Thermally stable bipolar host materials for high efficiency phosphorescent green and blue organic light-emitting diodes

Article abstract of DOI:10.1016/j.dyepig.2017.05.004

Two bipolar host materials H1 (N-linked carbazole) and H2 (C-3-linked carbazole) via changing the substitution position of triazine and carbazole moieties are reported. Their structural and physical properties were fully characterized. The thermal analysis demonstrated that both H1 and H2 are of high thermal stability. The triplet energy levels of H1 and H2 were determined to be 2.64 and 2.7?eV, respectively. Both green and blue OLEDs with H1 and H2 acting as host materials were fabricated to investigate their performances in PHOLED. The green PHOLED based on H1 exhibits the best performance with the maximum current efficiency, maximum power efficiency and maximum brightness of 51.7?cd/A, 37.8 lm/W and 132800?cd/m², respectively, while the blue PHOLED based on H2 shows the best performance with the maximum current efficiency, maximum power efficiency and maximum brightness of 21.6?cd/A, 17.2 lm/W and 16640?cd/m², respectively. The characteristic measurements of OLED devices reveal that both H1 and H2 are promising host materials for practical application. By comparing the commercial host materials of 4,4’-bis(carbazol-9-yl)biphenyl (CBP) and N,N-dicarbazoyl-3,5-benzene (mCP), the power efficiencies of green and blue PHOLED were boosted by 22.3% and 75.5%, respectively.

Full text of DOI:10.1016/j.dyepig.2017.05.004

Dyes and Pigments 143 (2017) 470e478
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Thermally stable bipolar host materials for high efficiency
phosphorescent green and blue organic light-emitting diodes
,
Qingchen Dong a *, Feifei Tai a, Hong Lian a, Zheng Chen a, Mingming Hu b,
,
, ***
Jinhai Huang b **, Wai-Yeung Wong a
a MOE Key Laboratory of Interface Science and Engineering in Advanced Materials and Research Center of Advanced Materials Science and Technology,
Taiyuan University of Technology, 79 Yingze West Street, Taiyuan, 030024, PR China
b Shanghai Taoe Chemical Technology Co., Ltd, Shanghai, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Two bipolar host materials H1 (N-linked carbazole) and H2 (C-3-linked carbazole) via changing the
substitution position of triazine and carbazole moieties are reported. Their structural and physical
properties were fully characterized. The thermal analysis demonstrated that both H1 and H2 are of high
thermal stability. The triplet energy levels of H1 and H2 were determined to be 2.64 and 2.7 eV,
respectively. Both green and blue OLEDs with H1 and H2 acting as host materials were fabricated to
investigate their performances in PHOLED. The green PHOLED based on H1 exhibits the best perfor-
mance with the maximum current efficiency, maximum power efficiency and maximum brightness of
51.7 cd/A, 37.8 lm/W and 132800 cd/m2, respectively, while the blue PHOLED based on H2 shows the best
performance with the maximum current efficiency, maximum power efficiency and maximum bright-
ness of 21.6 cd/A, 17.2 lm/W and 16640 cd/m2, respectively. The characteristic measurements of OLED
devices reveal that both H1 and H2 are promising host materials for practical application. By comparing
the commercial host materials of 4,4’-bis(carbazol-9-yl)biphenyl (CBP) and N,N-dicarbazoyl-3,5-benzene
(mCP), the power efficiencies of green and blue PHOLED were boosted by 22.3% and 75.5%, respectively.
© 2017 Elsevier Ltd. All rights reserved.
Received 21 March 2017
Received in revised form
2 May 2017
Accepted 2 May 2017
Available online 3 May 2017
Keywords:
Triazine derivatives
Carbazole derivatives
Bipolar host materials
Phosphorescent organic light-emitting
diodes
1. Introduction
triplet energy gap (ET) larger than that of the dopant to prevent
reverse energy transfer from the guest back to the host; secondly,
Phosphorescent organic light-emitting diodes (PHOLEDs) have
attracted huge attention from academic to industry in recent years
due to their appealing commercial interest in the field of flat panel
display, solid-sate lighting and virtual reality [1e4]. Due to the
challenges of materials and life-time faced in the PHOLED industry,
numerous works have been done to explore novel materials
including the carrier transport materials, emitting materials and
host materials, etc. to push the development of PHOLEDs [5e10].
