Dibenzothiophene-indolocarbazole-based bipolar material: Host for green phosphorescent OLEDs and non-doped fluorescent emitter

Article abstract of DOI:10.1016/j.dyepig.2018.10.060

Novel bipolar material 2-(5-phenylindolo[3,2-a]carbazol-12(5H)-yl)dibenzothiophene 5,5-dioxide (DBTO-IN/CAR) was designed and synthesized with a high yield of 64%. The synthesized material exhibited an excellent glass transition temperature of 156 °C, which manifests as a high thermal stability of DBTO-IN/CAR. We fabricated two different devices, namely a green phosphorescent OLED and a non-doped bipolar fluorescent emitter, with DBTO-IN/CAR as the bipolar material. The DBTO-IN/CAR host material based device exhibited excellent maximum power and current efficiencies of 45.36 lm/W and 51.98 cd/A, respectively. The efficiencies are higher than that of the reference CBP-based similar device, which exhibited a power efficiency of 42.01 lm/W. The DBTO-IN/CAR-based green phosphorescent device revealed a good external quantum efficiency of 19.03%. Moreover, the non-doped DBTO-IN/CAR fluorescent emitter related device also exhibited better current and external quantum efficiencies of 4.30 cd/A and 2.47%, respectively, with the emission of cyan color.

Full text of DOI:10.1016/j.dyepig.2018.10.060

Accepted Manuscript
Dibenzothiophene-indolocarbazole-based bipolar material: Host for green
phosphorescent OLEDs and non-doped fluorescent emitter
Ramanaskanda Braveenth, Il-Ji Bae, Kanthasamy Raagulan, Sohyeon Kim,
Sunghoon Kim, Miyoung Kim, Kyu Yun Chai
PII:
S0143-7208(18)31950-8
https://doi.org/10.1016/j.dyepig.2018.10.060
DYPI 7126
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 4 September 2018
Revised Date: 26 October 2018
Accepted Date: 26 October 2018
Please cite this article as: Braveenth R, Bae I-J, Raagulan K, Kim S, Kim S, Kim M, Chai KY,
Dibenzothiophene-indolocarbazole-based bipolar material: Host for green phosphorescent OLEDs
and non-doped fluorescent emitter, Dyes and Pigments (2018), doi: https://doi.org/10.1016/
j.dyepig.2018.10.060.
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to
our customers we are providing this early version of the manuscript. The manuscript will undergo
copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please
note that during the production process errors may be discovered which could affect the content, and all
legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT
Graphical abstract
60
50
40
30
20
10
0
CBP
DBTO-IN/CAR
DBTO-IN/CAR (Dopant)
50
0.03
0.02
0.01
0.00
40
30
20
10
0
0
10
20
30
40
50
450
500
550
600
650
700
Current density (mA/cm2)
Wavelength (nm)
400
450
500
550
600
650
700
750

ACCEPTED MANUSCRIPT
Dibenzothiophene-indolocarbazole-based bipolar material:
Host for green phosphorescent OLEDs and non-doped
fluorescent emitter
Ramanaskanda Braveenth†,1, Il-Ji Bae‡,1, Kanthasamy Raagulan†, Sohyeon
Kim†, Sunghoon Kim†, Miyoung Kim‡,* and Kyu Yun Chai†,*
†
Division of Bio-Nanochemistry, College of Natural Sciences, Wonkwang
University, Iksan City, Chonbuk, 570-749, Republic of Korea.
‡ Nano-Convergence Research Center, Korea Electronics Technology Institute,
Jeonju, Korea.
* Corresponding Authors: Email: miy1kim@keti.re.kr; Tel.: +82-632-190-011
(M. Kim), Email: geuyoon@wonkwang.ac.kr; Tel.: +82-63-850-6230; Fax:
+82-63-841-4893 (K.Y. Chai).
1 These authors contributed equally to this work
Abstract
Novel
bipolar
material
2-(5-phenylindolo[3,2-a]carbazol-12(5H)-
yl)dibenzothiophene 5,5-dioxide (DBTO-IN/CAR) was designed and
synthesized with a high yield of 64%. The synthesized material exhibited an
excellent glass transition temperature of 156 °C, which manifests as a high
thermal stability of DBTO-IN/CAR. We fabricated two different devices,
namely a green phosphorescent OLED and a non-doped bipolar fluorescent
emitter, with DBTO-IN/CAR as the bipolar material. The DBTO-IN/CAR host
material based device exhibited excellent maximum power and current
efficiencies of 45.36 lm/W and 51.98 cd/A, respectively. The efficiencies are
higher than that of the reference CBP-based similar device, which exhibited a
power efficiency of 42.01 lm/W. The DBTO-IN/CAR-based green
phosphorescent device revealed a good external quantum efficiency of 19.03 %.
Moreover, the non-doped DBTO-IN/CAR fluorescent emitter related device
also exhibited better current and external quantum efficiencies of 4.30 cd/A and
2.47%, respectively, with the emission of cyan color.
Keywords: organic light-emitting diodes; bipolar host; green phosphorescent;
fluorescent; indolocarbazole.
1. Introduction
Organic light emitting diode (OLED) based devices have been astonishingly
successful in the commercial market, with a variety of display products such as

