Electrochromic effect in WO3 thin films prepared by CVD

Solar Energy Materials 16 (1987) 55-65 North-Holland, Amsterdam

55

E L E C T R O C H R O M I C EFFECT IN W O 3 THIN FILMS P R E P A R E D BY CVD D. D A V A Z O G L O U and A. D O N N A D I E U Laboratoire de Spectroscopie 11, Unitd Associde au C N R S no. 790, UniL,ersit~ des Sciences et Techniques' du Languedoc, 34060 Montpellier Cedex, France

O. B O H N K E Laboratoire d'Electrochimie des Solides, UnitO Associ(e au C N R S no. 436, FacultO des Sciences et des Techniques, 25030 Besan~on Cedex, France

Polycrystalline WO3 thin films were produced by CVD on fused quartz or SnO2 coated pyrex substrates. The film structures were determined. The electrochromic phenomenon was observed in this kind of films in a two electrode electrochemical cell using protons as inserting ions. Electrochromic performances were investigated using a three electrode electrochemical cell in two different electrolytes.

1. Introduction Electrochromism has been investigated in tungsten oxide thin films for possible application in display devices [1-8] and in " s m a r t windows" [9-11]. Some problems, however, prevent its commercial viability: a m o n g them one finds the long response time, the stability of the oxide in the electrolyte, the choice of the inserting ion [10,12,13] the substrate coating [14] etc. T o d a y the research on the preparation and characterization of electrochemical materials, and particularly on W O 3, in order to improve its properties, is of great interest. In a previous paper [15] the existence of the electrochromic effect has been shown on one kind of W O 3 films prepared by annealing at 600 o C of black tungsten (BW) or reflective tungsten (RW) layers produced by C V D on SnO 2 coated pyrex substrates. As an expansion of these results, insertion of protons and lithium ions has been investigated in another kind of films, annealed at different temperatures. Some of their electrochromic performances are presented in the present paper.

2. Experimental procudure and results 2.1. P r e p a r a t i o n m o d e

The preparation of the electrochromic species has been obtained in two steps. The first step is the deposition of the BW or R W film, on a glass coated by a SnO 2 thin film, the square resistance of which is - 10 ~2. The deposition was m a d e by 0 1 6 5 - 1 6 3 3 / 8 7 / $ 0 3 . 5 0 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing Division)

56

D. Davazoglou et al. / WO~ thin films prepared by CVD

pyrolysis at 400 o C of W(CO) 6 either in the presence or in the absence of an oxygen bleed for BW and RW films respectively, in a radiatively heated quartz reaction chamber [15-19]. The second step is the annealing of the sample in a similar quartz chamber at 500 ° C, in air until it turns transparent [15,19]. The WO 3 film thicknesses deposited on quartz substrates were determined by an optical method [19] while those of the BW and RW films were obtained by weight gain of the substrate during deposition knowing the density of these compounds [18]. The value of the ratio WO 3 film thickness/initial compound thickness (dwo3/dBwor dRw), defined as swelling coefficient, depends on both the annealing conditions and the nature of the initial compound. For annealing in air at 500 ° C the values of the swelling coefficient were found to be about 1.3 and 3.4 [15,19] for BW and RW as initial compounds respectively produced on quartz substrate. As it was not easy to determine optically the thickness of the W O 3 film deposited on SnO 3 glass coating, it was considered that the value of the swelling coefficients remain equal to those determined for the films formed on quartz substrates. Then the WO 3 film thicknesses were obtained multiplying the initial c o m p o u n d thickness by the corresponding swelling coefficient. In this way it was possible to compare the samples between them.

~q 0

a)

o L

B

(D

t WO 3 I 5nO 2

b) O

8_.0~ ~ED c,4

20

40 °

"'~

7 a TII

30"

20*

Fig. 1. X-ray spectra taken from two WO 3 films of comparable thickness formed by annealing BW films deposited on fused quartz substrate (a) and SnO2 coated glass (b),

D. Davazoglou et al. / WO3 thin films prepared by CVD

57

Oxidation of BW films thicker than 250 n m and R W films thicker than 40 n m was impossible, because their adherence on the SnO 2 coating b e c a m e very bad and the oxidation times were very long. Thus the present study is relative to c o m p a r a tively thin W O 3 films. 2.2. S t r u c t u r e d e t e r m i n a t i o n

X-ray diffraction measurements were made with a Philips X-ray diffractometer using a m o n o c h r o m a t i z e d C u K ~ incident radiation (X = 1.54 A). The current and the power used were 20 m A and 800 W respectively. Fig. l a shows the X-ray spectrum taken from a W O 3 film prepared by annealing, at 500 o C, in air of a BW layer deposited on a fused quartz substrate. The film is polycrystalline and the apparent peaks correspond to those of W O 3 crystallized in the monoclinic form [20,21]. In fig. l b is shown the X-ray spectrum of a W O 3 layer prepared by the same method on SnO 2 coated substrate and of about the same thickness. As previously, we observe the same peaks, which reveal the presence of a W O s polycrystalline thin film and also the presence of the peaks corresponding to the SnO 2 coating [22]. In fig. 2a and 2b, are represented X-ray spectra taken from two W O 3 films of comparable thickness prepared b y annealing of R W layers at 500 ° C in air on fused

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•

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. . . .