The phosphorescent host materials are crucial which serve as the
recombination center for electrons and holes to generate the
electronically excited states [11e14]. Generally, an appropriate host
material should meet the following intrinsic requirements: firstly, a
good carrier transporting properties to balance the charge flux and
confine the triplet exciton within the emitting layer and reduce the
driving voltage; thirdly, good thermal and morphological stability
to prolong the operation lifetime of the OLED [15,16]. Among the
host materials, bipolar molecules, which consist of an electron-
donating moiety and an electron-withdrawing moiety, usually
exhibit excellent performance in PHOLEDs. This is because the bi-
polar molecules have admirable carrier transport ability for both
electron and hole, which thus could balance the carrier mobility.
Limitation of p
econjugation with silicon or sp3 carbon or increase
of distortion of the molecular structure could enhance the triplet
energy level of bipolar host materials [17e21]. Nowadays, various
organic functional groups have been investigated to explore suit-
able bipolar host materials. The carbazole derivatives exhibit suit-
able energy gap and high fluorescent quantum yield [22e26], and
the triazine derivatives are good electron-transport materials due
to their electron deficiency [27,28]. In 2015, Lee et al. reported one
host material TrzmPCz based on triazine and carbazole for blue
PHOLED, and the current efficiency and power efficiency are 32 cd/
* Corresponding author.
** Corresponding author.
*** Corresponding author.
E-mail addresses: dongqingchen@tyut.edu.cn (Q. Dong), dele12@163.com
(J. Huang).
http://dx.doi.org/10.1016/j.dyepig.2017.05.004
0143-7208/© 2017 Elsevier Ltd. All rights reserved.

Q. Dong et al. / Dyes and Pigments 143 (2017) 470e478
471
A and 21.5 lm/W, respectively.29 Lee and co-workers attributed the
excellent performance of TrzmPCz to the distorted molecular
structure through theoretical calculation comparison of TrzmPCz
and the control compound TrzPCz. However, the authors did not
conduct the synthesis of control compound TrzPCz, and the prac-
tical physical properties and OLED fabrication based on TrzPCz were
also not performed.
Hence, in this study, we designed and synthesized two bipolar
host materials H1 and H2 through different linkage mode of
triazine and carbazole moieties. The molecular structure of H1 is
equal to the TrzPCz as mentioned in the previous report by Lee et al.
Both green and blue phosphorescent OLEDs based on H1 and H2
were fabricated. H1 exhibited the best performance with the
maximum current efficiency and power efficiency of 51.5 cd/A and
37.8 lm/W for green PHOLED, while H2 displayed the highest cur-
rent efficiency and power efficiency of 21.6 cd/A and 17.2 lm/W for
blue PHOLED, respectively. By comparing with the commercial host
materials of 4,4’-bis(carbazol-9-yl)biphenyl (CBP) and N,N-dicar-
bazoyl-3,5-benzene (mCP), the power efficiency of green and blue
PHOLED was enhanced by 22.3% and 75.5%, respectively.
were dissolved in solution of THF (20 mL) and 2M K2CO3 (20 mL),
followed by addition of Pd(OAc)2 (16.1 mg, 0.072 mmol) and X-phos
(50.7 mg, 0.144 mmol) under a nitrogen atmosphere. The mixture
was heated to reflux for 6 h. The completion of the reaction was
verified by spot TLC. The resulting mixture was extracted by
dichloromethane for 3 times, then the organic phase was collected
and concentrated by a rotary evaporator. The crude material was
purified by silica gel column chromatography using a petroleum
and dichloromethane mixture (v:v ¼ 6:1) as the eluent to obtain
the desired product (H1) as a white solid (0.52 g, 66%). 1H NMR
(CDCl3, 400 MHz, d/ppm): 9.10 (s, 1H), 8.83e8.76 (m, 5H), 8.49 (d,
J ¼ 1.3 Hz, 1H), 8.26 (d, J ¼ 7.7 Hz, 1H), 7.97 (d, J ¼ 7.8 Hz, 1H), 7.81
(dd, J ¼ 8.5, 1.7 Hz, 1H), 7.72e7.55 (m, 13H), 7.46 (d, J ¼ 3.7 Hz, 2H),
7.36e7.32 (m, 1H); 13C NMR (CDCl3, 101 MHz,
d/ppm):171.75,
137.65, 136.79, 136.28, 133.16, 132.55, 131.58, 129.96, 129.09, 128.69,
127.84, 127.62, 127.19, 126.26 (s), 125.71, 123.97, 123.47, 120.52,
120.17, 119.13, 110.09; HRMS (ESI, m/z): [MþH]þ calcd for C39H26N4,
550.2200, found 551.2239; IR (KBr): 1366 (yC-N) cmꢁ1, 1595, 1528,
1498, 1450 (yC ¼ C) cmꢁ1, 1626 (yC
¼
N) cmꢁ1, 3056 (yC
¼
C-H) cmꢁ1
;
Tdecomp (ꢀC): 402.5 ꢀC for H1.
