Patent Publication Number: US-2010110328-A1

Title: Liquid crystal display device

Description:
CLAIM OF PRIORITY 
     The present application claims priority from Japanese application serial No. 2008-281569, filed on Oct. 31, 2008, the content of which is hereby incorporated by reference into this application. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a display device, and more particularly to a liquid crystal display device which is small-sized and can reduce a thickness thereof. 
     2. Description of the Related Art 
     A liquid crystal display device includes a thin film transistors (TFT) substrate on which pixel electrodes, TFT and the like are formed in a matrix array, a color filter substrate which faces the TFT substrate in an opposed manner and forms color filters and the like thereon at positions corresponding to the pixel electrodes formed on the TFT substrate, and liquid crystal which is sandwiched between the TFT substrate and the color filter substrate. An image is formed by controlling optical transmissivity of liquid crystal molecules for every pixel. 
     The liquid crystal display device is flat and light-weighted and hence, the application fields of the liquid crystal display device have been spreading. A small-sized liquid crystal display device has been widely used in a mobile phone, a DSC (Digital Still Camera) and the like. 
     A liquid crystal display device has been requested to satisfy a demand for the increase of a size of a screen while maintaining a size of a profile thereof small. 
     JP-A-2008-145668 (patent document 1) discloses the following constitution which aims at the reduction of a size of a profile of a liquid crystal display device. The liquid crystal display device is configured such that a backlight is arranged on a back surface of a liquid crystal display panel constituted of a TFT substrate, a color filter substrate and the like, the liquid crystal display panel and the backlight are housed in the inside of a resin mold, and the resin mold is housed in the inside of a metal frame. 
     To reduce the size of the profile of the whole liquid crystal display device, an outer frame of the resin mold which surrounds the liquid crystal display panel is eliminated and, for compensating for the elimination of the outer frame, a portion of a diffusion sheet which constitutes a part of the backlight is arranged between the liquid crystal display panel and the metal frame so as to prevent the occurrence of cracks which may be caused when the liquid crystal display panel made of glass is brought into contact with metal. 
     SUMMARY OF THE INVENTION 
     On the other hand, in applications such as a mobile phone and a DSC, there has been a strong demand for the reduction of thickness of a liquid crystal display device for reducing a thickness of the product per se. To satisfy such a demand, there has been proposed a technique which, after a TFT substrate and a color filter substrate are combined, makes these substrates thin by polishing outer surfaces of these substrates. There has been also proposed a technique which makes a thickness of a part which constitutes a backlight as small as possible. However, there exists a limit in efforts for reducing a thickness of the liquid crystal display device by merely reducing a thickness of a glass substrate of the liquid crystal display panel or by merely reducing a thickness of the part of the backlight such as a light guide plate, an optical sheet or the like. 
     Further, while a screen of a liquid crystal display device is requested to exhibit high brightness, the liquid crystal display device is also required to satisfy a demand for small power consumption since the liquid crystal display device is driven by a battery. An LED is used as a light source of a liquid crystal display device used in a mobile phone or a DSC. Accordingly, the increase of the LED light emitting efficiency satisfies the demand for low power consumption and the demand for high brightness. However, the enhancement of the light emitting efficiency of the LED cannot be achieved readily. 
     Accordingly, it is an object of the present invention to realize the enhancement of the brightness of a screen of a liquid crystal display device by enhancing the utilization efficiency of light emitted from an LED instead of solely relying on the light emitting efficiency of the LED. It is another object of the present invention to realize the reduction of thickness of a liquid crystal display device by changing the constitution of the liquid crystal display device instead of solely relying on the reduction of thicknesses of respective parts of the liquid crystal display device. 
     To explain means which are specifically provided for achieving the above-mentioned objects of the present invention, they are as follows. 
     (1) According to one aspect of the present invention, there is provided a liquid crystal display device comprising: a liquid crystal display panel including a TFT substrate on which pixels each of which includes a pixel electrode and a thin film transistor are formed in a matrix array, a color filter substrate on which color filters are formed, a lower polarizer which is adhered to the TFT substrate, and an upper polarizer which is adhered to the color filter substrate; and a backlight, wherein the backlight includes a light emitting diode and a light guide plate which has an edge portion thereof arranged to face the light emitting diode, a flexible printed circuit board is connected to a terminal portion of the liquid crystal display panel, the light emitting diode is mounted on the flexible printed circuit board, and the flexible printed circuit board is folded and extends over a back surface of the backlight, and the light emitting diode is mounted on the flexible printed circuit board at a surface thereof on a side opposite to a surface thereof which faces the edge portion of the light guide plate. 
