Patent Publication Number: US-2012038873-A1

Title: Reflection-type liquid crystal display device

Description:
BACKGROUND 
     1. Field 
     Embodiments relate to a liquid crystal display device, and more particularly, to a reflection-type liquid crystal display device. 
     2. Description of the Related Art 
     Liquid crystal display devices are display devices that display images by adjusting transmittance and reflectance of light by controlling the molecular alignment of liquid crystal. Due to low electric power consumption and relatively low weight of liquid crystal display devices, such liquid crystal display devices may be widely applied to cellular phones, digital cameras, portable information appliances, large size TVs, etc. 
     Liquid crystal display devices are classified into transmission-type liquid crystal display devices and reflection-type liquid crystal display devices according to a light source thereof. Transmission-type liquid crystal display devices realize images by transmitting light from a backlight through a liquid crystal panel. Reflection-type liquid crystal display devices realize images by reflecting external light at a liquid crystal panel. 
     The transmission-type liquid crystal display devices have relatively low efficiency of light utilization since about ⅕ of light from the backlight transmits through the liquid crystal panel. Transmission-type liquid crystal display devices also consume relatively high electric power since over ⅔ of the total electric power is generally consumed by the backlight. 
     Reflection-type liquid crystal display devices use external light rather than a light source. Thus, electric power consumption thereof may be reduced compared to transmission type liquid crystal display devices. In general, a reflection-type liquid crystal display device includes an array substrate including a switching device, a pixel electrode, and a reflective layer, an opposite substrate including a common electrode, a color filter, and a black matrix, and liquid crystal disposed between the array substrate and the opposite substrate. The black matrix of the opposite substrate may prevent light leakage at boundaries of pixels and may improve a reflection contrast ratio. 
     SUMMARY 
     It is a feature of an embodiment to provide a reflection-type liquid crystal display device having high aperture ratio and contrast ratio. 
     It is a separate feature of an embodiment to provide a liquid crystal display device having an improved aperture ratio, e.g., higher aperture ratio, which is a ratio of emission zone per pixel, in order to increase brightness of a screen of the liquid crystal display device and reduce electric power consumption. 
     It is a separate feature of an embodiment to provide a liquid crystal display device having a structure such that misalignment of the black matrix during assembly of the array substrate and the opposite substrate may be reduced so as to enable an improved aperture ratio. 
     At least one of the above and other features and advantages may be realized by providing a liquid crystal display device, including a first substrate, a thin-film transistor on the first substrate, the thin-film transistor including a gate electrode, a gate insulating layer, an active layer, and source and drain electrodes, an organic insulating layer on the thin-film transistor, a first electrode layer on the organic insulating layer, the first electrode layer extending between respective portions of the organic insulating layer to contact the source and drain electrode, a black layer on the first electrode layer and the organic insulating layer, a liquid crystal layer on the black layer, a second electrode layer on the liquid crystal layer, and a second substrate on the second electrode layer. 
     The organic insulating layer may include a transparent organic insulating layer. 
     The organic insulating layer may include a black organic insulating layer. 
     The organic insulating layer may include a surface having a convex lens shape. 
     The display device may further include a passivation layer between the thin-film transistor and the organic insulating layer. 
     The black layer may be arranged over an entire surface of the first substrate. 
     The black layer may only be arranged over a portion of a surface of the first substrate. 
     The black layer may be arranged over a portion of a surface of the first substrate corresponding to boundaries of pixels. 
     The black layer may include an organic insulating material or carbon black. 
     The first electrode layer may be a pixel electrode. 
     The second electrode layer may be a common electrode. 
     The liquid crystal layer may include a cholesteric liquid crystal layer. 
     The liquid crystal layer may include a polymer network liquid crystal (PNLC). 
     Dyes may be dispersed in the PNLC. 
