Utilizing triazine/pyrimidine acceptor and carbazole-triphenylamine donor based bipolar novel host materials for highly luminescent green phosphorescent OLEDs with lower efficiency roll-off

Article abstract of DOI:10.1016/j.dyepig.2018.05.022

In this work, two novel bipolar host materials were designed, synthesized and applied in green phosphorescent based OLEDs. Both the host materials, 4-(2-(4,6-diphenyl-1,3,5-triazin-2-yl)-9H-carbazol-9-yl)-N,N-diphenylaniline (TRZ 1) and 4-(2-(4,6-diphenylpyrimidin-2-yl)-9H-carbazol-9-yl)-N,N-diphenylaniline (PYR 1) exhibited high thermal stability, with decomposition temperatures of 425 °C and 400 °C, respectively. The triplet energy of PYR 1 (2.63 eV) was higher than that of TRZ 1 (2.44 eV), and facilitated suitable energy transfer to the green dopant. The PYR 1 based green device demonstrated an excellent maximum current efficiency of 48.7 cd/A and external quantum efficiency of 16.4%. Interestingly, the green device with PYR 1 showed an outstanding brightness of 95,870 cd/m², which is three times greater than that of the reference CBP based device (31,370 cd/m²). The bipolar host PYR 1 is a promising material for high luminescent and low efficiency roll off applications, especially for green PhOLEDs.

Full text of DOI:10.1016/j.dyepig.2018.05.022

Accepted Manuscript
Utilizing triazine/pyrimidine acceptor and carbazole-triphenylamine donor based
bipolar novel host materials for highly luminescent green phosphorescent OLEDs with
lower efficiency roll-off
Ramanaskanda Braveenth, Dae Hyun Ahn, Ji-Hun Han, Ji Su Moon, Si Woo Kim,
Hyun A. Lee, Wu Qiong, Jang Hyuk Kwon, Kyu Yun Chai
PII:
S0143-7208(17)32415-4
10.1016/j.dyepig.2018.05.022
DYPI 6751
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 23 November 2017
Revised Date: 19 April 2018
Accepted Date: 11 May 2018
Please cite this article as: Braveenth R, Ahn DH, Han J-H, Moon JS, Kim SW, Lee HA, Qiong W, Kwon
JH, Chai KY, 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 (2018), doi: 10.1016/j.dyepig.2018.05.022.
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ACCEPTED MANUSCRIPT
Graphical abstract
N
N
N
N
PYR 1

ACCEPTED MANUSCRIPT
Utilizing
triazine/pyrimidine
acceptor
and
carbazole-
triphenylamine donor based bipolar novel host materials for
highly luminescent green phosphorescent OLEDs with lower
efficiency roll-off
Ramanaskanda Braveenth†,1, Dae Hyun Ahn‡,1, Ji-Hun Han†, Ji Su Moon‡,
Si Woo Kim‡, Hyun A Lee‡, Wu Qiong†, Jang Hyuk Kwon‡,* and Kyu Yun
Chai†,*
† Division of Bio-Nanochemistry, College of Natural Sciences, Wonkwang
University, Iksan City, Chonbuk, 570-749, Republic of Korea. *E-mail:
geuyoon@wonkwang.ac.kr;
‡ Department of Information Display, Kyung Hee University, Dongdaemoon-gu,
Seoul 130-701, Republic of Korea. *E-mail: jhkwon@khu.ac.kr
* Corresponding Authors: Email: geuyoon@wonkwang.ac.kr; Tel.: +82-63-
850-6230; Fax: +82-63-841-4893 (K.Y. Chai), Email: jhkwon@khu.ac.kr;
Tel.: +82-2-961-0948; Fax: +82-2-961-9154 (J.H. Kwon).
1 These authors contributed equally to this work
Abstract
In this work, two novel bipolar host materials were designed, synthesized and
applied in green phosphorescent based OLEDs. Both the host materials, 4-(2-
(4,6-diphenyl-1,3,5-triazin-2-yl)-9H-carbazol-9-yl)-N,N-diphenylaniline (TRZ
1)
and
4-(2-(4,6-diphenylpyrimidin-2-yl)-9H-carbazol-9-yl)-N,N-
diphenylaniline (PYR 1) exhibited high thermal stability, with decomposition
temperatures of 425 °C and 400 °C, respectively. The triplet energy of PYR 1
(2.63 eV) was higher than that of TRZ 1 (2.44 eV), and facilitated suitable
energy transfer to the green dopant. The PYR 1 based green device
demonstrated an excellent maximum current efficiency of 48.7 cd/A and
external quantum efficiency of 16.4%. Interestingly, the green device with PYR
1 showed an outstanding brightness of 95,870 cd/m2, which is three times
greater than that of the reference CBP based device (31,370 cd/m2). The bipolar
host PYR 1 is a promising material for high luminescent and low efficiency roll
off applications, especially for green PhOLEDs.
Keywords: organic light emitting diodes, bipolar host, green phosphorescent,
triazine, pyrimidine.

