A symmetric nonpolar blue AIEgen as nondoped fluorescent OLED emitter with low efficiency roll-off

Article abstract of DOI:10.1016/j.orgel.2019.105574

Blue emitters are necessary for achieving full-color displaying OLEDs, however, most blue emitters show low efficiency, short lifetime or serious efficiency roll-off, hindering the development of OLED techniques. In this research, a nonpolar symmetric agg

Full text of DOI:10.1016/j.orgel.2019.105574

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A Symmetric Nonpolar Blue AIEgen as Nondoped Fluorescent OLED Emitter with
Low Efficiency Roll-Off
Nannan Fei, Qiang Wei, Liang Cao, Yongqi Bai, Honglei Ji, Ruixiang Peng, Like
Huang, Shiyou Hao, Ziyi Ge
PII:
S1566-1199(19)30601-9
DOI:
https://doi.org/10.1016/j.orgel.2019.105574
ORGELE 105574
Reference:
To appear in:
Organic Electronics
Received Date:
Accepted Date:
08 April 2019
21 November 2019
Please cite this article as: Nannan Fei, Qiang Wei, Liang Cao, Yongqi Bai, Honglei Ji, Ruixiang
Peng, Like Huang, Shiyou Hao, Ziyi Ge, A Symmetric Nonpolar Blue AIEgen as Nondoped
Fluorescent OLED Emitter with Low Efficiency Roll-Off, Organic Electronics (2019), https://doi.org
/10.1016/j.orgel.2019.105574
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A Symmetric Nonpolar Blue AIEgen as Nondoped Fluorescent OLED
Emitter with Low Efficiency Roll-Off
a,b
b*
b
b
b
b
Nannan Fei , Qiang Wei , Liang Cao , Yongqi Bai , Honglei Ji , Ruixiang Peng ,
b
a*
b*
Like Huang , Shiyou Hao and Ziyi Ge
a
b
Xingzhi College, College of Chemistry and Life Sciences, Zhejiang Normal
Universtity, Jinhua 321004, P. R. China
Ningbo Institute of Materials Technology & Engineering, Chinese Academy
Sciences, Ningbo 315201, P. R. China
E-mail addresses: weiqiang@nimte.ac.cn; sky54@zjnu.cn; geziyi@nimte.ac.cn
Abstract
Blue emitters are necessary for achieving full-color displaying OLEDs, however,
most blue emitters show low efficiency, short lifetime or serious efficiency roll-off,
hindering the development of OLED techniques. In this research, a nonpolar
symmetric aggregation-induced emission (AIE) emitter was designed and constructed
through facile steps, with the triphenylamine-end, anthracene-spacer and
tetraphenylethene (TPE)-center. This emitter exhibited good thermal stability and
aggregation-enhanced emission (AEE) characteristics, based on which non-doped
blue OLED device was readily fabricated with the maximum external quantum
-2
efficiency (EQE) of 2.7% and also with no efficiency roll-off even at 1000 cd.m ,
indicative of high efficiency and good stability as fluorescent emitter.
