DE4225637C1 - Electrochromic thin layer system - comprises substrate and cathodic and anodic electrochromic layers - Google Patents

Abstract

Electrochromic thin layer system (I) has a substrate and a cathodic and an anodic electrochromic layer arranged between two electrodes. The novelty is that the anodic electrochromic layer (5) has as low an electrical conductivity in a region next to the cathodic layer (4), that the total resistance of the system vertical to the layer, determined by the electrical resistance (R) of this region is measured by: RxF = (1x10 power 4) to (5x10 power 4) ohm cm.sq. F = surface of the layer system. The total resistance of the layer system without the region of low electrical resistance satisfies the condition R. F is below 1000 ohm.cm2. USE/ADVANTAGE - (I) can be used in car rear-view mirrors. An additional insulating layer is not reqd.

Description

The present invention relates to an electrochromic thin-film system, can be used in particular as an electrochromic rear-view mirror for motor vehicles, with a substrate and one that can be colored cathodically and anodically electrochromic layer that immediately adjoin and between two electrodes are arranged. Such a layer system is except for the optional feature z. B. from known from DE 33 41 384 A1. The cathodically colorable electrochromic layer can be WO₃.

From DE 33 41 384 A1 is also Ni (OH) ₂ as for State-of-the-art anodically dyeable electrochromic material known.

The anodic known from DE 33 41 384 A1 dyeable electrochromic layer homogeneous.

Electrochromic materials are materials that are used during application of an electric field its optical constants n and k (refractive index and Abbe number), keep this state after switching off the field and return to the initial state after reversing the polarity. It is the electrochromic material is involved in a redox process.

Electrochromic materials are electronic and ionically conductive, like from displays, April 1982, pp. 67-68.

Typical examples of electrochromic materials are WO₃ and MoO₃, which are applied in a thin layer on a glass substrate, are colorless and transparent. However, if a voltage of a suitable magnitude is applied to such a layer, then suitable ions, e.g. B. protons and from the other side electrons in this layer and form the blue tungsten or molybdenum bronze H _x WO₃ or H _x MoO₃. The intensity of the color is determined by the amount of charge that flows into the layer. Electrochromism is essentially restricted to sodium, lithium and hydrogen bronzes, which allow the reaction to be carried out reversibly electrochemically. Hydrogen is of greatest interest for kinetic reasons.

A distinction is made depending on whether the electrochromic material is reduced or changes to the colored state by oxidation, cathodically and anodically dyeable electrochromic substances. Examples of cathodic dyeable substances are the above-mentioned WO₃, MoO₃ and Nb₂O₅, TiO₂, V₂O₅ and. a. Examples of substances that can be colored anodically are IrO₂, NiO, Ir / Sn oxide and. a.

DE 28 46 101 C2 is an electrochromic device known in the electrochromic Layer a non-electrochromic layer from the same material as the electrochrome Layer is applied, but the substance e.g. B. tungsten oxide, is in a higher oxidation state.

Because of their special coloring and storage properties Electrochromic systems for a wide variety of applications developed, for example for display elements, light transmission control elements or storage elements.

The use of electrochromic is of particularly high commercial interest Systems in the automotive industry for the production of automatically dimmable Rearview mirror. The so-called All-solid systems prevailed against electrochromic mirrors with liquid (mostly sulfuric acid-glycerin mixtures) or plastic Various electrolytes (organic polymers with acid groups) Advantages such as B. the reduced thickness of the entire system, no splashing the acid used as the electrolyte in the event of damage and breakage of the system u. a.

There are various options for arranging the individual Layers to build an electrochromic mirror. A comprehensive one Overview gives z. B. F.G.K. Baucke in "Large-Area Chromogenics: Materials and Devices for Transmittance Control "; SPIE institutes Series, Vol. IS 4, 1990, pp. 518-538.

An electrochromic system generally consists of two electrodes, the electrochromic material and an electrolyte. The electrolyte can also be missing, see DE-PS 15 89 429, Fig. 1, is the electrochromic material as a reducing material (cathodic coloring) or oxidizable  Material (anodically colored) on the respective electrode applied, care must be taken at the corresponding counter electrode that the opposite reaction to the coloring process does not damage the Form of irreversible oxidation or corresponding reduction can.

A class of systems especially designed for the production of rear-view mirrors is used for this reason uses a cathodic and an anodically colorable electrochromic layer in a system structure. Such systems are e.g. B. in EP-A1 2 41 217. They have WO₃ as cathodically colorable and Ni (OH) ₂ as anodically dyeable substances. Different manufacturing processes for electrochromic layers, e.g. B. on nickel hydroxide known from GB 20 86 601.

