Negative type display in electrochromic display device

A transparent electrode is formed on a transparent substrate in a desired configuration. An electrochromic layer is formed on the entire surface of the transparent substrate inclusive of the surface of the transparent electrode. The electrochromic layer is initially placed in the coloration state. A back layer which exhibits color sensation different from that of the coloration state of the electrochromic layer is disposed above the electrochromic layer. A desired electrode is selected to bleach the electrochromic layer formed thereon, through which the back layer is recognized, thereby displaying a desired pattern in the negative type.

BACKGROUND OF THE INVENTION 
The present invention relates to an electro-optical display containing an 
electrochromic material which manifests reversible variations in its light 
adsorption properties upon application of a properly controlled voltage or 
current. This display is referred to as an "electrochromic display (ECD)" 
hereinafter. 
The present invention relates, more particularly, to an electrochromic 
display device which displays a desired pattern in the negative type. 
[DESCRIPTION OF PRIOR ART] 
Electrochromic materials are well known in the art. Typical materials are a 
film of amorphous tungsten (WO.sub.3), a film of amorphous molybdenum 
oxide (MoO.sub.3), and a thin film of transition metal oxide. Examples 
were disclosed in Talmey, U.S Pat. No. 2,319,765, and Deb et al. U.S. Pat. 
No. 3,521,941. 
Such electrochromic materials can be shaped in a desired pattern to display 
desired characters, symbols and patterns by reversibly selecting its light 
absorption properties through the use of the electric control. Examples 
were disclosed in U.S. Pat. No. 1,068,744 and the above-mentioned U.S. 
Pat. No. 3,521,941. 
There are three types of ECD cells which employ the thin film of transition 
metal oxide. The first one includes liquid electrolyte as the source of 
ions (the abovementioned U.S. Pat. No. 2,319,765). The second type 
includes an inorganic insulation film (the above-mentioned U.S. Pat. No. 
3,521,941). The last type includes a solid state electrolytic-film 
(Castellion et al, U.S. Pat. No. 3,712,710). The solid state ECD is not as 
stable as compared with the semiliquid state ECD. 
The electrolyte comprises a gel of sulfuric acid (M.D. Meyers et al, U.S. 
Pat. NO. 3,708,220), or .gamma.-Butyrolactone or propylene carbonate mixed 
with LiClO.sub.4 (L.C. Beagle, U.S. Pat. No. 3,704,057). 
Injection and extration of electric charges at the counter electrode are 
preferably conducted when the electrochromic layer is also formed on the 
counter electrode. Examples were disclosed in R.D. Giglia et sl, U.S. Pat. 
No. 3,819,252 or Witzke et al, U.S. Pat. NO. 3,840,287. In this case, the 
background of the display is formed by adding pigment to the electrolyte 
(R.D. Gilia et al, U.S. Pat. No. 3,819,252), or by disposing an opaque 
plate behind the display electrode (Giglia, U.S. Pat. No. 3,892,472, or 
Leibowitz, U.S. Pat. No. 3,944,333). 
Display electrodes of the seven-segmented type for displaying the numeral 
information were disclosed in Giglia et al, U.S. Pat. No. 3,827,784. 
Technique for protecting the edge portion of the electrochromic layer 
formed on the display electrode through the use of an insulation layer was 
disclosed in Eric Saurer, U.S. Pat. No. 3,836,229, Giglia, U.S. Pat. No. 
3,892,472, and Leibowitz, U.S. Pat. No. 3,944,333. 
OBJECTS AND SUMMARY OF THE INVENTION 
Accordingly, an object of the present invention is to provide an 
electrochromic display device which displays information in the negative 
type. 
Another object of the present invention is to provide an electrochromic 
display cell suited for mass production. 
Still another object of the present invention is to enhance the visibility 
of the information displayed in the electrochromic display device. 