ACCEPTED MANUSCRIPT
mobile phones, television, flat panels, and flexible displays. This success is
possible as a result of its high efficiencies compared to conventional devices.
The development of thermally activated delayed fluorescence (TADF) and
phosphorescence, organic light-emitting diodes in recent times has enabled
harvesting of both triplet and singlet excitons to achieve 100% internal quantum
efficiency through the inter system crossing (ISC) and reverse intersystem
crossing (RISC) mechanisms [1-13]. A suitable host material, proper holes, and
electron charge balance in the emission layer are essential requirements for
high-performance OLEDs. The triplet energy of the host materials plays a major
role in supplying energy to the dopant, while preventing energy flowback to the
host. Along with triplet energy, the frontier molecular orbital energies (FMOs)
of the host material are also important to enable proper charge transportation
between adjacent layers [14-18]. Additionally, host materials avoid several
triplet exciton quenching operations, such as triplet-triplet annihilation (TTA)
and aggregation-caused quenching (ACQ) [19-24]. Three types of host material
have been reported, namely hole-transporting (HT), electron-transporting (ET),
and bipolar host materials [25-28]. HT host materials are constructed using
electron-donating arylamine and carbazole moieties, whereas ET host materials
use electron-withdrawing derivatives, such as the triazine, pyridine, pyrimidine,
phosphine oxide, and cyano groups. 4,4′-Bis(9-carbazolyl)-1,1′-biphenyl (CBP)
and 1,3-Di(9H-carbazol-9-yl)benzene (mCP) are well-known HT host materials.
The triplet energies of CBP and mCP are 2.56 and 2.90 eV, respectively.
Unfortunately, CBP has exhibited a low thermal stability which manifested as a
low glass transition temperature of 62 °C [29-36]. Tris(8-
hydroxyquinolinato)aluminum (Alq3) was employed as an ET host material in
an initial work [33]. Phosphine oxide based 2,8-bis(diphenylphosphine
oxide)dibenzofuran (PODBF) exhibited a high triplet energy of 3.14 eV as an
ET host material; its device applications are limited owing to its poor photo-
oxidation stability of the furan ring which leads to air instability of the whole
molecule [6, 37].
For HT and ET host materials, the charge recombination zone extends towards
either the ET or HT layer. Thus, this nature causes an efficiency drop associated
with the improper charge balance in the devices. To overcome these issues, a
double emission layer with two types of hosts and a single emission layer with
bipolar host materials were introduced to enhance the device efficiencies while
improving the thermal stabilities. The bipolar host materials have both an
electron donor and electron acceptor present in one molecule; this supports a
good charge balance, charge transportation/movement, broad recombination
area, as well as proper highest occupied molecular orbital (HOMO) and lowest

ACCEPTED MANUSCRIPT
unoccupied molecular orbital (LUMO) energies with adjacent layers and stable
thermal stabilities [38-40]. The molecular design is crucial for bipolar host
materials to meet the aforementioned intrinsic requirements. However, it is
slightly challenging to achieve a high triplet energy in bipolar molecules owing
to the intra-molecular charge-transfer mechanism between the electron donor
and acceptor moieties [41-46].
In our study, we designed and synthesized a bipolar host and fluorescent
emitting material DBTO-IN/CAR by incorporating Dibenzothiophene 5,5-
dioxide as the electron-withdrawing and indolocarbazole as the electron-
donating derivatives in a single molecule. The experimental results show that
DBTO-IN/CAR is a suitable candidate for green phosphorescence OLED as
host material and non-doped bipolar fluorescence emitter. We fabricated two
different OLED devices to investigate the device performance.
2. Materials and methods
2.1. Materials
All reagents and solvents were purchased from commercial sources. DBT and
1-bromo-2-nitrobenzene were obtained from Fisher Scientific Korea Ltd. (Seoul,
Korea). 3-(2-nitrophenyl)-9-phenyl-9H-carbazole was collected from Green
Guardee Technology Co., Ltd. (Beijing, China). Anhydrous toluene was made
using sodium, benzophenone and toluene through a distillation process. Toluene,
n-hexane, and dichloromethane were purchased from SK chemicals (Gyeonggi-
do, Korea). O-dichlorobenzene was obtained from Samchun Pure Chemical Co.,
Ltd. (Gyeonggi-do, Korea). Triethyl phosphite and sodium tert-butoxide were
purchased from Sigma Aldrich (Seoul, Korea). Palladium-based catalysts were
acquired from TCI Chemicals (Seoul, Korea). Analytical thin-layer
chromatography (TLC) was performed using aluminum-backed Merck
Kieselgel 60 and silica gel with mesh size of 200–300 from Merck (Seoul,
Korea). Deionized water was used for all cases.
2.2. Instrumentation
A JEON JNM-ECP FT-NMR spectrometer (Peabody, MA, USA) operating at
1
13
500 MHz was used to analyze the H and C NMR data. Mass spectrometry
was evaluated using a Xevo TQ-S spectrometer (Waters, Milford, MA, USA).
UV-absorption spectra were obtained using a SINCO S-4100 UV-Vis
spectrophotometer (SINCO, Seoul, Korea). The band-gap (Eg) energy value was
obtained from the onset wavelength of the UV-visible absorbance spectra. An
HR800 spectrofluorometer (Horiba Jobin Yvon, Paris, France) was accessed to

ACCEPTED MANUSCRIPT
record the photoluminescence (PL) spectra. The triplet energy level (ET) was
calculated from the onset wavelength of the emission spectra in toluene. The
HOMO level was recorded by a photoelectron spectrometer (RIKEN, Saitama,
Japan). The LUMO level was estimated by adding the band-gap energy to the
recorded HOMO energy value. Thermal gravimetric analysis (TGA) and
differential scanning calorimetry (DSC) measurements were obtained by using
SDT Q600 V20.9 Build 20 and DSC Q200 V24.9 Build 121 (TA Instruments,
New Castle, DE, USA) instruments with a heating rate of 10 °C/min in a
nitrogen atmosphere. OLED devices with a frontier molecular orbital
distribution achieved using density functional theory in a Gaussian 09 program
(Wallingford, CT, USA) were fabricated on an indium-tin-oxide (ITO) coated
glass substrate using a thermal evaporating system (5 x 10-7 Torr pressure)
(Sunicel plus 200, Seoul, Republic of Korea). Current density-voltage-
luminescence (J-V-L) efficiencies were determined by an OLED I-V-L test
system (Polaronix M6100, Suwon, Republic of Korea). The
electroluminescence (EL) spectra analysis was conducted using a
spectroradiometer (Konica Minolta CS-2000, Japan).
2.3. Synthesis procedure
The complete set of synthetic steps is displayed in Scheme 1.
2.3.1. Synthesis of 2-bromodibenzothiophene 5,5-dioxide (DBTO)
A mixture of 2-bromodibenzothiophene (DBT, 5.0 g, 19 mmol) and acetic acid
(AcOH, 150 ml) was refluxed at 80 °C for 30 min in a two-neck round-bottom
flask equipped with a condenser. After 30 min, 30% of hydrogen peroxide
(H2O2, 100 ml) was added drop wisely through the top of the condenser, and the
mixture was stirred continuously at 80 °C for a further two hours. Then, a white
precipitate was filtered and washed with deionized water several times. Finally,
the white solid was kept inside a vacuum oven for 8 h to obtain 4.6 g of the
dried product DBTO.
2-bromodibenzothiophene 5,5-dioxide (DBTO)
Yield: 82%; White solid; 1H NMR (500 MHz, CDCl3) δ 7.92 (S, 1H), 7.82-7.83
(d, J= 8 Hz, 1H), 7.75-7.76 (d, J= 8 Hz, 1H), 7.64-7.69 (m, 3H), 7.54-7.57 (t, J=
13
7.5, 1H); C NMR (500 MHz, CDCl3) δ 138.1, 136.5, 134.1, 133.6, 133.3,
131.1, 130.3, 128.7, 125.0, 123.5, 122.4, 121.8.
2.3.2. Synthesis of 3-(2-nitrophenyl)-9-phenyl-9H-carbazole (C)

