Patent Publication Number: US-8525766-B2

Title: Method of driving liquid crystal display device using alternating current voltages as storage capacitor voltage

Description:
This application claims the benefit of Korean Patent Application No. 10-2007-0020483, filed on Feb. 28, 2007, which is hereby incorporated by reference for all purposes as if fully set forth herein. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a liquid crystal display device, and more particularly, to a method of driving a liquid crystal display device. 
     2. Discussion of the Related Art 
     Liquid crystal display (LCD) devices are driven based on optical anisotropy and polarization characteristics of a liquid crystal material. Liquid crystal molecules have a long and thin shape, and the liquid crystal molecules are regularly arranged along in an alignment direction. Light passes through the LCD device along the long and thin shape of the liquid crystal molecules. The alignment of the liquid crystal molecules depends on the intensity or the direction of an electric field applied to the liquid crystal molecules. By controlling the intensity or the direction of the electric field, the alignment of the liquid crystal molecules is controlled to display images. 
     A related art LCD device and a driving method of the same will be described with reference to the accompanying drawings. 
       FIG. 1  is an equivalent circuit diagram of a related art LCD device. 
     In  FIG. 1 , the related art LCD device includes gate lines G 1  to Gn, data lines D 1  to Dn, switching elements T, liquid crystal capacitors C LC  and storage capacitors Cst. The gate lines G 1  to Gn and the data lines D 1  to Dn cross each other to define pixel regions P. The switching element T, the liquid crystal capacitor C LC  and the storage capacitor Cst are disposed at each pixel region P. A capacitance of the liquid crystal capacitor C LC  is defined by a potential difference between a pixel voltage and a common voltage applied to liquid crystal. 
     In the LCD device of  FIG. 1 , scanning signals are sequentially applied to the gate lines G 1  to Gn with time intervals, and the switching elements T connected thereto turn on. According to this, data signals from the data lines D 1  to Dn are input to pixels through the switching elements. 
     More particularly, the scanning signals are sequentially applied to a first gate line G 1  to an nth gate line Gn. When the scanning signal is applied to the first gate line G 1 , switching elements T, gate electrodes of which are connected thereto, turn on. At this time, selected data signals flow through the data lines D 1  to Dn, and selected pixels become on states. 
     Here, the scanning signals are applied for a short time. To maintain charged amounts of the liquid crystal capacitors C LC  until next scanning signals are applied, capacitances of the storage capacitors Cst are used. 
     If voltages having the same polarities are continuously applied to liquid crystal capacitors C LC , the liquid crystal of the liquid crystal capacitors C LC  may be degraded to cause flickering or dimming of an image. According, to prevent the degradation of the liquid crystal and improve qualities of the image, the LCD device is driven by inversion driving methods, in which polarities of the liquid crystal capacitors C LC  are regularly inversed. 
     The inversion driving methods include a frame inversion driving method, in which the polarities of the liquid crystal capacitors C LC  are inversed every frame, a column inversion driving method, in which the polarities of the liquid crystal capacitors C LC  are inversed every vertical line, a line inversion driving method, in which the polarities of the liquid crystal capacitors C LC  are inversed every horizontal line, a dot inversion driving method, in which the polarities of the liquid crystal capacitors C LC  are inversed every pixel region P, and so on. 
       FIG. 2  is a view of illustrating signals for explaining operation of an LCD device of  FIG. 1  and shows a pixel voltage Vp and a common voltage Vcom. The LCD device may be driven by a dot inversion driving method. 
     In  FIG. 2 , the pixel voltage Vp and the common voltage Vcom are applied to the liquid crystal capacitor C LC  of  FIG. 1 . The common voltage Vcom is a direct current (DC) voltage. The pixel voltage Vp is an alternating current (AC) voltage having positive and negative polarities alternately with respect to the common voltage Vcom. 
     In the dot inversion driving method, voltages having opposite polarities are applied to respective pixels adjacent to each other along horizontal and vertical directions. Further, the polarities are changed every frame. Accordingly, flickers are offset in the pixels adjacent to each other along the horizontal and vertical directions, the degradation of the liquid crystal can be prevented. 
     A structure of an array substrate for an LCD device according to the related art will be described hereinafter with reference to accompanying  FIG. 3 . 
       FIG. 3  is a cross-sectional view of schematically illustrating an array substrate for a twisted nematic (TN) LCD device according to the related art, which is driven with a normally white mode. 
