Process for the electric control of electrochromic elements

The invention relates to a novel method for the electrical control of electrochromic arrangements, with the aid of which it is possible to control the light transmission of these electrochromic elements continuously reproducible. The new method allows over the prior art by the dynamic adjustment of the switching voltage to changing resistors in the system and changing switching currents and a strong acceleration of the switching operation of electrochromic elements with high internal resistances due to large areas and / or relatively low-conductivity electrolytes, without leading to loss of stability.

Electrochromism refers to the reversible color change of materials which is triggered by the flow of an electric current and / or the application of an electrical voltage. There are a variety of electrochromic substances from different classes of substances, eg. B. Metal oxides (example: tungsten oxide), inorganic complex compounds (example: Prussian blue) or electronically conductive organic polymers (example: polyaniline). All have in common that they are colored differently in different oxidation states and that you can reversibly between these oxidation states by electrochemical oxidation or Can switch reduction. For most practical applications, preference is given to substances which can be switched at least between an uncolored and a colored state.

For the practical use of the effect of electrochromic electrochromic arrangements or elements are used, in which the electrically controlled modulation of the incident light for various applications, such as for building and

Vehicle glazing with automatically changeable translucency (intelligent windows), large-area displays, switchable optical filters for cameras, microscopes, etc. or also for auto-dimming rear view mirror is used.

In electrochromic arrangements, the electrochromic materials are located between two electrodes of which at least one (eg in displays or mirrors) or both (eg in the case of intelligent windows or switchable optical filters) are transparent. The electrochromic substances may be in the form of a film on the electrodes and / or be dissolved in the electrolyte. The invention relates to electrochromic arrangements with solid electrochromic layers, between which there is an electrolyte. This variant of electrochromic elements has a battery-like structure and accordingly shows a battery-like electrical behavior, ie A current flow must only occur during the charging and discharging processes, which correspond to coloration and discoloration. The current flow for changing the light transmittance of the electrochromic devices is usually caused in the prior art by applying a constant DC voltage, the polarity between coloration and decolorization is changed.

The structure of such an electrochromic element can be schematically represented as follows:    Substrate 1 / TCO / EC₁ / Electrolyte / EC₂ / TCO / substrate 2 , where with TCO the transparent conductive layers and with EC₁ and EC₂ the two electrochemically switchable, electrochromic layers are called. EC₁ For example, tungsten oxide or polyethylenedioxythiophene and EC₂ for example, from Prussian blue, polyaniline or nickel oxide. The combination of an electrochromic layer EC₁ with a so-called ion storage layer which does not change its color during the switching (instead of EC₂) is also possible.

In the case of electrochromic elements, as the electrolyte is often a preferably mechanically stable polymeric solid electrolyte (such as in EP1227362 described) to a loadable, eg to obtain in the building glazing practical electrochromic element. Such polymeric solid electrolytes usually consist of at least one suitable polymer, at least one plasticizer and at least one conductive salt such as optionally further additives, such as UV absorbers, fillers or antioxidants. The ionic conductivity of such polymeric solid electrolytes is several orders of magnitude worse than that of liquid electrolytes. The larger the polymer content in the solid electrolyte, the better its mechanical properties, but the worse is its ionic conductivity. Typically, the ionic conductivity of liquid electrolytes consisting of a high boiling point organic solvent such as γ-butyrolactone and a lithium salt such as lithium perchlorate is about 10^-2 S / cm at + 25 ° C. In contrast, the ionic conductivity of a polymer solid electrolyte, for example an ion-conducting PVB film, as described in WO02 / 40578, is only 10^-5 S / cm at + 25 ° C. This greatly reduced ionic conductivity of polymeric solid electrolytes over liquid electrolytes results in a significant reduction in the switching speed of electrochromic devices when operated with the electrochromic element electrical driving techniques known in the art.

Various variants of methods for the electrical control of electrochromic elements are known in the prior art.

