Smart driving method for electrochromic displays

Abstract

A method for operating an electrochromic display, wherein the electrochromic display comprises a plurality of display segments, each display segment provided in connection with at least one working electrode, and wherein the plurality of display segments are provided in connection with a common counter electrode, the method comprising, for a plurality of display segments of the electrochromic display, the steps: measuring the open circuit potential between the working electrode of a respective display segment and the common counter electrode; and, based on the measured open circuit potential, providing a predetermined voltage to the working electrode of the respective display segment.

Description

[0001] SMART DRIVING METHOD FOR ELECTROCHROMIC DISPLAYS

[0002] Field of the Invention

[0003] The present disclosure relates to electrochromic displays comprising multiple display segments and methods for driving electrochromic displays comprising multiple segments. In particular, it relates to methods for driving electrochromic displays based on a measured segment open circuit potential.

[0004] Background of the invention

[0005] An electrochromic display is a type of display that uses electrochemical reactions to change the colour and / or opacity of the display. Reflective electrochromic displays reflect ambient light to create images. Electrochromic displays have several advantages over other display technologies, such as low power consumption, the ability to be provided on flexible substrates, wide viewing angle, high contrast ratio. However, electrochromic displays also face challenges, such as slow switching speed, limited colour range, and degradation over time.

[0006] Electrochromic displays may comprise a plurality of electrochromic segments. Each segment forms a separate region which can be coloured, or decoloured, commonly known as bleached, independently of the other segments. An electrochromic display may for example, be a seven-segment display such as is known within the field of LCDs. However, due to their manufacturability, an electrochromic display may have segments of various sizes and need not comprise segments of similar of equal sizes.

[0007] Electrochromic displays inherently possess memory characteristics or bistability, which maintains the visual state for several minutes after an applied voltage is removed. In the case where the electrochromic display device includes a plurality of segments, each of the various combinations of the display segments defining a different pattern, the required pulse length to achieve a switch in visual state varies a significantly between different segment designs. A segment smaller in size typically requires just a few hundred milliseconds of an applied voltage while a larger segment may require up to a few seconds in some cases. However, applying the voltage for longer durations than necessary to achieve the switched state leads to parasitic reactions causing degradation of the display. This effect seems to be especially present during oxidation, i.e., decolouring via the application of a negative voltage. Therefore, it is very important to find a solution that optimizes pulse length in order to enable long lifetime applications. A second challenge with time-based driving is that when a segment has been in reduced state, that is a coloured state via the application of a positive voltage, for a long time it normally requires more time to switch back to oxidized, decoloured, state. An additional challenge with driving the electrochromic display is that this bistability time and the switching speed varies with the size of the addressed segment, the display aging and temperature.

[0008] As electrochromic displays age and exhibit different switching characteristics over their lifetime, methods which improve, and adapt in order to, efficiently drive displays over an extended period of time would be advantageous.

[0009] Improved electrochromic displays and methods of operating electrochromic displays would be advantageous.

[0010] Summary of the invention

[0011] Accordingly, the present invention preferably seeks to mitigate, alleviate or eliminate one or more of the above-identified deficiencies in the art and disadvantages singly or in any combination and solves at least the above mentioned problems by providing a method for operating an electrochromic display, wherein the electrochromic display comprises a plurality of display segments, each display segment provided in connection with at least one working electrode, and wherein the plurality of display segments are provided in connection with a common counter electrode, the method comprising, for a plurality of display segments of the electrochromic display, the steps: measuring the open circuit potential between the working electrode of a respective display segment and the common counter electrode; and, based on the measured open circuit potential, providing a predetermined voltage to the working electrode of the respective display segment.

[0012] A method for determining an ideal open circuit potential range of an electrochromic display comprising at least one display segment is also provided. Furthermore, an electrochromic display comprising a plurality of display segments is provided.

[0013] Further advantageous embodiments are disclosed in the appended and dependent patent claims.

[0014] Brief description of the drawings

[0015] These and other aspects, features and advantages of which the invention is capable will be apparent and elucidated from the following description of embodiments of the present invention, reference being made to the accompanying drawings, in which

[0016] Figure 1 is a simplified electrical model of a single segment electrochromic display.

[0017] Figure 2 is a simplified electrical model of a three-segment electrochromic display.

[0018] Figure 3 is a schematic model of an electrochromic display comprising three display segments, each segment being a different size and shape.

[0019] Figure 4 is an electrical diagram of the display driver with open circuit potential (OCP) voltage measurement at the counter electrode.

[0020] Figure 5 is a flow chart representing a process of OCP voltage measurement at the counter electrode with previous state stored in memory.

[0021] Figure 6 is an electrical diagram of the display driver with OCP voltage measurement at the working electrode.

[0022] Figure 7 is a flow chart representing a process of OCP voltage measurement at the working electrode.

[0023] Figure 8 is a flow chart representing a process for colouring segment using upper coloured range limit.

[0024] Figure 9 is a flow chart representing a process for bleaching segment using lower bleached range limit.

