(Bi)phenyl substituted 9-(2,2-diphenylvinyl)carbazoles as low cost hole transporting materials for efficient red PhOLEDs

Article abstract of DOI:10.1016/j.dyepig.2018.06.019

Two low-cost 9-(2,2-diphenylvinyl)carbazole-based derivatives with aryl substitutions were synthesized by simple procedure and then investigated. The respective glass transition temperatures of the materials were estimated to be higher than 90 °C, which can provide morphologically-stable amorphous films for applications in organic light emitting diodes. The compounds possess adequate ionization potentials and suitable triplet energies, which make them suitable hole transporting materials for use in red phosphorescent organic light-emitting diodes. The respective peak efficiencies of the red devices with the p-type dopants were recorded at 8.7% (5.6 cd/A and 3.9 lm/W) and at 8.7% (5.4 cd/A and 3.8 lm/W), correspondingly, demonstrating high potential of the material for applications in light emitting diodes. The characteristics indicated that the devices with the aryl substituted 9-(2,2-diphenylvinyl)carbazoles exhibit better performance than those of widely used hole transporting 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) -based device.

Full text of DOI:10.1016/j.dyepig.2018.06.019

Accepted Manuscript
(Bi)phenyl substituted 9-(2,2-diphenylvinyl)carbazoles as low cost hole transporting
materials for efficient red PhOLEDs
Saulius Grigalevicius, Daiva Tavgeniene, Gintare Krucaite, Raimonda Griniene, Yen-
Po Wang, Shang-Ru Tsai, Chih-Hao Chang
PII:
S0143-7208(18)31028-3
10.1016/j.dyepig.2018.06.019
DYPI 6824
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 7 May 2018
Revised Date: 11 June 2018
Accepted Date: 12 June 2018
Please cite this article as: Grigalevicius S, Tavgeniene D, Krucaite G, Griniene R, Wang Y-P, Tsai S-
R, Chang C-H, (Bi)phenyl substituted 9-(2,2-diphenylvinyl)carbazoles as low cost hole transporting
materials for efficient red PhOLEDs, Dyes and Pigments (2018), doi: 10.1016/j.dyepig.2018.06.019.
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ACCEPTED MANUSCRIPT
(Bi)phenyl substituted 9-(2,2-diphenylvinyl)carbazoles as low cost hole transporting
materials for efficient red PhOLEDs
a,
a
a
Saulius Grigalevicius *, Daiva Tavgeniene , Gintare Krucaite ,
a
b
b
b,
Raimonda Griniene , Yen-Po Wang , Shang-Ru Tsai , Chih-Hao Chang *
a
Department of Polymer Chemistry and Technology, Kaunas University of Technology,
Radvilenu plentas 19, LT50254, Kaunas, Lithuania
b
Department of Photonics Engineering, Yuan Ze University, Chung-Li 32003, Taiwan
*
Corresponding authors: Saulius Grigalevicius (saulius.grigalevicius@ktu.lt) and
Chih-Hao Chang (chc@saturn.yzu.edu.tw)
Abstract
Two low-cost 9-(2,2-diphenylvinyl)carbazole-based derivatives with aryl substitutions
were synthesized by simple procedure and then investigated. The respective glass transition
temperatures of the materials were estimated to be higher than 90 ℃, which can provide
morphologically-stable amorphous films for applications in organic light emitting diodes. The
compounds possess adequate ionization potentials and suitable triplet energies, which make
them suitable hole transporting materials for use in red phosphorescent organic light-emitting
diodes. The respective peak efficiencies of the red devices with the p-type dopants were
recorded at 8.7 % (5.6 cd/A and 3.9 lm/W) and at 8.7 % (5.4 cd/A and 3.8 lm/W),
correspondingly, demonstrating high potential of the material for applications in light emitting
diodes. The characteristics indicated that the devices with the aryl substituted 9-(2,2-
diphenylvinyl)carbazoles exhibit better performance than those of widely used hole
transporting 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) -based device.
Keywords: carbazole derivative, amorphous material, ionization potential, light emitting diode,
brightness.
1