In the simplest case, coloring and decolorization are carried out by applying a temporally constant DC voltage, which is usually between 0.5 and 3 V and whose polarity is changed between dyeing and decolorization processes. To be able to adjust the degree of absorption of the electrochromic elements steplessly, various methods have been proposed. Thus, in EP78464 an optical control circuit for electrochromic elements is described, in which the absorption of the electrochromic element is continuously optically measured, the signal compared with a predetermined value and then electrical charges to the element or removed from the element until the measured optical Absorbance corresponds to the setpoint. However, such a method is complicated, in particular since a light source and a detector must be mounted on the opposite sides of the electrochromic element for measurement.

In DE3142909 a continuous charge control for electrochromic layers is described in which the continuous adjustment of the degree of absorption takes place via the counting of the electrical charges which are supplied or removed from the electrochromic layer. This method represents a significant improvement over the optical measurement of the absorbance, as the charge count can be integrated directly into the electronic circuit. However, it is also stated in DE3142909 that smaller amounts of charge are removed for the decolorization processes than were supplied during dyeing. However, a permanently reversible circuit is only possible if the charge quantities for coloring and decolorization are as equal as possible.

In WO02 / 17008 a method and apparatus for charge and temperature controlled switching of electrochromic elements are described. In this case, the temperature of the electrochromic element is measured by a temperature sensor, for switching then a temperature-dependent voltage is applied and also the switching operation is controlled by the amount of electric charge flowing.

EP445720 describes a method and an arrangement for applying an electrical operating voltage to an electro-optical layer system with the aim of faster switching of electrochromic elements. This accelerated circuit is achieved in that at the beginning of the switching process, a higher voltage than the actually maximum permissible internal potential difference between the two transparent conductive layers is applied to compensate for the voltage drop in these conductive layers. At the end of the switching process, the otherwise usual voltage is applied again. In this case, the course of the voltage can be time-controlled and adapted to a predetermined voltage-time curve or the measurement of an internal potential difference takes place directly between opposite points of the electrochromic layers (a previously defined limit value may not be exceeded here). A similar technique is described in U.S. Patent US5124833. Although these methods can compensate for the voltage drop in the transparent conductive layers, not the often considerable voltage drop in the electrolyte.

DE19706918 also describes a method for operating an electrochromic element. In this case, in a start-up phase of the Umfärbevorganges the voltage is continuously increased or decreased to a maximum to a temperature-dependent end value, wherein the voltage is guided depending on the current. The maximum permitted switching voltage U_Max depends linearly on the temperature of the disk to U_Max = A - B · T depends. A and B are constants that are characteristic of the system used and are to be determined experimentally. The temperature dependence of the switching voltage is caused by the strong temperature dependence of the conductivity of the polymer solid electrolyte. However, this is not really linearly dependent on temperature, but obeys a more complicated relationship, the so-called Vogel-Tamann-Fulcher relationship. A linear temperature dependence is therefore only allowed as an approximation for small temperature intervals. When used over longer temperature ranges, this leads to irreversible destruction of the electrochromic elements during long-term operation. Complex is also the necessary experimental determination of the constants A and B. For a temperature-adapted control voltage, a measurement of the plate temperature must be made. This can be done according to DE19839748 by means of a temperature sensor, which is arranged in the region of the spacer of the electrochromic insulating glass. However, this means an extra effort in the production and increases the cost of electrochromic glazing.

The state of the art can therefore be summarized as follows: It is known that electrochromic elements can be switched steplessly via an optical control of the degree of absorption or via the control of the electric charges that have flowed. In principle, each optical intermediate state is adjustable. Furthermore, it is known to use a temperature-dependent time-invariable control voltage, wherein usually a linearly dependent on the temperature voltage is used and a measurement of the temperature is necessary. An acceleration of the circuit of larger electrochromic elements by increasing the drive voltage as compensation for the voltage drop of the transparent conductive layers is also known.

