electrocolorimetry of electrochromic materials on flexible ito electrodes
TRANSCRIPT
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Solar Energy Materials & Solar Cells 92 (2008) 980– 985
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Solar Energy Materials & Solar Cells
0927-02
doi:10.1
$ The
on Elect� Corr
E-m
journal homepage: www.elsevier.com/locate/solmat
Electrocolorimetry of electrochromic materials on flexible ITO electrodes$
Carlos Pinheiro a,b, A.J. Parola a,�, F. Pina a, J. Fonseca c, C. Freire c
a Requimte, Dep. Quımica, FCT, Universidade Nova de Lisboa, 2829-516 Caparica, Portugalb YDreams, Madan Parque, Quinta da Torre, 2829-516 Caparica, Portugalc Requimte, Dep. Quımica, Faculdade de Ciencias, Universidade do Porto, Rua do Campo Alegre, 4169-007 Porto, Portugal
a r t i c l e i n f o
Article history:
Received 17 October 2007
Accepted 1 March 2008Available online 15 April 2008
Keywords:
Electrochromism
Electropolymerization
Colorimetry
Salen ligands
Prussian blue
48/$ - see front matter & 2008 Elsevier B.V. A
016/j.solmat.2008.03.001
original version of this paper was presented
rochromism (IME-7), Istanbul, Turkey, 3–7 S
esponding author.
ail addresses: [email protected] (A.J. Parola), a
a b s t r a c t
Electrochromic materials are characterized by their colour changes upon applied voltage. Colour can
mean many things: a certain kind of light, its effect on the human eye, or the result of this effect in the
mind of the viewer. Since the electrochromic materials are developed towards real life applications it is
relevant to characterize them with the usual commercial colour standards. A colorimetric study of
electrogenerated Prussian blue and electrogenerated polymers based on salen-type complexes of Cu(II),
Ni(II) and Pd(II) deposited over transparent flexible electrodes of polyethylene terephthalate coated
with indium tin oxide (PET/ITO electrodes) was carried out using the CIELAB coordinates. A cuvette with
a designed adapter to allow potentiostatic control was placed on an integrating sphere installed in the
sample compartment of a spectrophotometer to run the colorimetric measurements. The colour
evolution in situ was measured through the transmittance of the films by potentiostatic control.
Chronocoulometry/chronoabsorptometry was used to evaluate maximum coloration efficiencies for the
coloration step: 184 (Pd), 161 (Cu) and 83 cm2/C (Ni) and for bleaching: 199 (Pd), 212 (Cu) and 173 cm2/C
(Ni) of the Pd, Cu and Ni polymer films, respectively. The Prussian Blue/Prussian White states over the
PET/ITO films were relatively reversible while the reversibility and stability of the polymers based on
the metals salen-type complexes depends on the metal, Pd being the most stable.
& 2008 Elsevier B.V. All rights reserved.
1. Introduction
Chromogenic materials have shown a great potential for avariety of applications [1,2]. Typically, thermochromic and photo-chromic phenomena were exploited to produce dynamic colourfor funny products (t-shirts, cups, toys, etc.) [3,4], warningmessages for objects reaching high temperatures [5], securityinks for documents [6], photochromic lenses for sunglasses [7],etc. Electrochromism is another kind of chromogenic phenomenainvolving an electrochemical cell embedded in materials, attract-ing the attention of both the academic [8] and the industrialcommunity [9,10].
Electrochromic materials have the ability to change its opticalproperties (in the UV/Vis and/or the NIR spectral region) whensubmitted to a determined electric potential. Several materialshave shown electrochromic properties [11] like the well-knownPrussian blue (PB) inorganic polymer whose colour changes fromuncoloured (Prussian white, PW) to blue (PB) and finally to yellow(Prussian yellow, PY).
ll rights reserved.
at 7th International Meeting
eptember 2006.
[email protected] (C. Freire).
