Electrochromic thin films deposited onto polyester substrates

Thin Solid Films, 204 (1991) 123-131

123

PREPARATION AND CHARACTERIZATION

E L E C T R O C H R O M I C T H I N FILMS D E P O S I T E D O N T O P O L Y E S T E R SUBSTRATES C. ROUSSELOT Laboratoire d'Electrochimie des Solides ( U.A. 436 CNRS), Universitb de Franche-Comtb, Route de Gray, 25030 Besangon (France) P. A. GILLET Laboratoire des Fluorures (U.A. 449 CNRS), Universitk du Maine, Route de Laval, 72017 Le Mans (France) O. BOHNKE Laboratoire d'Electrochimie des Solides ( U.A. 436 CNRS) , Universitk de Franche-Comtk, Route de Gray, 25030 Besan¢on (France) (Received January 29, 1991; accepted April 2, 1991)

The results of in situ measurements of the electrochemical and optical properties of tungsten trioxide (WO3) thin films obtained by vacuum evaporation onto transparent and conductive plastic substrates are presented. Much attention is focused on the procedure used to obtain the electrochromic films. These electrochromic layers have been tested in lithium electrolyte. Moreover, the electrochemical behaviour of two transparent electrodes deposited onto polyester, and previously used as the electrochromic electrode substrates, has been investigated in lithium electrolyte in order to use them as a counterelectrode in a transmissive electrochrome device.

1. INTRODUCTION Much attention is being focused on the development of "smart windows" with dynamic control in order to improve energy efficiency in buildings. Many systems have been proposed using chromogenic materials 1-3. Among them, the electrochromic materials are very attractive, compared with liquid crystal materials (liquid crystal displays), because of both the possibility of large area devices and the memory effect. Variable light transmission electrochromic smart windows can be fabricated by depositing the electrochromic thin film onto a glass conductive substrate and by including it as a window in the building. Another way would be to make "curtains" by using very thin plastic conductive substrates and by depositing the electrochromic layer onto it in order to obtain a movable thin smart window. This device would be very thin and could be sealed onto the window as a curtain. No work has been reported on the procedure used to obtain electrochromic thin films on plastic substrate and on the behaviour of such obtained materials. This paper mainly deals with such films obtained by vacuum evaporation and with their electrochromic properties in lithium electrolyte. We also investigated the electrochemical properties, in lithium electrolyte, of 0040-6090/91/$3.50

© ElsevierSequoia/Printedin The Netherlands

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C. R O U S S E L O T , P. A. G1LLET, O. B O H N K E

commercial conductive thin films deposited onto plastic substrates in order to use them as a counterelectrode in a transmissive electrochromic device. Some results have been previously reported on the behaviour of indium tin oxide (ITO) electrodes deposited onto glass substrates in lithium electrolyte s 7 They clearly exhibit different electrochemical and optical behaviours, certainly as a result of the different oxide preparation methods used and the different crystallinities of the oxides obtained. We investigate in this paper the structure and the electrochemical properties of commercial conductive materials deposited onto polyester film, in I M LiC104 dissolved in propylene carbonate (PC). 2.

E X P E R I M E N T A L DETAILS

We used transparent conductive layers obtained from Bekaert: A L T A I R - O (55 f~,/[~) and A LTAIR-M (15 f2/[~). The substrate of these conductive layers was a 125 ~m thick polyester. These layers were all prepared by cathodic sputtering. A L T A I R - O is an I T O layer and A L T A I R - M is a metallic thin film (ALTAIR-O and ALTAI R-M are trademarks of Southwall Technologies). The cyclic voltammograms (CVs) of both the conductive layers and the electrochromic electrodes were obtained in I M LiCIO4-PC electrolyte with a classical three-electrode cell. The reference electrode was a silver electrode in AgC104 (10 2 M ) dissolved in PC (V = + 3 . 5 5 V with respect to Li/Li+) s. The counterelectrode was a platinum plate. The electrolyte was continuously deaerated to avoid oxidation by dissolved oxygen. The variations in both the current and the optical density of the electrochromic electrode were obtained simultaneously in the electrochemical cell using a Tektronix 2510 data acquisition system, an IBM PS/2 55SX computer for control and conventional PAR electrochemical instrumentation ( 173 potentiostat and 176 current follower), as described elsewhere ~. The changes in the optical density of the films were measured with an He Ne laser (2 = 633 rim). The responses of the electrodes were measured by both voltage sweep and voltage step. The experimental data were analysed with specific programs written in the laboratory. The optical spectra were obtained with a Monolight Spectrum Analyser with a 20 W tungsten halogen light source, a scanning m o n o c h r o m a t o r and a silicon photodiode detector. A full spectrum (350 1100 nm) is acquired in 8 ms. 3.