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1. Introduction
Phosphorescent organic light emitting diodes (PhOLEDs) have attracted much
attention because they use both singlet and triplet excitons for generation of light, making
00% internal quantum efficiency possible. Achieving the high level internal quantum
1
efficiency depends on several factors, including high quantum yield emitters, exothermic
energy transfer from host to emitter, effective exciton confinement as well as balanced carrier
transport [1, 2]. It is well known that carrier transporting materials are crucial to enable a
balance carrier transport from cathode and anode [3, 4]. Considerable exertion is needed for
the development of efficient red PhOLED devices, because the lower gap of red phosphors
usually induces serious carrier trapping, leading to higher operation voltages and carrier
imbalance [5]. Accordingly, it is desirable to exploit new hole transport materials to create red
PhOLEDs with reduced power consumption and improved efficiency.
Crystallization of low molar mass hole transporting materials during film-formation
process and during PhOLEDs operation at high temperature is the main obstacle for their
application in the devices [6, 7]. The low glass transition (T ) temperatures lead to the
g
formation of polycrystalline films resulting from thermal stress [8]. It is reported that OLED
devices with layers of the widely used commercial hole transporting materials: N,N,N',N'-
tetraphenyl-[1,10-biphenyl]-4,4'-diamine (TPD) and N,N'-diphenyl-N,N'-bis-(1-naphthyl)-
o
1
,1'-biphenyl-4,4'-diamine (NPD) showed irreversible failure when heated above 100 C due
to crystallization. The T values of organic semiconductors are determined by their molecular
g
structure, i.e. the larger molecular volume results in higher T [9, 10].
g
In this study, the new low cost 9-(2,2-diphenylvinyl)carbazole based derivatives with
aryl substitutions were synthesized and investigated. Our previous study found that
introducing the diphenylvinyl fragment in carbazole ring could increase spatial hindrance of
the moiety and the derivatives could be used for the preparation of thin and stable amorphous
layers on substrates [11]. We have examined the novel hole transporting materials in the
fabrication of red PhOLEDs. The respective peak efficiencies were recorded at 8.7 % (5.6
cd/A and 3.9 lm/W) and at 8.7 % (5.4 cd/A and 3.8 lm/W), correspondingly, for the devices
using
3,6-diphenyl-9-(2,2-diphenylvinyl)carbazole
(6)
and
3-(4-biphenyl)-9-(2,2-
diphenylvinyl)carbazole (7) as hole transporting materials. The high efficiencies of the red
PhOLEDs suggest great potential of the new (2,2-diphenylvinyl)carbazole based electroactive
materials for applications in OLED devices.
2

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2
. Experimental
2
.1. Instrumentation
1
H NMR spectra were recorded using a Varian Unity Inova (300 MHz) instrument. Mass
spectra were obtained on a Waters ZQ 2000 spectrometer. 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
o
curves were recorded in a nitrogen atmosphere at a heating rate of 10 C/min.
The ionization potentials of the layers of the compounds synthesized were measured in air
by the electron photoemission method, which was described earlier [12]. The measurement
method was, in principle, similar to that described by Miyamoto et al. [13]. The samples for
the ionization potential measurements were prepared as follows [14]. The materials were
dissolved in THF and the solution was coated on Al plates pre-coated with ~0.5 µm thick
adhesive layer of methylmethacrylate and methacrylic acid copolymer (MKM). The function
of this layer was improvement of adhesion as well as elimination of the electron
photoemission from Al layer. In addition, the MKM layer was sufficiently conductive to
avoid charge accumulation on it during the measurements.
UV-vis spectra of compounds were measured using a Shimadzu UV-1650PC
spectrophotometer. The PL spectra of compounds were measured using a CCD spectrograph
and the 325 nm line of a He-Cd laser as the excitation source. The hole drift mobility was
measured by xerographic time of flight technique [15].
The multilayer phosphorescent devices were fabricated on glass substrates and had the
typical structure with the organic layers sandwiched between a bottom indium tin oxide (ITO)
anode and a top metal cathode. After routine ultra-sonication cleaning of the ITO-coated glass
in deionized water and organic solvents, the substrate was also pre-treated with UV-Ozone for
5
minutes. The organic and metal layers were deposited onto ITO-coated glass by thermal
-6
evaporation in a vacuum chamber with a base pressure of < 10 Torr. Device fabrication was
completed in a single cycle without breaking the vacuum. The thicknesses of the organic and
metal layers were monitored using an in-situ method by a quartz-crystal microbalance (QCM)
equipped in the thermal evaporation chamber. The deposition rates of organic materials and
aluminium were respectively kept at around 0.1 nm/s and 0.5 nm/s. The active area was
2
defined by the shadow mask (2 × 2 mm ). Current density-voltage-luminance characteristics
were measured using a Keithley 238 current source-measurement unit and a Keithley 6485
3

