WO2005071655A2

WO2005071655A2 - Active matrix foil display

Info

Publication number: WO2005071655A2
Application number: PCT/IB2005/050173
Authority: WO
Inventors: Volker Schoellmann; Mark T. Johnson
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2004-01-21
Filing date: 2005-01-14
Publication date: 2005-08-04
Also published as: WO2005071655A3; TW200601260A

Abstract

A foil display device comprising an electrostatically operable foil (15) adapted to locally be brought into contact with a substrate (12) to thereby modulate light emitted from the display, a plurality of pixel electrodes arranged on one side of the foil, each connected to a pixel circuit, arranged to apply a pixel voltage to the pixel electrode during a time period, and a common electrode arranged on the other side of the foil. The display further comprises means (32) for providing said common electrode (30) with a gradually increasing or decreasing voltage during the time period, so that a switching point in time for the foil (15) in a particular pixel area corresponding to a particular pixel electrode (20) is determined by the pixel voltage applied to this pixel electrode (20). The display device according to the invention is addressed using active matrix addressing, by setting the voltage level of each pixel electrode by means of its respective pixel circuit. Then the common electrode voltage is changed within a given range. When the common electrode voltage reaches a given value, the force exerted by common electrode voltage will exceed (or drop below) the force exerted by the applied pixel voltage by a large enough value, and the pixel will switch.

Description

Active matrix foil display

The present invention relates to a foil display device comprising an electrostatically operable foil adapted to locally be brought into contact with a substrate to thereby modulate light emerging from the display. More specifically, the invention relates to an active matrix foil display, further having a plurality of pixel electrodes arranged on one side of the foil, each connected to a pixel circuit, arranged to apply a pixel voltage to the pixel electrode during at time period, and a common electrode arranged on the other side of the foil. The invention also relates to a method for addressing such a display.

A conventional emissive foil display is known from e.g. WO 00/38163, in which case its operation is based on the local extraction of light from a light guide by means of the scattering foil clamped between the light guide and a passive plate. The movement of the foil within each pixel can be controlled by means of voltages applied to different electrodes, arranged on the light guide and passive plate. The electrodes create two electrostatic fields that exert forces on the foil, one towards the light guide, and one toward the passive plate. When the foil is brought into contact with the light guide, it will frustrate the total internal reflection of the light guide, and light will be extracted. A different type of foil display is a reflective display, where the accurate control of the position of the foil with respect to the substrate determines the reflectivity of a pixel, and hence the reflected light which is emitted by the display, due to destructive or constructive intereference effects or frustrated reflection. This will modulate the amount of light reflected by the display, i.e. the amount of light emerging from the display. The switching curves of a pixel element in a foil display is shown in Fig. 1, where the x-axis represents the potential difference between the foil electrode and the electrodes on the passive plate, and the y-axis represents the potential difference between the foil electrode and the electrodes on the light guide. Indirectly, these potential differences represent the forces exerted on the foil away from and towards the light guide respectively. When in the bi-stable region 1, between the ON-curve 2 and the OFF-curve 3, a pixel will maintain its previous state. This creates a memory effect in the pixel element, making it possible to use a passive matrix addressing method to drive the display. Previously, a foil display device employing active matrix addressing has been proposed by the applicant, see WO 2004/088629-A1 (Appl. no. 03100870.9 / PHNL 030324). Such a device has an electrode layer structured into individual pixel electrodes on one side of the foil, each connected to a pixel circuit, and an electrode common for all pixels (or groups of pixels) on the other side of the foil. The common electrode is arranged to attract the foil, and the pixel circuits are arranged to provide the pixel electrodes with voltages overcoming this force. In using active matrix addressing the pixel memory is provided by the pixel circuit instead of by the dynamic foil itself. If a select pulse is given, a voltage can be stored on the pixel circuit, which defines whether a pixel is switched "on" or "off. Thus only two positions are needed in the switching curve diagram, a first position 4 in the ON region (i.e. below both the ON curve 2 and the OFF curve 3), and a second position 5 in the OFF region (i.e. above the ON curve 2 and the OFF curve 3). As a consequence, the drivers can be simplified. A problem with foil displays in general concerns generation of gray scales, as the pixel element is operated discretely (either ON or OFF). For this purpose, a number of addressing schemes have been developed, involving pulse width modulation of the ON-times of the pixel elements. However, in order to obtain a sufficient number of gray levels, such addressing schemes often require a dividing the frame period into a large number of addressing time slots, requiring extremely short switching times.

