Patent Publication Number: US-10779400-B2

Title: Flexible display device

Description:
This application is a continuation application of U.S. application Ser. No. 16/575,528 filed Sep. 19, 2019 and issued as U.S. Pat. No. 10,645,802 on May 5, 2020, which is a continuation application of U.S. application Ser. No. 16/383,986 filed Apr. 15, 2019 and issued as U.S. Pat. No. 10,462,896 on Oct. 29, 2019, which claims priority to Korean Patent Application No. 10-2018-0083080, filed on Jul. 17, 2018, and all the benefits accruing therefrom under 35 U.S.C. § 119, the disclosure of which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     1. Field 
     The present disclosure relates to a flexible display device, and more particularly, to a flexible display device having a driving member attached thereto. 
     2. Description of the Related Art 
     A flexible display device can be used not only in portable electronic devices such as a mobile phone, a smartphone, a tablet personal computer (“PC”), a smart watch, a watch phone, a mobile communication terminal, an electronic notebook, an electronic book, a portable multimedia player (“PMP”), a navigation device, or an ultra-mobile PC (“UMPC”), but also in various other products such as a television (“TV”), a notebook computer, a monitor, a billboard, or an Internet-of-Things (“IoT”) device. 
     A flexible display device, which is a display device having a display unit, signal wiring and the like formed on a flexible substrate, made of a flexible material such as plastic, so as to be able to display an image even when bent like paper, has attracted attention as a next-generation display device. 
     The range of application of the flexible display device has diversified from a static computer monitor and a TV to a mobile personal portable device, and research has been conducted on a flexible display device having a relatively large display area and a reduced volume and weight. 
     SUMMARY 
     Exemplary embodiments of the present disclosure provide a flexible display device capable of preventing defects by reducing or effectively preventing a flexible substrate or signal wiring from cracking even when bent. 
     However, exemplary embodiments of the present disclosure are not restricted to those set forth herein. The above and other exemplary embodiments of the present disclosure will become more apparent to one of ordinary skill in the art to which the present disclosure pertains by referencing the detailed description of the present disclosure given below. 
     According to an exemplary embodiment of the present disclosure, there is provided a display device including a flexible substrate including a bending area corresponding to an area at which the display device is bent, and a first area and a second area which is spaced apart from the first area by the bending area; a display element unit which is controlled to display an image, the display element unit disposed on the flexible substrate in the first area of the flexible substrate; and a buffer member disposed on the flexible substrate in the bending area of the flexible substrate. The buffer member in the bending area includes: a first buffer member having a first maximum thickness, and a second buffer member having a second maximum thickness which is smaller than the first maximum thickness, and among the first buffer member and the second member, the first buffer member disposed closer to the first area and the second member disposed closer to the second area. 
     In an exemplary embodiment, a display device may further include on a same side of the flexible substrate which is opposite to that on which the display element unit is disposed: a first supporting member coupled to the first area of the flexible substrate; and a second supporting member coupled to the second area of the flexible substrate, a first inner side of the first supporting member facing a second inner side of the second supporting member. The flexible substrate bent at the bending area may dispose along a thickness direction of the display device: the second supporting member overlapping the first supporting member, the first inner side of the first supporting member corresponding to a border between the first area of the flexible substrate and the first buffer member, and the second inner side of the second supporting member corresponding to a border between the second area of the flexible substrate and the second buffer member. 
     In an exemplary embodiment, the first buffer member may be in contact with the first inner side of the first supporting member, the second buffer member may be in contact with the second inner side of the second supporting member, and a thickness of a portion of the first buffer member which is directly adjacent to and in contact with the first inner side may be greater than a thickness of a portion of the second buffer member which is directly adjacent to and in contact with the second inner side. 
     In an exemplary embodiment, the first supporting member, the first buffer member, the second buffer member, and the second supporting member may include a same material and form one integral body with one another. 
     In an exemplary embodiment, the first and second buffer members may further include a moisture permeation preventing material. In the bending area, the first buffer member may cover a portion of the flexible substrate directly adjacent to the first inner side of the first supporting member, and the second buffer member may cover a portion of the flexible substrate directly adjacent to the second inner side of the second supporting member. 
     In an exemplary embodiment, each of the first and second supporting members may be thicker than each of the first and second buffer members. 
     In an exemplary embodiment, the flexible substrate bent at the bending area may further dispose: the first buffer member at a bent end of the display device, and a bottom surface of the first supporting member facing a top surface of the second supporting member along the thickness direction of the display device. The first buffer member may extend from the bent end of the display device to dispose a first extension member thereof overlapping the bottom surface of the first supporting member along the thickness direction of the display device. 
     In an exemplary embodiment, the first maximum thickness of the first buffer member extending to define the first extension member may be greater than a thickness of the first supporting member. 
     In an exemplary embodiment, the flexible substrate bent at the bending area may further dispose the first extension member of the first buffer member further overlapping the second supporting member along the thickness direction of the display device. 
     In an exemplary embodiment, on a same side of the flexible substrate on which the display element unit is disposed, the display device may further include a driving member which controls the display element unit to display the image, the driving member disposed in the second area of the flexible substrate; and a circuit board which provides a control signal to the driving member, the circuit board connected to the flexible substrate at the second area thereof to be spaced apart from the driving member in the second area. 
     In an exemplary embodiment, on a same side of the flexible substrate on which the display element unit is disposed, the display device may further include a third buffer in the first and second areas of the flexible substrate, the third buffer member facing the first and second buffer members in the bending area of the flexible substrate. 
     In an exemplary embodiment, a thickness of the third buffer member may be smaller than each of the first and second maximum thicknesses of the first and second buffer members, and a material of the third buffer member may differ from materials of the first and second buffer members. 
     In an exemplary embodiment, a display device includes a flexible substrate including a bending area corresponding to an area at which the display device is bent, and a first area and a second area which is spaced apart from the first area by the bending area; a display element unit which is controlled to display an image, the display element unit disposed on the flexible substrate in the first area of the flexible substrate; a buffer member disposed on the flexible substrate in the bending area of the flexible substrate, the buffer member in the bending area including: a first buffer member having a first maximum thickness, and a second buffer member having a second maximum thickness which is smaller than the first maximum thickness, and among the first buffer member and the second member, the first buffer member disposed closer to the first area and the second member disposed closer to the second area; and on a same side of the flexible substrate which is opposite to that on which the display element unit is disposed: a first supporting member coupled to the first area of the flexible substrate, the first supporting member including a first inner side closest to the bending area being curved to have a first curvature; and a second supporting member coupled to the second area of the flexible substrate, the second supporting member including a second inner side closest to the bending area. The flexible substrate bent at the bending area disposes along a thickness direction of the display device: the second supporting member overlapping the first supporting member, and the curved first inner side of the first supporting member protruding toward the first buffer member. 
     In an exemplary embodiment, the first buffer member may be in contact with the curved first inner side of the first supporting member, the second buffer member may be in contact with the second inner side of the second supporting member, and a thickness of a portion of the first buffer member which is directly adjacent to and in contact with the curved first inner side may be greater than a thickness of a portion of the second buffer member which is directly adjacent to and in contact with the second inner side. 
     In an exemplary embodiment, the first supporting member, the first buffer member, the second buffer member, and the second supporting member may include a same material and form one integral body with one another. 
     In an exemplary embodiment, the first and second buffer members may further include a moisture permeation preventing material. In the bending area, the first buffer member may cover a portion of the flexible substrate directly adjacent to the curved first inner side, and the second buffer member may cover a portion of the flexible substrate directly adjacent to the second inner side. 
     In an exemplary embodiment, each of the first and second supporting members may be thicker than each of the first and second buffer members. 
     In an exemplary embodiment, the flexible substrate bent at the bending area may further dispose: the first buffer member at a bent end of the display device, and a bottom surface of the first supporting member facing a top surface of the second supporting member along the thickness direction of the display device. The first buffer member may extend from the bent end of the display device to dispose a first extension member thereof overlapping both the bottom surface of the first supporting member and the top surface of the second supporting member along the thickness direction of the display device, and the first maximum thickness of the first buffer member extending to define the first extension member may be greater than a thickness of the first supporting member. 
     In an exemplary embodiment, on a same side of the flexible substrate on which the display element unit is disposed, the display device may further include a third buffer member in the first and second areas of the flexible substrate, the third buffer member facing the first and second buffer members in the bending area of the flexible substrate. 
     In an exemplary embodiment, a thickness of the third buffer member may be smaller than each of the first and second maximum thicknesses of the first and second buffer members, and a material of the third buffer member may differ from materials of the first and second buffer members. 
     According to the aforementioned and other exemplary embodiments of the present disclosure, a flexible display device capable of reducing or effectively preventing defects by reducing or effectively preventing a flexible substrate or signal wiring from cracking even when bent. 
