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Study and optimization of a flexible electrochromic device based on polyaniline
Electrochimica Acta 49 (2004) 2051–2055
Study and optimization of a flexible electrochromic device based on polyaniline A. Bessière a,b , C. Duhamel b , J.-C. Badot a,∗ , V. Lucas b , M.-C. Certiat b a
LCAES, UMR-CNRS 7574, ENSCP, 11 Rue P&M Curie, 05 75231 Paris Cedex, France b EADS CCR, 12 Rue Pasteur, BP 76, 92152 Suresnes Cedex, France
Received 4 July 2003; received in revised form 28 October 2003; accepted 25 December 2003
Abstract The synthesis of polyaniline (PANI) thin films was made onto commercially available 5 cm ×5 cm polyethylene terephthalate (PET)/indium tin oxide (ITO) substrates. By depositing a gold frame previously to the electrochemical PANI synthesis, homogeneous electrochromic PANI layers were obtained. Complete flexible cells could then be built by using a transparent gel electrolyte and a simple PET/ITO counter-electrode. Branched poly(ethyleneimine) (BPEI)-H3 PO4 and polymethylemethacrylate (PMMA)-PC-LiClO4 were both tested as electrolytes, but only the latter led to a non-degrading system when the device undergoes several switching potential steps. This flexible, middle-scale and inexpensive device enabled to get a 18% transmission contrast at 780 nm within 3 min. © 2004 Elsevier Ltd. All rights reserved. Keywords: Polyaniline; PET/ITO counter-electrode; Electrochromic WO3
1. Introduction Up to now, most of the studies devoted to the realization of flexible electrochromic displays has been oriented towards the use of the inorganic electrochromic WO3 [1,2]. Though the synthesis of this compound on glass substrates is now well understood and well controlled, the deposition of WO3 thin films on plastic substrates still raises great difficulties [3]. Actually, as far as a plastic temperature-sensitive substrate supports the device, annealing steps must be avoided. Hence WO3 thin films in plastic devices usually do not reach as good electrochromic properties as in solid ones. Moreover, the presently best WO3 -based systems have been realized by magnetron sputtering [4], which represents a quite expensive route of synthesis. Unlike inorganic semiconductors, organic semiconductors can be deposited easily in a thin film using inexpensive techniques and they can be applied to flexible support systems [5]. In the aim of realizing a middle-scale (5 cm × 5 cm) flexible electrochromic device, we therefore intended to use polyaniline (PANI). This conducting polymer presents ∗ Corresponding author. Tel.: +33-1-44-27-67-08; fax: +33-1-46-34-74-89. E-mail address: [email protected] (J.-C. Badot).
0013-4686/$ – see front matter © 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2003.12.034
the advantage of exhibiting good electrochromic properties without any need of annealing steps. Polyaniline is able to switch from a transparent-yellow state when reduced to a green one when oxidized. Its performances are excellent in terms of optical contrast and time response [6–8]. However, up to now very few PANI-based devices have been realized on plastic substrate. In this paper, we intended to show that a prototype of a flexible 25-cm2 wide electrochromic device can be realized thanks to the use of the PANI as the only electrochromic material. This work is based on the requirements of a cheap, middle-scale and flexible device, which is a currently stringent but hard to satisfy demand [9,10].
2. Experimental procedure Our goal was to work on 5 cm × 5 cm squares of commercially available flexible substrates. Good quality (R = 60 /sq.) PET/ITO rolls were provided by IST (Belgium). The deposition of PANI thin films was made by electropolymerization. The synthesis as well as the electrochromic tests was carried out along with in situ optical measurements, inside a 15-mm thick glass container. Thus the PET/ITO/PANI samples constituted the working electrodes
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A. Bessi`ere et al. / Electrochimica Acta 49 (2004) 2051–2055
15 50 40
10
30 20
5 I (mA)
10
2 1
0 -5 -10 -15
-0.2
0
0.2
0.4
0.6
0.8
E (V) vs. ECS
Fig. 1. Voltammograms corresponding to the electropolymerization of PANI on PET/ITO (25 cm2 ) carried out by 50 mV s−1 potential sweeping. The numbers of cycles are reported in the figure.
