Field of the invention

The present invention relates to an electrophoretic display. An example of a pixel of a known electrophoretic display is illustrated in figure 1.

Background of the invention

The pixel of the known display comprises a first transparent substrate 1, usually made from glass, onto which a transparent electrode 2 is arranged. This electrode is re- ferred to as the common electrode 2. The display further com ^¬ prises a second transparent substrate 3 and usually also made from glass, onto which a preferably transparent pixel elec ^¬ trode 4 is arranged. A liquid comprising dispersed charged pigment particles 5, 6 is arranged between first and second transparent substrates 1, 3. One or more types of charged pigment particles may be dispersed in the liquid.

The display comprises a plurality of pixels. To that end, an active matrix (not shown) is used to control the po ^¬ tential of pixel electrode 4 for each pixel individually. The active matrix comprises at least two metal layers. Addition ^¬ ally, the potential of common electrodes 2 of multiple pixels can be controlled at the same time using a common electrode driver (not shown) if so desired. In figure 1, common elec ^¬ trode 2 and pixel electrode 4 are illustrated for a single pixel. The display comprises a plurality of such pixels, ar ^¬ ranged in rows and columns. The pixels may or may not be identical. The active matrix is typically arranged on second substrate 3 on a side facing the liquid.

When light falls onto the display, as illustrated by arrow 7 in figure 1, light passes through first transparent substrate 1, common electrode 2 (if applicable), the liquid, pixel electrode 4 (if applicable), and second transparent substrate 3. The known display uses an external reflector 8 to diffusively reflect light as indicated by arrows 9.

The visual appearance of a pixel is determined by the distribution of charged pigment particles 5, 6. This dis ^¬ tribution depends on the voltage applied between pixel elec ^¬ trode 4 and common electrode 2. In the display shown in fig- ures 1 and 2, a viewable area can be identified that substan ^¬ tially corresponds with the common electrode. In general, the viewable area corresponds to that area of the pixel which does not comprise reflective materials and for which the vis- ual appearance is solely determined by the amount of charged particles. In figure 1, the viewable area is indicated by ar ^¬ row 10 whereas it is indicated by rectangle 11 in the top view of electrodes 2, 4 in figure 2.

Assuming negatively charged black pigment particles 5 only, when a pixel should turn white, a voltage is applied between pixel electrode 4 and common electrode 2 to attract charged pigment particles 5 towards pixel electrode 4 which functions as an accumulation electrode. Consequently, no pig ^¬ ment particles 5 are arranged underneath common electrode 2 and the viewable area becomes transparent. By using diffusive reflector 8, light is not only reflected but also diffused resulting in a white appearance of the viewable area. By en ^¬ suring that the viewable area is much larger than pixel elec ^¬ trodes 4 used for accumulating pigment particles 5, an over- all white appearance can be obtained. Similarly, when a black appearance is desired, a voltage is applied to move pigment particles 5 towards common electrode 2.

For each pixel of the electrophoretic display, the active matrix comprises at least two metal layers used for the interconnect of the thin film transistors of the active matrix. These metal layers are typically reflective. More in particular, these layers provide specular reflection. Consequently, these layers should not be arranged in the viewable area, or at least to a minimum extent.

Figure 3 illustrates a possible layer stack of the active matrix. As a first layer, a gate metal 12 is deposit ^¬ ed. This metal 12 is covered by a dielectric 13 such as Sili ^¬ con Nitride. Next, semiconductor islands 14 are defined on dielectric 13. These islands form the active material used for the thin film transistors and can for instance be made from amorphous Silicon. Next, a source and drain metal layer 15, hereinafter referred to as source metal, is deposited that defines the source and drain contacts of the thin film transistor. A final dielectric layer 16 is deposited on top of source metal 15. Then, an organic film 17 is deposited on top of dielectric layer 16. Both organic film 17 and dielec ^¬ tric layer 16 are etched locally, to allow the subsequent formation of an ITO pixel electrode 4 to contact source metal 15, more in particular to contact the drain of a thin film transistor of the active matrix.

Figure 4 illustrates an example of an equivalent circuit of the active matrix. A thin film transistor 18 has its gate 19 connected to a gate line, its source 20 to a source line, and its drain 21 to a storage capacitor 22. The latter is formed using dielectric layer 13, gate metal 12 and source metal 15 of the active matrix stack. One plate 23 of storage capacitor 22 is connected to ground and/or to a stor- age capacitor controller via a storage capacitor line. The combination of the liquid, common electrode 2 and pixel elec ^¬ trode 4 constitutes a pixel capacitor 24.

When the thin film transistor is not conducting, it is important to maintain the voltage of pixel electrode 4 to prevent the image from blurring due to leakage currents through parasitic resistor 25, which represents the non-zero resistance of the liquid. To that end, storage capacitor 22 should be sufficiently large. However, because storage capac ^¬ itor 22 is made from reflective metal layers, it cannot be- come arbitrarily large as this would reduce the viewable ar ^¬ ea .