     As shown in  FIG. 3 , the LCD device according to the related art includes a lower substrate  22  and an upper substrate  50 , with a liquid crystal layer  14  is interposed between the lower substrate  22  and the upper substrate  50 . Thin film transistors T, pixel electrodes  46 , gate lines  13  and data lines  42  are formed on the lower substrate  22 . A black matrix  52 , red, green and blue color filters  54   a ,  54   b  and  54   c  and a common electrode  56  are formed on the upper substrate  50 . The lower substrate  22  including the thin film transistors T, the pixel electrodes  46 , the gate lines  13  and the data lines  42  may be referred to as an array substrate. The upper substrate  50  including the black matrix  52 , the color filters  54   a ,  54   b  and  54   c , and the common electrode  56  may be referred to as a color filter substrate. 
     The gate lines  13  and the data lines  42  cross each other to define pixel regions P. The thin film transistors T are disposed near respective crossings of the gate and data lines  13  and  42  and are arranged in a matrix. 
     Each pixel electrode  46  is disposed at each pixel region P and is formed of a transparent conductive material such as indium tin oxide (ITO) that has relatively high transmittance of light. The pixel electrodes  46  are connected to the thin film transistors T, respectively. The pixel electrodes  46  are also arranged in a matrix. 
     Each thin film transistor T includes a gate electrode  30 , an active layer  34 , and source and drain electrodes  36  and  38 . The gate electrode  30  is connected to the gate line  13  and is supplied with pulse signals from the gate line  13 . The source electrode  36  is connected to the data line  42  and is supplied with data signals from the data line  42 . The data signals are provided to the pixel electrode  46  through the drain electrode  38  that is spaced apart from the source electrode  36  and that is connected to the pixel electrode  46 . The active layer  34  is disposed between the gate electrode  30  and the source and drain electrodes  36  and  38 . 
     In a TN LCD device, when voltages are not applied, liquid crystal molecules of the liquid crystal layer  14  are initially twisted with 90 degrees. 
     That is, the liquid crystal molecules adjacent to the upper substrate  50  have an angle of 90 degrees with respect to the liquid crystal molecules adjacent to the lower substrate  22 , and the liquid crystal molecules therebetween are arranged with gradually decreasing changed. 
     First and second polarizers  62  and  64  are disposed at outer surfaces of the upper substrate  50  and the lower substrate  20 , respectively. The first polarizer  62  has a light transmission axis perpendicular to a light transmission axis of the second polarizer  64 . The light transmission axes of the first and second polarizers  62  and  64  are parallel to the liquid crystal molecules adjacent to the upper substrate  50  and the lower substrate  20 , respectively. 
     In an off state when voltages are not applied, light from a backlight (not shown) passes through the second polarizer  64  and becomes linearly polarized light. The linearly polarized light is twisted with 90 degrees while passing through the liquid crystal layer  14  and transmits the first polarizer  62  to display white. 
     On the other hand, in an on state when voltages are applied, the liquid crystal molecules of the liquid crystal layer  14  are arranged perpendicularly to the upper and lower substrates  50  and  22 . 
     Accordingly, light from the backlight passes the second polarizer  64  and the liquid crystal layer  14 , but the light is blocked or absorbed by the first polarizer  62 , the light transmission axis of which is perpendicular to that of the second polarizer  64 , to thereby display black. 
     Meanwhile, in the LCD device of  FIG. 3 , an end portion of the pixel electrode  46  extends over the gate line  13 , which is previously disposed, and the storage capacitor Cst includes the gate line  13  as a first electrode and the pixel electrode  46  overlapping the gate line  13  as a second electrode. At this time, it is importance to make the storage capacitor Cst have a enough capacitance. 
     However, in the LCD device, since the gate line  13  is used an electrode of the storage capacitor Cst, there may be signal delay of the gate line  13 , and this lowers operation of the LCD device. 
     To solve the problem, another structure of an array substrate for an LCD device has been proposed, which further includes a storage line as the first electrode of the storage capacitor. 
       FIG. 4  is a plan view of an array substrate for an LCD device according to the related art. 
     In  FIG. 4 , gate lines  74  are formed on a substrate  70  along a first direction, and data lines  86  are formed along a second direction. The gate lines  74  and the data lines  86  cross each other to define pixel regions P. 
     A thin film transistor T is formed near by each crossing point of the gate and data lines  74  and  86 . The thin film transistor T includes a gate electrode  72 , an active layer  80 , a source electrode  82  and a drain electrode  84 . The gate electrode  72  is connected to the gate line  74  and receives scanning signals from the gate line  74 . The active layer  80  is formed over the gate electrode  72 . The source electrode  82  is connected to the data line  86  and receives image signals from the data line  86 . The drain electrode  84  is spaced apart from the source electrode  82 . 
