The present invention is related to an electrochromic device and,
more particularly, to such a device conceived for modulating light transmitted
or reflected by equipment items or apparati such as windows, display cases, windows
or rear view mirrors for automobiles, electrooptic shutters, display screens,
Electrochromic devices consisting of a liquid or solid layer of an
electrolytic material confined between two transparent electrodes are known which,
as the result of the application of a voltage between the two electrodes, take
on a coloration due to the formation of a metallic layer on one of the electrodes
or to the insertion of ions into an oxide layer. Thus, by varying electrically
the optical density of the device, it is possible to modulate the transparency
or the reflectance of the equipment items mentioned above for adaptation to variations
in the ambient lighting, for example.
In particular, from French Patent Application No. 91 00223, filed
January 10, 1991 by the applicants of the present application, an electrochromic
electrolytic material is known consisting of a homogeneous solution of at least
one organic solvent and of at least one salt of an electrodepositable metal, the
solution comprising, in addition, at least one organic acid and at least one salt
of a non-electrodepositable metal facilitating the dissolution of the electrodepositable
metal salt. As an example, this material can then be constituted from bismuth
bromide in solution in gamma-butyrolactone, and comprises, in addition, lithium
bromide to facilitate the solubilization of bismuth bromide. Such a material exhibits
an improved thermal stability and permits the formation of absorbing and/or reflecting
metal deposits in electrochromic devices for modulating light of large size and
Such a device generally exhibits the structure represented partially
in perspective in Figure 1 of the appended drawing. This Figure illustrates that
the device takes the form of cell 1 comprising a layer of electrochromic material
2 confined between two plates 3, 4 in a transparent material such as glass, which
plates carry on their two opposing faces conducting layers 5, 6, respectively.
A joint 7 of an adhesive material, for example, laterally confines electrochromic
layer 2, assuring the assemblage of plates 3, 4 with maintenance of a predetermined
space (several tens of µm) between them. Conventionally also, the application of
an electrical voltage to the two conducting layers 3, 4 is assured by elongated
and parallel electrodes 8, 9, respectively, disposed on the conducting layers 5,
6, respectively, along two opposite edges of the cell. Electrodes 8, 9 are prepared
from a material of much higher conductivity than that of conducting layers 5, 6
generally comprised of the oxide of tin and/or indium (ITO) and prepared by deposition
under vacuum. Electrodes 8, 9 thus improve the uniformity of the distribution
of an electrical voltage input delivered from a source (not shown) upon the entire
layer of electrochromic material.
One first difficulty that one encounters in the use of electrochromic
cells resides in the necessity to assure a rapid darkening of the cell, capable
of following the fluctuations of the ambient light, brutal and glaring when, for
example, the headlights of a vehicle strike to flash the reflecting surfaces of
a rear view mirror equipped with such a cell. There currently exists a need for
electrochromic devices having, from this point of view, a very short response
time, less than that possible to obtain in the cell with two electrodes disposed
as shown in Figure 1.
Among the other difficulties encountered in assuring good operation
of a cell of the type shown in Figure 1, one encounters those derived from the
existence of parasitic electric currents flowing transversely through the cell
from one electrode to the other in parallel to the different layers constituting
this cell. One ascertains experimentally the existence of these currents which
are added to the current perpendicular to the conducting layers which are necessary
for the generation of the electrochromic effect. The parasitic currents cause the
migration of ions from one side of a cell to the other, when one uses the input
configuration with two electrodes shown in Figure 1. Although these parasitic currents
have hardly any immediate visible effects, they cause over a long time, through
a cumulative effect, an asymmetry of coloration of the cell with applied voltage;
coloration which is then stronger in the vicinity of one of the electrodes whereas
one seeks, on the contrary, a uniformity as good as possible of this coloration.
Another difficulty involves the deterioration over time of one of
the conducting layers 5, 6, the one on which a metallic deposit is brought about
which assures the coloration of the cell. Indeed, physiochemical effects resulting
from such deposits on the conducting layer cause a diminution of its conductivity.
This diminution affects only one of the two conducting layers; the distribution
of current in one cell with two electrodes of the type shown in Figure 1 is affected
in an asymmetric manner, which reinforces again the harmful effects of the parasitic
currents mentioned above in the matter of losses of uniformity of the coloration
of the cell under voltage.
