Three new carbazole derivatives with high thermal stability as host for efficient green phosphorescent organic-light emitting diodes

Article abstract of DOI:10.1016/j.dyepig.2019.107670

Three host materials, DCzPh, DCzPy and DCzPm, were developed for phosphorescent organic light emitting devices (PhOLEDs). These three compounds exhibit high triplet energy level (E_T > 2.7 eV), suitable glass transition temperatures (T_g > 130 °C), appropriate highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels, and balanced charge-transporting properties. Green PhOLEDs were fabricated by utilizing these three materials as hosts and the efficiencies were satisfactory. Device G3 based on DCzPm exhibited high efficiency with the maximum external quantum efficiency (EQE) of 17.2%, and even at the brightness of 1000 cd m^?2, the EQE reached 17.1%, demonstrating its low efficiency roll-off. The above results indicate that these host materials possess great commercial potential in OLED applications.

Full text of DOI:10.1016/j.dyepig.2019.107670

Accepted Manuscript
Three new carbazole derivatives with high thermal stability as host for efficient green
phosphorescent organic-light emitting diodes
Jun Ji, Pengfei Li, Qifeng Tian, Weiliang Feng, Changjiang Wu
PII:
S0143-7208(19)31237-9
DOI:
https://doi.org/10.1016/j.dyepig.2019.107670
Article Number: 107670
Reference:
DYPI 107670
To appear in: Dyes and Pigments
Received Date: 29 May 2019
Revised Date: 24 June 2019
Accepted Date: 24 June 2019
Please cite this article as: Ji J, Li P, Tian Q, Feng W, Wu C, Three new carbazole derivatives with high
thermal stability as host for efficient green phosphorescent organic-light emitting diodes, Dyes and
Pigments (2019), doi: https://doi.org/10.1016/j.dyepig.2019.107670.
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ACCEPTED MANUSCRIPT
Three new carbazole derivatives with high thermal stability as
host for efficient green phosphorescent organic-light emitting
diodes
a, b
b
b
b
b
Jun Ji *, Pengfei Li , Qifeng Tian , Weiliang Feng , Changjiang Wu
a
(
School of Chemical Engineering and Technology, Tianjin University,
Tianjin 300350, P. R. China
b
Key Laboratory for Green Chemical Process of Ministry of Education, School of
Chemical Engineering and Pharmacy, Wuhan Institute of Technology, Wuhan
4
30205, P. R. China)
E-mail: junji@wit.edu.cn (J. Ji)

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Graphical abstract

ACCEPTED MANUSCRIPT
Abstract
Three host materials, DCzPh, DCzPy and DCzPm, were developed for
phosphorescent organic light emitting devices (PhOLEDs). These three compounds
exhibit high triplet energy level (E >2.7 eV), suitable glass transition temperatures
T
o
(
T >130 C), appropriate highest occupied molecular orbital (HOMO) and lowest
g
unoccupied molecular orbital (LUMO) levels, and balanced charge-transporting
properties. Green PhOLEDs were fabricated by utilizing these three materials as hosts
and the efficiencies were satisfactory. Device G3 based on DCzPm exhibited high
efficiency with the maximum external quantum efficiency (EQE) of 17.2%, and even
-2
at the brightness of 1000 cd m , the EQE reached 17.1%, demonstrating its low
efficiency roll-off. The above results indicate that these host materials possess great
commercial potential in OLED applications.
Keywords
Green phosphorescent organic light-emitting diodes; Carbazole derivatives; High
thermal stability; Host materials;

