CA1038071A - Acid wetted "solid" electrochromic structure - Google Patents

Abstract

ACID WETTED "SOLID ELECTROCHROMIC STRUCTURE

ABSTRACT OF THE DISCLOSURE

An electrochromic data display and light modulating device which may be formed by sandwich arrangement of the imaging or light modulating layer and the counter-electrode layer, with a suitable porous insulating layer between, and an electrolyte saturating the porous insulating layer.

Description

i038~71 BACKGROUND OF INVENTION
~ his invention relates to electro-optical devices whose electro-magnetic radiation absorption characteristics can be selectively altered by influence of a suitably controlled electric field. ~ore particularly, this invention is directed to a sandwich type cell in which a layer of electro-chromic material is separated by a solid, porous insulating layer from a porous counter-electrode, wherein the insulating layer i9 wetted with an acid electrolyte.
In United States Patents 3,521,941 July 28, 1970 Satyendra Kumar Deb et al, 3,578,843 May 18~ 1971 Paolo Della Porta et al, 3,~fo~,057 November 28, 1972 Lindley Clair Beegle, and 3,708,220 January 2, 1973 Marion Douglas Meyers et al. there are described electro-optical devices exhibiting a phenomenon known a~ persistent electrochromisrn wherein electromagnetic radiation ab~orption characteri~tic o~ a persi~tent electroohromic material i8 altered under the influence oP an electric field. Such devices were em-ployed in sandwich arrangement between two electrodes. Coloration was in-duced by chareing the electrochromic film negative with respect to the counter-electrode, employing an external potential. The counter-electrode can be the same as the persistent electrochromic material or different.
~y reversing the original polarity of the field or by applying a new field, it was also possible to cancel, erase or bleach the visible col-oration .
~hese steps of color induction and erasure are de~ined as cycling.
Although the devices described in the prior applications are ef-fective to change their electromagnetic radiation transmitting properties under the influence of an electric field, and have good cycle life and c~cle speed, there has been a compromi~e between these two factors.
It i6 there~ore an ob~ect of this invention to provide an electro-chromic light modulating device which has a ~aster switching speed.

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A further object is to provide an electrochromic light modulating device longer life.
These and other ob~ects of the invention will become apparent as the description thereof proceeds~
In accordance with this invention there is provided a variable ligbt transmission device which comprises light transmitting substrate having a persistent electrochromic material on a conductive support as a light modulating material, a counter-electrode, and a layer of a porous insulating material in contact with said persistent electrochromic material and counter-electrode and permeated with ion conductive electrolyte fluid, and a space containine electrolyte fluid to wet said porous insulator means.
r~he image display device i5 formed in a sandwich arrangement o~
an electrochromic l~yer as an Lmagine or l:Lght modulatln~ area and n counter-electrode with a porous insulating medium between the areas which is wetted with an electrolyte. Means are provided for supplying electric current to the light modulating layer and the counter-electrode layer. Any con-ventional electrode means is suitable. A particularly advantageous means for electrical connection is to deposit the electrochromic light modu-lating layer on a conductive surface, such as *~ESA glass.
; United States Patent ~umber 3,521,941 discloses a solid electro-chromic structure comprising a solid electrochromic film capable of cycling through a coloring mode and erased mode. Devices of this type require a small amount of ~ater to operate and must be driven at 2 to 7 volts DC
to operate at a useful switching speed. Unfortunately, at these switching voltages water is consumed giving off H2 gas and/or 2 gas which disrupts the overlying layers resulting in a decrease in cycle life. Otber structures have been prepared in which a liquid or paste or gel has been substituted for the solid "current carrier permeable insulator" a~d the counter-electrode is on a separate conductive substrate. These structures operate *Trade Mark ~ - 2 -.'~3 ~38~1 at lower voltages, typically about 1 volt, gas evolution is eliminated or suppressed and cycle life in 106 cycles is known. Life is believed to be parti~lly limited by the solubility of the electrochromic film.
rrhe proposed invention combined certain advantages of both structures. The structure includes the solid features - 2a -~' ~