in standard three-electrode cells. A reference electrode (ESC: electrode saturated calomel) and a counter-electrode (Pt-grid) were plunging as well into the liquid electrolyte. The glass container was placed into the cavity of a Cary 2400 (Varian) spectrophotometer. Transmission experiments could then have been conducted simultaneously to electrochemical measurements at 28 ◦ C. In a first attempt, PANI deposition was conducted by plunging the 5 cm × 5 cm PET/ITO samples in the acidic synthesis solution. PET/ITO pieces were cut up in a square shape comprising a strip to make the electrical connection. Due to the large scale of the sample combined with the low conductivity of the ITO, only slight PANI trails could have then been deposited onto the substrate whatever the cycling time was. In order to enhance the electronic conductivity over the whole substrate surface (ITO), a gold frame of 100 nm-thick was deposited by cathodic sputtering. The gold frame was then covered with an insulating spray in order to avoid contact between the gold film and the electrolyte solution. Thus, PANI could have been homogeneously synthesized over the ITO surface left in contact with the PANI electrolyte solution. The most homogeneous and still adherent PANI layer was synthesized by 50 mV s−1 potential cycling between −0.2 and +0.65 V/SCE. Such a high scan rate was found to be essential to get a homogeneous deposition process. The solution was made of HCl pH = 2, CaCl2 2 M and aniline 3 M. Due to the acid sensitivity of the ITO deposited on the PET substrate, the pH of the medium could not be lower than 2. CaCl2 is introduced in the synthesis solution to increase the concentration of the anion Cl− which is inserted and de-inserted during the synthesis cycle. Okomoto and Kotaka [11,12] showed that the polymerization rate was considerably increased by a high anion concentration whatever the pH for pH < 3–4. We will show in the following section that the optimal synthesis conditions could have been found out for the present device by means of the characterization of the prepared
films as well as by the in situ optical and electrochemical measurements. Our aim is to use two types of electrolytes: the first one a proton-based electrolyte and the second a lithium-based electrolyte. The proton-based one was prepared from an aqueous solution of H3 PO4 (pH = 2) and Cl-BPEI (crosslinked branched poly(ethyleneimine)). The liquid mix obtained changes into a gel under temperature. Its conductivity is 10−4 S cm−1 [13]. The lithium-based one was synthesized by mixing a 1 M PC (propylene carbonate)–LiClO4 solution with PMMA (polymethylemetacrylate) 30 wt.%. A gel is obtained after 24 h; it becomes smooth after 7 days. Its conductivity is 8 × 10−4 S cm−1 [14]. These two electrolytes were chose as they can easily form gels. Of course some others compounds could have been investigated.
3. Results and discussion The voltammograms of a typical PANI thin film synthesis are represented in Fig. 1. Compared to the synthesis conducted on small glass-based samples [15], the voltammograms are much more featureless since a high resistivity is introduced by the commercially available PET/ITO substrate. As far as the PANI film is not too thick, the current intensity still regularly grows from one cycle to the subsequent one, showing no sign of PANI detaching. An optimum number of 50 cycles was experienced. Beyond 60 cycles, the PANI layer tends to come unstuck from the ITO substrate, which can also be observed as sharp breaks in the current curve. The optical transmission of the PET/ITO/PANI sample at 780 nm was in situ recorded (see Fig. 2). The transmission regularly decreased, as the PANI film became thicker while it oscillated between a bleached and a colored state at each potential mid-cycle. The thickness of the 50 cycles-synthesized PANI film of about 3.3 m was measured on the sample cross-section by scanning electron microscopy. Thus, by using a thin film gold frame and by choosing conditions suited to the large PET/ITO substrate one can realize the deposition of homogeneous thin films of PANI on 5 cm × 5 cm commercially available PET/ITO substrates. The deposition process as the electrochromic efficiency of the growing PANI thin film can be easily controlled during the synthesis by the double recording of the current intensity and the optical transmission. Whole EC systems were then prepared by assembling the PET/ITO/PANI half-devices with PET/ITO samples cut off. Some cyclic voltammograms were recorded at 8.3 mV s−1 (50 mV min−1 ) showing the expected redox peaks for the PANI films. Those preliminary experiments conducted on both devices enabled us to choose the potential values suitable to switch between the green emeraldine and the colorless leucoemeraldine forms of PANI. Thus the reduction peak of the as-prepared PANI film (corresponding to the emeraldine–leucoemeraldine transition) was passed for a potential difference between the two electrodes by taking
A. Bessi`ere et al. / Electrochimica Acta 49 (2004) 2051–2055
2053
100 90 80
T (%)
70 60 50 40 30 20
0
5
10
15 t (min.)