The disadvantage of this prior art described above is that, in particular, the low switching speed of electrochromic elements can not be increased when using polymer solid electrolytes which are mechanically very stable but have comparatively low ionic conductivity.

The object of the invention is therefore primarily to provide a novel method for the electrical control of electrochromic elements, in which not only the voltage drops in the transparent conductive layers but also the voltage drop in the solid electrolyte between the electrochromic layers by a dynamic change of the driving voltage during the circuit can be compensated.

As a result, electrochromic elements with polymeric solid electrolytes can in principle be switched just as fast as electrochromic elements with liquid electrolytes without endangering the long-term stability of the system. Another object of the present invention is to provide simple methods of controlling the depth of staining of electrochromic elements.

Surprisingly, it has been found that an acceleration of the switching speed of electrochromic elements with comparatively poorly conducting polymer solid electrolytes and / or with relatively high resistances of the TCO layers due to the size of the electrochromic elements can not be achieved by the DC voltage for staining U_{Switching, coloring} and the DC voltage for decolorization U_{Switching, discoloration} of the element in each case from 2 partial voltages U₁ and U₂ be composed, wherein the partial voltage U₂ is dynamically adapted to the current flowing in the circuit i and to the strong temperature-dependent electrical resistance of the electrochromic element R.

According to the invention, the partial stresses for the coloring U_{1, coloring} and for the discoloration U_{1, decolorization} chosen so that the electrochromic elements are not damaged even with a long concern of this tension. In general, this requirement is met when the voltage for the coloring U_{1, coloring} amount less than or equal to the open circuit voltage of the fully colored and the voltage for the decolorization U_{1, decolorization} amount less than or equal to the open circuit voltage of the fully decolorized electrochromic element. Even voltages that are slightly above these no-load voltages are permissible. The open circuit voltages of the fully colored or fully discolored state of the electrochromic element may not be exceeded by more than a factor of 1.5. The magnitude of the no-load voltages of the fully colored and decolorized state are typical of the actually selected electrochromic layer combinations and the electrolytes used and can be temperature-dependent.

The electrical resistance R of the electrochromic element is composed, as indicated in equation 1, of 4 partial resistances, the resistance of the transparent conductive layer R depending on the area and the sheet resistivity of the TCO layer_TCO, the resistances of the two switchable layers (EC₁ and EC₂) R_EC1 and R_EC2 and the strongly temperature-dependent resistance of the electrolyte R_electrolyte,<maths id="math0001" num="1"><math display="block"><mrow><msub><mrow><mtext>R = R</mtext></mrow><mrow><mtext>TCO</mtext></mrow></msub><msub><mrow><mtext> + R</mtext></mrow><mrow><mtext>EC1</mtext></mrow></msub><msub><mrow><mtext> + R</mtext></mrow><mrow><mtext>EC2</mtext></mrow></msub><msub><mrow><mtext> + R</mtext></mrow><mrow><mtext>electrolyte</mtext></mrow></msub></mrow></math><img file="EP1517293A1_D0001.tif" /></maths>

The resistance of the transparent conductive layer R_TCO remains in an electrochromic element of predetermined geometry largely unaffected by the practically occurring disk temperatures (approximately between - 40 and + 90 ° C) and the switching state of the element. It can be made from the product of sheet resistance R_sq the TCO layer with the contact distance I₁ [cm] divided by the width I₂ [cm] of the electrochromic element are calculated (equation 2).<maths id="math0002" num="2"><math display="block"><mrow><msub><mrow><mtext>R</mtext></mrow><mrow><mtext>TCO</mtext></mrow></msub><msub><mrow><mtext> = R</mtext></mrow><mrow><mtext>sq</mtext></mrow></msub><msub><mrow><mtext>· I</mtext></mrow><mrow><mtext>1</mtext></mrow></msub><msub><mrow><mtext> / I</mtext></mrow><mrow><mtext>2</mtext></mrow></msub></mrow></math><img file="EP1517293A1_D0002.tif" /></maths> Surprisingly, it has been found that it is sufficient for the inventive method for the electrical control of electrochromic elements, the resistances of the transparent conductive layer R_TCO and the electrolyte R_electrolyte to evaluate because the resistances of the electrochromic layers R_EC1 and R_EC2 in practice-relevant electrochromic arrangements (building glazings, vehicle glazing, rear view mirrors, large-area information displays) with relatively low-conductivity electrolytes and / or large surfaces to be switched small compared to R_TCO and R_electrolyte are.

