Electrochromic devices having protective interlayers

The incorporation of one or more protective layers between the electrochromic material and the polymeric electrolyte in an electrochromic device produces longer lasting, more uniform devices. The further placing of a protective layer between the electrolyte and the counterelectrode yields still better devices.

The invention relates to improvements in electro-optical devices which 
contain a layer of persistent electrochromic material on one electrode in 
contact with a specific family of polymeric electrolytes which is also in 
contact with a counterelectrode within the device. The improvements 
involve placing one or more protective layers between the electrolyte and 
the electrochromic material and, optionally, between the electrolyte and 
the counterelectrode. Electrochromic devices operate by passing an 
electric current between the electrodes, through the electrochromic 
material, to change the photoabsorptive characteristics of said material 
so as to darken or lighten it. Such devices are provided with means both 
for applying the electric field to the device and for reversing the 
electric field. 
A variety of such devices having numerous uses have been described in the 
prior art. U.S. Pat. No. 3,708,220, for example, describes electrochromic 
devices in which a particular gelled electrolyte solution is used. 
U.S. Pat. No. 3,521,941, discloses the 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. 
U.S. Pat. No. 3,971,624 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. 
The present invention incorporates one or more protective layers between 
the electrochromic material and the polymeric electrolyte. The protective 
layers are either (1) an ion permeable insulator or (2) a 
non-electrochromic version of the electrochromic material plus the 
insulator with the insulator being in intimate contact with the 
electrolyte. Further improvement results from putting a second protective 
ion permeable insulator layer on the other side of the electrolyte. When 
the counterelectrode is gold still further improvement results from 
putting a thin palladium layer atop the second insulator layer and 
adjacent to the counterelectrode. 
The incorporation of the protective layer or layers yields an 
electrochromic device which is more stable than previous devices having 
polymeric electrolytes. 
It is thus an object of this invention to produce a device which has a 
longer life than was possible by prior devices.

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 wave 
length region, to a second persistent state in which it is absorptive of 
electromagnetic radiation in the given wave length 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, after 
removal of the electric field, in the absorptive state to which it is 
changed, as distinguished from a substantially instantaneous reversion to 
the initial state, as in the case of the Franz-Keldysh effect. 
The materials which form the electrochromic materials of the device in 
general are electrical insulators or semiconductors. Thus are excluded 
those metals, metal alloys, and other metal-containing compounds which are 
relatively good electrical conductors. 
The persistent electrochromic materials are further characterized as 
inorganic substances which are solid under the conditions of use, whether 
as pure elements, alloys, or chemical compounds, containing at least one 
element of variable oxidation state, that is, at least one element of the 
Periodic System which can exist in more than one oxidation state in 
addition to zero. The term "oxidation state" as employed herein is defined 
in "Inorganic Chemistry", T. Moeller, John Wiley & Sons, Inc., New York, 
1952. 
These include materials containing a transition metal element (including 
Lanthanide and Actinide series elements), and materials containing 
non-alkali metal elements such as copper. Preferred materials of this 
class are films of transition metal compounds in which the transition 
metal may exist in any oxidation state from +2 to +8. Examples of these 
are: transition metal oxides, transition metal oxysulfides, transition 
metal halides, selenides, tellurides, chromates, molybdates, tungstates, 
vanadates, niobates, tantalates, titanates, stannates, and the like. 
Particularly preferred are films of metal stannates, oxides and sulfides 
of the metals of Groups (IV)B, (V)B and (VI)B of the Periodic System, and 
Lanthanide series metal oxides and sulfides. Examples of such are copper 
stannate, tungsten oxide, cerium oxide, cobalt tungstate, metal 
molybdates, metal titanates, metal niobates, and the like. 
Additional examples of such compounds are as disclosed in U.S. Pat. No. 
3,521,941 incorporated herein by reference. 
While the exact mechanism of persistent electrochromism is unknown, the 
coloration is observed to occur at the negatively charged electrochromic 
layer. Generally, the phenomenon of persistent electrochromism is believed 
to involve transport of cations such as hydrogen or lithium ions to the 
negative electrode where color centers form in the electrochromic image 
layer as a result of charge compensating electron flow. 
When the persistent electrochromic materials are employed as films, 
thickness desirably will be in the range of from about 0.1-100 microns. 