20
25
30
Fig. 2. Evolution of the transmission at 780 nm for the PET/ITO/PANI half-cell as a function of time. Transmission is recorded simultaneously to the synthesis voltammograms of Fig. 1.
the positive electrode as the one with the PANI layer of −1.2 and −1.9 V by using, respectively, the BPEI-H3 PO4 and PMMA-PC-LiClO4 electrolytes. The reverse transition peak (from leucoemeraldine to emeraldine) was passed,
respectively, for a potential difference of 0.9 and 0.85 V. Both electrolytes enable to observe the required color changes by a 8.3 mV s−1 sweeping over a potential range of 2–3 V. This range is much larger than the one required to get the same (a)
4 min.
(b)
+0.9 V +0.85 V 1min. -1.2 V
-1.9 V
Potential step time Time of the transmission spectra recording
Fig. 3. Scheme of the potential steps applied to cells: (a) PET/ITO/PANI/BPEI-H3 PO4 /ITO/PET and (b) PET/ITO/PANI/PMMA-PC-LiClO4 /ITO/PET. The values in volts reported here correspond to potential differences between the two electrodes of the device. 70 Bleached states 60
T (%)
50
Colored states Time
40
30
20
10
400
450
500
550
600
650
700
750
Wavelength (nm)
Fig. 4. Transmission spectra of the device PET/ITO/PANI/PEI-H3 PO4 /ITO/PET recorded for the potential steps applied as described in Fig. 3. The bleached and colored states correspond, respectively, to −1.2 and +0.9 V.
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70 Bleached states
Time
60
T (%)
50
Colored states
40
30
lution can be attributed to a degradation of the PANI film by the H+ intercalation/deintercalation process [19]. When the PMMA-PC-LiClO4 electrolyte is used (see Fig. 5), the transmission spectrum of the first colored state presents less pronounced features. However the device is still green. When the potential difference is switched back to the first −1.9 V step, the device becomes transparent with a slight purple color. After many successive switches, the colored state spectra remain almost identical. Besides the bleached state tends to gain transparency at the higher wavelengths, which could not have been explained till now.
20
4. Conclusion 10
400
450
500
550
600
650
700
750
Wavelength (nm)
Fig. 5. Transmission spectra of the device PET/ITO/PANI/PMMA-PCLiClO4 /ITO/PET recorded for the potential steps applied as described in Fig. 3. The bleached and colored states correspond, respectively, to −1.9 and +0.85 V.
