WO2025133133A1 - Improved electrochromic multilayer electroactive reflective module, and associated system and method - Google Patents

Abstract

The invention relates to a multilayer electroactive reflective module (1) for an automotive part, comprising a substrate (10), a multilayer stack (18) which is arranged on the substrate (10) and is configured to reflect a reflected light beam (2') having a determined wavelength depending on a potential difference applied to the stack (18), a first electrode (12) and a second electrode (13) which are configured to apply the potential difference, the multilayer stack comprising a metal mirror (14), an electrochromic layer (15) of nanometric thickness (d15) comprising at least one solid polymer electrolyte (16) and at least one electrochromic molecule (17) so as to modify the absorption of the electrochromic layer (15) based on the potential difference applied to the stack (18), and a partially transparent metal bilayer (19) positioned on top of the electrochromic layer (15).

Description

Improved electrochromic multilayer electroactive reflective module, associated system and method

The present invention relates to the field of electroactive reflective multilayer modules. It finds particularly advantageous application in the field of motor vehicle cladding or signaling, in particular for front parts of vehicles or for the interior of such vehicles.

STATE OF THE ART

It is common to present a pattern or visual element on a motor vehicle part, for its decoration or for signaling purposes. For this, light sources are generally used that allow such a pattern to be displayed day and night. To limit the power consumption of this type of module, we can look towards more economical solutions, which use ambient light at least during the day.

For this purpose, there are electroactive reflective multilayer modules, configured to reflect part of the visible spectrum and thus return a particular color. Modules exploiting the effect of Fabry-Pérot cavities are particularly well known. In a Fabry-Pérot cavity, a reflected color called "structural color" appears when light is confined in a nanometric cavity delimited by two substantially parallel surfaces. These modules comprise a substrate, on which is formed a stack comprising at least one layer of reflective mirror and a layer of Fabry-Pérot absorber, for example a layer of conductive polymer. In existing solutions, the thickness of the absorber layer determines the wavelengths of the reflected light beam that will exit the polymer layer by the interference phenomenon. These specific wavelengths correspond to a color in the visible spectrum and arrive at the eyes of an observer. The latter therefore has the impression that the layer of material has changed color.

In order to modulate the thickness of the conductive polymer layer, there are modules incorporating a liquid electrolyte reservoir. The thickness of the conductive polymer layer can be modulated using a reversible redox process in the presence of an ion source, when the conductive polymer layer is subjected to a potential difference.

In practice, these systems remain limited, particularly because the use of a liquid electrolyte reservoir can cause leaks and problems connecting to the reservoir. In addition, these systems generally have a broad reflection in terms of reflected wavelength. The color perceived by the user is therefore dull and not very pronounced. This hinders their integration in certain applications, such as automotive.

An object of the present invention is therefore to propose a solution improving an electroactive reflective multilayer module compared to existing solutions, and in particular to make it better compatible with an application in automotive parts.

Other objects, features, and advantages of the present invention will become apparent from the following description and accompanying drawings. It is understood that other advantages may be incorporated.

SUMMARY

• un premier substrat,
• un empilement multicouche disposé sur le premier substrat et configuré pour recevoir un faisceau lumineux incident et réfléchir un faisceau lumineux réfléchi présentant une longueur d’onde déterminée, ladite longueur d’onde dépendant d’une différence de potentiel appliquée à l’empilement, l’empilement multicouche comprenant au moins une couche formant un miroir métallique,
• une première électrode et une deuxième électrode, reliant électriquement de part et d’autre l’empilement multicouche, et configurées pour appliquer ladite différence de potentiel.

To achieve this objective, according to a first aspect, a multi-layer electroactive reflective module is provided for an automotive part comprising:

• a first substrate,
• a multilayer stack arranged on the first substrate and configured to receive an incident light beam and reflect a reflected light beam having a determined wavelength, said wavelength depending on a potential difference applied to the stack, the multilayer stack comprising at least one layer forming a metal mirror,
• a first electrode and a second electrode, electrically connecting the multilayer stack on either side, and configured to apply said potential difference.

• une couche électrochromique présentant une épaisseur nanométrique et surmontant le miroir métallique, la couche électrochromique comprenant :
  • au moins un électrolyte polymère solide,
  • au moins une molécule électrochromique distincte de l’au moins un polymère électrolyte solide de façon à modifier l’absorption de la couche électrochromique, selon la différence de potentiel appliquée à l’empilement,
• une bicouche métallique partiellement transparente comprenant une première couche métallique à base ou faite d’un premier métal, et une deuxième couche métallique à base ou faite d’un deuxième métal distinct du premier métal, la bicouche métallique surmontant la couche électrochromique.

Advantageously, the multilayer stack further comprises:

• an electrochromic layer having a nanometric thickness and surmounting the metal mirror, the electrochromic layer comprising:
  • at least one solid polymer electrolyte,
  • at least one electrochromic molecule distinct from the at least one solid electrolyte polymer so as to modify the absorption of the electrochromic layer, depending on the potential difference applied to the stack,
• a partially transparent metal bilayer comprising a first metal layer based on or made of a first metal, and a second metal layer based on or made of a second metal distinct from the first metal, the metal bilayer overlying the electrochromic layer.

In this document, the reflective module as proposed may also be referred to as an electroactive reflective unit with a multi-layer structure, with a first substrate topped by a multi-layer stack. The term "module" is used to denote a modular element that is independent and self-sufficient to achieve the desired effects, including structural reflection, i.e., the return of specific wavelengths resulting from the structure of the modular element depending on stimuli applied to this element.

Here, the term “molecule” means an entity that is capable of existing in the free state regardless of the stimuli applied to the multilayer stack. According to an exemplary embodiment, the electrochromic molecule may designate a monomer, or a monomer molecule, which, by definition, consists of molecules of low molecular weight. The monomer molecules remain separate and do not polymerize with the presence of a voltage applied to the multilayer stack.

The electrochromic layer has a nanometric thickness, forming a Fabry-Pérot cavity. The metallic bilayer deposited on the electrochromic layer, together with the mirror, closes the formed Fabry-Pérot cavity. The properties of the electrochromic layer are modulated by the electrochromic molecule which, depending on the applied potential difference, is in a redox form with an absorption peak. The electrochromic molecule thus changes the absorption of the electrochromic layer according to the wavelength, and more particularly in the visible range. The electrical stimulation caused by the potential difference therefore modifies the absorption coefficient according to the applied potential difference. The unabsorbed wavelengths resulting from the constructive interference of the Fabry-Pérot layer will form the light beam which is reflected on the reflecting mirror. The reflective module therefore makes it possible to modify the wavelength of the reflected beam, and therefore the color perceived by a user.

This solution therefore differs from existing solutions in which only the thickness of the Fabry-Pérot layer is varied to modulate the wavelength of a beam. It is also possible to overcome the mechanical constraints resulting from this change in thickness. A wide variety of colors can be obtained depending on the electrochromic molecule(s) used.

The electrochromic bilayer deposited on the electrochromic layer also forms a broadband absorber that absorbs part of the visible light spectrum. This absorbed radiation will therefore not be reflected by the reflective module, and therefore will not contribute to the perceived color. The reflected radiation, synergistically with the electrochromic layer, will have a reduced wavelength range in the visible range. The perceived color will therefore be more vivid and more pronounced.

The electrochromic layer comprising a solid polymer electrolyte provides the ions required by the electrochromic molecule during the oxidation-reduction reactions induced by the potential difference. The redox form of the electrochromic molecule can thus be changed to modulate the reflected wavelength. Since the electrochemical layer comprises the solid polymer electrolyte and the electrochromic molecule, the reflective module is simplified and therefore easier to manufacture. This module thus distinguishes itself from solutions that could implement a multilayer assembly comprising an electrochromic layer and a separate electrolyte layer. The solid polymer electrolyte also improves the stabilization of the chemical species involved in the oxidation-reduction processes of the electrochromic molecule, which improves the reversibility of color changes and expands the range of electrochromic colors that can be obtained.

The stack is also in solid or semi-solid form, which prevents leaks and reduces the size and weight of the reflective module compared to existing solutions using liquid electrolytes. Solid polymer electrolytes also have better thermal stability. Solid polymer electrolytes have better flexibility. The reflective module can therefore have flexibility that facilitates its integration into an automotive application, for example on a curved surface, as well as good mechanical strength.