An object of the present invention is to overcome the above problem, and provide an improved way to achieve gray scales in a foil display. A further object of the present invention is to provide analogue gray scales.

According to a first aspect of the invention, these and other objects are achieved with a foil display device of the kind mentioned by way of introduction, further comprising means for providing said common electrode with a voltage that gradually changes during said time period, so that a switching point in time for the foil in a particular pixel area corresponding to a particular pixel electrode is determined by the pixel voltage applied to this pixel electrode. The expression "light emerging from the display" is intended to include light emitted from the display as well as light reflected by the display. The display device according to the invention is addressed using active matrix addressing, by setting the voltage level of each pixel electrode by means of its respective pixel circuit. Then the common electrode voltage is changed within a given range. When the common electrode voltage reaches a given value, the force exerted by common electrode voltage will exceed (or drop below) the force exerted by the applied pixel voltage by a large enough value, and the pixel will switch. This is quite different from the approach described in WO 2004/088629-A1 (Appl. no. 03100870.9 / PHNL 030324), where the common electrode applies a fixed voltage providing a fixed biasing force towards or against the active plate, and where the pixel electrode is provided with a pixel voltage during addressing to overcome this biasing force. In that case, the pixel is switched immediately during addressing. Due to the shape of the switching curves, a different voltage level provided by a pixel circuit to a pixel electrode will determine a different distance to the relevant switching curve (ON or OFF), and a different voltage will thus be required on the common electrode to switch the pixel. By selecting the voltage level provided by an individual pixel circuit during addressing, the time required for the common electrode voltage to reach this limit can thus be controlled, thereby offering analogue control of the switching times. The pixel electrodes (and the pixel circuits) can be arranged on the same side as the substrate, and in that case preferably on the surface of the light guide. This means that the pixel electrodes will attract the foil towards the substrate, while the common electrode will attract the foil away from the light guide. The common electrode can be arranged to provide a voltage that decreases from a high level. The foil is then initially held out of contact with the substrate by the force created by the common electrode voltage. Only after addressing, when the common electrode voltage falls below a given level will the force from the pixel voltage overcome the force from the common electrode, and the pixel will be switched ON. Such addressing is referred to as selective ON addressing. The common electrode can also be arranged to provide a voltage that increases from a low or zero level. During addressing, each pixel electrode which has been addressed ON will then exert a force which immediately will bring the foil in contact with the substrate. Only after addressing, when the common electrode voltage reaches a level high enough to overcome this attractive force will the pixel be switched OFF. Such addressing is referred to as selective OFF addressing. Alternatively, the pixel electrodes (and the pixel circuits) can be arranged on the other side as the substrate, and in that case preferably on the surface of a passive plate arranged to sandwich the foil between itself and the substrate. With this arrangement, the pixel electrodes will attract the foil away from the substrate, while the common electrode will attract the foil towards substrate. Again, the common electrode can be arranged to provide a voltage that decreases from a high level or a voltage that increases from a low or zero level. However, now a decreasing voltage will result in selective OFF addressing, and an increasing voltage in a selective ON addressing. According to a second aspect of the invention, these and other objects are achieved with a method for addressing a foil display device of the kind mentioned by way of introduction, comprising the steps of applying a pixel voltage to each pixel electrode during a time period, and providing the common electrode with a gradually increasing or decreasing voltage during this time period, so that a switching time for the foil in a particular pixel area corresponding to a particular pixel electrode is determined by the pixel voltage applied to this pixel electrode.

This and other aspects of the present invention will now be described in more detail, with reference to the appended drawings showing a currently preferred embodiment of the invention. Fig. 1 is a switching curve diagram for a typical foil display. Fig. 2 is a schematic side view of a display device according to a first embodiment of the invention. Fig. 3 is flow chart of an addressing method performed with the display device in Fig. 2. Fig. 4 is a switching curve diagram indicating the method in Fig. 3 for two different pixels. Fig. 5 is a time line of the addressing scheme in Fig. 3 and 4. Fig. 6 is a switching curve diagram indicating an alternative addressing method using the device in Fig. 2. Fig. 7 is a time line of the addressing scheme in Fig. 6. Fig. 8 is a time line illustrating how addressing according to the invention can be combined with time modulation. Fig. 9 is a schematic side view of a display device according to a second embodiment of the invention.