electrochromic result in an electrochemical cell comprising a liquid electrolyte and a Pt counter-electrode (i.e. about 0.5 V). Actually two resistances brought, respectively, by the gel electrolyte and by the ITO counter-electrode are both responsible for the ohmic drop here observed. These potential values were then used to submit the cells to suitable potential steps. The scheme of the applied potential steps is represented in Fig. 3. Each potential step is applied for 4 min. The recording of the transmission spectra starts 1 min before the end of each step and lasts 1 min. The transmission spectra of the electrochromic BPEI-H3 PO4 -based and PMMA-PC-LiClO4 -based cells are, respectively, represented in Figs. 4 and 5. In Fig. 4, the first colored state is characterized by a large and pronounced absorption band centered around 730 nm and a narrower one occurring around 420 nm, light before 400 nm being absorbed by the PET/ITO substrate. These absorption bands can readily be ascribed to the emeraldine state of PANI [16–18] and are responsible of an intense green color for the device. When the potential difference is reversed back to −1.2 V, the device displays a high transmission over the whole visible range. The slightly higher transmission at the longer wavelengths gives it a faint yellow hue. The contrast is then observed at 780 nm. At that wavelength, the transmission of the device switches between 33 and 68% (contrast T = 35% and contrast ratio CR = 2). As the switching operation between +0.9 and −1.2 V is repeated, the bleached state spectra remain identical whereas the colored-state ones undertake a clear evolution. The 730 nm absorption band becomes less and less intense as the number of switches increases, thus leading to a fainting green hue. The transmission at 780 nm changes from 33% at the first step to 46% at the last one. After six switches, the contrast at 780 nm is not more than 22% (contrast ratio CR = 1.5). This fast evo-
PANI films were electrochemically deposited as homogeneous layers onto 5 cm × 5 cm PET/ITO substrates thanks to a gold frame and optimized synthesis conditions. Flexible middle-scale PANI-based half-devices were then assembled with PET/ITO counter-electrodes via a transparent gel electrolyte. Two types of electrolytes were tested: (a) the branched poly(ethyleneimine) (BPEI)-H3 PO4 (proton electrolyte) and (b) the polymethylemetacrylate (PMMA)-PC-LiClO4 (lithium electrolyte). However, the latter led to a non-degrading system when the device undergoes several switching potential steps. Although the PMMA-PC-LiClO4 -based flexible device leads to a slightly lower contrast than the BPEI-H3 PO4 -based one, it presents the advantage to keep identical or even better performances over several switches (T = 18% at 780 nm). The advantages of using PANI instead of WO3 to realize flexible plastic devices are underlined in the introduction. Among the electrochromic conducting polymers available, PANI is very convenient for the aim of a large-scale cheap device. The synthesis is indeed very cheap to run: the monomer as well as any starting product is cheap and electrosynthesis does not require any expensive machine (on the contrary to sputtering for instance). In addition, it was shown that the response time of the PANI itself as well as the color contrasts obtainable under ideal conditions (1 cm × 1 cm-devices built on ITO/glass for instance) are quite attractive. The emeraldine state of PANI is also known to be very stable. In this study PANI is not used under these ideal conditions. Both the large scale and the plastic substrate lead to diminished properties. Acknowledgements It is a pleasure to acknowledge fruitful discussions with Dr. B. Viana (LCAES, Paris). This work was supported by EADS. References [1] M. Antinucci, A. Ferriolo, Proc. Soc. Photo-Opt. Instrum. Eng. 2255 (1994) 395.
A. Bessi`ere et al. / Electrochimica Acta 49 (2004) 2051–2055 [2] R. Lechner, L.K. Thomas, Sol. Energy Mater. Sol. Cells 54 (1998) 139. [3] P.M.S. Monk, R.J. Mortimer, D.R. Rosseinsky, Electrochromism, Fundamentals and Applications, VCH, Weinheim, Germany, 1995. [4] C. Brigouleix, P. Topart, E. Bruneton, F. Sabary, G. Nouhaut, G. Campet, Electrochim. Acta 46 (13–14) (2001) 1931. [5] J.C. Wittman, C. Straupé, S. Meyer, B. Lotz, P. Lang, G. Horowitz, F. Garnier, Thin Solid Films 311 (1997) 317. [6] T. Kobayashi, H. Yoneyama, H. Tamura, J. Electroanal. Chem. 161 (1984) 419. [7] M.C. Bernard, A. Hugot-Le Goff, Surf. Interf. Anal. 19 (1992) 27. [8] E.A.R. Duek, M.-A. De Paoli, M. Mastragostino, Adv. Mater. 4 4 (1992) 287. [9] N. O’Brien, J. Gordon, H. Mathew, B.P. Hichwa, Thin Solid Films 345 (1999) 312.
