency of 3.25 cd/A and
maximum brightness of 8300 cd/m2. The compounds were also applied as hole transporting layers in
phosphorescent OLEDs with green phosphorescent emitter of tris(2-phenylpyridine)iridium(III)
dispersed in N,N0-dicarbazolyl-4,40-biphenyl host. An efficient device using 3,6-diphenyl-9-
ethylcarbazole as hole transporting layer demonstrated a turn-on voltage of ca. 5 V, a maximum
brightness of 9800 cd/m2, and maximum current efficiency of 22.5 cd/A.
Received 7 January 2014
Received in revised form
7 February 2014
Accepted 8 February 2014
Available online 25 February 2014
Keywords:
Substituted carbazole
Electro-active material
Amorphous material
Ionization potential
Light emitting diode
Current efficiency
Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction
compounds containing unsubstituted heteroaromatic rings [12]. In
this work, we have designed and synthesized phenyl, naphthyl or
Organic light- emitting diodes (OLEDs) based on organic de-
rivatives have attracted much attention because of their potential
use in flat panel displays and lighting [1e4], however efficient
devices can be obtained only by building multilayer structures
[5,6]. One approach that has been widely used to improve efficiency
of the OLEDs is the appliance of effective hole transporting layers in
the multilayer devices.
biphenyl di-substituted 9-alkylcarbazoles, which were expected to
show enhanced hole injection and transport properties and could
be suitable as hole transporting (HT) materials for multilayer OLEDs.
2. Experimental
2.1. Instrumentation
Carbazole-based polymers and low-molecular-weight de-
rivatives are among the most studied materials for electronic ap-
plications due to high hole mobility in their layers and good
photoconductive properties [7]. Derivatives containing electroni-
cally isolated carbazole rings also have high triplet energies and are
widely used as host materials for electro-phosphorescent OLEDs [8].
We have previously synthesised a series of hole-transporting
carbazole-, [3,30]bicarbazole- and indolo[3,2-b]carbazole-based
derivatives [9e11]. It was observed that aryl substituted derivatives
demonstrate better charge transporting properties than the
1H NMR spectra were recorded using a Varian Unity Inova
(300 MHz) apparatus. Mass spectra were obtained on a Waters ZQ
2000 spectrometer. Differential scanning calorimetry (DSC) mea-
surements were carried out using a Bruker Reflex II thermosystem.
Thermogravimetric analysis (TGA) was performed on a Netzsch STA
409. The TGA and DSC curves were recorded in a nitrogen atmo-
sphere at a heating rate of 10 ꢀC/min.
The ionization potentials of the layers of the compounds syn-
thesized were measured by the electron photoemission method in
air, which was described earlier [13]. The measurement method
was, in principle, similar to that described by Miyamoto et al. [14].
The samples for the ionization potential measurements were pre-
pared as follows [15]. The materials were dissolved in THF and were
* Corresponding authors.
E-mail addresses: jjou@mx.nthu.edu.tw (J.H. Jou), saulius.grigalevicius@ktu.lt
(S. Grigalevicius).
http://dx.doi.org/10.1016/j.dyepig.2014.02.008
0143-7208/Ó 2014 Elsevier Ltd. All rights reserved.

2
G. Krucaite et al. / Dyes and Pigments 106 (2014) 1e6
coated on Al plates pre-coated with w0.5
mm thick methyl-
2.2.2. 3,6-Di(1-naphthyl)-9-(2-ethylhexyl)carbazole (7)
methacrylate and methacrylic acid copolymer (MKM) adhesive
layer. The function of this layer is not only to improve adhesion, but
also to eliminate the electron photoemission from Al layer. In
addition, the MKM layer is conductive enough to avoid charge
accumulation on it during the measurements. The thickness of the
3,6-Diiodo-9-(2-ethylhexyl)carbazole (5) (1 g, 1.8 mmol), 1-
naphthalene boronic acid (0.8 g, 4.6 mmol), PdCl2(PPh3)2 (0.05 g,
0.07 mmol) and powdered potassium hydroxide (0.53 g, 9.4 mmol)
were stirred in THF (15 mL) 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 ace-
tate and hexane (vol. ratio 1:5) as an eluent. Yield: 0.6 g of yellowish
crystals. M.p.: 206 ꢀC (DSC). MS (APCIþ, 20 V): 532.5 ([M þ H],
layers was 0.5e1 mm.
The electroluminescent devices were fabricated on glass sub-
strates and had a typical structure with the organic layers sand-
wiched between a bottom ITO anode and a top metal cathode.