The synthetic procedure of H2 is similar to that of H1, in which
(4-(9H-carbazol-9-yl)phenyl)boronic acid (3) was used to replace
compound 1. The final product (H2) was obtained as a white solid
2. Experimental
2.1. General information
(0.57 g, 72%). 1H NMR (CDCl3, 400 MHz,
d/ppm): 9.10 (s, 1H),
8.83e8.76 (m, 5H), 8.49 (d, J ¼ 1.3 Hz, 1H), 8.26 (d, J ¼ 7.7 Hz, 1H),
7.97 (d, J ¼ 7.8 Hz, 1H), 7.81 (dd, J ¼ 8.5, 1.7 Hz, 1H), 7.72e7.55 (m,
13H), 7.46 (d, J ¼ 3.7 Hz, 2H), 7.36e7.32 (m, 1H); 13C NMR (CDCl3,
All the solvents and chemicals that were used in the synthetic
route were of reagent grades and purchased from J & K Chemical Co.
and Aladdin Chemical Co. without further purification. The starting
compounds 1, 2 and 3 were purchased from Shanghai Taoe
Chemical Technology Co., Ltd for synthesis of target products
directly. Tetrahydrofuran (THF) was purified by distillation over
sodium under a N2 atmosphere prior to use. All reactions and
manipulations were carried out under a N2 atmosphere. All column
chromatography was performed on silica gel (300e400 mesh) as
the stationary phase in the column. All materials were purified
further by vacuum sublimation prior to fabrication of OLED devices.
The 1H and 13C NMR spectra were recorded on a Bruker AM 400
spectrometer with tetramethylsilane as the internal standard.
High-resolution mass spectra were measured on a Waters LCT
Premier XE spectrometer. The ultravioletevisible (UVeVis) ab-
sorption spectra were obtained on a Varian Cary 500 spectropho-
tometer. Photoluminescence (PL) spectra were recorded on a
Varian-Cary fluorescence spectrophotometer. The cyclic voltam-
metry experiments were performed using a Versastat II electro-
101 MHz,
d/ppm):171.85, 140.80, 136.16, 132.56, 129.07, 128.71,
127.45e127.25, 126.03, 123.61, 120.42, 119.99,109.87; HRMS (ESI, m/
z): [MþH]þ calcd for C39H26N4, 550.2200, found 551.2239; IR (KBr):
1357 (yC-N) cmꢁ1, 1596, 1520, 1482, 1450 (yC ¼ C) cmꢁ1, 1628 (yC
¼
)
N
cmꢁ1, 3056 (yC
¼
C-H) cmꢁ1; Tdecomp (ꢀC): 416.8 ꢀC for H2.
2.3. Fabrication and measurement of OLEDs
The OLED devices were fabricated through vacuum thermal
evaporation technology according to the method modified from our
previous approach [30]. Device with an area of 3 mm ꢂ 3 mm was
fabricated by vacuum deposition (at 1 ꢂ 10ꢁ6 Torr) of functional
organic layers on the indium-tin-oxide (ITO)/glass substrate which
has a sheet resistance of 25
U/square. The ITO/glass substrate was
cleaned sequentially by detergent, de-ionized water, acetone,
ethanol and then dried in the oven at 120 ꢀC. Afterwards, the clean
ITO glass substrate was treated with oxygen plasma for 8 min.
