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Infrared switching electroemissive devices based on highly conducting polymers
Thin Solid Films 352 (1999) 243±248 www.elsevier.nl/locate/tsf
Infrared switching electroemissive devices based on highly conducting polymers P. Topart*, P. Hourquebie CEA/Le Ripault, DMAT/CF, BP 16, 37260 Monts, France Received 18 February 1999; received in revised form 1 June 1999; accepted 17 June 1999
Abstract The use of highly conducting polyaniline ®lms as active layers in electrochromic devices operating in the mid-infrared, IR, range (8±12 mm) has been investigated. Upon reversible electrochemical doping, the specular re¯ectance of thin ®lms deposited on an IR transparent window and electrode switches from 0.2 to 0.65 around 12 mm. The complex refractive index of the conducting polymer has been determined by ellipsometry in the range 0.3 to 16 mm. Optical calculations of the multilayer structure demonstrate that the active layer thickness to maximum contrast should be close to 300 nm. Devices incorporating a lithium or protonic gel ionic conductor and WO3 as ion storage layer have been built. Optical performance in the infrared is similar to that of the layer in contact with a liquid electrolyte. The response time (t90) determined from optical density changes at 633 nm is of a few seconds. Moreover, devices incorporating the Li 1 conductor retained 81% of the original re¯ectance contrast after 900 cycles. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Electrochromism; Conducting polymers; Infrared
1. Introduction Electrochromic devices have been primarily investigated for optical display, automobile mirrors and solar control glazings for building windows. They are in some cases commercially manufactured [1]. For these aplications, devices operate in the visible and near infrared spectral region. Beside metal oxides, processable conducting polymers represent a particularly attractive class of electrochromic materials since they are easily deposited as thin ®lms. Their optical properties can be tailored by chemical synthesis so that they are either anodic or cathodic coloring [2]. Moreover, upon electrochemical doping they exhibit multicolor capability [3]. Large scale electrochromic devices based on polyaniline, PANI, have been developed [4]. Within conducting polymers, PANI has emerged as one of the most promising materials due to its excellent environmental and thermal stability. Besides, the processing of PANI protonated with camphor sulfonic acid, CSA, has enabled the fabrication of high quality, homogeneous thin ®lms exhibiting a disordered metal-like behavior [5]. For aerospace and military applications dynamic thermal emissivity control in the mid infrared region is required [6]. * Corresponding author. Tel.: 133-2-47-34-45; fax: 133-2-47-34-5154-42. E-mail address: [email protected], (P. Topart)
Such electroemissive devices based on crystalline WO3 and operating in the 3±5 mm range have been studied [7]. Recently, the use of conducting polymers for mid infrared electrochromic displays has been demonstrated [8]. For these devices, conducting polymer/dopant systems which show a high contrast in the transparent to absorbing transition have been developed. In this paper, the use of PANI-CSA as active layer in electroemissive devices is investigated. The conducting polymer switches from a re¯ecting (conducting) to an absorbing/transparent (insulating) state upon reversible electrochemical doping. By using infrared ellipsometric data of the active layer, the optical performance of the device has been optimized in the 8±12 mm band. Calculations are compared to in situ combined electrochemical and infrared re¯ectance measurements. An electroemissive system which also enables visible transmittance control has been built. The specular re¯ectance of devices can be continually adjusted between 0.2 and 0.65 at 12 mm by applying potentials lower than one volt. 2. Experimental details 2.1. Apparatus Electrochemical measurements were carried out with a
0040-6090/99/$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S 0040-609 0(99)00401-0
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PAR 273 potentiostat interfaced to a computer. A standard three electrode arrangement was used. The reference electrode was the saturated calomel electrode, (SCE). The applied potential, E, the current density, j, and the charge density passed, Q, were monitored with a home made Labview (National Instruments) data acquisition program. Specular re¯ectance, R, spectra in the range 0.4 to 25 mm were recorded with a Bruker IFS88 Fourier transform interferometer equipped with a variety of light sources, beam splitters and detectors for different overlapping frequency ranges. The absolute value of re¯ectance was determined by comparison with a gold (IR) or an Al (visible) mirror. The angle of detection is about 78. In-situ combined electrochemical and infrared re¯ectance experiments were performed by using a Kel-F cell placed in the spectrometer. A platinum disc served as counter electrode and the reference electrode was Ag/AgCl in 1.0 M HClO4 and 1.0 M LiClO4 in propylene carbonate, PC. The response time of devices was measured by monitoring optical density, O.D., changes at 633 nm (He±Ne laser). In situ conductivity measurements were carried out with four 10 mm long, 2 mm wide interdigital gold electrodes separated by 60 mm. The conductivity was calculated according to [9]. Ex situ electrical conductivity of samples was measured by the linear four probe technique. Infrared complex refractive index measurements (1.6±16 mm) were carried out with a SOPRA Ellipsometer. For visible, NIR measurements (0.3±1.7 mm), a VASE Woollam ellipsometer was employed. Angles of incidence were 60, 65, 70 and 758 for the visible and IR range, respectively. All optical measurements were carried out at room temperature. The modelisation of the optical behavior of the multilayer structure was performed with the Filmstar (FTG software, USA), thin ®lm optical design software. Film thicknesses were determined with a Dektak 3 ST pro®lometer. 2.2. Chemicals Propylene carbonate, metacresol, 2-acrylamido-2-methylpropane sulfonic acid, camphorsulfonic acid, potassium tungstate (Aldrich) and perchloric acid (Prolabo) were used as received. Aniline (Aldrich) was distilled under reduced pressure. LiClO4 (Aldrich) was vacuum dried at 1208C for 12 h and low molecular weight poly(methyl-methacrylate) (Aldrich) was dried at 808C overnight.
