Indenocarbazole based bipolar host materials for highly efficient yellow phosphorescent organic light emitting diodes

Article abstract of DOI:10.1016/j.orgel.2016.01.004

We report bipolar host materials with robust indenocarbazole and biphenyl moiety as hole-electron-transporting unit for phosphorescent yellow organic light-emitting diodes (OLEDs). New host materials demonstrated an excellent morphological stability with high glass transition temperature of 207 °C. Simultaneously, it also revealed appropriate triplet energy of about 2.6 eV for ideal triplet energy transfer to yellow phosphorescent dopant. A phosphorescent yellow OLED with new host ICBP1 (and ICBP2) and conventional yellow dopant iridium(III)bis(4-(4-t-butylphenyl)thieno[3,2-c]pyridinato-N,C2′)acetylacetonate (Ir(tptpy)₂acac) shows a low driving voltage of 3.4 (and 3.6 V) at 1000 cd/m², and maximum external quantum efficiency as high as 26.4%. Such efficient performance of phosphorescent yellow OLEDs is attributed to a good charge balance and high electron transport properties of host materials.

Full text of DOI:10.1016/j.orgel.2016.01.004

Organic Electronics 31 (2016) 11e18
Contents lists available at ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
Indenocarbazole based bipolar host materials for highly efficient
yellow phosphorescent organic light emitting diodes
Gyeong Heon Kim a, Raju Lampande a, Byoung Yeop Kang a, Hyeong Woo Bae a,
,
, **
Ju Young Lee a, Jang Hyuk Kwon a *, Jung Hwan Park b
a Department of Information Display, Kyung Hee University, 26, Kyungheedae-ro, Dongdaemun-gu, Seoul, 130-701, Republic of Korea
b Duksan Neolux Co., Ltd., 21-32, Ssukgol-gil, Ipjang-myeon, Seobuk-gu, Cheonan-si, Chungcheongnam-do, 331-821, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
We report bipolar host materials with robust indenocarbazole and biphenyl moiety as hole-electron-
transporting unit for phosphorescent yellow organic light-emitting diodes (OLEDs). New host mate-
rials demonstrated an excellent morphological stability with high glass transition temperature of 207 ꢀC.
Simultaneously, it also revealed appropriate triplet energy of about 2.6 eV for ideal triplet energy transfer
to yellow phosphorescent dopant. A phosphorescent yellow OLED with new host ICBP1 (and ICBP2) and
conventional yellow dopant iridium(III)bis(4-(4-t-butylphenyl)thieno[3,2-c]pyridinato-N,C20)acetylacet-
onate (Ir(tptpy)2acac) shows a low driving voltage of 3.4 (and 3.6 V) at 1000 cd/m2, and maximum
external quantum efficiency as high as 26.4%. Such efficient performance of phosphorescent yellow
OLEDs is attributed to a good charge balance and high electron transport properties of host materials.
© 2016 Elsevier B.V. All rights reserved.
Received 3 November 2015
Received in revised form
29 December 2015
Accepted 2 January 2016
Available online xxx
Keywords:
Bipolar host
Indenocarbazole
Phosphorescence
Organic light-emitting diodes
Triplet energy
1. Introduction
component in modern two stacks tandem white organic light
emitting diodes for solid state lighting applications [4]. Addition-
Since the discovery of organic electroluminescent devices,
several research papers have been investigated for the high per-
formance of phosphorescent organic light emitting diodes (OLEDs).
Phosphorescent materials can harvest both singlet and triplet ex-
citons, which helps to achieve theoretical 100% internal quantum
efficiency of OLED devices. The favorable attributes of OLEDs are
low manufacturing cost, easy fabrication process, flexibility, high
brightness, fast response time, wide viewing angle and good life-
time yield etc. for the solid state lighting [1,2]. Although, several
studies have been reported towards highly effectual host, proficient
interfacial layers and optimized architectures but still the intrinsic
limits of OLED upto their theoretical limit have not been reached
yet [3]. Therefore, further scientific efforts are needed in the di-
rection of new material design for host with suitable triplet energy,
charge transport and injection layers to reach the theoretical limit
of OLED performance.