The evolution of new technologies like transparent flexibleelectrodes [12,13] fit in a third wave of computing, usuallyreferred to as ubiquitous computing [14], and the limitationsreached in conventional ones like display technology lead to thedesign of new products concepts. Electronic paper, flexibledisplays, transparent displays, smart packaging, smart windowsare particularly attractive areas for the use of electrochromicmaterials [15].
Full colour characterization of electrochromic systems isessential towards applications. Usually the optical changes aredescribed as absorbance or transmittance spectral variations,although spectrophotometric data does not give friendly informa-tion about colour. Colour is a very subjective and environment-dependent property, influenced by object, light source spectralproperties, level of illumination, background lightness andpsychological factors [16]. Colorimetry is a science devoted tothe colour measurement/characterization trying to obtain themost reliable colours coordinates. The CIELAB coordinates arebased on colour matching (CIE system) by a linear combination ofthree ‘‘virtual’’ primary colours X, Y and Z—tristimulus values.Because this tristimulus values are not visually uniform, a non-linear transform follows, resulting in opponent-type coordinatesL*, a* and b*. The parameter L* represents lightness, a* redness-greenness and b* yellow-blueness. These coordinates can becalculated from reflectance or transmittance spectra [16]. Besides
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the colour characterization it is important to determine theefficiency of the electrochromic system—coloration efficiency(CE). Reynolds et al. [17] have proposed standard criteria of CEtrying to uniform values obtained in different ways by differentresearch groups and for different electrochromic materials. CE isdefined as the ratio of the absorbance variation (DA), at a specifiedwavelength (usually the maximum absorption wavelength, lm), tothe injected/ejected charge per unit electrode area (DQ0) [17]. Thiseffort to characterize the colour using standard coordinates andthe determination of CE can give very helpful data for possibleproduct applications:
CE ¼DAðlmÞ
DQ 0(1)
Late transition metal complexes (M ¼ Ni, Cu, Pd and Pt) withsalen-type ligands are oxidatively polymerized at electrodesurfaces, in moderately/weak donor solvents, to generate electro-active films [18]. Although based on bona fide coordinationcompounds, these films exhibit properties that cannot beattributed to an aggregation of individual complexes. Rather, theybehave like polyphenylene compounds, with the metal ion actingas a bridge between biphenylene moieties, analogous to conduct-ing organic polymers [18–22]. They show physicochemical andelectronic properties that are a function of electrochemicallycontrolled redox applied potential. In previous spectroelectro-chemical studies on poly[M(salen)], M ¼ Ni and Cu [21,22], itwas possible to verify that their electronic spectra showsignificant changes on going from the reduced (undoped) to theoxidized (doped) state, which anticipates important electrochro-mic properties.
In this paper we report on the deposition of a new family ofelectrochromic inorganic polymer films based on salen-typecomplexes (Scheme 1) on commercial flexible polyethyleneterephthalate films coated with indium tin oxide (PET/ITOelectrodes) and the deposition of the well-known electrochromicmaterial PB for comparative purposes. Colour and CE weredetermined by colorimetry and tandem chronocoulometry/chron-oabsorptometry techniques.
2. Experimental details
2.1. General
Iron(III) chloride hexahydrate (FeCl3 �6H2O, Merck), potassiumhexacyanoferrate (III) (K3[Fe(CN)6], Fluka), potassium chloride(KCl, Pronalab) and tetrabutylammonium perchlorate (TBAP,Fluka) were certified p. a. grade and used without furtherpurification. Acetonitrile (Riedel, p. a. grade) was dried followinga procedure reported in literature [23]: pre-drying over CaH2
during 2 days, followed by reflux and distillation.
N N
O
OMe
M
O
MeO
N N
O
Me
X
O
Me
Scheme 1. Salen-type complexes of Cu(II), Ni(II) and Pd(II): (A) [M(3-MeOsaltMe)],
M ¼ Ni or Cu and (B) [X(3-Mesalen)], X ¼ Pd.
The compounds 2-hydroxy-3-methylbenzaldehyde, ethylene-diamine and palladium acetate were obtained from Aldrich andwere used as received, except for ethylenediamine, which wasdistilled prior to use.