ture ionic conductivity measured by the ac impedance technique was 10 23 S/cm. The gel protonic conductor, poly(2acrylamido-2-methylpropanesulfonic acid), PAMPS has been synthesized according to Ref. [12]. The polymer was subsequently hydrated by exposure to an 80% R.H. ambient so that the mole ratio of water to sulfonate groups reached 5 [13]. For this composition, the room temperature conductivity was 1:7 £ 1022 S/cm. For in situ electro-re¯ectance measurements, the substrate was a 25 mm diameter ZnSe disc (Melles Griot, France). This CO2 laser window was coated on both sides by antire¯ection layers, AR, at 10.6 mm so that the re¯ectance was less than 0.5% per surface. The IR transparent electrode was a 300 nm thick gold on NiCr grid formed by photolithography (ACM, France). The line width was 20 mm and the period was 2 mm. In the electrochromic device, the ion storage material was electrochemically deposited WO3 [14] on 25 V/A indium tin oxide (ITO) on glass (ACM, France). Device assembly was carried out in a nitrogen ®lled glove box by pressing the PANI-CSA and WO3 supporting substrates with the ionic conductor by using a 600 mm spacer. Devices were subsequently sealed with epoxy. 3. Results and discussion The IR specular re¯ectance of PANI-CSA reaches 66% at 20 mm, Fig. 1. Film conductivities varied between 300 and 400 S/cm. The average surface roughness of ®lms deposited on glass, Ra, lies between 0.2 and 0.3 mm which results in a diffuse re¯ectance less than 10%. The dielectric function (11 2 j12 ) measured by ellipsometry shows two plasma frequencies at 1.5 mm (0.81 eV) and 9 mm (0.14 eV), Fig. 2. The real part of the dielectric function is negative beyond 1.5 mm which is expected for metals as described by the Drude model. However, it crosses zero again at 9 mm. Similar features have been observed for a dielectric function obtained from a Kramers±Kronig analysis of re¯ectance
2.3. Material synthesis and substrates The synthesis of camphorsulfonic acid-doped polyaniline, PANI-CSA, has been described elsewhere [10]. Films were cast onto substrates by spraying a few drops of a solution of 1 wt.% PANI-CSA in meta-cresol solution with a pipette. Films were formed after drying at 458C for 15 h on a hot plate. The poly(methylmethacrylate), PMMA, gel ionic conductor was prepared according to Ref. [11] in a nitrogen ®lled glove box. The gel was obtained by mixing 25 wt.% PMMA in 1.0 M LiClO4 in PC. The room tempera-
Fig. 1. Measured (solid line) and calculated (dashed line) specular re¯ectance, R, between 0.4 and 25 mm of a 1.2 mm thick PANI-CSA ®lm deposited on glass.
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Fig. 2. Real part, 11, and imaginary part, 12, of the dielectric function of PANI-CSA measured by ellipsometry between 0.3 and 16 mm.
