Optical and electrical properties of electrochromic devices depending on electrolyte concentrations and cell gaps

Article abstract of DOI:10.1002/bkcs.10980

The optical and electrical properties of electrochromic devices (ECDs) have been studied as a function of the electrolyte concentration and the cell gap. During tests, the electrolyte concentration was adjusted from 0.01 to 1 M LiClO₄, and the cell gaps between the positive and negative electrodes were set at 25, 60, and 100 μm. By varying the electrolyte concentrations and the cell gap, various parameters such as the transmittance, response time, charge consumption, coloration efficiency, and bistability of ECD were comparatively analyzed. Viologen (VO) was used as the reduction electrochromic material, and triarylamine (TAA) was used as the oxidation material.

Full text of DOI:10.1002/bkcs.10980

Article
DOI: 10.1002/bkcs.10980
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C. S. Ah et al.
KOREAN CHEMICAL SOCIETY
Optical and Electrical Properties of Electrochromic Devices Depending
on Electrolyte Concentrations and Cell Gaps
*
Chil Seong Ah, Juhee Song, Seong Mok Cho, Tae-Youb Kim, Hojun Ryu, Sanghoon Cheon,
Joo Yeon Kim, Chi-Sun Hwang, and Yong-Hae Kim
Smart I/O Platform Research Department, Electronics and Telecommunications Research Institute,
Daejeon 305-700, Republic of Korea. *E-mail: acs@etri.re.kr
Received April 30, 2016, Accepted September 5, 2016
The optical and electrical properties of electrochromic devices (ECDs) have been studied as a function of
the electrolyte concentration and the cell gap. During tests, the electrolyte concentration was adjusted from
0.01 to 1 M LiClO4, and the cell gaps between the positive and negative electrodes were set at 25, 60,
and 100 μm. By varying the electrolyte concentrations and the cell gap, various parameters such as the
transmittance, response time, charge consumption, coloration efficiency, and bistability of ECD were com-
paratively analyzed. Viologen (VO) was used as the reduction electrochromic material, and triarylamine
(TAA) was used as the oxidation material.
Keywords: Electrochromic, Transmittance, Response time, Charge consumption, Coloration efficiency,
Bistability
Introduction
when it is mixed in glass, smart window technology devel-
opment is under way that can enable selective adjustment of
the transmittance in the range of visible and infrared light.10
A sunglass technology is being developed to make available
selective adjustment of transmittance in the range of visible
light; this material will utilize diverse forms of organic poly-
mers.11 In addition, a reflective information display electro-
chromic technology is also being developed that can be
achieved through stacking up of three colors such as cyan,
magenta, and yellow.12 Furthermore, the following technolo-
gies are becoming available: a mirror technology that will
enable functions of both transmission and reflection by
means of reversible deposition and stripping of Ag+ ion on
electrodes13; an information display device manufactured
with a roll-to-roll process using polymers mixed with an
electrochromic material14; and an electrochromic iris for a
camera lens that can control its optical transmission.15 How-
ever, in spite of the variety of applied research, there have
been very few analysis results for the basic parameters of
ECDs. In this paper, depending on the electrolyte concentra-
tion and cell gap, all the following critical features were ana-
lyzed: transmittance, colored and decolored response time,
electrical charge consumed, coloration efficiency, and
bistability.
Electrochromism is a phenomenon in which reversible
color changes such as coloration and decoloration occur
during electrochemical redox reactions in some materials.