Before use in device fabrication, the ITO-coated glass substrates
were carefully cleaned and treated with UV/ozone immediately
before deposition of the organic layers. The hole transporting layers
were made by spin-coating ca 40 nm layer of the corresponding
material onto the substrates from chloroform solutions. Evapora-
tion of the electroluminescent tris(quinolin-8-olato)aluminium
(Alq3) layer (40 nm) and a LiF/Al electrode (1/150 nm) was per-
formed at a pressure of 1 ꢁ 10ꢂ5 torr in vacuum evaporation
equipment. The size of the emitting areas was 0.25 cm2.
100%). 1H NMR spectrum (CDCl3,
d, ppm): 8.22 (s, 2H, Ar), 8.01e7.84
(m, 6H, Ar), 7.67e7.34 (m, 12H, Ar), 4.31 (d, 2H, NCH2, J ¼ 7.2 Hz),
2.21e2.11 (m, 1H, CH(CH2)3), 1.51e1.28 (m, 8H, 4 ꢁ CH2), 1.04e0.88
(m, 6H, 2 ꢁ CH3).
Elemental analysis for C40H37N % Calc.: C 90.35, H 7.01, N 2.63; %
Found: C 90.38, H 7.04, N 2.61.
The electro-phosphorescent devices were also fabricated on
glass substrates having ITO anode. The device structure used
was ITO/PEDOT:PSS/HTM/CBP:Ir(ppy)3/TPBi/LiF/Al, where the
conducting polymer poly(3,4-ethylenedioxythiophene): poly(-
styrenesulfonate) (PEDOT:PSS) was used as the hole-injection layer
[16], synthesized derivatives 6 and 8 were tested as hole trans-
porting materials (HTM) and N,N0-dicarbazolyl-4,40-biphenyl (CBP)
host doped with the green phosphorescent tris(2-phenylpyridine)
iridium(III) (Ir(ppy)3) was used as an emitting layer. LiF/Al was
applied as the electron-injection layer and cathode,
correspondingly.
The luminance of the fabricated OLEDs was measured using a
Minolta CS-100 luminance-meter. A Keithley 2400 electrometer
was used to measure the currentevoltage characteristics of the
devices. All the measurements were performed at ambient condi-
tions in air.
2.2.3. 3,6-Di(4-biphenyl)-9-(2-ethylhexyl)carbazole (8)
3,6-Diiodo-9-(2-ethylhexyl)carbazole (5) (1 g, 1.8 mmol), 4-
biphenyl boronic acid (0.93 g, 4.6 mmol), PdCl2(PPh3)2 (0.05 g,
0.07 mmol) and powdered potassium hydroxide (0.53 g, 9.4 mmol)
were stirred in THF (15 mL) 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 ace-
tate and hexane (vol. ratio 1:5) as an eluent. Yield: 0.55 g of
yellowish crystals. M.p.: 165 ꢀC (DSC). MS (APCIþ, 20 V): 584.5
([M þ H], 100%). 1H NMR spectrum (CDCl3,
d, ppm): 8.43 (s, 2H, Ar),
7.84e7.38 (m, 22H, Ar), 4.24 (d, 2H, NCH2, J ¼ 7.2 Hz), 2.19e2.09 (m,
1H, CH(CH2)3), 1.51e1.24 (m, 8H, 4 ꢁ CH2), 1.04e0.86 (m, 6H,
2 ꢁ CH3).
Elemental analysis for C44H41N % Calc.: C 90.52, H 7.08, N 2.40; %
Found: C 90.55, H 7.04, N 2.43.
2.2. Materials
3. Results and discussion
9-Ethylcarbazole (1), 9H-carbazole (3), 2-ethylhexylbromide, 1-
naphthalene boronic acid, phenyl boronic acid, 4-biphenyl boronic
The synthesis of aryl-disubstituted 9-alkylcarbazoles (6e8) was
carried out by a multi-step synthetic route as shown in Scheme 1.
3,6-Diiodo-9-ethylcarbazole (2) as a key material was synthesized
from commercially available 9-ethylcarbazole by Tucker iodination
with KI/KIO3 in acetic acid [17]. 3,6-Diiodo-9-(2-ethylhexyl)carba-
zole (5) was prepared by two step procedure involving Tucker
iodination of 9H-carbazole (3) and alkylation of the obtained 3,6-
diiodo-9H-carbazole (4) by using 2-ethylhexylbromide. 3,6-
Diphenyl-9-ethylcarbazole (6) was prepared by Suzuki reaction
[20] of diiodo-derivative 2 with an excess of phenyl boronic acid.
Naphthyl- and 4-biphenyl- substituted derivatives 7 and 8 were
obtained by the Suzuki reaction of the 3,6-diiodo-9-(2-ethylhexyl)
carbazole (5) with an excess of 1-naphthalene boronic acid or 4-
biphenyl boronic acid, correspondingly.
The synthesized derivatives were all identified by mass spec-
trometry, elemental analyses and 1H NMR spectroscopy. The data
were found to be in good agreement with the proposed structures.
