WO1997028484A1

WO1997028484A1 - Electrochromic devices

Info

Publication number: WO1997028484A1
Application number: PCT/GB1997/000199
Authority: WO
Inventors: Richard Anders Batchelor
Original assignee: Pilkington Plc
Priority date: 1996-02-02
Filing date: 1997-01-24
Publication date: 1997-08-07
Also published as: EP0879440A1; AU1451397A

Abstract

Methods of driving an electrochromic device, of the construction shown in figure 1, having layers of electrochromic (16), electrolyte (18) and counter-electrode (6) materials interposed between electrically conducting films. A first method involves: (a) to colour the device (1), passing a constant current in a first direction through the layers (16, 18, 6) until the first of a limit on the amount of charge passed or a limit on the voltage applied is reached, whereupon the applied voltage on reaching that limit is maintained for a predetermined time period; or (b) to bleach the device, passing a constant current in a second, opposite, direction through the layers (16 18, 6) until the first of a limit on the rate of change of voltage, a limit on the voltage applied or a limit on the amount of charge is passed is reached, whereupon the applied voltage on reaching that limit is maintained for a predetermined time period. A second method involves: (a) applying an alternating current or voltage signal to the device (1); (b) determining the magnitude of the device impedance from the current/voltage behaviour of the device (1); (c) estimating the temperature of the device (1) from the determined impedance; (d) setting a maximum device (1) driving voltage and a voltage pause time in accordance with the estimated temperature; and (e) to colour or bleach the device (1), passing a constant current in an appropriate direction through the layers until the first of a limit on the amount of charge passed or the maximum driving voltage is reached, whereupon the applied voltage on reaching that limit is maintained for the voltage pause time.

Description

TTTLE:

Electrochromic Devices

DESCRIPTION: Technical Field

The invention relates to electrochromic devices as used, for example, in so-called variable transmission windows or variable reflection mirrors, and in particular to a method of driving such devices.

Background Art

Electrochromic devices are known to have successive layers of electrochromic, electrolyte and counter-electrode materials. The device may have first and second laminar substrates each covered on one side with an electrically conducting film, the layers interposed between the two substrates with the film covered sides innermost Alternatively, the device may have one laminar substrate covered on one side with an electrically conducting film, the layers being carried on this film covered side with a further electrically conducting film applied over the exposed layer. The most common substrate material is glass, but plastics materials, like acrylic, may also be used.

By way of example, the electrically conductive films may be indium doped tin oxide, the electrochromic material may be tungsten trioxide, the counter-electrode material may be cerium titanium oxide and the electrolyte material may be a suitable polymer to which lithium perchlorate has been added.

A tungsten trioxide/cerium titanium oxide device can be changed between bleached and coloured states by altering the applied electrical potential, that is, the potential applied via the electrically conductive films (acting as electrodes) across the electrochromic, electrolyte and counter electrode layers. The polarity of the potential dictates the direction of transfer of cations (provided by the lithium perchlorate) through the electrolyte material, between the electrochromic and the counter-electrode materials. The cation transfer is reversible. When reduced, or in other words when cations are inserted, the electrochromic material is coloured, whereas, when oxidised (when cations are de- inserted), it is virtually colourless. Conversely, the counter-electrode material is chosen because it is virtually colourless when either reduced or oxidised, or at least any colouring on reduction is indiscernible.

A tungsten trioxide/cerium titanium oxide device can be varied from a blue coloured state to a pale yellow "colourless state". The degree of colouration is related to the quantity of cations inserted into the electrochromic material, and hence the extent of reduction, which is dictated by the amount of charge passed through the device; the greater the charge passed, the deeper the colour.

Other electrochromic/counter-electrode material combinations may work in reverse, with the electrochromic layer colouring on oxidation, and different combinations can produce different colours and degrees of colour change. There are also devices wherein a single layer acts as both the counter-electrode and the electrically conducting film. Furthermore, there are devices, such as those available from the Gentex company, which have a single material which functions as the electrochromic, counter-electrode and electrolyte layers.

The changeable state of an electrochromic device lends itself to use in, amongst other applications, a window where variable transmission characteristics are required. For instance, by colouring the window during the hottest part of a summer's day, the amount of solar radiation entering a building can be πύnimised, and on dull winter days the window can be bleached so as to make best use of the available natural light As with all windows, however, variable transmission windows are exposed to a wide variety of external conditions and have to be able to operate over a range of temperatures, ideally something like -20°C to 90°C.

