A novel, bipolar host based on triazine for efficient solution-processed single-layer green phosphorescent organic light-emitting diodes

Article abstract of DOI:10.1016/j.dyepig.2013.09.017

A novel N-phenyl carbazole substituted 2, 4,6-trisphenyl-triazine host material(TPCPZ) for solution processed green phosphorescent organic light-emitting devices (PhOLEDs) was synthesized by a Suzuki-cross coupling reaction. The optical, electrochemical a

Full text of DOI:10.1016/j.dyepig.2013.09.017

Dyes and Pigments 101 (2014) 9e14
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
A novel, bipolar host based on triazine for efficient solution-processed
single-layer green phosphorescent organic light-emitting diodes
,
,
a
a
b
a
Bin Huang a b, Wei Jiang a , Jinan Tang , Xinxin Ban , Ruigang Zhu , Huange Xu ,
*
Wen Yang a, Yueming Sun a
,
*
a School of Chemistry and Chemical Engineering, Southeast University, Nanjing 211189, Jiangsu, PR China
b Department of Chemical and Pharmaceutical Engineering of Southeast University Chengxian College, Nanjing 210088, Jiangsu, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 1 July 2013
Received in revised form
16 August 2013
Accepted 11 September 2013
Available online 25 September 2013
A novel N-phenyl carbazole substituted 2, 4,6-trisphenyl-triazine host material(TPCPZ) for solution
processed green phosphorescent organic light-emitting devices (PhOLEDs) was synthesized by a Suzuki-
cross coupling reaction. The optical, electrochemical and thermal properties of TPCPZ have been char-
acterized. TPCPZ exhibits a high glass transition temperature of 165 ꢀC and a triplet energy of 2.63 eV. The
appropriate HOMO energy level (ꢁ5.39 eV) and LUMO energy level (ꢁ2.16 eV) matching with the HOMO
energy level of PEDOT:PSS(ꢁ5.35 eV) and the LUMO energy level of Cs2CO3/Al bilayer cathode (ꢁ2.2 eV),
facilitate the transfer of holes and electrons. The solution-processed single-layer device using TPCPZ as
the host for fac-tris(2-(4-phenylpyridine)iridium (Ir(ppy)3) exhibited a low turn-on voltage of 3.5 V, a
maximum current efficiency of 20.8 cd Aꢁ1 and a maximum luminance of 18,000 cd mꢁ2. These results
demonstrated that TPCPZ as a host material is advantageous for fabrication of highly efficient single-layer
green PhOLEDs.
Keywords:
OLEDs
Solution-processed
Electrophosphorescence
Triazine
Bipolar
Host material
Crown Copyright Ó 2013 Published by Elsevier Ltd. All rights reserved.
1. Introduction
12], oxadiazole [13e16], phenathroline [17e19], benzimidazole
[20e24], is well described. Triazines, are known to be good electron
Since efficient phosphorescent emitters for organic light-emit-
ting diodes (PhOLEDs) was discovered by Forrest’s group, tremen-
dous efforts have been made in the development of highly efficient
devices [1e5]. In PhOLEDs, the emitting layer consists of a phos-
phorescent dye which is doped into a host material to avoid con-
centration quenching and to optimize the charge balance. High
efficiencies are obtained only when the energy is efficiently
transferred from the host to the phosphorescent dye. To achieve
efficient electrophosphorescence, the choice of host is of vital
importance [6].Generally, the triplet energy of the host material is
higher than that of the guest. For efficient OLEDs, well balanced
charge carrier transport and a broad recombined zone are desir-
able. In general, the electron mobility of many host materials is
much lower than the hole mobility because of the fact that they
consist electron donors such as aromatic amines, carbazoles. This
task is mostly solved by a design of a bipolar material [7,8]. Intro-
duction of strong electron donors such as aromatic amines or car-
bazoles [9] to electron accepting N-heterocycles like pyridine [10e
conductors and their derivatives have been used as electron
transport layers in OLEDs [4,25e28].Recently, some bipolar host
materials based on triazines for PhOLEDs are reported [29e34]. A
series of donor substituted 1,3,5-triazine derivatives as host for blue
or green PhOLEDs show good performance.