     (2) In the liquid crystal display device having the constitution (1), the light emitting diode may have the surface thereof on a side opposite to the surface thereof which faces the edge portion of the light guide plate positioned outside the TFT substrate. 
     (3) According to another aspect of the present invention, there is provided a liquid crystal display device comprising: a liquid crystal display panel including a TFT substrate on which pixels each of which includes a pixel electrode and a thin film transistor are formed in a matrix array, a color filter substrate on which color filters are formed, a lower polarizer which is adhered to the TFT substrate, and an upper polarizer which is adhered to the color filter substrate; and a backlight, wherein the backlight includes a light emitting diode, a light guide plate which has an edge portion thereof arranged to face the light emitting diode, and an optical sheet which is arranged on the light guide plate, the light guide plate has a large plate thickness at a portion thereof which faces the light emitting diode and a small plate thickness at a position where the optical sheet is arranged, and the light emitting diode and the portion of the light guide plate which has the large plate thickness are positioned outside the TFT substrate. 
     (4) In the liquid crystal display device having the constitution (3), a flexible printed circuit board may be connected to a terminal portion of the liquid crystal display panel, the light emitting diode may be mounted on the flexible printed circuit board, and the flexible printed circuit board may be folded and may extend over a back surface of the backlight, and the light emitting diode may be mounted on the flexible printed circuit board at a surface thereof on a side opposite to a surface thereof which faces the edge portion of the light guide plate. 
     (5) According to still another aspect of the present invention, there is provided a liquid crystal display device comprising: a liquid crystal display panel including a TFT substrate on which pixels each of which includes a pixel electrode and a thin film transistor are formed in a matrix array, a color filter substrate on which color filters are formed, a lower polarizer which is adhered to the TFT substrate, and an upper polarizer which is adhered to the color filter substrate; and a backlight, wherein a portion of the TFT substrate corresponding to a terminal portion has a large plate thickness, and a portion of the TFT substrate to which the lower polarizer is adhered has a small plate thickness, the backlight includes a light emitting diode, a light guide plate which has an edge portion thereof arranged to face the light emitting diode, and an optical sheet which is arranged on the light guide plate, the light guide plate has a large plate thickness at a portion thereof which faces the light emitting diode and a small plate thickness at a position where the optical sheet is arranged, and the light emitting diode and the portion of the light guide plate which has the large plate thickness are positioned outside the TFT substrate. 
     (6) In the liquid crystal display device having the constitution (5), a flexible printed circuit board may be connected to a terminal portion of the liquid crystal display panel, the light emitting diode may be mounted on the flexible printed circuit board, and the flexible printed circuit board may be folded and extends over a back surface of the backlight, and the light emitting diode may be mounted on a surface of the flexible printed circuit board at a surface thereof on a side opposite to a surface thereof which faces the edge portion of the light guide plate. 
     According to the present invention, in the small-sized liquid crystal display device including the backlight in which the light emitting diode is arranged to face the edge portion of the light guide plate, it is possible to bring the light emitting diode and the edge portion of the light guide plate into close contact with each other. Due to such constitution, it is possible to increase the utilization efficiency of light emitted from the light emitting diode and hence, it is possible to reduce the power consumption of the light emitting diode. 