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1. Introduction
Organic light emitting diodes (OLEDs) have received great attention from both
the research community and industry for their excellent potential in display
technologies [1,2]. Charge recombination to produce singlet and triplet excitons
is determined by spin statistics. According to the statistics, the ratio of singlet to
triplet formation is 1:3, which corresponds to a theoretical internal quantum
efficiency (IQE) of 25%. However, the IQE can be increased by utilizing triplet
excitons. Phosphorescence-based emitters are highly efficient materials
compared to fluorescence emitters [3–5]. In 1998, Forrest et al. made a key
advance in the development of phosphorescent organic light emitting diodes
(PhOLEDs), attaining a higher IQE by using heavy metal complexes with
intersystem crossing (ISC) [6–8]. This mechanism requires the phosphorescence
emitters to be doped with appropriate host materials to prevent quenching
processes such as triplet-triplet annihilation (TTA) and aggregation quenching
(ACQ) [3,9–13]. The design of the host material is crucial to ensure proper
energy transfer from the host to the dopant in the PhOLEDs. The energy
difference between the highest occupied molecular orbital (HOMO) and lowest
unoccupied molecular orbital (LUMO) and the triplet energy of host materials
should be higher than those of the dopant in order to achieve effective transfer.
At the same time, the frontier molecular orbital (FMO) energy levels of adjacent
layers, such as the hole transporting layer (HTL) and electron transporting layer
(ETL), should match [14–23].
Additionally, maintaining a proper charge balance is key to obtaining better
efficiencies in PhOLEDs. Generally, the hole mobility of the HTL is higher than
that of the ETL. Thus, host materials should be modified to achieve charge
balance; this approach has been successfully implemented by introducing
bipolar host materials. Recently, many groups have reported work on bipolar
host strategies, using different electron donating and electron withdrawing
groups to develop materials that exhibit steady charge balance at the emission
layer (EML) [12,24–31].
In this study, we report two novel bipolar host materials 4-(2-(4,6-diphenyl-
1,3,5-triazin-2-yl)-9H-carbazol-9-yl)-N,N-diphenylaniline (TRZ 1) and 4-(2-
(4,6-diphenylpyrimidin-2-yl)-9H-carbazol-9-yl)-N,N-diphenylaniline (PYR 1)
containing a hole transporting carbazole-triphenyl amine derivative and an
electron withdrawing triazine or pyrimidine derivative for application in green
PhOLEDs. The carbazole moiety is widely used as a donating group because of
its high hole mobility and triplet energy (3.0 eV) [4, 32-34]. Triazine and
pyrimidine are good electron-withdrawing and hole-blocking moieties owing to

