Reversible full-color generation with patterned yellow electrochromic polymers

Article abstract of DOI:10.1002/anie.201205206

A different light: The yellow electrochromic copolymer, propylenedioxythiophene phenylene P(ProDOT-Ph) is patterned by the micromolding in capillaries (MIMIC) process to fabricate line gratings. Under illumination by white light, colors are generated by diffraction and can be changed by applying an external voltage (see picture). The diffracted color is blue shifted by electrochemical doping, a process that is completely reversible. Copyright

Full text of DOI:10.1002/anie.201205206

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DOI: 10.1002/anie.201205206
Color Modulation
Reversible Full-Color Generation with Patterned Yellow
Electrochromic Polymers**
Thiruvelu Bhuvana, Byeonggwan Kim, Xu Yang, Haijin Shin, and Eunkyoung Kim*
Structural colors in nature[1] are generally much brighter than
chemical colors and commonly found in butterflies, beetles,
and fish.[2] These colors change in some animals, such as the
chameleon,[3] which camouflages itself by blending in with the
surrounding colors. However in an artificial chameleon,[4]
typically known as chromogenic system, it is difficult to
achieve reversible color modulation as nature does, mainly
because of the lack of materials or systems that show
multicolor under different stimuli in a reproducible way.
The structural colors change with respect to the angle of the
structure to the eye, or with respect to the depth of the
pattern,[5] however, keeping these parameters intact makes
reversible color modulation is challenge.
or imprint method have been explored for monochromatic
diffraction intensity modulation[10] and in a recent report,
electroactive subwavelength gratings are used for color and
intensity modulation in reflective mode[11] however, there are
hardly any reports on the use of patterned conjugated
polymers as diffraction gratings for color modulation, despite
wide range of conjugated polymers.[12] Thus we considered it
challenging to explore electrochemical (EC) gratings as visual
color filters and artificial chameleons. Electrochemically
active polymers have a great advantage over other materials
because of their ability to change redox state under external
bias,[10–13] which results in a change in the refractive index.
Herein, we report a new method to obtain an artificial
chameleon effect on reversibly electroactive polymer gratings
without changing gratings parameters, employing multiple
gratings, or changing the surrounding medium. Considering
the light dispersion principles,[7] we hypothesized that a color
generated from the dispersion of light into a polymer grating
can be electrochemically modulated by changing the redox
state of the polymer in the grating. Thus, the aim of this study
is to demonstrate reversible color modulation by electro-
chemical reactions using a patterned film of propylenediox-
ythiophene phenylene copolymer P(ProDOT-Ph). This poly-
mer is cathodically coloring,[14] changes color from yellow to
pale blue depending upon applied voltage. We chose the
yellow electrochromic polymer because it has a faint color
and thus the optical absorption factor change, on applying
a voltage, is mainly a result of the change in the refractive
index because the extinction coefficient change in the
imaginary part is weak.[13d] Moreover it shows a greater
change in refractive index between the neutral and oxidized
state in comparison to P3HT.[11] The change in refractive
index was found to be almost two times more than that of
P3HT in the small voltage range of ꢀ2 to 2 V.
In passive gratings,[6] diffractive colors are produced, but
color switching is not possible since the refractive index of the
grating material is fixed. To generate multicolor and to switch
from one color to another, at least three or more gratings with
different grating parameters (period and thickness) are
required, which are then superimposed to produce a full
color scheme. For example, Knop reported multiple-phase
gratings using polyvinylchloride with different thickness over
400 nm to generate visible colors.[7] In a recent report,
microchannels of poly(dimethylsiloxane) (PDMS) were
used as a tunable visual color filter based on microfluidic
transmission and a shift from red to blue color was observed.[8]
The grating materials with different refractive index were
allowed to flow through the microchannels to obtain a differ-
ent colors. Ge et al.[9] employed colloidal photonic crystals
having reversible tunability in response to external magnetic
fields. Electroactive thin films patterned by photolithographic
[*] Dr. T. Bhuvana,[+] B. Kim,[+] X. Yang, H. Shin, Prof. E. Kim
Department of Chemical and Biomolecular Engineering, Yonsei
University
50 Yonsei-ro, Seodaemun-gu, Seoul 120-749 (South Korea)
and
Active Polymer Center for Pattern Integration (APCPI), Yonsei
University
50 Yonsei-ro, Seodaemun-gu, Seoul 120-749 (South Korea)
E-mail: eunkim@yonsei.ac.kr
Homepage: http://web.yonsei.ac.kr/APCPI
The yellow electrochromic polymer P(ProDOT-Ph) was
synthesized by a Suzuki coupling reaction following reported
procedures (see Supporting Information, Figure S1–S4).[14,15]
A solution with 0.4 wt% of the polymer P(ProDOT-Ph) in
chloroform was used for the preparation of the thin film and
patterning. The line grating patterns of the polymer were
fabricated with the micromolding in capillaries (MIMIC)
method.[16] The overall scheme for simple and large area (ca.
