U.S. patent number RE34,469 [Application Number 07/615,037] was granted by the patent office on 1993-12-07 for solid state electrochromic light modulator.
This patent grant is currently assigned to EIC Laboratories, Inc.. Invention is credited to Stuart F. Cogan, R. David Rauh.
United States Patent |
RE34,469 |
Cogan , et al. |
December 7, 1993 |
**Please see images for:
( Certificate of Correction ) ** |
Solid state electrochromic light modulator
Abstract
An all solid-state variable transmission electrochromic device
has a source of charge compensating ions. An inorganic oxide
counterelectrode film which on reduction with the accompanying
insertion of the charge compensating ions increases its
transmission of light of predetermined wavelength is separated from
a primary electrochromic film which on reduction with the
accompanying insertion of the charge compensating ions decreases
its transmission of light of predetermined wavelength by an
insulating electrolyte film that transports the charge compensating
ions. First and second electrodes are contiguous with the inorganic
oxide counter electrode film and the primary electrochromic film,
respectively, and separated by the three films.
Inventors: |
Cogan; Stuart F. (Sudbury,
MA), Rauh; R. David (Newton, MA) |
Assignee: |
EIC Laboratories, Inc.
(Norwood, MA)
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Family
ID: |
27370566 |
Appl.
No.: |
07/615,037 |
Filed: |
November 19, 1990 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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64069 |
Jun 18, 1987 |
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Reissue of: |
207496 |
Jun 16, 1988 |
04938571 |
Jul 3, 1990 |
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Current U.S.
Class: |
359/269; 359/267;
359/275 |
Current CPC
Class: |
G02F
1/1524 (20190101); G02F 2001/1555 (20130101) |
Current International
Class: |
G02F
1/15 (20060101); G02F 1/01 (20060101); G02F
1/155 (20060101); G02F 001/17 () |
Field of
Search: |
;350/357
;359/269,267,275 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0005300 |
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Jan 1977 |
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JP |
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8105126 |
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Jun 1983 |
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JP |
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59-159134 |
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Sep 1984 |
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JP |
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1240225 |
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Oct 1986 |
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JP |
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Primary Examiner: Hille; Rolf
Assistant Examiner: Saadat; Mahshid
Attorney, Agent or Firm: Fish & Richardson
Government Interests
This invention was made with Government support under Contract No.
DE-AC03-87SF16733 awarded by the Department of Energy. The
Government has certain rights to this invention.
Parent Case Text
This application is a .[.continuing application.].
.Iadd.continuation-in-part .Iaddend.of U.S. application Ser. No.
64,069 filed Jun. 18, 1987, of Stuart F. Cogan entitled Light
Modulating Device .Iadd.now abandoned. .Iaddend.
Claims
What is claimed is:
1. A solid-state variable transmission electrochromic device
comprising,
a source of charge compensating ions,
an inorganic oxide electrochromic counter electrode film composed
of a mixture of at least two oxides with a first of said oxides an
oxide of a metal from the group consisting of vanadium and chromium
and a second of said oxides an oxide of a different metal from the
group consisting of V, Cr, Nb, Ta and Ti which on reduction with
the accompanying insertion of said charge compensating ions
increases its transmission of light of predetermined
wavelength,
a primary electrochromic film which on reduction with the
accompanying insertion of said charge compensating ions decreases
its transmission of light of said predetermined wavelength,
an insulating electrolyte film contiguous with and separating said
inorganic oxide counter electrode film and said primary
electrochromic film for the transport of said charge compensating
ions therebetween,
first and second electrodes contiguous with said inorganic oxide
counter electrode film and said primary electrochromic film
respectively and separated by said inorganic oxide counter
electrode film, said insulating electrolyte film and said primary
electrochromic film,
said first and second electrodes being for receiving an electric
potential therebetween for producing a current flow such that
electrons flow into one of said electrodes and out of the other and
said charge compensating ions flow through said insulating
electrolyte film from that one of said inorganic oxide counter
electrode and said primary electrochromic film being oxidized to
that one thereof being reduced for modulating said device between
states of minimum and maximum transmission at said predetermined
wavelength with the direction of transmission change being
determined by the direction of current flow.
2. The device of claim 1 wherein one of said electrodes is a thin
film transmissive at said predetermined wavelength and the other of
said electrodes is reflective at said predetermined wavelength,
wherein the reflectivity of said device may be modulated between a
state of maximum reflectivity and a state of minimum reflectivity
by controlling the absorption of said device with a potential
applied between said first and second electrodes.
3. The device of claim 1 wherein said first and second electrodes
are thin films transmissive at said predetermined wavelength,
wherein the transmittance of said device may be controlled between
a state of maximum transmittance and a state of minimum
transmittance in response to a potential applied between said first
and second electrodes.
