Intense green-light emission from 9,10-bis (4-(1,2,2-triphenylvinyl)styryl)anthracene emitting electroluminescent devices

Article abstract of DOI:10.1039/c5tc01211b

Although 9,10-bis(4-(1,2,2-triphenylvinyl)styryl)anthracene (TPE-An) with excellent aggregation-induced emission (AIE) properties can effectively enhance the luminescence efficiency of light-emitting materials, the practical TPE-An-emitting electrolumines

Full text of DOI:10.1039/c5tc01211b

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DOI: 10.1039/C5TC01211B
(Submitted to J. Mater. Chem. C)
Intense Green-light Emission from 9,10-bis (4-(1,2,2-triphenylvinyl)styryl)
anthracene Emitting Electroluminescence Devices
a,
b,
Shunyu Jin, Yan Tian, Fei Liu *, Jun Chen, Shaozhi Deng *, Ningsheng Xu
a
b
Corresponding author: liufei@mail.sysu.edu.cn, stsdsz@mail.sysu.edu.cn
State Key Laboratory of Optoelectronic Materials and Technologies,
And Guangdong Province
Key Laboratory of Display Material and Technology,
Sun Yatꢀsen University, Guangzhou 510275, People's Republic of China
1

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Abstract
Although 9,10ꢀbis (4ꢀ(1,2,2ꢀtriphenylvinyl)styryl) anthracene (TPEꢀAn) with excellent aggregationꢀinduced
emission (AIE) property can effectively enhance the luminescence efficiency of lightꢀemitting materials, the
practical TPEꢀAnꢀemitting electroluminescence (EL) devices have not been successfully fabricated until
now. In our paper, the energyꢀband diagram of TPEꢀAn is determined by combination of ultravioletꢀvisible
(UVꢀVis)
spectroscope
and
cyclic
voltammetry
(CV)
techniques.
Subsequently,
spaceꢀchargeꢀlimitedꢀcurrent (SCLC) method is firstly used to obtain the hole mobility of TPEꢀAn and
design a better EL device structure. By the calculation, the maximum hole mobility of 50 nm TPEꢀAn film
ꢀ4
2
can arrive at 4.00×10 cm /V·s at 0.1 MV/cm field. Finally, TPEꢀAnꢀemitting EL devices have been
respectively fabricated on glass and flexible PET substrate. Both of these two EL devices exhibit an intense
greenꢀlight emission at 515 nm. Moreover, the maximum luminance of EL device on glass can reach
2
2
2
4721.8 cd/m at 10.0 V, and its maximum current efficiency is as high as 14.7 cd/A at 131.6 mA/cm . It is
noted that the flexible EL device on PET substrate maintains a stable emission and nice working
o
performance even if the bending angles arrives at 140 . Therefore, it suggests that TPEꢀAn should have a
promising future in EL device applications.
Key words: TPEꢀAn, SCLC technique, hole mobility, electroluminescence (EL), flexible device
2

