Method for making electrochromic films having improved etch resistance

Thin, amorphous electrochromic layers deposited on substrate electrodes by, for example, vacuum deposition techniques, are subjected to a special heat treatment at a selected high temperature for a selected short time to convert at least a free portion of each layer to be exposed to the electrolyte from the amorphous form to a crystalline form while preventing excessive water loss which might adversely affect the electrochromic properties of the layer for display purposes. Crystallization of at least the free surface portion of the electrochromic layer significantly increases its etch resistance whereas retention of water in the electrochromic layer maintains satisfactory electrochromic properties for display purposes.

FIELD OF THE INVENTION 
The present invention relates to electrochromic materials and to 
electrooptical displays which utilize such materials. More particularly, 
the present invention provides means in the form of a heat treatment for 
improving the etch resistance of electrochromic films or layers during 
operation of electrochromic displays, thereby increasing the useful life 
of such displays. The invention further relates to the incorporation of 
the heat treatment in the manufacture of electrochromic displays. 
DESCRIPTION OF THE PRIOR ART 
Thin, amorphous films of electrochromic materials, such as WO.sub.3, form 
the basis for a passive electrochromic display device in which the film is 
changed from transparent to a blue color by application of a negative 
electrical potential thereto. Typically, an electrochromic display 
comprises a first transparent substrate with selectively actuable 
transparent electrodes and first image-forming electrochromic layers 
thereon, a second spaced substrate with a counter-electrode and second 
electrochromic layer thereon and an ion-conducting or electrolyte layer 
between the substrates. Constructions of electrochromic displays are shown 
in U.S. Pat. Nos. 3,944,333 to Leibowitz issued Mar. 16, 1976; 4,006,966 
to Meyers et al issued Feb. 8, 1977; 4,059,341 to Zeller issued Nov. 22, 
1977; 4,060,311 to Green issued Nov. 29, 1977; 4,068,928 to Meyers et al 
issued Jan. 17, 1978 and 4,073,570 to Korinek issued Feb. 14, 1978 among 
others. 
Many electrochromic materials exist which change color according to 
oxidation state. These are well known in the art and are disclosed in part 
in the patents cited above. In the process of changing color, the 
electrochromic material passes through various intermediate valence or 
oxidation states where it can exist at non-stoichiometric compounds. For 
example, tungsten trioxide (WO.sub.3) passes through various 
non-stoichiometric valence states of tungsten oxide (also known as 
hydrogen tungsten oxide) and may approach a state very close to tungsten 
pentoxide. However, since particular oxidation states of the 
electrochromic material are soluble in the electrolyte of the display, the 
material suffers degradation with operating time and temperature, causing 
an adverse effect on the quality of the image produced and thus the 
service life of the display. This degradation is sometimes referred to as 
etching of the electrochromic layer. Although electrochromic displays 
exhibit highly useful display properties, such as excellent contrast and 
viewability from various angles, their usefulness has been somewhat 
limited as a result of the aforementioned degradation of the 
electrochromic layers with operating time and temperature. Attempts by 
prior art workers to alleviate this problem are exemplified in U.S. Pat. 
Nos. 3,970,365 to Giglia issued July 20, 1976; 3,957,352 to Leibowitz 
issued May 18, 1976; 4,012,831, to Leibowitz issued Mar. 22, 1977, the 
latter two of which teach that a reduction in degradation of the 
electrochromic layers can be achieved by adding certain components to the 
electrolyte with which the layers are in contact. 
SUMMARY OF THE INVENTION 
An object of the present invention is to reduce degradation of 
electrochromic layers in an electrochromic display by a heat treatment 
which imparts increased etch resistance to the layers. 
Another object of the invention is to impart said improved etch resistance 
to the layers without substantially harming the electrochromic properties 
thereof for display purposes. 
In manufacturing electrooptical displays of the electrochromic type, it is 
common to deposit the electrochromic layers on the substrate electrodes by 
vacuum deposition techniques, such as sputtering, thermal vapor deposition 
as well as others. Generally, the as-deposited electrochromic layers are 
thin, for example, from 0.1 to 1 micron in thickness, and are amorphous. 
Through research related to the present invention, the as-deposited 
electrochromic layers have been found to contain a relatively large 
percentage of water, classified as so-called surface water and bonded 
water, which seems to be necessary for development of satisfactory 
electrochromic properties in the layers under applied negative electric 
potentials. By surface water, we mean water than can be driven off at 
temperatures less than about 100.degree. C. whereas bonded water requires 
significantly higher temperatures in the area of 250.degree.-275.degree. 
