Tailoring the electrochromic properties of devices via polymer blends, copolymers, laminates and patterns

Tailoring the electrochromic properties of devices viapolymer blends, copolymers, laminates and patterns

Ian D. Brotherstona, Dhurjati S.K. Mudigondaa, Jessica M. Osborna,Julie Belka, Judy Chena, David C. Lovedaya, Je�rey L. Boehmea,

John P. Ferrarisa,*, David L. Meekerb

aDepartment of Chemistry, The University of Texas at Dallas, M/S BE 26, P.O. Box 830688, Richardson, TX 75083-0688, USAbUS Army Engineer Waterways Experiment Station, 3909 Halls Ferry Road, Vicksburg, MS 39180, USA

Received 7 September 1998; received in revised form 6 October 1998

Abstract

The ability to tune colour constitutes an important goal in the design of electrochromic devices. The majority ofsingle electrochromic materials, whether inorganic or organic have a very narrow spectral colour change, however.

Complex colours such as browns and tans that mimic natural vegetation and soils are not inherent to thesematerials. We have explored several strategies to achieve these colours including the use of copolymers that containtwo distinct moieties, blends of polymers that have di�erent colour changes, laminating two di�erent polymers on

the working electrode, and using ®ne patterns of two di�erent materials to confuse the eye. We have constructedsolid state electrochromic devices using ITO/Mylar substrates, a solid polymer electrolyte, a V2O5 counterelectrodeand working electrodes that were made via one of the four methods outlined above. The colour changes of these

devices were measured using spectrocolourimetry conforming to the CIE standard. The colour changes observedwere similar to those of natural vegetation and soils. # 1999 Elsevier Science Ltd. All rights reserved.

Keywords: Electrochromic; Polymer; Blends; Copolymers; Patterns; Laminates

1. Introduction

Electrochromism, the phenomenon whereby a ma-

terial changes colour with a change in oxidation state

which is induced by an applied electric ®eld or current,

has attracted world-wide attention since the seminal

papers of Deb [1,2]. Both inorganic and organic ma-

terials are capable of displaying electrochromism.

Although there are many di�erent categories of elec-

trochromic systems [3] the type that has attracted most

attention in the research literature is that of surface

con®ned thin ®lms consisting of metal oxides or poly-

mers deposited onto an electrode. The oxidation or re-

duction of these ®lms is nearly always associated with

reversible ion insertion and extraction across an elec-

trochromic material/electrolyte interface with comp-

lementary electron transfer across the electrochromic

material/electrode interface. Oxidation, the removal of

an electron from the material, is referred to as p-dop-

ing, while reduction, the addition of an electron to the

material, is referred to as n-doping. The majority of

organic electrochromic materials change colour with

reversible p-doping whereas inorganic electrochromic

materials undergo reversible n-doping.

Electrochimica Acta 44 (1999) 2993±3004

0013-4686/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved.

PII: S0013-4686(99 )00014-6

* Corresponding author. Tel.: +1 972 883 2905; fax: +1

972 883 2925.

E-mail address: [email protected] (J.P. Ferraris)

The ability to tune the colours observed in the oxi-dised/reduced states of electrochromic devices has

received considerable attention in recent years [4,5].Devices based upon organic conducting polymers haveshown particular promise in this respect compared to

devices based on inorganic materials due to the relativeease of ``molecular engineering'' that organic systemsa�ord. Organic based systems also have the added ad-

vantage that they are often more processable thaninorganic systems.All conducting polymers are potentially electrochro-

mic in thin ®lm form. Linear, conjugated polymersbased upon polythiophene, containing various substitu-ents, such as linear or branched alkyl chains, oxyalkylchains and aryl groups have received particular atten-

tion [6±8]. Without such groups the processibility ofsuch materials is limited. A limitation of these ma-terials is the p-doping induced colour change is often

red to blue and the achievement of other colours is noteasily accomplished. Another class of conducting poly-mers that have exhibited electrochromism are non-con-

jugated polymers that have pendant groups. Here thechromophores responsible for the change in theabsorption spectra of the oxidised material are well

de®ned and a tunable distinct colour change is possible[9±12].The aim of the work presented here was to control

the colour change observed in an electrochromic ma-

terial in a predictable and reproducible manner. Thecolours desired in this study were those of natural veg-etation, e.g. green, brown, tan etc. Other factors that