Figure 2 shows a foil display device 11 according to an embodiment of the invention. This display is an emissive display of the kind that comprises a light guide (active plate) 12, connected to a light source 13, such as a LED, a passive plate 14, and a flexible element clamped in between these plates. The flexible element can be a foil 15 of a flexible, light scattering material, such as parylene, with an unstructured electrode layer 16 disposed thereon. The electrode layer 16 is connected to a foil voltage, Vf₀π. Spacers 17 are arranged between the passive plate 14 and the foil 15, and between the light guide 12 and the foil, respectively. The spacers are adapted to keep the foil in place between the plates 12 and 14, and can be formed as sets of parallel lines on both sides of the foil, aligned with each other. On one side of the foil 15, here on the same side as the light guide 12, are arranged a plurality of pixel electrodes 20. As the name implies, each pixel of the display has a separate pixel electrode 20. In the illustrated example, the pixel electrodes 20, which can be formed by an ITO layer structured into pixel areas, are arranged on the light guide 12, on the side facing the foil 15, and optionally be covered by an insulating layer 21. Alternatively, the electrodes 20 can be arranged on an intermediate structure between the foil 15 and the light guide 12, or can be located on the other side of the light guide 12. Each pixel electrode 20 is connected to a pixel circuit 23, which also is connected to a select voltage line 24, and a data voltage line 25. Preferably, the pixels (i.e. the pixel electrodes 20 and their associated pixel circuits 23) are arranged in rows and columns, where pixel circuits 23 connected to pixels in the same row are connected to a common select voltage (a row select voltage), and pixel circuits 23 connected to pixels in the same column are connected to the same data voltage (column data voltage). A pixel circuit is typically arranged to apply a pixel data voltage v_Pi_xeι to a pixel electrode in response to the column data voltage V _ata present on the data voltage line 25 when there is a row select signal v_seι_e-t on the select voltage line 24. The row select and column data voltages are generated by row and column drivers 26 and 27, respectively, which are arranged to address the pixels in the display based on video data 28. The pixel circuits 23, which are only schematically shown in Fig. 3, can be single transistor switches, for example like the switches used in an AMLCD (active matrix LCD) active plate. The circuit may also include a storage capacitor in parallel with the pixel capacitance in order to maintain a stable voltage across the pixel. Such active matrix switches are realized by means of thin film transistors (TFT) which can be directly disposed on the light guide 12. Preferably the circuit 23 including the storage capacitor and the addressing lines 24, 25 are aligned with respect to the spacers 17, and may be located under the spacer structure 17 such as to maximize the light emitting efficiency. On the other side of the foil 15, here on same side as the passive plate 14, a common electrode 30 is arranged. In the illustrated example, the electrode 30, which can be an ITO layer, is disposed on the passive plate 14, facing the foil 15, and optionally covered by an insulating layer 31. Alternatively, the electrode can be arranged on an intermediate structure between the foil 15 and the passive plate 14, or can be located on the other side of the passive plate 14. The common electrode is connected to a common voltage driver 32, which is arranged to provide a common electrode voltage v_com o_n gradually increasing or decreasing in a specified voltage range. The addressing of the display will be described more in detail with reference to Figs. 3-5. With reference to Figs. 3 and 4, an addressing method performed with the display device in Fig. 2 comprises three basic steps. First (step SI), the display is set to the off state, position 41 in Fig. 4, by applying a zero voltage to the pixel electrodes 20, and a suitable voltage to the common electrode 30. (It is here assumed that the foil electrode is grounded, i.e. Vf₀ii = 0, so that the above voltage levels results in a zero voltage difference between foil 15 and pixel electrode 20.) Then, in step S2, data voltages are written to the pixel electrodes 20 row by row using the pixel circuits 23. The data voltages V_data applied to the pixel electrodes 20 are chosen such that each pixel reaches a state in the bistable region, illustrated in Fig. 4 by positions 42a, 42b relating to two different pixels. Consequently, the pixels do not switch during this step, and the display content does not change. Finally, in step S3, the common electrode voltage v_common is reduced as a function of time, which moves the state of each pixel to the right, along the dashed lines 43a and 43b in Fig. 4. When the state of a pixel crosses the ON-curve 2 (points 44a and 44b) the pixel is switched ON. Depending in the slope of the ON-curve, the different pixel voltages vj and v₂ will require different common electrode voltages v_common,ι and v_comm0n^ to switch the pixel, and the pixels are thus switched ON at different points in time depending on their individual data voltage. Fig. 5 shows the sweep of the common electrode voltage during the frame period. First, during the addressing period 51 (steps SI and S2) it is constant, and then, during the remaining frame period 52 (step S3), it decreases. At t=tl, the common electrode voltage passes v_COmmon,ι, and the first pixel is switched ON (point 44a in Fig. 4). Then, at t=t2, the common electrode voltage passes v_COmmon,2, and the second pixel is switched ON (point 44b in Fig. 4). At the end of the addressing period 52, at t=t3, the display is again reset, and all pixels are switched OFF. Each pixel will thus emit light from its ON-switching time until the end of the addressing period, in the example t3-tl and t3-t2 respectively. The