It has been found that electrochromic devices behave in a significantly temperature dependent manner. Experiments have shown that the conductivity of the electrolyte layer and the kinetics (electrochemical reactions) at the electrochromic/electrolyte/counter- electrode layer interfaces both increase considerably with temperature. This in turn results in larger currents and faster transitions between states, which can lead to an increased likelihood of uneven colouration, especially in large area devices, and/or degradation, particularly due to irreversible oxidation on bleaching. The unevenness of colouration in large area devices may be caused by significant voltage drops across the electrically conducting films as a result of large currents and high sheet resistance. Any such significant voltage drop will noticeably affect the uniformity of the potential difference to which the electrolyte layer is subjected. The degree of unevenness can be decreased, for instance, by reducing the conductive film sheet resistance or by increasing the resistivity of the electrolyte layer. However, reducing the conductive film sheet resistance can detrimentally affect light transmission characteristics and may increase cost The preferred option, therefore, is to increase the electrolyte layer resistivity, although this is not a complete solution as the resistivity may fall at elevated temperatures.

Thus, for variable transmission windows and the wide ranging conditions to which they are exposed, account has to be taken of the effect of the temperature dependency of the device on the performance of the window. It has been proposed to use sensors to continually monitor how the device is behaving and to provide feedback on the basis of which steps can be taken to allow for temperature induced variations. Such sensors, however, cannot give a truly representational view of performance without being located all over the device, which is not ideal for a window where visibility may be impaired, and the sensors may increase the hardware complexity.

The Invention

It is an object of the invention to provide a method of driving an electrochromic device capable of operating over a wide temperature range by allowing for temperature dependent variations in the behaviour of the device, without the need for sensors detecting, say, device temperature or light transmission.

In a first embodiment the invention provides a method of driving an electrochromic device having layers of electrochromic, electrolyte and counter-electrode materials interposed between electrically conducting films, characterised by:

(a) to colour the device, passing a constant current in a first direction through the layers until the first of a limit on the amount of charge passed or a limit on the voltage applied is reached, whereupon the applied voltage on reaching that limit is maintained for a predetermined time period; or (b) to bleach the device, passing a constant current in a second, opposite, direction through the layers until the first of a limit on the rate of change of voltage, a limit on the voltage applied or a limit on the amount of charge is passed is reached, whereupon the applied voltage on reaching that limit is maintained for a predetermined time period.

It has been found that by driving the device at constant current the driving voltage is automatically adjusted for the temperature dependent characteristics, that is to say, the voltage is automatically adjusted with temperature induced changes in the device conductivity and kinetics. For instance, at higher temperatures a smaller driving voltage is all that is required to achieve the specified constant current and the device is automatically driven at the required voltage, whereas at lower temperatures a larger voltage is required to achieve the same specified current and the device is automatically driven at the larger voltage. The automatic voltage adjustment maintains the constant current which in turn controls the rate of switch of the device. The rate of switch has an effect upon the evenness of colouration and high rates lead to uneven colouration. Consequently, controlling the rate of switch controls the evenness of colouration.

The device can be switched to coloured from the bleached state by passing a predetermined amount of charge at constant current On reaching the predetermined charge, the voltage is then held at the value reached for a preset time, before the switching is completed. A similar routine can be used for device bleaching. However, devices have a tendency to lose a proportion of the charge stored when stood for long periods at high temperature. When charge is lost, full bleaching of the device will be completed before the predetermined quantity of charge has been passed, and continuing to drive the device at constant current can lead to excessive voltages and significant device degradation.

It has been observed that the rate of increase of the applied voltage with time increases greatly when bleaching is complete, and this characteristic has been adopted as indicating the end of bleach and as a signal to limit the bleaching voltage reached. Again, because of the use of constant current, the bleaching routine automatically adjusts the final bleaching voltage to device temperature. In addition, the bleaching routine tends to compensate for the effect of the electrically conductive layer sheet resistance on the current voltage response, in that the detection of the bleach completion is measured as a rate of change, independent of the sheet resistance, rather than an absolute value, upon which the sheet resistance will have an effect

The magnitude of the constant current may be determined according to the acceptable current per unit area and the area of the device.

The charge limit, the voltage limit and the time period may be determined using cyclic voltammetry and empirical measurements.

