Patent Publication Number: US-11644868-B2

Title: Foldable display

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the priority benefit of the Korean Patent Application No. 10-2019-0107592 filed in the Republic of Korea on Aug. 30, 2019, the entire contents of which is hereby expressly incorporated by reference as if fully set forth herein into the present application. 
     BACKGROUND 
     Technical Field 
     The present disclosure relates to a foldable display apparatus. Especially, the present disclosure relates to a foldable display apparatus which may be folded in three steps to have a G-shape. 
     Discussion of the Related Art 
     As the information society develops, the demands for display apparatus are also developing in various manners. For example, various flat panel display apparatus such as a liquid crystal display (or LCD), and organic light emitting display (or OLED), and a mirco LED display have been developed. 
     In particular, a foldable display or a rollable display having a flexible display panel that can be bent or foldable is spotlinghted as the next generation technology due to the advantage of being able to provide a display area of large screen while maintaining the convenience of carrying. The flexible display may be applied to portable electric devices such as the mobile communication terminals, the electronic notebooks, the e-books, the portable multimedia players (PMPs), the untra mobile PCs (UMPCs), the mobile phones, the smart phones, and the tablet PCs (Personal Computers). Further, it can be applied to other various fields such as the television sets, the monitors and the instrument panels for automobiles. 
     A foldable display that folds to have a small area when carrying it, and unfold to have a wide display area when using it may be damaged easily than other display because it may be repeatedly folded and unfolded several times. Therefore, it is necessary to structurally impove by reducing the folding stress and/or bending stress caused by repeating the folding and unfolding operations so that no damage occurs even using for a long time. 
     SUMMARY 
     Accordingly, embodiments of the present disclosure are directed to a foldable display that substantially obviates one or more of the problems due to limitations and disadvantages of the related art. 
     The purpose of the present disclosure, as for solving the problems described above, is to provide a foldable display that can be folded to have a small size when not used, which is advantageous for carrying and storage, and can be expanded to provide a wide display area when used. The another purpose of the present disclosure is to provide a foldable display having a structure in which the damages caused by the bending and/or folding stress according to the repeated folding-unfolding operation is minimized. The still another purpose of the present disclosure is to provide a foldable display having a structure for relifing the folding and/or bending stresses at the beding portions having different bending radius as folded in a G-folding manner. 
     Additional features and aspects will be set forth in the description that follows, and in part will be apparent from the description, or may be learned by practice of the inventive concepts provided herein. Other features and aspects of the inventive concepts may be realized and attained by the structure particularly pointed out in the written description, or derivable therefrom, and the claims hereof as well as the appended drawings. 
     To achieve these and other aspects of the inventive concepts, as embodied and broadly described, a foldable display comprises: a display layer disposed on the top surface of a flexible substrate; an encapsulation layer covering the display layer; and a back plate disposed under the bottom surface of the flexible substrate, the back plate includes: a reference plane; a first folding plane disposed at one side of the reference plane; a second folding plane disposed at another side of the reference plane; a first bending part disposed between the reference plane and the first folding plane and having a first bending radius; and a second bending part disposed between the reference plane and the second folding plane and having a second bending radius, wherein the first folding plane is thinner than the reference plane. 
     In one example, the back plate further includes: an upper surface having flat condition; and a lower surface parallel with the upper surface and having level differences, wherein the flexible substrate is attached to the upper surface with an optical adhesive. 
     In one example, the first folding plane is thinner than the second folding plane. 
     In one example, the first folding plane is eqault to or thinner than the first thickness, and the second folding plane is equal to or thicker than the second thickness. 
     In one example, the first folding plane is thinner than the first thickness, the reference plane is thicker than the first thickness, the second thickness is thicker than the reference plane, and the second folding plane is thickner than the second thickness. 
     In one example, a thickness of the first folding plane is equal to the first thickness, a thickness of the reference plane is equal to the second thickness, and the second folding plane is thicker than the second thickness. 
     In one example, a thickness of the first folding plane is equal to the first thickness, and thicknesses of the reference plane and the second folding plane are equal to the second thickness. 
     In one example, a thickness of the the first folding plane is equal to the first thickness, a thickness of the second folding plane is equal to the second thickness, and the reference plane is thicker than the second thickness. 
     In one example, the first folding plane, the reference plane and the second folding plane are thicker than the first thickness and the second thickness. 
     In one example, the first folding plane is folded over the reference plane, and the second folding plane is folded over the first folding plane. 
     In another aspect, a foldable display comprises: a back plate including: an upper surface being coplanar; a lower surface parallel to the upper surface and having level differences; a first bending part having a first width on the lower surface; a second bending part being apart from the first bending part and having a second width larger than the first width on the lower surface; a reference plane between the first bending part and the second bending part; a first folding plane expanded from the first bending part to opposite side of the reference plane; and a second folding plane expanded from the second bending part to opposite side of the reference plane, wherein the reference plane is thicker than the first folding plane and the first bending part, and wherein the second folding plane is thicker than the first folding plane and the first bending part. 
     In one example, the foldable display further comprises: a flexible substrate disposed on the upper surface of the back plate; a display layer disposed on the flexible substrate; an encapsulation layer covering the display layer; and a cover film disposed on the encapsulation layer. 
     In one example, a thickness of the second bending part is equal to or thicker than the first bending part. 
     In one example, a thickness of the first folding plane is equal to or thinner than the first bending part, and a thickness of the second folding plane is equal to or thicker than the second bending part. 