[0025] Figure 10 is a flow chart representing a process for refreshing coloured segment using lower coloured range limit. Figure 11 is a flow chart representing a process for refreshing a bleached segment using upper bleached range limit.

[0026] Figure 12 is the representative measurement of Lightness value in relation to Applied Voltage across a single segment showing upper and lower colouring and bleaching range limits as the vertical lines.

[0027] Figure 13A is a schematic representation of a display comprising two segments, 101, 102 and a counter electrode.

[0028] Figure 13B is the applied voltage to the circuit of 13A over a period of time, 0 to 18, showing the voltage provided to 101 as the small-dashed line, the voltage provided to 102 as the large-dashed line, and the voltage applied to the counter electrode as a solid line. The voltage applied to the counter electrode is constant 1.5V.

[0029] Figure 13C is the measured current in 102, 101 and the counter electrode over the same period of time as in figure 13B.

[0030] Figure 14 shows a schematic representation of a display comprising a segment with working electrodes connected at multiple locations around the segment.

[0031] Detailed description

[0032] The present electrochromic displays 100 relate to, in general, thin-film flexible electrochromic displays. The electrochromic displays 100 comprise an electrochromic polymer layer. The electrochromic polymer layer may be, for example, a Poly(3,4- ethylenedioxythiophene) or PEDOT layer. However, the electrochromic layer may be any suitable electrochromic polymer, such as Poly(3,4-propylenedioxythiophene) or PProDOT, Poly(viologen) or PV, Poly(aniline) or PANI; Poly(3-hexylthiophene) or P3HT, Poly (3 -methylthiophene) or P3MT. Ideally, the electrochromic polymer is PEDOT:PSS as is known within the field of electrochromic displays and displays. The electrochromic displays 100 comprise, as is known within the field, an electrolyte. The electrolyte is adjacent to and in physical contact with the electrochromic polymer. The electrolyte and electrochromic polymer may collectively be referred to as the display stack. The display stack may comprise additional components such as substrates etc. As shown in figure 3, the electrochromic display 100 comprises a plurality of segments 110a, 110b, 110c. Electrochromic displays 100 comprising multiple segments 110a, 110b, 110c can be differentiated from electrochromic displays comprising a single segment in that the driving, that is the control of switching and refreshing of the segments, is significantly more complex. For example, the segments may have substantially different sizes compared to each other. The charge required to switch, or refresh a segment is dependent on the size and shape of the of the segment and therefore each segment will need to be driven independently and differently. In addition, as will be discussed later, the location of each segment on the display also causes complexities in the driving. The electrochromic display 100 may comprise two or more segments, such as three or more segments, such as five or more segments, such as seven segments, or more than seven segments. The electrochromic display 100 of figure 3 comprising three segments 110a, 110b, 110c is purely an example.

[0033] The electrochromic display 100 comprises electrical circuitry connecting each segment to a driving module 150. The driving module 150 comprises a processor for implementing the driving methods disclosed herein. The present electrochromic displays may be driven by any suitable driving module 150, such as a microcontroller, FPGA, and the like. The driving module 150 of the electrochromic display 100 is adapted to implement the driving methods disclosed herein. It is therefore to be understood that when referring to the electrochromic display 100 being adapted to implement a method or process it is generally the driving module 150 which functionally implements the method or methods unless otherwise indicated i.e., the term electrochromic display 100 refers to the physical display components such as the electrochromic polymer, electrolyte layer etc. and the driving module 150, unless otherwise indicated.

[0034] The challenges involved in driving an electrochromic display can be described by using a simple electrical model. Figure 1 shows a simplified electrical model of a single segment electrochromic display consisting of a capacitor C with one resistor Rs connected in series, one resistor Rpconnected in parallel, working electrode WE and common electrode CE. In this model the display segment is represented by the capacitor C and the internal resistance RP, and Rs represents the resistance in the conductors leading to the segment. The intensity of the colour of the display is determined by the charge stored in the capacitor C. The more charge stored in the capacitor per unit area the darker the colour of the segment. A darker coloured segment is generally referred to as coloured. A lighter coloured segment is generally referred to as bleached. The value of Rsis determined by the resistance in the conductors to the segment. The conductors may be for example silver and / or carbon connectors. The value of Rsis also based on contact resistance and resistance in the electrochromic polymer layer.

[0035] The value of RPis determined by the internal leakage in the display, which includes the electrical conductivity of the electrolyte. The value of Rsinfluences the switching speed; the lower Rsthe faster the capacitor C can be charged. Ideally, Rsshould be minimized, however it serves a purpose in some cases since it minimizes peak current leading to improved lifetime. The value of RPinfluences the bistability and power consumption and to improve bistability RPshould be as high as possible. Another reason to keep it high is to minimize leakage current that goes through Rpinstead of C. For example, a pinhole in the electrolyte causes RPto decrease, leading to a higher leakage current and shorter bistability. All three parameters (C, RPand Rs) are influenced by temperature, humidity and device / material ageing to some extent.

