WO2002097525A1

WO2002097525A1 - Electrochromic matrix device and method of controlling the same

Info

Publication number: WO2002097525A1
Application number: PCT/SE2002/001045
Authority: WO
Inventors: Mårten EDWARDS; Anders Hagfeldt; Jörgen OLSSON
Original assignee: Edwards Maarten; Anders Hagfeldt; Olsson Joergen
Priority date: 2001-05-31
Filing date: 2002-05-31
Publication date: 2002-12-05
Also published as: SE0101957L; SE0101957D0

Abstract

The present invention provides means for rapid grayscale operation of electrochromic active-matrix devices. This is achieved through a method of changing grayscale-states for a plurality of matrix-elements in an electrochromic matrix device. Said method comprising at least one switch cycle (LOAD) with a duration that is equal to or less than the switch time of the matrix elements (tswitch), wherein each switch cycle comprises the steps of selecting matrix-elements that are to be changed, connecting said selected matrix-elements to a drive-potential, and controlling the drive-potential for each said selected matrix-elements, such that they reach their respective desired grayscale states. The present invention further provides electrochromic matrix devices that meet the requirements of the method.

Description

Swedish Patent Application

Applicant/Inventor: Marten Edwards Applicant/Inventor: Anders Hagfeldt Inventor: Jδrgen Olsson

Electrochromic matrix device and method of controlling the same

Field of the invention

The present invention relates to a method and a device enhancing the performance of electrochromic devices, and more specifically a method and a device that shortens the over all switch time for electrochromic array devices.

Background

Electrochromic devices, such as information displays, smart windows, dimable mirrors, etc., comprise one or more electrochromic matrix-elements i.e. cells, segments, pixels, dots, etc. Each element is comprised of at least two electrodes that are electrically connected via an ion-conducting medium, i.e. an electrolyte. The elements may change color if the potential difference between the electrodes is changed. This color change is caused by redox reactions, i.e. changes in the distribution of electric charge within the element. The color changes are caused by changes in the spectral absorption and/or emission properties of electromagnetic radiation. With 'color' we mean also the spectral properties outside the visible region, e.g. in the infrared and the ultraviolet regions.

There are several types of electrochromic systems, such as:

1. Intercalation systems. Wherein thin metal-oxide films on conducting glass are commonly used as electrodes. The metal-oxide film (e.g. tungsten trioxide) on the color electrode colors at negative potentials by simultaneous injection of electrons and intercalation of small cations, i.e. small positive ions like protons and lithium ions. This reduction reaction is balanced by oxidation reactions at the counter electrode that in some cases contribute to the coloration.

2. Systems with dyed highly porous films. Wherein the color electrode is comprised of a highly porous film of sintered small metal-oxide particles on conductive glass. The surface of the particle network is covered by redox-active chromophore molecules (dye), which exhibit different colors in different oxidation states. Charge is guided to and from the dye molecules via the conductive particle network. As the film is highly porous the amount of dye molecules is high enough to cause strong coloration of the electrode. The redox reactions in the porous color electrode are balanced by reactions of redox-active species in the electrolyte or in a film at the counter electrode.

3. All-in-solution systems. Wherein both the color redox system and the counter redox system are dissolved in the electrolyte. The electrodes are in this case commonly made of plain conductive glass.

In an electrochromic system, a comparatively high electric charge is needed to switch between different color states. This situation differs from many other display technologies, e.g. liquid-crystal displays. Electrochromic devices gradually shift from one state to another over a system -related time period t__Witch. The switch time tswitch (or response time) for an electrochromic system is defined as the time required for a switch between the fully colored and the fully bleached state. Alternatively the switch time may be defined more rigorously in different ways, e.g. as the time when 90 % of the difference in transmission or reflectance between the fully bleached and colored states has been attained. This definition result in two switch times, from colored to bleached and from bleached to colored. Although these switch times may be different, it will be assumed in the following that they are equally long. The switch time may depend on the operation conditions, such as driving voltages, temperature, the number of matrix-elements that are switched simultaneously, etc. Furthermore the switch time may be different for different matrix-elements in an array due to manufacturing variations, different sizes of the matrix-elements, the resistance in the power lines, etc. As the present invention is mainly intended for electrochromic graphic displays and array devices of other kinds, it is useful to introduce the array switch time tan-ay and the matrix-element switch time t_m-eiement. an-ay is defined as the time required for a change between arbitrary images of the whole array, and tm-eiement is defined as the time under which a single matrix-element has to be electrically connected in order to switch color state. Note that the definitions of t_m- ement and t_SWitch are slightly different. The matrix-element switch time may be shorter than the real switch time tm-element ≤ ts itch) .

In active-matrix displays, each matrix-element circuit comprises at least one active component, such as a field-effect transistor (FET) or the like. A 'standard' active- matrix design with thin-film transistors as switches is shown in figure 1. The gates of the transistors in one row are connected to a common line, and the sources of the transistors in one column are connected to a common line. Each transistor drain is connected to a matrix-element electrode. In a simple row-by-row addressing scheme the rows are selected one at a time by signals on the respective select lines (row lines). Video information, that may contain grayscale information, is fed to the selected matrix-elements via the data lines (column lines). The array switch time for such a display is

tarray ⁼ IN X tm-element t -t]

where N is the number of rows. Throughout this application all intermediate color- states between fully bleached and fully colored are referred to as grayscale-states (or grayscale levels) irrespective of color.

