Note: Descriptions are shown in the official language in which they were submitted.
10461S6
BACKGROUND OF INVENTION
This invention relates to electro-optical devices whose electro-
magnetic radiation absorption characteristics can be selectively altered by
influence of a suitably controlled electric field. More particularly, the
invention is concerned with an electro-optical device which has a more
uniform state of electrical charge at the imaging and/or counter-electrode
areas. Still more particularly, this invention is directed to a sandwich
type cell in which two layers of electrochromic material are separated by
solid, semi-solid or liquid ion conducting media.
The following United States Patents de~ibeelectro-optical devices
exhibiting a phenomenon known as persistent electrochromism wherein the
electromagnetic radiation absorption characteristic of a persistent electro- -
chromic material is altered under the influence of an electric field:
Patent No.Patent Date Inventor
3,521,941 7/28/70 S.K. Deb
R.F. Shaw
3,578,843 5/18/71 G.A. Castellion
3,704,057 11/28/72 L.C. Beegle
3,708,220 1/2/73 M.D. Meyers
T.A. Augurt
Such devices were employed in sandwich arrangement between two
electrodesO Coloration was induced by charging the electrochromic film
negative with respect to the counter-electrode, employing an external
potential. The counter-electrode can be the same as the persistent electro-
chromic material or different.
By reversing the original polarity of the field or by applying
a new field, it was also possible to cancel, erase or bleach the visible
coloration.
The steps of color induction and erasure are defined as cyclingO
It is an object of this invention to provide electrochromic
imaging devices having more uniform operating characteristics.
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SUMMARY OF THE INVENTION
The image display device is formed in a sandwich arrangement of
an electrochromic layer as an imaging area and a counter-electrode with a
spacing of an ion conducting medium, e.g. an electrolyte, between the areas.
Means are provided for supplying electric current to the counter-electrode
layer. Any conventional means is suitable. In the present invention, the
state of charge on the electrochromic imaging layer or counter-electrode
is made uniform by applying appropriate current at the time of manufacture
to obtain a desired predetermined state of charge.
Thus, in accordance with the broadest aspect of the invention, there
is provided an electro-optical device which comprises a layer having persist-
ent electrochromic material as a light modulating layer, a counter-electrode
layer, and an electrolyte layer in contact with said light modulating layer
and said counter-electrode layer, wherein said counter-eleetrode layer
includes eleetroehromie material in a substantially greater proportion than in
said light modulating layer and has a uniform predetermined partial electrical
charge.
DETAILED DESCRIPTION OF INVENTION
As used herein, a "persistent electrochromic material" is defined
as a material responsive to the application of an electric field of a given
polarity to change from a first persistent state in which it is essentially
non-absorptive of electromagnetic radiation in a given wavelength region, to
a second persistent state in which it is absorptive of electromagnetic radi-
ation in the given wavelength region, and once in said second state, is
responsive to the application of an electric field of the opposite polarity
to return to its first state. Certain of such materials can also be
responsive to a short circuiting condition, in the absence of an electric
field, so as to return to the initial state.
By "persistent" is meant the ability of the material to remain in
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the absorptive state to which it is changed, after removal of the electric
field, as distinguished from a substantially instantaneous reversion to the
initial state, as in the case of the Franz-Keldysh effect.
Electlochromic Materials
The materials which form the electrochromic materials of the
device in general are electrical insulators or semi-conductors. Thus are
excluded those metals, metal alloys, and other metal-containing compounds
which are r~latively good electrical conductors, as described in United
States Patent Number 3,704,057.
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These include materials containing a tran~ition
metal element (including Lanthanide and Actinide series
clem~nts), and material~ containing non-alkali metal ele-
ments such as copper. Pre~erred materials o~ thi~ clasq are
5 ~ilms o~ transition metal compounds in which the transition
metal may exist in any oxidation state from +2 to +8. Exam-
ples o~ these are: transition metal oxides, transition metal
oxysul~ide~, transition metal halides, selenides, tellurides,
chromates, molybdates, tungstates, vanadates, niobates, tan~
talates, titanates, stannate~, and the like.
A particularly advantageous aspect o~ the pre~ent
invention is the use of two separate layers o~ identical
electrochromic materials one layer being employed as the
counter-electrode ~or the other layer. A prererred embodi-
ment conqists Or tungsten oxide as the electrochromic color
electrode and tungsten oxide and graphite as the counter-
electrode.
When the persi~tent electrochromic materials are
employed as ~ilms, thickness desirably will be in the range
Or rrom about 0.1-100 microns. However, since a small poten-
tial will provide an enormous field strength across very thin
~ilms, the latter, i.e., O.l-10 microns, are pre~erred over
thicker ones. Optimum thickness will also be determined by
the nature o~ the particular compound being laid down as a
~ilm and by the ~ilm-~orming method since the particular com-
pound and ~ilm-rorming method may place physical (e.g., non-
uni~orm film-sur~ace) and economic limitations on manufacture
o~ the devices,
The electrochromic films may be laid down on any
substrate wh~ch, relative to the film, is electrically conduc-
ting. The electrically conductive material is coated on a
pla~tic or glass substratej either transparent or translucent
~or the imaging area. For the counter-electrode area, the
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substrate may be pla~tic, glass or metal i~ desired. The
electrode material may be carbon or the electrochromic mate-
rial, or mixtures If carbon is not present, the electro-
chromlc material should bc laid down on a conductive material
previously deposited on the ~ubstrate, or on a conductive
substrate such as a metal.
