Electropolymerized AIE-active polymer film with high quantum efficiency and its application in OLED

Article abstract of DOI:10.1002/pola.28414

A carbazole functionalized electro-active AIE-activity molecule, TPE-DFCz, was designed, synthesized, and well characterized. The clear difference in oxidation potentials between tetraphenylethylene (TPE) unit and carbazole groups was found which guaranteed that polymerization occurred only at the peripheral carbazole groups and the TPE unit remained unchanged. Its luminescent network film was prepared conveniently by electrochemical polymerization (EP). The cross-linked film exhibited green emission with high quantum efficiency of 63%, relatively smooth surface, and good thermal stability. The effect of different scan cycles on the optical property was also investigated. The electroluminescent device using the optimized polymer film as active layer showed a maximum luminance of 3200 cd?m^?2 and a maximum luminance efficiency of 1.16 cd?A^?1 with very low roll-off of the efficiency. The AIE-active EP films afford more opportunities to develop polymer films with high quantum efficiency via a simple, effective method and promote the potential applications in display devices.

Full text of DOI:10.1002/pola.28414

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Electropolymerized AIE-Active Polymer Film with High Quantum
Efficiency and its Application in OLED
Jinyu Li,1 Xiao Han,1 Qing Bai,1 Tong Shan,1 Ping Lu,1 Yuguang Ma2
1State Key Laboratory of Supramolecular Structure and Materials, Jilin University, 2699 Qianjin Avenue, Changchun 130012,
China
2Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South
China University of Technology, Guangzhou 510640, China
Correspondence to: P. Lu (E-mail: lup@jlu.edu.cn)
Received 31 August 2016; accepted 18 October 2016; published online 00 Month 2016
DOI: 10.1002/pola.28414
ABSTRACT: A carbazole functionalized electro-active AIE-activity
molecule, TPE-DFCz, was designed, synthesized, and well char-
acterized. The clear difference in oxidation potentials between
tetraphenylethylene (TPE) unit and carbazole groups was found
which guaranteed that polymerization occurred only at the
peripheral carbazole groups and the TPE unit remained
unchanged. Its luminescent network film was prepared conve-
niently by electrochemical polymerization (EP). The cross-
linked film exhibited green emission with high quantum effi-
ciency of 63%, relatively smooth surface, and good thermal
stability. The effect of different scan cycles on the optical prop-
erty was also investigated. The electroluminescent device
using the optimized polymer film as active layer showed a
maximum luminance of 3200 cd m22 and a maximum lumi-
nance efficiency of 1.16 cd A21 with very low roll-off of the effi-
ciency. The AIE-active EP films afford more opportunities to
develop polymer films with high quantum efficiency via a sim-
ple, effective method and promote the potential applications in
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display devices.
2016 Wiley Periodicals, Inc. J. Polym. Sci.,
Part A: Polym. Chem. 2016, 00, 000–000
KEYWORDS: aggregation-induced emission; efficiency; electro-
chemical polymerization; electrochemistry; luminescence;
OLEDs; polymers; tetraphenylethylene
conducting substrates with various sizes and geometries by
electrochemically oxidizing suitable electro-active precur-
sors.23–25 The growth rate and thickness of the polymer films
are also controllable by conveniently regulating applied poten-
tial and total amount of charge passed through the cell, respec-
tively.26–28 All these features endow EP as an ideal method to
prepare cross-linked polymer films with good stability.
INTRODUCTION Great achievements have been made in organ-
ic light-emitting materials over the past decades due to their
wide applications in optoelectronic fields such as organic light-
emitting diodes (OLEDs),1–4 organic field-effect transistors
(OFETs),5–8 and organic photovoltaic devices (OPVs).9–11 Con-
jugated polymers are regarded as the most promising candi-
dates because they can be prepared conveniently by low-cost
technologies for the fabrication of optoelectronic devices.12,13
Several methods including spin-coating,14 ink-jet printing,15
and electrochemical polymerization (EP)16 are generally uti-
lized to prepare polymer films. Creation of polymer films with
long-term stability under device operating conditions is of sig-
nificant importance. In this case, EP method is a promising
approach to afford luminescent cross-linked network with ide-
al stability by fixing the positions and phase of polymer
chains,17 which has already shown great potentials in superca-
pacitors,18 fluorescence detection of TNT,19 and electrode
modifying interlayer.20–22 In addition, the in situ polymeriza-
tion avoids the complicated synthesis process of polymer
and reduces the waste of materials during film fabrication.