Prior art

The switch times of electrochromic displays range from 10 ms to several minutes. According to equation 1 the discussed row-by-row scheme would give an array switch time from 10 s up to several days for an electrochromic 1000x1000 matrix of the standard type. Fig 1 shows an example of a matrix element circuit 10 in an active matrix display, which comprises a drive transistor 12 having a source connected to a column line 14, a drain connected to a matrix element 16, and a gate connected to a row line 18. However, as shown in US 5,049,868, tan-ay is much shorter in existing modified active matrix displays 20 with one additional control transistor 22 and a control capacitor 24 in each matrix-element circuit 20, as shown in fig. 2. Such modified matrices are hereafter referred to as "active memory matrices". The thin-film transistors 12 in a standard active-matrix device 10 have both a selecting and a switching function. In active memory-matrices these functions are separated on two different transistors 12 and 22. The matrix-element electrodes 16 are, via drive-transistors 12, connected to a drive potential source that is common for all matrix-elements in the matrix. The drive-transistors 12 are either ON (fully conductive) or OFF (not conductive). The conduction of the drive- transistors 12 is controlled by the potentials of control-capacitors 24, and the charging states of the control- capacitors 24 are controlled by control-transistors 22. Matrix- elements 16 are selected row-by-row by gating all control-transistors 22 in one row and charging the control capacitors 24 in this row to either ON (1) or OFF (0) potentials via signals on the column lines 14, as is schematically shown in fig. 3.

The process of selecting matrix-elements in the whole matrix is denoted SELECT and compared to the matrix-element switch times in an electrochromic display it is very fast:

tsELECT < t_m-element. (2)

Deselecting all matrix-elements, DESELECT, is carried out by simultaneously gating all row lines with OFF potentials on the column lines. This process is even faster than SELECT:

tDESELECT < tsELECT « t_m-element. (3)

As a conclusion, the time it takes to arbitrarily change the state of all drive- transistors ( sELEcr) is negligible in comparison with the matrix-element switch time

(tm-element). One possible arrangement of a display comprising a common back electrode, connected to ground, is shown in fig. 4. This display 30 comprises a number of electrochromic matrix-element electrodes 16, which are connected to a back electrode 32 via " matrix-element control circuits ", i.e. drive-transistors 12. The electrolyte 34 and the front electrode 36 are common for all matrix-elements in this example. An external drive potential U is applied between the back electrode 32 and the front electrode 36 via an external "common switch" 38. The device is operated in two basic modes: BREAK and LOAD. In BREAK the common switch 38 and all drive-transistors 12 are OFF. There are no external connections between the electrodes 32 and 36. Thus the imaged information is preserved if the color memory is long, i.e. if the degree of self-discharging is low. In the second operation mode, LOAD, the external switch 38 is ON and the drive-transistors 12 of selected matrix- elements are ON. Matrix-elements are selected in the absolute beginning of LOAD by SELECT, and all matrix-elements are deselected by DESELECT in the absolute end of LOAD. A current will flow through all selected matrix-elements until their potentials U_m-eiement are equal to U or until the matrix-elements are deselected. To guarantee full charging the LOAD time is set equal to the matrix-element switch time:

tload ^— tm-element-

(4)

In this way the potentials U_m-eiement of all selected matrix-elements will attain the externally applied drive potential U during the LOAD period. It is not meaningful with longer LOAD-periods for switching purposes.

The process of arbitrarily changing the imaged information of the whole matrix is denoted ARRAY. An ARRAY is built up of several LOAD periods with different external drive potentials U. In the simplest case of only two gray shades, only black and white, an ARRAY consists of two LOAD periods with external drive potentials Ubieach and Udar (fig. 5). The array switch time becomes

tan-ay ⁼ - X tm-element, (,t⁾) which is independent of the array size N and a clear improvement as compared to the standard active-matrix (cf. eqn. 1). The general expression for n_gιay grayscale levels becomes

tanay ⁼ ngjay X tm-element- (θ)

Thus the existing active memory-matrix technology suffers from long array switch times for grayscale displays. For example, if the matrix-element switch time is 400 ms, which is normal for electrochromic displays of type 2 above, t_array becomes 6.4 s with 16 grayscale levels.

Therefore, it would in many situations be a great advantage if one could obtain means for significantly reducing the array switch time (t_array) for grayscale operation (eqn. 6).

Summary of the invention

The object of the invention is to provide a method that overcomes the drawbacks of the prior art methods. This is achieved by the method of changing grayscale-states for a plurality of matrix-elements in an electrochromic matrix device as defined in claim 1.

Another object of the invention is to provide a device that overcomes the drawbacks of the prior art devices. This is achieved by the electrochromic matrix devices as defined in the claims 15, 16 and 19.

One advantage with the method and the devices according to the invention is that the array switch time (t_aπay) may be reduced to the array switch time for electrochromic black-and-white arrays in prior art (eqn. 5) and preferably to the theoretical limit set by the matrix-element switch time (t_m element) . The array switch time (t_anay) may also be reduced for electrochromic color devices.

