Patent Publication Number: US-2007109465-A1

Title: Display device emitting light from both sides

Description:
This application claims priority to Korean Patent Application No. 2005-0109141, filed on Nov. 15, 2005, and all the benefits accruing therefrom under 35 U.S.C. § 119, the contents of which in its entirety are herein incorporated by reference.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a display device and a manufacturing method thereof, and more particularly, to a display device which displays an image on both sides of a display region using an organic light emitting diode (“OLED”) and a liquid crystal display (“LCD”).  
      2. Description of the Related Art  
      Recently, flat panel displays such as a liquid crystal displays (“LCDs”) or organic light emitting diode (“OLED”) displays have become widely used in the replacement of cathode ray tubes (“CRTs”).  
      The LCD comprises a liquid crystal display panel having a liquid crystal layer. As the liquid crystal display panel does not emit light by itself, a backlight unit is disposed on a rear part of a thin film transistor to emit light to the display.  
      The organic light emitting diode (“OLED”) display has recently become popular because it is driven with a low voltage; is thin and light; has a wide view angle; has a relatively short response time; and several other advantageous features. OLED displays are classified into passive matrix type and active matrix type according to their driving methods. The active matrix type is capable of realizing a wide screen and high resolution while the manufacturing process thereof may be complicated.  
      Meanwhile, a display device which displays images on both sides of a display region, like internal/external display windows of a folder type cell phone, has drawn attention. In the conventional display device displaying an image on both sides, a backlight unit is disposed between a pair of liquid crystal display panels.  
      With such a configuration, the conventional display device requires a single backlight unit to be disposed between the two liquid crystal display panels. However, the backlight unit may increase the thickness of the conventional display apparatus.  
     BRIEF SUMMARY OF THE INVENTION  
      Accordingly, it is an aspect of the present invention to provide a display device which is thin in thickness and displays an image on both sides.  
      The foregoing and/or other aspects of the present invention can be achieved by providing a display device including; a first insulating substrate, an organic light emitting layer which is formed on a first area of the first insulating substrate and emits a light, a second insulating substrate and a third insulating substrate which are sequentially disposed on the organic light emitting layer, and a liquid crystal layer which is formed on a second area between the second insulating substrate and the third insulating substrate with a smaller size than the first area, and is backlit by the light emitted from the organic light emitting layer.  
      According to another aspect of the present invention, the liquid crystal layer is overlapped with the organic light emitting layer.  
      According to another aspect of the present invention, the display device further includes a sealant which adheres the first insulating substrate and the second insulating substrate.  
      According to another aspect of the present invention, the display device further includes a moisture absorber which is provided on a surface of the second insulating substrate which faces the first insulating substrate.  
      According to another aspect of the present invention, the display device further includes a thin film transistor (“TFT”) which is formed on a surface of the second insulating substrate which faces the third insulating substrate.  
      According to another aspect of the present invention, the display device further includes a color provider which provides a color to the light which is emitted from the organic light emitting layer.  
      According to another aspect of the present invention, the color provider includes a color filter disposed between the first insulating substrate and the organic light emitting layer.  
      According to another aspect of the present invention, the color provider includes a color filter film attached to a surface of the first insulating substrate opposite the second insulating substrate.  
      According to another aspect of the present invention, the display device further includes a driving circuit substrate which is connected with the first insulating substrate, and a flexible printed circuit board which connects the second insulating substrate and the driving circuit substrate.  
      According to another aspect of the present invention, the display device further includes a first driving circuit substrate which is connected with the first insulating substrate, and a second driving circuit substrate which is connected with the second insulating substrate.  
      According to another aspect of the present invention, the display device further includes a pixel electrode which is formed under the organic light emitting layer and a common electrode which is formed on the organic light emitting layer.  
      According to another aspect of the present invention, the common electrode includes silver and at least one of magnesium and calcium.  
      According to another aspect of the present invention, the first insulating substrate is larger than the second insulating substrate, and the second insulating substrate is larger than the third insulating substrate.  