     The display device may further include a color filter on the second substrate. ,    
     The black layer may be arranged on the first substrate before the second substrate is arranged on the first substrate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other features and advantages will become more apparent to those of ordinary skill in the art by describing in detail exemplary embodiments with reference to the attached drawings, in which: 
         FIG. 1  illustrates a cross-sectional view of a pixel of a liquid crystal display device according to an exemplary embodiment; 
         FIG. 2  illustrates a cross-sectional view of a pixel of a liquid crystal display device according to another exemplary embodiment; 
         FIG. 3  illustrates a cross-sectional view of a pixel of a liquid crystal display device according to another exemplary embodiment; 
         FIG. 4  illustrates a cross-sectional view of a pixel of a liquid crystal display device according to another exemplary embodiment; 
         FIG. 5  illustrates a cross-sectional view of a pixel of a liquid crystal display device according to another exemplary embodiment; 
         FIG. 6  illustrates a cross-sectional view of a pixel of a liquid crystal display device according to another exemplary embodiment; 
         FIG. 7  illustrates a cross-sectional view of a pixel of a liquid crystal display device according to another exemplary embodiment; and 
         FIG. 8  illustrates a cross-sectional view of a pixel of a reflection liquid crystal display device according to another exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Korean Patent Application No. 10-2010-0077495, filed on Aug. 11, 2010, in the Korean Intellectual Property Office, and entitled: “Reflection-Type Liquid Crystal Display Device,” is incorporated by reference herein in its entirety. 
     Exemplary embodiments will now be described more fully hereinafter with reference to the accompanying drawings; however, they may be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. 
     In the drawing figures, the dimensions of layers and regions may be exaggerated for clarity of illustration. It will also be understood that when an element is referred to as being “on,” “above”, “below,” or “under” another element, it can be directly “on,” “above”, “below,” or “under” the other element, respectively, or intervening elements may also be present. In addition, it will also be understood that when an element is referred to as being “between” two elements, it can be the only element between the two elements, or one or more intervening elements may also be present. 
     It will be also be understood that although the terms first, second, third etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another element. Thus, a first element in some embodiments could be termed a second element in other embodiments without departing from the teachings of the present invention. Exemplary embodiments of aspects of the present inventive concept explained and illustrated herein include their complementary counterparts. Like reference numerals refer to like elements throughout the specification. 
       FIG. 1  illustrates a cross-sectional view of a pixel  100  of a reflection-type liquid crystal display device including a cholesteric liquid crystal according to an exemplary embodiment. 
     Referring to  FIG. 1 , the pixel  100  may include a first substrate  101 , a thin film transistor (TFT)  110  including a gate electrode  111 , a gate insulating layer  112 , an active layer  121 , and source/drain electrodes  123 , a passivation layer  124 , an organic insulating layer  126 , a first electrode  141 , a black layer  142 , a liquid crystal layer  151 , a second electrode  153 , and a second substrate  161 . 
     The gate electrode  111  may be formed on the first substrate  101 . The first substrate  101  may be a glass substrate, a quartz substrate, or a plastic substrate, etc. The gate electrode  111  may be a single layer or a stacked structure including aluminum (Al), copper (Cu), molybdenum (Mo), titanium (Ti), chromium (Cr), and/or an alloy thereof, etc. 
     The gate insulating layer  112  may be formed on the gate electrode  111 . The active layer  121  may be formed on the gate insulating layer  112 . The gate insulating layer  112  may be a single layer or a stacked structure including silicon oxide (SiOx) and/or silicon nitride (SiNx), etc. The active layer  121  may include semiconductor material, e.g., an amorphous silicon layer doped with impurities. 
     The source/drain electrodes  123  may be formed on the active layer  121 . The source/drain electrodes  123  may be a single layer or a stacked structure including aluminum (Al), copper (Cu), molybdenum (Mo), titanium (Ti), chromium (Cr), and/or an alloy thereof, etc. 