     Other features and exemplary embodiments may be apparent from the following detailed description, the drawings, and the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other features of the present disclosure will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings, in which: 
         FIG. 1  is a top plan view of an exemplary embodiment of a flexible display device according to the present disclosure; 
         FIG. 2  is a cross-sectional view of an exemplary embodiment of the flexible display device of  FIG. 1  in an unbent state; 
         FIG. 3  is a cross-sectional view an exemplary embodiment of the flexible display device of  FIG. 1  in a bent state; 
         FIG. 4  is a detailed cross-sectional view of the flexible display device of  FIG. 3 ; 
         FIGS. 5A and 5B  are cross-sectional views illustrating an exemplary embodiment of a method of forming a buffer member in a bending area of a flexible display device according to the present disclosure; 
         FIG. 6  is a cross-sectional view of another exemplary embodiment of a flexible display device in a bent state according to the present disclosure; 
         FIG. 7  is a cross-sectional view of still another exemplary embodiment of a flexible display device in a bent state according to the present disclosure; 
         FIG. 8  is an enlarged cross-sectional view of the flexible display device of  FIG. 7 ; 
         FIG. 9  is a cross-sectional view of yet another exemplary embodiment of a flexible display device in a bent state according to the present disclosure; 
         FIG. 10  is a cross-sectional view of yet another exemplary embodiment of a flexible display device in a bent state according to the present disclosure; and 
         FIG. 11  is a cross-sectional view of yet another exemplary embodiment of a flexible display device in a bent state according to the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The advantages and features of the invention and methods for achieving the advantages and features will be apparent by referring to the embodiments to be described in detail with reference to the accompanying drawings. However, the invention is not limited to the embodiments disclosed hereinafter, but can be implemented in diverse forms. The matters defined in the description, such as the detailed construction and elements, are nothing but specific details provided to assist those of ordinary skill in the art in a comprehensive understanding of the invention, and the invention is only defined within the scope of the appended claims. Like reference numerals refer to like elements throughout. 
     An element described as being related to another element such as being “on” another element or “located on” a different layer or a layer, includes both a case where an element is located directly on another element or a layer with no intervening element or layer therebetween and a case where an element is located on another element via another layer or still another element. In contrast, an element described as being is related to another element such as being “directly on” another element or “located directly on” a different layer or a layer, indicates a case where an element is located on another element or a layer with no intervening element or layer therebetween. 
     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 element, component, 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 herein. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting. As used herein, the singular forms “a,” “an,” and “the” are intended to include the plural forms, including “at least one,” unless the content clearly indicates otherwise. “At least one” is not to be construed as limiting “a” or “an.” “Or” means “and/or.” As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. 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 element 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 on 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 disclosure 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. 
     Exemplary embodiments are described herein with reference to cross section illustrations that are schematic illustrations of idealized embodiments. 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 described herein should not be construed as limited to the particular shapes of regions as 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 claims. 
     For a flexible display device having a display unit, signal wiring and the like formed on a flexible substrate, made of a flexible material such as plastic, so as to be able to display an image even when bent like paper, cracks may be generated in the flexible substrate and/or the signal wiring for various reasons. Such cracks may undesirably cause defects in a flexible display device. 
       FIG. 1  is a top plan view of an exemplary embodiment of a flexible display device according to the present disclosure, and  FIG. 2  is a cross-sectional view of the flexible display device of  FIG. 1  in an unbent or flat state. As an exemplary embodiment of being unbent or flat, the flexible display device and components thereof may be disposed extended in a single plane. 
     Referring to  FIGS. 1 and 2 , the flexible display device and components thereof may be disposed in a plane defined by two directions which cross each other. In  FIG. 1 , the vertical direction may define a first direction while the horizontal direction may represent a second direction which crosses the first direction. In  FIG. 2 , the vertical direction may define a third direction which crosses each of the first and second directions, to represent a thickness direction of the flexible display device and the components thereof, while the horizontal direction may define the first direction or the second direction. In  FIG. 1 , the thickness direction is defined into the page view. 
     Referring to  FIGS. 1 and 2 , a flexible display device  1  includes a display area DA in which an image is displayed and a non-display area NDA which is disposed at the periphery of the display area DA. The flexible display device  1  may have an overall rectangular shape with right-angled or rounded edges, such as at corners thereof in the top plan view. However, the shape of the display area DA is not particularly limited, and the display rea DA may have a circular shape, an elliptical shape, or various other shapes in the top plan view. 
     The non-display area NDA is disposed on the periphery of the display area DA. The non-display area NDA may be disposed adjacent to both of opposing relatively short sides of the display area DA. Also, the non-display area NDA may be disposed adjacent not only to both of the opposing relatively short sides, but also to both of opposing relatively long sides of the display area DA, and may be disposed at all the sides of the display area DA to surround the display area DA. That is, the non-display area NDA adjacent to the display area DA may form the borders of the display area DA. 
     The flexible display device  1  may include a display panel DP, which generates and displays an image such as with light, and a driving member  200 , which is attached to the display panel DP and forms pixel circuitry of the display panel DP. The display panel DP may define a display screen of the flexible display device  1 . In an exemplary embodiment, an outer surface of the display panel DP may define the display screen of the flexible display device  1 . The driving member  200  may include a driver integrated circuit (“IC”). 
     The display panel DP may include components or devices (not illustrated) which are controlled or driven to generate and/or display an image. These components or devices may be disposed in the display area DA. A control or driving signal may be applied to these components or devices to generate and/or display the image at the display area DA. 
     In an exemplary embodiment, for example, an organic light-emitting diode (“OLED”) display panel may be used as the display panel DP. In the description that follows, it is assumed that the display panel DP is an OLED display panel, but the present disclosure is not limited thereto. That is, various other display panels such as a liquid crystal display (“LCD”) panel, a field emission display (“FED”) panel, or an electrophoretic display panel may be used as the display panel DP. 
     In an exemplary embodiment, the display panel DP may include a main area MA and a bending area BA at which the display panel DP and components thereof may be bent from a flat state thereof. The main area MA may be flat. In the main area MA, the display area DA and a part of the non-display area NDA may be disposed. It will be understood that as used herein related to bending, a reference to “display panel DP” also applies to the components of the display panel DP. 
     The bending area BA may be disposed on at least one side of the main area MA.  FIGS. 1 and 2  illustrate that there is provided one bending area BA near the lower side of the main area MA in the top plan view. Alternatively, the bending area BA may be disposed near the left, right or upper side of the main area MA in the top plan view. Still alternatively, the bending area BA may be disposed near two or more sides of the main area MA. 
     The display panel DP at the bending area BA may be bent in an opposite direction to a display direction. As illustrated in  FIG. 2 , the display direction is indicated downward as ‘light direction’ in a case where the flexible display device  1  is of a top emission type. In an exemplary embodiment, the non-display area NDA may partially overlap with the main area MA in a thickness direction of the flexible display device  1 . By bending at least part of the display panel DP in the non-display area NDA in the opposite direction to the display direction, the bezel of the flexible display device  1  can be reduced when the display panel DP is bent. 
     The display panel DP may further include a sub-area SA extending from the bending area BA. The main area MA, the bending area BA and the sub-area SA may together define a whole planar area of the display panel DP. When the display panel DP is bent, a plane in which the sub-area SA is disposed may be parallel to a plane in which the main area MA is disposed. When the display panel DP is bent, the sub-area SA may overlap with the main area MA in the thickness direction. In an exemplary embodiment, when the display panel DP is bent, the entire sub-area SA may overlap with the main area MA in the thickness direction. The bending area BA and the sub-area SA may both be included into the non-display area NDA, but the present disclosure is not limited thereto. 
     The display panel DP may include a pad area disposed in the non-display area NDA. The pad area may be disposed in the sub-area SA, but the present disclosure is not limited thereto. Alternatively, the pad area may be disposed in the main area MA or the bending area BA. 
     The pad area may include a plurality of pad terminals (not illustrated) through which a control or driving signal is provided to the display panel DP from outside thereof. The pad terminals may be connected to signal wiring  150  extending from the display area DA and into the non-display area NDA. The signal wiring  150  may be connected to the components or devices of the display panel DP described above. The driving member  200  may be attached to the pad terminals. The control or driving signal may be provided through the driving member  200  to the display panel DP from outside thereof. In  FIG. 1 , the driving member  200  may be considered as corresponding to the pad area without being limited thereto. 
     The flexible display device  1  may further include a printed circuit board PCB. The printed circuit board PCB may be attached to a part of the non-display area NDA on the outside of the pad area. That is, the pad area to which the driving member  200  is attached may be disposed between the display area DA and the area to which the printed circuit board PCB is attached. The printed circuit board PCB may be attached to a lower part of the sub-area SA. The printed circuit board PCB may be a flexible printed circuit board, but the present disclosure is not limited thereto. The printed circuit board PCB may be connected to the display panel DP via a flexible film. 
     The flexible display device  1  may include the display panel DP, a back plate ( 310  and  320 ), and a buffer member ( 350  and  360 ). 
     The display panel DP may include a flexible substrate  110 , which is disposed across the entirety of the display panel DP, a pixel array layer  120  which is disposed on the flexible substrate  110  in the main area MA, and a thin encapsulation layer  130  having a relatively small thickness. The display panel DP may further include the driving member  200 , which is disposed on the flexible substrate  110  in the sub-area SA. 