light emitting time determines the amount of light emitted, and thus the gray level, which therefore can be analogue. This addressing scheme described in Figs. 3-5 is a "selective ON" addressing scheme, as the addressing relates to ON-switching of pixels. It is clear that the same analogue gray scale effect can be achieved by initially switching all pixels ON in step SI, then address the pixels with (different) data voltages in step S2, and finally in step S3 use an increasing common electrode voltage to switch the pixels OFF. Such "selective OFF addressing" is illustrated schematically in Fig. 6 - 7. First the display is set to the ON state, position 61 in Fig. 6, by applying a zero voltage to the common electrode 30, and a suitable voltage to the pixel electrodes 20. (It is here again assumed that the foil electrode is grounded, i.e. V_f0π = 0, so that the above voltage levels results in a zero voltage difference between foil 15 and common electrode 30.) Then, data voltages are written to the pixel electrodes 20 row by row using the pixel circuits 23. At the same time, the common electrode voltage is increased so that pixels can reach a state in the bistable region, illustrated in Fig. 4 by positions 62a, 62b relating to two different pixels. (Note that pixels that have not been addressed with any pixel voltage and thus have grounded pixel electrodes 20, will be switched OFF at this point, having reached position 65.) It may be advantageous to have the light guide inactivated up to this point, and only activate it when the addressing is completed. Finally, the common electrode voltage Vcommo_n is increased further as a function of time, which moves the state of each pixel to the left, along the dashed lines 63a and 63b in Fig. 4. When the state of a pixel crosses the OFF-curve 3 in points 64a and 64b, the pixel is switched OFF. Fig. 7 shows how the sweep of the common electrode voltage again is constant during the addressing period 71, but is then increased during the remaining frame period 72. When the common electrode voltage passes v_COmmon,ι and v_COmmon,2 at times tj and t₂, the respective pixels are switched ON (position 64a and 64b in Fig. 6). In this case, of course, the light emitting time will start in the beginning of the addressing period, and end at tj and t₂, respectively. The analogue gray scale generation described above can be combined with time modulation, as is illustrated in Fig. 8 in relation to selective ON addressing. According to this addressing scheme, the frame period 81 is divided into a plurality of sub-fields 82, and each pixel is active during a portion 83 of each sub-field. The length of this portion is determined by the data voltage applied to the pixel as described above. A scheme according to Fig. 7 will require several addressing periods (one for each sub-field), and will thus leave less time available for light generation in the frame period. According to a second embodiment of the invention, similar addressing schemes as described above can be realized by using active matrix switches 23 on the passive plate 14 and a common electrode 30 on the light guide 12, as is shown in Fig. 9. This will just reverse the function of the respective electrodes, so that the common electrode will exert a force on the foil 15 directed towards the light guide 12, and the data voltage applied by the pixel circuits 23 to the pixel electrodes 20 will exert a force on the foil 15 directed towards the passive plate 14. The principles of addressing will be identical as described above, with the difference that an increasing common electrode voltage v_COmmon will be used for selective ON addressing, and a decreasing common electrode voltage v_common will be used for selective OFF addressing. The person skilled in the art realizes that the present invention by no means is limited to the preferred embodiments described above. On the contrary, many modifications and variations are possible within the scope of the appended claims. For example, the common electrode in Fig. 3 and 10 extends over the entire panel area, and the scan in step S2 is thus performed over the entire display. Such single scan addressing has the disadvantage that motion artifacts may occur (sample and hold effect). The motion artifact can be reduced by shortening the light emitting time of the display, which can be achieved either by addressing the display more slowly, or by introducing a delay period between the completion of the addressing period and the start of the voltage sweep. In both cases, however, the display brightness is reduced (as no light is generated during the delay period). A different way to avoid motion artifacts is to divide the common electrode into a number of sections with separate drivers. In this manner it is feasible to realize a multi- block addressing scheme, i.e. to address one row section while generating light with another row section. Such a scheme will reduce motion artifacts without reducing brightness. While the common electrode voltage in the above description has been described as strictly increasing or decreasing, it is of course possible to generate a common electrode voltage which alternately increases and decreases between consecutive frame periods. For example, a first decreasing and then increasing sweep applied to the common electrode in Fig. 2 would allow control of both the ON-switching time AND the OFF switching time. This would gather all light emitting periods around the center of the frame period rather than in the beginning or end thereof. Further, in the above description the active matrix switches have been described as thin film transistors, while it is of course possible to use metal insulator metal (MIM) switches or diode switches to realize the above described display and addressing method. Finally, it should be noted that the above detailed description has been related to emissive foil displays coupling light out of a light guide, the invention is equally applicable to other types of foil displays.