1.5 cm2) patterning of P(ProDOT-Ph) using the elastomeric
PDMS stamp is shown in Figure 1a. An elastomeric PDMS
stamp with a line grating with a 10 mm period, 5.6 mm relief
feature, and 1 mm pattern depth used was confirmed by
scanning electron microscope (SEM) as shown in Figure 1b.
The SEM image in Figure 1c shows the polymer gratings in
large area created by MIMIC with an applied pressure of
0.0015 MPa. The patterns were of uniform thickness and were
[+] These authors contributed equally to this work.
[**] This study was financially supported by a National Research
Foundation (NRF) grant funded by the Korean government (MEST)
through the Active Polymer Center for Pattern Integration (R11-
2007-050-00000-0), the Pioneer Research Center Program (2011-
0001672), and the Converging Research Center Program through
the Ministry of Education, Science and Technology (2010K001430).
This work was supported (in part) by the Yonsei University Research
Fund of 2010.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201205206.
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sulfonimide/propylene carbonate (LiBTI/PC) was
filled between the two electrodes. Figure 2 shows the
effect of the external potential on the optical proper-
ties of the EC cell. The cell with the unpatterned
polymer film was yellow colored at ꢀ2 V at neutral
state,[11,14] with an absorption at 440 nm with a low
transmittance of 14% as shown in Figure 2a. When the
applied voltage was 1.5 V, the absorbance at 440 nm
dropped by 25% and the absorption over the rest of
the visible spectral range was slightly increased. As the
voltage was further increased to 2 V (oxidized state),
a small but broad absorption band appeared at around
620 nm along with a weak band at 440 nm and
transmittance of 50%. As shown in the inset of
Figure 2a and Figure S6 in the Supporting Informa-
tion, optical images of the EC cell were vivid yellow at
ꢀ2 V and greenish blue at 2 V. Such electrochromism
originates from the reversible redox reactions de-
scribed in Figure 2c, in good agreement with previous
report.[11,14]
For the EC cell with a patterned polymer film,
there was no distinct absorption peak at around
440 nm (Figure 2b) in comparison to the unpatterned
film. Instead, the absorption band was observed at
around 525 nm with transmittance of 38%. Upon
application of 2 V, the color of the cell was very weak
and the transmittance increased to 60%. As shown in
the inset of Figure 2b and Figure S6, optical images of
the EC cell were pale yellow at ꢀ2 V and transparent
greenish blue at 2 V. It would be expected that the
absorbance of the patterned film should be higher than
Figure 1. a) The MIMIC method for the preparation of P(ProDOT-Ph) patterns:
1) Patterned PDMS stamp placed firmly on the ITO glass. Polymer solution was
cast on one end of the PDMS stamp allowing the solution to fill the micro-
channels; 2) Solvent evaporation at room temperature; and 3) Detachment of
PDMS stamp. SEM images of b) the patterned elastomeric PDMS stamp and
c) the patterned P(ProDOT-Ph), inset: its cross-sectional profile. d) A plot of
the applied pressure on the PDMS stamp against the thickness of the polymer
pattern obtained by MIMIC method.
