LiClO4-PC electrochromic displays

Performance of a-WO3/LiCIO4-PC electrochromic displays E. ANDO, K. KAWAKA.MI, K. MATSUHIRO, Y. MASUDA This paper deals with the improvements in response time and reliability of the a-WOs/LiCIO4-PC (propylene carbonate) electrochromic display. Response time was improved by adopting the highly porous a-WOs film, the porous reflector and the high surface area non-polarizable counterelectrode. Results from the reliability tests indicated that the ECD had a sufficient lifetime for practical applications such as in watches, clocks and calculators. The mechanism of performance drift upon cycling the display is investigated using ESCA, AES and atomic absorption methods.

Keywords: display devices (computers); tungstic acid; electrochromism; perfornmnce; testing. Electrochromism is the property of a material to change colour reversibly by redox reaction. An electrochromic display (ECD) making use of this phenomenon is a matter of recent concern because of its possible application in flat panel systems. Since it is nonemissive, it does not wash out under bright light and it can be viewed over long periods without tiring the eyes. Compared with a liquid crystal display, the fabrication of a large-sized ECD panel is relatively easy, because the ECD needs no precise gap control. It has an excellent aesthetic appearance with a light background, and a wide viewing angle achieved since there is no need of polarizers. However, using an electrochemical reaction, a-WO3-based ECDs are known to have a slow response time and insufficient long-term reliability. There have been numerous studies on the characteristics of a-WO3-based ECDs 1. The reports up to now have dealt with the kinetics2-4 or the stability5-7 of a-WO3 films and also with the stability of counterelectrodes8'9. The object of this investigation was restricted to the individual characteristics, and there have been very few reports concerning the total device performances of practical ECDs 1°. The main purpose of this study is to describe some aspects of the performance of the ECD using a-WO3/ LiCIO4-PC. The improvements in response time, the high temperature storage life and the cycle life are reported. Another purpose of this study is to clarify the mechanism of the cycling degradation. The results from the analyses of the degraded a-WO3 films are also reported.

EXPERIMENTAL

a-WOs film Amorphous tungsten oxide films were prepared by electron beam evaporation of WO3 powder at a deposi1 tion rate of 5/~ s- onto ITO (indium tin oxide; 20 F~ per square) coated soda-lime-silica glass substrates.

The authors are in the Research and Development Division, Asahi Glass Company Ltd, 1150 Hazawa, Yokohama, Kanagawa, 221 Japan.

DISPLAYS. JANUARY 1 9 8 5

The evaporations were carried out at room temperature, and the pressure during the evaporation was controlled at between 8--20 × 10 -3 torr by dry N2 gas bleeding. The film thickness of about 5 000 A was determined in consideration of the aesthetic appearance of the electrode patterns.

Counter-electrode/porous reflector The counter-electrodes were made from a mixture of carbon powder, depolarizers and polymer binders. The mixture was kneaded into a sheet with a thickness of 0.2 mm. In the present study, WlsO49 and V6013 were used as depolarizers. The porous white reflectors were made by essentially the same method as used for the counter-electrodes, but a white pigment was mixed with the polymer binders. The thickness of the sheet was also 0.2 mm. The counter-electrode and the porous white reflector pair were then laminated and pressed. The thickness of the final sheet was controlled between 0.2 and 0.4 mm.

Electrochromic cell The EC cells were composed of the front glass substrate, the display electrode, the electrolyte, the reflector/counter-electrode, and a back cover glass substrate, as shown in Fig. 1. The inside of the cover glass was coated with ITO film. Two substrates were sealed with an organic material at elevated temperature. After the LiCIO4-PC electrolyte had been injected, a filling hole in the back cover glass was sealed with an organic sealant. Two different types of EC cells were prepared for the present experiment. One was a 3½-digit display with a digit area of 6.8 mm 2. The other was a test cell with an active area of 200 mm 2 for analysing the a-WO3 films. A n a l y s e s o f a - W O s films The a-WO3 films after cycling were analysed by ESCA, AES and atomic absorption method. The tungsten-tooxygen ratio in the a-WO3 films was determined by the ratio of ESCA W4f/Ols peak area with the aid of calibrations obtained from standard materials 11. Crystal-

0141-9382/85/010003-08 $03.00~) 1985Butterworth& Co (Publishers) Ltd

3

is a linear relationship between the inverse of the response time and the applied voltage. SiO /-Gloss /

/

/-o-W03/

Electrolyte /f-Indium-tin oxide

// //,///,-

counter-electrode paste

Fig. 1 Cross-section o f a-WO3/non-aqueous electrolyte electrochromic cell. F and R, the substrate thickness, were 0.3 m m and O. 7 m m respectively

line powders of W18049, W2005a, W03 and Li2W04 were used as the standard materials.

