Benzothiadiazole, hexylthiophen and alkoxy benzene based solution processable copolymer: Effect of the electron withdrawing substituents (fluorine atoms) on electrochemical, optical and electrochromic properties

Article abstract of DOI:10.1016/j.orgel.2018.06.048

4,7-Bis(4-hexyl-5-(trimethylstannane) -2-thienyl) ?2,1,3-benzothiadiazole (HTBT) coupled with 2,5-bis (decyloxy)-1,4-dibromobenzene to afford the donor-π-bridge-donor-acceptor conjugated poly (4,7-bis (4-hexyl-2-thienyl) ?2,1,3-benzothiadiazole-co-p-didec

Full text of DOI:10.1016/j.orgel.2018.06.048

Organic Electronics 61 (2018) 1–9
Contents lists available at ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
Benzothiadiazole, hexylthiophen and alkoxy benzene based solution
processable copolymer: Effect of the electron withdrawing substituents
(
fluorine atoms) on electrochemical, optical and electrochromic properties
Zhen Xu , Hongmei Du , Mengjie Yin , Bozhen Wang , Jinsheng Zhaob,∗, Yu Xiec,∗∗
a
b
a
a
a
School of Chemistry and Chemical Engineering, Shandong University of Technology, Zibo, 255049, China
Shandong Key Laboratory of Chemical Energy Storage and Novel Cell Technology, Liaocheng University, Liaocheng, 252059, China
College of Environment and Chemical Engineering, Nanchang Hangkong University, Nanchang, 330063, PR China
b
c
A R T I C L E I N F O
A B S T R A C T
Keywords:
4,7-Bis(4-hexyl-5-(trimethylstannane) -2-thienyl) −2,1,3-benzothiadiazole (HTBT) coupled with 2,5-bis (decy-
loxy)-1,4-dibromobenzene to afford the donor-π-bridge-donor-acceptor conjugated poly (4,7-bis (4-hexyl-2-
thienyl) −2,1,3-benzothiadiazole-co-p-didecyloxybenzene) (PTBTB). Further, copolymerization of fluorine-
functionalized HTBT moiety with alkoxyl benzene π-bridge using Stille crossing-coupling reaction yield PTBTB-
F. The optical, electrochemical, fluorescent, spectroelectrochemical and electrochromic switching properties of
two polymers with different acceptors are studied in detail. Both polymers exhibit excellent solubility in con-
ventional organic solvents, high thermal stability and favorable fluorescence properties. The combination of a
strong electron withdrawing pendant group (fluorine atoms) with benzothiadiazole (BT) backbone significantly
influences the electrochemical redox, optical and electrochromic behaviors of conducting polymers. The in-
troduction of fluorine atoms to BT lead to a higher oxidation onset potential, but an improved electrochemical n-
Solution processable
Donor-π-bridge-donor-acceptor
Conjugated polymer
Fluorine
Electrochromic
g
doping process. The optical band gaps (E ) of PTBTB and PTBTB-F films are calculated to be 2.02 eV and 1.91 eV,
respectively. PTBTB and PTBTB-F films exhibited obvious and reversible color switching between their neutral
and oxidized states with high optical contrasts as well as favorable response times, making the polymers unique
candidates for flexible organic electrochromic device applications.
1. Introduction
properties, particularly through π-stacking [10].
Recently efforts have focused on D-A type conjugated polymers.
Organic conjugated oligomers and polymers as kinds of important
Hybrid recombination between donor and acceptor unit lead to the
polymer with the higher HOMO level and the lower LOMO level, thus
resulting in a lower band gap. The donor and acceptor units are alter-
nately arranged and extended indefinitely along the backbone, and the
main chain of the molecule exhibits a superlattice structure similar to
inorganic semiconductor [11]. Besides, the molecular structure and
properties of the polymer can be modified. Change of the monomer
repeat unit by considering its electron withdrawing and donating
strengths or substitution of functional groups onto the polymer back-
bone could be used to control the magnitude of the band gap, and
further to regulate the color of neutral state and other electrochromic
properties.
