WO2005071653A1

WO2005071653A1 - Active matrix foil display

Info

Publication number: WO2005071653A1
Application number: PCT/IB2005/050210
Authority: WO
Inventors: Volker Schoellmann; Mark T. Johnson
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2004-01-21
Filing date: 2005-01-18
Publication date: 2005-08-04
Also published as: TW200529169A

Abstract

A foil display device comprising an electrostatically operable foil (15) arranged 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 pixel electrode being connected to a pixel circuit (23) for receiving a data voltage (Vdata) and for applying, in response to said data voltage, a pixel voltage to the pixel electrode, and means (10) for exerting a biasing force on the foil, said biasing force on the foil (15) defining a switching voltage for each pixel electrode. The pixel circuit (23) comprises means for controlling said pixel voltage to pass said switching voltage at a given time (t1, t2, t3) thus determining the light emitting period of the pixel.

Description

Active matrix foil display

The present invention relates to a foil display device comprising an electrostatically operable foil arranged 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 pixel electrode being connected to a pixel circuit for receiving a data voltage and for applying, in response to said data voltage, a pixel voltage to the pixel electrode, and means for exerting a biasing force on said foil, said biasing force defining a switching voltage for each pixel electrode.

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 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 bi-stability of the foil of a foil display creates a memory effect in each 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 EP XXXXXXX (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. In other technical fields attempts have been made to realize an analogue pulse width modulation of a digital switching process. For example, document EP 1017038 describes analogue pulse width modulation of a spatial light modulator. This solution consists of providing a reference signal together with the data signal, one of which is non-constant, and comparing these signals to determine the pulse length. This is a complex solution, requiring a circuit with a comparator, such as an OPAMP.

An object of the present invention is therefore to provide a device and a method for analogue gray scale addressing of an active matrix foil display.

According to a first aspect of the invention, this and other objects are achieved with a display device of the kind mentioned by way of introduction, wherein the pixel circuit comprises means for controlling said pixel voltage to pass said pixel voltage switching level at a given time, thereby determining the light emitting period of the pixel. Using this layout a proportionality between the applied potential and the light intensity is achieved, and it is thus possible to freely choose the exact time of switching of each individual pixel, and to enable analogue gray scale addressing. Further, as the entire display content can be set with a single addressing scan of the display, this provides the possibility to achieve a brighter display. The power consumption is lowered while the complexity of the required driver electronics is reduced. The biasing force can be an electrostatic force, provided by a common electrode on the other side of the foil. Preferably, the common electrode is connected to a voltage that is constant during the control of the pixel voltage. Hereby the switching voltage is also constant, and the control of the pixel voltage is facilitated. Alternatively, the biasing force is mechanical, e.g. an elastic force. Preferably, the pixel circuit is ananged to provide a gradually increasing or decreasing pixel voltage. The pixel electrodes can be arranged on the substrate, and thus provide an electrostatic force attracting the foil towards the substrate. In this case, a decreasing voltage will enable control of the OFF-switching moment. During addressing, selected pixel electrodes can be provided with a first pixel voltage to switch the pixel ON, and this voltage can then be decreased until the pixel is turned OFF. An increasing voltage will, on the other hand, enable control of the ON-switching moment. Selected pixel electrodes can be provided with a first voltage insufficient to switch the pixel ON, and this voltage can then be increased until the pixel is turned ON. The pixel will then stay ON until an OFF-switching action is performed, preferably in the form of a robust switch OFF before the consecutive addressing period. Alternatively, the pixel electrodes are instead arranged on the other side of the foil, e.g. on a passive plate arranged in parallel with the substrate. In this case, the force pattern will be reversed, and so will the switching sequence. With a decreasing pixel voltage, the pixel will first be switched OFF; and then switched ON when the pixel voltage falls below the switching voltage. With an increasing voltage, the pixel will first be switched ON, and then switched OFF when the pixel voltage exceeds the switching voltage. According to one embodiment of the pixel circuit according to the invention, it includes a controlled electrical leakage path for charging or discharging said pixel electrode. The leakage path can e.g. be a resistance or a transistor. According to another embodiment of the pixel circuit according to the invention, it includes a cwrent source for charging or discharging the pixel electrode. The charging/discharging cunent of the cunent source can be variable, and decided by the data voltage. Alternatively, the charging/discharging current of the cunent source is fixed, in which the data voltage can determine the charging starting point and thus the switching point in time. It is possible to control a group of lakage paths or cunent sources simultaneously, in order to dim or brighten at least a part of the display. The pixel circuit can further also include an additional transistor switch adapted to stop charging/discharging of the pixel electrode. Such a switch can preferably be used to avoid charging/discharging during addressing of the pixels. The foil display may be of the emissive kind, in which case the substrate is a light guide, adapted to emit light when brought in contact with the foil. According to a second aspect of the invention, the above object is achieved with a method for addressing a foil display of the kind mentioned by way of introduction, comprising the steps of exerting a biasing force on the foil, said biasing force defining a switching voltage for the pixel, receiving a data voltage, applying, in response to said data voltage, a pixel voltage to the pixel electrode, and controlling said pixel voltage to pass said switching voltage at a given time, thereby determining the light emitting period of the pixel.