[10] [11] [12] [13] [14] [15] [16] [17] [18]
[19]
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A. Agrawal, J.P. Cronin, R. Zhang, Sol. Energy Mater. 31 (1993) 9. H. Okamoto, T. Kotaka, Polymer 39 (18) (1998) 4349. H. Okamoto, T. Kotaka, Polymer 40 (1999) 407. F. Michalak, P. Aldebert, Solid State Ion. 85 (1996) 265. B. Scrosati, Applications of Electroactive Polymers, Chapman & Hall, London, 1993. E.M. Genies, C. Tsintavis, J. Electroanal. Chem. 195 (1985) 109. S. Stafstrom, J.L. Bredas, A.J. Epstein, H.S. Woo, D.B. Tanner, W.S. Huang, A.G. MacDiarmid, Phys. News Lett. 59 (13) (1987) 1454. A. Watanabe, K. Mori, Y. Iwasaki, N. Nakamura, S. Niizuma, Macromolecules 20 (1987) 1793. P.N. Prasad, et al. (Eds.), Polymers and Other Advanced Materials: Emerging Technologies and Business Opportunities, Plenum Press, New York, 1995. T. Kobayashi, H. Yoneyama, H. Tamura, J. Electroanal. Chem. 177 (1984) 293.
Study and optimization of a flexible electrochromic device based on polyaniline A. Bessière a,b , C. Duhamel b , J.-C. Badot a,∗ , V. Lucas b , M.-C. Certiat b a
LCAES, UMR-CNRS 7574, ENSCP, 11 Rue P&M Curie, 05 75231 Paris Cedex, France b EADS CCR, 12 Rue Pasteur, BP 76, 92152 Suresnes Cedex, France
Received 4 July 2003; received in revised form 28 October 2003; accepted 25 December 2003
Abstract The synthesis of polyaniline (PANI) thin films was made onto commercially available 5 cm ×5 cm polyethylene terephthalate (PET)/indium tin oxide (ITO) substrates. By depositing a gold frame previously to the electrochemical PANI synthesis, homogeneous electrochromic PANI layers were obtained. Complete flexible cells could then be built by using a transparent gel electrolyte and a simple PET/ITO counter-electrode. Branched poly(ethyleneimine) (BPEI)-H3 PO4 and polymethylemethacrylate (PMMA)-PC-LiClO4 were both tested as electrolytes, but only the latter led to a non-degrading system when the device undergoes several switching potential steps. This flexible, middle-scale and inexpensive device enabled to get a 18% transmission contrast at 780 nm within 3 min. © 2004 Elsevier Ltd. All rights reserved. Keywords: Polyaniline; PET/ITO counter-electrode; Electrochromic WO3
1. Introduction Up to now, most of the studies devoted to the realization of flexible electrochromic displays has been oriented towards the use of the inorganic electrochromic WO3 [1,2]. Though the synthesis of this compound on glass substrates is now well understood and well controlled, the deposition of WO3 thin films on plastic substrates still raises great difficulties [3]. Actually, as far as a plastic temperature-sensitive substrate supports the device, annealing steps must be avoided. Hence WO3 thin films in plastic devices usually do not reach as good electrochromic properties as in solid ones. Moreover, the presently best WO3 -based systems have been realized by magnetron sputtering [4], which represents a quite expensive route of synthesis. Unlike inorganic semiconductors, organic semiconductors can be deposited easily in a thin film using inexpensive techniques and they can be applied to flexible support systems [5]. In the aim of realizing a middle-scale (5 cm × 5 cm) flexible electrochromic device, we therefore intended to use polyaniline (PANI). This conducting polymer presents ∗ Corresponding author. Tel.: +33-1-44-27-67-08; fax: +33-1-46-34-74-89. E-mail address: [email protected] (J.-C. Badot).
