Abstract:
A liquid crystal display device is configured to prevent the appearance on its display of a black stain stemming from a drop in volume resistivity of liquid crystal caused by ions therein. The device includes a thin-film transistor (TFT) substrate and a counter substrate bonded together along the periphery thereof by a seal material. The TFT substrate and the counter substrate have liquid crystal sandwiched therebetween and include a display area. A third electrode is formed outside the display area of the TFT substrate. A concave portion is formed in an organic insulation film on the liquid crystal side of the third electrode.

Description:
CLAIM OF PRIORITY 
       [0001]    The present application claims priority from Japanese Patent Application JP 2015-229727 filed on Nov. 25, 2015, the content of which is hereby incorporated by reference into this application. 
       BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
       [0002]    The present disclosure relates to a display device. More particularly, the disclosure relates to a liquid crystal display device that provides against display unevenness attributable to ion aggregates. 
       2. Description of the Related Art 
       [0003]    Liquid crystal display devices are generally configured to have a thio-film transistor (TFT) substrate disposed opposite to a counter substrate with liquid crystal sandwiched therebetween, the TFT substrate typically having pixel electrodes and TFTs formed thereon in a matrix pattern. The display device forms an image suitably controlling the light transmittance of liquid crystal molecules per pixel. 
         [0004]    Liquid crystal includes impurities that tend to be ionized. Ions move in a particular direction inside a display area of the device and are liable to be localized near the display area periphery. The localized ions reduce the resistance of the liquid crystal. An image displayed on the liquid crystal display area will have a black stain appearing at a location where the liquid crystal resistance is low. 
         [0005]    JP-A-2014-142456 describes an organic electroluminescent (EL) display device in which upper transparent electrodes outside the display area are supplied with a cathode voltage via through-holes constituted by a two-layer structure made of a metal film and a transparent conductive film. JP-A-2014-206622 describes a configuration in which dummy electrodes formed outside the display area are impressed with a predetermined voltage to move ions out of the display area. JP-A-2009-265484 describes a configuration in which shielding electrodes are formed outside the display area on the side of the TFT substrate to protect the display area from being affected by a scanning voltage from scanning line leader lines formed in the periphery of the display area. JP-A-1996-328042 describes a configuration in which, with a scanning line driver circuit incorporated in a liquid crystal display panel, shielding electrodes are formed in a manner covering the scanning line driver circuit so as to shield the liquid crystal against a direct-current voltage from the scanning line driver circuit. 
       SUMMARY OF THE INVENTION 
       [0006]    There is a growing need to maximize the display area of the liquid crystal display panel while minimizing the external form of the panel. To do so involves minimizing the width between the edge of the display area and that of the liquid crystal display panel, i.e., the width of the frame. Meanwhile, particular pixel electrode patterns are characterized by their tendency to let ions aggregate in the frame area. In such cases, the ions collected outside the display area tend to move back to the periphery of the display area and are likely to trigger a black stain there. 
         [0007]    The present disclosure has been made in view of the above circumstances and provides arrangements for preventing the ions aggregated in the frame area from moving back to the display area, thereby preventing the appearance of a black stain in the screen periphery. 
         [0008]    The present disclosure proposes overcoming the above circumstances using the typical embodiments outlined below. 
         [0009]    (1) According to one embodiment of the present disclosure, there is provided a liquid crystal display device including a thin-film transistor (TFT) substrate and a counter substrate bonded together along the periphery thereof by a seal material. The TFT substrate and the counter substrate have liquid crystal sandwiched therebetween and include a display area. A third electrode is formed outside the display area of the TFT substrate. A concave portion is formed in an organic insulation film on the liquid crystal side of the third electrode. 
         [0010]    (2) Preferably in the liquid crystal display device described in paragraph (1) above, the organic insulation film may be layered over lines other than the third electrode. 