These and other aspects of the present invention will now be described in more detail, with reference to the appended drawings showing a cunently prefened embodiment of the invention. Fig. 1 is a schematic side view of a foil display device according to an embodiment of the present invention. Fig. 2 is a circuit diagram of a first embodiment of the pixel circuit in Fig. 1. Fig. 3 is a circuit diagram of a second embodiment of the pixel circuit in Fig. 1. Fig. 4 is a diagram of the pixel voltage generated by the pixel circuits in

Figs. 2 and 3. Fig. 5 is a circuit diagram of a third embodiment of the pixel circuit in Fig. 1. Fig. 6 is a diagram of the pixel voltage generated by the pixel circuit in Fig. 5. Fig. 7 is a circuit diagram of a fourth embodiment of the pixel circuit in Fig. 1. Fig. 8 is a diagram of the pixel voltage generated by the pixel circuit in Fig. 7. Figure 1 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, V_foπ. 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 same side as the passive plate 14, are ananged means for exerting a biasing force on the foil, to force it towards or away from the light guide 12. In the illustrated case, the biasing force is electrostatic, and the means comprise a common electrode 10, e.g. an ITO layer, disposed on the passive plate 14, facing the foil 15, and optionally covered by an insulating layer 18. Alternatively, the electrode can be ananged 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 19, which is ananged to provide a common electrode voltage Vco_mmon to the common electrode 10. This common electrode voltage applied to the common electrode results in an electrostatic force on the foil 15, thereby attracting it towards the passive plate 14. Therefore, the common electrode voltage level determines the voltage required on each pixel electrode in order to switch the pixel ON, and this required voltage will be refened to as a switching voltage v_SWi_tCh for each pixel. For reasons of simplicity, one single switching voltage has been assumed in the following description. However, it should be noted that a foil display normally has an inherent hysteresis with a bi-stable region between two switching curves, and that therefore the switching voltage typically is different for ON switching and OFF switching respectively. This does not effect the function of the present invention. On the other side of the foil 15, here on the same side as the light guide 12, are ananged 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 ananged on the light guide 12, on the side facing the foil 15, and optionally covered by an insulating layer 21. Alternatively, the electrodes 20 can be ananged 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 ananged in rows and columns, where pixel circuits 23 connected to pixels in the same row are connected to a common select voltage line (a row select voltage), and pixel circuits 23 connected to pixels in the same column are connected to the same data voltage line 24 (column data voltage). Each pixel circuit 23 is then typically ananged 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_eo_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 ananged to address the pixels in the display based on video data 28. The pixel circuits 23 are arranged to provide a varying pixel voltage to the pixel electrodes, in dependence of the data voltage applied to the pixel circuit. The pixel circuits 23, which are only schematically shown in Fig. 1, can be transistor switches, similar to switches used in some polyLED displays. 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. In the following, several different embodiments of such pixel circuits 23 according to the invention and their function will be described, with reference to the figures 2-8. The pixel circuit in Fig. 2 comprises a transistor switch 31, having its gate connected to a select line 33, and its emitter connected to a data voltage line 35. The transistor collector is connected to the pixel electrode 20, and to a reference voltage v_ref via a voltage leakage path, here in the form of a resistance 37, e.g. implemented in the form of a fixed resistance, such as a thin film resistive layer (TFR). The pixel electrode 20 and the common electrode 10 are indicated as plates of a capacitor 38, illustrating the capacitive behavior of a pixel element in a foil display. The pixel circuit in Fig. 3 is similar to the one in Fig. 2, and similar elements have been given identical reference numbers. The leakage path from the pixel electrode 20 to the common electrode 10 is here a second transistor 40, having a gate 41 connected to a control line 42, and a collector connected to the reference voltage v_ref. The gate voltage of transistor 40 in fact defines the resistance of this transistor, and