4. A device in accordance with claim 1 wherein said primary
electrochromic film is tungsten trioxide,
said inorganic oxide counter electrode film is a mixture (V.sub.2
O.sub.5).sub.1-x (Nb.sub.2 O.sub.5).sub.x (x.dbd.0.01-0.99)
prereduced with Li,
said insulating electrolyte film is sputtered from a composite
target of 42% Li.sub.2 O, 25% SiO.sub.2 and 32% ZrO.sub.2,
said charge compensating ions are lithium,
said first electrode is reflective aluminum, (X.dbd.
and said second electrode is transparent and tin-doped indium oxide
on a substrate.
5. A device in accordance with claim 2 wherein said primary
electrochromic film is tungsten trioxide,
said inorganic oxide counter electrode film is a mixture
.[.(V.sub.2 O.sub.5)1-x(Nb.sub.2 O.sub.5)x(x.dbd.0.01-0.99).].
(.Iadd.V.sub.2 O.sub.5).sub.1-x (Nb.sub.2 O.sub.5).sub.x
(x.dbd.0.01-0.99) .Iaddend.prereduced with Li,
said insulating electrolyte film is sputtered from a composite
target of 42% Li.sub.2 O, 26% SiO.sub.2 and 32% ZrO.sub.2,
said charge compensating ions are lithium,
said first electrode is reflective aluminum,
and said second electrode is transparent and tin-doped indium oxide
on a substrate.
6. A device in accordance with claim 1 wherein said primary
electrochromic film is tungsten trioxide.
said inorganic oxide counter electrode film is a mixture (V.sub.2
O.sub.5).sub.1-x (Nb.sub.2 O.sub.5).sub.x, X.dbd.0.01-0.99)
prereduced with Li,
said insulating electrolyte film is sputtered from a composite
target of 42% Li.sub.2 O, 26% SiO.sub.2 and 32% ZrO.sub.2,
said charge compensating ions are lithium,
said first electrode is reflective aluminum,
and said second electrode is transparent and tin-doped indium oxide
on a substrate,
wherein the reflectance of said device may be controlled between a
state of maximum transmittance and a state of minimum transmittance
in response to a potential applied between said first and second
electrodes.
7. A device in accordance with claim 1 wherein said first electrode
comprises tin-doped indium oxide.
8. A device in accordance with claim 1 wherein said primary
electrochromic film is tungsten trioxide,
said counter electrode is a mixture .[.(V.sub.2
O.sub.5)1-x(Nb.sub.2 O.sub.5) x (x.dbd.0.01-0.99).]. (.Iadd.V.sub.2
O.sub.5).sub.1-x (Nb.sub.2 O.sub.5).sub.x (x.dbd.0.01-0.99)
.Iaddend.prereduced with Li,
said insulating electrolyte film is poly
(bismethoxyethoxymethoxide) phosphazine doped with LiCF.sub.3
SO.sub.3,
and said first and second electrodes are tin-doped indium oxide
deposited upon glass.
9. A device in accordance with claim .[.1.]. .Iadd.3
.Iaddend.wherein said primary electrochromic film is tungsten
trioxide,
said counter electrode is a mixture (V.sub.2 O.sub.5).sub.1-x
(Nb.sub.2 O.sub.5).sub.x (x.dbd.0.01-0.99) prereduced with Li,
said insulating electrolyte film is poly(bismethoxyethoxymethoxide)
phosphazine doped with LiCF.sub.3 SO.sub.3,
and said first and second electrodes are tin-doped indium oxide
deposited upon glass,
.[.wherein one of said electrodes is a thin film transmissive at
said predetermined wavelength and the other of said electrodes is
reflective at said predetermined wavelength,.].
wherein the .[.reflectivity.]. .Iadd.transmissivity .Iaddend.of
said device may be modulated between a state of maximum
.[.reflectivity.]. .Iadd.transmissivity .Iaddend.and a state of
minimum .[.reflectivity.]. .Iadd.transmissivity .Iaddend.by
controlling the .[.adsorption.]. .Iadd.absorption .Iaddend.of said
device with a potential applied between said first and second
electrodes.
10. A device in accordance with claim 3 wherein said primary
electrochromic film is tungsten trioxide,
said counter electrode is a mixture .[.(V.sub.2).sub.5)1-x(Nb.sub.2
O.sub.5).sub.x (x.dbd.0.01-0.99).]. (.Iadd.V.sub.2 O.sub.5).sub.1-x
(Nb.sub.2 O.sub.5).sub.x (x.dbd.0.01-0.99) .Iaddend.prereduced with
Li,
said insulating electrolyte film is poly
(bismethoxyethoxymethoxide) phosphazine doped with LiCF.sub.3
SO.sub.3,
and said first and second electrodes are tin-doped indium oxide
deposited upon glass.
