Multi-Color Electrochromic Cells Having Solid Polymer Electrolyte Layer With Organic Electrochromic Material Dissolved Therein

1. Field of the Invention

This invention relates to multi-color electrochromic flat-panel displays embodying solid-state electrochromatic cells with solid polymer electrolytes. The electrochromic material, which is soluble in water associated with or a water emulsion of an ionically conducting polymer electrolyte, changes color in response to small applied anodic or cathodic currents providing writing and erasure processes as a result of anodically or cathodically applying voltages to the cell. On open-circuit, the cell retains its state of charge or memory. The multi-color electrochromic cells using solid polymer electrolytes according to the present invention provide a display which is thin, compact, lightweight, sunlight readable, possessing open circuit memory, operable over a wide operating temperature range requiring low power input for operation.

2. Description of the Prior Art

Electrochromic effects of various chemicals and various chemical systems wherein specific chemicals undergo chemical reactions in response to small applied anodic or cathodic currents changing their color are known. Color changes, rates, and performance characteristics, are greatly dependent upon the electrochromic chemical used and the entire electrochromic system including the electrolyte and cell configuration. Electrochromic reactions of rare earth diphthalocyanines have been reported by P. N. Moskalev and I. S. Kirin, Opt. Spectrosc., 29, 220 (1970) and P. N. Moskalev and I. S. Kirin, Russ. J. Phys. Chem., 47, 1019 (1972). Color changes in a lutetium diphthalocyanine film on tin oxide in an aqueous electrolyte of KCl or Na.sub.2 SO.sub.4 is described in M. M. Nicholson and F. A. Pizzarello, Charge Transport in Oxidation Product of Lutetium Diphthalocyanine, J. Electrochem Soc., 127, 2490 (1979) and M. M. Nicholson and F. A. Pizzarello, Galvanostatic Transients in Lutetium Diphthalocyanine Films. The cathodic and anodic electrochromism of lutetium diphthalocyanine films have been found dependent upon the specific aqueous and organic liquid electrolyte as well as other specific cell components as reported by M. M. Nicholson and F. A. Pizzarello, Cathodic Electrochromism of Lutetium Dipthalocyanine Films, J. Electrochem Soc., 128, 1740 (1981). Electrochromic action of other rare earth diphthalocyanines are recognized to be similar and electrochromic changes of lutetium and ytterbium diphthalocyanines are very similar in a number of aqueous and organic electrolyte liquids. D. Walton, B. Ely and G. Elliott, Investigations into the Electrochromism of Lutetium and Ytterbium Diphthalocyanines, J. Electrochem Soc., 128, 2479 (1981). The use of lutetium diphthalocyanine with aqueous and organic liquid electrolytes in electrochromic displays is described in M. M. Nicholson and T. P. Weismuller, Multicolor Electrochromic Display Technology, Technical Report No. 5, Office of Naval Research Contract N00014-77-C-0636, Task No. NR 359-667 (1983). Different electrochromic behavior of lutetium diphthalocyanine films in single boundary and dual boundary cell configurations using aqueous liquid electrolytes and emphasizing the need for aqueous electrolytes to promote ion transport is described in M. M. Nicholson and T. P. Wiesmuller, Evidence of Electronic Conduction Due to Mixed Oxidation States in Lutetium Diphthalocyanine Films, Report No. 7, Office of Naval Research Contract N00014-77-C-0636, Task No. NR 359-667 (1985).