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their high electron deficiencies, which makes them suitable for use in high-
efficiency green PhOLEDs. Consequently, we expect that our design will result
in stable physical properties such as thermal strength to enhance the lifetime of
the device. We have compared the performance of our green devices with that
of the well-known host material CBP based on the same device structure.
2. Materials and methods
2.1. Materials
All reagents and solvents were obtained from commercial suppliers. 3M Ph-
MgBr, CYC and toluene were purchased from Sigma Aldrich (Seoul, Korea).
PYR and Ph-BoA were obtained from TCI chemicals (Seoul, Korea). THF was
distilled from sodium/benzophenone before use. Methanol, chloroform, n-
hexane and dichloromethane were bought from SK chemicals (Gyeonggi-do,
Korea). Starting material 1 was obtained from Green Guardee Technology Co.,
LTD (Beijing, China). Analytical thin layer chromatography (TLC) was
performed using aluminum backed Merck Kieselgel 60 coated plates with both
254nm and 365nm UV visualization (Seoul, Korea). Column chromatography
was carried out using Silica gel with mesh size of 200- 300. All reactions were
stirred magnetically, unless otherwise mentioned and we used deionized water
for all purposes related to our current study.
2.2. Instrumentation
13
1H and C NMR spectra were recorded using a JEON JNM-ECP FT-NMR
spectrometer (Peabody, MA, USA) operating at 500 MHz. Absorbance spectra
were obtained using a SINCO S-4100 UV-Vis spectrophotometer (SINCO,
Seoul, Korea). The band gap (Eg) was estimated from the onset of the UV-
visible absorbance spectra. Photoluminescence (PL) spectra were measured
using a JASCO FP-8500 spectrofluorimeter (JASCO, Tokyo, Japan), and
tetrahydrofuran (THF) was used as the solvent. The triplet energy level (ET) was
determined from the onset wavelength of the emission spectra at 77 K in
toluene (10-4 M). To measure the HOMO (highest occupied molecular orbital)
and LUMO (lowest unoccupied molecular orbital) levels, cyclic voltammetry
(CV) was carried out using a BioLogic SP-50 (Biologic, Paris, France). The
HOMO level was calculated by subtracting the oxidation potential shift between
ferrocene and the HTMs. The LUMO level was estimated by subtracting the
band gap to the obtained HOMO level. The current density-voltage-luminance
(J-V-L) characteristics of each device were measured using a luminance color
meter (Konica Minolta CS-100A) and a source meter unit (Keithley 2635A,
USA). The electroluminescence (EL) spectra and CIE (Commission

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Internationale l’Eclairage) 1931 color coordinates were obtained using a Konica
Minolta CS-2000 spectroradiometer (Japan). Molecular simulations were
carried out using density function theory (DFT) calculations with the B3LYP
(Beck three parameter hybrid functional and Lee-Yang-Parr correlation
functional) and double numerical plus d- function (DND) atomic orbital basic
set with DMol3 module implemented in material studio 8.0 software (Accelrys
inc, San Diego, CA, USA). Thermal gravimetric analysis (TGA) and differential
scanning calorimetry (DSC) were conducted 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 under nitrogen atmosphere. Mass
analysis was carried out using a Xevo TQ-S spectrometer (Waters, Milford, MA,
USA). TOF MS data was collected using a Hybrid Tandem spectrometer
(SYNAPT G2-Si-HDMS, Waters, Herts, England). Elemental analysis (EA)
was recorded by a IT/flash/2000 elemental analyzer (Thermo Fisher Scientific,
Loughborough, England). IR spectra was obtained by a Fourier Transform
infrared spectrometer (Perkin Elmer, Boston, MA, USA).
2.3. Synthesis procedure
2.3.1. Synthesis of 2-chloro-4,6-diphenyl-1,3,5-triazine (2Ph-TRZ)
2,4,6-trichloro-1,3,5-triazine (CYC, 5 g, 27 mmol) was placed in 250 ml two
neck round bottom flask under vacuum conditions for 15 min. An inert
atmosphere was then introduced by using a nitrogen balloon, and 100 ml of
anhydrous THF was added. The mixture was then stirred at 0 °C, and 20ml of
3M phenyl magnesium bromide Ph-MgBr was injected dropwise. The mixture
was kept at room temperature for 12 h while stirring. After completion of the
reaction, then evaporated and recrystallized using the solvent methanol (100 ml)
to yield a pale white solid as the requisite product 2Ph-TRZ.
2-chloro-4,6-diphenyl-1,3,5-triazine
Yield: 89%; pale white solid; 1H NMR (500 MHz, CDCl3) δ 8.61-8.63 (d, J = 7
Hz, 4H), 7.61-7.64 (t, J = 7.5 Hz, 2H), 7.53-7.56 (t, J = 7.5 Hz, 4H); 13C NMR
(500 MHz, CDCl3) δ 173.4, 172.2, 134.4, 133.6, 129.4, 128.9.
2.3.2. Synthesis of 2-chloro-4, 6-diphenylpyrimidine (2Ph-PYR)
A mixture of 2, 4, 6-trichloropyrimidine (PYR, 1.3 ml, 7 mmol), phenylboronic
acid (Ph-BoA, 1.5 g, 12 mmol), Pd(PPh3P)4 (0.2 g, 0.17 mmol), 40 ml of 2M
aqueous K2CO3 solution , 40 ml of water, 25 ml of ethanol, and 80 ml of toluene

