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Electrochromic devices on polyester foil
Solid State Ionics 165 (2003) 1 – 5 www.elsevier.com/locate/ssi
Electrochromic devices on polyester foil A. Azens *, G. Gustavsson, R. Karmhag, C.G. Granqvist Department of Engineering Sciences, The A˚ngstro¨m Laboratory, Uppsala University, P.O. Box 534, SE-75121 Uppsala, Sweden
Abstract We report on a new flexible electrochromic foil device with sufficient optical modulation range, dynamics, and durability for practical uses. The device incorporates a film of W oxide made by sputtering in the presence of hydrogen, another film of Ni – V oxide made by sputtering and postdeposition treatment in ozone, and a polymer laminate. Applications as variable-transmittance visors in motorcycle helmets are discussed. D 2003 Elsevier B.V. All rights reserved. PACS: 78.20.Jq; 81.15.Cd Keywords: Electrochromic; Laminated polyester; Nickel – vanadium oxide; Hydrogen-containing WO3
1. Introduction Electrochromic materials are able to vary their optical properties between widely separated extrema through the action of an applied voltage [1]. The phenomenon was discovered in W oxide films more than 30 years ago through work by Deb [2]. Electrochromism can be used in devices such as ‘‘smart windows’’ for energy-efficient architecture [3], variable-reflectance mirrors, display devices of many different types, eyewear, and surfaces for variable thermal emittance [4]. Most of these applications employ electrochromic materials deposited onto rigid glass substrates. The present work takes a different route, however, and describes a flexible foil with electrochromic films deposited onto polyester. The device is color-neutral and durable enough for a
* Corresponding author. Tel.: +46-18-471-3782; fax: +46-18500-131. E-mail address: [email protected] (A. Azens). 0167-2738/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2003.08.009
variety of applications, one of them being variabletransmittance visors for motorcycle helmets. Our electrochromic devices are of the conventional five-layer configuration [1] with, essentially, a cathodically coloring W oxide film separated from an anodically coloring Ni oxide film by a layer of a polymer electrolyte. This three-layer stack is interposed between transparent and electrically conducting indium – tin oxide films backed by polyester foil. Bleaching occurs when charge is shuttled from the W oxide into the Ni oxide as a response to an applied DC voltage, and coloring occurs when charge is moved in the opposite direction. The device exhibits open-circuit memory, which leads to energy efficiency. This paper is organized as follows: Section 2 describes the deposition of the electrochromic thin films, and Section 3 reports on their preassembly treatment and on lamination to make devices. Section 4 contains results of a number of tests on properties such as optical modulation range, electrochromic switching speed, and cycling durability. Section 5 then summarizes the main results and treats device
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A. Azens et al. / Solid State Ionics 165 (2003) 1–5
aspects with a focus on variable-transmittance visors for motorcycle helmets.
2. Film deposition DC magnetron sputtering was performed in a boxtype deposition unit with 50 12.5 cm2 targets [5], specifically using 99.95% pure W and Ni (93%) – V (7%). The addition of V made the latter target nonmagnetic and, hence, convenient for magnetron sputtering. The substrates were of 0.175-mm-thick polyester (CP Films, type OC 80/7) pre-coated with indium – tin oxide having a resistance/square of 60 V. Typical substrates were roughly 30 10 cm2 in size and shaped to fit as visors in helmets. Sputtering took place in Ar/O2/H2 with optimized mixing ratio. The target-substrate distance typically was 20 to 25 cm, and the total gas pressure was in the 30 – 40-mTorr range. Fig. 2. Spectral transmittance of a 300-nm-thick W oxide film produced by sputtering in Ar/O2/H2 under conditions making it ready for device lamination.
Fig. 1. Chromaticity coordinate z, corresponding to blue light, versus luminous transmittance for 300-nm-thick W oxide films of three types: deposited in Ar/O2 and then colored by charge insertion from an electrolyte of LiCiO4 in propylene carbonate (PC), as deposited in Ar/O2 with different oxygen contents, and as deposited in Ar/O2/H2 with different hydrogen contents. The color specifications were made by integration of spectrophotometrically recorded transmittance. Symbols denote evaluated data and lines were drawn for convenience.