Other objects and further scope of applicability of the present invention 
will become apparent from the detailed description given hereinafter. It 
should be understood, however, that the detailed description and specific 
examples, while indicating preferred embodiments of the invention, are 
given by way of illustration only, since various changes and modifications 
within the spirit and scope of the invention will become apparent to those 
skilled in the art from this detailed description. 
To achieve the above objects, pursuant to an embodiment of the present 
invention, a transparent electrode is formed on a transparent glass 
substrate in a desired configuration. An electrochromic layer is formed on 
the entire surface of the transparent glass substrate inclusive of the 
surface of the transparent electrode. The electrochromic layer is 
initially placed in the coloration state. A back layer which exhibits a 
color sensation different from that of the coloration state of the 
electrochromic layer is disposed above the electrochromic layer. 
A desired transparent electrode, or a segment electrode, is selected to 
bleach the electrochromic layer formed thereon. The back layer can be seen 
through the segment electrode and the bleached electrochromic layer formed 
thereon. In this way, a desired pattern is displayed in the negative 
fashion. 
In another embodiment, the electrochromic layer is formed only on the 
segment portion of the transparent electrode. Lead electrode portions of 
the transparent electrode are coated with an insulation film to protect 
the transparent electrode from the electrolyte. A film which exhibits 
color sensation similar to that of the coloration state of the 
electrochromic layer is formed on the other surface of the transparent 
glass substrate at portions where the segment electrode is not formed. 
A desired transparent electrode, or a segment electrode is selected to 
bleach the electrochromic layer formed thereon. The back layer can be seen 
through the segment electrode portion and, therefore, a desired pattern is 
displayed in the negative fashion.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now in detail to the drawings, and to facilitate a more complete 
understanding of the present invention, a basic structure of the ECD will 
be first described with reference to FIGS. 1 through 4. 
FIGS. 1 and 2 show a basic structure of an electrochromic display cell. The 
ECD cell mainly comprises two transparent glass substrates 1 and 2. On the 
inner surface of the glass substrate 1, a transparent electrode 4 
including a segment electrode and a lead electrode is formed. An 
electrochromic layer 3 is formed on the segment electrode. On the inner 
surface of the other glass substrate 2, a counter electrode 7 coated with 
an electrochomic layer 6 is formed. A porous plate 5 impregnated with 
pigment is disposed between the two glass substrates 1 and 2 for providing 
the display background. Electrolyte 8 is filled in the cell and, 
therefore, the porous plate 5 is impregnated with the electrolyte 8. A 
spacer 9 is provided for determining the distance provided between the two 
glass substrates 1 and 2, and for sealing purposes. An insulator layer 10 
is formed on the lead electrode for protecting the lead electrode from the 
electrolyte 8. When the electrochromic layer 3 is placed in the bleached 
state, the operator can uniformly see the porous plate 5 through the front 
glass substrate 1. When a desired segment electrode 4 is selected, namely, 
when a desired segment electrode 4 is made negative with respect to the 
counter electrode 7 through the use of a power source E and a selection 
switch S, the electrochromic layer 3 formed on the thus selected segment 
electrode 4 is placed into the colorationstate. In the case where the 
electrochromic layer 3 comprises a WO.sub.3 film or a MoO.sub.3 film, the 
selected segment is colored blue. Accordingly, a desired pattern is 
displayed on the background, for example, the white background created by 
the porous plate 5 by properly selecting the segments. 
In the ECD cell shown in FIGS. 1 and 2, it is very difficult to exactly 
determine the edge portions of the electrochromic layer 3 and the 
insulator layer 10. 
In the case where the insulator layer 10 overlaps a portion a of the 
electrochromic layer 3 as shown in FIG. 3, the coloration state of the 
portion "a" can not be controlled. More specifically, the portion "a" is 
gradually colored when the electrochromic layer 3 is placed in the 
coloration state due to the distribution of electric charges. The thus 
colored portion "a" can not be bleached even when the electrochromic layer 
3 is placed in the bleached state, because the portion "a" is coated with 
the insulator layer 10. 