ally, phosphorescent yellow materials are getting important com-
ponents to have high color rendering index in modern proper
multiple peak white OLED for lighting applications [5]. To get a high
performance of white OLEDs, efficient phosphorescent yellow host
is equally important due to their great potential. Therefore, to
determine high quantum efficiency and current/power efficiency of
yellow devices, development of new host materials containing new
hole and electron moieties with suitable bipolar characteristics,
large triplet energy and high charge carrier mobility as well as
efficient hole and electron transport layers are needed. Another
performance decisive factor is the charge balance, which plays a
vital role in determining the external quantum efficiency and cur-
rent/power efficiencies of OLEDs [6]. Proper charge balance not
only widens the recombination zone in the emissive layer (EML)
but also leads to a high performance with minimization of tri-
pletetriplet annihilation [7]. To achieve a proper charge balance in
the EML, bipolar characteristics of host materials are very crucial for
efficient hole and electron movement. The strong bipolar charac-
teristics can be attained by using appropriate hole and electron
transport moiety in the host materials. Furthermore, proper charge
balance in the emissive layer can also be achieved by tuning the
energy levels of host and dopant system [8]. This helps to reduce
Phosphorescent yellow and blue host materials are crucial
* Corresponding author.
** Corresponding author.
E-mail addresses: jhkwon@khu.ac.kr (J.H. Kwon), jhpark@dshm.co.kr (J.H. Park).
http://dx.doi.org/10.1016/j.orgel.2016.01.004
1566-1199/© 2016 Elsevier B.V. All rights reserved.

12
G.H. Kim et al. / Organic Electronics 31 (2016) 11e18
the charge trapping emission by dopant molecules and provides a
complete energy transfer from host to dopant molecule.
(m, 4H), 7.37e7.25 (m, 6H), 1.51 (s, 6H).
Over the past few years, very limited studies have been reported
on developing new phosphorescent host materials for yellow OLED
devices. The 4,40-bis~9-carbazolyl-2,20-biphenyl (CBP) has been
one of the widely used phosphorescent yellow host material, which
consist of robust carbazole moiety and biphenyl group [9]. The CBP
has good triplet energy but shows a relatively low glass transition
temperature (Tg, 62 ꢀC) value, which could results in poor thermal
stability of the device. Additionally, it also has a high LUMO energy
value, which may directly effects on the electron injection prop-
erties from transport layer and form improper charge balance at the
emissive layer, results in average device performance [10]. There-
fore, to improve the performance of OLEDs, narrow bandgap host
materials with their good charge injection properties from the
charge transport are required. Recently, Hwang et al. reported a
phenylcarbazole and pyrimidine based phosphorescent host ma-
terial with a bandgap of 3.13 eV. They showed a maximum current
efficiency of 68.3 cd/A and EQE of 22.3% for yellow OLED device
[11]. Similarly, Yang et al. also designed and synthesized a phe-
nanthroimidazole/carbazole based hybrid bipolar host materials for
phosphorescent yellow devices and which has a bandgap of 3.11 eV
and high Tg value in the range of 113e243 ꢀC. They demonstrated a
maximum EQE and current efficiency of 19.3% and 57.2 cd/A for the
2-(4-(9H-carbazol-9-yl)phenyl)-1-phenyl-1H-phenanthro[9,10-d]
imidazole and (fbi)2Ir(acac) host dopant combination [6]. In our
previous report, we demonstrated a maximum current efficiency of
84.4 cd/A for the Bepp2: iridium(III)bis(4-(4-t-butylphenyl)thieno
2.1.2. 12,12-Dimethyl-11,12-dihydroindeno[2,1-a]carbazole (2) and
11,11-dimethyl-5,11-dihydroindeno[1,2-b]carbazole (3)
A mixture of 1 (56 g, 459.78 mmol) and triphenylphosphine
(301.49 g, 1149.44 mmol) was dried in vacuum and filled with ni-
trogen gas in
5 L round bottom flask. 1,2-Dichlorobenzene
(2200 mL) was added to dissolve the mixture and refluxed for
12 h in nitrogen atmosphere. After solvent removal with distilla-
tion, the residue was divided by column chromatography
(eluent
¼
CH2Cl2/hexane, 1:7) to 12,12-dimethyl-11,12-
and 11,11-dimethyl-5,11-
dihydroindeno[2,1-a]carbazole
dihydroindeno[1,2-b]carbazole, respectively. Two residues were
recrystallized with CH2Cl2/hexane and obtained white solid of
12,12-dimethyl-11,12-dihydroindeno[2,1-a]carbazole and 11,11-
dimethyl-5,11-dihydroindeno[1,2-b]carbazole, respectively (yield:
12,12-dimethyl-11,12-dihydroindeno[2,1-a]carbazole 42.5 g, 11,11-
dimethyl-5,11-dihydroindeno[1,2-b]carbazole 37.5 g, 73%). 1H
NMR (CDCl3, 300 MHz)
d
(ppm) (12,12-dimethyl-11,12-
dihydroindeno[2,1-a]carbazole) 8.14e8.10 (m, 3H), 7.85e7.83 (m,
1H), 7.71e7.69 (d, J ¼ 8.0 Hz, 1H), 7.55e7.50 (m, 2H), 7.48e7.29 (m,
4H), 1.73 (s, 6H) (11,11-dimethyl-5,11-dihydroindeno[1,2-b]carba-
zole) 8.11e8.06 (m, 2H), 7.75e7.70 (m, 1H), 7.67 (s, 1H), 7.49e7.44
(m, 1H), 7.40e7.32 (m, 5H), 7.26e7.22 (m, 1H), 1.60 (s, 6H). 13C NMR
(CDCl3, 500 MHz) d (ppm) 153.3, 140.3, 140.1, 137.4, 135.3, 134.2,
127.2, 126.8, 125.7, 123.9, 123.5, 122.2, 120.3, 120.0, 119.8, 117.4,
112.3, 110.7, 46.7, 25.7.
