Abstract:
A display device includes a display area having a plurality of pixels arranged in a matrix, each of the plurality of pixels including a light emitting element and a transistor, and a touch sensor provided over the display area. The touch sensor includes a plurality of first electrodes and a plurality of second electrodes, and the plurality of first electrodes has a shape of ring-shaped electrodes connected to each other.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    The present application claims priority from Japanese application JP2016-043910 filed on Mar. 7, 2016, the content of which is hereby incorporated by reference into this application. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a display device. Particularly the invention relates to a display device equipped with a touch sensor over a display area where an organic EL element is formed. 
         [0004]    2. Description of the Related Art 
         [0005]    It is demanded that a display device for a mobile device should be reduced in thickness and weight. In view of this, when a liquid crystal display device and an organic EL display device are compared, the organic EL display device is considered more advantageous in that it needs no backlight. Also, as the development of techniques for forming a pixel drive circuit and an organic EL element on a flexible substrate has been underway, a thinner and lighter display than a conventional display using a glass substrate has been realized. In this course of events, a reduction in thickness of members other than the display device, such as the touch sensor and the polarizer, is demanded as well. Particularly, the thickness increases if the touch sensor is bonded and mounted on the display device as a separate member. Therefore, a touch sensor as a built-in member of the display device is demanded. 
         [0006]    A method for providing a built-in touch sensor in the organic EL display device is disclosed in Japanese Patent No. 5,778,961. According to this invention, it is disclosed that one of the electrodes forming the organic EL element is formed in the shape of a band and used as an electrode of the touch sensor. Meanwhile, JP 2014-56566 A discloses a configuration in which a layer with a low dielectric constant is provided between a touch sensor and a display device. 
       SUMMARY OF THE INVENTION 
       [0007]    Providing the built-in touch sensor in the organic EL display device raises new problems. One of these problems is that, due to the shorter distance between the electrode of the touch sensor and the organic EL element, the noise caused by the signal input to and circuit operation of a pixel drive circuit which drives the organic EL element may increase. This causes a reduction in S/N ratio of the touch sensor and deterioration in sensing performance. The organic EL layer is a multilayer structure made up of a plurality of layers. It is common that a cathode or anode conductive film is uniformly formed on the top layer. The parasitic capacitance acting between this conductive film and the neighboring layer increases. 
         [0008]    Since the increase in parasitic capacitance leads to an increase in time constant and a reduction in detection signal level, sensing performance deteriorates due to an increase in detection time and a reduction in S/N ratio. Although it is possible to employ a configuration which reduces parasitic capacitance by inserting a layer with a low dielectric constant as in JP 2014-56566 A, problems remain with reducing thickness and an additional member is needed. 
         [0009]    In view of the foregoing circumstances, the invention is to propose a configuration which suitably reduces parasitic capacitance by improving the electrode structure of a touch sensor, and provide a display device having this configuration. 
         [0010]    A display device includes a display area having a plurality of pixels arranged in a matrix, each of the plurality of pixels including a light emitting element and a transistor, and a touch sensor provided over the display area. The touch sensor includes a plurality of first electrodes and a plurality of second electrodes, and the plurality of first electrodes has a shape of ring-shaped electrodes connected to each other. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  schematically shows a display device according to the invention. 