One observes again in the cells with two electrodes of the same type
a much weaker coloration in the center of the cell than that which one observes
in the vicinity of the electrodes; this phenomenon being much more pronounced as
the distance separating electrodes 8, 9, and therefore the width of the cell,
Besides the problems of non-uniformity of coloration mentioned above,
the current technology of electrochromic cells makes apparent various other needs
which are not yet perfectly satisfied. The operation of such a cell requires the
consumption of a certain electrical power. It appears desirable to conceive cell
structures permitting reduction thereof to a minimum. In another connection, many
current or potential applications of electrochromic devices concern automobile
vehicles. There exists a need for such devices incorporating the possibility of
electrical heating permitting a time of defrosting transparent plates 3, 4 and
a reheating of the electrochromic layer to an adequate operating temperature.
In this regard, it is clear that the cell with two electrodes of Figure 1 does
not permit passing a current from one side of one of the conducting layers 5,
6 to the other to heat it by the Joule effect. Document EP-A-O408427 describes
an electrochromic pane comprising a third electrode enabling such heating, but
only one of the two conducting layers.
The present invention therefore has a purpose of preparing an electrochromic
device with a short darkening time and a coloration under voltage of improved uniformity.
The present invention also has a purpose of preparing such a device
with reduced power consumption to obtain the electrochromic effect.
The present invention also has a purpose of preparing such a device
incorporating means of heating of improved efficacy.
Summary of the Invention
One achieves the purposes of the invention, as well as others which
will become apparent upon reading the following description, with an electrochromic
device comprising one layer of an electrochromic product held between two conducting
layers formed on each face of a transparent plate, electrodes formed on each of
the conducting layers, and means for selectively supplying these electrodes with
electrical energy. According to the invention, the electrodes are of elongated
shape and extend over facing surfaces of two conducting layers essentially symmetrically
with respect to the plane of the electrochromic layer.
Thanks to this configuration of electrodes, the circulation of current
in the electrochromic layer is symmetrical and the intensity thereof reinforced
for a given voltage input. As a result, the time of darkening of the layer is
shorter and the uniformity of this darkening improved over the entire surface of
this layer by the suppression of transverse parasitic currents mentioned above.
This symmetrization also permits, as will be seen later, a reduction of non-uniformities
of coloration of the layer which would otherwise result, in the long term, from
the degradation of the conducting layers.
According to one embodiment of the present invention, the electrochromic
layer is limited by at least two essentially straight long edges bounded by at
least four electrodes disposed face to face parallel and in proximity to each
of the two edges.
Advantageously, the distance 1 between these sides of the electrochromic
layer is such that 1<0.5.1c and preferably 1<0.3.1c,
1c being a critical width which will be defined in the rest of the present
description. According to another characteristic of the device, according to the
invention, one of the two parallel electrodes carried by the same conducting layer
is selectively connected to a source of electric power in order to heat the device
by the Joule effect developed in the conducting layer between the two electrodes.
One utilizes for this purpose a source of alternating current which is superimposed
on a common supply to the two electrodes for a direct current. One also advantageously
assures the possible heating and defrosting of the device.
The electrochromic device according to the invention can also comprise
a plurality of pairs of facing electrodes defining together an essentially closed
contour or only two electrodes, to develop accordingly an essentially closed contour,
rectangular or annular, for example. Such configurations of electrodes permit the
improvement also of the uniformity of the coloration of the electrochromic device
according to the invention when the latter is placed under voltage.
Other characteristics and advantages of the present invention will
become apparent upon reading the description which follows and examining the appended
Brief Description of the Drawings
Figure 1 is a partial perspective view of an electrochromic cell of the prior
art, discussed in the Background of the Invention;
Figure 2 is a partial perspective view of an electrochromic device according
to the present invention;
Figure 3 is a plan view of an annular electrochromic device according to the
present invention; and
Figure 4 is a view in cross section according to the line of section III-III
of the device of Figure 3.
