This invention relates to improvements in electrooptical devices which contain a layer of persistent electrochromic material on one electrode in contact with a specific polymeric electrolyte which, in addition, contacts a counterelectrode in the device. These devices operate by the passage of electric current between the electrodes through the polymeric electrolyte.
They are provided with means for applying an electric field to said electrodes and with means for reversing the polarity of the respective electrodes to reverse the electric field and thereby change the photo-absorptive character-; istics of the electrochromic layer.
A variety of such devices having numberous uses have been described in the prior art. U.S. Patent No. 3,708,220 issued January 2, 1973 to M. D. Meyers, et al, for example, describes electrochromic devices of the kind referred to in which a particular gelled electrolyte solution is used.
U.S. Patent No. 3,521,941, issued July 28, 1970 to S. K. Deb, et al,discloses thè potential use of plastics, e.g. polyesters, vinyl or like polymers, allylic or like polymers, polycarbonates, phenolics, amino resins polyamides, polyimides, and cellulosic resins for electrochromic devices.
The use of these polymers is not exemplified.
U.S. Patent No. 3,971,624 issued July 27, 1976 to P. Bruesch, et al, discloses the use of a perfluorated sulfonic acid polymer as an electrolyte for electrochromic devices, though there is no disclosure of how to incorporate such a polymer into a device.
~ lectrochromic devices are useful for digital displays and other image displays, and for variable light transmission applications such as electrochromic windows, mirrors, and the like.
Il . , --` 1077608 In a typical device embodying the present invention a film of a selected persistent electrochromic material is coated on the surface of an electron-conductive electrode which usually is a plate such as conductive glass, metal, a carbon plate, or the like. The layer of electrochromic material on the electrode will provide the base upon which the polymeric electrolyte is deposited such that the electro-chromic layer intimately contacts the polymeric electrolyte in the device. A counter-electrode is placed atop the de-posited polymeric electrolyte and across the device from the electrochromic layer. When an electric field is applied to -the electrodes, electric current flows through the polymeric electrolyte between the electrochromic layer and the counter-electrode, aausing either coloration or bleaching of the electrochromic material. The devices are provided with means for selectively changing the polarity of the electrodes to provide, alternatively, for coloration at one polarity and for bleaching at the opposite polarity.
The present invention is directed specifically to the method of incorporating a polymeric electrolyte into electrochromic devices which will be useful in any of the several kinds described above. The invention provides such devices with improved electrolytes resulting in faster switch-ing and longer llfe due to less deterioration of the electro-; 25 lyte. The polymeric electrolytes are incorporated into the devices by dlssolving the polymer in a suitable solvent then depositing the polymer on the layer of electrochromic material, and evaporating the solvent to produce a solid hard film of polymeric electrolyte.
- 2 -, `` 10776~)8 Attempts to prepare electrochromic devices using solid, self-supporting polymer films pressed between electrodes have resulted ~n contact problems at the interfaces between the solid electrolyte and the electrode. This contact problem can result in partial or total loss of operation. The use of liquid interfaces between the solid polymer and facing electrodes also produces defective operation due to inevitable variations in the liquid interface thickness. It also defeats the major purpose of using a solid polymer in the first place which is to prevent and/or minimize dissolu-tion or degradation of the electrochromic layer. If polymers are used with elastomeric characteristics so as to avoid the initial contact problems, the dissolution or degradation develops as the device ages and relaxation of the compressed polymer develops.
While any of a number of polymers can theoretically be used as the electrolyte in such devices, several important I
functions of the electrochromic devices vary significantly , ~; depending on the particular polymer selected. Among the functions that are so affected, the most important are (1) ~ .
the speed of switching of the electrochromic layer from one state to another, (2) the intensity of color attained in the electrochromic layer, (3) the electrical potential required for coloring the layer, and (4) the electrical efficiency of 1~
~the switching operation.
By the term "switching" is meant the change of 1`~ .
polarity of the electrodes and the resulting change in the radiation absorbing character of the electrochromic material s upon the application of an electric field. ~he transmission 30~ of ourrent in one direction through the device will increase ~ ~ .