2.2. Prussian blue film preparation
PB films were electrogenerated on flexible electrodes ofpolyethylene terephthalate films coated with indium tin oxide(PET/ITO, Aldrich with a resistivity of 60O/sq and useful areabetween 3 and 4 cm2 (0.9�4 cm2)). The electrochemical poly-merizations were performed in a 1 cm path quartz cuvette.A three-electrode holder was constructed to fit the electrochemi-cal cell. An aqueous solution of 5 mM K3[Fe(CN)6], 5 mMFeCl3 �6H2O and 0.2 M KCl was used as PB polymerizationsolution. Electropolymerization was conducted under a constantapplied voltage of 0.55 V (vs. Ag/AgCl), during 300 s. Afterdeposition, the electrode was gently rinsed with distilled water.
2.3. Salen-type complexes of Cu(II), Ni(II) and Pd(II) film preparation
The complexes N,N0-2,3-dimethylbutane-2,3-dyil-bis(3-methoxy-salicylideneiminate)metal(II), M ¼ Ni or Cu ([Ni(3-MeOsaltMe)]or [Cu(3-MeOsaltMe)]) were prepared as described elsewhere[24]. The complex N,N0-bis(3-methylsalicylideneiminate)Pd(II),[Pd(3-Mesalen)], was synthesized by refluxing an acetonitrilesolution containing stoichiometric amounts of palladium(II)acetate and the ligand H2(3-Mesalen), prepared previously bystandard procedures [25]; the complex was recrystalized fromdichloromethane.
[M(salen)], M ¼ Ni, Cu and Pd complexes were electropolymer-ized by potential cycling at a scan rate 0.02 V/s, using a flexiblePET/ITO (Aldrich with a resistivity of 60O/sq and useful areabetween 3 and 4 cm2 (0.9�4 cm2)) electrodes as workingelectrode in a 1 cm path quartz cuvette. The potential range usedin film polymerization depends on the metal complex: for thenickel complex was 0.0–1.3 V, for the copper complex was �0.15to 1.4 V and for the Pd complex was �0.1 to 1.2 V (vs. Ag/AgCl).
2.4. Electrochemical measurements
Electrochemical polymerization and choronocoulometry/chronoabsorptometry were performed with a conventionalthree-electrode cell using a computer-controlled computerizedpotentiostat-galvanostat Model 20 Autolab, from Eco-Chemie Inc.The collection of data was controlled by the GPES Version 4.9 EcoChemie B.V. Software (Utrecht, The Netherlands). The workingelectrodes were constructed as described above, auxiliary elec-trode was a platinum wire and the reference electrode was anAg/AgCl (BAS). Electrochemical measurements of PB films wereperformed in a 0.2 M KCl aqueous solution and for salen-typecomplexes were performed in 0.1 M TBAP salt in freshly dried/distilled acetonitrile.
2.5. Spectroscopic/colorimetric measurements
UV/Vis absorbance spectra and chronoabsorptometry wererecorded in a Shimadzu UV2501-PC spectrophotometer at 1 nmresolution. Colorimetric ‘‘titrations’’ were performed in a totaltransmittance geometry configuration (CIE illuminating/viewingstandards [16]) using an integrating sphere installed in the samplecompartment of the spectrophotometer. The illuminant used isthe C. Transmittance spectra were recorded after each potentialhad been applied for 50–60 s. Colorimetric data were calculatedwith the UVPC optional colour analysis software v2.72 version
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C. Pinheiro et al. / Solar Energy Materials & Solar Cells 92 (2008) 980–985982
from Shimadzu Scientific Instruments Inc. All spectroscopicmeasurements of the electrochromic films were done in situ in a1 cm path quartz cuvette with a 1 cm2 square top aperture and a
-4-202468
101214
-0.2V
ΔQ' (
mC
cm-2
)
0.00.20.40.60.81.01.21.4
Abs
orba
nce 6
90nm
(A.U
.)