semiconductor materials have attracted great attention in the field of
light emitting diodes [1], organic photovoltaic devices [2], emission
transistors [3], electrochromic devices [4–7] and sensors [8,9]. Opti-
mizing the performance of these devices are the major challenges, now
a large number of studies have shown that electronic and structural
features can be manipulated to improve the material's capabilities (e.g.,
desired HOMO/LUMO energy levels, low band gap, electron or hole
mobility, processability and stability.). For electrochromic polymers
(
ECPs), the drive is to design and synthesize electrochromic materials
with low driving voltage, high optical contrast, high mobility as well as
excellent ambient and operating stability. Exploiting liquid crystal
properties or incorporating inherently “flat” molecular structure, such
as benzene or thiophene derivatives, which was beneficial to in-
tramolecular charge transport and some other optic-electronic
Fluorinated, π-conjugated polymers are of great interest in the
electro-optic field. Incorporating fluorine onto electron-withdrawing
groups, which has been proven to be effective in lowering the HOMO
∗
Corresponding author.
∗
∗
Corresponding author.
E-mail addresses: zhaojinsheng@lcu.edu.cn (J. Zhao), xieyu_121@163.com (Y. Xie).
https://doi.org/10.1016/j.orgel.2018.06.048
Received 26 April 2018; Received in revised form 6 June 2018; Accepted 28 June 2018
Available online 30 June 2018
1566-1199/ © 2018 Elsevier B.V. All rights reserved.

Z. Xu et al.
Organic Electronics 61 (2018) 1–9
and LOMO energy levels due to the high pulling electronegativity of F
atom [12]. The fluorine substituents have great influence on the doping
properties. For p-doping, introduction of electronegative fluorine
caused significant reduction of the doping density, which is likely to
increase the energy barrier for p-doping and make it difficult for elec-
trons to remove from the π-conjugated system. As a result, the doping
potential shifted to more positive values, and the increase in oxidation
potential usually associated with the increase in the number of fluorine
atoms [13]. But for n-doping, the increased electronegativity associated
with the fluorine groups could result in a lower energy barrier for n-
doping, effectively inhibit the degradation reactions of the unstable
radical anion at negative voltages with oxygen and trace water in the
electrolyte, and improve the stability during the n-doping process.
Highly fluorinated systems, such as tetradecafluorosexithiophene [14],
perfluoropentacene [15] and perfluoroalkyl end-capped oligothio-
phenes [16,17] have been successfully prepared and considered to be
good n-type semiconductors. Besides, substitution of thiophenes with
fluorophenyl group [12,13] or its derivatives [18] has also been shown
to reveal improved n-doping property even under ambient conditions.
Further, the introduction of fluorine atoms could not cause deleterious
steric effects, on the contrary, the noncovalent interactions between F
atoms and the components on neighboring thiophene rings can facil-
itate planarization of the backbone structure and hence promote in-
tramolecular charge transfer [19].
polymers were dissolved in CHCl
3
. The light absorption spectra of the
polymers in solution or in films were measured with a Varian Cary 5000
UV-vis-NIR spectrophotometer. Cyclic voltammetry, redox reversibility
and stability were carried out using a CHI 760C electrochemical ana-
lyzer with a three-electrode cell using polymer film coated ITO glasses
2
(the covering area was approximate 1.0 × 2.0 cm ) as the working
electrode, a silver wire (0.02 V vs. SCE.) and a platinum wire as the
pseudo-reference electrode and the counter electrode, respectively. The
three-electrode system was fixed and placed in a colorimetric cell filled
6
with 0.2 M TBAPF /ACN electrolyte. Spectroelectrochemical experi-
ments were carried out using a Varian Cary 5000 UV-Vis-NIR spectro-
photometer connected to the above electrochemical measurement
system, as well as colorimetry analysis. Electrochromic film layers were
made by spraying the chloroform solution of polymers onto ITO glass,
and dried under nitrogen before use. All the photographs in the article
were taken with a canon Power Shot A3000 IS Camera.