     The flexible substrate  110  may be a substrate for supporting various elements of the display panel DP and/or the flexible display device  1 . The flexible substrate  110  may be disposed across the main area MA, the bending area BA, and the sub-area SA and may be understood as including areas respectively corresponding to those of the main area MA, the bending area BA and the sub-area SA. 
     The flexible substrate  110  may include or be formed of a soft material. In an exemplary embodiment, for example, the flexible substrate  110  may be a film including one or more material selected from a polyester polymer, a silicone polymer, an acrylic polymer, a polyolefin polymer, and a copolymer thereof. Specifically, the flexible substrate  110  may include one or more material selected from polyethylene terephthalate (“PET”), polybutylene terephthalate (“PBT”), polysilane, polysiloxane, polysilazane, polycarbosilane, polyacrylate, polymethacrylate, polymethylacrylate, polyethylacrylate, polyethylmethacrylate, a cyclic-olefin copolymer (“COC”), a cyclic-olefin polymer (“COP”), polyethylene (“PE”), polypropylene (“PP”), polyimide (“PI”), polymethyl methacrylate (“PMMA”), polystyrene (“PS”), polyacetal (“POM”), polyetheretherketone (“PEEK”), polyester sulfone (“PES”), polytetrafluoroethylene (“PTFE”), polyvinyl chloride (“PVC”), polycarbonate (“PC”), polyvinylidene fluoride (“PVDF”), a perfluoroalkyl polymer (“PFA”), a styrene acrylonitrile copolymer (“SAN”), and a combination thereof. 
     In some exemplary embodiments, in a case where the flexible display device  1  is implemented as a transparent flexible display device, the flexible substrate  110  may include or be formed of a transparent soft material. 
     In an exemplary embodiment of manufacturing a flexible display device, the flexible substrate  110  may be formed by applying a plastic material, to a predetermined thickness, on the top surface of a release layer which is disposed on a relatively thick carrier substrate, and curing the plastic material. Here, the carrier substrate may be separated from the flexible substrate  110  by releasing the release layer such as through a laser release process. 
     The pixel array layer  120  may be provided on the flexible substrate  110  and may be controlled or driven to display an image. Specifically, the pixel array layer  120  may be disposed on the top surface of the flexible substrate  110 . The top surface of the flexible substrate  110  may be a surface of the flexible substrate  110  in an emission direction in which light is transmitted in a thickness direction. The definition of the top and bottom surfaces of the flexible substrate  110  may apply throughout the description that follows. 
     The pixel array layer  120  may be disposed on the flexible substrate  110  in the main area MA and may include a plurality of pixels which are controlled or driven to display an image. Although not specifically illustrated, the pixel array layer  120  may include thin-film transistors (“TFTs”), anode electrodes, an organic light-emitting layer (“EL”) and cathode electrodes. 
     The thin-film transistors may be provided in transistor areas of the pixels disposed on the flexible substrate  110  and may include a plurality of conductive layers and a plurality of insulating layers. Here, the thin-film transistors may be a-Si thin-film transistors, poly-Si thin-film transistors, oxide thin-film transistors or organic thin-film transistors. 
     The anode electrodes may be provided on the thin-film transistors and may be connected to the thin-film transistors. The anode electrodes may include indium tin oxide (“ITO”), indium zinc oxide (“IZO”), zinc oxide (ZnO) or indium oxide (In 2 O 3 ). In a case where the flexible display device  1  is of a top emission type, the anode electrodes may further include silver (Ag), magnesium (Mg), aluminum (Al), platinum (Pt), palladium (Pd), gold (Au), nickel (Ni), neodymium (Nd), iridium (Ir), chromium (Cr), lithium (Li), calcium (Ca), or a combination thereof. 
     The organic light-emitting layer may be disposed or formed on the anode electrodes. The display panel DP may include a hole injection layer (“HIL”) and/or a hole transport layer (“HTL”) disposed between the organic light-emitting layer and the anode electrodes and may further include an electron transport layer (“ETL”) and/or an electron injection layer (“EIL”) disposed between the organic light-emitting layer and the cathode electrodes. 
     The cathode electrodes may be disposed on the organic light-emitting layer. In an exemplary embodiment, the cathode electrodes may be disposed or formed on the entire organic light-emitting layer. The cathode electrodes may include Li, Ca, LiF/Ca, LiF/Al, Al, Mg, Ag, Pt, Pd, Ni, Au Nd, Ir, Cr, BaF, Ba, or a combination thereof (e.g., the combination of Ag and Mg). In a case where the flexible display device  1  is of a top emission type, a relatively thin conductive material layer may be formed as the cathode electrodes. The cathode electrodes may further include a transparent conductive film disposed on the thin conductive material layer. The transparent conductive film may include indium tin oxide, indium zinc oxide, zinc oxide, or indium tin zinc oxide. 
     The thin encapsulation layer  130  may be disposed on the pixel array layer  120 . The thin encapsulation layer  130 , which is for reducing or effectively preventing the infiltration of moisture into the pixels, may be disposed or formed to cover the pixel array layer  120 . Although not specifically illustrated, the thin encapsulation layer  130  may be a layer in which at least one inorganic film and at least one organic film are alternately stacked. In an exemplary embodiment, for example, the thin encapsulation layer  130  may include a first inorganic film, an organic film, and a second inorganic film that are sequentially stacked. 
     The display panel DP may further include a barrier layer (not illustrated) provided on the thin encapsulation layer  130 . The barrier layer may reduce or effectively prevent the infiltration of moisture into the pixels. The barrier layer may include a material having a relatively low moisture permeability such as, for example, a polymer material. 
     The display panel DP may further include a polarization film (not illustrated) disposed on the barrier layer. The polarization film may improve the optical characteristics of the display panel DP by reducing or effectively preventing the reflection of external light. 
     As illustrated in  FIGS. 2 and 3 , the driving member  200  may be a driver integrated chip. In an exemplary embodiment, for example, the driver integrated chip may be attached to the display panel DP, which includes a plastic substrate, in a chip-on-plastic (“COP”) manner. The driver integrated chip may be attached to a flexible film in a chip-on-film (“COF”) manner to be connected to the display area DA. The driving member  200  may be attached to the pad area of the display panel DP. Specifically, in the pad area of the display panel DP, a plurality of pad terminals (not illustrated) may be provided, and bumps (not illustrated) of the driving member  200  may be electrically connected to the pad terminals of the display panel DP. 
     The flexible substrate  110  includes the signal wiring  150 , which transmits external signals to the display area DA, such as to the pixels of the display panel DP. The signal wiring  150  connected to the pixels of the display panel DP may extend from one side of the display area DA and may be connected to the driving member  200  corresponding to the pad area. That is, the display area DA and the driving member  200  may be electrically connected by the signal wiring  150 . 
     The signal wiring  150  may include or be formed of a conductive material, particularly, a conductive material with excellent ductility, to minimize cracks. In an exemplary embodiment, for example, the signal wiring  150  may include or be formed of a conductive material with excellent ductility such as gold, silver, or aluminum, but the material of the signal wiring  150  is not particularly limited. The signal wiring  150  may include or be formed of various conductive materials that can be used to fabricate components or devices of the display panel DP, like those within the display area DA of the display panel DP. Specifically, the signal wiring  150  may also include or be formed of molybdenum (Mo), chromium, titanium (Ti), nickel, neodymium, copper (Cu), or an alloy of silver and magnesium. The signal wiring  150  may be disposed or formed to have a multilayer structure including various conductive materials, for example, a triple-layer structure of titanium/aluminum/titanium, but the present disclosure is not limited thereto. 
     When the flexible substrate  110  is bent, the flexible substrate  110  may be stressed due to a tensile force. Also, when the flexible substrate  110  is bent, the flexible substrate  110  may be stressed by external shock, for example, by being placed in contact with the corners of the back plate ( 310  and  320 ), which will be described later. For these reasons, cracks may be generated in the flexible substrate  110 , and as a result, the signal wiring  150  on the flexible substrate  110  may be disconnected. 
     In an asymmetrical bending structure that will be described, the signal wiring  150  is more likely to be disconnected in the main area MA than in the sub-area. SA. This will be described later in detail. 
     A center line CL is defined for the flexible display device  1 . A center line of the display panel DP which will be bent to form the bent flexible display device  1  corresponds to the center line CL thereof. Referring to  FIG. 3 , for example, the display panel DP and the flexible display device  1  may be bent about the center line CL to dispose an upper thickness of the bent flexible display device  1  above the center line CL, and a lower thickness below the center line CL. The center line CL extends in a direction perpendicular to the flexible substrate  110  at a center of an open area (OP 1  and OP 2  together, hereinafter referenced as “OP”). The flexible substrate  110  may include first and second flexible substrate portions  111  and  112  opposite to each other with respect to the center line CL, which may hereinafter be referred to as first and second flexible substrates  111  and  112 . The first flexible substrate  111  may extend from one side of a portion of the flexible substrate  110  in the main area MA and into the bending area. BA to be in contact with the second flexible substrate  112 . The second flexible substrate  112  may be in contact with the first flexible substrate  111  and may extend from the first flexible substrate  111  in the bending area BA to be in contact with a portion of the flexible substrate  110  in the sub-area. SA. 