     In one example, the first folding plane is thinner than the first bending part, the reference plane is thicker than the first bending part, the second bending part is thicker than the reference plane, and the second folding plane is thicker than the second bending part. 
     In one example, a thickness of the first folding plane is equal to the first bending part, a thickness of the reference plane is equal to the second bending part, and the second folding plane is thicker than the second bending part. 
     In one example, a thickness of the first folding plane is equal to the first bending part, and thicknesses of the reference plane and the second folding plane are equal to the second bending. 
     In one example, a thickness of the first folding plane is equal to the first bending part, a thickness of the second folding plane is equal to the second bending part, and the reference plane is thicker than the second bending part. 
     In one example, the first folding plane, the reference plane and the second folding plane are thicker than the first bending part and the second bending part. 
     In one example, the first bending part is bent and the first folding plane is folded over the reference plane, and the second bending part is bent and the second folding plane is folded on the first folding plane. 
     The foldable display apparatus according to the present disclosure has a first bending part and a second bending part, and implements the G-folding structure, thereby minimizing the size when folded and maximizing the size when unfolded. The foldable display according to the present disclosure may have a G-folding structure including at least two folding areas having different radii of curvatures, and may minimize the stress caused from the repeated folding and unfolding operations at then bending portions by providing different thickness to the bending portions having different radii of curvatures. In detail, by providing different thicknesses to the bending portions having different curvatures and the flat portions extended thereto, the neutral plane (or neutral surface) are positioned differently according to the curvature, thereby minimizing the damage by the repeated bending operations. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the inventive concepts as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are included to provide a further understanding of the disclosure and are incorporated in and constitute a part of this application, illustrate embodiments of the disclosure and together with the description serve to explain principles of the disclosure. In the drawings: 
         FIG.  1 A  is a perspective view illustrating a fully unfolded state of a foldable display according to the present disclosure. 
         FIG.  1 B  is a perspective view illustrating a folded state in a G-folding manner of a foldable display according to the present disclosure. 
         FIG.  2    is a side view illustrating a structure of a foldable display according to the present disclosure. 
         FIG.  3    is a cross-sectional view illustrating a structure of a back plate of a foldable display according to a first embodiment of the present disclosure. 
         FIG.  4    is a cross-sectional view illustrating a structure of a back plate of a foldable display according to a second embodiment of the present disclosure. 
         FIG.  5    is a cross-sectional view illustrating a structure of a back plate of a foldable display according to a third embodiment of the present disclosure. 
         FIG.  6    is a cross-sectional view illustrating a structure of a back plate of a foldable display according to a fourth embodiment of the present disclosure. 
         FIG.  7    is a cross-sectional view illustrating a structure of a back plate of a foldable display according to a fifth embodiment of the present disclosure. 
         FIG.  8    is a cross-sectional view illustrating a structure of a back plate of a foldable display according to a sixth embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to the exemplary embodiments of the present disclosure, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. In the specification, it should be noted that like reference numerals already used to denote like elements in other drawings are used for elements wherever possible. In the following description, when a function and a configuration known to those skilled in the art are irrelevant to the essential configuration of the present disclosure, their detailed descriptions will be omitted. The terms described in the specification should be understood as follows. Advantages and features of the present disclosure, and implementation methods thereof will be clarified through following embodiments described with reference to the accompanying drawings. The present disclosure may, however, be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the present disclosure to those skilled in the art. Further, the present disclosure is only defined by scopes of claims. 
     A shape, a size, a ratio, an angle, and a number disclosed in the drawings for describing embodiments of the present disclosure are merely an example, and thus, the present disclosure is not limited to the illustrated details. Like reference numerals refer to like elements throughout. In the following description, when the detailed description of the relevant known function or configuration is determined to unnecessarily obscure the important point of the present disclosure, the detailed description will be omitted. 
     In the case that “comprise,” “have,” and “include” described in the present specification are used, another part may also be present unless “only” is used. The terms in a singular form may include plural forms unless noted to the contrary. 
     In construing an element, the element is construed as including an error range although there is no explicit description. 
     In describing a positional relationship, for example, when the positional order is described as “on,” “above,” “below,” and “next,” the case of no contact there-between may be included, unless “just” or “direct” is used. If it is mentioned that a first element is positioned “on” a second element, it does not mean that the first element is essentially positioned above the second element in the figure. The upper part and the lower part of an object concerned may be changed depending on the orientation of the object. Consequently, the case in which a first element is positioned “on” a second element includes the case in which the first element is positioned “below” the second element as well as the case in which the first element is positioned “above” the second element in the figure or in an actual configuration. 
     In describing a temporal relationship, for example, when the temporal order is described as “after,” “subsequent,” “next,” and “before,” a case which is not continuous may be included, unless “just” or “direct” is used. 
     It will be understood that, although the terms “first,” “second,” etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element, without departing from the scope of the present disclosure. 
     “X axis direction”, “Y axis direction” and “Z axis direction” may not be interpreted only as a geometric relationship in which the relationship between each other is perpendicular, and may mean that the configuration of the present disclosure has a wider scope to the extent that it may functionally work. 
     In describing the elements of the present disclosure, terms such as the first, the second, A, B, (a) and (b) may be used. These terms are only to distinguish the elements from other elements, and the terns are not limited in nature, order, sequence or number of the elements. When an element is described as being “linked”, “coupled” or “connected” to another element that element may be directly connected to or connected to that other element, but indirectly unless otherwise specified. It is to be understood that other elements may be “interposed” between each element that may be connected to or coupled to. 