0 50 100 150 200-6-4-20246
cycle (80s)
step (40s)
0.6V
I (m
A)
Time (s)
Fig. 1. In situ chronocoulometry/chronoabsorptometry data for PB film deposited
on PET/ITO in 0.2 M KCl aqueous solution as electrolyte. Square-wave switching
between�0.2 and 0.6 V (vs. Ag/AgCl), step duration of 40 s. Chronocoulometry (full
line), chronoabsorptometry recorded at 690 nm (dashed line) and chronoampero-
metry (full line+full circle). Inset: PB and PW films absorption spectra.
40 60 80 100 120
0
20
40
60
80
100
120
CE
(cm
2 C-1
)
Time (s)
0.6V -0.2V
0.00
0.15
0.30
0.45
Abs
orba
nce 6
90nm
(A.U
.)
0.60
0.75
Fig. 2. CE (open squares) calculated from in situ chronocoulometry/chronoab-
sorptometry (see Fig. 1). Chronoabsorptometry cycles (full line).
Table 1Collected data from in situ choronocoulometry/chronoabsorptometry experiments of th
Cycle no Redox transition % Full switch D%T
1 PW/PB 85 63.76
96 71.8
98 73.38
99 74.19
PB/PW 86 64.16
95 71.19
98 73.47
99 74.16
2 PW/PB 99 74.32
PB/PW 98 73.19
3 PW/PB 99 73.74
PB/PW 97 72.59
4 PW/PB 98 73.13
PB/PW 96 71.79
5 PW/PB 96 72.19
Applied potentials: PW/PB 0.6 V (vs. Ag/AgCl) and PB/PW �0.2 V (vs. Ag/AgCl).
% full switch calculated with Tmax (89.35%) and Tmin (14.49%) obtained during the first
designed adapter to allow potentiostatic control (see Image 1S inSupplementary Material).
3. Results and discussion
3.1. Prussian blue
Since Neff described the PB film deposition on a solid electrode[26] several studies have been done, including deposition ontransparent ITO-coated glass and complete characterization byMortimer and Reynolds [27]. However, spectroelectrochemicalstudies of PB deposited on transparent flexible electrodes werenever reported. Fig. 1 shows the choronocoulometry/chronoab-sorptometry study for the PB/PW transition for films deposited onflexible PET/ITO electrodes.
The choronocoulometry/chronoabsorptometry shows for thesame sample a larger amount of charge per unit area for thereduction process than for the oxidation one (DQ0 ¼ integration ofcurrent (I) over time per unit electrode area). Differences in the CEvalues for the reduction and oxidation process are also found (seeFig. 2).
The PB films deposited on flexible PET/ITO electrodes showrelative stability over a range of at least five oxidation/reductioncycles (9 steps of 40 s each one, the last cycle have only one step,see Table 1). The evolution of CE along one step potential, shownin Fig. 2, increases until its maximum than reaches a plateau. InTable 1, CE values along the in situ choronocoulometry/chron-oabsorptometry experiment at a specific percentage of full switchare reported (%Full switch) [28].
While the colour efficiency maintains approximately the samevalue, the calculated percentage of full switch for both PW/PB andPB/PW at the same time step (20 s) decreases with the number ofcycles. As already noted in Figs. 1 and 2, differences between thetwo processes (PB/PW and PW/PB) concerning the CE andswitching times needed to reach the same percentage of fullswitching are observed during all the in situ chronocoulometry/chronoabsorptometry experiments. Comparing results fromTable 1 with those described by Mortimer and Reynolds [27],concerning PB deposited on glass/ITO electrodes (non-flexibleelectrodes) several differences are found. The CE obtained inflexible electrodes is 25% inferior and the time expended to obtainthe same % of full switching almost doubles when compared to PBdeposited over a glass ITO electrode. Moreover, the D%T obtainedfor the same % of full switching are 11–13% greater, indicating a
e PB films deposited on PET/ITO in 0.2 M KCl aqueous solution
DA DQ0 (mC/cm2) CE (cm2/C) Time (s)
0.543 5.65 96 9
0.707 6.38 111 11
0.748 6.69 112 12
0.770 6.9 112 13
0.729 6.45 113 11
0.767 7.08 108 13
0.778 7.41 105 17
0.781 7.60 103 23
0.780 7.09 110 20
0.770 7.45 103 20
0.770 6.99 110 20
0.761 7.32 104 20
0.760 6.86 111 20
0.751 7.19 104 20
0.749 6.71 112 20
step of the first cycle.
