EP1936603A1

EP1936603A1 - Security document comprising an electrochromic element

Info

Publication number: EP1936603A1
Application number: EP06026309A
Authority: EP
Inventors: Andreas Georg; Franz Brucker
Original assignee: European Central Bank
Current assignee: European Central Bank
Priority date: 2006-12-19
Filing date: 2006-12-19
Publication date: 2008-06-25

Abstract

Security document comprising substrate means, an electrochromic element and at least two power supply points each electronically connected to the electrochromic element, but electronically isolated from each other, wherein the electrochromic element comprises at least two electrodes, at least one electrolyte and at least one electrochromic material, wherein the electrolyte and the electrochromic material are enclosed by the electrodes.

Description

BACKGROUND OF THE INVENTION

1. Field of the invention

The present invention relates to a security document, particularly a banknote comprising an electrochromic element as an electronic security means.

2. Description of the Related Art

The identification and authentication of security documents, and in particular banknotes is a long-standing problem. In order to solve this goal, security means for security documents have been developed and are being developed to allow users and/or machines to distinguish between genuine and forged security documents and/or to discriminate among different kind of security documents.
Among the techniques adopted are the use of the special papers, special inks and patterns, the inclusion of watermarks and security threads as well. For example, some of these techniques are disclosed in US-A-4 462 866 , US-A-4 652 015 , US-A-4 943 093 und US-A-5 161 829 .
Although these traditional security means have been derived from sophisticated technological developments, it must be realised that they are becoming dated and more vulnerable to technological advances. Therefore, new security means have to be developed.
For example, EP-A-1 431 062 discloses a banknote comprising on-board electrical power supply means for driving a new class of electronic security means utilizing on-board power supply. The electronic security means are means comprising a microchip, electroluminescent materials, electro-active polymers/materials, light-emitting diodes and polymeric electronic displays.
A very similar approach is disclosed in WO 00/07151 .
The security means as disclosed in EP-A-1 431 062 and WO 00/07151 have some very important disadvantages. First of all, it should be noted that the electrical power supply means and the microchips are widespread. Therefore, such elements are easily available. On the other hand, if very special chips are produced, these elements will cause enormous costs. Furthermore, the lifetime of the electric power supply means, the security means and the security document, especially the banknotes comprising these elements are limited, respectively.
The publication P. M. S. Monk et al. Electrochromic paper: utility of electrochromes incorporated in paper Electrochimica Acta 46 (2001) 2195 - 2202 teaches that the electrochemistry of electrochromic paper is similar to that of electrochromes without the paper. An electrochromic paper can be obtained by dispersing an organic or inorganic electrochrome in a paper. An image is generated when a stylus electrode touches it, generating an electrochromic image. Possible applications of the electrochromic paper comprise viable paper products having security implications, such as tickets, vouchers and banknotes, wherein the electrochromic paper product is merely placed between two electrodes and a voltage applied. Visual inspection for the formation of colour confirms (or otherwise disproves) the genuineness of the product.
However, major drawbacks of an electrochromic paper of that kind comprise the low contrast achievable; the limited long-term stability of the electrochromic paper because the electrochrome is directly exposed to the environment and because of the high voltages, which are needed to operate the electrochromic paper, the comparatively simple structure making it easy to counterfeit the paper, and the comparatively long time needed for carrying out a testing cycle, i. e. colouring and bleaching of the electrochromic image.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a security document, particularly a banknote with a new class of electronic security means for allowing users and/or machines to distinguish between genuine and counterfeited security documents solving the problems of the prior art. In particular, it is an object of the present invention to provide a security feature, which is difficult to counterfeit, which can be applied on flexible substrates, preferably banknotes, and which can be tested by means of a simple power supply machine together with an human operator. The security feature should be thin, preferably thinner than 100 µm, more preferably below 50 µm, and allow a short testing cycle, preferably shorter than thirty seconds, more preferably shorter than 10 seconds, especially shorter than 5 seconds. In addition, the security feature should have high long-term stability and its production should be possible in a simple and cost efficient way.
These problems are solved with the features of claim 1. Preferred embodiments of the invention are described in the dependent claims. The other claims describe particularly favourable ways of producing and using the security document of the present invention.
The present invention provides a security document with a new kind of a reversible security feature

➢ which allows users and/or machines to distinguish between genuine and counterfeited security documents in a very simple way;
➢ which is very difficult to counterfeit;
➢ which is particularly suitable for security documents requiring a thin and highly bendable security feature, such as a banknote;
➢ which allows for very short test cycles;
➢ which has a comparatively high contrast;
➢ which makes the use of low voltages possible, so that degradation processes due to high voltages can be avoided;
➢ which has a very high long-term stability; and
➢ which can be prepared in a comparatively simple and cost efficient way once the special and sophisticated equipment is on hand.