Keywords:
aggregation-induced emission; organic light-emitting devices; tetraphenylethene;
anthracene; roll-off;
1

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1
. Introduction
Organic light-emitting devices (OLEDs) have received significant research interests both
in academia and industry due to their applications in solid-state lighting and full-color
displaying with the evident advantages of low energy cost, high operating voltage, active
illumination and full-solid state.[1-4] The OLED emitters, as the core component of the
devices, can decide both the performance and the fabricating methods of the OLED
device.[5, 6] In order to obtain full-color displaying, the red, green and blue (RGB)
emitters are the three primary emitters. However, compared with other two primary
emitters, the blue emitters show low efficiency, short lifetime or serious efficiency roll-off,
due to their larger band gap and aggregation-caused quenching (ACQ) effects, which
strongly limited the development of OLED techniques.[7, 8] Therefore, most current blue
OLED devices take advantages of the doping method, however, the doped systems usually
suffer from the drawbacks of phase segregation, color aging and poor reproducibility.[9] In
contrast to ACQ emitters, the emitters with aggregation-induced emission (AIE) properties,
or AIEgens, such as tetraphenylethene(TPE), can overcome the quenching effects in solid
state, which are favored to prepare the high-efficient non-doped OLED devices, with the
advantages of simplified process and reduced cost in fabrication. [10, 11] On the other
hand, anthracene, as a typical polycyclic aromatic hydrocarbon (PAHs), usually serves as
building blocks for efficient blue emitters, due to high photoluminescence (PL), ready
chemical modification and good thermal stability, but the planar structure of anthracene in
aggregated-state tend to form close π–π stacking and further to deteriorate the emitters‘ EL
performances.[12, 13]
In previous research, various AIEgen were fabricated with high efficiency as OLED
emitters, but many of them have the moieties of strong polarity, which tend to cause
efficiency roll-off at high luminescence, due to intra-/intermolecular interactions. Herein,
in order to design high efficient and stable blue emitters, we linked the anthraces moiety to
the TPE core to suppress its molecular packing, and an important moiety of the
commercial triphenylamine (TPA) as the terminal group, due to its nonplanar
2

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propreller-shaped to prevent the aggregation of the anthraces and good hole-transporting
properties.[14-16] The symmetric TPE emitter can applied in the non-doped blue OLED
device with the maixmum external quantum efficiency (EQE) of 2.7%, and the device
-2
showed no efficiency roll-off even at 1000 cd.m .
2
2
. Experimental section
.1 General information
All chemicals applied into reaction were received without further purification. Only
titanium(IV) chloride was purchased from Alfa Aesar, while all the other chemicals,
including N,N-diphenyl-4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)aniline (CAS
No.: 267221-88-5), 9,10-dibromoanthracene ( CAS No. : 523-27-3) and
9
-phenyl-9H-carbazol-3-ylboronic acid (CAS No. : 854952-58-2), were supplied by
Energy Chemicals.
2
.2 Synthetic procedures
1
,2-bis(4-bromophenyl)-1,2-diphenylethene
(TPE-Br)
and1,2-diphenyl-1,2-bis(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)ethene
TPE-BE) in this research was synthesized according to previous research.[17] The
related data are listed as follows: 1,2-bis(4-bromophenyl)-1,2-diphenylethene
(
1
13
(
TPE-Br) : H NMR (400 MHz, CDCl ), δ (TMS, ppm): 6.87, 6.99, 7.10, 7.20. C
3
NMR (100 MHz, CDCl ), δ (TMS, ppm): 120.6, 126.9, 127.7, 128.0, 130.8, 131.2,
3
+
1
4
32.8, 140.2, 142.3, 142.7. ESI-TOF-MS (C H Br ): m/z 487.9794 (M calculated
2
6
18
2
87.9775).
1
,2-diphenyl-1,2-bis(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)ethene
1
13
(
TPE-BE): H NMR (400 MHz, CDCl ), δ (TMS, ppm): 1.32, 7.0–7.1, 7.53. C
3
NMR (100 MHz, CDCl ), δ (TMS, ppm): 25.1, 83.4, 126.7, 127.8, 127.9, 130.9,
3
3

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1
31.5, 134.2, 124.3, 141.4, 143.5, 146.9. ESI-TOF-MS (C H B O ): m/z 584.3314
3
8
42
2
4
+
(
M calculated 584.3269).