11. A device in accordance with claim 1 wherein said primary
electrochromic film is tungsten trioxide,
said counter electrode is a mixture .[.(V.sub.2
O.sub.5)1-x(Nb.sub.2 O.sub.5)x (x.dbd.0.01-0.99).]. (.Iadd.V.sub.2
O.sub.5).sub.1-x (Nb.sub.2 O.sub.5).sub.x (x.dbd.0.01-0.99)
.Iaddend.prereduced with Li,
said insulating electrolyte film is poly
(bismethoxyethoxymethoxide) phosphazine doped with LiCF.sub.3
SO.sub.3,
and one of said electrodes is tin-doped indium oxide deposited onto
glass and the other of said electrodes is a reflective aluminum
film on glass.
12. A device in accordance with claim 2 wherein said primary
electrochromic film is tungsten trioxide,
said counter electrode is a mixture .[.(V.sub.2
O.sub.5)1-x(Nb.sub.2 O.sub.5)x (x.dbd.0.01-0.99).]. (.Iadd.V.sub.2
O.sub.5).sub.1-x (Nb.sub.2 O.sub.5).sub.x (x.dbd.0.01-0.99)
.Iaddend.prereduced with Li,
said insulating electrolyte film is poly
(bismethoxyethoxymethoxide) phosphazine doped with LiCF.sub.3
SO.sub.3,
and one of said electrodes is tin-doped indium oxide deposited onto
glass and the other of said electrodes is a reflective aluminum
film on glass.
13. A device in accordance with claim 1 wherein said primary
electrochromic film is tungsten trioxide,
said counter electrode is a mixture (V.sub.2 O.sub.5).sub.1-x
(Nb.sub.2 O.sub.5).sub.x (x.dbd.0.01-0.99) prereduced with Li,
said insulating electrolyte film is poly(bismethoxyethoxymethoxide)
phosphazine doped with LiCF.sub.3 SO.sub.3,
and one of said electrodes is tin-doped indium oxide, deposited
onto glass .[.and the other of said electrodes is a reflective
aluminum film on glass,.].
wherein the reflectance of said device may be controlled between a
state of maximum transmittance and a state of minimum transmittance
in response to a potential applied between said first and second
electrodes. .Iadd.14. A device in accordance with claim 1 wherein
said primary electrochromic film is tungsten trioxide,
said inorganic oxide counter electrode film is a mixture
(V.sub.2 O.sub.5).sub.1-x (Nb.sub.2 O.sub.5).sub.x
(x.dbd.0.01-0.99) prereduced with Li,
said insulating electrolyte film is sputtered from a composite
target of 42% Li.sub.2 O, 26% SiO.sub.2 and 32% ZrO.sub.2,
said charge compensating ions are lithium
said first electrode is reflective aluminum,
said second electrode is transparent tin-doped indium oxide,
one of said first and second electrodes being on a
substrate..Iaddend.
Description
This invention relates to electrochromic devices for modulation of
light transmission.
Electrochromic materials are materials whose optical properties can
be reversibly altered in response to an applied potential in a
process involving the simultaneous insertion or extraction of
electrons and charge compensating ions. These materials have been
used, e.g., in display devices, variable reflectance mirrors, and
in windows for controlling light transmission.
Prior art information displays comprising electrochromic devices
typically hide the counter electrode, if one is designated, behind
a light scattering material dispersed in the electrolyte. Thus, the
color/bleach processes of the counter electrode do not enter into
the optical display effect. Conversely, the materials and
structures for variable transmittance devices depend on the optical
properties of all the layers, including both the primary and
counter electrode layers, as well as their complementary
color/bleach behaviors during oxidation and reduction. Effective
optical transmission modulation typically requires a wide dynamic
range, which in turn depends on the extent to which both electrodes
taken together can be bleached to achieve a high degree of
transmissivity and colored to achieve a high optical density.
In a typical prior art variable transmission electrochromic device
the counter electrode reaction is not defined, and judging from the
conditions of operation involving large polarizations, seem to
involve the decompositions of electrolyte, which would hinder the
long term stability and color/bleach reproducibility. In other
prior art examples, devices utilize a weakly coloring counter
electrode, which still compromises the bleached state
transmittance. Several examples of prior art devices utilizing
WO.sub.3 in conjunction with oxidatively coloring IrO.sub.2 have
been described, but these devices are uneconomical for most
applications.
Devices have been described in which both electrodes are initially
present in their fully oxidized state. The initial coloration
therefore typically involves undesirable electrolyte oxidation to
depolarize the electrochemical cell. If the electrolyte contains
water, then this reaction would give rise to oxygen bubble
formation, disrupting the thin film structures. Some structures
have been put forth utilizing liquid electrolytes, but these
structures would provide formidable sealing problems for large area
architectural applications. U.S. Pat. No. 4,278,329 discloses
prereduction of the primary electrochomic layer in liquid
electrolytes.