The electrochromic action of 4,4'-dipyridinium compounds known as viologens in aqueous and organic liquid electrolyte systems is known. The action of n-heptylviologen in various salt liquid solutions showed dependence upon specific anions, cations and metal presence. R. J. Jasinski, The Electrochemistry of Some n-Heptylviologen Salt Solutions, J. Electrochem Soc., 124, 637 (1977). Large differences in redox potentials between ethylviologen and benzylviologen in aqueous and organic liquid solvents have been noted. H. T. van Dam and J. J. Poujee, Electrochemically Generated Colored Films of Insoluble Viologen Radical Compounds, J. Electrochem Soc., 121, 1555 (1974). The electrochromic changes of diheptylviologen film on tin oxide electrodes in an aqueous electrolyte are described in J. Bruinink and C. G. A. Kregting, The Voltammetric Behavior of Some Viologens at SnO.sub.2 Electrodes, J. Electrochem Soc., 125, 1397 (1978). Photoreduction of aqueous solutions of heptylviologen bromide on p-GaAs in the form of photoelectrochemical cells is described in B. Reichman, F. F. Fan and A. J. Bard, Semiconductor Electrodes, J. Electrochem Soc., 127, 333 (1980). The differing conductance of helptylviologen in aqueous and organic solvents is described in H. T. van Dam, The Conductance of Heptylviologen Dibromide in Water and Methanol, J. Electrochem Soc., 123, 1181 (1976).

Solid-state electrochromic displays are taught by U.S. Pat. No. 4,550,982 wherein an organic electrochromic material and an ionic material are provided in a polymeric layer. The organic electrochromic material is associated with the polymeric material either as integrated macromolecules or polymer molecules, or as pendant macromolecules by covalent bonding, or is dispersed in the polymeric layer.

Other electrochromic displays using solid polymer electrolytes are described in Japanese Patent Application Publication No. 84/37804, Matsushita Electric Industrial Co. Ltd., using pyridinium materials and in Japanese Patent Application No. 82/223451, Publication No. 84/113422, teaching a solid acrylic resin tetrathiofulvalene having a lithium borofluoride polymer display.

The electrochromic cells of this invention have a water soluble organic electrochromic material dissolved in water associated with a water containing ionically conducting polymer electrolyte or in the water of emulsion of a non-water containing ionically conducting polymer electrolyte adjacent the display electrode substrate and may be generally described as shown in FIG. 4.

The electrochromic half cell has a display electrode formed of tin oxide or a mixture of indium oxide and tin oxide as a film on a substantially transparent plate. According to the present invention, the organic electrochromic chemical is in solution in water associated with a water soluble ionically conducting polymer solid electrolyte or in solution in a water emulsion of water insoluble ionically conducting polymer electrolyte. A supporting electrolyte within the ionically conducting polymer layer is to provide high overall ionic conductivity in the final cell, thereby providing low internal resistance losses in the final cell. The ionically conducting polymer with a supporting electrolyte and electrochromic chemical forms a solid layer having one side immediately adjacent the display electrode. Any water soluble organic electrochromic chemical may be used, such as the alkylviologens. The solid electrolyte comprises a water soluble organic electrochromic chemical in solution in water soluble or water containing ionically conducting polymer layer or in solution in a water emulsion of a water insoluble or non-water containing ionically conducting polymer layer, either containing a supporting electrolyte. The other side of the solid electrolyte layer is adjacent an insoluble cation conducting membrane, such as Nafion 117, used as a separator between the electrochromic half cell and the counter half cell.

The counter half cell has a counter electrode which may be any conductive material and may be the same as or different than the display electrode. An ionically conducting polymer layer containing a supporting electrolyte is between the counter electrode and the separator layer. An electroactive redox material is either in association with the surface of the counter electrode as a discrete layer or is associated with the ionically conducting polymer layer between the counter electrode and the separator to maintain redox electrochemistry in this compartment upon the passage of either anodic or cathodic currents. Thus, the electrochromic cells of this invention provide all solid-state cells.

It is an object of this invention to provide all solid-state multi-color electrochromic cells.

It is a further object of this invention to provide all solid-state multi-color electrochromic display cells having an organic electrochromic chemical in solution in water associated with an ionically conducting polymer electrolyte layer.

Still another object of this invention is to provide all solid-state multi-color electrochromic display cells suitable for sunlight readable multi-color flat-panel display operable with low input voltage and power.