     According to another aspect of the present invention, the light emitting diode and the portion of the light guide plate which faces the light emitting diode can be positioned outside the TFT substrate and hence, it is possible to reduce a thickness of the whole liquid crystal display device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view showing a liquid crystal display device according to an embodiment 1 of the present invention; 
         FIG. 2  is a cross-sectional view showing a liquid crystal display device according to an embodiment 2 of the present invention; 
         FIG. 3  is a cross-sectional view showing a liquid crystal display device according to an embodiment 3 of the present invention; 
         FIG. 4  is a cross-sectional view showing a liquid crystal display device according to an embodiment 4 of the present invention; 
         FIG. 5  is a plan view of a liquid crystal display device; 
         FIG. 6  is a back view of the liquid crystal display device; 
         FIG. 7  is a back developed view of the liquid crystal display device; 
         FIG. 8  is a cross-sectional view of a conventional liquid crystal display device; and 
         FIG. 9  is an exploded perspective view of a group of optical sheets. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Prior to the explanation of the specific constitution of the present invention, the constitution of a liquid crystal display device of a related art which includes a backlight is explained.  FIG. 5  is a plan view of a mobile-phone-use liquid crystal display device. Hereinafter, in this specification, the term “liquid crystal cell” implies the structure which is constituted of a TFT substrate  10 , a color filter substrate  20  and liquid crystal sealed in a gap defined between the TFT substrate  10  and the color filter substrate  20 . The term “liquid crystal display panel” implies the structure which is formed by adhering an upper polarizer  21  and a lower polarizer  11  to the liquid crystal cell. The term “liquid crystal display device” means the structure which is formed by combining a backlight with the liquid crystal display panel. 
     In  FIG. 5 , the liquid crystal display panel is placed in a resin mold. The liquid crystal display panel is fixed to a stepped portion (not shown in the drawing) of the resin mold  50  using a pressure-sensitive adhesive double coated tape or an adhesive light blocking film. The upper polarizer  21  is adhered to a portion of the liquid crystal display panel which corresponds to a display region. 
     The TFT substrate  10  is formed larger than the color filter substrate  20  in size, and a portion of the TFT substrate  10  projecting from the color filter substrate  20  constitutes a terminal portion  12 . An IC driver  30  is formed on the terminal portion  12 . A flexible printed circuit board  40  is connected to terminals formed on the terminal portion  12 . The flexible printed circuit board  40  surrounds an edge portion of the resin mold  50  and, thereafter, extends toward a back side of the resin mold  50 . 
       FIG. 6  is a back view of the liquid crystal display device shown in  FIG. 5 . In  FIG. 6 , the flexible printed circuit board  40  which starts from the terminal portion  12  of the TFT substrate  10  (not shown in the drawing) surrounds the edge portion of the mold and extends toward a back surface side of the resin mold  50 . A reflection sheet  81  which constitutes a portion of the backlight is arranged on the back surface of the resin mold  50 . As explained later, the reflection sheet  81  directs light radiated from the backlight toward a liquid crystal display panel side. The backlight not shown in the drawing is arranged in the inside of a frame of the resin mold  50 . 
       FIG. 7  is a view obtained by developing the flexible printed circuit board  40  from the liquid crystal display panel shown in  FIG. 6  which is a back view for illustrating a surface of the flexible printed circuit board  40  and portions of the inside of the resin mold  50 . In  FIG. 7 , light emitting diodes  70  and electronic parts  41  such as resistances and capacitors are arranged on the flexible printed circuit board  40 . Three light emitting diodes  70  are mounted on the flexible printed circuit board  40 . 
     In  FIG. 7 , an electronic part accommodating space  51  for accommodating the electronic parts  41  mounted on the flexible printed circuit board  40  is formed in the resin mold  50 . Three light emitting diode accommodating spaces  52  for accommodating the light emitting diodes  70  are formed in the resin mold  50  corresponding to the number of the light emitting diodes  70 . When the light emitting diodes  70  are arranged in the light emitting diode accommodating spaces  52 , the light emitting diodes  70  are arranged to face an edge portion of a light guide plate  80  not shown in  FIG. 7  in an opposed manner. 
     In  FIG. 7 , a pressure-sensitive adhesive double coated tape  61  is, for fixing the flexible printed circuit board  40  which is folded to the back surface of the resin mold  50 , mounted on the resin mold  50  at a position between the electronic part accommodating space  51  and the light emitting diode accommodating spaces  52 . In this manner, all of the electronic parts  41  mounted on the flexible printed circuit board  40  or the light emitting diodes  70  are accommodated in the electronic part accommodating space  51  or the light emitting diode accommodating spaces  52  formed on the resin mold  50 . Accordingly, there is no possibility that a thickness of the liquid crystal display device is increased due to the electronic parts  41  and the light emitting diodes  70 . 
       FIG. 8  is a cross-sectional view of a mobile-phone-use liquid crystal display device. In  FIG. 8 , a liquid crystal layer not shown in the drawing is sealed in a gap defined between a TFT substrate  10  on which pixel electrodes and TFTs are formed and a color filter substrate  20  on which color filters are formed. The TFT substrate  10  and the color filter substrate  20  are adhered to each other using a sealing material not shown in the drawing which is formed on a periphery of these substrates. Since liquid crystal can control only polarized light, a lower polarizer  11  is adhered to a TFT substrate  10  side on which light from a backlight is incident so as to polarize the light from the backlight to linear polarized light. 