The use of a solid polymer electrolyte, particularly in the form of a gel or semi-solid material, also avoids some of the assembly difficulties associated with the use of liquid electrolytes. Liquid electrolytes are generally introduced by surface capillary action into a previously assembled functional cell, typically with openings placed in opposite corners. Once the liquid electrolyte is brought into contact with one of the openings, it rises through the internal cavity of the cell. However, since the rise becomes more difficult as filling progresses, due to the increasing potential energy of the electrolyte, this method limits the size of the device and increases the risk of bubbles.

In addition, solid polymer electrolytes, especially in the form of a gel or semi-solid material, facilitate the assembly of the electrochromic layer with the substrate. This facilitates electronic contact between the electrode substrates and the electrochromic layer, minimizing the risk of colorless areas appearing in the electrochromic layer, even when a potential difference is applied. This is especially advantageous in the case of large-area reflective modules.

A solid polymer electrolyte, particularly in the form of a gel or semi-solid material, is therefore easier to handle than liquid electrolytes and offers better interaction with the substrate and, where applicable, the electrode, particularly due to their stickiness and/or adhesion. This therefore facilitates the industrial-scale manufacture of the reflective module.

The architecture of the reflective module is also simplified. A single electrochromic layer may be sufficient, without requiring the addition of other layers, including a conductive polymer layer and a counter-electrode, as would be the case for a module with a Fabry-Pérot layer of variable thickness depending on the applied potential difference.

Furthermore, the reflective module has reduced energy consumption. Indeed, a low potential difference, typically of the order of ± 2 V, is sufficient to modulate the redox form of the electrochromic molecule and change the reflected color, particularly in the visible range. The reflective module is made more versatile. The reflective module is therefore particularly suitable for automotive applications.

A second aspect relates to an electroactive reflective system for an automotive part comprising at least one reflective module according to the first aspect. The system comprises the effects and advantages of the reflective module, and is thus particularly suitable for automotive applications.

• Une fourniture du premier substrat comprenant la première électrode, et une fourniture de la deuxième électrode,
• Une formation de l’empilement multicouche comprenant :
  • Un dépôt de la bicouche métallique,
  • Un dépôt de la couche électrochromique,
  • Un dépôt de l’au moins une couche formant le miroir métallique,
• Un assemblage du premier substrat et de la deuxième électrode, de sorte que l’empilement multicouche soit relié de part et d’autre aux première et deuxième électrodes.

A third aspect relates to a method of manufacturing the reflective module according to the first aspect or the reflective system according to the second aspect, comprising:

• A supply of the first substrate comprising the first electrode, and a supply of the second electrode,
• A formation of the multi-layer stack comprising:
  • A deposit of the metallic bilayer,
  • A deposit of the electrochromic layer,
  • A deposit of at least one layer forming the metallic mirror,
• An assembly of the first substrate and the second electrode, such that the multilayer stack is connected on either side to the first and second electrodes.

A fourth aspect relates to a motor vehicle part comprising a reflective module according to the first aspect or a reflective system according to the second aspect.

BRIEF DESCRIPTION OF THE FIGURES

The aims, objects, as well as the characteristics and advantages of the invention will emerge more clearly from the detailed description of an embodiment thereof which is illustrated by the following accompanying drawings in which:

Figures 1A and 1B represent a reflective module according to two exemplary embodiments.

There is a diagram illustrating the change in redox form of an electrochromic molecule in the electrochromic layer, during an oxidation-reduction reaction and according to an exemplary embodiment.

Figures 2B and 2C are graphs of the reflectivity R as a function of the thickness of a solid polymer electrolyte layer of a Fabry-Pérot cavity as a function of the wavelength, respectively in 2B without gold and chromium metal bilayer, and in 2C topped with a gold and chromium metal bilayer.

There represents an automobile part comprising a reflective system, according to an exemplary embodiment.

There represents a reflective module according to another exemplary embodiment.

There represents a reflective system according to an exemplary embodiment comprising a waveguide.

Figures 6 to 12 represent steps in the module manufacturing process, according to several embodiment examples.

There schematically represents a top view of an example of a distribution of nanometric-sized holes produced within a metal layer of the metal bilayer, according to an exemplary embodiment.

The drawings are given as examples and are not limiting of the invention. They constitute schematic representations of principle intended to facilitate the understanding of the invention and are not necessarily to the scale of practical applications. In particular, the relative dimensions of the substrates and layers, the thickness of a layer or a substrate in relation to its other dimensions, are not necessarily representative of reality.

DETAILED DESCRIPTION

Before commencing a detailed review of embodiments of the invention, optional features are set out below which may optionally be used in combination or alternatively.

In one example, in the metal bilayer, the first metal layer is based on or made of gold and the second metal layer is based on or made of chromium. The use of these two metals in the metal bilayer makes it possible to achieve the broadband absorber function in a distributed manner over the visible spectrum.

According to one example, in the metal bilayer, the first metal layer based on or made of gold overcomes the electrochromic layer and the second metal layer based on or made of chromium overcomes the first metal layer. The chemical stability of the materials in the stack is thus improved.

According to one example, the metal bilayer has a non-zero thickness substantially less than or equal to 10 nm.

According to one example, in the metal bilayer, the first metal layer has a non-zero thickness substantially less than 10 nm, preferably substantially less than or equal to 7 nm, preferably substantially less than or equal to 5 nm. According to one example, the first metal layer has a non-zero thickness substantially greater than or equal to 3 nm, preferably a thickness substantially equal to 3 nm.

According to one example, in the metal bilayer, the second metal layer has a non-zero thickness substantially less than 10 nm, preferably substantially less than or equal to 7 nm, preferably substantially less than or equal to 5 nm. According to one example, the second metal layer has a non-zero thickness substantially greater than or equal to 3 nm, preferably a thickness substantially equal to 3 nm.

According to one example, in the metal bilayer, at least one of the first and second metal layers has a plurality of nanometric holes along the main extension plane of the metal bilayer. Thus, the module has these nanometric holes near the surface receiving and re-emitting the light. The extraction of the reflected light is therefore improved, which increases the efficiency of light reflection by the module.

In one example, each of the first and second metal layers has the plurality of nanometer-sized holes along the main extension plane of the metal bilayer. In one example, the holes are continuous between the first metal layer and the second metal layer of the bilayer.

• Un ionogel comprenant une matrice polymère et un liquide ionique, et/ou
• Un liquide ionique polymérique.

According to one example, the solid polymer electrolyte comprises:

• An ionogel comprising a polymer matrix and an ionic liquid, and/or
• A polymeric ionic liquid.

The solid polymer electrolyte thus exhibits good ionic conductivity and allows for improved charge transfer for redox compositions of the electrochromic molecule. This makes it easier to modify the wavelength of the reflected beam. Ionogels and solid polymer electrolytes based on one or more polymeric ionic liquids exhibit good chemical and mechanical stability. They are also sufficiently deformable to facilitate their incorporation into an automotive part, for example, on a curved surface. This increases the lifespan of the reflective module. These examples are therefore particularly suitable for automotive applications. Furthermore, the solid polymer electrolyte can be in gel form. Gel electrolytes overcome the disadvantages of liquid and solid electrolytes, such as the poor chemical stability of liquids and the risk of leakage, and the slow switching speed and lack of transparency of solids. They are also easier to process and are better suited to flexible substrates. The improved stabilization of the chemical species involved in the redox processes is further enhanced, which allows for improved reversibility of the color change and opens up vast possibilities for expanding the electrochromic color palette.

In one example, the solid polymer electrolyte comprises a polymeric ionic liquid in admixture with a polymer matrix and/or the polymeric ionic liquid is crosslinked.

According to one example, the electrochromic layer, and preferably the solid polymer electrolyte, has an ionic conductivity substantially greater than or equal to 10 ^-4 S/cm at room temperature (substantially 25°C), for example substantially between 10 ^-4 S/cm and 10 ^-2 S/cm. These ranges of values can more particularly be achieved when the solid polymer electrolyte comprises an ionogel.

According to one example, the electrochromic layer, and preferably the solid polymer electrolyte, has an ionic conductivity substantially less than or equal to 10 ^-4 S/cm, at room temperature (substantially 25°C). This range of values can more particularly be achieved when the solid polymer electrolyte comprises a polymeric ionic liquid.

According to one example, the solid polymer electrolyte is based on at least one polymer chosen from the group consisting of polyethers, polycarbonates, polyesters, polynitriles, polyalcohols, polyamines, polysiloxanes, fluoropolymers, biopolymers and their derivatives.

For example, the electrochromic molecule is an organic molecule. This makes the deposition process easier and reduces manufacturing costs. A wide variety of colors can also be obtained. Indeed, organic electrochromic molecules, particularly viologens, have many redox forms with different colors.