Response time The response times were measured electrically 12. The colouring time and the bleaching time were defined as the time to inject a colouring charge of Q , and as the time to extract 90 per cent of Q,, respectively. 6 mC cm -2 was the standard colouring charge. The colour density of 6 mC cm -2 corresponded to a contrast ratio of 3:1 measured in the reflective mode by visual photometry. An interval of a second was taken between colouring and bleaching. The applied voltages were defined as the potential of the display electrode against the counter-electrode. As a standard voltage, - 1.5 V was used for colouring and 0 V for bleaching.

Figures 3a and 3b show the temperature dependence of the colouring time and the bleaching time respectively. The response times with the load resistances of 100 fl, 200 ~2, 490 f~ and 1050 f} connected to the EC cell are also shown in Fig. 3. When the' response times are plotte d against the load resistance, there is a linear relationship between them at each temperature, and the extrapolated value gives the cell resistance. The temperature dependence of the cell resistance obtained in this way is shown in Fig. 4. Figure 4 also shows the temperature dependence of the electrolyte resistivity. The higher the temperature or the larger the load resistance, the less the temperature-dependence of the response time. In addition, in the high temperature region, the electrolyte resistance calculated is very small compared with the cell resistance. The ITO resistance does not change significantly 13, and consequently, in the high temperature region, the response time of the EC cell is mainly restricted by the lead ITO resistance. In the low temperature region, on the other hand, the cell resistance is much greater than the ITO resistance, which is estimated at less than 100 Q. Therefore, in the low temperature region, the response

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Life test Reliability of the EC cell was examined in a high temperature storage test, and its performance evaluated by the change in Qw for a given colouring time. Cycle lifetime of the EC cell was tested by driving at 1 Hz square wave. To display the figures changing from 0 through 9 and back to zero, each segment was driven as follows. The segments which would be coloured in the following state were either addressed or open-circuited in accordance with the previous states. The segments which would be bleached were short-circuited with the counter-electrode irrespective of their previous conditions. The test was carried out at 25°C. The average contrast during the test was kept constant; 6 mC cm-L The applied voltage was - 1 . 5 V for colouring. The degradation of the EC cell was estimated by the change in Qw for a given colouring time. RESULTS A N D D I S C U S S I O N Response time Figure 2 shows the relationship between the response time and the applied voltage of the EC cell. The response time is strongly dependent on the applied voltages as well as on the colouring charge density. There

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200 500 -2

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Voltagewlth respect to counter-electrode (V)

Fig. 2 Inverse o f the response time plotted against applied voltage. The lines on the left side and on the right side represent the colouring time and the bleaching time respectively. Each line corresponds to the charge densit7 of." x - - 3 m C cm-2; + - - 6 m C cm-2; [] - - 9 m C cm-~; o - - 12 m C cm -2. The electrode area o f the E C D was 6.8 m m 2, using a digit area o f 6.8 m m 2 at 25°C. Voltages were applied with respect to the counter-electrode

D IS P LA Y S . J A N U A R Y 1985

time of the EC cell is mainly governed by factors other than the ITO resistnce.

Temperoture,

80

As mentioned above, the response time of the practical EC cell is controlled not only by the a-WO3 film, but also by the counter-electrode, the electrolyte resistance and the lead ITO resistance. The contributions of these materials to the response time vary with the cell geometry, the operating display area and the temperature.

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In the present study, the improvements of the response time were carded out on the a-WO3 film, the counterelectrode and also the reflector, which was necessary for the reflective type cells. i01

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The electrochromism of the a-WO3 is one of the most significant factors in obtaining a fast responding EC cell. The relationship between the electrochromic properties and the packing density or the background gas pressure during evaporation have already been reported 14. Here, the evaporation condition was chosen so as to obtain the best colouring and bleaching response time after the heat treatment necessary to fabricate the EC cell.