     A common line is further formed. The common line includes a first portion  76   a , a second portion  76   b , a third portion  76   c , a fourth portion  76   d , and a fifth portion  76   e  corresponding to each pixel region P. The first portion  76   a  and the second portion  76   b  are parallel to the data line  86  and positioned at both sides of the data line  86 , respectively, such that the data line  86  is disposed between the first and second portions  76   a  and  76   b . The third portion  76   c  and the fourth portion  76   d  are parallel to the gate line  74  and cross the data line  86  in upper and lower areas of the pixel region P, respectively. The third and fourth portions  76   c  and  76   d  connect the first portion  76   a  and the second portion  76   b . The fifth portion  76   e  connects the second portion  76   b  and another first portion  76   a , i.e., a first portion of a next pixel region, across the pixel region P. The fifth portion  76   e  may be disposed near by the thin film transistor T. Therefore, the first portion  76   a , the second portion  76   b  and the fifth portion  76   e  have one-united shape at each pixel region P. 
     A pixel electrode  88  is formed at each pixel region P and is connected to the drain electrode  84 . The pixel electrode  88  overlaps the fifth portion  76   e  of the common line. The overlapped fifth portion  76   e  functions as a first electrode and the overlapped pixel electrode  88  functions as a second electrode to thereby form a storage capacitor. The pixel electrode  88  may partially overlap the first and second portions  76   a  and  76   b.    
       FIG. 5  is a view of illustrating signals for explaining operation of an LCD device of  FIG. 4  and shows a pixel voltage Vp and a common voltage Vcom. 
     In  FIG. 5 , the pixel voltage Vp is applied to the pixel electrode  88 , and the common voltage Vcom is applied to a common electrode (not shown), which is formed on a substrate opposite to the array substrate of  FIG. 4 . A storage capacitor voltage Vstg, which is applied to the common line  76   a ,  76   b ,  76   c ,  76   d  and  76   e  of  FIG. 4 , has the same value as the common voltage Vcom. 
     The thin film transistor T of  FIG. 4  turns on by a scanning signal applied to the gate electrode  72  of  FIG. 4 , and the pixel voltage Vp is applied to the pixel electrode  88  of  FIG. 4  through the thin film transistor T from the data line  86  of  FIG. 4 . The pixel voltage Vp alternates with respect to the common voltage Vcom. 
     By the way, in manufacturing the LCD device, there may be problems that the common line  76   a ,  76   b ,  76   c ,  76   d  and  76   e  and the pixel electrode  88  may short-circuit and particles may exist on a surface of a channel of the thin film transistor T. When a normally white mode LCD device displays black, pixels having the problems are shown white. Accordingly, these problems cause bright defects on a black image. 
     More detail explanation will be followed with reference to accompanying  FIG. 6 . 
       FIG. 6  is a cross-sectional view of an LCD device according to the related art and corresponds to the line VI-VI of  FIG. 4 . 
     In  FIG. 6 , the LCD device according to the related art includes a lower substrate  70  and an upper substrate  90 , with a liquid crystal layer  98  is interposed between the lower substrate  70  and the upper substrate  90 . Thin film transistors (not shown), pixel electrodes  88 , gate lines (not shown), and data lines  86  are formed on the lower substrate  70 . A black matrix  92 , red, green and blue color filters  94   a ,  94   b  and  94   c  and a common electrode  96  are formed on the upper substrate  90 . 
     As stated before, a common line is further formed on the lower substrate  70 . The common line includes a first portion  76   a , a second portion  76   b , a third portion  76   c  of  FIG. 4 , a fourth portion  76   d  of  FIG. 4 , and a fifth portion  76   e  of  FIG. 4  corresponding to each pixel region P. The pixel electrode  88  overlaps the fifth portion  76   e  of  FIG. 4  to form a storage capacitor. The pixel electrode  88  also overlaps the first and second portions  76   a  and  76   b.    
     By the way, during a fabrication process, the pixel electrode  88  may short-circuit with the second portion  76   b  of the common line as shown in an area F of  FIG. 6 . Although shown in the figure, the pixel electrode  88  may short-circuit with the first portion  76   a  of the common line. 