[15]. Our values are in good agreement with those reported in that paper e.g. 0.8 and 0.2 eV. The deviation from the metallic behavior has been explained by a disorder-induced localization model [15]. Moreover, a fairly good agreement
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is found between the measured and calculated re¯ectance from (11, 12) values, Fig. 1. The electrochemical behavior of a 1 mm thick PANI-CSA deposited on a gold grid on ZnSe has been investigated in 1.0 M HClO4 and 1.0 M LiClO4 in PC. For potentials lower than 0.45 and 0.6 V in the acidic and non-aqueous media, respectively, cyclic voltammograms exhibited one reversible redox process [16], Fig. 3A. In 1.0 M HClO4 the oxidation peak was 0.187 V and the reduction peak was 20.016 V. In 1.0 M LiClO4 in PC, oxidation and reduction potentials were 0.12 and 20.015 V vs. SCE, respectively. This demonstrates that the metallic grid while being highly IR transparent also allows for electrochemical switching. The in-situ conductivity of PANI-CSA varies by at least four orders of magnitude when the potential is increased from 20.2 to 0.45 V in 1.0 M HClO4 (pH 0), Fig. 3B. Beyond 0.45 V, the polymer is overoxidized and the conductivity decreases. This has been identi®ed as the pernigraniline form of the polymer [16]. A similar curve is obtained in 1.0 M LiClO4 in PC with a conductivity maximum at 0.6 V vs. SCE. In order to evaluate the effect of PANI-CSA ®lm thickness on the infrared response, the re¯ectance, transmittance, T and absorptance, A of the following multilayer structure was calculated at 10.6 mm: AR/ZnSe/AR 3 mm/ PANICSA/ 100 mm ionic conductor. The antire¯ection layer, AR, was one quarter wave optical thickness Y2O3 [17]. The theoretical total transmittance of the gold mesh was 98% i.e. fraction of total open area, with 96% transmitted at the central order [18]. Since the period e.g. 2 mm was much greater than the wavelength, most of the diffracted energy is detected within the cone angle of our spectro-
Fig. 3. (A) Cyclic voltammogram of a 1 mm thick PANI-CSA ®lm deposited on a gold grid on glass. (B) Open circuit potential, OCP, dependence of the in-situ conductivity of the same PANI-CSA ®lm deposited on four interdigital electrodes. The background electrolyte was 1.0 M HClO4, the scan rate was 5 mV/s and the reference electrode was SCE.
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Fig. 4. PANI-CSA ®lm thickness dependence of the re¯ectance R, Transmittance, T and absorptance, A of the multilayer stack: AR/ZnSe/AR 3 mm/ PANI-CSA/ 100 mm Ionic conductor. AR 1 quarter wave Y2O3.
meter. This fact was con®rmed by optical measurements. Thus, in a ®rst approximation, the in¯uence of the grid was neglected. The optical constants of the ionic conductor (1:49 2 0:3j) were chosen so that the layer was fully absorbing. The results of Fig. 4 show that a re¯ectance maximum Ê . This at 10.6 mm is obtained for a ®lm thickness of 4650 A corresponds to a quarter wave optical thickness. Beyond this thickness the ®lm behaves as a massive layer. Moreover, the penetration depth given by l /4p k where l is the wavelength and k is the imaginary part of the refractive index, Ê . Thus, thicknesses greater than this will allow equals 2130 A for maximum re¯ectance in the doped state and high transparency in the undoped state. Moreover, thin conducting polymers layers favor a fast redox switching usually limited by the diffusion of ions [19]. The potential dependence of the specular re¯ectance of PANI-CSA in 1.0 M HClO4 in the transparency window of the ZnSe substrate is shown Fig. 5. Upon p-doping i.e. oxidation, the re¯ectance increases. This is attributed to free carrier absorption, Fig. 2. Indeed values of k at 1.7 mm measured for PANI-CSA and insulating deprotonated PANI ®lms deposited from N-methyl pyrrolidone decrease from 1.40 to 2:62 £ 1023 while n equals 1.30 and 1.38, respectively. When the ®lm is reduced at 2 0.2 V vs. Ag/ AgCl, the re¯ectance reaches a minimum which depends on ®lm thickness. For a ®lm thickness close to the penetration Ê , the re¯ectance minimum is lowered as depth e.g. 3000 A compared to that of a thicker ®lm e.g. 1.5 mm. Moreover, both layers exhibit a re¯ectance maximum close to 70% at
12 mm. The curves show that the re¯ectance of the reoxidized ®lm (after a ®rst reduction) at 0.45 V is close to the value obtained for the ®lm in air. This observation proves that the redox process is reversible. The re¯ectance of the as grown ®lm in air is lower than that in contact with the acidic medium. This is explained by a complete protonation of the ®lm. The potential dependence of the re¯ectance of a 1.5 mm thick PANI-CSA ®lm in 1.0 M HClO4 and 1.0 M LiClO4 in PC is shown in Fig. 6. The shape of these curves is similar to that of Fig. 3B. Moreover, in agreement with the conductivity data, a decrease of re¯ectance (not shown here) has been observed for anodic potentials greater than 0.45 and 0.6 V in the acidic and organic media, respectively. Electrochromic devices containing polymer-gel electrolytes were assembled according to the schematic representation of Fig. 7. Re¯ectance changes of a device incorporating the H 1 conductor, PAMPS, submitted to a constant applied current of 15.4 mA/cm 2 is shown in Fig. 8. The re¯ectance can be modulated between 0.2 and 0.65 at 13 mm. These results con®rm those obtained for the PANICSA ®lm in contact with a liquid electrolyte, Fig. 5. Moreover the optical ef®ciency de®ned as DR/DQ reaches 20 cm 2/C at 13 mm. The in¯uence of the ionic conductor on the cyclability of devices has been investigated by submitting them to potential pulses of 40 s. The system incorporating the Li 1 conductor showed a better long term stability than that containing the proton conductor. After applying potential pulses between 1.2 and 20.8 V for 20 h, e.g. 900 cycles, the maximum re¯ectance of the PMMA/ LiClO4 based device
Fig. 5. Potential dependence of the specular re¯ectance, R, of 0.3 mm and 1.5 mm thick PANI-CSA ®lms in 1.0 M HClO4. The substrate was a ZnSe window at 10.6 mm coated with a gold grid. The reference electrode was Ag/AgCl.
