A Dual electrochrome of Poly-(3,4-Ethylenedioxythiophene) doped by N,N'-Bis(3-sulfonatopropyl)-4-4'-bipyridinium-redox chemistry and electrochromism in flexible devices

Article abstract of DOI:10.1002/cssc.200900202

An electrochromic zwitterionic viologen, N,N'-bis(3-sulfonatopropyl)-4-4'-bipyridinium, has been used for the first time for doping poly (3,4-ethylenedioxythiopene) (PEDOT) films during electropolymerization. Slow and fast diffusional rates for the monomer at deposition potentials of +1.2 and +1.8 V, respectively yielded the viologen-doped PEDOT films with granular morphology and with dendrite-like shapes. The dual electrochrome formed at +1.8 V, showed enhanced coloration efficiency, larger electrochemical charge storage capacity, and superior redox activity in comparison to its analogue grown at +1.2 V, thus demonstrating the role of dendritic shapes in amplifying electrochromism. Flexible electrochromic devices fabricated with the viologen-doped PEDOT film grown at +1.8 V and Prussian blue with an ionic liquid-based gel electrolyte film showed reversible coloration between pale and dark purple with maximum coloration efficiency of 187cm²C^-1 at λ = 693 nm. The diffusional impedance parameters and switching kinetics of the device showed the suitability of this dual electrochrome formed as a single layer for practical electrochromic cells.

Full text of DOI:10.1002/cssc.200900202

DOI: 10.1002/cssc.200900202
A Dual Electrochrome of Poly-(3,4-Ethylenedioxy-
thiophene) Doped by N,N’-Bis(3-sulfonatopropyl)-
4-4’-bipyridinium—Redox Chemistry and Electrochromism
in Flexible Devices
Shweta Bhandari,[a] Melepurath Deepa,*[a] Suman Pahal,[a] Amish G. Joshi,[a] Avanish
Kumar Srivastava,[a] and Rama Kant[b]
An electrochromic zwitterionic viologen, N,N’-bis(3-sulfonato-
propyl)-4-4’-bipyridinium, has been used for the first time for
doping poly (3,4-ethylenedioxythiopene) (PEDOT) films during
electropolymerization. Slow and fast diffusional rates for the
monomer at deposition potentials of +1.2 and +1.8 V, respec-
tively yielded the viologen-doped PEDOT films with granular
morphology and with dendrite-like shapes. The dual electro-
chrome formed at +1.8 V, showed enhanced coloration effi-
ciency, larger electrochemical charge storage capacity, and su-
perior redox activity in comparison to its analogue grown at
+1.2 V, thus demonstrating the role of dendritic shapes in am-
plifying electrochromism. Flexible electrochromic devices fabri-
cated with the viologen-doped PEDOT film grown at +1.8 V
and Prussian blue with an ionic liquid-based gel electrolyte
film showed reversible coloration between pale and dark
purple with maximum coloration efficiency of 187 cm2 Cꢀ1 at
l=693 nm. The diffusional impedance parameters and switch-
ing kinetics of the device showed the suitability of this dual
electrochrome formed as a single layer for practical electro-
chromic cells.
Introduction
For decades, there has been an enormous research interest in
economically viable materials capable of modulating reflection
or transmission through multiple hues. Electrochromic materi-
als are best suited for different applications, such as high con-
trast displays, antiglare rearview mirrors, and energy-saving
smart windows, owing to their ability to dynamically modulate
incoming radiation through low power inputs.[1,2] Conducting
polymers are attractive electrochromes by virtue of their high-
contrast ratios, fast switching speeds, ease of color tuning, and
high redox stability.[3] Amongst electrochromic polymers, poly-
(3,4-ethylenedioxythiophene) (PEDOT) and its derivatives have
always been forerunners because of their ease of processing,
ease of switching between two redox states, and high chemi-
cal stability provided by an electron-donating ethylenedioxy
group.[4–6] However, in terms of performance, there always re-
mains a scope for further improvement to enhance electro-
chromic contrast and stability of materials and electrolytes for
device applications.
Longchamp et al.[13] also reported an increased electrochromic
contrast by fabricating a layer by layer assembly of a PEDOT-
polyviologen coating. However, in this method too, repeated
coatings were cumbersome to carry out and charge trapping
effects within the layers tended to influence the device opera-
tion.[10] Earlier, polypyrroles have been doped with electrochro-
mic molecules, such as indigo carmine,[14] phthalocyanines,[15,16]
and porphyrins.[17] Song et al.[18] studied electrochromism in
detail for polypyrrole by incorporating 2,2’-azinobis (3-ethyl-
benzothiazoline-6-sulfonate) as a dopant which itself is electro-
chromic. However, to the best of our knowledge, no protocol
involving the incorporation of an electrochromic zwitterionic
viologen, as the charge compensating ion during electropoly-
merization of EDOT, has been reported to date. The usual viol-
ogen salts could not be used for fulfilling this objective as the
bipyridinium moiety is positively charged and therefore, upon
electropolymerization, only the negatively charged anion will
associate with the polymer chain. Only in the case of a zwitter-
ionic viologen, similar to the one used herein, the bipyridinium
For contrast improvement, derivatization of the monomer
EDOT has already been accomplished by many researchers.[7–9]
Ko et al.[10] associated an electrochromic viologen with EDOT;
however, the route of derivatizing the monomer and its further
polymerization are rather complicated processes. Similarly,
post-polymerization functionalization of PEDOT has also been
done[11] but there is a paucity of reports wherein electrochro-
mic efficiency of such derivatized polymers has been studied.