A limit on the rate of change of voltage may also be used as part of the colouring routine.

In a second embodiment the invention provides a method of driving an elecuOchromic device having layers of electrochromic, electrolyte and counter-electrode materials interposed between electrically conducting films, characterised by:

(a) applying an alternating current or voltage signal to the device;

(b) determining the magnitude of the device impedance from the current voltage behaviour of the device;

(c) estimating the temperature of the device from the determined impedance;

(d) setting a maximum device driving voltage and a voltage pause time in accordance with the estimated temperature; and

(e) to colour or bleach the device, passing a constant current in an appropriate direction through the layers until the first of a limit on the amount of charge passed or the maximum driving voltage is reached, whereupon the applied voltage on reaching that limit is maintained for the voltage pause time. The method according to the second embodiment ofthe invention is particularly suited to use with large area devices in low temperature environs. At higher temperatures (above room temperature), the electrolyte layer resistance will be much smaller than the electrically conducting film resistance, especially in large area devices, and the total window impedance may not be a good guide to temperature. It is therefore envisaged that the method according to the second embodiment of the invention may be used to control a device in combination with the method according to the first embodiment of the invention which is particularly suited to higher temperatures. It is thought that some form of control could be utilised which would switch from one method to the other according to device temperature.

The alternating current or voltage signal may be in the form of a square wave or a sinusoidal wave.

The temperature of the device may be estimated using a look-up table of values pre-calculated using instruments which measure the device impedance as the device is repeatedly cycled in varying temperatures.

The charge passed, the maximum driving voltage and the voltage pause time may be determined using cyclic voltammetry and empirical measurements.

Preferably, applying an alternating current or voltage signal to the device and determining the magnitude of the device impedance from the current/voltage behaviour of the device includes using a stopped current method.

Either embodiment of the invention may be implemented using only the standard driving current supply wires to the device and requires no additional connections, and may be implemented with conventional device driving hardware such as a suitably programmed personal computer and an electrochemical interface or dedicated integrated circuitry.

A device driven according to an embodiment of the invention is not only useable in a variable transmission window wherein the electrically conducting films will be translucent but also in a variable reflection mirror (now used particularly for automobile rear view mirrors) wherein one of the electrically conducting films will be reflective.

The Drawings

Figure 1 is an exploded view of an electrochromic device which can be driven by a method according to the invention;

Figure 2 is a partial transverse cross section of the device shown in figure 1;

Figure 3 is a graph illustrating the behaviour of the device similar to that shown in figure 1, subjected to constant current cycling; Figure 4 is a block diagram of hardware which may be used to drive the device shown in figure 1 by a method according to the invention;

Figure 5 is a generalised flow diagram of the steps involved in a method according to a first embodiment of the invention;

Figures 6 and 7 are graphs illustrating the behaviour of the device similar to that shown in figure 1 driven by a method according to a first embodiment of the invention.

Figure 8 is a generalised flow diagram of the steps involved in a method according to a second embodiment of the invention;

Figure 9 is a graph illustrating the variation in the impedance with temperature of the device shown in figure 1;

Figure 10 is a graph illustrating the behaviour of the device shown in figure 1, subjected to cyclic voltammetry;

Figure 11 is a graph illustrating the behaviour of the device shown in figure 1, subject to the removal of driving current; and

Figures 12 and 13 are graphs illustrating the behaviour of the device shown in figure 1 driven by a method according to a second embodiment of the invention.

Best Mode

With reference to figures 1 and 2, the device indicated generally at 1 has first and second sheets of glass 2,4 each 100mm x 100mm, separated by a 1mm thick translucent interlayer of polymer electrolyte 6, the composition of which is disclosed in PCT/EP95/01861. Each of the sheets is sputter coated on its inner face 8,10 with an electrically conductive film 12,14 of indium doped tin oxide (ITO). Applied over the top of the ITO film 12, also by sputtering, is an electrochromic layer 16 of tungsten trioxide, and applied over the top of the ITO film 14, again by sputtering, is a counter-electrode layer 18 of cerium titanium oxide. Also applied over each film of ITO 12,14, along one vertical edge, is an elongate electrical contact, commonly known as a bus bar 20,22. These are in the form of copper strips stuck on to the ITO films 12,14 with conductive adhesive. Power supply wires 24,26 are connected to each of the bus bars 20,22. The device 1 is put together as a cast-in-place laminate, using a conventional technique. First of all the two sputter coated sheets 2,4 are formed into a cell by bonding them together at their edges by double-sided adhesive tape, with the electrochromic and counter-electrode layers 16,18 innermost Liquid electrolyte, previously degassed by stirring under vacuum, is poured into the cell. The electrolyte interlayer 6 is then cured and the cell is sealed with an epoxy resin (not shown). The device 1 is preconditioned (initiation of cation transfer) by cyclically driving it between voltages of ±3V for gradually increasing periods of time.