In the case where the insulator layer 10 does not reach the edge of the 
electrochromic layer 3 as shown in FIG. 4, that is, when a portion "b" of 
the transparent electrode 4 is exposed to the electrolyte, the portion "b" 
is damaged by the electrolyte due to electro-chemical reaction. 
FIG. 5 shows an embodiment of the ECD cell of the present invention. Like 
elements corresponding to those of FIGS. 1 and 2 are indicated by like 
numerals. 
The transparent electrode 4 including the segment electrodes and the lead 
electrodes is formed on the transparent glass substrate 1 in a desired 
configuration. An electrochromic layer 3a is formed on the entire surface 
of the glass substrate 1. An insulator film 10a is formed on the 
electrochromic layer 3a at selected positions corresponding to the lead 
electrodes. In this embodiment, the electrically controllable portion, or, 
the display portion is determined by the transparent electrode 4 and the 
insulator film 10a. The porous plate 5 exhibits a color sensation 
different from that of the coloration state of the electrochromic layer 
3a. 
The electrochromic layer 3a is initially placed in the coloration state. In 
the case where the electrochromic layer 3a comprises a WO.sub.3 film or a 
MoO.sub.3 film, the ECD cell is uniformly blue when the display is in the 
OFF state. To display a desired pattern, desired segment electrodes are 
selected to bleach the electrochromic layer 3a formed thereon. Through the 
segment portion where the electrochromic layer 3a is bleached, the 
operator can see the porous plate 5 which exhibits color sensation 
different from that of the coloration state of the electrochromic layer 
3a. When the application of the voltage is reversed, the electrochromic 
layer 3a formed on the selected segment electrode is colored or returned 
to the initial condition. Since the coloration degree of the 
electrochromic material is proportional to the amount of the electric 
current flowing therethrough, the segment portion can be colored to a 
degree identical with that of the remaining portions by properly 
controlling the applied voltage and the application period of the 
coloration voltage. 
The thus formed ECD cell does not have the portion b shown in FIG. 4. The 
distribution of the coloration species will not create any deterioration 
when the segment is in the coloration state, because the entire 
electrochromic layer 3a is in the coloration state. Moreover, the 
distribution of the coloration species toward the segment portion, under 
the condition where the segment is placed in the bleached state, will not 
create any troubles because the segment portion is electrically 
controllable. 
EXAMPLE I (for the embodiment of FIG. 5) 
The transparent glass substrates 1 and 2 are flat glass plates having a 1 
mm through 3 mm thickness. The glass substrate 1 carries a preferably 
shaped transparent conductive layer 4, the electrochromic layer 3a and the 
insulator film 10a formed thereon. 
The transparent conductive layer 4 comprises an In.sub.2 O.sub.3 film doped 
with SnO.sub.2. The transparent conductive layer 4 is formed through the 
use of an electron beam evaporation method to a thickness of 1800 through 
2000 A, and has a resistance value of 20-30 5/8/sq. On the thus formed 
conductive layer, a WO.sub.3 film is formed to a thickness of 4000 A 
through the use of the conventional mask evaporation method. The thus 
formed layers are shaped in a desired configuration through the use of a 
photo-etching method by using the positive type resist, for example, 
AZ-1350 manufacture by Shipley Co. The positive type resist is developed 
by the alkaline liquid and, therefore, the WO.sub.3 film is etched at the 
same time when the resist is developed. The In.sub.2 O.sub.3 film is 
etched by a mixture liquid comprising ferric chloride and hydrochloric 
acid. 
On the entire surface of the thus formed substrate, a WO.sub.3 film is 
formed to a thickness of 1000 A through the use of vacuum evaporation 
techniques, thereby forming the electrochromic layer 3a. The insulator 
film 10a is formed on the electrochromic layer 3a at the position where 
lead electrodes are formed. The insulator film 10a is made of SiO.sub.2 
and formed to a thickness of 3500 A through the use of masked electron 
beam evaporation techniques. 