           [0012]      FIGS. 2A and 2B  schematically show built-in touch electrodes in the display device. 
           [0013]      FIG. 3  shows the cross-sectional structure of the display device. 
           [0014]      FIGS. 4A and 4B  show an example of the shape of electrodes of a touch sensor according to the invention. 
           [0015]      FIG. 5  shows the relation between the shape of the detection electrode and electrostatic capacitance. 
           [0016]      FIG. 6  shows an example of the shape of electrodes of a touch sensor according to the invention. 
           [0017]      FIG. 7  shows an example of the shape of electrodes of a touch sensor according to the invention. 
           [0018]      FIG. 8  shows an example of the shape of electrodes of a touch sensor according to the invention. 
           [0019]      FIG. 9  shows an example of the shape of electrodes of a touch sensor according to the invention. 
           [0020]      FIG. 10  shows an example of the shape of electrodes of a touch sensor according to the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0021]    Hereinafter, each embodiment of the invention will be described with reference to the drawings. In the drawings, the width, thickness, shape and the like of each part may be schematically illustrated, compared with the actual configuration, in order to clarify the explanation. However, such illustrations are simply an example and should not limit the interpretation of the invention. Also, in the specification and the drawings, components similar to those described before in drawings that are already mentioned may be denoted by the same reference signs and not described further in detail. 
         [0022]    In the invention, when describing a configuration in which one structure is arranged over another structure, if simply the term “over” is used, it includes both the case where one structure is arranged directly upward from and in contact with another structure and the case where one structure is arranged above the another structure with still another structure in-between, unless stated otherwise. 
         [0023]      FIG. 1  shows an example of the configuration of a display device according to the invention. In a display device  100 , a display area  102  and scanning line drive circuits  103 ,  104  are formed over a substrate  101 , and a drive IC  105 , a display FPC (flexible printed circuit board)  106 , and a touch FPC  107  are connected to the substrate  101 . In  FIG. 1 , the drive IC  105  is mounted over the substrate  101 . However, the drive IC  105  may also be mounted over the display FPC  106 . Also, a counter substrate  108  may be provided in such a way as to cover the display area  102 . In the display area  102 , a plurality of scanning lines laid in the direction of row (in  FIG. 1 , horizontal direction) and a plurality of video signal lines laid in the direction of column (in  FIG. 1 , vertical direction) are arranged. A subpixel  109   a  is arranged at the intersection of a scanning line and a video signal line. Each subpixel  109   a  has a light emitting element which emits light in a different color from other subpixels. A plurality of such subpixels  109   a  is gathered to form one pixel  109  (in  FIG. 1 , indicated by dotted-line frames), thus performing full-color display. In this example, three scanning lines  110  (g 1 , g 2 , g 3 ) are arranged per line of pixels, and three video signal lines  120  (R, G, B) are arranged per column of pixels. Although not illustrated, wirings such as a power supply line for supplying a predetermined voltage to the light emitting elements are provided in the display area  102 . In each subpixel  109   a,  a pixel circuit which controls the luminance of the light emitting element is provided so as to emit light with a luminance corresponding to a signal supplied from the drive IC  105  via the video signal line  120 . 
         [0024]    The display device  100  has a touch sensor in addition to the display function. Although the touch sensor is omitted from  FIG. 1  in order to explain the display function in particular, the touch sensor is arranged in an upper layer of the light emitting element, that is, closer to the display surface side than the light emitting element, as shown in  FIG. 2A . The touch sensor is made up of two kinds of electrodes, for example. One is a drive electrode  201  laid in the direction of row, and the other is a detection electrode  202  laid in the direction of column. 