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1
. Introduction
Since electroluminescence (EL) device of organic materials were firstly fabricated in the 1960s [1], much
effort has been devoted to improving the work efficiency of EL devices by different methods, such as
optimizing the device structures or finding excellent lightꢀemitting materials [2ꢀ6]. But for most luminescent
materials with conventional πꢀconjugated planar chromophores, the aggregationꢀcaused quenching (ACQ)
effect usually emerges when they form thin film. In this situation, the light emissions from the film of
common luminescent material are weakened to great extent, which has hold back their applications in EL
devices. This problem has not been solved until a new aggregationꢀinduced emission (AIE) material, i.e.,
silole molecule, emerged in 2001 [7]. Because AIE materials successfully overcame the shortage of the
aggregationꢀcaused quenching (ACQ) effect, they got a rapid development in the last decade [8ꢀ11]. The
luminescence efficiency of EL device will be significantly improved if AIE material is used as emitting
layer [12ꢀ15]. Among the reported AIE fluorophores, tetraphenylethene (TPE) is very unique because it
possesses obvious AIE behaviors, which reveals TPEꢀbased AIE materials have potential applications in EL
device [15, 16]. As a famous member of TPE derivatives, 9,10ꢀbis(4ꢀ(1,2,2ꢀtriphenylvinyl)styryl) anthracene
(TPEꢀAn) exhibits very fascinating AIE characteristics, which has attracted considerable attention [14].
It is known to us all that the emitting layer with high carrier mobility is indeed needed for high EL
performance. So determining the carrier mobility of TPEꢀAn film at varied field is the prerequisite for
designing excellent TPEꢀAnꢀemitting EL device. To our knowledge, many techniques have been used to
measure the carrier mobility of organic film, such as time of flight (TOF), field effect transistor (FET) and
transient EL methods [17, 18]. TOF and FET methods are only suitable for measuring thick film (2 ꢁm~8
ꢁm) rather than thin film (10 nm~1ꢁm). The common thickness of organic emitting layer is usually smaller
than 100 nm in EL device. Therefore, TOF or FET methods are not applied to measure the carrier mobility
of thin film. And transient EL technique is commonly used to measure the mobility of minor carriers, which
is also not suitable for determining the major carriers in organic emitting materials. Compared with other
methods, spaceꢀchargeꢀlimited current (SCLC) technique is a more effective method to determine the
mobility of major carrier in organic emitting layers with subꢀmicrometer thickness [19ꢀ21]. Most
importantly, SCLC measurement can be used to calculate the carrier mobility at varied electric field, which
is more helpful for simulating the practical performance of thin film under working conditions. However,
SCLC method has not been applied to investigate the hole mobility of TPEꢀAn until now. As a result,
3