Furthermore, polymer film can be deposited directly onto the
Another factor that needs to take into consideration is the
high efficiency in solid state, which is very important to pro-
mote the application in OLEDs. Most traditional light-
emitting materials suffer from the aggregation-caused
quenching (ACQ) effect in high-concentration solution or in
solid state due to the strong p-p interactions.29 A particular
phenomenon that materials show almost no emission in
solution but emit strongly in condensed media, named as
aggregation-induced emission (AIE), was firstly discovered
by Luo et al. in 2001.30 This new phenomenon provides a
unique way to obtain organic conjugated materials with high
quantum efficiency in aggregated state. Tetraphenylethylene
(TPE) is a classical chromophore which possesses a simple
structure but shows a splendid AIE property.31,32 The facile
Additional Supporting Information may be found in the online version of this article.
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synthesis and ready functionalization have made it a mostly
investigated building block catering to the needs of a wide
range of high technological applications.33 Up to date, efficient
fluorescent blue, green, red, and white OLEDs materials have
been prepared using TPE as a versatile constructing unit.34
Carbazole is a commonly utilized organic luminescent material
with good hole-injection ability and high thermal stability.35,36
Carbazole also behaves as a highly electro-active group with a
relatively low oxidation potential.37–39 Very effective coupling
between its oxidative species of carbazyl radical cation, and
structurally well-defined coupling products of dimeric carba-
zyl have been reported, which demonstrates that carbazole
plays a crucial role for the precise control of the structure and
properties of EP films.40–43 These advantageous properties
arouse interests in constructing electro-active AIE molecules.
Applying AIE-active derivative as a precursor to prepare the
luminescent network film was recently reported by Liu et al.,44
in which OLEDs using these films as light emitting layers
exhibited a maximum luminance of 489 cd m22 and a maxi-
mum luminance efficiency (LE) of 0.38 cd A21. In present
work, we further develop an electro-active carbazole function-
alized TPE derivative, TPE-DFCz, and a cross-linked electro-
chemically polymerized film with high quantum efficiency,
smooth surface, and good thermal stability is obtained through
EP method. A significantly improved performance in OLEDs
using TPE-DFCz-based EP film as active layer is achieved with
a maximum luminance of 3200 cd m22 and a maximum LE
of 1.16 cd A21. The result indicates that electrochemical
synthesis can be a new facile route for constructing the cross-
linked polymer luminescent films with AIE-activity and afford
high performance in OLEDs.
system at a heating rate of 10 8C min21 and a nitrogen flow rate
of 80 mL min21. The atomic force microscopy (AFM) images
were recorded by a Seiko SPA 300 in contact mode (AFM mode).
The transmission electron microscopy studies (TEM) were per-
formed by JEM-2100F TEM instrument. The cyclic voltammetry
(CV) were performed using a standard one-compartment, three-
electrode electrochemical cell attached to an Electrochemical
Workstation named Model CHI-660C, Shanghai, Chenhua. ITO
(1.2 cm 3 2.5 cm) was used as the working electrode, an Ag/Ag1
electrode was used as the reference electrode and titanium metal
was used as the counter electrode with area of 2 cm 3 4 cm.
TPE-DFCz (1 mg mL21) and TBAPF6 (0.1 M) were dissolved in a
mixed solvent of acetonitrile and CH2Cl2 (v/v 5 1:1). The EP films
were prepared by the CV mode, in which the thickness of the EP
film can be easily controlled by scan cycles and scan rates, such
as about 2.7 nm per cycle and about 27 nm as 10 cycles at a scan
rate of 100 mV s21. After the EP process, the resulting EP film
was washed with
a mixture of acetonitrile and CH2Cl2
(v/v 5 1:1) to remove any unreacted precursors and supporting
electrolytes and then dried in a vacuum oven.
Device Fabrication
The ITO-coated glass substrates were cleaned in an ultrason-
ic bath with purified water, isopropylalcohol, acetone, tolu-
ene, acetone, isopropyl alcohol, respectively, and cleaned for
20 min by exposure to UV-ozone. The EP films were used as
emitting layers, then TPBi, as both a hole-blocking layer and
an electron-transporting layer, was thermally deposited onto
the emitting layers under vacuum with a back pressure of
1025 Pa. Finally, LiF buffer layer and an Al cathode were
deposited onto TPBi. The luminance-current characteristics
of devices were measured using a PR650 spectroscan spec-
trometer, and current-voltage characteristics were recorded
by a Keithley 2400 sourcemeter. All of procedure was per-
formed at room temperature under ambient conditions.