In yet another report, Stepp et al.[12] constructed electrochro-
mic polyelectrolyte multilayers from a polyviologen and poly-
(styrenesulfonate) by using alternating polyion solutions. De-
[a] S. Bhandari, Dr. M. Deepa, S. Pahal, Dr. A. G. Joshi, Dr. A. K. Srivastava
National Physical Laboratory,
Dr. K. S. Krishnan road, New Delhi-110012 (India)
Fax: (+91)-11-45609310
E-mail: m_deepa@mail.nplindia.ernet.in
[b] Dr. R. Kant
Department of Chemistry,
University of Delhi, New Delhi - 110007 (India)
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cssc.200900202.
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M. Deepa et al.
ion is covalently attached to the negatively charged propane
sultone, and therefore the viologen (as a whole) will be incor-
porated as the dopant in PEDOT. Hence, in order to achieve
the above target, we adopted a simple method wherein a salt
of an organic viologen N,N’-bis(3-sulfonatopropyl)-4-4’-bipyridi-
nium served as a dopant during polymerization of EDOT and
also acted as a cathodic electrochrome.
We deposited films at two different constant potentials of
+1.2 and +1.8 V in order to study the effect of applied poten-
tial on PEDOT nanostructures and compared the optical and
redox properties of the resulting films. The films deposited at
+1.2 and +1.8 V are referred to as PEDOT-Vio (1.2) and PEDOT-
Vio (1.8), respectively. The motivation for growing films at high
potentials stemmed from previous work by Lee et al.[19–21] in
which nanotubular PEDOT was formed at high oxidation po-
tentials. In addition, flexible electrochromic devices were also
fabricated by employing viologen-doped PEDOT and Prussian
blue (PB) deposited over conducting plastic substrates, as the
coloring layers and an ionic liquid (IL)-based gel film as the
electrolyte. Device performance evaluation demonstrated the
applicability of these films to smart windows.
Results and Discussion
Deposition profiles
Current–time transients for the electrodeposition of the PEDOT
films by subjecting the electrode from 0 V at t=0, to +1.2 V
and +1.8 V for the same duration of 120 s are shown in
Figure 1. The double layer capacitance charge[22,23] causes the
abrupt rise in the initial current under both potentials, and
peak heights are 0.0080 and 0.0004 A under +1.8 and +1.2 V,
respectively. An exponential decay follows (inset of Figure 1a).
This decay is gradual in the film deposited at +1.2 V and con-
trasts from the current response of the film deposited under
+1.8 V in which the current decay is rapid. The subsequent in-
crement in the current after decay, to acquire plateau behavior,
is attributed to the diffusion limited oxidation of the monomer
and it is rather small in the film deposited at +1.2 V, which is
indicative of a slower diffusion rate. As the monomer diffuses
slowly, due to a slow reaction rate (a function of an applied
potential), the deposition occurs uniformly on the whole sur-
face at +1.2 V[19] manifesting in a regular granular morphology.
Contrary to this response, at +1.8 V, the diffusion of the mono-
mer and the polymerization reaction are very fast (driven by
the high potential), as a result of which the diffused monomer
does not have sufficient time to deposit uniformly and there-
fore it is consumed immediately to form elongated fibers from
different single nucleation sites. The SEM micrographs of the
two films: PEDOT-Vio (1.2) and PEDOT-Vio (1.8), reflect the con-
clusions drawn on the basis of this hypothesis.
Figure 1. a) Current–time transients for oxidative electropolymerization of
3,4-(ethylenedioxythiophene) (0.1m ) in a Tween 20 (0.1m) and N,N’-bis(3-
sulfonatopropyl)-4-4’-bipyridinium (0.1m) solution at two different constant
potentials of +1.8 and +1.2 V for 120 s to yield PEDOT (1.8) (&) and PEDOT
(1.2) (~) films, respectively, and b,c) their corresponding I versus tꢀ1/2 plots.
Inset of (a) shows the deposition profiles in the early stages.
Further support is obtained from the apparent diffusion co-
efficient (D) for the monomeric species, determined using the
Cottrell equation [Equation (1)]:[24]
where I is the current in amperes, n is the number of electrons,
F is the Faraday constant, Cj is the initial concentration in
molcmꢀ3, t is the time in seconds, and A is the area in cm2.