         [0011]    (3) Preferably in the liquid crystal display device described in paragraph (1) above, the third electrode may be impressed with the same voltage as that of scanning lines. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    Further objects and advantages of the present disclosure will become apparent upon a reading of the following description and appended drawings in which: 
           [0013]      FIG. 1  is a plan view showing a typical crystal display device to which the present disclosure is applied; 
           [0014]      FIG. 2  is a cross-sectional view taken on line A-A in  FIG. 1  and showing a first embodiment of the present disclosure; 
           [0015]      FIG. 3  is a cross-sectional view taken on line A-A in  FIG. 1  and showing a variation of the first embodiment; 
           [0016]      FIG. 4  is a cross-sectional view taken on line A-A in  FIG. 1  and showing another variation of the first embodiment; 
           [0017]      FIG. 5  is a cross-sectional view taken on line A-A in  FIG. 1  and showing still another variation of the first embodiment; 
           [0018]      FIG. 6  is a cross-sectional view taken on line A-A in  FIG. 1  and showing a second embodiment of the present disclosure; 
           [0019]      FIG. 7  is a plan view showing a third embodiment of the present disclosure; 
           [0020]      FIG. 8  is a plan view showing one variation of the third embodiment; 
           [0021]      FIG. 9  is a plan view showing another variation of the third embodiment; 
           [0022]      FIG. 10  is a plan view showing how ions move in a liquid crystal display device; 
           [0023]      FIG. 11  is another plan view showing how ion move in the liquid crystal display device; and 
           [0024]      FIG. 12  is a plan view showing how aggregated ions move back to the display area of the liquid crystal display device. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0025]    The behavior of ions dissolved in liquid crystal is described first, prior to the description that follows of specific embodiments of the present disclosure. The ions in liquid crystal tend to be charged positively. The liquid crystal is driven by a voltage impressed between pixel electrodes and a common electrode. The ions dissolved in the liquid crystal aggregate in a particular direction under the influence of the shape of the pixel electrodes, for example. 
         [0026]      FIG. 10  is a schematic plan view showing how ions behave in liquid crystal. In.  FIG. 10 , the liquid crystal is sandwiched between a TFT substrate  100  and a counter substrate  200 .  FIG. 10  shows the ions to be attracted toward the top left corner of the screen. Under the influence of the pixel electrode shape in particular, the ions tend to aggregate in a specific direction of the screen. In  FIG. 10 , a light shielding film  201  is formed inside a seal material  150 , with ions aggregating under the light shielding film  201 . Plus marks in  FIG. 10  indicate positive ions.  FIG. 11  shows a state in which the ions further aggregate over operation time in the periphery of the screen. In  FIG. 11 , the ions are still outside a display area  50 , i.e., under the light shielding film  201 , so that images on the screen are left unaffected. 
         [0027]      FIG. 12  shows a state in which upon elapse of a further operation time period, the ions aggregating in the top left corner of the display area  50  move from under the light shielding film  201  and into the display area  50 .  FIG. 12  depicts a black stain being formed in the top left corner of the display area  50  to which the ions have moved. 
         [0028]    The phenomenon of ions moving to the display area periphery is more likely to occur the narrower the width of the frame. An object of the present disclosure is to prevent the black stain from appearing in the display area periphery in that manner. The present disclosure will now be described below in detail using embodiments. 
       First Embodiment 
       [0029]      FIG. 1  is a plan view showing a liquid crystal display panel to which the present disclosure is applied and which is used on a mobile phone for example. In.  FIG. 1 , the TFT substrate  100  and the counter substrate  200  are bonded together in the periphery thereof by the seal material  150 , the two substrates having liquid crystal sandwiched therebetween. The TFT substrate  100  is formed to be larger than the counter substrate  200 . That portion of the TFT substrate  100  which is not covered by the counter substrate  200  constitutes a terminal area  160 . A driver integrated circuit (IC)  60  is disposed in the terminal area  160 . A flexible wiring substrate for externally feeding power and signals to the liquid crystal display panel is connected to the terminal area  160 . 