In recent years, many groups are devoted to improve the per-
formance of solution-processed PhOLEDs which are highly desir-
able to simplify the fabrication process and reduce the cost of larger
area displays [35e44]. Even though polymer light-emitting diodes
can be easily fabricated by solution casting, they are generally
difficult to synthesize and purify for electronic devices. Therefore, it
is a good strategy to develop OLEDs based on solution-processed
small molecules. However, the largest challenge for small mole-
cules lies in the poor solubility accompanied by the tendency to
crystallize and failure to form high quality films. Consequently, it is
of significant importance to design and synthesize amorphous
small molecules with high purity and solubility. To date, few works
reported on bipolar host based on 1, 3,5-triazine for solution-
processed PhOLEDs.
In this work, a bipolar host composed of electron-transporting 2,
4,6-trisphenyl-triazine and hole-transporting N-phenyl carbazole,
* Corresponding authors. Tel./fax: þ86 25 52090621.
E-mail address: 101011462@seu.edu.cn (W. Jiang).
0143-7208/$ e see front matter Crown Copyright Ó 2013 Published by Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2013.09.017

10
B. Huang et al. / Dyes and Pigments 101 (2014) 9e14
namely 2, 4,6-tris (3-((9-phenyl) carbazol-3-yl)-phenyl) -tri-
azine(TPCPZ) is designed and synthesized. The dipolar nature of
selected as the internal standard. The solutions were bubbled with
a constant argon flow for 10 min before measurements.
DFT calculations of TPCPZ were performed using the Gaussian
03 program package. The calculation was optimized at the B3LYP/6-
31G(d) level of theory. The molecular orbitals were visualized using
Gaussview [45].
TPCPZ can promote the strong
pep intermolecular stacking of
molecules in the solid state, which can facilitate high charge
transport. Moreover, the electron-donating N-phenyl carbazole
moieties are conjugated to the electron accepting 2, 4,6-trisphenyl-
triazine core; the donor-acceptor intramolecular interaction tends
to decrease the energy gap. TPCPZ has a triplet energy of 2.62 eV,
which make it suitable as host material for green phosphorescent
emitters. The material exhibits high thermal stability and good
sublimation properties. The glass transition temperature of 165 ꢀC
ensures morphological stability of the host-guest emission layer
during the operation of the device. Additionally, TPCPZ has
good solubility in common solvents such as CH2Cl2, CHCl3, 1,2-
dichloroethane et al., which make it suitable for the fabrication of
solution-processed PhOLEDs. The single-layer device with TPCPZ as
host has low turn-on voltage of 3.5 V, a high maximum luminance
efficiency of 20.8 cd Aꢁ1 and maximum luminance of 18,000 cd mꢁ2
in a solution-processed green phosphorescent OLED using an
emitter of fac-tris(2-(4-phenylpyridine)iridium (Ir(ppy)3)).
2.2. Synthesis of 2,4,6-tris(3-((9- phenyl)carbazol-3-yl)-phenyl)-
triazine(TPCPZ)
To a solution of 2,4,6-tris(3-bromophenyl)-triazine (TBrPZ)
(0.546 g, 1.0 mmol) and N-phenyl-3-(4,4,5,5-tetramethyl-1,3,2-
dioxaborolane-2-yl)-6H-carbazole (1.107 g, 3.0 mmol) in 20 mL
of toluene and 4 mL of ethanol was added 2 mL of 2.0 M aqueous
K2CO3 solution. The reaction mixture was then purged with ni-
trogen for 10 min before adding tetrakis(triphenylphosphine)
palladium(0) (0.055 g, 0.048 mmol). After refluxing for 24 h under
nitrogen, the resulting mixture was cooled to room temperature
and then poured into water and extracted with 60 mL (3 ꢂ 20 mL)
CH2Cl2. The combined organic phase was then washed with 20 mL
(2 ꢂ 10 mL) saturated aqueous NaCl solution and dried with