In the systems known from EP-A1-2 41 217, a corresponding cathodic coloring reaction (WO₃ + H⁺ + e.) Is running simultaneously with the anodic coloring reaction (e.g. Ni (OH) ₂ → NiOOH + H⁺ + e ^- ) ^- → H _x WO₃). This not only leads to increased dyeing efficiency, but both reactions also complement one another if the layer thicknesses are selected appropriately in such a way that undesirable side reactions are largely suppressed and systems which are extremely cycle and long-term stable are obtained.

In the systems known from EP-A1-2 41 217 there is between the two electrochromic layers arranged a solid electrolyte layer, the has the task of electrically isolating both layers from one another, i.e. H. Allow ions to pass through, but prevent electron flow.

The switching speed of the electrochromic systems is significantly different from affects the ionic conductivity of this dielectric insulator layer. When using metal oxides for the insulator layer (according to DE-PS 30 23 836 e.g. B. Ta₂O₅ or SiO₂ or TiO₂) is the ion conductivity essentially tied to the amount of water absorbed. The  System stability affected by the fact that at higher temperatures the water can escape irreversibly from the solid state system. The Long-term temperature stability of the systems is limited.

Another disadvantage is that caused by the complex system structure high manufacturing and material costs.

In a generic layer system, the two electrochromic boundaries Layers immediately next to each other. The one with the arrangement of an insulator layer Disadvantages associated between the two electrochromic layers are therefore eliminated.

Such a layer system is not only but also in DE 33 41 384 A1 mentioned at the beginning, in DE-OS 35 14 281, page 9, 1st paragraph.

A disadvantage of an electrochromic layer system without an insulator layer however, that to generate and maintain the coloring permanent current flow is required when using the layer system for an electrochromic rearview mirror for those for rearview mirrors usual dimensions with an area of about 100 cm² and the usual Operating voltage of 1.5 V can already be about 150 mA.

The potential reduction associated with the permanent current flow has the consequence that the inking efficiency is low and thus the switching speed is very limited. This is particularly noticeable when fast switching systems are necessary.

In addition, such systems have when the external electrical is switched off No storage function.  

The object of the invention is an electrochromic thin-film system of the generic type Kind in such a way that it without additional insulating Layer between the two electrochromic layers the above Disadvantages described of a system without an insulator layer do not have. In particular, that should electrochromic thin-film system for rear-view mirrors for motor vehicles be suitable.

This object is achieved by the electrochromic described in claim 1 Thin film system solved.

It has surprisingly been found that in electrochromic thin-film systems with one cathodically and one anodically colored Layer on an additional dielectric intermediate layer, which is usually the job is to make the two electrochromic layers electrical isolate from each other and thus z. B. that for coloring maintaining the necessary electrical field can be dispensed with entirely can, if the anodically colorable electrochromic layer itself in a directly on the cathodically colorable electrochromic layer adjacent area is designed so that in this area a has only low electrical conductivity, see in detail the Claim 1.  

If according to claim 1 by the electrical resistance of the Area of the anodically colorable electrochromic layer with reduced electrical conductivity determined total resistance R of the layer system perpendicular to the layer, measured at the contacts on the Electrodes, in any color state of the equation

10⁴ Ω cm² R · F 5 · 10⁴ Ω cm² (1)

is sufficient, where F is the area of the layer system, and where the Total resistance of the layer system without the area with reduced electrical conductivity satisfies the condition R · F <10³Ωcm², that shows Electrochromic layer system according to claim 1 a satisfactory electrochromic response and sufficient storage function.

The layer system according to the invention is comparatively powerful compared to that Systems with dielectric interlayer easier and Can be produced more cost-effectively and also points over them a higher long-term temperature stability.

Increase in total electrical resistance over that given by (1) has upper limit due to decreasing electron mobility reduced in the area of the anodic electrochromic layer electrical conductivity a decrease in the depth of dyeing and Switching speed. On the other hand, if the one defined by (1) below the lower limit, the coloring efficiency and thus decrease the switching speed and the memory properties are too strong from.  

The claim made in claim 1 of a size formed from area and electrical resistance as a measure of the electrical properties of the anodically colorable electrochromic layer is useful for two reasons.

On the one hand there is the coloring tension that is applied to the system at most can be limited for electrochemical reasons and, provided that it has the electrode contact surfaces there is no voltage drop, depending on the size of the ink to be colored Area largely independent. In the present case, the maximum dyeing voltage at approx. 1.5 V. If you increase this voltage, it happens for water decomposition and thus for irreversible electrochemical processes in the shift system.