I ¼ nFACj ðDjÞ1=2=ðpÞꢀ1=2ðtÞ
1=2
ð1Þ
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Redox Chemistry and Electrochromism in Flexible Devices
From the slopes of current versus tꢀ1/2 plots, D was found to
be 9.3ꢁ10ꢀ13 cm2 sꢀ1 in 0.3–6.0 s and 3.4ꢁ10ꢀ10 cm2 sꢀ1 in the
7.0–117.0 s range for PEDOT-Vio (1.8) (Figure 1b). Similarly, for
the PEDOT-Vio (1.2) (Figure 1c), apparent D values were 1.6ꢁ
10ꢀ13 cm2 sꢀ1 in 0.2–2.3 s and 9.5ꢁ10ꢀ11 cm2 sꢀ1 in 2.5-124.2 s. It
is evident that in both time zones, during film deposition, the
apparent D value is larger for PEDOT-Vio (1.8), thus experimen-
tally proving that monomer to polymer transformation and
deposition is faster in PEDOT-Vio (1.8).
Scanning electron microscopy (SEM) and transmission elec-
tron microscopy (TEM) studies
The SEM image of the PEDOT-Vio (1.2) (Figure 2a) is character-
ized by typical globular agglomerates usually found in surfac-
tant-assisted polymerized films.[25–27] In contrast, the SEM
images (Figure 2b and c) of PEDOT-Vio (1.8) reveals dendrite-
like shapes (the individual strands are few microns in diameter)
that are distributed randomly on the surface of the film depos-
ited at +1.8 V (Figure 2b). Each individual dendrite (Figure 2c),
has a common point of origin from where the elongated fibers
spread out in all directions. The slow diffusion of the monomer
and slow polymerization rate allow polymer deposition to
occur uniformly without any preference at +1.2 V; whereas at
+1.8 V, the process is so fast that the monomer molecules are
immediately consumed for extending the polymer chain,
which leads to the formation of the dendrite-like shapes.
Figure 3 shows the formation of the different morphologies at
the two potentials.
The bright field TEM image of the PEDOT-Vio (1.2) (Fig-
ure 4a) shows the featureless morphology of the film. Fine
nanocrystallites trapped in an amorphous matrix are clearly
visible in the corresponding high magnification image (Fig-
ure 4b). The crystalline component of the film is evident from
the bright spotty rings in the selected area electron diffraction
pattern (SADP) (inset of Figure 4b). The microstructure of
PEDOT-Vio (1.8) contains similar attributes (Figure 4c); howev-
er, the level of crystallinity is much higher in this film which
can be judged in absolute terms on the basis of the greater
number of regions with lattice fringes prevalent in the film
(Figure 4d). The bright spots superimposed over the diffuse
rings confirm the crystalline component in the PEDOT-Vio (1.8)
film (inset of Figure 4d). The interplanar spacings deduced
from the HRTEM images were 9.7, 4.6, 3.0, and 2.3 ꢂ for the
PEDOT-Vio (1.2) film and 1.7 and 3.6 ꢂ for the PEDOT-Vio (1.8)
film. A chain stacking distance of 3.4 ꢂ has been reported pre-
viously for PEDOT doped by tosylate ions,[28] which is rather
close to d=3.6 ꢂ for the PEDOT-Vio (1.8) film. This short inter-
chain distance of 3.4 ꢂ was previously shown to increase con-
ductivity by the virtue of orbital overlap, in the case of PEDOT-
tosylate.[28] A similar line of reasoning accounts for the higher
dc conductivity observed for the as-fabricated PEDOT-Vio (1.8)
(2.6 mScmꢀ1) in comparison to 1.1 mScmꢀ1 for the PEDOT-Vio
(1.2) film. These dc conductivities were obtained from the
quasi-ohmic (straight line) I–V plots recorded for the films in a
configuration described earlier in more detail.[29]
Figure 2. SEM images of a) PEDOT-Vio (1.2) film showing the globular sur-
face morphology, b) and c) PEDOT-Vio (1.8) film, where (b) shows the
random distribution of dendrite like shapes on the surface and (c) gives an
enlarged view of a dendritic shape. Insets of (a) and (b) are the correspond-
ing high magnification images at 25000ꢁ and 20000ꢁ, respectively.
FTIR and X-ray photoelectron spectroscopy (XPS) studies
The FTIR reflectance spectra of the two films: PEDOT-Vio (1.8)
and PEDOT-Vio (1.2) are shown in Figure 5. The difference in
deposition potentials results in different resonant conforma-
tions of the PEDOT chains in the two films which are clearly
observable in the spectral profiles of the two. The band at
1430 cmꢀ1, corresponding to the symmetric CaꢀCb (ꢀO) stretch-
ing mode is observed only in the spectrum of the PEDOT film
grown at +1.8 V.[30] This peak is not seen in the spectrum of
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M. Deepa et al.
panded coil conformation and that of the benzoid form with
the compact coiled form has been reported.[31,32] The CbꢀCb
stretching vibration[30] at 1362 cmꢀ1 is only present in the film
deposited at +1.8 V. Furthermore, the CaꢀCa’ inter-ring stretch
frequency upshifts from 1247 to 1255 cmꢀ1, ongoing from the
deposition potential of +1.2 to +1.8 V, which suggests the for-
mation of Ca=Ca’ in PEDOT-Vio (1.8). Neither the film deposited
at +1.2 V nor the one grown at +1.8 V show any peak due to
the n(C=O) mode, thereby providing clear evidence for the fact
that both the films are not overoxidized. Deposition at high
potentials, therefore, is not necessarily accompanied by over
oxidation or ring-scission.