However, since a small potential will provide an enormous field strength 
across very thin films, the latter, i.e., 0.1-10 microns, are preferred 
over thicker ones. Optimum thickness will also be determined by the nature 
of the particular compound being laid down as a film and by the 
film-forming method, since the particular compound and film-forming method 
may place physical (e.g., non-uniform film surface) and economic 
limitations on manufacture of the devices. 
When tungsten oxide is employed as the electrochromic imaging material and 
an electric field is applied between the electrodes, a blue coloration of 
the previously colorless electrochromic layer occurs, i.e., the persistent 
electrochromic layer becomes absorptive of electromagnetic radiation over 
a band initially encompassing the red end of the visible spectrum, thereby 
rendering the imaging layer blue in appearance. Prior to the application 
of the electric field, the electrochromic imaging layer is essentially 
non-absorbent and thus colorless. 
The electrodes used herein may be any material which, relative to the 
electrochromic film, is electrically conducting. These electrically 
conductive materials are generally coated on a suitable substrate material 
such as glass, wood, paper, plastics, plaster and the like, including 
transparent, translucent, opaque or other optical quality materials. At 
least one of the electrode-substrate combinations is transparent, though 
both may be. 
Suitable polymers for use as the electrolytes herein are those of U.S. Pat. 
No. 3,521,941 as well as any other electrically conducting polymers. 
Preferably the polymers are polymers and copolymers containing acidic or 
basic groups or salts thereof. These groups are generally covalently 
bonded to the polymer chain. Most preferably the polymers with acidic or 
basic groups are soluble. 
The acid type polymers exchange cations while the basic polymers exchange 
anions. The main groups of cation exchangers of the strong acid type are 
--SO.sub.3 H and --PO.sub.3 H.sub.2, while those of the weak type are 
--COOH. An example of the strong basic type is --CH.sub.2 
N(CH.sub.3).sub.3 OH and an example of the weak basic type is &gt;NH.sub.2 
OH. Among these four types, sulfonic acid and quarternary ammonium 
hydroxide contain strongly ionized functions and, consequently, have high 
ionic conductivity resulting from migration of H.sup.+ or OH.sup.- ions. 
The nature of the ionic group greatly affects the ionic conductivity of 
the ion exchange polymers. The most conductive polymers are those in which 
the mobile ion is a proton. The functional group --SO.sub.3 H should 
consequently be preferred to its salts, such as --SO.sub.3 Na, or to weak 
acids, such as --COOH. The extent of sulfonation will also have an effect 
on the ionic conductivity of the polymer. 
Examples of polymeric electrolytes include such as: polystyrene sulfonic 
acid, polyethylene sulfonic acid, and perfluorated sulfonic acid 
(Nafion.RTM.). 
The polymeric electrolytes may be incorporated into the electrochromic 
device by dissolving the polymer in a suitable solvent, depositing the 
polymer on the layer below in the device, and evaporating the solvent to 
produce a solid film of polymeric electrolyte. The electrolyte preferably 
has a thickness of about 10,000 A to 100,000 A or more, the optimum level 
varying with the type of polymer, the number and type of the various 
protective layers, as well as the use to which the device is put. A more 
complete description of depositing the polymer is disclosed and claimed in 
copending U.S. Ser. No. 841,630 filed Oct. 13, 1977, of Robert D. Giglia, 
incorporated herein by reference. 
The protective layer or layers, when they are insulators, are used in 
intimate contact with the polymeric electrolyte and are believed to 
provide an electronic insulation of the polymeric electrolyte while still 
maintaining ionic conduction or permeability from and between the adjacent 
electronically conductive layers of the device. Suitable materials for the 
insulating layers, which may be the same or different though preferably 
the same, include silicon oxide, calcium fluoride, and magnesium fluoride. 
Also included are other metal oxides or sulfides prepared by oxidizing or 
sulfiding a metal surface so that the insulator is formed directly in the 
device. Examples include the above materials as well as aluminum oxide and 
other inorganic insulators, such as selenide, arsenide, nitride, chloride, 
fluoride, bromide, and carbide materials. 
The insulator layers must be thick enough to offer the requisite electronic 
insulation, but not so thick as to impair the ionic permeability and/or 
conduction. Generally, thicknesses of about 100 to 1500 Angstroms are 
usable. The preferred thickness varies depending upon the actual insulator 
used. For silicon oxide, the preferred thickness is about 350-450 
Angstroms; for magnesium fluoride, about 200-300 Angstroms. As the 
thickness increases above the preferred ranges, the speed of switching is 
reduced if the driving voltage is held constant. 