ACCEPTED MANUSCRIPT
pico-ammeter equipped with a calibrated Si-photodiode. The electroluminescent spectra were
recorded using an Ocean Optics spectrometer.
2
.2. Materials
9
H-Carbazole (1), KI, KIO , 2,2-diphenylacetaldehyde, (±)-camphor-10-sulfonic acid, 4-
3
biphenyl boronic acid, phenyl boronic acid, bis(triphenylphosphine)palladium(II) dichloride
(
Pd(PPh ) Cl ) and potassium hydroxide were purchased from Aldrich and used as received.
3 2 2
4
4
,4'-N,N'-dicarbazole biphenyl, 3,5,3',5'-tetra(m-pyrid-3-yl)-phenyl[1,1']biphenyl, 1,1-bis[(di-
-tolylamino)phenyl]cyclohexane and tris(1-phenylisoquinoline)iridium(III) were purchased
from Lumtec and were subjected to temperature-gradient sublimation before use.
,6-Diiodo-9H-carbazole (2) and 3-iodo-9H-carbazole (3) were obtained by a procedure of
Tucker [16].
,6-Diiodo-9-(2,2-diphenylvinyl)carbazole (4) and 3-iodo-9-(2,2-diphenylvinyl)carbazole (5)
were prepared by earlier described procedure [17].
,6-Diphenyl-9-(2,2-diphenylvinyl)carbazole (6). 3,6-Diiodo-9-(2,2-diphenylvinyl)carbazole
2, 0.6 g, 1 mmol), phenyl boronic acid (0.37 g, 3 mmol), PdCl (PPh ) (0.042 g, 0.06 mmol)
3
3
3
(
2
3 2
and powdered potassium hydroxide (0.56 g, 10 mmol) were stirred in THF (6 mL) containing
degassed water (1 mL) at 80 °C under nitrogen for 24 h. After thin layer chromatography
(TLC) control the reaction mixture was cooled and quenched by the addition of ice water. The
product was extracted by chloroform. The combined extract was dried over anhydrous Na SO .
2
4
The crude product was purified by silica gel column chromatography using the mixture of
ethyl acetate and hexane (vol. ratio 1:35) as an eluent. Yield: 0.29 g (58 %) of white crystals.
+
1
M.p.: 220ºC (DSC). MS (APCI , 20 V): 498.1 ([M+1], 100 %). H NMR spectrum (CDCl , δ,
3
ppm): 8.28 (d, 2H, J = 1.2 Hz); 7.67 (d, 4H, J = 6.9 Hz); 7.53 (d, 2H, J = 1.8 Hz, J = 9.0 Hz);
2
7
.50 -7.40 (m, 9H, Ar); 7.31 (t, 2H, J = 7.1 Hz); 7.25 (d, 2H, J = 6.0 Hz); 7.24 (s, 1H, N-CH);
-
1
7
.14 – 7.06 (m, 5H, Ar). IR (KBr), (cm ) : 3053, 3027 (C-H, Ar); 2951, 2922 (=C-H); 1617,
599, 1457, 1456 (C=C, Ar); 1357, 1297, 1281 (C-N, Ar); 814 (C-H, >C=CH); 876, 840, 762,
96 (C-H, Ar). Elemental analysis for C H N % Calc.: C 91.72, H 5.47, N 2.81; % Found:
1
6
3
8
27
C 91.69, H 5.44, N 2.83.
4

