GB2126365A

GB2126365A - A method of heating an electrooptical device

Info

Publication number: GB2126365A
Application number: GB8319391A
Authority: GB
Inventors: Dr Werner Thoni
Original assignee: Balzers AG
Current assignee: OC Oerlikon Balzers AG
Priority date: 1982-07-29
Filing date: 1983-07-18
Publication date: 1984-03-21
Also published as: FR2531300A1; DE3321032A1; GB8319391D0

Abstract

A method of heating electrochromic or liquid crystal devices comprises supplying an A.C. heating current in addition to the D.C. voltage needed to operate the device, the frequency of the A.C. current being such as not to disturb the operation of the device. Preferably the heating current is supplied to the two flat operating electrodes (12,13) of the device. <IMAGE>

Description

SPECIFICATION A method of heating an electrooptical andlor electrochemical element The invention relates to a method of heating an electrooptical andlor electrochemical element with a thin layer system consisting of a plurality of layers applied on to a carrier, at least one layer being formed as a flat electrode for supplying DC voltage needed for the function of the element. Such elements are e.g. electrochromic layer systems, in which a layer of electrochromic material is embedded between two (transparent) electrically conductive flat electrodes, which material changes its optical transmission (absorption) underthe influence of a DC voltage applied to the flat electrodes.

Transparent electrochromic layer systems have been suggested e.g. for spectacle glasses having adjustably variable absorption. They are also known for mirrors in which reflectivity may be adjusted to a desired value by the variable absorption of an electrochromic layer system arranged before the reflecting surface. Other such known layer systems are sensors in which a layer is embedded between electrodes which is of a substance which reacts with or absorbs other substance to be detected passing through one of the (porous) electrodes, e.g. a noxious substance in the atmosphere which changes the electric resistance of the sensor layer; the DC current which flows across the electrode through the layer when DC voltage has been applied, can then be used as a measure of the influence of the substance to be detected.Other sensors produce themselves a DC voltage variable by the influence of a substance to be detected, which voltage can be carried away via the flat electrodes orf the system.

Further examples of electrooptical layer systems of the initially-mentioned kind are various displays, e.g.

such in which a layer of a so-called liquid crystal is positioned between two translucent flat electrodes, where long molecules of the liquid crystal are by the influence of an electric field produced by the electrodes orientated in a preferred direction in which they have a smaller ability to scatter light than in an unordered state. The difference in the ability to scatter light at different places, at which different voltages were applied to differently shaped partial electrodes, serves for the representation of characters, e.g. numerals and letters.

In most applications of electrooptical and electrochemical layers temperature plays an important part because nearly in all cases the properties of the system, e.g. the electric conductivity of a sensor layer or the optical absorptivity of an electrochromic spectacle glass layer, are so much influenced by temperature that satisfactory functioning is achieved only if the temperature can be suitably controlled.

It is known to heat surfaces by the passage of current through electrically conductive foils or thin layers applied thereto; known are for instance spectacles for skiers and rear view mirrors for motor vehicles heated by layers applied by evaporation coating. Usually currents supplying electrodes are situated on two mutually opposite sides of a heating field formed by a conductive thin layer and heating current, flowing across these, flows in the layer parallel to their surface. With electrooptical and electrochemical elements this heating of the layer system suffers from various difficulties because the heating current can disturb the function of the element. For that reason heating had to be disensed with in many cases.

The aim of the invention is to devise a method-of heating and thereby controlling the temperature of electrooptical and electrochemical elements which allows their heating without disturbing theirfunction. This aim is achieved in that alternating current is simultaneously supplied to the flat electrode as a heating current not disturbing the function of the element.

This enables complete separation of the heating function from the other functions; as is known, the heating current may be supplied and carried away most simply on mutually opposite sides of the flat electrode. A method according to the invention is particularly suitable for the heating of such electrooptical and electrochemical elements the thin layer system of which has two or more layers formed as flat electrodes with dielectric layers arranged therebetween, the alternating heating current being preferably supplied at one end of a first flat electrode and carried away at the opposite end of a second flat electrode.Due to the very small thickness of the thin layers the capacitor formed by the two flat electrodes and the dielectric situated between them has a relatively high capacity so that even with the use of a relatively low frequency of several kHz a capacitive-AC resistance is obtained which is very small compared with the electric DC resistance of the dielectric layer between them, so that the two flat electrodes are practically shortcircuited for the alternating current. It was found that the arrangement described below with reference to one embodiment provides a particularly uniform heating, while the alternating field between the flat electrodes is practically equal to zero and consequently does not disturb the electrooptical and/or electrochemical function.