All the other organic layers were deposited at a rate of 1.0 Å/s
sequentially. The cathode was completed through thermal depo-
sition of LiF (1 nm) at a deposition rate of 0.1 Å/s, then Al metal
(200 nm) was deposited through thermal evaporation at a rate of
5.0 Å/s. The EL spectra were measured by a Spectrascan PR655
photometer. The current-voltage-luminance characteristics (I-V-L)
were measured by a computer-controlled Keithley 2400 source-
meter integrated with a TOPCOM BM-7 luminance meter. Current
efficiency and power efficiency were calculated from the plot of I-V-
L. All samples were characterized immediately after thin films
deposition without encapsulation at room temperature. AFM was
measured to investigate the surface morphology of the thin films of
CBP, mCP, H1 and H2.
chemical work station (Princeton applied research) using
a
conventional three-electrode configuration with a glassy carbon
working electrode, a Pt wire counter electrode, and a regular
calomel reference electrode in saturated KCl solution. The oxidation
and reduction potentials were measured in dichloromethane/
acetonitrile (7:3, v/v) solution containing of 0.1 M tetra-n-buty-
lammonium perchlorate (TBAP) as the supporting electrolyte at a
scan rate of 100 mV/s. The differential scanning calorimetry (DSC)
analysis was performed on a DSC Q2000 V24.11 Build 124 instru-
ment with a heating scan rate of 10 ꢀC/min from 0 ꢀC to 250 ꢀC
under nitrogen atmosphere. Thermogravimetric analysis (TGA) was
carried out on the TGA instrument by measuring weight loss of
samples with a heating scan rate of 10 ꢀC/min from 50 ꢀC to 800 ꢀC
under nitrogen.
3. Results and discussion
2.2. Preparation of 3-(3-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-9-
phenyl-9H-carbazole (H1) and 9-(3'-(4,6-diphenyl-1,3,5-triazin-2-
yl)-[1,1'-biphenyl]-4-yl)-9H-carbazole (H2)
3.1. Synthesis and characterization
Chemical structures and the detailed synthetic protocols of
target compounds H1 and H2 are summarized in Scheme 1. Syn-
thesis of H1 and H2 is very simple and can be achieved by one step:
In a 100 mL one-neck flask, (9-phenyl-9H-carbazol-3-yl)boronic
acid (1) (0.50 g, 1.74 mmol) and compound 2 (0.56 g, 1.44 mmol)

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Q. Dong et al. / Dyes and Pigments 143 (2017) 470e478
Scheme 1. Synthetic routes of bipolar host materials H1 and H2.
(9-phenyl-9H-carbazol-3-yl)boronic acid (1) and (4-(9H-carbazol-
9-yl)phenyl)boronic acid (3) were allowed to react with 2-(3-
bromophenyl)-4,6-diphenyl-1,3,5-triazine, respectively, through
Suzuki coupling to give rise to the target compounds in good yield.
Flash column chromatography was performed to purify the final
products. All the compounds were well characterized by 1H NMR,
13C NMR as well as the high-resolution mass spectrometry (HRMS).
absorption spectra in THF are calculated to be 3.78 eV, and 3.62 eV,
respectively. From the PL spectra of H1 and H2, we can see that,
there are two emission maxima located at 416 nm and 441 nm.
3.2. Thermal properties
The thermal properties of compounds H1 and H2 were char-
acterized by thermal gravimetric analyses (TGA) and differential
scanning calorimetry (DSC) under a nitrogen stream. As shown in
Fig. 1 and Table 1, the TGA data reveal that the onset decomposition
temperatures (Td, corresponding to 5% weight loss) of these three
materials are close to 400 ꢀC implying that they are thermally
stable. By analysis of the DSC traces, we found that, the endo-
thermic glass transition temperatures (Tg) for H1 and H2 are
determined to be 95 and 100 ꢀC which is desirable for high-
performance OLEDs. The high thermal stability of the new host
materials is crucial for enhancing the film morphology and device
lifetime by reducing the crystallization and phase separation dur-
ing device operation.
3.3. Photophysical properties
To investigate the photophysical properties of compounds H1
and H2, their UVeVis absorption spectra, photoluminescence (PL)
spectra (at room temperature in THF) and phosphorescence spectra
(at 77 K in 2-methyl THF) were measured and presented in Fig. 2.