      According to another aspect of the present invention, the display device further includes a fourth insulating substrate which is disposed between the first insulating substrate and the second insulating substrate, and a sealant which adheres the first insulating substrate to the fourth insulating substrate.  
      According to another aspect of the present invention, the display device further includes an optical film which is disposed between the second insulating substrate and the fourth insulating substrate.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The above and/or other aspects, features and advantages of the present invention will become apparent and more readily appreciated from the following description of exemplary embodiments, taken in conjunction with the accompany drawings of which:  
       FIG. 1  is a cross-sectional view of a first exemplary embodiment of a display device according to the present invention;  
       FIG. 2  is an enlarged cross-sectional view of the first exemplary embodiment of a display device according to the present invention;  
       FIGS. 3-5  are cross-sectional views of second through fourth exemplary embodiments of a display device according to the present invention;  
       FIG. 6A  is a front perspective view of an exemplary embodiment of an electronic device utilizing the exemplary embodiment of a display device according to the present invention; and  
       FIG. 6B  is a front perspective view of an alternative configuration of an exemplary embodiment of an electronic device utilizing the exemplary embodiment of a display device according to the present invention.  
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      The invention now will be described more fully hereinafter with reference to the accompanying drawings, in which embodiments of the invention are shown. This invention may, however, be embodied in many 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. Like reference numerals refer to like elements throughout.  
      It will be understood that when an element is referred to as being “on” another element, it can be directly on the other element or intervening elements may be present therebetween. In contrast, when an element is referred to as being “directly on” another element, there are no intervening elements present. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items.  
      It will be understood that, although the terms first, second, third etc. may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer or section from another region, layer or section. Thus, a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of the present invention.  
      The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” or “includes” and/or “including” when used in this specification, specify the presence of stated features, regions, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, regions, integers, steps, operations, elements, components, and/or groups thereof.  
      Furthermore, relative terms, such as “lower” or “bottom” and “upper” or “top,” may be used herein to describe one element&#39;s relationship to another elements as illustrated in the Figures. It will be understood that relative terms are intended to encompass different orientations of the device in addition to the orientation depicted in the Figures. For example, if the device in one of the figures is turned over, elements described as being on the “lower” side of other elements would then be oriented on “upper” sides of the other elements. The exemplary term “lower”, can therefore, encompasses both an orientation of “lower” and “upper,” depending of the particular orientation of the figure. Similarly, if the device in one of the figures is turned over, elements described as “below” or “beneath” other elements would then be oriented “above” the other elements. The exemplary terms “below” or “beneath” can, therefore, encompass both an orientation of above and below.  
      Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and the present disclosure, and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.  
      Embodiments of the present invention are described herein with reference to cross section illustrations that are schematic illustrations of idealized embodiments of the present invention. As such, variations from the shapes of the illustrations as a result, for example, of manufacturing techniques and/or tolerances, are to be expected. Thus, embodiments of the present invention should not be construed as limited to the particular shapes of regions illustrated herein but are to include deviations in shapes that result, for example, from manufacturing. For example, a region illustrated or described as flat may, typically, have rough and/or nonlinear features. Moreover, sharp angles that are illustrated may be rounded. Thus, the regions illustrated in the figures are schematic in nature and their shapes are not intended to illustrate the precise shape of a region and are not intended to limit the scope of the present invention.  
      Hereinafter, the present invention will be described in detail with reference to the accompanying drawings.  
      Hereinafter, a display device according to a first exemplary embodiment of the present invention will be described in detail with reference to  FIGS. 1 and 2 . An exemplary embodiment of a display device  1  according to the present invention comprises an organic light emitting diode (“OLED”) display  2  and a liquid crystal display (“LCD”)  3 . The OLED display  2  and the LCD  3  share a second insulating substrate  210 . The LCD  3  receives light from the OLED display  2 .  
      The OLED display  2  comprises a first insulating substrate  110  and the second insulating substrate  210  which are in contact with each other through a first sealant  510 ; a first thin film transistor  121 ; and an organic layer  150  which are formed on the first insulating substrate  110 .  
      Hereinafter, the OLED display  2  will be described in detail.  