Prior art devices in which the counter electrode is an
electrochromic organic polymer, such as polyaniline, generally
suffer from sensitivity to irreversible oxidation processes over
long term operation. In one example of prior art (U.S. Pat. No.
4,294,520), an electrochromic display device is disclosed which
comprises a vapor deposited layer of an electrochromic material and
a contiguous layer comprising chromium (III) oxide and the oxide of
a transition metal or silicon, sandwiched between a pair of
electrodes at least one of which is light transmitting. Such a
device is deficient in relying on electrolysis of water of
hydration in the layers to produce the hydrogen inserted form of
one of the layers in the first polyarization cycle. Furthermore,
this dehydration continues over subsequent cycles if the voltage is
not carefully controlled to avoid water electrolysis, resulting in
eventual deceleration of device response, or necessitating
increased polarizations, which exacerbates the problem. In
addition, this device has no well-defined electrolyte layer. Hence
some current occurs as direct electronic current through the
structure, making it difficult to control the state of coloration
by the applied voltage or duration of current flow. Such electronic
current creates an internal short circuit of the device, causing
the coloration to fade under open circuit.
Accordingly, an object of the present invention is a variable
transmission electrochromic device with high dynamic range, open
circuit memory and longevity, such advantages being realized by
counter electrode materials with low residual reduced state
coloration and with a long cycle life.
Another object of this invention is a device with controllable,
reproducible optical modulation and dynamic range, such features
rendered possible by preparing one or both electrodes with a known
state of reduction during device assembly.
According to the invention there is a composite whose transmittance
may be varied in response to an applied electrical potential. The
composite includes a variably transmissive electrochromic layer
that is normally colorless, but when reduced by the insertion of an
electron and charge compensating ion becomes colored by absorption,
reflectance or a combination of both. The composite also includes a
second electrochromic layer, a counter electrode, which is colored
when oxidized and colorless when reduced, thus forming a complement
with the first electrochromic layer. The oxidation and reduction of
the counter electrode occurs by electron injection and insertion of
the same charge compensating ion as the first electrochromic layer.
The charge compensating ions are transported by an ion conducting
but electron blocking layer, such as an electrolyte, separating the
two electrochromic layers.
The transmittance of the composite is at a maximum when the first
electrochromic layer is fully oxidized and the counter electrode is
fully reduced, while it is at a minimum when the first
electrochromic layer is fully reduced and the counter electrode is
fully oxidized. The transmittance is changed by applying a voltage
across the composite such that a current flows changing the
oxidation state of the two electrochromic layers.
In preferred embodiments, both the primary electrochromic element
and the counter electrode are in the form of thin films. The
primary electrochromic element is tungsten trioxide, which becomes
either blue (light absorbing) or blue-bronze (light
absorbing/reflecting) on reduction with compensation ion insertion,
depending on its degree of crystallinity. The preferred insertion
ion is Li.sup.+, due to its relatively high mobility in tungsten
oxide. Li.sup.+ is preferred over H.sup.+ because H.sup.+ can also
participate in a side reaction forming H.sub.2 gas, which is
undesirable.
The ion conducting element must transport insertion ions, such as
Li.sup.+, and must be transparent over the optical range being
modulated. Examples of suitable ion conducting materials are
LiNbO.sub.3 and mixtures of poly-N-vinyl pyrrolidone and
polyethylene glycol doped with LiClO.sub.4.
The counter electrode material has the general formula A.sub.x
(MO), where MO is a mixture of vanadium oxide or chromium oxide
together or with oxides of one of the following metals: Nb, Ti or
Ta; and A is an atom identical to the insertion ion in the primary
electrochromic layer and the transported ion in the electrolyte.
The mixed oxides have superior reduced state visible light
transmission to V.sub.2 O.sub.5 and may be oxidized and reduced
with Li.sup.+ insertion many times, reversibly, without loss of
activity or without change in optical properties. Copending
application Ser. No. 64,069 filed 18th June of 1987, entitled LIGHT
MODULATING DEVICE discloses V.sub.2 O.sub.5.
The electrical potential and electron current flow are supplied
through a pair of electrodes. Where the device is used for
transmittance modulation, both electrodes are transparent over the
optical range being modulated. Tin-doped indium oxide (ITO) is a
suitable transparent thin film electrode material. Where the device
is used for reflectivity modulation, one electrode is transparent
while the other is reflective over the optical range being
modulated. Examples of reflective materials include aluminum,
nickel, or gold.