Various types of materials are used, including cathodic
materials such as viologens (VOs) and WO3, which display
coloration through a reduction process by receiving elec-
trons. There are also anodic materials, such as phenothia-
zine and NiOx, which work through an oxidation process
by losing electrons.1,2 Unlike liquid crystal displays
(LCDs) or organic light-emitting diodes (OLEDs), which
are emissive type of devices, an electrochromic device
(ECD) is a nonemissive information display device that
provides high visibility even in outdoor settings and has an
excellent contrast ratio under strong lighting conditions,
such as in sunlight.3 A wide range of benefits of ECDs
have been found, such as easy control of light transmittance
using a driving voltage, low driving voltage, bistability
under open-circuit status, and small dependence on the
viewing angle.1–6
ECDs have been the subject of intensive study all around
the world.1–6 In spite of their long history of research, there
are very few commercialized ECD products using any sort
of advanced level of this technology. Some products that
have been developed are an automotive electrochromic rear-
view mirror by Gentex,7 a crash helmet by Chromogenics,8
and a smart window by Sage Glass.9 Despite the overall lack
of developed products, electrochromic technology can still
provide a variety of potential applications, and therefore
remains a subject of substantial research. Taking advantage
of the nanocrystalline structure of an electrochromic material
Experimental
An ECD consists of a three-dimensional (3-D) nanostruc-
tured reduction electrochromic layer, an oxidation electro-
chromic layer, and an electrolyte layer (Figure 1(a)).
Figure 2(b) and (c) show the coloration and decoloration
images of a viologen–triarylamine (VO–TAA) full cell and
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(b)
(c)
(a)
glass
FTO
VO
decoloration (0.5 V)
cell
gap
Li +
Li+
Li +
Li +
Li +
V
TAA
coloration (-1.5 V)
VO-TAA full cell
FTO
glass
100
80
60
40
20
0
+
+
(VO)2(HO)OP
(TAA)
0.5 V
PO(OH)2
2 Cl-
-1.5 V
400
500
600
700
800
wavelength (nm)
Figure 1. (a) Overall structure of an ECD and the molecular structure of the utilized ECD. (b) Coloration and decoloration images of a
VO–TAA full-cell, and (c) the corresponding spectra of coloration at –1.5 V and decoloration at 0.5 V.
the corresponding spectra of coloration at –1.5 V and deco-
loration at 0.5 V. The coloration image at –1.5 V is black-
ish, and the measured transmittance has a low value. The
reduction and oxidation electrochromic materials were VO
and TAA, which were immobilized on 3-D TiO2 nanostruc-
tures. LiClO4 dispersed in propylene carbonate was the
electrolyte. An FTO (fluorine-doped SnO2, sheet resistance
of 15 Ω/cm2; TEC-15, Pilkington Co. (Tokyo, Japan)) sub-
strate was used as a conducting substrate, which was
washed with methanol, acetone, and water before use. The
phosphate-anchored VO (N,N0-4,4’-bis(2-phosphonylethyl)-
bipyridium dichloride) and TAA (3-[4-(diphenylamino)
phenyl]methylphosphonic acid) were synthesized in the
laboratory (Figure S1, Supplementary Information).1,5,16
The nanostructured reduction and oxidation electrochromic
layers were fabricated as follows: High-temperature TiO2
thin films were formed on FTO glass by the doctor-blade
method. High-temperature TiO2 paste was purchased from
ENB Co. (Gumi-si, Korea). The TiO2 paste was prepared
by mixing TiO2 nanoparticles (with diameter <20 nm), ter-
pineol, lauric acid, and ethyl cellulose. The thin films were
placed in a furnace for heat treatment after they were dried
at room temperature for 20 min and at 95ꢀC for 30 min.
The heat treatment was performed in the furnace for 2 h
after the temperature was increased from room temperature
to 450ꢀC at a heating rate of 5ꢀC/min (thin-film thickness
~6 μm).5 The substrates were then dipped into 0.5 mM VO
and TAA–ethanol solution for 20 h. When the reaction was
complete, the substrates were washed with ethanol and then
dried with N2 gas.