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1
. Introduction
In the past few decades, PhOLEDs have received huge attention because of their
application potential in high-resolution and flexible displays. [1-4] Compared with
traditional fluorescent OLED, PhOLEDs are more favored due to their 100%
theoretical internal quantum efficiencies (IQEs),[2,5-6] which is because
phosphorescent emitter can utilize both singlet and triplet excitons via the heavy-atom
effect. To achieve better performance, PhOLEDs usually adopt host-guest system for
the emitting layer (EML).[7-10] The host-guest system can reduce unfavorable factors
such as triplet-triplet annihilation (TTA) and concentration quenching. An ideal host
material should have a high triplet energy level (E ) to ensure that the energy can be
T
transferred from host to the emitter. Besides, host materials should have appropriate
HOMO/LUMO levels to improve carrier injection into the EML.[8,11-12] More
importantly, to achieve charge-balances in the EML, host materials must possess a
bipolar nature. Bipolar host materials composed of hole-transporting unit and
electron-transporting unit can balance the hole and electron transport in the EML and
realize better electroluminescence (EL) performance as well as lower efficiency
roll-off.[13-14] Thus, the development of high-performance bipolar host materials is
significantly important for PhOLEDs.
In this work, three bipolar host materials, 3,5-bis(9-phenyl-9H-carbazol
-3-yl)-1,1'-biphenyl (DCzPh), 3,3'-(5-(pyridin-3-yl)-1,3-phenylene)bis(9-phenyl-9H
-carbazole) (DCzPy) and 3,3'-(5-(pyrimidin-5-yl)-1,3-phenylene)bis(9-phenyl-9H
-carbazole) (DCzPm) were designed and synthesized. The carbazole moiety was
chosen as the hole-transporting unit, because of its high triplet energy level and good
hole-transporting ability.[15-17] Biphenyl, phenylpyridine and phenylpyrimidine
were selected as electron-transporting units, respectively.[18] Carbazole was linked to
the electron-transporting units at its 3-site C atom, this link mode is beneficial for
hole transporting.[19-20] These three compounds exhibit high triplet energy levels
(all higher than 2.7 eV), good thermal stability, and appropriate HOMO/LUMO
energy levels. Green PhOLEDs based on the above three hosts exhibit excellent

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performance, for example, for device G3 hosted by DCzPm, the turn-on voltage is
3
.1V, and the maximum EQE can achieve as high as 17.2%. At the brightness of 1000
-2
cd m , EQE of G3 can still reach 17.1%, which is almost the same as the maximum
value.
2
. Experimental
2
.1. Materials and measurements
All reagents used in this work were of analytical grade and obtained via commercial
source without further purification. The silica gel used in the column purification was
2
00 to 300 mesh.
1
13
H and C NMR spectra were characterized on a Bruker AM-400 spectrometer.
Mass spectra were analyzed using a Waters LCT Premier XE spectrometer. Elemental
analysis was measured by Vario EL� . The ultraviolet-visible (UV-Vis) spectra were
measured on a Nicolet CARY-100 spectrophotometer. Photoluminescence (PL)
spectra were carried out on a Varian Cary fluorescence spectrophotometer. The
electrochemical property was investigated on a VersaSTAT II workstation using 0.1
-1
mol L
tetrabutylammonium hexafluorophosphate (TBAPF ) as supporting
6
electrolyte in dichloromethane. Differential scanning calorimetry (DSC) analysis was
measured on a NETZSCH STA 409 PC/PG instrument.
2
2
3
.2. Synthesis
.2.1. Synthesis of (9-phenyl-9H-carbazol-3-yl)boronic acid
-Bromo-9-phenyl-9H-carbazole (10.0 g, 31 mmol) was dissolved in anhydrous
o
tetrahydrofuran (80 mL) at -78 C and n-BuLi (23 mL, 2.0 M in n-hexane, 36.8 mmol)
was added dropwise under N protection and stirred for 3 h. Afterwards, triisopropyl
2
borate (8.8 g, 46.8 mmol) was added dropwise and maintained for 0.5 h before
warming to ambient temperature slowly to stir overnight before pouring into aq. HCl
(
2M, 80 mL) and extraction with DCM twice. Removal of the dried Na SO solvent
2 4
and recrystallisation with DCM and n-hexane (v/v = 1/1) to deliver solid (7.2g, yield
1
8
0%). H NMR (CDCl , 400 MHz, δ): 9.12 (s, 1H), 8.37 (d, 2H), 7.60-7.66 (m, 4H),
3