The invention will now be described, by way of example, with reference to the accompanying diagrammatic drawing, in which: Figure 1 shows a first embodiment of an electrooptical element the translucency of which could be controlled by electrical DC voltage; and Figure 2 shows a second embodiment of such an element in which the reflection is controllable by DC voltage.

Figure 1 shows a transparent carrier 1 (e.g. a spectacle glass), on which is as a first flat electrode applied a layer 2 of an alloy of indium oxide and tin oxide the thickness of which is approximately 280 nm. This flat electrode has a so-called surface resistivity of 20 ohms per square, i.e. between the opposite sides of a square of any size of such a layer is an electric resistance of 20 ohms (with a large square the width of the current path between the two sides# is larger, but at the same time also the distance is larger, so thatthere is always the same resistance). The layer system has an identical second layer 3 which also represents a transparent electrically conductive flat electrode.Between the two flat electrodes 2 and 3 are embedded, in the illustrated example, two further layers, namely a first nonmetallic layer 4 of tungsten oxide which is 500 nm thick, and a second non-metallic layer 5 of silicon oxide which is 150 nm thick. Both these layers 4, 5 form, together with the flat electrodes 2 and 3, a so-called electrochromic layer system which changes its optical transmissivity under the influence of a DC voltage applied to said electrode, i.e.

increases or reduces it according to polarity. Further details about electrochromic layer systems may be found in literature.

For supplying DC voltage from a voltage source 6 serve the two connections 7 and 8, as shown in the drawing. In the illudstrated example increased absorption can be obtained with a DC voltage of +2.5 V (positive pole on the upper flat electrode 3 in Figure 1), while a reduced absorption of the electrochromic interposed layers may be obtained with a voltage of -1.5 V on the same electrode. This effect was, however, rather dependent on temperature so that when such layers are used it is often desirable to ensure corresponding temperature by controlled heating.As is further shown in Figure 1, the flat electrode 3 is for this purpose provided, at the end opposite to that at which the connection 8 is situated, with a further connection 9 and through these two connections 8, 9 alternating heating current can be supplied from the source 10 which flows through the flat electrode 3 in a direction parallel thereto and heats it corresponding to the ohmic resistance of this current path. The heating output can be controlled by the height of the AC voltage of the source 10, and the frequency, as mentioned, is so chosen that impairment of the function of the electrochromic layer system is reliably avoided. It was found that this is always possible, while on the contrary when heating with a DC current functional disturbances were observed even if the heating current - as in Figure 1 - does not flow through the electrochromic layers of the system.

The embodiment shown in Figure 2 relates to a similar electrochromic layer system, but in this case the first electrode layer 12, which is adjacent to the substrate or carrier 11, is in the form of an aluminium mirror the reflectivity of which is influenced by the more or less strong absorptivity of the electrochromic layer system positioned thereon. A transparent gold layer serves as a second electrode layer 13, and between these electrode layers 12 and 13 are further layers, namely a WOB- layer 14 which is 500 nm thick and a zirconium oxide layer 15 which is 150 nm thick.

For the use of the method according to the invention for the heating of a layer system according to Figure 2 a DC voltage is applied to the connections 17 and 18 as an operational voltage for the electrochromic layers, and simultaneously an AC voltage of corresponding frequency is superimposed as a heating voltage. It could be found by measurement that in this embodiment the capacitor, formed by the two flat electrodes 12 and 13 and the interposed layers 14 and 15, had a capacitance of 0.1 microfarad per square centimetre, so that even with a frequency of 1000 Hz the resistance to alternating current was hardly of any significance compared with the ohmic resistance of the dielectric; one could speak practically about a capacitive short circuiting between the flat electrodes.The technical operational data of this system are similarto the first embodiment, namely +2.8 V for maximum colouration of the electrochromic layer system (minimum reflectivity of the mirror) and -1.5 V for minimum absorption (maximum reflection).