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. . . .

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13)

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Fig. 2. X-ray spectra taken from two polycrystalline WO3 films obtained by annealing RW films deposited on fused quartz (a) and S n O 2 coated glass (b).

D. Davazoglou et al. / WO3 thin films prepared by CVD

58

quartz substrate (a) and SnO 2 coating glass (b). It is obvious that both are polycrystalline and the peaks correspond to those of the monoclinic forms of WO 3. 2.3. Electrochromic measurements

Two kinds of electrochromic configurations were used: the first one, a two electrode cell, allowed us to study the transmittance spectra when the samples were colored or not; the second one, a three electrode cell, allowed us to determine, for a given wavelength, the electrochromic characteristics of the samples, without kinetics influence of the counter electrode. 2.3.1. The two electrode cell

A typical electrochemical cell, in quartz, was used for the electrochromic measurements. The working electrode was formed of the electrochromic compound. A platinum disk was used as counter electrode. The cell was filled with a normal aqueous H2SO 4 solution. The working electrode was maintained parallel to the cell wall by a system of two plastic grooves fixed on the cover of the cell. The voltage was applied between the Pt disk and the SnO2 film. A Beckman UV 5240 double beam spectrometer was used to measure the transmittance of the electrochemical configuration; this was 0.5 at 1.3 .~m. An analogous cell, containing electrolyte but no glass sample was placed in the reference beam. Before all optical measurements, a + 1 V voltage was applied until the m a x i m u m transmittance was obtained. After that the WO 3 layers were successively colored

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Fig. 3. Measured spectral transmittance as a function of voltage for an electrochromic sample formed by a WO3 film obtained by oxidation of a BW layer deposited on SnO2 coated glass and in contact with a normal H2SO 4 solution.

59

D. Davazoglou et aZ / WO3 thin films prepared by C VD

T 1 ~non

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v

02 ;

0 0.3

0.4

05

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Fig. 4. Spectral transmittance as a function of voltage from a two electrode cell using as working electrode an electrochromic sample formed by a WO3 film produced by oxidation of a RW la~cr deposited on SnO2 coated glass. The used electrolyte was a normal H2SO 4 solution. and bleached by reversing the applied voltage. The transmittance of the colored film was recorded with the coloring voltage applied continuously. In the figs. 3 and 4, typical spectral transmittances are drawn for two samples, with a similar W O 3 layer thickness, formed by oxidizing at 500 ° C in air BW and R W films respectively. It is obvious, in both cases, that the transmittance can be varied gradually and reversibly between wide limits particularly in the near infrared by the use of voltages less than 2 V. There are no significant differences concerning the electrochromic effect between the two spectra. Nevertheless the better transmittance before coloration in the near infrared region for the sample prepared from R W c o m p o u n d is in agreement with previous results [19]. 2.3.2. T h e three electrode cell

The electrochromic performances of W O 3 films (coloring and bleaching times, injected charge, optical density) were measured in situ in a three electrode cell. The variations of the electrical and optical parameters were determined simultaneously in the cell through a computer control. The synopsis of the setup is shown in fig. 5. The electrical measurements were carried out using a P A R electrochemical instrumentation (Model 173 potentiostat, Model 176 current follower) controlled by a 8096 C o m m o d o r e computer. The voltage V applied to the electrochromic electrode (WE) was measured against a reference electrode (RE). The optical variations of the film were carried out with a H e - N e laser (?t = 633 nm). The laser b e a m crossed the electrochromic cell and the transmitted light was measured through a photocell. The acid electrolyte was a normal aqueous H z S O 4 solution. The reference electrode was then a saturated sulfate electrode (SSE). The organic electrolyte was made of 1M LiC104 dissolved in a hydroorganic solvant: propylene carbonate (PC)

D. Davazoglou et a L / WO3 thin films prepared by CVD

60

I

,I

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0 T

....