were important included the need for the device to be¯exible, light weight, and easily fabricated. The life-times of the devices were also of concern but for the

envisaged application are much less demanding thanthose required for electrochromic windows. Finally,device switching times could be in the range of minutesand uniformity of the colour change was not an issue;

in fact a speckled/blotchy e�ect could be a distinct ad-vantage.Polymer blends have been suggested as a simple way

of engineering the speci®ed colour change in an elec-trochromic device [4]. There have been numerousreports on the electrochromic properties of polymer

blends combining thermoplastics or elastomers withpolymers such as polypyrole, polythiophene and polya-niline, but most of these focus on improved processi-

bility and/or mechanical properties [13]. In contrastthere have been a rather limited number of studieswhere a systematic change in the electrochromic proper-ties has been attempted using blends. We prepared

blends of known electrochromic materials in di�erentratios and investigated the compositional dependenceof the colour changes. Poly(N-vinylcarbazole), PVK,

an electrochromic material that is colourless in its neu-tral state and green in its p-doped state [14] and

poly(N-phenyl-2-(2 '-thienyl)-5-(50-vinyl-20-thienyl)pyr-ole), PSNPhS, whose colour change is from yellow in

its neutral state to a reddish brown in its p-doped state[12] were blended in three ratios. Both of these systemsare non-conjugated polymers containing pendant elec-

trochromic groups.An alternative approach to the one outlined above is

to use copolymers. The colour of the ®nal material is

controlled by the ratio of the respective comonomers.The materials we investigated using this approach werea series of copolymers containing N-vinylcarbazole

(NVC) and (N-phenyl-2-(2 '-thienyl)-5-(50-vinyl-20-thie-nyl)pyrole (SNPhS) and another series of copolymerscontaining 5-vinylterthiophene (SSS) and SNPhS.A third approach that can be used to tailor the elec-

trochromic response is to layer homopolymers withwell de®ned electrochromic properties on top of eachother. Here the neutral and doped colours of such

laminated electrodes would follow basic colour sub-traction theories so that by careful choice of the homo-polymers facile colour control of the device can be

achieved. We have fabricated devices using poly(ethyle-nedioxythiophene), PEDOT, a polymer that undergoesa transition from blue to colourless upon oxidation

[15], and poly(N-methylpyrole), PN-MeP, a polymerthat changes from transparent to brown upon oxi-dation [16].The ®nal technique we employed relies on ®nely pat-

terning electrodes with two di�erently coloured ma-terials in order to confuse human colour perception.When the patterns consist of at least one electrochro-

mic material, electrochromic devices in which the col-our change observed by the human eye di�ers fromthe colour change observed on the microscopic level

are possible. Electrodes were constructed via a screenprinting process that produced ®ne patterns of V2O5

and PEDOT with di�erent patterns and materialratios. In these electrodes only cation movement was

facilitated upon doping/dedoping.We constructed solid state devices comprised of

working electrodes made via one of the four processes

outlined above, deposited onto a Mylar/ITO support,a V2O5 or plain Mylar/ITO counterelectrode and asolid polymer electrolyte. The colour changes of these

devices were measured using spectrocolourimetry con-forming to CIE standards.

2. Experimental

2.1. Blends

The preparation of the monomers and polymerswere described previously [4]. Polymer blends were pre-

pared by mixing various mass ratios of PVK andPSNPhS in chloroform. The composition of the blends

I.D. Brotherston et al. / Electrochimica Acta 44 (1999) 2993±30042994

were as follows: Blend I: PSNPhS:PVK (1:4); Blend II:PSNPhS:PVK (3:2); Blend III: PSNPhS:PVK (4:1).

These solutions (2 ml at 4% w/v in chloroform)were cast onto 15 cm� 6 cm ITO/Mylar strips (AltairO2, Southwall Technologies) using an Acculab Jr.2

30 casting rod and the polymer ®lms were air driedbefore device assembly. The electrochromic materialcast onto ITO serves as the working electrode (anode)in the device. The blank ITO serves as the counterelec-

trode (cathode). The polymer electrolyte was preparedby mixing poly(methylmethacrylate) (Mw 120,000) (500mg), propylene carbonate (1 ml), ethylene carbonate (2

g), and tetrabutylammonium tetra¯uoroborate (300mg) in acetonitrile (3 ml). The electrolyte was spuncast onto the blank ITO and the device was assembled

in a sandwich composite (Fig. 1).