~4 min after the Surlyn film was placed between them. Elec-
trolyte was injected into the joined electrodes after evacuating
the air from the cells. Solutions of various concentrations
(0.01, 0.02, 0.05, 0.1, 0.2, 0.5, and 1 M) of LiClO4 in propyl-
ene carbonate were used as electrolytes. Cyclic voltammetry
for the half-cell ECDs was performed using a CHI1030
potentiostat (CH Instruments, Inc., (Austin, TX, USA) USA),
and the optical properties of the ECDs were measured using
an optical measurement system consisting of a light source
(DH-2000-BAL, Ocean Optics, Inc. (Winter Park, FL, USA)
USA), optical fibers, and a spectrometer (USB2000 + UV–
vis, Ocean Optics, Inc., USA). A driving bias was applied
using a function generator (Hewlett Packard Co., (Palo Alto,
CA, USA) 33120A), a DC power supply (Protek Co.,
(Tempe, AZ, USA) OPE-303Q), and an isolation board with
an operational amplifier.
Results and Discussion
Cyclic Voltammetry. Figure 2(a) shows the cyclic voltam-
metry (CV) data for a VO-anchored TiO2 half-cell. As the
electrolyte concentration increases, the reduction current
increases and the reduction potential increases negatively.
A CV curve for the TiO2 half-cell without VO at 0.1 M
LiClO4 is shown as a dotted line in Figure 2(a). The TiO2
layer participated in the oxidation–reduction reaction in the
range of negative potentials only. However, the oxidation–
reduction current of the TiO2 half-cell without VO was four
times lower than that of the VO-anchored TiO2 half-cell at
the same electrolyte concentration of 0.1 M LiClO4; this
was even lower than the oxidation–reduction current at the
lowest electrolyte concentration of 0.01 M LiClO4. Based
on these results, it can be concluded that the oxidation–
reduction current of VO predominates in the overall
oxidation–reduction current of the half-cell. Figure 2
(b) shows a reduction coloration image of the VO-anchored
The full cell was prepared as follows: The electrodes of the
reduction and oxidation electrochromic layers were bonded
using a Surlyn film (separation ~25, 60, and 100 μm, respec-
tively). The devices were 2.0 × 3.0 cm2 in size, of which
1.4 × 1.9 cm2 was the active area. The two electrodes were
joined by subjecting them to heat treatment at 115ꢀC for
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(d)
(a)
0.6
0.4
0.01 M
0.05 M
0.2 M
1 M
0.02 M
0.1 M
0.5 M
0.1 M
0.0
0.2
-0.1
-0.2
-0.3
0.0
0.01 M
0.05 M
0.2 M
1 M
0.02 M
0.1 M
0.5 M
-0.2
-0.4
-0.6 -0.4 -0.2 0.0 0.2 0.4
voltage (V)
0.0
0.2
0.4
voltage (V)
0.6
0.8
(b)
(e)
VO half cell
TAA half cell
(cv: -0.65 V)
(cv: 0.75 V)
(c)
(f)
VO half cell
0.5 V
TAA half cell
90
90
60
30
0
-0.5 V
60
30
0
-0.5 V
-0.7 V
0.75 V
0.85 V
600
400
500
600
700
800
400
500
700
800
wavelength (nm)
wavelength (nm)
Figure 2. (a) CV spectra (–0.7 to 0.5 V, 5 mV/s) of VO as a function of the electrolyte of concentration of LiClO4, (b) the corresponding
image of coloration at –0.65 V, and (c) the spectra of the VO half-cell at 0.5 V (decoloration), –0.5 V (coloration), and –0.7 V (colora-
tion). (d) CV spectra (0–0.85 V, 5 mV/s) of TAA as a function of the electrolyte concentration of LiClO4, (e) the corresponding image of
coloration at 0.75 V, and (f ) the spectra of the TAA half-cell at –0.5 V (decoloration), 0.75 V (coloration), and 0.85 V (coloration). The
dotted line in (a) indicates the CV of TiO2 (0.1 M LiClO4).
TiO2 half-cell at –0.65 V reduction potential. When the
half-cell became colored, it turned purple. The spectra of
the VO half-cell ECD were taken at 0.5 and –0.7 V for
decoloration and coloration during measurement of CV, as
shown in Figure 2(c), which shows two transmittance peaks
at 550 and 600 nm. The transmittance decreases with the
increase in applied voltages.