     It should be understood that the term “at least one” includes all combinations related with any one item. For example, “at least one among a first element, a second element and a third element” may include all combinations of two or more elements selected from the first, second and third elements as well as each element of the first, second and third elements. 
     Features of various embodiments of the present disclosure may be partially or overall coupled to or combined with each other, and may be variously inter-operated with each other and driven technically as those skilled in the art can sufficiently understand. The embodiments of the present disclosure may be carried out independently from each other, or may be carried out together in a co-dependent relationship. 
     The display panel according to the embodiments of the present disclosure may include a liquid crystal display panel, an organic light emitting diode display panel, and an electroluminescent display panel, but it is not limited thereto. In addition, the display panel applied to the display apparatus according to the embodiment of the present disclosure is not limited to the shape or size of the display panel. 
     Hereinafter, an example of a display apparatus according to the present disclosure will be described in detail with reference to the accompanying drawings. In designating reference numerals to elements of each drawing, the same components may have the same reference numerals as much as possible even though they are shown in different drawings. Scale of the elements shown in the accompanying drawings have a different scale from the actual for convenience of description, it is not limited to the scale shown in the drawings. 
     Hereinafter, referring to  FIGS.  1 A and  1 B , a structure of the foldable display according to the present disclosure will be explained.  FIG.  1 A  is a perspective view illustrating a fully unfolded state of a foldable display according to the present disclosure.  FIG.  1 B  is a perspective view illustrating a folded state in a G-folding manner of a foldable display according to the present disclosure. 
     Referring to  FIGS.  1 A and  1 B , a foldable display according to the present disclosure may be a display in which it is easy to carry, store and move in a small size by folding a specific portion of the display, and it provide a wide display area by unfolding in a large area when using the display. In one example, the foldable display may be formed such that the central portion of the display panel may be folded and unfolded in half. In another example, the foldable display may be implemented by a double folding type in which the ⅓ point line and the ⅔ point line may be folded in the longitudinal direction of the display panel. 
     In detail, the G-folding type foldable display may be implemented by that a first ⅓ part of the display panel may be bent upward with respect to the ⅓ point line of the display panel, and a second ⅓ part of the display panel may be bent upward with respect to the ⅔ point line of the display panel, so that the second ⅓ part may be folded over the first ⅓ part. For another example, even though it is not described in this disclosure, a Z-folding type foldable display may be implemented by that a first ⅓ part of the display panel may be bent upward with respect to the ⅓ point line of the display panel, and a second ⅓ part of the display panel may be bent downward with respect to the ⅔ point line of the display panel. 
     It will be described with reference to  FIG.  1 A . A G-folding type foldable display may comprise a flexible substrate FS, a displaying layer DSL, an encapsulation layer EN, a cover film CP and a back plate BP, in a cross-sectional view. Further, the G-folding type foldable display may comprise a reference plane  100 , a first folding plane  200 , a second folding plane  300 , a first bending part  210  and a second bending part  310 . 
     At first, the planar structure will be briefly described. The reference plane  100  may be a reference region defined in the central area of the display which is always in the same state whether folded or unfolded. The first folding plane  200  and the second folding plane  300  may be folded upward over the reference plane  200  under the folded state, and may be disposed at the left side and the right side of the reference plane  200 , respectively to form a flat plane with the reference plane  200  under the unfolded state. The first bending part  210  may be a region that is bent in a semicircle shape when the first folding plane  200  is folded. The second bending part  310  may be a region that is bent in a semicircle shape when the second folding plane  300  is folded. 
     Hereinafter, the structure of the foldable display according to the present disclosure will be described in detail in the cross-sectional view. The flexible substrate FS may be a base layer for forming the display elements thereon. For example, the flexible substrate may be formed by depositing an organic material having excellent flexibility such as polyimide on a rigid glass substrate (not shown). After completing the display elements on the flexible substrate FS, the glass substrate may be removed from the flexible substrate FS. 
     The flexible substrate FS may be made of a plastic material such as a polyimide and may be formed in a thin film type, so that the display elements formed thereon may be damaged by the moisture and/or oxygen intruding through the flexible substrate FS. In the present disclosure, attaching a back plate BP made of a material having a property of preventing moisture and oxygen, i.e., including a styrene-isobutylene copolymer, on the lower surface of the flexible substrate FS, the damage to the display element may be prevented. The flexible display may be completed by attaching the back plate BP having a flexible property while maintaining a certain degree of strength on the lower surface of the flexible substrate FS where a glass substrate was disposed before and removed later. It is preferable that the back plate BP may be made of a flexible material having the elasticity to maintain the flatness of the display. Further, it is preferable that the back plate BP may be a film type having a constant thickness. 
     The foldable display according to the present disclosure may be completed by thinning some portions where the display panel is bent, after forming a flexible display. For example, some parts of the back frame BP corresponding to the folding part of the foldable display panel may be removed to implement the folding operation. The thickness of the folding frame FF may be thinned by removing some portions of the rear surface at the first bending part  210  and the second bending part  310 . In one example, when the bending curvature of the first bending part  210  is smaller than the bending curvature of the second bending part  310 , it is preferable that the thickness of the first bending part  210  of the display may be thinner than the thickness of the second bending part  310  of the display. 
     The display layer DSL may include a plurality of display elements formed on the upper surface of the flexible substrate FS. For example, after forming the driving elements includes thin film transistors, the organic light emitting elements including organic light emitting layer may be stacked thereon. 
     On the display layer DSL an encapsulation layer EN may be formed for protecting the organic light emitting elements. The encapsulation layer EN may have a three layered structure in which an inorganic layer, an organic layer and an inorganic layer are sequentially stacked. 