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There is one more important property of the counterelectrode. This is the rest potential, which decides the applied voltages for colouring and bleaching. The rest potential of the counter-electrode was adjusted at - 2 0 0 mV with respect to Ag/Ag + by mixing WlsO49 with V6013 SO as to bleach the ECD by short-circuiting as well as to colour the ECD quickly, as shown in Fig. 5. This reduces the power consumption of the ECD to a minimum, and therefore is very profitable for smallsized portable applications where batteries are used.

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Fig. 3 Temperature dependence o f the response time for: a - - colouring; b - - bleaching. The load resistance was connected to the cell: o - - 0 t); D - - 100 fi; A - - 200 fi; + - - 490 t); x - - 1050 t). The colouring voltage and the bleaching voltage were - 1 . 5 V a n d 0 V against the counter-electrode, respectively. The charge density was 6 m C c m - 2 f o r a digit area o f 6.8 m m 2

DISPLAYS.

JANUARY

1985

The counter-electrode properties are no less important than those o f the a-WO3 film. Reversibility of the redox reaction and little transient polarization upon colouring and bleaching are required for the counter-electrode. The capacitance of the counter-electrode prepared in the present study was so large that the difference between the rest potential and the transient potential was very small.

In case of the reflective type ECDs, a diffusible reflector is necessary. This is generally placed between the display electrode and fhe counter-electrode. Conventional porous alumina ceramics with a porosity of about 50 per cent doubled the electrolyte resistance. The white reflector prepared in the present experiment was so porous as to reduce the increase of the electrolyte resistance to half. In practical applications the total thickness of the displays is generally restricted. This is especially true for a

5

the resistance of the counter-electrode and the lead electrode of the counter-electrode. Providing that the contribution of the common resistance to the cell resistance becomes small, the response time will be independent of the operating display area. Table 1 shows the effect of the load resistance connected to each digit of the display electrode.

0 ÷~

0

The larger the load resistance, the less the difference between the injected charge densities. In case of the load resistance of 490 f~, the difference of the injected charge density for a given colouring time was within - 10 per cent for 3 to 1 digit driving.

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Mixing ratio (real %)

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Fig. 5 Potential plotted against mixing ratio of W18049 and 1/6013 watch application. The laminated reflector and counter-electrode as thin as 0.2 mm enabled the ECD to be fabricated thinner than 1 mm, as shown in Fig. 1.

Simple constant voltage driving An EC cell is coloured by applying a constant current or a constant voltage for a given time between the a-WO3 film and the counter-electrode. Using constant current driving, each segment can be equally coloured. Here, maximum applied voltage must be restricted within the voltage where no side-reaction occurs. At lower temperatures, the applied voltage reaches the maximum voltage allowed because of the increase of the cell resistance. Therefore, a longer and smaller current pulse has to be applied even at high temperatures in order to maintain the colour density constant at low temperatures. Conversely, the colouring time is shorter for a constant voltage driving at moderate and high temperatures. However, the response times of the EC cell depend not only on the temperature but also on the operating display area. Therefore, in addition to the temperature compensation, another compensation is necessary for the driving circuit. A brief experiment was carried out for a constant voltage driving in order to reduce the temperature dependence and the response time difference which arises from operating at a different display area. As mentioned before, the cell resistance of the ECD is the sum of the resistance of the a-WO3 film, the counterelectrode, the electrolyte and the ITO electrode. Providing that the contribution of the ITO resistance to the cell resistance becomes large, the temperature dependence of the response time will be greatly reduced. When the load resistance of 490 f~ was connected to the cell, the changes of injected charge for a given colouring time were within + 15 per cent between 0°C and 60°C, whereas the colouring time and the bleaching time were still below 500 ms.

Simple constant voltage driving seems to be possible for the EC cell prepared here without significantly sacrificing the response time, when the ITO resistance of the display electrode and the output resistance of the driving circuit are properly adjusted.

Storage test Figure 6 shows the result of the storage test at 80°C. The injected charge for a given colourig time is maintained almost constant up to 1500 hours. There are two important factors in obtaining a stable EC cell. One is the air-tightness of the seal. The other is the electrochemical stability of the a-WO3 film and the counter-electrode in the electrolyte. When oxygen gas penetrates into the EC cell through the seal, the open-circuited memory becomes worse and the response time changes greatly due to the potential shift of the electrodes. In case of water, the a-WO3 film dissolves in the electrolyte and afterwards disappears from the substrate.

Table 1. Ratio of injected charge density for 3 to 1 digit operation at 20°C RL* (~)

Ratio

0 100 200 490 1050

0.75 0.83 0.87 0.90 0.94

* Load resistance was connected to each digit.