EXPERIMENTAL
Instrumentation
All the reagents were purchased from Acros, J&K and Aldrich
Chemical Co. and used as received. THF was dried and puri-
fied by fractional distillation over sodium in the presence of
benzophenone. The supporting electrolyte, tetrabutylammo-
nium hexafluorophosphate (TBAPF6, 99%) was purchased
from J&K and dried for 24 h under vacuum before use. Sol-
vents for electrochemical experiments were the mixture of
acetonitrile and CH2Cl2 (v/v 5 1:1), which were carefully
purified and purged by dry nitrogen prior to the electro-
chemical measurements.
Synthesis of 1,2-Bis(4-bromophenyl)-1,2-Diphenylethene (2)
4-Bromobenzophenone (1) (11.50 g, 19.20 mmol), zinc dust
(2.50 g, 38.20 mmol) and THF (100 mL) were put into a
250 mL two-necked round-bottom flask equipped with a
reflux condenser. TiCl4 (2.1 mL, 19.20 mmol) was dropped
slowly using a syringe under nitrogen protection. The mix-
ture was slowly warmed to room temperature and then
refluxed for 12 h. Water was added to quench the reaction.
The organic layer was washed and extracted with chloro-
form. And then compound 2 (3.60 g) was isolated by column
chromatography using hexane as eluent. Yield: 76%.
The 1H NMR were recorded on a Bruker AVANCE 500 spectrome-
ters at 500 MHz at 298 K using deuterated dimethyl sulfoxide
(DMSO-d6) and CDCl3 as solvents and tetramethylsiane (TMS) as
internal standard. All of the compounds were characterized by a
Flash EA 1112, CHNS elemental analysis instrument. The MALDI-
TOF-MS mass spectra were recorded using an AXIMA-CFRTM
plus instrument. UV-Vis absorption spectra were measured on
Shimadzu UV-3100PC spectrophotometer, and fluorescence
recorded by using a RF-5301PC. Differential scanning calorimetry
(DSC) analysis was carried out using a TAQ100 instrument at
10 8C min21 while flushing with nitrogen. Thermal gravimetric
analysis (TGA) was recorded on a Perkin Elmer thermal analysis
1H NMR (500 MHz, CDCl3): d 7.29 (d, J 5 2.4 Hz, 1H), 7.28 –
7.27 (m, 1H), 7.26–7.23 (m, 3H), 7.16 (dd, J 5 5.9, 2.5 Hz, 4H),
7.14–7.11 (m, 4H), 7.02 (dd, J 5 5.0, 4.2, 1.9 Hz, 5H), 6.94–6.88
(m, 5H). MALDI-TOF-MS: m/z: [M 1 H]1 calcd. for: C26H18Br2
490.24; found: 491.50.
Synthesis of 9,9’-((2-(4,4,5,5-tetramethyl-1,3,2-
dioxaborolan-2-yl)29H-fluorene-9,9-diyl)
bis(hexane-6,1-diyl))bis(9H-carbazole) (6)
A mixture of 5 (3.72 g, 5.00 mmol), potassium acetate (3.92 g,
40 mmol), 4,4,4’,4’,5,5,5’,5’-octamethyl-2,2’-bi(1,3,2-dioxaborolane)
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1H NMR (500 MHz, CDCl3): d 8.09 (d, J 5 7.7 Hz, 1H), 8.06
(d, J 5 7.7 Hz, 1H), 7.71 (dd, J 5 7.6, 4.7 Hz, 1H), 7.66 (d,
J 5 6.1 Hz, 1H), 7.64 (d, J 5 3.4 Hz, 1H), 7.62 (s, 1H), 7.55 (d,
J 5 7.9 Hz, 1H), 7.49 (t, J 5 8.9 Hz, 1H), 7.47–7.43 (m, 1H),
7.43–7.39 (m, 1H), 7.39–7.35 (m, 1H), 7.33–7.30 (m, 2H),
7.25 (dd, J 5 8.9, 4.0 Hz, 2H), 7.23 (dd, J 5 7.9, 3.3 Hz, 2H),
7.20 (s, 1H), 7.17 (dd, J 5 12.7, 7.2 Hz, 3H), 7.15–7.07 (m,
1H), 4.17 (t, J 5 7.2 Hz, 1H), 4.10 (t, J 5 7.2 Hz, 1H), 1.94 (d,
J 5 8.0 Hz, 1H), 1.91 (d, J 5 5.5 Hz, 1H), 1.88 (d, J 5 8.3 Hz,
1H), 1.69 (dd, J 5 14.5, 7.3 Hz, 1H), 1.63 (dd, J 5 15.5, 8.4
Hz, 1H), 1.57 (s, 1H), 1.15 (d, J 5 6.9 Hz, 1H), 1.10 (dd,
J 5 18.3, 11.5 Hz, 2H), 1.05 (d, J 5 7.2 Hz, 1H), 0.64 (s, 1H),
SCHEME 1 The synthetic routes to TPE-DFCz.