According to a first embodiment the device 1 is driven by applying a current of 1.2mA (which equates to 20μAcm^"2 for a device active area of 64 cm²) through the tungsten trioxide layer 16, the cerium titanium oxide layer 18 and the electrolyte layer 6, via the power supply wires 24,26 and the ITO films 12,14. Applying a negative voltage to the tungsten trioxide layer 16, so as to generate a current flowing in a first direction, causes lithium ions from the electrolyte layer 6 to be inserted into the tungsten trioxide layer 16, which produces a visible blue colouration. Applying a positive voltage has the opposite effect generating a current flowing in a second, opposite, direction, and the device 1 is bleached. Any reference herein to a positive or negative voltage can be taken also to mean a "more positive" or "more negative" voltage as appropriate, for example a device may in some instances be "coloured" by driving it from a fully bleached to a less bleached state without actually applying a negative voltage.

Figure 3 is a graphical illustration of how the a device of the same construction as device 1, but of slightly smaller dimensions, behaves in response to constant current cycling. The plot is of applied potential against time, with the device 1 varying alternately between a fully bleached state (+3V) and a fully coloured state (-3V), indicated by spikes B and C respectively. From observing the device 1 under such constant current cycling it is possible to determine the charge quantities required to achieve any particular state of either colour or bleach. In the bleaching cycle, one of which is indicated as X and having a duration of about 200 seconds, the majority of charge passes within a small voltage window, and then the voltage rises very rapidly with time. The device 1 is controlled by means of a personal computer 30 (figure 4) which, in combination with an electrochemical interface 32, such as those available from the Schlumberger company under the designation 1286, can both monitor the behaviour of the device and in response apply appropriate driving currents and voltages. The personal computer 30 operates in accordance with a routine as depicted by the flow chart of figure 5, monitoring, from the output through wires 24,26, charge, voltage and rate of change of voltage, and adjusting the voltage applied to the device 1 accordingly. The routine follows one of two different algorithms depending on whether the device 1 is to be coloured or bleached.

Using cyclic voltammetry and empirical measurements, it is predetermined that driving the device 1 from a fully bleached to a fully coloured state at room temperature requires 300 mC of charge, and that -3V is the safe applied voltage limit on colouring. Therefore, to fully colour the device 1 from a fully bleached state, a current of 1.2 mA is applied under the control of the personal computer 30 which then monitors when either 300 mC is passed or the voltage limit of -3V is attained, whichever comes first. On reaching the limit the voltage applied to the device 1 is held for a duration which is also predetermined and in this case is 20 seconds. The holding time period is calculated empirically. The -3V voltage limit is intended to provide a fallback position particularly at lower temperatures. At higher temperatures, the charge limit is reached for only a small voltage increase and so the charge limit predominates. It is only at lower temperatures that the voltage approaches the voltage limit before the required charge is passed. Subsequently holding the voltage has the effect of allowing the current to drop and enabling the remaining charge needed to complete the colouration to pass without driving the device 1 to higher voltages than necessary. The time of the period is determined to be the time necessary for the device to complete the final part of its colouring cycle.

To fully bleach the device 1, a similar routine is followed except that different predeteirnined values are adopted and additional steps are involved, these being the monitoring and comparison of the rate of change of voltage to a specified limit. A bleaching current of 1.2 mA is applied until either the rate of change of voltage exceeds 0.1 Vs^'1, the voltage limit of 3V is attained or a charge limit of 350 mC is passed, whichever occurs first In comparison to the colouring routine, for the bleaching routine it is the charge limit which provides the fallback position. At higher temperatures, it is the rate of change of voltage measurement which is used to determine the end of bleach whereas at lower temperatures the voltage limit is reached first On reaching the limit the applied voltage is maintained for the predetermined duration of 40 seconds. To ensure the bleach is completed across the whole device. The time period is larger than when colouring because incomplete bleaching is more easily discernible than incomplete colouring so it is more important to ensure the complete bleaching.