[0036] As described previously, the present electrochromic display generally comprises multiple segments. An electrochromic display comprising multiple segments can be described by expanding the single segment model to comprise multiple segments connected in parallel. Figure 2 shows simplified electrical model for a three-segment display with the capacitors Co, Ci, C2 and internal resistances RPO, RPI, RP2 representing the display segments. Resistors Rso, Rsi, Rs2 represent the resistance in the conductors leading to the individual segments. Ideally, the counter electrode CE is shared between all three segments and RCE represents the resistance in the counter electrode. It is important to note that in a typical display the segments differ in size, shape, and location on the display. This means that Rso Rsi Rs2, RPO RPI RP2 and Co Ci C2. This makes driving more challenging since these values influence the time it takes to charge the capacitor, i.e., switch speed, meaning that different segments require different timings. In the light of the challenges described above it is necessary to develop a driving method that compensates for this complexity. In order to overcome this challenge, a method of operating an electrochromic display comprising multiple segments is provided. Figure 3 shows an electrochromic display 100 comprising multiple segments 110a, 110b, 110c. The electrochromic display 100 may comprise any number of segments. The electrochromic display comprises a driving module 150. The driving module 150 comprises both the electronic circuitry and the programmable components for driving the electrochromic display 100.

[0037] The method comprises measuring the open circuit potential (OCP) across each individual segment 110a, 110b, 110c of the electrochromic display 100. Each display segment 110a, 110b, 110c has at least one working electrode I l la, 111b, 111c for connecting the display segment 1101, 110b, 110c to a driving voltage. A common counter electrode 120 is connected to each segment 110a, 110b, 110c. The common counter electrode 120 is connected to each of the plurality of display segments 110 of the electrochromic display 100. That is, each of the display segments 110a, 110b, 110c are electrically connected through electrical circuitry to the same counter electrode 120. Each of the display segments 110a, 110b, 110c are ionically connected via the display stack, e.g., the electrolyte, to the common counter electrode 120. The working electrode I l la, 111b, 111c of each respective display segment 110a, 110b, 110c is separated from the common counter electrode 120 by at least each display segment’s 110a, 110b, 110c respective electrolyte layer. The provision of a common counter electrode 120 enables, as will be discussed below, improved measurement of OCP for a display with multiple display segments 110a, 110b, 110c and furthermore, enables simultaneous colouring and decolouring of different display segments 110a, 110b, 110c.

[0038] Based on the measured OCP, a driving voltage is provided to the display segment 110. The driving voltage is provided to the working electrode 111 of the display segment 110.

[0039] The driving voltage is typically and ideally provided as a sequence of bursts or pulses. The pulses are discontinuous, that is, the voltage is not provided as a continuous voltage. The pulses are provided for a duration of time and at an interval. The voltage of each pulse, the duration of each pulse, and the interval between pulses, i.e., the duty cycle, and the total number of pulses defines the total charge provided to each segment 110a, 110b, 110c. The driving voltage is ideally a fixed predetermined voltage, or generally one of a plurality of fixed predetermined voltages. A fixed predetermined voltage refers to the voltage being fixed and set to a specific value. Generally, the driving voltage is one of a plurality of fixed values. The voltage values corresponding to either a colouring voltage value or a bleaching voltage value. The voltage is generally not adjustable across a span, such as a ramped voltage. The voltage pulses may therefore be considered as step voltage pulses. Having a fixed voltage corresponding to either a colouring or bleaching voltage simplifies the electronic circuitry required to provide the driving voltage to the segment 110a, 110b, 110c. The application voltage, or driving voltage, is provided as pulses and is not continuously supplied to the segments 110a, 110b, 110c. This is possible due to the semi-bistable nature of the segments 110a, 110b, 110c and reduces power consumption.

[0040] The OCP may be calculated by measuring the potential between the working electrode I l la, 111b, 111c and a reference and subtracting the known or measured potential between the common electrode 120 and the reference. The OCP may be measured by setting a working electrode I l la to a known voltage, while the other working electrodes 111b, 111c are set to a high impedance mode. The potential between the 120 and the reference is measured and the known or measured potential between the working electrode I l la and the reference is subtracted. The reference may be a virtual ground set to a predetermined voltage. The predetermined voltage is non-zero with respect to absolute ground, i.e, the surface of the earth. The OCP for each segment 110a, 110b, 110c is measured sequentially by repeating the process for each working electrode 111 a, 11 lb, 111 c. As would be understood, and is inherent and necessary when measuring open circuit potential, during OCP measurement the driving voltage is not applied to the display segment 110a, 110b, 110c.

[0041] The relationship between OCP and the colour of the segment makes it useful to use the OCP as a feedback parameter for driving. An important aspect of this approach is that the measurement of OCP is not meaningfully influenced by the series resistance (Rs) in the display. That is because during an OCP measurement, there is close to zero current flowing through the circuit and there will not be any significant voltage drop across these resistors. For this reason, the voltage can be approximated to the actual segment voltage, that is the capacitive charge across segment. Further, the driving approach is centred around the definition of two voltage ranges that corresponds to a coloured and bleached state of the segment. Given these boundary conditions, the driving method and driving device may be configured to ensure that all segments are within the coloured voltage range and / or bleached voltage range.