[3,2-c]pyridinato-N,C20)acetylacetonate
(Ir(tptpy)2acac)
host
2.1.3. 11-(4-bromophenyl)-12,12-dimethyl-11,12-dihydroindeno
[2,1-a]carbazole (4)
dopant system [12]. Recently, we reported series of new green host
materials with indenocarbazole as hole transporting moiety, and
exhibited an excellent OLED device performances [7]. Encouraging
from these results, we developed host materials using inden-
ocarbazole moiety to further enhance the yellow OLED perfor-
mances in terms of efficiency and lifetime.
In this work, we modified the carbazole based CBP compound by
adding efficient indolocarbozole moiety with dimethyl group at
inner and outer position of the compound to decrease the band gap
and to improve the charge balance property as well as thermal
stability of the new bipolar host materials. The new compound was
successfully synthesized and used as a host for phosphorescent
yellow OLEDs. The photo-physical, electro-chemical and thermal
characteristics of new materials were successfully examined by
using UVevis absorption, photoluminescence (PL), cyclic voltam-
metry (CV), differential scanning calorimetry (DSC) and thermog-
ravimetric analysis (TGA) respectively.
A mixture of 2 (30 g, 105.87 mmol), 1-bromo-4-iodobenzene
(89.85 g, 317.61 mmol), copper powder (7.40 g, 116.46 mmol), po-
tassium carbonate (14.63 g, 105.87 mmol) and 18-crown-6 (2.80 g,
10.59 mmol) was refluxed for 24 h in nitrogen atmosphere in 2 L
round bottom flask. After reaction, the residue was divided by
column chromatography (eluent ¼ CH2Cl2/hexane, 1:3). And the
residue was recrystallized using CH2Cl2/hexane (yield: 31.5 g, 68%).
1H NMR (CDCl3, 300 MHz)
d
(ppm) 8.19e8.17 (d, J ¼ 8.1 Hz, 1H),
8.13e8.10 (d, J ¼ 7.2 Hz, 1H), 7.96e7.93 (d, J ¼ 8.4 Hz, 1H), 7.80e7.73
(m, 3H), 7.43e7.40 (d, J ¼ 8.4 Hz, 1H), 7.37e7.25 (m, 6H), 6.81e6.78
(d, J ¼ 7.2 Hz, 1H), 1.19 (s, 6H). 13C NMR (CDCl3, 500 MHz) d (ppm)
154.4, 144.44, 144.42, 139.5, 139.3, 138.9, 133.2, 133.0, 132.9, 127.0,
126.9,125.9,125.2,123.8,122.0,120.4,119.7,119.5,119.4,113.2,110.7,
110.6, 48.0, 26.5.