Claims

CLAIMS:

1. A foil display device comprising: an electrostatically operable foil (15) adapted to locally be brought into contact with a substrate (12) to thereby modulate light emerging from the display, a plurality of pixel electrodes (20) arranged on one side of the foil , each connected to a pixel circuit (23) arranged to apply a pixel voltage (v_p)xeι) to the pixel electrode during a time period (52), and a common electrode (30) arranged on the other side of the foil, characterized by a voltage driver (32) for providing said common electrode (30) with a gradually increasing or decreasing voltage during said time period, so that a switching point in time for the foil (15) in a particular pixel area corresponding to a particular pixel electrode (20) is determined by the pixel voltage (v_pιxeι) applied to this pixel electrode (20).

2. The display device according to claim 1, wherein the common electrode (30) is arranged on the same side of the foil as the substrate (12).

3. The display device according to claim 1, wherein the pixel electrodes (20) are arranged on the same side as the substrate (12).

4. The display according to any one of the preceding claims, wherein each pixel circuit (23) is arranged to receive, during an addressing period (51) preceding the time period (52), a data voltage (V_data), and to apply said pixel voltage (v_pιxeι) in response to said data voltage.

5. The display according to claim 4, wherein said pixel circuits (23) comprise transistor switches.

6. The display according to any one of the preceding claims, wherein said time period (52) is a sub-period of a frame period of a video data.

7. The display according to any one of the preceding claims, wherein spacers (17) are arranged between the foil (15) and the substrate (12) to define a pixel structure, and wherein said pixel circuits (23) are located under these spacers (17).

8. The display device according to any one of the preceding claims, wherein the substrate is a light guide (12) such that light is extracted when the foil (15) is brought into contact with the light guide (12).

9. A method for addressing a foil display having an electrostatically operable foil

(15) adapted to locally be brought into contact with a light guide (12) to thereby extract light from the display, a plurality of pixel electrodes (20) arranged on one side of the foil, and a common electrode (30) arranged on the other side of the foil, comprising the steps of: applying a pixel voltage (v_Pi_xeι) to each pixel electrode (20) during a time period (52), and providing said common electrode (30) with a gradually increasing or decreasing voltage during said time period, so that a switching point in time for the foil in a particular pixel area corresponding to a particular pixel electrode is determined by the pixel voltage applied to this pixel electrode. .

10. A method according to claim 9, further comprising: receiving, during an addressing period (51) preceding the time period (52), a data voltage (Vdata), and to apply said pixel voltage (v_Pi_xeι) in response to said data voltage.