According to one example, the electrochromic molecule has a molar mass less than or equal to 600 g/mol. The solubility of the electrochromic molecule in the solid polymer electrolyte can thus be improved, particularly for an organic molecule. In addition, a low potential difference, typically of the order of ± 2 V, is sufficient to modulate the redox form of the electrochromic molecule and change the reflected color, and more particularly in the visible range. The response time of the electrochromic molecule following the application of a potential difference is also rapid, for example of the order of a second or a few seconds.

According to one example, the electrochromic molecule is chosen from the group consisting of: viologens, spiropyrans, bipyridines, carbazoles, methoxyphenyls, quinones, tetrathiafulvalene, phenylenediamine, pyrazoline, porphyrinoids, and in particular thiophene-porphyrinoids, furan-porphyrinoids, triphenylamines, and their derivatives. These molecules are particularly suitable for modulation of the wavelength of the reflected beam in the visible range, in a reliable and repeatable manner over time. These molecules indeed have good chemical stability and a reversible transition between different redox forms, for a large number of successive cycles. The lifetime of the reflective module is thus improved.

In one example, the electrochromic layer comprises several different electrochromic molecules. Thus, depending on the absorption peaks of the different redox forms of each electrochromic molecule, a wide variety of color shades can be obtained when perceiving the reflected beam.

In one example, the electrochromic molecule is in a colorless form without applying a potential difference. Thus, without applying a potential difference, only the Fabry-Pérot effect can contribute to the wavelengths of the reflected beam. Applying the potential difference allows the redox form of the electrochromic molecule to be modified. Thus, the transmission of the electrochromic layer is maximal without applying a potential difference. This is particularly the case for small electrochromic molecules (whose molar mass is, for example, less than or equal to 600 g/mol), and in particular small organic electrochromic molecules.

According to one example, the electrochromic molecule has at least one colorless form and at least one colored form.

According to one example, the electrochromic layer is in a colorless form without application of a potential difference, in which the electrochromic layer does not exhibit an absorption peak in the visible range. In the colored form, the electrochromic molecule preferably exhibits at least one absorption peak in the visible range.

According to one example, the electrochromic layer has a transmittance greater than or equal to 80%, preferably at least without application of a potential difference. This transmittance makes it possible to further improve the transmission of the incident light beam to the electrochromic layer and, following its reflection from the mirror, the transmission of the beam reflected out of the module.

According to one example, the electrochromic layer has a thickness substantially greater than or equal to 50 nm. According to one example, the electrochromic layer has a thickness substantially less than or equal to 300 nm. According to one example, the electrochromic layer has a thickness substantially between 50 nm and 300 nm, preferably between 100 nm and 200 nm. This thickness range allows the construction by constructive interference by the Fabry-Pérot effect of a beam reflected in the visible range, by the electrochromic layer. These thickness ranges are also particularly suitable for modulating the wavelength in a synergistic manner between the Fabry-Pérot effect and the electrochromic molecule.

According to one example, the reflective module and more particularly the multilayer stack does not comprise an electrolyte layer additional to the electrochromic layer. According to one example, the at least one solid polymer electrolyte and the at least one electrochromic molecule are mixed in the electrochromic layer.

According to one example, the reflective module and more particularly the multilayer stack comprises a single electrochromic layer.

In one example, the multi-layer stack is topped by a second substrate. Thus, the reflective module is protected by this substrate, which is particularly advantageous for applications in the automotive sector.

Preferably, the first, and where appropriate the second, substrate are flexible substrates. This further minimizes the risk of poor electronic contact between the electrode substrates and the electrochromic layer, and therefore limits the risk of colorless areas appearing in the electrochromic layer, even under the application of a potential difference.

Preferably, the first and, where appropriate, the second substrate are based on polyethylene terephthalate or its derivatives.

According to one example, the first and second electrodes each form a layer, the first electrode and the second electrode being arranged on either side of the multilayer stack.

In one example, the system includes an electrical source configured to apply the applied potential difference to the stack.

According to one example, the system comprises a plurality of said reflective modules juxtaposed in at least one direction parallel to, and preferably coincident with, a main extension direction of said reflective modules. The plurality of modules thus forms a plurality of pixels whose reflected wavelength can be modulated according to the potential difference applied to each reflective module. It is therefore understood that the system allows a dynamic module-by-module display of the reflected wavelength. Due to the presence of a solid polymer electrolyte in the electrochromic layer, the system makes it possible to dispense with complex fluid connections, especially since the system comprises a plurality of reflective modules. To dispense with these connections, the skilled person would rather have sought to modulate the thickness of the same layer of conductive polymer in the same reflective module, in order to modify the reflected wavelength from existing solutions. However, this does not allow dynamic modulation of the wavelength pixel by pixel.

In one example, the system is configured to apply a potential difference independently between each reflective module.

According to one example, the system further comprises a lateral light source and a waveguide surmounting the at least one reflective module, the waveguide being configured to transmit a light beam from the light source to the at least one reflective module. When the ambient light is not sufficient to obtain a visible reflection of the desired wavelength, for example at night, the system is thus provided with its own light source to inject a beam into the reflective module and emit a reflected beam of the desired wavelength. Thus, the system has reduced consumption compared to existing systems using active lighting modules, while allowing good visibility at night.

For example, the waveguide is equipped with decoupling elements, such as prisms or suspended particles, making it possible to return the light rays propagating within it to at least one of the reflective modules.

According to one example, the multilayer stack consists of the at least one layer forming the metal mirror, the electrochromic layer, or a plurality of electrochromic layers, and the metal bilayer.

According to one example, the multilayer stack is in direct contact with the first substrate and, where appropriate, the second substrate. According to one example, the multilayer stack is in direct contact with the first and second electrodes.

• le dépôt de l’au moins une couche formant le miroir métallique est réalisé sur le premier substrat,
• le dépôt de la bicouche métallique est réalisé sur la deuxième électrode,
• le dépôt de la couche électrochromique est effectué sur l’un parmi le miroir métallique et la bicouche métallique,
• l’assemblage du premier substrat et de la deuxième électrode comprend une mise en contact de la couche électrochromique avec l’autre parmi le miroir métallique et la bicouche métallique.

According to an example of the process:

• the deposition of at least one layer forming the metal mirror is carried out on the first substrate,
• the deposition of the metallic bilayer is carried out on the second electrode,
• the deposition of the electrochromic layer is carried out on one of the metal mirror and the metal bilayer,
• the assembly of the first substrate and the second electrode comprises contacting the electrochromic layer with the other of the metal mirror and the metal bilayer.

The reflective module is thus manufactured in two sub-modules that can be more easily assembled. This allows for parallel manufacturing of the two sub-modules, which reduces manufacturing time. In addition, the formation of the electrochromic layer is thus decoupled from the formation of a layer above it. The risk of damage to the electrochromic layer by depositing another layer above it is thus avoided.

In one example, the method includes providing a second substrate, the second substrate including the second electrode.

According to one example, the method further comprises, after deposition of the electrochromic layer on one of the metal mirror and the metal bilayer, assembling the first and second substrates by the electrochromic layer and the other of the metal mirror and the metal bilayer, to form the multilayer stack.

A substrate or layer “based on” a species A means a substrate or layer comprising this species A only or this species A and possibly other species.

Several embodiments of the invention implementing successive steps of the manufacturing process are described below. Unless explicitly stated, the adjective “successive” does not necessarily imply, even if this is generally preferred, that the steps follow one another immediately, intermediate steps being able to separate them.

Furthermore, the term “step” means the carrying out of a part of the process, and can designate a set of sub-steps.

Furthermore, the term "step" does not necessarily mean that the actions carried out during a step are simultaneous or immediately successive. Certain actions of a first step may in particular be followed by actions linked to a different step, and other actions of the first step may be repeated subsequently. Thus, the term "step" does not necessarily mean unitary and inseparable actions in time and in the sequence of the phases of the process.

It is specified that in the context of the present invention, the thickness of a layer or a substrate is measured in a direction perpendicular to the surface along which this layer or this substrate has its maximum extension. The thickness is thus taken in a direction perpendicular to the main faces of the substrate on which the different layers rest.

It is specified that, in the context of the present invention, the terms “on”, “overcomes”, “covers”, “underlying”, “facing” and their equivalents do not necessarily mean “in contact with”. Thus, for example, the deposition, transfer, bonding, assembly or application of a first layer on a second layer does not necessarily mean that the two layers are in direct contact with each other, but means that the first layer at least partially covers the second layer by being either directly in contact with it, or by being separated from it by at least one other layer or at least one other element. By “in contact”, it is meant that a fine interface may exist, for example caused by manufacturing variability.