Although monitoring and comparing the rate of change of voltage are described as additional steps in the bleaching cycle, these may also be applied in the colouring cycle but in the example given the limit was set to such a value that it was never attained and was effectively therefore of no consequence.

Figures 6 and 7 are graphical illustrations of the above colouring and bleaching routines, in accordance with the first embodiment of the invention, carried out on the device of the same construction as device 1, but of slightly smaller dimensions. Figure 6 is a plot of the final applied voltage against cycle number for the device as it was continuously cycled at a constant current between the fully coloured and fully bleached states whilst at the same time the environment temperature was increased. This plot shows how the final voltage which the device reaches decreases with temperature because of the effects upon the device conductivity and kinetics. On the colouring switch, the required charge passes at lower voltages as the temperature increases. On bleaching, the end of the bleach is detected by monitoring when the rate of change of voltage, dV/dt exceeds 0.1 Vs^"1. Figure 7 is a plot of charge passed for the device going through the same schedule of cycling as shown in figure 6, the temperature increasing as the cycles progress. The plot shows that a steady amount of charge is cycled despite the changes in temperature.

The description above is all in the context of driving a device to its extreme coloured or bleached states. An intermediate state is achieved by knowing the quantity of charge required to take the device from say 80% fully coloured to 35% fully coloured, and a constant current of 1.2 mA is applied until the required quantity of charge is passed. However, the device 1 is continually monitored to ensure that it is operating within safe limits, and that because of the device temperature, the applied voltage is not becoming detrimentally excessive.

According to a second embodiment of the invention, the device 1 is controlled by means of the personal computer 30, in combination with an electrochemical interface 32, in accordance with a routine as depicted by the flow chart of figure 8, monitoring, from the output through wires 24,26, charge, voltage and current and adjusting the voltage applied to the device 1 accordingly as well as applying an alternating voltage signal.

The first step in the second method is the determination of the device impedance. This involves measuring the open circuit dc voltage of the device 1, and then applying a 25Hz square wave voltage signal superimposed upon a dc voltage equal to the open circuit voltage. The 25Hz current response is measured and used to determine the magnitude of the impedance of the device 1. If the initial measurement shows that the impedance is too large or small for the settings used, then the gain resistance is adjusted or the square wave amplitude changed. It has been found that changes in modulation amplitude cause no significant change in the measured impedance. The initial voltage amplitude is 50 mV, increased for lower temperatures to a maximum of 3V.

The next step in the routine is to estimate the device temperature from the determined impedance. This is done by reference to a look-up table of values calculated from previous cycling carried out on the device. Quantitative impedance measurements can be made with an impedance analyser and a potentiostat of the types available in the UK from the Schlumberger company under the designations 1255 and 1286 respectively, controlled by a personal computer (not shown).

During testing of the cycling routine, the device 1 is continually cycled back and forth between bleached and coloured states whilst the ambient temperature is varied from - 20°C to 70°C and back to -20°C. The impedance of the device 1 is continually measured and a plot is made of cycle number against device impedance showing the variation with temperature (figure 9). From this plot it is possible to infer the changes in device impedance with temperature, and to draw up a look-up table on which the routine can be based. Cyclic voltammetry is also used to calculate the device limiting voltages and the pause times are calculated empirically, by observing the device 1 under test conditions. Figure 10 is a graphical illustration of the response of the device 1 when in an environment at 80°C, the voltage being swept at 20mV per second. The sudden increase in current beyond an applied voltage of 2V (A) suggests the start of a breakdown of the electrolyte on the tungsten trioxide electrochromic material. A similar current onset occurs beyond - 2V (B). The parameters obtained from the cyclic voltammetry are then further refined by cycling tests to ensure that the device is being safely coloured and bleached under continuous cycling at the voltage limits and pause times selected.