      The first thin film transistor  121  is formed on the first insulating substrate  110 , exemplary embodiments of which comprise an insulating material such as glass, quartz, ceramic or various plastics. Exemplary embodiments of the first TFT  121  of the OLED display  2  according to the present invention may be made of amorphous silicon as a semiconductor layer. Alternative exemplary embodiments include configurations wherein the first TFT  121  of the OLED display  2  according to the present invention may be made of a thin film transistor which employs poly-silicon or an organic semiconductor. In the exemplary embodiment of an OLED display  2 , a single pixel comprises two or more thin film transistors including a switching thin film transistor and a driving thin film transistor. The first thin film transistor  121  shown in  FIG. 2  comprises a driving thin film transistor.  
      A first passivation layer  131  is formed on the first thin film transistor  121 . Exemplary embodiments of the first passivation layer  131  may be made of silicon nitride (“SiNx”)  
      A first color filter  132  is disposed on the first passivation layer  131 . The first color filter  132  is disposed as an island between adjacent first thin film transistors  121 . The first color filter  132  is disposed having a predetermined pattern, and comprises sub layers  132   a ,  132   b  and  132   c  which differ in color.  
      A first organic layer  133  is formed on the first passivation layer  131  and the first color filter  132 . Exemplary embodiments of the first organic layer  133  may comprise one of benzocyclobutene, olefin, acrylic resin, polyimide and fluoropolymer. A part of the first passivation layer  131  and the first organic layer  133  are either not formed on the first thin film transistor  121 , or are formed thereon but are subsequently removed therefrom, thereby forming a contact hole  134 .  
      A first pixel electrode  141  is formed on the first organic layer  133 . The first pixel electrode  141  may also be referred to as an anode and supplies a hole to an organic layer  150 . Exemplary embodiments of the first pixel electrode  141  are made of a transparent conductive material such as indium tin oxide (“ITO”) or indium zinc oxide (“IZO”). The first pixel electrode  141  is connected with the first thin film transistor  121  through the contact hole  134 .  
      A partition wall  142  is formed on the first pixel electrode  141  and the first organic layer  134 , surrounding the first pixel electrode  141 . The partition wall  142  partitions the pixel electrode  141  and forms a pixel region. Exemplary embodiments of the partition wall  142  may be made of a photosensitive material having thermal resistance and solvent resistance such as acrylic resin or polyimide, or an inorganic material such as SiO2 or TiO2. Alternative exemplary embodiments include configurations wherein the partition wall  142  may comprise a double layer configuration including an organic layer and an inorganic layer.  
      The organic layer  150  is formed on region of the first pixel electrode  141  which is left uncovered by the partition wall  142 . The organic layer  150  comprises a hole injecting layer  151  and an organic light emitting layer  152  which are made of a polymer.  
      Exemplary embodiments of the hole injecting layer  151  are made of a hole injecting material such as poly (3,4-ethylenedeoxythiophene) (“PEDOT”) and polystyrene sulfonic acid (“PSS”). In one exemplary embodiment the hole injecting material is mixed with water and formed by an inkjet method in an aqua suspension state.  
      The organic light emitting layer  152  emits a white light and according to one exemplary embodiment of the present invention may be formed by the inkjet method. In one exemplary embodiment the organic light emitting layer  152  emits white light and is formed of the same material in every pixel. Alternative exemplary embodiments include configurations wherein the organic light emitting layer  152  emits light of various colors. In such an alternative exemplary embodiment the color filters  132   a - 132   c  may be excluded. In such an alternative exemplary embodiment the second color filters  321  to be described below may also be omitted.  
      The hole transmitted by the first pixel electrode  141  and an electron supplied by a first common electrode  160  are combined as an exciton in the organic light emitting layer  152 , thereby generating light through the de-excitation of the exciton.  
      The first common electrode  160  is formed on the partition wall  142  and the organic light emitting layer  152 . The first common electrode  160  is also referred to as a cathode and supplies an electron to the organic light emitting layer  152 .  
      A transparent layer  143  is formed on top of the first common electrode  160  to allow for a uniform bonding surface to be applied to the second insulating substrate  210 .  