In the following detailed description, terms such as "longitudinal", "transverse", "upper", "lower" may be used. These terms must be interpreted relatively in relation to the position of the elements of the reflective module or of the system once assembled, by assimilating the direction normal to the main extension plane of the layers of the stack, to the vertical direction. A lateral or transverse dimension is understood as a dimension in a plane parallel or coincident with the main extension plane of the layers of the stack.

By “juxtaposed” elements we mean here that these elements are arranged side by side according to their main extension plane or arranged one above the other according to the direction of stacking, this direction being perpendicular to the main extension plane.

A parameter that is "substantially equal to/greater than/less than" a given value means that this parameter is equal to/greater than/less than the given value, within plus or minus 10% of this value. A parameter that is "substantially between" two given values means that this parameter is at least equal to the smallest given value, within plus or minus 10% of this value, and at most equal to the largest given value, within plus or minus 10% of this value.

By "nanometric", and more particularly "nanometric thickness" or "nanometric size", we mean a dimension, for example a thickness, greater than or equal to 1 nm and strictly less than 1 µm.

In the context of the invention, the visible range, or spectrum, corresponds to the wavelength range between 350 and 900 nm, and preferably between 400 and 800 nm.

The expression "A and/or B" means (A), (B), or (A and B). The expression "A, B and/or C" means (A), (B), (C), (A and B), (A and C), (B and C), or (A, B and C).

The multilayer electroactive reflective module 1 and the reflective system 3 comprising it are now described according to several exemplary embodiments.

As illustrated for example in Figures 1A and 1B, the reflective module 1 comprises a first substrate 10, on which a multilayer stack 18 is deposited. The substrate has a lower surface 10a and an upper surface 10b. The stack 18 can be arranged on the upper surface 10b, if necessary on the electrode 12 itself arranged on the upper surface 10b. The multilayer stack 18 is configured to receive an incident light beam 2 and reflect a reflected light beam 2'. The incident light beam 2 has a wavelength spectrum. By Fabry-Pérot effect, a part of this wavelength spectrum will be transmitted by constructive interference then reflected on a metal mirror 14 to form the reflected beam 2'. Furthermore, the multilayer stack 18 is configured to modulate the wavelengths of the reflected beam 2' with a modification of the optical properties of the multilayer stack 18 under the application of a potential difference. It is therefore understood that the wavelength spectrum of the reflected light beam 2' is at least distinct, and can be reduced in wavelength, compared to the spectrum of the incident beam 2.

To enable this reflection, the multilayer stack 18 comprises at least one reflective metal mirror 14 and an electrochromic layer 15. This layer 15 is configured to let out a wavelength determined by the Fabry-Pérot effect by constructive interference, and is configured to have optical properties modulated according to the applied potential difference. More particularly, the absorbance spectrum of the electrochromic layer 15 can be modified according to the applied potential difference. Note that the term “a wavelength” for the reflected beam 2’ is not limited to an isolated wavelength but can designate a range of wavelengths.

The Fabry-Pérot effect is first presented. The electrochromic layer 15, of nanometric thickness and typically of the order of one or several hundred nm, forms a Fabry-Pérot cavity in which the incident beam 2 is confined. This cavity produces, from the light it receives, interferences of a determined wavelength. These interferences result in multiple reflections of rays of a given wavelength propagating inside the cavity.

In addition and synergistically with this constructive interference phenomenon, the electrochromic layer 15 comprising at least one electrochromic molecule 17, the electrochromic layer 15 can exhibit absorption of certain wavelengths as when pigments or dyes are used. The electrochromic molecule(s) 17 are molecules forming part of at least one redox couple, which can pass from one redox form to another under the application of a potential difference in a conductive medium.

At least one redox form of an electrochromic molecule 17 is said to be “colored,” that is, the absorbance spectrum of the molecule in this redox form has an absorption peak in the visible range. The absorbed wavelength is thus removed from the spectrum of the light beam, which modifies the constructive interference by the Fabry-Pérot effect and the spectrum of the reflected beam 2’ at the output of the module 1. An electrochromic molecule 17 may have several colored forms, as seen in more detail later with reference to particular examples. At least one redox form of an electrochromic molecule 17 may be “colorless,” that is, the absorbance spectrum of the molecule in this redox form does not have an absorption peak in the visible range.

Depending on the potential difference applied to the electrochromic layer 15, it is understood that the electrochromic molecule(s) 17 may be in one redox form or another, and thus modulate the optical properties of the layer 15. Once the electrochromic molecule 17 has passed into a redox form after application of a potential difference, the electrochromic molecule 17 may remain in this form. Electrochromism may also benefit from a certain “memory effect”. It is therefore not necessary to maintain a potential difference to obtain a particular color rendering, even if it is possible, or even advantageous, to apply the potential difference at regular intervals to improve the maintenance of the color of the module. The consumption of the reflective module may therefore be reduced.

In order to modify the optical properties of the electrochromic layer 15, the module 1 comprises two electrodes 12, 13 configured to apply a potential difference to the stack 18, and more particularly to the electrochromic layer 15. A first electrode 12 and a second electrode 13 electrically connect the multilayer stack 18 on either side. These two electrodes 12, 13 can each form a layer arranged on either side of the stack 18, as illustrated in FIGS. 1A, 1B. Alternatively, it can be provided that these electrodes are connected to the stack 18 without each forming a layer, for example being formed on the edge of the stack 18 and electrically connecting electrically conductive parts of the stack.

To apply this potential difference, the reflective system 3 may comprise an electrical source 30 electrically connected to the first 12 and second 13 electrodes, for example an electronic controller. According to one example, the potential difference applied by the source 30 is in absolute value between 0 V excluded and 2 V, preferably between 0 V excluded and 1 V.

An electrochromic molecule 17 is capable of being modified by an oxidation-reduction reaction under application of a potential difference. During this reaction, for example illustrated by the , the redox form of the electrochromic molecule 17 is modified. The represents the electrochromic layer 15 comprising a mixture of the solid polymer electrolyte 16 and the electrochromic molecule 17. For example, the redox form of the molecule without application of a potential difference, for example its neutral form, is colorless. An electrolyte 16 comprises ions 160 to form an ionic conductive medium. Under the application of a potential difference, the electrochromic molecule can be oxidized and pass into a colored redox form. The electrochromic molecule 17 having a colored form, it absorbs a part of the visible spectrum and lets another part of the visible spectrum pass, giving the perceived color. It is therefore understood that the wavelength of the beam 2' leaving the module 1 can therefore be modulated according to the applied potential difference. For example, under the application of another potential difference, the electrochromic molecule can be transformed towards its initial redox form or pass into another redox form. Note that depending on the nature of the electrochromic molecule 17, the molecule can be oxidized and/or reduced in a plurality of redox forms.

When applying this potential difference, the electrochromic layer 15 may have a small or even no variation in its thickness d15, unlike existing solutions implementing a variation in the thickness to modify the wavelength transmitted by interference by the Fabry-Pérot effect. For example, when applying this potential difference, the thickness d15 of the electrochromic layer 15 has a variation of less than 1% of its initial thickness, preferably the thickness of the electrochromic layer 15 remains constant.

In order to form an ionic conductive medium in which the redox reactions will take place, and therefore to ensure ionic transport between the two electrodes and balance the charges resulting from the redox processes, the electrochromic layer 15 comprises a solid polymer electrolyte 16. The solid polymer electrolyte has good ionic conductivity for this purpose. The electrochromic layer 15 is therefore in the solid or semi-solid state, for example in the form of a gel or semi-solid material. This prevents leaks and reduces the size and weight of the reflective module 1. The architecture of the reflective module 1 is also simplified. The reflective module 1 is thus more easily incorporated into existing assemblies, for example in car parts in the context of an automotive application, as described in more detail later. Many solid polymer electrolytes can be manufactured from commercially available products, facilitating the manufacture of the module 1 and reducing its manufacturing cost.

In order to improve the reflection properties of the reflective module 1, the module further comprises a partially transparent metal bilayer 19. The metal bilayer 19 surmounts the electrochromic layer 15. The metal bilayer 19 can therefore be arranged between the electrochromic layer 15 and the second electrode 13. The metal bilayer acts as a broadband absorber in order to absorb a portion of the incident light radiation 2 to carry out a first selection of the wavelengths reaching the electrochromic layer.