DETAILED DESCRIPTION OF THE INVENTION

The following is a detailed description of the present invention. It provides a security document comprising an electrochromic element and at least two power supply points.
The term "security document", as used herein, refers to all kind of documents that contain at least one feature that can be used to prevent counterfeiting by providing authentication, identification, verification and/or classification of the document. In particular, they include banknotes, passports, chequebooks, identity cards, credit cards and/or debit cards, wherein banknotes are especially preferred.
According to the present invention, the security document comprises an electrochromic element. Elements of that kind are known in the art and usually contain at least one electrochromic material. The electrochromic material changes its colour upon oxidation or reduction reversibly. Preferred classes of electrochromic materials comprise a pair of redox substances, which form coloured, positively or negatively charged, chemically reactive free radicals after reduction or oxidation. The pair of redox substances preferably used is in each case a reducable and oxidizable substance. Both are preferably colourless or have only a slight colour. Under the action of an electric potential, one substance is reduced and the other is oxidized, at least one becoming coloured. For some preferred classes of electrochromic materials in parallel to the electron injection and extraction, which causes the reversible colour change, an injection and extraction of positive ions, like protons or Li⁺ takes place. Depending on the detailed materials, after switching off the potential, the two original redox substances can be reformed with their coloration or lightening of colour occurring.
Electrochromic substances particularly suitable for the purposes of the present invention are tungsten oxide, niobium oxide, titanium oxide, iridium oxide, nickel oxide, molybdenum oxide, antimon-tin oxide, prussian blue, polyaniline, polythiophene, viologene, polypyrrole, iodide or mixtures of two or more of these materials. Preferred polythiophene compounds are 3,4 polyethylene dioxythiophene (PEDOT) and poly(3,4-ethylenedioxythiophene-didodecycloxybenzene) (PEB).
Furthermore, as mentioned above, the electrochromic element may comprise a second redox couple, which can be considered as an ion-storage component or as a redox couple in form of ions, which are comprised in an electrolyte as mentioned above. This second redox couple may be electrochromic by itself. Therefore, the materials described above as electrochromic substances may be used. Particularly useful components for a redox couple in form of ions dissolved in a solvent are iodide/triodide, bromide/tribromide or fluoride/trifluoride.
The electrochromic element of the present invention comprises at least two electrodes, at least one electrolyte and at least one electrochromic material. The electrode is the area, which conducts the electrons parallel to the substrate from the power supply points to the different locations of the electrochromic material. In general, it may be formed of transparent conducting materials, as will be described below, or of non-transparent materials, like metals. A high conductivity is desired to allow short switching times. Therefore, the sheet resistance of a square geometry is preferably below 1 kΩ, more preferably below 100 Ω.
The term "electrolyte" refers to a substance containing free ions, which behaves as an electrically conductive medium. It is not limited to ionic solutions, i. e. ions in solutions, but also comprises ionic liquids (molten electrolytes) and solid electrolytes. The specific conductivity of the electrolyte at 20°C is preferably greater than 0.1 mS/cm. The vapour pressure of the electrolyte at 20°C is preferably lower than 0.1 Pa. In a preferred embodiment, the electrolyte comprises a redox salt, like lithium iodide, optional together with additional iodide and/or iodine. The solubility of this redox salt in the electrolyte is preferably at least 0.01 mol/l, more preferably at least 0.1 mol/l at 25°C.
Particularly suitable ionic liquids are present in the form of a liquid at 20°C and preferably have a dynamic viscosity, measured at 20°C at 1 Hz, of 1000 mPas or less.
In the present invention, the electrodes enclose the electrochromic material and the electrolyte and protect them from the environment. At least one of the electrodes preferably has a light transmittance allowing to see through. Its transparency at 550 nm is preferably at least 25 %, more preferably at least 50 % and most preferably at least 75 %. The transparent electrodes preferably comprise a chemically inert support material, such as glass or a polymer substrate, which preferably has a thickness not exceeding 50 µm, more preferably not exceeding 30 µm. The use of polymer foils, such as PET foils, is particularly preferred in that context.
Furthermore the electrodes preferably comprise a transparent electronically conducting layer. Such layers are known in the art. These layers preferably comprise highly doped metal oxides, such as SnO₂:F, SnO₂:Sb, In₂O₃:Sn (ITO), Cd₂SnO₄, ZnO:Al and ZnO:In.
In a first particular preferred embodiment of the present invention, the electrochromic material is dissolved in the electrolyte. A particular suitable example of that kind is an electrochromic element having the so-called "solution setup", i. e. an electrochromic element comprising the following layers in the following order:

electrode / electrochromic electrolyte solution / electrode.