     The cover film CP may be an outermost layer stacked on the encapsulation layer EN. The cover film CP may be a protection layer for protecting the display from the environment. The cover film CP may further include a polarization element for eliminating the reflection of external lights. 
     The structure of the flexible display for implementing the foldable display will be described in detail. The flexible display may include a display area AA and a non-display area IA. The display area AA may be provided at the middle portions of the display and defined as an area for representing the video information. The non-display area IA surrounding the display area AA may be defined as an area not presenting video information or an circumference area. 
     The display area AA may include a plurality of pixels P arrayed in a matrix manner. The pixel P may be defined by a scan line SL, a data line DL and a pixel driving power line PL. 
     The scan lines SL may be extended along a first direction (X-axis) and arrayed with a predetermined distance along a second direction (Y-axis). The display area AA of the flexible substrate FS may include a plurality of scan lines SL which are parallel with the first direction (X-axis) and arrayed along the second direction (Y-axis) with certain distance. Here, the first direction (X-axis) may be defined the lateral (or horizontal) direction on the flexible substrate FS, and the second direction (Y-axis) may be defined the portrait (or vertical) direction on the flexible substrate FS. However, it is not limited thereto, and they are defined vice versa. 
     The data lines DL may be extended along the second direction (Y-axis) and arrayed with a predetermined distance along the first direction (X-axis). The display area AA of the flexible substrate FS may include a plurality of data lines DL which are parallel with the second direction (Y-axis) and arrayed along the first direction (X-axis) with certain distance. 
     The pixel driving power line PL may be disposed on the flexible substrate FS as being parallel with the data line DL. The display area AA of the flexible substrate FS may include a plurality of pixel driving power lines PL parallel with the data lines DL. In some cases, the pixel driving power line PL may be disposed as being parallel with the scan line SL. The pixel P may include a pixel circuit PC electrically connected to the scan line SL, the data line DL and the pixel driving power line PL, and an emission element ED electrically connected to the pixel circuit PC. 
     The pixel circuit PC may control the electric current Ted flowing to the emission element ED from the pixel driving power line PL according to the data voltage supplied from the data line DL by responding to the scan signal supplied from the scan line SL. 
     The emission element ED may be operated by the data current Ted supplied from the pixel circuit PC to emit the lights having a brightness (or luminescence) corresponding to the data current Ted. In this case, the data current Ted may flow from the pixel driving power line PL to the common power line CPL via the driving thin film transistor and the emission element ED. 
     The common power line CPL may be disposed at the non-display area IA of the flexible substrate FS, and electrically connected to the common electrode disposed on the display area AA. Hereinafter, the cross-sectional structure of the display will be explained. 
     A buffer layer (not shown) may be further formed on the upper surface of the flexible substrate FS. The buffer layer may be formed on the upper surface of the flexible substrate FS for preventing the moisture from intruding into the display layer DSL through the flexible substrate FS vulnerable to moisture permeation. In one example, the buffer layer may include a plurality of inorganic layer alternately stacked. For example, the buffer layer may be have a multi-layered structure in which at least one inorganic layer including silicon oxide (SiOx) layer, silicon nitride (SiNx) layer and silicon oxynitride (SiON) layer are stacked alternately stacked. In some cases, the buffer layer may be not included. 
     The display layer DSL may include a thin film transistor layer, a planarization layer PLN, a bank BN and an emission element ED. The thin film transistor layer may be provided in the pixel P defined within the display area AA of the flexible substrate FS. 
     In one example, the thin film transistor layer may include a thin film transistor T, a gate insulating layer GI and an intermediate insulating layer ILD. Here, the thin film transistor T shown in  FIG.  1   a    may be a driving thin film transistor electrically connected to the emission element ED. 
     The thin film transistor T may include a semiconductor layer A formed on the flexible substrate FS or buffer layer, a gate electrode G, a source electrode S and a drain electrode D. The thin film transistor T shown in  FIG.  1   a    illustrates a top gate structure in which the gate electrode G is disposed over the semiconductor layer A, but it is not limited thereto. In another example, the thin film transistor T may have a bottom gate structure in which the gate electrode G is disposed under the semiconductor layer A, or a double gate structure in which two gate electrodes G are disposed under and over the semiconductor layer A, respectively. 
     The semiconductor layer A may be formed on the flexible substrate FS or the buffer layer. The semiconductor layer A may include one of a silicon-based semiconductor material, an oxide-based semiconductor material or an organic-based semiconductor material, and have a single-layered structure of a multi-layered structure. A light shielding layer (not shown) may be further included between the buffer layer and the semiconductor layer A for protecting the external light from intruding into the semiconductor layer A. 
     The gate insulating layer GI may be formed over the whole surface of the flexible substrate FS as covering the semiconductor layer A. The gate insulating layer GI may include an inorganic layer such as silicon oxide (SiOx) layer, silicon nitride (SiNx) layer or multi-layer of them. 
     The gate electrode G may be formed on the gate insulating layer GI as overlapped with the semiconductor layer A. The gate electrode G may be formed with the scan line SL. In one example, the gate electrode G may be formed of any one metal material including molybdenium (Mo), aluminum (Al), chromium (Cr), gold (Au), titanium (Ti), nickel (Ni), neodymium (Nd) and copper (Cu) or alloy of them in a single layer or in a multiple layer. 
     The intermediate insulating layer ILD may be formed on the whole surface of the flexible substrate FS as covering the gate electrode G and the gate insulating layer GI. The intermediate insulating layer ILD may provide a flat surface over the gate electrode G and the gate insulating layer GI. 