Synergistically between the Fabry-Pérot effect, electrochromism and broadband absorption of the metal bilayer, the wavelengths in the visible range are selected to reduce the wavelength range of the reflected beam. The resulting perceived color is therefore more vivid and appears more intense.

Figures 2A and 2B illustrate this. In these figures, the reflectivity R of a layer of conductive polymer, and more particularly of polymethylmethacrylate PMMA, between 400 and 800 nm is measured, depending on the thickness 5 of the polymer layer (in nm). The corresponds to an example without metallic bilayer deposited on the polymer layer, the corresponds to an example with a 19 metal bilayer of 190 gold (3 nm) and 191 chromium (3 nm) deposited on the polymer layer. For a given polymer thickness, it can be observed that the reflectivity peak is much narrower in terms of wavelengths with the 19 metal bilayer. Depending on the thickness of the polymer layer, a reflectivity peak greater than 0.90 can be observed over the entire visible range.

The reflective module 1 may further comprise a second substrate 11 surmounting the multilayer stack 18, and where appropriate surmounting the second electrode 13. The second substrate 11 has a lower surface 11a and an upper surface 11b. The stack 18 may be arranged on the lower surface 11a, where appropriate on the electrode 13 itself arranged on the lower surface 11a. The multilayer stack 18 and the electrodes 12, 13 may therefore be enclosed by the first 10 and second 11 substrates. The second substrate 11 may form an entry diopter for the incident light beam 2, and an exit diopter for the reflected beam 2'. The second substrate 11 is therefore preferably configured to allow these beams 2, 2' to pass. The substrate 11 preferably has a transmittance greater than or equal to 75%, preferably substantially equal to 80%.

The reflective module 1, or the system 3 comprising it, can be incorporated into parts such as car parts. illustrates by way of example a front part of a car front hood comprising in the center the reflective system 3. It is possible to provide for the reflective module or the reflective system 3 to be incorporated into other parts for example inside the passenger compartment or on other parts of the bodywork.

Preferably, the reflective module 1 has a reaction time of the order of a second, preferably less than or equal to a few seconds, preferably less than or equal to 1 second, and more preferably less than or equal to 200 ms. This is notably linked to the nature of the electrochromic molecule 17, and to the ionic conductivity of the solid polymer electrolyte 16, as well as to the thickness of the electrochromic layer 15.

According to one example, the reflective module 1 has a reflection rate of an incident light beam 2, greater than 50%, for example between 50% and 95%.

Particular examples of system 3 are now described with reference to Figures 4 and 5.

The system 3 may comprise at least one reflective module 1, and preferably several reflective modules 1. As illustrated in , the reflective module 1, and preferably each reflective module 1, may comprise a mask 111 configured to partially mask the transmission of the incident beams 2 and reflected beams 2'. This mask 111 may for example be placed on the upper surface 11b of the second substrate 11. The mask 111 may define zones 111 blocking the transmission of light and zones 111a allowing the incident beams 2 and reflected beams 2' to pass. For a reflective module 1, the mask 111 therefore makes it possible to reveal a pattern. Note that this mask 111 may be common to several juxtaposed reflective modules 1.

According to one example, the system 3 may comprise a plurality of reflective modules 1 juxtaposed along at least one so-called “juxtaposition” direction, parallel to or coincident with a main extension direction of these modules 1. Preferably, the reflective modules 1 are juxtaposed along at least two so-called “juxtaposition” directions of a plane parallel to or coincident with a main extension plane of these modules 1. The system 3 thus forms a matrix of pixels, each reflective module 1 being able to form a pixel. The system 3 allows a dynamic module-by-module display of the reflected wavelength.

The fact that the electrolyte is in solid or semi-solid form makes it possible to obtain more complex architectures with a plurality of reflective modules 1. This in fact makes it possible to do without a reserve of liquid electrolyte accompanying each pixel, and therefore limiting the filling factor of the pixel matrix formed. This also simplifies the system compared to the use of a remote reserve of liquid electrolyte, the fluid connections of which to each reflective module 1 would be complex.

The system 3 may for example comprise at least five juxtaposed reflective modules 1, preferably at least five juxtaposed reflective modules 1 per direction of juxtaposition.

In order to be able to modulate the wavelength of the light beam 2' leaving the module, and this module by module, the electrical source 30, or equivalently voltage source 30, can be configured to apply a potential difference to each reflective module 1, independently of each other. There can be one electrical source 30 per reflective module 1. Thus, the modules 1 can be controlled independently of each other for a pixelated animation. Alternatively, it is possible to provide for there to be a single electrical source 30, for example which applies the same voltage to all the reflective modules to simplify the control circuit.

From the above description, it is understood that the reflective module 1 can reflect an incident beam 2 coming from the external environment to the reflective module 1, for example ambient light. In a dimly lit environment, for example at night, it may be advantageous to retain a display function by the reflective module(s) 1. For this, and as illustrated in , the reflective system 3 may comprise at least one light source 31, preferably lateral. The light source 31 is configured to emit a light beam 2''. This light beam 2'' will then play the role of the incident light beam 2 described previously. According to this example, it is therefore understood that the reflective module 1 can reflect an incident beam 2 coming from the environment and/or a light beam 2'' coming from the light source 31. The system 3 may further comprise a waveguide 32 configured to transmit the light beam 2'' coming from the light source 31 to one or more reflective modules 1. For this, the waveguide 32 may comprise internal total reflection elements configured to conduct the light beam 2'' coming from the source. Depending on the angle of reflection of the beam in the waveguide, the beam 2'' from the source 31 can be transmitted to a reflecting module 1 or continue its propagation in the waveguide 32.

For example, the waveguide 32 may comprise prisms 320 configured to modify the optical path of a portion of the beam 2'' coming from the source 31 to send it to the corresponding reflecting module 1. A person skilled in the art is able to produce a waveguide in accordance with the arrangement of one or more reflecting modules 1. The prisms 320 may for example be arranged at regular intervals along the waveguide 32, in accordance with the juxtaposition of the reflecting modules 1. Other structures may be provided as an alternative or in addition to the prisms 320 by a person skilled in the art, for example suspended particles.

As for example illustrated in , the reflective module 1 may have a part 1a offset relative to the multilayer stack 18, comprising a portion 110, 130, 120, 100 respectively of the substrates 10 and/or 11 and/or of the electrodes 12 and/or 13. The surfaces 10b and 11a of the first 10 and/or second 11 substrates, and where appropriate the electrodes 12, 13, are thus only partly covered by the stack 18, as illustrated by FIGS. 4, 7B. represents a top view of the first substrate 11 covered by the mirror 14, illustrating this. This makes it possible to facilitate the electrical connection of the reflective module 1 to the electrical source 30. For example, the first 10 and/or the second 11 substrates, preferably with the associated electrode 12, 13, may extend in at least one direction of the main extension plane of the layers of the stack 18, over a distance greater than a corresponding distance of the costs of the stack 18. The part 1a may be connected to the electrical source 30. The part 1a may further facilitate the integration of the stack 18 into the system 3, without necessarily being connected to the electrical source 30.

Examples of dimensions of the reflective module 1 are now given. Each module 1 can extend in the main extension plane of the layers of the stack 18. Each reflective module 1 can in this plane have lateral dimensions, in directions perpendicular to each other, in the ranges of values of the order of a few millimeters for small surfaces, or even a few meters for large surfaces. The thickness of the multilayer stack 18 can be substantially greater than or equal to 50 nanometers (nm), and substantially less than or equal to 350 nanometers (nm). It is therefore understood that the reflective module is compact and therefore more easily integrated into existing parts, for example automobile parts, in particular compared to existing solutions using liquid electrolytes.

As illustrated by figures 7B and 7C, the first 10 and/or the second 11 substrates can extend in at least one direction of the main extension plane of the layers of the stack 18, over a distance d1 less than or equal to 5 mm, preferably 3 mm, relative to the layers of the stack 18.

Reflective module 1 is now written in more detail element by element.

• ts est l'épaisseur de la couche du ou des substrats 10, 11 ;
• tp est l'épaisseur totale des couches de l’empilement 18
• tf est l'épaisseur totale des couches d’électrodes 12, 13,
• rc est le rayon de courbure.

The first substrate 10 and/or the second substrate 11 are preferably flexible substrates. This facilitates the incorporation of the reflective modules into existing parts, and increases the mechanical strength of the reflective module 1. A material or layer is considered to be flexible if the mechanical and electrical properties of the film remain unchanged even under a significant strain of 2.5% with a concave and convex curvature radius of 0.5 mm. The deformation (flexibility) of the reflective module 1 can be evaluated using the following equation: deformation = (ts - tp - tf ) / (2.rc), where:

• ts is the thickness of the layer of the substrate(s) 10, 11;
• tp is the total thickness of the layers in the stack 18
• tf is the total thickness of the electrode layers 12, 13,
• rc is the radius of curvature.