Another advantage with the method and the devices according to the invention is that they makes it possible to use a power supply with only two drive potentials for color changes in a monochrome grayscale device, and preferably to operate such devices without changing the applied drive potential.

Still another advantage is that the whole display is changed simultaneously, whereby enhanced visibility is achieved.

Embodiments of the invention are defined in the dependent claims.

Brief description of the figures

Fig. 1 is a schematic representation of the circuit arrangement of an active matrix device.

Fig. 2 is a schematic representation of the circuit arrangement of an active memory matrix device according to prior art.

Fig. 3 shows selection of matrix-elements in an active memory matrix device.

Fig. 4 is a schematic representation of an active memory matrix device.

Fig. 5 illustrates the basic steps of controlling an active memory matrix device according to prior art.

Fig. 6 illustrates the basic steps of controlling an active memory matrix device according to the present invention.

Fig. 7 is a schematic representation of the circuit arrangement of an active memory matrix device according to the present invention.

Fig. 8 is a schematic representation of the circuit arrangement according to one embodiment of an active memory matrix device according to the present invention.

Fig. 9 is a schematic representation of the circuit arrangement according to another embodiment of an active memory matrix device according to the present invention. Fig. 10 is a schematic representation of the circuit arrangement according to another embodiment of an active memory matrix device according to the present invention.

Fig. 11 is a schematic representation of the circuit arrangement according to another embodiment of an active memory matrix device according to the present invention.

Fig. 12 is a schematic representation of the circuit arrangement according to another embodiment of an active memory matrix device according to the present invention.

Detailed description of the invention

The active memory-matrix technique described in prior art operates with basic periods (LOAD) of t_m-eiement to ensure that the potentials of all selected matrix- elements reach the same final potential. The drive transistor is ON during the whole LOAD period, meaning that the matrix-element current Im-eiement (the current to the matrix-element electrodes) of selected matrix- elements is not actively regulated by the matrix-element circuit. The function of the control capacitors is only to store information about what matrix-elements are selected. The present invention provides means for actively regulating the current to the matrix- element electrodes during LOAD periods by controlling the charge in the control capacitors. Thus, compared to prior art, wherein the control capacitors only represent the selection status, i.e. selected/not selected, a new functionality is added to the control capacitors. The capacitor charges are individually controlled by either charging them differently in the absolute beginning of LOAD, or by changing their charges during LOAD (figure 6). The method according to the present invention comprises at least one LOAD (switch cycle), depending on embodiment, which LOAD as above has a duration that is equal to or less than tswitch- In a general representation each LOAD of the embodiments presented below comprises the steps of: selecting matrix- elements that are to be changed, including at least one matrix- element that is to be changed between different grayscale -states than the others

connecting said selected matrix-elements to a drive-potential, and

controlling the drive-potential for each of said selected matrix- elements, such that they reach their respective desired grayscale- states.

A more detailed description of the method and embodiments thereof are found below.

Fig. 7 shows a schematic representation of an electrochromic matrix device that may be controlled by the method according to the invention. In the electrochromic matrix device 40 the step of selecting matrix-elements 46 is performed by a matrix control unit 42, and the steps of connecting and controlling the drive-potential for a selected matrix-element 46 is performed by a matrix-element control-unit 44 connected to and associated with said selected matrix-element 46, each matrix- element control-unit 44 being connected to and controlled by the matrix control unit 42. The matrix-element control-unit 44 may be a matrix-element circuit of the type shown in fig. 2, but it may also be formed in alternative ways. As shown in fig 8, such a matrix- element control- unit 44 generally comprises:

at least one drive transistor 12 having a source connected to a drive potential source 50, a drain connected to a matrix element 16, and a gate connected to a first node 52, said drive transistor 12 having an associated switch potential U_SWitciι at which it changes conduction state,

at least one control capacitor 24 connected between said first node 52 and ground, and

a control-circuit 48 connected to the matrix control 42 unit and to said first node 52, wherein the steps of connecting and controlling the drive-potential for a selected matrix-element comprise controlling of the control circuit 48 to change the charging- state of said control capacitor 24 by changing the potential at the first node 52 such that the conduction state of the drive transistor 12 is changed. The control circuit 48 may, as shown in fig. 2, be comprised of a transistor having its drain coupled to said first node 52, and its gate and source coupled to row and column control lines, 18 and 14, respectively.

In a first embodiment, the method comprises two successive LOAD periods, one for each direction of change in grayscale state (i.e. bleaching and coloring). For each LOAD period the step of selecting matrix-elements comprises selecting all matrix- elements that are to be changed in that grayscale direction, the step of connecting said selected matrix-elements comprises connecting all selected matrix- elements to a common drive-potential (Ubieach or Udark), and the step of controlling the drive- potential for each of said matrix-elements, comprises disconnecting matrix- elements from the common drive-potential as they reach their respective desired grayscale - states .