0013-4686/$ – see front matter © 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2003.12.034
the advantage of exhibiting good electrochromic properties without any need of annealing steps. Polyaniline is able to switch from a transparent-yellow state when reduced to a green one when oxidized. Its performances are excellent in terms of optical contrast and time response [6–8]. However, up to now very few PANI-based devices have been realized on plastic substrate. In this paper, we intended to show that a prototype of a flexible 25-cm2 wide electrochromic device can be realized thanks to the use of the PANI as the only electrochromic material. This work is based on the requirements of a cheap, middle-scale and flexible device, which is a currently stringent but hard to satisfy demand [9,10].
2. Experimental procedure Our goal was to work on 5 cm × 5 cm squares of commercially available flexible substrates. Good quality (R = 60 /sq.) PET/ITO rolls were provided by IST (Belgium). The deposition of PANI thin films was made by electropolymerization. The synthesis as well as the electrochromic tests was carried out along with in situ optical measurements, inside a 15-mm thick glass container. Thus the PET/ITO/PANI samples constituted the working electrodes
2052
A. Bessi`ere et al. / Electrochimica Acta 49 (2004) 2051–2055
15 50 40
10
30 20
5 I (mA)
10
2 1
0 -5 -10 -15
-0.2
0
0.2
0.4
0.6
0.8
E (V) vs. ECS
Fig. 1. Voltammograms corresponding to the electropolymerization of PANI on PET/ITO (25 cm2 ) carried out by 50 mV s−1 potential sweeping. The numbers of cycles are reported in the figure.
in standard three-electrode cells. A reference electrode (ESC: electrode saturated calomel) and a counter-electrode (Pt-grid) were plunging as well into the liquid electrolyte. The glass container was placed into the cavity of a Cary 2400 (Varian) spectrophotometer. Transmission experiments could then have been conducted simultaneously to electrochemical measurements at 28 ◦ C. In a first attempt, PANI deposition was conducted by plunging the 5 cm × 5 cm PET/ITO samples in the acidic synthesis solution. PET/ITO pieces were cut up in a square shape comprising a strip to make the electrical connection. Due to the large scale of the sample combined with the low conductivity of the ITO, only slight PANI trails could have then been deposited onto the substrate whatever the cycling time was. In order to enhance the electronic conductivity over the whole substrate surface (ITO), a gold frame of 100 nm-thick was deposited by cathodic sputtering. The gold frame was then covered with an insulating spray in order to avoid contact between the gold film and the electrolyte solution. Thus, PANI could have been homogeneously synthesized over the ITO surface left in contact with the PANI electrolyte solution. The most homogeneous and still adherent PANI layer was synthesized by 50 mV s−1 potential cycling between −0.2 and +0.65 V/SCE. Such a high scan rate was found to be essential to get a homogeneous deposition process. The solution was made of HCl pH = 2, CaCl2 2 M and aniline 3 M. Due to the acid sensitivity of the ITO deposited on the PET substrate, the pH of the medium could not be lower than 2. CaCl2 is introduced in the synthesis solution to increase the concentration of the anion Cl− which is inserted and de-inserted during the synthesis cycle. Okomoto and Kotaka [11,12] showed that the polymerization rate was considerably increased by a high anion concentration whatever the pH for pH < 3–4. We will show in the following section that the optimal synthesis conditions could have been found out for the present device by means of the characterization of the prepared
films as well as by the in situ optical and electrochemical measurements. Our aim is to use two types of electrolytes: the first one a proton-based electrolyte and the second a lithium-based electrolyte. The proton-based one was prepared from an aqueous solution of H3 PO4 (pH = 2) and Cl-BPEI (crosslinked branched poly(ethyleneimine)). The liquid mix obtained changes into a gel under temperature. Its conductivity is 10−4 S cm−1 [13]. The lithium-based one was synthesized by mixing a 1 M PC (propylene carbonate)–LiClO4 solution with PMMA (polymethylemetacrylate) 30 wt.%. A gel is obtained after 24 h; it becomes smooth after 7 days. Its conductivity is 8 × 10−4 S cm−1 [14]. These two electrolytes were chose as they can easily form gels. Of course some others compounds could have been investigated.