anhydrous Na2SO4. After removal of the solvent by rotary evapo-
ration, the residue was purified by silica gel column chromatog-
raphy to afford TPCPZ as a white solid. Yield: 0.60 g (58.08%).1H
2. Experimental
2.1. General
NMR(300 MHz, CDCl3): d(ppm) 9.17 (s, 3H), 8.81e8.78 (d, 3H), 8.50
(s, 3H), 8.16e8.19 (d, 3H), 7.96e7.98(d, 3H), 7.80e7.83(m, 3H),
7.67e7.73 (t, 3H), 7.65e7.56 (m, 12H), 7.52e7.43 (m, 6H), 7.40e7.35
(m, 6H), 7.16e7.20 (m, 3H). 13C NMR (75 MHz, CDCl3)
All reactants and solvents were purchased from commercial
sources and used without further purification. 1H NMR and 13C
NMR spectra were recorded on a Bruker ARX300 NMR spectrom-
eter with Si(CH3)4 as the internal standard. Elemental analysis was
performed on an Elementar Vario EL CHN elemental analyzer. Mass
spectra were obtained using a Thermo Electron Corporation Fin-
nigan LTQ mass spectrometer. UVevis absorption spectra were
recorded with a spectrophotometer (Agilent 8453) and PL spectra
d
(ppm):109.50, 109.78, 118.67, 119.71, 120.03, 123.06, 123.62,
125.18, 125.80, 126.71, 126.97, 127.14, 127.39, 128.78, 129.54, 131.16,
132.68, 136.51, 137.29, 140.18, 141.02, 142.18, 171.57. MS (MALDI-
TOF) [m/z]: calcd for C75H48N6, 1033.22; found, 1033.48. Anal.
Calcd. for C75H48N6 (%): C, 87.18; H, 4.68; N 8.13. Found: C, 87.34; H,
4.72; N 8.38.
were recorded with
a fluorospectrophotometer (Jobin Yvon,
FluoroMax-3). Thermogravimetric analysis (TGA) was performed
using a Netzsch simultaneous thermal analyzer (STA) system (STA
409PC) under dry nitrogen atmosphere at a heating rate of
10 ꢀC minꢁ1. Glass transition temperature was recorded by differ-
ential scanning calorimetry (DSC) at a heating rate of 10 ꢀC minꢁ1
with a thermal analysis instrument (DSC 2910 modulated calo-
rimeter). Cyclic voltammetry measurements were performed on a
Princeton Applied Research potentiostat/galvanostat model 283
voltammetric analyzer in CH2Cl2 solutions (10ꢁ3 M) at a scan rate of
100 mV sꢁ1 with a platinum plate as the working electrode, a silver
wire as the pseudo-reference electrode, and a platinum wire as the
counter electrode. The supporting electrolyte was tetrabuty-
lammonium hexafluorophosphate (0.1 M) and ferrocene was
2.3. Device fabrication and performance measurements
The solution-processed single-layer device using TPCPZ as host
with a configuration ITO/PEDOT:PSS (40 nm)/TPCPZ: Ir(ppy)3
(90 wt%:10 wt%,100 nm)/Cs2CO3 (2 nm)/Al (120 nm) has been
fabricated by spin-coating. In a general procedure, indium-tin oxide
(ITO)-coated glass substrates were pre-cleaned carefully and
treated by UV ozone for 4 min. A 40 nm Poly(3,4-ethylenedioxythio
phene) doped with Poly(styrene-4-sulfonate)(PEDOT:PSS) aqueous
solution was spin coated onto the ITO substrate and baked at 210 ꢀC
for 10 min. The substrates were then taken into a nitrogen glove
box, where Ir(ppy)3-doped TPCPZ layer was spin coated onto the
Scheme 1. Synthetic route of TPCPZ.

B. Huang et al. / Dyes and Pigments 101 (2014) 9e14
11
PEDOT:PSS layer from 1,2-dichloroethane solution and annealed at
1.1
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
120 ꢀC for 30 min. The substrate was then transferred into an
evaporation chamber, where the Cs2CO3/Al bilayer cathode was
UV-Vis
PL
ꢀ
evaporated at evaporation rates of 0.2 and 10 A/s for Cs2CO3 and Al,
respectively, under a pressure of 1 ꢂ 10ꢁ3 Pa. The current-voltage-
brightness characteristics of the device were characterized with
Keithley 4200 semiconductor characterization system. The elec-
troluminescent spectra were collected with a Photo Research
PR705 Spectrophotometer. All measurements of the devices were
carried out in ambient atmosphere without further encapsulations.