The electrochromic material undergoes an electron-transfer reaction on the surface of the first electrode, and changes its colour. On the counter electrode, a similar or different reaction takes place. A preferred example comprises methyl viologen in water as electrochromic electrolyte solution.
In a second particular preferred embodiment of the present invention, the electrochromic element comprises a first electrochromic layer between the first electrode and an electrolyte layer. A particular suitable example of that kind is an electrochromic element having the so-called "solution-to-solid setup", i. e. an electrochromic element comprising the following layers in the following order:

electrode / electrochromic layer / electrolyte / electrode.

Here, an electrochromic layer is formed during the colouring or bleaching cycle as a consequence of a redox reaction on the electrolyte-electrode surface (example: heptyl viologen), or it may be a fixed electrochromic layer, like WO₃, which changes its colour upon electron and ion (e. g. H⁺ or Li⁺) uptake from the electrolyte. On the counter electrode, a second redox reaction takes place, which is favourably catalysed by a thin Pt layer.
In a third particular preferred embodiment of the present invention, the electrochromic element comprises a first electrochromic layer between the first electrode and an electrolyte layer and a second electrochromic layer between the electrolyte and the second electrode. A particular suitable example of that kind is an electrochromic element having the so-called "battery setup", i. e. an electrochromic element comprising the following layers in the following order:

electrode / first electrochromic layer / electrolyte / second electrochromic layer /electrode

Here, ions (e. g. H⁺ and Li⁺) are shifted from the first electrochromic layer to the second electrochromic layer and vice versa, similar to the charging and decharging of a battery.
The electrochromic layers are again formed during the colouring or bleaching cycle as a consequence of a redox reaction on the electrolyte-electrode surface, or may be fixed electrochromic layers, which changes its colour upon electron and ion (e. g. H⁺ or Li⁺) uptake from the electrolyte.
For the purposes of the present invention, the use of the "solution-to-solid setup" with just one electrochromic layer has proven of particular advantage, since the problem of precharging the electrochromic device can be avoided. A "battery setup" needs a method to create a first charge in the device before it can be shifted between the electrodes. The "solution-to-solid setup" solves this problem, as the charge is present from the beginning in the electrolyte.
This problem of precharging is important especially for the application of security features like banknotes, as here typically there is a long time between two colour cycles (typically several days up to months). This is in contrast to other applications, like windows or displays. During such a long time of not being cycled, a charge having been incorporated before can get lost easily. This is intensified by the special usage of thin foils as substrates, as these typically show a limited but non avoidable permeability to water and/or oxygen, which accelerates the self-discharging.
According to a particularly preferred embodiment of the present invention, the security document comprises an electrochromic element, in which one electrode is at least partially coated with an electrochromic material and the other electrode is at least partially coated with a catalytic layer, and wherein the electrolyte comprises at least one redox salt. Especially suitable the electrolyte comprises an iodide, bromide or fluoride and positive ions of the first group of the periodic table of elements, in particular H⁺ or Li⁺.
In the present invention, the outer surface of the electrochromic element is preferably at least partially covered with a support material. Preferably at least 50 % of the total outer surface, more preferably at least 75 % of the total outer surface, and most preferably the entire outer surface are/is covered with a support material. The support material is not particular restricted and, in principle, can be chosen freely from papers, polymer foils, e. g. polyethylene terephthalate foils, and similar materials. However, it has proven of particular advantage to use materials with a low water and oxygen permeability to increase the long-term stability of the electrochromic element. The best results are achieved by the use of thin polymer foils having a thickness of less than 50 µm, preferably less than 25 µm, and comprising a barrier coating on one side, such as SiO₂ or Al₂O₃ and an additional barrier coating on the other side, which simultaneously acts as an transparent electrode of the electrochromic cell, such as ITO. Advantageously, this layer is deposited by sputtering or evaporation, as this leads to good barrier properties.
Preferably, the electrochromic element comprises a reflector layer, more preferably a reflecting electrode, reflecting substrate, a reflecting support or a reflecting electrolyte. Preferred reflector layers have a white colour and do essentially not attack the organic substances contacting the reflector layer or disturb the ionic or electronic conductivity. Particular useful materials are ZnO and TiO₂. These may be printed onto the substrates or dispersed in the electrolyte in form of small particles with typical sizes of 10 µm down to 10 nm, preferably 1 µm down to 100 nm. Surprisingly, by using a reflector layer the visibility of the colour change of the electrochromic element can be further improved.
For a security feature it is important to make the device as difficult as possible for counterfeiting. For this aim the following setup is especially advantageous:

Reflecting substrate (e. g. paper) with printed symbols / additional substrate (e. g. foil) / preferably transparent electrode / first electrochromic layer / electrolyte with redox agent / catalytic layer / transparent electrode /substrate (e. g. foil)

The printed symbols may be any symbol, which shows a high resolution, which is difficult to incorporate in the electrochromic device, e. g. by applying a reflecting electrolyte or a reflecting electrode.
In a very preferred embodiment of the present invention, the electrochromic material and/or the catalytic material is designed as a symbol, for example by using masks during the deposition process, or by etching processes or by a printing process. This also makes it more difficult to counterfeit the device.
Therefore, anybody, who tries to counterfeit the device, has to use a similar setup as the original, which makes it more difficult.
According to a very preferred embodiment of the present invention, the electrochromic element is flexible. Flexibility means that the electrochromic element can be bend with a radius of curvature of 5 cm, more preferably 1 cm, most preferably 0.5 cm. The radius is measured according to the "simple support ends" method, top part of fig. 5, in "A mechanical assessment of flexible optoelectronic devices", Zhong Chen, Braina Cotterell, Wei Wang, Ewald Guenther, Soo-Jin Chua, Thin Solid Films vol. 394, pages 202-206 of the year 2001.
In addition, the electrochromic element preferably comprises a bend protection. Bend protection means that the security document, especially a banknote can be bended without destroying the electrochromic element. A bend protection can be achieved by a stable metal frame surrounding the electrochromic element or a metal sheet below the element. Furthermore, the bend protection may be achieved by producing a electrochromic element having a small dimension. Preferably, the maximum dimension of the electrochromic element does not exceed 3 cm.
The switching time of the coloration of the electrochromic element, i. e. the time needed to switch the electrochromic element from the bleached state to the coloured state is preferably short. The switching time of the electrochromic element is preferably below thirty seconds, more preferably below 10 seconds and most preferably below 5 seconds. The switching time is defined as follows: For the colouring process, the security document is connected to a power source with voltages below 3V, wherein the use of 1.5V is particularly preferred. Then the change in the visual reflectance is measured. The "switching time" is defined here to be the time, in which 63% of the total change of the visual reflectance is reached.
In the present invention, the switching time of the electrochromic element can be further improved by using an assembly, wherein in a top view at least one of the electrodes, and more preferably both electrodes, has (have) a greater surface than the electrochromic layer. This special geometry reduces the effective resistance and in consequence, the switching time. One should note that for electrochromic layers, the charge and in consequence for a determined switching time the current is proportional to the area of the electrochromic layer. As the current defines the ohmic voltage losses, and the voltage is limited for keeping the device stable, a small area of the electrochromic layer is helpful for a limitation of the voltage. This may be achieved by a simple circle of the electrochromic layer, surrounded by a larger area of the electrode, or by structuring the electrochromic area in form of a symbol, which reduces the effective area, but which keeps the symbol large in its diameter and therefore keeps it clearly visible for a human observer.
Preferably, the surface of at least one electrode, more preferably of both electrodes is at least 25% greater than, more preferably at least 50% greater than, even more preferably at least 100% greater than and most preferably at least 200% greater than the surface of the electrochromic layer in the top view. Thereby a ratio of electrode surface : electrochromic layer surface in the range of 2:1 to 20:1 has proven of particular advantage.
The quality of the security document can be further improved by using two or even more electrochromic elements. Thereby the electrochromic elements preferably have different switching times to make counterfeiting even more difficult.
The bleaching time of the electrochromic element, i. e. the time needed to switch the electrochromic element from the coloured state to the bleached state is preferably short. The bleaching time of the electrochromic element is preferably below thirty seconds, more preferably below 10 seconds and most preferably below 5 seconds. The switching time for the bleaching process is defined as the time, in which 63% of the total change of the visual reflectance is reached after switching of the illumination.
The bleaching and/or the coloration time can be adjusted by the use of catalysts accelerating the reaction kinetics. Useful catalysts are Pt, Pd, Ni, Au, Os, Re, Ir, Ru, and Rh. The catalyst can be arranged as a separate layer, for example, onto a layer, which comprises an electrochromic material, or onto a transparent electronically conducting layer. If these layers are made in such a way, that they are porous, the catalyst can also be arranged below these layers. Furthermore, the catalyst can be arranged as an additive in the electrolyte.