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large amount of PB deposited. This could explain the lower rate ofcolour variation and the lower CE values.
The CIELAB coordinates observed for the PB and PW filmsdeposited over flexile PET/ITO are comparable with thoseobtained with PB deposited over glass ITO-coated electrodes(see Table 1S in Supplementary Material).
050
100150200
0.00.10.20.30.40.50.6
Abs
orba
nce 7
65nm
(A
.U.)
0 100 200 300 400
-4-2024 cycle = 100s
step= 50s-0.1V
1.3V
I (m
A)
Time (s)
CE
(cm
2 C-1
)
Fig. 3. In situ cronocoulometry/cronoabsorptometry data for Pd film in acetonitrile
0.1 M TBAP as supporting electrolyte. Square-wave switching between �0.1 and
1.3 V (vs. Ag/AgCl), step duration of 50 s. Chronoabsorptometry recorded at 765 nm
(full line), CE (open squares) and chronoamperometry (full line+full circle). (See
Figs. 2S and 3S in Supplementary Material for data concerning the other metals.)
Table 2Collected data from choronocoulometry/chronoabsorptometry experiments for metal s
Complex Redox transition % Full switch D%T
Pd Coloration 66 38.24
93 53.69
98 56.64
Bleaching 51 29.25
99 56.89
Cu Coloration 92 71.06
96 74.09
98 75.75
Bleaching 93 71.74
95 73.46
98 75.85
Ni Coloration 90 60.01
94 62.56
98 65.09
Bleaching 90 60.16
95 63.09
96 63.93
400 500400 500 600 700 8000.00
0.25
0.50
0.75
1.00
Abs
orba
nce
(A.U
.)
λλ (nm)
A B
Fig. 4. Absorbance spectra recorded in situ for the different salen-based metal electrochr
supporting electrolyte. (A) Palladium film, 2 deposition cycles (dashed line: oxidized stat
deposition cycles (dashed line: oxidized state 1.4 V (vs. Ag/AgCl); full line: reduced state�
1.3 V (vs. Ag/AgCl); full line: reduced rate 0 V (vs. Ag/AgCl)).
3.2. Salen-type complexes of Cu(II), Ni(II) and Pd(II)
3.2.1. Coloration efficiency
Using the same methods described for the PB films, the CEvalues were calculated for the modified electrodes with twodepositions cycles (Fig. 3).
CE shows a maximum immediately after the potential pulseand than it decreases all along the pulse period for both theoxidation process (anodic coloration) and the reduction process(cathodic bleaching). The values of CE calculated at half height ofthe pulse of each step are constant relatively to the valuesobtained in the first cycle. This behaviour is followed for all thesalen-type complexes tested (see Figs. 2S and 3S in SupplementaryMaterial). Table 2 summarizes the data obtained from thecronocoulometry/cronoabsorptometry results for the first cycle.
3.2.2. Colorimetric analysis
In previous work, optical variations for the [Ni(3-MeOsaltMe)]and [Cu(3-MeOsaltMe)] at their different redox states (oxidizedstate—colored and reduced state—bleached) were reported [24].Their potential applications as electrochromic materials are thereason for a detailed colorimetric characterization. A comparativestudy with the different metal complexes films deposited bypotential cycling with increasing number of cycles (2–10) wasperformed. In Fig. 4, their full visible absorbance spectra in thecoloured and bleached states recorded under potentiostaticcontrol are depicted.