(1.90 g, 7.50 mmol), and Pd(dppf)Cl2 (82.00 mg, 0.10 mmol) were
refluxed in 1,4-dioxane (30 mL) under nitrogen. After 48 h, the
mixture was cooled and the reaction was stopped. The organic
layer was extracted with dichloromethane, washed with water
and dried over anhydrous MgSO4 overnight. After evaporating the
solvent, the crude product was purified by column chromatogra-
phy using petroleum ether/CH2Cl2 as the eluent to afford a white
solid (3.16 g). Yield: 80%.
1H NMR (500 MHz, CDCl3): d 8.10 (d, J 5 7.7 Hz, 1H), 7.84
(d, J 5 7.5 Hz, 1H), 7.73 (dd, J 5 13.0, 6.8 Hz, 1H), 7.45 (dd,
J 5 7.9, 7.3 Hz, 1H), 7.33 (dd, J 5 13.4, 7.6 Hz, 1H), 7.26 (t,
J 5 6.0 Hz, 1H), 7.23 (dd, J 5 13.1, 5.7 Hz, 1H), 4.17 (t, J 5 7.3
Hz, 1H), 3.74 (s, 1H), 2.04–1.85 (m, 1H), 1.74–1.63 (m, 1H),
1.55 (d, J 5 30.5 Hz, 1H), 1.43 (d, J 5 33.2 Hz, 2H), 1.33–1.22
(m, 1H), 1.21–1.01 (m, 1H). MALDI-TOF-MS: m/z: [M 1 H]1
calcd. for: C55H59BN2O2 790.90; found: 791.20.
Synthesis of TPE-DFCz
2 (980.48 mg, 2.00 mmol), 6 (3.16 g, 4.00 mmol), and Pd(PPh3)4
(92.40 mg, 0.08 mmol) were added to a solution of Na2CO3
(2.0 M) in a 3:2 (v/v) mixture of toluene/water. The mixture
was stirred at 90 8C for 48 h under a nitrogen atmosphere. Sub-
sequently, water was added to quench the reaction. The organic
phase was extracted several times with chloroform, and then
was dried over anhydrous MgSO4 overnight. After filtration and
solvent evaporation, the liquid was purified by column chroma-
tography using petroleum ether/CH2Cl2 as the eluent to afford a
yellow-green solid (1.82 g). Yield: 55%.
FIGURE 1 (A) PL spectra of TPE-DFCz in THF-water mixtures
with different water fractions (fw); (B) plot of the relative PL
intensity (I/I0) versus the compositions of THF-water mixtures
of TPE-DFCz. I0 and I were the maximum PL intensity of TPE-
DFCz in pure THF and in THF-water mixtures (10 lM), respec-
tively. Excitation wavelength: 345 nm.
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Subsequently, monomer 6 was synthesized starting from com-
mercially available fluorene and carbazole through a three-
step reaction according to our reported literatures.45,46 With
monomer 2 and 6 in hand, TPE-DFCz was obtained through
Suzuki coupling reaction in a heterogeneous system using
Pd(PPh3)4 as a catalyst. The crude product was washed with
acetic acid and then purified through chromatography to give
target compound as yellow-green solid with good yield of
FIGURE 2 Thermal properties of TPE-DFCz. (A) The TGA graph
of TPE-DFCz (heating rate: 10 8C min21 under a nitrogen flow);
(B) DSC graph of TPE-DFCz (second heating curve, heating
rate: 10 8C min21 under a nitrogen flow).