From a manufacturing point of view, it is preferable to be able to deposit the films under conditions that make them ready for device lamination or to have convenient processes for making the as-deposited films ready for this. In the case of W oxide, one can carry out the deposition in Ar/O2/H2 according to principles delineated by Giri and Messier [6]. The hydrogen produces a blue color for the as-deposited films, and the key issue with regard to optimizing the gas mixture is to incorporate hydrogen into the film without creating oxygen deficiency. In practice, the optimization of the W oxide films can be made by measuring their color in the asdeposited state. Fig. 1 compares the chromaticity coordinate z, corresponding to blue light [7], with the variation of the luminous transmittance for films deposited in Ar/O2 and Ar/O2/H2. Data for a ‘‘standard’’ film deposited in Ar/O2 and colored electrochemically in an electrolyte of 1 M LiClO4 in propylene carbonate is shown for reference. As-deposited films, produced in Ar/O2, displayed a reduction in transmittance when the oxygen partial pressure was lowered. It can be seen from Fig. 1 that these underoxidized films exhibit a color that is very
A. Azens et al. / Solid State Ionics 165 (2003) 1–5
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different from the one in the electrochemically colored films. When sputtering was done in Ar/O2/H2, however, the transmittance of the as-deposited films was decreased when the hydrogen partial pressure was increased. Such films showed the same color rendering as the electrochemically treated films for transmittance levels down to about 20% in 300-nm-thick films. The evolution of z at still lower transmittance levels is most likely associated with oxygen deficiency. The optimum gas mixture for making blue W oxide films ready for device lamination was Ar/O2/ H2 = 4/2/13 at a total gas pressure of 40 mTorr. Fig. 2 illustrates the spectral transmittance for such a film.
3. Preassembly film treatment and device lamination The Ni– V oxide films are transparent in as-deposited state and require pretreatment prior to device lamination. Charge insertion via an electrochemical procedure is possible, in principle, but obviously unwieldy and not suited for practical fabrication. An
Fig. 3. Luminous transmittance versus postdeposition ozone exposure time for 220-nm-thick Ni – V oxide films produced by sputtering in Ar/O2/H2.
Fig. 4. Spectral transmittance of a 220-nm-thick Ni – V oxide film produced by sputtering in Ar/O2/H2 and posttreated in ozone under conditions making it ready for device lamination.
alternative method for charging was developed in earlier work of ours [8]. Essentially, this new method employs exposure to ozone obtained by ultraviolet irradiation of the film in the presence of oxygen. Fig. 3 displays the decrease of the luminous transmittance as a function time when a conventional ozone photo-reactor was used to illuminate a 220nm-thick Ni – V oxide film. The transmittance drops by some 50% during a few minutes, and a further decrease takes place for extended exposure times. Fig. 4 illustrates a typical spectral transmittance plot for a Ni –V oxide film prior to device lamination. Polyester-based foils—one with a W oxide film colored by sputtering in the presence of hydrogen (cf. Fig. 2) and another with a Ni – V oxide film colored by postdeposition ozone exposure (cf. Fig. 4)—were laminated together by a proprietary PMMA-based electrolyte of the type described in Ref. [9] using roll-pressing at a temperature of 80 jC. The edges of the double foil were then sealed by a two-component epoxy glue, electrical contacts were attached, and the electrochromic device was ready for testing and use.
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A. Azens et al. / Solid State Ionics 165 (2003) 1–5
4. Device tests Devices of the type described above were cycled between colored and bleached states using trapezoidal voltage pulses between 1.6 and + 1.5 V, respectively. Durability tests are currently going on, as discussed next. Fig. 5 shows the transmittance at three different wavelengths for cycling with one full color-bleach cycle each 200 s. The optical modulation is pronounced, especially for mid-luminous and red light. Most of the changes take place within a few tens of seconds after application of the pulse, but the coloration has not reached saturation even after several minutes. Fig. 6 reports on the transmittance in fully colored and bleached states after cycling in the same way as in Fig. 5. The foils are seen to switch between a high level of about 70% and a low level of 35 – 40% at a wavelength of 550 nm, which is in agreement with the data in Fig. 5. The span between the colored and bleached states remains practically unchanged for at least 5000 cycles. Lower transmittance levels, down
Fig. 5. Transmittance at three wavelengths versus time for electrochromic switching of a laminated device as described in the main text.
