Thin Film Ion Conducting Coating

Thin film transparent coatings for modulating optically active materials are currently available for providing "smart" windows whereby the reflective and transmissive properties of the windows can be varied by controlling the transport of electrons and ions into and out of the optically variable layer. U.S. patent application Ser. No. 642,956, filed Aug. 21, 1984 and entitled "Light Modulating Device" describes materials used to form such coatings.

Thin film materials are also known for providing electrical storage batteries having large areas for storing electric energy. The use of specific thin film electro-optically active materials for a solid state battery is described, for example, in a publication entitled "Electrochromic `Smart` Windows" by Ronald B. Goldner, which appeared in the IEEE Reflector Publication, Vol. 35, May, 1987.

For purposes of this disclosure, a thin film electro-optical device has a thickness in the order of several thousand Angstroms in contrast with so-called "thick film" solid state electronic circuits which are in excess of several microns. The aforementioned U.S. Patent Application, which is incorporated herein for purposes of reference, teaches a five layer device deposited on glass for controlling the reflective properties of one of the layers to selectively transmit optical energy through a transparent substrate. Although such thin film devices have been known for some time, commercial utilization of such thin film electro-optic devices has not heretofore been made economically feasible.

One of the modulating layers, the ion-conducting layer, that is used to transport ions into the electrochromic layer must exhibit and maintain two electrically opposing properties. The ion-conducting layer must readily transmit ions upon application of an electric field yet remain electrically insulative to electron transport. Such electrically insulative properties are found to vary over wide ranges of climatic conditions. Materials currently used to form the ion-conducting layer in electrochromic devices are slightly hydroscopic as well as water soluble. Increased moisture content in the air surrounding the electrochromic devices, besides causing variations in the insulative properties, can actually dissolve and hence destroy the coating unless the device is placed in a humidity controlled environment. The mechanical properties of such materials have been found to vary with increasing temperature. For temperatures in excess of 100.degree. C., these materials tend to crystallize, crack and thereby lose adherence to their immediate substrate.

For architectural applications as well as for use within the automotive, aircraft and military applications, rigid performance specifications are required with respect to mechanical strength, resistance to humidity and temperature stability. Temperature specifications for most commercial applications require good mechanical strength at temperatures in excess of 200.degree. C., for example.

One purpose of this invention therefore is to describe methods and materials for providing an electrically active layer having good ionic transport properties along with high electronic resistance, while retaining high physical integrity over wide ranges of temperature and humidity.

A transparent thin film ion-conductive and electronically resistive layer is formed by deposition of mixed oxides of lithium and tantalum or lithium and niobium on electro-optically active substrates for controlling the optical properties thereof. The same materials are used for providing the electrolyte layer in thin film storage batteries.

FIGS. 1A and 1B are cross-sectional views of a five layer electro-optic device depicting the electronically controlled optically reflective properties;

FIG. 2 is a graphic representation of the transmittance through the device depicted in FIGS. 1A and 1B, as a function of incident wavelength;

FIG. 3 is a side view of a three layer thin film storage battery according to the invention and

FIG. 4 is a side view of a four-layer thin film storage battery according to the invention.

A deposited five layer structure 18 on a glass substrate 20 is depicted in FIG. 1A wherein the layered structure 18 includes an ion-conducting layer 14. The layered structure is hereinafter referred to as a "smart window", whereby an electric field provided by means of a low voltage battery 16 and switch 17 is applied to the layered structure by means of indium or other metal electrodes 19 and wire conductors 21. As described in the aforementioned U.S. Patent Application, the polarity of the battery 16 governs the nature of the electric charge transporting between the first transparent electron conducting layer 11, counter-electrode layer 13, ion-conducting layer 14, electrochromic layer 15, and the second transparent conducting layer 12. When switch 17 in FIG. 1A is closed, the field across the deposited layered structure 18 conducts ions from the electrochromic layer 15 to the counter-electrode layer 13 through the ion-conducting layer 14, resulting in the "bleached"state depicted in FIG. 2 whereby the transmittance through the glass substrate 20 is a maximum. Representative light ray 26 represents wavelengths larger than 0.7 micron, defined as the "near-infrared" region of the optical spectrum. As described within the aforementioned Patent Application, the infrared region of the solar spectrum is that region whereby most of the usuable heat energy from the sun is transmitted.