ACCEPTED MANUSCRIPT
A mixture of (9-phenyl-9H-carbazol-3-yl)boronic acid (A, 5.0 g, 17.41 mmol),
1-bromo-2-nitrobenzene
(B,
3.9
g,
19.15
mmol),
Tetrakis(triphenylphosphine)palladium(0) (Pd(PPh3)4, 0.02 g, 0.17 mmol),
potassium carbonate (K2CO3, 2.0 M) in 50 ml water, 100 ml toluene, and 30 ml
ethanol was refluxed at 110 °C under an inert condition while stirring for 12 h
in a two-neck round-bottom flask equipped with a condenser. After completion
of the reaction, the mixture was worked up using dichloromethane (100 ml) and
water (70 ml). The organic layer was separated and dried over anhydrous
sodium carbonate. After filtration, the organic layer was concentrated using the
rotary evaporation method. The residues were separated using a silica column
with a n-Hexane: dichloromethane (2:1) solvent system to obtain an
intermediate C of 4.2 g.
3-(2-nitrophenyl)-9-phenyl-9H-carbazole (C)
1
Yield: 66.2 %; Yellow solid; H NMR (500 MHz, CDCl3) δ 8.15-8.16 (d, J= 8
Hz, 2H), 7.87-7.88 (d, J= 8 Hz, 1H), 7.57-7.62 (m, 6H), 7.44-7.51 (m, 5H),
13
7.31-7.36 (m, 2H); C NMR (500 MHz, CDCl3) δ 149.8, 141.4, 140.7, 137.5,
137.0, 132.6, 132.2, 130.0, 129.1, 127.8, 127.7, 127.2, 126.5, 126.0, 124.1,
123.8, 123.2, 120.6, 120.3, 119.9, 110.2, 110.1.
2.3.3. Synthesis of 5,12-dihydroindolo[3,2-a]carbazole (IN/CAR)
3-(2-nitrophenyl)-9-phenyl-9H-carbazole (C, 4.0 g, 10.97 mmol), 40 ml of
triethyl phosphite, and 40 ml of o-dichlorobenzene were vacuumed for 15 min.
Then, a nitrogen atmosphere was generated through a nitrogen balloon. The
mixture was refluxed at 180 °C for 24 h. After completion of the reaction, the
crude mixture was evaporated using a rotary evaporator. The work-up was
continued after concentration using water (150 ml) and dichloromethane (200
ml), and the organic layer dried over anhydrous magnesium sulfate. The crude
residues were separated by a silica column with a gradient mobile system of n-
hexane: dichloromethane. Finally, recrystallization (n-Hexane) was carried out
to achieve an intermediate IN/CAR of 3.3 g.
1
Yield: 89 %; White solid ; H NMR (500 MHz, CDCl3) δ 8.66 (s, 1H), 8.22-
8.23 (d, J= 7.5 Hz, 1H), 8.08-8.13 (m, 2H), 7.60-7.64 (m, 5H), 7.49-7.53 (m,
13
2H), 7.39-7.47 (m,3H), 7.28-7.36 (m, 2H); C NMR (500 MHz, CDCl3) δ
140.7, 140.3, 139.2, 137.9, 134.3, 130.0, 127.8, 127.6, 124.8, 124.3, 124.2,
122.1, 120.6, 120.2, 120.0, 119.9, 118.6, 116.6, 110.9, 110.2, 107.2, 103.1.
2.3.4.
2-(5-phenylindolo[3,2-a]carbazol-12(5H)-yl)dibenzothiophene
5,5-
dioxide (DBTO-IN/CAR)