Fig. 6. Transmittance at a wavelength of 550 nm versus number of color/bleach cycles for a laminated electrochromic foil device in colored and bleached states.
to 25% or less in the colored state, could be reached for longer coloration times. The devices have open-circuit memory, which is an asset since electrical power must be drawn only to effect changes in the optical properties. Fig. 7 illus-
Fig. 7. Transmittance at a wavelength of 550 nm versus time for a laminated electrochromic foil device kept under open-circuit conditions.
A. Azens et al. / Solid State Ionics 165 (2003) 1–5
trates the transmittance at a wavelength of 550 nm for a device colored to a transmittance equal to 20%. It is found that the transmittance has increased by only some 10% after a time as long as 160 h. Keeping the device in the colored state for an extended time influenced the bleaching time, and a device with an initial bleaching time of about 1 min attained a bleaching time of some 20 min after being maintained in the colored state for 160 h.
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moderate substrate temperatures [11]. The bleachedstate transmittance can be significantly enhanced with additions such as Al or Mg to the Ni oxide or Ni – V oxide [12,13]. Acknowledgements Parts of this work were financed by the Swedish Foundation for Strategic Environmental Research and the National Energy Administration of Sweden.
5. Summary and applications aspects This work has demonstrated techniques to make flexible electrochromic foils with sufficient optical modulation range, dynamics, and durability for making them interesting for a range of applications. Facile manufacturing is possible by a combination of different technologies, and the potentially cumbersome prelamination treatments of the films was avoided by sputtering the W oxide film in the presence of hydrogen and by postdeposition treatment of the Ni-based film in ozone for a few minutes. In particular, we have considered the application of the electrochromic foils in variable-transmittance visors for motorcycle helmets. Driving or riding safety requires different transmittance levels for varying lighting conditions, and the properties illustrated in Figs. 5 and 6 are excellent for such applications, as concluded in a recent study [10]. Further refinements of the electrochromic foils are underway. Thus, the switching dynamics can be made speedier once polyester foils with more conducting coatings become readily available. It should be noted in this context that intense work is going on to improve the performance of transparent and electrically conducting coatings that can be applied at
References [1] C.G. Granqvist, Handbook of Inorganic Electrochromic Materials, Elsevier, Amsterdam, 1995, reprinted 2002, Solar Energy Mater. Solar Cells 60 (2000) 201. [2] S.K. Deb, Appl. Opt. Suppl. 3 (1969) 192. Philos. Mag. 27 (1973) 801. [3] A. Azens, C.G. Granqvist, Proc. Soc. Photo-Opt. Instrum. Eng. 4458 (2001) 104. [4] A. Larsson, G.A. Niklasson, L. Stenmark, Proc. Soc. PhotoOpt. Instrum. Eng. 4102 (2000) 69. [5] T.S. Eriksson, C.G. Granqvist, J. Appl. Phys. 60 (1986) 2081. [6] A.P. Giri, R. Messier, Mater. Res. Soc. Symp. Proc. 24 (1984) 221. [7] S.J. Williamson, H.Z. Cummins, Light and Colour in Nature and Art, Wiley, New York, 1983. [8] A. Azens, L. Kullman, C.G. Granqvist, Solar Energy Mater. Solar Cells 76 (2003) 147. [9] J. Stevens, U.S. Patent 5,288,43. [10] S. Cook, C. Quigley, R. Tait, Quality and field of vision—a review of the needs of drivers and riders. Final Report PPAD 9/33/39/TT1130, Dept. Transport, Local Government and Regions, UK, 2002, unpublished. [11] C.G. Granqvist, A. Hulta˚ker, Thin Solid Films 411 (2002) 1. [12] A. Azens, J. Isidorsson, R. Karmhag, C.G. Granqvist, Thin Solid Films 422 (2002) 1. [13] E. Avendan˜o, A. Azens, G.A. Niklasson, C.G. Granqvist, Journal of Solid State Electrochemistry, in press.