Figure 2(d) shows the CV curve for the TAA-anchored
TiO2 half-cell. The oxidation potential is observed to be in
the range of positive scale; the oxidation current increased
with increasing electrolyte concentration, and the
oxidation–reduction potential increased positively. Figure 2
(e) shows an oxidation coloration image of the TAA-
anchored TiO2 half-cell when the potential was equal to
0.75 V. The colorized half-cell appeared in moderate
golden yellow. The spectra of the TAA half-cell at –0.5 V
for decoloration and at 0.75 and 0.85 V for coloration were
taken during measurement of the CV, as can be seen in
Figure 2(f ), which shows two transmittance peaks at
480 and 690 nm. The transmittance decreases with the
increase in the applied voltage.
It was found that the first reduction of VO led to a color
change from transparent to purple; the first oxidation led to
a color change of TAA from transparent to yellowish. The
VO half-cell showed low transmittance in the wavelength
region 550–600 nm and high transmittance in the wave-
length regions 450 and 750 nm. In contrast, TAA showed
high transmittance in the wavelength region 500–600 nm
and low transmittance both at 450 and 750 nm. The combi-
nation of these two electrochromic materials led to the
blackish appearance of the ECD (Figure 1(b)) and resulted
in an evenly low transmittance over the entire wavelength
region (Figure 1(c)).
Transmittance. Figure 3(a) shows the transmittance spec-
tra in the colored and decolored states measured for a range
of electrolyte concentrations of 0.01–1 M LiClO4 for
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TAA did not engage in coloration at 0.01 M LiClO4.
Meanwhile, as the electrolyte concentration increased, a
remarkable decrease of the transmittance appeared in the
shorter wavelength region of ~480 nm and in the longer
wavelength region of ~690 nm. With more than 0.02 M
LiClO4, TAA engaged in coloration, and the decrease of
the transmittance became obvious as the electrolyte concen-
tration was increased more and more. With some excep-
tions for 0.01 M and 1 M LiClO4, the transmittance in the
decolored state remained at almost the same level as in the
decoloration transmittance spectra, although a slight differ-
ence was observed at the wavelength of ~400 nm. For
0.01 M LiClO4, the spectrum did not recover to the deco-
lored transmittance level due to the incomplete decoloration
of the reduced VOs. Also, for the 1 M LiClO4, the deco-
loration spectrum showed a transmittance peak at ~690 nm,
which was considered to be due to the incomplete decolora-
tion of the oxidized TAA molecules.
(a)
0.01 M
0.05 M
0.5 M
0.01 M
0.1 M
0.5 M
0.02 M
0.1 M
1 M
0.02 M
0.2 M
1 M
0.05 M
0.2 M
80
60
40
20
25 µm
0
0.01 M
0.05 M
0.5 M
0.01 M
0.1 M
0.5 M
0.02 M
0.1 M
1 M
0.02 M
0.2 M
1 M
0.05 M
0.2 M
(b)
80
60
40
20
Figure 3(b) and (c) show the transmittance spectra as a
function of the electrolyte concentration for 60 and 100 μm
cell gaps, respectively. These patterns are similar to those
shown in Figure 3(a). The degree of coloration became
higher with increase in electrolyte concentration. Compared
to the 25 μm cell gap case, a decrease of the transmittance
in 0.01 M LiClO4 for 100 μm cell gap was distinctly
observed in the whole wavelength region. Specifically, the
decrease of the transmittance turned out to be larger in the
shorter (~480 nm) and longer (~690 nm) wavelength
regions, meaning that TAA engaged more effectively with
the increase of the electrolyte concentration. It appears that,
despite the identical electrolyte concentration of 0.01 M
LiClO4, the TAA coloration could be achieved because the
number of Li+ ions became four times more at 100 μm cell
60 µm
0
(c)
0.01 M
0.05 M
0.5 M
0.01 M
0.1 M
0.5 M
0.02 M
0.1 M
1 M
0.02 M
0.2 M
1 M
0.05 M
0.2 M
80
60
40
20
0
100 µm
+
gap (the number of Li ions: 0.1 μmol/cm2) than at 25 μm
+