A general equation representing the operation of the invention in
which WO.sub.3 is the primary electrochromic layer can be written
as: ##STR1##
Here, b denotes that the two electrode materials need not be
present in any special molar ratio. The degree to which the counter
electrode MO is reduced in the bleached state is given by the
stoichiometric parameter x. Coloration is accomplished by the
transfer of A from MO to WO.sub.3. Thus, assuming that MO is the
limiting electrode and that therefore sufficient WO.sub.3 is
present to accept all of the available A, the dynamic optical range
of the device may be determined by the amount of A present.
The inserting species, A, must be introduced during fabrication.
This can be accomplished by several methods, including direct vapor
deposition of A.sub.x (MO), reduction of MO by elemental A in a
separate steps, or electrochemical reduction of the MO layer in the
electrolytic solution of A.sup.+. Alternatively, A may be
introduced initially into the primary electrochromic layer
(WO.sub.3 in the above equation) using similar methods; or A may be
distributed between both electrodes by the same means, the
resulting as-fabricated structure being in some intermediate state
of coloration.
Dynamic range of the device will degrade if some irreversible
process consumes A during device operation, such as H.sub.2 gas
formation if A=H, or a phase transformation in one of the electrode
materials. The reversibility of H and Li insertion/extraction in
WO.sub.3 is wellknown. The preferred counter electrodes, MO, in
this invention also have high reversibility, so that the device may
be cycled many thousands of times without optical degradation.
When the device is colored on the first cycle, the A species may be
derived from an irreversible electrochemical decomposition of the
electrolyte, if it was not introduced initially into the counter
electrode. Typically this A-species is hydrogen, produced by
electrolysis of water comprising or trapped in the electrolyte.
Controlling the quantity of A in the device facilitates fabricating
the device with a reproducible and knowable dynamic range.
Other features, objects and advantages of the invention will be
apparent from the following detailed description and appended
claims when read in connection with the accompanying drawings, in
which:
FIG. 1 is a fragmentary cross-section view of an electrochromic
device embodying the invention incorporating a polymeric or
semi-solid gel electrolyte;
FIG. 2 is a fragmentary cross-sectional view of another embodiment
of the device with a solid electrolyte and all thin film
construction;
FIG. 3 shows a light transmission spectrum for the two extreme
optical states of one embodiment of the device; and
FIG. 4 shows the electrochromic coloration efficiency vs.
wavelength of two suitable counter electrode materials and of
tungsten oxide.
With reference now to the drawings and more particularly FIG. 1,
there is shown a device 10 for modulating transmittance or
reflectivity, and which employs a polymer electrolyte to impart a
"safety glass" configuration. Device 10 has a pair of electrodes 12
and 12' connected to a conventional dc current source (not shown),
each deposited onto a transparent substrate, 14 and 14'. Substrate
14 is typically glass, but may also be a flexible plastic film
(e.g., mylar), or a rigid plastic sheet (e.g., polyacrylic). Where
device 10 is used for infrared optical modulation, suitable
infrared-transparent materials for substrate 14 include ZnSe, ZnS,
Si, Ge, glasses of metal fluorides, and sheets of organic polymers
sufficiently thin to have a low absorption cross section over the
integrated infrared spectrum.
Electrodes 12 and 12' are both transparent over the optical range
being modulated when transmittance modulation is desired. Examples
of suitable electrically conductive, optically transparent
materials include ITO, fluoride-doped tin oxide, and doped ZnO.
When reflectivity modulation is desired, one of the electrodes may
be made optically reflective over the desired wavelength range, and
is typically a metal such as aluminum, stainless steel, nickel,
gold or silver. Preferred transparent and reflective electrode
materials are ITO and Ni, respectively. The thicknesses of both
electrodes 12 and 12' are preferably within the range 0.05-1.0
.mu.m, preferably 0.2 .mu.m. They are deposited as thin films using
conventional vacuum deposition techniques, e.g., sputtering.
Layers 16 and 18 are deposited onto layers 12 and 12' also as thin
films, preferably by sputtering. Layer 16 is the counter electrode
and layer 18 is the primary electrochromic element. Layer 18 is an
electrochromic material which in its fully oxidized state is
transmissive over the modulated wavelength range and which becomes
absorbing and/or reflecting over the same range when reduced.
Examples of suitable materials for layer 18 include WO.sub.3,
MoO.sub.3, and mixtures thereof. The preferred material is
WO.sub.3. Layer 16 is a mixed oxide of a material such as vanadium
or chromium, either together or with an oxide of Nb, Ta, or Ti. The
mixed oxides of layer 16 preferably have the following
properties:
1. Like the primary electrochromic layer 18, they can be reversibly
reduced electrochemically by adding electrons and charge
compensating ions (e.g., H.sup.+, Li.sup.+, Na.sup.+, K.sup.+,
Ag.sup.+, Cu.sup.+).