ACCEPTED MANUSCRIPT
A mixture of 2-bromodibenzothiophene 5,5-dioxide (DBTO, 2.0 g, 6.77 mmol),
5-phenyl-5,12-dihydroindolo[3,2-a]carbazole (IN/CAR, 2.6 g, 7.79 mmol),
Pd(OAc)2 (0.05 g, 0.20 mmol), 10% t-Bu3P in toluene (0.98 ml, 4.06 mmol),
and 100 ml of anhydrous toluene was added in a two-neck round-bottom flask
equipped with a condenser, and refluxed at 110 °C for 18 h in a nitrogen
atmosphere while stirring. The crude residues were separated by a silica column
and n-hexane- dichloromethane (2:1) solvent system to obtain the target
molecule DBTO-IN/CAR as a yellow solid.
Yield: 64.3 %; Yellow solid; FT-IR (KBr pellet): ʋmax 3052.8, 1595.2, 1500.6,
1474.2, 1442.3, 1421.5, 1368.9, 1308.4, 1243.9, 1218.0, 1167.7, 1119.8 and
1060.3 cm-1 ; 1H NMR (500 MHz, CDCl3) δ 8.14-8.17 (m, 2H), 8.10-8.11 (d, J=
8.0 Hz, 1H), 8.00-8.02 (m, 1H), 7.90-7.92 (m, 1H), 7.78-7.79 (d, J= 7.5 Hz, 1H),
7.64-7.67 (m, 3H), 7.53-7.59 (m, 5H), 7.36-7.44 (m, 4H), 7.30-7.31 (d, J= 8.0
Hz, 1H), 7.20-7.26 (m, 1H), 6.84-6.87 (t, J= 7.5 Hz, 1H), 6.28-6.29 (d, J= 8.5
13
Hz, 1H); C NMR (500 MHz, CDCl3) δ 145.8, 141.9, 141.3, 140.7, 138.6,
137.6, 137.0, 136.1, 134.0, 133.8, 131.1, 130.8, 130.3, 130.0, 128.2, 128.1,
127.1, 125.2, 124.9, 124.8, 123.8, 123.1, 122.4, 122.1, 121.6, 121.5, 120.8,
119.9, 119.3, 118.5, 118.2, 109.9, 109.6, 107.9, 104.7; MS (APCI) m/z: 546.76
for C36H22N2O2S [(M+H)+]. TOF MS (ES+) m/z: cal. For C36H22N2O2S 546.14,
found 546.14; Anal. Calcd for C36H22N2O2S (%): C, 79.10; H, 4.06; O, 5.85; N,
5.12; S, 5.86. Found: C, 78.95; H, 4.03; O, 5.79; N, 5.08; S, 6.11.
2.4. OLED fabrication and characterization
To fabricate the OLEDs, indium-tin-oxide (ITO) coated glass substrates (150
nm) were cleaned in an ultrasonic bath with solvent isopropyl alcohol and
deionized water for 10 and 20 min, respectively. The substrates were then dried
using nitrogen gas and each subjected to a UV-ozone treatment for 6 min. All of
the organic layers and the metal cathode were deposited on the pre-cleaned ITO
glass under a pressure of ~5×10-7 Torr using a Sunic organic evaporator. Finally,
all of the devices were encapsulated using a glass cover and all deposition
processes were conducted inside a nitrogen-filled glove box. The OLED area
was 2 mm2 for all of the samples fabricated in this work.

ACCEPTED MANUSCRIPT
Scheme 1. Synthetic route of target molecule DBTO- IN/CAR.
3. Results and discussion
The thermal properties of DBTO-IN/CAR were characterized by thermal
gravimetric analysis (TGA) and differential scanning calorimetry (DSC). All
measurements were reordered under a nitrogen atmosphere. The DSC data of

ACCEPTED MANUSCRIPT
DBTO-IN/CAR is shown in Figure 1 and Table 1. We noticed that the glass
transition temperature of DBTO-IN/CAR was 156 °C, which is higher than
those of the CBP and mCP host materials [29-32]. The thermal decomposition
of our material was recorded at approximately 435 °C for a 5% weight
reduction. The melting point was noticed at 254 °C. These good thermal
stabilities can enhance the morphological properties during device fabrication
and operation.
100
DBTO-IN/CAR
-0.5
DBTO/IN/CAR
90
80
70
60
50
40
30
20
10
0
-1.0
-1.5
-2.0
-2.5
-3.0
-3.5
-4.0
0
-2
-4
-6
-8
200
300
400
Temperature (o C)
0
100
200
300
400
500
600
700
800
900 1000
0
25
50
75
100
125
150
175
200
Temperature (oC)
Temperature (oC)
Figure 1. Differential scanning calorimetry and thermal gravimetric analysis curves of
DBTO-IN/CAR.
Table 1. Thermal, photophysical, and electrochemical properties of DBTO-IN/CAR.
a
b
g
h
Material Tg
Td
UV-Vis PL max HOMO
LUMO f
(eV)
Eg
ET
d
(°C) (°C)
c (nm)
(nm)
e (eV)
(eV)
(eV)
156
435
376
458
5.68
2.38
3.30
2.71
DBTO-
IN/CAR
a Glass transition temperature
b Thermal decomposition temperature at 5% weight reduction
c Onset of UV-absorption wavelength
d Photoluminescent maximum emission
e Highest occupied molecular orbital energy
f Lowest unoccupied molecular orbital energy
g Energy band gap
h Triplet energy

ACCEPTED MANUSCRIPT
Figure 2 shows the UV-vis absorption and photoluminescent (PL) spectra of
DBTO-IN/CAR at room temperature. The absorption and emission data are
summarized in Table 1. The absorption at approximately 300 nm attributed to
the π- π* transition of the indolocarbazole (IN/CAR) moiety. The charge
transfer (CT) absorption from the electron-donating IN/CAR to the electron-
withdrawing DBTO was noticed at 340 nm. The band-gap energy of DBTO-
IN/CAR was recorded at 3.30 eV, which relates to the onset absorption
wavelength of 376 nm. The maximum PL emission was found at 458 nm, which
relates to an energy of 2.71 eV. The energy value is suitable for a host material
for green phosphorescent dopant because the host material is responsible for
supplying energy to the dopant and preventing energy flow back from the
dopant. The triplet energy of the CBP reference material is 2.58 eV; thus, we
believed DBTO-IN/CAR will exhibit a greater efficiency improvement than
CBP-based green phosphorescent OLED.
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
1.2
1.0
0.8
0.6
0.4
0.2
DBTO-IN/CAR
DBTO-IN/CAR
280
300
320
340
360
380
400
420
400
450
500
550
600
650
Wavelength (nm)
Wavelength (nm)
Figure 2. UV-Vis absorption and photoluminescence spectra of DBTO-IN/CAR.
The HOMO energy level was calculated as -5.68 eV using the AC2
photoelectron spectrometer. The LUMO energy level was achieved by adding
the band-gap energy to the HOMO energy; this is summarized in Table 1. The
LUMO value is -2.38 eV, which is higher than that of the green dopant (-2.90
eV). The proper frontier molecular orbital energy levels support the effective
transportation of holes and electrons from the anode and cathode, respectively.
Moreover, this can enhance the device efficiencies by providing a suitable
environment for charge recombination. To understand the molecular orbital
distribution, we calculated the density functional theory DFT using a Gaussian
09 program with the basic set of 6-31G. The HOMO-LUMO distributions are
depicted in Figure 3. The HOMO is distributed over the electron-donating
indolocarbazole moiety, whereas the LUMO is distributed over the electron-
withdrawing DBTO group. We notice a small overlapping of the aromatic
benzene of the donor and acceptor units. This explains the bipolar nature of the
DBTO-IN/CAR molecule.