The layered structure 18 is depicted in FIG. 1B with the polarity of battery 16 reversed with respect to that in FIG. 1A such that when switch 17 is depressed, a field is generated across the layered structure such that ions are transmitted from the counter-electrode layer into the electrochromic layer 15, changing the structure of the electrochromic layer to the metallic or so-called "colored" state depicted in FIG. 2. In this state a large portion of the infrared energy is reflected by the electrochromic layer 15, as indicated by the reflection of the representative light ray 26. The phenomenon of reflection by the polycrystalline tungsten oxide material (WO.sub.3) comprising the electrochromic layer 15 is described, for example, in the article entitled "Further Evidence for Free Electrons as Dominating the Behavior of Electrochromic Polycrystaline WO.sub.3 Films", which appeared in the Applied Physics Letters, 47 (5) Sept. 1, 1985, which paper is incorporated herein for purposes of reference.

To provide a durable, economically feasible electrochromic device, the following materials were selected for layers 11-15 in the layered structure depicted in the previous Figures. Layers 11 and 12 were formed from RF sputtered tin-doped indium oxide; layer 13 was formed from RF sputtered In.sub.2 O.sub.3 ; layer 14 was formed from RF sputtered LiO.sub.2 : Nb.sub.2 O.sub.5 ; and layer 15 was formed from RF sputtered WO.sub.3. Layers 11, 12, 13 and 15 were similar to those described in the aforementioned U.S. Patent Application. Layer 14, the ion-conducting or electrolyte layer, was formed by RF sputtering LiNbO.sub.3 to form a mixed phase oxide having a "glassy" configuration i.e., durable and transparent. Although it was earlier known that such mixed-oxide glasses exhibited high ionic conductivity and high electronic resistivity per se, as described within an article entitled "Ionic Conductivity of Quenched Alkali Niobate and Tantalate Glasses", which appeared in the Journal of Applied Physics 49 (9) September, 1978, such use was not heretofore considered as an ion conducting layer within a light modulating layered structure.

One of the difficulties involved in attempting to RF sputter a mixed phase oxide resulting in stoichiometric Li.sub.2 O:Nb.sub.2 lO.sub.5 is that, when stoichiometric quantities of LIO.sub.2 and Nb.sub.2 O.sub.5 are prepared for RF sputtering onto the electrochromic layer 15, the resulting mixed phase oxides of lithium and tantalium are less than stoichiometric and hence do not exhibit the optimum ionic conductivity and maximum electronic resistivity, as required within electrochromic layered structures. It has since been observed that when the mixed oxides of lithium and niobium are prepaared in excess of stoichiometric quantities, the resulting sputter-deposited mixed oxides become stoichiometric since some material is lost in the sputtering process. When electronic layered structures 18 are formed by means of a continuous RF sputtering process within an inert gas-oxygen atmosphere, durable water resistant layered structures are formed that also exhibit stability at temperatures in excess of 200.degree. C. to 300.degree. C.

When the "smart window" device 10 is operated in its colored state, as depicted in FIG. 1A, the switch 17 to battery 16 is closed for a sufficient time to cause the lithium ions to transport from the ion conductive-electron resistive layer 14 to the electrochromic layer 15 to bring the electrochromic layer to its metallic or colored state. The switch 17 is then opened and the electrochromic layer remains in its colored state until such time as the polarity is reversed and the lithium ions are removed from the electrochromic layer 15 to thereby oxidize the electrochromic layer to its bleached state. When the polarity of battery 16 is reversed, to that depicted in FIG. 1A, and switch 17 is closed, the charge stored within the electrochromic layer 15 returns to the battery by means of conductors 21 to restore the battery to its original charge, since very little energy is lost in ion transition to the electochromicc layer 15. Should a rectified voltage source, other than battery 16, be employed to charge the "smart window" device, the "smart window" itself then behaves like a storage battery and, depending upon the thickness of the electrochromic layer 15 and the total surface area of the "smart window" device, could store a considerable amount of electrical energy.

The explanation for the transport of lithium ions is believed due to the glassy structure of the LiO.sub.2 : Nb.sub.2 O.sub.5 mixtures. The lithium ion is capable of compensating the unsaturated oxygen bond and thereby becomes the mobile ion. The random nature of the glassy mixture readily provides an excess of available sites for the mobile lithium ions. A good description of the ion transport mechanism is found in a paper by H.L. Tuller and D.P. Button entitled "The Role of Structure in Fast Ion Conducting Glasses", presented in 1985 at the Proceedings of the Sixth International Symposium on Metallurgy and Material Science. This paper is incorporated herein for purposes of references and should be reviewed for a good description of the mechanism of the lithium ion transport.