      The exemplary embodiment of an OLED display  2  according to the present invention displays an image toward the first insulating substrate  110  and at the same time supplies light to the LCD  3 . Accordingly, the light emitted from the organic light emitting layer  152  is supplied to both the first insulating substrate  110  and the second insulating substrate  210 . In order to do so the first common electrode  160  is transparent.  
      Exemplary embodiments of the first common electrode  160  may be made of a mixture of magnesium and silver or of calcium and silver. Exemplary embodiments of the first common electrode  160  may be about 50 nm to about 200 nm in thickness. When the first common electrode  160  is less than about 50 nm in thickness, a resistance thereof may increase to the point that common voltage supply becomes inefficient. When the first common electrode  160  is about 200 nm or more in thickness, the first common electrode  160  may become opaque. In one exemplary embodiment light transmittivity of the first common electrode  160  is about 50% or more.  
      Exemplary embodiments of the second insulating substrate  210  connected with the first insulating substrate  110  through the first sealant  510  may comprise an insulating material such as glass, quartz, ceramic or plastic. The second insulating substrate  210  may be smaller than the first insulating substrate  110  to allow for the easy mounting of a driving circuit substrate  171  to an end part of the first insulating substrate  110 .  
      Since the organic layer  150  is susceptible to moisture, exemplary embodiments of the first sealant  510  have a moisture transmittivity of about 60 g/m 2  day or less. Exemplary embodiments of the first sealant  510  may be made of UV curing resin. The first sealant  510  may comprise a spacer (not shown) to keep intervals between the first insulating substrate  110  and the second insulating substrate  210  uniform.  
      A moisture absorber  211  is provided on the second insulating substrate  210  in the space between the second insulating substrate  210  and the first insulating substrate  110 . Exemplary embodiments of the moisture absorber  211  may employ Ca, CaO, Ba, BaO, Mg, MgO, AlO or other similar substances. Exemplary embodiments of the moisture absorber  211  may be formed by a sputtering method or by an electron beam deposition method. Alternative exemplary embodiments include configurations wherein the moisture absorber  211  may be formed on the first insulating substrate  110 .  
      The driving circuit substrate  171  is attached to a first end of the first insulating substrate  110 . The driving circuit substrate  171  receives a signal from the outside to control the first thin film transistor  121  and to display an image. In one exemplary embodiment the driving circuit substrate  171  is connected with the first insulating substrate  110  through an anisotropic conductive film  172 .  
      The LCD  3  comprises the second insulating substrate  210  and a third insulating substrate  310  which are in contact with each other by a second sealant  520 ; a second thin film transistor  221  which is formed on the second insulating substrate  210 ; a second color filter  321  which is formed on the third insulating substrate  310 ; and a liquid crystal layer  410  which is disposed between the second and third insulating substrates  210  and  310 . In one exemplary embodiment the size of the liquid crystal layer  410  is smaller than that of the organic layer  150 . The liquid crystal layer  410  is provided to be overlapped by the organic layer  150  or the organic light emitting layer  152 . That is, the liquid crystal layer  410  is made to have a display region which is smaller than the OLED display  2 .  
      Hereinafter, the LCD  3  will be described in detail.  
      The second thin film transistor  221  is formed on the second insulating substrate  210 . Exemplary embodiments of the second insulating substrate  210  may be made of an insulating material such as glass, quartz, ceramic or various plastics. Exemplary embodiments of the second thin film transistor  221  are made of amorphous silicon as a semiconductor layer, but the present invention is not limited thereto. Alternative exemplary embodiments of the second thin film transistor  221  may be made of poly-silicon or an organic semiconductor.  
      A second passivation layer  231  is formed on the second thin film transistor  221 . Exemplary embodiments of the second passivation layer  231  may be made of silicon nitride (“SiNx”).  
      A second organic layer  241  is formed on the second passivation layer  231 . Exemplary embodiments of the second organic layer  241  may comprise one of BCB, olefin, acrylic resin, polyimide and fluoropolymer. A part of the second passivation layer  231  and the second organic layer  241  is either not laid down on or is subsequently removed from the second thin film transistor  221 , thereby forming a contact hole  232 .  