In this embodiment operational voltage is used for the heating of the layer system which consists of superposition of the DC voltage needed for the operation and of the AC voltage needed for the heating. For this purpose can be used the circuit shown in Figure 2 by means of which can be connected to the flat electrodes 12 and 13 via the connections 17 and 18 the source 16 of DC voltage with always the desired polarity and atthe same time via the switch 22 a source 20 of AC voltage. In the illustrated circuit a capacitor 19 is provided in the AC circuit and a choke 21 in the DC circuit to block the relevant circuits for direct current and alternating current, respectively.It is obvious to an electrical engineer that for the superposition of a DC voltage and an AC voltage other connections are possible which could be used within the framework of this invention; the superposition circuit per so is not subject matter of the invention.

With such a superposition of the DC voltage, needed for the operation of an element, and AC voltage, serving for heating, a particularly uniform heating of elements with two flat electrodes can be achieved, and in addition, contrast to the embodiment in Figure 1, one connection is safe. Experience showed that without disturbance of the electrochromic function heat flow rates of up to several watts per square centimetre may be readily used for the electrochromic layer systems. By suitable control of the heatoutputthetemperature of the elements can thereby be maintained at a desired value within wide limits independently on the ambient temperature.

Claims

1. A method of heating an electrooptical and/or electrochemical element with athin layer system formed by a plurality of layers on a carrier, at least one said layer being formed as a flat electrode for supplying DC voltage needed for the function of the element, wherein, in operation, simultaneously with the direct current alternating current is supplied to the flat electrode as a heating current, the current being of a frequency not disturbing the function of the element.

2. A method according to Claim 1, wherein the heating current is supplied and carried away at opposite ends of the flat electrode.

3. A method according to Claim 1 of heating an electrooptical element, the thin layer system of which has at least two said layers, formed as flat electrodes, with at least one dielectric layer arranged between the two flat electrodes, wherein the alter nating heating current is supplied at one end of one of the flat electrodes and carried away at the opposite end of the second flat electrode.

4. A method according to Claim 1, wherein the heating current has a frequency of at least 1 kHz.

5. A method of heating an electrooptical and/or electrochemical element substantially as herein described with reference to the accompanying drawing.

6. A product including an element according to Claim 4 or Claim 5.

6. An electrooptical element formed as an electrochromic layer system with two flat electrodes and an electrochromic layer arranged between them, wherein each of the flat electrodes has an electrode for supplying the heating current arranged at opposite ends of the flat electrodes.

7. An electrooptical and/or electrochemical element constructed, arranged and adapted to operate substantially as herein described with reference to, and as shown in, Figure 1 or Figure 2 of the accompanying drawing.

8. A product including an element according to Claim 6 or

7.

New claims or amendments to claims filed on

8.11.83 Superseded claims 1 to 8 New or amended claims:

1. A method of heating an electrooptical and/or electrochemical element with a thin layer system having at least two said layers, formed as flat electrodes for supplying DC voltage required for the operation of the element, and at least one dielectric layer being arranged between said at least two flat electrodes, wherein simultaneously with the direct current, alternating heating current is supplied at one end of a first said flat electrode and carried away at tuhe opposite end of a second said flat electrode, the current being of a frequency such as not to disturb the operation of the element.

2. A method according to Claim 1, wherein the heating current has a frequency of at least 1 kHz.

3. A method of heating an electrooptical and/or electrochemical element, the method being substantially as herein described with reference to Figure 2 of the accompanying diagrammatic drawings.

4. An electrooptical and/or electrochemical element formed as an electrochromic layer system with an electrochromic layer arranged between a first and a second flat electrode, wherein each of said flat electrodes if provided with a supply electrode for supplying heating current to said flat electrodes, said supply electrodes being arranged at opposite ends of said flat electrodes.

5. An electrooptical and/or electrochemical element constructed, arranged and adapted to operate substantially as herein described with reference to, and as shown in, Figure 2 of the accompanying drawings.