General Description of the Invention
Reference is made to Figure 2 of the drawings wherein the identical
numerical references to the cells are utilized as in Figure 1, repeating identical
or similar elements or devices. Thus, the electrochromic element according to
the invention represented in Figure 2 takes on, like the one of Figure 1, the form
of a cell in the heart of which one finds a layer of an electrochromic product
2 confined between two transparent plates 3, 4 covered on their facing sides with
conducting layers 5, 6, respectively. An adhesive joint 7 laterally confines the
layer 2 between plates 3, 4 whereby the assemblage is assured besides.
The electrochromic layer 2 can be comprised of any material susceptible
to be colored under the effect of the application of an electrical voltage and,
in particular, by the electrolytic material described in French Patent Application
No. 91 00223 cited above. The conducting layers 5, 6 may be comprised conventionally
of the oxide of tin and/or indium (ITO) or of fluorine (FTO) formed by well known
techniques of deposition under a vacuum, such as cathodic sputtering (ITO), vapor
phase deposition (FTO), or pyrolysis.
According to an essential characteristic of the cell according to
the invention, the latter no longer comprises two electrodes disposed asymmetrically
with respect to the plane of electrochromic layer 2, as in the cell of Figure
1, but four electrodes are disposed symmetrically with respect to the plane of
this layer and are divided in two pairs 8, 10 and 9, 11 parallel and in the vicinity
of two opposite edges of layer 2. The electrodes of the same pair are essentially
straight and disposed facing one another. Electrodes 8, 11 on the one hand and
9, 10 on the other hand are thus formed on facing surfaces of conducting layers
5, 6. A source of direct current (not shown) may be connected selectively by a
terminal to electrodes 8, 11 and by another terminal to electrodes 9, 10. The points
of connection of these terminals and of these electrodes can be established on
extensions (not shown) of electrodes, not positioned facing the other electrode
of the same pair, in order to facilitate access to these points of connection.
By appropriate choice of the sign and amplitude of the voltage thus established
in layer 2, one can either cause a coloration of the layer to appear (darkening)
or to cause this coloration to disappear (fading).
The disposition of electrodes 8, 9, 10, 11 according to the invention
assures a perfect symmetry of the lines of current in the electrochromic layer,
those lines traversing the latter perpendicularly to its plane, which avoids any
generation of transverse parasitic currents capable of creating inhomogeneities
of coloration of the layer in the darkening phase, as noted in the Background of
Furthermore, a reinforcement of the current in the cell, for a given
voltage supply, with respect to that observed in a cell of two electrodes of the
type shown in Figure 1 has been established experimentally. This reinforcement
is favorable for reducing the darkening time of the cell for a given voltage, as
the display of measurements brings together in the following table:
Type of formulation Resistivity of layers (5, 6) Darkening time at λ = 633 nm Cell of Figure 2 Cell of Figure 1 Rear view mirror10 Ω/square1.4 s2.4 s Rear view mirror4 Ω/square1.3 s1.5 s Large surface4 Ω/square11.9 s17.4 s
This table brings together measurements made with an experimental
cell with a surface of 2.5 x 4 cm², the distance between electrodes 8, 10 on the
one hand and 9,11 on the other hand being slightly greater than 4 cm. For the
electrochromic layer, two different formulations of electrolytic material described
in French Patent No. 91 00223 cited above were used. One of the formulations is
suitable for equipment items of small surface, such as automobile rear view mirrors,
while the other formulation suits equipment items of large surface such as windows.
A clear reduction of darkening time with the two formulations is
ascertained from this table.
The reinforcement of the current in the cell, for a given supply
of direct current, which has been mentioned above, permits, in the other direction,
the use of sources of lower voltage in order to obtain the same current. In this
manner one obtains cell currents essentially equal by using a source of voltage
of 1.8 volts with a cell of the type of Figure 1 and a source of voltage of 1.5
volts with a cell according to the invention.
A modeling of the cell according to this invention has confirmed
the symmetry of the current in the cell and the suppression of transverse currents.