     The back plate ( 310  and  320 ) may be disposed on the bottom surface of the flexible substrate  110  and may support the display panel DP including the flexible substrate  110 . In an exemplary embodiment of manufacturing a flexible display device, the back plate ( 310  and  320 ) may include or be formed by a lamination process following a laser release process for separating a carrier substrate from the flexible substrate  110 . The back plate ( 310  and  320 ) may include a thermoplastic material. In an exemplary embodiment, for example, the back plate ( 310  and  320 ) may include a polyacrylic material such as polyethylene terephthalate, polycarbonate, polyether sulfone, polyethylene naphthalate (“PEN”), or polynorbornene (“PNB”). In an exemplary embodiment, for example, the back plate ( 310  and  320 ) may be a thermoplastic film. 
     The back plate ( 310  and  320 ) may include first and second back plates  310  and  320 , respectively. The first and second back plates  310  and  320  may be flat plates. The first back plate  310  may be disposed on a part of the bottom surface of the flexible substrate  110  overlapping with the main area MA in the thickness direction. In an exemplary embodiment, for example, the first back plate  310  may overlap with the main area MA and may support the pixel array layer  120  in the main area MA, thereby maintaining the pixel array layer  120  to be flat. 
     The second back plate  320  may be disposed on a part of the bottom surface of the flexible substrate  110  overlapping with the sub-area SA in the thickness direction. The second back plate  320  may be disposed below the first back plate  310  along the thickness direction when the flexible substrate  110  is bent as shown in  FIG. 3 . In an exemplary embodiment, for example, the second back plate  320  may overlap with the sub-area SA and may support the signal wiring  150  and the driving member  200  in the sub-area SA, thereby maintaining one end of the flexible substrate  110  in the sub area SA to be flat. 
     The first and second back plates  310  and  320  are illustrated as not overlapping with the bending area BA, but the present disclosure is not limited thereto. Alternatively, a part of the first back plate  310  may overlap with the bending area BA. 
     The first back plate  310  may include a first inner side  311  overlapping or aligned with the border between the main area MA and the first flexible substrate  111  in the thickness direction. The second back plate  320  may include a second inner side  321  overlapping or aligned with the border between the second flexible substrate  112  and the sub-area SA in the thickness direction. 
     The first and second inner sides  311  and  321  of the first and second back plates  310  and  320  are illustrated as being straight, rather than curved, but the present disclosure is not limited thereto. Alternatively, one or more among the first and second inner sides  311  and  321  may be curved with a predetermined curvature. 
     The back plate ( 310  and  320 ) may include or define the open area OP. The open area OP may be an area of the flexible substrate  110  where the back plate ( 310  and  320 ) is not disposed. The open area OP may overlap with the bending area BA in the thickness direction. The open area OP may have the same size as the bending area BA, such as along the first and/or second directions. The open area OP may be disposed between the first and second back plates  310  and  320 . That is, the first and second back plates  310  and  320  may be spaced apart from each other by the open area OP. 
     The open area OP may include first and second open areas OP 1  and OP 2 . The first open area OP 1  may be closer to the main area MA than to the sub-area SA, and the second open area OP 2  may be closer to the sub-area SA than to the main area MA. The first open area OP 11  may be adjacent to the first back plate  310 . The second open area OP 2  may be adjacent to the second back plate  320 . The first and second back plates  310  and  320  may be the same distance apart from the center line CL. 
     At the open area OP, the flexible display device  1  may be bent to have a predetermined curvature. The open area OP of the back plate ( 310  and  320 ) is illustrated as having the same size as the bending area BA, but the present disclosure is not limited thereto. Alternatively, the open area OP may have a larger or smaller size than the bending area BA. 
     The buffer member ( 350  and  360 ) may be disposed to overlap with a part of the bottom surface of the flexible substrate  110  in the bending area BA. The buffer member ( 350  and  350 ) may include first and second buffer members  350  and  360 . The first buffer member  350  may be disposed on the bottom surface of the first flexible substrate  111 , such as being disposed directly on the bottom surface of the first flexible substrate  111 , and may be provided in the first open area OP 1 . The first buffer member  350  may be disposed adjacent to the main area MA. 
     The second buffer member  360  may be disposed on the bottom surface of the second flexible substrate  112 , such as being disposed directly on the bottom surface of the second flexible substrate  112 , and may be provided in the second open area OP 2 . The second buffer member  360  may be disposed adjacent to the sub-area SA. 
     The first buffer member  350  may be in contact with the first back plate  310 . The second buffer member  360  may be in contact with the second back plate  320 . Specifically, the first buffer member  350  may be in contact with the first inner side  311  of the first back plate  310  and with the first flexible substrate  111 . The second buffer member  360  may be in contact with the second inner side  321  of the second back plate  320  and with the second flexible substrate  112 . As will be described later, a cross-sectional thickness of a part of the first buffer member  350  in contact with the first inner side  311  of the first back plate  310  may be greater than a cross-sectional thickness of a part of the second buffer member  360  in contact with the second inner side  321  of the second back plate  320 . 
     The first inner side  311  of the first back plate  310  may be provided at the open area OP and may overlap with the border between the main area MA and the bending area BA of the flexible substrate  110 , and the second inner side  321  of the second back plate  320  may be provided in the open area OP to be spaced apart from the first inner side  311  of the first back plate  310  in a direction toward the sub-area SA and may overlap with the border between the sub-area SA and the bending area BA of the flexible substrate  110 . However, the present disclosure is not limited to this. That is, the first inner side  311  of the first back plate  310  and the second inner side  321  of the second back plate  320  may be disposed in the main area MA and the sub-area SA, respectively, or may both be disposed in the bending area BA. 
     The first and second buffer members  350  and  360  may include a thermoplastic material. In an exemplary embodiment, for example, the first and second buffer members  350  and  360  may include a polyacrylic material such as polyethylene terephthalate, polycarbonate, polyether sulfone, polyethylene naphthalate, or polynorbornene. In an exemplary embodiment, for example, the first and second buffer members  350  and  360  may be thermoplastic films. The material of the first and second buffer members  350  and  360  may be the same as the material of the first and second back plates  310  and  320 . That is, the first and second buffer members  350  and  360  may be formed as one integral body with the first and second back plates  310  and  320 , but the present disclosure is not limited thereto. As one integral body, one or more among the first and second buffer members  350  and  360  and the first and second back plates  310  and  320  may extend or protrude to define one or more of the others among the first and second buffer members  350  and  360  and the first and second back plates  310  and  320 . Alternatively, the material of the first and second buffer members  350  and  360  may be different from the material of the first and second back plates  310  and  320 . Also, the first and second buffer members  350  and  360  may include different materials or the same material. 
     The cross-sectional thickness of the first and second buffer members  350  and  360  may be smaller than those of the first and second back plates  310  and  320  so as not to increase the total thickness of the flexible display device  1 , but the present disclosure is not limited thereto. That is, the maximum cross-sectional thickness of the first and second buffer members  350  and  360  may be smaller than the minimum cross-sectional thickness of the first and second back plates  310  and  320 . Alternatively, the cross-sectional thickness of the first and/or second buffer members  350  and  360  may be greater than the thickness of the first and second back plates  310  and  320 . In an exemplary embodiment, for example, the thickness of the first buffer member  350  may be greater than the thickness of the back plate ( 310  and  320 ), and the thickness of the second buffer member  360  may be smaller than the thickness of the back plate ( 310  and  320 ). Such thicknesses may be considered across an entirety of the buffer members  350  and  360 , or at portions thereof closest to and directly adjacent to the back plate ( 310  and  320 ). 
     The first and second buffer members  350  and  360  may have the same width taken along the first and/or second directions. The width of the first and second buffer members  350  and  360  may be taken between the center line CL and a respective back plate ( 310  or  320 ). At a portion in contact with the first back plate  310 , the first buffer member  350  may have a first thickness T 1  defined by a surface furthest from the first flexible substrate  111 . At a portion in contact with the second back plate  320 , the second buffer member  360  may have a second thickness T 2  defined by a surface furthest from the second flexible substrate  112 . The first thickness T 1  may be the maximum thickness of the overall first buffer member  350 . The second thickness T 2  may be the maximum thickness of the overall second buffer member  360 . The first thickness T 1  may be greater than the second thickness T 2 . 
     Along the first and/or second directions, the average thickness of the first buffer member  350  may be greater than the average thickness of the second buffer member  360 . Also, the volume of the first buffer member  350  may be greater than the volume of the second buffer member  360 , where volumes are defined by a product of a thickness dimension, a first direction dimension and a second direction dimension. The contact area between the first buffer member  350  and the bottom surface of the flexible substrate  110  is the same as the contact area between the second buffer member  360  and the bottom surface of the flexible substrate  110 , such as considering the buffer members  350  and  360  symmetrically disposed about the center line. Since the contact area between the first buffer member  350  and the bottom surface of the flexible substrate  110  is the same as the contact area between the second buffer member  360  and the bottom surface of the flexible substrate  110 , the volume of the first and second buffer members  350  and  360  may be defined by or comparable with the average thickness of the first and second buffer members  350  and  360 . The volume (or average thickness) of the first buffer member  350  in the first open area OP 1  may be asymmetrical with those of the second buffer member  360  in the second open area OP 2 , with respect to the center line CL. 