To separately determine the electrical properties of an ion conducting layer of both Li.sub.2 O:Nb.sub.2 O.sub.5 and Li.sub.2 O: Ta.sub.2 O.sub.5, their mixed oxides were prepared by RF sputtering in a vacuum chamber under a controlled atmosphere including argon and oxygen having a total pressure of 5 to 15 militorr onto a glass substrate which is coated with WO.sub.3 land tin-doped In.sub.2 O.sub.3. To compensate for the loss of X moles of lithium during the deposition process, ##EQU1## were prepared.

Another method for compensating for the loss of lithium ions during deposition is to insert X moles of lithium into the coated glass substrate prior to deposition of the mixed oxies. The compensation process for evolving stoichiometric mixed oxides of Li.sub.2 O: Nb.sub.2 O.sub.5 land Li.sub.2 O:Ta.sub.2 O.sub.5 is an important part of this invention since resultant mixed oxide layers in stoichiometric composition were heretofore unattainable. The resulting thickness of the mixed oxide layers ranged from 140 to 800 Nanometers, the electronic resistivity measured in excess of 10.sup.10 Ohm-centimenters and the lithium ionic conductivity was in excess of 10.sup.-7 Ohms-centimeters.sup.-1. Other materials which are believed to form mixed oxides with Li.sub.2 O are B.sub.2 lO.sub.3, MgO, A1.sub.2 O.sub.3, SiO.sub.2, MnO, ZnO, Ga.sub.2 O.sub.3, WO.sub.3 l, MoO.sub.3, GeO.sub.2 , SrO, Y.sub.2 O.sub.3, BaO and ZrO.sub.2. The metal oxides which form mixed oxides with lithium are hereafter defined as "lithiated oxides".

Other transparent substrates, such as a flexible Mylar or Kapton, DuPont's tradename for polyimide plastic, could also be used in place of the glass. This flexible substrate containing the "smart window" layered structure would then allow plate glass windows in existing buildings to be converted to "smart windows" by adhering the flexible "smart window" layered plastic material either to the interior or the exterior surface of the plate glass, in situ.

A simplified three-layered battery device 25 is depicted inFIG. 3, wherein an electron conductive-ion conductive substrate 24 is coated with an ion conductive-electron resistive layer 23. An electron conductive-ion conductive layer 22 is then applied over the ion conductive-electron resistive layer 23. Electrical connection with layers 22, 24 is made by means of indium solder 19 and wire conductors 21. As described earlier, an external battery 16 and switch 17 provide the requisite voltage across the battery device whereby the polarity of the external battery determines the direction of ion transport into and out of the ion conductive-electron insulative layer 23. With the external battery replaced by a voltage generator, and with switch 17 in its closed position, charge is readily stored in the electron conductive-ion conductive layers 22, 24. The electron conductive-ion conductive layers comprise any of the lithiated oxides of manganese, chromium, iron, cobalt and nickel, whereas the ion conductive-electron resistive layer comprises mixed metal oxides of lithium oxide and tantalum, niobium or zirconium, similar to the ion conductive-electron resistive layers described earlier.

A four-layered battery device 27 is shown in FIG. 4, which is similar to the "smart window" device 10 of FIGS. 1A, 1B with the electrochromic layer 15 eliminated and with the substrate 28 made of an electronically conductive material, such as aluminum. A first storage electrode 29 of ion conductive-electron conductive material such as the lithiated oxides of manganese, chromium, iron, cobalt and nickel is deposited over the electronically conductive substrate. An ion conductive-electron resistive layer 30 is deposited over the first storage electrode and is made from the mixed-oxides described earlier for layer 14 of FIGS. 1A, 1B. A second storage electrode 31 of similar materials as the first storage electrode 29 is next deposited over the ion conductive-electron resistive layer 30. An electronically conductive layer 32 of tin-doped dindium oxide is deposited over the second electrode layer and an external battery 16 is connected with the electronically conductive substrate 28 and the electronically conductive layer 32 by means of switch 17, conductors 21 and indium solder 19. When the switch is closed, energy is stored in the first and second storage electrodes in the manner described for the three-layered device wherein the electronically conductive substrate 28 and the first storage electrode 29 provide the functions of layer 24 and wherein the electronically conductive layer 32 and the second storage electrode 31 provide the functions of layer 22.

It has thus been shown that electrochromic reflective devices, which are environmentally stable, can be provided by incorporating mixed oxides which include lithium oxide as one of the constituents. The stoichiometry of the resultant mixed oxide layers are insured by either excess lithium in the initial lithium oxide or by pre-depositing excess lithium onto the substrate. The stable electrochromic devices are useful for controlled optically reflective devices as well as storage batteries.