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Fig. 6. Potential dependence of the specular re¯ectance, R, of 1.5 mm thick PANI-CSA ®lms at 8, 10.6 and 12 mm. (A) 1.0 M LiClO4 in PC and (B) in 1.0 M HClO4. Same conditions as in Fig. 5.
decreased by 5 and 81% of the initial contrast was retained. For PAMPS, only 50% of the initial contrast remained after the same amount of cycles between 0.85 and 2 0.6 V. The combination of an anodic coloring layer, PANI-CSA (pale yellow to green) with a cathodic coloring layer WO3 (transparent to blue) enabled an additional control of visible transmittance. The change of optical density at 633 nm of
the electrochromic device incorporating the Li 1 conductor submitted to potential pulse of 50 s between 1.2 and 2 0.8 V is shown in Fig. 9. The charge and OD curves indicate that the redox switching is reversible. The transition from bleached to colored state reaches a steady-state. It is also faster than that from the colored to bleached state. The characteristic switching time, t90, de®ned for 90% of the total steady-state response is 9 s for coloration. Moreover the optical ef®ciency de®ned as DDO=DQ 60 cm 2/C. The
Fig. 7. Schematic representation of infrared/ visible electrochromic devices containing protonic (PAMPS) or lithium conducting (PMMA/LiCLO4/PC) polymer-gel electrolytes.
Fig. 8. Potential dependence of the specular re¯ectance, R of the device: [AR/ZnSe/AR3mm/Augrid/PANI±CSA400nm/600mmPAMPS/ 300nmWO3/ITO25V/A/glass]. The applied current was 15.4 mA/cm 2.
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PMMA/Li 1 conductor confers better long term stability to the device than PAMPS. However, faster switching was observed with the latter. Further work is under way to characterize the device performance in the 3±5 mm band as well as to improve long term stability. Acknowledgements The technical help of B. Blondel for the chemical synthesis of PANI is gratefully acknowledged. We would like to thank Dr. E. Monterrat for ellipsometric measurements and P. Coquard for assistance in re¯ectance measurements. References
Fig. 9. Time dependence of the applied potential, E, current density, j, charge density, Q and optical density at 633 nm for the device: [AR/ZnSe/ AR 3 mm/Augrid/PANI±CSA400nm/600mmPMMAgel/300nmWO3/ ITO25V/A/glass] submitted to potential pulses of 50 s at 1.2 and 20.8 V.
response time, t90, of devices incorporating PAMPS was 4.5 s. 4. Conclusions The 'metallic-like' behavior as well as the good cyclability of PANI-CSA ®lms make them adequate candidates for mid-infrared thermal emissivity control devices. In situ combined electrochemical and IR re¯ectance measurements demonstrate that the re¯ectance of PANI-CSA ®lms can be adjusted between 0.2 and 0.65 at 12 mm for potentials from 2 0.2 to 0.45 V vs. SCE. These results, in agreement with optical calculations, indicate that the optimum layer thickness is close to 300 nm. Electroemissive devices comprising WO3 as the cathodic coloring ion storage layer and Li 1 or H 1 gel ionic conductors exhibit an IR response similar to that observed for the active layer alone. Moreover the
[1] S.P. Sapers, M.J. Cumbo, R.B. Sargent, V.P. Raksha, L.S. Wang, R.B. Lahademe, B.P. Hichwa, Proc. 39th SVC Conf. (1996) 248±255. [2] S.A. Sapp, G.A. Sotzing, J.L. Reddinger, J.R. Reynolds, Adv. Mater. 8 (1996) 808. [3] C. Arbizzani, M. Mastragostino, B. Scrosati, in: H.S. Nalwa (Ed.), Handbook of Organic Conductive Molecules and Polymers 4, Wiley, New York, 1997. [4] T. Meisel, R. Braun, Proc. SPIE 1728 (1992) 200. [5] R.S. Kohlman, A.J. Epstein, in: T.A. Skotheim, R.L. Elsenbaumer, J.R. Reynolds (Eds.), Handbook of Conducting Polymers, Marcel Dekker, New York, 1998, pp. 85. [6] D.E. Schmieder, G.W. Walker, in: J.S. Acceta, D.L. Shumaker (Eds.), The Infrared and Electro-optics Systems Handbook 7, , SPIE Press, Bellingham, 1993, pp. 182. [7] S.F. Cogan, R.D. Rauh, J.D. Klein, T.D. Plante, Proc. SPIE 1149 (1989) 2. [8] P. Chandrasekhar, T.J. Dooley, Proc. SPIE 2528 (1995) 169. [9] E.M. Genies, P. Hany, M. Lapkowski, C. Santier, L. Olmedo, Synth. Met. 25 (1988) 29. [10] P. Hourquebie, B. Blondel, S. Dhume, Synth. Met. 85 (1997) 1437. [11] O. Bohnke, G. Frand, M. Rezrazi, C. Rousselot, C. Truche, Solid State Ionics 66 (1993) 97. [12] R.D. Giglia, S.Y. Huang, US patent 4,375,318, 1983. [13] J.-P. Randin, J. Electrochem. Soc., 129 (1982) 1215. [14] K. Yamanaka, Jpn. J. Appl. Phys. 26 (1987) 1884. [15] K. Lee, A.J. Heeger, Y. Cao, Phys. Rev. B 48 (1993) 14884. [16] W.W. Focke, G.E. Wnek, Y. Wei, J. Phys. Chem. 91 (1987) 5813. [17] W.J. Tropf, M.E. Thomas, in: E.D. Palik (Ed.), Handbook of Optical Constants in Solids II, Academic Press, New York, 1991, pp. 1088. [18] M. Kohin, S.J. Wein, J.D. Traylor, R.C. Chase, J.E. Chapman, Opt. Eng. 32 (1993) 911. [19] P. Chandrasekhar, A.M. Masulaitis, R.W. Gumbs, Synth. Met. 36 (1990) 303.