the voltage applied to the control line 42 will thus control the size of the leakage cunent running through the leakage path. By controlling this control voltage collectively for all pixels, or a group of pixels, a simple "dimming" feature can be provided to the display (increasing v_COnt_roi will increase the rate of charge/discharge for all pixels and lengthen/shorten all light pulse times, therefore dimming the display or making it more bright). The same effect could also be used if changing from normal TV (50Hz) to improved TV (100Hz), where now all light pulse lengths should be halved (as the frame time will be reduced from 20 msec to 10 msec). The pixel circuits in Figs. 2 and 3 can be related to either increasing or decreasing pixel voltages. Depending on whether the voltage v_ref is higher or lower than the voltage cunently applied to the pixel electrode, the leakage path can act to charge or discharge the pixel electrode. The function of the circuits in Figs. 2 and 3 will now be described with reference to Fig. 4 for the case of a decreasing pixel voltage. First, an initially applied voltage vi, v₂, v₃ is applied to the pixel electrode 20. This initial voltage is larger than the switching voltage Vswitch of the pixel, and the pixel is thus switched ON. The switch 31 is then opened again, and due to the presence of the leakage path 37 or 40 the applied pixel voltage decreases as a function of time. After a time ti, t₂, t₃ the voltage across the pixel drops below the switching voltage v_switciι_> and the pixel is turned OFF. The time span can be adjusted by changing the applied voltage, which is clear from the figure where a larger voltage v₃ results in a longer time span t₃. The brightness of the pixel is proportional to the time integral of the light intensity, which can be assumed to be constant while the pixel is switched ON. Using this approach it is thus feasible to generate analogue grey scales. By connecting the pixel electrode via the leakage path 37, 40 to a higher reference voltage v_ref the pixel voltage will increase as a function of time. In that case, the pixel is initially switched OFF, and then switched ON at a controlled moment in time. As an alternative to the pixel circuits in Figs. 2 and 3, it is also possible to design the pixel circuit 23 in Fig. 1 such that the applied data voltage is used to control the charging or discharging of the pixel as a function of time by means of a pixel circuit that constitutes a cunent source. There are two modes of operation whereby such a circuit can operate: either with a fixed or with a variable charging cunent. Figure 5 shows an embodiment of a thin film pixel circuit layout with a fixed charging/discharging cunent. As in Figs. 2 and 3, the gate of a switching transistor 31 is connected to a select voltage line 33, while its emitter and collector are connected to a data voltage line 35 and to the pixel electrode 20. The pixel electrode 20 is further connected to the collector of a second transistor 45, having its emitter connected to a voltage supply line 46, and its gate connected to a control line 47. Depending on the voltage on the supply line 46, the circuit will act to charge or discharge the pixel voltage from an initial voltage. Transistor 45 preferably operates in its saturation region as a cunent source, and may for example be of p-type polarity for the case of a charging circuit, or of n-type for a discharging cunent. The operation of the pixel circuit 23 in Fig. 5 will be described for the case of an increasing pixel voltage with reference to Fig. 6. The pixel data voltage vj, v₂, v₃ supplied by the data voltage line 35 via transistor 31 will set an initial voltage level of the pixel electrode 20. A control voltage v_COnt_roi applied to the gate of transistor 45 will determine a charging cunent flowing through the transistor 45, leading to an increase of the pixel voltage. The voltage v_COntroi is equal for all pixels, resulting in equal charging cunents, and thus equal slopes in Fig. 6. Therefore, the data voltage supplied by the data voltage line 35 will determine the time ti, t₂, t₃ at which the pixel voltage exceeds the switching voltage v_S itch, and the pixel is switched ON. The pixel remains ON until an OFF switching action is performed at time t_reSet- This can be achieved by a robust OFF switching action (e.g. by increasing the common electrode voltage to such a level that the foil is attracted to the passive plate). Alternatively, the collector of an additional transistor switch 50 can be connected to the pixel electrode 20, having its gate connected to a reset select line 51 and its emitter connected to a reset voltage line 52, e.g. being connected to ground. Compared to the non-linear voltage decrease in Fig. 4, the voltage increase