According to the invention, therefore, the dynamic partial stresses for the coloring U_{2, staining} and for the discoloration U_{2, discoloration} to the partial resistors R_electrolyte and R_TCO and to the current flowing in the circuit i continuously<maths id="math0003" num="3"><math display="block"><mrow><msub><mrow><mtext>U</mtext></mrow><mrow><mtext>2, staining</mtext></mrow></msub><msub><mrow><mtext> = i · (a · R</mtext></mrow><mrow><mtext>electrolyte</mtext></mrow></msub><msub><mrow><mtext> + b · R</mtext></mrow><mrow><mtext>TCO</mtext></mrow></msub><mtext>)</mtext></mrow></math><img file="EP1517293A1_D0003.tif" /></maths> or.<maths id="math0004" num="4"><math display="block"><mrow><msub><mrow><mtext>U</mtext></mrow><mrow><mtext>2, discoloration</mtext></mrow></msub><msub><mrow><mtext> = i · (c · R</mtext></mrow><mrow><mtext>electrolyte</mtext></mrow></msub><msub><mrow><mtext> + d · R</mtext></mrow><mrow><mtext>TCO</mtext></mrow></msub><mtext>)</mtext></mrow></math><img file="EP1517293A1_D0004.tif" /></maths> (with a, b, c and d ... constants), whereby for the constants the values ​​a = 1, c = 1 and b = ²/₃ and d =²/₃ be used. Through this dynamic adaptation of the partial voltage U₂ a strong acceleration of the circuit of electrochromic elements is possible, since thereby voltage drops, which are caused by the corresponding resistors and the flowing current, can be at least partially compensated.

The constants a to d are between 0 and 1 and indicate to what degree the corresponding resistance of the transparent conductive layer R_TCO or of the electrolyte R_electrolyte enter into the equation. If the constants a and c are equal to 1, the calculation of the dynamic partial voltage U₂ according to Equations 3 and 4, the full voltage drop across the electrolyte is taken into account. If a and c are greater than 1, there is a risk of applying too high effective voltages to the electrochromic element, which could lead to damage and ultimately to the destruction of the electrochromic element. It has also been shown that the resistance of the TCO layer can not be completely compensated (this would mean: b or d = 1), if the effective voltages in the edge region of the electrochromic element should not be too high. According to our own model calculations, the permissible value for b and d in the continuous converter is a maximum of 0.666 since damage to the electrochromic elements does not occur even in the case of high switching cycle numbers.

The total voltage for coloration or decolorization of an electrochromic element is thus calculated according to the method according to the invention for the electrical control of electrochromic elements, as indicated in Equations 5 and 6:<maths id="math0005" num="5"><math display="block"><mrow><msub><mrow><mtext>U</mtext></mrow><mrow><mtext>Switching, coloring</mtext></mrow></msub><msub><mrow><mtext> = U</mtext></mrow><mrow><mtext>1, staining</mtext></mrow></msub><msub><mrow><mtext> + i · (a · R</mtext></mrow><mrow><mtext>electrolyte</mtext></mrow></msub><msub><mrow><mtext> + b · R</mtext></mrow><mrow><mtext>TCO</mtext></mrow></msub><mtext>)</mtext></mrow></math><img file="EP1517293A1_D0005.tif" /></maths> or.<maths id="math0006" num="6"><math display="block"><mrow><msub><mrow><mtext>U</mtext></mrow><mrow><mtext>Switching, discoloration</mtext></mrow></msub><msub><mrow><mtext> = U</mtext></mrow><mrow><mtext>1</mtext></mrow></msub><msub><mrow><mtext>, Discoloration + i · (C · R</mtext></mrow><mrow><mtext>electrolyte</mtext></mrow></msub><msub><mrow><mtext> + d · R</mtext></mrow><mrow><mtext>TCO</mtext></mrow></msub><mtext>)</mtext></mrow></math><img file="EP1517293A1_D0006.tif" /></maths>