3. Results and discussion The voltammograms of a typical PANI thin film synthesis are represented in Fig. 1. Compared to the synthesis conducted on small glass-based samples [15], the voltammograms are much more featureless since a high resistivity is introduced by the commercially available PET/ITO substrate. As far as the PANI film is not too thick, the current intensity still regularly grows from one cycle to the subsequent one, showing no sign of PANI detaching. An optimum number of 50 cycles was experienced. Beyond 60 cycles, the PANI layer tends to come unstuck from the ITO substrate, which can also be observed as sharp breaks in the current curve. The optical transmission of the PET/ITO/PANI sample at 780 nm was in situ recorded (see Fig. 2). The transmission regularly decreased, as the PANI film became thicker while it oscillated between a bleached and a colored state at each potential mid-cycle. The thickness of the 50 cycles-synthesized PANI film of about 3.3 m was measured on the sample cross-section by scanning electron microscopy. Thus, by using a thin film gold frame and by choosing conditions suited to the large PET/ITO substrate one can realize the deposition of homogeneous thin films of PANI on 5 cm × 5 cm commercially available PET/ITO substrates. The deposition process as the electrochromic efficiency of the growing PANI thin film can be easily controlled during the synthesis by the double recording of the current intensity and the optical transmission. Whole EC systems were then prepared by assembling the PET/ITO/PANI half-devices with PET/ITO samples cut off. Some cyclic voltammograms were recorded at 8.3 mV s−1 (50 mV min−1 ) showing the expected redox peaks for the PANI films. Those preliminary experiments conducted on both devices enabled us to choose the potential values suitable to switch between the green emeraldine and the colorless leucoemeraldine forms of PANI. Thus the reduction peak of the as-prepared PANI film (corresponding to the emeraldine–leucoemeraldine transition) was passed for a potential difference between the two electrodes by taking
A. Bessi`ere et al. / Electrochimica Acta 49 (2004) 2051–2055
2053
100 90 80
T (%)
70 60 50 40 30 20
0
5
10
15 t (min.)
20
25
30
Fig. 2. Evolution of the transmission at 780 nm for the PET/ITO/PANI half-cell as a function of time. Transmission is recorded simultaneously to the synthesis voltammograms of Fig. 1.
the positive electrode as the one with the PANI layer of −1.2 and −1.9 V by using, respectively, the BPEI-H3 PO4 and PMMA-PC-LiClO4 electrolytes. The reverse transition peak (from leucoemeraldine to emeraldine) was passed,
respectively, for a potential difference of 0.9 and 0.85 V. Both electrolytes enable to observe the required color changes by a 8.3 mV s−1 sweeping over a potential range of 2–3 V. This range is much larger than the one required to get the same (a)
4 min.
(b)
+0.9 V +0.85 V 1min. -1.2 V
-1.9 V
Potential step time Time of the transmission spectra recording
Fig. 3. Scheme of the potential steps applied to cells: (a) PET/ITO/PANI/BPEI-H3 PO4 /ITO/PET and (b) PET/ITO/PANI/PMMA-PC-LiClO4 /ITO/PET. The values in volts reported here correspond to potential differences between the two electrodes of the device. 70 Bleached states 60
T (%)
50
Colored states Time
40
30
20
10
400
450
500
550
600
650
700
750
Wavelength (nm)
Fig. 4. Transmission spectra of the device PET/ITO/PANI/PEI-H3 PO4 /ITO/PET recorded for the potential steps applied as described in Fig. 3. The bleached and colored states correspond, respectively, to −1.2 and +0.9 V.
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70 Bleached states
Time
60
T (%)
50
Colored states
40
30
lution can be attributed to a degradation of the PANI film by the H+ intercalation/deintercalation process [19]. When the PMMA-PC-LiClO4 electrolyte is used (see Fig. 5), the transmission spectrum of the first colored state presents less pronounced features. However the device is still green. When the potential difference is switched back to the first −1.9 V step, the device becomes transparent with a slight purple color. After many successive switches, the colored state spectra remain almost identical. Besides the bleached state tends to gain transparency at the higher wavelengths, which could not have been explained till now.
