High performance black-to-transmissive electrochromic device with panchromatic absorption based on TiO2-supported viologen and triphenylamine derivatives

Article abstract of DOI:10.1016/j.orgel.2016.04.028

A novel black-to-transmissive electrochromic device based on TiO₂-supported viologen and triphenylamine derivatives was designed and constructed via the absorption-complementary approach. In the device, cathodically coloring electrochromic mate

Full text of DOI:10.1016/j.orgel.2016.04.028

Organic Electronics 34 (2016) 139e145
Contents lists available at ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
High performance black-to-transmissive electrochromic device with
panchromatic absorption based on TiO2-supported viologen and
triphenylamine derivatives
Duo Weng, Yuchen Shi, Jianming Zheng*, Chunye Xu*
Hefei National Laboratory for Physical Sciences at Microscale, CAS Key Laboratory of Soft Matter Chemistry, Department of Polymer Science and
Engineering, University of Science and Technology of China, Hefei 230026, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel black-to-transmissive electrochromic device based on TiO2-supported viologen and triphenyl-
amine derivatives was designed and constructed via the absorption-complementary approach. In the
device, cathodically coloring electrochromic material 1,4-bis[((N-phosphono-2-ethyl)-4,40-bipyr-
idinium)-methyl]-benzene tetrachloride acted as working electrode and novel anodically coloring elec-
trochromic material (4-((4-(dimethylamino)-phenyl)(4-methoxyphenyl)-amino)-benzyl) phosphonic
acid acted as counter electrode. The assembled electrochromic device achieved panchromatic absorption
Received 21 February 2016
Received in revised form
16 April 2016
Accepted 16 April 2016
Available online 22 April 2016
over entire visible spectrum with almost zero transmittance in colored state. The optical contrast (DT) of
the device realized in this work was comparable to the highest value (60%) among all reported black-to-
transmissive ECDs. Furthermore, excellent cycling stability was achieved, which maintained almost 80%
Keywords:
Black-to-transmissive
Electrochromic device
Viologen
of the initial DT value at 570 nm after continuous 100,000 switchings. These outstanding comprehensive
electrochromic performances potentially make this device a promising candidate for electrochromic
device applications.
Triphenylamine
Stability
Chemisorption
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
three primary colors (RGB) [11e13], purple [14] and magenta [15]
have been investigated so far, achievement of black-to-
Electrochromism, which refers to a reversible optical absor-
bance/transmittance change in response to an externally applied
potential, has been developing over several decades. In the elec-
trochromic system, electrochemical redox reactions result in a
reversible variation of transmitted/reflected light and color change
of electroactive materials [1e3]. Past few years have witnessed
great advances and gradual applications of electrochromic mate-
rials especially for electrochromic devices (ECDs), such as auto-
matic anti-glazing mirror [4], sunglasses [5] and switchable
displays [6,7], as well as e-paper [8] and “smart windows” [8e10].
Electrochromic devices (ECDs) with efficient controllability and
excellent stability are particularly desirable for practical applica-
tions. Color change is of great significance among the properties of
ECDs which can be used in many fields. Although much study about
different colored-to-transmissive electrochromic devices such as
transmissive switchable electrochromic devices still remains a
challenging task. The reason for that is partly due to the complexity
for rendering materials absorbing efficiently over visible spectrum
(400e750 nm) in colored state, and fully bleaching out in the same
region in bleached state. To date, two primary strategies can be
used to achieve this goal. One is to synthesize an individual poly-
mer or copolymer, which serves as a single working electrode
absorbing across the whole visible region. Typical example by this
method would be a chemical copolymer with substituted donor-
acceptor system developed by Reynolds group [16,17]. All re-
ported polymers exhibit good black-to-transmissive switching with
high optical contrast, yet the stability of the fabricated ECD is
insufficient. The other strategy is based on “color-mixing” theory
bearing the blends or multilayers to achieve complementary ab-
sorption.