     The source electrode S and the drain electrode D may be formed on the intermediate insulating layer ILD as overlapped with the semiconductor layer A and at both sides of the gate electrode D, respectively. The source electrode S and the drain electrode D may be formed with the data line DL, the pixel driving power line PL and the common power line CPL. That is, the source electrode S, the drain electrode D, the data line DL, the pixel driving power line CPL and the common power line CPL may be formed at the same time by the patterning the source-drain material deposited on the intermediate insulating layer ILD. 
     The source electrode S and the drain electrode D may contact the semiconductor layer A via a contact hole penetrating the intermediate insulating layer ILD and the gate insulating layer GI. The source electrode S and the drain electrode D may be formed of any one metal material including molybdenium (Mo), aluminum (Al), chromium (Cr), gold (Au), titanium (Ti), nickel (Ni), neodymium (Nd) and copper (Cu) or alloy of them in a single layer or in a multiple layer. Here, the source electrode S of the thin film transistor T shown in  FIG.  1    may be electrically connected to the pixel driving power line PL. 
     The thin film transistor T provided in the pixel P of the flexible substrate FS may configure the pixel circuit PC. The planarization layer PLN may be formed over the whole surface of the flexible substrate FS as covering the thin film transistor layer including the pixel circuit PC. The planarization layer PLN may provide a flat surface condition over the thin film transistor layer. In one example, the planarization layer PLN may be formed of an organic layer such as acryl resin, epoxy resin, phenolic resin, polyamide resin or polyimide resion. In another example, the planarization layer PLN may include a pixel contact hole PH for exposing some portions of the drain electrode D of the driving thin film transistor disposed in the pixel P. 
     The bank BN may be disposed on the planarization layer PLN and define an aperture area (or emission area) within the pixel P disposed in the display area AA. The bank BN may be expressed as a ‘pixel defining layer’. 
     The emission element ED may include a pixel driving electrode AE, an emission layer EL and common electrode CE. The pixel driving electrode AE may be formed on the planarization layer PLN and electrically connected to the drain electrode D of the driving thin film transistor via the pixel contact hole PH provided at the planarization layer PLN. In this case, the circumference edge area excepting most of middle portions of the pixel driving electrode AE defined as the aperture area of the pixel P may be covered by the bank BN. Covering the circumferences of the pixel driving electrode AE, the bank BN may defined the aperture area of the pixel P. 
     The pixel driving electrode AE according on example may include a metal material having high reflectiveness property. For example, the pixel driving electrode AE may be formed as a multiple layered structure such as titanium-aluminum stack (Ti/Al/Ti) structure, aluminum-ITO stack (ITO/Al/ITO) structure, APC alloy (Ag/Pd/Cu) structure and APC-ITO stack (ITO/APC/ITO) structure, or a single layered structure including at least one material including silver (Ag), aluminum (Al), molybdenium (Mo), gold (Au), magnesium (Mg), calcium (Ca) and barium (Ba) or alloy including at least two of them. 
     The emission layer EL may be formed at the whole surface of the display area AA of the flexible substrate FS as covering the pixel driving electrode AE and the bank Bn. In one example, the emission layer EL may include at least two emission portions vertically stacked for emitting the white light. For example, the emission layer EL may include a first emission portion and a second emission portion for emitting the white light by mixing a first color light and a second color light. Here, the first emission portion emitting the first color light may include any one of blue emission layer, green emission layer, red emission layer, yellow emission layer and yellow-green emission layer. The second emission portion may include an emission layer emitting the second color light having a complementary color relationship of the first light among the blue emission layer, green emission layer, red emission layer, yellow emission layer and yellow-green emission layer. 
     In another example, the emission layer EL may include any one of blue emission layer, green emission layer and red emission layer for emitting color light corresponding to the allocated at each pixel P. For example, the emission layer EL may include any one of organic light emission layer, inorganic light emission layer and quantum dot emission layer, or a multiple structure of organic light emission layer and quantum dot emission layer or mixing structure of them. 
     The common electrode CE may be formed as connecting the emission layer EL electrically. The common electrode CE may be formed on the whole surface of the display area AA of the flexible substrate FS as commonly connected to the emission layer EL provided at each pixel P. 
     In one example, the common electrode CE may include a transparent conductive material passing though the light or semi-transparent conductive material. In the case that the common electrode CE is made of semi-transparent conductive material, the light emission efficiency of light from the emission element may be improved by applying a micro cavity structure. In one example, the semi-transparent conductive material may include magnesium (Mg), silver (Ag) or alloy of magnesium and silver. In addition, a capping layer may be formed on the common electrode CE for enhancing the emission efficiency of the light by controlling the refractive index of the light emitted from the emission element. 
     A plurality of spacers (not shown) may be arrayed in the non-emission area where the emission element ED is not disposed in the display area AA. The spacer may be an element for preventing direct contact between the screen mask and the substrate in the process of depositing the emission layer EL. The spacer may be disposed on the bank BN so that the emission layer EL and the common electrode CE may be deposited as covering the spacers disposed within the display area AA. In some cases, the emission layer EL and/or the common electrode CE may not cover the spacers. As the spacers are disposed at some portions of the bank BN within the display area AA, even though the common electrode CE does not cover the spacers, the common electrode CE may be connected over the whole of the display area AA. 
     The encapsulation layer EN may be formed as covering the upper surface and the side surfaces of the display layer DSL. The encapsulation layer EN may play role of preventing the oxygen or moisture from intruding into the emission element ED. 