This modeling equally permits an explanation of another advantage provided by the
invention; that is, the reduction of the inhomogeneity of the coloration of the
cell resulting from a degradation over the long term of one of the conducting layers
by a metal deposit which takes place at the level of this layer, as was noted
in the Background of the Invention. If, therefore, the resistance of the surfaces
of the two conducting layers take on, over the long term, different values Rs
at R's, the model shows that the distribution of current in the cell
takes place as if these layers presented uniformly a surface resistance equal to
(Rs + R's)/2
. This behavior suppresses or reduces the asymmetry, over the long term, of the
optical density of the electrochromic layer.
The same modeling has permitted establishing one condition for obtaining
a coloration of homogeneous density over all of the surface of the cell without
decreasing this density in the center of the cell and this continually.
Indeed, according to the invention, the critical width 1c
is defined such that:
Z is the real impedance of the electrochromic layer;
S its surface; and
Rs the surface resistance of the conducting layers
The model and the measurements have permitted establishing that one
obtains a density of cell coloration truly uniform over the entire surface thereof
if the width 1 of the electrochromic layer (see Figure 2) is such that:
The dispositions of the electrodes other than those illustrated
in Figure 2 are used in order to best satisfy this condition of width. Thus, one
can arrange more than two pairs of rectilinear electrodes over the periphery of
the electrochromic layer; for example, four pairs around one rectangular layer
thereby essentially surrounded on its four sides by electrodes. One could dispose
an even greater number of rectilinear electrodes over an essentially closed periphery
surrounding an electrochromic layer in applications where the form of this layer
must deviate from a rectangular shape: rear view mirrors, eyeglasses, etc.
According to the invention, one can also prepare a cell comprising
only two electrodes extending over facing surfaces outlining a closed or essentially
closed periphery, rectangular, annular, or other. In this manner is shown in Figures
3 and 4, where references identical to the above references are used to mark identical
or similar devices, a cell comprising two essentially circular electrodes 8, 9.
The supply of electricity to the two electrodes takes place through contacts formed
on lugs 14, 15 much of which provides access to one of the electrodes. The modeling
mentioned above confirmed that the geometry of the electrodes makes the lines of
current in the electrochromic layer perpendicular. The homogeneity of the coloration
of the cell can be reinforced through an appropriate choice of the diameter of
the electrochromic layer, the diameter being assimilated to a "width" respecting
the critical width defined above.
If the configuration of the two electrodes of the cell of Figures
3 and 4 does not lend itself to the addition of heating of the cell, the configuration
with four electrodes of the cell of Figure 2, and all configurations which flow
therefrom, advantageously allow such heating by the Joule effect developed in
conducting layers 5 and 6. The supply of electricity required can even be regulated,
according to the invention, in a manner so as not to upset the electrochromic
effect, if the latter is produced during heating of the cell.
For this purpose it is proposed, according to the invention, to supply
electrodes 8, 9, 10, 11 with voltages V&sub8;, V&sub9;, V&sub1;&sub0;, V&sub1;&sub1;
Vc2-Vc1 = voltage required to obtain
the electrochromic effect.
Voltages (V&sub1;&sub1;-V&sub8;) and (V&sub1;&sub0;-V&sub9;) permit
the development of heating by Joule effect of layers 5 and 6, respectively; the
frequency w/2π of the alternating component Vc2 sinωt being
chosen sufficiently high (typically >50Hz) in order to not cause a parasitic
modulation of the optical density of the electrochromic layer. One notes that through
this choice of the supply of voltages to the electrodes, differences in voltages
(V&sub8;-V&sub1;&sub0;) and (V&sub9;-V&sub1;&sub1;) of facing electrodes remain
equal. Heating at that time does not upset the electrochromic effect at all. Such
heating is particularly useful when a window or a rear view mirror has to be defrosted,
as is the case in an automobile. It permits at the same time bringing the electrochromic
layer to an appropriate operating temperature.
The manufacture of an electrochromic cell according to the invention
calls for known techniques. The assemblage of transparent plates 3 and 4 can be
produced by pouring onto one of the plates around electrochromic layer 2, if the
latter is solid, a ribbon of an adhesive product. If the electrochromic product
is liquid, the ribbon of adhesive is deposited first, plates 3, 4 are assembled
with the aid of this ribbon and liquid electrochromic product is introduced into
the enclosure thus formed by aspiration, for example. The adhesive ribbon constitutes,
at the same time, after hardening, joint 7 laterally confining the electrochromic
product. Numerous adhesive products are available for this use, such as epoxy or
acrylic resins, polyurethane or silicone glues, thermoplastic glues (ethylene/vinyl
acetate, polyamide), etc. One can arrange two parallel adhesive ribbons in order
to improve the tightness of the cell. The adhesive product chosen must assure
good protection of the electrochromic product against humidity and oxidation while
assuring a good mechanical and thermal strength of the assembly.