On the other hand is in that of interest for the present application Layer thickness range for the anodically colorable electrochromic layer from 50 to 500 nm a dependence of the conductivity or the electrical Resistance from the layer thickness cannot be determined. Be justified can this fact with the fact that defects are independent of the  Form layer thickness and how even if the layer thickness increases before a short circuit line.

The electrical resistance of those integrated in the overall layer system An anodic electrochromic layer per se is very difficult to measure accessible. However, the electrical resistance is one according to the invention designed anodic electrochromic layer so large that the Resistance of the remaining layers can be neglected; the system resistance is essentially determined by the resistance of the area anodic electrochromic layer with reduced electrical Conductivity determined.

The electrical conductivity of the known cathodically electrochromic substances ranges from 10¹ (Ωcm) ^-1 in the bleached state to 5 × 10⁴ (Ωcm) ^-1 in the colored state (Shanks, HR; Sidles, PH and Danielson, depending on the coloration) , GC; Advancesin Chemistry Series, Vol. 39, (1962), p. 237-245).

The specific conductivity of a transparent electrode is about 10⁴ (Ωcm) ^-1 , and that of an aluminum electrode designed as a reflector is even greater.

For a rearview mirror with the layer structure according to subclaim 2 and with for The usual dimensions of the rear-view mirror (≈100 cm² area) result for example with careful contacting according to the above conductivity values a contact resistance upon contact with the electrodes and a Sheet resistance in the other layers of <1Ω. According to Eq. (1) an electrical resistance of the anodic electrochromic layer of <100Ω compared.  

The example above shows that the contact on the electrodes measured system resistance essentially the electrical resistance the area of the anodic electrochromic layer with reduced corresponds to electrical conductivity. The electrical properties an anodic electrochromic layer according to claim 1 thus determined in a simple manner by measuring the system resistance will.

The anodically electrochromic layer preferably has only one narrow area at the interface to the cathodic electrochromic layer reduced electrical conductivity.

This has the advantage that at least in the remaining area of the anodic electrochromic layer due to the unchanged high electrical Conductivity maintain the high coloring depth and switching speed stay.

The change in resistance in the anodic electrochromic layer can both in one step and continuously.

A steady transition of the electrical layer properties, e.g. B. so that the electrical resistance from the electrode-side interface the anodic electrochromic layer to the cathodic interface electrochromic layer rises continuously, becomes manufacturing technology Reasons preferred.

Regarding the choice of suitable materials for the electrochromic layers there are only limitations in that it must be possible to use the anodic to produce dyeable layer such. B. by influencing the Composition, porosity, etc. that in the case of sufficient transparency the layer thicknesses to be maintained are sufficiently high electrical resistance is achievable.

This is particularly the case with the electrochromic Ni (OH) ₂- known per se, Ir (OH) ₂ and Co (OH) ₂ layers possible. For the cathodic electrochromic Layer all known cathodically electrochromic materials can be used.

A layer structure with WO₃ as cathodic and Ni (OH) ₂ as anodic inkable Substance is preferred because such a layer system is simple and is inexpensive to produce as a stable thin-film system. About that In addition, the layer system has a high coloring efficiency.

For the substrate and the electrodes are those for electrochromic thin-film systems known materials used. Are as substrate material e.g. B. glass, transparent plastics, etc. suitable. The electrode materials are depending on the use of the thin film system, e.g. B. for transmissive or reflective systems.  

Transparent electrodes (e.g. the electrode denoted by ( 1 ) in FIG. 1) can consist of almost transparent metals (Au, Rh etc.) or semiconductors (indium / tin oxide, tin oxide, zinc oxide also doped). Transparent electrodes preferably consist of undoped indium / tin oxide.

All metals which have sufficient reflectivity and conductivity (for example Rh, Ag, Cr, Al) are suitable for the electrodes formed as reflection layers (the electrode denoted by ( 2 ) in FIG. 1). Al electrodes are preferably used because they have a high reflectivity (cf. Rh, Ag) and good electrochemical stability and are inexpensive. All layers are produced in a manner known per se by means of PVD processes (vapor deposition, sputtering) with customary process parameters.

So a WO₃ layer is preferably made in a thermal evaporator (WO₃ layers are difficult to produce in the electron beam evaporator).

The coating rate is preferably about 1 nm / sec. A higher coating rate is limited with conventional thermal evaporators because of the Durability of the tungsten layer used cannot be achieved. Lower coating rates should be avoided for economic reasons.

The vapor deposition pressure should be between 0.5 × 10 ^-5 and 5 × 10 ^-4 mbar, preferably around 2 × 10 ^-4 mbar, in the WO₃ coating and can be, for example, by gas inlet (e.g. O₂, N₂ or Ar ; The type of gas does not matter, only the gas pressure is important or the throttling of the pump output can be set.