2.2. Synthesis of polymers (PTBTB and PTBTB-F)
HTBT (4) and HTBT-F (5) were purchased from Derthon
Optoelectronic Materials Science Technology Co., Ltd, 2,5-didecyloxy-
1,4-dibromobenzene (3) was synthesized according to the methods
from literature [22].
Compound 4 or 5 (1.3 mmol), 2,5-didecyloxy-1,4-dibromobenzene
To investigate the effect of fluorine atoms, we incorporated two F
atoms onto the 4,8-position of benzothiadiazole (BT) unit to synthesize
the difluoro-substituted BT units as acceptor, and used 4-hex-
ylthiophene as donor unit and 2,5-didecyloxybenzene as π-bridge to
build the backbone of the target donor-π-bridge-donor-acceptor (D-π-D-
A) copolymers. D-π-D-A type polymers, as a new family of π-conjugated
polymers, have enhanced planar structures, high carrier transport ef-
ficiencies and optimized band gaps, which should be paid attention by
researchers. Xu group has synthesized several novel D-π-A polymers
3 2 2
(1.3 mmol), Pd(PPh ) Cl (0.05 mmol) and 100 mL dry toluene were
successively added into a 250 mL single-necked flask in a short time,
then heated at 110 °C and stirred for 48 h in argon. After reaction fin-
ished, the solution was naturally cooled to room temperature, and then
vacuum rotatory evaporator was used for the concentration. By adding
appropriate amount of methanol into the concentrate, crude product
that was readily precipitated from the solution. The precipitate was
filtered, and repeatedly reflux extracted by Soxhlet extractor to give
final products, in which methanol and acetone were used as medium
respectively. After vacuum drying, both polymers were collected as
[
20,21] as EC materials in the last few years, yet the shortcomings, such
as low transmittance change, unconspicuous n-doping characteristic or
slow response speed, need to be further resolved. So, substitution of
fluorine atom onto the BT unit to build the new similar D-π-D-A system
is expected to obtain the desired EC materials with the stable p- and n-
doping process, low band gap, favorable fluorescent and electrochromic
properties, together with excellent solubility. The synthetic route of
PTBTB and PTBTB-F is shown in Scheme 1. The structure, thermo-
gravimetry, fluorescence, electrochemical, optical and electrochromic
properties of target polymers are characterized in detail.
deep red solids.
1
PTBTB (Yield, 75.5%). H NMR(CDCl
3
, 400 MHz, ppm): δ = 8.08 (s,
), 2.69 (t,
), 1.18–1.38 (m, 44H, –CH ),
). (see Supplementary Materials Fig. S1a). GPC:
= 32.5 kDa, PDI = 1.10. Elem. Anal. Calcd. For C52 : C,
2H, ArH), 7.91 (s, 2H, ArH), 7.05 (s, 2H), 3.97 (t, 4H,–O–CH
4H, SF-CH ), 1.72 (m, 4H,–O–CH –CH
0.84 (t, 12H, –CH
2
2
2
2
2
3
M
n
74 2 2 3
H N O S
72.85%; H, 8.93%; N, 3.27%; O, 3.73%; S, 11.22%. Found: C, 72.41%;
H, 9.03%; N, 3.32%; O, 3.80%; S, 11.44%.
1
PTBTB-F (Yield, 73.9%). H NMR(CDCl
3
, 400 MHz, ppm): δ = 8.23
), 2.70 (t, 4H, SF-CH ),
), 1.17–1.36 (m, 44H, –CH ), 0.84 (t, 12H,
). (see Supplementary Materials Fig. S1b). GPC: M = 40.3 kDa,
PDI = 1.03. Elem. Anal. Calcd. For C52 : C, 69.91%; H,
.35%; F, 4.25%; N, 3.14%; O, 3.58%; S, 10.77%. Found: C, 70.28%; H,
8.53%; F, 4.06%; N, 3.07%; O, 3.79%; S, 10.27%.
(
s, 2H, ArH), 7.06 (s, 2H), 3.95 (t, 4H,–O–CH
1.70 (m, 4H,–O–CH –CH
CH
2
2
2. Experimental
2
2
2
–
3
n
2.1. Materials and instrumentation
72 2 2 2 3
H F N O S
8
All chemicals used in this study were purchased from commercial
sources (e.g., Aladdin Chemical, Sinopharm Chemical Reagent and
Aldrich Chemical.) and used as received without further purification.