- 3 -~077608 the light absorptive character (color) of the layer. After the current is stopped, the same absorptivity tends to remain in the layer, i.e. the color is persistent, until current of the reverse polarity is applied whereupon the photo absorptivity of the layer decreases. The application of current in the direction which decreases absorptivity is called bleaching. The electro-chromic material is persistent in both its colored and its bleached state whenever no current is being applied.
According to the present invention, there is now provided in an electrochromic device which comprises a layer of persistent electrochromic material on an electrode surface in electrical contact with a polymeric electrolyte, a counter-electrode also electrically contacting said polymeric electrolyte and electrical means for selectively applying electrical fields -of opposite polarity across said electrodes, the improvement wherein said :~ , . .
polymeric electrolyte is incorporated into the device by the steps of (1) dissolving the polymer in a solvent, (2) depositing the resultant solution upon the electrochromic material, and ~3) evaporating the solvent from the coating.
Preferably the electrochromic material is W03.
The depositing may be performed by a method selected from spin coating, dipping, roller coating, and spraying.
¦ Suitable polymers for use as the electrolytes of the present invention are soluble polymers and copolymers containing acidic or basic groups. These groups are generally covalently bonded to the polymer chain. ;
¦ The acid types exchange cations while the basic types exchange ~ -J : anions. The main groups of cation exchangers of the strong acid type are -S03H and -P03H2, while those of the weak acid type are -COOH. An example of the strong basic type is -CH2N(CH3)3NOH and an example of the weak basic j~ type is > NH20H. Among these four types, sulfonic acid and quaternary ~ -¦~ ammonium hydroxide contain strongly ionized functions and, consequently, ¦ ~ 30 have high ionic conductivity resulting from migration of H or OH ions.
The nature of the ionic group greatly effects the ionic conductivity of the
4 -~ ~ C ~
1~177608 ion exchange polymers. The most conductive polymers are those in which the mobile ion is a proton. The functional group -S03H should consequently be preferred to its salts, such as -S03Na, or to weak acids, such as -COOH.
The extent of sulfonation will also have an effect on the ionic conductivity of the polymer. Suitable ion conducting polymers include such as polystyrene sulfonic acid, polyethylene sulfonic - 4a -, ~` . , .
.. . , ~ . - , - . ~ , . : , -10776~8 acid, perfluorosulfonic acid, as well as any other polymers of the same general class.
The solvents suitable for use herein to dissolve the polymers to produce the desired solution include water and essentially all common organic solvents such as methanol, ethanol, acetone, methyl ethyl ketone, methylene chloride, dimethyl formamide, ethyl acetate, as well as homologues and analogues thereof. Thus any solvent which will dissolve the polymer and then evaporate at room temperature may be used.
The electrolyte solutions may be conveniently prepared by simply placing the specified amount of the polymer into the solvent and then stirring the mixture with or with-out heating. The concentration of the polymer solution may vary from lO 4 moles/liter to a saturated solution, though the actual concentration does not appear to be critical.
; The polymeric electrolyte is incorporated into the electrochromic device by depositing the polymer atop the electrochromic layer by spin coating, dipping, roller coating, or spraying techniques. Preferably spin coating is used to make small devices due to its resulting in a more uniform and even layer of the polymer. The use of these coating techniques yields a polymeric electrolyte adhered ¦~ to and ooated on the electrochromic film wherein the possibility of deleterious gaps or air spaces is eliminated, or, at least, greatly reduced.
The present deposition procedure entails the , .
preparation of polymer and solvent solution suitable for the coating method selected and the application of the coating ~ to the eleatrode surface. Generally the concentration of .,, :
. .~ . . ., .,. , .
the solution is not critical and may vary widely.
In the case of spin coating, the final film thick-ness is affected by the viscosity of the solution and the rate of spinning. Solid polymer films of 1 to lOOy thick may be obtained from polymer-solvent solutions with a visco-sity approximately 30 centistokes and at spinning speeds between 500 rpm and 5000 rpm.
Dip coating involves immersing the electrode in the polymer'solution, withdrawing, and hanging to dry in a dust free area. Thin polymer films are formed by slow with-, drawal rates; thicker coatings are made by increasing the withdrawal speed.
Roller coating is another method of applying the i~ polymer solution to the electrode. In this method the electrode passes tkrough a set of rollers which transfers ' the polymer solution to the electrode. A typical roller , coater machine employs a drive roller to move the electrode 1, beneath an applicator roller. The applicator roller applies I ; , the polym,er solution to the electrode. A doctor roller , controls the amount of polymer solution on the applicatorroller so that an exact amount of polymer is applied to the surface of the electrode.