20
4. Conclusion 10
400
450
500
550
600
650
700
750
Wavelength (nm)
Fig. 5. Transmission spectra of the device PET/ITO/PANI/PMMA-PCLiClO4 /ITO/PET recorded for the potential steps applied as described in Fig. 3. The bleached and colored states correspond, respectively, to −1.9 and +0.85 V.
electrochromic result in an electrochemical cell comprising a liquid electrolyte and a Pt counter-electrode (i.e. about 0.5 V). Actually two resistances brought, respectively, by the gel electrolyte and by the ITO counter-electrode are both responsible for the ohmic drop here observed. These potential values were then used to submit the cells to suitable potential steps. The scheme of the applied potential steps is represented in Fig. 3. Each potential step is applied for 4 min. The recording of the transmission spectra starts 1 min before the end of each step and lasts 1 min. The transmission spectra of the electrochromic BPEI-H3 PO4 -based and PMMA-PC-LiClO4 -based cells are, respectively, represented in Figs. 4 and 5. In Fig. 4, the first colored state is characterized by a large and pronounced absorption band centered around 730 nm and a narrower one occurring around 420 nm, light before 400 nm being absorbed by the PET/ITO substrate. These absorption bands can readily be ascribed to the emeraldine state of PANI [16–18] and are responsible of an intense green color for the device. When the potential difference is reversed back to −1.2 V, the device displays a high transmission over the whole visible range. The slightly higher transmission at the longer wavelengths gives it a faint yellow hue. The contrast is then observed at 780 nm. At that wavelength, the transmission of the device switches between 33 and 68% (contrast T = 35% and contrast ratio CR = 2). As the switching operation between +0.9 and −1.2 V is repeated, the bleached state spectra remain identical whereas the colored-state ones undertake a clear evolution. The 730 nm absorption band becomes less and less intense as the number of switches increases, thus leading to a fainting green hue. The transmission at 780 nm changes from 33% at the first step to 46% at the last one. After six switches, the contrast at 780 nm is not more than 22% (contrast ratio CR = 1.5). This fast evo-
PANI films were electrochemically deposited as homogeneous layers onto 5 cm × 5 cm PET/ITO substrates thanks to a gold frame and optimized synthesis conditions. Flexible middle-scale PANI-based half-devices were then assembled with PET/ITO counter-electrodes via a transparent gel electrolyte. Two types of electrolytes were tested: (a) the branched poly(ethyleneimine) (BPEI)-H3 PO4 (proton electrolyte) and (b) the polymethylemetacrylate (PMMA)-PC-LiClO4 (lithium electrolyte). However, the latter led to a non-degrading system when the device undergoes several switching potential steps. Although the PMMA-PC-LiClO4 -based flexible device leads to a slightly lower contrast than the BPEI-H3 PO4 -based one, it presents the advantage to keep identical or even better performances over several switches (T = 18% at 780 nm). The advantages of using PANI instead of WO3 to realize flexible plastic devices are underlined in the introduction. Among the electrochromic conducting polymers available, PANI is very convenient for the aim of a large-scale cheap device. The synthesis is indeed very cheap to run: the monomer as well as any starting product is cheap and electrosynthesis does not require any expensive machine (on the contrary to sputtering for instance). In addition, it was shown that the response time of the PANI itself as well as the color contrasts obtainable under ideal conditions (1 cm × 1 cm-devices built on ITO/glass for instance) are quite attractive. The emeraldine state of PANI is also known to be very stable. In this study PANI is not used under these ideal conditions. Both the large scale and the plastic substrate lead to diminished properties. Acknowledgements It is a pleasure to acknowledge fruitful discussions with Dr. B. Viana (LCAES, Paris). This work was supported by EADS. References [1] M. Antinucci, A. Ferriolo, Proc. Soc. Photo-Opt. Instrum. Eng. 2255 (1994) 395.
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