[0042] The driving module 150 of the electrochromic display 100 may perform measurement of OCP across the segment 110 by analogue or digital methods. Preferably, the driving module uses an Analog to Digital Converter (ADC) to measure the OCP at the counter electrode 120 and the measurement is made one segment 110a, 110b, 110c at a time. Figure 4 shows electrical diagram of the display driver with OCP voltage measurement at the counter electrode 120.

[0043] Typically, ADCs on, for example microcontrollers, are only capable of measuring voltages between 0V and the supply voltage, for example 3V or 5V. As the open circuit potential may be negative, for example, when the display segment 110 is bleached, the driving module 150 is adapted to measure both negative and positive open circuit potentials. Figure 5 shows the logic flow used to measure the OCP. The voltage measurement across a segment is enabled by setting one working electrode to a known voltage (typically HIGH or LOW). The voltage selection should be based on the previously read segment voltage Vs, corresponding to the previous read open circuit potential, and selected in a way that makes it as likely as possible to achieve a voltage in the counter electrode 120 that is within the measurement range of the ADC i.e., generally positive. In this state the ADC measurement on the counter electrode is performed and the previously described calculations are made to determine segment voltage Vs. In a situation where the determined voltage is zero or close to zero the voltage in the working electrode should be adjusted to the opposite voltage level to that previously applied i.e., if the previous voltage applied was HIGH, then the subsequent applied voltage should be LOW and vice versa. Such a process ensures that the voltage in the counter electrode is within the range of the ADC. In this state the ADC measurement on the counter electrode is performed and the calculations are made to determine segment voltage Vs, corresponding to the open circuit voltage of the display segment 110. The resulting Vs is stored for future usage and used to drive the segment 110 as necessary. This is done for all the segments enabling individual open circuit potential measurement. The electrochromic display 100 may comprise multiple ADCs or an ADC with multiple channels to measure the OCP on the working electrode I lla, 111b, 111c of the individual display segments 110a, 110b, 110c. Figure 6 shows electrical diagram of the display driver with OCP voltage measurement at the working electrodes.

[0044] Figure 7 shows the logic flow used to measure the OCP with a circuit as shown in figure 6. The voltage measurement across a segment 110a, 110b, 110c is enabled by setting the counter electrode 120 to VCE. In this state the segment voltage Vs is measured at each segment 110a, 110b, 110c on the working electrode I lla, 111b, 111c. For example ADCi measures OCP across SEGi, with reference 110a and the previously described calculations are made to determine segment voltage Vs. The resulting Vs is then used to define the driving of the segment 110a. This approach allows that the segments in any state, that is, bleached, or coloured can be measured without having to switch between different voltage levels as necessary described previously. However, multiple ADCs connected to each working electrode I l la, 111b, 111c, or an ADC comprising multiple channels connected to each working electrode I lla, 111b, 111c is necessary. This introduces additional hardware costs compared to having a single ADC connected to the counter electrode 120.

[0045] The colouring and bleaching of a single segment 110a, 110b, 110c is performed based on the result of the OCP measurement across the segment 110a, 110b, 110c. A critical parameter for this driving method to work is to understand the relationship between OCP and segment colour. Generally, the relationship between colour and OCP will be similar to the relationship between Lightness Value and Applied Voltage and can be used to optimize driving schemes. A representative plot of Lightness Value versus Applied Voltage is shown in figure 12. In the representative plot in figure 12 VUCRL is 1.0V, VLCRL is 0.8V, VLBRL is -1.0V and VUBRL is -0.8V, however these will be different for different electrochromic polymers and are only provided as examples. Based on these measurements an upper coloured range limit voltage VUCRL, a lower coloured range limit voltage VLCRL, a lower bleached range limit voltage VLBRL and an upper bleached range limit voltage VUBRL are recorded as references to apply the appropriate voltage scheme. Figure 8 shows the logic for colouring a segment 110a, 110b, 110c using upper coloured range limit. It starts with measuring the OCP of the segment, designated Vs, and comparing it to VUCRL and if Vs is higher, then there is no further driving voltage is applied, at this time. If the Vs is lower, then a specific positive colouring voltage is applied as a short sequential burst and the process is repeated until Vs is equal to or larger than VUCRL to colour the segment 110a, 110b, 110c.

[0046] Figure 9 shows the logic for bleaching a segment using the lower bleached range limit. It starts with measuring the OCP of the segment, Vs, and comparing it to VLBRL and if Vs is lower, then there is no further driving voltage is applied, at this time. If the Vs is higher, then a specific negative bleaching voltage is applied as short sequential burst and the process is repeated until Vs is equal to or less than VLBRL to bleach the segment.

[0047] As would be understood from figures 8 and 9, which of the upper and lower limits is relevant to compare to during a switching cycle is based on whether the segment 110a, 110b, 110c should be coloured or bleached as colouring and bleaching require positive and negative driving voltages respectively.