parallel to each other. The period of the polymer grating was
10 mm with a relief feature of 4.2 mm separated by a distance
of 5.8 mm. The thickness of the pattern (ca. 980 nm)
was found to reach near the depth of the stamp
itself. This patterning process was modified by
application of different pressures on the elasto-
meric PDMS stamp during patterning. The appli-
cation of pressures ranging from 0.0015 to
0.03 MPa resulted in a modification of the thick-
ness profile on the patterned polymer. Figure 1d
shows the effect of pressure on the thickness of the
pattern. At higher applied pressure, the elasto-
meric stamp was compressed more, resulting in
a decrease in the thickness of the channels in the
stamp itself. When approximately 0.01 MPa of
pressure was applied, the feature size decreased
to half of the original thickness (Supporting
Information, Figure S5). This method was pre-
ferred as it resulted in isolated thick strips of the
polymer on the substrate, whereas surface pattern-
ing or imprinting methods resulted in a continuous
residual polymer layer on the substrate between
the gratings.
that of the unpatterned films owing to its greater thickness
despite the fill factor being 0.42, but the absorption spectra
Figure 2. Absorption spectra recorded at different voltages from ꢀ2, 0, 1, and 2 V
(from 1 V, with 0.2 V increase) for a) a 180 nm thick unpatterned P(ProDOT-Ph)
film and b) a 770 nm thick patterned P(ProDOT-Ph) film. Inset showing the optical
images taken at ꢀ2 V (neutral) and 2 V (oxidized). Scale bar, 0.5 cm. c) The
electrochromism arises from the reversible redox reactions in the EC cell (BTI=bis-
An electrochemical (EC) device was fabricated
with the polymer film on indium-tin oxide (ITO)
glass as a working electrode and bare ITO glass as
a counter electrode. A liquid electrolyte consisting
of a 0.2m solution of lithium bistrifluoromethane- trifluoromethanesulfonimide).
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revealed that the patterned film is more transparent and
uniform over the visible range as a result of diffraction
through the patterned polymer.
a 635 nm laser. Such a large variation in DE value is ascribed
to the large grating thickness (770 nm) and is important for
color modulation in a cell. From the DEm=1, the refractive
index difference between the polymer pattern and electrolyte
(Dn) was estimated 0.313 and 0.144 at ꢀ2 V to 2 V,
respectively, implying the maximum Dn of 0.169 within this
potential ranges.
Results on color generation and modulation through the
polymer grating are shown in Figure 3. A white light emitting
diode (LED) light was used to illuminate the EC cell and the
When conducting polymers undergo reversible redox
reactions, this process is usually accompanied by a change in
the refractive index of the polymer which is a result of doping
and de-doping of counterions from the electrolyte medi-
um.[12b] Another important phenomenon that is also associ-
ated with the electrochemical reaction in these polymers is
the volume change. The volume change is as high as 30–40%
for polypyrorrole derivatives[13b] whereas for poly(3,4-ethyl-
enedioxythiophene) (PEDOT) derivatives it is less than
1%,[13f] and hence in the present study the volume change
in the polymeric matrix can be ignored. We opted to measure
the change in refractive index by measuring the change in
diffraction efficiency (DE). The DE of a grating is determined
from the ratio of the total diffracted intensity (Idiff) and the
incident light intensity (I0) as DE = Idiff/I0 where Idiff is the sum
of the intensity for all of the diffracted light. For a square-
wave phase grating, the DE of the zeroth order (DEm=0) (m
refers to the diffraction order) and first order (DEm=1) can be
calculated based on the Equations (1)–(3)
DEm¼0 ¼ cos2ðx=2Þ
DEm¼1 ¼ f2=p sinðx=2Þg
x ¼ 2pt ðn0ꢀn1Þ=l
ð1Þ
ð2Þ
ð3Þ
2
where x is the phase difference between the grating material
and the surrounding medium, t is the thickness of the grating,
and n0 and n1 are the refractive index of the grating material
and the surrounding medium, respectively. The line grating
creates a spatially periodic modulation of the refractive index,
and x depends on the peak-to-peak amplitude path of the
light. From Equations (2) and (3), by knowing DEm=1 for
a given l, the change in the refractive index, Dn =j n0ꢀn1 j ,
can be calculated. In the present study, the refractive index of
the polymer can be reversibly changed by electrochemical
doping and de-doping process. For a given thickness, the color
of the polymer grating can be modulated conveniently in an
EC cell, without changing the thickness of the polymer
grating. When Dn changes reversibly, it is possible to tune the
color of the pattern reversibly and generate new color in the
intermediate value of Dn.