A
reported device combines four absorption-
complementary electrochromic polymers (red, green, blue, and
yellow), and is capable of generating dark color [18]. However,
relatively low optical contrast of 25% poses a big obstacle to its
practical application. These two strategies mentioned above are
both based on electrochromic polymers, while the stability of ECD
* Corresponding authors.
E-mail addresses: jmz@ustc.edu.cn (J. Zheng), chunye@ustc.edu.cn (C. Xu).
http://dx.doi.org/10.1016/j.orgel.2016.04.028
1566-1199/© 2016 Elsevier B.V. All rights reserved.

140
D. Weng et al. / Organic Electronics 34 (2016) 139e145
is insufficient or optical contrast is not satisfactory.
round-bottomed flask and heated under stirring at 50 ꢂC until the
solid precipitate was formed. The precipitate was filtered. The
filtrate was heated again and the process was repeated until no
more solid formed. The as-obtained precipitate was washed with
ether and dried under vacuum to pale yellow product (3 g, 58%).
Then, 1,4-bis[((N-diethylphosphono-2-ethyl)-4,40-bipyridinium)-
methyl]-benzene tetrabromide (2) was prepared by mixing the
above product (2 g, 4.99 mmol) and 1,4-bis(bromomethyl) benzene
(0.657 g, 2.49 mmol) in acetonitrile (50 ml) in a 100 ml round
bottomed flask and refluxed under stirring for 24 h. The resulting
precipitate was filtered and washed with ether and hot acetonitrile,
then dried under vacuum to yield 2.2 g of yellow product. Finally,
1,4-bis[((N-Phosphono-2-ethyl)-4,40-bipyridinium)-methyl]-ben-
zene tetrachloride (3) was obtained by adding the above product
(2 g, 1.88 mmol) to hydrochloric acid solution (40 ml) and allowed
to reflux for 24 h under stirring. The solvent was removed under
vacuum and the compound was crystallized with ethanol, filtered
and dried in the vacuum to yield 1.7 g of pale yellow product. The
main synthesis process was shown in Scheme 1.
In this work, we designed a black-to-transmissive ECD based on
viologen and triphenylamine derivatives with excellent optical
performance and outstanding stability. Our group has been devoted
ourselves in EC field for a long time [19,20]. It is found that viologen
and triphenylamine are two commonly used small organic mole-
cules with distinct electrochromic performances. Viologen is one
type of electrochromic cathodic materials with reversible optical
color change between dication state (V2þ, bleached) and radical
cation state (Vþꢀ, colored). The color of viologen can be controlled
through modifying substituent group [21e25]. Comparing with
viologen, triphenylamine as anodic electrochromic material, can be
easily oxidized to form stable polarons attributed to electron-rich
characteristics, while resulting in a noticeable change in color
[26e28]. Based on the “color mixing” theory, it claims that if two
color stimuli are mixed, the resulting color will lie somewhere
along a straight line connecting two points on the chromaticity
diagram [29]. Herein, we utilized the coloring mixing strategy and
selected blue and yellow-green as complementary colors. Specif-
ically, we synthesized two color-complementary viologen and tri-
phenylamine derivatives, 1,4-bis[((N-phosphono-2-ethyl)-4,40-
bipyridinium)-methyl]-benzene tetrachloride (PBT) and a new
material (4-((4-(dimethylamino)-phenyl) (4-methoxyphenyl)-
amino)-benzyl) phosphonic acid (DBP). They exhibited blue and
yellow-green respectively and the absorption of PBT and DBP might
complement in visible region. Then the black-to-transmissive
switching electrochromic device was produced by the combina-
tion of PBT and DBP materials.
1: 1H NMR (300 MHz, D2O, ppm):
d 1.09e1.13 (t, 6H), 2.62e2.73
(t, 2H), 3.95e4.05 (q, 4H), 4.81e4.91 (t, 2H), 7.82e7.84 (d, 2H),
8.35e8.37 (d, 2H), 8.66e8.68 (d, 2H), 8.92e8.94 (d, 2H).
