Abstract:
A transflective liquid crystal display having a plurality of pixels, each pixel having a plurality of color sub-pixels. Each sub-pixel comprises a reflective electrode, a transmissive electrode connected to a secondary reflective electrode. The transmissive electrode is associated with a color filter, while one only of the reflective electrode and the secondary reflective electrode is associated with a color filter. The transmissive electrode is associated with a first charge storage capacitance. The reflective electrode is associated with a second charge storage capacitance which is adjustable depending on the operating states of the liquid crystal display.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   The present application is related to U.S. patent application Ser. No. 11/146,568, filed Jun. 7, 2005, assigned to the assignee of the present invention. 
   FIELD OF THE INVENTION 
   The present invention relates generally to a liquid crystal display panel and, more particularly, to a transflective-type liquid crystal display panel. 
   BACKGROUND OF THE INVENTION 
   Due to the characteristics of thin profile and low power consumption, liquid crystal displays (LCDs) are widely used in electronic products, such as portable personal computers, digital cameras, projectors, and the like. Generally, LCD panels are classified into transmissive, reflective, and transflective types. A transmissive LCD panel uses a back-light module as its light source. A reflective LCD panel uses ambient light as its light source. A transflective LCD panel makes use of both the back-light source and ambient light. 
   As known in the art, a color LCD panel  1  has a two-dimensional array of pixels  10 , as shown in  FIG. 1 . Each of the pixels comprises a plurality of sub-pixels, usually in three primary colors of red (R), green (G) and blue (B). These RGB color components can be achieved by using respective color filters.  FIG. 2  illustrates a plan view of the pixel structure in a conventional transflective liquid crystal panel, and  FIGS. 3   a  and  3   b  are cross sectional views of the pixel structure. As shown in  FIG. 2 , a pixel can be divided into three sub-pixels  12 R,  12 G and  12 B and each sub-pixel can be divided into a transmission area (TA) and a reflection area (RA). In the transmission area as shown in  FIG. 3   a , light from a back-light source enters the pixel area through a lower substrate  30 , and goes through a liquid crystal layer, a color filter R and the upper substrate  20 . In the reflection area, light encountering the reflection area goes through an upper substrate  20 , the color filter R and the liquid crystal layer before it is reflected by a reflective layer  52 . Alternatively, part of the reflective area is covered by a non-color filter (NCF), as shown in  FIG. 3   b.    
   As known in the art, there are many more layers in each pixel for controlling the optical behavior of the liquid crystal layer. These layers may include a device layer  50  and one or two electrode layers. The device layer is typically disposed on the lower substrate and comprises gate lines  31 ,  32 , data lines  21 - 24  ( FIG. 2 ), transistors, and passivation layers (not shown). 
   In a single-gap transflective LCD, one of the major disadvantages is that, the transmissivity of the transmission area (the V-T curve) and the reflectivity in the reflection area (the V-R curve) do not reach their peak values in the same voltage range. As shown in  FIG. 3   c , the V-R curve is peaked at about 2.8v, while the “flat” section of the V-T curve is between 3.7 - 5v. The reflectivity experiences an inversion while the transmissivity is approaching its higher value. 
   SUMMARY OF THE INVENTION 
   The present invention provides a method and a pixel structure to improve the viewing quality of a transflective-type liquid crystal display. The pixel structure of a pixel in the liquid crystal display comprises a plurality of sub-pixel segments. Each of the sub-pixel segments comprises a transmission area and a reflection area. In the sub-pixel segments, a data line, a first gate line, a second gate line and a common line are used to control the operational voltage on the liquid crystal layer areas associated with the sub-segments. The transmission area has a transmissive electrode associated with a first charge storage capacity and the reflection area has a reflective electrode associated with a second storage capacity. The first and second gate lines can be separately set at a first control state and a second control state. The ratio of the first charge storage capacitor to the second charge storage capacity can be adjusted by an adjustment storage capacitor and controlled according to the states of the gate lines. By adjusting and controlling the adjustment storage capacitor, the potential on the reflective electrode is reduced so as to shift the reflectivity curve toward the higher voltage end. With such a charge refreshing approach, the transmissivity and reflectivity of a single-gap LCD can reach their optimal values at about the same applied voltage. However, the shifting of the reflectivity curve causes a major discrepancy between the transmissivity and reflectivity in the low brightness region and this discrepancy significantly affects the color and contrast of displayed image. 