In this embodiment, just like in the prior art system, an ARRAY-process comprises of two LOAD-periods, one for bleaching (U=Ubieach) and one for coloring (U=Udark), but in contrast to prior art the present invention the second LOAD-period is subdivided into n_gray-l grayscale periods, where n_gray is the number of grayscale levels. Matrix elements that are to be colour- changed are fully bleached in the first LOAD and colored to the desired grayscale level in the second LOAD. To give a certain matrix- element a desired grayscale level, the drive transistor 12 is only ON during a predefined number of grayscale periods, whereby the charge of the control capacitor 24 may be varied in a controlled manner, and thus also the color (grayscale state) of the matrix-element. This method is applicable to a matrix device of the type shown in fig. 3 and described in detail above. However, the method of controlling the device is adapted to handle grayscale operation in a more efficient way. The array switch time (tanay) for this embodiment is equal to:

tanay ⁼ 2 X tm element — X ts itc { • ) irrespective of the number of grayscale levels.

For some applications the visual impression of the above embodiment might be considered disturbing, as all matrix elements that are to be changed first are bleached during the first LOAD. In an alternative embodiment both the first and the second LOAD periods are divided into (n_g,-ay-l) grayscale periods. During the first LOAD the matrix elements that are to be bleached are bleached to their desired grayscale state whereas matrix elements that are to be colored are fully bleached. During the second LOAD the matrix elements that are to be colored are colored to their desired grayscale state whereas matrix elements that are to be bleached are fully colored. Thus, matrix elements that are to be colored and bleached are first fully bleached and fully colored, respectively. Alternatively these first steps of full bleaching and coloration, respectively, may be eliminated in an operation scheme that is more demanding on the matrix control unit. In such a scheme only matrix elements that are to be bleached and colored are operated on in the first and second LOAD periods, respectively. That is, the matrix elements are not fully bleached or colored before they are charged to their desired grayscale states. This requires that information about the actual grayscale states of the matrix elements is accessible to the matrix control unit and, further, that the matrix control unit is able of processing this information such that the proper number of grayscale periods is calculated for arbitrary changes between different grayscale states. This may also mean that more than (n_gray-l) grayscale periods are needed. The present invention does not comprise the details of how such grayscale information is accessed and processed.

The drive transistor 12 in fig. 3 is either ON or OFF, i.e. it is never in intermediate conduction states. The control capacitors 24 are also charged by one of two different potentials (0,1) via the column lines. The difference from prior art is that the drive transistor 12 of selected matrix-elements is not necessarily ON during the whole LOAD period. The ON-time is varied by SELECT processes for each grayscale period during LOAD (fig. 6). It is the ON-time that determines the final grayscale level. To have n_gray grayscale states, (n_gray - 1) grayscale periods are needed, each comprising one SELECT process. As discussed above there are several variants of the operation scheme for the first embodiment of the invention. This is the case also for the following embodiments below, although the operation schemes in most cases are not discussed in detail. We emphasize however that similar variants as for the first embodiment are in several cases possible also for the following embodiments.

In an alternative embodiment of the method according to the invention, grayscale operation is achieved without several SELECT processes during LOAD, as the grayscale information is transferred to the matrix-element control-unit as a duration parameter. To achieve this, the matrix-element control-unit of fig. 2 is modified in that it has a resistance 54 added in parallel to the control capacitor 24 (fig. 9). The LOAD periods contain only one SELECT, but the control capacitors 24 are charged to different potentials via the column lines 14. The charges decrease because of the parallel resistances 54. In this way the drive transistors may change gradually between the ON and OFF conduction states during LOAD. The initial potentials after SELECT determine the final grayscale levels of the matrix-elements. n_gray potential levels are needed on the column lines.

An alternative way to provide a matrix- element control-unit with the ability to control the potential on the matrix-element in accordance with a duration parameter, is to modify the matrix-element control-unit of fig. 2 such that the the gate-drain potential of the drive-transistors 56 determines the conduction state instead of the gate-substrate potential (fig 10). That is, the potential difference (Uca acitor- Um-eiement) between the control capacitor 24 and the matrix-element electrode 16 determines if the drive transistor 56 is ON or OFF or between these states. Just like above, one ARRAY consists of two LOAD periods that contain only one SELECT each. In the first LOAD the control capacitors 24 of the selected matrix elements are charged such that the drive transistor 56 is fully conducting throughout the LOAD period, meaning that the selected matrix elements are fully bleached (or fully colored). In the second LOAD the selected matrix elements are colored (or bleached) to different grayscale states by charging the control capacitors 24 such that the conduction states of the drive transistors initially are ON but that they approach the OFF state during the LOAD period. The final grayscale state is determined by the pre-set potential of the capacitor 24, Ucapacitor- In the examples the electrolyte is common for all matrix-elements, but it may also be separated in electrolyte baths for single matrix elements or groups of matrix elements to e.g. avoid or reduce cross-talk between matrix elements.

The back electrode may be segmented or replaced by row or column lines, whereby e.g. color displays may be constructed (see below). One such "back line" may further be shared by two neighbouring rows or columns (using N/2 lines for a NxN array). The front electrode may also be segmented in lines for e.g. color operation.

Color display operation may be achieved in a number of ways that are obvious to one skilled in the art. One way is to use color filters on different groups (e.g. rows or columns) of matrix elements. Alternatively the composition of the matrix elements may vary such that the "electrochromic color" of matrix elements in different groups is different. This may require that the drive potential source (e.g. Ue^reen, U^blue, U^red) is different for the different color groups. One way of achieving this is to segment either the front or the back electrode in column or row lines, and by using different power sources for the different lines. It is also possible to use complex electrochromic systems (e.g. mixes of dyes in electrochromic systems of type 2) with several electrochromic colors in each matrix element. In this case the grayscale does not change monotonically between "fully colored" and "fully bleached". In spite of this we refer to the different color states of the matrix elements as grayscale states.