The incorporation of the viologen dopant, N,N’-bis(3-sulfona-
topropyl)-4-4’-bipyridinium in PEDOT-Vio (1.8), is further con-
firmed from the elemental composition deduced from the XPS
core level spectra of C, O, S, and N (Figure 6). For PEDOT-Vio
(1.8), the C(1s) photoemission, after fixing the full width at half
maximum (FWHM) at 2.2 eV, has been deconvoluted into mul-
tiple components with peaks at 284.1, 285.0, 285.7, and
287.6 eV (Figure 6a). The first two lower energy peaks arise
from the CꢀC and CꢀS linkages in PEDOT and the viologen
dopant. Since these bonds exist in both the polymer backbone
and the dopant ion, it is difficult to identify the independent
contributions of the dopant and polymer from the C(1s) spec-
trum. The carbon component at 285.7 eV is due to CꢀN, sug-
gesting viologen inclusion and the peak of the highest binding
energy is due to CꢀO in PEDOT. The ICꢀN contribution was
18%. The O(1s) signal (FWHM=2.43 eV) is constituted of two
components (Figure 6b): the lower energy one with a higher
intensity centered at 530.4 eV corresponds to S=O of the
dopant (contribution is 61%), and the second peak at 531.9 eV
is ascribed to the oxygen of the CꢀOꢀC bonds in the polymer
chain.
Figure 3. The formation of the PEDOT-Vio (1.8) and PEDOT-Vio (1.2) films
with distinctly different morphologies from the solution containing EDOT,
Tween 20, and N,N’-bis(3-sulfonatopropyl)-4-4’-bipyridinium at two dc poten-
tials of +1.8 and +1.2 V, respectively.
Sulfur, which is omnipresent in both the polymer backbone
and the dopant, produces the spin–spin doublet S(2p1/2) and
S(2p3/2) with a energy split of 1.2 eV and a relative intensity of
1:2 (Figure 6c).[33,34] Peaks at lower binding energies of 158.2
and 159.4 eV correspond to spin–spin doublets of the sulfur
atoms in PEDOT whereas the higher binding energy peaks at
160.2 and 161.4 eV correspond to the sulfur atoms of the viol-
ogen dopant. The core level spectrum of N(1s) (Figure 6d) in-
dubitably affirms that the viologen dopant is incorporated in
the polymer chain. The deconvolution of the broad envelope
yielded independent contributions from the ꢀN= and the posi-
tively charged N+ species of the viologen at 399.8 and
401.1 eV, respectively. This result is in accordance with the as-
signments made for a nitrogen-containing compound formed
as a result of a reaction between vinyl benzyl chloride-graft co-
polymerized films with 4-4’-bipyridine and benzyl chloride.[35]
Here, the level of doping in the PEDOT-Vio (1.8) film by N,N’-
bis(3-sulfonatopropyl)-4-4’-bipyridinium as deduced from the
Figure 4. HRTEM images of a,b) PEDOT-Vio (1.2) and c,d) PEDOT-Vio (1.8)
films and corresponding SADPs with spotty halo rings in insets of (b) and
(d).
the film deposited at +1.2 V. Instead, an absorption at
1465 cmꢀ1 is observed, which is characteristic of the symmetric
Ca=Cb (-H) stretch vibration, thus illustrating the prominence of
the benzoid states in this film. Greater delocalization of the
conjugated p-electrons over the PEDOT chain in the expanded
coil conformation, as compared to compact coiled one, results
in higher charge mobility. A correlation of the quinoid with ex-
I
N(viologen)/IC(total) atomic ratio was 0.28, using areas under nitro-
gen and carbon peaks after applying a baseline correction
from the survey spectrum (see the Supporting Information,
Figure S1).
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Redox Chemistry and Electrochromism in Flexible Devices
cating the electrochromic activity of the viologen
dopant. On raising the negative potential from ꢀ0.6
to ꢀ1.5 V in steps of 0.1 V, the bipolaronic peak is
lost and the peak due to p-p* transition at 573 nm
gains intensity (Figure 7b). With increasing magni-
tude of the reduction potential, even the viologen
dopant’s peak at 301 nm experiences an intensity in-
crease (inset of Figure 7b). This peak at lmax increases
as the neutral viologen reduces to form the radical
cation. As PEDOT is reduced and darkens with in-
crease in negative potential, the cathodically coloring
viologen dopant also gets reduced and complemen-
tarily enhances the level of purple coloration of the
film. The redox chemistry in the electrolyte for the
films is expressed in Equation (2) where (N+)(SO3)ꢀ is
the abbreviation for N,N’-bis(3-sulfonatopropyl)-4-4’-
bipyridinium.
oxidation
þ
ðPEDOTÞðxþy Þ : ðNþÞðxꢀy ÞðSOꢀÞx : ðClO4Þꢀ
ƒƒƒƒ!