     The gate electrode  111 , the gate insulating layer  112 , the active layer  121 , and the source and drain electrodes  123  may constitute the TFT  110 . The TFT  110  may transfer driving voltages to the liquid crystal. In embodiments, data lines (not shown) may be electrically connected to the source/drain electrodes  123  and/or gate lines (not shown) may be electrically connected to the gate electrode  111 . Such data lines and gate lines may be formed on the first substrate  101 . The data lines (not shown) and the gate lines (not shown) may include a metal layer, e.g., a same material as the gate electrode  111  and/or the source/drain electrodes  123 . In addition to the TFT  110 , circuit devices such as a storage capacitor (not shown) may further be formed on the first substrate  101 . 
     The passivation layer  124  may be formed on the source/drain electrodes  123 . The passivation layer  124  may be a single layer or a stacked structure including a silicon oxide (SiOx) and/or a silicon nitride (SiNx), etc. The passivation layer  124  may protect the TFT  110  from impurities, moisture, etc. In some embodiments, the passivation layer  124  may not be provided. 
     The organic insulating layer  126  may be formed on the passivation layer  124 . The organic insulating layer  126  may include a transparent organic material, e.g., polyimide, polyvinyl alcohol, poly(vinyl phenol-maleimide), and/or a photoacryl material, etc. The organic insulating layer  126  may insulate the source/drain electrodes  123  from a pixel electrode, e.g., the first electrode  141 . In embodiments in which the passivation layer  124  is not provided, the organic insulating layer  126  may be formed on the source/drain electrodes  123  and may function as the passivation layer  124 . In some embodiments, a surface of the organic insulating layer  126  may include a convex lens shape portion (see, e.g., dotted line C) in order to collect externally reflected light. 
     The first electrode  141  may be formed on the organic insulating layer  126 . The first electrode  141  may extend between respective portions of the organic insulating layer  126  and the passivation layer  124 , and may contact one of the source/drain electrodes  123 . The first electrode  141  may be a transparent conductive oxide layer, e.g., indium tin oxide (ITO) and/or indium zinc oxide (IZO), etc. The first electrode  141  may include a non-transparent material, e.g., aluminum (Al), silver (Ag), an Al alloy, an Ag alloy, etc., since the black layer  142  absorbs light that penetrates the liquid crystal layer  151  and prevents light reflection at the first electrode  141 . The first electrode  141  may be a pixel electrode that is independently formed for each pixel. 
     The black layer  142  may be formed on the first electrode  141  and the organic insulating layer  126 . The black layer  142  may extend over a surface, e.g., an entire surface, of the first substrate  101 . The black layer  142  may include an organic material in which a black colorant is dispersed. The black layer  142  may include, e.g., carbon black. In this regard, e.g., the organic material may include polyimide, polyvinyl alcohol, poly(vinyl phenol-maleimide), and/or a photoacryl material, etc. The black layer  142  may function as a light-absorbing layer and a black matrix. 
     If a cholesteric liquid crystal is used, the liquid crystal layer  151  may reflect light having a predetermined wavelength due to Bragg reflection (λ=n×P, n: refractive index, and P: pitch of cholesteric liquid crystal) to realize color. If the liquid crystal layer  151  reflects light with a predetermined wavelength and light penetrating the liquid crystal layer  151  is reflected by a metal layer, e.g., the source/drain electrodes  123  stacked on the first substrate  101 , all visible light may be reflected, and thus color cannot be realized. However, since the light penetrating the liquid crystal layer  151  may be absorbed in the black layer  142 , the absorbed light is not reflected by a metal layer, e.g., the source/drain electrodes  123 , on the first electrode  141 . Thus, e.g., light of predetermined wavelengths may be selectively reflected by the liquid crystal layer  151 . Accordingly, color may be realized without requiring a color filter. In should be understood that in some embodiments a color filter may still be employed. 