The counter electrode 7 is formed in a same manner that is conducted for 
forming the transparent conductive layer 4. That is, the counter electrode 
7 is made of the In.sub.2 O.sub.3 film and has a thickness of 1800 through 
2000 A. The WO.sub.3 film 6 is formed on the counter electrode 7 to a 
thickness of 5000 A. 
The spacer 9 mainly comprises a thin glass plate of 1 mm thickness, and is 
sealed through the use of epoxy resin such as R.2401/HC.160 manufactured 
by Somal Kogyo KK by maintaining the cell at 120.degree. C. for 30 
minutes. 
The porous plate 5 is a porous alumina ceramic plate, for example, C-3 
manufactured by Nippon Toki Co. The electrolyte is impregnated into the 
porous plate 5. The electrolyte includes .gamma.-butyrolactone and litium 
perchlorate by 1.0 M/.gamma. both manufactured by Kishida Chemical Co. The 
electrolyte is injected into the cell under the conditions of a pressure 
of 1.times.10.sup.-2 mmHg and a temperature of -40.degree. C. After 
completion of the impregnation, the injection inlet is sealed with epoxy 
resin, for example, "Quick Set" manufacture by Konishi Co., Ltd. 
The thus formed ECD cell is exposed to the sun beam for two days to conduct 
the optical write-in operation, whereby the entire electrochromic layer is 
placed in the coloration state. The transmission factor of the thus formed 
electrochromiclayer in the air is 40% when the detection is carried out 
through the use of a 590 mm wave-length beam. This value corresponds to 
the saturated transmission factor of the WO.sub.3 of 1000 A thickness. 
To bleach the thus colored electrochromic layer, the electric charge of 16 
mc/cm.sup.2 is required. Therefore, the driver circuit connected to the 
ECD cell of this example develops constant current of 32 mA/cm.sup.2 for 
0.5 seconds to change the state of the electrochromic layer. The 
above-mentioned ECD cell is now under life test for more than five million 
cycles. 
FIG. 6 shows another embodiment of the ECD cell of the present invention. 
Like elements corresponding to those of FIG. 5 are indicated by like 
numerals. 
In this embodiment, an electrochromic layer 3b is formed only on the 
segment electrodes in such a manner that the transparent electrode 4 is 
covered by the electrochromic layer 3b, or, the electrochromic layer 3b 
has a greater size than the segment electrodes. An insulator layer 10b is 
formed on the substrate to cover the entire surface except the segment 
portions. The insulator layer 10b exhibits a color sensation similar to 
that of the coloration state of the electrochromic layer 3b. The insulator 
layer 10b is preferably an epoxy resin doped with pigment. The insulator 
layer 10b must be stable against the electrolyte. 
EXAMPLE II (for the embodiment of FIG. 6) 
An In.sub.2 O.sub.3 film is formed on the 1 mm thick glass plate in a 
desired configuration to provide the segment electrodes and lead 
electrodes. The WO.sub.3 film of 5000 A thickness is formed threon and 
shaped to cover the segment electrodes, or more specifically, to have edge 
portions greater than that of the segment electrodes by 2 mm length. 
The insulator layer 10b comprises epoxy resin mixed with blue pigment, for 
example, "Prussian Blue" manufactured by Dainichi Seika Kogyo KK. by 5 
wt%. The insulator layer 10b is formed to 20 .mu.m thickness through the 
use of a screen printing method. Remaining portions are similar to that of 
the EXAMPLE I. 
FIG. 7 shows still another embodiment of the ECD cell of the present 
invention. Like elements corresponding to those of FIG. 5 are indicated by 
like numerals. 