2. In their fully reduced state, they are transmissive over the
wavelength range of modulation.
3. As they become oxidized, they decrease their transmission over
the wavelength range of modulation.
Layers 16 and 18 are separated by an electrolyte layer 20 for
transporting ions between layers 16 and 18. Layer 20 is transparent
over the wavelength range being modulated. It allows ion conduction
but not electron conducting or tunnelling between layers 16 and 18.
Layer 20 is a polymer or semi-solid gel electrolyte in the present
embodiment, preferably polymer, and is preferably within the range
1 to 1000 .mu.m thick, preferably 50 .mu.m. Layer 20 may either
conduct protons or one of the possible inserting metal ions listed
above. Because of their high mobilities in candidate materials for
layers 16, 18 and 20, Li.sup.+ and H.sup.+ are preferred. Suitable
materials for layer 20 include Li.sup.+ conducting polymers, e.g.,
poly (bis-methoxyethoxyethoxide) polyphosphazine (MEEP) or mixtures
of polyethylene glycol and N-methyl pyrrolidone, either doped with
LiCF.sub.3 SO.sub.3 or other suitable Li salt. Where layer 16 is
compatible with protic acid electrolytes, as in mixed oxides of
vanadium and niobium for example, then a proton conducting polymer
may be used for layer 20. Possible proton conducting polymers
include poly-2-acrylamido-2 -methylpropane sulfonic acid
(polyAMPS), and perfluorinated sulfonated ionomers such as Nafion.
Layer 20 may be sputtered from a composite target of 42% Li.sub.2
O, 26% SiO.sub.2 and 32% ZrO.sub.2.
Prior to assembly of the device, either layer 16 or 18 may be
converted to its fully reduced form. This may be accomplished
electrochemically by reduction in a nonaqueous Li.sup.+ electrolyte
if the Li-inserted form is to be used with Li.sup.+ conducting
electrolyte or by reduction in a protic electrolyte if a proton
conducting electrolyte is to be used. It may also be accomplished
chemically by treating either layer with a suitable reducing agent,
e.g., n-butyl lithium for Li insertion, or aqueous .[.S.sub.2
O.sub.4 .dbd..]. .Iadd.S.sub.2 O.sub.4.sup..dbd. .Iaddend. for the
H-inserted form. Alternatively, it may be accomplished by vacuum
processing in a separate step, such as sputtering from a Li target
or a target which decomposes to give Li atoms in the vapor phase,
or exposing to an H.sub.2 plasma for H-insertion. Further, the
layer may be produced by vapor deposition of the reduced material
directly using a source or target of the desired composition, or a
reducing reactive atmosphere to transform the source or target to
the desired composition during the deposition process.
The above procedure enables the device to be assembled with one
electrode in fully oxidized state and the other fully reduced,
which would represent either the extreme colored or extreme
bleached state of the device. Alternatively, layers 16 and 18 may
be prepared in some intermediate state of oxidation, e.g., by
shorting out fully oxidized and fully reduced layers in the
presence of electrolyte prior to assembly. This procedure reduces
the chemical activity of either layer, rendering either less
susceptible to chemical reaction during the assembly procedure.
The structures are assembled by laminating layer 20 between the
substrates coated with contiguous layers 12 and 16 on one substrate
and 12' and 18 on the other.
In operation, layers 12 and 12' are connected to an external
current source. If electrons are supplied through layer 12, layer
16 will become reduced, with the accompanying insertion of a charge
compensating cation from the electrolyte. Simultaneously, electrons
will be withdrawn from layer 18, with the accompanying expulsion of
a charge compensating cation into the electrolyte. The electrolyte
conducts ions from layer 18 to layer 16. If the polarity of the
current is reversed, layer 16 .[.18.]. .Iadd.16 .Iaddend.and layer
.[.16.]. .Iadd.18 .Iaddend.become oxidized and reduced,
respectively, and the flow of charge compensating cations through
the electrolyte is reversed in direction. When all of the charge
compensating ions from layer 16 have been transferred to layer 18,
or when layer 18 is reduced as far as possible and has accepted as
many charge compensating ions as possible, the device is in its
fully colored state. Similarly, the device is in its fully
colorless state when the maximum charge compensating ions have been
transferred from layer 18 to layer 16.
Since device 10 has a well-defined relationship between voltage and
state of charge, a voltage may be imposed between 12 and 12' and a
current be allowed to flow to adjust to that voltage difference,
the resulting transmittance being predictable from that voltage
difference alone. This feature of truly reversible electrochromic
devices allows for simplified control electronics, especially with
the relationship between transmittance and voltage difference.