Electrochromic devices on polyester foil A. Azens *, G. Gustavsson, R. Karmhag, C.G. Granqvist Department of Engineering Sciences, The A˚ngstro¨m Laboratory, Uppsala University, P.O. Box 534, SE-75121 Uppsala, Sweden
Abstract We report on a new flexible electrochromic foil device with sufficient optical modulation range, dynamics, and durability for practical uses. The device incorporates a film of W oxide made by sputtering in the presence of hydrogen, another film of Ni – V oxide made by sputtering and postdeposition treatment in ozone, and a polymer laminate. Applications as variable-transmittance visors in motorcycle helmets are discussed. D 2003 Elsevier B.V. All rights reserved. PACS: 78.20.Jq; 81.15.Cd Keywords: Electrochromic; Laminated polyester; Nickel – vanadium oxide; Hydrogen-containing WO3
1. Introduction Electrochromic materials are able to vary their optical properties between widely separated extrema through the action of an applied voltage [1]. The phenomenon was discovered in W oxide films more than 30 years ago through work by Deb [2]. Electrochromism can be used in devices such as ‘‘smart windows’’ for energy-efficient architecture [3], variable-reflectance mirrors, display devices of many different types, eyewear, and surfaces for variable thermal emittance [4]. Most of these applications employ electrochromic materials deposited onto rigid glass substrates. The present work takes a different route, however, and describes a flexible foil with electrochromic films deposited onto polyester. The device is color-neutral and durable enough for a
* Corresponding author. Tel.: +46-18-471-3782; fax: +46-18500-131. E-mail address: [email protected] (A. Azens). 0167-2738/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2003.08.009
variety of applications, one of them being variabletransmittance visors for motorcycle helmets. Our electrochromic devices are of the conventional five-layer configuration [1] with, essentially, a cathodically coloring W oxide film separated from an anodically coloring Ni oxide film by a layer of a polymer electrolyte. This three-layer stack is interposed between transparent and electrically conducting indium – tin oxide films backed by polyester foil. Bleaching occurs when charge is shuttled from the W oxide into the Ni oxide as a response to an applied DC voltage, and coloring occurs when charge is moved in the opposite direction. The device exhibits open-circuit memory, which leads to energy efficiency. This paper is organized as follows: Section 2 describes the deposition of the electrochromic thin films, and Section 3 reports on their preassembly treatment and on lamination to make devices. Section 4 contains results of a number of tests on properties such as optical modulation range, electrochromic switching speed, and cycling durability. Section 5 then summarizes the main results and treats device
2
A. Azens et al. / Solid State Ionics 165 (2003) 1–5
aspects with a focus on variable-transmittance visors for motorcycle helmets.
2. Film deposition DC magnetron sputtering was performed in a boxtype deposition unit with 50 12.5 cm2 targets [5], specifically using 99.95% pure W and Ni (93%) – V (7%). The addition of V made the latter target nonmagnetic and, hence, convenient for magnetron sputtering. The substrates were of 0.175-mm-thick polyester (CP Films, type OC 80/7) pre-coated with indium – tin oxide having a resistance/square of 60 V. Typical substrates were roughly 30 10 cm2 in size and shaped to fit as visors in helmets. Sputtering took place in Ar/O2/H2 with optimized mixing ratio. The target-substrate distance typically was 20 to 25 cm, and the total gas pressure was in the 30 – 40-mTorr range. Fig. 2. Spectral transmittance of a 300-nm-thick W oxide film produced by sputtering in Ar/O2/H2 under conditions making it ready for device lamination.
Fig. 1. Chromaticity coordinate z, corresponding to blue light, versus luminous transmittance for 300-nm-thick W oxide films of three types: deposited in Ar/O2 and then colored by charge insertion from an electrolyte of LiCiO4 in propylene carbonate (PC), as deposited in Ar/O2 with different oxygen contents, and as deposited in Ar/O2/H2 with different hydrogen contents. The color specifications were made by integration of spectrophotometrically recorded transmittance. Symbols denote evaluated data and lines were drawn for convenience.