cell gap (the number of Li ions: 0.025 μmol/cm2). For
decoloration, the transmittance values all remained similar,
except for the 0.01 and 1 M LiClO4 samples with the gap
of 60 μm and for the 1 M LiClO4 sample with the gap of
100 μm. The decoloration spectrum in 0.01 M LiClO4 for
the 100 μm cell gap was different from that with 25 μm
cell gap. In contrast to the case of the 25 μm cell gap, the
decoloration was completed in the 0.01 M LiClO4 for the
100 μm cell gap. It could be that, although the electrolyte
concentration was the same at 0.01 M LiClO4, the larger
cell gap led to an increased number of Li+ ions, which in
turn caused sufficient oxidation of the reduced VO mole-
cules. As previously reported,17 the optical density (OD) of
VO for 6 μm TiO2 layer thickness is ~3.84 at 528 nm (the
total number of VO molecules is 0.18 μmol/cm2). From the
transmittance spectra shown in Figure 3(a), we can calcu-
late that the optical density of VO molecules is about
0.54 at 0.01 M LiClO4 concentration, which corresponds to
0.025 μmol/cm2 of VO molecules. This number is very
close to the number of Li+ ions, namely 0.025 μmol/cm2
for 0.01 M LiClO4 and a 25 μm cell gap; this sample may
be incompletely reduced and oxidized.
450
600
wavelength (nm)
750
Figure 3. Transmittance spectra as a function of the electrolyte
concentration for (a) 25, (b) 60, and (c) 100 μm cell gaps, 9 s after
coloration and decoloration. The coloration and decoloration vol-
tages were –1.5 V (dotted line) and 0.5 V (solid line),
respectively.
25 μm cell gap. All coloration and decoloration voltages
were fixed at –1.5 and 0.5 V, respectively. The transmit-
tance in the colored states decreased with increasing elec-
trolyte concentrations, meaning that the coloration at higher
electrolyte concentrations was more efficient than at lower
concentrations. At a concentration of 0.01 M LiClO4, the
coloration spectrum showed transmittance bands in the
wavelength region 550 and ~600 nm. This is similar to
the coloration spectrum of the reduced VOs (Figure 2(c)).
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Incomplete decoloration was observed at the electrolyte
Figure 4(b) shows how the transmittance changed in
response to the variation of the electrolyte concentration
during coloration and decoloration for each cell gap. Except
for the 1 M LiClO4 concentration, the higher the electrolyte
concentration, the larger was the transmittance change. This
appears to be consistent with the notion that the coloration
transmittance decreases as the electrolyte concentration is
increased for the same coloration voltage (–1.5 V).
Response Time. Figure 5 shows the coloration and deco-
loration time at the applied voltages of –1.5 and 0.5 V for
each of the electrolyte concentrations and for cell gaps of
25, 60, and 100 μm. The coloration time was ~2 to 3 s,
which was not significantly affected by the cell gap
(Figures S2–S4) as shown in Figure 5(a). The decoloration
time did not vary significantly with the cell gap either, but
was affected by the electrolyte concentration, as shown in
Figure 5(b). Except for the electrolyte concentration of
0.01 M LiClO4 for the cell gaps of 25 and 60 μm, the
decoloration time was less than 1 s for most of the electro-
lyte concentrations (0.02–1 M LiClO4) (Figures S2–S4).
As the electrolyte concentration was increased, the colora-
tion time decreased, showing that at higher electrolyte
concentration of 0.01 and 1 M LiClO4. Given the implica-
tions of the data shown in Figure 2(c) and (f ), the incom-
plete decoloration at 0.01 M was caused by VO and the
incomplete decoloration at 1 M was caused by TAA. Other
than this, over the range 0.02–0.5 M, decoloration was
completely effected. Contrary to the case of decoloration,
the coloration transmittance decreased evenly over the
entire range 0.01–1 M as the electrolyte concentration
increased.