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The lack of colour uniformity when different sizes of electrodes or different numbers of segments are operated is due to the common resistance which consists of

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Storage time (h)

Fig. 6 Ratio of injected charge to the initial value plotted against storage time at 80°C

DISPLAYS. J A N U A R Y 1985

Amorphous WO3 films are known to exhibit ion exchange when kept in an alkali electrolyte (eg 1M LiCIO4-PC) 15. - - W - - O H + Li + ~ - - - - W - - O - L i + + H ÷ A large potential shift of the a-WO3 film is observed with this ion exchange, causing the considerable change of response time for a given voltage. When acid is added to the electrolyte, this ion exchange can be suppressed. Something similar to this may take place in the practical EC cell. Various metal oxides as well as a-WO3 films show ion exchange in the electrolyte 16. The metal oxide in the counter-electrode is also considered to show the ion exchange in the electrolyte. In the practical EC cell, the quantity of the metal oxide in the counter-electrode is at least three orders of magnitude greater than in the a-WO3 and is also large compared with the total of lithium ions in the electrolyte. Therefore, in the practical EC cell, ion exchange of the a-WO3 film seems to be affected by that of the metal oxide in the counter electrode.

Table 2. Change of oxygen content analysed by ESCA Number of cycles

Ooxide/W

OoH-/Ooxide

0 5 x 104 1.6 x 10 s 0* 2.3 x l0 s*

2.63 2.91 2.97 2.79 3.12

<0.05 0.11 0.19 <0.05 0.20

* Heat treatment (300°C 15 min)

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19.5 Sputter time (minutes)

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39.0

The EC cell prepared in the present experiment had long-term stability over a wide temperature range. This is due to the air-tightness of the seal and to the stability of WlsO49 and V6013 added to the counter-electrode. A n a l y s e s o f d e g r a d e d a - W O a films t3.

Many authors have reported the cycling degradation of a-WOa films in an electrolyte using LiCIO4-PC 17'18. The mechanism of this degradation seems not to be fully understood.

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Figure 7 and Table 2 show the results of analyses of the a-WO3 films after cycling. An increase of oxygen and O H - was observed in the a-WO3 films. There was also

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Sputter time (minutes)

Fig. 8 Depth profile o f a-WO3 film by A E S : a - - before cycling; b - - after 1.6 x 105 cycles. In," o - - Na (×4); e - - W ( x 4)

Table 3. Li and Na content in a-W03 film analysed by atomic absorption

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Binding energy (eV)

Fig. 7 E S C A Ols spectra o f a-W03 film: a - - before cycling; b - - after 1.6 x 10~ cycles

DISPLAYS. J A N U A R Y 1985

Number of cycles

Na (#gcm -2)

Li ~g cm -2)

0 2.6 x l0 s 1.7 x 106

1.3 3.1 14.7

0.89 0.46 0.51

an increase in sodium content. The depth profile of a-WO3 films by AES, shown in Fig. 8, indicates that the sodium has come from electrolyte side. Lithium content in the a-WO3 film does not increase with cycling, whereas the sodium content increases gradually as shown in Table 3. The change of the auger spectra before and after cycling indicates that there was more than one peak generated, as shown in Fig. 9a and 9b. The peak at 262.8 eV in Fig. 9c is due to NaKL23L23 and the peak at 260.9 eV in Fig. 9a is due to W4d3r2. The unknown peak at 258.9 eV is due to the chemical shift of the sodium in the a-WO3 film, for there was no change in W4dsn spectra. The energy difference between two generated peaks, 258.9 eV and 262.8 eV,

7

Mechanism of degradation The electrochromic phenomenon in a-WO3 films can be expressed by the following reaction: WO3 (colourless) + xM ÷ + xe- = MxWO3 (blue) where M ÷ is a proton or metal ion. The above formula exhibits 'the formation of pseudo-tungsten bronze by the double injection of a positive ion and an electron 19.