[0048] The upper coloured range limit, lower coloured range limit, upper bleached range limit, and lower bleached range limit may be referred to in general as the ideal OCP range. The range between the upper and lower coloured range limits may be referred to as the ideal coloured OCP range. The range between the upper and lower bleached range may be referred to as the ideal bleached OCP range. The ideal open circuit potential range is generally and initially a predetermined range set based on the specific electrochromic segment 110a, 110b, 100c, or the specific electrochromic display 100. However, as will be described in the following section, the ideal open circuit potential range may be adjusted based on measured open circuit potential.

[0049] The electrochromic display 100 may be adapted to maintain the state, coloured or bleached, of a single segment 110a, 110b, 110c based on the measuring result of the OCP across the segment 110a, 110b, 110c. Maintaining the state is generally referred to as refreshing the display, or refreshing the state. That is, providing a voltage to the segment 110a, 110b ,110c which ensures it maintains its present state. As the present electrochromic display 100 is generally semi-bistable, that is a segment 110a, 110b, 110c possesses bistability for a duration of time, but is not indefinitely bistable, refreshing the state is necessary. Figure 10 shows the logic for refreshing a previously coloured segment 110a, 110b, 110c, using upper coloured range limit. It starts with measuring the OCP of the segment Vs and comparing it to VLCRL and if Vs is higher, then a predetermined sleep timer is activated to count down to the next measurement for the same segment. If the Vs is lower, that is, if the OCP is lower than the lower bound of the ideal colouring OCP range, then a specific colouring voltage is applied as short sequential bursts (in accordance with figure 8) until VUCRL is reached. Figure 11 shows the logic for refreshing a previously bleached segment using lower bleached range limit. It starts with measuring the OCP of the segment, i.e., Vs, and comparing it to VUBRL and if Vs is lower, then a predetermined sleep timer is activated to count down to the next measurement for the same segment. If the Vs is higher, then a specific bleaching voltage is applied as short sequential bursts (in accordance with figure 9) to bleach until VUBRL is reached.

[0050] By providing upper and lower colouring and bleaching limits it is possible to maintain, or switch the segment 110a, 110b, 110c to a desired state without needing to reach a single predetermined value. Such upper and lower limits have been shown to improve the lifetime and reduce power consumption of the electrochromic display 100. As described in the logic flows above, this is especially relevant when refreshing the state of a segment 110a, 110b, 110c, as provided the segment is within the upper / lower bound, i.e., whichever is a lower absolute value, the segment 110a, 110b, 110c need not be provided with a voltage pulse.

[0051] The driving module 150 of the electrochromic display 100 is adapted to measure OCP across several segments and optimized to minimize the time to complete the measurements. The predictive nature of the electrochromic displays makes it possible to adapt the measurement frequency based on predictions. This is relevant both in between switches, passive measurement frequency, and during the switch, active measurement frequency. Tracking the time derivative of the OCP over time can help predict when the OCP will reach a certain level and adjust the measurement frequency. If the measured OCP is far away from the target OCP, the measurement frequency can be reduced and when the measured OCP is close to the target OCP, the measurement frequency can be increased. Additionally, the present method for driving based on OCP may be intermittently used based on a number of completed switching cycles. For example, the OCP measurement need not be performed every switching cycle, but may be performed every n switching cycles, such as every 2nd, every 10th, every 100th, or switching cycles. The method for driving the electrochromic display 100 may comprise updating the ideal open circuit potential range, that is, the upper coloured range limit, the upper bleached range limit, the lower coloured range limit, and the lower bleached range limit, collectively referred to the as the upper and lower bounds of the ideal open circuit potential range, based on the measured OCP. The method may comprise adjusting the upper bound and / or the lower bound based on the difference between the measured open circuit potential and the upper and / or lower bound of the ideal open circuit potential range. For example, if the measured open circuit potential is substantially lower than the ideal lower bound of the ideal open circuit potential range, the lower bound of the ideal open circuit potential range may be adjusted downwards, for example in order to compensate for aging or ambient temperature changes. The method may comprise updating the ideal open circuit potential range i.e., the upper and lower bounds based on a sequence of OCP measurements. The open circuit potential range limits may be adjusted based on a difference between the measured OCP to the ideal OCP range limits. The open circuit potential range limits may be adjusted based on the time derivative, that is the rate of change of the sequence of measured OCPs. Advantageously, the upper and or lower bounds of the ideal open circuit potential range may be adjusted based on if the time derivative of the measured open circuit potential is less than a value corresponding to an expected time derivative. For example, if the rate of change is less than for example 10 mV, such as less than 5 mV, such as less than 2 mV, or such as less than 1 mV the ideal the upper and / or lower bounds of the ideal open circuit potential range may be adjusted to compensate for a measured lack of response in the display segment. Such adjustment of the OCP range limits is especially ideal as displays age. The electrochromic polymer generally displays environmental and age-dependent switching characteristics. Such environmental and age-dependent switching characteristics can be accounted for and overcome by adjusting the ideal / target OCP range limits based on measured OCP.