     In one example, the encapsulation layer EN may include a first inorganic layer PAS 1 , an organic layer PCL on the first inorganic layer PAS 1  and a second inorganic layer PAS 2  on the organic layer PCL. The first inorganic layer PAS 1  and the second inorganic layer PAS 2  may play role of preventing the moisture or oxygen from intruding thereinto. In one example, first inorganic layer PAS 1  and the second inorganic layer PAS 2  may include an inorganic material such as silicon nitride, aluminum nitride, zirconium nitride, titanium nitride, hafnium nitride, tantalum nitride, silicon oxide, aluminum oxide or titanium oxide. The first inorganic layer PAS 1  and the second inorganic layer PAS 2  may be formed by a chemical vapor deposition process or atomic layer deposition process. 
     The organic layer PCL may be surrounded by the first inorganic layer PAS 1  and the second inorganic layer PAS 2 . The organic layer PCL may be formed as having relatively thicker thickness than the first inorganic layer PAS 1  and the second inorganic layer PAS 2  in order to adsorb and/or block particles that may occur during the manufacturing process. The organic layer PCL may be formed of an organic material such as silicon oxycarbon (SiOCz) acrylic or epoxy-based resin. The organic layer PCL may be formed by the ink jet coating process or the slit coating process. 
     The cover film CP may be disposed on the encapsulation layer EN over the flexible substrate FS. The cover film CP may be a topmost film for protecting whole elements formed there-under. Therefore, the cover film CP may have a size suitable for covering the display area AA and the non-display area IA. 
     The cover film CP may include an optical film (not shown). The optical film may be an element for preventing the color deterioration when the external light is reflected from the metal layer disposed in the display area AA. Therefore, it is preferable that the optical film may cover the whole area of the display area AA, at least. 
     Hereinafter, referring to  FIG.  2   , the folding structure of the foldable display according to the present disclosure will be explained.  FIG.  2    is a side view illustrating a structure of a foldable display according to the present disclosure. In the following description, the folding function will be mainly described. In  FIG.  2   , the upper drawing is a side view illustrating the fully folded state of the foldable display according to the present disclosure, and the lower drawing is a side view illustrating the G-folding state. 
     The foldable display according to the present disclosure may have a rectangular plate shape. The foldable display may have a G-folding structure. For example, the foldable display according to the present disclosure may include a reference plane  100 , a first folding plane  200 , a second folding plane, a first bending part  210  and a second bending part  310 . 
     For example, the first bending part  210  may be defined at  ⅓  portion of the lateral direction on the display. Further, the second bending part  310  may be defined at ⅔ portion of the lateral direction on the display. As shown in  FIG.  1 A , the first bending part  210  may be a region defined to be extended to both sides based on a first bending axis FX 1 . The second bending part  310  may be a region defined to be extended to both sides based on a second bending axis FX 2 . 
     The region between the first bending part  210  and the second bending part  310  may be defined as the reference plane  100 . The outer region of the first bending part  210 , that is an outer region disposed at the opposite side to the reference plane  100  based on the first bending part  210 , may be defined as the first folding plane  200 . In addition, the outer region of the second bending part  210 , that is an outer region disposed at the opposite side to the reference plane  100  based on the first bending part  210 , may be defined as the second folding plane  300 . 
     The reference plane  100  may be a base region that does not relatively fold in the foldable display. The folding and unfolding operations may be relative, so it may not be defined that one region is absolutely fixed while only the other region is folded and unfolded. In describing this relative motion, the present disclosure defines a reference plane at the central region of the display. 
     The foldable display according to the present disclosure may be switched between the folded state and the unfolded state as the first bending part  210  and the second bending part  310  perform the folding and unfolding operations. In one example, as the first bending part  210  is folded, the first folding plane  200  may be in folded state over the reference plane  100 . As the first bending part  210  is unfolded, the first folding plane  200  may be in unfolded state while forming the same plane with the reference plane  100 . 
     With the same manner, as the second bending part  310  is folded, the second folding plane  300  may be in folded state over the reference plane  100 . As the second bending part  310  is unfolded, the second folding plane  300  may be in unfolded state while forming the same plane with the reference plane  100 . 
     Here, when both the first folding plane  200  and the second folding plane  300  are folded, the first folding plane  200  may be firstly folded over the reference plane  100 , and then the second folding plane  300  may be folded over the back surface of the first folding plane  200 . On the other hand, the second folding plane  300  may be firstly folded over the reference plane  100 , and then the first folding plane  200  may be folded over the back surface of the second folding plane  300 . The bending curvature (or radius) of the bending part varies depending on which folding plane is folded first. Therefore, in order to minimize damage as the folding and unfolding operations of the foldable display are repeated, it is preferable to set the folding plane to be folded first. In this disclosure, the first folding plane  200  is defined to be folded first, as shown in  FIG.  2   . 
     The first bending part  210  may include a first width W 1  and a first thickness T 1 . Here, the first width W 1  may be defined by the first bending radius R 1  corresponding to the curvature of the first bending part  210 . For example, when the first bending radius R 1  is 4 mm, the first width W 1  may be corresponding to half of the circumference length of the 4 mm radius circle. That is, the first width W 1  may be 4 πmm. The first thickness T 1  may be defined as a critical thickness that may absorb the folding stress or the bending stress when the back plate BP is folded with the first bending radius R 1 . 