Figure 4(a) shows the transmittance for the coloration
and decoloration voltages of –1.5 and 0.5 V in each elec-
trolyte concentration. The transmittance for the coloration
and decoloration was not greatly affected by the cell gap,
but was rather significantly affected by the electrolyte con-
centration. The coloration transmittance decreased as the
electrolyte concentration increased; that is, the degree of
coloration increased effectively as the electrolyte concentra-
tion increased. Except for 0.01 and 1 M LiClO4 electrolyte
concentrations, in the region 0.02–0.5 M the decoloration
transmittance appeared to have almost the same value; the
best transmittance was found in the region 0.02–0.2 M.
(a)
(a)
75
3
60
25 µm cell gap and -1.5 V
25 µm cell gap and 0.5 V
60 µm cell gap and -1.5 V
2
45
60 µm cell gap and 0.5 V
100 µm cell gap and -1.5 V
30
25 µm cell gap
1
100 µm cell gap and 0.5 V
60 µm cell gap
15
100 µm cell gap
0
0
(b)
(b)
75
60
45
30
15
0
4
25 µm cell gap
60 µm cell gap
25 µm cell gap
60 µm cell gap
100 µm cell gap
2
0
100 µm cell gap
0.0
0.2
0.4
0.6
0.8
1.0
0.0
0.2
0.4
0.6
0.8
1.0
[LiClO4 (M)]
[LiClO4 (M)]
Figure 4. (a) Transmittance in coloration (–1.5 V) and decolora-
tion (0.5 V), and (b) transmittance changes for various cell gaps
and electrolyte concentrations at the wavelength 550 nm.
Figure 5. (a) Coloration time at –1.5 V and (b) decoloration time
at 0.5 V (within 10–90% transmittance change after coloration or
decoloration) at the wavelength 550 nm.
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(0.01 M LiClO4), the number of ions becomes absolutely
insufficient for ion exchange, so that coloration and deco-
loration work ineffectively and the resulting speed becomes
low (the number of VO molecules for 6 μm TiO2 thickness:
0.025 μmol/cm2; the number of Li+ ions for 0.01 M LiClO4
and 25 μm cell gap: 0.025 μmol/cm2).
8
6
4
2
0
Charge. Figure 6 shows the variation of the consumed
charge per unit area versus the electrolyte concentration at
the coloration and decoloration voltage of –1.5 and 0.5 V
for the cell gaps 25, 60, and 100 μm. The difference of
consumed charge was more heavily dependent on the elec-
trolyte concentration than on the cell gap. It should be
noted that as the electrolyte concentration was increased,
the charge consumption increased very rapidly. Then, satu-
ration of the consumed charge was reached at 0.5 M
LiClO4. It was, however, observed that the charge con-
sumed decreased at a higher concentration of 1 M LiClO4.
The charge consumption was not much affected by the cell
gap, but it was significantly affected by the electrolyte con-
centration. In decoloration, except for the electrolyte con-
centration of 0.01 M LiClO4 with cell gaps of 25, 60, and
100 μm, the electrical charge was rapidly consumed in the
early stage (within 1 s) for most electrolyte concentrations
(0.02–1 M LiClO4) (Figure S5). In coloration, on the other
hand, saturation was not reached even after 9 s for most
electrolyte concentrations, and as a result the charge contin-
ued to be consumed (Figure S5). This can be compared
to the 2–3 s of coloration time measured from the
25 µm and -1.5 V
60 µm and -1.5 V
100 µm and -1.5V
25 µm and 0.5 V
60 µm and 0.5 V
100 µm and 0.5 V
0.0
0.2
0.4
0.6
0.8
1.0
[LiClO4 (M)]
Figure 6. Amount of charge being consumed (9 s after coloration
or decoloration) per unit area as a function of the cell gaps and
electrolyte concentration at the coloration and decoloration voltage
of –1.5 and 0.5 V, respectively.