DISK

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Fig. 5. Synopsis of the e x p e r i m e n t a l setup for e l e c t r o c h r o m i c m e a s u r e m e n t s in a three electrode cell configuration. W E = w o r k i n g electrode ( S n O 2 / W O 3 ) , R E = reference electrode, C E = c o u n t e r electrode (Pt), V = a p p l i e d voltage, I = current, a n d O T = o p t i c a l t r a n s m i s s i o n . S

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Fig. 6. C o l o r a t i o n time versus the o v e r p o t e n t i a l applied to W O 3 electrode in 1N H 2 S O 4.

D. Dauazoglou et al. / WO 3 thin films prepared by C VD

61

and 1 wt.% of bidistilled water. The reference electrode was then Ag/A1CIO 4 (0.01M) in PC. Previous studies [7,8] have shown that such an electrolyte gives a fast response time and keeps amorphous evaporated WO 3 film stable. All the experiments were performed under N 2 atmosphere to avoid both the electrochemical reduction of dissolved oxygen and the reoxidation of the hydrogen tungsten bronze

[23]. Fig. 6 shows the relationship between the coloration time of both RW and BW films and the overpotential applied to this film in the sulfuric acid electrolyte. The overpotential is defined as the applied potential (V) minus equilibrium potential of the film in the electrolyte. In sulfuric acid, the equilibrium potentials of both RW and BW are - 4 0 0 mV versus SSE. The overpotential is the driving force which leads to the electrochemical reaction and therefore is the significant parameter. In this figure, the coloration time is relative to a coloration of the film up to a 0.3 optical density. We can observe that the response time is strongly dependent on the

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cycling.

62

D. Davazoglou et al. / W O ¢ thin films prepared by CVD .5

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preparation procedure used to obtain the WO 3 film. The presence of oxygen during pyrolysis enhances the electrochromic effect. Moreover, coloration-bleaching cycling also enhances response time as shown in fig. 7 for BW samples. A coloration time as low as 300 ms is obtained with an overpotential of - 0 . 8 V applied to the electrochromic electrode. Fig. 8 shows the relationship between the optical density of the film and the injected charge Q during coloration. The slope of the obtained straight lines gives the coloring efficiency of the film. It is found to be 38 and 41 cm 2 C 1 for RW and BW respectively in transmissive mode. This low efficiency if compared with amorphous films, i.e. 120 cm 2 C 1 for anodic oxidized films obtained by potentiostatic technics [24], 60 cm 2 C 1 for evaporated films [26] and 70 cm 2 C 1 for anodic oxidized films obtained under pulsed currents [25], may be attributed to the polycrystalline nature of the CVD film. Indeed, crystallized anodic oxidized films show an efficiency of 34 cm 2 C-1 [24] instead of 120 cm 2 C - 1 as mentioned above. 7.5 mC cm -2 is found to be the coloring charge which corresponds to an optical

63

D. Davazoglou et al. / WO3 thin films prepared by CVD 10

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[VOLTS]

Fig. 9. Coloration time versus the overpotential applied to a BW film in LiC104 (M) 1% H20-PC electrolyte.

density of 0.3 for BW film. The behavior of R W film for injected charge greater than 6 m C cm -2 may be attributed either to a degradation or to a structural modification of the film cycling. The reversibility of the electrochromic p h e n o m e n o n is good over tens of cycles even if the decoloration time is long, of the order of 30 to 60 s. The electrochromic effect has also been measured in hydroorganic electrolyte for BW films. The coloration time was longer than in acid electrolyte as shown in fig. 9 and a coloration efficiency of 25.5 cm 2 C - 1 was found as shown in fig. 10. However, reversible electrochromic effect has been detected in this medium. Further studies are necessary to determine the best electrolyte to use in order to obtain better electrochromic reponse and faster bleaching kinetics. Indeed, complete decoloration occurred in 6 0 - 1 2 0 s.

64

D. Davazoglou et al. / WO3 thin films prepared by C VD .5

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Fig. 10. Optical density versus the injected charge into a BW film in LiCIt4 (M) 1% if20-P(I electrolyte.

3. Conclusion We have shown the presence of the electrochromic effect in WO 3 thin films prepared by oxidation at high temperature of tungsten compounds obtained by CVD. The results are very encouraging since they show a good reversibility of the electrochromic process with a fast coloration time and a good efficiency for crystallized materials. However, the influence of many parameters (stoichiometry, substrate coating, electrolyte, etc.) remains to be studied to optimize these CVD films. Nevertheless we hope that this kind of materials will find interest in electrochromic devices for smart window applications. Ackowledgements The authors thank Professor J.C. Manifacier for providing the SnO 2 coated glass substrates, Dr. R. Fourcade for his numerous and stimulating discussions about the structure determination, and Dr. L. Martin for the optical measurements.

D. Davazoglou et al. / WO 3 thin films prepared by CVD

65

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