     The second bending part  310  may have a second width W 1  and a second thickness T 2 . Here, the second width W 2  may be defined by the second bending radius R 2  corresponding to the bending curvature of the second bending part  310 . For example, when the second bending radius R 2  is 6 mm, the second width W 2  may be corresponding to half of the circumference length of the 6 mm radius circle. That is, the second width W 2  may be 6 πmm. The second thickness T 2  may be defined as a critical thickness that may absorb the folding stress or the bending stress when the back plate BP is folded with the second bending radius R 2 . 
     For example of G-folding structure, it is preferable that the first bending radius R 1  may be smaller than the second bending radius R 2 . In other word, it is preferable that the first width W 1  of the first bending part  210  may be smaller than the second width W 2  of the second bending part  310 . Further, it is preferable that the first thickness T 1  may be thinner than the second thickness T 2 . In some cases, the first thickness T 1  may be same with the second thickness T 2 . However, it is preferable that the first thickness T 1  is not thicker than the second thickness T 2 . 
     In the G-folding structure according to the present disclosure, the first folding plane  200  is folded at first and then the second folding plane  300  is folded. The second bending radius R 2  of the second bending part  310  may be larger than the first bending radius R 1 , and the the second bending part  310  is folded over the first folding plane  200 . Therefore, it is preferable that the first folding plane  200  may be formed as having thin thickness as possible. For example, the first folding plane  200  may have thinner thickness than thicknesses of the reference plane  100  and the second folding plane  300 . 
     Considering the manufacturing process, the back plate BP may be supplied in a state in which a film material is wound in a roll form. After forming a back plate BP by cutting as needed length from the film roll, the back plate BP may be attached to the rear surface of the flexible substrate FS separated from the glass substrate. 
     Some portions of the thickness of the back plate BP may be removed from the rear surface to form a desired thickness in the desired region. For example, at the first bending part  210 , some thickness portions of the folding frame FF may be removed to have the first thickness T 1  within the first width W 1 . At the second bending part  310 , some thickness portions of the folding frame FF may be removed to have the second thickness T 2  within the second width W 2 . In addition, the first folding plane  200  may be thinned to have thinner thickness than the reference plane  100  and the second folding plane  300 . For example, the first folding plane  200  may be thinned at the processing for removing some thickness portions of the first bending part  210  to have the same thickness with the first bending part  210 . 
     The description so far has focused on the case where the display is folded or unfolded. Therefore, the explanation is described in the view of the whole thickness of the display. It is one of the main features of the present disclosure that the thickness of the entire display has a different thickness for each region. In order to have different thickness in the display according to the present disclosure, the back plate BP may be formed to have different thickness for each region. Therefore, hereinafter, the explanation will be described based on the back plate BP. For example, the first thickness T 1  and the second thickness T 2  may be the thicknesses of the display in the above description. Hereinafter, the first thickness T 1  and the second thickness T 2  may be the thickness of the back plate BP. 
     The back plate BP according to the present disclosure may include an upper surface UPS and a lower surface DNS. The upper surface UPS may be a flat surface without any level difference, so all regions are coplanar. On the upper surface UPS, the flexible substrate FS may be disposed. For example, using an optical adhesive (not shown), the back plate BP may be attached on the bottom surface of the flexible substrate FS which having the display layer DSL on the top surface. 
     The lower surface DNS may be a stepped surface having level difference, so overall area of the lower surface is not coplanar. That is, some portions may have a different thickness from other portions. The first bending part  210  and the second bending part  310  may be thinner than the reference plane  100 . Further, the first folding plane  200  may have thinner thickness than the reference plane  100 . 
     Forming the first bending part  210  and the second bending part  310  to have different thicknesses, the neutral plane of the first bending part  210  and the neutral plane the second bending part  310  may be set at different position each other. As the result, the bending stress or the folding stress at each bending part may be minimized, the reliability of folding state may be enhanced. 
     Hereinafter, various embodiments of forming the back plate BP to have different thicknesses will be described. In the following embodiments, the explanation will be described in the view of the thickness variations of the back plate BP which is a core configuration for implementing the G-folding structure. 
     First Embodiment 
     Hereinafter, referring to  FIG.  3   , a structure of a back plate BP according to the first embodiment of the present disclosure will be explained.  FIG.  3    is a cross-sectional view illustrating a back plate structure of a foldable display according to the first embodiment of the present disclosure. The back plate BP according to the first embodiment of the present disclosure may include a reference frame  100 , a first folding plane  200 , a second folding plane  300 , a first bending part  210  and a second bending part  310 . The first bending part  210  may have a first width W 1  and a first thickness T 1 . The second bending part  310  may have a second width W 2  and a second thickness T 2 . The second thickness T 2  may thicker than the first thickness T 1 . 
     The first folding plane  200  may have a thickness same with the first thickness T 1 . The reference plane  100  may have a thickness thicker than the first thickness T 1  and thinner than the second thickness T 2 . The second folding plane  300  may have a thickness same with the second thickness T 2 . 
     Second Embodiment 
     Hereinafter, referring to  FIG.  4   , a structure of a back plate BP according to the second embodiment of the present disclosure will be explained.  FIG.  4    is a cross-sectional view illustrating a back plate structure of a foldable display according to the second embodiment of the present disclosure. The back plate BP according to the second embodiment of the present disclosure may include a reference frame  100 , a first folding plane  200 , a second folding plane  300 , a first bending part  210  and a second bending part  310 . The first bending part  210  may have a first width W 1  and a first thickness T 1 . The second bending part  310  may have a second width W 2  and a second thickness T 2 . The second thickness T 2  may be thicker than the first thickness T 1 . 