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7
2
.52 (m, 2H), 7.45 (m, 2H), 7.39 (m, 1H).
.2.2. Synthesis of 3,3'-(5-chloro-1,3-phenylene)bis(9-phenyl-9H-carbazole) (DCzCl)
(9-phenyl-9H-carbazol-3-yl)boronic acid (7.0 g, 24.4 mmol), 1,3-dibromo-5-
chlorobenzene (3.0 g, 11.1 mmol) and K CO (4.7 g, 34.0 mmol) were charged to
2
3
THF (30 mL) and deionized water (15mL), Pd(PPh ) (0.06 g, 0.52 mmol) was
3
4
charged after bubbling with N for 15 min. Reflux the mixture under N protection for
2
2
6
h, cooled down and pour into mL H O (100 mL) and filter. The crude solid was
2
purified by recrystallization using THF/methanol (v/v = 1/2) to deliver 3.9 g solid
1
(
60%). H NMR (400 MHz, CDCl , δ): 8.45-8.46 (m, 2H), 8.25 (d, J = 7.6 Hz, 2H),
3
7
2
.94-7.95 (m, 1H), 7.62-7.76 (m, 12H), 7.46-7.52 (m, 8H), 7.33-7.38 (m, 2H).
.2.3. Synthesis of 3,5-bis(9-phenyl-9H-carbazol-3-yl)-1,1'-biphenyl (DCzPh)
Charge DCzCl (1.0 g, 1.68 mmol), phenylboronic acid (0.22 g, 1.80 mmol) and
K CO (0.7 g, 5.06 mmol) into THF (12 mL) and deionized water (6 mL), then
2
3
Pd(OAc) (0.02 g, 0.09 mmol) and X-phos (0.04 g, 0.08 mmol) were charged after
2
bubbling with N for 15 min. Reflux the mixture for 12 h under protection of N , cool
2
2
down to ambient temperature, pour into H O (50 mL) and filter. The crude product
2
was refined by recrystallization using THF/methanol (v/v = 1/1.5) to deliver solid
1
(
0.68 g, yield 63%). H NMR (400 MHz, CDCl , δ): 8.53 (d, J = 1.2 Hz, 2H), 8.26 (d,
3
J = 8.0 Hz, 2H), 8.06-8.07 (m, 1H), 7.94 (d, J = 1.6 Hz, 2H), 7.82-7.85 (m, 4H),
13
7
.63-7.70 (m, 8H), 7.43-7.58 (m, 11H), 7.33-7.38 (m, 2H). C NMR (100 MHz,
CDCl , δ): 143.14, 142.44, 141.60, 141.40, 140.53, 137.67, 133.54, 129.96, 128.88,
3
1
27.57, 127.51, 127.11, 126.20, 125.70, 125.62, 124.78, 123.97, 123.49, 120.48,
+
1
20.14, 119.10, 110.12, 109.96. HRMS (ESI, m/z): [M+H] calcd for C H N ,
48
33
2
6
37.2638, found, 637.2647. Elemental analysis calcd for C H N : C, 90.54; H, 5.07;
48 32 2
N, 4.40. Found: C, 90.93; H, 4.43; N, 4.14.
2
.2.4. Synthesis of 3,3'-(5-(pyridin-3-yl)-1,3-phenylene)bis(9-phenyl-9H-carbazole)
DCzPy)
Charge DCzCl (1.0 g, 1.68 mmol), pyridin-3-ylboronic acid (0.23 g, 1.87 mmol) and
(

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0
.7 g K CO (5.06 mmol) into THF (12 mL) and deionized water (6 mL), then
2
3
Pd(OAc) (0.02 g, 0.09 mmol) and X-phos (0.04 g, 0.08 mmol) were charged after
2
bubbling with N for 15 min. Reflux the mixture for 12 h under protection of N , cool
2
2
down to ambient temperature, pour into water and filter. The crude solid was purified
by recrystallization using THF/methanol (v/v = 1/1.5) to deliver solid (0.62 g, yield
1
5
8%). H NMR (400 MHz, CDCl , δ): 9.00 (s, 1H), 8.60 (s, 1H), 8.43 (d, J = 1.2 Hz,
3
2
H), 8.17 (d, J = 7.6 Hz, 2H), 8.00-8.03 (m, 2H), 7.82 (d, J = 1.2 Hz, 2H), 7.72-7.75
1
3
(
m, 2H), 7.37-7.60 (m, 17H), 7.24-7.28 (m, 2H). C NMR (100 MHz, CDCl , δ):
3
1
1
1
48.70, 148.66 143.54, 141.44, 140.63, 139.04, 137.61, 137.06, 134.73, 133.12,
29.99, 127.64, 127.12, 126.38, 126.30, 125.63, 124.63, 124.03, 123.67, 123.43,
+
20.50, 120.21, 119.10, 110.22, 110.01. HRMS (ESI, m/z): [M+H] calcd for
C H N , 638.2591, found, 638.2598. Elemental analysis calcd for C H N : C,
47
32
3
47 31
3
8
2
8.51; H, 4.90; N, 6.59. Found: C, 88.37; H, 4.45; N, 6.71.
.2.5. Synthesis of 3,3'-(5-(pyrimidin-5-yl)-1,3-phenylene)bis(9-phenyl-9H-carbazole)
(DCzPm)
Charge DCzCl (1.0 g, 1.68 mmol), pyrimidin-5-ylboronic acid (0.23 g, 1.86 mmol)
and 0.7 g K CO (5.06 mmol) into THF (12 mL) and water (6 mL), then Pd(OAc)
2
2
3
(0.02 g, 0.09 mmol) and X-phos (0.04 g, 0.08 mmol) were charged after bubbling
with N for 15 min. Reflux the mixture for 10 h under N protection, cool down to
2
2
ambient temperature, pour into H O (50 mL) and filter. The crude product was refined
2
by recrystallization using THF/methanol (v/v = 1/1) to deliver solid (0.65 g, yield
1
6
1%). H NMR (400 MHz, CDCl , δ): 9.28 (s, 1H), 9.16 (s, 2H), 8.49 (d, J = 1.6 Hz,
3
2
H), 8.24 (d, J = 7.6 Hz, 2H), 8.14-8.15 (m, 1H), 7.87 (d, J = 1.6 Hz, 2H), 7.77-7.81
(m, 2H), 7.60-7.67 (m, 8H), 7.48-7.55 (m, 4H), 7.43-7.46 (m, 4H), 7.31-7.36 (m, 2H).
1
3
C NMR (100 MHz, CDCl , δ): 157.69, 155,24, 143.96, 141.47, 140.72, 137.55,
3
1
1
35.46, 134.81, 132.70, 130.01, 127.69, 127.19, 127.12, 126.37, 125.54, 124.32,
+
24.07, 123.36, 120.49, 120.26, 119.10, 110.30, 110.05. HRMS (ESI, m/z): [M+H]
calcd for C H N , 639.2543, found, 639.2548. Elemental analysis calcd for
4
6
31
4
C H N : C, 86.49; H, 4.73; N, 8.77. Found: C, 86.87; H, 4.52; N, 8.37.
46
30
4