1
ƒƒƒƒ
3
y1
2
reduction
ð2Þ
þ
ðPEDOTÞðxÞ : ðNþÞxðSO3ꢀÞx þ y1ðClO4ꢀÞy þ y2eꢀ
1
Figure 5. FTIR spectra of a) PEDOT-Vio (1.8) and b) PEDOT-Vio (1.2) films in the 400–
2000 cmꢀ1 wavenumber region in reflectance mode; a) oxyethylene ring deformation,
b) symmetric [CꢀSꢀC] deformation, c) [CꢀOꢀC] deformation, d) [CaꢀCa’] inter-ring sym-
metric stretching, e) [CbꢀCb] symmetric stretching, f) [CaꢀCb(-O)] symmetric stretching,
and g) [Ca=Cb(-H)] symmetric stretching vibrations.
The redox chemistry of the neat viologen along
with the detailed description of oxidation and reduc-
tion phenomena in the PEDOT-Vio (1.8) and PEDOT-
Figure 6. Core level spectra of functionalized PEDOT-Vio (1.8) film with solid
lines signifying the deconvoluted contributions of a) C(1s), b) O(1s), c) S(2p),
and d) N(1s).
Figure 7. a) Absorbance versus wavelength plots of PEDOT-Vio (1.8) and
PEDOT-Vio (1.2) films under a reduction potential of ꢀ1.5 V, inset of
(a) shows a magnified view of the viologen absorption; b) absorption spec-
tra of PEDOT-Vio (1.8) film recorded under different reduction potentials
varied from ꢀ0.6–ꢀ2.0 V under steps of 0.1 V and under an oxidation poten-
tial of +1.0 V. Optical density change (DOD) as a function of charge density
at a’) l=550 and b’) l=633 nm for the PEDOT-Vio (1.8) (&) and PEDOT-Vio
(1.2) (~) films.
Electro-optical studies
The impact of electrochromic zwitterionic viologen dopant on
the optical properties was examined by studying the spectro-
electrochemistry of the film in LiClO4-PC (1m) under different
cathodic potentials varied from ꢀ0.6 to ꢀ1.5 V in steps of
0.1 V (Figure 7a and b). In PEDOT-Vio (1.8) at +1.0 V (Fig-
ure 7b), apart from the broad oxidation peak in the 800–
900 nm region ascribed to the formation of bipolarons, a sharp
strong peak at wavelengths below 350 nm also appeared, indi-
Vio (1.2) films can be found in the Supporting Information (Fig-
ure S2).
Under an applied E=ꢀ1.5 V (this corresponds to the poten-
tial for maximum reduction), in PEDOT-Vio (1.2) (Figure 7a), the
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M. Deepa et al.
characteristic viologen charge transfer peak shows a red shift
to 307 nm and has a lower intensity as compared to the re-
sponse of PEDOT-Vio (1.8) (lmax =301 nm) film. The intensity of
p-p* transition peak (Figure 7a) corresponding to PEDOT-Vio
(1.2) film is lower than that of the PEDOT-Vio (1.8) film for the
same area and thickness, which clearly indicates that in
PEDOT-Vio (1.8) film, a larger optical density change can be re-
alized for the same value of applied potential.
The coloration efficiencies (CE; h) of the films is defined as
the change in optical density (DOD) for the charge (q) con-
sumed per unit electrode area (A) [Equation (3)].[36]
Figure 8. Digital photographs of the flexible PEDOT-Vio (1.8)-IL gel-PB device
in the a) colored (E=ꢀ1.5 V) and b) bleached (E=+1.5 V) states.
CE ðhÞ ¼ DODðlÞ=q=A ¼ log ðTb=TcÞ=q=A
ð3Þ
transmittance minima in Figure 9a and b for both the devices
(ca. 750 nm) at ꢀ1.5 V arises due to the oxidized PEDOT. This
lmin shows a red shift as the negative potential is decreased
due to a progressive reduction in the dedoping level. However,
for the PEDOT-Vio (1.2)-IL gel-PB device (Figure 9b), the lmin at
ꢀ1.5 V has a unusually higher intensity in comparison to its
amplitude at ꢀ0.5 and ꢀ1.0 V, which suggests that film degra-
dation is activated at high reduction potentials. The maximum
transmission modulation (DT) of 20% at 568 nm in the PEDOT-
Vio (1.8)-IL gel-PB device is observed under ꢁ1.5 V, whereas
for the PEDOT-Vio (1.2)- IL gel-PB device, it is lower (ca. 12% at
501 nm). Here, DT varies from 17 to 20% in the visible region
and then gradually drops to 8% at 1100 nm for the PEDOT-Vio
(1.8)-IL gel-PB device. A similar trend, albeit lower modulation
values, is observed at other potentials. In PEDOT-Vio (1.2)- IL
gel-PB device, the value drops to zero (l=1100 nm) at ꢁ1.5 V.
These values are slightly lower than the DT of 27% of 2,5-bis-
(trimethylsilyl)-PEDOT and poly(3,6-bis(3,4-ethylenedioxythio-
phene)-N-methylcarbazole) devices and 32% of PEDOT and
poly(3,6-bis(3,4-ethylenedioxythiophene)-N-eicosylcarbazole)
devices.[4]
Figure 7a’ and b’ show the DOD at two different wave-
lengths of 550 and 633 nm as a function of charge density. The
reference state was set at +1.0 V (oxidized state) with charge
densities of 1.2 and 1.8 mCcmꢀ2 for PEDOT-Vio (1.8) and
PEDOT-Vio (1.2) films, respectively. As can be gauged from Fig-
ure 7a’ and b’, a lower charge density suffices to impart a
higher coloration efficiency to the PEDOT-Vio (1.8) film as com-
pared to the PEDOT-Vio (1.2) film. At 550 nm, the PEDOT-Vio
(1.8) film exhibits a coloration efficiency of 336 cm2 Cꢀ1 while
PEDOT-Vio (1.2) film has a coloration efficiency of 257 cm2 Cꢀ1.