2.1.4. 12,12-Dimethyl-11-(4-(4,4,5,5-tetramethyl-1,3,2-
dioxaborolan-2-yl)phenyl)-11,12-dihydroindeno[2,1-a]carbazole (5)
A mixture of 4 (20.0 g, 45.62 mmol), 4,4,40,40,5,5,50,50-octa-
methyl-2,20-bi(1,3,2-dioxaborolane) (12.74 g, 50.19 mmol), [1,10-
Bis(diphenylphosphino)ferrocene]dichloropalladium(II) (1.12 g,
1.37 mmol) and Potassium acetate (13.43 g,136.87 mmol) was dried
and filled with nitrogen gas in 1 L round bottom flask. DMF
(230 mL) was added to dissolve the mixture and refluxed for 3 h in
nitrogen atmosphere. After reaction, the solution was extracted
with ethyl acetate and dried over anhydrous MgSO4. The dried
solution was filtered. And finally the solution was purified with
column chromatography (eluent ¼ ethyl acetate/hexane, 1:3) and
recrystallized with CH2Cl2/hexane to get white solid (yield: 13.7 g,
2. Experimental section
2.1. Synthesis
2.1.1. 9,9-Dimethyl-2-(2-nitrophenyl)-9H-fluorene (1)
2-(9,9-dimethyl-9H-fluoren-2-yl)-4,4,5,5-tetramethyl-1,3,2-
dioxaborolane (150 g, 468.41 mmol), 1-bromo-2-nitrobenzene
(89.89 g, 444.99 mmol), tetrakis(triphenylphosphine)palladium(0)
(27.07 g, 23.42 mmol) and potassium carbonate (2.2 M, 640 mL,
1405.24 mmol) with tetrahydrofuran (2200 mL) and water(500 mL)
solvents were refluxed for 4 h in 5 L round bottom flask. The so-
lution was extracted with dichloromethane, and dried over anhy-
drous MgSO4. After removal of solvent under reduced pressure,
final residue was purified by column chromatography
(eluent ¼ dichloromethane/hexane,1:3) to get the pale yellow solid
of 9,9-dimethyl-2-(2-nitrophenyl)-9H-fluorene (yield: 147.7 g,
62%). 1H NMR (CDCl3, 300 MHz)
d
(ppm) 8.21e8.19 (d, J ¼ 7.9 Hz,
1H), 8.14e8.11 (m, 1H), 8.08e8.05 (d, J ¼ 8.3 Hz, 2H), 7.81e7.76 (m,
2H), 7.57e7.54 (d, J ¼ 8.2 Hz, 1H), 7.37e7.25 (m, 5H), 6.80e6.78 (m,
1H), 1.45 (s, 12H), 1.18 (s, 6H). 13C NMR(CDCl3, 500 MHz) d (ppm)
154.6, 144.6, 144.0, 139.6, 139.3, 135.94, 135.91, 130.6, 126.8, 126.7,
125.7, 125.1, 123.7, 121.9, 120.1, 119.5, 119.3, 119.3, 112.9, 110.8, 84.2,
48.0, 26.3, 24.9.
84%). 1H NMR (CDCl3, 300 MHz)
7.79e7.74 (m, 2H), 7.66e7.61 (m, 1H), 7.55e7.42 (m, 3H), 7.39e7.30
d (ppm) 7.88e7.85 (m, 1H),

G.H. Kim et al. / Organic Electronics 31 (2016) 11e18
13
2.1.5. 11-[40-(12,12-Dimethyl-11a,12-dihydro-6aH-11-aza-indeno
[2,1-a]fluoren-11-yl)-biphenyl-4-yl]-12,12-dimethyl-11,12-dihydro-
11-aza-indeno[2,1-a]fluorene (ICBP1)
(2.41 g, 7.72 mmol), Tris(dibenzylideneacetone)dipalladium(0)
(0.35 g, 0.38 mmol), Tri-tert-butylphosphine (0.156 g, 0.77 mmol)
and sodium tert-butoxide (2.24 g, 23.18 mmol) was dried and filled
with nitrogen in 250 mL round bottom flask. Toluene (100 mL) was
added to dissolve the mixture and refluxed for 4 h in nitrogen at-
mosphere. After reaction, the product was worked up and extracted
with CH2Cl2. The solution was dried over MgSO4 and filtered. The
filtered solution was divided by column chromatography
(eluent ¼ dichloromethane/hexane, 1:3) and recrystallized with
dichloromethane/hexane to get white solid (yield: 4.2 g, 76%). 1H
A mixture of 5 (5.00 g, 10.30 mmol), 4 (4.52 g, 10.30 mmol),
Tetrakis(triphenylphosphine)palladium(0) (0.56 g, 0.52 mmol) and
Potassium Carbonate (1 M, 30 mL, 30.90 mmol) was refluxed for
14 h in THF (80 mL) solution of 250 mL round bottom flask. After
reaction, solid product was filtered and washed with water/
acetone. White and solid compound was melted with heating in
toluene and filtered by silica. The solution was recrystallized with
acetone (yield: 2.4 g, 32.5%). 1H NMR (CDCl3, 300 MHz)
d
(ppm)
NMR (CDCl3, 300 MHz)
4H), 7.86 (s, 2H), 7.83e7.75 (m, 6H), 7.55e7.45 (m, 6H), 7.41e7.34
(m, 6H), 1.19 (s, 12H); 13C NMR (CDCl3, 500 MHz)
(ppm) 154.7,
d (ppm) 8.26e8.25 (m, 4H), 7.98e7.95 (m,
8.24e8.21 (d, J ¼ 7.9, 2H), 8.16e8.14 (m, 2H), 8.01e7.97 (m, 4H),
7.82e7.78 (m, 4H), 7.71e7.68 (m, 4H), 7.38e7.26 (m, 10H),
6.90e6.88 (m, 2H), 1.55 (s, 4H), 1.27 (s, 8H); 13C NMR (CDCl3,
d
146.8, 141.7, 141.1, 139.7, 139.5, 138.4, 137.7, 128.8, 127.9, 127.5, 127.1,
125.9, 123.9, 123.6, 122.9, 120.31, 120.30, 120.1, 114.3, 110.0, 101.2,
46.5, 28.3. Found: C, 90.1; H, 5.5; N, 3.9. Calc. for C54H42N2: C, 90.2;
H, 5.9; N, 3.9%.