2: 1H NMR (300 MHz, D2O, ppm):
d 1.16e1.21 (t, 12H), 2.70e2.81
(t, 4H), 4.02e4.11 (q, 8H), 5.25e5.38 (t, 4H), 6.25 (s, 4H), 7.48 (s, 4H),
7.72e7.81 (d, 2H), 8.25e8.32 (d, 2H), 8.76e8.83 (d, 2H), 9.62e9.70
(d, 2H).
3: 1H NMR (300 MHz, D2O, ppm):
d 2.30e2.41 (t, 4H), 4.77e4.86
(t, 4H), 5.87 (s, 4H), 7.51 (s, 4H), 8.41e8.45 (m, 8H), 9.04e9.06 (m,
The absorption, optical, redox behaviors and electrochemical
properties of PBT and CBP electrodes were characterized and dis-
cussed below. An electrochromic device (ECD) with these two
materials was fabricated, and its electrochromic performance and
cycle stability were investigated as well.
8H).
2.2.2. Preparation of (4-((4-(dimethylamino)-phenyl) (4-
methoxyphenyl)-amino)-benzyl) phosphonic acid (DBP)
The synthesis route of a new compound DBP is described as
follows. In the first step, 4-bromo-N, N-dimethylphenylamine
(3.0 g, 15 mmol), p-anisidine (2.02 g, 16.5 mmol), sodium tert-
butoxide (2.016 g, 20.97 mmol), tris(dibenzylideneacetone)dipal-
ladium (0.27 g, 0.299 mmol) and tri-tert-butyphosphine (0.12 g,
0.59 mmol) were added to a three-neck flask, followed by 60 ml dry
toluene. The mixture was heated at 90 ꢂC with stirring for 24 h in
argon atmosphere. Then the solvent was removed and the crude
product was dissolved by chloroform. The mixture was washed
with sodium chloride solution and the organic phase was dried by
MgSO4. Finally, 1.5 g of orange oily liquid (4) was obtained by col-
umn chromatograghy. (Yield: 47%)
In the second step, compound 4 (1.5 g, 6.19 mmol), 4-
bromobenzaldehyde (1.65 g, 8.97 mmol), tris(dibenzylideneace-
tone)dipalladium (0.11 g, 0.12 mmol), tri-tert-butylphosphine
(0.054 g, 0.25 mmol) and cesium carbonate (2.7 g, 8.30 mmol) were
mixed to dry toluene (50 mL) and heated under agitation at 90 ꢂC
for 24 h in argon atmosphere. Then the solvent was removed and
the product was dissolved by chloroform. After filtering the
mixture, the filtrate was washed with saturated sodium chloride
solution and the organic phase was dried by MgSO4. Finally, 1.8 g of
orange-yellow oily liquid (5) was obtained by column chromatog-
raghy. (Yield: 72%)
2. Experiment
2.1. Materials and instrumentation
All chemicals used in this work were commercial products and
used as received without further purification. The material for the
electrolyte, propylene carbonate (PC), ethylene carbonate (EC) and
lithium hexafluorophosphate (LiPF6) were purchased from Sigma
Aldrich Chemical. Toluene was dried by refluxing with CaH2.
NMR spectra were measured by Bruker Avance AV400. Infrared
(IR) spectra were recorded on a Nicolet 8700 Fourier-transform
infrared (FT-IR) spectrometer. Ultraviolet-visible NIR (UV-vis-NIR)
spectra were obtained by JASCO V-670 spectrophotometer. Cyclic
voltammograms were collected using CHI 660D electrochemical
analyzer with a three-electrode cell using a film (2 cm ꢁ 2 cm) on
fluorine-doped indium oxide glasses (FTO) as working electrode, a
silver wire as reference electrode and a platinum wire as counter
electrode. Spectroelectrochemical analysis was performed by the
combination of CHI 660D electrochemical analyzer and UV-vis-NIR
spectrophotometer. Photographs of the films and devices were
taken with a Canon (IXUS 125 HS) digital camera.