Regulation of the matrix-element current in prior art is obtained by changing the (constant) potential between the front and back electrodes at the same time as new matrix-elements are selected. The matrix-elements always obtain the applied potential. In the present invention the drive potentials of selected matrix elements are individually controlled. However, the matrix-element current can be regulated also by applying different kinds of applied potential functions (ramps, square waves, ...) in combination with the manipulation of the matrix-element transistor conduction state. One example is to have initially high potentials for decreasing the matrix-element switch time. Other examples may combine the potential function (e.g. ramp) and the matrix-element circuitry to attain the current regulation. In another embodiment the array switch time (tarray) is reduced from 2xt_m-eiement (cf. eq. 7) to the matrix-element switch time (tm-eiement):

tarray ~ tm-element,

(9)

An ARRAY then consists of only one LOAD period of simultaneous bleaching and coloring. This is possible by further modifying the matrix-element control-unit of fig. 2 by adding a second drive transistor 58 and a second drive potential source 60 as shown in fig 11. Two external drive potentials, 50 and 60, (Ubieach and Udar ) are connected to the matrix-element electrodes 16 via the two drive transistors 12 and 58. The additional external drive potential 60 requires additional "back lines" to the matrix-elements. In figure 11 each back line is shared between two adjacent rows of matrix-elements. In this way N/2 lines are needed for Ubieach and N/2 lines are needed for U ar , i.e. in total N back lines are needed for the drive potential connections in an NxN matrix. The external drive potentials, 50 and 60, may be held constant. Thus the drivers for the power supply to the matrix-elements become very simple. The first drive transistor 12 is ON if ΔU (Ucapacitor-Usubstrate) is higher than a certain limit, whereas the second drive transistor 58 is ON if ΔU is lower than a certain limit. The limits are chosen so that both transistors never conduct (significantly) at the same time. The control capacitors have three logic levels (-1, 0, 1), corresponding to: (-1) the first drive transistor 12 is in a conducting state, (0) neither one of the drive transistors, 12 and 58, are in a conducting state, and (1) the second drive transistor 58 is in a conducting state. The described operation scheme requires (as already discussed for one of the operation schemes for the first embodiment) that the matrix control unit can handle changes between arbitrary grayscale states in a single LOAD period. Note that the usage of two drive transistors, 12 and 58, and the simultaneous bleaching and coloring also reduces the array switch time for operation with only two grayscale levels (cf. eqns. 5 and 9).

In still a further embodiment, the array switch time (tarray) s equal to or less than the switch time of a single matrix-element (t_m-eiement). In this embodiment the step of selecting matrix-elements comprises selecting all matrix- elements that are to be changed, and

the step of connecting said selected matrix-elements comprises connecting selected matrix- elements that are to be changed to the same grayscale-state to a common drive-potential.

Whereby all selected matrix elements reach their desired grayscale-states simultaneously, and the step of controlling the drive-potential for each of said matrix-elements, comprises disconnecting all matrix-elements from their respective drive-potentials at the end of the switch cycle.

Fig. 12 shows a possible arrangement by which the method according to this embodiment may be performed. In this arrangement each matrix-element control- unit 100 is comprised of equally many matrix-element sub control-units 110 as the number of grayscales that are to be provided. Each of these matrix-element sub control-units 110 being comprised of:

at least one drive transistor 12, 70, 72 and 74 having a source connected to a drive potential source 50, 82, 84 and 86, a drain connected to a matrix element 16, and a gate connected to a node 52, 90, 92 and 94

at least one control capacitor 24, 76, 78 and 80 connected between said node 52, 90, 92 and 94 and ground, and

a control- circuit 48, 96, 98 and 99 connected to the matrix control unit 42 and to said node 52, 90, 92 and 94.

Wherein the drive transistor 12, 70, 72 and 74 of each matrix- element sub control- unit 110 is connected to a drive potential different from drive potentials connected to drive transistors 12, 70, 72 and 74 of other matrix-element sub control-units 110. And wherein the step of connecting selected matrix elements 16 is performed by the matrix control unit 42, which is arranged to connect no more than one matrix-element sub control-unit 110 in each matrix-element control-unit 44 at a time.

This last embodiment combines fast switching, high grayscale accuracy, and a simple arrangement for the matrix control unit (no need of e.g. storing information about gray scale levels).

In fig. 4 the external drive potential U constitutes the drive potential source for all matrix elements. In this case it is not important if the back electrode 32 or the front electrode 36 is chosen as ground. However, if there are more than one external drive potential the choice of grounding point is less obvious. One can distinguish between two basic cases: Firstly, there may be more than one external drive potential U connected to each matrix-element control circuit (figs. 11-12). Then the natural choice of grounding is either at only one of these connections (like in fig. 11) or at the front electrode. Secondly, the front electrode 36 or the back electrode 32 may be segmented in different groups with different external drive potentials U, as has been discussed above. If the front electrode is segmented, then the natural choice of grounding point is the back electrode, and vice versa.