The materials are soluble in common organic solvents. Transparent
thin films of these materials could be prepared by spin coating from
solutions or by vacuum evaporation.
acid,
bis(triphenylphosphine)palladium(II)
dichloride
(Pd(PPh3)2Cl2), Alq3, and potassium hydroxide were purchased
from Aldrich and used as received.
3,6-Diiodo-9-ethylcarbazole (2), 3,6-diiodo-9H-carbazole (4)
and 3,6-diiodo-9-(2-ethylhexyl)carbazole (5) were synthesized
according to the procedures outlined in literature [17e19].
2.2.1. 3,6-Diphenyl-9-ethylcarbazole (6)
3,6-Diiodo-9-ethylcarbazole (2) (1.5 g, 3.4 mmol), phenyl
boronic acid (1.2 g, 9.8 mmol), PdCl2(PPh3)2 (0.09, g 0.1 mmol) and
powdered potassium hydroxide (0.94 g, 16.7 mmol) were stirred in
THF (15 mL) 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 1:15) as aþn eluent. Yield: 0.6 g of white crystals.
M.p.: 124 ꢀC (DSC). MS (APCI , 20 V): 348.5 ([M þ H],100%). 1H NMR
spectrum (CDCl3,
d, ppm): 8.37 (s, 2H, Ar), 7.77e7.71 (m, 6H, Ar),
The behaviour under heating of the materials 6e8 was studied
by DSC and TGA under a nitrogen atmosphere. The compounds
demonstrated high thermal stability. The onsets of mass loss were
at 372 ꢀC for 6, 355 ꢀC for 7 and at 404 ꢀC for 8, as confirmed by TGA
with a heating rate of 10 ꢀC/min.
7.51e7.06 (m, 8H, Ar), 4.42 (q, 2H, NCH2, J ¼ 7.5 Hz), 1.49 (t, 3H, CH3,
J ¼ 7.5 Hz).
Elemental analysis for C26H21N % Calc.: C 89.88, H 6.09, N 4.03; %
Found: C 89.85, H 6.11, N 4.06.

G. Krucaite et al. / Dyes and Pigments 106 (2014) 1e6
3
Scheme 1.
All the compounds 6e8 were obtained as crystalline materials
as confirmed by DSC. The thermograms of material 7 are shown in
Fig. 1. When the crystalline sample was heated during the DSC test,
the endothermic peak due to melting was observed at 206 ꢀC.
When the melt sample was cooled down and heated again, the
glass-transition was observed at 57 ꢀC and on further heating no
peaks due to crystallization and melting appeared.
Materials 6 and 8 demonstrated slightly different behaviour in
the DSC measurements. The DSC thermograms of material 8 are
depicted in Fig. 2. The crystalline sample of 8 melted at 165 ꢀC on
first heating and formed glass upon cooling. When the amorphous
sample was heated again, the glass-transition was observed at
62 ꢀC, and on further heating an exothermic peak due to
crystallisation was observed at 131 ꢀC to give the same crystals,
which were obtained from solution and melted at 165 ꢀC. The
crystalline sample of compound 6 demonstrated similar behaviour
as material 8. It melted on first heating at 187 ꢀC, demonstrated
glass transition upon second heating at 50 ꢀC and then crystallized
at 98 ꢀC. The investigations confirmed that the crystalline de-
rivatives 6e8 could be converted into amorphous materials and
used for preparation of thin amorphous layers on substrates. X-ray
scattering analyses also confirmed that thin layers of the de-
rivatives 6e8 are stable in the amorphous state.
The ionization potentials (Ip) of layers of the materials synthe-
sized were measured by the electron photoemission method. The
photoemission spectra of thin amorphous layers of compounds 6e
cooling
cooling
=131 oC
T =57 oC
g
2nd heating
1st heating
T
T
=62 oC
cr
g
2nd heating
1st heating
T
=206 oC
=165 oC
m
T
m
0
40
80
120
160
o
200
240
0
30
60
90
120
150
180
o
Temperature ( C)
Temperature ( C)
Fig. 1. DSC curves of compound 7. Heating rate: 10 ꢀC/min.
Fig. 2. DSC curves of compound 8. Heating rate: 10 ꢀC/min.

4
G. Krucaite et al. / Dyes and Pigments 106 (2014) 1e6
The HT materials 6 and 8, which demonstrated suitable film
forming properties by spin-coating, were also applied as HT layers
in electro-phosphorescent OLEDs of the form: ITO/PEDOT:PSS/6 or
8/CBP:Ir(ppy)3/TPBi/LiF/Al. Emitting layers of the devices consisted
of a green phosphorescent guest of tris(2-phenylpyridine)iridiu-
m(III) (Ir(ppy)3) dispersed in N,N0-dicarbazolyl-4,40-biphenyl (CBP)
host. Formation of the OLEDs is described in the Experimental
Section.