Further details regarding electrochromic elements can be found in Pail S. Monk, Roger J. Mortimer, David S. Rosseninsky "Electrochromism: Fundamentals and Applications", VCH Weinheim, 1995, or in C.G. Granqvist, "Handbook of inorganic electrpchromic materials", Elsevier Science B.V. Amsterdam, 1995 which is incorporated herein by reference.
According to the present invention, the security document comprises at least two power supply points each electronically connected to the electrochromic element, but electronically isolated from each other. The present invention distinguishes between two kinds of charge transport: charge transport by electrons and by ions. The first shall be referred as "electronical" and the second as "ionical". The term "electronical connection" refers to a connection via a material preferably having a resistivity ρ of less than 10^-1 Ω · cm, very preferably of less than 5*10^-3 Ω · cm, when measured at 25°C. By the way of contrast, two articles will be "electronically isolated one from another" if there is no electronical connection between the articles, in particular via a material having a resistivity ρ of less than 10² Ω · cm, when measured at 25°C.
In principle, there are no particular restrictions on the material of the power supply points used in the present invention. However, they preferably comprise an electrically conductive material preferably having a resistivity ρ of less than 10^-1 Ω · cm, very preferably of less than 10^-3 Ω · cm, when measured at 25°C.
Particular suitable materials are metals, such as silver, gold, aluminium, messing, iron, chromium, and stainless steel.
The surfaces of the power supply points are preferably left exposed to the environment. In such circumstances, it will be apparent to those skilled in the art that materials will be preferably chosen that suffer minimal tarnishing or corrosion during everyday use of the secure document.
The power supply points are each electronically connected to the electrochromic element, preferably via one or more electronically conducting tracks. Thereby the electronically conducting tracks can be made of any electronically conducting material, but preferably have a resistivity ρ of less than 10^-1 Ω · cm, very preferably of less than 5* 10^-3 Ω · cm, when measured at 25°C.
For preferred embodiments, a sealing is added, which encloses an inner area, which comprises electrolyte, electrochromic material, and not wholly the electrodes. In these embodiments, the power supply points are preferably outside of this inner area, and the electrodes electronically connect the power supply points optionally via additional electronically conducting tracks with the inner area. In particularly preferred cases, the sealing is applied by a thermal step, like welding of the supporting material or application of a hot melt, which does not destroy the conductivity of the electrodes, especially the electronical contact from the inner area to outside of the inner area.
The security document of the present invention is preferably adapted for checking its authenticity by electronically connecting the power supply points to a power source and observing a status change of the electrochromic element, preferably a change in the optical status.
The positions of the power supply points and of the electrochromic element on the security document can principally be chosen freely. It is merely important that the power supply points are electronically isolated from each other and that they are electronically connected to the electrochromic element so that a reversible status change of the electrochromic can be induced by connecting the power supply points to a power source.
However, the electrochromic element is preferably located nearby a color reference to ease the visibility of a correct color change of the electrochromic element. Such color reference may comprise different tints, which the electrochromic element passes through.
The kind of the substrate means used in the present invention is not critical. However the use of substrate means comprising paper, plastic, polymer, elemental metallic foils, metallic alloy foils and/or synthetic paper is preferred.
The security document of the present invention is comparatively thin and its thickness is preferably smaller than 100 µm. In one especially preferred embodiment, the overall thickness of the power supply points, not including the substrate thickness, is between approximately 10 to 50 µm. The thickness of the interconnects between the power source and the electrochromic element is preferably within the range from approximately 1 to 30 µm. The thickness of the electrochromic element, not including the substrate means is preferably in the range from 10 to 100 µm.
Preferred methods for the production of a security document of the present invention are described in the following sections. The substrate means is preferably provided with the power supply points and the electrochromic element, wherein the power supply points and the interconnects are preferably printed onto the substrate and the complete electrochromic element is preferably subsequently attached to the substrate. In this case, electronical connection will be made by ensuring that exposed printed power supply points on the substrate align with power supply points on the electrochromic element. Alternatively, the power supply points, optionally the interconnects, and the electrochromic element is preferably arranged together in one element, which is then attached to the substrate means.