   In order to improve the viewing quality of the display in the low brightness region, the transmissive electrode is connected to a further reflective electrode so as to retain part of the unshifted reflectivity curve. 
   The present invention will become apparent upon reading the description taken in conjunction of  FIGS. 4 to 17   b.    

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic representation showing a typical LCD display. 
       FIG. 2  is a plan view showing the pixel structure of a conventional transflective color LCD display. 
       FIG. 3   a  is a cross sectional view showing the reflection and transmission of light beams in the pixel as shown in  FIG. 2 . 
       FIG. 3   b  is a cross sectional view showing the reflection and transmission of light beams in another prior art transflective display. 
       FIG. 3   c  is a plot of transmissivity (T) and reflectively (R) against applied voltage (V) in a prior art single-gap transflective LCD. 
       FIG. 4  is a plan view illustrating a sub-pixel segment in a liquid crystal display, according to the present invention. 
       FIG. 5   a  is a plan view illustrating the color filtering arrangement in a sub-pixel segment, according to one embodiment of the present invention. 
       FIG. 5   b  is a plan view illustrating the color filtering arrangement in a sub-pixel segment, according to another embodiment of the present invention. 
       FIG. 6   a  is a schematic presentation of a cross sectional view showing the color filtering arrangement of  FIG. 5   a.    
       FIG. 6   b  is a schematic presentation of a cross sectional view showing the color filtering arrangement of  FIG. 5   b.    
       FIG. 7  is a circuit diagram showing the equivalent circuit of the sub-pixel segment of  FIG. 4 . 
       FIG. 8   a  is the equivalent circuit of the transmission area in the sub-pixel segment of  FIG. 4 . 
       FIG. 8   b  is the equivalent circuit of the reflection area in the sub-pixel segment of  FIG. 4 . 
       FIG. 9   a  is the equivalent circuit of the transmission area of  FIG. 8   a  when the gate lines are set at a first control state. 
       FIG. 9   b  is the equivalent circuit of the reflection area of  FIG. 8   b  when the gate lines are set at the first control state. 
       FIG. 9   c  is the equivalent circuit of the adjustment storage capacitor of  FIG. 8   b  when the gate lines are set at the first control state. 
       FIG. 10   a  is the equivalent circuit of the transmission area of  FIG. 8   a  when the gate lines are set at a second control state. 
       FIG. 10   b  is the equivalent circuit of the reflection area of  FIG. 8   b  when the gate lines are set at a second control state. 
       FIG. 11   a  is a plot of transmissivity (T) and reflectively (R) against applied voltage (V) showing the shifting of the R-V curve as a result of the adjustment of charge storage capacity associated with the reflection area. 
       FIG. 11   b  is a plot of transmissivity and reflectivity as a function of gamma level. 
       FIG. 11   c  is a plot of transmissivity and reflectivity against applied voltage showing an approach to multi-threshold harmonization, according to the present invention. 
       FIG. 11   d  is a plot of transmissivity and reflectivity against applied voltage showing as a result of multi-threshold harmonization. 
       FIG. 11   e  is a plot of transmissivity and reflectivity as a function of gamma level, as a result of multi-threshold harmonization. 
       FIG. 12  is a plan view illustrating a sub-pixel segment in a liquid crystal display, according to a different embodiment of the present invention. 
       FIG. 13  is a circuit diagram showing the equivalent circuit of the sub-pixel segment of  FIG. 12 . 