ACCEPTED MANUSCRIPT
Figure 3. Frontier molecular orbital distribution of DBTO-IN/CAR.
To investigate the electroluminescent properties of our DBTO-IN/CAR, we
fabricated OLED devices with green phosphorescent dopant Ir(ppy)3. The
device structure was indium tin oxide (ITO) (150 nm)/ 1,4,5,8,9,11-
Hexaazatriphenylenehexacarbonitrile
(HATCN)
(7
nm)/
4,4′-
Cyclohexylidenebis[N,N-bis(4-methylphenyl)benzenamine] (TAPC) (43 nm)/
DBTO-IN/CAR: 5 wt % Tris[2-phenylpyridinato-C2,N]iridium(III) (Ir(ppy)3)
(20 nm)/ 1,3,5-Tri(m-pyridin-3-ylphenyl)benzene (TmPyPB) (35 nm)/ 8-
Quinolinolato lithium (Liq) (1.5 nm)/ Aluminum (Al) (100 nm), and is shown in
Figure 4. In the given configuration, ITO is the anode, Al the cathode, HATCN
the hole-injecting layer (HIL), TAPC the hole-transporting layer (HTL),
TmPyPB the electron-transporting layer (ETL), and Liq the electron-injecting
layer (EIL) (Figure 4). We fabricated a similar device with CBP as the host
material for our reference study, and compared the device efficiencies.
Figure 4. Green phosphorescent OLED device structure.
The current density-voltage (J-V) and luminescence-voltage (L-V) properties
are shown in Figure 5 and summarized in Table 2. The turn-on voltage of the
DBTO-IN/CAR-based device was 3.6 V, whereas the driving voltage was 4.9 V.
These values are lower than those of the CBP-based reference device, which
had turn-on and driving voltages of 3.8 and 5.2 V, respectively. The lower turn-
on and driving voltages enhance the device efficiencies. The maximum current
efficiency of the DBTO-IN/CAR-based device was 51.98 cd/A, which is higher

ACCEPTED MANUSCRIPT
than that of the reference-based device (50.81 cd/A) owing to its lower turn-on
voltage. The power efficiency of the DBTO-IN/CAR-based device was
determined as 45.36 lm/W at maximum and 27.56 lm/W at 1000 cd/m2.
Moreover, the power efficiency of the reference device was determined as 42.01
and 25.88 lm/W for the maximum and at 1000 cd/m2, respectively. This
indicates that better charge recombination occurred in the DBTO-IN/CAR-
based device than in the reference CBP device. Consequently, the DBTO-
IN/CAR host material supplied sufficient energy to the green dopant to prevent
energy flow back to the host material. The DBTO-IN/CAR-based device
exhibited a high external quantum efficiency (EQE) of 19.03%, but the CBP-
based reference device exhibited a lower EQE of 17.72 % (Figure 6). The lower
HOMO energy level of DBTO-IN/CAR attributed to the hole transportation and
charge recombination into the emission layer. Additionally, the DBTO-IN/CAR
material based green OLED revealed a slightly lower efficiency roll-off, which
is an important phenomenon for long-operation devices.
50
104
103
102
101
100
60
50
40
30
20
10
0
50
40
30
20
10
0
CBP
DBTO-IN/CAR
CBP
DBTO-IN/CAR
40
30
20
10
0
3
4
5
6
7
0
10
20
30
40
50
Current density (mA/cm2)
Voltage (V)
Figure 5. Current density-voltage (J-V) and luminescence-voltage (L-V) characteristics of
DBTO-IN/CAR host material and reference CBP.
20
CBP
DBTO-IN/CAR
18
16
14
12
10
8
6
4
2
0
3.5
4.0
4.5
5.0
5.5
6.0
6.5
7.0
Voltage (V)
Figure 6. External quantum efficiencies of DBTO-IN/CAR and CBP based green PhOLEDs.

ACCEPTED MANUSCRIPT
We fabricated another OLED device using DBTO-IN/CAR as a bipolar
fluorescent emitting material without doping (Figure 7). The device structure
was ITO (150 nm)/ MoO3 (7 nm)/ NPB (80 nm)/ TCTA (5 nm)/ non-doped
DBTO-IN/CAR (20 nm)/ TPBI (40 nm)/ Liq (1 nm)/ Al (100 nm). The turn-on
voltage of the DBTO-IN/CAR bipolar emitter was 4.5 V. The maximum current
efficiency was 4.3 cd/A, which is one of the higher values among non-doped
fluorescent emitter-based OLEDs. The bipolar DBTO-IN/CAR fluorescent
emitter revealed a good maximum power efficiency of 3.0 lm/W. Consequently,
the fluorescent OLED device exhibited an excellent external quantum efficiency
of 2.47%. These details are depicted in Figure 8.
Figure 7. Device structure of DBTO-IN/CAR based non-doped fluorescent emitting OLED.
104
103
102
101
100
100
80
60
40
20
0
5
4
3
2
1
0
5
4
3
2
1
0
DBTO-IN/CAR (Dopant)
DBTO-IN/CAR (Dopant)
10
20
30
40
50
60
70
4
5
6
7
8
9
Current density (mA/cm2)
Voltage (V)
Figure 8. Current density-voltage (J-V) and luminescence-voltage (L-V) characteristics of
DBTO-IN/CAR as non-doped fluorescent emitter.
The electroluminescent spectra of the DBTO-IN/CAR non-doped OLED device
are shown in Figure 9. The maxima peak wavelength is 492 nm with CIE
coordinate values of 0.23 and 0.41 for x and y, respectively.