Phos
3. Results and discussion
3.1. Synthesis and characterization
Scheme 1 shows the synthetic routes and structure of the newly
synthesized TPCPZ. 2,4,6-tris(3-bromophenyl)-triazine (TBrPZ) was
synthesized according to the literature procedure4. N-phenyl-3-
(4,4,5,5-tetramethyl-1,3,2-dioxaborolane-2-yl)-6H-carbazole was
synthesized according to the literature procedure with carbazole as
start material [46]. Subsequently, Suzuki-cross coupling reaction of
TBrPZ with N-phenyl-3-(4, 4, 5,5-tetramethyl-1, 3, 2-dioxabor-
olane-2-yl)-6H-carbazole led to TPCPZ with a yield of 58.08%.
Finally, the product was purified by the silica column method and
recrystallization from n-hexane/CH2Cl2, yielding the very pure
white powders. 1H NMR, 13C NMR, matrix-assisted laser desorption
ionization time-of-flight (MALDI-TOF) mass spectrometry, and
elemental analysis were employed to confirm the chemical struc-
ture of TPCPZ in the experimental section.
200 250 300 350 400 450 500 550 600 650
Wavelength (nm)
Fig. 2. Normalized UVevis absorption (Abs, green curve), photoluminescence (PL, blue
curve) and phosphorescence (77 K) (PH, red curve) spectra of TPCPZ.
3.3. Optical properties of the host materials
Fig. 2 depicts the UVevis absorption and photoluminescence
(PL) spectra of TPCPZ in hexane. Four major absorption peaks locate
at 223, 252, 289, 335 nm in the absorption spectra, while the
emission peaks appear at 405 nm in the PL spectra. The absorption
peaks at 223, 252 nm can be attributed to the
pep* and nep*
transitions of the central 2, 4,6-trisphenyl-triazine and outer N-
phenyl. The absorptionpeaks at 289 nm, 335 nm can be attributed to
3.2. Thermal properties of the host materials
the pep* and nep* transitions of the outer carbazole of TPCPZ.
From the absorption edge of the UVevis absorption, the optical
bandgap (Eg) of TPCPZ can be estimated to be 3.23 eV. The phos-
phorescence spectra measured from a frozen 2-methyltetrahy
drofuran matrix at 77 K are also shown in Fig. 2. The triplet en-
ergy of TPCPZ was determined to the value of 2.63 eV by the highest
energy 0e0 phosphorescent emission, which is sufficiency high
enough to serve as the appropriate host for Ir(ppy)3.
The thermal properties TPCPZ were investigated by thermal
gravimetric analyses (TGA) and differential scanning calorimetry
(DSC) under nitrogen atmosphere at a heating rate of 10 ꢀC minꢁ1
.
As shown in Fig.1, TGA measurement reveals its high thermal
decomposition temperature (Td), corresponding to 5% weight-loss
of 559 ꢀC. The DSC trace exhibits a clear glass transition temper-
ature(Tg) of 165 ꢀC during the second heating scans. The thermal
analysis results clearly demonstrate the very high thermal stability
of TPCPZ, which facilitates the forming of amorphous films through
solution processing.
3.4. Electrochemical analysis and theoretical calculations of the
host material
The electrochemical properties TPCPZ were studied in solution
through cyclic voltammetry (CV) using tetrabutylammonium
TPCPZ
1.0
0.1
0.8
TPCPZ
0.0
-0.1
o
Tg= 165
C
0.6
0.4
0.2
0.0
-0.2
-0.3
-0.4
-0.5
150
160
170
180
o
190
200
Temperature ( C)
200
300 400
500
600
0.5
1.0
1.5
2.0
2.5
o
Temparature ( C)
+
Potential(V vs Fc/Fc )
Fig. 1. TGA traces (blue curve) of TPCPZ recorded at a heating rateꢁo1f 10 ꢀC minꢁ1, DSC
(red curve) measurement recorded at a heating rate of 10 ꢀC min
.
Fig. 3. Oxidation part of the CV curves of TPCPZ in CH2Cl2 solutions (10ꢁ3 M).