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Scheme 1. Synthetic routes for DCzPh, DCzPy and DCzPm.
3
. Results and discussion
3
.1. Synthesis and characterization
As presented in Scheme 1, the key intermediate DCzCl was synthesized between
9-phenyl-9H-carbazol-3-yl)boronic acid and 1,3-dibromo-5-chlorobenzene, and the
(
titled molecules were coupled between DCzCl and corresponding arylboronic acid.
All three compounds were obtained in satisfactory yield, and their structures were
1
13
fully characterized by H, C NMR and mass spectrometry (HRMS).
.2. Thermal properties
3
Good thermostability is beneficial to prolong the operation lifetime of OLEDs.
Consequently, thermogravimetric analysis (TGA) and differential scanning
calorimetry (DSC) analysis were adopted to evaluate the thermal properties of DCzPh,
DCzPy and DCzPm. Seen from Fig. 1 and Table 1, the glass-transition temperature
o
(
T ) of DCzPh, DCzPy and DCzPm were 132, 137 and 140 C, respectively, while
g
the thermal decomposition temperatures (T ) were 434, 418 and 433 °C, respectively,
d

ACCEPTED MANUSCRIPT
demonstrating excellent thermostability of all these newly designed molecules.
Fig. 1. Thermal properties of DCzPh, DCzPy and DCzPm.
Table 1 Properties of DCzPh, DCzPy and DCzPm.
λmax abs (nm)
λmax em
(nm)
378
Q.Y.
Stokes Shift
HOMO
LUMO
ET
Tg
c
o
g
Compound
Eg (eV)
T ( C)
d
-
1
-1 a
b
-1
d
d
e
o
f
(
ε/ M cm )
296 (7791)
297 (8608)
295 (6505)
(%)
(cm )
(eV)
(eV)
(eV)
( C)
DCzPh
DCzPy
DCzPm
11.32
10.45
10.68
7382
7355
8518
3.41
3.43
3.38
-5.13
-5.33
-5.22
-1.67
-1.87
-1.78
2.74
2.72
2.75
132
137
140
434
418
433
380
394
a
-5
Determined in dichloromethane solution (10 M), ε = molar extinction coefficient.
b
Emission quantum yield, measured in the dichloromethane solution using an
integrated sphere.
c
opt
Estimated from absorption spectrum onset (Eg = 1241/λonset).
Estimated from CV curve and the equation ELUMO = EHOMO + Eg.
Estimated from phosphorescence spectrum at 77 K.
Measured from the DSC.
d
e
f
g
Measured from the TGA.
3
.3. Photophysical properties
Initially, the photophysical properties of the titled materials were investigated by the
absorption and fluorescence spectra, and the results were displayed in Fig. 2a. It can
be seen that they exhibit similar maximum absorption bands at around 296 nm, which