     At this time, since the pixel electrode  88  is influenced by a storage capacitor voltage of the common line, the same voltage as the common electrode  96  is applied to the pixel electrode  88  to thereby transmit light. Accordingly, there exist bright defects on a black image when voltages are applied. 
     In addition, although not shown in the figure, there may be particles on a surface of a channel of the thin film transistor. At this time, the thin film transistor including particles should be separated, and the pixel corresponding to the thin film transistor results in a bright defect on the black image. 
     Recently, zero defects have been highly required, and it is essential to zero bright defects in the LCD device. 
     By the way, as mentioned above, since the TN LCD device is driven with the normally white mode, it is difficult to minimize the bright defects. Furthermore, low cell gap has been demanded due to needs of fast response, and the short circuit between electrodes causes loss of productivity. 
     SUMMARY OF THE INVENTION 
     Accordingly, embodiments of the present invention are directed to a method of driving a liquid crystal display device that substantially obviates one or more problem due to limitations and disadvantages of the related art. 
     An advantage of embodiments of the invention is to provide a method of driving a liquid crystal display device that solves bright defects on a black image. 
     Another advantage is to provide a method of driving a liquid crystal display device that improves image qualities and productivity. 
     Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings. 
     To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, a method of driving a liquid crystal display device, which includes first and second substrates, gate lines on the first substrate, data lines crossing the gate lines to define pixel regions, a thin film transistor connected to each gate line and each data line, a common line between adjacent gate lines, a pixel electrode in each pixel region and overlapping the common line, and a common electrode on the second substrate, includes steps of sequentially applying scanning signals to the gate lines, applying data signals to the data lines to supply the pixel electrode with pixel voltage, applying a common voltage to the common electrode, and applying a storage capacitor voltage to the common line, wherein the pixel voltage and the storage capacitor voltage are alternating current (AC) voltages having positive and negative polarities alternately with respect to the common voltage. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of embodiments of the invention as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate an embodiment of the invention and together with the description serve to explain the principles of the invention. 
       In the drawings: 
         FIG. 1  is an equivalent circuit diagram of a related art LCD device; 
         FIG. 2  is a view of illustrating signals for explaining operation of an LCD device of  FIG. 1 ; 
         FIG. 3  is a cross-sectional view of schematically illustrating an array substrate for a twisted nematic (TN) LCD device according to the related art, which is driven with a normally white mode; 
         FIG. 4  is a plan view of an array substrate for an LCD device according to the related art; 
         FIG. 5  is a view of illustrating signals for explaining operation of an LCD device of  FIG. 4 ; 
         FIG. 6  is a cross-sectional view of an LCD device according to the related art and corresponds to the line VI-VI of  FIG. 4 ; 
         FIG. 7  is a plan view of an array substrate for an LCD device according to the present invention; and 
         FIGS. 8A to 8C  are views of illustrating signals for explaining operation of an LCD device of  FIG. 7 . 
     
    
    
     DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS 
     Reference will now be made in detail to an embodiment of the present invention, an example of which is illustrated in the accompanying drawings. 
     In a normally white mode TN LCD device of the present invention, an alternating current (AC) voltage is applied to a common line, which is formed on an array substrate. Accordingly, a pixel having a pixel electrode short-circuited with a common line becomes a dart defect. 
       FIG. 7  is a plan view of an array substrate for an LCD device according to the present invention. 
     In  FIG. 7 , gate lines  104  are formed on a substrate  100  along a first direction, and data lines  116  are formed along a second direction. The gate lines  104  and the data lines  116  cross each other to define pixel regions P. 
     A thin film transistor T is formed near by each crossing point of the gate and data lines  104  and  116 . The thin film transistor T includes a gate electrode  102 , an active layer  110 , ohmic contact layers (not shown), a source electrode  112  and a drain electrode  114 . The gate electrode  102  is connected to the gate line  104  and receives scanning signals from the gate line  104 . The active layer  110  and the ohmic contact layers overlap the gate electrode  102 . The source electrode  112  and the drain electrode  114  are formed over the ohmic contact layers. The source electrode  112  is connected to the data line  116  and receives image signals from the data line  116 . The drain electrode  114  is spaced apart from the source electrode  112 . 