ACCEPTED MANUSCRIPT
DBTO-IN/CAR (Dopant)
0.03
0.02
0.01
0.00
450
500
550
600
650
700
Wavelength (nm)
400
450
500
550
600
Figure 9. Electroluminescence (EL) spectra of non-doped DBTO-IN/CAR fluorescent dopant.
Table 2. Device characteristics of green phosphorescent and non-doped fluorescence OLEDs.
DBTO-IN/CAR
(Non-doped
Emitter)
CBP
(Reference Host)
DBTO-IN/CAR
(Green Host)
Characteristic
3.8 V
5.2 V
3.6 V
4.9 V
4.5 V
8.1 V
Turn-on voltage
Driving voltage a
Current density b
0.10 mA/cm2
0.11 mA/cm2
1.25 mA/cm2
Current
efficiency
(Max)
50.81 cd/A
51.98 cd/A
4.30 cd/A
42.01 lm/W
17.72 %
45.36 lm/W
19.03 %
3.00 lm/W
2.47 %
Power efficiency
EQE
(Max)
52.89 cd/m2
0.34, 0.61
57.97 cd/m2
0.36, 0.61
53.63 cd/m2
0.23, 0.41
Luminescence b
CIE color b
a At 1000 cd/m2
b At turn-on voltage
4. Conclusion
A bipolar host and fluorescent emitting DBTO-IN/CAR material was designed
and synthesized using 2-bromodibenzo[b,d]thiophene 5,5-dioxide
withdrawing moiety and 5-phenyl-5,12-dihydroindolo[3,2-a]carbazole donor
a

ACCEPTED MANUSCRIPT
units. The DBTO-IN/CAR-based green phosphorescent OLED device exhibited
a low turn-on voltage of 3.6 V with an excellent external quantum efficiency of
19.03%. The maximum current and power efficiencies were 51.98 cd/A and
45.36 lm/W, respectively. These efficiencies are higher than those of the CBP-
based reference device, the external quantum and power efficiencies of which
were noted as 17.72% and 42.01 lm/W, respectively. The DBTO-IN/CAR-
based non-doped fluorescence emitting OLED device exhibited excellent power
and current efficiencies of 3.0 lm/W and 4.30 cd/A, respectively. The maximum
external quantum efficiency of the DBTO-IN/CAR fluorescence emitter was
noted as 2.47%. The CIE color coordinate values were 0.03 and 0.41 (x,y) with
an emission wavelength of 490 nm. The bipolar nature of DBTO-IN/CAR
enhanced the green phosphorescence OLED as well as the non-doped
fluorescent emitter.
Acknowledgement
This research was supported by Wonkwang University in 2017.
References
1. Jang, S.E., Joo, C.W., Yook, K.S., Kim, J.W., Lee, C.W. and Lee, J.Y.,
2010. Thermally stable fluorescent blue organic light-emitting diodes
using spirobifluorene based anthracene host materials with different
substitution position. Synthetic Metals, 160(11-12), pp.1184-1188.
2. Kim, G.H., Lampande, R., Park, M.J., Bae, H.W., Kong, J.H., Kwon,
J.H., Park, J.H., Park, Y.W. and Song, C.E., 2014. Highly Efficient
Bipolar Host Materials with Indenocarbazole and Pyrimidine Moieties
for Phosphorescent Green Light-Emitting Diodes. The Journal of
Physical Chemistry C, 118(49), pp.28757-28763.
3. Kawamura, Y., Goushi, K., Brooks, J., Brown, J. J., Sasabe, H. and
Adachi, C. (2005). 100% phosphorescence quantum efficiency of Ir (III)
complexes in organic semiconductor films. Applied Physics
Letters, 86(7), 071104.
4. Baldo, M.A., O'brien, D.F., You, Y., Shoustikov, A., Sibley, S.,
Thompson, M.E. and Forrest, S.R., 1998. Highly efficient
phosphorescent
emission
from
organic
electroluminescent
devices. Nature, 395(6698), p.151.
5. Köhler, A., Wilson, J.S. and Friend, R.H., 2002. Fluorescence and
phosphorescence in organic materials. Advanced Materials, 14(10),
pp.701-707.

ACCEPTED MANUSCRIPT
6. Tsai, M.H., Ke, T.H., Lin, H.W., Wu, C.C., Chiu, S.F., Fang, F.C., Liao,
Y.L., Wong, K.T., Chen, Y.H. and Wu, C.I., 2009. Triphenylsilyl-and
trityl-substituted
carbazole-based
host
materials
for
blue
electrophosphorescence. ACS applied materials & interfaces, 1(3),
pp.567-574.
7. Tao, Y., Yang, C. and Qin, J., 2011. Organic host materials for
phosphorescent organic light-emitting diodes. Chemical Society
Reviews, 40(5), pp.2943-2970.
8. Holmes, R.J., Forrest, S.R., Tung, Y.J., Kwong, R.C., Brown, J.J., Garon,
S. and Thompson, M.E., 2003. Blue organic electrophosphorescence
using exothermic host–guest energy transfer. Applied Physics
Letters, 82(15), pp.2422-2424.
9. Holmes, R.J., D’Andrade, B.W., Forrest, S.R., Ren, X., Li, J. and
Thompson,
M.E.,
2003.
Efficient,
deep-blue
organic
electrophosphorescence by guest charge trapping. Applied Physics
Letters, 83(18), pp.3818-3820.
10. Ren, X., Li, J., Holmes, R.J., Djurovich, P.I., Forrest, S.R. and
Thompson, M.E., 2004. Ultrahigh energy gap hosts in deep blue organic
electrophosphorescent devices. Chemistry of materials, 16(23), pp.4743-
4747.
11. Nakanotani, H., Masui, K., Nishide, J., Shibata, T. and Adachi, C., 2013.
Promising operational stability of high-efficiency organic light-emitting
diodes based on thermally activated delayed fluorescence. Scientific
reports, 3, p.2127.
12. Wang, S., Zhang, Y., Chen, W., Wei, J., Liu, Y. and Wang, Y., 2015.
Achieving high power efficiency and low roll-off OLEDs based on
energy transfer from thermally activated delayed excitons to fluorescent
dopants. Chemical Communications, 51(60), pp.11972-11975.
13. Zhang, Q., Tsang, D., Kuwabara, H., Hatae, Y., Li, B., Takahashi, T.,
Lee, S.Y., Yasuda, T. and Adachi, C., 2015. Nearly 100% internal
quantum efficiency in undoped electroluminescent devices employing
pure organic emitters. Advanced Materials, 27(12), pp.2096-2100.
14. Masui, K., Nakanotani, H. and Adachi, C., 2013. Analysis of exciton
annihilation in high-efficiency sky-blue organic light-emitting diodes
with
thermally
activated
delayed
fluorescence. Organic
Electronics, 14(11), pp.2721-2726.