In the third step, compound 5 (1.8 g, 5.20 mmol) and sodium
borohydride (0.26 g, 7.26 mmol) were dissolved in 50 mL THF in
argon atmosphere and refluxed at 70 ꢂC for 24 h. Then the mixture
was extracted by diethyl ether and the collected organic phase was
dried by MgSO4. Next, the compound 6 was obtained by evapo-
rating the solvent. (Yield: 86%)
In the fourth step, 1.6 g of compound 6, 5 mL triethyl phosphate,
and 0.8 g ZnBr2 were added into a flask and stirred at room tem-
perature under argon atmosphere. After 24 h, HCl/CH3OH was
added into the mixture and stirred for 10 h. The solution was
2.2. Materials synthesis
2.2.1. Preparation of 1,4-bis[((N-phosphono-2-ethyl)-4,40-
bipyridinium)-methyl]-benzene tetrachloride (PBT)
The synthesis route of compound PBT was performed following
sequential process reported by Taya, M. group [25]. First, 1-diethyl
ethylphosphonate-4,40-bipyridinium bromide (1) was prepared by
mixing 4,40-bipyridine (2 g, 12.8 mmol) and diethyl bromoethyl
phosphonate (3.14 g, 12.8 mmol) in acetone (20 ml) in a 100 ml

D. Weng et al. / Organic Electronics 34 (2016) 139e145
141
Scheme 1. Synthesis route of PBT and DBP.
filtered and washed by H2O and CH2Cl2. We obtained 0.5 g almost
2.3. Fabrication of ECD
2.3.1. Electrodes
colorless viscous liquid (8). (Yield: 37%) The main synthesis process
was shown in Scheme 1. The 1H NMR and FT-IR characterizations of
PBT and DBP were detailed in supporting information (see Fig. S1-
S10).
TiO2 paste was spread onto fluorine-doped tin oxide glass (FTO,
/,, Nippon Sheet Glass, Japan) with doctor-blade method and
then baked at 450 ꢂC for 30 min to obtain nanoparticle TiO2 films,
with thickness of 1 m and transmittance above 80% on light
14
U
4: 1H NMR (300 MHz, CDCl3, ppm):
d
2.81e2.83 (s, 6H),
3.50e3.55 (d, 3H), 4.30e4.35 (s, 1H), 6.59e6.92 (m, 8H).
5: 1H NMR (300 MHz, CDCl3, ppm):
2.94 (s, 6H), 3.74 (s, 3H),
m
d
wavelength of 400e800 nm TiO2 Films were immersed in either
PBT/H2O (0.03 M) or DBP/CH2Cl2 (0.03 M) solution for 24 h for
chemisorption. After removing the solution, the prepared PBT/
TiO2 and DBP/TiO2 electrodes were all stored in a glovebox before
use. The SEM characterizations of electrodes were provided in
Fig. S11.
6.62e6.65 (d, 2H), 6.74e6.77 (d, 2H), 6.80e6.83 (d, 2H), 6.90e7.02
(d, 2H), 7.05e7.09 (d, 2H), 7.52e7.55 (d, 2H), 9.663 (s, 1H).
6: 1H NMR (300 MHz, CDCl3, ppm):
d 2.85 (s, 6H), 3.71 (s, 3H),
4.50 (s, 2H), 6.60e6.63 (d, 2H), 6.70e6.75 (d, 2H), 6.82e6.85 (d, 2H),
6.93e7.02 (m, 4H), 7.06e7.09 (d, 2H).
7: 1H NMR (300 MHz, CDCl3, ppm):
d 1.09e1.15 (t, 6H), 2.83 (s,
2H), 3.36e3.43 (m, 6H), 3.68 (s, 3H), 3.61e3.65 (m, 4H), 6.57e6.63
(d, 2H), 6.80e6.85 (d, 2H), 6.90e6.97 (m, 4H), 7.04e7.07 (d, 2H).
2.3.2. Electrolyte
8: 1H NMR (300 MHz, CDCl3, ppm):
d
2.78e2.89 (s, 6H),
PC/EC/LiPF6 solution was chosen as electrolyte. The concentra-
tion of LiPF6 was 0.1 M, and the volume ratio of PC: EC was 1:1. The
electrolyte was bubbled with argon for 4 h and stored in a glovebox.