0.64–0.59 (m, 1H), 0.57 (d, J 5 7.3 Hz, 1H). 13C NMR (126
MHz, CDCl3): d 148.33, 144.85, 143.12, 140.27, 138.30,
131.98, 129.69, 126.22, 124.24, 123.61, 121.64, 121.32,
118.16, 110.74, 107.58, 106.48, 104.19, 101.03, 97.87, 63.13,
59.42, 56.02, 52.86, 34.46, 33.04, 28.23, 25.62, 24.20, 22.15.
MALDI-TOF-MS: m/z: [M 1 H]1 Calcd. for: C124H112N4
1658.29; found: 1659.90. ANAL. CALCD. for C124H112N4: C,
89.81; H, 6.81; N, 3.38; found: C, 89.66; H, 6.82; N, 3.36.
FIGURE 3 (A) CV profile of 1st cycle of TPE-DFCz in acetoni-
trile/CH2Cl2 (v/v 5 1:1) in the presence of TBAPF6 (0.1 M) elec-
trolyte at room temperature under nitrogen protection. Scan
rate: 100 mV/s; Scan range: 20.5 to 1.4 V. (B) CV profiles (1st to
10th cycles) of TPE-DFCz in acetonitrile/CH2Cl2 (v/v 5 1:1) in the
presence of TBAPF6 (0.1 M) electrolyte at room temperature
under nitrogen protection. Scan rate: 100 mV/s; Scan range:
20.7 to 0.9 V. Inset: EP film on the electrode under UV light.
RESULTS AND DISCUSSION
Scheme 1 describes the synthesis routes of TPE-DFCz. To
begin with, dibromotetraphenylethylene (2) was prepared via
McMurry coupling reaction using the 4-bromobenzophenone
(1) as the starting material in the presence of zinc and TiCl4.
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calorimetry (DSC) under a nitrogen atmosphere. DSC result
indicated that TPE-DFCz exhibited glass transition tempera-
ture (Tg) at 96 8C in the experimental temperature range
(Fig. 2).
This is
a relatively high value among the solution-
processable small molecule containing flexible dihexyl side
chains. Specially, no crystallization or phase transition was
observed. The decomposition temperature (Td) was mea-
sured to be at 453 8C due to its high molecular weight.
The TGA and DSC characterization demonstrated TPE-DFCz
possessed high thermal stability which is desirable for its
application in solid state.
FIGURE 4 The normalized UV-Vis spectra and PL spectra of
TPE-DFCz in dilute THF solution (concentration: 10 lM) and in
thin film spin-coated from 1023 M THF solution at 1500 rpm.
Excitation wavelength: 350 nm.
55%. TPE-DFCz was characterized by MS, NMR spectra, ele-
mental analysis, and corresponded well with its expected
structure.
TPE-DFCz showed typical AIE-active spectral behavior, that
is, it had no emission when dissolved in THF but exhibited
intense emission in the aggregated state. Only a weak PL sig-
nal was recorded even when the water fraction (fw) was as
high as 40% because the molecules still remained dissolved
in the mixture (Fig. 1). However, when fw was higher than
50%, addition of even a small amount of water gave signifi-
cant increase in the PL intensity. After reaching a maximum
intensity at 60% water fraction, the PL intensity decreased
with increasing water content. Then, a further increase of
the water fraction was observed which resulted in a continu-
ous increase of PL intensity. This phenomenon was often
found in the characterization of the AIE effect, which might
be related to the various kinds of nanoparticle suspen-
sions.47 Generally speaking, crystal particles lead to an
enhancement in the PL intensity, while the amorphous par-
ticles result in a reduction in intensity (Supporting Informa-
tion Fig. S1).48 The observed overall PL intensity data would
depend on the combined results of the two kinds of nano-
particles, which manifested no regularity in high water con-
tents of TPE-DFCz. From the THF solution to the aggregated
suspension in the 90% aqueous mixture, the maximum PL
peak intensity was increased up to 84 times due to the
restriction of molecular motions confirming its AIE-active
nature. This process was also reflected by the corresponding
PL spectrum at low temperature of 77 K, in which the emis-
sion intensity was 437-folded higher than the emission
recorded at room temperature (Supporting Information Fig.
S2).
FIGURE 5 The absorption (A) and PL (B) spectra of EP films on
ITO using different scan cycles. The EP films are prepared
using TBAPF6 (0.1 M) as the supporting electrolyte at a scan
rate of 100 mV/s and a scan range of 20.7 to 0.9 V.