Referring to FIG. 2, there is shown a device substantially similar
to that of FIG. 1, but with a thin film solid electrolyte, 0.01 to
10 .mu.m thick, preferably 0.1 .mu.m. This device comprises a
multilayer thin film stack. Corresponding reference symbols
identify identical elements in FIGS. 1 and 2. Device 30 is prepared
by sequential deposition of the layers by thin film processes,
preferably vacuum sputtering. The solid electrolyte 32 is a ceramic
which conducts ions, but not electrons. Suitable Li+ conductors for
layer 32 are ternary mixtures of Li.sub.2 O, ZrO.sub.2 and
SiO.sub.2, and simple compounds of Li which behave as electrolytes,
such as LiNbO.sub.3, LiTaO.sub.3, Li.sub.3 N, LiI, Li.sub.2
WO.sub.4 and LiA1F.sub.4, and variants and mixtures thereof.
Suitable proton conductors are partially hydrated electronically
insulating inorganic oxides or fluorides, such as SiO.sub.2,
A1.sub.2 O.sub.3, Ta.sub.2 O.sub.5, Nb.sub.2 O.sub.5, and
MgF.sub.2. The device in FIG. 2 also may be made with layers 16 and
18 reversed.
In the Li-based device, one of the layers is prepared in the
Li-inserted, reduced form while the other layer is prepared in the
fully oxidized form. For Example Li.sub.x WO.sub.3 may be prepared
by sputtering from a Li.sub.2 WO.sub.4 target in a reducing
atmosphere, from a Li/W alloy target in a partially oxidizing
atmosphere, or by sputtering metallic Li directly onto a WO.sub.3
layer. In the H-based device, hydrogenated electrode layers may be
produced by introducing the fully oxidized layer into a hydrogen
plasma. By preparing the device in this way, the finished device
will be either in its fully colored or fully bleached state,
depending on whether the primary electrochromic layer 18 on the
counter electrode layer 16 is initially reduced.
The operation of the embodiment in FIG. 2 is identical to that in
FIG. 1.
FIG. 3 shows a spectrum of transmittance versus wavelength for a
typical structure in its two extreme states, also referred to in
the examples. The difference in transmission at the wavelength of
modulation between these two extreme states represents the dynamic
range of the device at that wavelength. All intermediate values of
light transmission within that dynamic range are possible. When any
coloration state is reached, the current source may be removed and
that state is retained until current is resumed in either
direction.
Applications of devices 10 and 30 include windows with adjustable
transmittance for glare reduction and energy efficiency, and solar
panels. They can be used as light attenuators for active
information displays, such as electroluminescent displays. They can
be incorporated into photographic equipment, e.g., as variable grey
scale filters and as lens diaphragms. If applied to reflective
substrates, these devices may be used as a variable reflectance
mirror, e.g., for rearview automotive mirrors. By eliminating
electrolyte 32, which will allow under some conditions switching
but eliminate substantial open circuit optical memory, device 30
can be used for high frequency (<1 Hz) as light modulation under
the excitation of an ac electrical current. The invention may thus
be useful for analog or digital modulation of optical frequency
carriers.
EXAMPLES
EXAMPLE 1
The first example demonstrates the effect of increased
transmittance during reduction and decreased transmittance during
oxidation for two members of a group of counter electrode materials
according to the invention. The effect is described by the use of
colorization efficiencies which relate the optical density change
(.DELTA.OD) to the quantity of lithium intercalated into the film
(q, in coulombs/cm.sup.2) through the equation,
wherein CE(.lambda.) is the wavelength dependent coloration
efficiency. In equation (1), a negative value of q is defined to
represent deintercalation of lithium (i.e., oxidation) from the
electrochromic layer. A negative value of CE(.lambda.) represents a
film which becomes increasingly transmissive during reduction with
lithium.
An ITO-coated glass substrate was coated further with a thin film
of the mixed oxide counter electrode material. These coatings were
deposited by reactive radio frequency (rf) magnetron sputtering
from a composite target of the parent metals. In this way, one
counter electrode film was prepared from a mixed target containing
50 mole percent Cr and 50 mole percent V, while another was
prepared from a 35:65 Nb, V target. The metal composition of the
deposit reflected that of the targets, as determined by surface
spectroscopic analysis. Conditions for the desired reduced-state
visible transmittance were similar for both oxides: a sputter gas
composition of 10% oxygen in argon was introduced into the vacuum
chamber at a flow rate of 10 sccm and maintained at a total
pressure of 35 .mu.m. An rf power density of 5 watts/cm.sup.2 at
the metal target was employed during the deposition and the
substrate was allowed to thermally equilibrate with the plasma to a
typical substrate temperature of 120.degree. C. The distance
between the target and the substrate was 5 cm. A typical
as-deposited film thickness is 0.15 .mu.m, with a reversible
lithium capacity of 15 mC/cm.sup.2.