In addition, further structure characterization of the polymers were
carried out by FT-IR and XPS as well as thermal stability by TGA, which
were displayed in Supporting Information in detail.
4,7-Bis(4-hexyl-5-(trimethylstannane) -2-thienyl) −2,1,3-benzothia-
diazole (HTBT, > 98%) and 5,6-difluoro-4,7-Bis(4-hexyl-5-(trimethyl-
stannane) -2-thienyl) −2,1,3-benzothiadiazole (HTBT-F, > 98%) were
from Derthon Optoelectronic Materials Science Technology Co., Ltd.
3. Results and discussion
(
Shenzhen, China). 2,5-Didecyloxy-1,4-dibromobenzene was prepared
3.1. Electrochemical properties
in exactly the same way as the previous work [22].
1
H NMR spectra were measured on Varian AMX 400 with CDCl
3
as
The electrochemical properties of two polymers films which were
solvent and tetramethylsilane (TMS) as the internal standard. Average
molecular weights and molecular weight distributions were detected by
Waters 515 HPLC Pump using HPLC-grade THF as eluent and poly-
styrene (PS) as standard. The content of each element persisted in the
polymer was determined by Vario EL Ш. TGA were performed in pure
nitrogen flow with a Netzsch STA 449C thermogravimetric analyzer
with the heating rate of 10 °C/min. The fluorescence spectra were re-
corded on an F-4500 fluorescence spectrophotometer, where the
spray coated on ITO/glass, were investigated by cyclic voltammetry.
−
1
The resulting single-cycle CV curves at a scan rate of 100 mV s
with
the potentials range between −2.0 V and 1.8 V for PTBTB, and between
−2.0 V and 2.0 V for PTBTB-F can be seen in Fig. 1, in which two
polymers showed similar oxidation and reduction behaviors with re-
versible redox peaks. PTBTB and PTBTB-F in thin films exhibited the
unconspicuous oxidation peaks appeared at 1.32 V and 1.50 V, and also
the broad reduction peaks with maximum reduction potential of 0.80 V
2

Z. Xu et al.
Organic Electronics 61 (2018) 1–9
Scheme 1. Synthetic route of the conjugated polymers.
backbone. As shown in Fig. 1, the reduction peak onset potential for n-
doping shifted to more positive values, indicating the improved stabi-
lity of radical anion and the enhanced n-type doping properties. Be-
sides, the HOMO and LUMO energy levels of PTBTB, which were de-
termined by cyclic voltammetry results, were calculated to be −5.47 eV
and −3.51 eV, respectively, with ferrocene as reference. By compar-
ison, the HOMO of PTBTB-F was 0.24 eV deeper, and the LUMO was
0.23 eV deeper than that of PTBTB. The almost identical decrease of the
HOMO and LUMO energy levels were found through the introduction of
fluorine groups onto the BT unit, thus, leading to the similar electro-
chemical band-gap.
Cyclic voltammetry studies of p-doping/neutralization and n-
doping/neutralization at different scan rates were also performed to
prove electroactivity of polymer films (Fig. 2 and Fig. 3). From the
figures, the broad oxidation/reduction peaks in the p-doped state could
be always observed at different scan rates, indicating the reversible
redox behavior during the p-doping/dedoping process. The oxidation
potentials of two polymers in the cathodic region might be not obvious,
but sharp reduction peaks proved the n-dopable character of the poly-
mers. In addition, for the same polymer film, along with the increase of
the scan rate, the current response steadily increased. And analyses
state that the anodic and cathodic peak current densities versus scan-
ning rates relationship appears to be linear, which appears to signal that
the polymer films can be well adhered to the surface of the ITO/glass
electrode, and also the electrochemical redox processes were non-dif-
fusion controlled [25]. By contrast, linear correlation in the n-doped
state of PTBTB-F was significantly higher, which further illustrates that
substitution of BT unit with F atoms is expected to yield improved n-
doping performance.