       FIG. 14  is the equivalent circuit of the reflection area in the sub-pixel segment of  FIG. 12 . 
       FIG. 15   a  is the equivalent circuit of the reflection area of  FIG. 14  when the gate lines are set at the first control state. 
       FIG. 15   b  is the equivalent circuit of the control storage capacitor of  FIG. 14  when the gate lines are set at the first control state. 
       FIG. 16  is the equivalent circuit of the reflection area of  FIG. 14  when the gate lines are set at a second control state. 
       FIG. 17   a  is a schematic representation of a cross section view showing the color filtering arrangement in a sub-pixel segment of a double-gap transflective LCD, according to one embodiment of the present invention. 
       FIG. 17   b  is a schematic representation of a cross section view showing the color filtering arrangement in a sub-pixel segment of a double-gap transflective LCD, according to another embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   A sub-pixel segment, according to one embodiment of the present invention, is shown in  FIG. 4 . The sub-pixel  100  has a transmission area (TA) and a reflection area (RA). The reflection area (RA) has a reflector or reflective electrode  180 . The transmission area (TA) in the sub-pixel  100  has a transparent electrode  190 , electrically connected to a secondary reflector  192  in a secondary reflection section (RS). As can be seen in  FIG. 5   a , the sub-pixel has a color filter  152  to filter the light beam encountering the liquid crystal layer in the transmission area and also in the secondary reflection section. The reflection area has a non-color filter  150 . The non-color filter  150  can be made of a clear optical material or a neutral-color filter or a very light color filter. 
   Alternatively, the color filter  152  only covers the transmission area, as shown in  FIG. 5   b . The secondary reflection section does not have a color filter or only has a non-color filter  153 . The reflection area has a color filter  151 . It is understood that the color filter  151  and the color filter  152  can have substantially the same color and same transmissivity. However, the color filters  151 ,  152  can have the same color but different transmissivity—that is, one color filter is lighter than the other. The sub-pixel  100  has a data line  202 , a first gate line  212 , a second gate line  214  and a common line  210 . As shown in  FIG. 4 , the transmission area is associated with a first storage capacitor  232  (C 1 ), while the reflection area is associated with a second storage capacitor  234  (C 2 ) and a charge refreshing capacitor or adjustment storage capacitor  236  (C 3 ). The capacitor  232  is electrically connected to the transparent electrode  190  and the secondary reflector  192  through a via  188 . The capacitor  232  is also electrically connected to the data line  202  and the first gate line  212  through a first semiconductor switching element  240  (TFT- 1 ). The second capacitor  234  is electrically connected to the reflector  180  through a via  184 . The second capacitor  234  is also electrically connected to the data line  202  and the first gate line  212  through the second switching element  249  (TFT- 2 ). The second capacitor  234  is further connected to the adjustment capacitor  236  through a second semiconductor switching element  250  (TFT- 3 ). The adjustment capacitor  236  is electrically connected to the common line  210  through a fourth switching element  260  (TFT- 4 ). The first switching element  240  has a first switch end  241 , a second switch end  243  and a control end  242 . The second switching element  249  has a first switch end  241 , a second switch end  244  and a control end  242 . The first switch end  241  is electrically connected to the data line  202 , and the control end  242  is electrically connected to the first gate line  212 . The third switching element  250  has a first switch end  251 , a second switch end  253  and a control end  252 . The control end  252  is electrically connected to the second gate line  214 . The fourth switching element  260  has a first switch end  261 , a second switch end  263  and a control end  262 . The second switch end  263  is electrically connected to the common line  210  via an electrically conductive segment  284 , and the control end  262  is electrically connected to the first gate line  212 . 