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3-(4-Biphenyl)-9-(2,2-diphenylvinyl)carbazole (7). 3-Iodo-9-(2,2-diphenylvinyl)carbazole
(
5, 0.7 g, 1.5 mmol), 4-biphenyl boronic acid (0.44 g, 2.2 mmol), PdCl (PPh ) (0.031 g, 0.044
2 3 2
mmol) and powdered potassium hydroxide (0.41 g, 7.3 mmol) were stirred in THF (7 mL)
containing degassed water (1 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 chloroform. The combined extract was dried over anhydrous Na SO . The crude
2
4
product was purified by silica gel column chromatography using the mixture of ethyl acetate
and hexane (vol. ratio 1:35) as an eluent. Yield: 0.46 g (63 %) of white crystals. M.p.: 188ºC
+
1
(
DSC). MS (APCI , 20 V): 498.2 ([M+1], 100 %). H NMR spectrum (CDCl , δ, ppm): 8.29
3
(d, 1H, J = 1.6 Hz); 8.10 (d, 1H, J = 7.6 Hz); 7.77 (d, 2H, J = 8.4 Hz); 7.72 – 7.65 (m, 5H, Ar);
7
.59 (dd, 1H, J = 1.8 Hz, J = 8.6 Hz); 7.51 – 7.22 (m, 12H, Ar); 7.14 – 7.07 (m, 5H, Ar). IR
1 2
-1
(
KBr), (cm ) : 3078, 3051, 3027 (C-H, Ar); 1619, 1598, 1574, 1494 (C=C, Ar); 1355, 1332,
236, 1231 (C-N, Ar); 828, 846 (C-H, >C=CH); 812, 767, 745, 697 (C-H, Ar). Elemental
analysis for C H N % Calc.: C 91.72, H 5.47, N 2.81; % Found: C 91.67, H 5.49, N 2.82.
1
3
8
27
3
. Results and discussion
The synthesis of phenyl or 4-biphenyl substituted 9-(2,2-diphenylvinyl)carbazole based
derivatives (6 and 7) was carried out by synthetic route, which is shown in Scheme 1. The key
starting compounds: 3,6-diiodo-9H-carbazole (2) and 3-iodo-9H-carbazole (3) were
synthesized firstly from commercially available 9H-carbazole (1) by Tucker iodination
procedure, which is described in the literature [16]. 3,6-Diiodo-9-(2,2-diphenylvinyl)carbazole
(4) and 3-iodo-9-(2,2-diphenylvinyl)carbazole (5) were than prepared in reactions of the iodo-
derivatives 1 and 3 with an excess of 2,2-diphenylacetaldehyde under acidic conditions. The
objective electroactive materials: 3,6-diphenyl-9-(2,2-diphenylvinyl)carbazole (6) and 3-(4-
biphenyl)-9-(2,2-diphenylvinyl)carbazole (7) were finally synthesized in Suzuki reaction
conditions of the intermediates 4 and 5 by using an excess of phenyl boronic acid or biphenyl
boronic acid, correspondingly.
5

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H
N
1
H
H
N
N
( a )
I
I
I
2
3
(
b )
( b )
N
N
4
5
I
I
I
(
c )
(
c )
N
N
7
6
Scheme 1. a) KI, KIO , acetic acid; b) 2,2-diphenylacetaldehyde, (±)-camphor-10-sulfonic
3
acid, toluene; c) Pd(PPh ) Cl , KOH, THF, phenyl boronic acid for compound 6 or 4-biphenyl
3
2
2
boronic acid for compound 7.
The synthesized objective derivatives 6 and 7 were identified by mass spectrometry,
1
elemental analysis, infrared (IR) and H NMR spectroscopy. The data were found to be in good
agreement with the proposed structures. The materials were readily soluble in common organic
solvents. Transparent thin films of these materials could be prepared by spin coating from
solutions or by vacuum evaporation.
The behaviour under heating of the compounds 6 and 7 was investigated by DSC and TGA
under a nitrogen atmosphere. It was confirmed by TGA that the materials demonstrate very
o
o
high thermal stability. The 5 % mass loss was established at 377 C for 6 and at 392 C for 7,
o
as confirmed during the analyses with a heating rate of 10 C/min.
6