The coloration efficiencies of the films were determined as a
function of wavelength by electrochemical reduction and oxidation
with lithium in a spectrophotometer. The coloration efficiencies
are shown in FIG. 4. The coloration efficiencies of both materials
are small and negative over the entire wavelength range of
interest, indicating that reduction with lithium causes them to
become increasingly transmissive over a 350 to 1400 nm wavelength
range. Furthermore, multicycle electrochemical reduction-oxidation
testing of the mixed oxides over 2000 cycles has demonstrated that
these materials undergo a large number of optical switching cycles
without degradation or accompaniment of irreversible side
reactions.
EXAMPLE 2
The second example comprises electrochromic light modulators
employing a Li.sup.+ -conducting polymer electrolyte and the mixed
oxide counter electrodes. The primary electrochromic layer,
amorphous WO.sub.3, was first deposited onto ITO-coated glass by
reactive rf magnetron sputtering. A reactive sputter gas
composition of 10% oxygen in argon was employed in the vacuum
chamber at a flow rate of 10 sccm and maintained at a total
pressure of 100 .mu.m. The deposition was carried out at an rf
power density of 5 watts/cm.sup.2 at the target with a substrate to
target distance of 5 cm. The temperature of the substrate was
controlled by thermal equilibrium with the plasma and is typically
120.degree. C. The deposition was carried out for a sufficient
length of time that the thickness of the WO.sub.3 film was 0.2
.mu.m, and the film was capable of reversible reduction and
oxidation with the alkali insertion ion to a level of 15
mC/cm.sup.2.
A second ITO-coated glass substrate was coated with a counter
electrode material comprised of a thin sputtered film of amorphous
(Cr.sub.0.5 V.sub.0.5)O.sub.y or (Nb.sub.0.35 V.sub.0.65)O.sub.z,
as described in the first Example. The counter electrode film was
electrochemically reduced with lithium by the double injection
process (Li.sup.+ and electron) in an electrolyte of 1N LiClO.sub.4
/propylene carbonate so that the film contains 15 mC/cm.sup.2 of
intercalated lithium. The resulting counter electrode film thus had
the composition Li.sub.x (Cr.sub.0.5 V.sub.0.5)O.sub.y or Li.sub.x
(Nb.sub.0.35 V.sub.0.65)O.sub.z, with x approximately equal to
unity in the fully reduced state. The lithium acts as the charge
compensating ion during electrochemical reduction of the primary
and counter electrode layers.
An electrochromic light modulator was then fabricated by laminating
the substrates containing the WO.sub.3 and reduced counter
electrode films together with a Li.sup.+ conducting polymer that is
transparent over the wavelength range of desired transmittance
modulation. The Li.sup.+ conducting polymer in this example, was a
mixture of N-methyl pyrrolidone (PVP) and polyethylene glycol (PEG)
doped with LiCF.sub.3 SO.sub.3. The PVP/PEG mixture had the weight
ratio of 40/60 and the ratio of LiCF.sub.3 SO.sub.3 /PVP-mer was
3:1. The application of a voltage between the ITO electrodes caused
the transmittance of the light modulator to be controlled as a
unique function of the applied voltage. FIG. 3 shows the
transmittance spectra for the two extreme optical states of the
device described with the V:Nb:O counter electrode. The device with
the V:Cr:O counter electrode gave nearly identical performance. In
both cases, the most transmissive state was obtained with an
applied voltage of 1.5 volts with respect to the WO.sub.3 layer and
the least transmissive at a voltage of -2.0 volts with respect to
the WO.sub.3 layer.
EXAMPLE 3
In the third example, an all solid-state electrochromic light
modulator was fabricated by sequential RF sputtering of an active
electrochromic material (a-WO.sub.3), an oxide-based Li.sup.+
conductor (Li.sub.2 O.SiO.sub.2.ZrO.sub.2), and a V:Nb:O counter
electrode as described in the previous examples. Lithium was
electrochemically intercalated into the a-WO.sub.3 layer and an
aluminum top-contact was evaporated onto the counter electrode
layer. The light modulator has the following structure,
glass/ITO/aLi.sub. x WO.sub.3 /Li.sub.2 O.SiO.sub.2.ZrO.sub.2
/(Nb.sub.0.035 V.sub.0.65)O.sub.y /aluminum In the as-fabricated
condition the light modulator exhibited a deep blue coloration and
low specular reflectance when viewed through the glass side. When a
potential of -2.0 volts with respect to the aluminum layer was
applied between the aluminum and ITO contacts the WO.sub.3 layer,
the modulator became highly reflecting as viewed through the glass.
A charge transfer of only 10 mC/cm.sup.2 was required to achieve
this optical modulation.
Other embodiments are within the appended claims.
* * * * *