6
Fig. 1. Cyclic voltammogram curves of PTBTB and PTBTB-F in 0.2 M TBAPF /
−
1
ACN solution at a scan rate of 100 mV s
.
and 1.12 V, respectively. The oxidation onset potentials (Eonset) for p-
doping increased from 1.05 to 1.29 V with the incorporation of strongly
electronegative fluorine onto electron-withdrawing benzothiadiazole
group. The increase in electronegativity of the acceptor caused un-
desirable inductive effect and increased energy barrier for p-doping,
which would hinder electron transfer involved in the oxidation of
polymers, resulting in a relatively higher oxidation potential [23,24].
However, the increase in electron withdrawing character associated
with the fluorine groups could reduce the energy barrier for n-doping,
which is beneficial for electron injecting into the π-conjugated
Long-term switching cyclability of the polymer films were char-
acterized by repeated potential scanning between neutral and oxidized
3

Z. Xu et al.
Organic Electronics 61 (2018) 1–9
−
1
Fig. 2. CV curves of the PTBTB film at different scan rates between 50 and 300 mV s
in 0.2 M TBAPF
6
/ACN solution, respectively.(a) during the p-type doping
process; (b) during the n-type doping process.
states at a scan rate of 100 mV s−1. Calculated from the CV curves in
Fig. S7, PTBTB and PTBTB-F exhibited favorable cycling stability with
328 nm and 345 nm, respectively, in solution and in film, which ex-
hibited about 2 nm or 10 nm bathochromic-shift compared with that of
PTBTB. The red-shifted ICT peak and absorption onset of PTBTB in thin
film are 523 nm and 650 nm, respectively, corresponding to a band-gap
of 1.91 eV. So, the replacement of hydrogens with fluorine atoms onto
the BT units resulted in the narrow band-gap of PTBTB-F. The in-
troduced fluorine can enhance the planarity of polymer backbones via
F⋯S non-covalent attractive interaction, and also the additional C-H⋯F
81.2% and 85.5% of the electroactivity still retained after 1000 cycles
in the glove box, implying promising potential for electrochromic ap-
plications.
3.2. Optical properties
The UV-Vis absorption spectra of PTBTB and PTBTB-F are shown in
and C-F···π interchain interactions, thus improving closer π-π polymer
F
Fig. 4 for comparison, both in chloroform solutions and in thin films.
Both polymers showed two similar characteristic absorption peaks due
to similar polymer main chains, and the two distinctly separated
characteristic peaks were resulted from π-π* transition and the in-
tramolecular charge transfer, respectively. In chloroform solution, as
shown in Fig. 4a, PTBTB was shown in red with main absorption peaks
at 326 nm and 489 nm. The absorption spectra of PTBTB in thin film
chain stacking and the conjugate effect [19].
In order to further study the molecular structure and optoelectronic
properties of the polymers in essence, the optimized geometries, the
electron cloud distribution and the theoretical HOMO and LOMO en-
ergy levels of a single repetition structure unit were calculated by using
the frontier molecular orbital theory. From the molecular orbital profile
in Fig. 5, it can be seen that PTBTB and PTBTB-F showed similar
electron cloud configuration of the HOMO and LUMO, respectively. The
electron cloud of the HOMO orbital delocalized on the adjacent con-
jugate aromatic skeleton along the polymer backbone; However, the
LUMO wave functions were mainly localized on the BT acceptor moi-
eties, indicating the efficient charge transfer from donor to acceptor
[27,28]. The HOMO and LUMO energy levels of PTBTB and PTBTB-F
were determined by using Gaussian 05 software according to DFT,
which were listed in Table 1. The HOMO and LUMO energy levels of
PTBTB were calculated to be −5.23 and −3.00 eV, respectively,
(
Fig. 4b), however, showed significant red-shift peaks compared with
those in solution. The red-shift and the broad absorption spectra in the
film state, might be assigned to the agglomerate of polymer molecules
and enhanced π-π* stacking effect, resulting in an increase in interchain
interaction [26]. The main peaks of PTBTB film are 339 nm and
5
17 nm, respectively, with the optical band gap of 2.02 eV, which was
calculated based on absorption onset. In both chloroform and thin film,
the absorption spectra of PTBTB-F showed red-shift peaks compared to
those of PTBTB. The π-π* transition absorption peaks of PTBTB-F were
−
1
6
Fig. 3. CV curves of the PTBTB-F film at different scan rates between 100 and 300 mV s in 0.2 M TBAPF /ACN solution, respectively.(a) during the p-type doping
process; (b) during the n-type doping process.