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were added in a two neck round bottom flask equipped with condenser and
refluxed at 110 °C for 8 h under nitrogen atmosphere. After completion of the
reaction, the mixture was worked up using ethyl acetate (60 ml) and water (50
ml). The organic layer was dried over anhydrous magnesium sulfate and then
filtered and concentrated. The crude residues were separated using a silica
column and an n-hexane: dichloromethane (4:1) solvent system to obtain the
intermediate 2P-PYR as a white solid.
2-chloro-4, 6-diphenylpyrimidine
Yield: 74%; white solid; 1H NMR (500 MHz, CDCl3) δ 8.13-8.14 (m, 4H), 8.01
(s, 1H), 7.51-7.55 (m, 6H); 13C NMR (500 MHz, CDCl3) δ 167.7, 162.1, 135.7,
131.7, 129.1, 127.5, 111.0.
2.3.3. Synthesis of 4-(2-(4,6-diphenyl-1,3,5-triazin-2-yl)-9H-carbazol-9-yl)-
N,N-diphenylaniline (TRZ 1)
A mixture of 2-chloro-4,6-diphenyl-1,3,5-triazine (2Ph-TRZ, 1 g, 3.7 mmol),
(9-(4-(diphenylamino)phenyl)-9H-carbazol-2-yl)boronic acid (1, 2.2 g, 4.9
mmol), Pd(PPh3P)4 (0.01 g, 0.01 mmol), 40 ml of 2M aqueous K2CO3 solution ,
40 ml of water, 30 ml of THF, and 80 ml of toluene were added in a two neck
round bottom flask equipped with condenser. The mixture was then stirred at
110 °C for 12 h under nitrogen atmosphere. The residues were extracted with
dichloromethane (3 * 30 ml) and deionized water (80 ml). The organic layer
was collected and dried over anhydrous magnesium sulfate. Finally, the crude
was separated using a silica column with an n-hexane: dichloromethane (6:1 to
4:1) gradient-based mobile phase to achieve the pure TRZ 1 target molecule.
4-(2-(4,6-diphenyl-1,3,5-triazin-2-yl)-9H-carbazol-9-yl)-N,N-
diphenylaniline
Yield: 86%; Greenish yellow solid; FT-IR (KBr pellet): ʋmax 3057, 1622, 1588,
1522, 1510, 1472, 1444, 1372, 1358, 1337, 1311, 1279, 1226, 1174, 1154, 1100,
1
1069, 1024, 1001 and 931 cm-1 ; H NMR (500 MHz, CDCl3) δ 8.87 (s, 1H),
8.73-8.78 (m, 5H), 8.28-8.30 (d, J = 10 Hz, 1H), 8.21-8.23 (d, J = 10 Hz, 1H),
7.56-7.64 (m, 6H), 7.48-7.52 (m, 4H), 7.31-7.37 (m, 7H), 7.26-7.28 (m, 4H),
13
7.09-7.21 (t, J = 7.5 Hz, 2H); C NMR (500 MHz, CDCl3) δ 172.2, 171.6,
147.6, 140.3, 136.4, 132.5, 129.6, 128.1, 127.1, 124.8, 124.2, 123.5, 121.1,
120.7, 120.2, 110.9, 110.3, 100.0; MS (APCI) m/z: 641.83 for C45H31N5
[(M+H)+]. TOF MS (ES+) m/z: cal. for C45H31N5 641.25, found 641.26; Anal.