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In order to understand the role of the positive ion, quantitative investigtions have been carried out here for the first time. A sodium ion as well as a lithium ion was found to be injected into the a-WO3 film in proportion to the charge, as shown in Fig. 10 and Fig. 11. In both cases, the efficiency, which is the ratio of the net increase of Na or Li content to the injected charge, is not unity. This is considered to be due to the proton. In case of the Na-Li mixed electrolyte, sodium ions exist in the a-WO3 film of the coloured state more than lithium ions. 20

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injected charge (mC crn-z)

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Fig. 10 Lithium content in a-W03 film plotted against injected charge. Water content in 0.95 M LiClO4-PC electrolyte was: o 30 ppm; • 13 000 p p m

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Fig. 9 Na K L L line o f a-WO3film by E S C A : a - - before cycling; b - - after 1.6 x 10z cycles; c - - Na2WO4powder

is in good agreement with that between sodium metal and sodium ion. Therefore, the peak at 258.9 eV was concluded to be due to the existence of metallic sodium. Summarizing the results, there was the increase of oxygen, OH-, sodium ion and metallic sodium in the degraded a-W03 films.

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0

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Injected ch0rge (me cm -2)

Fig. 11 Sodium content in a-WO 3 film plotted against injected charge. Water content in 0.98 M NaClO4-PC electrolyte was: o - - 40 ppm; • - - 14 000 p p m

DISPLAYS.

JANUARY

1985

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metallic sodium and also combine with the oxygen dissolved in the electrolyte to form sodium tungstate. The degradation of the a-WO3 film is caused by the formation of these undesirable materials.

_

I m p r o v e m e n t s o f c y c l i n g life Cycle lifetime is strongly dependent on the operating charge density. The less the charge density, the longer the cycle lifetime. The operating charge density of 6 mC cm -2 was chosen so as to give a satisfactory contrast to ECDs.

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The result of the cycling test is shown in Fig. 13. The injected charge was maintained within + 10 per cent up to 107 cycles. This indicates that the ECD using the a-WO3/LiCIOa-PC has a sufficient cycle lifetime for practical applications.

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Voltage withrespect to counter-electrode(V)

Fig. 12 Cyclic voltammogram of the ECD. The potential was scanned at a rate of 30 m V s -t between 0.5 V and - 1 . 0 V to the counter-electrode for a digit area of 6.8 mm 2. The polarization of the counter-electrode was negligibly small below 5 mV: a - - 1M LiCIO4-PC; b - - 1M NaCIO4-PC Table 4. Ratio of injected charge to the initial value after 1.4 x l0 s cycles

Electrolyte

Bubbling gas/Time (}1)

Ratio

1M LiCIO4-PC 1M LiCIO4-PC 1M NaCIO4-PC 1M NaCIO4-PC

N2/15 02/15 N2/15 02/15

0.97 0.62 0.61 0.44

Figure 12 shows a cyclic voltammogram of the ECD. Since the resistivity of the Na electrolyte and that of the Li electrolyte are almost the same, the shapes of the cyclic voltammogram reflect the cation transport in the a-WO3 films. The colouring time and the bleaching time of the 1M NaCIO4-PC electrolyte cell were both slower than those of 1M LiCIO4-PC electrolyte cell bY about 35 per cent. The results of the cyclic voltammogram and the response time indicate that a sodium ion is difficult to extract from the a-WO3 film compared with a lithium ion. The effects of the Na electrolyte and the dissolved oxygen on the cycling life are shown in Table 4. It is clear that both sodium and oxygen reduce the cycling life considerably. As a result, the cycling degradation can be explained as follows. Sodium ions dissolved in the electrolyte as an impurity are injected into the a-WOa film in colouring cycle. Some of these ions then combine with electrons to form

DISPLAYS.

JANUARY

1985

Several things have been done to improve the cycling life. The purification of materials in order to get rid of sodium and to minimize the dissolved oxygen was effective. The quality of the a-WO3 film was no less important than these. For this it was found that the a-WO3 film of fast bleaching or good reversibility of charge had the longer cycling life. Bleaching speed of the a-WO3 film is known to be controlled by proton diffusion for a proton electrolyte 2°, but it is not clear for a lithium electrolyte 21'22. Figures 10 and 11 show the fact that water in a non-aqueous electrolyte does not act as a proton donor, but as a promoter of other cation injection. Addition of water to a non-aqueous electrolyte could speed up the bleaching but not the colouring if there were no change in electrolyte conductivity. In other words, the bleaching speed of the non-aqueous electrolyte cell became much worse when the water content was reduced to less than 100 parts per million. These facts imply that proton diffusion may occur in addition to lithium or sodium diffusion in the a-WO3 film even in non-aqueous electrolyte ECDs. If protons, which exist in the a-WO3 films originally, are diffused to the colour site in favour of injected lithium or sodium ions, the bleaching speed will be faster and the cycling degradation will be reduced. This can explain the fact that the cycling degradation depends on the properties of a-WO3 films. 1.2 "--¢/