These and other objects and advantages of this invention will become apparent upon reading the preferred embodiments with reference to the drawing wherein:

FIG. 1 is a section of one embodiment of a multi-color electrochromic cell of this invention;

FIG. 2 shows cyclic voltammetry measurements on a cell of the embodiment of this invention as described in Example I;

FIGS. 3a, 3b, and 3c show cycling of a cell of one embodiment of this invention as described in Example II; and

FIG. 4 is a simplified schematic showing an electrochemical cell according to this invention.

Referring to FIG. 1, an electrochromic cell of one embodiment of this invention is shown schematically in cross section wherein the organic electrochromic half cell 20 is separated from counter half cell 30 by insoluble cation conducting membrane 10. Electrochromic half cell 2 comprises transparent plate 5, display electrode layer 6, solid electrolyte layer 9 comprising organic electrochromic chemical in solution in a water containing ionically conducting polymer or in solution in a water emulsion of a non-water containing ionically conducting polymer and a supporting electrolyte. The electrochromic cell may be encased in any suitable casing to enclose a sealed space and any suitable means for exterior electrical contact with the electrodes may be provided. In FIG. 1, the thickness of the layers are not meant to convey actual thicknesses, but are generally exaggerated for illustration purposes.

Transparent plate 5 may be fabricated from any substantially transparent material such as glass or a transparent plastic sheet. Display electrode layer 6 is a tin oxide film or an indium oxide and tin oxide film which may be applied to transparent plate 5 by spraying, CVD technique, or by precipitation. Methods of fabrication of such display electrodes are known to the art.

The solid electrolyte layer 9 comprises a water soluble organic electrochromic material in solution in the water of a water containing ionically conducting polymer or in the water of a water emulsion of a non-water containing ionically conducting polymer, and a supporting electrolyte. The electrolyte layer with water dissolved organic electrochromic chemical is thus immediately adjacent the electronically conducting film of the display electrode providing high efficiency and rapid response time. Suitable thickness of the electrolyte layer used in the cells of this invention is about 40 .ANG. to about 20,000 .ANG., about 200 .ANG. to about 10,000 .ANG. being preferred. The organic electrochromic chemical may include known organic electrochromic materials such as tetrathiafulvalen, arylpyrazoline, fluorene, benzoquinone, naphthoquinone, anthraquinone, pyrylium, pyridium, methylene blue, furil, other aromatic or heterocyclic compounds, and their derivatives, ferroine, ferrocene, complexes of diphthalocyanine and rare earth metals, and other organometallic compounds, and viologens. Preferred electrochromic chemicals for use in the cells of this invention are selected from the group consisting of water soluble alkylviologens. A particularly preferred electrochromic chemicals for use in the cells of this invention is heptylviologen. The electrochromic chemical used in the cells of this invention will be water soluble. Suitable concentration for the organic electrochromic chemical in the water is about 0.001M to about 0.2M.

Any alkali ion conducting polymer is suitable for use as the ionically conducting polymer associated with the supporting electrolyte to form the solid electrolyte layer 9. It is preferred that the polymer have a functional group with a strong polarity. Suitable polymer materials include melamine resin, silicon resin, xylene resin, vinyl acetate resin, copolymer resin of vinyl chloride and vinyl acetate, polycarbonate resin, resin of cellulose derivatives, polyvinyl carbazole resin, polyether resin, such as polyethylene oxide and polypropylene oxide, acrylic resin, such as polyacrylonitrile and polymethacrylonitrile, and fluororesin, such as polyvinylidene fluoride. Preferred ionically conducting polymers include poly(phosphazenes), poly(ethylene oxide), poly(propylene oxide), perfluorosulfonic acid sold under registered trademark NAFION.RTM. by E.I. duPont de Nemours and Company, poly(styrene sulfonic acid) known as PSSA, poly-2-acrylamido-2-methyl propane sulfonic acid known as poly-AMPS, poly(ethylene sulfonic acid) known as PESA, poly(oxyethylene), poly(methacrylate), poly(ethylene succinate), poly(vinyl acetate), polyacrylonitrile, poly(vinylidene fluoride) and poly(tetramethylene oxide). When a water insoluble or non-water containing polymer is used a water emulsion of the polymer is formed using very little water which provides a medium for the water soluble organic electrochromic chemical. This still provides a solid state electrolyte, the water being present in less than 20 wt. percent, preferably about 1 to about 8 wt. percent. When a water soluble or water containing polymer is used, the water soluble organic electrochromic chemical is dissolved in water associated with the polymer. In a preferred embodiment, water containing polymers are used including poly-2-acrylamido-2-methyl propane sulfonic acid, poly(styrene sulfonic acid), poly(ethylene sulfonic acid), and perfluorosulfonic acid. By the term "water containing polymer", I mean polymers such as described by Jean-Paul Randin in Ion-Containing Polymers as Semi-Solid Electrolytes in WO.sub.3 -based Electrochromic Devices, J. Electrochem. Soc. 129, 1215 (1982).