2.2. Copolymers

N-vinylcarbazole (NVC) was purchased fromAldrich and was used without further puri®cation. The

syntheses of SNPhS and SSS are described elsewhere[9,17]. The copolymers of NVC with SNPhS and SSSwith SNPhS were prepared via a cationic polymeris-

ation from feedstock ratios of 1:4 (CO-1), 3:2 (CO-2)and 4:1 (CO-3) (SNPhS:NVC) and 4:1 (CO-4), 1:1(CO-5) and 1:4 (CO-6) (SNPhS:SSS) respectively. Solidstate electrochromic devices were prepared as described

above.

2.3. Laminates

A solution containing 0.05 M PEDOT and 0.01 Msodium polystyrenesulfonate (Na-PSS) in an aqueous

suspension was obtained from Bayer2. This solutionwas then painted onto a 3 cm� 3 cm square section ofITO/Mylar (Altair O2, Southwall Technologies). The

resulting polymer ®lms were dried under atmosphericconditions. N-MeP was then electrochemically poly-merized onto the PEDOT painted electrodes in a

three-electrode cell using an aqueous solution of 0.05M N-MeP containing 0.01 M Na-PSS. Using anEG&G PARC 273A potentiostat, a constant potential

of 1.0 V was applied with respect to a Ag/AgCl refer-ence electrode. The resulting laminate electrode was

air-dried overnight.A solution of vanadium triisopropoxide (Alfa) and

isopropyl alcohol was made in a 1:150 volumetric

ratio. 1 ml of this solution was then spun onto a 3 cm� 3 cm blank ITO/Mylar substrate at 2000 rpm. Theresulting ®lm was then dried in a vacuum oven at 508Covernight and was used as the counterelectrode in theelectrochromic device. The solid state device wasassembled as described above.

Three di�erent solid state electrochromic deviceswere fabricated: the ®rst consisted of only PEDOT asthe working electrode, the second contained only PN-MeP as the working electrode, and the third device

consisted of a layer of PN-MeP laminated on top of aPEDOT layer which together formed the working elec-trode.

2.4. Patterns

The PEDOT/polystyrene sulfonate aqueous suspen-sion (Bayer), described previously was used. A sol±gelsolution of V2O5 was prepared by mixing vanadium

triisopropoxide (Alfa), isopropyl alcohol and water inthe ratio 1:50:50. These mixtures were then applied tothe ITO/mylar substrate via a screen printing process.

This allowed ®ne patterns of the PEDOT/V2O5 to befabricated with ease. These electrodes were then left toair dry. The patterns chosen were mainly based onstripes and checkerboards with di�erent spatial resol-

ution and varying amounts of the two materials. Solidstate devices incorporating these electrodes were thenconstructed using the V2O5 counterelectrode and the

solid polymer electrolyte described previously.

2.5. Colourimetric theory

Colour perception of an object is clearly di�erentfrom observer to observer. The retina located at the

back of the eye is composed of layers of photo-receptors called rods and cones. Rods allow us to dis-tinguish the brightness or darkness of an object. Cones

on the other hand are chromatic in nature, and permitthe observer to see an object's hue. The cones createelectrochemical signals that are then transported to thevisual cortex via the optic nerve. According to op-

ponent colour theory, specialized cells in the visualcortex can either interpret red/green signals or blue/yel-low signals [18]. This chromatic information acquired

by the visual cortex, along with the rod's achromaticvalue allows one to describe an object's observed col-our stimulus as a vector, Q, in a three-dimensional

space.In 1931 the Commission Internationale de

l'Eclairage (CIE) standardized the tristimulus values X,

Fig. 1. Typical electrochromic device construction.

I.D. Brotherston et al. / Electrochimica Acta 44 (1999) 2993±3004 2995

Y, and Z whose unit vectors map a three-dimensional

metric vector space. Usually, the colour of an objectsuch as an electrochromic device is evaluated in termsof three tristimulus primaries called red, green, and

blue. The device's colour can be de®ned by com-ponents along the R, G, and B axes which map thisRGB colour space. Problems occur when one or more

components are negative. To avoid this, the CIE intro-duced the XYZ tristimulus colour space where a colourstimulus vector, Q, is represented by Eq. (1).