concentration the coloration became faster. The decolora-
tion time (<1 s) was shorter than the coloration time
(2–3 s). The reason for the slower speed of coloration com-
pared to that of decoloration is that there is natural deco-
loration in the process of coloration, and so the actual
saturation of coloration takes longer to offset the natural
decoloration. If the electrolyte concentration is very low
(a)
(b)
240
160
80
60 µm cell gap
25 µm cell gap
0
450
600
750
450
600
750
wavelength (nm)
wavelength (nm)
(c)
(d)
240
160
80
25µm cell gap
60 µm cell gap
100 µm cell gap
100 µm cell gap
0
450
600
750 0.0
0.2
0.4
0.6
0.8
1.0
wavelength (nm)
[LiClO4 (M)]
Figure 7. Coloration efficiency as a function of the electrolyte concentration for (a) 25, (b) 60, and (c) 100 μm cell gaps for 0.01 M (dark
yellow), 0.02 M (magenta), 0.05 M (cyan), 0.1 M (blue), 0.2 M (green), 0.5 M (red), and 1 M (black) LiClO4 9 s after coloration. The
coloration and decoloration voltage were –1.5 and 0.5 V, respectively. (d) Coloration efficiency 9 s after coloration at the wavelength
550 nm with different cell gaps and electrolyte concentrations.
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revealed a deep green color absorbed at 690 nm (Figures 2
(f ) and S7). With low electrolyte concentration in the range
0.02–0.1 M, the coloration efficiency peaked at ~518 nm
and the golden yellow color of the TAA molecules was
dominant, the first reduction; with high electrolyte concen-
tration in the range 0.2–1 M, the coloration efficiency
peaked at ~720 nm and the green color of the TAA mole-
cules was dominant (Figures 7 (a)–(c) and S7), the second
reduction. As can be seen in Figure 7(b) and (c), the colora-
tion efficiency according to the electrolyte concentration
appeared similar to that shown in Figure 7(a). It is under-
standable that as the electrolyte concentration increased, the
coloration efficiency increased. The coloration efficiency
(ηC) of an electrochromic device can be expressed as fol-
lows: it is proportional to the absorbance, but inversely pro-
portional to the amount of injected charge (with ΔA:
absorbance change, Selectrode: electrode area, Q: charge).
12
25 µm cell gap
60 µm cell gap
100 µm cell gap
9
6
3
0
0.0
0.2
0.4
0.6
0.8
1.0
[LiClO4 (M)]
Figure 8. Tansmittance change (bistability) 60 s after coloration
for various cell gaps and electrolyte concentrations, measured after
turning power off 9 s after coloration at –1.5 V.
ηC = ΔA × Selectrode=Q
transmittance change, as shown in Figure 5(a). The reason
why the saturation of the charge consumed did not reach in
2–3 s might be that there was natural decoloration in the
process of coloration, which in turn affected the amount of
charge being consumed in coloration. Even though satura-
tion in the coloration transmittance change spectrum was
reached, offsetting the natural decoloration, the charge con-
sumed in coloration was continuously included by the
charge consumed by the natural decoloration, which was
the reason why the charge consumed did not saturate.
In driving the ECD, the consumed charge was not signifi-
cantly affected by the cell gap, but it was significantly
affected by the electrolyte concentration (Figure 6). The
lower the electrolyte concentration, the less the charge con-
sumed. This can be compared with the case of the coloration
transmittance and the transmittance change, as shown in
Figure 4. At the coloration voltage of –1.5 V, the coloration
transmittance increased as the electrolyte concentration
decreased; then the transmittance change also decreased
(Figure 4). In other words, it was found that more charge
had to be consumed in order to decrease the coloration trans-
mittance and increase the transmittance change (Figure 6).
Coloration Efficiency. As can be seen in Figure 7(a), the
coloration efficiency in 0.01 M LiClO4 for a cell gap of
25 μm was similar to that with VO alone (Figure S6). For
the concentration of 0.01 M LiClO4, TAA hardly engaged
in the coloration. The coloration spectrum of VO typically
showed absorption peaks in the wavelength region
550–600 nm.