(charging) is linear, whereby the slope of the curve is steeper when passing through v_S i_tc_h- This will reduce sensitivity to variations in the switching voltage. According to the embodiment in Fig. 5, all brightness levels (pulse widths) will further be equally sensitivity to variations in the switching voltage (as the charging slope is constant). Figure 7 shows an embodiment of a thin film pixel circuit layout with a variable charging cunent. Some parts of the circuit are similar to the circuit in Fig. 5, and similar elements have been given identical reference numerals. Here, the collector of the switching transistor 31 forms the control line 47, connected to the gate of the second transistor 45. Transistors 31 and 45 thus form a programmable current source that is controlled by the voltage difference between Vdata and v_suppiy. A capacitor 54 is ananged between the gate of transistor 45 and the voltage supply line 46 to ensure that cunent flows during the hold period of the addressing. Just as in Fig. 5, depending on the voltage on the supply line 46, the circuit will act to charge or discharge the pixel voltage from an initial voltage. Transistor 45 preferably operates in its saturation region as a cunent source, and may for example be of p- type polarity for the case of a charging circuit, or of n-type for a discharging cunent. The operation of the pixel circuit 23 in Fig. 7 will be described for the case of an increasing pixel voltage with reference to Fig. 8. The pixel voltage is initially set to a reference reset voltage using the reset transitory 50, and then increases with time. The charging current, and thus the slope of the voltage increase, is here decided by the voltage difference between Vdata and v_SUppi_V) where the data voltage is supplied via the transistor 31 to the gate of transistor 45. The greater the difference between Vdata and v_suppiy , the steeper the slope, and the data voltage will thus determine the time ti, t₂, t₃ at which the pixel voltage exceeds the switching voltage v_switch, and the pixel is switched ON. The pixel remains ON until an OFF switching action is performed at time t_reSet_s which again can be achieved by a robust OFF switching action or by the reset switch 50, 51, 52. Again, the voltage increase (charging) is linear, whereby the slope of the curve is steeper when passing through v_switch, reducing the sensitivity to variations in the switching voltage. However, in this case the lower brightness levels (pulse widths) are less sensitive to variations in the switching voltage (as the charging slope is dependent on V_data). Of course, while the circuits in Figs. 5 and 7 have been described with reference to increasing pixel voltages, as mentioned above they can equally well be implemented to provide a decreasing voltage, in which case the pixel voltages will fall from an initial level, and pass the switching voltage at a certain moment in time, thus switching the pixel OFF (much like the situation in Fig. 4 above). In both the pixel circuits in Figs. 5 and 7, an additional switching transistor 55 could be added immediately below the transistor 45 as shown schematically in Fig. 5 and 7. The additional transistor 55 has a gate connected to an ON/OFF line 56, and offers the possibility to disconnect the pixel from the cunent source during the addressing period, and thereby avoid intensity inhomogeneities due to the charging/discharging of the pixel electrode during the addressing period. 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, while the above described embodiments have been illustrated with the most simple cunent source (voltage addressed, with 2 transistors), the invention is equally applicable to any of the prior art cunent sources, including the so called cunent minor circuits, which are addressed using a data cunent (instead of a data voltage). It is also possible to omit the common electrode 10, and to provide the biasing force mechanically, e.g. using the elastic force due to the deflection of the foil towards or away from the light guide. 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) ananged to locally be brought into contact with a substrate (12) to thereby modulate light emerging from the display, a plurality of pixel electrodes (20) ananged on one side of the foil, each pixel electrode being connected to a pixel circuit (23) for receiving a data voltage (Vd_ata) and for applying, in response to said data voltage, a pixel voltage (v_pj_xeι) to the pixel electrode, and means (10) for exerting a biasing force on the foil, said biasing force defining a switching voltage (v_swj_tCh) or each pixel electrode, characterized in that said pixel circuit (23) comprises means for controlling said pixel voltage (Vpi_xei) to pass said switching voltage (v_SWj_tCh) at a given time (ti, t₂, t₃), thereby determining the light emitting period of the pixel.