Consequently, the personal computer 30, once having estimated the temperature of the device 1, sets the appropriate applied voltage limit and pause time, and then controls the driving of the device 1 in accordance with these. It is predetermined, also using cyclic voltammetry and empirical measurements, that driving the device 1 from a fully bleached to a fully coloured state at room temperature requires 300mC of charge and that -3 V is the safe voltage limit on colouring. Therefore, to fully colour the device from fully bleached, a current of 1.2mA is applied until 300mC is passed, as monitored by the personal computer 30. However, at the same time, the personal computer 30 monitors the voltage applied to the device 1. As soon as either the required charge is passed or the voltage limit is reached, the applied voltage on reaching that limit is held at that value for the set voltage pause time, in this case 20 seconds at room temperature. Holding the voltage has the effect of allowing the current to drop and enabling the remaining charge needed to complete the colouration to pass without driving the device 1 to higher voltages than necessary.

To fully bleach the device 1, the same routine is followed except that values for the voltage limit and pause time appropriate to bleaching are adopted.

Rather than driving the device at constant current, a similar effect can be achieved by ramping the applied voltage from 0V to the predetermined voltage limit over a given time period, for example 1 minute.

An alternative way of determining electrolyte resistance is to use the so-called stopped current mediod. The device 1 is coloured or bleached at constant current The current source is then disconnected and the jump in voltage across the device 1, which corresponds only to the voltage across the device's ohmic resistances, is measured. Figure 11 illustrates a stopped current measurement on the device 1 colouring at 40mA. The 800mV jump in voltage on disconnecting the current source suggests that the window has an ohmic resistance of 20Ω (compared with the more precise value of 18.5Ω measured on the aforementioned impedance analyser).

Figures 12 and 13 are graphical illustrations of a test of the above driving routine carried out on the device 1. The device 1 was moved from cycling under very cold conditions to progressively warmer conditions and then back to very cold conditions. The plots show how the voltage maxima (figure 12) and the total driving times (figure 13) were automatically adjusted by the routine to fully colour and bleach the device at all temperatures, without harsher than necessary conditions being applied.

Claims

Qaims

1. A method of driving an electrochromic device having successive layers of electrochromic, electrolyte and counter-electrode materials interposed between electrically conducting films, characterised by:

(a) to colour the device, passing a constant current in a first direction through the layers until the first of a limit on the amount of charge passed or a limit on the voltage applied is reached, whereupon the applied voltage on reaching that limit is maintained for a predetermined time period; or

(b) to bleach the device, passing a constant current in a second, opposite, direction through the layers until the first of a limit on the rate of change of voltage, a limit on the voltage applied or a limit on the amount of charge is passed is reached, whereupon the applied voltage on reaching that limit is maintained for a predetermined time period.

2. A method according to claim 1 wherein the magnitude of the constant current is determined according to the acceptable current per unit area and the area of the device.

3. A method according to claim 1 or claim 2 wherein the charge limit the voltage limit and the time period are determined using cyclic voltammetry and empirical measurements.

4. A method according to any of claims 1 to 3 wherein to colour the device, a constant current is passed in a first direction through the layers until a limit on the rate of change of voltage is reached.

5. A method of driving an electrochromic device having successive layers of electrochromic, electrolyte and counter-electrode materials interposed between electrically conducting films, characterised by: (a) applying an alternating current or voltage signal to the device;

(b) determining the magnitude of the device impedance from the current/voltage behaviour of the device;

(c) estimating the temperature of the device from the determined impedance;

(e) to colour or bleach the device, passing a constant current in an appropriate direction through the layers until the first of a limit on the amount of charge passed or the maximum driving voltage is reached, whereupon the applied voltage on reaching that limit is maintained for the voltage pause time.

6. A method according to claim 5 wherein the alternating current or voltage signal is in the form of a square wave or a sinusoidal wave.

7. A method according to claim 5 or claim 6 wherein the temperature of the device is estimated using a look-up table of values pre-calculated using instruments which measure the device impedance as the device is repeatedly cycled in varying temperatures.

8. A method according to any of claims 5 to 7 wherein the charge passed, the maximum driving voltage and the voltage pause time are determined using cyclic voltammetry and empirical measurements.

9. A method according to claim 5 wherein applying an alternating current or voltage signal to the device and determining the magnitude of the device impedance from the current/voltage behaviour of the device includes using a stopped current method.

10. A method of driving an electrochromic device having successive layers of electrochromic, electrolyte and counter-electrode materials interposed between electrically conducting films, characterised in that the method switches between a method according to claim 1 or a method according to claim 5 depending upon the device temperature.

11. Apparatus for carrying out a method according to claim 1 or claim 5.

12. Apparatus according to claim 11 including a personal computer and an electrochemical interface or dedicated integrated circuitry.