4

Z. Xu et al.
Organic Electronics 61 (2018) 1–9
Fig. 4. UV-vis spectra of the polymers in solution (a) and in films (b).
Fig. 5. Optimized geometries and orbit contours of the HOMO and LUMO for
the polymers.
Fig. 6. Fluorescence photos and fluorescence emission spectra of the polymers
dissolved in CHCl3.
corresponding to a theoretical band-gap of 2.23 eV. The HOMO energy
level of PTBTB-F was −5.37 eV, which is 0.14 eV deeper than that of
PTBTB. Similarly, 0.27 eV decrease in LUMO energy level was found
due to the introduction of two electronegative F atoms onto electron-
withdrawing BT group. So, the stronger effect of fluorine on the LUMO
energy level leaded to the larger decrease in LUMO value than that of
the HOMO, thus resulting in a lower band-gap (2.10 eV) of PTBTB-F,
which is corresponding to the spectra data. Besides, the HOMO energy
levels of two polymers are below −4.90 eV, and at this level the elec-
trochemical reactions with wet oxygen could be effectively restrained
under exciting light of 254 nm, respectively, with maximum photo-
luminescence peaks at 613 nm and 600 nm. The maximum fluorescence
emission wavelength of the polymer in solution showed obvious Stokes
red-shift compared with the maximum absorption wavelength, in-
dicating that the geometrical structure of the polymer changed sig-
nificantly from the ground state to fluorescence excited state [32].
Besides, the excitation and emission peaks were located in the visible
region. All these illustrate that EC polymers PTBTB and PTBTB-F with
favorable fluorescence effect, may hold great potential for electro-
chemical fluorescence applications such as fluorescence sensors [33].
The fluorescence quantum yields (Φ) of the two polymers were
measured and calculated based on the well-established method as fol-
lows [34]:
[
29]. As for the larger calculation band-gaps than optical results, dif-
ferent measuring forms could explain this [30,31].
The solutions of PTBTB and PTBTB-F exhibited novel fluorescence
characteristics, which can be proved by the fluorescence photos and
fluorescence emission spectra in Fig. 6. The solutions of PTBTB and
PTBTB-F emitted vivid orange-red and bright orange fluorescent light
Table 1
Optical and electrochemical data of the polymers.
Polymer
λ
max (solution)
λ
max (film)
λ
onset (film)
E
g,op
E
ox, onset
HOMO
(eV)b
LUMO
(eV)c
HOMO
(eV)d
LUMO
(eV)d
(nm)
(nm)
(nm)
(eV)a
(V)
PTBTB
PTBTB-F
489
493
517
523
614
650
2.02
1.91
1.05
1.29
−5.47
−5.71
−3.45
−3.80
−5.23
−5.37
−3.00
−3.27
a
E
g,op (eV) = 1240/λonset
HOMO = –(Eox, onset + 4.4) (Eox, onset vs. SCE).
LUMO = HOMO + Eg,op
.
b
c
.
d
Calculated by DFT calculations.
5

Z. Xu et al.
Organic Electronics 61 (2018) 1–9
Fig. 7. Step profiler images of (a) PTBTB and (b) PTBTB-F spray coated on ITO/glass. inset: SEM images of the corresponding polymers films.
6
Fig. 8. The spectroelectrochemistry curves of PTBTB (a) and PTBTB-F (b) films on ITO/glass electrode in 0.2 M TBAPF /ACN at different applied potentials.
Fig. 9. Relative luminance as a function of increasing applied potential for PTBTB (a) and PTBTB-F (b) film with three different thickness.