In both cases, the large coloration efficiency of PEDOT reflects
the contribution of the viologen dopant to the electrochromic
contrast. The values of coloration efficiency are not only great-
er than the reported values of 180 cm2 Cꢀ1 (l=585 nm) of pris-
tine PEDOT but also than that of some of the derivatives such
as poly(propylenedioxythiophene) (PProDOT) (h=285 cm2 Cꢀ1,
l=578 nm) and comparable to PProDOT-Me2 (h=375 cm2 Cꢀ1
,
l=585 nm).[37] However, at 633 nm, coloration efficiencies are
lower: 179 cm2 Cꢀ1 in PEDOT-Vio (1.8) and 157 cm2 Cꢀ1 in
PEDOT-Vio (1.2) films but a large differential continues to per-
sist between the h633 values.
The wavelength dependence of the coloration efficiencies of
two devices is shown in Figure 9a’ and b’. For the PEDOT-Vio
(1.8)-IL gel-PB device, the coloration efficiency maximum
shows a monotonic increase with increase in the applied po-
tential from ꢁ0.5 to ꢁ1.5 V (Figure 9a’). A similar trend is ob-
served for the PEDOT-Vio (1.2)-IL gel-PB device. A sharp lmax in
the UV region is also observed in the spectra owing to the oxi-
dation–reduction of the viologen dopant in PEDOT. In the
PEDOT-Vio (1.8)-IL gel-PB device, this maximum is observed
under all dc conditions at 382 nm, whereas in the PEDOT-Vio
(1.2)-IL gel-PB device, it is present only under ꢁ1.5 V. The max-
imum coloration efficiency for the PEDOT-Vio (1.8)-IL gel-PB
device is 187 cm2 Cꢀ1 at 693 nm whereas the PEDOT-Vio (1.2)-IL
gel-PB device registered a hmax =117 cm2 Cꢀ1 at 528 nm. The
complex capacitance response of the devices measured at dif-
ferent dc oxidation and reduction potentials are shown in the
Supporting Information (Figure S4). From the complex capaci-
tance curves of the two devices measured under different dc
oxidation and reduction potentials, larger magnitudes of Cw!0
were observed for the PEDOT-Vio (1.8)- IL gel-PB device during
coloring and bleaching, which indicates a larger internal sur-
face area or a greater number of redox active sites in the
PEDOT-Vio (1.8) film that are in contact with the electrolyte
anions.
Flexible devices of PEDOT-Vio (1.8), PEDOT-Vio (1.2 V), and
PB films
The practical utility of the PEDOT-Vio (1.8) and PEDOT-Vio (1.2)
films was established by fabricating flexible PEDOT-IL gel-PB
devices. Photographs of the PEDOT-Vio (1.8)-based device in
dark and transparent states are shown in Figure 8. The trans-
mittance of the electrolyte film was recorded independently
(see the Supporting Information, Figure S3). The electrolyte
showed a transmittance greater than 85% in most of the visi-
ble region, indicating its suitability for electrochromic windows.
Ionic conductivity of the electrolytic solution prior to gelation,
recorded on a dc conductivity meter, was found to be 2.8ꢁ
10ꢀ3 Scmꢀ1 at 258C; upon gelation, it was measured by ac im-
pedance analysis by sandwiching the electrolyte between the
two SnO2:F electrodes. The value reduced marginally to 1.2ꢁ
10ꢀ3 Scmꢀ1, which is acceptable for device application.
The transmittance of the devices in the 300–1100 nm wave-
length range was measured under different potentials applied
to the PEDOT-Vio (1.8) and PEDOT-Vio (1.2) films: ꢁ0.5, ꢁ1.0,
and ꢁ1.5 V, in colored and bleached states (Figure 9). The
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Redox Chemistry and Electrochromism in Flexible Devices
Figure 9. Transmission spectra of a) PEDOT (1.8)-IL gel-PB device and b) PEDOT-Vio (1.2)- IL gel-PB device and coloration efficiency plots as a function of wave-
length of a’) PEDOT-Vio (1.8)- IL gel-PB and b’) PEDOT-Vio (1.2)- IL gel-PB devices calculated from changes in transmission, under potentials of ꢁ1.5, ꢁ1.0, and
ꢁ0.5 V. Variation of transmittance as a function of time at l=632.8 nm for a’’) PEDOT-Vio (1.8)- IL gel-PB and b’’) PEDOT-Vio (1.2)-IL gel-PB devices under a
square wave dc potential of ꢁ1.5 V at a frequency of 0.009 Hz.