     The first thickness T 1  may be smaller than the thickness of the first back plate  310 . The second thickness T 2  may be smaller than the thickness of the second back plate  320 . In a case where the second buffer member  360  is not disposed in the second open area OP 2 , the second thickness T 2  may be zero, but the present disclosure is not limited thereto. Alternatively, the first thickness T 1  may be greater than the thickness of the first back plate  310 , in which case, the first buffer member  350  may be disposed on the top surface of the first back plate  310  to be in contact with, and cover, the top surface of the first back plate  310  at the first inner side  211 . 
     The first and second buffer members  350  and  360  may further include a moisture permeation preventing material. The moisture permeation preventing material may be disposed or formed as an inorganic film or an organic film, but the invention is not limited to thereto. Alternatively, the moisture permeation preventing material may be formed as a stack of an inorganic film and an organic film. The first buffer member  350  may cover an area in which the first inner side  311  and the first flexible substrate  111  are in contact with each other, and the second buffer member  360  may cover an area in which the second inner side  321  and the second flexible substrate  112  are in contact with each other. That is, the penetration of moisture into the flexible substrate  110  can be effectively prevented by the first and second buffer members  350  and  360 , and peeling of the back plate ( 310  and  320 ) from the flexible substrate  110  can be reduced or effectively prevented. In an exemplary embodiment, for example, the moisture permeation preventing material may include a silicone material with excellent thermal stability, excellent adhesiveness, and an excellent moisture permeation prevention function. 
     The arrangement of the first and second back plates  310  and  320  and the change of the curvature of the flexible substrate  110  when the flexible display device  1  is bent will hereinafter be described with reference to  FIGS. 3 and 4 . 
       FIG. 3  is a cross-sectional view of an exemplary embodiment of the flexible display device of  FIG. 1  in a bent state, and  FIG. 4  is a detailed cross-sectional view of the bent flexible display device of  FIG. 3 . The bent flexible display device of  FIGS. 3 and 4  are each formed by the flexible display device of  FIG. 2  which is bent. For convenience of illustration, the signal wiring  150  shown in  FIG. 2  is omitted from  FIGS. 3 and 4 . 
     Referring to  FIG. 3 , when the flexible display device  1  is bent, a part of the bending area BA may overlap with the main area MA in the thickness direction. That is, dead space in the bent flexible display device  1  can be reduced by reducing the size of the bending area BA disposed outside of the main area MA of the flexible display device  1  along the direction in which the bending area BA and the main area MA are arranged, and making the bending area BA overlap with the main area MA along the thickness direction. 
     In the bent flexible display device  1 , the second back plate  320  may be disposed below the first back plate  310  in the thickness direction. 
     Although not specifically illustrated, the bent flexible display device  1  may further include heat dissipation sheets, cushion sheets, a graphite layer, and a plurality of adhesive layers between the first and second back plates  310  and  320  which are disposed facing each other as illustrated in  FIG. 3 . 
     The heat dissipation sheets may be disposed on the back plate ( 310  and  320 ). Each of the heat dissipation sheets may have a stack of multiple layers. Each of the heat dissipation sheets may include two layers, and the two layers may be disposed on the first and second back plates  310  and  320 . The heat dissipation sheets may include a metal material. In an exemplary embodiment, for example, the heat dissipation sheets may be formed as relatively thin copper sheets, but the present disclosure is not limited thereto. The material of the heat dissipation sheets may be appropriately chosen. Since a metal material has excellent thermal conductivity, the heat dissipation sheets, which include or are formed of a metal material, can improve the thermal conductivity of the flexible display device  1 . Accordingly, heat generated in the flexible display device  1  can be efficiently released, and as a result, the heat dissipation efficiency of the flexible display device  1  can be improved. 
     The cushion sheets may be disposed on the heat dissipation sheets. The cushion sheets may be disposed between the heat dissipation sheets. Two cushion sheets may be provided and may be disposed to be spaced apart from each other in a direction in which the flexible substrate  110  extends. The cushion sheets may include or be formed as elastic members. In an exemplary embodiment, for example, the cushion sheets may include or be formed of a material such as polyurethane and may thus be able to absorb shock from the outside or the inside thereof. In an exemplary embodiments, the cushion sheets may be shock mitigation layers. Accordingly, the durability of the flexible display device  1  can be further improved. However, the material of the cushion sheets is not particularly limited. That is, the material of the cushion sheets may be appropriately chosen from among a variety of elastic materials. 
     The graphite layer may be disposed between the two cushion sheets, but the present disclosure is not limited thereto. The graphite layer may be disposed on the cushion sheets. The graphite layer may include carbon powder and may thus improve thermal conductivity. Accordingly, the thermal conductivity of the flexible display device  1  can be improved, and heat generated in the flexible display device  1  can be efficiently released. 
     The heat dissipation sheets, the cushion sheets, and the graphite layer may be bonded to one another via adhesive layers, but the present disclosure is not limited thereto. The heat dissipation sheets, the cushion sheets, and the graphite layer may be placed in contact with one another without any layers disposed therebetween. The heat dissipation sheets may be attached to the first and second back plates  310  and  320  via adhesive layers. 
     As illustrated in  FIG. 3 , an end of the bent flexible display device  1  is disposed at the left of the view. With reference to the end of the bent flexible display device  1 , the second inner side  321  of the second back plate  320  may be disposed further inside than the first inner side  311  of the first back plate  310 . That is, a portion the flexible display device  1  at the first inner side  311  of the first back plate  310  may define a maximum dimension of the bending area BA which is visible at an emission side of the flexible display device  1 , since remaining portions of the flexible display device  1  are disposed further inside this portion of the flexible display device  1 . This maximum dimension of the bending area BA which is visible at an emission side of the flexible display device  1  may define a bezel width of the flexible display device  1 . 
     Specifically, as illustrated in  FIG. 4 , the first inner side  311  of the first back plate  310  may include a first contact point  111 P where the first inner side  311  of the first back plate  310  is in contact with the first flexible substrate  111 , and the second inner side  321  of the second back plate  320  may include a second contact point  112 P where the second inner side  321  of the second back plate  320  is in contact with the second flexible substrate  112 . The flexible substrate  110  may include a first intersection point CP 1  where the flexible substrate  110  intersects the center line CL. 
     As a dotted line feature in  FIGS. 3 and 4 , the flexible substrate  110  may be bent symmetrically when the inner side  311  of the first back plate  310  and the second inner side  321  of the second back plate  320  overlap or align with each other in the thickness direction. When the flexible substrate  110  is bent symmetrically, as illustrated by the dotted line feature in  FIGS. 3 and 4 , the flexible substrate  110  may include a second intersection point CP 2  where the flexible substrate  110  intersects the center line CL. 
     An extension line connecting the first contact point  111 P and the first intersection point CP 1  may form a first line Line 1 . An extension line connecting the second contact point  112 P and the first intersection point CP 1  may form a second line Line 2 . An extension line connecting the first contact point  111 P and the second intersection point CP 2  may form a third line Line 3 . 
     The first line Line 1  may form a first average bending angle θ 1  with the center line CL. The second line Line 2  may form a second average bending angle θ 2  with the center line CL. The third line Line 3  may form a third average bending angle θ 3  with the center line CL. The first, second, and third average bending angles θ 1 , θ 2 , and θ 3  may all be acute angles. The tangent values of the first, second, and third average bending angles θ 1 , θ 2 , and θ 3  may form an average bending slope. Accordingly, the first flexible substrate  111  may have the first average bending angle θ 1  and a first average bending slope (SLOPE  1 ), and the second flexible substrate  112  may have the second average bending angle θ 2  and a second average bending slope (SLOPE  2 ). 
     In an exemplary embodiment, the first average bending angle θ 1  may be greater than the second and third average bending angles θ 2  and θ 3 , the second average bending angle θ 2  may be smaller than the first and third average bending angles θ 1  and θ 3 , and the third average bending angle θ 3  may be smaller than the first average bending angle θ 1  and may be greater than the second average bending angle θ 2 . Similarly, the first average bending slope (SLOPE  1 ) may be greater than the second and third average bending slopes (SLOPE  2  and SLOPE  3 ), the second average bending slope (SLOPE  2 ) may be smaller than the first and third average bending slopes (SLOPE  1  and SLOPE  3 ), and the third average bending slope (SLOPE  3 ) may be smaller than the first average bending slope (SLOPE  1 ) and may be greater than the second average bending slope (SLOPE  2 ). However, the present disclosure is not limited to this. Alternatively, the second average bending slope (SLOPE  2 ) may become greater than the third average bending slope (SLOPE  3 ) depending on the degree to which the second back plate  320  is recessed in a direction away from the end of the bent flexible display device  1 . 