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Fig. 13 Ratio of the injected charge to the initial value plotted against number of cycles. Seven-segment electrodes were operated to display the figures from 0 through 9 and back to zero. The average contrast was maintained at 6 m C cm -2 at 25°C

9

CONCLU~ON The response time of the ECD in the present study was controlled not only by the a-WO3 film, but also by other constitutive materials. In the high temperature region, the response time was mainly governed by the ITO resistance, whereas in the low temperature region it was governed by other factors. The response time was improved by adopting a system which was composed of a highly porous a-WO3 film prepared under low vacuum, and a laminated layer of porous reflector and a high surface area nonpolarizable counter-electrode~ The potential of the counter-electrode was optimized at - 2 0 0 mV with respect to Ag/Ag + by using a mixture of W18049 and V6013 so as to bleach the ECD by short-circuiting. As a result, response times as fast as 100 ms at 25°C for the charge density of 6 mC cm -2 were obtained using a single battery as a power source. The a-WOa/LiCIO4-PC ECDs were stable for more than 1500 hours at 80°C. This indicates that the a-WO3 ECD using the non-aqueous Li electrolyte is basically stable as long as the seal of the ECD is perfectly air-tight. From ESCA, AES and atomic absorption analyses of a-WO3 films, it was found that the cycling degradation was mainly caused by the accumulation of sodium and oxygen, which were dissolved in the electrolyte as impurities. This strongly suggests that the formation of sodium tungstate might be possible in the degraded aWO3 film. The purification of materials, the diminution of oxygen and the improvement of the a-WO3 film led to a lifetime of more than 107 cycles at the charge density of 6 mC cm -2 at 25°C.

Acknowledgement The authors gratefully acknowledge the invaluable assistance of the staffs of the Electronic Display Group and the Analytical Group of the Research and

10

Development Division of the Asahi Glass Company Ltd.

References 1 Dautremont-Smith, W.C. Displays 3 (1982) 3 2 Crandali, R.S., Faughnan, B.W. Phys Rev Lett 39 (1977) 232 3 Crandall, R.S., Faughnan, B.W. Appl Phys Lett 28 (1976) 95 4 Ho, C., Raistriek, I.D., Hu~ins, R.A. J Electrochem Soc 127 (1980) 343 5 Randin, J.P. J Electron Mater 7 (1978) 47 6 Arnoldussen, T.C. J Electrochem Soc 128 (1981) 117 7 Yoshiike, N., Kondo, S. ibid 130 (1983) 2283 8 Giglia, R.D., Haacke, G. Proc Soc Inf Disp 23 (1982) 41 9 Yamanaka, K. Jap J Appl Phys 21 (1982) 926 10 Matsuhiro, K., Ando, E., Kawakami, K. Res Rep 34 (Asahi Glass Company Ltd, Japan, 1984) to be published 11 Kawakami, K., Ando, E., Matsuhiro, K., Matsu-

moto, K., Sugizaki, M., Nishimura, H., Noshiro, M. Res Rep 33 (Asahi Glass Company Ltd, Japan, 1983) 12 Matsuhiro, K., Ando, E., Kawakami, K., Masuda, Y. Proc third int display res conf (Kobe, Japan, 1983) 62 13 Mizuhashi, M. Shinka 22 (1979) 253 (in Japanese) 14 Matsuhiro, K., Masuda, Y. Proc Soc Inf Disp 21 (1980) 101 15 Sehlotter, P., Piekeimann, L. J Electron Mater 11 (1982) 207 16 Kozawa, A. Denki Kagaku 11 (1979) 642 (in Japanese) 17 Knowles, T.J. Appl Phys Lett 31 (1977) 817 18 Morita, H., Washida, H. Oy6 Butsuri 51 (1982) 488 19 Faughnan, B.W., Crandall, R.S., Heyman, P.M. RCA Rev 36 (1975) 177 20 Faughnan, B.W., Crandall, R.S. 'Topics in Applied Physics' (Springer, 1980) 181 21 Zeller, H.R., Beyeler, H.U. Appl Phys 13 (1977) 231 22 Mohapatra, S.K. J Electrochem Soc 125 (1978) 284

DISPLAYS. JANUARY 1985