Optical switching studies of the devices performed at a
monochromatic wavelength of 632.8 nm, by applying alternate
square wave potentials of +1.5 and ꢀ1.5 V, are shown in Fig-
ure 9a’’ and b’’. At an optimized frequency of 0.009 Hz, the
transmission modulation of PEDOT-Vio (1.2)-IL gel-PB device is
smaller as compared to PEDOT-Vio (1.8)-IL gel-PB device owing
to a higher charge storage capacity and facile ion transfer. The
modulation of PEDOT-Vio (1.2)-IL gel-PB device is only 66% of
the total modulation shown by the PEDOT-Vio (1.8)-IL gel-PB
device. Response times to attain colored and bleached states
were determined from the time taken by the device to under-
go a decrease in its transmittance from 90% to 10% and an in-
crease from 10% to 90%, respectively, of its full response. The
PEDOT-Vio (1.2)-IL gel-PB device requires 30 s to color and 8 s
to bleach whereas PEDOT-Vio (1.8)-IL gel-PB device shows a
faster kinetics; it colors in 15 s and bleaches in 6 s, which indi-
cates an easy diffusion of the anions due to a more porous
morphology of the PEDOT-Vio (1.8) film. Furthermore, the am-
plitude of the transmission modulation also decreases for
PEDOT-Vio (1.2)-IL gel-PB device upon repetitive cycling, which
suggests poor endurance to cycling. The switching times are
larger than previously reported values for PEDOT based
charge compensating anions during electropolymerization of
PEDOT, has been demonstrated. The resulting thin-film nano-
structures of viologen-doped PEDOT comprised of 1) a regular
particulate structure and (2) dendritic shapes superimposed on
a granular phase, when grown at oxidation potentials of +1.2
and +1.8 V, respectively. Distinct signals due to nitrogen in the
core level XPS spectrum and the characteristic absorptions of
the bipyridinium moiety in the electro-optical response of the
film, affirmed that the viologen is not an adventitious entity
but is an inherent part of the polymer chain. The dendritic
morphology achieved in the PEDOT-Vio (1.8) film, imparted a
higher coloration efficiency and improved electrochemical ac-
tivity as compared to the PEDOT-Vio (1.2) film. Redox switching
between pale to deep purple and vice-versa, achieved in a
flexible electrochromic device with the PEDOT-Vio (1.8) film
and an IL gel polymer electrolyte, showed fast response times,
high diffusional capacitance, and a coloring efficiency of
187 cm2 Cꢀ1 in the visible region; these performance attributes
unambiguously justify the applicability of PEDOT-Vio (1.8) films
to smart windows.
films[20,38] and devices.[39,40] The high sheet resistance of the un- Experimental
derlying conducting substrate (<350 Wsqꢀ1), the interfacial re-
Materials
sistance of the PEDOT/electrolyte, or PB/electrolyte interface
are possible reasons for slow kinetics.
3,4-ethylenedioxythiophene (EDOT), 4-4’-bipyridine, 1,3-pro-
pane sultone, and polyvinyl alcohol (MW: 86000–124000, dried
for 24 h at 808C prior to use) were obtained from Aldrich and
ferric chloride was obtained from Qualigens. Ionic liquid: 1-
butyl-1-methylpyrrolidiniumtrifluoromethanesulfonate,
Conclusions
A simple route for synthesizing a dual electrochrome of
PEDOT-doped by a viologen by using a zwitterionic viologen
salt N,N’-bis(3-sulfonatopropyl)-4-4’-bipyridinium as a source of
Tween 20 (polyoxyethylene sorbitan monolaurate), potassium
ferricyanide, and reagents dimethyl sulfoxide and ethanol were
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103

M. Deepa et al.
obtained from Merck and used as received. Deionized water
(resistivity ca. 18.2 MWcm) was obtained through Milli-Q
system. Inorganic transparent electrodes of SnO2:F coated
glass (Pilkington, sheet resistance: 14 Wsqꢀ) were cleaned in a
soap solution, 30% HCl solution, double distilled water, and
acetone prior to use. For devices, polyethylene terephthalate
(PET)-based transparent substrates coated with a conductive
layer of PEDOT-poly(styrenesulfonate) (PEDOT:PSS) (Orgacon,
sheet resistance <350 Wsqꢀ1, base thickness of about 125 mm)
were washed with deionized water and dried in air prior to
use.
This assembly was left undisturbed at room temperature for
48 h for gelation and self-supporting film formation. Subse-
quent to electrolyte film formation, PEDOT film deposited over
PEDOT:PSS-coated PET substrate was placed over the PB film
such that the electrolyte film lied in between. The device com-
ponents were held together firmly with binder clips to avoid
bubble formation and to allow the acrylic tape to adhere with
the two films; the clips were removed only after 24 h. Finally, a
cyanoacrylate adhesive was used for sealing the device.