     The linear polarized light is modulated by liquid crystal which is controlled by video signals for every pixel. On the color filter substrate  20 , the color filter is formed for every pixel thus forming a color image. Light which passes through the color filter substrate  20  is polarized (analyzed) by the upper polarizer  21  and is viewed as an image with naked eyes. 
     There has been a demand for the reduction of the thickness of the whole liquid crystal display device. For this end, the reduction of thickness of the glass substrate such as the TFT substrate  10  or the color filter substrate  20  is effective. In an example shown in  FIG. 7 , both a thickness of the TFT substrate  10  and a thickness of the color filter substrate  20  are 0.18 mm respectively. The use of such a thin glass substrate, however, gives rise to drawbacks in manufacturing steps with respect to a mechanical strength, deflection and the like thus lowering a manufacturing yield. Further, a glass substrate which is available from markets is standardized in thickness such as 0.5 mm or 0.4 mm and hence, it is difficult to obtain a thin glass plate having a thickness of 0.18 mm from markets. Accordingly, after the liquid crystal cell is completed, the thicknesses of the TFT substrate  10  and the color filter substrate  20  are reduced by mechanical polishing or chemical polishing. 
     In  FIG. 8 , the TFT substrate  10  is formed larger than the color filter substrate  20  in size, and a portion of the TFT substrate  10  which projects from the color filter substrate  20  constitutes a terminal portion  12 . An IC driver  30  is mounted on the terminal portion, and terminals which are connected to a flexible printed circuit board  40  are formed on the terminal portion  12 . Since the terminal portion  12  is constituted of only the TFT substrate, a plate thickness of the terminal portion  12  is 0.18 mm. Accordingly, a mechanical strength of the terminal portion  12  is low compared to mechanical strengths of other portions. Although the liquid crystal display panel is placed on a resin mold  50 , for preventing the liquid crystal display panel from being separated from the resin mold  50 , the liquid crystal display panel is fixed to the resin mold  50  using an adhesive light blocking film. A light blocking tape  60  plays a role of preventing leaking of light emitted from light emitting diodes  70  toward a TFT substrate  10  side. 
     In  FIG. 8 , the liquid crystal display panel is placed on the resin mold  50  which accommodates a light source, optical parts and the like therein. The flexible printed circuit board  40  is connected to the terminal portion  12  of the TFT substrate  10 . Electronic parts  41  such as resistances and capacitors are arranged on the flexible printed circuit board  40  and, further, light emitting diodes  70  which constitute the backlight are arranged on the flexible printed circuit board  40 . 
     The flexible printed circuit board  40  is folded so as to surround an edge portion of the resin mold  50  and extends toward a back side of the resin mold  50 . The flexible printed circuit board  40  is fixed to the resin mold  50  using a pressure-sensitive adhesive double coated tape  61  on a back surface of the resin mold  50 . The electronic parts  41  arranged on the flexible printed circuit board  40  are accommodated in an electronic part accommodating space  51  which is formed in the resin mold  50 . Further, the light emitting diodes  70  are accommodated in a light emitting diode accommodating space  52  which is formed in the resin mold  50 . Accordingly, there exists no possibility that the liquid crystal display device has a large thickness as a whole due to the presence of the electronic parts  41  and the light emitting diodes  70 . 
     The backlight is arranged on a back side of the liquid crystal display panel. The backlight is housed in the resin mold  50 . In  FIG. 8 , a light guide plate  80  is arranged to face the light emitting diodes  70 . A portion of the light guide plate  80  which faces the light emitting diodes  70  has a large thickness corresponding to a size of the light emitting diodes  70 . However, other portions of the light guide plate  80 , that is, portions on which optical sheets are placed have a small thickness for reducing the thickness of the whole liquid crystal display device. The light guide plate  80  has a function of directing light which is incident on the light guide plate  80  from the light emitting diodes  70  in the lateral direction toward the liquid crystal display panel side. 