Infrared switching electroemissive devices based on highly conducting polymers P. Topart*, P. Hourquebie CEA/Le Ripault, DMAT/CF, BP 16, 37260 Monts, France Received 18 February 1999; received in revised form 1 June 1999; accepted 17 June 1999
Abstract The use of highly conducting polyaniline ®lms as active layers in electrochromic devices operating in the mid-infrared, IR, range (8±12 mm) has been investigated. Upon reversible electrochemical doping, the specular re¯ectance of thin ®lms deposited on an IR transparent window and electrode switches from 0.2 to 0.65 around 12 mm. The complex refractive index of the conducting polymer has been determined by ellipsometry in the range 0.3 to 16 mm. Optical calculations of the multilayer structure demonstrate that the active layer thickness to maximum contrast should be close to 300 nm. Devices incorporating a lithium or protonic gel ionic conductor and WO3 as ion storage layer have been built. Optical performance in the infrared is similar to that of the layer in contact with a liquid electrolyte. The response time (t90) determined from optical density changes at 633 nm is of a few seconds. Moreover, devices incorporating the Li 1 conductor retained 81% of the original re¯ectance contrast after 900 cycles. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Electrochromism; Conducting polymers; Infrared
1. Introduction Electrochromic devices have been primarily investigated for optical display, automobile mirrors and solar control glazings for building windows. They are in some cases commercially manufactured [1]. For these aplications, devices operate in the visible and near infrared spectral region. Beside metal oxides, processable conducting polymers represent a particularly attractive class of electrochromic materials since they are easily deposited as thin ®lms. Their optical properties can be tailored by chemical synthesis so that they are either anodic or cathodic coloring [2]. Moreover, upon electrochemical doping they exhibit multicolor capability [3]. Large scale electrochromic devices based on polyaniline, PANI, have been developed [4]. Within conducting polymers, PANI has emerged as one of the most promising materials due to its excellent environmental and thermal stability. Besides, the processing of PANI protonated with camphor sulfonic acid, CSA, has enabled the fabrication of high quality, homogeneous thin ®lms exhibiting a disordered metal-like behavior [5]. For aerospace and military applications dynamic thermal emissivity control in the mid infrared region is required [6]. * Corresponding author. Tel.: 133-2-47-34-45; fax: 133-2-47-34-5154-42. E-mail address: [email protected], (P. Topart)
Such electroemissive devices based on crystalline WO3 and operating in the 3±5 mm range have been studied [7]. Recently, the use of conducting polymers for mid infrared electrochromic displays has been demonstrated [8]. For these devices, conducting polymer/dopant systems which show a high contrast in the transparent to absorbing transition have been developed. In this paper, the use of PANI-CSA as active layer in electroemissive devices is investigated. The conducting polymer switches from a re¯ecting (conducting) to an absorbing/transparent (insulating) state upon reversible electrochemical doping. By using infrared ellipsometric data of the active layer, the optical performance of the device has been optimized in the 8±12 mm band. Calculations are compared to in situ combined electrochemical and infrared re¯ectance measurements. An electroemissive system which also enables visible transmittance control has been built. The specular re¯ectance of devices can be continually adjusted between 0.2 and 0.65 at 12 mm by applying potentials lower than one volt. 2. Experimental details 2.1. Apparatus Electrochemical measurements were carried out with a
0040-6090/99/$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S 0040-609 0(99)00401-0