In the embodiment of FIG. 7, both a transparent segment electrode 4 and an 
electrochromic layer 3c are formed to have a slightly larger size than 
that is required. A color film 11 which exhibits color sensation similar 
to that of the coloration state of the electrochromic layer 3c is formed 
on the outer surface of the transparent glass substrate 1. The film 11 is 
provided with openings 12 at positions corresponding to the segment 
electrodes. The glass substrate 1 must be thin to minimize the parallax of 
the film 11 and the electrochromic layer 3c. 
EXAMPLE III (for the embodiment of FIG. 7) 
The glass substrate 1 is a glass plate of 0.5 mm thickness. The In.sub.2 
O.sub.3 film and the WO.sub.3 film of 5000 A thickness are formed on the 
glass plate and shaped in such a same manner that is conducted in the 
EXAMPLE I. An insulator layer 10c is formed in such a manner that is 
conducted in the EXAMPLE I. The film 11 is formed on the other surface of 
the glass plate through the use of a silk screen. The film 11 comprises 
blue paint, for example, epoxy resin (R.2401/HC.11 manufactured by Somal 
Kogyo KK) doped with dye such as "DEANT" of the following formula by 0.25 
wt%. 
##STR1## 
The thus obtained blue paint is painted to have 20 .mu.m thickness through 
the use of the screen printing method. The thus formed film 11 has the 
transmission factor of 40%. The printing machine used to form the film 11 
is "LS-20N" manufactured by Newlong Seimitsu Kogyo KK, and the screen used 
to form the film 11 is tetron monomulti fiber screen, 180 mesh, emulsion 
of 5 .mu.m thickness manufactured by Mesh Kogyo KK. 
Remaining portions are similar to that of the EXAMPLE I. 
The dye suited for the film 11 is as follows: 
##STR2## 
FIGS. 8 and 9 show a front substrate of yet another embodiment of the ECD 
cell of the present invention. The rear substrate and the porous plate 5 
are similar to the embodiments of FIGS. 5 through 7. 
The front substrate mainly comprises a glass substrate 16, an 
electrochromic layer 17 comprising a WO.sub.3 film or a MoO.sub.3 film, a 
first transparent conductive film 13, a second transparent conductive film 
14, and an insulator layer 15 for electrically separating the second 
transparent conductive film 14 from the first transparent conductive film 
13. The first transparent conductive film 13 is shaped to form the segment 
electrodes and the lead electrodes, and the second conductive film 14 is 
formed at positions where the sgement electrode are not formed for 
controlling the background of the display. 
Fabrication steps of the front substrate of FIGS. 8 and 9 will be described 
with reference to FIGS. 10(A) through 10(D). 
(I) The transparent conductive film 13 is formed on the flat glass plate 16 
in a desired configuration as shown in FIG. 10(A). The transparent 
conductive film 13 functions as the segment electrodes and the lead 
electrodes connected to the segment electrode. 
(II) A mask 21 comprising photo resist or etching resist is formed on the 
transparent conductive film 13 at positions corresponding to the segment 
electrodes and terminals of the lead electrodes as shown in FIG. 10(B). 
(III) Through the use of the resist mask formed by the above method, the 
insulator layer 15 is formed on the entire surface of the substrate. 
Thereafter, the second transparent conductive film 14 is formed on the 
entire surface of the insulator layer 15 as shown in FIG. 10(C). The 
insulator layer 15 must be dense to ensure the electrical isolation 
between the conductive films 13 and 14. 
(IV) The resist mask 21 is removed to shape the second transparent 
conductive film 14 and the insulator layer 15 as shown in FIG. 10(D). 
(V) The electrochromic layer 17 is formed on the entire surface of the 
substrate except the terminal portions of the lead electrodes through the 
use of a metal mask as shown in FIG. 9. 
The coloration state of the electrochromic layer 17 at the segment portions 
is controlled by the segment electrodes or the first conductive film 13. 
The coloration state of the electrochromic layer 17 at the background 
portions is controlled by the second conductive film 14. 