12
B. Huang et al. / Dyes and Pigments 101 (2014) 9e14
Fig. 4. Optimized geometries and calculated HOMO and LUMO density maps for TPCPZ according to DFT calculations at B3LYP/6-31* level.
hexafluorophosphate (TBAPF6) as the supporting electrolyte and
ferrocene as the internal standard. The highest occupied molecular
orbital (HOMO) energy level of TPCPZ was characterized by the
electrochemical cyclic voltammetry (CV) (Fig.3). During the anodic
scan in CH2Cl2, TPCPZ exhibited reversible oxidation process, which
can be assigned to the oxidation of electron-donating carbazole
moiety, with the onset potential of 1.18 eV. No reduction wave was
detected. On the basis of the onset potential for oxidation, the
HOMO energy level was estimated to be ꢁ5.39 eV, the HOMO en-
ergy level approach the work function of PEDOT(ꢁ5.2 eV), which
allows in a low barrier of hole injection. The LUMO energy level of
TPCPZ estimated from the HOMO energy level and Eg is ꢁ2.16 eV,
which matches the LUMO energy level of Cs2CO3/Al bilayer cathode
(ꢁ2.2 eV), facilitates electrons injection.
with inner 2, 4,6-trisphenyl-triazine unit, resulting in a non-planar
structure in the molecule. These geometrical characteristics can
effectively prevent intermolecular interactions between systems
and thus suppress molecular recrystallization and limit the extent
of conjugation between the central core and branches, which im-
proves the morphological stability of thin film of TPCPZ. The LUMO
level of TPCPZ is localized predominantly on the 2, 4,6-trisphenyl-
triazine unit, while the HOMO level is distributed over the outer
layer N-phenyl carbazole fragments.
3.5. Electroluminescent properties
The single-layer green electrophosphorescent device with
TPCPZ as the host and Ir(ppy)3 as the dopant with the configura-
tion of ITO/PEDOT:PSS/TPCPZ:Ir(ppy)3/Cs2CO3/Al has been fabri-
cated by spin-coating. As shown in Fig.5, the electroluminescence
(EL) spectra of TPCPZ based device is identical with the CIE
DFT calculations were performed to understand the physical
properties of the TPCPZ at themo-Lecular level. As shown in Fig.4,
the outer layer N-phenyl carbazole units are significantly twisted
(a)
(b)
10
TPCPZ
10
10
0
2
4
6
8
10
12
14
Voltage (V)
(d)
(c)
1.0
TPCPZ
0.8
0.6
0.4
0.2
0.0
10
TPCPZ
1
400
500
600
700
0
500 1000 1500 2000 2500 3000
Current Density (mA/cm2)
Wavelength (nm)
Fig. 5. (a) Current-voltage (b) Luminance-voltage characteristics (c) Luminance efficiency versus current density plots of the devices. (d) The normalized EL spectra of devices at a
driving voltage of 10 V.

B. Huang et al. / Dyes and Pigments 101 (2014) 9e14
13
[11] Su SJ, Sasabe H, Takeda T, Kido J. Pyridine-containing bipolar host mate-
rials for highly efficient blue phosphorescent OLEDs. Chem Mater
2008;20:1691e3.
[12] Su SJ, Cai C, Kido J. RGB phosphorescent organic light-emitting diodes by using
host materials with heterocyclic cores: effect of nitrogen atom orientations.
Chem Mater 2011;23:274e84.
[13] Tao Y, Wang Q, Ao L, Zhong C, Qin J, Yang C, et al. Molecular design of host
materials based on triphenylamine/oxadiazole hybrids for excellent deep-
red phosphorescent organic light-emitting diodes. J Mater Chem 2010;20:
1759e65.
[14] Tao Y, Wang Q, Ao L, Zhong C, Qin J, Yang C, et al. Multifunctional triphe-
nylamine/oxadiazole hybrid as host and exciton-blocking material: high ef-
ficiency green phosphorescent OLEDs using easily available and common
materials. Adv Funct Mater 2010;20:2923e9.
[15] Tao Y, Wang Q, Yang C, Wang Q, Zhang Z, Zou T, et al. A simple carbazole/
oxadiazole hybrid molecule: an excellent bipolar host for green and red
phosphorescent OLEDs. Angew Chem Int Ed 2008;47:8104e7.