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PAR 273 potentiostat interfaced to a computer. A standard three electrode arrangement was used. The reference electrode was the saturated calomel electrode, (SCE). The applied potential, E, the current density, j, and the charge density passed, Q, were monitored with a home made Labview (National Instruments) data acquisition program. Specular re¯ectance, R, spectra in the range 0.4 to 25 mm were recorded with a Bruker IFS88 Fourier transform interferometer equipped with a variety of light sources, beam splitters and detectors for different overlapping frequency ranges. The absolute value of re¯ectance was determined by comparison with a gold (IR) or an Al (visible) mirror. The angle of detection is about 78. In-situ combined electrochemical and infrared re¯ectance experiments were performed by using a Kel-F cell placed in the spectrometer. A platinum disc served as counter electrode and the reference electrode was Ag/AgCl in 1.0 M HClO4 and 1.0 M LiClO4 in propylene carbonate, PC. The response time of devices was measured by monitoring optical density, O.D., changes at 633 nm (He±Ne laser). In situ conductivity measurements were carried out with four 10 mm long, 2 mm wide interdigital gold electrodes separated by 60 mm. The conductivity was calculated according to [9]. Ex situ electrical conductivity of samples was measured by the linear four probe technique. Infrared complex refractive index measurements (1.6±16 mm) were carried out with a SOPRA Ellipsometer. For visible, NIR measurements (0.3±1.7 mm), a VASE Woollam ellipsometer was employed. Angles of incidence were 60, 65, 70 and 758 for the visible and IR range, respectively. All optical measurements were carried out at room temperature. The modelisation of the optical behavior of the multilayer structure was performed with the Filmstar (FTG software, USA), thin ®lm optical design software. Film thicknesses were determined with a Dektak 3 ST pro®lometer. 2.2. Chemicals Propylene carbonate, metacresol, 2-acrylamido-2-methylpropane sulfonic acid, camphorsulfonic acid, potassium tungstate (Aldrich) and perchloric acid (Prolabo) were used as received. Aniline (Aldrich) was distilled under reduced pressure. LiClO4 (Aldrich) was vacuum dried at 1208C for 12 h and low molecular weight poly(methyl-methacrylate) (Aldrich) was dried at 808C overnight.
ture ionic conductivity measured by the ac impedance technique was 10 23 S/cm. The gel protonic conductor, poly(2acrylamido-2-methylpropanesulfonic acid), PAMPS has been synthesized according to Ref. [12]. The polymer was subsequently hydrated by exposure to an 80% R.H. ambient so that the mole ratio of water to sulfonate groups reached 5 [13]. For this composition, the room temperature conductivity was 1:7 £ 1022 S/cm. For in situ electro-re¯ectance measurements, the substrate was a 25 mm diameter ZnSe disc (Melles Griot, France). This CO2 laser window was coated on both sides by antire¯ection layers, AR, at 10.6 mm so that the re¯ectance was less than 0.5% per surface. The IR transparent electrode was a 300 nm thick gold on NiCr grid formed by photolithography (ACM, France). The line width was 20 mm and the period was 2 mm. In the electrochromic device, the ion storage material was electrochemically deposited WO3 [14] on 25 V/A indium tin oxide (ITO) on glass (ACM, France). Device assembly was carried out in a nitrogen ®lled glove box by pressing the PANI-CSA and WO3 supporting substrates with the ionic conductor by using a 600 mm spacer. Devices were subsequently sealed with epoxy. 3. Results and discussion The IR specular re¯ectance of PANI-CSA reaches 66% at 20 mm, Fig. 1. Film conductivities varied between 300 and 400 S/cm. The average surface roughness of ®lms deposited on glass, Ra, lies between 0.2 and 0.3 mm which results in a diffuse re¯ectance less than 10%. The dielectric function (11 2 j12 ) measured by ellipsometry shows two plasma frequencies at 1.5 mm (0.81 eV) and 9 mm (0.14 eV), Fig. 2. The real part of the dielectric function is negative beyond 1.5 mm which is expected for metals as described by the Drude model. However, it crosses zero again at 9 mm. Similar features have been observed for a dielectric function obtained from a Kramers±Kronig analysis of re¯ectance
2.3. Material synthesis and substrates The synthesis of camphorsulfonic acid-doped polyaniline, PANI-CSA, has been described elsewhere [10]. Films were cast onto substrates by spraying a few drops of a solution of 1 wt.% PANI-CSA in meta-cresol solution with a pipette. Films were formed after drying at 458C for 15 h on a hot plate. The poly(methylmethacrylate), PMMA, gel ionic conductor was prepared according to Ref. [11] in a nitrogen ®lled glove box. The gel was obtained by mixing 25 wt.% PMMA in 1.0 M LiClO4 in PC. The room tempera-
Fig. 1. Measured (solid line) and calculated (dashed line) specular re¯ectance, R, between 0.4 and 25 mm of a 1.2 mm thick PANI-CSA ®lm deposited on glass.