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TPEꢀAnꢀemitting EL device has not been successfully fabricated to our knowledge though they have
excellent AIE properties, which has hold back the development of TPEꢀAn in EL area. Therefore, it has
provided challenges for researchers to apply SCLC technique to determine the major carrier mobility of
TPEꢀAn and fabricate TPEꢀAnꢀemitting EL device with high performance.
In this paper, the SCLC technique is firstly used to investigate the hole mobility of TPEꢀAn film at varied
fields. Based on the energyꢀband diagram of TPEꢀAn and the SCLC results, we have successfully fabricated
TPEꢀAnꢀemitting EL device on glass and polyethylene terephthalate (PET) substrates. Finally, the working
performances of these two EL devices were respectively investigated to value their application feasibilities
in display areas.
2
. Experimental section
As shown in Fig. S1, the known WittigꢀHorner reaction route was adopted to synthesize pure TPEꢀAn
powders, which is the same with the synthesis route in Ref. [14]. To confirm their chemical compositions,
1
the midꢀinfrared spectrum, H nuclear magnetic resonance (NMR) spectrum and photoluminescence (PL)
spectrum of TPEꢀAn powders are respectively provided in Figs. S2ꢀS4. CV measurement was carried out in
dichloromethane (DCM) solution (AR, ＞99%). The SCLCꢀtesting devices were fabricated on indium tin
oxide (ITO) glass with a sheet resistance of 14 ꢂ/sq, and the detailed fabrication procedure of SCLC device
can be seen as follows. Firstly, oxygen plasma was employed to remove excess moisture on the surface of
ITO glass. Subsequently, TPEꢀAn film was deposited on ITO glass substrate by a simple thermal vapor
ꢀ4
deposition way. The base pressure of the chamber was kept at 5×10 Pa during the growth process. The
morphology and work function of TPEꢀAn (15 nm) film on ITO glass substrate was provided in Figs. S5 and
S6, respectively. The multilayered SCLCꢀtesting device was consisted of ITO/C60 (2.5 nm)/TPEꢀAn (d
nm)/Al (100nm), in which the thickness (d) of TPEꢀAn film was respectively 25, 40, 50, 60, 80, 100, 150,
200, 300 and 400 nm. The growth rate of C60 (Nichem Fine Tech., 99%), TPEꢀAn and Al film was adopted
to be 1 Å/sec, 1 Å/sec and 5 Å/sec, respectively. The currentꢀvoltage (IꢀV) characteristics of the devices
were recorded by a programmable picoammeter (Keithley 236).
The fabrication process of TPEꢀAnꢀemitting EL devices on glass and PET substrate are depicted as follows.
ITO substrate was respectively cleaned by acetone, ethanol and deionized water in ultrasonic cleaner before
it was handed into the chamber. Subsequently, ITO substrate was treated by ten minutes` oxygen plasma to
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get a clean and dry surface. The multilayered EL device was fabricated on glass substrate with the
configuration of ITO/NPB (40 nm)/TPEꢀAn (15 nm)/TPBi (10 nm)/Alq (20 nm)/ LiF (1 nm)/Al (100 nm).
3
In this device structure, N,Nꢀbis(1ꢀnaphtyl)ꢀN,N´ꢀdiphenylꢀ1,1,1´ꢀbiphenylꢀ4,4´ꢀdiamine (NPB, Aglaia
Tech., 99.9%), TPEꢀAn, 1,3,5ꢀtris(nphenylbenzimidazolꢀ2ꢀyl)benzene (TPBi, Aglaia Tech., 99.5%) and
3
tris(8ꢀhydroxyquinolinato) (Alq , Aglaia Tech., 99.9%) served as holeꢀtransporting layer (HTL), emitting
layer (EL), holeꢀblocking layer (HBL) and electron transporting layer (ETL), respectively. And LiF/Al layer
was used as the cathode, in which LiF was used as the electron injection layer of EL device. All the
deposition rates of organic materials were 1 Å/sec. The deposition rate of LiF (Aladdin Industrial Inc.,
99.99%) and Al was respectively kept at 0.2 Å/sec and 5 Å/sec. After deposition, the active area of the EL
2
device was calculated to be 0.3 cm (0.5 cm×0.6 cm). The device structure of flexible EL device on PET
substrate is very similar with that of the EL device on glass substrate, and the only difference was that PET
was chosen as the substrate instead of ITO glass. The currentꢀvoltageꢀluminescence (IꢀVꢀL) characteristics
and EL spectra of TPEꢀAnꢀemitting devices were studied in ambient condition by a programmable
picoammeter (Keithley 236) and photometer (STꢀ86LA). Photoluminescence (PL) test was performed on
HORIBA JY Fluorologꢀ3 spectrometer (λ=365 nm), and the fluorescence quantum yield of TPEꢀAn in the
solid state was determined by using a calibrated integrating sphere system.
3
. Results and discussion
High carrier mobility is a very important criterion for choosing an excellent emitting layer of EL device.
Before the SCLC technique was performed on the TPEꢀAn devices, it is necessary to explore the
energyꢀband diagram in advance for designing a favorable configuration of SCLCꢀtesting device. Here,
UVꢀVis absorption spectroscope and CV method were combined together to ascertain the band diagram of