In a specific embodiment of the present invention, the conducting tracks and/or the power supply points are deposited onto the substrate using an electroless deposition technique. In this technique, specially formulated catalytic ink is printed onto the substrate in a desired pattern. The substrate is then immersed in a chemical solution containing ions of the metal to be deposited. Over time, electroless deposition of the metal onto the substrate areas printed with catalytic ink occurs. This technique is advantageous compared to other methods for producing the desired electrically conducting tracks and the power supply points, such as printing of metal-loaded inks, since the technique produces deposited material with a density that is very close to that of the bulk material. Furthermore, this technique is advantageous over standard printing of loaded inks in that the adhesion of the deposited material to the substrate is superior. The aforementioned electroless deposition technique is described in detail in Patent Application WO 02/099163 and is suitable for a range of substrates (such as polyester, polypropylene, synthetic paper, fine-weave cloths and polycarbonate) and a range of deposited metals (including copper, nickel, cobalt, iron, tin and a variety of magnetic and non-magnetic alloys). However, the present invention does not preclude other methods of depositing electrically conducting materials. Other methods include printing (such as screen printing and gravure printing) of particle-loaded inks, electroplating methods, chemical vapour deposition, sputtering, evaporation and etch-resist methods, all of which are known to those skilled in the art.
The manufacture of the electrochromic element can be achieved in a conventional manner. The electrochromic layer, the preferably transparent conducting layer, the metallic conducting layers, and/or the catalytic layer are preferably prepared by deposition techniques, more preferably by physical vapour deposition (PVD), which includes magnetron sputtering and evaporation. Here, masks can be used to allow the preparation of specific shapes, area ratios and symbols.
At the end, the space between the electrodes is preferably filled with the electrolyte and the electrochromic element is sealed to avoid leakage, evaporation or impurification of the components. Said sealing is preferably achieved by encapsulating the electrolyte, the electrochromic material and part of the electrodes in a suitable support material, such as a polymeric film or a flexible glass foil, wherein the support material preferably has high oxygen and/or water barrier properties. The sealing is preferably achieved by a heating step, preferably by welding the electrodes together, wherein the electronically conducting layer is preferably not destroyed.
In a particularly preferred embodiment of the present invention, at least two electrodes each comprising a support layer, preferably a PET foil, and an electronically conducting layer, preferably an ITO-layer, are placed in front of each other in a way, that the electronically conducting layers look at each other. At least one of these electronically conducting layers is provided with an electrochromic layer. Thereafter, an electrolyte, preferably comprising a redox couple, is applied onto one of the electronically conducting layers, for example by printing or means of a microdispenser. Then, the other electrode is applied onto this, followed by an application of a sealing, for example by using a hot melt or melting the substrates together.
Surprisingly, it was found, that the conductivity of the electronically conducting layer is not destroyed by heating up the support during the sealing process. Both approaches, to seal the device with a hotmelt or by melting the foils, require a heating step of the foil. As the device has to be contacted from outside of the sealing, an electrode has to be lead through the sealing. It was found that a too long time of heating up the foil during the sealing process or a too high temperature destroy the conductivity of the electronically conducting, especially the ITO-layer, whereas a too short time or a too low temperature does not lead to a tight sealing. Preferred values for the temperature were in the range of 200°C up to 300°C and preferred times in the range of 1sec up to 10 sec.
On the other hand it was found, that by melting two support foils coated with an electronically conducting layer together, a electronically conducting connection can be produced from the one electronically conducting layer on the first support foil to the other electronically conducting layer on the second support foil.