Q � XX� YY� ZZ �1�X, Y, and Z represent components along the unit vec-

tor axes which map the tristimulus colour space. Thethree, positive tristimulus values are de®ned by the fol-lowing equations:

X � k

�lb�l�S�l�x�l� dl �2�

Y � k

�lb�l�S�l�y�l� dl �3�

Z � k

�lb�l�S�l�z�l� dl �4�

where b(l )dl is the re¯ectance of the sample at l+dl,

S(l ) is the spectral power distribution of the illumi-

nant, x(l ), y(l ), and z(l ) are the colour matchingfunctions, and k is a normalization constant. The col-our matching functions and the spectral power distri-

bution of the illuminant are standardized by the CIE.Thus, the tristimulus values are calculated by an inte-gration of the re¯ectance over a de®ned wavelength

range, typically 380±770 nm [19].The representation of colour stimuli in XYZ tristi-

mulus space as vectors is informative, but usually not

convenient in practice. Thus, a two-dimensional rep-resentation is commonly preferred, although some in-formation is lost. A useful representation is obtainedin the unit plane, X+Y+Z=1, of the XYZ tristimulus

space. All colour stimulus vectors or their extensionsmust intersect this plane. The location of this pointusually determines the vector's direction (hue and

chroma) but not magnitude (value). Convenient coor-dinate axes are obtained when the XYZ tristimulusaxes are orthogonal to each other and the intersection

point is transposed to the XY-plane. This two-dimen-sional plot of colour stimuli is called an (x,y )-chroma-ticity diagram. The chromaticity coordinates, x and y,are related to the XYZ tristimulus values by the fol-

lowing equations:

x � X

X� Y� Z�5�

Fig. 2. CIE 1931 (x,y )-chromaticity diagram with spectral locus and purple line.


y � Y

X� Y� Z�6�

There is a region, or gamut, in a chromaticity diagramwhere all colour stimuli must reside. This region's

boundary is called the spectral locus and is dependenton the colour matching function of the human eye.The ``purple'' line is one that connects the smallest

wavelength detectable with the eye (blue) to the largestobservable wavelength (red). Fig. 2 shows the spectrallocus and purple line with corresponding wavelengths

in an (x,y )-chromaticity diagram of a CIE 1931 stan-dard colourimetric observer.Some important predictability information can be

acquired through the investigation of the (x,y )-

chromaticity diagram. When two independent colourstimuli are present, they are represented by two distinctpoints on the chromaticity diagram. When one of

these colours is subtracted from the other, the resultingchromaticity coordinates of the colour stimulus mustlie on the line joining the two original chromaticity

coordinate pairs. Therefore, if one knows the chroma-ticity coordinates of two electrochromic polymers, one

could determine the approximate hue of a device fabri-cated from laminate layers of these polymers.

The re¯ectance spectra and colour measurementswere performed on a Hunter Lab MiniScan XE spec-trocolourimeter with the large view area option to

record di�use re¯ectance using a D65 Daylight illumi-nant. The MiniScan system is controlled through acomputer interface using HunterLab Universal soft-

ware. The colour data are displayed in CIE 1976L�a�b� coordinates using the CIE 1964 10 degree stan-dard observer model. In this opponent colour scale

coordinate system L� indicates the intensity of there¯ected light, the a� coordinate indicates a changefrom red (positive direction) to green (negative direc-tion) while the b� coordinate indicates a colour change

from yellow (positive direction) to blue (negative direc-tion) (Fig. 3). This data can be also be converted toCIE 1931 XYZ tristimulus values. The MiniScan XE

records the spectral re¯ectance over an area of 5.06cm2 with a wavelength range of 400 to 700 nm, with aresolution of 10 nm and a wavelength accuracy of 1

nm. The CIE coordinates are then extracted from thecomponents of the re¯ected light. The data can thenbe plotted in several formats, viz; a two or three

dimensional plot where either a pair of or all three ofthe variables L�, a� and b� form an orthogonal coor-dinate system; the total colour, represented by the vec-tor Q in Fig. 3, is calculated using Eq. (7),

Q ��L �2 �a �2 �b�2�

p�7�

or in an (x,y )-chromaticity coordinate system de®nedearlier.