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Calcd for C45H31N5 (%): C, 84.22; H, 4.87; N, 10.91.Found: C, 83.64; H, 4.82;
N, 11.11.
2.3.4. Synthesis of 4-(2-(4,6-diphenylpyrimidin-2-yl)-9H-carbazol-9-yl)-N,N-
diphenylaniline (PYR 1)
A mixture of 2-chloro-4,6-diphenylpyrimidine (2Ph-PYR, 1 g, 3.7 mmol), (9-
(4-(diphenylamino)phenyl)-9H-carbazol-2-yl)boronic acid (1, 2.2 g, 4.9 mmol),
Pd(PPh3P)4 (0.01 g, 0.01 mmol), 40 ml of 2M aqueous K2CO3 solution , 40 ml
of water, 30 ml of THF, and 80 ml of toluene were added in a two neck round
bottom flask equipped with condenser. The mixture was then stirred at 110 °C
for 12 h under nitrogen atmosphere. The residues were extracted with
dichloromethane (2 * 30 ml) and deionized water (40 ml). The organic layer
was collected and dried over anhydrous magnesium sulfate. Finally, the crude
was separated using a silica column and an n-hexane: dichloromethane (8:1 to
3:1) gradient-based mobile phase to achieve the pure PYR 1 target molecule.
4-(2-(4,6-diphenylpyrimidin-2-yl)-9H-carbazol-9-yl)-N,N-diphenylaniline
Yield: 77%; yellow solid; FT-IR (KBr pellet): ʋmax 3433, 3034, 1586, 1563,
1528, 1510, 1491, 1438, 1400, 1358, 1333, 1311, 1270, 1228, 1179, 1103, 1072,
1024 and 999 cm-1 ; 1H NMR (500 MHz, CDCl3) δ 8.83 (s, 1H), 8.70-8.71 (d, J
= 8.5 Hz, 1H), 8.22-8.26 (m, 5H), 8.20-8.21 (d, J = 8.5 Hz, 1H), 8.01 (s, 1H),
7.45-7.58 (m, 10H), 7.26-7.35 (m, 7H), 7.24-7.26 (m, 4H), 7.08-7.10 (t, J = 8.5
13
Hz, 2H); C NMR (500 MHz, CDCl3) δ 165.1, 164.7, 147.7, 147.2, 142.2,
141.5, 137.7, 136.0, 130.8, 129.5, 129.0, 128.1, 127.3, 126.5, 125.4, 124.7,
124.4, 123.3, 120.8, 120.5, 120.1, 120.0, 110.3, 110.1, 110.1; MS (APCI) m/z:
640.76 for C46H32N4 [(M+H)+]. TOF MS (ES+) m/z: cal. for C46H32N4 640.26,
found 640.27; Anal. Calcd for C46H32N4 (%): C, 86.22; H, 5.03; N, 8.74.Found:
C, 86.08; H, 4.95; N, 8.60.

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Scheme 1. Synthesis of the host materials TRZ 1 and PYR 1. (a) Anhydrous THF, 0 °C to
room temperature, 12 h. (b) Pd(PPh3P)4, 2 M K2CO3, toluene + ethanol + water, 110 °C, 8 h.
(c) Pd(PPh3P)4, 2 M K2CO3, toluene+ THF + water, 110 °C, 12 h.
2.4. OLED fabrication and characterization
To fabricate the OLEDs, indium-tin-oxide (ITO) coated glass substrates (50 nm,
sheet resistance of 10 Ω/□) were cleaned in an ultrasonic bath with acetone and

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isopropyl alcohol for 10 min and rinsed with deionized water. The substrates
were then dried using nitrogen, followed by UV-ozone treatment for 10 min. All
the organic layers and the metal cathode were deposited on the pre-cleaned ITO
glass by vacuum evaporation under a pressure of ~1×10-7 Torr. The deposition
rate of all the organic layers was about 0.5 Å/s. Similarly, the deposition rates of
LiF and Al were maintained at 0.1 Å/s and 4.0 Å/s, respectively. Finally, all the
devices were encapsulated using a glass cover and UV curable resin inside a
nitrogen filled glove box. The OLED area was 4 mm2 for all the samples studied
in this work. The J-V and L-V characteristics of the fabricated OLED devices
were measured using a Keithley 2635A SMU and a Konica Minolta CS-100A,
respectively. The EL spectra and the Commission International de l'Eclairage
(CIE) 1931 color coordinates were obtained using a Konica Minolta CS-2000
spectroradiometer.
3. Results and discussion
3.1. Synthesis
The synthetic procedures for both the host materials are depicted in Scheme 1.
Both TRZ 1 and PYR 1 were synthesized using Suzuki cross coupling reactions
between either 2-chloro-4,6-diphenyl-1,3,5-triazine 2Ph-(TRZ) or 2-chloro-4,6-
diphenylpyrimidine (2Ph-PYR) and (9-(4-(diphenylamino)phenyl)-9H-
carbazol-2-yl)boronic acid (1) with a palladium catalyst and the base potassium
carbonate. The desired bipolar host materials were achieved in good yields of
86% and 77%, respectively. The synthesized materials were purified using
column chromatography, and further purification was carried out prior to device
fabrication through the sublimation process. Finally, all the synthesized new
materials were confirmed using NMR (1H, 13C), IR, elemental analysis and mass
spectrometry analysis.
3.2. Thermal Analysis
The thermal stabilities of the host materials were appraised by TGA and DSC
measurements; these data are shown in Table 1 and depicted in Figure 1. TRZ 1
and PYR 1 exhibited high melting point of 252 °C and 239 °C, Both of our host
materials demonstrated high decomposition temperatures of over 400 °C for 5%
weight reduction, which can enhance the morphological and thermal stabilities
of the thin films in the devices.