According to one example, the substrate 10 and/or the second substrate 11 are manually deformable without tools. As seen previously, at least the second substrate 11 may have a transmittance greater than or equal to 75% in the visible spectrum. According to one example, the first substrate 10 and/or the second substrate 11 are based on or made of polymer. More particularly, the first substrate 10 and/or the second substrate 11 are based on or made of polyethylene terephthalate (PET), PMMA or their derivatives. Note that other polymers can be envisaged.

In the case of a plurality of juxtaposed reflective modules 1, the first 10 and/or the second 11 substrates may be common to a plurality of modules 1. Alternatively, it may be provided that each module comprises its own substrate(s), distinct between different modules 1. This makes it possible to facilitate the manufacture of each reflective module 1, which will then be assembled together for example on a common support 33, as illustrated in .

The electrodes 12, 13 may be in the form of a layer deposited on the first 10 and second 11 substrates respectively. For example, the electrodes 12, 13 are based on or made of indium tin oxide (ITO).

The metal mirror 14 may be formed from at least one metal layer 140, for example based on or made of aluminum. Good reflection of the incident beam 2 is thus obtained. The metal mirror 14 may further comprise layers allowing better chemical compatibility with the electrochromic layer 15. For this, the metal mirror 14 may comprise a layer based on or made of gold 142. The gold layer 142 may thus be in contact with the electrochromic layer 15 without risking degradation of this layer or of the metal mirror 14. Note that several metal mirrors 14 may be used in the stack 18, or even in the reflective module 1. For example, it is possible to provide a superposition of one or more substrate(s) 10 and one or more mirror(s) 14. In order to attach the gold layer 142 to the aluminum layer 140, the mirror 14 may comprise a bonding layer based on or made of chromium 141 between these layers 140, 142. The metal mirror is preferably of nanometric thickness, that is to say of thickness less than 1 µm. For example, the aluminum layer 140 may have a thickness d140 substantially between 40 nm and 80 nm, for example between 50 nm and 70 nm, and preferably substantially equal to 50 nm. The chromium layer may have a thickness d141 substantially equal to 5 nm. The gold layer may have a thickness d142 substantially equal to 7 nm.

In the electrochromic layer 15, the solid polymer electrolyte 16 may comprise an ionogel comprising a polymer matrix and an ionic liquid, and/or a polymeric ionic liquid, for example with a polymer matrix.

• Chlorure de 1-éthyl-3-méthylimidazolium ;
• 1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (abrégé EMIM TFSI) ;
• 1-Ethyl-3-methylimidazolium triflate (abrégé EMIM Triflate) ;
• Liquide ionique (IL) bio-compatible à base de choline.

In the example of the ionogel, the polymer matrix can be chosen from poly(vinyl alcohol) (abbreviated PVA); polyethers - for example polyethylene glycol (abbreviated PEG) and poly thio-ether. The ionic liquid can be chosen from the following components:

• 1-Ethyl-3-methylimidazolium chloride;
• 1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (abbreviated EMIM TFSI);
• 1-Ethyl-3-methylimidazolium triflate (abbreviated EMIM Triflate);
• Biocompatible choline-based ionic liquid (IL).

As known to those skilled in the art, a polymeric ionic liquid (in English polymeric ionic liquid ) is an ionic conductive polymer obtained from the polymerization of ionic liquid monomers. According to some examples, and in a manner known to those skilled in the art, the polymeric ionic liquids may be solid and have sufficient mechanical strength to form the layer 15. According to other examples, the polymeric ionic liquids may have insufficient strength to form the layer 15 on their own, they are then typically soluble when exposed to an organic solvent. In order to provide the polymeric ionic liquids with mechanical properties, the electrolyte may comprise a polymeric ionic liquid and a polymer matrix forming a mechanical support. It is possible, as an alternative or in addition, to crosslink a solid polymer liquid with crosslinkable chemical bonds (for example C=C bonds). After crosslinking, the polymer network formed is insoluble.

The solid polymer electrolyte thus exhibits good ionic conductivity, allows redox reactions in the electrochromic layer 15 and improves the stabilization of the chemical species involved in redox processes. Ionogels and solid polymer electrolytes based on polymeric ionic liquids exhibit good chemical and mechanical stability. In particular, they exhibit a wide electrochemical stability window. For example, the solid polymer electrolyte exhibits an electrochemical stability window greater than or equal to 3 V, for example substantially equal to 3.2 V.

Furthermore, the ionogels and solid polymer electrolytes based on the polymeric ionic liquids are sufficiently deformable and stretchable to accommodate deformations of the module and mechanical stresses during use, which is particularly advantageous for automotive applications. According to one example, the Young's modulus of the solid polymer electrolyte is substantially between 0.2 and 4 MPa. The elongation at break may be substantially greater than or equal to 100%, for example substantially between 150% and 160%. The reflective module 1 thus has a long service life despite the stresses that may be exerted on the module. The ionogels and solid polymer electrolytes based on the polymeric ionic liquids also allow the production of patterns, for example by photolithography. Patterns may in particular be used to manufacture decorative films. Preferably, the electrochromic layer 15 has a transmittance greater than or equal to 80%.

According to one example, the electrochromic layer 15 may further have an ionic conductivity greater than or equal to 1.10 ^-4 S/cm at room temperature. The ionic conductivity may be substantially between 10 ^-4 S/cm and 10 ^-2 S/cm at room temperature. The ionic conductivity may be of the order of 1.10 ^-3 S/cm. These ionic conductivities are notably achievable due to the use of an ionogel. In the case of a polymeric ionic liquid, the solid polymer electrolyte layer may further have an ionic conductivity less than or equal to 1.10 ^-4 S/cm at room temperature.

The solid polymer electrolyte 16 may be based on at least one polymer chosen from polyethers, polycarbonates (for example polybutylene glutarate abbreviated PBG), polyesters (for example polymethylmethacrylate, abbreviated PMMA), polynitriles (for example polyacrylonitrile, abbreviated PAN), polyalcohols (for example polyvinyl alcohol, abbreviated PVA), polyamines (for example polyethyleneimine, abbreviated PEI), polysiloxanes (for example polydimethylsiloxane, abbreviated PDMS), fluoropolymers (for example polyvinylidene fluoride, abbreviated PVDF, and poly(vinylidene fluoride-co-hexafluoropropylene), abbreviated P(VDF-co-HFP)), biopolymers (for example lignin, chitosan and cellulose) and their derivatives. The solid polymer electrolyte may, for example, comprise a copolymer of which at least one of the monomer units corresponds to the polymers cited above.

For the formation of ionic liquids, and in a manner known to those skilled in the art, the cations that can be used are, for example, 1,3-dialkylimidazolium, N-alkylpyridinium, tetraalkylammonium, tetraalkylphosphonium and N-alkylpyrrolidinium. The anions that can be used are, for example, bis(trifluoromethylsulfonyl)imide, hexafluorophosphate, tetrafluoroborate, trifluoromethanesulfonate, chloride ion, bromide ion, iodide ion, nitrate ion, acetate. These ions can have different hydrophobic or hydrophilic properties. Depending on the nature of the polymer matrix, for example, or the desired hydrophobic or hydrophilic properties, it is understood that the ions used to form an ionic liquid, whether polymeric or not, can be adapted.

The electrochromic layer 15 preferably has a thickness of between 50 and 300 nm, preferably between 100 nm and 200 nm. Since the electrochromic molecules are distributed in the matrix formed by the solid polymer electrolyte 16, it is not necessary to have a greater thickness of the electrochromic layer 15. The compactness of the reflective module 1 is therefore improved. Limiting the thickness d15 of the electrochromic layer 15 also promotes high transmittance, and therefore the transmission of the incident 2 and reflected 2' beams. Preferably, the electrochromic molecules 17 are distributed homogeneously in the solid polymer electrolyte 16.

According to one example, the electrochromic molecule is an organic molecule, for example a polymer molecule or macromolecule. Preferably, the electrochromic molecule has a molar mass less than or equal to 600 g/mol. Many molecules are thus commercially available to reduce the manufacturing cost of the module 1. By way of example, the electrochromic molecule 17 can be chosen from: viologens, spiropyrans, bipyridines, carbazoles, methoxyphenyls, quinones, tetrathiafulvalene, phenylenediamine, pyrazoline, porphyrinoids, and in particular thiopheneporphyrinoids, furanporphyrinoids, triphenylamines, and their derivatives. For example, viologens result from the quaternization of bipyridine.