ACCEPTED MANUSCRIPT
cooling
nd
2
heating
o
C
st
Tg= 92
o
C
1
heating
T = 171
cr
o
C
Tm= 162
Material
6
o
C
T = 220
m
40
80
120
160
o
200
240
Temperature, C
cooling
nd
2
heating
st
o
C
1
heating
Tg= 91
Material 7
o
C
T =188
m
50
75
100
125
150
175
200
o
Temperature, C
o
Figure 1. DSC curves of the materials 6 and 7. Heating rate: 10 C/min.
Compounds 6 and 7 were obtained as crystalline materials after synthesis; however they
could form also amorphous materials with rather high glass transition temperatures (T ) as it
g
was demonstrated by DSC experiment. The DSC thermograms are shown in Figure 1. When
the crystalline sample of 6 was heated, first endothermic peak due to melting was observed at
o
o
1
78 C followed by an exothermic crystallization at 171 C to obtain new crystals, which
o
melted at 220 C. When the melted sample was cooled down and heated again, only glass-
o
transition was observed at 92 C and on further heating no peaks due to crystallization and
melting appeared.
7

ACCEPTED MANUSCRIPT
Derivative 7 was also obtained after synthesis as crystalline material, however it
demonstrated slightly different behaviour in the DSC measurements. The crystalline sample
o
of 7 melted at 188 C on first heating without following crystallization. The melt formed
amorphous material upon cooling. When the amorphous sample was heated again, the glass-
o
transition was observed at 91 C and on further heating no peaks due to crystallization and
melting appeared. The investigations confirmed that the crystalline derivatives 6 and 7 could
be converted into amorphous state and could be used for preparation of thin amorphous layers
on substrates.
The ionization potentials (I ) of thin layers of the compounds 6 and 7 were measured by
p
the electron photoemission method. The photoemission spectra of thin amorphous films of
these compounds are presented in Figure 2. The values of I in eV were 5.75 eV for layer of 6
p
and 5.7 eV for layer of 7. It could be observed that biphenyl substituted derivative 7 has a
slightly lower value of I due to longer conjugated electron system in the molecule. It is also
p
evident that the values of I of the newly synthesized compounds 6 and 7 are lower than that of
p
9
-(2,2-diphenylvinyl)carbazole (I = 5.94 eV) [18] as well as of derivatives having un-
p
substituted carbazole rings (I > 5.9 eV) [19, 20]. The characteristics demonstrate that thin
p
layers of the newly synthesized derivatives could be suitable as hole transporting layer
materials for application in optoelectronic devices. The layers should demonstrate better hole-
injecting and transporting properties in multilayer electroluminescent devices than that of
widely used poly(9-vinylcarbazole) (PVK) [19].
6
7
(Ip = 5.75 eV)
(Ip = 5.7 eV)
5
,5 5,6 5,7 5,8 5,9 6,0 6,1 6,2 6,3 6,4 6,5
hν (eV)
Figure 2. Electron photoemission spectra of layers prepared using the derivatives 6 and 7.
8