Variations in the thickness of the electroactive poly-mer ®lms within the individual devices led to some¯uctuations in CIE L�a�b� coordinates. To reduce the

e�ect of these ¯uctuations, three devices of each copo-lymer were constructed and a series of experimentswere performed. Typically, ten separate measurements

were acquired and averaged for the doped and neutralstate of each device.

Fig. 3. CIE 1976 L�a�b� opponent colour scale with total col-

our vector Q.

Table 1

CIE 1976 L�a�b� colour coordinates and total colour for the homopolymers and polymer blends of PVK and PSNPhS in both the

doped and neutral states

Polymer Neutral Doped

L� a� b� Total colour L� a� b� Total colour

PVK 82.97 ÿ2.31 16.55 84.64 73.74 ÿ5.78 24.16 77.82

Blend I 84.32 ÿ1.35 17.43 86.11 76.55 2.99 20.33 79.26

Blend II 78.53 3.16 25.52 82.63 69.63 7.01 24.22 74.05

Blend III 75.51 5.68 26.65 80.24 55.42 10.16 26.13 62.11

PSNPhS 78.79 4.73 27.47 83.58 57.69 16.12 33.33 68.55


3. Results and discussion

3.1. Blends

The CIE 1976 L�a�b� coordinates for the homopo-lymers and the blends in the doped and neutral statesare given in Table 1. To the observer the neutral statesof PVK and PSNPhS are colourless and yellow, re-

spectively. The observed colour of the neutral ordedoped polymer is dependent on the amount ofPSNPhS present, as expected. The increase of a� and

b� in the doped state qualitatively indicates a colourchange towards orange (yellow and red mixture) onthe L�a�b� scale. Fig. 4 shows the that the total colour

of the doped device can be correlated (R 2=0.849) withthe percentage of PSNPhS in the blend. The negativeslope indicates that in the doped state the dominantchromophore is the PVK moiety.

Re¯ectance spectra of the polymer blends provideanother characterisation technique for these systems.In the neutral state these blends show little di�erences

in the re¯ectance spectra in the visible region but exhi-bit distinct di�erences in the doped states (Fig. 5) forwhich the greatest changes are the appearance of a

peak in the green near 450 nm and a broad peak inthe red region from 600 to 700 nm. The change in there¯ectance in the green region arises from the doped

state of PVK and the formation of PSNPhS derived

moieties in¯uence the red colours. The combination of

these species create the subtle shades of green, brown

and tan colours observed in the devices.

Fig. 4. Total colour vs. percentage of PSNPhS for doped electrochromic devices where the working electrodes were blends of

PSNPhS and PVK.

Fig. 5. Di�erences in re¯ectance spectra for devices where the

working electrodes were either the homopolymers PSNPhS

and PVK or the blends thereof, referenced to their doped

states: (A) PVK; (B) PSNPhS; (C) blend III; (D) blend II; (E)

blend I.


3.2. Copolymers

The stoichiometries of the copolymers of NVC withSNPhS and SSS with SNPhS were estimated fromtheir respective FTIR spectra. These indicated that the

compositions of the copolymers were in reasonableagreement with their monomer feedstock ratios (Table2 and Table 3). Electrochemistry and spectroelectro-chemistry of the copolymers con®rmed this obser-

vation.The CIE 1976 L�a�b� colour coordinates for both

the neutral and doped devices of the copolymers of

NVC with SNPhS and SSS with SNPhS are given inTable 4 and Table 5, respectively. As expected the col-our observed is directly related to the copolymer com-

position, with the a� value (a measure of the redcomponent) increasing with the increasing amount ofSNPhS for both copolymer systems investigated. The

data for the NVC with SNPhS copolymer is in excel-lent agreement with that obtained for the blends of thehomopolymers outlined above where the value of a� isseen to change with respect to the value of b�. Fig. 6and Fig. 7 show the total colour in the doped stateversus the percentage of SNPhS for the copolymers ofNVC with SNPhS and SSS with SNPhS respectively.