The detailed data are listed in Table 1. As shown in Fig. 1, both H1
and H2 exhibit one major absorption bands at around 280 nm in
their UVeVis absorption spectra, which can be derived from the
p/p* transitions. The secondary, weak absorption peaks that are
located at around 340 nm can be interpreted as arising from the CT
states of the compounds [31]. The optical energy band gaps (Eg) of
H1 and H2 that can be obtained from the threshold of the
Fig. 1. (A) DSC and (B) TGA curves of H1 and H2.

Q. Dong et al. / Dyes and Pigments 143 (2017) 470e478
473
Table 1
The physical properties of compound H1 and H2.
lmax absa
(nm)
lmax ema
(nm)
Eg
HOMOc
(eV)
LUMOd
(eV)
ET
Tg
Td
Fh
ti
b
e
f
g
Compound
(eV)
(eV)
(ꢀC)
(ꢀC)
(%)
(ns)
H1
H2
279
273
416
416
3.22
3.29
ꢁ6.09
ꢁ6.09
ꢁ2.87
ꢁ2.70
2.64
2.70
94.74
100.05
383.93
386.47
9
16
2.84
2.42
a
Measured in THF.
b
Estimated from onset of the absorption spectra (Eg ¼ 1240/lonset).
c
d
e
f
Calculated from cyclic voltammetry.
Calculated by the equation EHOMO ¼ ELUMO e Eg.
Calculated by the first peak of phosphorescent spectra measured at 77 K.
Measured by DSC.
g
h
i
Measured by TGA.
Fluorescence quantum yield are measured in CH2Cl2 relative to quinine sulphate.
Measured at room temperature in 2-methyltetrahydrofuran solution with the excitation wavelength at 375 nm.
From the phosphorescence spectra in the frozen 2-methyl THF at
77 K, the triplet energies (ET) of H1 and H2 are determined to be
2.64 eV and 2.70 eV, respectively, which were calculated on the
basis of the short wavelength peaks. Moreover, we also investigated
the quantum yield and lifetime of these two compounds, the results
of which were listed in Table 1. These photophysical properties of
H1 and H2 allow them suitable in fabricating green and blue
PHOLEDs [32].
3.4. Electrochemical properties and energy levels
The electrochemical properties of H1 and H2 were investi-
gated by cyclic voltammetry (CV) and the first oxidation poten-
tials were used to determine the highest occupied molecular
orbital (HOMO) and lowest unoccupied molecular orbital (LUMO)
energy levels (Fig. 3). The CV curves were measured in acetoni-
trile (0.1 M TBAP as the supporting electrolyte; scan rate:
100 mV
s
ꢁ1), with SCE reference electrode (ferrocene/
Fig. 2. Absorption spectra, photoluminescence spectra (at room temperature in THF)
and phosphorescence spectra (at 77 K in THF) of H1 and H2 at 1.0 ꢂ 10ꢁ5 mol/L.
Fig. 3. Cyclic voltammograms of H1 and H2 in toluene with tetra(n-butyl)ammo-
niumhexafluorophosphate (0.1 M) as the supporting electrolyte for the oxidation scan.
Fig. 4. Current density-voltage curves of the (A) hole-only and (B) electron-only de-
vices for CBP, mCP, H1 and H2.

474
Q. Dong et al. / Dyes and Pigments 143 (2017) 470e478
Fig. 5. (A) Schematic drawing illustrating the device structure and (B) energy level diagram of green PHOLEDS; (C) Schematic drawing illustrating the device structure and (D)
energy level diagram of blue PHOLEDS; (E) molecular structures of organic materials as-used in the PHOLEDs. (For interpretation of the references to colour in this figure legend, the
reader is referred to the web version of this article.)
ferrocenium (Fc/Fcþ) redox couple as the external standard), a
glassy-carbon working electrode and a platinum-wire counter
electrode. The onset oxidation potentials of both H1 and H2 were
measured to be 1.29 eV from the cyclic voltammograms. The
HOMO values for H1 and H2 were calculated to be ꢁ6.09 eV.
According to the formula ELUMO ¼ EHOMO þ Eg, the corresponding
LUMO values of H1 and H2 were determined to be ꢁ2.87
and ꢁ2.70 eV. These data can guarantee the outstanding carriers
injection ability of H1 and H2 in green and blue phosphorescent
OLEDs [33,34].