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could be ascribed to the intramolecular charge transfer (ICT) effect from the
electron-rich phenylcarbazole to the electron-deficient pyridine or pyrimidine
unit.[21-22] The band gaps (E ) of DCzPh, DCzPy and DCzPm estimated by
g
absorption edge were 3.41, 3.43 and 3.38 eV, respectively. Besides, these three
compounds show a single emission band with maximum peaks positioned at 378, 380
and 394 nm, respectively, and the slightly red-shift of emission peaks was caused by
the increasing electrophilic property of electron accepting moiety which enhanced the
ICT effect.
Additionally, the triplet levels (E ) of DCzPh, DCzPy and DCzPm were
T
calculated to be 2.74, 2.72 and 2.75 eV, respectively, by analyzing the low
temperature phosphorescent spectrum at 77 K, which qualify them to be hosts in the
green and red PhOLEDs.
Fig. 2. (a) UV-Vis absorption and room temperature emission spectra of DCzPh,
−5
DCzPy and DCzPm in DCM at 10 M. (b) The corresponding phosphorescence
spectra recorded in 2-methyltetrahydrofuran at 77 K.

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3
.4. Electrochemical properties and theoretical calculation
The suitability of energy levels between different functional layers is critical to the
performance of OLEDs devices. Therefore, cyclic voltammetry (CV) was adopted to
evaluate the electrochemical properties of the titled compounds and the pertinent date
are depicted in Table 1 and Fig. 3. The HOMO levels could be estimated with the first
oxidizing potentials through the following equation: EHOMO = −Εox − 4.4 eV.
Afterwards, the LUMO levels can be determined by EHOMO and E . As a consequence,
g
the HOMO/LUMO levels of DCzPh, DCzPy and DCzPm were calculated to be
−5.13/−1.67 eV, −5.33/−1.87 eV and −5.22/−1.78 eV, respectively.[23]
Fig. 3. Cyclic voltammograms of DCzPh, DCzPy and DCzPm in DCM solution with
-1
0
.1 mol L TBAPF electrolyte, scanning rate: 100 mV/s.
6
To get an in-depth into the electronic structures of these compounds, density
functional theory (DFT) computations for DCzPh, DCzPy and DCzPm were
*
implemented at B3LYP/6-31G level.[24] It can be learnt from Table 2 that the
HOMO of DCzPh is mostly situated at one of the carbazole moieties and the core
phenyl, and the LUMO lies on the biphenyl and slightly extended to carbazole unit.
Meanwhile, the HOMOs of DCzPy and DCzPm are mostly dispersed on the
biscarbazole moieties and the core phenyl group, while the LUMOs are totally located
on the core phenyl and pyridine/pyrimidine moieties. Compared to DCzPh, DCzPy

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and DCzPm exhibit more separated HOMO/LUMO distributions, which could be
ascribed to the enhanced ICT effect that improved their charge carriers transporting
property. Besides, the HOMO/LUMO energy levels of DCzPh, DCzPy and DCzPm
were calculated to be −5.38/−1.44 eV, −5.53/−1.25 eV and −5.61/−1.47 eV,
respectively.
Table 2. Calculated molecular orbits and HOMO/LUMO energy levels of DCzPh,
DCzPy and DCzPm.
DCzPh
DCzPy
-5.53 eV
-1.25 eV
DCzPm
-5.61 eV
-1.47 eV
HOMO
-5.38 eV
LUMO
-1.44 eV
3
.5 EL performance
To directly assess the EL performance of DCzPh, DCzPy and DCzPm, these
materials were fabricated in green PhOLEDs. The device structure is ITO/ TAPC (40
nm)/ TCTA (5 nm)/Ir(ppy) -host (20 nm)/ TmPyPB (50 nm)/LiF (0.6 nm)/Al (80 nm).
3
Herein, a classical green phosphorescent Ir-complex (Ir(ppy) ) was selected as the
3
emitter, and the common doping concentration of 8% was chosen. TAPC and TCTA
were used as the hole-transporting layer (HTL), meanwhile, TCTA also acts as
electron-blocking layer (EBL). TmPyPB was used as the electron transporting layer
(ETL).[13] Fig. 4 shows the device structure details and the chemical structures of
organic molecules in each layer.