A current trend in display technology is to reduce the pixel size and to increase the pixel resolution of the display. If the pixel size decreases, the storage capacitors must decrease in size as well to maintain a useful viewable area. Consequently, for current high resolution displays, problems are encountered when trying to maintain a displayed image in case the thin film transistors are inactive.

Some prior art documents recite pixel layouts, such as US2009/206339 Al , WO2005/012987 Al , WO2011/012499 Al , US 6,621,541 Bl and WO2001/007961 Al (relating to so-called E- ink) . US2009/206339 Al recites a flat display device with a substrate divided into an active region for displaying an im- age and a peripheral region that does not display the image, and includes: a gate line that crosses a data line to define a pixel region in the active region; a thin film transistor in a region near a crossing of the gate line and the data line; a first common electrode in the pixel region; a storage electrode on the first common electrode to provide storage capacitance; a pixel electrode electrically connected with the storage electrode and overlapping the pixel region, the data line, and the gate line; and an ink film covering the active region and the peripheral region, and having microcap ^¬ sules including charged particles; the microcapsules indicate an E-ink device which does not suffer from various drawbacks of the pixels having a pixel electrode and a common elec ^¬ trode; movement of pigment particles is totally different. Therefore a combination of any of documents US2009/206339 Al and WO2001/007961 Al, relating to microcapsule devices, any of WO2005/012987 Al , and US 6,621,541 Bl, relating to liquid crystal layers having an on-off modus and layout, and

WO2011/012499 Al , is inherently complex, as it is not clear which of the elements should be incorporated and which should be left out; further the drawbacks and advantages of the var ^¬ ious types of devices are so different that a combination of various types results in unpredictable results with respect to these advantages and drawbacks and therefore would not be considered.

To solve this problem, the thin film transistors must regularly be made active to re-apply a given voltage, thereby increasing the power consumption of the display. If the time between refreshes becomes too short, the pixel can not retain a sufficiently high voltage.

An object of the present invention is to provide a solution to the abovementioned problem.

Summary of the invention

This object has been achieved with a display as de- fined in claim 1. This display is characterized in that the reflector is formed by at least one of the at least two metal layers of the active matrix and in that the electrophoretic display further comprises a scattering film configured to diffusively scatter light reflected by the reflector.

The applicant has realized that by avoiding the com ^¬ mon approach to minimize the arrangement of metals layers of the active matrix into the viewable area, but instead to pur- sue the opposite and to use a scattering element (e.g. a scattering foil) in combination with the reflector formed by the metal layers, the viewable area can be maintained or even increased while at the same time achieving high storage ca ^¬ pacitor values, even for relatively small pixels.

Detailed description of the invention

In an embodiment, a scattering film is arranged on the first transparent substrate on a side facing away from the liquid. The scattering film can for instance be made from Polyethylene terephthalate with an appropriate surface struc- ture . However, the invention does not exclude that the scat ^¬ tering element is incorporated into the active matrix layer stack for instance in the form of a dedicated layer or by us ^¬ ing a different type of metal or structured metal for the at least two metals. In the latter example, the reflector and scattering film are integrated in the same layer.

The scattering element is configured to diffusively scatter light that exits the scattering film. When the scattering film is spaced apart from the reflector, light passes the scattering film twice.

For each pixel, a preferably transparent pixel elec ^¬ trode can be arranged on the second preferably transparent substrate on a side facing the liquid and a transparent com ^¬ mon electrode can be arranged on the first transparent sub ^¬ strate on a side facing the liquid, wherein the common elec- trodes of the pixels are electrically connected. These elec ^¬ trodes are typically made from Indium Tin Oxide (ITO) .

In an embodiment, the pixel electrode and common electrode are laterally offset from each other for each pix ^¬ el, each pixel having a viewable area that substantially de- termines a visual appearance of the pixel, wherein the pixel electrode is arranged outside the viewable area and wherein the common electrode is arranged at least partially inside the viewable area. The visual appearance can be determined by an amount of charged pigment particles being arranged inside the viewa ^¬ ble area and an amount of charged pigment particles being ac ^¬ cumulated at the pixel electrode in dependence of a voltage between the pixel electrode and the common electrode.

In an embodiment, the liquid comprises charged pig ^¬ ment particles of only one polarity and of only one pigment. For instance, only black negatively charged pigment particles are used. It should however be noted that the invention does not exclude the use of multiple polarities and pigments. For instance, positively charged red or green pigment particles could be added. Typically, the electrophoretic liquid further comprises non-pigment molecules or ions of opposite polarity.