4
-(10-bromoanthracen-9-yl)-N,N-diphenylaniline (TPA-An-Br): A mixture of
CAS
67221-88-5) (2.02 g, 5.39 mmol) and 9,10-dibromoanthracene ( CAS : 523-27-3)
2.20 g, 6.46 mmol), Pd(PPh ) (0.48 g, 0.42 mmol) and K CO (9.60 g, 69.7 mmol),
N,N-diphenyl-4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)aniline
(
:
2
(
3
4
2
3
were added into a 250 mL two-neck bottle under nitrogen. After then, 150 mL mixed
solvent of THF and H O (v/v = 4:1) was injected into the bottle, and the mixture was
2
refluxed for 24 h. After cooling to room temperature, the mixture was poured into
water and extracted with dichloromethane. The combined organic layers were then
dried over anhydrous magnesium sulfate, filtrated, evaporated and finally purifed by
silica-gel column chromatography using hexane/dichloromethane as eluent. 2.0g of
1
bright yellow solid Cz-An-Br was obtained with yield of 74.2 %. H NMR (400 MHz,
CDCl ), δ (TMS, ppm): 8.60, 8.58, 7.81, 7.79, 7.60, 7.58, 7.56, 7.43, 7.41, 7.39, 7.34,
3
1
3
7
.32, 7.30, 7.26, 7.25, 7.24, 7.23, 7.09, 7.07, 7.05. C NMR (100 MHz, CDCl ), δ
3
(TMS, ppm): 147.80, 147.51, 137.81, 132.06, 131.83, 131.35, 130.40, 129.54, 127.98,
1
27.59, 127.04, 125.60, 124.91, 123.36, 122.93, 122.70, 77.16. ESI-TOF-MS
+
(
C H BrN): m/z 499.0944 (M calculated 499.0936).
3
2
22
4
,4'-(((1,2-diphenylethene-1,2-diyl)bis(4,1-phenylene))bis(anthracene-10,9-diyl))bis
(N,N-diphenylaniline) (TPA-An-TPE): The compound TPA-An-TPE was also
prepared through Suzuki reaction. In this preparation, TPE-BE (1.0 g, 1.75mmol),
TPA-An-Br (1.370 g, 5.22 mmol), Pd(PPh ) (329 mg, 0.285 mmol) and K CO
3
4
2
3
(6.55g, 47.4 mmol) were placed into the mixed solvent of THF and water (v/v = 4:1)
to reflux for 24 h under nitrogen, and finally 0.32 g of bright yellow solid
1
TPA-An-TPE were obtained in the yield of 15.3 %. H NMR (400 MHz, CDCl ), δ
3
1
3
(
TMS, ppm): 7.86, 7.85, 7.83, 7.81, 7.79, 7.68, 7.45-7.29, 7.28-7.20, 7.10-7.04. C
NMR (100 MHz, CDCl ), δ (TMS, ppm): 147.85, 147.13, 143.27, 137.43, 132.73,
3
1
32.15, 131.51, 131.34, 130.76, 130.03,129.89, 129.40, 127.92, 126.91, 125.21,
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+
1
1
24.69, 123.09, 76.70. ESI-TOF-MS (C H N ): m/z 1170.4915 (M calculated
90 62 2
170.4913).
2
.3 Device fabrication and characterization
The materials to fabricate OLED devices were purchased from Jinlin Optical and
Electronic Materials Co. The Indium-doped tin oxide (ITO) glass sheet (1.2 mm thick,
≤
15 Ω/square, transmittance > 90%) was purchased from Nippon Sheet Glass
Company, Ltd, and cleaned by a routine cleaning procedure which includes initial
manual washed in aqueous detergent, and then sequentially sonication in acetone,
isopropanol and ethanol for 30 minutes each. Then, the pre-cleaned ITO glass was
moved into an ultraviolet chamber for UV-ozone treatment of 20 minutes. The
synthesized emitting material TPA-An-TPE were further purified through
o
-3
sublimation at 220 C under the vacuum of 1.5×10 Pa in the sublimation system
CSe solar ETS-60T-S) before fabricating the OLED devices. The organic layers were
deposited consecutively on the pre-cleaned ITO glass substrates in a vacuum chamber
(
-4
-1
(
5×10 Pa). The deposition rate of all the organic materials was 0.9-1.1Å S , while
-1
-1
that of the LiF was 0.1 Å S and aluminum was 5 Å S . The electrical characteristics
of the devices were measured with a Keithey 2400 Source meter at the range of 0-15V.
The electroluminescence spectra and luminance of the devices were obtained on a
PR655 spectrometer in ambient conditions at room temperature.