500 MHz) d (ppm) 158.0, 154.7, 144.8, 141.3, 141.1, 139.9, 132.1, 128.4,
127.1, 127.0, 126.0, 125.3, 124.0, 122.1, 120.4, 119.8, 119.6, 119.6, 116.5,
113.2, 111.0, 105.5, 48.3, 29.9, 26.6. Found: C, 90.0; H, 5.5; N, 3.8.
Calc. for C54H42N2: C, 90.2; H, 5.9; N, 3.9%.
2.2. Material characterization
2.1.6. 6-[40-(12,12-Dimethyl-10a,12-dihydro-6aH-6-aza-indeno
[1,2-b]fluoren-6-yl)-biphenyl-4-yl]-12,12-dimethyl-6,12-dihydro-6-
aza-indeno[1,2-b]fluorene (ICBP2)
1H and 13C NMR spectra were recorded using Bruker Avance 400
NMR spectrometer. The differential scanning calorimetry was done
using TA Instruments DSC Q2000. UVevis and PL spectra were
A mixture of 3 (4.38 g, 15.45 mmol), 4,40-dibromo-1,10-biphenyl
Scheme 1. Synthetic route of ICBP1 and ICBP2.

14
G.H. Kim et al. / Organic Electronics 31 (2016) 11e18
measured using SCINCO S-4100 spectrometer and JASCO FP6500
spectrometer, respectively. CV was performed using EC epsilon
electrochemical analysis equipment. The optical band-gap was
determined by using the tailing edge of absorption spectra. The
HOMO level was obtained from the CV characteristics. The LUMO
level of each material was measured from the optical band gap and
HOMO value.
Minolta CS-2000 spectroradiometer. All measurements were per-
formed in ambient condition.
3. Results and discussion
3.1. Molecular design and simulation
To design new bipolar host materials, indenocarbazole moiety
was utilized as hole transporting unit in the commercially available
CPB host. Two chemical structures of indenocarbazole were
considered, where position of dimethyl functional group is located
in the inner and outer directions. The detailed synthetic routes of
ICBP1 and ICBP2 are explained in Scheme 1.
2.3. Device fabrication and characterization
To fabricate yellow phosphorescence OLEDs, Indium-Tin-Oxide
(ITO) coated glass substrates (50 nm, sheet resistance of 10
U/,)
were sequentially cleaned in ultrasonic bath with acetone, and
isopropyl alcohol for 10 min each, and then rinsed with deionized
water. Finally, substrates were dried using nitrogen followed by UV-
ozone treatment for 10 min. All organic layers and metal cathode
were deposited on the pre-cleaned ITO glass by vacuum evapora-
tion technique under a vacuum pressure of ~1 ꢁ 10ꢂ7 Torr. The
deposition rate of all organic layers in was about 0.5 Å/s. Similarly
the deposition rate of LiF and Al were maintained at 0.1 Å/s, 5 Å/s,
respectively. Finally, all devices were encapsulated using glass
cover and UV curable resin inside the nitrogen filled glove box. The
OLED area was 4 mm2 for all the samples studied in this work. JeV
and LeV characteristics of fabricated OLED devices were measured
by using Keithley 2635A SMU and Konica Minolta CS-100A,
respectively. EL spectra and Commission Internationale de
l'Eclairage (CIE) 1931 color coordinate were obtained using Konica
In order to understand the structural characteristic and geom-
etry optimization of ICBP1 and ICBP2, molecular simulations were
performed using density functional theory (DFT) calculation with
B3LYP (beck three parameter hybrid functional [13e15] and Lee-
eYangeParr correlation functional [16]) and 6-31G (d) basis set
implemented in Gaussian 09 [17]. The molecular orbital dispersion
of bipolar host materials in the highest occupied molecular orbital
(HOMO) and lowest unoccupied molecular orbital (LUMO) states
are shown in Fig. 1. It is clearly observed that HOMO and LUMO
orbitals of ICBP1 are well separated whereas ICBP2 show small
coupling between both orbitals. In particular, the HOMO orbitals
are mainly situated over the indenocarbazole moiety, whereas the
LUMO orbitals are localized on the biphenyl moiety. Such well
separated HOMO and LUMO orbitals are needed for the efficient
charge transport and high triplet energy. In order to gain more
insight into the rotation angle between indenocarbazole and
biphenyl functional group, we calculated their respective rotation
angles for the inner and outer dimethyl group configuration. The
rotation angles of these two moieties in ICBP1 are 77.8ꢀ and 86.1ꢀ
respectively. However, in case of ICBP2 it is 56.1ꢀ and 45.0ꢀ, which is
quite lower than the ICBP1. The strong steric hindrance by inner
dimethyl groups of ICBP1 resulting into bigger rotated angle as
compare to the outer dimethyl group in ICBP2. This phenomenon
seems to provide a little wider band gap and high T1 characteristics.