At the beginning of such a switching operation according to the invention, the partial voltage U is applied to the electrochromic element_{1, staining} or when decolorizing U_{1, discoloration} created. The then flowing current i is measured and the new voltage U to be applied_{Switching, coloring} or U_{Switching, discoloration} calculated according to equations 5 and 6 above and applied to the element. Then again, measurement of the current, calculation and application of the new switching voltage. These operations are repeated until the desired color depth is reached. According to the invention, the time intervals for the adaptation of the drive voltage are preferably between 10 milliseconds and 5 seconds.

While the resistance of the transparent conductive layers R_TCO remains largely unchanged under the operating conditions of an electrochromic element, the resistance of the electrolyte R_Elektrotyt due to temperature changes, which in practical applications may be between about -40 and + 90 ° C, to change several orders of magnitude.

In an embodiment of the method according to the invention for the electrical control of electrochromic elements, therefore, the strongly temperature-dependent resistor R_electrolyte of the ion-conducting electrolyte measured at certain time intervals during the switching operation. It has surprisingly been found that the determination of the resistance R of the electrochromic element very simply by measuring the open circuit voltage of the disc U_Neutral, Applying a switching voltage U_switching and measurement of the thereby flowing current I and subsequent calculation according to equation 7 can take place:<maths id="math0007" num="7"><math display="block"><mrow><msub><mrow><mtext>R = | U</mtext></mrow><mrow><mtext>switching</mtext></mrow></msub><msub><mrow><mtext>- U</mtext></mrow><mrow><mtext>Neutral</mtext></mrow></msub><mtext>) / |</mtext></mrow></math><img file="EP1517293A1_D0007.tif" /></maths> To measure the open circuit voltage, the switching process must be interrupted briefly. This novel method according to the invention for determining the actual total resistance of the electrochromic disks can be carried out both during the dyeing and during the decolorization. From this total resistance determined and the calculated according to equation 2 TCO resistance, the resistance of the electrolyte according to equation 1, ignoring the resistances of the two switchable layers (EC₁ and EC₂) to calculate.

The electrolyte resistance determined in this way can be used to dynamically adapt the switching voltage according to Equations 5 and 6. In this procedure, a measurement of the temperature of the electrochromic element is no longer necessary. An advantage of this method is that no temperature sensor has to be integrated into the glazing area.

Important for a permanently reproducible circuit of electrochromic elements is the control of their degree of coloration, which takes place according to the prior art via an optical measurement or via the control and storage of the charges flowing during switching. Surprisingly, it has been found that the depth of coloration or the amount of injected charges of open-circuit voltage U_Neutral proportional to the electrochromic element. Accordingly, in an inventive embodiment of the electronic control for electrochromic elements, the stepless circuit via the control of the open circuit voltage. For this purpose, the switching operation is interrupted in short successive intervals for a short time and here the open circuit voltage is measured, which is compared with the resulting for the desired color depth open circuit voltage. If this is reached, the dyeing or Entfärbevorgang is interrupted. In this procedure can be dispensed with the control of the electric charges flowed.