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Fig. 4 The energy diagram of green PhOLED and the chemical structures of organic materials
used in the PhOLEDs.
The EL performances of these three host materials are summarized in Table
3
5
. The current density (
J)–voltage (V)–luminance (L) curves are shown in Fig.
a. All devices exhibit relatively low turn-on voltages (< 4 V), and the Von of D
CzPm based device G3 is the lowest among the three devices because of the suitable
energy levels and bipolar charge transport properties. Fig. 5b exhibits the current
efficiency (η ) and the external quantum efficiency (EQE) versus J of G1, G2 and G3.
c
Both the η and EQE of these three devices are satisfactory. Specifically speaking,
c
Device G1 based on DCzPh exhibits the maximum EQE of 14.5% and maximum ηc
-1
of 49.1 cd A , and the device G2 based on DCzPy exhibits higher maximum
-1
efficiencies with maximum EQE of 15.6% and maximum η of 52.6 cd A . The
c
performance of Device G3 is the best among these three devices with the maximum
-1
EQE of 17.2% and maximum η of 58.0 cd A , respectively. It can be inferred from
c
the above results that the maximum efficiencies of these devices are closely related to
their turn-on voltages, as well as the electron-transporting units. The strong EL
performance of pyrimidine-based host material DCzPm can be explained by its
outstanding bipolar charge transport property. Moreover, as shown in Fig. 5c, all
devices exhibit pure green emission originated from the Ir(ppy) . Besides, these three
3
-2
devices exhibit low efficiency roll-off, at the luminance of 1000 cd m , the EQE of
device G3 can still reach up to 17.1% with almost no change. Even at the extremely

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-2
high luminance of 5000 cd m , the EQE still maintains at 16.9%. The ultralow
efficiency roll-off merit can be explained by the balanced carrier–transport property of
DCzPm, which broadens the exciton recombination region and reduces the density of
triplet exciton, thus decreasing the annihilation of triplet excitons.
Fig.5. (a) Current density-voltage-luminance (J-V-L) characteristics of green
PhOLEDs G1, G2 and G3; (b) η and η versus current density of G1, G2 and G3; (c)
c
ext

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EL spectra of G1, G2 and G3 at 6 V.
Table 3. Electroluminescence performance of PhOLEDs
a
-2 b
-1 c
-1 d
e
f
devices
hosts
V (V)
on
Lmax(cd m )
ηc (cd A )
ηp (lm W )
EQE (%)
CIE(x, y)
G1
G2
DCzPh
DCzPy
DCzPm
3.9
3.5
3.1
49832
28602
34280
49.1 48.9 45.3
52.6 50.7 40.8
58.0 58.0 56.7
32.1
38.9
14.5 14.2 13.3
15.6 15.2 12.0
17.2 17.1 16.9
0.28, 0.62
0.29, 0.62
0.28, 0.62
G3
a
45.6
-2
V , turn-on voltage, the voltage at luminance of 1 cd m ;
on
b
c
d
e
f
Lmax, maximum luminance;
-2
-2
Current efficiency, order: maximum, at 1 000 cd m , at 5 000 cd m ;
Maximum power efficiency;
-2
-2
External quantum efficiency, order: maximum, at 1 000 cd m , at 5 000 cd m ;
CIE coordinates (x,y), measured at 6 V.
4
. Conclusion
In summary, three novel host materials DCzPh, DCzPy and DCzPm were
developed by utilizing carbazole as the electron-donating unit,
biphenyl/phenylpyridine /phenylpyrimidine as the electron-acceptor units,
respectively. These three compounds exhibit high triplet energy level, good thermal
stability, suitable HOMO/LUMO energy levels and balanced charge-transporting
properties. Green PhOLEDs based on these hosts were fabricated and characterized in
detail to evaluate their EL performances. Notably, DCzPm-based device exhibited the
best performance with the maximum EQE of 17.2%, and with negligible efficiency
-2
roll-off even at 1000 cd m , demonstrating its great commercial application potential
in OLEDs.
Acknowledgements
The authors acknowledge financial support from the Key Project of Educational
Commission of Hubei Province (D20151501), the Research Fund of

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Independent Innovation Base of Hubei Province (HBIR01) and the National
Natural Science Foundation of China (51474160).
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Highlights
ò Three bipolar host materials DCzPh, DCzPy and DCzPm were designed and
synthesized.
o
ò The above three hosts exhibit good thermal stability with T > 130 C.
g
ò DCzPm-based green PhOLED exhibits excellent performance with maximum
EQE of 17.2%.