The thermal properties of TPE-DFCz were investigated using
thermogravimetric analysis (TGA) and differential scanning
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FIGURE 6 AFM images showing 5 lm 3 5 lm areas of EP films of TPE-DFCz on ITO. The EP films were prepared at the scan rate
of 100 mV/s using TBAPF6 with different scan cycles. (a–d were 5, 10, 15, and 20 scan cycles, respectively, under room tempera-
ture; e–h after heating at 120 8C for 20 min.).
Cyclic voltammetry (CV) was employed to investigate the elec-
trochemical behaviour of TPE-DFCz. Film of TPE-DFCz was
fabricated by multicycled cyclic voltammetry (CV) in a mixture
of acetonitrile and CH2Cl2 (v/v 5 1:1) containing the electro-
Recurrent sweeps resulted in a progressive increase of the
peak current, indicating the occurrence of the coupling reac-
tion between the carbazole units and the growth of the EP
film on the electrode. After 10 cycles, a transparent film was
formed on the electrode which exhibited green emitting col-
or under UV light. The resulting cross-linked film was highly
robust and insoluble in any organic solvents.
lyte with TPE-DFCz concentration of 6.03 3 1024 mol L21
.
From the first cycle of a positive CV scan in the potential sweep
between 20.5 and 1.4 V [Fig. 3(A)], the onset oxidative poten-
tial of TPE-DFCz appeared at 0.81 V, which was attributed to
the oxidation of carbazole. As the scan potential higher than
0.81 V, the anodic current increased rapidly and yielded a
potential peak at 0.98 V, which indicated that more carbazole
groups were oxidized. Previous studies have shown that the
carbazole transformed into cationic radicals during oxidation
and then couples effectively with each other to form dimeric
carbazole cations.12,49 During the negative scan, an obvious
reductive peak was observed at the potential of 0.47 V, assign-
ing to the reduction of the dimeric carbazole cation. In con-
trast, the oxidation and reduction peaks of the TPE core were
observed at potentials of 1.05 and 0.98 V, respectively. This
clear difference in oxidation potentials between TPE unit and
carbazole groups was of vital importance because it guaran-
teed that polymerization could occur only at the peripheral
carbazole groups, whereas the TPE unit remained untouched.
Figure 4 shows the absorption spectra of TPE-DFCz in dilute
THF solution and in thin films formed on quartz substrates
by spin-coating method. The absorption spectrum of TPE-
DFCz in THF solution was characterized by several absorp-
tion peaks at 345, 327, 292, 264, and 231 nm, which was
the combination of TPE, carbazole and dihexylfluorene
groups. The 345 nm low-energy band was attributed to the
p-p* electronic transitions in the backbone, which was simi-
lar as that observed for other fluorene-based oligomers.46,50
The absorption peak at 327 nm could be assigned to the
p-p* transitions of TPE. The high-energy band emerged at
292, 264, and 231 nm was from the peripheral carbazole
groups.51 Similar results were also observed in the absorp-
tion spectrum of the film. The film of TPE-DFCz exhibited
strong blue emission peaking at 487 nm with high quantum
efficiency of nearly 93%.
From the second scan in the continuous CV curves as shown
in Figure 3(B), a new peak appeared at 0.68 V, which was
owing to the oxidation of the formed dimeric carbazoles.
The UV-Vis spectra of TPE-DFCz EP films with increased
scan cycles were presented in Figure 5(A). EP films were
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FIGURE 7 (A) EL spectra of the solid thin film of TPE-DFCz recorded in the voltage from 6 to 12 V. (B) Luminance efficiency-
voltage-current density characteristics of multilayer EL devices of TPE-DFCz. (C) Luminance efficiency-Voltage-EQE characteristics
of multilayer EL devices of TPE-DFCz.
prepared at the scan rate of 100 mV/s using TBAPF6 with
various scan cycles at the voltage from 20.7 to 0.9 V. For
the spin-coated film, three absorption peaks locating at 296,
333, and 348 nm were observed. Notably, the absorption
profiles of EP films became broad and red-shifted due to the
p-p* transition of dimeric carbazolyl.37 In the EP films, a
broad tail band extending from 450 to 800 nm with relative-
ly low intensities were also appeared. This low-energy band
had been already observed in other carbazole-based EP films
and been thought to be relative to the dications of dimeric
carbazolyl.12,37,52 With the increased number of scan cycles,
the absorption intensity was also enhanced, demonstrating
that the thickness of EP films could be controlled by scan
cycles.
of the molecules originating from the closer molecular pack-
ing. Similarly, the PL intensity was also enhanced with the
increased number of scan cycles. The EP film showed high
quantum efficiencies due to the AIE nature. The quantum
efficiencies scanned by 5 to 20 cycles were measured to be
63%, 40%, 32%, 30%, respectively. It can be seen that the
efficiency was gradually reduced with the continuously
increased scan cycles. This might be due to the left residual
charged ion species within the films during electrochemical
deposition process and would decrease the quantum efficien-
cy. This effect became more severe in the resultant thicker
films when scanned with more cycles.