T H I N FILM D E P O S I T I O N P R O C E D U R E

Tungsten trioxide thin films were deposited onto transparent and conductive polyester substrates by vacuum evaporation of WO3 powder (purchased from Johnson Matthey) from a tungsten boat. The rate of evaporation was controlled by a quartz thickness controller (Inficon). The procedure we used to obtain an electrochromic coating is schematically shown in Fig. 1. Evaporation is performed through different steps, at a pressure of 10-5 mbar. The deposition rate and the intensity of the current flowing through the tungsten boat are reported in Fig. 1. The procedure used consists of different steps of successive heating, evaporation and cooling: (i) heating the tungsten boat for 20 min with a current increasing from 0 to

125

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150 A, leading to the degassing of the WO3 powder; (ii) deposition, at a constant rate of 1.5/~ s-1 for 10 min, onto the substrate (the current intensity is continuously adjusted to maintain the evaporation rate); (iii) decreasing the intensity of the current to 80 A to produce a zero deposition rate and to cool the substrate for 15 min; (iv) heating the tungsten boat by increasing the current from 80 A to 160 A for 5 rain without deposition. Steps (ii), (iii) and (iv) are repeated three times. The substrates are maintained in rotation above the evaporation source to obtain homogeneous films. Finally, a thin film (4200/~) is obtained, which is very adherent to the conductive polyester substrate and which is electrochromic. The adherence of the film, which is very dependent on the evaporation procedure used, was tested with a 3M Scotch ® band. 4. RESULTS AND DISCUSSION 4.1. Conductive layers The conductive layers display a low square resistance, i.e. for ALTAIR-O, 55f~/r-], and for ALTAIR-M, 15Q/r--I as measured with a four-point probe

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c. ROUSSELOT, P. A. GILLET, O. BOHNKE

instrument (SQOHM-1, Solems Technol). Figure 2 shows the optical transmittance spectra of A L T A I R - O (Fig. 2(a)) and A L T A I R - M (Fig. 2(b)). The films are very thin, of the order of 1500 ~, as shown by the interference pattern of the spectra. This leads to a resistivity of A L T A I R - O around 10 3 ~ cm. A substantial difference between A L T A I R - O and A L T A I R - M electrodes is the low transmission in the near-lR region of the latter material. Both materials have a high transmission in the region 400-600nm. Such a behaviour is important for electrochromic smart window design. I00

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Figures 3 and 4 show the CVs of A L T A I R - O and A L T A I R - M respectively in LiC104 (1 M ) - P C electrolyte. The sweep rates are 5 0 m V s i and 100mV s 1 respectively and the potential is measured v s . an Ag/Ag + reference electrode. The potential range used, i.e. (from 3 V to + 3 V) v s . Ag/Ag + is such that lithium metal plating cannot occur (Li/Li + is at - 3.55 V v s . the Ag/Ag + reference electrode) 8. The CVs show that Li + ions undergo insertion into the oxide layer (under cathodic polarization) and extraction (under anodic polarization), either in A L T A I R - O or in ALTAIR-M. The CVs obtained with A L T A I R - O are reproducible and reversible:

ELECTROCHROMIC THIN FILMS ON POLYESTER

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Fig. 4. Voltammograms of an ALTAIR-M electrode in LiCIO4 (1 M)-PC electrolyte (sweep rate, 100 mV s- 1): curve 1, first cycle; curve 2, second cycle; curve 3, third cycle. n o d e g r a d a t i o n of the e l e c t r o d e is d e t e c t a b l e after several cycles. Figure 3 presents a C V after 10 cycles. H o w e v e r , it is clear that, if the kinetics of insertion is rapid, the

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C. ROUSSELOT, P. A. GILLET, O. BOHNKE

kinetics of extraction is much slower, as shown by the values of the current density flowing through the electrode during cathodic and anodic polarizations. O n the contrary, the CV obtained with A L T A I R - M (Fig. 4) clearly indicates a rapid degradation of the electrode during the first few cycles. This drastic decrease in the current, also observed in the case of a gold electrode deposited onto a plastic substrate, m a y be due to the reduction of Li + ions and alloying with the metal. The degradation of the electrode is marked by its brown coloration. It is then obvious that such an electrode c a n n o t be used as a counterelectrode in a transmissive device. However, A L T A I R - O seems usable although extraction presents a slow kinetics. 4.2. Electrochromic electrodes After deposition of thin films of W O 3 onto conductive plastic substrates, the response of the electrochromic electrode was measured by voltage sweep and voltage step in LiCIO 4 (1 M)--PC. The CVs and optical density varitions of W O 3 electrodes are shown in Fig. 5 for A L T A I R - O (curves a) and A L T A I R - M (curves bt E

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Fig. 5. Voltammograms and coloration of a W O 3 electrochromic thin film deposited onto ALTAIR-O (curves aJ and ALTAIR-M (curves bt substrates in LiC10,t (1 M}-PC electrolyte. Sweep rate, 50 mV s 1.