While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not to be limited to the disclosed embodiments, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.

Claims

1. Method of changing grayscale-states for a plurality of matrix-elements in an electrochromic matrix device, wherein at least one of said matrix-elements is to be changed between different grayscale-states than the others, and said matrix- elements have an associated switch time t_SWιtch, said method comprising at least one switch cycle (LOAD) with a duration that is equal to or less than t_SWιtch, characterized in that each switch cycle comprises the steps of:

selecting matrix-elements that are to be changed, including at least one matrix- element that is to be changed between different grayscale-states than the others

connecting said selected matrix-elements to a drive-potential, and

controlling the drive-potential for each of said selected matrix-elements, such that they reach their respective desired grayscale-states.

2. Method according to claim 1 characterized in that:

the step of selecting matrix-elements comprises selecting all matrix-elements that are to be changed in one grayscale direction.

3. Method according to claim 2 characterized in that

the step of connecting said selected matrix-elements comprises connecting all selected matrix-elements to a common drive-potential, which provides a change of grayscale-state in said one direction, and

the step of controlling the drive-potential for each of said matrix- elements, comprises disconnecting matrix-elements from the common drive-potential as they reach their respective desired grayscale-states.

4. Method according to claim 2 or 3 characterized in that it comprises two successive switch cycles, wherein the first cycle comprises a changing of matrix- elements in a first grayscale direction, and the second cycle comprises a changing of matrix-elements in a second grayscale direction, opposite to the first.

5. Method according to claim 2 or 3 characterized in that each switch cycle further comprises the steps of:

selecting all matrix- elements that are to be changed in a second grayscale direction opposite to the first, and

connecting all said selected matrix- elements to be changed in said second grayscale direction to a common drive-potential, which provides a change of grayscale-state in said second direction,

whereby a simultaneous changing of matrix-elements in two opposite grayscale directions is achieved during a single switch cycle.

6. Method according to claim 1 characterized in that:

the step of selecting matrix-elements comprises selecting all matrix- elements that are to be changed, and

the step of connecting said selected matrix-elements comprises connecting selected matrix-elements that are to be changed to the same grayscale- state to a common drive-potential,

whereby all selected matrix elements reach their desired grayscale-states simultaneously, and the step of controlling the drive-potential for each of said matrix-elements, comprises disconnecting all matrix-elements from their respective drive -potentials at the end of the switch cycle.

7. Method according to any of the claims 1 to 6, characterized in that the step of selecting matrix-elements is performed by a matrix control unit (42), and the steps of connecting and controlling the drive-potential for a selected matrix- element is performed by a matrix-element control-unit (44) connected to and associated with said selected matrix-element (16), each matrix-element control- unit (44) being connected to and controlled by the matrix control unit (42).

8. Method according to claim 7, characterized in that the step of connecting a selected matrix-element (16) is performed by the matrix-element control-unit (44) upon a first signal from the matrix control unit (42), and the step of controlling the drive-potential for said selected matrix-element (16) is performed by the matrix-element control-unit (44) upon a second signal from the matrix control unit (42).

9. Method according to claim 7, characterized in that the step of connecting a selected matrix-element (16) is performed by the matrix-element control-unit (44) upon a first signal from the matrix control unit (42), and the step of controlling the drive-potential for said selected matrix- element (16) is performed by the matrix- element control-unit (44) in accordance with a duration parameter comprised in said first signal from the matrix control unit (42).

10. Method according to claim 9, characterized in that the duration parameter is defined as the potential of said first signal.

11. Method according to claim 7 or 8, wherein each matrix-element control-unit (44) comprises:

at least one drive transistor (12) having a source connected to a drive potential source (50), a drain connected to a matrix element (16), and a gate connected to a first node (52), said drive transistor (12) having an associated switch potential switch at which it changes conduction state,

at least one control capacitor (24) connected between said first node (52) and ground, and

a control-circuit (48) connected to the matrix control unit (42) and to said first node (52),

characterized in that the steps of connecting and controlling the drive-potential for a selected matrix-element (16) comprise controlling of the control circuit (48) to change the charging- state of said control capacitor (24) by changing the potential at the first node (52) such that the conduction state of the drive transistor (12) is changed.

12. Method according to claim 9 or 10, wherein each matrix-element control-unit (44) comprises:

at least one drive transistor (12) having a source connected to a drive potential source (50), a drain connected to a matrix element (16), and a gate connected to a first node (52), said drive transistor (12) having an associated switch potential Uswitch at which it changes conduction state,

characterized in that the value of the duration parameter is stored in the control capacitor (24) and that the stored value thereafter gradually is changed due to a slow discharge through a resistor (54) connected in parallel with each said control capacitor (24), such that the switch potential U_swιtch for the drive potential is reached at the same time as the matrix- element (16) reaches the desired grayscale-state.

13. Method according to claim 9 or 10, wherein each matrix-element control-unit (44) comprises:

at least one drive transistor (12) having a source connected to a drive potential source (50), a drain connected to a matrix element (16), and a gate connected to a first node (52),

at least one control capacitor (24) connected between said first node (52) and ground, and a control- circuit (48) connected to the matrix control unit (42) and to said first node (52),

characterized in that the value of the duration parameter is stored in the control capacitor (24), and that the conduction state for said drive transistor (12) is determined by the difference in potential between the gate and the drain.