[0052] The ideal OCP range limits may be adjusted based on the response of a specific segment 110a, 110b, 110c to an application / driving voltage pulse. For example, after the provision of a driving voltage pulse to a segment 110a at a specific predetermined voltage level, the measured OCP may indicate that the segment 110a has not substantially increased / decreased. The voltage pulse may be provided at a first predetermined voltage level and at a first predetermined pulse duration and interval. Subsequently, the OCP of the segment 110a is measured. The measured OCP is then compared to the ideal OCP range. After comparing the measured OCP to the ideal OCP range, the ideal OCP range may be adjusted based on the measured OCP. Based on the comparison between the measured OCP, or sequence of measured OCP values, and the ideal OCP range, the predetermined voltage of the voltage pulse may be adjusted. That is, it may be adjusted higher or lower depending on the measured OCP value. Additionally, the pulse interval or pulse duration may be adjusted based on the measured OCP value or values. Additionally, or alternatively, the voltage level of the voltage pulse may be adjusted based on the measured OCP value. Adjusting the ideal OCP range additionally reduces energy consumption as if voltage pulses at a first predetermined level result in greater than expected changes to OCP, the voltage pulse may be adjusted to reduce energy consumption.

[0053] The electrochromic display 100 may be adapted to not measure OCP on each switching cycle. The electrochromic display 100 may intermittently measure OCP of a display segment 110a, 110b, 110c to minimize the time to complete the measurements and reduce the power consumption. This approach involves storing the switching time of each segment combined with intermittent OCP measurement. A baseline switching time is measured during initial switching cycles and during the following switches a predetermined time interval is used to trigger switching instead of measuring the OCP for each switching cycle. Periodically the baseline of stored switching times can be updated by once again using the measured OCP. The switching time is the time it takes for the measured OCP to be within the ideal OCP range.

[0054] The electrochromic display 100 may use the OCP across the segment as feedback to optimize driving scheme applied to control the display and to compensate for segments 110a, 110b, 110c that are showing bleeding effect. Bleeding effect refers to the phenomenon where the state of the polymer spreads over time to outside the intended area of the segment and into areas not designed to be activated. Since the reduced state and the oxidized state of the segment have different conductivity, bleeding tends to negatively impact the electrical characteristics of the segment in the form significant changes to Rs. Such changes in Rs make it very hard to reliably activate the display segments impacted by bleeding effect using a time-based voltage application. Since OCP measurements are not significantly influenced by value of Rs, switching display segments based on OCP values will remain stable regardless of bleeding effect. Particularly, displays that use the electrochromic layer as the electrical conductor in place of a separate conductive layer, such as comprise a transparent conductive oxide (TCO) layer, for example, an indium tin oxide (ITO) layer, can overcome bleeding effect only by using the OCP based driving method. The present electrochromic device 100 does not comprise a separate conductive layer, but rather use the electrochromic layer as the electrical conductor.

[0055] The driving voltage to each segment 110a, 110b, 110c is optimized to provide less bleeding effect and extend the lifetime of the display. Accordingly, a refreshing voltage that is lower than the typical switching voltage can be applied to minimize bleeding effect and extend the lifetime of the display. Since the refreshing action can be slower and still visually unnoticeable, a lower refreshing voltage does not compromise the functionality of the display.

[0056] The OCP measurement and feedback control of the driving voltage may be used to colour and bleach separate display segments 110a, 110b, 110c simultaneously. This is typically possible where the absolute value of the colouring voltage and the absolute value of the bleaching voltage adds up to the operating voltage of the system. For example, the a microcontroller, MCU, may have an operating voltage of 3V, whilst the colouring voltage may be 1.5V, and the bleaching voltage -1.5V; Abs(1.5V) + Abs(-1.5V) = 3V. It is beneficial and preferable that the colouring and bleaching take place in parallel. This reduces the time for a switching cycle compared to sequentially bleaching segments to be bleached, and subsequently colouring segments to be coloured. Parallel colouring and bleaching is possible with the introduction of a virtual ground connected to the common electrode 120. With this driving method the total switching time is equal to the bleaching or colouring time, dependent on which one is longer, generally this results in a significant time reduction compared to sequential driving. In addition, the switching appearance is more visually appealing to the observer. An additional benefit with parallel driving is that it reduces the overall energy consumption of the display. This is possible since energy can be redistributed across the segments 110a, 110b, 110c from one state to the next state. That means that parts of the energy that was used to set one state in the display can be reused to set the next state. The amount of reused energy depends on how large segment area that is coloured in one state in proportion to the next state. The reduction in energy consumption with parallel switching can be seen with respect to the schematic circuit shown in figure 13 A. The circuit in figure 13A comprises two segments, 101, 102 each comprising a respective working electrode I l la, 111b. Figure 13B shows the application voltage to the two segments 101, 102 provided to the working electrodes I l la, 111b respectively over a period of time. Figure 13C shows the measured current during the switching cycles for the same period. The current is shown separately for each working electrode and the counter electrode, labelled CE in the charts. Referring to both figure 13B and 13C it can be seen that during the period 0 to 3 both segments are bleached. During the period 3 to 6 both segments are coloured. During the period 6 to 9 both segments are bleached again. At period 9 to 12 segment 102 is coloured via the provision of the 1.5V to 102, and segment 101 remains bleached by the provision of -1.5V. During period 12 to 15 the segments are simultaneously switched i.e., in parallel. During period 15 to 18 the segments are simultaneously switched i.e., switched in parallel to their opposite states i.e., segment 101 is coloured and segment 102 is bleached. The measured current through the counter electrode during the parallel colouring and bleaching of segments, between time period 12 and 15, and 15 and 18 is shown in figure 13B to be close to zero. This can be compared to the measured current for the proceeding switching actions. The charge from a segment 101 is redistributed to another segment 102. As less current flows through the circuit, losses, in particular losses through the representative resistance RCE are minimised.