P. Topart, P. Hourquebie / Thin Solid Films 352 (1999) 243±248
Fig. 2. Real part, 11, and imaginary part, 12, of the dielectric function of PANI-CSA measured by ellipsometry between 0.3 and 16 mm.
[15]. Our values are in good agreement with those reported in that paper e.g. 0.8 and 0.2 eV. The deviation from the metallic behavior has been explained by a disorder-induced localization model [15]. Moreover, a fairly good agreement
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is found between the measured and calculated re¯ectance from (11, 12) values, Fig. 1. The electrochemical behavior of a 1 mm thick PANI-CSA deposited on a gold grid on ZnSe has been investigated in 1.0 M HClO4 and 1.0 M LiClO4 in PC. For potentials lower than 0.45 and 0.6 V in the acidic and non-aqueous media, respectively, cyclic voltammograms exhibited one reversible redox process [16], Fig. 3A. In 1.0 M HClO4 the oxidation peak was 0.187 V and the reduction peak was 20.016 V. In 1.0 M LiClO4 in PC, oxidation and reduction potentials were 0.12 and 20.015 V vs. SCE, respectively. This demonstrates that the metallic grid while being highly IR transparent also allows for electrochemical switching. The in-situ conductivity of PANI-CSA varies by at least four orders of magnitude when the potential is increased from 20.2 to 0.45 V in 1.0 M HClO4 (pH 0), Fig. 3B. Beyond 0.45 V, the polymer is overoxidized and the conductivity decreases. This has been identi®ed as the pernigraniline form of the polymer [16]. A similar curve is obtained in 1.0 M LiClO4 in PC with a conductivity maximum at 0.6 V vs. SCE. In order to evaluate the effect of PANI-CSA ®lm thickness on the infrared response, the re¯ectance, transmittance, T and absorptance, A of the following multilayer structure was calculated at 10.6 mm: AR/ZnSe/AR 3 mm/ PANICSA/ 100 mm ionic conductor. The antire¯ection layer, AR, was one quarter wave optical thickness Y2O3 [17]. The theoretical total transmittance of the gold mesh was 98% i.e. fraction of total open area, with 96% transmitted at the central order [18]. Since the period e.g. 2 mm was much greater than the wavelength, most of the diffracted energy is detected within the cone angle of our spectro-
Fig. 3. (A) Cyclic voltammogram of a 1 mm thick PANI-CSA ®lm deposited on a gold grid on glass. (B) Open circuit potential, OCP, dependence of the in-situ conductivity of the same PANI-CSA ®lm deposited on four interdigital electrodes. The background electrolyte was 1.0 M HClO4, the scan rate was 5 mV/s and the reference electrode was SCE.
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Fig. 4. PANI-CSA ®lm thickness dependence of the re¯ectance R, Transmittance, T and absorptance, A of the multilayer stack: AR/ZnSe/AR 3 mm/ PANI-CSA/ 100 mm Ionic conductor. AR 1 quarter wave Y2O3.
meter. This fact was con®rmed by optical measurements. Thus, in a ®rst approximation, the in¯uence of the grid was neglected. The optical constants of the ionic conductor (1:49 2 0:3j) were chosen so that the layer was fully absorbing. The results of Fig. 4 show that a re¯ectance maximum Ê . This at 10.6 mm is obtained for a ®lm thickness of 4650 A corresponds to a quarter wave optical thickness. Beyond this thickness the ®lm behaves as a massive layer. Moreover, the penetration depth given by l /4p k where l is the wavelength and k is the imaginary part of the refractive index, Ê . Thus, thicknesses greater than this will allow equals 2130 A for maximum re¯ectance in the doped state and high transparency in the undoped state. Moreover, thin conducting polymers layers favor a fast redox switching usually limited by the diffusion of ions [19]. The potential dependence of the specular re¯ectance of PANI-CSA in 1.0 M HClO4 in the transparency window of the ZnSe substrate is shown Fig. 5. Upon p-doping i.e. oxidation, the re¯ectance increases. This is attributed to free carrier absorption, Fig. 2. Indeed values of k at 1.7 mm measured for PANI-CSA and insulating deprotonated PANI ®lms deposited from N-methyl pyrrolidone decrease from 1.40 to 2:62 £ 1023 while n equals 1.30 and 1.38, respectively. When the ®lm is reduced at 2 0.2 V vs. Ag/ AgCl, the re¯ectance reaches a minimum which depends on ®lm thickness. For a ®lm thickness close to the penetration Ê , the re¯ectance minimum is lowered as depth e.g. 3000 A compared to that of a thicker ®lm e.g. 1.5 mm. Moreover, both layers exhibit a re¯ectance maximum close to 70% at