14. Method according to claim 7, characterized in that each matrix-element control-unit (44) is comprised of:

equally many matrix- element sub control-units (110) as the number of grayscales that are to be provided, each matrix-element sub control-unit (110) being comprised of:

at least one drive transistor (12, 70, 72 and 74) having a source connected to a drive potential source (50, 82, 84 and 86), a drain connected to a matrix element (16), and a gate connected to a node (52, 90, 92 and 94),

at least one control capacitor (24, 76, 78 and 80) connected between said node (52, 90, 92 and 94) and ground, and

a control-circuit (48, 96, 98 and 99) connected to the matrix control unit (42) and to said node (52, 90, 92 and 94),

in that the drive transistor (12, 70, 72 and 74) of each matrix-element sub control-unit (110) is connected to a drive potential (50, 82, 84 and 86) different from drive potentials connected to drive transistors (12, 70, 72 and 74) of other matrix-element sub control-units (110), and

in that the step of connecting selected matrix elements is performed by the matrix control unit (42), which is arranged to connect no more than one of the matrix-element sub control-units (110) in each matrix-element control-unit (44) at a time.

15. An electrochromic matrix device characterized in that it is controlled by a method according to any of the claims 1 to 14.

16. An electrochromic matrix device having a plurality of matrix-elements (16), each matrix-element (16) being connected to and associated with a matrix-element control-unit (44), each matrix-element control-unit (44) being connected to and controlled by a matrix control unit (42), characterized in that each matrix- element control-unit (44) comprises:

at least two drive transistors (12, 58), each having a drain connected to the matrix element (16), and a gate connected to a common first node (52), and each having a source connected to an associated drive potential source (50);

at least one control capacitor (24) connected between said first node (52) and ground;

17. Device according to claim 16, characterized in that the drive transistors (12, 58) of said matrix-element control-units (44) are arranged such that only one of them is in a conducting state at a time.

18. Device according to clai 16 or 17, characterized in that that each matrix- element control-unit comprises two drive transistors (12, 58) having their sources connected to a first and a second drive potential source (50, 60) respectively, wherein the first drive potential source (50) provides a bleach potential Ubie ch and the second potential source (60) provides a dark potential

Udark-

19. An electrochromic matrix device having a plurality of matrix-elements (16), each matrix-element (16) being connected to and associated with a matrix-element control-unit (44), each matrix-element control-unit (44) being connected to and controlled by a matrix control unit (42), wherein each matrix-element control- unit (44) comprises:

characterized by

a resistor (54) connected in parallel with said control capacitor (24).

20. An electrochromic matrix device having a plurality of matrix-elements (16), each matrix- element (16) being connected to and associated with a matrix-element control-unit (44), each matrix-element control-unit (44) being connected to and controlled by a matrix control unit (42), wherein each matrix-element control- unit (44) comprises:

at least one drive transistor (56) having a source connected to a drive potential source (50), a drain connected to a matrix element (16), and a gate connected to a first node (52),

characterized in that the conduction state of said drive transistor (56) is determined by the difference in potential between the gate and the drain.

21. An electrochromic matrix device having a plurality of matrix-elements (16), each matrix-element (16) being connected to and associated with a matrix-element control-unit (44), each matrix-element control-unit (44) being connected to and controlled by a matrix control unit (42), wherein each matrix-element control- unit (44) is comprised of:

equally many matrix-element sub control-units (110) as the number of grayscales that are to be provided, each matrix-element sub control-unit (110) being comprised of:

a control-circuit (48, 96, 98 and 99) connected to the matrix control unit and to said node (52, 90, 92 and 94),

characterized in that the drive transistor (12, 70, 72 and 74) of each matrix- element sub control-unit (110) is connected to a drive potential (50, 82, 84 and 86) different from drive potentials connected to drive transistors (12, 70, 72 and 74) of other matrix-element sub control-units (110), and in that the matrix control unit (42) is arranged to solely connect one of the matrix-element sub control-units (110) in each matrix-element control-unit (44) at a time.

22. Device according to any of the claims 16 to 21, characterized in that each matrix-element control circuit (48, 96, 98 and 99) is comprised of a transistor having its drain coupled to said first node (52, 90, 92 and 94), and its gain and source coupled to row (18) and column (14) control lines respectively.

23. A display device, characterized in that it comprises an electrochromic matrix device according to any of the claims 16 to 22.

24. An electronic book, characterized in that it comprises an electrochromic matrix device according to any of the claims 16 to 22.

25. A mobile telephone, characterized in that it comprises an electrochromic matrix device according to any of the claims 16 to 22.

26. A computer system, characterized in that it comprises an electrochromic matrix device according to claim any of the claims 16 to 22.

27. An information board, characterized in that it comprises an electrochromic matrix device according to claim any of the claims 16 to 22.