C. for removal. The present invention relates to the further discovery 
that the resistance to dissolution or etching of the as-deposited 
electrochromic layers, although dependent to some extent upon the 
electrolyte environment with which the layers are in contact in the 
display, can be considerably improved while retaining satisfactory 
electrochromic properties for display purposes by heat treating each layer 
at a selected high temperature for a selected short time to convert at 
least a free surface portion thereof from the amorphous form to the 
crystalline form while preventing excessive water loss which might 
adversely affect the electrochromic properties. In the present invention, 
the temperature of the heat treatment generally is selected to be equal to 
or above the crystallization temperature determined for the particular 
electrochromic material whereas the time of heat treatment is selected as 
sufficient to assure crystallization through the desired portion of the 
electrochromic layer but insufficient to produce deleterious water loss 
from the layer. In general, the heat treatment temperature and time are 
selected in inverse relation to one another. When exposed to typical 
electrochromic display electrolytes under typical applied potentials, an 
electrochromic layer of the invention having at least a crystallized free 
surface portion in contact with the electrolyte is observed to be 
significantly more etch resistant than an as-deposited electrochromic 
layer having an amorphous free surface in contact with the electrolyte. 
Of course, as a result of the increased etch resistance of the 
electrochromic layer of the invention, displays incorporating such layers 
will exhibit a substantially increased service life while exhibiting many 
of the benefits and advantages of electrochromic displays.

DETAILED DESCRIPTION OF THE INVENTION 
Referring now to FIG. 1 of the drawings, the electrochromic display is a 
sandwich construction of a first transparent substrate 1 and a second 
spaced substrate 2, which need not be transparent, and electrolyte layer 
15 therebetween. Substrate 1 has a conductive pattern of transparent 
electrodes on the underside thereof such as segments 3,4 of a pattern 
which can be selectively actuated to form digits or other optical patterns 
via conductive leads 5,6 leading to terminals 7,8. Substrate 1 may be of 
transparent glass or plastic with a selected pattern of transparent 
electrodes 3,4 thereon of a material such as, for example, tin oxide. The 
pattern may be etched on the substrate by using a commercially available 
material known as NESA glass and removing the conductive coating except 
for electrodes 3,4. 
The second substrate has a conductive back electrode 9 thereon. Substrate 2 
may be made of glass, ceramic or plastic coated with a suitable conductive 
layer to form back electrode 9 connected to terminal 10. Coated on the 
transparent electrode segments 3,4 and also on back electrode 9, if 
desired, are layers of electrochromic material indicated as 11, 12, 13. 
The electrochromic layers 11, 12, on segments 3,4 respectively are applied 
by suitable masking techniques to cover a smaller area than the electrodes 
so as to give good edge definition. 
Electrolyte layer 15 may take various forms including, but not limited to, 
liquid electrolytes, gelled electrolytes, paste electrolytes, 
ion-conducting solids and ion-exchange resins. These and other types of 
electrolytes are generally well known in the art; for example, see U.S. 
Pat. Nos. 3,521,941 to Kumar et al; 3,827,784 to Giglia et al; 3,840,287 
to Witzke et al; 3,995,943 to Jasinski; 4,009,936 to Kasia and 4,012,831 
to Leibowitz. A preferred electrolyte comprises a solid membrane of a 
soluble polymer cationic ion exchange resin in acid form and chemically 
wetted, such as soluble polystyrene sulfonic acid polymer cationic 
exchange resin chemically wetted with water, more fully described in 
copending Leibowitz patent application U.S. Ser. No. 791,536 entitled 
"Electrochromic Device With Cationic Exchange Resin Separator" filed Apr. 
27, 1977. In some situations, a porous separator sheet (not shown) may be 
utilized as a carrier for the electrolyte, for example, as shown in the 
Leibowitz patent, U.S. Pat. No. 3,944,333, also of common assignee 
herewith. 
Reference to FIG. 2 shows the assembled display. The two substrates 1 and 2 
are attached to one another by an adhesive 16, such as epoxy, and the 
electrolyte layer is incorporated into the display in conventional manner. 
Then the substrates are sealed with adhesive around the remaining sides. 
Suitable well-known means for actuating the display element includes means 
for applying an electric field from a battery 17 to a selected segment 4 
via terminal 8 and the back electrode 9 via terminal 10. Means for 
reversing the polarity of the applied voltage to erase the image is 
indicated symbolically by a two-pole double throw switch 18. 
As mentioned hereinbefore, electrochromic layers 11, 12 are formed on 
substrate electrodes 3,4 generally by vacuum deposition techniques, 
although others such as chemical vapor deposition and spraying may 
sometimes be utilized. Thermal vapor deposition and sputtering are 
exemplary of useful vacuum deposition techniques. In accordance with one 
embodiment of the present invention, the resistance to dissolution or 
etching of the thin amorphous as-deposited electrochromic layers is 
significantly improved by subjecting each such layer to a heat treatment 
at a selected high temperature at or above the crystallization temperature 
of the particular electrochromic material employed for a selected short 
time to convert at least a free surface portion of each layer which will 
ultimately be exposed to electrolyte layer 15 from the amorphous state to 
a crystalline state while preventing excessive loss of water which loss 
might adversely affect the electrochromic properties for display purposes. 