Examples of chemical formulas of these compounds are given below for illustration.

A person skilled in the art is perfectly capable of identifying derivatives of these molecules that may have electrochromic properties, as described for example in Stolar, Monika. " Organic electrochromic molecules: synthesis, properties, applications and impact" Pure and Applied Chemistry , vol. 92, no. 5, 2020, pp. 717-731. https:/doi.org/10.1515/pac-2018-1208. A derived molecule may, for example, be a molecule having a substituent group, for example a group or a carbon chain carrying or not at least one heteroatom.

The redox equilibria below illustrate as examples redox forms of a viologen during successive redox reactions:

Viologen can typically exhibit 3 different redox forms: colorless, blue/purple, orange/red.

The redox equilibrium below illustrates as an example two redox forms of a spiropyran:

A spiropyran can typically exhibit two different redox forms at the following potential differences: colorless (0 V), orange-pink when ΔV = 2 V (at -1.2 V and 0.8 V for example).

The electrochromic layer 15 may further comprise other chemical compounds, and in particular to participate in the redox reactions of the electrochromic molecule 17. For example, the electrochromic layer 15 comprises a redox mediator. The redox mediator may for example be the potassium ferrocyanide and potassium ferricyanide pair, and/or the TEMPO radical (2,2,6,6-Tetramethylpiperidin-1-yl)oxyl or (2,2,6,6-tetramethylpiperidin-1-yl)oxidanyl).

The metal bilayer 19 is now described in more detail, for example with reference to Figures 1A and 1B. The metal bilayer 19 comprises a first metal layer 190 and a second metal layer 191. The first 190 and second 191 layers are preferably in contact with each other. The first layer 190 is preferably in contact with the electrochromic layer 15. The second metal layer 19 is preferably in contact with the second electrode 13.

According to one example, the first metal layer 190 is based on or made of gold and the second metal layer 191 is based on or made of chromium or vice versa. Chromium has an absorption peak mainly in the blue and little in the red, whereas gold has an absorption peak mainly in the red and little in the blue. The use of these two metals in the metal bilayer 19 makes it possible to obtain the broadband absorber function in a manner distributed over the visible spectrum. Other metals can be considered, such as silver for example.

Preferably, the first metal layer 190 is based on or made of gold. Gold in fact has better chemical compatibility with the electrochromic layer 15.

The thickness of the metal layers 190 and 191 is chosen so as to partially absorb the incident light radiation 2 while being sufficiently transparent to allow good reflection by the reflective module 1. In order to allow the part of the incident radiation 2 not absorbed to pass through, the metal bilayer 19 preferably has a transmittance greater than or equal to 75%, preferably substantially equal to 80%.

For this, the first 190 and second 191 metal layers have a non-zero thickness, preferably greater than or equal to 3 nm. To maintain good transparency, as for example illustrated by FIGS. 1A and 1B, the first 190 and second 191 metal layers may each have a thickness d ₁₉₀ , d ₁₉₁ preferably less than 10 nm, preferably substantially less than or equal to 7 nm, preferably substantially less than or equal to 5 nm, and preferably substantially equal to 3 nm. The metal bilayer 19 may have a total thickness substantially less than or equal to 20 nm, preferably substantially less than or equal to 10 nm.

According to an example, illustrated by the , the metal bilayer 19 is free of nano-sized holes 192. The metal bilayer 19 can be continuous without interruption.

As illustrated for example by the , the metal bilayer 19 may alternatively comprise holes of nanometric size. This makes it possible to improve the extraction of the radiation reflected by the module 1, this radiation passing through fewer layers at the level of these holes 192. For example, their largest dimension in the main extension plane of the metal bilayer 19 is nanometric. This dimension may be their diameter d ₁₉₂ .

At least one of, and preferably each of, the first 190 and the second 191 metal layers have these holes 192. These holes 192 can pass through at least 90%, and preferably substantially 100%, of the layer 190, 191 in question, and preferably both layers 190, 191, in a direction substantially perpendicular to the main extension plane of the bilayer 19.

According to one example, the holes 192 represent, in projection onto the surface of the layer considered, 20% to 40% of the surface of the corresponding layer(s).

As illustrated by the , these holes 192 may have a circular section, this section being more particularly taken in the main extension plane of the metal bilayer 19. The distribution of these holes 192 in the corresponding layer(s) is preferably carried out regularly in the center and at the four corners of squares subdividing these layers. Each of the squares has for example a side with a length L ₁₉₂ of 600 nm. The diameter d ₁₉₂ of each of the holes 192 may be substantially equal to 200 nm.

The method for manufacturing the reflective module 1 is now described according to several exemplary embodiments with reference to figures 6 to 12. Note that the method can comprise any step allowing the characteristics of the reflective module 1 described above to be obtained. Particular examples of manufacturing recipes are also given. The deposition parameters and techniques can be configured to obtain the thicknesses described previously.

As illustrated by the , the method comprises providing the first substrate 10 comprising the first electrode 12. The method may comprise a step of depositing the first electrode 12 on the first substrate 10, and more particularly on its upper face 10b, for example by depositing a layer of ITO on the first substrate 10.

The method then comprises forming the multilayer stack 18 as introduced above.

For this, the method may comprise the deposition of the metal mirror 14 on the first substrate 10, and more particularly on the first electrode 12, as illustrated in FIGS. 6 to 7C. As seen previously, the mirror 14 may comprise several successive layers of metals. These layers 140, 141, 142 may be formed by any physical deposition technique, for example by cathode sputtering, by electron beam evaporation, by flash evaporation or by induction evaporation.

On the metal mirror 14 formed, the method can comprise the deposition of the electrochromic layer 15. For this, numerous deposition techniques can be envisaged, depending in particular on the nature of the solid polymer electrolyte 16. Note that as an alternative or in combination, according to a variant not illustrated, it can be provided that the electrochromic layer 15 is deposited on the second electrode 13, and in particular on the metal bilayer 19.

For this, numerous deposition techniques can be envisaged, depending in particular on the nature of the solid polymer electrolyte 16. Deposition is carried out drop by drop, with a blade, by centrifugation (generally referred to as spin-coating). For example and as illustrated by FIGS. 8A and 8B, the deposition of the electrochromic layer can comprise the deposition of a precursor solution to form a layer 15'. According to one example, the precursor solution comprises between 1% and 10% by mass, preferably substantially 5% of electrochromic molecule. This deposited layer 15' can then form the electrochromic layer 15 by heat treatment and/or by UV radiation and/or by drying.

• la préparation d’une solution aqueuse de 4 % de PVA, de viologène et d’un mélange de ratio molaire 1 :1 de ferrocyanure de potassium et de ferricyanure de potassium,
• le mélange de la solution aqueuse à un ratio volumique 4 :1 avec une solution aqueuse de borax jusqu’à obtenir un gel,
• le dépôt du mélange sur le miroir métallique 14,
• le séchage à température ambiante pour obtenir la couche 15 sans bulle.

According to a first particular example, the deposition of the electrochromic layer 15 can comprise:

• the preparation of a 4% aqueous solution of PVA, viologen and a mixture of potassium ferrocyanide and potassium ferricyanide with a molar ratio of 1:1,
• mixing the aqueous solution at a volume ratio of 4:1 with an aqueous borax solution until a gel is obtained,
• the deposition of the mixture on the metal mirror 14,
• drying at room temperature to obtain layer 15 without bubbles.

• une préparation d’une solution de spiropyrane à 2 mM et de TEMPO ((2,2,6,6-Tetramethylpiperidin-1-yl)oxyl ou (2,2,6,6-tetramethylpiperidin-1-yl)oxidanyl)) à 4 mM, dans du BMIM-TFSI (1-Butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide),
• le mélange de la solution préparée avec une solution de P(VDF-co-HFP), la solution de P(VDF-co-HFP) étant à 1 :20 en ratio massique avec de l’acétone, pour obtenir un ratio de 83, 3 % en masse de BMIM-TFSI,
• le dépôt du mélange sur le miroir métallique 14,
• le séchage à température ambiante pendant 24h pour obtenir la couche 15.