     Cholesteric liquid crystal may be significantly influenced by a fringe field by the first electrode  141  patterned on the first substrate  101 . More particularly, when an electric field is applied to the liquid crystal layer  151  by the first electrode  141  and the second electrode  153 , the liquid crystal layer  151  between the first electrode  141  and the second electrode  153  may have a homeotropic texture. However, the liquid crystal layer  151  near boundaries of pixels may not have the homeotropic texture due to the fringe field and may have, e.g., a mixed texture of a planar texture and a focal conic texture. Accordingly, when the electric field is removed, the liquid crystal layer  151  may not have a desired texture and/or may not represent a desired gray scale. 
     In embodiments, the black layer  142  may block light having an undesired gray scale from the fringe field region, e.g., around boundaries of the pixels. By providing the black layer  142  on the first electrode  141 , light leakage as a result of, e.g., the fringe field, may be prevented and/or reduced. In addition, embodiments in which the black layer  142  is formed on the first substrate  101  on which the TFT is formed may enable improved alignment of the first substrate  101  and the second substrate  161  during, e.g., assembly of the first substrate  101  and the second substrate  161 . More particularly, e.g., embodiments may reduce and/or prevent a reduction in aperture reduction as a result of misalignment of the first substrate  101  and the second substrate  161  during assembly. As a result of, e.g., a reduction in light leakage around boundaries of the pixels and/or improved alignment of the first substrate  101  and the second substrate  161 , embodiments may provide improved aperture ratios relative to comparable conventional devices. 
     The liquid crystal layer  151  may be formed on the black layer  142 . The liquid crystal layer  151  may include cholesteric liquid crystal. 
     The second electrode  153  may be formed on the liquid crystal layer  151 . The second electrode  153  may include a transparent conductive oxide layer, e.g., ITO and/or IZO, etc., as the first electrode  141 . The second electrode  153  may be a common electrode that is common to a plurality of pixels. 
     A second substrate  161  may be formed on the second electrode  153 . The second substrate  161  may include a glass substrate, a quartz substrate, and/or a plastic substrate, etc., as the first substrate  101 . 
     An alignment layer (not shown) may be disposed between the black layer  142  and the liquid crystal layer  151  and/or between the liquid crystal layer  151  and the second substrate  161 . 
       FIG. 2  illustrates a cross-sectional view of a pixel  200  of a reflection-type liquid crystal display device including cholesteric liquid crystal according to another exemplary embodiment. In general, only differences between the exemplary pixel  100  of  FIG. 1  and the exemplary pixel  200  of  FIG. 2  will be described below. Referring to  FIG. 2 , the pixel  200  includes a black organic insulating layer  226  as an organic insulating layer. In such embodiments, the black organic insulating layer  226  may be formed on the passivation layer  124 . The black organic insulating layer  226  may include an organic insulating material including a black colorant. In some embodiments, a surface of the black organic insulating layer  226  may include a convex lens shape portion in order to collect light that is externally reflected. 
     The black layer  142  may formed on the first electrode  141  and the black organic insulating layer  226 . In such embodiments, by providing the black organic insulating layer  226  on the first substrate  201  as the organic insulating layer, the black layer  242  and the black organic insulating layer  226  may together function more efficiently as a light-absorbing layer and a black matrix. 
       FIG. 3  illustrates a cross-sectional view of a pixel  300  of a reflection-type liquid crystal display device including a cholesteric liquid crystal according to another exemplary embodiment. In general, only differences between the exemplary pixel  100  of  FIG. 1  and the exemplary pixel  300  of  FIG. 3  will be described below. Referring to  FIG. 3 , the pixel  300  includes a black layer  342 , which formed over a portion, i.e., not over an entire surface, of the first substrate  101 . More particularly, the black layer  342  may be formed on the first electrode  141  and the organic insulating layer  126  on a portion of the first substrate  101 . The black layer  342  may be patterned so as to screen, i.e., overlap, regions other than an active region through which light is emitted. That is, the black layer  342  may be patterned to screen, i.e., overlap, regions where metal wirings such as TFTs, gate lines, data lines, etc., are aligned at, e.g., boundaries of pixels. 