   The color filtering arrangement and the electrical components of the sub-pixel segment  100  are schematically illustrated in  FIGS. 6   a  and  6   b . As can be seen in  FIGS. 6   a  and  6   b , the sub-pixel segment  100  has a pair of polarizers  110 ,  112 , a pair of half-wave plates  120 ,  122  and a pair of quarter-wave plates  130 ,  132 . The upper component layers are disposed on the upper side of the transparent substrate  140 . The lower component layers are disposed on the lower side of the transparent substrate  142 . Disposed on the upper side of the transparent substrate  142  is a device layer  160 . The device layer  160  comprises the storage capacitors  232 ,  234 ,  236  and the switching elements  240 ,  250 ,  260 . The reflectors  180 ,  192  and the transparent electrode  190  are generally electrically insulated from the device layer  160  by a passivation layer  165 , but electrically connected to device layer through vias  184  and  188 . 
   As shown in  FIG. 6   a , a light beam encountering the sub-pixel segment  100  are filtered by the color filter  152  associated with the secondary reflector  192  in the secondary reflection section (RS) and the transparent electrode  190  in the transmission area (TA). The filter  150  associated with the reflector  180  in the reflection area (RA) is a non-color filter as illustrated in  FIG. 5   a . The filters  150  and  152  are disposed between the transparent substrate  140  and a common transparent electrode  170 . 
   As shown in  FIG. 6   b , the non-color filter  153  is associated with the secondary reflector  192  in the secondary reflection section (RS). The color filter  152  is associated with the transparent electrode  190  of the transmission area (TA). The color filter  151  is associated with the reflector  180  in the reflection area (RA). The transparent electrodes  170 , 190  are usually made from indium-tin oxide (ITO). 
   The equivalent circuit for the electronic components in the sub-pixel segment  100  is shown in  FIG. 7 . As shown, the transparent electrode  190  and the secondary reflector  192  together have a capacitance CT connected through the via  188  to the first storage capacitor  232  (C 1 ) in parallel. These capacitors are connected to the data line  214  via the switch ends  243 ,  241  of the first switching element  240 . The reflector  180  has a capacitance CR separately connected to the second storage capacitor  234  (C 2 ) in parallel. These capacitors are also connected through the via  184  to the data line  214  via the switch ends  244 ,  241  of the second switching element  249 . The capacitor  234  is also connected to the adjustment capacitor  236  in parallel via the switch ends  253 ,  251  of the second switching element  250 . The adjustment capacitor  236  is also connected to the common line  210  through the switch ends  261 ,  263  of the fourth switching element  260 . 
   As shown in  FIG. 8   a , the charging and discharging of the capacitors CT and C 1  is controlled by gate-line  1  through the control end  242  (see  FIG. 7 ) of the first switching element  240 . As shown in  FIG. 8   b , the charging and discharging of the capacitors CR, C 2  and C 3  are controlled by gate-line  2  through the control end  252  (see  FIG. 7 ) of second switching element  250 , and by gate-line  1  through both the control end  242  of the second switching element  249  and the control end  262  of the fourth switching element  260 . 
   In the first control state, gate-line  1  is set to high and gate-line  2  is set to low. When gate-line  1 =high, the switching element  240  and the switching element  260  are “ON”. When gate-line  2 =low, the switching element  250  is “OFF”. In this control state, the capacitors CT and C 1  are connected to the data line  202 , as shown in  FIG. 9   a . Thus, the transparent electrode  190  and the reflector  192  have the same potential (V data ) of the data line  202 . The capacitors CR and C 2  are operatively connected to the data line  202 , but disconnected from the adjustment capacitor C 3 , as shown in  FIGS. 9   b  and  9   c . Thus, the reflector  180  has the same potential (V data ) of the data line  202 . The adjustment capacitor C 3  is discharged, but its potential is in equilibrium with the voltage on common line  210 . 