         [0030]    In the display area  50  of  FIG. 1 , scanning lines  1  extend in a crosswise direction and are arrayed in a longitudinal direction. Video signal lines  2  extend in the longitudinal direction and are arrayed in the crosswise direction. A region enclosed by two scanning lines  1  and two video signal lines  2  constitutes a pixel  3 . The pixels  3  are formed in a matrix pattern in the display area  50 . A scanning line driver circuit is formed outside the long side of the display area  50 . 
         [0031]    In recent years, the distance between the edge of the display area and that of the liquid crystal display panel, i.e., the so-called frame width, has been narrowing. 
         [0032]    For example, the frame width wfa on the long side in  FIG. 1  is about 0.5 mm or less. The distance wfc between the edge of the seal material  150  and that of the display area  50  can be about 0.1 mm. Sometimes, the frame width on the long side of the liquid crystal display panel may be different from the frame width on its short side. For example, the short-side frame width wfb in  FIG. 1  is about 0.9 mm or less, which may be wider than the long-side frame width. In some cases, the short-side frame width on the terminal area side is different from the short-side frame width on the opposite side. 
         [0033]    In  FIG. 1 , a third electrode  10  for trapping ions is formed inside the seal material  150  and outside the display area  50 . A voltage lower than a common voltage is impressed to the third electrode  10 . The role of the third electrode  10  impressed with the voltage is to let positive ions aggregate toward the third electrode  10  or to retain the aggregated ions in the periphery of the third electrode  10 . 
         [0034]    In.  FIG. 1 , the third electrode  10  is formed in a dual structure, when viewed in a plan view, surrounding the display are  50 . The area where the third electrode  10  is not disposed is furnished with lines impressed with a voltage higher than that of the third electrode  10 . The positive ions aggregating outside the display area  50  are trapped by the third electrode  10 , so that the ions do not move into the display area  50 . 
         [0035]    The liquid crystal exists inside the seal material  150 . In order to let the third electrode  10  affect the liquid crystal more strongly than the other lines, no organic insulation film is disposed over the third electrode  10 . On the other hand, an organic insulation film is disposed over the lines other than the third electrode  10 . The third electrode  10 , thus located closer to the liquid crystal than the other electrodes or lines, influences the liquid crystal more vigorously. 
         [0036]      FIG. 2  is a cross-sectional view taken on line A-A in  FIG. 1 . A layered structure depicted in  FIG. 2  represents a cross-section of a so-called in-plane-switching (IPS) liquid crystal display device. What matters with the liquid crystal display device are its viewing angle characteristics. The IPS method involves driving liquid crystal molecules by electric fields parallel to the substrates, thereby offering good viewing angle characteristics. The IPS method is characterized by the pixel electrodes and the common electrode being formed on the side of the TFT substrate  100 . That is, a signal voltage is impressed between a first electrode  106  such as an indium-tin-oxide (ITO) transparent electrode formed flat and a second electrode  108  such as an ITO transparent electrode, with a capacitance insulating film  107  interposed therebetween. The impressed signal voltage rotates liquid crystal molecules to control their transmittance per pixel, thereby forming an image. The first electrode  106  may be constituted by the pixel electrodes or by the common electrode. The second. electrode  108  may also be formed by the pixel electrodes or by the common electrode. 
         [0037]    In  FIG. 2 , a first base film  101  formed by silicon nitride (SiN) is disposed over the TFT substrate  100 . A second base film  102  formed by silicon dioxide (SiO 2 ) is disposed over the first base film  101 . The first base film  101  and the second base film  102  are intended to protect the semiconductors formed in the display area from being contaminated by impurities from a glass substrate  100 . 
         [0038]    A gate insulating film  103  is formed over the second base film  102 . The third electrode  10  is formed over the gate insulating film  103 . The third electrode  10  is formed with the same layer as that of the scanning lines. The third electrode  10  may be supplied with a gate voltage or fed with a low voltage (VL) given to the shift transistors of the scanning line driver circuit formed in the display area periphery. The third electrode  10  may be constituted by scanning line leader lines or by VL lines. That is, the third electrode  10  may or may not be provided solely for its purported purpose. 