     A reflection sheet  81  is arranged below the light guide plate  80 . The reflection sheet  81  reflects light which extends downwardly from the light guide plate  80  and directs the light toward the liquid crystal display panel, and enhances the utilization efficiency of light emitted from the light emitting diodes. A group of optical sheets is arranged on the light guide plate  80 . The group of optical sheets is sequentially arranged in order of a lower diffusion sheet  82 , a lower prism sheet  83 , an upper prism sheet  84 , and an upper diffusion sheet  85  from a light guide plate  80  side. 
       FIG. 9  is an exploded perspective view of the group of optical sheets. A lowermost sheet described in  FIG. 9  is the lower diffusion sheet  82 . A light which is radiated toward the liquid crystal display panel side from the light guide plate  80  has brightness irregularities such that the light radiated from a portion of the light guide plate  80  in the vicinity of the light emitting diodes  70  is relatively brighter than the light radiated from other portions of the light guide plate  80 . The lower diffusion sheet  82  plays a role of making the brightness of light emitted from the backlight uniform by alleviating such brightness irregularities. 
     The lower prism sheet  83  is arranged on the lower diffusion sheet  82 . As shown in  FIG. 9 , for example, prisms each of which has a triangular cross section are formed on the lower prism sheet  83  in such a manner that the prisms extend in the lateral direction and are arranged in the longitudinal direction. An arrangement pitch of the respective prisms is set to approximately 50 μm. In  FIG. 9 , the lower prism sheet  83  plays a role of enhancing the utilization efficiency of light by directing light which tends to spread in the direction “a” toward the direction perpendicular to the lower prism sheet  83 . 
     The upper prism sheet  84  is arranged on the lower prism sheet  83 . As shown in  FIG. 9 , for example, prisms each of which has a triangular cross section are formed on the upper prism sheet  84  in such a manner that the prisms extend in the longitudinal direction and are arranged in the lateral direction. An arrangement pitch of the respective prisms is set to approximately 50 μm. In  FIG. 9 , the upper prism sheet  84  plays a role of enhancing the utilization efficiency of light by directing light which tends to spread in the direction “b” toward the direction perpendicular to the upper prism sheet  84 . 
     In  FIG. 9 , the upper diffusion sheet  85  is arranged on the upper prism sheet  84 . The upper diffusion sheet  85  has a function of suppressing the generation of moiré on the display screen of the liquid crystal display device. That is, the brightness of a light emitted from the lower prism sheet  83  or the upper prism sheet  84  is microscopically changed periodically corresponding to the prism pitch. 
     On the other hand, on the TFT substrate  10  of the liquid crystal display panel, scanning lines which extend in the lateral direction and are arranged in the longitudinal direction are formed, for example. Accordingly, portions which allow the transmission of light and portions which block light are periodically formed in the longitudinal direction on the TFT substrate  10  due to the presence of the scanning lines. Further, on the TFT substrate  10  of the liquid crystal display panel, video signal lines which extend in the longitudinal direction and are arranged in the lateral direction are formed. Accordingly, portions which allow the transmission of light and portions which block light are periodically formed in the lateral direction on the TFT substrate  10  due to the presence of the video signal lines. 
     Due to such a constitution, the interference of light is generated between light which passes through the lower prism sheet  83  and the upper prism sheet  84  and the TFT substrate  10  of the liquid crystal display panel, and this interference of light generates moiré. The upper diffusion sheet  85  plays a role of suppressing the generation of moiré by alleviating the interference between light and the scanning lines or the video signal lines formed on the TFT substrate  10  due to alleviation of the difference in intensity of light which passes through the prism sheets. 
     As described above, the explanation of the group of optical sheets is made by taking a case where the group of optical sheets includes four sheets, that is, the lower diffusion sheet  82 , the lower prism sheet  83 , the upper prism sheet  84 , and the upper diffusion sheet  85  as an example. However, in the liquid crystal display device to which the present invention is applied, four sheets are not always necessary. For example, the upper diffusion sheet  85  may be omitted or one of the prism sheets may be omitted. 
     Returning to the constitution shown in  FIG. 8 , the group of optical sheets overlaps with the light guide plate  80 . A gap is formed between the upper diffusion sheet  85  which constitutes an uppermost optical sheet out of the group of optical sheets and the lower polarizer  11  of the liquid crystal display panel. This gap is formed for preventing the generation of flaws on the lower polarizer  11  or the upper diffusion sheet  85  due to rubbing between the lower polarizer  11  and the upper diffusion sheet  85 . 