3
3
. Result and discussion
.1 Synthesis of materials
The molecular structure of TPE-An-TPA and its synthetic route were illustrated in
Scheme 1. To be specific, the AIEgen of TPE-En-TPA was prepared through Suzuki
coupling between the two precursor of TPE-bearing boric acid ether TPE-BE and the
Br-containing precursor TPA-An-Br in high yield. The precursor of TPA-An-Br was
obtained through Suzuki coupling between the commercially available chemicals of
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9
,10-dibromoanthracene
and
N,N-diphenyl-4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)aniline, while The AIE
precursor of TPE-BE was prepared through the reported methods of McMurry
reaction of 4-bromobenzophenone and later nucleophilic substitution. Thanks to the
non-planar structure of TPE unit, the objected AIEgen of TPA-An-TPE, despite of its
rigid structure, exhibited good solubility in common organic solvents, such as THF
(tetrahydrofuran), dichloromethane and chloroform, which enabled the wet-method
for structure characterization.
Br
O
O
o
Zn, TiCl4
1. n-BuLi, THF, -78 C
Br
B
O
O
O
Br
B
O
THF, Reflux
2.
O
B
o
,
-78 C
O
TPE-Br
TPE-BE
TPE-BE
Pd(Ph ) , K CO
3
O
B
Pd(Ph ) , K CO
3 4 2 3
3 4
2
N
+ Br
Br
N
Br
O
THF/H O, Reflux
2
THF/H O, Reflux
2
TPA-An-Br
N
N
TPA-An-TPE
Scheme 1. The synthetic route to the AIEgen of TPA-An-TPE.
3
.2 Thermal properties
High thermal stability is a perquisite condition for both fabrication and application of
the OLED devices. Therefore, thermogravimetric analysis (TGA) and differential
scanning calorimetry (DSC) were conducted on TPA-An-TPE. Generally, the fully
aromatic components of anthracene, TPE and TPA endowed the compound with high
rigidity and therefore high thermal stability. As shown in the Figure 1, this emitter
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o
exhibited high glass transitional temperature (Tg) of 168.3 C, high decomposition
o
temperature of 537 C at 5% weight loss (Td) and high mass of residual char (nearly
o
4
4%) at 800 C, all of which indicated this compound was both thermally and
morphologically stable for fabrication and application as the OLED emitters.
Figure 1. Thermogravimetric analysis (TGA) (a) and differential scanning
calorimeter (DSC) (b) curves of TPA-An-TPE, with the scanning rate at 10 k.min-1
under N2.
3
.3 Photophysical Properties
Figure 2. The absorbance and photoluminescence of TPA-An-TPE (Blue line)
-1
recorded in THF solution at the concentration of 1 mg.mL .
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The absorption and emission spectra of this emitter were also recorded in THF
solutions are shown in Figure 2. The compound exhibited much fine absorption
spectrum, with one peak of 398 nm quite close to the absorption peak of anthracene.
The photoluminescence (PL) behaviors of TPA-An-TPE were also investigated in the
solution and aggregated state. When dissolved into THF, the emitter of TPA-An-TPE
exhibited weak emission with the peaks of 477 nm, because its singlet excitons tend
to nonradioactive decay through vigorously intramolecular motion in good solvent as
a result of its twisted conformation. Since the emitters is hydrophobic effect, their PL
spectra are measured in THF/water mixtures with increasing water fractions (f ) to
w
form aggregated states (Figure 3 a-c). Both the intensity and the emission peak were
quite complexed, which only showed the increasing tendency, but not simply
monotonic increasing. However, similar phenomena were also observed and
mentioned in previous research. Similarly, we also attributed the complicated PL
behaviors in the mixed solvent to the formation of various kinds of nanoparticle
suspensions, such as, amorphous particles and crystal particles. As for this AIEgen,
water exhibited worse solubility and higher polarity compared with THF, and
therefore it caused red shifted emission of the AIEgen and also an increased PL
intensity with the water fraction increasing from 0% to 30%. At higher ratio of water
(>30%), the aggregates appeared, which could reduce the polarity of molecular
environment and therefore induce a blue shift. To be exact, from the water fraction
from 40% to 70%, the amorphous aggregates appeared to decrease the PL intensity,
while at the ratio of 80-90%, the water fraction would provide crystalline aggregates
to increased PL intensity. However at the ratio of 99% for water, most of the AIEgens
were precipitated from the solvent, and the PL intensity of the solvent have a sharp
decrease.[18, 19]
Meanwhile, the powder of TPA-An-TPE show blue color, which exhibited strong
blue emission under UV 365(Figure 4a-b), with the photoluminiscence quantum
yield (PLQY) of 25.8%. Therefore, the emitter of TPA-An-TPE, thanks to the
AIE-active group of TPE moiety, exhibited the AEE (aggregation-enhanced emission)
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nature which is much favored in the OLED devices, as most OLED emitters were
applied as solid state.