      A second pixel electrode  251  is formed on the second organic layer  241 . Exemplary embodiments of the second pixel electrode  251  are made of a transparent conductive material such as indium tin oxide (“ITO”) or indium zinc oxide (“IZO”). The second pixel electrode  251  is connected with the second thin film transistor  221  through the contact hole  232 .  
      The third insulating substrate  310  is connected with the second insulating substrate  210  through the second sealant  520 . Exemplary embodiments of the third insulating substrate  310  comprise an insulating material such as glass, quartz, ceramic or various plastics. The third insulating substrate  310  is smaller in size than the second insulating substrate  210 , allowing the easy mounting of a flexible printed circuit board (“FPCB”)  261  to an end part of the second insulating substrate  210 . Also, the third insulating substrate  310  needs not be larger than the liquid crystal layer  410 .  
      A black matrix  311  is formed on the third insulating substrate  310 . The black matrix  311  is formed in a matrix pattern and prevents external light from being supplied to a channel area of the second thin film transistor  221 . Exemplary embodiments of the black matrix  311  may comprise chrome oxide or organic matter including a black pigment.  
      The second color filter  321  is formed in the openings of the black matrix  311 . The second color filter  321  comprises three sub layers  321   a ,  321   b  and  321   c  which are different in color.  
      An overcoat layer  331  is formed on the second color filter  321 . The overcoat layer  331  provides a flat surface on which a second common electrode  341  is disposed.  
      The second common electrode  341  is formed on the overcoat layer  331 . Exemplary embodiments of the second common electrode  341  are made of a transparent conductive material such as indium tin oxide (“ITO”) or indium zinc oxide (“IZO”). The second common electrode  341  supplies a voltage to the liquid crystal layer  410  together with the second pixel electrode  251 , thereby adjusting an arrangement of the liquid crystal layer  410 . The orientation of liquid crystal molecules in the liquid crystal layer  410  may be modified by an electric field to either transmit or block polarized light from passing therethrough. It is also possible for the liquid crystal molecule to allow a fraction of the light to pass through while blocking the rest depending on the voltage difference between the second common electrode  341  and the second pixel electrode; in this manner a gray scale may be established for each pixel.  
      The flexible printed circuit board  261  is attached to a first end of the second insulating substrate  210 . In one exemplary embodiment the flexible printed circuit board  261  is bent and connected with the driving circuit substrate  171 . Between the flexible printed circuit board  261  and the second insulating substrate  210  and between the flexible printed circuit board  261  and the driving circuit substrate  171  are provided anisotropic conductive films  262  and  530 .  
      In the LCD  3 , the second insulating substrate  210  and the third insulating substrate  310  may respectively comprise a polarization plate (not shown) which is formed on a surface of each substrate closest to the liquid crystal layer.  
      Hereinafter, a process of operating the first exemplary embodiment of a display device  1  according to the present invention will be described.  
      When the OLED display  2  operates, the driving circuit substrate  171  supplies a driving signal to the OLED display  2 , but not to the LCD  3 . The white light emitted by the organic light emitting layer  152  according to the driving signal passes through the first insulating substrate  110  and the first color filter  132 , and in passing through the color filter is tinted a color to display an image. The white light is transmitted to the second insulating substrate  210 , but the image is not displayed since the LCD  3  does not receive the driving signal. When the LCD  3  is in a black mode, e.g., when the liquid crystal molecules are arranged so as to block the passage of light, the white light is not emitted to the outside of the LCD  3 .  
      When the LCD  3  operates, the driving circuit substrate  171  supplies the driving signal to the LCD  3  to display the image, and to the OLED display  2  so that the organic light emitting layer  152  emits light uniformly. The uniform light generated by the organic light emitting layer  152  is incident to the liquid crystal layer  410  through the first common electrode  160  and the second insulating substrate  210 . The transmittivity of the light incident to the liquid crystal layer  410  is adjusted by the liquid crystal layer  410  as described above. Then, the light passes through the second color filter  321  and receives the colored light to display the image. The light emitted by the organic light emitting layer  152  and transmitted to the first insulating substrate  110  receives the color when passing through the first color filter  132 . Here, the OLED display  2  does not display the image, and instead displays a white picture.  