The entire electrochromic layer 17 is initially placed in the coloration 
state through the use of conductive films 13 and 14 by maintaining the 
conductive films 13 and 14 at a negative potential with respect to the 
counter electrode 7 (see FIGS. 5 through 7). The coloration state is 
memorized even when the coloration voltage is removed. A desired segment 
is placed into the bleached state by applying a positive voltage with 
respect to the counter electrode 7 to a desired segment electrode or the 
conductive film 13. Through the thus selected segment, the operator can 
see the porous plate 5 (see FIGS. 5 through 7) which exhibits a color 
sensation different from that of the coloration state of the 
electrohcromic layer 17. The thus selected segment can be returned to the 
initial coloration state by applying the coloration voltage to the segment 
electrode. It is preferable to regenerate the coloration state of the 
entire electrochromic layer 17 at a predetermined time interval. One 
method to regenerate the entire electrochromic layer 17 is to electrically 
connect the second conductive layer 14 to the segment electrode which is 
in the coloration state. 
EXAMPLE IV (for the embodiment of FIGS. 8 and 9) 
The transparent glass substrate 16 is a flat glass plate having 1 mm 
through 3 mm thickness. The transparent conductive film B comprises an 
In.sub.2 O.sub.3 film doped with SnO.sub.2. The transparent conductive 
film 13 is formed through the use of the electron bean evaporation method 
to a thickness of 1800 through 2000 A, and has a resistance value of 20-30 
.OMEGA./sq. The thus formed transparent conductive film 13 is shaped in a 
desired configuration through the use of a photo-etching method by using 
the positive type resist, for example, AZ.1350 manufactured by Shipley Co. 
The In.sub.2 O.sub.3 film is etched by mixture liquid comprising 42Be' 
ferric chloride and 12 normal hydrochloric acid at 40.degree. C. 
The mask 21 is formed through the use of the screen printing method. The 
printing machine is "LS-20" manufactured by Newlong Seimitsu Kogyo KK, and 
the screen plate is "Tetron 300-mesh" manufactured by Mesh Kogyo KK. The 
etching resist is "Naz-dar #300" manufactured by Naz-dar Co. The etching 
resist is maintained at 120.degree. C. for twenty minutes for drying 
purposes. Therefore, the insulator layer 15 is formed through the use of 
vacuum evaporation techniques. The insulator layer 15 is a SiO.sub.2 film 
of 2 .mu.m thickness. The conductive film 14 is formed on the entire 
surface of the insulator layer 15 through the use of a vacuum evaporation 
method. 
The thus formed substrate is disposed within trichloroethylene manufactured 
by Kishida Chemical Co. Ultrasonic vibrations are applied to the substrate 
to remove the resist. The WO.sub.3 film is formed on the thus formed 
substrate to a thickness of 5000 A through the use of the vacuum 
evaporation method except the portions of the terminals for the lead 
electrodes. Remaining portions are similar to that of the EXAMPLE I. 
The coloration signal applied to the segment electrode is -1V.500 msec, and 
the bleaching signal is =2.3V.1 sec. The regeneration is conducted once 
every 60 minutes for one second. The device is under the above-mentioned 
life test for more than one year. 
EXAMPLE V (for the embodiment of FIGS. 8 and 9) 
The resist mask is "Screenable Maskant MSN42B" manufactured by Minetch Co. 
The insulator layer 15 is formed through the use of the CVD method (CVD 
furnance manufactured by Watkins-Johnson Co. under the condition of the 
substrate temperature of 450.degree. C. The formed insulator layer 15 is a 
SiO.sub.2 film of 1000 A thickness. Instead of the porous ceramic plate 5, 
a yellow Tefron filter manufactured by Sumitomo Electric Industries Ltd. 
is used. Remaining portions are similar to the EXAMPLE IV. 
The invention being thus described, it will be obvious that the same may be 
varied in many ways. Such variations are not to be regarded as a departure 
from the spirit and scope of the invention, and all such modifications are 
intended to be included within the scope of the following claims.