     The first flexible substrate  111  may have a first average curvature (CV 1 ). The second flexible substrate  112  may have a second average curvature (CV 2 ). Since the first average bending slope (SLOPE  1 ) is greater than the second and third average bending slopes (SLOPE  2  and SLOPE  3 ), the first average curvature (CV 1 ) may be greater than the second and third average curvatures (CV 2  and CV 3 ). The second average curvature (CV 2 ) may be smaller than the first and third average curvatures (CV 1  and CV 3 ). The third average curvature (CV 3 ) may be smaller than the first average curvature (CV 1 ) and may be greater than the second average curvature (CV 2 ). 
     Since the first and second flexible substrates  111  and  112  have asymmetrical average bending angles, asymmetrical average bending slopes, and asymmetrical average curvatures, the flexible substrate  110  may be bent asymmetrically. When the flexible substrate  110  is bent asymmetrically, a part of the bending area BA which is outside of the main area MA in a flat state of the flexible display device  1 , may be disposed to overlap the main area MA in a bent state of the flexible display device. As a result, the bezel width of the flexible display device  1  can be reduced. Accordingly, an essentially bezel-less flexible display device can be provided. 
     However, the stress applied to the first flexible substrate  111  may be greater when the flexible substrate  110  is bent asymmetrically than when the flexible substrate  110  is bent symmetrically. Also, the stress applied to the first flexible substrate  111  may be greater than the stress applied to the second flexible substrate  112 . 
     Since the average bending angle, the average bending slope, and the average curvature of the first flexible substrate  111  are greater than the average bending angle, the average bending slope, and the average curvature of the second flexible substrate  112 , the first flexible substrate  111  may be bent to a greater extent than the second flexible substrate  112 . Accordingly, when the flexible substrate  110  is bent, the first flexible substrate  111  may receive more stress than the second flexible substrate  112 . Also, since the contact force between the first flexible substrate  111  and the first contact  111 P of the first inner surface  311  of the first back plate  310  is greater than the contact force between the second flexible substrate  112  and the second contact point  112 P of the second inner surface  321  of the second back plate  320 , the first flexible substrate  111  may receive more stress than the second flexible substrate  112 . That is, the stress applied to the flexible substrate  110  may be greater in the first flexible substrate  111  than in the second flexible substrate  112 . As a result, cracks may be generated in the signal wiring  150  on the first flexible substrate  111 , and eventually, the signal wiring  150  may be disconnected. 
     However, since the buffer member ( 350  and  360 ) is applied asymmetrically to the flexible substrate  110 , the stress caused by the asymmetric bending of the flexible substrate  110  can be alleviated. In addition, the buffer member ( 350  and  360 ) can reduce or effectively prevent peeling of the flexible substrate  110  and the back plate ( 310  and  320 ) from each other, and can also reduce or effectively prevent the penetration of moisture between the flexible substrate  110  and the back plate ( 310  and  320 ). Specifically, since the average thickness of the first buffer member  350  and the volume of the first buffer member  350  which is in contact with the flexible substrate  110  are greater than the average thickness of the second buffer member  360  and the volume of the second buffer member  360  which is in contact with the flexible substrate  110 , the first buffer member  350  can properly perform a buffer function while guiding the bending of the first flexible substrate  111 , which receives more stress than the second flexible substrate  112 . Accordingly, a reliable flexible display device can be provided by reducing or effectively preventing the disconnection of the signal wiring  150 . 
       FIGS. 5A and 5B  are cross-sectional views illustrating the formation of the buffer member ( 350  and  360 ) in the bending area BA. The step of forming the buffer member ( 350  and  360 ) in the bending area BA will hereinafter be described with reference to  FIGS. 5A and 5B . 
     Referring to  FIG. 5A , a heating block  400  may be disposed on a preliminary buffer member in the bending area BA. The heating block  400  may include first, second, and third heating areas  410 ,  420 , and  430 . The first heating area  410  may be an area on the left side of the heating block  400 . The second heating area  420  may be an area on the right side of the heating block  400 . The third heating area  430  may be an area between the first and second heating areas  410  and  420 . The preliminary buffer member may include first, second, and third buffer areas  341 ,  342  and  343 . The first buffer area  341  may be an area on the left side of the preliminary buffer member. The second buffer area  342  may be an area on the right side of the preliminary buffer member ( 341 ,  342  and  343 ). The third buffer area  343  may be an area between the first and second buffer areas  341  and  342 . The first, second, and third buffer areas  341 ,  342 , and  343  of the preliminary buffer member ( 341 ,  342  and  343 ) may have substantially the same thickness taken from the flexible substrate  110 , before being in contact with the heating block  400 . After contacting by the heating block  400 , the first, second, and third buffer areas  341 ,  342 , and  343  of the preliminary buffer member may have different thicknesses. 
     The first and second heating areas  410  and  420  may have a smaller thickness than the third heating area  430 . That is, the third heating area  430  may protrude toward the flexible display device  1  further than the first and second heating areas  410  and  420 . The third heating area  430  may have a flat surface area having a predetermined width. The average thickness of the first heating area  410  may be smaller than the average thickness of the second heating area  420 . The average thickness of the second heating area  420  may be greater than the average thickness of the first heating area  410 . 
     The first heating area  410  may contact with the first buffer area  341 . The second heating area  420  may contact with the second buffer area  342 . The third heating area  430  may contact with the third buffer area  343 . Specifically, after contacting the heating block  400 , the first buffer area  341  may become thicker than the second buffer area  342 . Also, the first buffer area  341  may have a greater width along the horizontal direction in  FIG. 5A  than the second buffer area  342 . 
     When placed in contact with the heating block  400 , which has a predetermined shape, the preliminary buffer member ( 341 ,  342  and  343 ) may be melted by the thermal energy of the heating block  400  and may be molded into a shape corresponding to the shape of the heating block  400 . 
     Referring to  FIG. 5B , which shows a process of placing the heating block  400  and the preliminary buffer member ( 341 ,  342  and  343 ) in contact with each other, parts the preliminary buffer member ( 341 ,  342  and  343 ) may be melted in the order they are placed in contact with the heating block  400 . The melted preliminary buffer member ( 341 ,  342  and  343 ) may have fluid mobility, and as a result, a fine flow may be generated in the preliminary buffer member ( 341 ,  342  and  343 ). The fluid mobility of the melted preliminary buffer member ( 341 ,  342  and  343 ) may vary depending on the pressing force of the heating block  400  against the preliminary buffer member ( 341 ,  342  and  343 ) and/or the temperature of the heating block  400 . Specifically, the greater the pressing force of the heating block  400 , the greater the fluid mobility of the melted preliminary buffer member ( 341 ,  342  and  343 ). On the contrary, the smaller the pressing force of the heating block  400 , the smaller the fluid mobility of the melted preliminary buffer member ( 341 ,  342  and  343 ). Also, the higher the temperature of the heating block  400 , the greater the fluid mobility of the melted preliminary buffer member ( 341 ,  342  and  343 ). On the contrary, the lower the temperature of the heating block  400 , the smaller the fluid mobility of the melted preliminary buffer member ( 341 ,  342  and  343 ). 
     In this manner, the melted preliminary buffer member ( 341 ,  342  and  343 ) may be formed into the shape of the buffer member ( 350  and  360 ) of  FIG. 2 . 
       FIG. 6  is a cross-sectional view of another exemplary embodiment of a flexible display device in a bent state according to the present disclosure. 
     Referring to  FIG. 6 , a flexible display device  2  differs from the flexible display device  1  of  FIGS. 2 through 5  in that a first buffer member  350   a  is disposed to extend over to a first back plate  310   a  when a flexible substrate  110   a  is bent. 
     Specifically, the first buffer member  350   a  may include or define a first extension member  351   a  extending over to the bottom of the first back plate  310   a  from an end of the bent flexible display device  2 . That is, the first extension member  351   a  may extend from the first buffer member  350   a  and may be disposed on the bottom surface of the first back plate  310   a , such as being disposed directly on the bottom surface of the first back plate  310   a . The first extension member  351   a  may have a thickness T 3  in a thickness direction of the flexible display device  2 . A maximum thickness T 1  of the first buffer member  350   a , which includes or extends to define the first extension member  351   a , may be greater than a maximum thickness T 2  of a second buffer member  360   a  and may also be greater than the thickness of the first back plate  310   a . In this case, the first extension member  351   a  may overlap with a part of a second back plate  320   a  in the thickness direction. When the flexible substrate  110   a  is bent, the first extension member  351   a  can minimize interference between the bottom surface of the first back plate  310   a  and the bottom surface of the second back plate  320   a , which is disposed at the bottom of the first back plate  310   a , and can mitigate shock transmitted between the first and second back plates  310   a  and  320   a.    
     In the exemplary embodiment of  FIG. 6 , like in the exemplary embodiment of  FIGS. 2 through 5 , an asymmetric buffer member ( 350   a  and  351   a ) may be used. That is, the stress applied to the first flexible substrate  111   a  may be greater when the flexible substrate  110   a  is bent asymmetrically than when the flexible substrate  110   a  is bent symmetrically. Also, the stress applied to the first flexible substrate  111   a  may be greater than the stress applied to the second flexible substrate  112   a.    