     As shown in  FIG. 8 , the light emitting diodes  70  are mounted on the flexible printed circuit board  40  and are accommodated in the light emitting diode accommodating space  52 . It is ideal that the light emitting diodes  70  and the edge portion of the light guide plate  80  are brought into close contact with each other. However, in an actual liquid crystal display device, a gap “g” is formed between the light guide plate  80  and the light emitting diodes  70  due to the deflection, the spring-back or the like of the flexible printed circuit board  40 . 
     When the gap “g” is formed, the utilization efficiency of light emitted from the light emitting diodes  70  is extremely lowered. For example, when a gap “g” of approximately 0.1 mm exists between the light guide plate  80  and the light emitting diodes  70  in  FIG. 8 , the utilization efficiency of light emitted from the light emitting diodes  70  is lowered by approximately 10%. The present invention aims at, as explained in the following embodiments, the enhancement of the utilization efficiency of light emitted from the light emitting diodes  70  by making the gap “g” between the edge portion of the light guide plate  80  and the light emitting diodes  70  as small as possible. 
     Further, the light emitting diode is requested to have a size or a height to some extent. As shown in  FIG. 8 , conventionally, the light emitting diodes  70  are arranged below the TFT substrate  10  and the light blocking tape  60 . Accordingly, a thickness of the liquid crystal display device is determined by taking all of a height of the light emitting diodes  70 , a thickness of the light blocking tape  60 , and a thickness of the TFT substrate  10  into consideration. Here, the edge portion of the light guide plate  80  is aligned with the height of the light emitting diodes  70  and hence, the thickness of the edge portion of the light guide plate  80  also influences the thickness of the liquid crystal display device in the same manner. 
     The present invention aims at, as explained in the following embodiments, the reduction of a thickness of the liquid crystal display device by arranging the TFT substrate  10 , the light emitting diodes  70  and the light guide plate  80  such that the TFT substrate  10  does not overlap with the light emitting diodes  70  or the edge portion of the light guide plate  80 . Hereinafter, specific embodiments of the present invention are explained. 
     Embodiment 1 
     The embodiment 1 is directed to an example which enhances the utilization efficiency of light emitted from light emitting diodes  70  by improving the close contact between the light emitting diodes  70  and an edge portion of a light guide plate  80 .  FIG. 1  is a cross-sectional view of a liquid crystal display device showing the embodiment 1 of the present invention. In  FIG. 1 , the edge portion of the light guide plate  80  is present in the vicinity of an edge portion of a TFT substrate  10 . 
     In  FIG. 1 , a flexible printed circuit board  40  is connected to a terminal portion  12  of the TFT substrate  10 . With respect to this flexible printed circuit board  40 , a light-emitting-diode-use flexible printed circuit board  46  which supplies electricity and the like to the light emitting diodes  70  is folded and extends to and along a back surface of a liquid crystal display panel and a back surface of a backlight. In  FIG. 1 , a main flexible printed circuit board  45  on which electronic parts  41  and the like are mounted is not bent or folded and extends frontwardly. The main flexible printed circuit board  45  is bent or the like at the time of assembling the liquid crystal display device into an apparatus and is connected to a power source or a signal source of the apparatus. 
     This embodiment is characterized in that a connection point where the light emitting diodes  70  and the light-emitting-diode-use flexible printed circuit board  46  are connected with each other is arranged at a surface of the light emitting diodes  70  on a side opposite to a surface of the light emitting diodes  70  which faces the light guide plate  80 . Due to such an arrangement, when the light-emitting-diode-use flexible printed circuit board  46  is pulled in the direction indicated by an arrow shown in  FIG. 1 , the light emitting diodes  70  are brought into close contact with the edge portion of the light guide plate  80 . 
     That is, the technical feature of this embodiment lies in that when the light-emitting-diode-use flexible printed circuit board  46  is pulled in the direction indicated by the arrow, the light emitting diodes  70  are pressed against the light guide plate  80  in the direction perpendicular to the light guide plate  80  as indicated by a white arrow. Due to such a constitution, the light emitting diodes  70  are pressed against the edge portion of the light guide plate  80  uniformly without inclination with respect to the edge portion of the light guide plate  80 . 