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1
.2
.0
.8
.6
.4
.2
.0
1.2
1.0
0.8
0.6
0.4
0.2
0.0
6
7
Abs.
Abs.
Fluo.
Fluo.
Phos.
Phos.
1
0
0
0
0
0
2
50 300 350 400 450 500 550 600
Wavelength (nm)
Figure 3. Absorption and fluorescence spectra of the compounds 6 and 7 in hexane at room
temperature. Phosphorescence spectra of 6 and 7 in hexane at 77 K.
Table 1. Photo-physical properties of compounds 6 and 7 measured in solutions.
a
b
c
d
e
Material Absorbance λmax. (nm)
Fluo. λpeak (nm)
Phos. λpeak (nm)
Eg (eV)
3.27
ET (eV)
2.56
6
7
256, 294, 338
400, 425, 449, 482(sh)
537
482
297, 334
369, 381
3.34
2.78
a
b
c
d
Absorption maximum; emission peaks of fluorescence; emission peaks of phosphorescence; singlet
e
energy gap (E ) was calculated from the absorption onset; triplet energy gap (E ) was estimated from
g
T
the phosphorescence onset.
Figure 3 shows absorption spectra, fluorescence spectra, and phosphorescence spectra of
compounds 6 and 7 measured in hexane. The respective solution concentrations of compounds
-5
-6
6
and 7 were selected to be 1.0 x 10 M and 8.5 x 10 M for the UV-vis measurements;
-5
whereas the respective solution concentrations of compounds 6 and 7 were set to 2.0 x 10 M
-4
and 1.7 x 10 M for both fluorescence and phosphorescence. The numeric data of the
corresponding photo-physical properties extracted from the figure are also summarized in
Table 1. The absorption peaks of both 6 and 7 were similar to those of the carbazole moiety
[
21]. Nevertheless, the absorption spectral profile of 6 differs slightly from that of 7,
indicating that the different moieties end-capped to the carbazole core influence the Franck-
Condon factor and thus alter the absorption intensity. The energy bandgaps calculated from
the absorption onset for 6 and 7 are respectively 3.27 and 3.34 eV. On the other hand, both
compounds possess quite different fluorescence spectra. The fluorescence of 6 exhibited a
9

ACCEPTED MANUSCRIPT
multi-peak spectral profile with clear vibronic features, while 7 possessed a narrower
fluorescence spectrum with obscure vibronic peaks. The relatively lower energy gap of 6
implied that the conjugation length was extended as compared to that of the carbazole
monomer. The Stoke’s shifts of compounds 6 and 7 were estimated to be 61 nm and 35 nm,
respectively. The phosphorescence peaks of 6 and 7 measured at 77K were respectively
recorded at 537 and 428 nm. Thus, the corresponding triplet energy gaps of 6 and 7 could
respectively be determined to be 2.56 eV and 2.78 eV, respectively. The values of triplet
energies of 6 and 7 make them both suitable for the use as the hole transport layer (HTL) in
red phosphorescent OLEDs [22]. In addition, a widely used hole transport material, 1,1-
bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC), possesses an adequate HOMO level of 5.5
-2
2
eV, a wide triplet energy gap of 2.87 eV, and a high hole mobility of 10 cm /Vs, and thus
could be used as a reference for evaluating the potential of 6 and 7 for EL applications [23].
Herein, we selected phosphorescent Ir(piq) as the emitter because its saturated red emission
3
could expand the colour gamut in display applications [ 24 ]. Furthermore, 4,4'-N,N'-
dicarbazolebiphenyl (CBP) with bipolar transport capability was used to combine with the red
emitter for constructing an exothermic host-guest system for the emissive layer (EML) [25].
On the other hand, a wide triplet energy gap electron transport material, 3,5,3',5'-tetra(m-
pyrid-3-yl)-phenyl[1,1']biphenyl (BP4mPy), was used as the electron transport layer (ETL)
due to its high carrier/exciton confinement capability [26]. We constructed the device using a
configuration of ITO (120 nm)/ TAPC or 6 or 7 (40 nm)/ CBP doped with 8 wt.% of Ir(piq)3
(30 nm)/ BP4mPy (40 nm)/ LiF (0.8 nm)/ Al (150 nm), where LiF and aluminum were
respectively used as the electron injection layer and the reflecting cathode. Figure 4(a) shows a
structural drawing of the materials used, while the energy level diagram of the tested OLEDs
is shown in Figure 4(b).
1
0