2. A foil display according to claim 1, wherein a common electrode (10) is ananged on the other side of the foil (15) for providing an electrostatic force constituting said biasing force.

3. A foil display according to claim 2, wherein said common electrode (10) is connected to a voltage (v_COmmon) that is constant, during the control of the pixel voltage (v_pi_xeι)-

4. A foil display device according to claim 1 - 3, wherein the pixel circuit (23) is arranged to provide a gradual charge or discharge of the pixel electrode, and thus a gradually increasing or decreasing of the pixel voltage.

5. A foil display device according to claims 1-4, wherein the pixel circuit (23) includes a controlled electrical leakage path (37; 41) for charging or discharging said pixel electrode (20).

6. A foil display according to claim 5, wherein said leakage path comprises a transistor (41).

7. A foil display according to claims 5 or 6, wherein a group of leakage paths (37; 40) in the display are controlled simultaneously, in order to dim or brighten at least a part of the display.

8. A foil display device according to claims 1-4, wherein said pixel circuit includes a cunent source (45, 46, 47) for charging or discharging the pixel electrode.

9. A foil display device according to claim 8, wherein said cunent source (45, 46, 47) provides a constant charging cunent.

10. A foil display according to claims 8 or 9, wherein a group of cunent sources (45, 46, 47) in the display are controlled simultaneously, in order to dim or brighten at least a part of the display.

11. A foil display device according to claim 8, wherein said cunent source (45, 46, 47) provides a variable charging cunent.

12. A foil display according to one of claims 5-11, wherein said pixel circuit (23) further comprises an additional transistor switch (55) adapted to stop charging/discharging of the pixel electrode.

13. The display according to any one of the preceding claims, wherein spacers (17) are ananged 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).

14. 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).

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

(15) ananged to locally be brought into contact with a light guide (12) to thereby extract light from the display, a common electrode (10) arranged on one side of the foil, and a plurality of pixel electrodes (20) ananged on the other side of the foil, comprising the steps of: exerting a biasing force on the foil, said biasing force defining a switching voltage (Vswitch) for the pixel, receiving a data voltage (Vdata), applying, in response to said data voltage, a pixel voltage (v_Pj_xeι) to the pixel electrode (20), and controlling said pixel voltage (v_Pj_xeι) to pass said switching voltage (v_swjtc_h) at a given time (ti, t₂, t₃), thereby determining the light emitting period of the pixel.

16. A method according to claim 15, wherein said biasing force is exerted as an electrostatic force by a common electrode (10) ananged on the other side of the foil (15).

17. A method according to claim 16, wherein a constant voltage is applied to said common electrode (10) during the control of the pixel voltage (v_Pi_Xeι).

18. A method according to claims 15-17, wherein the pixel voltage is controlled to be gradually increasing or decreasing.

19. A method according to claims 15-18, wherein a rate of change of the pixel voltage is controlled simultaneously for a number of pixels, in order to dim or brighten at least a part of the display.