The gas pressure of the WO₃ essentially controls the porosity and thus the Switching speed. The specified limits put the optimum between Switching properties and stability. Increasing the pressure at Evaporation leads to higher switching speeds, but to a reduction the temperature stability. A reduction in pressure at WO₃ leads not for absorption, but the switching properties become worse (Speed).

To produce a layer that is preferably used as an anodic electrochromic layer Ni (OH) ₂ layer can also vapor deposition and sputtering are used, the process parameters being controlled so that the desired conductivity is established in the layer.

Preferably, an electrochromic Ni (OH) ₂ layer by reactive evaporation manufactured in an O₂ atmosphere. It has been shown that this Process the electrical conductivity in the deposited layer in simply set via the substrate temperature in the coating process can be: With increasing substrate temperature, the electrical Conductivity in the deposited layer.

In particular, it has been used to produce a desired conductivity gradient proven in the layer to be advantageous with evaporators unshielded evaporation source to work. That from the evaporation source radiated heat can be used to advantage during the substrate of the coating process continue to heat up. The continuously increasing substrate temperature causes a as the layer thickness increases, the electrical power decreases continuously Conductivity.

Like the process parameters, in particular the substrate temperature, in detail to achieve a desired conductivity profile in the  The layer can be controlled by a person skilled in the art in a simple manner without inventive Action to be determined experimentally. The process parameters depend on the geometric dimensions of the vacuum chamber and its individual structure (e.g. screens, distance between evaporation source and substrate).

For the production of an electrochromic rearview mirror for motor vehicles, whose area is usually around 100 cm², it has been achieved a desired resistance curve in the layer as appropriate proven the coating process after reaching a substrate temperature starting from 90 ° C and during the coating the substrate temperature slowly increase to about 200 ° C.

With a suitable geometric arrangement of the vapor deposition source and substrate Given the vapor deposition parameters, this temperature increase alone can by heating the substrate by the one emitted by the vapor deposition source Heat can be achieved.

This procedure gives an anodically electrochromic Ni (OH) ₂ layer, whose electrical resistance from the electrode interface to Interface continuously increases towards the cathodic electrochromic layer, with the usual layer thicknesses the total resistance of the anodic electrochromic layer is around 100 Ω.

The evaporation pressure in the recipient should be set in the production of the Ni (OH) ₂ layer after pumping to <2 · 10 ^-5 mbar through O₂ inlet (reactive evaporation!) To 1 · 10 ^-4 - 8 · 10 ^-4 mbar . Higher pressures lead to a loose layer structure with negative effects on the temperature stability. Too low pressure leads to less oxidation, ie to increased absorption in the layer.

The coating rates should be as high as possible for economic reasons be. An upper limit for the coating rate of Ni (OH) ₂ is 1 nm / s, however layers of this type are no longer very much temperature stable. To increase the stability of the layers (i.e. layer thickness) the rate should not be <0.2 nm / s.

The layer thicknesses of the individual layers are in the usual ranges. In the case of electrochromic layers in particular, the suitable layer thickness ranges are in a manner known per se on the absorption capacity of the electrochromic Material in bleached condition and at the color depth align.

In the interest of a high dyeing depth, the layer thickness of the Ni (OH) ₂ layer should be as large as possible. Layer thicknesses of more than 500 nm should however be avoided as the absorption in the layer increases becomes strong. With layer thicknesses below 200 nm, the Contrast too weak.

The thickness of the WO₃ layer is advantageously based on the thickness of the Ni (OH) layer 0 aligned. It should be approximately 3 times the Ni (OH) ₂ layer thickness be. This harmful side reactions such. B. irreversible coloring, minimized. In general, however, are for the WO₃ layer layer thicknesses in the range from 100 to 800 nm are also suitable, the upper one and the lower limit as in the case of the Ni (OH) ₂ layer by the coloring depth and the absorption of the electrochromic material are determined.

The transparent indium / tin oxide electrodes should be as small as possible Have surface resistance, since this is essentially the speed of dyeing co-determined. Indium / tin oxide electrodes are particularly suitable with layer thicknesses around 200 nm and a sheet resistance of 10 Ω.  

The Al electrode used in reflective systems must form the Ni (OH) ₂ have sufficient permeability to H₂O. But it must sufficient conductivity and reflectivity have. The best results are obtained with layer thicknesses around 200 nm reached.

The advantages that can be achieved with the invention are in particular that Coloring and storage properties achieved with a simple system structure as they are otherwise only with complex layer systems, e.g. B. with at least one additional dielectric intermediate layer are to be found.