     The first folding plane  200  may be thinner than the first thickness T 1 . The reference plane  100  may be thicker than the first thickness T 1  and thinner than the second thickness T 2 . The second folding plane  300  may be thicker than the second thickness T 2 . 
     Third Embodiment 
     Hereinafter, referring to  FIG.  5   , a structure of a back plate BP according to the third embodiment of the present disclosure will be explained.  FIG.  5    is a cross-sectional view illustrating a back plate structure of a foldable display according to the third embodiment of the present disclosure. The back plate BP according to the third embodiment of the present disclosure may include a reference frame  100 , a first folding plane  200 , a second folding plane  300 , a first bending part  210  and a second bending part  310 . The first bending part  210  may have a first width W 1  and a first thickness T 1 . The second bending part  310  may have a second width W 2  and a second thickness T 2 . The second thickness T 2  may be thicker than the first thickness T 1 . 
     The first folding plane  200  may have a thickness same with the first thickness T 1 . The reference plane  100  may have a thickness same with the second thickness T 2 . The second folding plane  300  may be thicker than the second thickness T 2 . 
     Fourth Embodiment 
     Hereinafter, referring to  FIG.  6   , a structure of a back plate BP according to the fourth embodiment of the present disclosure will be explained.  FIG.  5    is a cross-sectional view illustrating a back plate structure of a foldable display according to the fourth embodiment of the present disclosure. The back plate BP according to the fourth embodiment of the present disclosure may include a reference frame  100 , a first folding plane  200 , a second folding plane  300 , a first bending part  210  and a second bending part  310 . The first bending part  210  may have a first width W 1  and a first thickness T 1 . The second bending part  310  may have a second width W 2  and a second thickness T 2 . The second thickness T 2  may be thicker than the first thickness T 1 . 
     The first folding plane  200  may have a thickness same with the first thickness T 1 . The reference plane  100  may have a thickness same with the second thickness T 2 . The second folding plane  300  may be thicker than the second thickness T 2 . That is, the reference plane  100 , the second bending part  310  and the second folding plane  300  may have the same thickness as the second thickness T 2 . 
     Fifth Embodiment 
     Hereinafter, referring to  FIG.  7   , a structure of a back plate BP according to the fifth embodiment of the present disclosure will be explained.  FIG.  7    is a cross-sectional view illustrating a back plate structure of a foldable display according to the fifth embodiment of the present disclosure. The back plate BP according to the fifth embodiment of the present disclosure may include a reference frame  100 , a first folding plane  200 , a second folding plane  300 , a first bending part  210  and a second bending part  310 . The first bending part  210  may have a first width W 1  and a first thickness T 1 . The second bending part  310  may have a second width W 2  and a second thickness T 2 . The second thickness T 2  may be thicker than the first thickness T 1 . 
     The first folding plane  200  may have a thickness same with the first thickness T 1 . The reference plane  100  may be thicker than the second thickness T 2 . The second folding plane  300  may have a thickness same with the second thickness T 2 . That is, the reference plane  100  may have the thickest thickness. Further, the first folding plane  200  and the first bending part  210  may have the first thickness T 1 , the thinnest thickness. The second folding plane  300  and the second bending part  310  may have the second thickness T 2  which is thicker than the first thickness T 1  and thinner than the reference plane  100 . 
     Sixth Embodiment 
     Hereinafter, referring to  FIG.  8   , a structure of a back plate BP according to the sixth embodiment of the present disclosure will be explained.  FIG.  8    is a cross-sectional view illustrating a back plate structure of a foldable display according to the sixth embodiment of the present disclosure. The back plate BP according to the sixth embodiment of the present disclosure may include a reference frame  100 , a first folding plane  200 , a second folding plane  300 , a first bending part  210  and a second bending part  310 . The first bending part  210  may have a first width W 1  and a first thickness T 1 . The second bending part  310  may have a second width W 2  and a second thickness T 2 . The second thickness T 2  may be equal to or thicker than the first thickness T 1 . 
     The first folding plane  200  may be thicker than the first thickness T 1 . The reference plane  100  may be thicker than the first thickness T 1  and the second thickness T 2 . The second folding plane  300  may be thickener than the second thickness T 2 . The reference plane  100  may be thicker than the first folding plane  200  and the second folding plane  300 . Further, the second folding plane  300  may be thicker than the first folding plane  200 . 
     The foldable display according to the various embodiments of the present disclosure may comprise the first bending part and the second bending part having different curvature each other. By setting the different thicknesses to the parts of the back plate BP having different curvatures, the position of the neutral plane may be defined depending on the different curvatures. Accordingly, the bending stress or folding stress at each bending part may be minimized, and the reliability of the folding and unfolding operations may be improved. In order to change the thickness of the back plate BP at each part, the thickness may be easily adjusted by using a cutting/carving method in which the back plate BP is cut or carved. In addition, as the emitted light from the emission element may be radiated to the opposite surface to the back plate BP, the thickness variations of the back plate BP may not affect to the light efficiency at all. 
     Features, structures, effects and so on described in the above described examples of the present disclosure are included in at least one example of the present disclosure, and are not necessarily limited to only one example. Furthermore, features, structures, effects and so on exemplified in at least one example of the present disclosure may be implemented by combining or modifying other examples by a person having ordinary skilled in this field. Therefore, contents related to such combinations and modifications should be interpreted as being included in the scope of the present application. 
     It will be apparent to those skilled in the art that various modifications and variations can be made in the present disclosure without departing from the spirit or scope of the disclosures. Thus, it is intended that the present disclosure covers the modifications and variations of this disclosure provided they come within the scope of the appended claims and their equivalents. These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.