         [0039]    An interlayer insulating film  104  is formed to cover the third electrode  10 . The interlayer insulating film  104  is intended to insulate the scanning lines  1  from the video signal lines  2  in the display area  50 . Lines  20  formed with the same layer as that of the video signal lines  2  are disposed over the interlayer insulating film  104 . The lines  20  are made up of common electrode lines, common electrode, video signal line leader lines, and lines attached to the scanning line driver circuit, among others. As such, the lines  20  generically represent the electrodes and lines supplied with a voltage equal to or higher than the common voltage. The liquid crystal display device is driven by an alternate current, so that the video signal lines  2  are supplied with positive and negative voltages. In this context, each voltage refers to an average voltage. 
         [0040]    An organic passivation film  105  is formed to cover the lines  20 . The organic passivation film  105  is formed as thick as about 2 to 3 μm to double as a planarizing film in the display area  50 . The organic passivation film  105  is removed where the third electrode  10  is present, so that a concave portion  30  is formed over the third electrode  10 . Meanwhile, the electrodes other than the third electrode  10  are covered by the organic passivation film  105 . As a result, the third electrode  10  is located closer to the liquid crystal and thus tends to affect the liquid crystal more strongly than the other electrodes. The organic passivation film  105 , generally constituted by a photosensitive resin such as acrylic resin or silicone resin, can be patterned without the use of a photoresist. The concave portion  30  may be formed at the same time as the through-holes for feeding the video signal to the pixel electrodes in the display area. That means the formation of the concave portion  30  does not increase process load. 
         [0041]    A pixel area is partially shown to the left in  FIG. 2 . In  FIG. 2 , the first electrode  106  is formed over the organic passivation film  105 . The capacitance insulating film  107  constituted by SiN is formed over the first electrode  106 . The second electrode  108  is formed over the capacitance insulating film  107 . The video signal impressed between the first and the second electrodes causes electric lines of force to extend in liquid crystal  300 , rotating liquid crystal molecules to control the transmittance of the liquid crystal. The second electrode  108  is covered by an alignment film. The insulating film  107  insulating the first electrode  106  from the second electrode  108  is called the capacitance insulating film because the insulating film  107  constitutes a medium that forms a holding capacity between the first electrode  106  and the second electrode  108 . In the display area  50 , the scanning lines are formed over the gate insulating film  103 , and the video signal lines are formed over the interlayer insulating film  104 . 
         [0042]    Returning to the outside of the display area in  FIG. 2 , the capacitance insulating film  107  is formed to cover the organic passivation film  105 . An alignment film  109  is formed over the capacitance insulating film  107 . Because the third electrode  10  strongly affects the inside of the concave portion  30  formed in the organic passivation film  105 , the ions aggregating outside the display area are attracted into the concave portion  30 . Also, the concave portion  30  serves as a container to accommodate positive ions. The concave portion  30  thus prevents the ions aggregating outside the display area from moving back into the display area. Outside the display area, the alignment film  109  is disposed over the capacitance insulating film  107 . The alignment film  109  is intended to define the direction of the initial alignment of liquid crystal molecules in the display area  50 . 
         [0043]    In  FIG. 2 , the TFT substrate  100  and the counter substrate  200  are bonded opposite to each other with the seal material  150  interposed therebetween. A black matrix  201  is formed over the counter substrate  200 . Color filters  202  are formed on the side of the display area  50 . The black matrix  201  in the da splay area  50  is interposed between the color filters  202  to enhance the contrast of the screen. In the seal portion of  FIG. 2 , the black matrix  201  serves as a light shielding film. An overcoat film  203  is formed to cover the black matrix  201  and the color filters  202 . The alignment film  109  is formed over the overcoat film  203 . 