12 mm. The curves show that the re¯ectance of the reoxidized ®lm (after a ®rst reduction) at 0.45 V is close to the value obtained for the ®lm in air. This observation proves that the redox process is reversible. The re¯ectance of the as grown ®lm in air is lower than that in contact with the acidic medium. This is explained by a complete protonation of the ®lm. The potential dependence of the re¯ectance of a 1.5 mm thick PANI-CSA ®lm in 1.0 M HClO4 and 1.0 M LiClO4 in PC is shown in Fig. 6. The shape of these curves is similar to that of Fig. 3B. Moreover, in agreement with the conductivity data, a decrease of re¯ectance (not shown here) has been observed for anodic potentials greater than 0.45 and 0.6 V in the acidic and organic media, respectively. Electrochromic devices containing polymer-gel electrolytes were assembled according to the schematic representation of Fig. 7. Re¯ectance changes of a device incorporating the H 1 conductor, PAMPS, submitted to a constant applied current of 15.4 mA/cm 2 is shown in Fig. 8. The re¯ectance can be modulated between 0.2 and 0.65 at 13 mm. These results con®rm those obtained for the PANICSA ®lm in contact with a liquid electrolyte, Fig. 5. Moreover the optical ef®ciency de®ned as DR/DQ reaches 20 cm 2/C at 13 mm. The in¯uence of the ionic conductor on the cyclability of devices has been investigated by submitting them to potential pulses of 40 s. The system incorporating the Li 1 conductor showed a better long term stability than that containing the proton conductor. After applying potential pulses between 1.2 and 20.8 V for 20 h, e.g. 900 cycles, the maximum re¯ectance of the PMMA/ LiClO4 based device
Fig. 5. Potential dependence of the specular re¯ectance, R, of 0.3 mm and 1.5 mm thick PANI-CSA ®lms in 1.0 M HClO4. The substrate was a ZnSe window at 10.6 mm coated with a gold grid. The reference electrode was Ag/AgCl.
P. Topart, P. Hourquebie / Thin Solid Films 352 (1999) 243±248
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Fig. 6. Potential dependence of the specular re¯ectance, R, of 1.5 mm thick PANI-CSA ®lms at 8, 10.6 and 12 mm. (A) 1.0 M LiClO4 in PC and (B) in 1.0 M HClO4. Same conditions as in Fig. 5.
decreased by 5 and 81% of the initial contrast was retained. For PAMPS, only 50% of the initial contrast remained after the same amount of cycles between 0.85 and 2 0.6 V. The combination of an anodic coloring layer, PANI-CSA (pale yellow to green) with a cathodic coloring layer WO3 (transparent to blue) enabled an additional control of visible transmittance. The change of optical density at 633 nm of
the electrochromic device incorporating the Li 1 conductor submitted to potential pulse of 50 s between 1.2 and 2 0.8 V is shown in Fig. 9. The charge and OD curves indicate that the redox switching is reversible. The transition from bleached to colored state reaches a steady-state. It is also faster than that from the colored to bleached state. The characteristic switching time, t90, de®ned for 90% of the total steady-state response is 9 s for coloration. Moreover the optical ef®ciency de®ned as DDO=DQ 60 cm 2/C. The
Fig. 7. Schematic representation of infrared/ visible electrochromic devices containing protonic (PAMPS) or lithium conducting (PMMA/LiCLO4/PC) polymer-gel electrolytes.
Fig. 8. Potential dependence of the specular re¯ectance, R of the device: [AR/ZnSe/AR3mm/Augrid/PANI±CSA400nm/600mmPAMPS/ 300nmWO3/ITO25V/A/glass]. The applied current was 15.4 mA/cm 2.
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PMMA/Li 1 conductor confers better long term stability to the device than PAMPS. However, faster switching was observed with the latter. Further work is under way to characterize the device performance in the 3±5 mm band as well as to improve long term stability. Acknowledgements The technical help of B. Blondel for the chemical synthesis of PANI is gratefully acknowledged. We would like to thank Dr. E. Monterrat for ellipsometric measurements and P. Coquard for assistance in re¯ectance measurements. References
Fig. 9. Time dependence of the applied potential, E, current density, j, charge density, Q and optical density at 633 nm for the device: [AR/ZnSe/ AR 3 mm/Augrid/PANI±CSA400nm/600mmPMMAgel/300nmWO3/ ITO25V/A/glass] submitted to potential pulses of 50 s at 1.2 and 20.8 V.
response time, t90, of devices incorporating PAMPS was 4.5 s. 4. Conclusions The 'metallic-like' behavior as well as the good cyclability of PANI-CSA ®lms make them adequate candidates for mid-infrared thermal emissivity control devices. In situ combined electrochemical and IR re¯ectance measurements demonstrate that the re¯ectance of PANI-CSA ®lms can be adjusted between 0.2 and 0.65 at 12 mm for potentials from 2 0.2 to 0.45 V vs. SCE. These results, in agreement with optical calculations, indicate that the optimum layer thickness is close to 300 nm. Electroemissive devices comprising WO3 as the cathodic coloring ion storage layer and Li 1 or H 1 gel ionic conductors exhibit an IR response similar to that observed for the active layer alone. Moreover the
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