Fig. 3. Electron photoemission spectra of layers prepared using materials 6e8.
8 are presented in Fig. 3. It could be observed that values of Ip of the
newly synthesized compounds depend on aromatic substituents
attached to carbazole core. Biphenyl substituted derivative 8
demonstrated the lowest Ip of 5.55 eV. The layers of phenyl
substituted derivative 6 showed the highest Ip of 5.8 eV. It is
evident that Ip’s of the new compounds 6e8 are slightly lower than
that of derivatives having electronically isolated carbazole rings
(Ip > 5.9 eV) [21,22]. The investigations demonstrate that thin
layers of 6e8 could be suitable for application in optoelectronic
devices as hole transporting materials. Their layers should
demonstrate better hole injecting and transporting properties in
multilayer electroluminescent devices than that of widely used PVK
[21].
The compounds 6e8 were all tested in electroluminescent
OLEDs as hole transporting (HT) materials. An HT layer of PEDOT
containing device was also prepared for comparison. The two layer
OLED devices were prepared using Alq3 for the electro-
luminescent(EL)/electron transporting layers. Aluminium was used
as a cathode with a thin electron injection layer of LiF. When a
positive voltage was applied to the device a bright green electro-
luminescence of Alq3 was observed with an emission maximum at
around 520 nm. This confirmed that hole injection and charge
mobility in the HT layers of 6e8 is fully sufficient for an effective
charge carrier recombination occurring within the Alq3 layer.
Fig. 4 shows current density-voltage (a), luminance e current
density (b) and current efficiency e current density (c) character-
istics for the OLEDs containing the HT layers of 6e8 or PEDOT. It is
seen that current density of the devices with layers of 6e8 is higher
than that of the device with PEDOT. The OLEDs in general exhibit
turn-on voltages of 2.5e3.0 V and a maximum brightness of 1270e
8300 cd/m2. Maximal current efficiency of 3.25 cd/A was obtained
in device containing HT layer of biphenyl-substituted carbazole (8).
It is evident that incorporation of HT layers of compounds 6e8 to
the devices leads to better performance in current efficiency at the
same current density in comparison with that of PEDOT-based
device. It seems that layers of the compound 8 containing the
lowest Ip (5.55 eV) are better suited for hole injection from ITO
anode into Alq3 layer than those of materials 6 and 7 containing
slightly higher Ip’s. It should be pointed out that these character-
istics were obtained in a non-optimized test device. The perfor-
mance may be further improved by an optimization of the layer
thicknesses and processing conditions [23].
Fig. 4. OLED characteristics of the devices with the configuration: ITO/6, 7, 8 or
PEDOT/Alq3/LiF/Al.

G. Krucaite et al. / Dyes and Pigments 106 (2014) 1e6
5
Current density-voltage (a), luminance e current density (b) and
current efficiency e current density (c) characteristics for the
OLEDs are presented in Fig. 5. It could be observed that the electro-
phosphorescent devices with HT layers of 6 or 8 both demonstrate
rather similar characteristics. It seems that the hole injection layers
of PEDOT reduce barrier for charge injection into layer of HT ma-
terial 6, which has higher Ip than that of 8. The both devices exhibit
turn-on voltage of ca. 5 V (defined as the voltage where electro-
phosphorescence becomes detectable) and a maximum bright-
ness of 9800e11600 cd/m2. Maximal current efficiencies of the
OLEDs are about 22.5 cd/A and 20 cd/A for the materials 6- and 8-
based devices. The efficiencies show noticeable drop in the
observed current density window; however for the technically
important brightness of 100 cd/m2, an efficiency about 20 cd/A is
detected. These characteristics were also obtained in non-
optimized test devices and may be improved by an optimization.
In conclusion, phenyl, naphthyl or biphenyl disubstituted 9-
alkylcarbazoles have been synthesized and characterized as mate-
rials for hole transporting layers. The derivatives show high ther-
mal stability and form amorphous layers with glass transition
temperatures of 50e62 ꢀC. The electron photoemission spectra of
the layers showed ionisation potentials of about 5.55e5.8 eV. The
materials have been tested as the hole transporting layers in OLEDs.
One Alq3-based device exhibited good overall performance (turn-
on voltage: 2.5 V; maximum photometric efficiency: w3.25 cd/A;
maximum brightness: 8300 cd/m2). These OLED properties are
rather promising among Alq3-based two-layer devices. An green
electro-phosphorescent device using 3,6-diphenyl-9-ethylcarba
zole as hole transporting layer demonstrated a turn-on voltage of
ca. 5 V, a maximum brightness of 9800 cd/m2, and maximum
current efficiency of 22.5. For the technically important brightness
of 100 cd/m2, an efficiency exceeding 20 cd/A was detected in the
OLED.
Acknowledgements
This research was funded by a grant No. MIP-024/2013 from the
Research Council of Lithuania.
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