In like manner, the electrodes can be formed on the conducting layers
through various known techniques such as serigraphy or the deposition of a ribbon
of an epoxy glue doped with silver, such as the glues sold under the names Epotek
H21D, H22, H24 by the Epotekny Company or a conducting paste with platinum or gold,
with palladium and with silver or with silver alone, such as the pastes sold under
the references 9710, 9110, 7115 by the Degussa Company, for example, with the aid
of a robot-commanded syringe; the deposition being followed by a thermal treatment,
as is well known. One seeks to obtain low resistivity electrodes.
It now appears that the electrochromic device according to the invention
does indeed produce the promised advantages; that is, a decreased time of darkening
phase, a maintenance of this homogeneity in case of degradation over time of a
conducting layer, a reduced voltage consumption, and a possibility of heating and
defrosting the device.
It will be well understood that the invention is not limited to the
described and presented embodiments which are given only as examples. Therefore,
the invention extends to electrochromic cells provided with an electrolyte with
ionic or protonic conduction, for example; such cells also having the advantage
of symmetry of current of the cell obtained by the present invention.
An electrochromic device comprising a layer (2) of an electrochromic product
held between two conducting layers (5,6) each formed on one face of a transparent
plate (3,4), electrodes formed on each of the conducting layers and means for
selectively supplying these electrodes with electrical energy, characterized in
that the electrodes (8,9,10,11) are of elongated shape and extend over facing
surfaces of the two conducting layers (5,6) essentially symmetrical with respect
to the plane of the electrochromic layer (2).
A device according to claim 1 in which electrochromic layer (2) is bounded
by at least two essentially straight edges, characterized in that it comprises
at least four electrodes (8,9,10,11) disposed facing two by two in parallel and
in the vicinity of each of these edges.
A device according to claim 2, characterized in that the distance (2) between
said edges of the electrochromic layer is such that 1<0.21c and,
preferably, 1<0.31c where (1c) is a critical width defined
by the equation:
Z is the real impedance of the electrochromic layer;
S its surface; and
Rs the resistance of the surface of the conducting
layers in ohms/square.
A device according to claim 2 or 3 characterized in that the facing electrodes
of a pair of such electrodes are selectively connectable to a source of direct
current of identical magnitude for all the pairs of electrodes.
A device according to claim 3 or 4 characterized in that one of the two parallel
electrodes carried by the same conducting layer is selectively connectable to a
source of electrical energy appropriate for heating the device by Joule effect
developed in the conducting layer between these two electrodes.
A device according to claim 5 characterized in that said source is a source
of alternating current which is superimposed on a supply common to the two electrodes
by a direct current, optionally the frequency of the source of alternating current
being higher than a predetermined frequency chosen in order not to induce any electrochromic
effect in the electrochromic layer.
A device according to claim 6 or 7 characterized in that the electrode situated
facing the one which is supplied by a source of alternating current is simultaneously
supplied by this same source.
A device according to any one of claims 1-7 characterized in that it comprises
a plurality of pairs of facing electrodes, or only two facing electrodes, said
electrodes defining together an essentially closed contour, optionally said contour
being rectangular or annular.
A device according to any one of claims 1-8 characterized in that the electrodes
are constituted of a deposit of a ribbon of an epoxy glue doped with silver or
a conducting paste comprising at least one metal.
A device according to any one of claims 1-9, characterized in that the electrochromic
layer is constituted of an electrolytic material comprising at least one organic
solvent, at least one salt of an electrodepositable metal, at least one organic
acid, and at least one salt of metal non-electrodepositable and facilitating the
solubilization of the metal salt, optionally said electrochromic layer being confined
laterally by a joint (7), adhesively assembling the two transparent plates into