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ELECTROCHROMIC THIN FILMS ON POLYESTER

conductive layer substrates. The sweep rate was 50 mV s-~. Both electrochromic electrodes display a coloration and a bleaching during the voltage sweep. However, the behaviour of these electrodes is different for cycling. The influence of the conductivity of the layer is clearly shown on the CV curves by the slope of the curves: the higher the conductivity, the higher the current density for a given potential, and the smaller the ohmic drop in the electrode. The voltage steps used to characterize the electrochromic electrodes have been chosen according to the CV curves, i.e. from - 1 . 5 V to - 3 . 5 V v s . Ag/Ag ÷ for coloration, in order to avoid lithium plating onto WO3, and from + 1.5 V to + 3.5 V v s . Ag/Ag ÷ for bleaching in order to avoid perchlorate oxidation onto WO3. Pulses 1 s in length have been applied to 1 cm 2 electrodes. Figure 6 shows both the electrical and the optical responses of an electrochromic electrode with ALTAIR-O (55 ~/I-]) as a substrate, at 633 nm. By switching the potential between - 2.5 V and + 2.5 V v s . Ag/Ag ÷ (curves a) or between - 3.5 V and + 3.5 V v s . Ag/Ag ÷ (curves b), a variation

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130

C. ROUSSELOT, P. A. GILLET, O. BOHNKE

in the optical density of 0.35 and 0.5 respectively may be obtained in 1 s in coloration and in ! s for bleaching. The coloration efficiency is found to be 40 cm 2 C - 1. The electrochromic process is reversible and no degradation is observed during cycling. The same potential steps have been applied to a WO3/ALTAIR-M (15 D/D) electrode (1.5 cm2). Figure 7 shows the electrical and optical responses of this electrode at 633 nm. By switching the potential between - 1.5 V and + 1.5 V vs. Ag/Ag + (curves a), between - 2 . 5 V and + 2 V vs, Ag/Ag + (curves b) and between 3.5 V and + 3 V vs. Ag/Ag + (curves c), optical density variations of 0.38, 0.6 and 0.70 respectively may be obtained in 1 s for both coloration and bleaching. The coloration efficiency decreases as the electrode is cycled because of the degradation of the electrochromic electrode, as mentioned in the CV study above. We obtained the same decrease in the performance ofa WO3/Au electrochromic electrode. This is mainly due to a reaction between the inserted species and the metal substrate, leading to a decrease in the coloration efficiency as cycling proceeds. -

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Fig. 7. C u r r e n t density a n d optical density v a r i a t i o n s v s . time for a W O 3 / A L T A I R - M e l e c t r o c h r o m i c electrode (1.5 cm 2). The electrode p o t e n t i a l is switched between - 1.5 V and + 1.5 V vs. Ag/Ag* (curves a), between - 2.5 V and + 2 V vs. A g / A g + (curves b) a n d between - 3.5 V and + 3 V t~s. A g / A g ÷ (curves c).

ELECTROCHROMIC THIN FILMS ON POLYESTER

131

5. CONCLUSIONS

These results show that electrochromic WO3 may be deposited onto polyester substrates by using a procedure of evaporation combining evaporation and cooling steps. The electrochromic performances, measured in LiC104 (1 M)-PC electrolyte, are good. The commercial ITO material (ALTAIR-O) may be conveniently used either as a counterelectrode in a device or as an electrochromic layer substrate. Good electrochemical and electrochromic reversibilities have been observed during cycling. However, the commercial metal-type material (ALTAIR-M) exhibits a rapid degradation during the first cycles in the electrolyte. It cannot be used either as a counterelectrode or as the electrochromic substrate. Generally, it seems really difficult to use a metallic transparent substrate for an electrochromic electrode. REFERENCES 1 C. M. Lampert and C. G. Granqvist, Large-area chromogenics: materials and devices for transmittance control, Proc. Soc. Photo-Opt. Instrum. Eng., IS4 (1988). 2 M.K. Carpenter and D. A. Corrigan (eds.), Proc. Syrup. on Electrochromic Materials, Electrochemical Society, Pennington, N J, 1990. 3 C.M. Lampert, Sol. EnergyMater.,11(1984) l 27. 4 S.F. Cogan, E.J. Anderson, T.D. PlanteandR. D. Rauh, AppLOpt.,24(15)(1985)2282. 5 R.B. Goldner, G. Foley, E. L. Goldner, P. Norton, K. Wong, T. Haas, G. Seward and R. Chapman, Appl. Opt., 24 (15) (1985) 2283, 6 J . S . E . M . Svensson and C. G. Granqvist, Appl. Opt., 24 (15) (1985) 2284. 7 A. Corradini, A. M. Marinangeli and M. Mastragostino, Electrochim. Acta, 35 (11 12) (1990) 1757. 8 O. Bohnke, Doctoral Thesis, Besanqon, 1984.