When only one insulator protective layer is used, it should be placed 
between the polymeric electrolyte and the electrochromic film layer to 
minimize unwanted reactions between the polymer and the film. 
When the protective layer, between the electrochromic material and the 
electrolyte further contains a non-electrochromic layer of the same 
material as the electrochromic material it is put in intimate contact with 
the electrochromic layer. The material is deemed non-electrochromic in 
that it does not color under the influence of an electric field in an 
essentially dry (less than about 5% water) device under normal 
electrochromic operating voltages. The non-electrochromic layer is 
essentially a more highly oxidized version of the same material which 
serves as the electrochromic material. It may be prepared by vacuum 
depositing the material at a slower rate than the electrochromic layer and 
in an oxidizing atmosphere, as opposed to a reducing or neutral 
atmosphere. While any of the materials which may be used for the 
electrochromic layer may also be used for the non-electrochromic layer, 
preferably tungstic oxide is used for both. 
When a gold counterelectrode is used in the present devices which have 
insulating layers on both sides of the polymeric electrolyte, the 
incorporation of a very thin "nucleating" layer between the insulator 
layer and the gold counterelectrode results in a still more improved 
device. Suitable nucleating layer materials include palladium, platinum, 
and rhodium. Preferably, palladium is used due to its protonic conduction 
characteristics. 
The devices of the present invention may be conveniently built by 
depositing one layer upon the other until the desired structure is 
created. 
The following specific examples are given to illustrate the invention 
further and to show specific embodiments and modes of practice of the 
invention and are not intended to be limitative. 
EXAMPLE 1 
An electrochromic device was prepared as follows: 
A 500 Angstrom layer of electrochromic tungstic oxide was deposited on a 
sheet of indium oxide conductive glass. Atop the electrochromic layer, 400 
Angstroms of silicon oxide (almost completely SiO.sub.2) was deposited by 
thermal evaporation in an O.sub.2 environment. Then 0.07 gms polystyrene 
sulfonic acid (PSSA) polymer was dissolved in 1.0 ml of methanol so that 
the polymeric electrolyte could be incorporated in the device. 25,000 
Angstroms of PSSA was deposited by a spin coating technique. A 120 
Angstrom layer of gold was deposited upon the dry polymer layer and used 
as the counter-electrode. 
A second device was prepared in the same manner as the first except 
omitting the silicon oxide layer. 
The two devices were tested by alternately coloring and clearing the 
devices at DC potentials of about 2 volts to color and 1 volt to clear. A 
15 second coloration time produced about 20% transmission of light over a 
14 cm. area. Upon reversing the potential for 15 seconds the transmission 
increased to 55%. A cycle of switching includes one coloring period and 
one clearing period. 
The two devices gave the following comparative results: 
______________________________________ 
Silicon Oxide 
No Silicon Oxide 
______________________________________ 
Cycle Life 240.sup.1 10.sup.2 
______________________________________ 
.sup.1 Failure due to development of residual absorption in clear state. 
.sup.2 Failure due to excessive erase charge necessary to return device t 
clear state. 
EXAMPLE 2 
The procedure for the preparation of the first device of Example 1 was 
repeated except that a second 400 Angstrom layer of silicon oxide was 
deposited atop the polymeric electrolyte before the counterelectrode was 
incorporated. 
The device was tested as in Example 1 with the following results: 
______________________________________ 
Two Silicon Oxide Layers 
______________________________________ 
Cycle Life 
5,000 cycles.sup.3 
______________________________________ 
.sup.3 Failure due to loss of conductivity in counterelectrode. 
EXAMPLE 3 
The procedure of Example 2 was repeated except that a non-electrochromic 
layer of tungstic oxide was incorporated between the electrochromic layer 
and the first silicon oxide layer. The non-electrochromic layer was 
deposited at 1.7.times.10.sup.-4 torr, using an O.sub.2 bleed, at a rate 
of 3 A/sec. to a thickness of about 650 A. 
The device was tested as in Example 1 and gave the following results: 
______________________________________ 
Non-Electrochromic Layer plus 
Two Silicon Oxide Layers 
______________________________________ 
Cycle life 7,000 cycles.sup.4 
______________________________________ 
.sup.4 Failure due to loss of conductivity in counterelectrode. 
EXAMPLE 4 
The procedure of Example 1 was repeated except that magnesium fluoride (250 
A) was used in place of the silicon oxide. 
______________________________________ 
Cycle life 200 cycles.sup.5 
______________________________________ 
.sup.5 Failure due to development of residual absorption in clear state.