Over a longer period of operation of an electrochromic element, creepage currents and / or traces of oxidizing and / or reducing agents present in the polymer electrolyte can influence the state of charge independently of the controlled charge quantities. In order to bring the electrochromic element back into a defined, controllable initial state, therefore, in an embodiment according to the invention, at certain larger time intervals, which may preferably be between 1 hour and 3 months, deep discoloration or Deep discharge of the element. For this purpose, a deinking voltage is applied for a period of time which may be between 5 minutes and 2 hours, which is 0.3 to 2.4 V in magnitude over the no-load voltage of the fully decolorized electrochromic element. This can, in order not to impair the functionality of the discs during the day, for example, occur at night in the above-mentioned intervals, when the switching function of the electrochromic element is not used anyway.

embodiment

An electrochromic element of the structure:

Glass / FTO / tungsten oxide / ion conductive PVB film / Prussian blue / FTO / glass with an active button of 30cm x 8.5cm was prepared and sealed as described in EP1227365. The ionic conductivity of the ion-conductive PVB film (PVB ... polyvinyl butyral) was about 7 × 10 4^-6 S / cm, the sheet resistance of the FTO layers approx. 18 Ω / sq. The combination glass / FTO is commercially available K-glass of the Fa. Pilkington (FTO is a transparent conductive layer and stands for fluorine-doped tin dioxide). After the preparation of the element, the basic decolorization was carried out by applying a DC voltage of 2.4 V for 2 hours, with the Prussian blue as the negative pole and the tungsten oxide as the positive pole. After the end of the basic decolorization, the open-circuit voltage of the decolorized state was determined. It was -720 mV. Thereafter, the electrochromic element was dyed by applying a voltage of 1.4 V (tungsten oxide: negative pole, Prussian blue: positive pole). After a charge of 15 mC / cm² was flown, the dyeing process was terminated and determines the open circuit voltage of the colored state. It was +1100 mV. .

Thereafter, the relationship between injected charge amount and open circuit voltage was determined. For this purpose, the fully colored electrochromic element was decolorized by applying a direct voltage of 1.4 V (tungsten oxide: positive pole, Prussian blue: negative pole) for 3 minutes. Thereafter, the element was stained again step by step by applying 1.4 V of reverse polarity and, in the meantime, the no-load voltage was measured. And the charges flowed per area calculated The relationship thus determined between injected charges and open circuit voltage is shown in Figure 1. It can be seen that this relationship can be used to evaluate and control the amount of injected charges and thus the depth of coloration of an electrochromic element.

This electrochromic element was then switched once with constant voltage according to the prior art and then with the inventive continuous adjustment of the switching voltage and in each case recorded the current density-time characteristic. Both characteristics are shown in FIG. With 1 here is the current density-time characteristic when coloring and decolorization with ± 1.4 V and with 2 is the characteristic for switching with continuous adjustment of the switching voltage with the above-determined open-circuit voltages (U_{1, decolorization} = -720 mV, U_{1, coloring} = +1.1 V) and values ​​for (a · R_electrolyte + b · R_TCO) and (c · R_electrolyte + d · R_TCO) of 40 ohms each. Curve 1 shows the current density-time curve for staining and decolorization within the illustrated time of 480 seconds. At the same time, with dynamic adjustment of the switching voltage, 8 coloring and decoloring cycles are carried out (curve 2). The faster circuit in the method according to the invention can also be recognized by the higher current density and the charge connected per time. By means of the continuous adjustment of the switching voltage, approximately as much charge per area is switched in 30 seconds as in the case of constant voltage switching in 4 minutes.

3 shows the time profile of the switching voltage for a circuit according to the prior art with constant voltage (here ± 1.4 V, curve 1) and for the inventive circuit with continuous adjustment of the switching voltage to the resistor and the flowing current (curve 2) , In the range of high current densities, high switching voltages of approx. 12 V reaches, but which do not have a detrimental effect on the electrochromic element. Towards the end of a circuit, the applied voltages in the method according to the invention are even smaller than in the case of the constant-voltage circuit, so that the electrochromic element is switched on altogether more gently.