         [0025]      FIG. 2B  shows an enlarged view of a dotted-line frame  210  shown in  FIG. 2A . In  FIG. 2B , an X-direction corresponds to the direction of row, and a Y-direction corresponds to the direction of column. The drive electrodes  201  and the detection electrodes  202  are provided over the display area of the display device  100  and therefore formed of a transparent conductive film of ITO (indium tin oxide), IZO (indium zinc oxide) or the like. Other materials forming the transparent conductive film may include an Ag nanowire or the like. The Ag nanowire is a material formed by dispersing Ag in the form of fine fiber into a solvent, and can be formed by coating. Moreover, the space between electrodes of one kind is arranged over the electrodes of the other kind and therefore these electrodes are connected by a bridge wire  203  or the like. In  FIG. 2B , the electrodes are rectangular. However, the shape of the drive electrodes and the detection electrodes is not limited to this. When this touch sensor is touched at a predetermined position, the capacitance between the drive electrode and the detection electrode at that position changes, and this change in capacitance is detected, thus detecting the touched position. Each electrode is connected to a touch drive circuit and a detection circuit by the touch FPC  107 . 
         [0026]    The touch sensor shown in  FIG. 2B  is a mutual capacitance-type touch sensor. A touch drive circuit inputs a drive signal to the drive electrode. The drive signal is a pulse-like signal which rises and falls. With such rises and falls, the potential of the detection electrode fluctuates via the coupling with the drive electrode. The fluctuation in potential of the detection electrode is amplified and detected by a detection circuit, thus determining whether there is a touch or not. 
         [0027]      FIG. 3  shows an example of the cross-sectional structure of the display device equipped with the touch sensor. From the bottom in  FIG. 3 , the substrate  101 , a TFT array  301 , a light emitting element layer  302 , a sealing layer  303 , a touch sensor  304 , a circular polarizer  305 , and a cover glass  306  are arranged. An adhesive layer that is needed in the case of bonding the respective layers is not described. The cover glass  306  extends not only over the display area but also over the area where the driver IC  105  and the display FPC  106  are mounted. 
         [0028]    In this structure, the touch sensor  304  is arranged over the TFT array  301  and the light emitting element layer  302  via the sealing layer  303 . In the case where the substrate on which the touch sensor  304  is formed is reduced in thickness or in the case where the drive electrode and the detection electrode of the touch sensor are formed directly on the sealing layer  303 , the touch sensor  304 , and the electrodes included in the TFT array  301  and the light emitting element layer  302  are arranged very closely to each other. Consequently, an electrically strong capacitive coupling is formed between the touch sensor  304  and these electrodes. With a display operation, various signals are inputted to the TFT array  301  and the internal circuit operates. However, these signals and changes in potential at the time of circuit operation cause a noise, thus lowering the S/N ratio of the touch sensor  304 . Moreover, with this parasitic capacitance, the time constants of the drive electrode and the detection electrode increase and therefore the touch detection operation itself takes a longer time. 
         [0029]    A detection signal of the touch sensor is the result of detecting a change in potential occurring at the detection electrode by capacitive coupling when a drive signal is applied to one drive electrode. The amount of change ΔVsense in the detection signal of the touch sensor is expressed by the following equation, where Cp is the parasitic capacitance with respect to the detection electrode, Cxy is the coupling capacitance between the drive electrode and the detection electrode, n is the number of drive electrodes intersecting with the detection electrode, and Vin is the amplitude of the drive signal applied to the drive electrode. 
         [0000]    
       