Generally, if some of the so-called surface water is driven off during the 
heat treatment, it will normally be replenished when the electrochromic 
layers are brought in contact with the electrolyte during final assembly. 
FIG. 3 illustrates electrochromic layer 12 after subjection to this type 
of heat treatment wherein it is evident that a free surface case 20 of 
crystalline structure is formed through the layer to a selected depth and 
more or less encloses the amorphous portion 21 of the layer and separates 
it from actual contact with the electrolyte layer 15. In general, it will 
be apparent to those skilled in the art that the temperature of the heat 
treatment and the duration thereof will depend upon the type of 
electrochromic material being treated, the thickness of the electrochromic 
layer, the water content of the layer, the amount of crystallization 
thickness required through the layer and the atmosphere maintained in the 
heating chamber. Usually, the temperature and time will be varied in 
inverse relation to one another. By way of example, amorphous layers of 
WO.sub.3 (tungsten trioxide) deposited on tin oxide electrodes by vacuum 
deposition technique to a thickness of 0.30 microns have been heat treated 
at 525.degree. C. for 1 minute in air with satisfactory results in terms 
of providing considerably enhanced etch resistance to electrolytes, such 
as a 10% by volume H.sub.2 SO.sub.4 solution, a chemically wetted soluble 
polystyrene sulfonic acid polymer cationic exchange resin and phosphoric 
acid, while retaining sufficient water in the layer (i.e. reducing 
excessive water loss during the heat treatment) to provide acceptable 
coloring and bleaching rates for display purposes. For example, etch 
resistance was measured by exposure of the treated electrochromic layer to 
a dilute base solution (Ph 10) and recording the time for dissolution. It 
was found that the electrochromic layer treated at 525.degree. C. for 1 
minute exhibited a degradation time of 10 minutes whereas an amorphous 
layer exhibited a degradation time of less than 30 seconds. Coloring rate 
was determined by measuring the time required to inject 5mC/CM of charge 
in a layer with an indium wire dipped in a 10% by volume H.sub.2 SO.sub.4 
solution which rests on the free surface of the WO.sub.3 film. An external 
wire was connected to the indium wire through a charge integrator and then 
to the electrode upon which the WO.sub.3 film was deposited. The WO.sub.3 
layer heat treated at 525.degree. C. for 1 minute colored in 3 seconds 
under these conditions whereas an amorphous film colored in 2 seconds. 
Typically, for amorphous WO.sub.3 layers, the crystallization temperature 
has been determined to be about 265.degree. C. Thus, according to the 
invention, the temperature of the heat treatment is selected at or above 
this value to insure that crystallization takes place. The time, as 
already mentioned, is selected in relation to the temperature such that 
crystallization occurs to the desired thickness without causing a harmful 
loss in water from the as-deposited layer. Generally, for WO.sub.3 layers 
heat treated in a conventional tube furnace having an air atmosphere, 
times from about 1/2 to 15 minutes are satisfactory. Times of about 1 to 2 
minutes are preferred when the temperature is at least about 500.degree. 
C. Of course, if steam or water bearing gas is introduced into the furnace 
to inhibit moisture removal from the layer during crystallization, the 
time of the heat treatment may be extended, or possibly higher 
temperatures utilized. Of course, the temperature-time parameters for 
electrochromic materials other than WO.sub.3 can be readily determined by 
those skilled in the art with accepted heat treatment techniques. Tungsten 
trioxide, molybdenum trioxide and admixtures thereof are preferred 
electrochromic materials for use in the invention, however. 
Although the invention has been described thus far with respect to the 
formation of a crystallized case 20 on the amorphous electrochromic layer, 
it is considered within the scope of the invention to crystallize the 
layer through its entire thickness so long as water loss from the layer is 
minimized as described hereinabove. Crystallization through the entire 
electrochromic layer thickness may be the end result after subjection to 
the inventive heat treatment described herein although this has not yet 
been verified. Nevertheless, the important feature of the invention is to 
convert at least the free surface portion of the electrochromic layer to 
be in contact with the electrolyte from the amorphous to the crystallized 
form with said minimized water loss. 
While the invention has been explained by a detailed description of certain 
specific embodiments, it is understood that various modifications and 
substitutions can be made in them within the scope of the appended claims 
which are intended to include equivalents of such embodiments.