         [0044]    In  FIG. 2 , the third electrode  10  may be formed at the same time as, and with the same layer of, the scanning lines in the display area  50 . A negative voltage impressed to the third electrode  10  traps positive ions outside the display area and keeps the trapped positive ions where they are. The negative voltage to be impressed here may be the same as the potential impressed to the scanning lines. That is, the scanning lines are impressed with a large positive potential only when turned on; they are impressed with a voltage (negative) smaller than the common voltage while being turned off. Consequently, when the same potential as that of the scanning lines is in use, the third electrode  10  is impressed with the negative voltage during periods other than when the scanning lines are turned on, i.e., the third electrode  10  is fed with the negative voltage almost all the time. Generally, the negative potential ranges from −5 V to −7 V. 
         [0045]    Outside the display area, there is also provided the scanning line driver circuit made of shift registers supplied with a low voltage. The low voltage used for the shift register may be impressed to the third electrode  10 . In any case, the third electrode  10  is fed with a voltage lower on average than the common voltage. The structure shown in  FIG. 2  traps positive ions near the third electrode  10  and keeps the trapped ions inside the concave portion  30  or in its vicinity in the organic passivation film  105 . This forestalls the phenomenon of positive ions moving into the display area. 
         [0046]      FIG. 3  is a cross-sectional view showing a variation of the first embodiment. What makes the structure of  FIG. 3  different from that of  FIG. 2  is that the third electrode  10  is formed with the same layer as that of the video signal lines. The voltage impressed to the third electrode  10  is the gate voltage, or the low voltage fed to the shift registers constituting the scanning line driver circuit, as described above is reference to  FIG. 2 . In  FIG. 3 , the third electrode  10  may be impressed with the gate voltage when connected to the scanning line leader lines, for example, via through-holes formed in the interlayer insulating film  104 . 
         [0047]    In the structure of  FIG. 3 , the third electrode  10  is topped by only the capacitance insulating film  107  and the alignment film  109 . This allows the third electrode  10  to be still closer to the liquid crystal  300  than in the structure of  FIG. 2 . That means the third electrode  10  affects positive ions still more strongly. Positive ions are trapped and retained in the concave portion  30  in the organic passivation film  105  in the same manner described above in reference to  FIG. 2 . 
         [0048]      FIG. 4  is a cross-sectional view showing another variation of the first embodiment. What makes the structure of  FIG. 4  different from that of  FIG. 2 or 3  is that only the alignment film  109  is interposed between the third electrode  10  and the liquid crystal  300 . This structure allows the third electrode  10  to exert its influence still more strongly. 
         [0049]    In.  FIG. 4 , as in  FIG. 3 , the third electrode  10  is formed with the same layer as that of the video signal lines. However, in  FIG. 4 , the capacitance insulating film  107  is removed from the concave portion  30  in the organic passivation film  105 . This allows the third electrode  10  to be still closer to the liquid crystal  300 . It is preferred that the capacitance insulating film  107  cover the side wails of the concave portion  30  in the organic passivation film  105 . This structure prevents moisture and other impurities from seeping to the liquid crystal side through the organic passivation film  105 . 
         [0050]      FIG. 5  is a cross-sectional view showing still another variation of the first embodiment. What makes the structure of  FIG. 5  different from that of  FIG. 2, 3 or 4  is that the organic passivation film  105  is removed from where the seal material  150  is formed. The seal material  150  in the seal portion can be made thicker by as much as the mass of the missing organic passivation film  105 . This structure contributes in improving the reliability of the seal material being bonded. The structure also prevents moisture and other impurities from seeping in from the outside through the organic passivation film  105 . 
         [0051]    In  FIG. 5 , as in  FIG. 3 , the third electrode  10  is formed with the same layer as that of the video signal lines. Positive ions are trapped in the concave portion  30  of the organic passivation film  105 . In the structure of  FIG. 5 , positive ions are also trapped in the concave portion  30  formed between the organic passivation film  105  and the seal material  150 . 
         [0052]    In  FIG. 5 , as in  FIG. 2 , the third electrode  10  is topped by the capacitance insulating film  107  and the alignment film  109 . Alternatively, as in  FIG. 2 , the third electrode  10  may be topped by the interlayer insulating film  104 , capacitance insulating film  107 , and alignment film  109 . In another alternative, as in  FIG. 4 , the third electrode  10  may be topped by only the alignment film  109 . The effect of trapping ions using the structure in  FIG. 5  is the same as described above in reference to the structures in  FIGS. 2, 3 and 4 . 