For checking authenticity of the security document of the present invention, the power supply points are electrically connected to a power source and a status change of the electrochromic element is observed. Thereby a voltage below 3V, more preferably below 1.5V, is preferably used. The quality of verification can be further increased by simultaneously measuring optical and electrical properties of the electrochromic element and/or by measuring changes of optical and/or electrical properties of the electrochromic element with time.
The accompanying drawings show particularly preferred embodiments of the present invention for the purpose of further illustrating the present invention; it being understood that the invention is not limited to the precise arrangements shown.
FIG. 1 shows a schematic top view of a first particularly preferred electrochromic element, which has been opened in the middle. The element comprises a first and a second electrode comprising a first and a second support layer (1, 3), e. g. a PET foil, on top of which a first and a second electronically conducting layer (5, 7), e. g. ITO, are located. Each of the electrodes also comprises a power supply point (11, 13).
The electrodes are mechanically connected to each other by a sealing (19). Said sealing (19) can be achieved by the use of a suitable sealing material, such as an inert polymer material, or by welding together the different layers of the electrochromic element.
The sealing (19) encloses an inner area (21) which is preferably filled with an electrolyte. This electrolyte preferably contains a redox couple, e.g. iodide and triodide. The inner area of the first electrode comprises an image made of electrochromic material (9), e.g. tungsten oxide, which is placed on top of the first electronically conducting layer (5) and only covers a small area of the first electrically conducting layer (5) in order to ensure a comparatively short switching time. The inner area of the second electrode, i. e. the area of the second electrode, which is enclosed by the sealing (19), partially comprises the second electronically conducting layer, which is at least partially covered with a catalytic layer (17), such as Pt.
In a closed state, both electronically conducting layers (5, 7) extend to opposite directions and merely overlap in the middle, i. e. the inner area of the second electrode. The design avoids carefully that there are locations, where first and second electrode and sealing fall together, as this should lead to short circuits. Both power supply points (11, 13) are easily accessible, since they are not covered neither by the opposite electrode (7, 5) nor by the opposite substrate layer (3, 1). In addition, the second electrode (7) and the second substrate layer (3) comprise a hole (15) at the location of the second power supply point (13), so that the second power supply point (13) is also easily accessible from the same side as the first power supply point (11).
FIG. 2 shows a schematic top view of a second particularly preferred electrochromic element, which has been opened in the middle. The element comprises a first and a second electrode comprising a first and a second support layer (101, 103), e. g. a PET foil, on top of which a first and a second electronically conducting layer (105, 107), e. g. ITO, are located. In addition, the first substrate layer (101) comprises an extension of the second electrode (106), which is electronically connected to the second electrode (107) via a welding (120) and which is electronically isolated from the first electrode (105).
The first electrode and the extension of the second electrode (106) each comprise a power supply point (111, 113). The electrodes are mechanically connected to each other by a sealing (119). Said sealing (119) can be achieved by the use of a suitable sealing material, such as an inert polymer material, or by welding together the different layers of the electrochromic element.
The sealing (119) encloses an inner area (121), which is preferably filled with an electrolyte. This electrolyte preferably contains a redox couple, e.g. iodide and triodide. The inner area of the first electrode comprises an image made of electrochromic material (109), e.g. tungsten oxide, which is placed on top of the first electronically conducting layer (105) and only covers a small area of the first electronically conducting layer (105) in order to ensure a comparatively short switching time. The inner area of the second electrode, i. e. the area of the second electrode, which is enclosed by the sealing (119), partially comprises the second electronically conducting layer (107), which is at least partially covered with a catalytic layer (117), such as Pt.
In a closed state, both electronically conducting layers (105, 107) extend to opposite directions and merely overlap in the middle, i. e. the inner area of the second electrode. Again, the design avoids carefully that there are locations, where first (105) and second electronically conducting layer (107) and sealing (119) fall together, as this should lead to short circuits. The second power supply point (113) is connected to the second electrode (107) via an additional melting step, for example by melting the two substrate layers together in form of a ring (120). Here, the same technology can be applied as for the sealing ring (119). Both power supply points (111, 113) are easily accessible from the top, since they are not covered neither by the opposite electrode (107) nor by the second substrate layer (103).