3.71e3.75 (s, 3H), 3.94e4.07 (d, 2H), 6.71e6.83 (m, 4H), 6.91e7.00
(m, 6H), 7.15e7.19 (m, 2H).

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D. Weng et al. / Organic Electronics 34 (2016) 139e145
2.3.3. Assembly
Fig.1 illustrates an exploded schematic of the ECD design, which
is assembled as follows. The cell was fabricated in a glove box under
argon atmosphere in order to avoid any moisture and oxygen
contamination. A parafilm (100 mm thickness) was cast onto the
DBP electrode and pressed together with the PET electrode. The
parafilm was employed as a spacer for sealing the device tightly and
keeping 100
mm thickness of the electrolyte. After gelation, the
edges of the device were sealed by UV-curable sealant.
3. Results and discussion
3.1. Visible absorption spectra of electrodes
Absorbance spectral analysis was performed to investigate
detailed absorption properties of PBT and DBP electrodes in visible
region. As illustrated in Fig. 2, the absorption of cathodically col-
oring blue PBT material effectively covered the range of
500e650 nm and the absorption peak was distributed on 556 nm.
As for anodically coloring yellow green DBP material on the
opposite electrode, it revealed two absorption peaks at 456 nm and
715 nm respectively, which maximally compensated absorption
regions less than 500 nm and greater than 650 nm. Thereby, the
total absorption spectra of PBT and DBP electrodes nearly covered
the entire range of visible spectrum, which indicated that the
combination of PBT and DBP inside an ECD could achieve the goal
with broad and effective absorption in visible region.
Fig. 2. Visible absorption spectra of PBT and DBP electrodes of their colored states in
PC/EC/LiPF6 (0.1 M).
and match well to fabricate an ECD.
Furthermore, detail optical properties of PBT and DBP electrodes
were characterized by spectroelectrochemical experiments when
0.3 V and 1.0 V had been applied. Fig. 3(c) and (d) present the
transmittance curves of PBT and DBP electrodes in different state,
respectively. As shown, in their transparent states, PBT electrode
(ꢃ0.3 V) exhibited high transmittance of 85% from 500 to 750 nm
and DBP electrode (0.3 V) displayed satisfactory transmittance of
65% from 560 to 750 nm as well. In their dark states, the PBT
(ꢃ1.0 V) and DBP (1.0 V) electrodes both demonstrated effective
absorption in visible region with relatively low transmittance (deep
color), with the minimums of T% below 10%. Here, we defined
3.2. Electrochemical properties of electrodes
The electrochemical properties of PET and DBP electrodes were
investigated with cyclic voltammetry (CV). As displayed in Fig. 3(a)
and (b), the PBT electrode was reduced to blue state when the
voltage reached ꢃ1.0 V, and was oxidized to transparent state as the
potential increased to ꢃ0.3 V. Conversely, the DBP counter elec-
trode was oxidized to yellow green state when the voltage reached
1.0 V, and was reduced back to transparent at the voltage of 0.3 V.
Besides, charge-matching is essential between working electrode
and counter electrode for a complementary ECD. The two elec-
trodes PBT and DBP demonstrated the matching charge capacity,
which can be justified by integrating the current density in the CV
curves. Thus, the CV curves exhibited that the similar redox po-
tential value with opposite polarity and matching charge capacity
of two electrodes, representing they could complement each other
transmittance change (DT %) of EC device:
D
%T ¼ Ttð
l
Þ ꢃ Tdð
Þ
l
Tt
wavelength
calculated in Fig. 3(c) and (d), the
(l
) and Td
(
l
) represent the light transmittance of certain
in the transparent and dark states, respectively. As
T% of PBT electrode located at
l
D
570 nm reached up to 75% and the DBP electrode attained 60% at
the wavelength of 570 nm. These high values further indicated that
the fabricated ECD with PBT and DBP electrodes could achieve high
optical contrast and effective absorption in visible region with deep
transmittance in colored state.