[0057] As shown in figure 14, to improve the visual uniformity when colouring or bleaching a segment 110a the working electrode I l la may be connected at multiple locations around the segment 11 Oa. During colouring or bleaching of a segment 11 Oa the switching appearance is not uniform, especially if the segment is large in size. The switch typically starts close to the working electrode I l la connection and then diffuses across the segment producing a “curtain effect”. This effect can be reduced or eliminated by connecting the working electrode I l la at multiple locations around the segments 110a and ideally all around the segment. This approach further reduces the resistance in the segment and leads to faster switching speeds. Further, the OCP voltage in the measured position is more representative of the overall voltage of a respective segment 110a. Whilst figure 14 shows a single segment, an electrochromic display 100 comprising multiple segments 110a, 110b, 110c can be provided with multiple working electrodes 11 la, 11 lb, 111c for each segment 110a, 110b, 110c. Although, the present invention has been described above with reference to specific embodiments, it is not intended to be limited to the specific form set forth herein. Rather, the invention is limited only by the accompanying claims.

[0058] In the claims, the term “comprises / comprising” does not exclude the presence of other elements or steps. Furthermore, although individually listed, a plurality of means, elements or method steps may be implemented by e.g. a single unit or processor. Additionally, although individual features may be included in different claims, these may possibly advantageously be combined, and the inclusion in different claims does not imply that a combination of features is not feasible and / or advantageous. In addition, singular references do not exclude a plurality. The terms “a”, “an”, “first”, “second” etc do not preclude a plurality. Reference signs in the claims are provided merely as a clarifying example and shall not be construed as limiting the scope of the claims in any way.

Claims

CLAIMS1. A method for operating an electrochromic display (100), wherein the electrochromic display (100) comprises a plurality of display segments (110a, 110b, 110c), each display segment (110a, 110b, 110c) provided in connection with at least one working electrode (Illa, 111b, 111c), and wherein the plurality of display segments are provided in connection with a common counter electrode (120), the method comprising, for a plurality of display segments (110a, 110b, 110c) of the electrochromic display (100), the steps: measuring the open circuit potential between the working electrode (Il la, 111b, 11c) of a respective display segment (110a, 110b, 110c) and the common counter electrode (120); and, based on the measured open circuit potential, providing a predetermined voltage to the working electrode (Il la, 111b, 111c) of the respective display segment (1101, 110b, 110c).

2. The method according to claim 1, wherein the method comprises: comparing the measured open circuit potential with a predetermined ideal open circuit potential range comprising separate and distinct upper and lower bounds; and, providing the predetermined voltage to the working electrode (Il la, 111b, 111c) of the display segment (110a, ,110b, 110c) such that the measured open circuit potential is within the ideal open circuit potential range.

3. The method according to claim 1 or 2, wherein during measurement of the open circuit potential, a reference voltage provided to the common counter electrode (120) is adjusted based on whether the measured open circuit potential was zero such that both positive and negative open circuit potentials are measurable.

4. The method according to any of claims 1 to 3, wherein the predetermined voltage provided to the working electrode (I lla, 111b, 111c) of the display segment (110a, 110b, 110c) is provided as a pulsed voltage, each pulse having a duration, and each pulse provided at an interval.

5. The method according to claim 4, wherein the voltage, duration of and / or the interval between the voltage pulses is controlled based on the measured open circuit potential.

6. The method according to claim 4 or 5, wherein a sequence of measurements of the open circuit potential between the working electrode (I l la, 111b, 111c) of the respective display segment (110a, 110b, 110c) and the common counter electrode (120) are measured, and wherein the ideal open circuit potential range is adjusted based on the sequence of open circuit potential measurements.

7. The method according to claim 6, wherein the adjustment of the ideal open circuit potential range is based on the difference between the measured open circuit potential and the upper or lower bound of the ideal open circuit potential range; and / or the rate of change of the measured open circuit potential.

8. The method according to claim 7, wherein the measurement of the open circuit potential between the working electrode (I l la, 111b, 111c) of the respective display segment (110a, 110b ,110c) occurs after the provision of the predetermined voltage, such that the response of the respective display segment (110a, 110b, 110c) to the predetermined voltage is measurable.