The triplet energy of the molecules are calculated from the differ-
ence between the total energy of the molecule from singlet ground
state (S0) to triplet lowest excited state (T1). The calculated T1
values from the molecular simulations of ICBP1 and ICBP2 are 2.76
and 2.74 eV, which is little higher than the experimental value but
shows an expected tendency and valid agreement with the exper-
imental results.
The calculated HOMO values of ICBP1 and ICBP2 are 5.30 and
5.27 eV, which is slightly deeper, but correlate well with the
experimental values. On the other hand, calculated LUMO values of
ICBP1 and ICBP2 are 1.49 (experimental: 2.49 eV) and 1.24 eV
(experimental: 2.53 eV), respectively, which is quite higher than
Fig. 1. Spatial distribution of the HOMO and LUMO from DFT calculation.
Table 1
Physical properties of ICBP1 and ICBP2.
Host
Td0.5/5.0b [ꢀC]
labsc [nm]
lPLc [nm]
Egd (Cal.e) [eV]
HOMOf (Cal.g) [eV]
LUMOh (Cal.g) [eV]
T1c,i (Cal.g) [eV]
a
Tg [ꢀC]
a
Tm [ꢀC]
ICBP1
ICBP2
NAj
207
424
380
408/438
390/436
319
325
377
380
3.38 (3.81)
3.24 (4.03)
5.86 (5.30)
5.77 (5.27)
2.49 (1.49)
2.53 (1.24)
2.63 (2.76)
2.58 (2.74)
a
Measured from DSC.
Measured from TGA.
Measured in 2-Methyl THF.
Measured by a band edge of absorption spectra.
Values from the calculated HOMO and LUMO.
Determined from the onset of the oxidation potentials.
Values from DFT calculation B3LYP/6-31G(d).
Calculated from the HOMO and Eg.
b
c
d
e
f
g
h
i
Determined from peak value of low temperature PL spectra (77 K).
Not detected.
j

G.H. Kim et al. / Organic Electronics 31 (2016) 11e18
15
the experimental values. However, theoretical HOMO and LUMO
values of all two materials show contrast tendency as compare to
the experimental data even after the multiple runs of simulation,
hence exact reason behind such tendency is unclear.
The calculated band-gap of ICBP1 and ICBP2 are 3.81 eV, 4.03 eV,
respectively. Though, these values are contrast to the experimental
values. In particular, the band gap of ICBP1 should be higher than
the ICBP1 due to their high conjugation length and high rotation
angle. However, simulated bandgap values are almost contrasted
for both compounds even after the multiple runs of simulations.
The exact reason for such type of bandgap values is unclear, hence
further study is required. The complete physical properties are
summarized in Table 1.
3.2. Photo-physical, electro-chemical and thermal properties
The UVeVis absorption and PL spectra of new compounds in
dilute dichloromethane (CH2Cl2) solution were investigated. The
relevant values of the maximum absorption and PL peaks are dis-
played in Table 1. The absorption and PL spectra of ICBP1 and ICBP2
in solution state are shown in Fig. 2. Both compound ICBP1 and
ICBP2 show absorption peaks at around ~260 nm and ~325 nm. The
Fig. 4. CV analysis of ICBP1 and CBP2 for oxidation potential.
of indenocarbazole, whereas relatively lower energy absorption
peak at 319e325 nm could be assigned to * of indenocarbazole.
strong absorption peaks at 260 nm are ascribed to nꢂ
p* transition
pꢂp
Relatively longer conjugation length of ICBP2 due to their outer
dimethyl group is well matched with the simulated lower torsional
angle results. The maximum PL peak of ICBP1 was observed at
374 nm which is blue shifted by 6 nm as compare to the ICBP2
(380 nm).