The schematic of the experimental set up for DE
measurement is shown in Figure S7a in the Supporting
Information. For calculation of the refractive index for the
polymer in the neutral and oxidized states, the first order DE
was measured. The diffraction pattern at ꢀ2 and 2 V of the
EC cell with a 770 nm thick polymer pattern is shown in
Figures S7b,c. The central zeroth-order diffracted light and
the adjacent first-order and second-order diffraction at ꢀ2 V
were stronger than at 2 V. The first-order spots were equi-
distant from the zeroth-order spot implying the diffraction
pattern is completely symmetric. The intensity of the two
spots arising from the first-order diffraction was measured
and they were found to be of similar intensity (with deviation
less than 10 mW). DEm=1 was determined as 0.35 and 0.11 for
the polymer pattern at ꢀ2 and 2 V, respectively, using
Figure 3. a) The experimental set up for the color generation in trans-
mission mode. A white LED light illuminated the EC cell to generate
the various colors under controlled applied voltage. b) Optical images
of colors obtained at different voltages for 1) a 770 nm thick polymer
grating and 2) a 180 nm thick unpatterned film under a white LED
light source. Scale bar, 0.3 cm. c) Plot of diffracted light intensity (I)
for a 770 nm thick polymer grating, at different applied voltages; ꢀ2,
0, 1, and 2 V (with 0.1 V increase between 1 V and 2 V) under a white
LED light source. Inset: Plot for the normalized diffracted light
intensity (I) at different voltages.
zeroth-order diffracted light was collected in transmission
mode (see Figure 3a) at applied voltage varying from ꢀ2 to
2 V. The spectrum of the white light source employed is
provided in Figure S8 (see Supporting Information). The
colors obtained for a 770 nm thick grating device at fixed
angle of incidence and a 180 nm thick unpatterned film device
at different voltages are shown in Figure 3b and Figure S9.
The color obtained for the grating device (Figure 3b(1) and
S9) was bright green at ꢀ2 V and it remained green until 0 V
and upon application of 1 V, it turned to blue. In the
intermediate doped states, different color mixing occurred
such as blue color at 1.5 Vand finally purple upon application
of 2 V. In the case of the unpatterned film (Figure 3b(2)), the
transmitted light appeared as orange brown (ꢀ2 V) and
bluish purple (2 V), which is different from the structural
colors observed from the patterned film. This experiment
distinguishes the color change by diffraction from electro-
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chromism. Different colors are generated on application of
external potential on patterned polymer which is otherwise
not possible with mere thin polymer film. Figure 3c shows the
spectral change of the EC cell with a 770 nm thick polymer
pattern by the voltage variation from ꢀ2 to 2 V at fixed angle
(558). Upon application of a voltage of ꢀ2 V, the lmax of the
diffracted light of the EC cell was 536 nm and it changed to
460 nm at 2 V. The normalized diffracted light intensity
obtained for ꢀ2 to 2 V are shown in inset Figure 3c. Indeed
vivid colors were generated from the EC grating device when
transmitted from a white LED light source.
With the change in the redox (doping) state of the
polymer by the application of external voltage, Dn changes
causing n0 to change. Color modulation with grating devices
of different thickness is demonstrated in Figure 4. A plot of
change in lmax value and Dn on application of external
potential for 770 nm thick grating device is shown in Fig-
ure 4a. The lmax and Dn values did not change significantly
between ꢀ2 and 0 V as the polymer was in neutral state, but
they changed drastically as the voltage was varied from 1 to
2 V, since the polymer was electrochemically doped and this
resulted in the change of refractive index of the polymer. At
voltages from ꢀ2 to 2 V, the lmax value decreased from 536
and 460 nm. The Dn change is correlated well with the lmax
values at different voltages and especially the maximum
change was observed between 1 and 2 V. Thus, the variation in
lmax values is primarily due to Dn change, which being
a complex function shows little variation in the trend in Dn
change in comparison to lmax values (Figure 4a, inset). This
emphasizes that Dn change can be achieved by application of
external potential and this can be utilized efficiently for color
modulation in diffraction gratings without changing any
grating parameters.