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of the so called "Solid Electrochromic Structure" but in-cludes an ion carrying liquid acid electrolyte, which per-meates the solid in~ulating la~er resulting in improved ion mobility between closely spaced electrodes thereby permits raster switching and use of lower voltages to improve life.
Also, the liquid electrolyte may be held mainly in the porous insulating layer (capillary action) limiting the total elec-trolyte volume and reducing solubility o r the electrochromic rilm.
This invention is expected to be userul in appli-cations ror electrochromic devices requiring long lire and rast switchine speeds. Elimination of an electrolyke column may make~large electrochromic window~ a po~lbility a~ the leakage haz~rd lq ~reatl~ reduced with thlq conoept.
The foregoing and other rea-tures, obJects and ad-vantages Or the present invention will become more apparent from the rollowing detailed de~cription, D~TAILED DESCRIPTION OF INVENTION
The invention consists of a ~ransparent, conductive electrode which has deposited on it an electrochromic film, a rigid~ porous insulating layer and a porous counter-electrode la~er. The insulating layer-electrode structure is wetted b~
an acid electrolyte or other proton source and the assembly is sealed to exclude oxygen and water.
As used hereinJ a ~'persis-ten-t electrochromic mate-rial" is derined as a material responsive to the application Or an electric field o r a given polarity to change rrom a ~irst persistent state in which it is essentially non-absorptive Or electromagnetic radiation in a given wavelength region, to a second persistent state in which it is absorptive Or electro-magnetic radiation in the given wavelength regionJ and once in said ~econd state, is responsive to the application Or an electric ~ield of the opposite polarit~ to return to its rirst 1~3~
state. Certain of such materials can al30 be responsive to a short circuiting condition, in the absence o~ an electric field, so as to return to the initial state.
By "persistent" is meant the ability o~ the mate-rial to remain in the absorptive state to which it is changed,a~ter removal of the electric ~ield, as distinguished rrom a substantiall~ instantaneous reversion to the initial state, as in the case o~ the Franz-Keldysh effect.
Electrochromic Materials The materials which form the electrochromic mate-rials o~ the device in general are electrical insulators or semiconductors, Thus are exoluded tho~e metals, metal allo~sJ
ancl other metal-oontainlng compouncls whlch aro rolablvel~
good electrical concluotors, a~ de~cribed in United States Patent Number 3,704,057~
These include materials containing a transition metal element (including Lanthanide and Actinide series ele-ments), and materials containing non-alkali metal elements such as copper, Preferred materials of this class are films o~ transition metal compound~ in which the transition metal may exist in any oxidation state from +2 to ~8. Examples of these are: transition metal oxides, transition metal oxysul-fides, transition metal halides, ~elenides, tellurides, chro-mates, molybdates, tungstatesJ vanadates, niobates, tantalates, titanates, stannates, and the like.
A particularly advantageous aspect o~ the present invention is in the use of two separate layers of identical electrochromic materials one layer being employed as the counter~electrode ~or the other layer. A pre~erred embodiment consists of tungsten oxide as the electrochromic color elec-trode and tungsten oxide and graphite as the counter-electrode.
When the persi~tent electrochromic materials are employed as ~ilms, thickne~s desirably will be in the range L~ -3~0~7~