     In  FIG. 1 , after pulling the light-emitting-diode-use flexible printed circuit board  46  in the direction indicated by the arrow, a stopper may be provided to a distal end of the light-emitting-diode-use flexible printed circuit board  46  so as to maintain the close contact between the light emitting diodes  70  and the light guide plate  80 . 
     The liquid crystal display device shown in  FIG. 1  is housed in a resin mold  50  or a frame. In housing the liquid crystal display device, there may be a case where the light emitting diodes  70  are pressed by the resin mold  50  or a wall of the frame. Also in this case, the light emitting diodes  70  receive a pressing force as indicated by the white arrow in  FIG. 1  and hence, it is possible to enhance the close contact between the edge portion of the light guide plate  80  and the light emitting diodes  70 . 
     Even when the light guide plate  80  and the light emitting diodes  70  are spaced-apart from each other by a distance of mere approximately 0.1 mm, the utilization efficiency of light emitted from the light emitting diodes  70  is lowered by approximately 10%. Further, when the inclination is present between the light emitting diodes  70  and the edge portion of the light guide plate  80 , the utilization efficiency of light emitted from the light emitting diodes  70  is further lowered. According to this embodiment, the close contact between the light emitting diodes and the edge portion of the light guide plate is increased and, at the same time, the inclination of the light emitting diodes is eliminated. Accordingly, it is possible to enhance the utilization efficiency of light emitted from the light emitting diodes  70  in the liquid crystal display device by 10% or more thus enhancing the brightness of a display screen. When it is sufficient for a display screen to maintain the same brightness, electricity supplied to the light emitting diodes  70  can be reduced by 10% or more thus prolonging a time until a battery is recharged. 
     Embodiment 2 
     This embodiment provides the constitution which can reduce a thickness of the liquid crystal display device.  FIG. 2  is a cross-sectional view showing the constitution of this embodiment. In  FIG. 2 , a portion of a light guide plate  80  in the vicinity of an edge portion of the light guide plate  80  which faces light emitting diodes  70  has a large plate thickness. Further, the portion is positioned outside an edge portion of a TFT substrate  10 . The light emitting diodes  70  are arranged to be in close contact with the edge portion of the light guide plate  80 . 
     The light emitting diodes  70  require a predetermined height. The edge portion of the light guide plate  80  which faces the light emitting diodes  70  also requires a height substantially equal to the height of the light emitting diodes  70 . As shown in  FIG. 7 , when the light emitting diodes  70  or the edge portion of the light guide plate  80  which faces the light emitting diodes  70  are arranged below the TFT substrate  10 , a thickness of the liquid crystal display device inevitably includes a sum of a thickness of the TFT substrate  10  and a thickness of the light emitting diode  70  and hence, the reduction of the thickness of the liquid crystal display device is limited. 
     According to the present invention, by arranging the light emitting diodes  70  and the edge portion of the light guide plate  80  which faces the light emitting diodes  70 , that is, a portion of the light guide plate  80  having a large thickness outside the TFT substrate  10 , the above-mentioned limit can be eliminated thus realizing the large reduction of the thickness of the liquid crystal display device. 
     As shown in  FIG. 2 , in the same manner as the embodiment 1, a connection point where the light emitting diodes  70  and the light-emitting-diode-use flexible printed circuit board  46  are connected with each other is arranged at a surface of the light emitting diodes  70  on a side opposite to a surface of the light emitting diodes  70  which faces the light guide plate  80 . Due to such an arrangement, when the light-emitting-diode-use flexible printed circuit board  46  is pulled in the direction indicated by an arrow, the light emitting diodes  70  are brought into close contact with the edge portion of the light guide plate  80 . Accordingly, also in this embodiment, in the same manner as the embodiment 1, it is possible to enhance the utilization efficiency of light emitted from the light emitting diodes  70  thus reducing the power consumption of the light emitting diodes  70 . 
     Embodiment 3 
       FIG. 3  is a cross-sectional view of a liquid crystal display device showing an embodiment 3 of the present invention. In the embodiment 2, as shown in  FIG. 2 , the thickness of the whole liquid crystal display device is reduced by arranging the light emitting diodes  70  and the edge portion of the light guide plate  80  outside the TFT substrate  10 . In the constitution of the embodiment 2, as shown in  FIG. 2 , parts of the liquid crystal display device which define the thickness of the whole liquid crystal display device are the light guide plate  80 , the group of optical sheets which is placed on the light guide plate  80 , the lower polarizer  11 , the TFT substrate  10 , the color filter substrate  20 , and the upper polarizer  21 . 