28. An electrochromic matrix device having a plurality of matrix- elements (16), each matrix-element (16) being connected to and associated with a matrix-element control-unit (44), each matrix-element control-unit (44) being connected to and controlled by a matrix control unit (42), wherein said matrix control unit (42) together with said matrix-element control-unit (44) are arranged for changing grayscale-states of matrix-elements in the electrochromic matrix device during at least one switch cycle (LOAD), characterized in that

the matrix control unit (42) is arranged to select matrix-elements including at least one matrix-element that is to be changed between different grayscale- states than the others,

the matrix-element control-unit (44) is arranged to connect the selected matrix- elements to a drive-potential, and

the matrix-element control-unit (44) is arranged to control the drive-potential for each of said selected matrix-elements, such that they reach their respective desired grayscale-states.

29. An electrochromic matrix device according to claim 28, characterized in, that at least one of said matrix- elements is to be changed between different grayscale- states than the others, and said matrix-elements have an associated switch time tswitch, wherein said at least one switch cycle (LOAD) has a duration that is equal to or less than t_switch

30. An electrochromic matrix device according to claim 28 or 29, characterized in that:

the matrix control unit (42) is arranged to select all matrix- elements that are to be changed in one grayscale direction.

31. An electrochromic matrix device according to claim 30, characterized in that

the matrix-element control-unit (44) is arranged to connect all selected matrix- elements to a common drive-potential, which provides a change of grayscale- state in said one direction, and

the matrix-element control-unit (44) is arranged to disconnec matrix-elements from the common drive-potential as they reach their respective desired grayscale-states.

32. An electrochromic matrix device according to claim 30 or 31, characterized in that said matrix control unit (42) together with said matrix-element control-unit (44) are arranged to change grayscale-states of matrix-elements in the electrochromic matrix device during two successive switch cycles, wherein the first cycle comprises a changing of matrix-elements in a first grayscale direction, and the second cycle comprises a changing of matrix-elements in a second grayscale direction, opposite to the first.

33. An electrochromic matrix device according to claim 30 or 31 , characterized in that, under each switch cycle,

said matrix control unit (42) is arranged to select all matrix-elements that are to be changed in a second grayscale direction opposite to the first, and said matrix- element control-unit (44) is arranged to connect all said selected matrix-elements to be changed in said second grayscale direction to a common drive-potential, which provides a change of grayscale-state in said second direction,

34. An electrochromic matrix device according to claim 28 or 29, characterized in that:

the matrix control unit (42) is arranged to select all matrix-elements that are to be changed, and

the matrix-element control-unit (44) is arranged to connect selected matrix- elements that are to be changed to the same grayscale-state to a common drive- potential, whereby all selected matrix elements reach their desired grayscale- states simultaneously, and

the matrix-element control-unit (44) is arranged to disconnect all matrix- elements from their respective drive-potentials at the end of the switch cycle.

35. An electrochromic matrix device according to any of claims 28 - 34, characterized in that the matrix-element control-unit (44) is arranged to connect a selected matrix-element (16) upon a first signal from the matrix control unit (42), and matrix-element control-unit (44) is arranged to control the drive-potential for said selected matrix-element (16) upon a second signal from the matrix control unit (42).

36. An electrochromic matrix device according to any of claims 28 - 34, characterized in that the matrix-element control-unit (44) is arranged to connect a selected matrix-element (16) upon a first signal from the matrix control unit (42), and the matrix-element control-unit (44) is arranged to control the drive-potential for said selected matrix-element (16) in accordance with a duration parameter comprised in said first signal from the matrix control unit (42).

37. An electrochromic matrix device according to claim 36, characterized in that the duration parameter is defined as the potential of said first signal.

38. An electrochromic matrix device according to claims 28 - 35, characterized in that each matrix-element control-unit (44) comprises:

wherein the steps of connecting and controlling the drive-potential for a selected matrix-element (16) comprise controlling of the control circuit (48) to change the charging- state of said control capacitor (24) by changing the potential at the first node (52) such that the conduction state of the drive transistor (12) is changed.

39. An electrochromic matrix device according to claims 28 - 34 and 36 - 37, characterized in that each matrix-element control-unit (44) comprises:

wherein the value of the duration parameter is stored in the control capacitor (24) and that the stored value thereafter gradually is changed due to a slow discharge through a resistor (54) connected in parallel with each said control capacitor (24), such that the switch potential Us itch for the drive potential is reached at the same time as the matrix-element (16) reaches the desired grayscale-state.

40. An electrochromic matrix device according to claims 28 - 34 and 36 - 37, characterized in that each matrix-element control-unit (44) comprises:

wherein the value of the duration parameter is stored in the control capacitor (24), and that the conduction state for said drive transistor (12) is determined by the difference in potential between the gate and the drain.

41. An electrochromic matrix device according to claim 28, characterized in that each matrix-element control-unit (44) is comprised of:

equally many matrix-element sub control-units (110) as the number of grayscales that are to be provided, each matrix- element sub control-unit (110) being comprised of: at least one drive transistor (12, 70, 72 and 74) having a source connected to a drive potential source (50, 82, 84 and 86), a drain connected to a matrix element (16), and a gate connected to a node (52, 90, 92 and 94),

a control- circuit (48, 96, 98 and 99) connected to the matrix control unit (42) and to said node (52, 90, 92 and 94),

wherein the matrix control unit (42) is arranged to connect no more than one of the selected matrix-element sub control-units (110) in each matrix-element control-unit (44) at a time.