When tungsten oxide i9 employed as the electro-
chromic imaging material and an electric field is applied be-
tween the electrodes, a blue coloration of the previously
transparent electrochromic layer occurs, i.e., the persistent
electrochromic layer becomes absorptive of electromagnetic
radiation over a band initially encompassing the red end Or
the visible spectrum, thereby rendering the imaging layer
blue in appearance. Prior to the application of the electric
~ield, the electrochromic imaging layer was essentially non-
absorbent and thus transparent.
sPacin~ La~er
A semi-solid ion conductive gel may be employed as
disclosed in United States Patent Number 3,708,220.
A preferred embodiment employs H2S04 in combination
with polyvinyl alcohol.
A fluid layer containing an acid may al~o be u~ed in
place of the gel, as disclosed in United States Patent Number
3,704,057 or a solid inorganic layer as disclosed in United
States Patent Number 3,521,941
Counter-Electrode
As previously indicated, the counter-electrode may
be any electrically conductive material. Particularly advan-
tageous is a layer of electrochromic material, as described
previously. It i9 also advantageous to use the same electro-
chromic material for the imaging area and counter-electrode.
A mixture of graphite and an electrochromic material, or
graphite alone may be used as the counter-electrode. Other
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metallic counter-electrodes are disclosed in United States
Patent Number 3,701~,057.
Control Or the state of charge o~ the electro-
chromic electrode and more importantly the counter-electrode
at assembly has re~ulted in electrochromic devices which have
more uniformly good switching speed and cycle life. Most
notable is the narrowing in the range of switching character-
istic ~rom device to device which facilitates the use of
simple, low cost integrated circuits to drive the elect-ro-
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chromic devices.
At a given applied voltage, the switching speed ofan electrochromic device'having electrochromic material in the
counter-electrode is very dépendent upon the proportion o~
colored to cleared electrochromic material in the counter-
electrode. For example, a normal electrochromic numeric di~-
play counter-elffctrode may contain 5 to lO times the weight
of tungsten oxide as the electrochromic electrode. Tungsten
oxide blued (charged) is several orders Or magnitude more
~ electronically conductive than tungsten oxide clear (uncharged).
In order ~or the electrochromic electrode to color, it is im-
'portant that there be tungsten oxide blued (charged) in the '
counter-electrode to provide a ready source o~ protons to ~a-
cilitate the ionic process and to reduce resistive losses.
Although a completely "blue~' counter-electrode results in rapid
coloring o~ the el'ectrochromic electrode, it also results in
very slow clearing speed. The counter-èlectrode should have
a residual charge at all times to ~acilitate both electronic
and ionic processes and the electr'ochromic electrode in turn
should be the limiting electrode in terms Or capacity
Previous electrochromic devices required precise
control of water content and evaporation step to establish
adequate residual charge in the counter-electrode so as to
result in uni~orm switching speed. Previously counter-
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-electrodes could contain rrom 0 to 15~ of the tungsten
oxide in the blue state, The disclosed invention eliminates
this problem and is a means of correcting unwanted conditions
whi¢h can develop as a result of normal variations in manu-
5 ~'acturing steps.
This invention relates to electrochemical methods
of controlling the charge 3tate Or the electrochromic and/or
counter-electrode, The electrochromic electrode or counter- `
electrode may be precharged be~ore assembly into a working
device by placing the electrode to be charged in a suitable
electrolyte with a second electrode and applying a DC voltage,
second electrode positive. Another method permits the charg-
ing to be accomplished during or after assembly of the elec-
trochromic device, In this case a third electrode may be
placed in contact with the electrolyte layer and a potential
(DC) applied across the electrochromic electrode or the
counter-electrode and the third electrode for a short period
of time, Generally it is convenient to use the counter-
electrode and the third electrode as the control need be les~
critical to avoid damage to the device, It is preferable
to have about 50% o~ the counter-electrode electrochromic
deposit in the blue state at time of manufacture.
Thus in the case Or an image o~ one square Genti-
meter of electrochromic film, the charge to color or erase
is about 5 millicoulombs (mc.) The coulombic capacity of the
counter-electrode should exceed the coulombic capacit~ of the
imaging electrochromic area, For example, a 5 mc, capacity
electrochromic image area should be matched with a 50 mc.
electrochromic counter-electrode, The counter-electrode is
charged by application of direct current to 25 to 30 mc. to
obtain good overall performance in coloring and erasing.