The spray coating method requires a polymer solution more dilute than the aforementioned processes.
2~51 This equipment may include a spray gun and reservoir and a supply of clean compressed air. The spray coa~ing is applied witk the electrodé held vertically. One or more coats may ~ be applied to assure coverage. Immediately after spraying k~
the electrode is placed in a horizontal position to allow the ;30 ~ coating to level off whlle the solvent evaporates.
~ 6 - ' ~ (-~ 1077608 '~ The following non-limiting examples illustrates the advantage of the present invention. ' EX-AMPLE I
An electrochromic device was prepared in accordancP
with the present invention as follows:
~ A variable transmission, electrochromic window was - prepared starting with a transparent, conductive indium oxide coated glass as supplied by Optical Coating Laboratory. -'' The conductive glass substrate was coated with a layer of electrochromic tungsten oxide. A thin protective layer of silicon oxide was deposited upon the WO3 layer. National Starch Co. Versa-TL-500 polystyrene sulfonic acid polymer - was dialyzed and freeze dried to remove impurities. A
;; solution of the polymer in methanol was prepared at the ~ , 15 concentration 70 mg/ml. This solution was used to coat the silicon oxide layer by spinning. The substrate was placed ;i .
upon the spinner chuck in a horizontal position and polymer ,~ solution added to the surface. The spinner was started and ',,' ' rotational speed increased to about 1000 rpm causing excess ', 20 polymer solution to be carried off the edges of the substrate leaving a thin layer of polymer which solidified in a few seconds due to solvent evaporation. Following the spinning, , ' '~ a second thin la,yer of silicon oxide was deposited upon the ~, polymer layer. iNext a thin, transparent layer of palladium was deposited followed by a layer of gold'approximately i ,~
~125A;thick. The de~lce was~,sea'lediwith a;second,glass plate an'd.aN,epoxy e',d~ge;aeal.
A second de~ice was,pre,pared as aboue ~ith~the , ~ exceptïon that the po~yst,yrene sùI'fonïc acia~po~lymer~wa~s '~
30~ incorporated into the device as a solid thin sheet (7~ thick). '- -. ~ , :.;:
~ * Trade~Mark ' ~ ' . ' ' ~ ' '' " ' ` 10776~8 Each device was colored and cleared by the appli-cation of electricity at an average current density of about 1 milliamp/cm2 using a cycle of Cl) lS sec. to color, (2) 15 sec. on open circuit, (3) 15 sec. to clear, and (4) 15 sec. on open circuit through a transmission range of 50 to 20%.
The results were as follows:
Device of Invention Comparison Cycles to failure over 8000 less than 10 It is thus readily apparent that depositing the polymeric electrolyte as specified results in a greatly improved device.
.i - , ~ EXAMPLE II
i~ The procedure of Example I is repeated using the i lS following polymer~solvent combinations which are then deposited as above. The resultant devices show superiority to those - prepared without the specified deposition method.
A. perfluorosulfonic acid/methanol ~ B. polystyrenesulfonic acid/ethanol 1 20 C. polyethylenesulfonic acid/water D. perfluorosulfonic acid/isopropanol .; -EXAMPLE III
An electrochromic, numeric information display was prepared starting with a transparent, conducting tin oxide 25~ ~ glass. Tungsten oxide was deposited onto the tin oxide using a mask so as to leave a deposit in the form of digits. The exposed tin oxide and tungsten oxide surfaces were coated with polystyrenesulfonic acid polymer by the spin coating method.
Flnal1y a layer of palladium was deposited to serve as the 3Q~ counter-electrode and decorative background for the electro--chromic image. The electrochromic digits were colored and ~ade transparent by the application of DC pulses as in Exa~ple I.
A second display was prepared as above with the exception that the polystyrene sulfonic acid polymer was incorporated in the device as a solid thin sheet of essentially the same thickness as the coated layer above.
The two devices were tested as in Example I and gave the following results: .
Device of Invention Comparison Cycles to failure 1800 O *
Non operative due to non-uniform coloration during first cycle s~ 1 5 ~, ~
~20 `~ .