Fig. 2. UVevis absorption and PL spectra of ICBP1 and ICBP2.
Fig. 3. Phosphorescence PL spectra of ICBP1 and ICBP2 in 2-methyl tetrahydrofuran
solution (1 ꢁ 10ꢂ4 M) at 77 K.
Fig. 5. JeV characteristics of (a) HOD and (b) EOD.

16
G.H. Kim et al. / Organic Electronics 31 (2016) 11e18
The triplet energies of ICBP1 and ICBP2 were evaluated from the
phosphorescence PL spectra measured at low temperature (77 K, in
liquid nitrogen) in 2-methyl tetrahydrofuran solution (Fig. 3). The
measured triplet energy of ICBP1 and ICBP2 using initial phos-
phorescent emission peak are 2.63 (
¼ 471 nm) and 2.58 eV
¼ 480 nm), respectively. These values are suitable for triplet
l
(l
Fig. 6. (a) Energy level diagram of fabricated yellow phosphorescent OLEDs. (b) Molecular structures of Ir(tptpy)2acac, HATCN, TAPC and TmPyPB.

G.H. Kim et al. / Organic Electronics 31 (2016) 11e18
17
perchlorate (TBAP) 10ꢂ4 M in acetonitrile solution. The cyclic vol-
tammogram of compounds are shown in Fig. 4 and their relative
values are listed in Table 1. Platinum wire, synthesized material on
ITO coated glass substrate and Ag wire in 0.1 M AgNO3 was used as
counter, working and reference electrodes, respectively. Similarly,
tetrabutyl ammonium perchlorate (Bu4NClO4) was used as a sup-
porting electrolyte. HOMO values of ICBP1 and ICBP2 were obtained
from the onset point of oxidation potential and assuming the en-
ergy level of ferrocene/ferrocenium (Fc/Fcþ) (i.e., 4.80 eV). Both
host materials show HOMO level of 5.86 eV (ICBP1) and 5.77 eV
(ICBP2). The LUMO energy level was calculated from the HOMO
level and energy band-gap value, which is measured from the
tailing edge of UVeVis absorption spectrum. The LUMO value of
ICBP1 and ICBP2 are 3.38 and 3.24 eV, respectively.
TGA and DSC were used to inspect the thermal properties of
new host materials. The thermal properties of both compounds
(ICBP1 and ICBP2) are listed in Table 1. The decomposition tem-
peratures (Td) of ICBP1 and ICBP2 (corresponding to 5% loss) are
found to be 438 and 436 ꢀC, respectively. Moreover, Tg of ICBP2 was
noticed at 207 ꢀC, which is very high and appropriate for the
fabrication of OLED devices. However, the Tg value of ICBP1 was not
detected but we expect that the Tg of ICBP1 could be similar or close
to ICBP2 due to their similar chemical structure.
3.3. Charge transport properties
In order to study the bipolar characteristics of ICBP1 and ICBP2
host materials, electron only device (EOD) and hole only device
(HOD) were fabricated. The (4, 40-cyclohexylidenebis[N,N-bis(4-
methylphenyl)benzenamine]) TAPC and 3,30-[50-[3-(3-pyridyl)
phenyl][1,10:30,100-terphenyl]-3,300-diyl]bispyridine (TmPyPB) were
used as hole and electron transporting materials to block the in-
jection of electrons and holes from the respective electrodes. The
charge transporting devices were fabricated with the following
structure: ITO/TAPC (40 nm)/Host (30 nm)/TAPC (40 nm)/Al
(100 nm) as HOD and ITO/TmPyPB (60 nm)/Host (30 nm)/TmPyPB
(60 nm)/LiF (1.5 nm)/Al (100 nm) as EOD. The current density
versus-voltage (JeV) characteristics of both HOD and EOD devices
are shown in Fig. 5. The excellent charge transport properties of
both devices clearly indicate the bipolar behavior of new host
materials. Moreover, ICBP1 shows high electron current density for
the measured voltage range as compare to the ICBP2 host. However,
hole current densities of new compounds are almost identical. Such
improved electron transporting properties are attributed to
reduced LUMO level of host material. We believe that the high
electron and proper hole transporting properties of ICBP1 could
provide a proper charge balance in the emissive layer of OLEDs.