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Figure 1. TGA curves of TRZ 1 and PYR 1.
3.3. Photophysical properties
Figure 2 shows the UV-vis absorption and photoluminescence (PL) spectra of
TRZ 1 and PYR 1 at room temperature, and the results are summarized in Table
1. Both the UV-vis absorption peaks exhibited similar shapes. A sharp increase
in the absorption intensity at 325 nm was observed for both TRZ 1 and PYR 1,
and was attributed to a π- π* charge transition from the electron-donating
carbazole-triphenylamine moiety to the electron-withdrawing triazine or
pyrimidine moiety. Band gap values of 2.89 eV and 3.06 eV were calculated for
TRZ 1 and PYR 1 from their absorption edge values of 429 nm and 405 nm,
respectively. The high band gap values of TRZ 1 and PYR 1 reflected the meta
position attachment between the donor and the acceptor, which strongly reduces
π-conjugation and widens the band gap by affecting the ICT process. The PL
spectra at room temperature for TRZ 1 and PYR 1 exhibited maxima at 508 nm
and 446 nm, respectively. The triplet energy (ET) is the key factor in preventing
energy transfer from the guest to host in host materials, and ensuring the
efficiency of the resulting devices. The triplet energies of TRZ 1 and PYR 1
were estimated to be 2.44 eV and 2.63 eV from the low temperature PL spectra.
The calculated values are suitable for green phosphorescence based OLEDs.
The difference in the triplet energies of TRZ 1 and PYR 1 arises from the
different withdrawing moieties attached to the common donor group. The triplet
energy of PYR 1 was also higher than that of the reference material CBP (2.58
eV), and thus, we believed that PYR 1 based devices would show greater
efficiency enhancement than TRZ 1 or CBP based devices.

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Figure 2. UV-Vis absorption and photoluminescence spectra of the hosts TRZ 1 and PYR 1.
3.4. Electrochemical properties
The HOMO energy levels were determined from the cyclic voltammetry (CV)
measurements, which are depicted in Figure 3. TRZ 1 and PYR 1 were found to
have very similar HOMO levels (5.59 eV and 5.63 eV, respectively). The
LUMO energy levels were calculated by adding the band gap energy to the
HOMO energy, and are listed in Table 1. The LUMO energy levels of TRZ 1
and PYR 1 were 2.70 eV and 2.57 eV. PYR 1 exhibited a higher LUMO level
than TRZ 1, and both host materials had higher LUMO energies than the dopant
Ir(ppy)3 (2.90 eV). These favorable energy levels contributed to the efficient
recombination of charges at the emission layer, indicating their promise for
fabricating efficient devices.
0.5
0.4
0.3
0.2
0.1
0.0
-0.1
-0.2
2.5
2.0
1.5
1.0
0.5
0.0
-0.5
-1.0
TRZ 1
PYR 1
-1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
Figure 3. Cyclic voltammetry measurements of TRZ 1 and PYR 1.