TPEꢀAn. Figure 1(a) shows the UVꢀVis spectrum of TPEꢀAn. The onset of the absorption peak was found to
hc
locate at 479 nm. Based on the Einstein equation of
λ
=
g
, bandgap (E ) can be obtained to be 2.59 eV.
E
Based on our previous research, the HOMOꢀLUMO gap of TPEꢀAn in the powders and in the film are
comparable [24]. The CV curve of TPEꢀAn is given in Figure 1(b). From Fig. 1(b), TPEꢀAn can be
confirmed to be electrochemically active. Referred by the reference potential of ferrocene [22, 23], the
HOMO of TPEꢀAn can be worked out to be 5.25 eV. Based on the relationship among the HOMO, LUMO
g g
and E , the LUMO level of TPEꢀAn can be deduced to be 2.66 eV because E has been worked out to be
2.59 eV by UVꢀVis technique. Combined the obtained HOMO with LUMO results, the band diagram of
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TPEꢀAn is given in Fig. 1(c).
Formation of an ohmic or quasiꢀohmic contact between TPEꢀAn film and ITO substrate was very necessary
for accurately determining the carrier mobility in the following SCLC experiments. Herein, 2.5 nm
buckminsterfullerene (C60) film was chosen as the buffer layer to form a quasiꢀohmic contact between
TPEꢀAn and substrate by because the use of C60 can effectively decrease the work function difference
between ITO and TPEꢀAn film [25, 26]. Moreover, the use of C60 will do good for enhancing the hole
injection because C60 has a similar carrier mobility with TPEꢀAn. As seen in Fig. S7, the IꢀV curves of the
ITO/C60 (2.5 nm)/TPEꢀ An (100 nm) are nearly linear, which proves the quasiꢀohmic contact has been
established between the ITO and TPEꢀAn layer as we expected. Al was chosen as the cathode of TPEꢀAn
SCLCꢀtesting devices because its Fermi level is far lower than that of the LUMO of TPEꢀAn. In this
situation, the existence of large barrier between the Fermi level of Al and the LUMO of TPEꢀAn prevents
the electrons from injecting into TPEꢀAn film based on the classical energyꢀband theory. Therefore, ITO/C60
(2.5 nm)/TPEꢀAn (d nm)/Al (100nm) configuration was finally chosen as the holeꢀinjection device for
SCLC measurements. In our experiments, TPEꢀAn film was grown in different thickness to study the effect
of thickness on the carrier mobility. The working mechanism of the SCLC technique is based on the
classical SCLC equation, which can be described as [27]:
2
9
E
d
J = ε0εꢀ
8
(1)
,
where E is the applied electric field, d is the thickness of the organic layer, ꢀ is the carrier mobility, ε
0
is the
permittivity of free space and ε is the relative dielectric constant. According to Eq. (1), the relationship
between lnJ and lnE can be deduced as:
9
ε0εꢀ
ln J = ln
+ 2ln E
(2)
8
d
From Eq. (2), lnJ is proportional to lnE and the slope of lnJꢁlnE curve is 2, which is the key proof for SCLC
behaviors of organic film.
Figure 2(a) shows the current densityꢀvoltage (JꢀV) characteristics of TPEꢀAn film with various thickness. It
is obviously seen that the JꢀV curves of all SCLC devices can be divided into two different regions. At low
voltage, it exhibits a clear Schottkyꢀdiode behavior, in which J has an exponential relationship with voltage.
But with the increase of applied voltage, a sudden change occurred on the JꢀV curves of TPEꢀAn, as found
in Fig. 2(a). As shown in Fig. 2 (b), all the lnJꢁlnE curves at high voltage (≥1 V) are proven to be almost
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linear by calculating the JꢀV data in Fig. 2(a). In addition, the average slope of lnJꢁlnE curves can be
calculated to be 2.14 0.09, which meets the key proof of SCLC behaviors in Eq. (2). So it suggests that
±
relationship between current density and applied field should conform to the SCLC model in our
experiments.
It is known that the carrier mobility is dependent on the applied electric field. Some reports have referred
that carrier mobility is also affected by the energy disorder due to the interaction of each hopping charge
with randomlyꢀoriented or randomlyꢀlocated dipoles in the organic film [28, 29]. As a result, the effect of
the energy disorder on hole mobility has to be considered to obtain more accurate mobility in our
experiments. Here, we need to resort to the PooleꢀFrenkel (PF) equation:
ꢀ
(E) = ꢀ0 exp(β E)
(3)
,
where ꢀ is the zeroꢀfield mobility, E represents the applied field, and β is PooleꢀFrenkel factor. The
0
magnitude of β reflects the degree of disorder, and a smaller β suggests a lower energy disorder, which
corresponds to a more ordered distribution of the energy level. Combined Eq. (1) with Eq. (3), the
fieldꢀdependent SCLC equation can be easily derived as:
2
9
8
E
d
JSCLC
=
ε0ε
ꢀ0 exp(β E)
(4)
2
Then, the expression of ln(J / E )can be easily obtained, which can be described as:

J 
E 
 9εε ꢀ0

0
ln
= β E + ln


2




(5)

8d
2
Based on Eq. (5), ln(J / E )is observed to have a linear relationship with
E
, and the corresponding
ꢀ12
curve is shown in Fig. 2(c). Because the permittivity ε
0
of vacuum is 8.85×10 F/m, the PooleꢀFrenkel
factor β and zeroꢀfield mobility ꢀ
0
can be respectively determined by calculating the slope and the intercept
2
of ln(J / E ) − E curve, as summarized in Table I.
To comprehend the effect of TPEꢀAn’s thickness d on ꢀ
0
, the ꢀ
0
ꢀd curve is given in Fig. 2(d). And the device
has a nonlinear relationship
ꢀd curve. When d is smaller than 50 nm, ꢀ
increases from 7.92×10 to 3.99×10 cm /V·s. But when d is bigger than 50 nm, ꢁ will continuously
configuration for SCLC measurements was shown in the inset. It is found that ꢀ
0
with d and there exist two critical d values in the whole ꢀ
0
0
ꢀ5
ꢀ4
2
0
decrease with the increase of the thickness d of TPEꢀAn film until d arrives at 200 nm. If d is over 200 nm,
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ꢀ6
2
0
ꢀ reaches a saturation value of 4.33×10 cm /V·s and becomes insensitive to d.
Determining the carrier mobility of TPEꢀAn at varied field is very essential for their practical applications in
EL devices. Based on Eq. (3), we can calculate the hole mobility at different electric fields. Figure 3(a) gives
the curve of hole mobility versus the thickness of TPEꢀAn at 0.1 MV/cm field (a commonlyꢀused working
field of EL device). And typical atomic force microscopy (AFM) images of TPEꢀAn film with 50 nm is
provided in the inset. The hole mobility ꢀ at various field can be deduced based on Eq. (3) because ꢀ
has been obtained by Eq. (5). In Fig. 3(a), the hole mobility ꢀ was seen to have a clear upꢀdownꢀflat
tendency, which is the same with the ꢀ ꢀd curve (Fig. 2(d)). The hole mobility ꢀ of TPEꢀAn with different
thickness at 0.1 MV/cm field is also provided in Table I. It is found that TPEꢀAn film has the maximum hole
0
and β
0
ꢀ4
2
mobility of 4.00
×10 cm /V·s when its thickness d is 50 nm. To better comprehend the relationship
between carrier mobility and applied field, the curve of mobility versus electric field is shown in Fig. 3(b).
When the thickness of TPEꢀAn film ranges from 25 nm to 60 nm, the mobility is found to be insensitive to
0
electric field whereas ꢀ is dependent on the thickness. But when d increases from 60 nm to 400 nm, the
mobility becomes fieldꢀdependent once again. It is obviously seen that the hole mobility of TPEꢀAn
ꢀ4
2
ꢀ4
2
(
3.14×10 cm /V·s) can be comparable to that of another TPE derivative pꢀBTPATPE (5.2×10 cm /V·s) at
the same thickness of 60 nm [30]. In recent years, pꢀBTPATPEꢀemitting EL device has exhibited nice
2
working behaviors due to its high mobility, such as a low turnꢀon voltage (at 1 cd/m ) of 3.2 V, high
2
luminance of 33700 cd/m and power efficiency of 11.0 lm/W [31]. Also, the mobility of holeꢀtransporting
ꢀ5
ꢀ4
2
ꢀ4
2
layer NPB (1.63
×10 ꢀ7.64×10 cm /V·s) is close to that of TPEꢀAn film (4.00×10 cm /V·s) [32, 33],
which is helpful to transport the hole from anode to TPEꢀAn layer. Therefore, one can expect that
TPEꢀAnꢀemitting EL device should have an excellent working performance.
EL devices of TPEꢀAn were respectively fabricated on glass and PET substrate. Depending on the energy
level and mobility of every film in EL device, TPEꢀAn, NPB, TPBi, Alq
respectively. The EL device on glass substrate adopted the configuration of ITO/NPB(40 nm)/TPEꢀAn(15
nm)/TPBi (10 nm)/Alq (20 nm)/LiF(1 nm)/Al(100 nm). Figure 4(a) schematically gives the energyꢀband
3
are served as HTL, HBL and ETL,
3
diagram of TPEꢀAnꢀemitting EL device on glass substrate. The currentꢀvoltageꢀluminescence (IꢀVꢀL) curve
of EL device on glass substrate is shown in Fig. 4(b), and the inset gives the luminescence image at 8.0 V.
One can see that EL device on glass substrate exhibited a very uniform luminescence image in the whole
2
surface of 0.3 cm (referred by the red dash line in the inset). It is seen in Figure 4(b) that the turnꢀon voltage
2
(
at 1 cd/m ) of EL device on glass substrate was 3.0 V. Moreover, the maximum luminance of EL device on
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2
glass substrate was as high as 24721.8 cd/m at 10.0 V. The current efficiencyꢀpower efficiencyꢀcurrent
density curve of TPEꢀAnꢀemitting EL device on glass is demonstrated in Figure 4(c) to reveal the ability of
converting electric energy into light. And the corresponding device configuration is provided in the inset.
2
The maximum current efficiency was found to be 14.7 cd/A at 131.6 mA/cm . Also, the maximum power
2
efficiency of the EL device arrived at 6.9 lm/W at 3.7 mA/cm . In our future works, the effect of isomers on