ACCEPTED MANUSCRIPT
15. Shih, P.I., Chien, C.H., Chuang, C.Y., Shu, C.F., Yang, C.H., Chen, J.H.
and Chi, Y., 2007. Novel host material for highly efficient blue
phosphorescent OLEDs. Journal of Materials Chemistry, 17(17),
pp.1692-1698.
16. Lyu, Y.Y., Kwak, J., Jeon, W.S., Byun, Y., Lee, H.S., Kim, D., Lee, C.
and Char, K., 2009. Highly Efficient Red Phosphorescent OLEDs based
on Non‐Conjugated Silicon‐Cored Spirobifluorene Derivative Doped
with Ir‐Complexes. Advanced Functional Materials, 19(3), pp.420-427.
17. Shih, P.I., Chien, C.H., Chuang, C.Y., Shu, C.F., Yang, C.H., Chen, J.H.
and Chi, Y., 2007. Novel host material for highly efficient blue
phosphorescent OLEDs. Journal of Materials Chemistry, 17(17),
pp.1692-1698.
18. Tokito, S., Iijima, T., Suzuri, Y., Kita, H., Tsuzuki, T. and Sato, F., 2003.
Confinement of triplet energy on phosphorescent molecules for highly-
efficient organic blue-light-emitting devices. Applied physics
letters, 83(3), pp.569-571.
19. Tanaka, I., Tabata, Y. and Tokito, S., 2004. Energy-transfer and light-
emission mechanism of blue phosphorescent molecules in guest-host
systems. Chemical physics letters, 400(1-3), pp.86-89.
20. Liu, X.Y., Liang, F., Yuan, Y., Jiang, Z.Q. and Liao, L.S., 2016.
Utilizing 9, 10-dihydroacridine and pyrazine-containing donor–acceptor
host materials for highly efficient red phosphorescent organic light-
emitting diodes. Journal of Materials Chemistry C, 4(33), pp.7869-7874.
21. Han, C., Zhu, L., Li, J., Zhao, F., Zhang, Z., Xu, H., Deng, Z., Ma, D.
and Yan, P., 2014. Highly efficient multifluorenyl host materials with
unsymmetrical molecular configurations and localized triplet states for
green and red phosphorescent devices. Advanced Materials, 26(41),
pp.7070-7077.
22. Braveenth, R., Ahn, D.H., Han, J.H., Moon, J.S., Kim, S.W., Lee, H.,
Qiong, W., Kwon, J.H. and Chai, K.Y., 2018. Utilizing
triazine/pyrimidine acceptor and carbazole-triphenylamine donor based
bipolar novel host materials for highly luminescent green
phosphorescent OLEDs with lower efficiency roll-off. Dyes and
Pigments, 157, pp.377-384.

ACCEPTED MANUSCRIPT
23. Xu, H., Chen, R., Sun, Q., Lai, W., Su, Q., Huang, W. and Liu, X., 2014.
Recent progress in metal–organic complexes for optoelectronic
applications. Chemical Society Reviews, 43(10), pp.3259-3302.
24. Chen, Y.M., Hung, W.Y., You, H.W., Chaskar, A., Ting, H.C., Chen,
H.F., Wong, K.T. and Liu, Y.H., 2011. Carbazole–benzimidazole hybrid
bipolar host materials for highly efficient green and blue phosphorescent
OLEDs. Journal of Materials Chemistry, 21(38), pp.14971-14978.
25. Ting, H.C., Chen, Y.M., You, H.W., Hung, W.Y., Lin, S.H., Chaskar, A.,
Chou, S.H., Chi, Y., Liu, R.H. and Wong, K.T., 2012. Indolo [3, 2-b]
carbazole/benzimidazole hybrid bipolar host materials for highly
efficient red, yellow, and green phosphorescent organic light emitting
diodes. Journal of Materials Chemistry, 22(17), pp.8399-8407.
26. Kim, S.H., Jang, J. and Lee, J.Y., 2007. Relationship between host
energy levels and device performances of phosphorescent organic light-
emitting diodes with triplet mixed host emitting structure. Applied
Physics Letters, 91(8), p.083511.
27. Xie, W., Zhao, Y., Li, C. and Liu, S., 2007. High efficiency
electrophosphorescent red organic light-emitting devices with double-
emission layers. Solid-State Electronics, 51(8), pp.1129-1132.
28. Adamovich, V.I., Cordero, S.R., Djurovich, P.I., Tamayo, A., Thompson,
M.E., D’Andrade, B.W. and Forrest, S.R., 2003. New charge-carrier
blocking materials for high efficiency OLEDs. Organic electronics, 4(2-
3), pp.77-87.
29. Takizawa, S.Y., Montes, V.A. and Anzenbacher Jr, P., 2009.
Phenylbenzimidazole-based new bipolar host materials for efficient
phosphorescent
organic
light-emitting
diodes. Chemistry
of
Materials, 21(12), pp.2452-2458.
30. Chou, H.H. and Cheng, C.H., 2010. A highly efficient universal bipolar
host for blue, green, and red phosphorescent OLEDs. Advanced
Materials, 22(22), pp.2468-2471.
31. Kim, Y., Moon, B. and Ha, C.S., 2006. Doping effect of blue light-
emitting electron transport molecule in blue organic light-emitting
devices. Journal of applied physics, 100(6), p.064511.
32. Ho, C.L., Wong, W.Y., Gao, Z.Q., Chen, C.H., Cheah, K.W., Yao, B.,
Xie, Z.Y., Wang, Q., Ma, D.G., Wang, L.X. and Yu, X.M., 2008. Red‐
Light‐Emitting Iridium Complexes with Hole‐Transporting 9‐