3.3. Performance of ECD
3.3.1. Electrochemical properties of ECD
To investigate the electrochemical properties and appropriate
operation parameters of ECD, an electrochemical analysis with
cyclic voltammetry (CV) was performed. As shown in Fig. 4. The
device changed to its colored state during the cathodic scan. When
the voltage reached ꢃ1.5 V, the device was fully colored and
exhibited deep black color. As the opposite potential increasing, the
device began to bleach and oxidized to its transparent state at the
voltage of 0.5 V. Consequently, the fabricated ECD could be dark-
ened by applying a potential of ꢃ1.5 V on the working electrode of
PBT and be bleached at a potential of 0.5 V, which is an appropriate
potential applied to the assembled ECD.
3.3.2. Electrochromic properties of ECD
The prepared PBT and DBP electrode materials were employed
to fabricate an ECD. Fig. 5(a) and (b) represent the photos of ECD in
its transparent and dark states. Applied with - 1.5 V, the ECD
transformed to dark state resulting a deep black color, with PBT and
Fig. 1. (a) Chemical structure of PBT and DBP, and (b) schematic of device based on PBT
and DBP electrodes.

D. Weng et al. / Organic Electronics 34 (2016) 139e145
143
Fig. 3. Cyclic voltammograms of (a) PBT and (b) DBP electrode with PC/EC/LiPF6 (0.1 M) as supporting electrolyte. (Scan rate ¼ 50 mV sꢃ1). Transmittance spectra of (c) PBT electrode
and (d) DBP electrode in transparent and dark states at different voltage.
Fig. 4. Cyclic voltammograms of ECD with PC/EC/LiPF6 (0.1 M) as electrolyte. (Scan
rate ¼ 50 mV sꢃ1). Insert: the photos of ECD in transparent and dark state.
CBP materials in their reduced state (deep blue) and oxidized state
(yellow green) respectively. With the opposite potential of 0.5 V
Fig. 5. Photos of ECD (2 cm ꢁ 2 cm) in (a) transparent state, (b) dark state, and (c) trans-
applying, the ECD became transparent immediately, accompanied
with the PBT and DBP materials returned to their oxidized states
and reduced state. Apparently, the character “USTC” under the ECD
became invisible completely when the ECD switched to its dark
state as exhibited in Fig. 5(b).
mittance spectra of ECD in extreme states of highly absorptive and highly transmissive.
measured as illustrated in Fig. 5(c). High transmittance of over 50%
from 500 to 800 nm could be observed in transparent state.
Conversely, the device demonstrated complete spectra absorption
approaching almost zero transmittance in dark state. The
The transmittance spectra of the fabricated ECD was also

144
D. Weng et al. / Organic Electronics 34 (2016) 139e145
closed to D55 illuminant (x ¼ 0.332, y ¼ 0.348, %YL ¼ 100%) [30],
which demonstrated that the ECD was capable to reach a highly
transmissive state. Moreover, the point of transparent state located
at light yellow region according to the chromaticity coordinates,
which corresponded to the photo of ECD. In dark state, the value of
x, y, and %YL (x ¼ 0.278, y ¼ 0.425, %YL ¼ 1.4%) all decreased, which
indicated the color of ECD had changed. Besides, the point of
colored state located in the mixed region of blue and yellow green,
because the ECD was composed by blue colored PET electrode and
yellow green colored DBP electrode. Furthermore, the value of %YL
decreased as low as 1.4%, which suggested the ECD could achieved a
deep black color. This provides further evidence that the color
change of the ECD is nearly from transparent to black.
3.3.3. Electrochromic switching properties
Electrochromic switching parameters of the ECD were investi-
gated by the combination of a double step chronoamperometry
technique and an UV-vis-NIR spectrophotometer. The results were
revealed in Fig. 7. We defined response time as the time needed by
ECD to achieve 90% of its transmittance changes. The two-step
potentials, the response time and the
DT were exhibited in Fig. 7.