Characterization techniques
FTIR spectra for the PEDOT films were recorded in the reflec-
tion mode with a specular reflectance accessory on a Perkin–
Elmer Spectrum BX spectrophotometer at 288C; the angle of
incidence was fixed at 358. The surface morphological features
of the films were investigated using a SEM (LEO 440) after
sputtering a thin layer of gold on the film surface. TEM was
carried out by transferring the sample onto a carbon-coated
copper grid on a HRTEM Tecnai G2 F30 STWIN with a FEG
source at 300 kV. XPS spectra were recorded, for the as-synthe-
sized PEDOT films, on a Perkin–Elmer 1257 model operating at
a base pressure of 2.4x 10ꢀ8 Torr at 300 K with a non-mono-
chromatized AlKa line at 1486.6 eV, an analyzer pass energy of
60 eV, and a hemispherical sector analyzer capable of 25 meV
resolution. The overall instrumental resolution was about
0.3 eV. The core level spectra were deconvoluted by using a
nonlinear iterative least squares Gaussian fitting procedure. For
all fitting doublets, the FWHMs were fixed accordingly. Correc-
tions due to the charging effects were taken care of by using
C(1s) as an internal reference and the Fermi edge of a gold
sample. Cyclic voltammetry (CV) for the films was performed in
a classical three-electrode electrochemical cell, wherein the
PEDOT film acted as the working electrode, an Ag/Ag+ was
employed as the reference electrode, and a Pt rod was used as
the counter electrode in a liquid electrolyte (1m LiClO4-propyl-
ene carbonate (PC)). The CV response of the viologen was re-
corded for a 0.2m aqueous solution with Ag/Ag+ as the refer-
ence electrode and two Pt rods were used as the working and
counter electrodes. Absorbance spectra for colored and
bleached PEDOT films were recorded ex situ in the 300–
1100 nm wavelength range on a Perkin–Elmer Lambda 25
spectrophotometer as films were found to exhibit an open-cir-
cuit memory of about 60–120 s, in a controlled temperature
and humidity chamber (approximately 22–258C, relative hu-
midity ca. 48–52%). The films were colored under different re-
duction potentials varying from ꢀ0.6 to ꢀ2.0 V for a fixed du-
ration of 60 s in a liquid electrolyte (1m LiClO4-PC) at each po-
tential. Electrochemical impedance spectroscopy measure-
ments were performed on the PEDOT-IL gel-PB device under
different dc bias potentials superimposed over an ac amplitude
of 10 mV with PEDOT as the working electrode. Switching time
characteristics for the device between colored and bleached
states were recorded with an automated setup consisting of a
He–Ne laser (l=632.8 nm), a Si photodetector, and a custom
made microprocessor controlled versatile unit. The device was
illuminated with the laser beam and a photodiode was used to
Synthesis of dopant N,N’-bis(3-sulfonatopropyl)-4-4’-bipyri-
dinium
N,N’-bis(3-sulfonatopropyl)-4-4’-bipyridinium was prepared by
refluxing 4,4’-bipyridine (1.5 g) with 1,3-propane sultone (8.0 g)
for 15 min at 1208C without solvent under nitrogen. To the re-
sulting semi-solid mixture, dimethyl sulfoxide (60 mL) was in-
jected and heated at 1208C while being stirred continuously
for 3 h. After cooling, the white precipitate of the viologen
N,N’-bis(3-sulfonatopropyl)-4-4’-bipyridinium was filtered and
washed several times with methanol, dried over Whatman 42
filter papers, and stored at temperatures less than 108C (yield
1
ca. 71%). H NMR (300 MHz, D2O): d (ppm)=9.1 (d, J=1.0 Hz,
4H), 8.5 (d, J=1.02 Hz, 4H), 4.8 (t, J=1.0 Hz, 4H), 2.9 (t, J=
1.02 Hz, 4H), 2.5 (m, 4H).
Deposition of PEDOT and PB films
PEDOT films were electropolymerized from an aqueous formu-
lation containing EDOT (0.1m), Tween 20 (0.1m) as surfactant
and N,N’-bis(3-sulfonatopropyl)-4-4’-bipyridinium (0.1m), po-
tentiostatically at two different potentials of +1.2 and +1.8 V
for 120 s at room temperature. Optically transparent SnO2:F-
coated glass and PEDOT:PSS coated PET substrates were used
as working electrodes, Ag/AgCl/KCl as a reference and a plati-
num sheet as a counter electrode. The purple films were first
rinsed in deionized water and dried in air prior to use. Prussian
blue (PB) films were grown galvanostatically on conductive
PEDOT:PSS-coated PET substrates in a two electrode configura-
tion with a platinum sheet as a counter electrode from a solu-
tion of ferric chloride (10 mM) and potassium ferricyanide
(10 mM) in HCl (0.01N in deionized water) for 8 min under a
constant current of 10 mAcmꢀ2 applied by a Keithley 2400 cur-
rent source. The films were rinsed in deionized water and
dried in air.
Electrolyte synthesis and fabrication of flexible devices
To synthesize polymer electrolyte, 6 wt% of polyvinyl alcohol
was dissolved in a solution of 1-butyl-1-methylpyrrolidiniumtri-
fluoromethanesulfonate (0.2m) in dimethyl sulfoxide at 708C
with rigorous stirring for 8 h. The resulting transparent viscous
solution was poured into a prefabricated cavity made on the
PB film created using a spacer (transparent acrylic tape, 1 mm
thick and 5 mm wide) placed along the periphery of the film.
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Redox Chemistry and Electrochromism in Flexible Devices
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Published online on November 18, 2009
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ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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105