Figure 3. The emission photograph (a) and PL spectra (b) of TPA-An-TPE in
THF/water mixtures, (c) and the PL intensity as a function of the ratio water fraction
mixtures under 365 nm UV illumination
Figure 4. The photograph of the TPA-An-TPE powder under sunlight (a) and under
UV light (b).
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3
.4 Theoretical calculations
Figure 5. The HOMO and LUMO energy levels and distributions of TPA-An-TPE.
To investigate the electronic structure, density function theory (DFT) calculation was
performed on TPA-An-TPE using the B3LYP/6-31G (d, p) set. The highest occupied
molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO) and
optimized geometries were displayed in Figure 5. This emitter adopted a quite
twisted conformation between the three moieties of anthracene, TPE and TPA, with
o
o
the torsional dihedral angles of 104.1 for TPA and anthracene, and 103.9 for TPE
and anthracene. The HOMO and LUMO energy levels were calculated to be -4.95 eV
and -1.61 eV respectively, with relatively high energy gap of 3.34 eV. The HOMO
electron cloud distributions was localized on both the TPA and anthracene units,
while the LUMO level was mainly localized on the anthracene core, indicating weak
electronic attraction between the anthracene and TPA due to their both
electron-donating properties. And the central core of TPE moiety just served as the
block to link the peripheral substituents through C–C bond, but with less contribution
to the HOMO and LUMO distribution.
3
.5 Electrochemical Behaviors
In order to calculate the specific HOMO and LUMO level, the electrochemical
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properties of the TPA-An-TPE was further measured by cyclic voltammetry (CV).
This AIEgen displayed onset oxidation potentials (Eox) at 0.72 eV (in Figure 6).
Further, the HOMO energy level were calculated to be -5.12 eV, based on the
equation of HOMO = - (4.4+Eox) eV[20, 21], while the LUMO energy levels are
-2.22 eV from the equation of LUMO = (HOMO + Eg) eV, with Eg obtained from the
on-tail absorption wavelength of 438 nm (2.90 eV). And these energy level were in
accordance with the value calculated through DFT method.
-
1.0 -0.5 0.0
0.5
1.0
Potential (V, vs Ag/AgCl)
Figure 6. Cyclic voltammogramic behavior of TPA-An-TPE.