The invention is based on an exemplary embodiment in the drawing explained in more detail.

It shows

FIG. 1 shows an electrochromic thin-layer system;

Fig. 2 shows a suitable, known measuring arrangement for determining the electrical resistance of the overall layer system.

Two electrodes ( 1, 2 ) can be seen on a transparent substrate ( 3 ), which include two electrochromic layers, a cathodically colored layer ( 4 ) and an anodically colored layer ( 5 ). The order of the two electrochromic layers is in itself arbitrary; However, in the case of reflective systems, it is preferred to arrange the layer which can be colored cathodically, seen from the substrate side, in front of the layer which can be colored anodically, since such a layer structure achieves a higher reflectivity in the uncolored state.

It may be advantageous to coat the electrochromic thin-film system, as shown in the figure, on the anode side with a protective layer, e.g. B. to provide another transparent substrate. In the figure, ( 6 ) and ( 7 ) respectively designate an adhesive and a protective layer. However, such a protective layer is not absolutely necessary, since the system can also be adequately sealed off from the outside by selecting a suitable material for the rear electrode ( 2 ).

In one exemplary embodiment, a layer system with the layer structure shown in FIG. 1 was tested.

The individual layers were designed as follows:

With the exception of the Ni (OH) ₂ layer, all layers were known Process produced with common process parameters.  

The Ni (OH) ₂ layer was produced by reactive evaporation with an evaporation pressure of 2 · 10 ^-2 Pa and an evaporation rate of 0.2 nm / s. The substrate temperature was approximately 60 ° C. at the beginning of the vapor deposition process and increased to approximately 200 ° C. over the course of time as a result of heat radiated from the evaporation source.

The total resistance measured at the contacts on the electrodes the layer system was 120 Ω. With the area of 100 cm², R · F becomes 12 x 10³ Ω cm²; Equation (1) is fulfilled.

A measure of the functionality of the layer system is the residual current, which is a measure after the reaction is complete for the electrical short circuit between anodic and cathodic coloring Substance is. In the layer system described above, the Residual current in the bleached state approx. 30 mA, in the colored state approx. 50 mA. These are definitely for the applicability in a technical system sufficient values.

As a change in transmission, values between 70 and 7% and thus achieved more than with all other known solid state systems. This is mainly due to the fact that on the dielectric insulation layer has been completely dispensed with using the currently known materials and methods are not completely free of absorption can and therefore always a reduction in the maximum reflection with brings itself.

Maximum reflection, color depth and switching speed are in the Area of what is usually only with layer systems dielectric intermediate layer is reached. The coloring depth was 10%, the dyeing and bleaching time, which is a measure of the Switching speed represent 10s or 5s (with Λ OD = 1).

Λ OD = Ig T _max - Ig T _min = Ig (T _max / T _min )

With
T _max = transmission in the bleached state
T _min = transmission in the colored state

Claims (5)

1. Electrochromic thin-film system, in particular usable as an electrochromic rear-view mirror for motor vehicles, with a substrate and a cathodic and an anodically colorable electrochromic layer, and immediately adjacent to one another and arranged between two electrodes, characterized in that the anodically colorable electrochromic layer ( 5 ) in one the area directly adjacent to the cathodically colorable electrochromic layer ( 4 ) has such a low electrical conductivity that the total resistance R of the layer system determined by the electrical resistance of this area perpendicular to the layer, measured at the connection contacts on the electrodes ( 1, 2 ), in each coloring state of the equation 10⁴ Ω cm² R · F 5 · 10⁴ Ω cm² is sufficient, where F is the area of the layer system, and the total resistance of the layer system without the area with reduced electrical conductivity satisfies the condition R · F <10³Ωcm² . 2. Electrochromic thin-film system according to claim 1, characterized in that the rear electrode ( 2 ) seen from the substrate ( 3 ) is designed as a reflective layer. 3. Electrochromic thin-film system according to claim 1 or 2, characterized in that the electrical conductivity of the anodically colorable electrochromic layer ( 5 ) to the interface with the cathodic electrochromic layer ( 4 ) decreases continuously. 4. Electrochromic thin film system according to at least one of claims 1-3, characterized in that the cathodically colorable electrochromic layer ( 4 ) from WO₃ and the anodically colorable electrochromic layer ( 5 ) consists of Ni (OH) ₂. 5. Electrochromic thin-film system according to claim 4, characterized in that the cathodically colorable WO₃ layer ( 4 ) directly adjoins the substrate-side electrode ( 1 ) and the anodically colorable Ni (OH) ₂ layer ( 5 ) immediately formed as a reflective layer Electrode ( 2 ).