of ~rom about 0 1-100 microns. ~owever, ~ince a small poten-tial will provide an enormou~ field ~trength acros~ very thin films, the latter, i.e., 0.1-10 microns, are pre~erred over thicker ones. Optimum thickness will also be determined by 5 the nature of the particular compound being laid down as a ~ilm and by the film-forming method ~ince the particular com-pound and film-forming method ma~ place physical (e.g., non-uni~orm film sur~a¢e) and economic limitations on manufacture of the devices, The ~ilms may be laid down on any substrate which, relakive to the ~ilm, i9 electrically conducting~ The elec-trically conductive material may be coated on another 3uit-able ~ubstrate material includin~ glass, woocl, papor, pla~ticsJ
plaster, and the lik0, including tran~parent, translucent, opaque or other optical quality materials. A pre~erred em-bodiment in the instant device would employ at least one trans-parent electrode, When tungsten oxide is employed a~ the electrochromic imaging material and an electric field i~ applied between the electrodes, a blue coloration o~ the previously transparent electrochromic layer occurs~ i.e., the persistent electro-çhromic layer becomes absorptive of electromagnetic radiation over a band inltiaIly encompassing the red end of the visible ~pectrum, thereby rendering the imaging layer blue in appear-: 25 ance. Prior to the application o~ the electric ~ield, theelectrochromic imaging layer was essentially non-abso~bent and thus transparent.
Electrol~te A ~luid layer containing an acid may al90 be used as disclosed in United States Patent Number 3,70l~,057. A pre-~erred embodiment employs H2SO
Porous Insulating Layer The porous insulating layer may be an inorganic or ~031~
other solid material a3 disclosed in United States Patent Number 33578,843, e,g. silicon oxide or aluminum oxide. Al~o contemplated is a layer of porou~ glass as described in Prod-uct Engineering, February 1973, page 33, Counter-Electrode As previously indicated, the counter-electrode may be any electrically conductive material which i~ light trans-mitting. Suitable metallic counter-electrode materials are di~closed in United States Patent Number 3J704,057. A par-ticularly good counter-electrode material is palladium.
The invention may be further understood by re~er-ence to the drawings in which Fig. 1 1~ a f'ront view Or a light modulating devlac according to the invention, Fig. 2 is a cro~s section o~ the electrochromic display device, As shown in Figs. 1 and 2, the light modulating device consists of a ~ront plate 1, consisting o~ a trans-parent or translucent material 2 such as glass or plastic 3 with a lîght transmitting layer 3, which is electrically con-ductive, e.g. tin oxide. The structure of 2 and 3 is avail-able commercially as NESA glass. As shown, a layer of tung-sten oxide 4 is depo~ited upon the tin oxide 3, and the tung-sten oxide ~, has deposited on it sequentially a la~er o~ a porous insulating material 5 such as A1203, and on this a layer o~ a oonductive metal 6 as a counter-electrode. A ~pac0 7 is provided to contain an acid electrolyte to wek the porous insulator 5. A non-conducting backing plate 8 and seal 9 close the device. The ~ront electrode 1 is connected to a power 3Q source by means of connector 10, and the counter-electrode is connected to the power source by connector 11. The current modes ror coloring and erasing are shown at switch 12. As illustrated, a viewer at station 13 could modulate transmission ~6~38~
or sunlight (or other radiation)'from point 14.
: The device discussed and illustrated in Figs. 1 and

2 could also operate in the reflecting mode by making the counter-electrode opaque. In this way the light to be modu-late,d would come from the front of the device as seen by the viewer.
Moreover, an information display devipe could be fabricated with the structure of this invention, if the tung-sten oxide layer is in the form of discrete dots and segments 10 with the required electrioal circuitry and logic mean~. .

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Claims (7)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A variable light transmission device which comprises light trans-mitting substrate having a persistent electrochromic material on a conductive support as a light modulating material, a counter-electrode, and a layer of a porous insulating material in contact with said persistent electrochromic material and counter-electrode and permeated with ion conductive electrolyte fluid, and a space containing electrolyte fluid to wet said porous insulator means.

2. A variable light transmission device as in claim 1, which comprises two layers, one with said electrochromic material, and the other of said counter-electrode separated by said porous insulating layer, disposed between a pair of conductive electrodes.

3. The device of claim 2, wherein said counter-electrode is a metallic layer.

4. The device of claim 2, wherein said counter-electrode is opaque.

5. The device of claim 1, wherein said electrochromic material is WO3.

6. The device of claim 4, wherein said counter-electrode is a metallic layer.

7. The device of claim 6, wherein said counter-electrode is palladium.