3
.6 Electroluminescence
In order to evaluate the potential application of the AIEgen in OLEDs, the multilayer
nondoped OLED was prepared with the architecture of indium tin oxide (ITO)/
N,N'-bis(1-naphthalenyl)-N,N'-bisphenyl-(1,1'-biphenyl)-4,4'-diamine(NPB)(40 nm)/
4
,4',4'' -tri-9-carbazolyltriphenylamine (TCTA) (5 nm)/ emitting layer (EML) (20
nm)/ 2,2',2''-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) (40 nm)/
LiF (1 nm)/ Al (100 nm), where EML was fabricated merely by non-doped AIEgen
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TPA-An-TPE, NPB and TCTA served as hole injection layer, while TPBi played the
role of hole-blocking and electron transport layers, respectively. The structure and the
energy-level diagram of the OLED devices were shown in Figure. 7a. The
characterization of the EL properties of these devices is summarized in Figure 7b-7d
and Table 1. It can be seen that the non-doped OLED turned on at low voltage of 3.1
eV, and radiated electroluminescence close to their PL in solid film, suggesting the
exciton recombination occur in the EML layer and the EL results from the radiative
decay of singlet excitons, which showed blue emission (with Full Width at Half
Maximum of 100 nm), ranging from 420 to 620 nm with single peak at 474 nm, and
-2
this EL were stable from 100 to 1000 cd m , confirming the emission spectrum
showed minimal dependence on the current density. Unlike some emitters with the
mechanism of high exciton utilization efficiency (EUE) [22-37], the OLED device
based on this AIEgen experienced neither structural optimization nor out-coupling
enhancement. From this angle, it is interesting to notice that the nondoped AIE
emitter exhibited the maximum EQE of 2.7%. Moreover, this EQE did not see any
-2
efficiency roll-off even at 1000 cd.m , which can be attributed to the fact that
nonpolar AIE emitter can largely reduce quenching of emission, such as
singlet–triplet and triplet–triplet annihilation.
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Figure 7. (a) Device configuration: ITO/ NPB/ TCTA/TPA-An-TPE/ TPBi/ LiF/ Al,
(b) EL spectra, (c) changes in luminance and current density with the applied voltages,
and (d) plots of luminance versus current efficiency and power efficiency.
Table1. The EL performance of non-doped blue OLED based on the emitter
TPA-An-TPE.
CE
PE
EQE
Voltage
CIE
item
-
1
(lm W-1)
(
cd A )
(%)
(V)
(x,y)
elemental data a)
7.1
6.1
5.9
4.9
2.7
2.7
2.7
3.1
3.6
4.5
-
1
00 cd m-2
6.7
(0.18, 0.28)
(0.19, 0.29)
1
000 cd m-2
7.1
a)
PE: maximum power efficiency; EQE: maximum external quantum efficiency,
-2
Voltage: turn-on voltage at 1 cd m .
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4
. Conclusion
In this study, a nonpolar symmetric AIEgen TPA-An-TPE was prepared, with
TPA-ends and anthracene spacer and TPE-core. Due to its symmetric structure, this
molecule TPA-An-TPE was prepared through quite facile steps. Thanks to its all
o
o
rigid structure, TPA-An-TPE exhibited high Tg of 168.3 C and Td of 537 C, and
the twisted structure enabled TPA-An-TPE to have blue photoluminescence and AEE
performance, both of which were favored as OLED emitter. The blue non-doped
OLED devices were therefore fabricated base on the emitter of TPA-An-TPE, with
-2
maximum EQE of 2.7 %, which exhibited no roll-off at1000 cd.m .
Acknowledgment
This work was financially supported by the National Key R&D Program of China
(2017YFE0106000, 2016YFB0401000), National Natural Science Foundation of
China (21805296, 21876158, 51773212, 21574144 and 21674123), China
Postdoctoral Science Foundation (2017M621987), Zhejiang Provincial Natural
Science Foundation of China (LR16B040002), Natural Science Foundation of Ningbo
City (2018A610134), Ningbo Municipal Science and Technology Innovative
Research Team (2015B11002 and 2016B10005), CAS Interdisciplinary Innovation
Team, CAS Key Project of Frontier Science Research (QYZDB-SSW-SYS030) and
CAS Key Project of International Cooperation (174433KYSB20160065).
Contributors
Nannan Fei, Liang Cao, Honglei Ji, Yongqi Bai performed data acquirement, data
analysis and wrote the manuscript; Like Huang and Ruixiang Peng helped perform the
analysis with constructive discussions; Qiang Wei designed the molecule and revised
the manuscript; Shiyou Hao and Ziyi Ge contributed to the conception of the study.
1
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Highlights



This emitter with good thermal stability and aggregation-enhanced emission
(AEE) characteristics
non-doped blue OLED device was fabricated with the maximum external
quantum efficiency (EQE) of 2.7%.
-2
non-doped blue OLED device with no efficiency roll-off even at 1000 cd.m .