alen-type films, after one cycle
DA DQ0 (mC/cm2) CE (cm2/C) Time (s)
0.260 1.46 177 2
0.434 2.59 168 4
0.477 2.59 184 6
0.315 1.58 199 2
0.488 2.64 185 4
0.571 3.63 157 6
0.624 3.88 161 10
0.657 4.10 160 18
0.664 3.13 212 4
0.672 3.29 204 6
0.683 3.58 191 22
0.591 7.21 82 9
0.646 7.77 83 11
0.711 9.27 77 19
0.718 4.14 173 6
0.735 6.5 113 52
0.739 6.55 113 65
400 500 600 700 800600 700 800 (nm) λ (nm)
C
omic films deposited on PET/ITO electrode in acetonitrile containing 0.1 M TBAP as
e 1.3 V (vs. Ag/AgCl); full line: reduced state �0.1 V (vs. Ag/AgCl)). (B) Copper film, 2
0.15 V (vs. Ag/AgCl)). (C) Nickel film, 4 deposition cycles (dashed line: oxidized state
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Table 3In situ cronocoulometry/cronoabsorptometry of the salen-type metal complexes deposited on PET/ITO flexible electrode in acetonitrile 0.1 M TBAP
E (V) vs. Ag/AgCl Pd Cu Ni
L* a* b* L* a* b* L* a* b*
1.4 – – – 35.23 9.93 �18.46 – – –
1.3 60.3 �23.78 16.43 – – – 49.85 9.06 �9.86
1.2 66.95 �28.76 24.42 50.66 10.83 0.99 51.39 8.15 �7.8
1.15 73.22 �32.92 33.2 52.46 10.48 3.06 51.81 8.1 �7.4
1 75.41 �32.94 36.96 61.39 7.23 12.35 55.45 6.4 �5.09
0.9 78.12 �33.26 41.86 72.82 0.44 21.5 62.4 �0.76 7.28
0.85 79.43 �33.3 43.94 74.27 �0.81 22.11 74.92 �6.05 21.33
0.8 80.47 �32.85 45.35 75.61 �1.93 22.56 83.92 �8.7 31.13
0.75 81.32 �32.08 46.24 76.78 �2.83 22.35 86.38 �9.16 33.57
0.65 83.1 �29.82 47.39 79.75 �5.19 22.14 89.11 �9.38 35.94
0.55 85.11 �26.73 47.8 82.93 �6.21 23.01 90.77 �8.71 35.86
0.45 87.41 �22.68 47.85 84.5 �5.75 23.27 92.26 �7.84 34.54
0.4 88.86 �20.6 48.08 85.08 �5.26 23.13 92.85 �7.64 34.1
0.30 91.81 �16.66 48.35 85.43 �5.07 23.19 93.28 �7.4 33.89
0.25 94.4 �14.19 47.64 – – – – – –
0.20 96.05 �13.28 46.25 85.98 �4.8 23.08 – – –
0.15 97.9 �12.39 44.18 – – – 93.63 �7.28 33.22
0 99.36 �12.01 41.32 86.28 �4.83 23.15 93.76 �6.98 32.96
�0.1 99.57 �11.77 41.48 – – – – – –
�0.15 – – – 86.53 �4.4 22.94 – – –
Films deposited with 5 cycles.
0.27 0.30 0.33 0.36 0.39 0.420.27 0.30 0.33 0.36 0.39 0.420.27 0.30 0.33 0.36 0.39 0.42
0.240.270.300.330.360.390.420.450.48
0V
1.3V
x
-0.15V
1.4V
x
1.3V
-0.1Vy
x
Fig. 5. Chromaticity coordinates (x, y) obtained by in situ chronocoulometry/chronoabsorptometry: (A) Pd, (B) Cu and (C) Ni. Films with 10 (open circles), 5 (open triangles)
and 2 (open square) cycles deposition. Crossed open circle represents the illumination source x, y coordinates.
Table 4Dominant wavelength (lm) for the electrochromic inorganic polymers films upon
10 deposition cycles on PET/ITO in 0.2 M KCl aqueous solution
Complex Redox state ld (nm)
Pd Ox 545
Red 570–575
Cu Ox 455–460
Red 570–575
Ni Ox 560a
Red 570–575
See Fig. 4S in Supplementary Material for more details.a Value refers to complementary-dominant wavelength (lc).