As illustrated, the device could switch rapidly between its trans-
parent and dark states. As calculated in Fig. 7(b), the switching time
of the device (2 cm ꢁ 2 cm) was less than 4 s (3.0 s) during the
coloring process and 5 s (4.8 s) during the bleaching process. The
switching time was affected by the diffusion rate of Liþ dissolved in
electrolyte and the amount of electron, which was driven by the
applied voltage. The higher applied voltage, the shorter switching
time. In our experiment, the bleached voltage required for the ECD
was relatively lower than the colored voltage from the CV curve, so
we chose 0.5 V for bleaching and ꢃ1.5 V for coloring. Thus, the time
for bleaching process was relatively longer than the coloring pro-
cess. Overall, the quick speed of the response time was due to the
well matching of two electrodes. In addition, chemisorptions be-
tween phosphonic acid and TiO2 nanoparticles made electro-
chromic materials tightly adhesive to conductive substrates,
consequently leading electrons to transfer into the FTO glass-TiO2
nanoparticles-electrochromic materials quickly.
Fig. 6. Colorimetry (xey diagram) of ECD (1) in the bleached state (0 V) and (2) in the
colored states (- 1.5 V).
Table 1
Numerical chromaticity coordinates CIE (x, y and %YL) for ECD in transparent and
dark state.
ECD state
x
y
%YL
Transparent state
Dark state
0.384
0.278
0.461
0.425
56%
1.4%
transmittance curves confirmed that the ECD achieved a satisfac-
tory optical contrast of 60% located at 570 nm and made a black-to-
transmissive device a reality.
To scientifically evaluate the color change of ECD occurring on
electrochemical switching, the ECD was subjected to colorimetric
analysis by using CIE 1931 system of colorimetry. The color of ECD is
represented quantitatively using three attributes “x” (the red-blue
ratio), “y” (the green-blue ratio) and %YL (the relative or percentage
luminance). Fig. 6 depicts the CIE color space plots for the device
described in this research and the CIE 1931 numerical data (x, y, %
YL) was given in Table 1. When the device was in transparent state,
the chromaticity coordinates (x ¼ 0.384, y ¼ 0.461, %YL ¼ 56%) were
3.3.4. Stability properties
The long-term stability upon cycling played a key role on the
electrochromic performance of the devices. Therefore, the elec-
trochemical stability of device was investigated by switching be-
tween the two-step of ꢃ1.5 V (5 s) and 0.5 V (10 s) for 120 K cycles.
We recorded the light transmittances at 570 nm after 10 K, 30 K,
60 K, 90 K and 120 K cycles, respectively. As shown in Fig. 8, no
Fig. 7. Electrochromic switching and transmittance changes of ECD (ꢃ1.5 V to color, 0.5 V to bleach) at its D l max on FTO glass-TiO2 nanoparticles substrates in PC/EC/LiPF6 (0.1 M).

D. Weng et al. / Organic Electronics 34 (2016) 139e145
145
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In this paper, a novel black-to-transmissive switching electro-
chromic device based on viologen and triphenylamine derivatives
via the absorption-complementary approach was successfully
designed, fabricated and characterized by our group. The ECD was
assembled with viologen-modified TiO2 electrode and a new
triphenylamine-modified TiO2 electrode. The new designed anod-
ically material DBP realized the matching spectra absorption and
redox potential with cathodically material PBT. The combination of
PBT and DBP materials inside an ECD achieved panchromatic ab-
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satisfactory
DT% through the visible region with
D
T
of 60% at
€
max
€
€
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570 nm (Tb ¼ 60.1% and Td ¼ 0.1%) and excellent stability up to
120,000 cycles. These outstanding properties endow the assembled
ECD with great potential to incorporate into both window-type and
display-type electrochromic devices.
~
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~
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Acknowledgements
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to blue by increasing the potential, Sol. Energy Mater. Sol. Cells 93 (2009)
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We gratefully acknowledge financial supports of this work by
the National Natural Science Foundation of China (21274138,
21474096 and 21273207). The authors are also grateful to Dr.
Chihlong Chiang and Dr. Fengyu Su, former postdoctoral fellows for
their pioneering work on this research.
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Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.orgel.2016.04.028.
ꢀ
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