Claims

Security document comprising substrate means, an electrochromic element and at least two power supply points each electronically connected to the electrochromic element, but electronically isolated from each other, characterized in that the electrochromic element comprises at least two electrodes, at least one electrolyte and at least one electrochromic material, wherein the electrolyte and the electrochromic material are enclosed by the electrodes.
Security document according to claim 1, wherein the security document is a banknote.
Security document according to claim 1 or 2, wherein the electrochromic material comprises tungsten oxide, niobium oxide, titanium oxide, iridium oxide, nickel oxide, molybdenum oxide, antimon-tin oxide, prussian blue, polyaniline, polythiophene, viologene, polypyrrole, iodide or mixtures of two or more of these materials.
Security document at least one of the preceding claims, wherein the electrochromic material is dissolved in the electrolyte.
Security document at least one of the preceding claims, wherein the electrochromic element comprises a first electrochromic layer between the first electrode and an electrolyte layer.
Security document according to claim 5, wherein the electrochromic element additionally comprises a second electrochromic layer between the electrolyte layer and the second electrode.
Security document according to at least one of the preceding claims, wherein at least one of the electrodes is transparent.
Security document according to claim 7, wherein one electrode is transparent and the other electrode is reflecting.
Security document according to at least one of the preceding claims, wherein the electrolyte comprises an ionic liquid.
Security document at least one of the preceding claims, wherein the electrolyte has a specific conductivity of greater than 0.1 mS/cm at 20°C.
Security document according to at least one of the preceding claims, wherein the vapour pressure of the electrolyte at 20°C is lower than 0.1 Pa.
Security document according to at least one of the preceding claims, wherein the electrolyte comprises a reflecting material.
Security document according to at least one of the preceding claims, wherein the electrochromic element has a solution-to-solid-setup.
Security document according to at least one of the preceding claims, wherein one electrode is at least partially coated with an electrochromic material and the other electrode is at least partially coated with a catalytic layer, and wherein the electrolyte comprises at least one redox salt.
Security document according to claim 14, wherein the electrolyte comprises an iodide, bromide or fluoride and positive ions of the first group of the periodic table of elements.
Security document according to claim 15, wherein the positive ions comprise H⁺ or Li⁺.
Security document according to at least one of the preceding claims, wherein the outer surface of the electrochromic element is at least partially covered with a water and/or oxygen barrier coating.
Security document according to at least one of the preceding claims, wherein the switching time of the electrochromic element is below 30 seconds.
Security document according to at least one of the preceding claims, wherein the electrochromic element comprises at least one electrochromic layer and wherein in a top view at least one of the electrodes has an at least 25% greater surface than the electrochromic layer.
Security document according to claim 19, wherein in a top view both electrodes have an at least 25% greater surface than the electrochromic layer.
Security document according to at least one of the preceding claims, wherein the electrodes are not electronically connected to each other.
Security document according to at least one of the preceding claims, wherein the security document comprises at least two electrochromic elements.
Security document according to claim 22, wherein the electrochromic elements have different switching times and/or colours.
Security document according to at least one of the preceding claims, wherein the electrochromic element is flexible.
Security document according to at least one of the preceding claims, wherein the electrochromic element comprises a bend protection.
Security document according to at least one of the preceding claims, wherein the maximum length of the electrochromic element does not exceed 3 cm.
Security document according to at least one of the preceding claims, wherein the power supply points comprise an electronically conductive material.
Security document according to claim 27, wherein the electronically conductive material is selected from the group consisting of electronically conductive metals, electronically conductive metal alloys and electronically conductive polymers.
Security document according to at least one of the preceding claims, wherein the power supply points exhibit resistance to oxidation and/or tarnishing from exposure to the atmospheric environment.
Security document according to at least one of the preceding claims, wherein the substrate means comprises paper, plastic, polymer, elemental metallic foils, metallic alloy foils and/or synthetic paper.
Security document according to at least one of the preceding claims, wherein its thickness is smaller than 100 µm.
Security document according to at least one of the preceding claims, wherein, the electrochromic element comprises a sealing, which encloses the electrochromic material, the electrolyte and partially, but not fully each of the electrodes.
Security document according to at least one of the preceding claims, wherein the security document is adapted for checking its authenticity by electrically connecting the power supply points to a power source and observing a status change of the electrochromic element.
Method for the production of a security document according to at least one of the preceding claims, wherein the power supply points and the electrochromic element are provided on the substrate means.
Method according to claim 34, wherein the power supply points are printed onto the substrate means.
Method according to claim 34 or 35, wherein the power supply points are deposited onto the substrate means by using electroless deposition, sputtering and/or vacuum deposition.
Method according to at least one of the claims 34 to 36, wherein the electrochromic element is provided before or after the provision of the power supply points.
Method according to at least one of the claims 34 to 37, wherein a sealing is applied, which encloses the electrochromic material, the electrolyte and partially, but not fully each of the electrodes, wherein the electronically conducting layer of the electrodes are not destroyed.
Use of a security document according to at least one of the claims 1 to 33 for checking its authenticity, wherein the power supply points are electrically connected to a power source and a status change of the electrochromic element is observed.
Use according to claim 38, characterized in that optical and electrical properties of the electrochromic element are measured simultaneously.
Use according to claim 37, characterized in that changes of optical and/or electrical properties of the electrochromic element are measured with time.