      When the LCD  3  operates, the OLED display  2  may operate in other modes. For example, the OLED display  2  may supply the white light to a region of the display device  1  which overlaps the liquid crystal layer  410 . Also, the organic light emitting layer  152  corresponding to a dark region of the image displayed by the LCD  3  may supply a light at a low brightness and the organic light emitting layer  152  corresponding to a bright region supplies a light at a high brightness.  
      The first exemplary embodiment of an OLED display  2  according to the present invention operates as a backlight of the LCD  3 . Accordingly, an additional backlight unit is not required, thereby reducing the thickness of the display device  1 . Particularly, the OLED display  2  and the LCD  3  share the second insulating substrate  210 , thereby further reducing the thickness of the display device  1 .  
       FIGS. 3-5  are cross-sectional views of second through fourth exemplary embodiments of a display device according to the present invention.  
      As shown in  FIG. 3 , in a second exemplary embodiment of a display device  200  according to the present invention, a first insulating substrate  110  and a second insulating substrate  210  are connected with driving circuit substrates  171  and  271 , respectively. Anisotropic conductive films  172  and  272  are provided between the driving circuit substrate  171  and the first insulating substrate  110 , and between the driving circuit substrate  271  and the second insulating substrate  210 , respectively.  
      As shown in  FIG. 4 , an OLED display  2  and an LCD  3  of a third exemplary embodiment of a display device  300  according to the present invention do not share an insulating substrate. The OLED display  2  comprises a first insulating substrate  110  and a fourth insulating substrate  610 . The LCD  3  comprises a second insulating substrate  210  and a third insulating substrate  310 . A driving circuit substrate  271  and an anisotropic conductive film  272  are provided on an end part of the second insulating substrate  210 . A moisture absorber  611  is provided on the fourth insulating substrate  610  on the surface nearest to the first insulating substrate  110 .  
      An optical film  630  is disposed between the second insulating substrate  210  and the fourth insulating substrate  610 . The optical film  630  may comprise one of a diffusion film, a prism film and a reflection polarization film. The optical film  630  improves uniformity and planarization of light emitted by the OLED display  2 , thereby improving overall display picture quality of the LCD  3 .  
      As shown in  FIG. 5 , in a fourth exemplary embodiment of a display device  400  according to the present invention, a color filter film  640  is attached to the outside of a first insulating substrate  110 . The color filter film  640  may comprise a black matrix which is formed on a film in a matrix pattern, and plurality of sub layers which are formed in the holes of the black matrix and have differing colors. A manufacturing process of the OLED display  2  can thereby be simplified.  
       FIGS. 6A and 6B  are front perspective views of an exemplary embodiment of an electronic device utilizing the exemplary embodiment of a display device according to the present invention.  FIG. 6A  shows the exemplary embodiment of a display device in a closed configuration and  FIG. 6B  shows the exemplary embodiment of a display device in an open configuration.  
      An electronic device  700  comprises a folding-type cell phone. The electronic device  700  comprises a main body  710  which has a key pad  711 , as shown in  FIG. 7B ; and a display part  720  which is rotatably connected with the main body  710  and displays an image thereon.  
      The display part  720  comprises an internal display window  721  which is relatively large-sized and displays an image when the electronic device  700  is unfolded. The internal display window may be made of the OLED display  2 . The display part  720  also comprises an external display window  722  which is relatively small-sized and displays an image when the electronic device  700  is folded. The external display window may be made of the LCD  3 .  
      The display device  1  according to the present invention may be mounted to the display part  720 . The internal display window  721  may employ the OLED display  2  and the external display window  722  may employ the LCD  3 . The display device  1  according to the present invention is thin without an additional backlight unit, thereby reducing the thickness of the display part  720 .  
      As described above, the present invention provides a display device which is thin in thickness and displays an image on both sides.  
      Although a few exemplary embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the appended claims and their equivalents.