       Second Embodiment 
       [0053]      FIG. 6  is a cross-sectional view showing a second embodiment of the present disclosure. The cross-sectional view in  FIG. 6  is also taken on line A-A in  FIG. 1 . What is shown in  FIG. 6  is a cross section structure corresponding to that of the first embodiment in  FIG. 3 . That is, the third electrode  10  is formed with the same layer as that of the video signal lines. What makes the structure of  FIG. 6  different from that of  FIG. 3  is that a fourth electrode  40  is formed between the third electrode  10  and the display area  50 . The fourth electrode  40  may be formed using the same transparent conductive film as that of the second electrode  108 . 
         [0054]    A negative voltage impressed to the third electrode  10  causes positive ions in the liquid crystal to be attracted toward the third electrode  10 . This protects the display area  50  from being adversely affected by positive ions. However, especially when the frame area is appreciably narrowed, the third electrode  10  located where it is may let its potential affect the display area  50 . That in turn may affect the behavior of the liquid crystal, disturbing the formation of an image in the screen periphery. 
         [0055]    With the second embodiment, as shown in  FIG. 6 , the fourth electrode  10  interposed between the third electrode  10  and the display area  50  prevents a trapping voltage impressed to the third electrode  10  from reaching the display area  50 . The structure in  FIG. 6  is substantially the same as the structure in  FIG. 3  except that the fourth electrode  40  is formed between the display area  50  and the third electrode  10 . That is, the ions migrating from the display area  50  are captured by the third electrode  10  and stay in the concave portion  30  or in its vicinity in the organic passivation film  105 . Meanwhile, the effect of the third electrode  10  on the display area  50  is minimized by the presence of the fourth electrode  40 . 
         [0056]    The fourth electrode  40  in  FIG. 6  is formed with the same transparent conductive film and in the same layer as that of the second electrode  108  in the display area  50 . Alternatively, the fourth electrode  40  may be formed with the same transparent conductive film and in the same layer as that of the first electrode  106 . Whereas the structure in  FIG. 6  corresponds to that in  FIG. 3 , the fourth electrode  40  may also be formed in the structure of  FIG. 2, 4 , or  5  in the same manner as in the structure of  FIG. 6 . The fourth electrode  40  thus disposed minimizes the effect of the third electrode  10  of the display area  50 . 
       Third Embodiment 
       [0057]    A third embodiment of the present disclosure features an area that has a trap structure formed including the third electrode  10  as viewed in a plan view of the liquid crystal display panel. The structure in  FIG. 1  of the first embodiment has a trap structure formed including the third electrode  10  surrounding the entire display area  50 . However, ions aggregate not necessarily along the entire periphery but often in a particular region of the frame area. The third electrode  10  is impressed with the gate voltage or a low voltage for the shift registers constituting the scanning line driver circuit. This electrode arrangement is sometimes difficult to form depending on where it is to be located. 
         [0058]      FIG. 7  shows an example in which the difficulty above is bypassed by forming at intervals a trap structure that includes the third electrode  10 .  FIG. 8  shows an example in which, where ions tend to aggregate in the corners of the liquid crystal display panel, the trap structure  10  is formed only in the corners. As shown in  FIGS. 10 to 12 , ions often aggregate in particular corners. In such a case, as shown in  FIG. 9 , the trap structure including the third electrode  10  need only be formed in specific corners.  FIG. 9  shows an example that has the trap structure formed in two corners opposite to the terminal area  160 , the trap structure including the third electrode  10 . Although the example of  FIG. 9  has the trap structure formed in two corners, only one corner may need to be furnished with the trap structure in some cases. 
         [0059]    The foregoing description was based on the assumption that the IPS liquid crystal display device is in use. However, the present disclosure is not limited to IPS devices but is also applicable to liquid crystal display devices operating on the twisted nematic (TN) method or the vertical alignment (VA) method, for example.