In an embodiment, the at least two metal layers comprise a first metal layer and a second metal layer, where ^¬ in the first metal layer is used to define gates of thin film transistors of the active matrix, and wherein the second met ^¬ al layer is used to define sources and drains of the thin film transistors. The same metal layers may be used to define the interconnect of the active matrix. For instance, gate lines may be formed to connect the gates of thin film tran ^¬ sistors of pixels that are adjacent in a first direction and/or source lines may be formed to connect the sources of thin film transistors of pixels that are adjacent in a direc- tion perpendicular to the first direction. Additionally, storage capacitor lines and/or ground lines may be formed us ^¬ ing the same metal layers.

The storage capacitor may comprise a parallel plate capacitor, wherein the first metal layer and the second metal layer define respective plates of the parallel plate capaci ^¬ tor. Here, the first plate of the capacitor is preferably connected to the drain of a thin film transistor of the ac ^¬ tive matrix and the second plate of the capacitor is kept at a predefined voltage. The second plates of the capacitors of adjacent pixels may be electrically connected using a storage capacitor line. When a storage capacitor line is used defined by the first metal layer, the line typically runs parallel to gate lines inside the active matrix to avoid a short circuit between the gate line and the storage capacitor line. Conse ^¬ quently, the active matrix may comprise a plurality of gate lines and storage capacitor lines.

It is noted that within the concept of the present invention, a parallel plate capacitor should be construed as any capacitor having two planar electrodes separated by a di ^¬ electric layer.

The predefined voltage may be ground. Additionally or alternatively, the electrophoretic display may further comprise a storage capacitor voltage controller for control ^¬ ling a voltage applied at the storage capacitor line.

The electrophoretic display may further comprise a backlight unit arranged near the second transparent substrate on a side facing away from the liquid, wherein the reflector further comprises one or more openings for allowing light transmitted from the backlight unit to pass through the re ^¬ flector. This allows a backlight to be used for instance to facilitate reading in dark environments.

The reflector preferably covers at least 50% of a pixel area, more preferably at least 75% of a pixel area, and even more preferably at least 90% of a pixel area.

Next, the invention will be described in more detail using the appended drawings, wherein:

Figure 1 illustrates a cross section of a pixel of a known display;

Figure 2 provides a top view of the electrode layout of the pixel of figure 1 ;

Figure 3 illustrates a layer stack of the active ma ^¬ trix used in the display of figure 1 ;

Figure 4 illustrates an equivalent circuit of a pix ^¬ el of the known display of figure 1 ;

Figure 5 depicts the arrangement of the metal layers in the active matrix of the pixel of figure 1 ;

Figure 6 depicts a possible arrangement of the metal layers in the active matrix of a pixel of an embodiment of a display according to the present invention; and

Figure 7 illustrates a cross section of a pixel of an embodiment of a display according to the present inven- tion; and

Throughout the description of the figures, identical numbers will be used to indicate identical or similar fea ^¬ tures .

According to the invention, the distribution of metal layers of the active matrix in a pixel 100 is not mini ^¬ mized but maximized, as illustrated in figure 6. Here, a gate metal is used to define a gate contact 101 for the thin film transistor made using an island of amorphous silicon 102. The gates of pixels adjacent in the horizontal direction are con ^¬ nected. The gate metal is also used for one 103 of the plates of the storage capacitor and to define storage capacitor lines 104 that connect the plates of pixels adjacent in the horizontal direction.

The other plate 105 of the capacitor is formed using source metal, which metal is also used to define the source contact 106 and drain contact 107 of the thin film transis ^¬ tor, and to define source lines 108 that connect the sources of thin film transistors in pixels adjacent in the vertical direction. One or more openings 109, 110 may be provided in the gate metal and source metal at the position of the capac ^¬ itor to allow light from a possible backlight to pass.

By comparing figure 6 to a typical electrode layout of the known display as illustrated in figure 5, it can read- ily be appreciated that the electrodes in the layout accord ^¬ ing to the invention occupy a significantly larger area than the electrodes in the known layout. It should be noted that the active matrix in figure 6 can be combined with the elec ^¬ trode layout in figure 2. In fact, it can be combined with many more electrode layouts because the pixel and common electrode are preferably transparent and therefore do not in ^¬ fluence the reflective behavior of the pixel. To connect the pixel electrode to the active matrix, a connection can be made from the pixel electrode to the drain of the thin film transistor, typically using a via from the pixel electrode layer to the second metal layer. Such via is known in the art and is not illustrated in figure 6.

The cross section in figure 7 illustrates that light 7 from the first substrate 1 is reflected by the source metal 30 in active matrix 31 and that reflected light is diffusive ^¬ ly scattered by a scattering film 32 arranged on first sub ^¬ strate 1. In addition, a backlight unit 33 is provided to emit light through an opening 34 in source metal 30 towards first substrate 1.

It should be apparent to a skilled person in the art that various modifications can be made to the invention or the embodiments thereof without deviating from the scope of protection defined by the appended claims. For instance, the invention is not limited to the type of transistor used in the active matrix. It is also not limited to the particular manner in which the thin film transistor is connected. Source and drain contacts of such transistors may be exchanged with ^¬ out departing from the inventive concept of the present in ^¬ vention .