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Table 1. Thermal, photophysical and electrochemical properties of TRZ 1 and PYR 1.
a
b
Host
Tm
Td
UV-
PL
HOMO LUMO
Eg
ET
Es
(°C)
(°C) Vis c max d
(nm) (nm)
e(eV)
f(eV)
g(eV) h(eV) i(eV)
TRZ 1
PYR 1
253
240
425
400
429
405
508
446
5.59
5.63
2.70
2.57
2.89
3.06
2.44
2.63
2.62
2.88
a Melting point
b Decomposition temperature at 5% weight reduction
c
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
i
Singlet energy
3.5. Molecular design and distribution
To study the electronic structures and molecular levels of the hosts, DFT
calculations were performed with DMol3 module at double numerical plus d-
functions (DND) atomic orbital basic set level. The FMO distributions are
shown in Figure 4. The HOMOs of both the host materials are delocalized over
the triphenylamine moiety. The LUMO of PYR 1 is clearly localized on the
pyridine moiety, while that of TRZ 1 is localized on the triazine and carbazole
derivatives. The host material TRZ 1 showed little extended distribution
compared to the host material PYR 1. This distribution revealed that its HOMO-
LUMO separation was favorable for the hole and electron transport properties
of the single molecule. The only difference between TRZ 1 and PYR 1 is the
withdrawing group, which was responsible for the greater distribution of the
LUMO over TRZ. PYR 1 showed clearer HOMO-LUMO separation, which can
enhance device efficiency by providing proper charge balance at the emission
layer.

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TRZ 1
PYR 1
HOMO
LUMO
Figure 4. Spatial distribution of the HOMO and LUMO of TRZ 1 and PYR 1.
3.6. Device performances
To investigate the properties of the synthesized bipolar host materials, we
fabricated green OLED based phosphorescent devices using Ir(ppy)3 as a green
dopant. The configuration of the device structure was ITO/ 1,4,5,8,9,11-
Hexaazatriphenylenehexacarbonitrile
(HATCN)
(7
nm)/
4,4′-
Cyclohexylidenebis[N,N-bis(4-methylphenyl)benzenamine]
(TAPC)
(43
nm)/HOST:5 wt.% Tris[2-phenylpyridinato-C2,N]iridium(III) (Ir(ppy)3) (20
nm)/ 1,3,5-Tris(3-pyridyl-3-phenyl)benzene (TmPyPB) (35 nm)/ Lithium
fluoride (LiF) (1.5 nm)/ Aluminum (Al) (100 nm), and is shown in Figure 5. In
the above configuration, ITO and Al act as the anode and cathode, respectively.
TAPC is used as a HTL, and TmPyPB is the ETL in the device. The triplet
energies of the HTL and ETL were 2.9 eV and 2.8 eV, respectively, which were
sufficiently high to block triplet exciton quenching from the green emitting
EML.
The current density-voltage (J-V) and luminescence-voltage (L-V)
characteristics are shown in Figure 6 and Table 2. The PYR 1 based device
showed a lower turn on voltage of 2.4 V than the CBP based reference device
(2.6 V). This indicates that PYR 1 has excellent bipolar characteristics to
establish proper charge balance at the EML. The lower driving voltage of the
PYR 1 based device (3.0 V) further enhanced the device efficiency, while the
TRZ 1 based device exhibited a higher driving voltage of 3.6 V. The device
based on PYR 1 demonstrated a maximum current efficiency of 48.7 cd/A,
which was higher than those of the TRZ 1 (45.3 cd/A) and CBP (45.8 cd/A)

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based devices. Consequently, we noted that the maximum luminescence of the
TRZ 1 and PYR 1 devices were 54,170 cd/m2 and 95,870 cd/m2, respectively.
The PYR 1 based device exhibited a maximum luminescence three times
greater than that of the device based on the well-known host CBP (31,370
cd/m2). This represents solid evidence that our PYR 1 bipolar host material has
high potential for application in green PhOLEDs. The maximum external
quantum efficiency (EQE) of the PYR 1 based device (16.4 %) was higher than
that of the reference CBP (15.7 %) and TRZ 1 (15.3 %) based devices. As the
bipolar characteristics of PYR 1 were better than those of TRZ 1, it was
expected to provide superior device efficiency. We observed that the CBP based
device showed rapid efficiency roll-off compared to the TRZ 1 and PYR 1
based bipolar host materials. Our synthesized host materials can be used for
long term operation, which is a key factor in improving the efficiency of such
devices.
To understand the electron and hole transporting abilities, a hole-only device
(HOD) and an electron-only device (EOD) were fabricated. The configuration
of the HOD was ITO (50 nm)/TAPC (20 nm)/HOST (50 nm)/TAPC (20 nm)/Al
(100 nm) and the structure of the EOD was ITO (50 nm)/TmPyPB (20
nm)/HOST (50 nm)/TmPyPB (20 nm)/LiF (1.5 nm)/Al (100 nm); these
configurations are depicted in Figure 7. The HOD results showed that the hole-
transporting properties of TRZ-1 were superior to those of the PYR-1 based
device, which was in agreement with the higher HOMO energy levels and also
the energy barrier with TAPC layer is smaller than that of PYR 1. The higher
LUMO energy value of PYR 1 resulted in greater electron transport ability than
TRZ 1. The bipolar characteristics of the synthesized host materials enhanced
the device properties and reduced the efficiency roll off.
The electroluminescence spectra are shown in Figure 8. The maxima of the EL
peaks were observed at 513 nm, 512 nm, and 512 nm for CBP, TRZ 1, and PYR
1, respectively. All the peaks were identical and no chromic shift was observed,
indicating perfect emission from the green dopant [35,36].