The polymers may include a cross-linking or bridge-bonding additive to serve as a stabilizer and to enhance adhesion to adjacent layers. Such additives are known to the polymer art and include functional groups of oxygen, sulfur, nitrogen, halogen and the like capable of reacting with the macromolecules upon application of light or heat to form cross-linking or bridge-bonding of the macromolecules. The polymers may also include a plasticizer to aid in dissociation of ionic material. Plasticizers are known to the polymer art including phthalic diesters such as dioctyl phthalate and dipentyl phthalate, esters of aliphatic dibasic acids such as dioctyl adipate, phosphoric esters such as dibutyl phosphate, ether compounds such as o-nitrophenyloctyl ether and diphenyl ether, glycol ester, epoxy compounds, and polymers of such plasticizers. Other suitable plasticizers include propylene carbonate, ethylene carbonate, .gamma.-butylolactone, like materials with high dielectric constant, and organic compounds which are used as liquid crystals such as N-(4-ethoxybenzilidene-4'-n-butylaniline). The polymers may also include an antioxidant and/or an ultraviolet ray absorbing material as known to the polymer art as a stabilizer. The electrolyte layer may also include a fine reflective powder. The reflective powder may be white, such as alumina or titanium oxide powder, or may be colored and capable of selectively reflecting incident rays.

The supporting electrolyte may comprise any ion conductive materials as known to the art such as alkali metal halides, alkaline earth metal halides, rare earth metal halides, halogenoalkyl ammonium, their solid solutions, a complex of an alkali metal halide and crown ether, halogenopyridines exemplified by iodo-1-n-butyl pyridinum, and like halogen ion conductive material, its hydrates, and alkali ion conductive materials, such as oxides of transition metals serving as alkali ion conductive materials, alkali metal halides which serve also as halogen ion conductive materials, alkali metal perchlorates, alkali metal tetrafluoroborates, alkali metal hexafluorophosphates, alkali metal nitrides, alkali metal aluminum tetrafluorides, alkali metal aluminum tetrachlorides, alkali metal thiocyanates, alkali metal trifluoroacetates, M.sub.14 Zn(GeO.sub.4), and M-beta-alumina, where M represents an alkali metal, silver conductive materials, such as silver halides, and copper conductive materials, such as copper halides, and surfactants. Particularly preferred are sodium sulfate, potassium chloride, sodium hexafluorophosphate, lithium and sodium methanetrifluorosulfonate. Suitable concentrations for the supporting electrolyte are about 0.001M to about 0.2M.

Separator 10 between electrochromic half cell 20 and counter half cell 30 may be any cation conducting membrane which is insoluble in the ionically conducting polymers and supporting electrolytes on each side of it or other materials in the cell environment. Suitable cation conducting membranes for use as separators in cells of this invention include perfluorinated carboxylic and sulfonic cationic exchange copolymers (the Nafions), poly(ethylene oxide), poly(propylene oxide), and poly(phosphazenes). In addition the solid-state cell can be effectively operated using anionic polymeric conductors such as cross linked copolymers of vinyl monomers possessing quaternary ammonium anion exchange groups (Ionics 103-PZL-386). The separator must possess sufficient ionic conductivity to result in low internal resistance (IR) losses in the total cell and to permit migration of the conducting ions from one cell compartment to another.