Figure 7(d) shows the coloration efficiencies for various
cell gaps and electrolyte concentrations at 550 nm. The col-
oration efficiency showed spectra of a similar pattern
regardless of the size of the cell gap, but was strongly
affected by the electrolyte concentrations. As the electrolyte
concentration of LiClO4 decreased, the coloration efficiency
dropped. It can therefore be concluded that the concentra-
tion of LiClO4 can greatly affect the oxidation–reduction
reactions of VO and TAA. It can be seen that, although the
transmittance change increases and more charge is con-
sumed as the electrolyte concentration increases, the colora-
tion efficiency increases. The reason for this is that the
coloration efficiency is calculated by dividing not the trans-
mittance but the absorbance change by the consumed
charge. Also, it should be noted that the higher concentra-
tion of Li+ ions increases significantly the efficiency of the
oxidation–reduction reaction of the electrochromic material.
Bistability. Figure 8 shows the bistability versus the elec-
trolyte concentrations for various cell gaps. Power was
turned off right after the ECD became colored, which, 60 s
later, was followed by measurement of the bistability from
the transmittance change ratio. In terms of cell gap, the
electrochromic device with a gap of 100 μm showed the
best bistability (within 3%). As the cell gap became smal-
ler, the bistability (5–12% for 25 μm) decreased. In terms
of electrolyte concentration, the bistability improved as the
electrolyte concentration increased. With a low electrolyte
concentration, the ion concentration in the solution
decreased after coloration, so decoloration proceeded very
rapidly, resulting in low bistability. Furthermore, with a
narrow cell gap, the bistability appeared low because the
number of ions in the solution decreased as the cell gap
became smaller. To increase the bistability, it was necessary
to enlarge the cell gap and to raise the electrolyte concen-
tration (to a level higher than 0.1 M LiClO4). It appears
that the bistability is related to the number of Li+ ions
As the LiClO4 concentration was increased, the overall
coloration efficiency increased, and new absorption peaks
were detected at both 518 and 720 nm. This means that
TAA engaged more strongly in coloration as the electrolyte
concentration increased. TAA coloration proceeded through
two stages: the first reduction ended up showing a golden
yellow color absorbed at 480 nm, while the second
Bull. Korean Chem. Soc. 2016
© 2016 Korean Chemical Society, Seoul & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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BULLETIN OF THE
Article
Optical and Electrical Properties of Electrochromic Device
KOREAN CHEMICAL SOCIETY
within the cell and it improves as the number of the electro-
lyte ions within the cell increases. Therefore, to increase
bistability, it is necessary to enlarge the cell gap, and
thereby increase the electrolyte concentration.
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Conclusion
In this study, overall, it was demonstrated that the optical and
electrical properties of ECDs were less affected by the cell
gap than by the electrolyte concentration. Coloration turned
out to be more effectively achieved by an increase of the elec-
trolyte concentration, while decoloration proceeded in a stable
manner with an electrolyte concentration of in the range
0.2–0.5 M. As measured from the transmittance change, the
coloration time was 2–3 s while the decoloration time was
less than 1 s. As measured from the amount of charge con-
sumed, the decoloration time was less than 1 s, while the col-
oration time was more than 5 s due to the natural
decoloration process that took place. The coloration ratio
increased as the electrolyte concentration increased. With a
low electrolyte concentration, VO was the main contributor to
the spectrum; TAA appeared to make an additional contribu-
tion to the spectrum as the electrolyte concentration increased.
Bistability was better both when the electrolyte concentration
increased and as cell gap increased. Based on all these find-
ings, it was concluded that cell gap and electrolyte concentra-
tion have great effects on the performance of ECDs.
Acknowledgment. This work was supported by Institute
for Information & Communications Technology Promotion
(IITP) grant funded by the Korean government (MSIP)
(B0101-16-0133 “The core technology development of
light and space adaptable energy-saving I/O platform for
future advertising service”).
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Bull. Korean Chem. Soc. 2016
© 2016 Korean Chemical Society, Seoul & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.bkcs.wiley-vch.de
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