ACCEPTED MANUSCRIPT
Arylcarbazole Moieties for Electrophosphorescence Efficiency/Color
Purity Trade‐off Optimization. Advanced Functional Materials, 18(2),
pp.319-331.
33. Tsai, M.H., Hong, Y.H., Chang, C.H., Su, H.C., Wu, C.C., Matoliukstyte,
A., Simokaitiene, J., Grigalevicius, S., Grazulevicius, J.V. and Hsu, C.P.,
2007. 3‐(9‐Carbazolyl) carbazoles and 3, 6‐Di (9‐carbazolyl) carbazoles
as Effective Host Materials for Efficient Blue Organic
Electrophosphorescence. Advanced Materials, 19(6), pp.862-866.
34. Thoms, T., Okada, S., Chen, J.P. and Furugori, M., 2003. Improved host
material design for phosphorescent guest–host systems. Thin Solid
Films, 436(2), pp.264-268.
35. Holmes, R.J., Forrest, S.R., Tung, Y.J., Kwong, R.C., Brown, J.J., Garon,
S. and Thompson, M.E., 2003. Blue organic electrophosphorescence
using exothermic host–guest energy transfer. Applied Physics
Letters, 82(15), pp.2422-2424.
36. Tokito, S., Iijima, T., Suzuri, Y., Kita, H., Tsuzuki, T. and Sato, F., 2003.
Confinement of triplet energy on phosphorescent molecules for highly-
efficient organic blue-light-emitting devices. Applied physics
letters, 83(3), pp.569-571.
37. Han, W.S., Son, H.J., Wee, K.R., Min, K.T., Kwon, S., Suh, I.H., Choi,
S.H., Jung, D.H. and Kang, S.O., 2009. Silicon-based blue
phosphorescence host materials: structure and photophysical property
relationship on methyl/phenylsilanes adorned with 4-(N-carbazolyl)
phenyl groups and optimization of their electroluminescence by
peripheral 4-(N-carbazolyl) phenyl numbers. The Journal of Physical
Chemistry C, 113(45), pp.19686-19693.
38. Vecchi, P.A., Padmaperuma, A.B., Qiao, H., Sapochak, L.S. and
Burrows, P.E., 2006. A dibenzofuran-based host material for blue
electrophosphorescence. Organic letters, 8(19), pp.4211-4214.
39. Chaskar, A., Chen, H.F. and Wong, K.T., 2011. Bipolar host materials: a
chemical approach for highly efficient electrophosphorescent
devices. Advanced Materials, 23(34), pp.3876-3895.
40. Choi, S., Godumala, M., Lee, J.H., Kim, G.H., Moon, J.S., Kim, J.Y.,
Yoon, D.W., Yang, J.H., Kim, J., Cho, M.J. and Kwon, J.H., 2017.
Optimized structure of silane-core containing host materials for highly

ACCEPTED MANUSCRIPT
efficient blue TADF OLEDs. Journal of Materials Chemistry C, 5(26),
pp.6570-6577.
41. Kim, J., Kim, S.H., Lee, H.W., Lee, S.E., Kim, Y.K. and Yoon, S.S.,
2015. Efficient bipolar host materials based on carbazole and 2-methyl
pyridine for use in green phosphorescent organic light-emitting
diodes. New Journal of Chemistry, 39(7), pp.5548-5552.
42. Ding, J., Wang, Q., Zhao, L., Ma, D., Wang, L., Jing, X. and Wang, F.,
2010. Design of star-shaped molecular architectures based on carbazole
and phosphine oxide moieties: towards amorphous bipolar hosts with
high triplet energy for efficient blue electrophosphorescent
devices. Journal of Materials Chemistry, 20(37), pp.8126-8133.
43. Chaskar, A., Chen, H.F. and Wong, K.T., 2011. Bipolar host materials: a
chemical approach for highly efficient electrophosphorescent
devices. Advanced Materials, 23(34), pp.3876-3895.
44. Chang, C.H., Kuo, M.C., Lin, W.C., Chen, Y.T., Wong, K.T., Chou,
S.H., Mondal, E., Kwong, R.C., Xia, S., Nakagawa, T. and Adachi, C.,
2012. A dicarbazole–triazine hybrid bipolar host material for highly
efficient green phosphorescent OLEDs. Journal of Materials
Chemistry, 22(9), pp.3832-3838.
45. Su, S.J., Sasabe, H., Takeda, T. and Kido, J., 2008. Pyridine-containing
bipolar host materials for highly efficient blue phosphorescent
OLEDs. Chemistry of Materials, 20(5), pp.1691-1693.
46. Chaskar, A., Chen, H.F. and Wong, K.T., 2011. Bipolar host materials: a
chemical approach for highly efficient electrophosphorescent
devices. Advanced Materials, 23(34), pp.3876-3895.

ACCEPTED MANUSCRIPT
Highlights
•
•
•
•
Current efficiency of bipolar host reached 51.98 cd/A
Green phosphorescent OLED showed outstanding power efficiency of 45.36 lm/W
Non-doped cyan emitting fluorescent OLED showed current efficiency of 4.30 cd/A
OLED devices showed lower efficiency roll off