[16] He Z, Kan CW, Ho CL, Wong WY, Chui CH, Tong KL, et al. Light-emitting dyes
derived from bifunctional chromophores of diarylamine and oxadiazole:
synthesis, crystal structure, photophysics and electroluminescence. Dyes
Pigments 2011;88:333e43.
coordinates of (0.315, 0.605), corresponding to the emission of
Ir(ppy)3, and indicate the efficient energy transfer from the host to
Ir(ppy)3. The JeV characteristic curves of the device demonstrate
the turn-on voltage is only 3.5 V. On one hand, the low turn-on
voltage of TPCPZ based device is most likely due to their high-
lying HOMO energy level and low-lying LUMO energy level,
which matched with the HOMO energy level of
PEDOT:PSS(ꢁ5.35 eV) and the LUMO energy level of Cs2CO3/Al
bilayer cathode (ꢁ2.2 eV), thereby facilitated holes and electrons
injection of the device. On the other hand, the bipolar nature of
TPCPZ make it have good and balanced charge carrier transport
properties for the holeeelectron recombination process and
confinement of the exciton formation zone in the emissive layer.
As revealed in Fig. 5, the maximum LE value of this single-layer
green-emitting device was 20.8 cd Aꢁ1 and a maximum lumi-
nance was 18,000 cd mꢁ2
.
[17] Gao ZQ, Luo M, Sun XH, Tam HL, Wong MS, Mi BX, et al. New host containing
bipolar carrier transport moiety for high-efficiency electrophosphorescence at
low voltages. Adv Mater 2009;21:688e92.
4. Conclusions
[18] Ge ZY, Hayakawa T, Ando S, Ueda M, Akiike T, Miyamoto H, et al. Synthesis
and properties of 3,8-bis(4-(9H-carbazol-9-yl)phenyl)-1,10-phenanthroline
for phosphorescent OLEDs. Chem Lett 2008;37:262e3.
[19] Ge ZY, Hayakawa T, Ando S, Ueda M, Akiike T, Miyamoto H, et al. Novel bipolar
bathophenanthroline containing hosts for highly efficient phosphorescent
OLEDs. Org Lett 2008;10:421e4.
[20] Li YQ, Fung MK, Xie Z, Lee ST, Hung LS, Shi J. An efficient pure blue organic
light-emitting device with low driving voltages. Adv Mater 2002;14:
1317e21.
[21] Ge ZY, Hayakawa T, Ando S, Ueda M, Akiike T, Miyamoto H, et al. Spin-coated
highly efficient phosphorescent organic light-emitting diodes based on bi-
polar triphenylamine-benzimidazole derivatives. Adv Funct Mater 2008;18:
584e90.
[22] Lai MY, Chen CH, Huang WS, Lin JT, Ke TH, Chen LY, et al. Benzimidazole/
amine-based compounds capable of ambipolar transport for application in
single-layer blue-emitting OLEDs and as hosts for phosphorescent emitters.
Angew Chem Int Ed 2008;47:581e5.
[23] Chen CH, Huang WS, Lai MY, Tsao WC, Lin JT, Wu YH, et al. Versatile,
benzimidazole/amine-based ambipolar compounds for electroluminescent
applications: single-layer, blue, fluorescent OLEDs, hosts for single-Layer,
phosphorescent OLEDs. Adv Funct Mater 2009;19:2661e70.
[24] Huang H, Yang X, Pan Biao, Wang Lei, Chen J, Ma D, et al. Benzimidazole-
carbazole- based bipolar hosts for high efficiency blue and white electro-
phosphorescence applications. J Mater Chem 2012;22:13223e30.
[25] Richard A, Klenklera HA, Tranc A, Popovic DZ, Xu G. High electron mobility
triazine for lower driving voltage and higher efficiency organic light emitting
devices. Org Electron 2008;9:285e90.
[26] Matsushima T, Takamori M, Miyashita Y, Honma Y, Tanaka T, Aihara H, et al.
High electron mobility layers of triazines for improving driving voltages,
power conversion efficiencies, and operational stability of organic light-
emitting diodes. Org Electron 2010;11:16e22.