The morphologies of EP films prepared with different scan
cycles under room temperature and after thermal annealing
120 8C for 20 min were studied by AFM, respectively. The
root mean square roughness (RMS) of the EP films prepared
in the optimal potential became gradually smaller with the
increased scan cycles as shown in Figure 6. For example, the
RMS is 9.31 nm when scanned for five cycles and 5.176 nm
for 20 cycles under room temperature. The values for these
EP films were relatively higher than other system as
reported by Liu et al.44 It might have the same origin that
The PL spectra of TPE-DFCz EP films with increased scan
cycles were presented in Figure 5(B). When excited by a
350 nm light, the maximum emission wavelengths (kem, max
)
of TPE-DFCz EP film appeared at 477, 480, 490, and
505 nm, respectively. Comparing with TPE-DFCz spin-coated
film, the emission maximum of the EP films were continu-
ously red-shifted, which implied that cross-linking by EP
would cause a significant increase in the conjugation extent
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electrochemical deposition would leave residual charged ion
species within the film and resulted in rougher surface. After
thermal annealing, the films became more uniform with
decreased RMS values, which was comparable to the results
reported before. Thermal annealing process is an efficient
and simple method for suppressing the negligible effect of
counter ions in the EP films, which was also applied for the
OLED fabrication in this work.
increased scan cycles. The results illustrate that the efficien-
cies and surface smoothness of the films can be conveniently
tuned by the scan cycles. The double-layer OLED using EP
film as active layer achieves a maximum luminance of 3200
cd m22, a LE of 1.16 cd A21 and an external quantum effi-
ciency of 0.46%. The electrochemical synthesis could be a
novel way to fabricate the cross-linked polymer films with
AIE molecules.
The high luminescence of the cross-linked TPE-DFCz ED
films ensured the fabrication of ED-film-based PLEDs. The
optimized double-layer device was prepared with the follow-
ing structure of ITO/ED film (27 nm)/TPBi (40 nm)/LiF
(0.5 nm)/Al (100 nm), in which ITO-coated glass was used
as both the substrate and anode, TPBi as both a hole-
blocking layer and an electron-transporting layer, and LiF as
the modificatory layer of the cathode Al. All the characteriza-
tion data for devices were summarized in Supporting Infor-
mation Table S1. The EP film scanned by 10 cycles was
applied as the active layer due to the appropriate thickness,
optimized quantum efficiency and surface roughness. Figure
7(B) presents the EL spectra and the luminous-efficiency—
voltage–current-density characteristics of the device. TPE-
DFCz EP device showed green emission with an emissive
peak at 500 nm in the EL spectra and CIE coordinates of
(0.237, 0.430). The EL spectrum of device was consistent
with its PL spectrum of EP film. The emission spectra were
kept unchanged when the voltages varied from 6 to 12 V
indicating the good stability of cross-linked film as active lay-
er. The EP device exhibited a turn-on voltage of 5.4 V, a max-
imum brightness of 3200 cd m22, an external quantum
efficiency of 0.46% and a LE of 1.16 cd A21, which was a
relatively high level in the double-layer PLEDs. The efficien-
cies showed very low roll-off indicating the excellent stability
of the EP film. The performance of the device in the present
work was significantly improved as compared to previously
reported data.44 The result indicated that cross-linked poly-
mer network films of based on TPE can be used as good
emitters in the construction of EL devices. We also believe
further optimization of the TPE-DFCz EP film by reducing
the residue of cations can lead to a better device perfor-
mance. Besides such polymer EP films can also have poten-
tial applications in solar cells to be acted as the active layers
or interfacial layers.
ACKNOWLEDGMENTS
This research is supported by the Ministry of Science and Technol-
ogy of China (2013CB834801, 2016YFB0401001), the National
Science Foundation of China (21374038), and the Jilin Provincial
Science and Technology Department (20160101302JC).
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