2
F
F
r
x
A
r
n
n
x
r
r
(standard, Φ = 0.38) was used as a reference. The Φ values of the
polymers were calculated to be 0.18 and 0.21, respectively, for PTBTB
and PTBTB-F.
⎛
⎜
⎞⎛ ⎞⎛
⎟⎜ ⎟⎜
⎞
Φ
x
= Φ
r
⎟
A
x
(1)
⎝
⎠⎝ ⎠⎝
⎠
In the equation, Φ
x
and Φ
r
are the Φ values of the polymer samples
and the reference sample, respectively. The values of n, A, and F denote
the refractive index of the solvent, the optical density at the excitation
wavelength and the measured integrated emission intensity, respec-
tively. The subscript “r” denotes the reference, and “x” denotes the
sample. The absorbances of the polymer solutions and the reference
sample should be comparable. For the measurements, the Φ value of 4-
methlamino-7-nitro-1,2,3-benzoxadiazole (NBD) in ACN solution
3.3. Morphology
Research indicates that the surface morphology, roughness and
thickness exert important influence on the electrochemical, optical and
even electrochromic performance, which could be measured by SEM
and Step Profiler. The SEM images and the thickness curves of the
polymer films that spray coated on ITO/glass, are presented in Fig. 7.
6

Z. Xu et al.
Organic Electronics 61 (2018) 1–9
Fig. 10. Electrochromic switching of the polymer films with different time intervals: (a) PTBTB film measured at 518 nm; (b) PTBTB film measured at 815 nm; (c)
PTBTB film measured at 1600 nm.
The illustrations we see in Fig. 7 show that both the films are comprised
of a large number of nano-scale fragments which are spreaded uni-
formly over the glass surface, but occasionally some hollows distributed
at the surface in the film. The saw-toothed undulating film thickness
peaks given by the thickness gauge also implied the irregular and dis-
continuous surfaces of the polymer films. This membrane structure may
increase the surface area of the polymer, and facilitate the doping and
dedoping of the supporting electrolyte under electric potential, which
can explain for the good electrochromic properties of the polymer films
to some extent. Besides, the average thickness was calculated to be
Further increasement of voltage turned the red film to a light blue, by
weakening the π-π* transition bands. By applying the voltage up to
1.50 V, the original absorption completely disappeared, and new broad
absorption bands were observed at around 815 nm and 1700 nm owing
to polaron and bipolaron charge carriers, respectively [35]. Upon ap-
plied potential, PTBTB film revealed a light cyan color at the fully
oxidized states.
Similarly, the pristine PTBTB-F film was light red with absorption at
345 nm and 523 nm in the neutral state. Unlike the PTBTB film, the
electrochemical oxidation leaded to the decrease in π-π* transition, but
also the emergence of new absorption peak at 476 nm, with an im-
mediate color change to light blue. When the voltage was increased to
284 nm for PTBTB and 353 nm for PTBTB-F, respectively.
1
.57 V, the polymer was completely oxidized. Full oxidation turned the
3
.4. Spectroelectrochemistry
intermediate blue state to the final light gray state, giving rise to a new
polaron band with λmax = 810 nm and a new broad bipolaron band in
the near infrared region along with the disappearance of the band at
The spectroelectrochemical properties of the polymer films were
obtained by in-situ UV scanning, and through continuously varying the
applied external voltage to obtain the changes in the color and spectral
absorption curves of the polymer film. The results were shown in Fig. 8.
PTBTB and PTBTB-F films both have two distinct characteristic ab-
sorption peaks at the neutral state, however, due to the steric hindrance
effect of the long alkyl side chain, the π-π* transition and the in-
tramolecular charge transfer were hindered, and the two absorption
peaks were located in the ultraviolet region and the short-wave visible
region respectively. The maximum absorption peak was observed at
λ
max = 523 nm.
To accurately quantify and compare the colors, each of the ab-
sorption spectra at different applied potentials corresponding to spec-
troelectrochemistry tests was converted to CIELAB L*a*b* coordinate.