         
           
             
               Δ 
                
               
                   
               
                
               Vsense 
             
             = 
             
               Vin 
               · 
               
                 Cxy 
                 
                   ( 
                   
                     nCxy 
                     + 
                     Cp 
                   
                   ) 
                 
               
             
           
         
       
     
         [0030]    Since the parasitic capacitance Cp is the denominator of the equation, the detection signal drops as the parasitic capacitance increases. 
         [0031]    According to the invention, a new structure of the detection electrode is proposed in order to suitably reduce the parasitic capacitance at the detection electrode.  FIGS. 4A and 4B  show an example of the configuration according to the invention.  FIG. 4A  shows a planar configuration of touch sensor electrodes, similarly to  FIG. 2B .  FIG. 4B  shows the cross-sectional structure taken along Z-Z′ shown in  FIG. 4A . 
         [0032]    In  FIG. 4B , the substrate  101 , the TFT array  301 , the light emitting element layer  302 , and the sealing layer  303  are similar to those shown in  FIG. 3 .  FIG. 4B  shows the structure of the touch sensor  304  more in detail. Detection electrodes  401 ,  402  and a drive wire  404  are arranged in the same layer. The detection electrodes  401  and  402  are connected by a bridge wire  403  laid over the drive wire  404 . 
         [0033]    As shown in  FIG. 4A , the detection electrodes  401 ,  402  are ring-shaped. Specifically, a ring-shape with a hollow inner area and with the outer peripheral shape of the electrode being left as it is, is employed. Since the electrode area is smaller than that of a detection electrode with a solid inner area as in the conventional technique, the parasitic capacitance Cp between the detection electrode and the underlying light emitting element layer  302  and the like can be reduced. 
         [0034]    Now, the shape of the detection electrodes  401 ,  402  will be described. When reducing the area of the detection electrode in order to reduce parasitic capacitance, simply reducing the shape has a similar effect. However, the coupling capacitance Cxy between the drive electrode and the detection electrode, which is important in the touch detection operation, contributes significantly in the area where the two electrodes come most closely to each other, that is, in peripheral edge parts of the electrodes. Therefore, by making hollow the inner area of the detection electrode and thus reducing the area, it is possible to suitably reduce the parasitic capacitance Cp without reducing the coupling capacitance Cxy. 
         [0035]      FIG. 5  shows changes in the parasitic capacitance Cp and the coupling capacitance Cxy in the case where the detection electrode is ring-shaped and in the case where the detection electrode has a conventional shape. In  FIG. 4A , the width of the ring in the case where the detection electrode is ring-shaped is expressed by a, and the full width of the detection electrode is expressed by b. In  FIG. 5 , the ratio of the two a/b is taken on the horizontal axis. In the case of the conventional shape without any hollow part, a/b=1/2 is the maximum. Also, the ratio of the coupling capacitances (Chollow/Csolid) in the case where the detection electrode is ring-shaped and in the case where the detection electrode has the conventional shape is taken on the vertical axis. If the two coupling capacitances are equal, (Chollow/Csolid)=1 is the maximum. 
         [0036]    If the width a of the ring is increased while the full width b of the detection electrode is kept constant, the parasitic capacitance Cp increases as the area of the detection electrode increases. Meanwhile, the ratio of the coupling capacitances reaches substantially 1:1 with respect to the conventional shape, when the width a of the ring reaches a certain value. That is, if the width al of the ring at this time is defined as a minimum value and the shape of the detection electrode is decided in such a way as to achieve this value or above, the parasitic capacitance Cp can be suitably reduced while the coupling capacitance Cxy is maintained. Thus, large amplitude of the detection signal can be realized. 
         [0037]    As an example, in a system where a cover glass with a dielectric constant of 5.7 and a thickness of 700 μm is provided over the touch sensor shown in  FIG. 4A , if the full width b of the detection electrode is 3 mm, the result of the calculation is a 1 =800 μm. That is, with a ring-shaped structure in which a 1.4-mm-diameter hole is provided inside a 3-mm-diameter detection electrode, a configuration which suitably reduces parasitic capacitance while maintaining the coupling capacitance with the drive electrode at a level equal to that in the conventional configuration can be realized. 
         [0038]    This structure also has the effect of reducing a noise from the TFT array  301  driving the light emitting element layer  302 , in addition to the reduction in parasitic capacitance. The noise due to the drive signal of the TFT array  301  is transmitted to the detection electrodes  401 ,  402  via the light emitting element layer  302 . However, with the reduction in the electrode area, the capacitive coupling can be reduced and the noise can be reduced. 
         [0039]    Now, a method for forming the touch sensor shown in  FIGS. 4A and 4B  will be described. Here, the process of forming the TFT array  301 , the light emitting element layer  302 , and the sealing layer  303  is omitted. 