3.5. Carrier transport properties
Bipolar charge transport property of host material is one of the
most important factors to contribute to the high performance of
PhOLEDs. To evaluate the charge-transporting character of H1 and

Q. Dong et al. / Dyes and Pigments 143 (2017) 470e478
475
Fig. 6. (A) Current density-voltage-luminance curves, (B) current efficiency-current density curves, (C) power efficiency-current density curves and (D) EQE-brightness charac-
teristics measured for Control-G, H1-G and H2-G, (E) electroluminescence spectra of green PHOLEDs at 6 V and (F) electroluminescence spectra of green PHOLED based on H1 at
different applied voltages.
H2, hole-only devices with configuration of ITO/MoO3 (3 nm)/NPB
(10 nm)/host (40 nm)/NPB (10 nm)/Al (200 nm)and electron-only
devices with configuration of ITO/TPBi (10 nm)/host (40 nm)/TPBi
(10 nm)/LiF (1 nm)/Al (200 nm) were fabricated, respectively. In
hole-only devices, the N,N’-diphenyl-N,N’-bis(1,1’-biphenyl)-4,4’-
diamine (NPB)/Al interface was designed to prevent electron in-
jection due to the large electron barrier of 1.8 eV. In the electron-
only devices, a thin layer of 2,20,2’’-(1,3,5-benzinetriyl)-tris(1-
phenyl-1-H-benzimidazole) (TPBi) was inserted between the ITO
anode and the host layer to prevent hole injection due to the deep
HOMO level (ꢁ6.2 eV) of TPBi and thus the large hole barrier
(1.5 eV). As shown in Fig. 4, both hole-only and electron-only de-
vices of H1 and H2 exhibit significantly high current densities in the
voltage range typically suitable for OLEDs, indicating the bipolar
charge-transport feature of them. Moreover, we can learn that both
H1 and H2 are good at electron transportation by comparing with
CBP and mCP. It should be noted that the hole-transporting ability
of H1 is better than H2, which may be attributed to the more
coplanar conformation of H1 due to the steric effect. As reported
previously, the more coplanar conformation is favored in the
forming of high quality film, which thus benefits charge hopping,
transportation and higher current densities in devices [35].
3.6. Phosphorescent OLEDs
To investigate the electroluminescent performances of H1 and
H2 serving as the host materials, green and blue PHOLED devices
based on H1 and H2 were fabricated. The device structures (Fig. 5a
and c) were as follows: ITO/MoO3 (3 nm)/NPB (40 nm)/TCTA
(10 nm)/Host: Ir(ppy)3 (20 nm, 8 wt%)/TPBi (35 nm)/LiF (1 nm)/Al

476
Q. Dong et al. / Dyes and Pigments 143 (2017) 470e478
Table 2
(200 nm) for green OLEDs (denoted as H1-G and H2-G, respec-
tively) and ITO/MoO3 (3 nm)/NPB (40 nm)/TCTA (10 nm)/Host:
FIrpic (20 nm, 8 wt%)/TPBi (35 nm)/LiF (1 nm)/Al (200 nm) for blue
OLEDs (denoted as H1-B and H2-B, respectively). MoO3, NPB and
4,4,4-tris(N-carbazolyl)triphenylamine (TCTA) served as the hole
injection layer (HIL) and hole transport layers (HTL), respectively,
the emitting layer (EML) was composed of 8 wt% Ir(ppy)3 or bis
[(4,6-difluorophenyl)-pyridinato-N,C2’]c(picolinate)iridium(III)
(Firpic) doped into host materials, TPBi acted as the electron
transport layer (ETL) and LiF was employed as the electron injection
layer (EIL). For comparison, the control devices of green OLED and
blue OLED with CBP and mCP as host materials were also fabricated,
denoted as Control-G and Control-B, respectively. Fig. 5b and
d displays the relevant HOMO and LUMO energy levels of these
materials, while the molecular structures of all organic materials
A summary of parameters measured for a set of structurally identical green and blue
PHOLEDs with different host materials, the performance of the relative control
devices are also listed for comparison.
a
Device
Von
(V)
CEb
(cd/A)
PEc
(lm/W)
Max. Luminance
(cd/m2)
EQEd
(%)
lEL
(nm)
Control-G
H1-G
H2-G
Control-B
H1B
H2-B
3.0
2.9
2.7
3.3
3.2
3.1
512
512
512
472
476
476
44.2
51.7
45.5
14.3
19.0
21.6
30.9
37.8
34.5
9.80
14.7
17.2
115400
132800
108600
15730
20360
16640
12.9
15.0
13.0
6.4
8.5
8.8
a
Turn-on voltage, recorded at luminance of 1 cd mꢁ2
Maximum current efficiency.