   In the second control state, gate-line  1  is set to low and gate-line  2  is set to high. When gate-line  1 =low, the switching element  240  and the switching element  260  are “OFF”. When gate-line  2 =high, the switching element  250  is “ON”. In this control state, the capacitors CT and C 1  are disconnected from the data line  202 , as shown in  FIG. 10   a . The capacitors CT and C 1  maintain their voltage potential for a period of time. Thus, the transparent electrode  190  and the secondary reflector  192  substantially maintain their original potential V data . The capacitors CR and C 2  are now connected to the adjustment capacitor C 3  in parallel as shown in  FIG. 10   b . The overall capacitance associated with the reflector  180  is increased from (CR+C 2 ) to (CR+C 2 +C 3 ). As a result, the potential on the reflector  180  is reduced. As such, the reflectivity curve is shifted toward the higher voltage end. The shifted-reflectivity curve is shown in  FIG. 11   a . As shown in  FIG. 11   a , the reflectivity curve (R — 0) is peaked at about 2.8 v, whereas the shifted-reflectivity curve (R_m) is peaked at about 4 v. In this illustrative example, C 3 /(CR+C 2 +C 3 )=⅖. With charge refreshing, the transmissivity and reflectivity of a single-gap LCD can be peaked at about the same applied voltage. The inversion in the reflectivity relative to the transmissivity can be avoided. 
   However, while the transmitivity starts to increase rapidly at about 2.2 v, the reflectivity remains low until about 2.8 v. In this low brightness region, the discrepancy in the transmissivity and reflectivity also causes the discrepancy between the gamma curve associated with the transmissivity and the gamma curve associated with the reflectivity, as shown in  FIG. 11   b .  FIG. 11   b  shows the transmissivity and reflectivity as a function of gamma level. In order to reduce the discrepancy between transmissivity and reflectivity, a multi-threshold harmonization (MTH) approach is used. 
   According to the present invention, the discrepancy between transmissity and reflectivity in the low brightness region can be reduced by combining the reflectivity with charge-refreshing and the reflectivity without charge-refreshing. For example, it is possible to combine 80% of the reflectivity with charge refreshing and 20% of the reflectivity without charge refreshing in order to carry out multi-threshold harmonization. 
   As shown in  FIG. 4 , the reflective electrode  192  is electrically connected to the transmissive electrode  190  in the secondary reflection section. Electrically, the reflective electrode  192  is separated from the reflective electrode  180 . Thus, the reflectivity curve associated with the reflective electrode  192  does not shift toward the higher voltage end. As shown in  FIG. 11   c , the reflectivity curve (R−0*20%) associated with the reflective electrode  192  is peaked at the same applied voltage as the reflectivity curve (R — 0) without charge refreshing. The reflectivity curve (R_m*80%) associated with the reflective electrode  180  is peaked about 4 v. The combined reflectivity (R_MTH) of these two reflectivity curves is shown in  FIG. 11   d . As can be seen from  FIG. 11   d , the matching between the transmissivity curve (Gamma_T) and the combined gamma curve (Gamma_R_MTH) in the multi-threshold harmonization is much better than the matching without multi-threshold harmonization ( FIG. 11   b ). As such, the color and brightness quality at the low brightness end is significantly improved. 
   The matching between the transmissivity and reflectivity can be further adjusted by changing the non-charge refreshing reflectivity relative to charge refreshing reflectivity—the area ratio between reflector  192  and reflector  180  and by adding more charge refreshing stages—one or more reflective electrodes connected to different charge refreshing capacitors. 
   In another embodiment of the present invention, the adjustment capacitor  236  is directly connected through the via  185  to the reflector  180  in parallel, and the second storage capacitor  234  is connected to the reflector  180  through the third switching element  250 . The equivalent circuit is shown in  FIG. 13 . The charging and discharging of the capacitor CT and C 1  remains the same as that shown in  FIG. 8   a . The charging and discharging of the capacitors CR, C 2  and C 3  is shown in  FIG. 14 . 
   In the first control state, gate-line  1  is set to high and gate line  2  is set to low. In this control state, CT, C 1  and C 2  are connected to the data line  202  and have the same potential (V data ) of the data line  202  ( FIGS. 9   a  and  15   a ). C 3  and CR are discharged but their potential is in equilibrium with the voltage on the common line  210  ( FIG. 15   b ). 