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N
(a)
N
N
N
N
N
Ir
N
N
N
N
N
TAPC
CBP
Ir(piq)3
BP4MPy
(b)
HTL
EML ETL
30 nm 40 nm
40 nm
1.8
2.33
2.46
2.57
2
.9
Cathode
Anode
5.5
5.75
5.7
6.0
6
.6
A B C
Figure 4. (a) Structural drawings of the materials used in PhOLEDs; (b) energy level
diagram of the red phosphorescent OLEDs with different HTLs.
Figure 5 and Table 2, respectively, provide the EL characteristics and the corresponding
numeric data of the tested red phosphorescent OLEDs. Figure 5(a) shows the normalized EL
2
spectra of devices A, B and C measured at 100 cd/m . All the devices presented an identical
spectral profile of Ir(piq) , indicating that the doping concentration of the emitter was
3
appropriate and the exciton formation zone was located within the EML [27]. Moreover, no
additional emission peak was observed in the EL spectra, implying that both 6 and 7 could
provide effective exciton confinement like TAPC. Thus, the tested devices with the pure
Ir(piq) saturated red emission resulted in the corresponding CIE coordinates being
3
superimposed on the NTSC vertices. In addition, almost no color variation occurred as the
voltage increased, verifying the thermal stability of these newly developed hole transport
materials for EL applications.
1
1

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(
a)
(b)
4
6
10
10
10
10
2
1
0
0
0
0
0
.0
100 cd/m
2
TAPC
10
4
6
7
.8
.6
.4
.2
.0
0
10
-2
2
1
0
0
0
0
-4
-6
1
1
1
TAPC
6
7
0
0
1
-8
-2
10
450
550
650
750
850
0
2
4
6
8
10 12 14 16 18
Wavelength (nm)
Voltage (V)
(
c)1
(d)6
0
6
5
4
3
2
1
0
5
4
3
2
1
0
8
6
4
2
0
TAPC
6
7
TAPC
6
7
-1
0
0
10
1
10
2
10
3
4
10
-1
0
10
1
10
2
10
3
4
10
1
10
10
10
2
2
Luminance (cd/m )
Luminance (cd/m )
Figure 5. EL characteristics of the red phosphorescent OLEDs: (a) normalized EL
spectra; (b) current density-voltage-luminance (J-V-L) curves; (c) external quantum
efficiency vs luminance; (d) luminance efficiency and power efficiency of devices A,
B, and C.
1
2

ACCEPTED MANUSCRIPT
Table 2. EL Characteristics of red phosphorescent OLEDs with different HTLs.
Device
A
B
C
HTL
TAPC
6
8.7
7
8.7
Max.
8.3
External Quantum
Efficiency (%)
2
1
00 cd/m
6.8
8.0
8.2
2
1
000 cd/m
Max.
5.1
6.6
6.9
5.2
5.6
5.4
Luminance Efficiency
2
1
00 cd/m
4.2
5.2
5.1
(
cd/A)
2
1
000 cd/m
3.2
4.2
4.3
Max.
4.7
1.7
3.9
3.8
Power Efficiency
lm/W)
2
1
00 cd/m
1.7
1.7
(
2
1
000 cd/m
0.9
1.0
1.0
2
Turn-on voltage (V)
1 cd/m
3.9
5.0
5.0
2
Max. Luminance (cd/m ) [V]
12223 [17.2]
10740 [18.0]
10951 [18.6]
2
1
00 cd/m
(0.682, 0.316) (0.682, 0.317) (0.682, 0.316)
(0.681, 0.316) (0.682, 0.317) (0.682, 0.317)
CIE coordinates
x, y)
(
2
1
000 cd/m
As shown in the current density-voltage-luminance (J-V-L) curves (Figure 5(b)),
device A with TAPC exhibited a higher current density than devices B and C, due to
the lower hole mobilities of 6 and 7. Furthermore, comparing the HOMO level of
TAPC, the slightly higher HOMO levels of 6 and 7 produced higher energy barriers at
the HTL/EML interface and thus obstructed hole injection into the EML. Hence, the
turn-on voltages of both devices B and C were recorded at 5.0 V, while device A
exhibited lower value of 3.9 V. Similarly, the maximum luminance of devices A, B,
2
and C was respectively recorded at 12223, 10740, and 10951 cd/m .
The efficiency curves of the tested devices are shown in Figures. 5(c) and 5(d).
As indicated, devices B and C presented similar external quantum efficiency and
luminance efficiency curves, and were both higher than those of device A. The
respective maximum efficiencies of devices B and C were respectively recorded at 8.7
%
(5.6 cd/A and 3.9 lm/W) and 8.7 % (5.4 cd/A and 3.8 lm/W), while device A
achieved a peak efficiency of 8.3% (5.2 cd/A and 4.7 lm/W). In addition, the
efficiency curve of device A achieved a peak value in a low luminance range and
decreased immediately thereafter. Significant efficiency roll-off was observed in
1
3