Maximum power efficiency.
.
b
c
d
External quantum efficiency.
Fig. 7. (A) Current density-voltage-luminance curves, (B) current efficiency-current density curves, (C) power efficiency-current density curves and (D) EQE-brightness charac-
teristics measured for Control-B, H1-B and H2-B, (E) electroluminescence spectra of blue PHOLEDs at 6 V and (F) electroluminescence spectra of blue PHOLED based on H2 at
different applied voltages.

Q. Dong et al. / Dyes and Pigments 143 (2017) 470e478
477
used in the device are listed in Fig. 5e. The obtained electrolumi-
nescence performances for green OLEDs are shown in Fig. 6, and
the data are collected in Table 2. It can be seen from the current
density-voltage-luminance curves (Fig. 6a) that, the turn-on volt-
ages and maximum luminance of the Control-G, H1-G and H2-G
green devices are determined to be 3.0, 2.9 and 2.7 V, 115400,
132800 and 108600 cd/m2, respectively [24]. Furthermore, from
the current efficiency-current density curves (Fig. 6b) and power
efficiency-current density curves (Fig. 6c), we can learn that the
maximum current efficiencies for Control-G, H1-G and H2-G green
devices were 44.2, 51.7 and 37.8 cd/A, respectively, and the corre-
sponding maximum power efficiency are 30.9, 37.8 and 34.5 lm/W,
respectively. Fig. 6d exhibits the comparison of EQE of the Control-
G, H1-G and H2-G green devices, showing the maximum EQE of
12.9, 15.0 and 13.0% for the three devices, respectively. The EL
spectra of the above green devices at 6 V are shown in Fig. 6e. The
maximum emission peak is found at 512 nm with the same Com-
mission Internationale de L'Eclairage (CIE) coordinates of (0.31,
0.61). The EL spectra of device based on H1 at different applied
voltages are shown in Fig. 6f. We can see that, the shape of the EL
spectra does not show much change with the increase of the
applied voltages which implies the high EL stability of device.
Therefore, all the data as depicted above manifest that compounds
H1 and H2 are excellent green host materials. Additionally, the
green device based on host material H1 exhibits the best perfor-
mance according to the aforementioned data. This could be
attributed to the excellent thermal stability and high glass transi-
tion temperature of H1 and H2, which enhances the capability of
forming a stable amorphous thin film. Besides, by comparing to H2
and CBP, the higher LUMO and HOMO energy level of H1 can also
block the hole and electron into the emitting layer, which could
enhance the recombination chance of hole and electron and then
benefits the improvement of device performance.
For blue OLEDs, the obtained electroluminescence perfor-
mances are shown in Fig. 6 and the data are also collected in Table 2.
It can be seen from the current density-voltage-luminance curves
that (Fig. 7a), the turn-on voltages and maximum luminance of the
Control-B, H1-B and H2-B blue devices are determined to be 3.3,
3.2 and 3.1 V,15730, 20360 and 16640 cd/m2, respectively. From the
current efficiency-current density curves (Fig. 7b) and power
efficiency-current density curves (Fig. 7c), it can be learnt that the
maximum current efficiencies for Control-B, H1-B and H2-B blue
devices were 14.3, 19.0 and 21.6 cd/A, respectively, and the corre-
sponding maximum power efficiency are 9.8, 14.7 and 17.2 lm/W,
respectively. Fig. 7d exhibits the comparison of EQE of the Control-
B, H1-B and H2-B blue devices, showing the maximum EQE of 6.4,
8.5 and 8.8% for the three devices, respectively. The EL spectra
comparison of the above blue devices at 6 V are shown in Fig. 7e.