In CIE coordinates, the L* denotes the lightness or darkness from white
to black, and the a*b* representation shows us the hue and saturation of
a given color where a* is concerned with the red-green balance (+a*
represents red) and b* the yellow-blue balance (+b* represents
yellow). Fig. 9 showed the changes in L*a*b* values for two polymers
with three different thickness between the neutral and oxidized state.
No variation in brightness of the PTBTB polymer film could be found in
the beginning. Until the potential was increased to 1.2 V, the brightness
517 nm with a 339 nm shoulder, resulted in a red color neutral state for
PTBTB. Then a voltage can be applied to thin polymer film, when in-
creased to 1.03 V, significant changes in the absorption spectrum curves
could be observed, indicating that the polymer began to be oxidized.
7

Z. Xu et al.
Organic Electronics 61 (2018) 1–9
Fig. 11. Electrochromic switching of the polymer films with different time intervals: (a) PTBTB-F film measured at 520 nm; (b) PTBTB-F film measured at 810 nm; (c)
PTBTB-F film measured at 1600 nm.
Table 2
film thickness, which is valuable for practical applications, such as
The optical contrasts (ΔT%), response times and coloration efficiencies (CE) of
PTBTB and PTBTB-F at maximum absorption wavelengths with the time in-
terval of 5 s.
high-resolution displays and smart windows.
3.5. Electrochromic contrast and switching speed
Polymer
λ(nm) Optical contrast
ΔT%)
Response time
(s)
Response time
(s)
a
b
(
The optical contrast and the response time of the polymers are two
very important parameters to decide whether they can be applied in
electrochromic devices [36]. Fig. 10 and Fig. 11 showed the changes in
transmittance by switching between the neutral and oxidized states at
the given wavelengths with the absorption maximum (λmax). Besides,
double potential step chronoamperometry was carried out by con-
tinuously increasing the switching rate, i.e., with different time inter-
vals from 10 to 5, 3, 2 s and finally to 1 s. As for PTBTB (Fig. 10), when
switched between the potentials of 0 V and 1.50 V, the optical contrasts
were recorded as 25% at 518 nm, 28% at 815 nm and 45% at 1600 nm
for a frequently-used interval of 5 s. Then for PTBTB-F (Fig. 11), as the
polymer was switched between 0 V and 1.57 V, the contrasts were 17%
at 520 nm, 28% at 810 nm and 65% at 1600 nm for a 5 s hold. But, we
still believe that further improvement in switching ability should be
favored, because of the significant decrease in electrochromic contrast
with increasing the switching rate. By comparison, the recently re-
ported solution-processable polymers in our group [22], showed similar
optical contrasts to the films in this article in the visible region, but
without high transmittance changes offered in near infrared region. The
high optical contrasts of PTBTB and PTBTB-F films in NIR region, as an
important optical property, which suggested the promising potential for
various NIR applications [37]. Response time, which need for 95% of
full optical switching, is listed in Table 2.
PTBTB
518
25
28
45
17
28
65
4.3
1.4
2.2
4.4
1.0
3.4
1.5
2.1
3.1
1.1
1.9
0.4
8
15
1
600
PTBTB-F 520
8
10
1
600
a
Time to reach 95% of full optical contrast from neutralized to fully oxidized
states.
b
Time to reach 95% of full optical contrast from fully oxidized to neutralized
states.
decreased to some extent but has not been a significant change, but
then displayed the trend of rapid growth. Apart from these, the a* and
b* values varied greatly from neutral to oxidized state, especially
parameter a*. For example, the a* value of PTBTB with absorbance
∼
0.8 a. u dropped from 43.9 to −5.0, and simultaneously combined
with the certain alteration of b*, indicating the well-reasoned color
changing from red to light cyan. Colorimetry analysis of PTBTB-F film
was analogous to the previous one, but the lightness change curves
were relatively smooth and regular. Besides, as can be seen from Fig. 9,
the increase in thickness of the film is expected to result in lower L*
value. Therefore, the brightness can be manipulated by changing the
8

Z. Xu et al.
Organic Electronics 61 (2018) 1–9
4. Conclusion
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