TPEꢀAn’s EL performance will be further studied because Z/E isomers of TPE derivatives may affect the
emission behaviors of EL devices [16].
To ensure TPEꢀAn is the emitting layer of EL device on glass substrate, the PL spectrum of TPEꢀAn film
and EL spectra of TPEꢀAnꢀemitting device on glass substrate are compared in Fig. 5. Typical PL spectrum
of TPEꢀAn film is shown in Fig. 5(a). It is seen that TPEꢀAn film has a clear greenꢀlight emission peak with
a quantum yield of 75%, which appears at 515 nm. From the EL spectrum of device on glass substrate (Fig.
5
(b)), the emission peak (515 nm) of EL device on glass substrate (515 nm), which coincides with that of
TPEꢀAn film well. It proves that the emitting layer of EL device on glass substrate should be TPEꢀAn film.
Table II lists the working performance of some other TPE derivativeꢀemitting EL devices on glass substrate
[
31, 34ꢀ37]. As found in Table II, TPEꢀAnꢀemitting EL device on glass substrate has a comparable working
performance with other AIE EL devices with excellent emission behaviors.
Subsequently, we further explore the application feasibility of TPEꢀAn’s on flexible EL device because
TPEꢀAn film has nice mechanical properties. In contrast with other flexible substrates, PET has a low
thermal expansion coefficient, high transmittance and low water absorption [38], and thus it was chosen as
the flexible substrate of TPEꢀAnꢀemitting EL device. The flexible EL device is composed of ITO/NPB (40
3
nm)/TPEꢀAn (15 nm)/TPBi (10 nm)/Alq (20 nm)/ LiF (1 nm)/Al (100 nm). All thin films in the flexible EL
device were deposited on PET substrate by a simple thermal evaporation way, which is close to the
fabrication process of EL device on glass substrate. The EL spectra of the flexible device before and under
deformation (the bending angle is 100°) are shown in Fig. 5(b). It is clearly seen that the emission peak of
5
13 nm keeps unvaried before and under deformation. Figure 6(a) demonstrates that the IꢀVꢀL curve of the
flexible EL device. The inset gives the typical luminescence photograph of the flexible EL device under
deformation, which works at 14.0 V. As shown in Fig. 6(b), the maximum luminance and current efficiency
2
2
of the flexible EL device is 338.0 cd/m (at 16.0 V) and 0.4 cd/A (at 68.3 mA/cm ), respectively. Moreover,
it is worth noting that the flexible device exhibits a uniformed and excellent EL behavior at the bending
angle of 33°.
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The working stability of the flexible EL device under deformation is another essential task, which cares
about its application developments. Typical photographs of the flexible EL device at various bending angles
are provided in Fig. 7(a), which consists of a working cycle. It is found that the flexible EL device can
recover the original shape after a working cycle is over. Although the maximum bending angle of the
flexible EL device reaches about 140°, no damages is found in the whole measurement, which reveals that
flexible TPEꢀAnꢀemitting EL device has a strong enough endurance to large bending stress. The same cycles
repeat 50 times to further evaluate the working stability of the flexible EL device. Luminanceꢀbending cycle
and turnꢀon voltageꢀbending cycle curves are respectively given in Fig. 7 (b). It is clearly seen that the
emission luminance and turnꢀon voltage of the flexible EL device almost keeps unvaried after 50 cycles.
Moreover, TPEꢀAnꢀemitting EL device on glass or PET substrate has a promising half lifetime, as seen in
2
Fig. S8. The half lifetime at an initial luminance of 1000 cd/m is 120 h for EL device on glass and the half
2
lifetime at an initial luminance of 200 cd/m is 38 h, respectively. It suggests that flexible TPEꢀAnꢀemitting
EL device remains a good and stable working performance after undergoing repeatedly large deformation,
which will be beneficial for its practical applications in flexible electronic devices.
Compared with the TPEꢀAnꢀemitting EL device on glass substrate, the working performance of flexible
device on PET substrate is relatively worse at the same voltage, and the possible reasons are explained as
follows. The sheet resistance of ITO/PET (182 ꢂ/sq) is found to be far higher than that of ITO/glass (14
ꢂ/sq) in our experiments, which may cause flexible EL device has a larger contact resistance than EL device
on glass substrate. Under this circumstance, the effective voltage loss at EL device on glass substrate is
much higher than flexible device at the same applied voltage because less voltage drops at the interface