Fig. 7. Performances of bipolar host ICBP1, ICBP2 devices. (a) JeVeL characteristics (b)
EQE-current density characteristics and (c) EL spectra.
energy transfer from host to phosphorescent yellow dopant.
The electro-chemical properties of both new host materials
were evaluated by CV measurements using tetra butyl ammonium
3.4. Electroluminescent properties
The performances of indenocarbazole based modified CBP with
Table 2
Electroluminescence properties of CBP, ICBP1 and ICBP2 devices.
Host
Turn-on voltagea [V]
Driving voltageb [V]
Efficiencyb (max.)
CIE 1931 (x, y)b
CEc [cd/A]
PEd [lm/W]
EQEe [%]
CBP
ICBP1
ICBP2
2.8
2.8
2.7
3.4
3.4
3.6
79.4 (79.1)
85.9 (94.1)
64.8 (69.4)
77.9 (73.1)
80.4 (92.4)
57.1 (68.2)
21.4 (22.3)
22.6 (26.4)
17.7 (18.9)
(0.49, 0.51)
(0.49, 0.50)
(0.49, 0.50)
a
Measured at 1 cd/m2.
b
c
Measured at 1000 cd/m2.
Current efficiency.
Power efficiency.
External quantum efficiency.
d
e

18
G.H. Kim et al. / Organic Electronics 31 (2016) 11e18
dimethyl functional group in same and opposite directions as host
materials were examined by fabricating phosphorescent yellow
OLEDs. The fabricated device structure is as follows: ITO/
1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile (HATCN, 7 nm)/
TAPC (75 nm)/Host: 7 wt% Ir(tptpy)2acac (20 nm)/TmPyPB (60 nm)/
LiF (1.5 nm)/Al (100 nm). The energy level diagram of the fabricated
OLED and the molecular structures of studied materials are shown
in Fig. 6. TAPC and TmPyPB were used as hole and electron trans-
port layers due to their appropriate triplet energy to prevent triplet
exciton quenching from the emissive layer [18,19]. Ir(tptpy)2acac
was as a yellow dopant in the emissive layer. Both yellow devices
show almost identical current density however ICBP1 based device
shows low turn on voltage of 2.8 V as compare to ICBP2 (2.9 V) at
1 cd/m2. However, the driving voltages of ICBP1 and ICBP2 at
brightness of 1000 cd/m2 are 3.4 and 3.6 V, respectively. In
particular, such a low turn on voltages of both devices are attrib-
uted to the low LUMO level of the host compounds. The current
density versus voltage characteristics, device efficiencies and
electroluminescence (EL) spectra of the fabricated OLED devices
with new host are shown in Fig. 7. The phosphorescent yellow
device with ICBP1 exhibits a maximum current and power effi-
ciencies of 94.1 cd/A and 92.4 lm/W. Similarly, ICBP2 based device
shows the maximum current and power efficiencies of 69.4 cd/A
and 68.2 lm/W. The performances of the fabricated phosphorescent
yellow OLEDs are summarized in Table 2. The ICBP1 based device
also shows an EQE as high as 26.4%, whereas ICBP2 has EQE of 18.9%
(Fig. 7. (b)). To the best of our knowledge this is one of the best EQE
value reported for phosphorescent yellow device. As shown in
Fig. 5, HOD, EOD devices fabricated three host materials (CBP, ICBP1,
and ICBP2) show similar hole-transport property, but electron-
transport property follows in order of ICBP1 > CBP > ICBP2. We
also compared fabricated OLED device performances made with
these three host materials. Measured current efficiency perfor-
mances exactly correlate with the order of electron transporting
property (See Table 2). These results indicate that enhanced elec-
tron transporting property of ICBP1 results in better charge balance
characteristic than other hosts.
yellow phosphorescent OLEDs demonstrated a very high EQE of
26.4%. Similarly, both host materials are thermally stable due to
their high Tg of 207 ꢀC. These new host materials will be an alter-
native to the widely used CBP host material for the future highly
efficient yellow phosphorescent light-emitting diodes.
Acknowledgments
This work was supported by the Human Resources Develop-
ment program (No. 20154010200830) of the Korea Institute of
Energy Technology Evaluation and Planning (KETEP) and grants
funded by the Korea government Ministry of Trade, Industry and
Energy. And this work was also supported by the Industrial Stra-
tegic Technology Development Program of MKE/KEIT [No.
10048317, Development of red and blue OLEDs with external
quantum efficiency over 20% using delayed fluorescent materials].
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In conclusion, we demonstrated an efficient bipolar host mate-
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porting properties to provide a proper charge balance, high triplet
energy and appropriate LUMO, HOMO energy levels. Fabricated
€
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