According to a second particular example, the deposition of the electrochromic layer 15 may comprise:

• a preparation of a solution of 2 mM spiropyran and 4 mM TEMPO ((2,2,6,6-Tetramethylpiperidin-1-yl)oxyl or (2,2,6,6-tetramethylpiperidin-1-yl)oxidanyl)) in BMIM-TFSI (1-Butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide),
• mixing the prepared solution with a P(VDF-co-HFP) solution, the P(VDF-co-HFP) solution being at 1:20 in mass ratio with acetone, to obtain a ratio of 83.3% by mass of BMIM-TFSI,
• the deposition of the mixture on the metal mirror 14,
• drying at room temperature for 24 hours to obtain layer 15.

In this example, the solid polymer electrolyte layer has a transmittance of 83%.

Preferably, the reflective module 1 is manufactured in two sub-modules 1' and 1'' which can be more easily assembled. The formation of the electrochromic layer 15 is thus decoupled from the formation of either the mirror 14 or the formation of the metal bilayer 19, depending on which layer the layer 15 is deposited. According to this example, following the deposition of the electrochromic layer 15, a first sub-module 1' is obtained. A second sub-substrate 1'' is then manufactured for their subsequent assembly.

Note that it is possible to provide as an alternative that the reflective module 1 is formed layer by layer starting from the first substrate 10 by successively stacking the layers to be deposited to form the reflective module 1, according to the same techniques described. However, this risks damaging the electrochromic layer 15 during the deposition of the mirror 14 or the metal bilayer 19 and the second electrode 13 which would then surmount the electrochromic layer 15.

The fabrication of the second sub-module 1'' is now described. The second substrate 11 may be provided, comprising the second electrode 13 deposited on the lower surface 11a, as illustrated by the The method may comprise a step of depositing the second electrode 13 on the second substrate 11, for example by depositing a layer of ITO on the second substrate 11.

The metallic bilayer 19 can be deposited on the second electrode 13, according to the same techniques as those described for the deposition of the mirror 14, as illustrated by the . Alternatively, it can be provided that the metal bilayer 19 is deposited on the electrochromic layer 15, as illustrated in . It is further possible to provide that the deposition of the electrochromic layer 15 is carried out on the metallic bilayer 19, according to a variant not illustrated.

As illustrated by the , according to the example in which one and/or the other of the metal layers 190, 191 of the bilayer comprises nanometric holes 192, the holes 192 can be produced for example by a colloidal lithography process or by electron beam lithography (more commonly referred to in English as “ e-beam lithography ”). These techniques offer good precision and good regularity in the creation of a homogeneous distribution of the holes 192 in a material, freeing themselves from the use of a mask.

The reflective module 1 can then be obtained by an assembly or equivalently a transfer, of the sub-modules 1' and 1''. For example and as illustrated by the , the exposed surfaces 15a, 19a respectively of the electrochromic layer 15 and of the metal bilayer 19 can be brought into contact. In order to secure the sub-modules 1', 1'', it can for example be provided that this assembly is carried out when the electrochromic layer 15 is not entirely solidified, this solidification being finalized after assembly of the sub-modules 1', 1''.

The method for manufacturing the reflective system 3 may comprise, for each module, the manufacturing steps previously stated. The method for manufacturing the reflective substrate 3 may further comprise the electrical connection of the electrical source 30 to the first 12 and the second 13 electrodes. This method may further comprise steps for mounting the plurality of reflective modules 1, for example on a common support 33, as illustrated in This method may further comprise steps of mounting additional elements of the system, for example the lateral light source and the waveguide or even the mask 111.

The invention is not limited to the embodiments previously described and extends to all embodiments covered by the invention. The present invention is not limited to the examples previously described. Many other variant embodiments are possible, for example by combining previously described characteristics, without departing from the scope of the invention. For example, it is possible for the reflective module to comprise a plurality of mirrors and substrates, for example alternating with each other. Other deposition techniques may also be envisaged by those skilled in the art depending on the nature of the deposited layer. In addition, the characteristics described in relation to one aspect of the invention may be combined with another aspect of the invention.

Claims (18)

Multilayer electroactive reflective module (1) for an automotive part comprising:

• a first substrate (10),
• a multilayer stack (18) arranged on the first substrate (10) and configured to receive an incident light beam (2) and reflect a reflected light beam (2') having a determined wavelength, said wavelength depending on a potential difference applied to the stack (18), the multilayer stack (18) comprising at least one layer forming a metal mirror (14),
• a first electrode (12) and a second electrode (13), electrically connecting the multilayer stack (18) on either side, and configured to apply said potential difference,

characterized in that the multilayer stack (18) further comprises:

• an electrochromic layer (15) having a nanometric thickness (d15) and surmounting the metal mirror (14), the electrochromic layer comprising:
  • at least one solid polymer electrolyte (16),
  • at least one electrochromic molecule (17) distinct from the at least one solid electrolyte polymer (16) so as to modify the absorption of the electrochromic layer (15), according to the potential difference applied to the stack (18),
• a partially transparent metallic bilayer (19) comprising a first metallic layer (190) based on a first metal, and a second metallic layer (191) based on a second metal distinct from the first metal, the metallic bilayer (19) overlying the electrochromic layer (15).

Module (1) according to the preceding claim, in which, in the metallic bilayer (19), the first metallic layer (190) is based on gold and the second metallic layer (191) is based on chromium. Module (1) according to the preceding claim, in which, in the metallic bilayer (19), the first metallic layer (190) based on gold overcomes the electrochromic layer and the second metallic layer (191) based on chromium overcomes the first metallic layer (190). Module (1) according to any one of the preceding claims, in which the metallic bilayer (19) has a thickness less than or equal to 10 nm. Module (1) according to any one of the preceding claims, wherein, in the metal bilayer (19), at least one of the first (190) and the second (191) metal layers has a plurality of holes (192) of nanometric size along the main extension plane of the metal bilayer (19). Module (1) according to any one of the preceding claims, wherein the solid polymer electrolyte (16) comprises:

• An ionogel comprising a polymer matrix and an ionic liquid, and/or
• A polymeric ionic liquid.

Module (1) according to any one of the preceding claims, in which the solid polymer electrolyte (16) is based on at least one polymer chosen from the group consisting of polyethers, polycarbonates, polyesters, polynitriles, polyalcohols, polyamines, polysiloxanes, fluoropolymers, biopolymers and their derivatives. Module according to any one of the preceding claims, in which the electrochromic molecule (17) is an organic molecule. Module according to any one of the preceding claims, in which the electrochromic molecule (17) has a molar mass less than or equal to 600 g/mol. Module (1) according to any one of the preceding claims, in which the electrochromic molecule (17) is chosen from the group consisting of: viologens, spiropyrans, bipyridines, carbazoles, methoxyphenyls, quinones, tetrathiafulvalene, phenylenediamine, pyrazoline, porphyrinoids, and in particular thiophene-porphyrinoids, furan-porphyrinoids, triphenylamines, and their derivatives. Module (1) according to any one of the preceding claims, wherein the electrochromic molecule (17) is in a colorless form without application of a potential difference. Module (1) according to any one of the preceding claims, in which the electrochromic layer (15) has a thickness (d15) of between 50 nm and 300 nm. Electroactive reflective system (3) for an automotive part comprising at least one reflective module (1) according to any one of the preceding claims. System (3) according to the preceding claim, comprising a plurality of said reflective modules (1) juxtaposed in at least one direction parallel to a main direction of extension of said reflective modules (1). System (3) according to any one of the two preceding claims, further comprising a lateral light source (31) and a waveguide (32) surmounting the at least one reflective module (1), the waveguide being configured to transmit a light beam (2'') from the light source (31) to the at least one reflective module (1). Method of manufacturing the reflective module (1) according to any one of claims 1 to 12, comprising:

• A provision of the first substrate (10) comprising the first electrode (12), and a provision of the second electrode (13),
• A formation of the multilayer stack (18) comprising:
  • A deposit of the metallic bilayer (19),
  • A deposit of the electrochromic layer (15),
  • A deposition of at least one layer forming the metallic mirror (14),
• An assembly of the first substrate (10) and the second electrode (13), such that the multilayer stack (18) is connected on either side to the first (12) and second (13) electrodes.

Method according to the preceding claim, in which:

• the deposition of at least one layer forming the metal mirror (14) is carried out on the first substrate (10),
• the deposition of the metallic bilayer (19) is carried out on the second electrode,
• the deposition of the electrochromic layer (15) is carried out on one of the metal mirror (14) and the metal bilayer (19),
• the assembly of the first substrate (10) and the second electrode (13) comprises bringing the electrochromic layer (15) into contact with the other of the metal mirror (14) and the metal bilayer (19).

Motor vehicle part (4) comprising a reflective module (1) according to any one of claims 1 to 12 or a reflective system (3) according to any one of claims 13 to 15.