between multilayers and substrate. Consequently, the working performance of EL device on glass substrate
should be better than that of EL device on PET substrate. Although flexible TPEꢀAnꢀemitting device cannot
have the same excellent working performance with its corresponding EL device on glass substrate, it still
exhibits a good enough performance in flexible EL devices. Therefore, TPEꢀAn film should have a more
promising future in flexible EL devices if the interface resistance can be effectively decreased.
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4
. Conclusion
The CV, UVꢀvis and SCLC techniques are combined together to design the configuration of
TPEꢀAnꢀemitting EL device. It is found that the hole mobility of TPEꢀAn exhibits an upꢀdownꢀflat tendency
with the increasing film thickness. The maximum hole mobility of 50 nm TPEꢀAn thin film is calculated to
ꢀ4
2
arrive at 4.00
×10 cm /V·s at 0.1 MV/cm field. TPEꢀAnꢀemitting EL devices were respectively fabricated
on glass and PET substrates by a simple thermal evaporation method. The EL device on glass demonstrated
2
a very fascinating greenꢀlight emission performance. It has a maximum luminance of 24721.8 cd/m (at 10.0
2
V) and its current efficiency is as high as 14.7 cd/A (at 131.6 mA/cm ), which is comparable to many other
TPE derivativeꢀemitting EL devices with excellent performances. And the flexible EL device still maintains
a stable and excellent luminance after 50 continuous measurement cycles, in which the maximum bending
o
angle arrives at 140 . It is suggested that TPEꢀAn can be a promising candidate in greenꢀlight EL
applications. In addition, this research method can provide a beneficial reference for designing other
highꢀefficiency EL device structures.
Acknowledgements
The authors are very thankful for the support of the National Basic Research Program of China (973
Program, Grant No. 2010CB327703, the National Natural Science foundation of China, Grant No.
5
0802117), Program for New Century Excellent Talents in University (NCETꢀ12ꢀ0573), National Project for
the Development of Key Scientific Apparatus (2013YQ12034506), the Natural Science foundation of
Guangdong Province (No. S2012010010519), China Scholarship Council Fund for Young Backbone Teacher,
the Science and Technology Department of Guangdong Province, the Education Department of Guangdong
Province, and the Science and Technology Department of Guangzhou City.
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DOI: 10.1039/C5TC01211B
Table and Figure Captions
Table I. Zeroꢀmobility, PoolꢀFrenkel factor and simulation mobility of TPEꢀAn thin film at various thickness
at electric field of 0.1 MV/cm).
(
Table II. EL performance of various tetraphenyletheneꢀemitting EL devices.
Figure 1. (a) Ultravioletꢀvisible absorption spectrum of TPEꢀAn powders. (b) Cyclic voltammetry curve of
TPEꢀAn in dichloromethane. Inset: molecular structure of TPEꢀAn. (c) Electronic structure of TPEꢀAn.
Figure 2. (a) Current densityꢀvoltage characteristics, (b) lnJꢁlnE curves and (c) spaceꢀchargedꢀlimited
currents of electron injection devices at various thicknesses of TPEꢀAn. (d) Zeroꢀfield mobility (ꢀ0) for the
hole mobility estimated by SCLC for ITO/C60/TPEꢀAn(d) /Al devices. Inset: scheme of SCLCꢀtesting device
configuration.
Figure 3. (a) Simulation curve of hole mobility versus TPEꢀAn`s thickness at electric field of 0.1 MV/cm.
Inset: 1 ꢁm×1 ꢁm AFM morphology images of ITO/C /TPEꢀAn multilayer with 50 nm TPEꢀAn layer. Scale
60
bar is 200 nm. (b) Mobilityꢀsquare root of simulated electric field for different TPEꢀAn thicknesses.
Figure 4. (a) Energy level diagram of multilayer EL Device of TPEꢀAn. (b) Current
densityꢀvoltageꢀluminance characteristics of TPEꢀAnꢀemitting EL device on glass. Inset: luminescence
photograph of EL device on glass taken at 8 V. The luminescing area is referred by red dash line. Scale bar:
2
mm. (c) Changes in efficiencies with the current density based on EL device on glass. Inset: configuration
of EL device on glass.
Figure 5. (a) Photoluminescence spectrum of TPEꢀAn thin film and (b) electroluminescence spectra of
TPEꢀAnꢀemitting EL device on glass and on PET substrates at 5 V and 14 V, respectively.
Figure 6. (a) Current densityꢀvoltageꢀluminance characteristics of flexible TPEꢀAnꢀemitting EL device. Inset:
photograph of flexible EL device taken at 14 V. (b) Changes in efficiencies with the current density in
flexible EL device. Inset: configuration of flexible EL device.
Figure 7. (a) TPEꢀAnꢀemitting flexible EL device images of backward and forward deformation at different
bending states. (b) Maximum luminance and turnꢀon voltage obtained after different bending cycles.
14