     As described above, the black layer  342  may absorb light penetrating a liquid crystal layer  151  such that light is not reflected by a metal layer formed on the first substrate  101  and may prevent and/or reduce light leakage at boundaries of pixels where the alignment of the liquid crystal layer  151  is difficult to control as a result of, e.g., fringe field. In such embodiments, the black layer  342  may function as a light-absorbing layer and a black matrix. 
       FIG. 4  illustrates a cross-sectional view of a pixel  400  of a reflection-type liquid crystal display device including a cholesteric liquid crystal according to another exemplary embodiment. In general, only differences between the exemplary pixel  100  of  FIG. 1  and the exemplary pixel  400  of  FIG. 4  will be described below. Referring to  FIG. 4 , the pixel  400  may include a black organic insulating layer  426  formed over the passivation layer  124 . The black organic insulating layer  426  may correspond to an organic insulating layer. The pixel  400  may include the black layer  342  formed over a portion of the first substrate  101 , i.e., not over an entire surface of the first substrate  101 . The black organic insulating layer  426  may include an organic insulating material including a black colorant. Also, a surface of the organic insulating layer  426  may have a convex lens shape in order to collect light that is externally reflected. In such embodiments, the black organic insulating layer  426  may be formed on the first substrate  101  as an organic insulating layer and the black layer  342  together with the black organic insulating layer  426  may efficiently function as a light-absorbing layer and a black matrix. 
       FIG. 5  illustrates a cross-sectional view of a pixel  500  of a reflection-type liquid crystal display device including a polymer network liquid crystal (PNLC) according to another exemplary embodiment. Referring to  FIG. 5 , the pixel  500  may include a liquid crystal layer  551  including a PNLC. In general, only differences between the exemplary pixel  100  of  FIG. 1  and the exemplary pixel  500  of  FIG. 5  will be described below. 
     More particularly, the liquid crystal layer  551  may be formed on the black layer  142 . The liquid crystal layer  551  may include a PNLC. The PNLC may include a network structure from polymer in the liquid crystal. When voltages are not applied thereto, molecular alignment of liquid crystal is irregular, and liquid crystal and polymer have different refractive index. As a result, when voltages are not applied thereto, light may be scattered at an interface between liquid crystal and polymer so as to realize white light. When voltages are applied thereto, the liquid crystal may be aligned and a refractive index of the liquid crystal may match with that of the polymer and light may be transmitted to realize black light. Accordingly, in some embodiments, to realize colors, a color filter (dotted line  5 ) may be disposed on the second substrate  161 . Alternatively, in some embodiments, colors may be realized by dispersing dyes in the polymer network liquid crystal layer  551 . When voltages are not applied thereto, dyes may be randomly aligned to realize colors due to randomly aligned liquid crystal. When voltages are applied thereto, liquid crystal may be aligned between two upper and lower electrodes in a direction perpendicular to the electrodes, and the dyes may be aligned in the same direction, and thus, black color may be realized. 
       FIG. 6  illustrates a cross-sectional view of a pixel  600  of a reflection-type liquid crystal display device including a PNLC according to another exemplary embodiment. In general, only differences between the exemplary pixel  600  of  FIG. 6  and the exemplary pixel  500  of  FIG. 5  will be described below. Referring to  FIG. 6 , the pixel  600  may include a black organic insulating layer  626  as an organic insulating layer. The black organic insulating layer  626  may be formed on the passivation layer  124 . More particularly, in such embodiments, the black organic insulating layer  626  may be formed on the first substrate  601  as the organic insulating layer so that both the black layer  142  and the black organic insulating layer  626  may efficiently function as a light-absorbing layer and a black matrix. The liquid crystal layer  551  may be formed on the black layer  142 . The liquid crystal layer  551  may include a PNLC. 