The counter half cell 30 counter electrode 13 may be made of any suitable electrode material providing desired electrical conductance such as gold, silver, copper, carbon, aluminum, nickel and iron, or may be the same material as the display electrode comprising transparent plate 5 and display electrode layer 6. Ionically conducting polymer layer 11 with supporting electrolyte is between counter electrode 13 and separator 10. Electroactive chemical may be any suitable redox electroactive chemical known to the art and may be in association with the surface of counter electrode 13 as a discrete layer, or may be dissolved in the supporting electrolyte or dispersed in the supporting electrolyte. Suitable redox materials for facilitating transfer of electric charges to and from counter electrode 13 include Cr.sup.3+ /Cr.sup.2+, Cu.sup.2+ /Cu.sup.2+, Ce.sup.3+ /Ce.sup.4+ and Fe.sup.2+ /Fe.sup.3+. While inorganic redox couples are preferred, organic redox couples such as quinones are suitable. Redox materials should be present in concentrations of about 0.01M to about 0.2M.

One or both electrolyte layers 9 and 11 may contain a conductive powder additive such as tin oxide, a mixture of indium and tin oxides (ITO), titanium oxide coated with stannic oxide, and carbon. Powder particle sizes are generally in the range of about 0.1 to about 100 microns.

The following examples set forth specific materials and embodiments in detail and are intended to exemplify the invention only and not to limit it in any way.

A solid state electrochromic cell using heptyl viologen and solid poly-AMPS in the electrochromic half cell polymer electrolyte was prepared and voltammograms showed successful cyclic reversibility.

Heptyl viologen to form 0.04M in the associated water was dissolved into a 10 wt. percent solution of poly-AMPS containing 0.001M NaPF.sub.6 as supporting electrolyte. Solid-state cells were prepared by applying a thin film, approximately 2 microns, of the poly-AMPS/heptyl viologen/NaPF.sub.6 onto the surface of an SnO.sub.2 conducting glass substrate. In a similar manner, a counter half cell was prepared of poly-AMPS/0.04MCeCl.sub.3 /0.001M NaPF.sub.6 which was applied to the surface of another SnO.sub.2 conducting glass substrate. These half-cells were dried for one hour and each pressed to the opposite side of an Na.sup.+ exchanged Nafion 117 membrane. After drying for a further half hour, cyclic voltammetry measurements were conducted at a sweep rate of 50 mV/second with results as shown in FIG. 2. The electrochromic working electrode potentials were recorded as a function of the counter half cell. Upon subjecting the electrochromic half cell to an initial cathodic scan, two reduction peaks showing a change from colorless to violet was observed at -1.74 V and a darker violet was observed at -1.90 V versus the counter half cell. In an anodic scan two oxidation peaks were observed at -1.82 V and -1.0 V resulting in complete removal of the violet color. Further anodic scan past the initial open-circuit potential of about 100 mV showed an oxidation peak at 1.6 V, probably associated with the Ce.sup.+3 /Ce.sup.+4 redox couple in the counter half cell.

The electrochemical reversibility of the electrochromic effect noted in Example I was ascertained by monitoring, as a function of applied potential, changes in the relative absorption of an initially 1 mV He/Ne laser beam (583 nm) after being passed directly through a cell as described in Example I. The transmitted laser beam intensity was detected using a Hamamatsu PIN photodiode amplified by an RCA 3140 Op.Amp. FIG. 3 summarizes the electrochemical cycling characteristics of the electrochromic cell as a function of varying potentiostatted charge/discharge voltages: FIG. 3a .+-.1.0 V; FIG. 3b .+-.1.25 V; and FIG. 3c .+-.1.50 V. The cells demonstrated excellent reversibility in excess of a hundred cycles.

While in the foregoing specification this invention has been described in relation to certain preferred embodiment thereof, and many details have been set forth for purpose of illustration, it will be apparent to those skilled in the art that the invention is susceptible to additional embodiments and that certain of the details described herein can be varied considerably without departing from the basic principles of the invention.