         [0040]    The detection electrodes  401 ,  402  and the drive electrodes  404  are formed on a sealing film surface. Here, after depositing a transparent conductive material such as ITO or IZO by sputtering, these electrodes are formed by a photolithography process. Since the sealing layer  303  formed over the light emitting element layer  302  has sufficient coatability and contactability, it is possible to apply the process as described above, even after the light emitting element layer  302  is formed. Instead of the foregoing transparent conductive material, a material containing silver nanowires may be printed to form the detection electrodes  401 ,  402  and the drive electrodes  404 . Next, after an insulation film is formed, a contact hole reaching the detection electrodes  401 ,  402  is formed and the bridge electrode  403  is formed. The bridge electrode  403  has a small area and therefore is not very visible. Thus, after a metal such as aluminum, silver, or copper is deposited in order to prioritize a reduction in resistance, the bridge electrode  403  is formed by a photolithography process. After that, the electrode pattern may be protected further by forming an insulation film or by bonding a film or the like, if necessary. By these processes, the touch sensor can be formed over the display area. 
         [0041]    As other examples of the invention, structures as shown in  FIGS. 6 and 7  may be employed as well. In  FIG. 6 , a cut-out is provided at a part of a ring-shaped detection electrode  601 , and a drive electrode  602  has a protruding part  610  via this cut-out. The protruding part  610  enters the inside of the ring. The parasitic capacitance of the detection electrode  601  can be reduced and the coupling capacity can be increased between the protruding part  610  and the detection electrode  601 .  FIG. 7  shows an example in which, in addition to a detection electrode  701 , a drive electrode  702  is ring-shaped as well. The drive electrode is driven with low impedance and therefore is not so susceptible to the influence of external electric field fluctuations than the detection electrode. However, in the drive electrode is formed of a transparent conductive material, it has a higher resistance than metal. Therefore, a central area within the plane, that is, an area distant from the circuit which drives the drive electrode, is more susceptible to the influence of a noise from the TFT array or the like. By forming the drive electrode  702  in a ring-shape, it is possible to reduce the influence of the noise and to perform stable touch detection in the entire area within the plane. 
         [0042]      FIG. 8  shows a still another configuration example that is different from the above. A rib  802  is provided on a diagonal line in ring-shaped detection electrode  801 . The time constant can be reduced, compared with a ring-shaped detection electrode. 
         [0043]    As described above, if the detection electrode or the drive electrode is ring-shaped, significant improvement in electrical functions can be expected, whereas the area where the drive electrode is provided and the area where the drive electrode is not provided are separated and a difference in refractive index may occur between these areas, causing the ring-shape of the detection electrode to be visible in some cases. Thus, on the inner side of a ring-shaped detection electrode  901 , an internal electrode  902  is formed of the material in the same layer as the detection electrode  901 , as shown in  FIG. 9 . As the internal electrode  902  is provided, the refractive index within the plane can be made uniform and therefore the visibility of the detection electrode can be lowered. 
         [0044]    The internal electrode  902  is insulated from both of the detection electrode  901  and a drive electrode  903  and is in a floating state. However, if the distance between the internal electrode  902  and the detection electrode  901  is short, a parasitic capacitance may be generated between the detection electrode  901  and the light emitting element layer  302  via the internal electrode  902  in some cases. 
         [0045]    If the distance between the ring-shaped detection electrode  901  and the drive electrode  903  is gap 1  and the distance between the ring-shaped detection electrode  901  and the internal electrode  902  is gap 2 , it is desirable that gap 1  is narrow since gap 1  influences the coupling capacitance between the detection electrode and the drive electrode. In view of the visibility of the ring-shaped detection electrode, it is desirable that gap 2  is narrow. However, if gap 2  is narrowed, the parasitic capacitance increases between the ring-shaped detection electrode  901  and the light emitting element layer  302  via the internal electrode  902 . Therefore, it is preferable that gap 2  is broader than gap 1 . 
         [0046]      FIG. 10  shows an example in which the ring-shape and the internal electrode provided on the detection electrode are applied to the drive electrode side as well. An internal electrode  1002  is formed on the inner side of a ring-shaped detection electrode  1001 , and an internal electrode  1004  is formed on the inner side of a ring-shaped drive electrode  1003 . 
         [0047]    The distance between the ring-shaped drive electrode  1002  and the internal electrode  1004  is gap 3 . Since the drive electrode  1002  receives less influence of the noise from the TFT array  301  or the light emitting element layer  302  than the detection electrode  1001 , gap 3  may be smaller than gap 2 . The relation between these distances may be gap 1 &lt;gap 3 ≦gap 2  or the like, for example. 
         [0048]    While there have been described what are at present considered to be certain embodiments of the invention, it will be understood that various modifications may be made thereto, and it is intended that the appended claims cover all such modifications as fall within the true spirit and scope of the invention.