     A common line is further formed between adjacent gate lines  104 . The common line includes a first portion  106   a , a second portion  106   b , a third portion  106   c , a fourth portion  106   d , and a fifth portion  106   e  corresponding to each pixel region P. The first portion  106   a  and the second portion  106   b  are parallel to the data line  116  and positioned at both sides of the data line  116  such that the data line  116  is disposed between the first and second portions  106   a  and  106   b . The third portion  106   c  and the fourth portion  106   d  are parallel to the gate line  104  and cross the data line  116  in upper and lower areas of the pixel region P in the context of the figure, respectively. The third and fourth portions  106   c  and  106   d  connect the first portion  106   a  and the second portion  106   b . The fifth portion  106   e  crosses the pixel region P along the first direction and connects the second portion  106   b  and another first portion  106   a , i.e., a first portion of a next pixel region P. The fifth portion  106   e  may be disposed near by the thin film transistor T. 
     A pixel electrode  122  is formed at each pixel region P. The pixel electrode  122  is connected to the drain electrode  114 . The pixel electrode  122  overlaps the fifth portion  106   e  of the common line. 
     Operation of an LCD device including the array substrate will be explained with reference to accompanying  FIGS. 8A to 8C . 
       FIGS. 8A to 8C  are views of illustrating signals for explaining operation of an LCD device of  FIG. 7  and show a pixel voltage Vp, a common voltage Vcom and a storage capacitor voltage Vstg. The LCD device may be driven with a normally white mode. 
     More particularly, when a scanning signal is applied to the gate line  104 , the thin film transistor T connected thereto turns on. An image signal, that is, the pixel voltage Vp is applied to the pixel electrode  122  through the thin film transistor T from the data line  116 . 
     The pixel voltage Vp is an AC voltage changing from a positive polarity to a negative polarity or from a negative polarity to a positive polarity when a frame is changed. The LCD device may be driven by a dot inversion, column inversion, line inversion or frame inversion driving method. 
     At this time, a common voltage Vcom is applied to a common electrode (not shown), which is formed on a substrate opposite to the array substrate, and the storage capacitor voltage Vstg is applied to the common line  106   a ,  106   b ,  106   c ,  106   d  and  106   e  of  FIG. 7 . 
     The storage capacitor voltage Vstg is an AC voltage and is not the same as the common voltage Vcom. The storage capacitor voltage Vstg is applied by another power source differently from the related art. 
     The storage capacitor voltage Vstg may have the same period and the same polarity as the pixel voltage Vp as shown in  FIG. 8A . The storage capacitor voltage Vstg may have the same period as and an opposite polarity to the pixel voltage Vp as shown in  FIG. 8B . The storage capacitor voltage Vstg may have a different period from the pixel voltage Vp as shown in  FIG. 8C . 
     In the LCD device, when voltages are applied and the LCD device displays a black image, normal pixels without defects accomplish black states by changing an arrangement of liquid crystal molecules by a difference between the pixel voltage Vp and the common voltage Vcom. On the other hand, an abnormal pixel, in which the pixel electrode  122  of  FIG. 3  short-circuit with the common line  106   a ,  106   b ,  106   c ,  106   d  and  106   e  at a point M of  FIG. 7 , for example, attains a black state by changing an arrangement of the liquid crystal molecules by a difference between the storage capacitor voltage Vstg and the common voltage Vcom. 
     At this time, even though the abnormal pixel may have different black color purity from the normal pixel, the abnormal pixel becomes a dark defect not a bright defect on a black image. Therefore, there is no bright defect, and a contrast ratio of the LCD device is improved to achieve high qualities. 
     The above-mentioned driving method, in which an AC voltage is applied to the common line, is advantageous to solving a problem that the pixel electrode and the common line short-circuit. 
     Meanwhile, in a pixel, particles CON may exist on a channel of the thin film transistor T pixel as shown in  FIG. 7 . Or a line corresponding to the pixel region P may short-circuit with the pixel electrode  122 . At this time, the thin film transistor T or a short-circuit portion may be separated from the pixel electrode  122  along the line CL, and the pixel electrode  122  may be welded with and connected to the common line  106   a ,  106   b ,  106   c ,  106   d  and  106   e.    
     Then, in the pixel, liquid crystal molecules (not shown) are arranged by a difference between the common voltage Vcom and the storage capacitor voltage Vstg, and a black state is attained. 
     Like this, in the normally white mode LCD device according to the present invention, when a black image is displayed, abnormal pixels become black states by applying an AC voltage to the common line, and thus bright defects can be overcome. 
     According to this, the LCD device has high qualities. 
     Moreover, since an array substrate having the abnormal pixels is not disused and can be used for the LCD device, the productivity is increased. 
     It will be apparent to those skilled in the art that various modifications and variations can be made in the array substrate for a liquid crystal display device and a method of manufacturing the same of the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.