The maximum emission peak is found at around 476 nm with the
Commission Internationale de L'Eclairage (CIE) coordinates of (0.18,
0.28), (0.18, 0.37) and (0.21, 0.41) for Control-B, H1-B and H2-B,
respectively. The EL spectra of device based on H2 at different
applied voltages are displayed in Fig. 7f. We can observe clearly
that, the shape of the EL spectra does not show much change with
the increase of the applied voltages which implies the high EL
stability of device H2-B. Therefore, all the data as depicted above
manifest that compounds H1 and H2 are proven to be also excellent
blue host materials. Additionally, blue PHOLED device based on H2
exhibits the best performance according to the above data. This
might be ascribed to the higher triplet energy level of H2, which
facilitate the effective energy transfer from host to emitter and then
benefits the improvement of device performance [29,36].
Fig. 8. AFM images of the thermally evaporated thin films of (a) CBP, (b) mCP, (c) H1
and (d) H2, respectively.
and mCP. Therefore, combining the triazole and carbazole de-
rivatives is a good method to enhance the performance of host
materials in OLEDs, which also enlarges the category of organic
functional groups to serve as host materials. It should be pointed
out that the electroluminescent performance was obtained under
ordinary laboratory condition where the device is not packaged.
The device performance could be further improved by optimization
of the processing conditions [37].
3.7. Morphology study
To further understand the mechanism underlying electrolumi-
nescent efficiency enhancement of green and blue OLEDs with H1
and H2 as the host, we then performed atomic force microscopy
(AFM) measurements to investigate the morphology of thin films of
H1 and H2 by comparing with the thin films of commercial host
materials CBP and mCP. As shown in Fig. 8, it can be seen that, the
surface roughness of H1 and H2 are lower than those of the com-
mercial host materials, especially that of CBP. Moreover, the surface
roughness of H2 is much smaller than that of mCP, which can also
explain its outstanding performance in blue PHOLED. Hence, the
morphology study reveals a crucial influence factor responsible for
the improvement of PHOLEDs with H1 and H2 as the host.
4. Conclusions
To conclude, two novel bipolar host materials H1 and H2 based
on triazole/carbazole derivatives were designed and synthesized.
Their thermal, photophysical, electrochemical and electrolumi-
nescent properties were fully investigated. Both H1 and H2 exhibit
good thermal properties with the decomposition temperatures (Td)
close to 400 ꢀC. Due to their high thermal stability, favorable pho-
tophysical, electrochemical and bipolar charge-transport proper-
ties, excellent electroluminescent performances for both green and
blue PHOLED were obtained with H1 and H2 as the host materials.
As discussed above, the bipolar host materials H1 and H2 exhibit
higher current efficiency and power efficiency for both green and
blue PHOLEDs as compared to the commercial host materials CBP

478
Q. Dong et al. / Dyes and Pigments 143 (2017) 470e478
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H1 exhibits the best device performances with the maximum
current efficiency, maximum power efficiency and maximum
brightness of 51.7 cd/A, 37.8 lm/W and 132800 cd/m2, respectively,
for green PHOLEDs, while H2 shows the best device performances
with the maximum current efficiency, maximum power efficiency
and maximum brightness of 21.6 cd/A, 17.2 lm/W and 16640 cd/m2,
respectively, for blue PHOLED due to its higher triplet energy level.
It was of important scientific significance to discover novel host
materials for high efficiency OLEDs by combining the triazole and
carbazole derivatives. Further investigation along this direction is
in progress.
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Acknowledgements
[20] Kim GW, Yang DR, Yong CK, Fan CH, Chai KY, Kwon JH, et al. Di(biphenyl)
silane and carbazole based bipolar host materials for highly efficient blue
phosphorescent OLEDs. Dyes Pigments 2016;136:8e16.
We acknowledge the financial support from the National Nat-
ural Science Foundation of China (Grant No.: 61307030, 21303117).
This work was also supported by the Program for the Outstanding
Innovative Teams of Higher Learning Institutions of Shanxi (OIT),
Fund Program for the Scientific Activities of Selected Returned
Overseas Professionals in Shanxi Province, the National Science
Foundation for Young Scientists of Shanxi Province, China (Grant
No.: 2014021019-2), the Outstanding Young Scholars Cultivating
Program, Research Project Supported by Shanxi Scholarship
Council of China (Grant No.: 2014-02) and the Key R&D Project of
Shanxi Province (International cooperation program, No.
201603D421032). W. Y. Wong thanks for the financial support from
the Hong Kong Polytechnic University.
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