From a manufacturing point of view, it is preferable to be able to deposit the films under conditions that make them ready for device lamination or to have convenient processes for making the as-deposited films ready for this. In the case of W oxide, one can carry out the deposition in Ar/O2/H2 according to principles delineated by Giri and Messier [6]. The hydrogen produces a blue color for the as-deposited films, and the key issue with regard to optimizing the gas mixture is to incorporate hydrogen into the film without creating oxygen deficiency. In practice, the optimization of the W oxide films can be made by measuring their color in the asdeposited state. Fig. 1 compares the chromaticity coordinate z, corresponding to blue light [7], with the variation of the luminous transmittance for films deposited in Ar/O2 and Ar/O2/H2. Data for a ‘‘standard’’ film deposited in Ar/O2 and colored electrochemically in an electrolyte of 1 M LiClO4 in propylene carbonate is shown for reference. As-deposited films, produced in Ar/O2, displayed a reduction in transmittance when the oxygen partial pressure was lowered. It can be seen from Fig. 1 that these underoxidized films exhibit a color that is very
A. Azens et al. / Solid State Ionics 165 (2003) 1–5
3
different from the one in the electrochemically colored films. When sputtering was done in Ar/O2/H2, however, the transmittance of the as-deposited films was decreased when the hydrogen partial pressure was increased. Such films showed the same color rendering as the electrochemically treated films for transmittance levels down to about 20% in 300-nm-thick films. The evolution of z at still lower transmittance levels is most likely associated with oxygen deficiency. The optimum gas mixture for making blue W oxide films ready for device lamination was Ar/O2/ H2 = 4/2/13 at a total gas pressure of 40 mTorr. Fig. 2 illustrates the spectral transmittance for such a film.
3. Preassembly film treatment and device lamination The Ni– V oxide films are transparent in as-deposited state and require pretreatment prior to device lamination. Charge insertion via an electrochemical procedure is possible, in principle, but obviously unwieldy and not suited for practical fabrication. An
Fig. 3. Luminous transmittance versus postdeposition ozone exposure time for 220-nm-thick Ni – V oxide films produced by sputtering in Ar/O2/H2.
Fig. 4. Spectral transmittance of a 220-nm-thick Ni – V oxide film produced by sputtering in Ar/O2/H2 and posttreated in ozone under conditions making it ready for device lamination.
alternative method for charging was developed in earlier work of ours [8]. Essentially, this new method employs exposure to ozone obtained by ultraviolet irradiation of the film in the presence of oxygen. Fig. 3 displays the decrease of the luminous transmittance as a function time when a conventional ozone photo-reactor was used to illuminate a 220nm-thick Ni – V oxide film. The transmittance drops by some 50% during a few minutes, and a further decrease takes place for extended exposure times. Fig. 4 illustrates a typical spectral transmittance plot for a Ni –V oxide film prior to device lamination. Polyester-based foils—one with a W oxide film colored by sputtering in the presence of hydrogen (cf. Fig. 2) and another with a Ni – V oxide film colored by postdeposition ozone exposure (cf. Fig. 4)—were laminated together by a proprietary PMMA-based electrolyte of the type described in Ref. [9] using roll-pressing at a temperature of 80 jC. The edges of the double foil were then sealed by a two-component epoxy glue, electrical contacts were attached, and the electrochromic device was ready for testing and use.
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A. Azens et al. / Solid State Ionics 165 (2003) 1–5
4. Device tests Devices of the type described above were cycled between colored and bleached states using trapezoidal voltage pulses between 1.6 and + 1.5 V, respectively. Durability tests are currently going on, as discussed next. Fig. 5 shows the transmittance at three different wavelengths for cycling with one full color-bleach cycle each 200 s. The optical modulation is pronounced, especially for mid-luminous and red light. Most of the changes take place within a few tens of seconds after application of the pulse, but the coloration has not reached saturation even after several minutes. Fig. 6 reports on the transmittance in fully colored and bleached states after cycling in the same way as in Fig. 5. The foils are seen to switch between a high level of about 70% and a low level of 35 – 40% at a wavelength of 550 nm, which is in agreement with the data in Fig. 5. The span between the colored and bleached states remains practically unchanged for at least 5000 cycles. Lower transmittance levels, down
Fig. 5. Transmittance at three wavelengths versus time for electrochromic switching of a laminated device as described in the main text.