C. Pinheiro et al. / Solar Energy Materials & Solar Cells 92 (2008) 980–985984
Table 3 reports the CIELAB coordinates corresponding totandem cronocoulometry/cronoabsorptometry experiments foreach metal salen-type complex.
Colour as described in the CIE system can be plotted on achromaticity diagram x, y. The chromaticity coordinates x and y
are derived from the ratios of the tristimulus values (X, Y and Z)[16]. Since the chromaticity has only two-dimensional variables(instead of three) information is lost and must only be used tomatch two colours from different samples [16] (see Fig. 5).
The evolution of xy coordinates along the potential shows avery peculiar curvature when compared with those obtained withPB both in PET/ITO (see Fig. 1S in supplementary material) andglass/ITO electrodes [27]. The colour changes spread over a largerxy region, indicating more saturated colour on alternatingbetween undoped and doped state. The modified electrodes with10 deposition cycles have larger xy values than those obtainedwith 5 and 2 deposition cycles, showing a more saturated colourapproximating the spectral locus coordinates. From the plot inFig. 5 and the spectrum locus (the line representing thechromaticity of the spectrum colours) it is possible to calculatethe dominant wavelength (ld) and the excitation purity (pc) thatcorrelates with the visual aspects of hue and chroma of perceivedcolour [16]. The ld calculated for the electrodes with 10 depositioncycles are presented in Table 4.
The Cu and Ni electrochromic films present the larger variationof ld among the three samples. The coloured state of the nickelfilm presents a non-spectral colour closest to the mauve colour,curiously the first industrial artificial dye (William Henry Perkin,1856).
The variation of relative luminance (L*) at electrochemicallycontrolled redox applied potential shows intervals of greatvariation over a narrow potential range corresponding to pseudopotential of reduction of the inorganic polymer (see Fig. 6).Working in transmittance, the CIELAB coordinate L* stands for
ARTICLE IN PRESS
-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.430405060708090
100
-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.430405060708090100
L* (A
.U.)
E (V vs Ag/AgCl) E (V vs Ag/AgCl) E(V vs Ag/AgCl)
Fig. 6. Relative luminance (L*) for the electrochromic inorganic polymers deposited with 5 cycles deposition on PET/ITO in 0.2 M KCl aqueous solution, as a function of
applied potential: (A) Pd, (B) Cu and (C) Ni.
C. Pinheiro et al. / Solar Energy Materials & Solar Cells 92 (2008) 980–985 985
perceived transparency indicating greater levels of transparencyfor the Cu complex films.
The new family of electrochromic inorganic polymers showsgood values of CE when comparing with other inorganic materialslike WO3 (40 and 50 cm2/C) [17] or IrO2 (15–18 cm2/C) [17] and PB.When compared with organic conducting polymers, which haveshown the highest CE results [17], the salen-type metal complexesinorganic polymer films studied here, lay between the perfor-mance of PEDOT and PProDOT [17].
4. Conclusion
PB films were successfully deposited over flexible electrodes.The film stability and the performance are, however, inferiorrelatively to those deposited over glass ITO-coated electrodes [27].Salen-type metal complex polymer were also successfully depos-ited as films over flexible PET/ITO electrodes and fully character-ized. CE results can be compared with those for conductingorganic polymers [17]. Colorimetry of the electrochromic salen-type complexes revealed colour states between yellow, green andpurples as dominant chroma.
The chronocoulometry/chronoabsorptometry study of electro-chromic materials deposited over flexible electrodes permits theevaluation of their potentialities for special purposes like flexibledisplays. Colour and CE characteristics must be basic and uniforminformation for electrochromic materials, constructing a wideelectrochromic materials database.
Acknowledgements
C.P. acknowledges FCT-MCTES and YDreams for the Ph.D. GrantSFRH/BDE/15563/2005.
J.F. acknowledges FCT-MCTES for the Ph.D. Grant RH/BD/22385/2005.
Appendix A. Supplementary data
The online version of this article contains additional supple-mentary data. Please visit doi:10.1016/j.solmat.2008.03.001.
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