     Since the average bending angle, the average bending slope, and the average curvature of the first flexible substrate  111   a  are greater than the average bending angle, the average bending slope, and the average curvature of the second flexible substrate  112   a , the first flexible substrate  111   a  may be bent to a greater extent than the second flexible substrate  112   a . Accordingly, when the flexible substrate  110   a  is bent, the first flexible substrate  111   a  may receive more stress than the second flexible substrate  112   a . Also, since the first flexible substrate  111   a  has a relatively strong contact force with a first contact  111 P of a first inner surface  311   a  of the first back plate  310   a , the first flexible substrate  111   a  may receive more stress than the second flexible substrate  112   a . That is, the stress applied to the flexible substrate  110   a  may be greater in the first flexible substrate  111   a  than in the second flexible substrate  112   a . As a result, cracks may be generated in signal wiring on the first flexible substrate  111   a , and eventually, the signal wiring may be disconnected. 
     However, since the buffer member ( 350   a  and  360   a ) is applied asymmetrically to the flexible substrate  110   a , the stress caused by the asymmetric bending of the flexible substrate  110   a  can be alleviated. In addition, the buffer member ( 350   a  and  360   a ) can reduce or effectively prevent peeling of the flexible substrate  110   a  and a back plate ( 310   a  and  320   a ) from each other, and can also reduce or effectively prevent the penetration of moisture between the flexible substrate  110   a  and the back plate ( 310   a  and  320   a ). Specifically, since the average thickness and the volume of the first buffer member  350   a  are greater than the average thickness and the volume of the second buffer member  360   a , the first buffer member  350   a  can properly perform a buffer function while guiding the bending of the first flexible substrate  111   a , which receives more stress than the second flexible substrate  112   a . Accordingly, a reliable flexible display device can be provided by reducing or effectively preventing the disconnection of the signal wiring. 
       FIG. 7  is a cross-sectional view of still another exemplary embodiment of a flexible display device in a bent state according to the present disclosure, and  FIG. 8  is an enlarged cross-sectional view of the flexible display device of  FIG. 7 . 
     Referring to  FIGS. 7 and 8 , a flexible display device  3  differs from the flexible display device  1  of  FIGS. 2 through 5  in that it further includes a third buffer member  370   b  disposed on the top surface of a flexible substrate  110   b  in a bending area BA. 
     Specifically, the flexible display device  3  may include first and second buffer members  350   b  and  360   b  and may further include the third buffer member  370   b . The third buffer member  370   b  may be disposed on the flexible substrate  110   b  to face the first and second buffer members  350   b  and  360   b . The third buffer member  370   b  may be disposed on the top surface of the flexible substrate  110   b . Specifically, the third buffer member  370   b  may cover the top surfaces of first and second flexible substrates  111   b  and  112   b , but the present disclosure is not limited thereto. Alternatively, the third buffer member  370   b  may further extend to the outside of the first and second flexible substrates  111   b  and  112   b.    
     When the flexible substrate  110   b  is bent, a plane where strain caused by the bending of the flexible substrate  110   b  substantially becomes zero may be defined as a neutral plane CS. The neutral plane CS may be formed by the third buffer member  370   b , which faces the first and second buffer members  350   b  and  360   b  disposed on the top surface and the bottom surface, respectively, with respect to the flexible substrate  110   b.    
     When the third buffer member  370   b  is used, signal wiring disposed in a display area DA and a part of a non-display area NDA of the flexible substrate  110   b  can be essentially placed on the neutral plane CS by the first, second, and third buffer members  350   b ,  360   b , and  370   b.    
     The first, second, and third buffer members  350   b ,  360   b , and  370   b  may include or be formed of the same material, but the present disclosure is not limited thereto. 
     Alternatively, the first, second, and third buffer members  350   b ,  360   b , and  370   b  may include or be formed of different materials. In this case, the lengths, thicknesses, and elasticity moduli of the first, second, and third buffer members  350   b ,  360   b , and  370   b  may be adjusted so as to place the signal wiring on the neutral plane CS. In an exemplary embodiment, for example, the first and second buffer members  350   b  and  360   b  may be thicker than the third buffer member  370   b , but may have a lower elasticity modulus than the third buffer member  370   b  so that the signal wiring in the non-display area NDA of the flexible substrate  110   b  can be placed on the neutral plane CS by the first, second, and third buffer members  350   b ,  360   b , and  370   b . Thicknesses of the first, second, and third buffer members  350   b ,  360   b , and  370   b  may be taken in a substantially normal direction to the flexible substrate  110   b  at respective positions along the flexible substrate  110   b.    
     The length and the thickness of the third buffer member  370   b  may be designed in various manners in comparison to the lengths and the thicknesses of the first and second buffer members  350   b  and  360   b . In an exemplary embodiment, for example, the third buffer member  370   b  may be thinner than the first and second buffer members  350   b  and  360   b , and the maximum thickness of the third buffer member  370   b  may be smaller than the maximum thicknesses of the first and second buffer members  350   b  and  360   b . In this example, the third buffer member  370   b  may have a higher elasticity modulus than the first and second buffer members  350   b  and  360   b , but the present disclosure is not limited thereto. 
     In the exemplary embodiment of  FIGS. 7 and 8 , like in the exemplary embodiments of  FIGS. 2 through 5  and  FIG. 6 , an asymmetric buffer member ( 350   b  and  360   b ) may be used. That is, the stress applied to the first flexible substrate  111   b  may be greater when the flexible substrate  110   b  is bent asymmetrically than when the flexible substrate  110   b  is bent symmetrically. Also, the stress applied to the first flexible substrate  111   b  may be greater than the stress applied to the second flexible substrate  112   b.    
     Since the average bending angle, the average bending slope, and the average curvature of the first flexible substrate  111   b  are greater than the average bending angle, the average bending slope, and the average curvature of the second flexible substrate  112   b , the first flexible substrate  111   b  may be bent to a greater extent than the second flexible substrate  112   b . Accordingly, when the flexible substrate  110   b  is bent, the first flexible substrate  111   b  may receive more stress than the second flexible substrate  112   b . Also, since the first flexible substrate  111   b  has a relatively strong contact force with a first contact  111 P of a first inner surface  311   b  of the first back plate  310   b , the first flexible substrate  111   b  may receive more stress than the second flexible substrate  112   b . That is, the stress applied to the flexible substrate  110   b  may be greater in the first flexible substrate  111   b  than in the second flexible substrate  112   b . As a result, cracks may be generated in signal wiring on the first flexible substrate  111   b , and eventually, the signal wiring may be disconnected. 
     However, since the buffer member ( 350   b  and  360   b ) is applied asymmetrically to the flexible substrate  110   b , the stress caused by the asymmetric bending of the flexible substrate  110   b  can be alleviated. In addition, the buffer member ( 350   b  and  360   b ) can reduce or effectively prevent peeling of the flexible substrate  110   b  and a back plate ( 310   b  and  320   b ) from each other, and can also reduce or effectively prevent the penetration of moisture between the flexible substrate  110   b  and the back plate ( 310   b  and  320   b ). Specifically, since the average thickness and the volume of the first buffer member  350   b  are greater than the average thickness and the volume of the second buffer member  360   b , the first buffer member  350   b  can properly perform a buffer function while guiding the bending of the first flexible substrate  111   b , which receives more stress than the second flexible substrate  112   b . Accordingly, a reliable flexible display device can be provided by reducing or effectively preventing the disconnection of the signal wiring. 
       FIG. 9  is a cross-sectional view of yet another exemplary embodiment of a flexible display device in a bent state according to the present disclosure. 
     Referring to  FIG. 9 , a flexible display device  4  differs from the flexible display device  1  of  FIGS. 2 through 5  in that it includes a first back plate  310   c  including a first curved surface S 1  on a first inner side  311   c  thereof, but does not include a buffer member. 
     Specifically, the first back plate  310   c  may include the first curved surface S 1 , to which a first curvature (CV 1 ) is applied, on the first inner side  311   c , which protrudes toward a first flexible substrate  111   c .  FIG. 9  illustrates that the first inner side  311   c  of the first back plate  310   c  includes the first curved surface S 1 , to which the first curvature (CV 1 ) is applied, and no curvature is applied to the second inner side  321   c  of the second back plate  320   c , but the present disclosure is not limited thereto. Alternatively, the second back plate  320   c  may include a second curved surface, to which the second curvature (CV 2 ) is applied, on the second inner side  321   c , which protrudes toward a second flexible substrate  112   c.    
     The first curved surface S 1  of the first inner side  311   c  is illustrated as having a semicircular shape, but the present disclosure is not limited thereto. Alternatively, the first curved surface S 1  of the first inner side  311   c  may have a quadrant shape to which the first curvature (CV 1 ) is applied, and the second curved surface of the second inner side  321   c  may have a quadrant shape to which the second curvature (CV 2 ) is applied. 
     The first and second curvatures (CV 1  and CV 2 ) may be designed in consideration of various factors such as the type of the flexible substrate  110   c , the degree to which the flexible substrate  110   c  is bent, and the material, the thickness, and the elasticity modulus of a buffer member. Thus, the first and second curvatures (CV 1  and CV 2 ) may be the same or may be different. In the exemplary embodiment of  FIG. 9 , the first curvature (CV 1 ) may be greater than the second curvature (CV 2 ). 