ACCEPTED MANUSCRIPT
device A, possibly due to the serious triplet-triplet annihilation (TTA) [28]. At more
2
2
practical luminance values of 100 cd/m and 1000 cd/m , device A only gave external
quantum efficiencies of 6.8% and 5.1%, respectively. In contrast, at a luminance of
2
2
1
00 cd/m (1000 cd/m ), devices B and C maintained higher efficiency values, with
corresponding forward efficiencies of 8.0% (6.6%) and 8.2% (6.9%). The different
tendencies of the carrier balance between devices was due to the difference in hole
mobility and the energy barrier at the HTL/EML interface, leading to different exciton
formation zone location and size in the EML. Since the electron mobility of BP4mPy
-4
2
was estimated to be about 10 cm /V·s, both 6 and 7 possessed adequate hole
mobility and could match BP4mPy, providing carrier balance in a broad luminance
5
-4
range [27]. At an electric field of 4×10 V/cm, hole mobility value of 4.5×10
2
cm /(V·s) was obtained for compound 6, while slightly lower hole mobility value of
-4
2
2
×10 cm /(V·s) was obtained for the compound 7 as it was demonstrated by time of
flight measurements. In addition, one could expect an enlarged exciton formation
zone achieved in devices B and C, which could mitigate the TTA. Furthermore, the
HOMO levels of either 6 or 7 were close to that of the EML, preventing hole
accumulation at the HTL/EML interface and thus mitigating the polaron quenching.
As the result, efficiency drops from the peak to the value recorded at a luminance
2
level of 1000 cd/m for devices B and C were only estimated to be 24% and 21%,
respectively. Overall, the outcome indicated that the simple tri-layer architecture
device incorporating 6 or 7 rendered very good performance with high luminance
levels and demonstrated the high potential of these fabricated hole transport materials.
4
. Conclusions
New low-cost aryl substituted 9-(2,2-diphenylvinyl)carbazole-based derivatives aiming to
facilitate hole transport function in phosphorescent organic light emitting diodes were
synthesized and investigated. The electroluminescent characteristics of the tested OLEDs
indicated that devices with layers of the new hole transporting materials exhibited better
efficiencies to that of commercially available 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane
(TAPC)-based device. By introducing new p-type 3,6-diphenyl-9-(2,2-diphenylvinyl)carbazole
(6) for hole transporting layer, the respective peak efficiency of a red phosphorescent organic
light emitting diode was respectively recorded at 8.7 % (5.6 cd/A and 3.9 lm/W) and at 8.0 %
2
(
5.2 cd/A and 1.7 lm/W) for 100 cd/m brightness. By introducing hole transporting layer of 3-
(4-biphenyl)-9-(2,2-diphenylvinyl)carbazole (7), the peak efficiency of the analogues device
1
4

ACCEPTED MANUSCRIPT
2
was 8.7 % (5.4 cd/A and 3.8 lm/W) and 8.2 % (5.1 cd/A and 1.7 lm/W) for 100 cd/m
brightness.
The good results of the devices with the aryl substituted 9-(2,2-
diphenylvinyl)carbazoles indicated a new path of molecular designs for the hole transporting
materials for the red phosphorescent organic light emitting diodes.
Acknowledgements
This research was funded by a grant No. S-MIP-17-64 from the Research Council of
Lithuania. Prof. C.-H. Chang gratefully acknowledge financial support from the Ministry of
Science and Technology of Taiwan (MOST 106-2221-E-155-035-). Dr. habil. Valentas
Gaidelis is acknowledged for measurements of electronic properties of layers of the material.
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9
-(2,2-diphenylvinyl)carbazole based hole transporting materials with aryl substitutions
were synthesized.
The hole transporting materials possess adequate ionization potentials and triplet energies.
Red PhOLEDs using the hole transporting materials were formed.
Peak external quantum efficiencies of the devices reached 8.7 %