Both graphs illustrate a good correlation between thetotal colour and the stoichiometric ratio of the como-nomers. The negative value of the slope seen for the

PVK/SNPhS copolymer is again an indication that inthe doped state the dominant chromophore is thePVK. However, the positive slope seen for the SSS/

SNPhS copolymer indicates that the SNPhS is thedominant chromophore here.The homopolymer, PSSS, changes from yellow

orange in the neutral state to bluish purple in the

doped state, while the homopolymer, PSNPhS, changesfrom yellow to reddish brown upon oxidation. CO-4

was brown in the neutral state turning to reddishbrown upon doping, CO-5 turned from dark yellow to

grey-brown upon doping, and CO-6 turned purple-brown upon doping from brown-yellow in the neutralstate.

The di�erences in re¯ectance spectra of the dedopedstate referenced to the doped state of the copolymersof PVK with SNPhS are shown Fig. 8. These data sup-port the trend observed for the a� and b� coordinates

discussed above. The ®gure illustrates the decreasedre¯ectance in the range of 500 to 700 nm with increas-ing amount of SNPhS in the copolymer. The doped

state of the device is usually darker and therefore lessre¯ective than the dedoped state. This is especially truefor PVK, as it is colourless in the neutral dedoped

state but appears green when doped. This leads to alarge o�set from the origin in the di�erence spectra forPVK. The copolymer spectra show a combination of

PVK and PSNPhS characteristics in accordance withtheir respective comonomer ratios. This con®rms theconcept that when the electrochromic behaviour is dis-tinctly di�erent for the homopolymers, as is the case

here for PVK and PSNPhS, then copolymers based onthese electrochromes can be used to compositionallycontrol the colour change. This is almost certainly due

to the discrete nature of the electrochromic moietiesthat constitute the copolymers. These data are also inagreement with the re¯ectance data obtained for

blends of these polymers.

3.3. Laminates

Colour measurements of the PEDOT device weretaken in the doped and dedoped states. This wasachieved by varying the applied potential from 1.0 to

ÿ1.4 V in 0.2 V increments using an EG&G PARC273A potentiostat. Similarly, the colour coordinates of

Table 2

Feedstock ratio and estimated stoichiometry based on FTIR spectra for the copolymers of NVC with SNPhS

Polymer Feedstock ratio (SNPhS:NVC) Estimated stoichiometry (SNPhS:NVC)

CO-1 20:80 30:70

CO-2 60:40 61:39

CO-3 80:20 72:28

Table 3

Feedstock ratio and estimated stoichiometry based on FTIR spectra for the copolymers of SSS with SNPhS

Polymer Feedstock ratio (SNPhS:SSS) Estimated stoichiometry (SNPhS:SSS)

CO-4 80:20 71:29

CO-5 50:50 60:40

CO-6 20:80 11:89


Table 4

CIE 1976 L�a�b� colour coordinates and total colour for the homopolymers and copolymers of blends of NVC with SNPhS in

both the doped and neutral states



PVK 82.97 ÿ2.31 16.55 84.46 73.74 ÿ5.78 24.16 77.81

CO-1 71.88 3.91 25.25 76.29 67.08 4.86 17.58 69.52

CO-2 71.95 3.14 37.19 81.05 61.17 5.98 19.12 64.37

CO-3 62.44 8.83 45.67 77.86 46.69 7.52 20.00 51.35

PSNPhS 78.79 4.73 27.47 83.58 57.69 16.12 33.33 68.55

Table 5

CIE 1976 L�a�b� colour coordinates and total colour for the homopolymers and copolymers of SSS with SNPhS in both the

doped and neutral states



PSNPhS 68.58 5.02 26.97 73.87 65.03 9.77 17.76 68.12

CO-4 65.38 3.43 35.38 74.42 54.05 5.57 18.57 57.42

CO-5 57.94 14.37 38.01 70.77 48.58 8.69 20.06 53.27

CO-6 54.63 14.66 10.10 57.46 51.94 12.96 1.82 53.56

PSSS 56.04 18.17 10.31 59.81 51.73 12.17 ÿ3.30 53.24

Fig. 6. Total colour vs. percentage of SNPhS for doped electrochromic devices where the working electrodes were copolymers of

PSNPhS and PVK.


the PN-MeP device was measured at applied potentials

ranging from 2.0 to ÿ1.2 V in 0.2 V increments.