9. The method according to any of claims 1 to 8, wherein the predetermined voltage provided to the display segment is a fixed voltage corresponding to either a colouring voltage, or a bleaching voltage.

10. The method according to any of claims 1 to 9, wherein the method comprises the step of, prior to the measuring of the open circuit potential:- providing a predetermined voltage pulse to the respective display segment (110a, 110b, 110c).

11. The method according to any of claims 1 to 10, wherein the step of measuring the open circuit potential of a respective display segment (110a) comprises:providing a reference voltage to the respective display segment (110a) working electrode (I lla) or to the common counter electrode (120); setting each of the plurality working electrodes (111b, 111c) for the remaining display segments (110b, 110c) to a high-impedance mode; and, measuring the open circuit potential between the common counter electrode (120) and the respective working electrode (1 Ila).

12. The method according to any of claims 1 to 11, wherein the method is for colouring at least a first display segment (110a) and bleaching at least a second display segment (110b, 110c), wherein the method comprises, in parallel:- providing a predetermined colouring voltage to the first display segment (110a), and- providing a predetermined bleaching voltage to the second display segment (110b, 110c).

13. The method according to claim 12, wherein charge from the first display segment (110a) is redistributed to the second display segment (110b, 110c).

14. The method according to any of claims 1 to 13, wherein the common counter electrode (CE) of the electrochromic display (100) is a virtual ground at a non-zero voltage with respect to absolute ground.

15. A method for determining an ideal open circuit potential range of an electrochromic display (100) comprising at least one display segment (110a), the ideal open circuit potential comprising a lower bound and an upper bound, the method comprising the steps:- providing a predetermined voltage pulse at a first voltage level, at a first predetermined pulse duration and interval, to the at least one display segment (110a),- measuring the open circuit potential between a working electrode (I lla) of the at least one display segment (110a) and a counter electrode (120),- comparing the measured open circuit potential to the upper bound and / or lower bound of the ideal open circuit potential range,- adjusting the upper bound and / or the lower bound of the ideal open circuit potential range based on the measured open circuit potential.

16. The method according to claim 15 , wherein the adj usting of the upper bound and / or the lower bound is based on the difference between the measured open circuit potential, and the upper bound and / or lower bound of the ideal open circuit potential range.

17. The method according to claim 15 or 16, wherein the steps of providing the predetermined voltage pulse, and measuring the open circuit potential are performed a plurality of times for the display segment (110a).

18. The method according to claim 17, wherein the adjusting of the upper and / or lower bound of the ideal open circuit potential is based on the time derivative of the measured open circuit potential.

19. The method according to claim 18, wherein the adjusting of the upper and / or lower bound of the ideal open circuit potential range is based on if the time derivative of the measured open circuit potential is less than a value corresponding to an expected time derivative.

20. The method according to any of claims 15 to 19, wherein the electrochromic display (100) comprises a plurality of display segments (110a, 110b, 110c) each comprising a working electrode (I l la, 111b, 111c) and in electrical connection with a common counter electrode (120) and wherein the steps are performed for a plurality of the display segments (110a, 110b, 110c).

21. The method according to any of claims 15 to 20, wherein the method comprises, based on the measured open circuit potential, and / or the time derivative of the of the measured open circuit potential:- providing a predetermined voltage pulse at a second, different, voltage level; and / or,- providing a predetermined voltage pulse at a second, different, pulse interval and / or duration.

22. An electrochromic display (100) comprising a plurality of display segments (110a, 110b, 110c) and a driving module (150), each display segment (110a, 110b, 110c) comprising a working electrode (I lla, 111b, 111c), wherein each display segment (110a, 110b, 110c) is connected to a common counter electrode (120), wherein the driving module (150) is configured to, for a plurality of the display segments (110a, 110b, 110c):- measure the open circuit potential between the working electrode (I lla, 111b, 11c) of a respective display segment (110a, 110b, 110c) and the common counter electrode (120); and,- based on the measured open circuit potential, provide a predetermined voltage to the working electrode (Il la, 111b, 111c) of the respective display segment (1101, 110b, 110c).

23. The electrochromic display (100) according to claim 22, wherein the driving module is configured to:- compare the measured open circuit potential with a predetermined ideal open circuit potential range comprising separate and distinct upper and lower bounds; and- provide the predetermined voltage to the working electrode (I lla, 111b, 111c) of the display segment (110a, ,110b, 110c) such that the measured open circuit potential is within the ideal open circuit potential range.

24. The electrochromic display (100) according to claim 23, wherein the driving module (150) is configured to:- adjust the upper bound and / or the lower bound based on the measured open circuit potential between the working electrode of a respective display segment (110a, 110b, 110c) and the common counter electrode (120).

25. The electrochromic display (100) according to any of claims 22 to 24, wherein the common counter electrode (CE) of the electrochromic display (100) is a virtual ground at a non-zero voltage with respect to absolute ground.

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