[27] Sun C, Hudson ZM, Helander MG, Lu ZH, Wang S. A polyboryl-functionalized
triazine as an electron transport material for OLEDs. Organometallics 2011;30:
5552e5.
[28] Chen HF, Wang TC, Lin SW, Hung WY, Dai HC, Chiu HC, et al. Peripheral
modification of 1,3,5-triazine based electron-transporting host materials for
sky blue, green, yellow, red, and white electrophosphorescent devices. J Mater
Chem 2012;22:15620e7.
[29] Inomata H, Goushi K, Masuko T, Konno T, Imai T, Sasabe H, et al. High-effi-
ciency organic electrophosphorescent diodes using 1, 3, 5-triazine electron
transport materials. Chem Mater 2004;16:1285e91.
[30] Son K, Yahiro M, Imai T, Yoshizaki H, Adachi C. Analyzing bipolar carrier
transport characteristics of diarylamino-substituted heterocyclic compounds
in organic light-emitting diodes by probing electroluminescence spectra.
Chem Mater 2008;20:4439e46.
[31] Rothmann MM, Haneder S, Como ED, Lennartz C, Schildknecht C, Strohriegl P.
Donor-substituted 1,3,5-triazines as host materials for blue phosphorescent
organic light-emitting diodes. Chem Mater 2010;22:2403e10.
[32] Rothmann MM, Fuchs E, Schildknecht C, Langer N, Lennartz C, Münster I, et al.
Designing a bipolar host material for blue phosphorescent OLEDs: phenoxy-
carbazole substituted triazine. Org Electron 2011;12:1192e7.
[33] An ZF, Chen RF, Yin J, Xie GH, Shi HF, Tsuboi T, et al. Conjugated asym-
metric donor-substituted 1,3,5-triazines: new host materials for blue
In summary, we have designed and synthesized a novel bipolar
solution-processable triazine-based host material. TPCPZ exhibits
high glass transition temperature and high thermal stability. Re-
sults from CV and computational calculations show that TPCPZ has
high-lying HOMO energy level and low-lying LUMO energy level.
The triplet energy of TPCPZ is 2.63 eV. Utilizing TPCPZ as host
material, highly efficient solution-processed single-layer green
PhOLED has been achieved with low turn-on voltage of 3.5 V, a
maximum current efficiency of 20.8 cd Aꢁ1 and a maximum
luminance of 18,000 cd mꢁ2
.
Acknowledgments
The authors thank Dr. Zhao Yan for OLEDs fabrication and
measurement. This work was supported by the National Natural
Science Foundation of China (Grant No.51103023 and 21173042),
the National Basic Research Program of China (Grant
No.2013CB932900), the Research Fund for Graduate Innovation
Project of Jiangsu Province (No. CXZZ13_0090) and the science and
technology support program (industry) project of Jiangsu province
(BE 2013118).
References
[1] Baldo MA, O’Brien DF, You Y, Shoustikov A, Sibley S, Thompson ME, et al.
Highly efficient phosphorescent emission from organic electroluminescent
devices. Nature 1998;395:151e4.
[2] Reineke S, Lindner F, Schwartz G, Seidler N, Walzer K, Lüeesem B, et al. White
organic light-emitting diodes with fluorescent tube efficiency. Nature
2009;459:234e9.
[3] Xiao LX, Chen ZJ, Qu B, Luo JX, Kong S, Gong QH, et al. Recent progresses on
materials for electrophosphorescent organic light-emitting devices. Adv
Mater 2011;23:926e52.
[4] Su SJ, Sasabe H, Pu YJ, Nakayama K, Kido J. Tuning energy levels of electron-
transport materials by nitrogen orientation for electrophosphorescent de-
vices with an ‘ideal’ operating volta. Adv Mater 2010;22:3311e6.
[5] Han CM, Zhang ZS, Xu H, Li J, Xie GH, Chen RF, et al. Controllably tuning
excited-state energy in ternary hosts for ultra low-voltage-driven blue elec-
trophosphorescence. Angew Chem Int Ed 2012;51:10104e8.
[6] Tao YT, Yang CL, Qin JG. Organic host materials for phosphorescent organic
light-emitting diodes. Chem Soc Rev 2011;40:2943e70.