Colour data for the laminate device were obtained at

potentials from 1.4 to ÿ1.0 V in 0.2 V increments.

Devices subjected to applied potentials above or below

the amounts indicated above showed no apparent

change in their colour. Thus, the maximum and mini-

mum potentials for each device correlate to fully dop-ing and fully dedoping the devices, respectively.

Fig. 9 shows a CIE 1931 (x,y )-chromaticity diagramfor the three devices. The data show that the (x,y )coordinates of the laminate device lie between thePEDOT and PN-MeP devices as expected. The non-

linearity in each of the device's chromaticity plots arediscussed by Hyodo in his investigation of the chroma-ticity values observed as devices were cycled from the

dedoped to the doped state [20].Fig. 10 shows the (x,y )-chromaticity coordinates for

PEDOT, PN-MeP and laminate devices in their fully

doped state. These coordinate pairs vary linearly witha correlation coe�cient of 0.995. Similarly, Fig. 11 is aplot of (x,y )-chromaticity coordinates for the corre-

sponding devices in their fully dedoped state. Again,these points vary linearly with a correlation coe�cientof 0.988. These ®gures indicate that under potentialcontrol, the laminated device's observed colour can be

considered as an apparent subtractive colour combi-nation of the two individual homopolymers. Thus bymaking a laminated device with polymers of known

electrochromic properties, the apparent colour of thelaminated device could be predicted using an (x,y )-chromaticity diagram.

3.4. Patterns

Fig. 12 shows the templates used for the screen

Fig. 7. Total colour vs. percentage of SNPhS for doped electrochromic devices where the working electrodes were copolymers of

SNPhS and SSS.

Fig. 8. Di�erences in re¯ectance spectra for devices where the

working electrodes were either the homopolymers PSNPhS

and PVK or the copolymers thereof, referenced to their

doped states: (A) PVK; (B) PSNPhS; (C) CO-6; (D) CO-5;

(E) CO-4.


Fig. 9. CIE 1931 (x,y )-chromaticity diagram for devices held at a series of potentials where the working electrodes were PEDOT,

PN-MeP and a laminate.

Fig. 10. CIE 1931 (x,y )-chromaticity coordinates for doped devices where the working electrodes were PEDOT, PN-MeP and a

laminate.


printing process. Four di�erent patterns were investi-

gated; a thin stripe/thick stripe pattern, a pattern con-

taining stripes of identical size, a checkerboard pattern

and a pattern containing ®ne dots. By carefully prepar-

ing the solutions for the screen printing process, the

patterns could be produced with excellent spatial resol-

ution. The amount of PEDOT and V2O5 deposited

was varied by altering the number of applications ofeach material. The devices appear yellow/green to theeye due to the interaction of the blue PEDOT and the

yellow V2O5. The data obtained for a device basedupon the checkerboard pattern where the respectiveratios of PEDOT and V2O5 are 44:56 are shown in

Fig. 13. Upon doping the devices became lighter (L�decreased) and more yellow in appearance (a�increased). The value of b� remained essentiallyunchanged after the ®rst cycle due to the relatively

thick PEDOT deposit. Although the changes recordedfor this device are small at present, re®nement of thescreen printing technique to deposit thinner layers with

more careful control of the relative amounts ofPEDOT and V2O5 is expected to lead to excellent col-our control.

4. Conclusions

Four di�erent techniques have been demonstrated totailor the colour of an electrochromic device. Devices

with prede®ned colour changes that are distinct fromthe parent polymers can be obtained from blends,copolymers and laminates. These methods have

resulted in devices that have the colours of natural veg-etation: green, brown and tan, which was the aim ofthis project. The use of patterns of electrochromic

Fig. 11. CIE 1931 (x,y )-chromaticity coordinates for dedoped devices where the working electrodes were PEDOT, PN-MeP and a

laminate.

Fig. 12. The templates used for the screen printing process

used in the creation of patterned devices.


materials has also been demonstrated and holds prom-

ise for achieving an even greater spectral range of col-our.

Acknowledgements

This work was supported by the US Army EngineerWaterways Experiment Station and the O�ce of theSecretary of Defense Augmentation Award for Scienceand Engineering Research Training.

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Documents

Tailoring the electrochromic properties of devices via polymer blends, copolymers, laminates and patterns