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Parameters
Zeroꢀfield mobility
Estimated mobility ꢁ
PoolꢀFrenkel factor β
2
2
ꢁ
0
(cm /V·s)
(cm /V·s)
Thickness
ꢀ
5
ꢀ4
ꢀ5
2
5 nm
0 nm
7.92×10
4.00×10
6.27×10
4.82×10
4.33×10
1.14×10
7.92×10
ꢀ
4
ꢀ4
ꢀ4
5
2.76×10
4.00×10
ꢀ5
ꢀ3
ꢀ5
100 nm
200 nm
400 nm
1.73×10
6.28×10
ꢀ5
ꢀ3
ꢀ5
2.1×10
4.83×10
ꢀ5
ꢀ3
ꢀ5
2.1×10
4.33×10
Table I
S. Y. Jin et al., submitted to J. Mater. Chem. C
(
)
15

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ꢀ
2
ꢀ1
Group
Emitter
2TPATPE
TIPE
V
on/V
Lmax/cdm
ηmax/cdA
Y. Liu et al [31]
S. Odabas et al [34]
W.L.Gong et al [35]
W. Z. Yuan et al [36]
L. Chen et al [37]
Our work
3.2
5
33770 (@15 V)
18000 (@15 V)
2585 (@9 V)
13 (@4.2 V)
2
5.2 (@50 mA/cm )
3.5 (@7 V)
Czꢀ1TPE
4TPETPA
TPEꢀPNPB
TPEꢀAn
3.5
4.1
3.2
10726 (@15 V)
49993 (@20 V)
24721.8 (@1 V)
8 (@5 V)
15.7(@3ꢀ4 V)
14.7 (@10 V)
3
2
Abbreviations: V = turnꢀon voltage at 1 cd /m , Lmax= the maximum luminance,
on
η
max =the maximum current efficiency, respectively.
Table II
(S. Y. Jin et al., submitted to J. Mater. Chem. C)
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Figure 1
(S. Y. Jin et al., submitted to J. Mater. Chem. C)
17

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Figure 2
(S. Y. Jin et al., submitted to J. Mater. Chem. C)
18

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Figure 3
(S. Y. Jin et al., submitted to J. Mater. Chem. C)
19

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Figure 4
(S. Y. Jin et al., submitted to J. Mater. Chem. C)
20

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Figure 5
(S. Y. Jin et al., submitted to J. Mater. Chem. C)
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Figure 6
(S. Y. Jin et al., submitted to J. Mater. Chem. C)
22

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Figure 7
(S. Y. Jin et al., submitted to J. Mater. Chem. C)
23