Fig. 6. Transmittance at a wavelength of 550 nm versus number of color/bleach cycles for a laminated electrochromic foil device in colored and bleached states.
to 25% or less in the colored state, could be reached for longer coloration times. The devices have open-circuit memory, which is an asset since electrical power must be drawn only to effect changes in the optical properties. Fig. 7 illus-
Fig. 7. Transmittance at a wavelength of 550 nm versus time for a laminated electrochromic foil device kept under open-circuit conditions.
A. Azens et al. / Solid State Ionics 165 (2003) 1–5
trates the transmittance at a wavelength of 550 nm for a device colored to a transmittance equal to 20%. It is found that the transmittance has increased by only some 10% after a time as long as 160 h. Keeping the device in the colored state for an extended time influenced the bleaching time, and a device with an initial bleaching time of about 1 min attained a bleaching time of some 20 min after being maintained in the colored state for 160 h.
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moderate substrate temperatures [11]. The bleachedstate transmittance can be significantly enhanced with additions such as Al or Mg to the Ni oxide or Ni – V oxide [12,13]. Acknowledgements Parts of this work were financed by the Swedish Foundation for Strategic Environmental Research and the National Energy Administration of Sweden.
5. Summary and applications aspects This work has demonstrated techniques to make flexible electrochromic foils with sufficient optical modulation range, dynamics, and durability for making them interesting for a range of applications. Facile manufacturing is possible by a combination of different technologies, and the potentially cumbersome prelamination treatments of the films was avoided by sputtering the W oxide film in the presence of hydrogen and by postdeposition treatment of the Ni-based film in ozone for a few minutes. In particular, we have considered the application of the electrochromic foils in variable-transmittance visors for motorcycle helmets. Driving or riding safety requires different transmittance levels for varying lighting conditions, and the properties illustrated in Figs. 5 and 6 are excellent for such applications, as concluded in a recent study [10]. Further refinements of the electrochromic foils are underway. Thus, the switching dynamics can be made speedier once polyester foils with more conducting coatings become readily available. It should be noted in this context that intense work is going on to improve the performance of transparent and electrically conducting coatings that can be applied at
References [1] C.G. Granqvist, Handbook of Inorganic Electrochromic Materials, Elsevier, Amsterdam, 1995, reprinted 2002, Solar Energy Mater. Solar Cells 60 (2000) 201. [2] S.K. Deb, Appl. Opt. Suppl. 3 (1969) 192. Philos. Mag. 27 (1973) 801. [3] A. Azens, C.G. Granqvist, Proc. Soc. Photo-Opt. Instrum. Eng. 4458 (2001) 104. [4] A. Larsson, G.A. Niklasson, L. Stenmark, Proc. Soc. PhotoOpt. Instrum. Eng. 4102 (2000) 69. [5] T.S. Eriksson, C.G. Granqvist, J. Appl. Phys. 60 (1986) 2081. [6] A.P. Giri, R. Messier, Mater. Res. Soc. Symp. Proc. 24 (1984) 221. [7] S.J. Williamson, H.Z. Cummins, Light and Colour in Nature and Art, Wiley, New York, 1983. [8] A. Azens, L. Kullman, C.G. Granqvist, Solar Energy Mater. Solar Cells 76 (2003) 147. [9] J. Stevens, U.S. Patent 5,288,43. [10] S. Cook, C. Quigley, R. Tait, Quality and field of vision—a review of the needs of drivers and riders. Final Report PPAD 9/33/39/TT1130, Dept. Transport, Local Government and Regions, UK, 2002, unpublished. [11] C.G. Granqvist, A. Hulta˚ker, Thin Solid Films 411 (2002) 1. [12] A. Azens, J. Isidorsson, R. Karmhag, C.G. Granqvist, Thin Solid Films 422 (2002) 1. [13] E. Avendan˜o, A. Azens, G.A. Niklasson, C.G. Granqvist, Journal of Solid State Electrochemistry, in press.