     When the first and second back plates  310   c  and  320   c  having the first and second curvatures (CV 1  and CV 2 ), respectively, are used, stress can be considerably reduced when the flexible substrate  110   c  is placed in contact with the first and second back plates  310   c  and  320   c . Particularly, in an asymmetric bending structure, the stress of the first flexible substrate  111   c  can be reduced by making the first curvature (CV 1 ) greater than the second curvature (CV 2 ). 
     Specifically, by making the first curvature (CV 1 ) of the first back plate  310   c  greater than the second curvature (CV 2 ) of the second back plate  320   c , the stress of the first flexible substrate  111   c , which is more stressed than the second flexible substrate  112   c , can be reduced, and the disconnection of signal wiring can be reduced or effectively prevented. Accordingly, a reliable flexible display device can be provided. 
       FIG. 10  is a cross-sectional view of yet another exemplary embodiment of a flexible display device in a bent state according to the present disclosure. 
     Referring to  FIG. 10 , a flexible display device  5  differs from the flexible display device  1  of  FIGS. 2 through 5  in that it includes a first back plate  310   d  including a first curved surface S 1  on a first inner side  311   d  thereof. 
       FIG. 10  illustrates that the first inner side  311   d  of the first back plate  310   d  includes the first curved surface S 1 , to which the first curvature (CV 1 ) is applied, and no curvature is applied to the second inner side  321   d  of the second back plate  320   d , but the present disclosure is not limited thereto. Alternatively, the second back plate  320   d  may include a second curved surface, to which the second curvature (CV 2 ) is applied, on the second inner side  321   d , which protrudes toward a second flexible substrate  112   d . The first curved surface S 1  of the first back plate  310   d  and the second curved surface of the second back plate  320   d  are as described above with reference to  FIG. 9 , and thus, detailed descriptions thereof will be omitted. 
     The flexible display device  5  may further include a buffer member ( 350   d  and  360   d ) in contact with the first and second back plates  310   d  and  320   d , to which first and second curvatures (CV 1  and CV 2 ), respectively, are applied. 
     The thickness of a part of a first buffer member  350   d  that is in contact with the first curved surface S 1  of the first inner side  311   d  of the first back plate  310   d  may be greater than the thickness of a part of a second buffer member  360   d  that is in contact with the second inner side  321   d  of the second back plate  320   d.    
       FIG. 10  illustrates that the first inner side  311   d  of the first back plate  310   d  includes the first curved surface S 1 , to which the first curvature (CV 1 ) is applied, and no curvature is applied to the second inner side  321   d  of the second back plate  320   d , but the present disclosure is not limited thereto. Alternatively, the second back plate  320   d  may include a second curved surface, to which the second curvature (CV 2 ) is applied, on the second inner side  321   d , which protrudes toward a second flexible substrate  112   d.    
     The first and second buffer members  350   d  and  360   d  and the first and second back plates  310   d  and  320   d  may include the same material and may be formed in one integral body with one another. 
     As described above with reference to  FIG. 6 , the first buffer member  350   d  may include or define a first extension member ( 351   a  in  FIG. 6 ) extending over to the bottom of the first back plate  310   d . That is, the first extension member may extend from the first buffer member  350   d  and may be disposed on the bottom surface of the first back plate  310   d , such as being disposed directly on the bottom surface of the first back plate  310   d . The first extension member may have a thickness (T 3  in  FIG. 6 ) in a thickness direction of the flexible display device  5 . A maximum thickness (T 1  in  FIG. 6 ) of the first buffer member  350   d , which includes or defines the first extension member, may be greater than a maximum thickness (T 2  in  FIG. 6 ) of the second buffer member  360   d  and may also be greater than the thickness of the first back plate  310   d.    
     As described above with reference to  FIGS. 2 through 5 , the buffer member ( 350   d  and  360   d ) may be applied asymmetrically. 
     Due to the asymmetric bending of a flexible substrate  110   d , the stress applied to the first flexible substrate  111   d  may be greater when the flexible substrate  110   d  is bent asymmetrically than when the flexible substrate  110   d  is bent symmetrically. Also, the stress applied to the first flexible substrate  111   d  may be greater than the stress applied to the second flexible substrate  112   d.    
     Since the average bending angle, the average bending slope, and the average curvature of the first flexible substrate  111   d  are greater than the average bending angle, the average bending slope, and the average curvature of the second flexible substrate  112   d , the first flexible substrate  111   d  may be bent to a greater extent than the second flexible substrate  112   d . Accordingly, when the flexible substrate  110   d  is bent, the first flexible substrate  111   d  may receive more stress than the second flexible substrate  112   b . Also, since the first flexible substrate  111   d  has a relatively strong contact force with a first contact  111 P of the first inner surface  311   d  of the first back plate  310   d , the first flexible substrate  111   d  may receive more stress than the second flexible substrate  112   d . That is, the stress applied to the flexible substrate  110   d  may be greater in the first flexible substrate  111   d  than in the second flexible substrate  112   d . As a result, cracks may be generated in signal wiring on the first flexible substrate  111   d , and eventually, the signal wiring may be disconnected. 
     When the first and second back plates  310   d  and  320   d  having the first and second curvatures (CV 1  and CV 2 ), respectively, are used, stress can be considerably reduced when the flexible substrate  110   d  is placed in contact with the first and second back plates  310   d  and  320   d . Particularly, in an asymmetric bending structure, the stress of the first flexible substrate  111   d  can be reduced by making the first curvature (CV 1 ) greater than the second curvature (CV 2 ). 
       FIG. 11  is a cross-sectional view of yet another exemplary embodiment of a flexible display device in a bent state according to the present disclosure. 
     Referring to  FIG. 11 , a flexible display device  6  differs from the flexible display device  1  of  FIGS. 2 through 5  in that it includes a first back plate  310   e  including a first curved surface S 1  on a first inner side  311   e  thereof where a second back plate  320   e  includes a second inner side  321   e  thereof, and further includes a third buffer member  370   e  on the top surface of a flexible substrate  110   e . The third buffer member  370   e , which is disposed on the top surface of the flexible substrate  110   e  to face first and second buffer members  350   e  and  360   e , is as described above with reference to  FIGS. 7 and 9 , and thus, a detailed description thereof will be omitted. 
     As described above with reference to  FIGS. 7 and 8 , when the third buffer member  370   e  is used, signal wiring disposed in a display area DA and a part of a non-display area NDA of the flexible substrate  110   e  can be placed on a neutral plane CS by the first, second, and third buffer members  350   e ,  360   e , and  370   e.    
     In the exemplary embodiment of  FIG. 11 , like in the exemplary embodiments of  FIGS. 2 through 5 ,  FIG. 6 , and  FIGS. 7 and 8 , an asymmetric buffer member ( 350   e  and  360   e ) may be used. That is, the stress applied to a first flexible substrate  111   e  may be greater when the flexible substrate  110   e  is bent asymmetrically than when the flexible substrate  110   e  is bent symmetrically. Also, the stress applied to the first flexible substrate  111   e  may be greater than the stress applied to a second flexible substrate  112   e.    
     Since the average bending angle, the average bending slope, and the average curvature of the first flexible substrate  111   e  are greater than the average bending angle, the average bending slope, and the average curvature of the second flexible substrate  112   e , the first flexible substrate  111   e  may be bent to a greater extent than the second flexible substrate  112   e . Accordingly, when the flexible substrate  110   e  is bent, the first flexible substrate  111   e  may receive more stress than the second flexible substrate  112   e . Also, since the first flexible substrate  111   e  has a relatively strong contact force with a first contact  111 P of a first inner surface  311   e  of the first back plate  310   e , the first flexible substrate  111   e  may receive more stress than the second flexible substrate  112   e . That is, the stress applied to the flexible substrate  110   e  may be greater in the first flexible substrate  111   e  than in the second flexible substrate  112   e . As a result, cracks may be generated in signal wiring on the first flexible substrate  111   e , and eventually, the signal wiring may be disconnected. 
     However, since the buffer member ( 350   e  and  360   e ) is applied asymmetrically to the flexible substrate  110   e , the stress caused by the asymmetric bending of the flexible substrate  110   e  can be alleviated. In addition, the buffer member ( 350   e  and  360   e ) can reduce or effectively prevent peeling of the flexible substrate  110   e  and a back plate ( 310   e  and  320   e ) from each other, and can also reduce or effectively prevent the penetration of moisture between the flexible substrate  110   e  and the back plate ( 310   e  and  320   e ). Specifically, since the average thickness and the volume of the first buffer member  350   e  are greater than the average thickness and the volume of the second buffer member  360   e , the first buffer member  350   e  can properly perform a buffer function while guiding the bending of the first flexible substrate  111   e , which receives more stress than the second flexible substrate  112   e . Accordingly, a reliable flexible display device can be provided by reducing or effectively preventing the disconnection of the signal wiring  150   e.    
     Although the exemplary embodiments of the invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.