[7] Chaskar A, Chen HF, Wong KT. Bipolar host materials: a chemical approach for
highly efficient electrophosphorescent devices. Adv Mater 2011;23:3876e95.
[8] Kim DW, Coropceanu V, Bredas JL. Design of efficient ambipolar host materials
for organic blue electrophosphorescence: theoretical characterization of hosts
based on carbazole derivatives. J Am Chem Soc 2011;133:17895e900.
[9] D’Andrade BW, Datta S, Forrest SR, Djurovich P, Polikarpov E, Thompson ME.
Relationship between the ionization and oxidation potentials of molecular
organic semiconductors. Org Electron 2005;6:11e20.
phosphorescent organic light-emitting diodes. Chem
e Eur J 2011;17:
10871e8.
[34] Su SJ, Cai CH, Tankmatsu J, Kido J. A host material with a small singletetriplet
exchange energy for phosphorescent organic light-emitting diodes: guest,
host and exciplex emission. Org Electron 2012;13:1937e47.
[35] Arias AC, MacKenzie JD, McCulloch I, Rivnay J, Salleo A. Materials and appli-
cations for large area electronics: solution-based approaches. Chem Rev
2010;110:3e24.
[10] Williams EL, Haavisto K, Li J, Jabbour GE. Excimer-based white phosphores-
cent organic light-emitting diodes with nearly 100% internal quantum effi-
ciency. Adv Mater 2007;19:197e202.

14
B. Huang et al. / Dyes and Pigments 101 (2014) 9e14
[36] Hou LD, Duan L, Qiao J, Li W, Zhang DQ, Qiu Y. Efficient single layer solution-
processed blue-emitting electrophosphorescent devices based on a small-
molecule host. Appl Phys Lett 2008;92(26):263301.
[37] Ye SH, Liu YQ, Lu K, Wu WP, Du CY, Liu Y, et al. An alternative approach to
constructing solution processable multifunctional materials: their structure,
properties, and application in high-performance organic light-emitting di-
odes. Adv Funct Mater 2010;20:3125e35.
[38] Ye SH, Chen JM, Lu K, Wu WP, Du CY, Liu Y, et al. Solution-processed
solid solution of a novel carbazole derivative for high-performance blue
phosphorescent organic light-emitting diodes. Adv Mater 2010;22:
4167e71.
[39] Li W, Qiao J, Duan L, Wang LD, Qiu Y. Novel fluorene/carbazole hybrids with
steric bulk as host materials for blue organic electrophosphorescent devices.
Tetrahedron 2006;63:10161e8.
[41] Lee CW, Lee JY. High quantum efficiency in solution and vacuum processed
blue phosphorescent organic light emitting diodes using novel
benzofuropyridine-based bipolar host material. Adv Mater 2013;25:596e600.
[42] Jiang W, Duan L, Qiao J, Dong GF, Wang LD, Qiu Y. Tuning of charge balance in
bipolar host materials for highly efficient solution-processed phosphorescent
devices. Org Lett 2011;13:3146e9.
[43] Gong SL, Fu Q, Wang Q, Yang CL, Zhong C, Qin JG, et al. Highly efficient deep-
blue electrophosphorescence enabled by solution-processed bipolar tetraar-
ylsilane host with both a high triplet energy and a high-lying HOMO level.
Adv Mater 2011;23:4956e9.
[44] Jiang W, Duan L, Qiao J, Dong GF, Zhang DQ, Wang LD, et al. Novel carbazole/
pyridine-based host material for solution-processed blue phosphorescent
organic light-emitting devices. Dyes Pigments 2012;92:891e6.
a
[45] Gaussian 03 (Revision B.05). Wallingford CT: Gaussian, Inc.; 2004.
[46] Cho I, Kim SH, Kim JH, Park S, Park SY. Highly efficient and stable deep-blue
emitting anthracene-derived molecular glass for versatile types of non-
doped OLED applications. J Mater Chem 2012;22:123e9.
[40] Jiang W, Ge ZJ, Cai PY, Huang B, Dai YQ, Sun YM, et al. Star-shaped dendritic
hosts based on carbazole moieties for highly efficient blue phosphorescent
OLEDs. J Mater Chem 2012;22:12016e22.