   In the second control state, gate-line  1  is set to low and gate-line  2  is set to high. The capacitor CT and C 1  are disconnected from the data line  202 , as shown in  Figure 10   a  and their potential maintains the same for a period of time. The capacitors CR and C 2  are now connected to the adjustment capacitor C 3  in parallel as shown in  FIG. 16 . The overall capacitance associated with the reflector  180  is increased from (CR+C 2 ) to (CR+C 2 +C 3 ). As a result, the potential on the reflector  180  is reduced. 
   It is possible to extend the present invention from a single-gap design to a double-gap design, as shown in  FIG. 17   a  and  17   b . As shown, while the arrangement of reflector  192  in the reflection section (RS) and the color filtering arrangement for the upper substrate is the same as those shown in  FIGS. 6   a  and  6   b , the gap between the reflector  180  and the upper electrode  170  is reduced. The electrical connection between the reflector  180  and the device  160  can be the same as that shown in  FIGS. 4 and 12  so as to allow the potential on the reflector  180  to be adjusted by the adjustment capacitor C 3 . 
   In sum, the use of the adjustment capacitor C 3  for shifting the reflectivity curve toward the higher voltage end is referred to as charge refreshing and the adjustment capacitor is referred to as a charge refreshing capacitor. Charge refreshing is used to avoid the reflectivity inversion problem. In order to further improve the viewing quality of a single-gap LCD, a combination of charge-refreshing and non-charge-refreshing is used. In the embodiments as shown in  FIGS. 4 and 12 , only one charge-refreshing stage is used. However, one or more additional charge-refreshing stages can also be implemented. By combining the non-charge-refreshing reflectivity and the charge-refreshing reflectivity, it is possible to reduce the discrepancy between the gamma curve associated with the transmissivity and the gamma curve associated with the charge-refreshing reflectivity. Thus, according to the present invention, at least one reflective electrode is electrically connected to the transmissive electrode in a transflective LCD and at least one reflective electrode is electrically connected to a charge-refreshing capacitor. 
   The present invention provides a method for improving viewing quality of a transflective liquid crystal display. The liquid crystal display is operable in a first state and in a second state for controlling optical behavior of the liquid crystal layer, wherein at least one further reflective electrode is electrically connecting to the transmissive electrode, the further reflective electrode disposed spaced from the reflective electrode in the lower side of the liquid crystal display, allowing a further part of the light entering the sub-pixel from the upper side of the liquid crystal display through the liquid crystal layer to be reflected by the further reflective electrode through the liquid crystal layer back to the upper side, and wherein a first charge capacitance is provided to the transmissive electrode and the further reflective electrode, and a second charge capacitance is provided to the reflective electrode, the second charge capacitance having a relative capacitance value compared to the first charge capacitance, and wherein at least one of the first capacitance and the second capacitance is controlled such that the relative capacitance value when the liquid crystal display is operated in the first state is different from the relative capacitance value when the liquid crystal display is operated in the second state. 
   Effectively, the liquid crystal display is operable in a first voltage mode and in a second voltage mode for controlling optical behavior of the liquid crystal layer, wherein at least one further reflective electrode is electrically connecting to the transmissive electrode, the further reflective electrode disposed spaced from the reflective electrode in the lower side of the liquid crystal, allowing a further part of the light entering the sub-pixel from the upper side of the liquid crystal display through the liquid crystal layer to be reflected by the further reflective electrode through the liquid crystal layer back to the upper side, and wherein the reflective electrode is operated at the first voltage mode, and the transmissive electrode and said at least one further reflective electrode are operated at the second voltage mode. 
   Thus, although the invention has been described with respect to one or more embodiments thereof, it will be understood by those skilled in the art that the foregoing and various other changes, omissions and deviations in the form and detail thereof may be made without departing from the scope of this invention.