According to Equation (3), the color modulation should
be more enhanced for thicker gratings. To elucidate this fact,
the gratings devices with different thickness namely 100, 210,
430, 630, 770, and 980 nm were fabricated with a constant
period of 10 mm. The change in DE between the redox states
was higher for thicker gratings, as indicated from the plot for
the DE change for the first-order diffraction, as a function of
thickness at ꢀ2 and 2 V (Figure S10, see Supporting Infor-
mation). The color ranges covered by the EC devices of
different thickness are compared by colorimetric a*b* (CIE
1976 L*a*b* Color Space)[17] color coordinates at different
voltages as seen in Figure 4b. In this plot, x-axis represents
the change from red to green moving from a positive to
a negative value of a* and y-axis represents the color change
from yellow to blue color moving from a positive to a negative
value of b*. It becomes very clear that unlike the unpatterned
film (Figure 4b, black diamond), which only covered the
yellow range, the patterned polymer (770 nm thick, green
circle) had a wide range of color generation covering yellow-
ish green to blue and to purple. The EC device with a 980 nm
thick grating showed the widest range of colors than the
others. Interestingly, all the patterns gave
reddish magenta color, which clearly distin-
guished from the EC color of the unpatterned
film.
Figure 4c illustrates the change in lmax
value on application of external voltage
between ꢀ2 and 2 V for devices with different
thickness. As expected, there was no significant
change in lmax value in the voltage range of ꢀ2
to 0 V. The change in lmax value on application
of positive bias was maximum for 980 nm thick
device, change in lmax value of 77 nm. Thus, lmax
change was correlated to the applied voltage
between ꢀ2 and 2 V for the EC device with
different grating thickness. As the thickness of
the grating decreased, the color range covered
by the grating at the different voltage also
decreased, as expected from Equation (3).
Simulated spectra for DEm=0 were plotted for
the visible spectra of the EC device at ꢀ2 V
with different grating thicknesses using Equa-
tion (1) knowing the refractive indices of the
Figure 4. Comparison of the color change for the patterned and unpatterned device at
different applied voltages. a) A plot of variation in lmax (green) and Dn (red) for
a 770 nm thick grating device as a function of applied voltage. Inset: Plot of lmax (in
energy unit eV and wavelength unit nm, as a function of Dn). b) Colorimetric a*b* color
coordinates plot corresponding for the grating device of different thickness
(orange 100 nm, brown 210 nm, magenta 430 nm, blue 630 nm, green 770 nm, and
red 980 nm) compared to the unpatterned film with a thickness of 180 nm (black
diamond) for different applied voltages. c) A plot of lmax for the grating devices of
different thickness as a function of applied voltage. d) A plot of change in diffracted
intensity (I) and lmax value for a 770 nm thick patterned EC device as a function of time
illustrating the memory effect. The device was held at ꢀ2 V before the power supply
was turned off.
polymer and electrolyte (Figure S10a). The
experimental results were quite comparable
to that of the theoretical plot shown as a dotted
line in Figure S10b, supporting the validity of
Equation (3).
The experimental intensities were higher
than the calculated values in the lower wave-
length range (Figure S10b). This is probably
due to the absorption peak in the yellow color
range. The mechanism of color modulation in
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reactions of the polymer, which offers stability at different
voltages. The oxidized or neutral states are stable even after
removal of electric power supply. As shown in Figure 4d, the
diffracted color and intensity of the EC grating device at ꢀ2 V
was maintained even after the electric power supply was
turned off. In the power-off state, the decrease in diffracted
intensity was less than 10% even after 40 h. This memory
effect is important for display devices which will allow less
consumption of power. Thus, the present study clearly
elucidates that the EC devices using patterned electroactive
polymers film can induce a reversible color change along with
a refractive-index change through electrochemical doping/de-
doping. The application of external potential gives control
over the modulation of desired color. This also demonstrates
a simple method for vivid color generation in the EC device
which is otherwise not possible by mere use of electroactive
thin film without any patterns.
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fabricate large-area thick-line gratings. Under the illumina-
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the 770 nm thick grating, the color varied from green to
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Received: July 3, 2012
Revised: August 21, 2012
Published online: December 6, 2012
Keywords: color modulation · diffraction · doping ·
.
electrochemistry · electrochromic polymer
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