     According to the embodiment 3 shown in  FIG. 3 , a stepped portion is formed on the TFT substrate  10 . That is, while a plate thickness of a portion of the TFT substrate  10  corresponding to the terminal portion  12  is maintained as it is, a plate thickness of a portion of the TFT substrate  10  to which the lower polarizer  11  is adhered is reduced by chemical polishing. Then, the lower polarizer  11  is adhered to a small plate-thickness portion  13  of the TFT substrate  10  whose thickness is reduced. 
     Due to such a constitution, it is no more necessary to take a thickness of the lower polarizer  11  into consideration in determining the thickness of the whole liquid crystal display device. Accordingly, it is possible to realize the further reduction of the thickness of the whole liquid crystal display device. In this embodiment, as shown in  FIG. 3 , a lower surface of the lower polarizer  11  is made coplanar with a lower surface of the TFT substrate  10 . It is needless to say, however, that even when the thickness of the small plate-thickness portion  13  of the TFT substrate  10  is not reduced to such an extent, the above-mentioned advantageous effects can be acquired by reducing the thickness of the small plate-thickness portion  13  to an extent that the lower surface of the lower polarizer  11  is positioned closer to a backlight side than the lower surface of the TFT substrate  10 . 
     Although the thickness of the portion of the TFT substrate  10  to which the lower polarizer  11  is adhered is reduced, a mechanical strength of the liquid crystal display panel is not lowered as a whole. That is, a portion of the liquid crystal display panel having a weakest mechanical strength is the terminal portion  12  of the TFT substrate  10 , which is constituted of only one sheet of TFT substrate  10 . In this embodiment, a plate thickness of the terminal portion  12  is not reduced. Further, the color filter substrate  20  is adhered to the portion of the TFT substrate  10  whose plate thickness is reduced and hence, such a portion exhibits a larger mechanical strength than the terminal portion  12  in a liquid crystal display panel state. Due to such a constitution, this embodiment can reduce the thickness of the whole liquid crystal display device without substantially decreasing the mechanical strength of the liquid crystal display panel. 
     Embodiment 4 
       FIG. 4  shows the constitution according to an embodiment 4 of the present invention. As shown in  FIG. 4 , in the same manner as the embodiment 2, light emitting diodes  70  and an edge portion of a light guide plate  80  which faces the light emitting diodes  70  are arranged outside a TFT substrate  10 . In  FIG. 4 , both a main flexible printed circuit board  45  which is connected to the TFT substrate  10  and a light-emitting-diode-use flexible printed circuit board  46  which is connected to the light emitting diodes  70  extend outward from a liquid crystal display device without being folded toward a back surface side of a backlight. These flexible printed circuit boards  45 ,  46  are connected to an external power source or an external signal source respectively when the liquid crystal display device is assembled into a set in such a manner that the respective flexible printed circuit boards  45 ,  46  are folded so as to prevent the increase of the thickness of the whole liquid crystal display device. 
     In  FIG. 4 , the liquid crystal display panel and the backlight are housed in a resin mold  50  or a frame. Here, the light emitting diodes  70  are pressed by the resin mold  50  or a wall of the frame so that these parts are brought into close contact with the edge portion of the light guide plate  80 .  FIG. 4  schematically shows such a state using a spring. A coil spring is schematically shown in  FIG. 4 , and the spring may be made of any material provided that the material exhibits spring property. 
     As described above, also in this embodiment, the light emitting diodes  70  are brought into close contact with the edge portion of the light guide plate  80  and hence, it is possible to enhance the utilization efficiency of light emitted from the light emitting diodes  70  thus suppressing the power consumption of the light emitting diodes  70 . Further, the light emitting diodes  70  and the edge portion of the light guide plate  80  which faces the light emitting diodes  70  are arranged outside the TFT substrate  10  and hence, it is possible to reduce the thickness of the liquid crystal display device as a whole. 
     Here, also in the constitution shown in  FIG. 4 , by setting the thickness of the TFT substrate  10  in two stages such that a plate thickness of a portion of the TFT substrate  10  to which the lower polarizer  11  is adhered is reduced, it is possible to decrease the thickness of the whole liquid crystal display device.