       FIG. 7  illustrates a cross-sectional view of a pixel  700  of a reflection-type liquid crystal display device including a PNLC according to another exemplary embodiment. In general, only differences between the exemplary pixel  700  of  FIG. 7  and the exemplary pixel  500  of  FIG. 5  will be described below. Referring to  FIG. 7 , the pixel  700  may include a black layer  742  that is formed over a portion, i.e., not over an entire surface, of the first substrate  101 . The black layer  742  may be formed on the first electrode  141  and the organic insulating layer  126  over a portion of the first substrate  101 . The black layer  742  may be patterned so as to screen, i.e., overlap, regions other than an active region through which light is emitted. That is, the black layer  742  may be patterned to screen, i.e., overlap, regions where, e.g., metal wirings such as TFTs, gate lines, data lines, etc., may be aligned at boundaries of pixels. 
     As described above, the black layer  742  may absorb light penetrating the liquid crystal layer  551  such that light may not be reflected by a metal layer formed on the first substrate  101  and may prevent light leakage at boundaries of pixels where the alignment of the liquid crystal layer  551  may be difficult to control. That is, the black layer  742  may function as a light-absorbing layer and a black matrix. 
       FIG. 8  illustrates a cross-sectional view of a pixel  800  of a reflection-type liquid crystal display device including a PNLC according to another exemplary embodiment. In general, only differences between the exemplary pixel  800  of  FIG. 8  and the exemplary pixel  700  of  FIG. 7  will be described below. Referring to  FIG. 8 , the pixel  800  may include a black organic insulating layer  826  formed on the passivation layer  124  as an organic insulating layer. 
     The black layer  742  may be formed on the first electrode  841  and the black organic insulating layer  826  so as to overlap a portion of the first substrate  101 . The black layer  742  may be patterned so as to screen, e.g., overlap, regions other than an active region through which light is emitted. For example, the black layer  742  may be patterned to screen regions where, e.g., metal wirings such as TFTs, gate lines, data lines, etc., may be aligned at boundaries of pixels. 
     As described above, the black layer  742  may absorb light penetrating a liquid crystal layer  551  such that light may not be reflected by a metal layer formed on the first substrate  101  and may prevent and/or reduce light leakage at boundaries of pixels where the alignment of the liquid crystal layer  551  may be difficult to control. That is, the black layer  742  may function as a light-absorbing layer and a black matrix. 
     More particularly, referring to  FIG. 8 , the black organic insulating layer  826  may be formed on the first substrate  101  as the organic insulating layer so that both the black layer  742  and the black organic insulating layer  826  may efficiently function as a light-absorbing layer and a black matrix. 
     In embodiments a black layer may be disposed between a first electrode layer and a liquid crystal layer so that light leakage at boundaries of pixels where the alignment of liquid crystal may be difficult to control. In embodiments, light reflection by a metal layer may be reduced and/or prevented so as to increase aperture ratio and/or reflection contrast ratio. In embodiments, the black layer may be disposed on a first substrate on which a TFT is formed so that a reduction in aperture ratio caused by misalignment when the first substrate and a second substrate are adhered to each other may be reduced and/or prevented. 
     Embodiments may provide a liquid crystal display device having an improved aperture ratio, e.g., higher aperture ratio, which is a ratio of emission zone per pixel in order to increase the brightness of a screen of the liquid crystal display device and reduce electric power consumption. More particularly, embodiments may provide a liquid crystal display device having a structure such that misalignment of the black matrix during assembly of the array substrate and the opposite substrate may be reduced so as to enable an improved aperture ratio. 
     Exemplary embodiments have been disclosed herein, and although specific terms are employed, they are used and are to be interpreted in a generic and descriptive sense only and not for purpose of limitation. Accordingly, it will be understood by those of ordinary skill in the art that various changes in form and details may be made without departing from the spirit and scope of the present invention as set forth in the following claims.