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Figure 5. Device configuration of the TRZ 1, PYR 1, and CBP host material devices.
103
102
100
CBP
105
104
103
102
101
101
100
TRZ 1
PYR 1
80
60
40
20
0
10-1
10-2
10-3
10-4
10-5
10-6
10-7
10-8
CBP
TRZ 1
PYR 1
-2
0
2
4
6
8
10
0
2
4
6
8
10
Voltage (V)
Voltage (V)
102
101
100
60
50
40
30
20
10
0
CBP
TRZ 1
PYR 1
CBP
TRZ 1
PYR 1
10-1
100
101
102
103
0
20000
40000
60000
80000 100000
Luminance (cd/m2)
Current Density (mA/cm2)
Figure 6. Current density-voltage (J-V) and luminescence-voltage (L-V) characteristics of
TRZ 1 and PYR 1, as well as those of the reference host CBP.

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300
250
200
150
100
50
300
CBP
TRZ 1
PYR 1
CBP
TRZ 1
PYR 1
250
200
150
100
50
0
0
0
2
4
6
8
10
12
14
0
2
4
6
8
10
12
Voltage (V)
Voltage (V)
Figure 7. Efficiency of the HOD and EOD of the two bipolar host materials.
Table 2. Device performances.
Characteristic
CBP
TRZ 1
PYR 1
2.6 V
2.6 V
2.4 V
Turn-on Voltage
Driving Voltage a
Max. luminance
3.3 V
3.6 V
3.0 V
31,370 cd/m2
54,170 cd/m2
95,870 cd/m2
Current
efficiency a
(Max)
47.1 / 45.8 cd/A
45.6 / 45.3 cd/A
48.7 / 48.7 cd/A
EQE a
(Max)
15.8 / 15.7 %
(0.28, 0.62)
513 nm
15.4 / 15.3 %
(0.28, 0.63)
512 nm
16.4 / 16.4 %
(0.28, 0.63)
512 nm
CIE color b
b
λmax
a At 1000 cd/m2
b At 10 mA/cm2

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(@ 10 mA/cm2)
1.2
1.0
0.8
0.6
0.4
0.2
0.0
CBP
TRZ 1
PYR 1
400
500
600
700
Wavelength (nm)
Figure 8. Electroluminescence spectra of the green phosphorescent OLEDs.
4. Conclusion
Two novel host materials (TRZ 1 and PYR 1) were designed and synthesized
using a carbazole-triphenylamine donor moiety and triazine or pyrimidine based
withdrawing groups to provide bipolar characteristics. The synthesized host
materials exhibited high thermal stability due to their bulky structures. The
triplet energies of both the bipolar host materials were suitable to ensure
effective energy flow for green phosphorescent OLEDs. The PYR 1 based green
OLED device registered a lower turn on (2.4 V) and driving voltage (3.0 V).
The maximum luminescence of the PYR 1 containing device (95,870 cd/m2)
was three times higher than that of the similar CBP based device. Consequently,
the PYR 1 based device showed an excellent maximum current efficiency of
48.7 cd/A while exhibiting a maximum external quantum efficiency of 16.4%
with lower efficiency roll off. Our new bipolar host materials can be used to
enhance the efficiency of thermally stable and highly luminescent green OLED
devices.
Acknowledgement
This research was supported by the Basic Science Research Program through
the National Research Foundation of Korea (NRF), funded by the education
(NRF-2016R1D1A3B01015531).
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Highlights
•
•
•
•
•
Current efficiency reached 48.7 cd/A
Lower turn on voltage of 2.4 V recorded
Green phosphorescent OLED showed outstanding luminescence of 95,870 cd/m2
Both devices showed lower efficiency roll off
Achieved EQE of 16.4%